METHANOL PRODUCTION BY DIRECT OXIDATION
OF METHANE IN A PLASMA REACTOR
by
RICK MOODAY, B.S.
A DISSERTATION
IN
CHEMICAL ENGINEERING
Submitted to the Graduate Faculty of Texas Tech University in
Partial Fulfillment of the Requirements for
the Degree of
DOCTOR OF PHILOSOPHY
Approved
Accepted
August, 1998
ACKNOWLEDGEMENTS
I would like to thank Phillips Petroleum Company for their constant support
during the course of my work. This research project was made possible by their generous
financial assistance. I would like to extend my gratitude to Dr. Uzi Mann, for his
guidance, creative ideas, encouragement, and unlimited patience. Dr. Mann ensured that
the project never strayed off the correct path and often contributed well beyond the call of
duty as my committee chairman. Sincere thanks are also extended to Dr. Richard W.
Tock, Dr. Dominick J. Casadonte, Jr., and Dr. Lynn L. Hatfield, for their active
participation in the project and technical guidance while serving on my committee. Dr.
Raghu S. Narayan was extremely kind and made me feel more than welcome in the
Chemical Engineering Department at Texas Tech.
I would like to extend special thanks to Dr. Robert M. Bethea. My experience
working for Dr. Bethea in the Unit Operations Lab re-introduced me to education and
chemical engineering after being away for a time. His example and guidance have been
invaluable and will continue to serve me well throughout my career.
Many people contributed to making this time a success for me. Dr. James Riggs
is a highly respected chemical engineer and faculty member at Texas Tech, but I
acknowledge him here for his ability to play the sport of golf I must admit that he taught
me much about course etiquette, scoring, foreign golf traditions ("Aussie Rules"), and
temperament. I will miss our morning outings but I will not forget the significance of a
"tainted par" or a "fully legitimate birdie."
11
Thanks must go out to a few of the people who helped me through the difficult
times during my pursuh of this degree. My good friends Ravishankar Sethuraman,
Mahesh Rege, Aashish Ahuja, Robert Ellis, Steve Tsai, Johnson Fung, Siva Natarajan,
Scott Hurowitz, Coby Crawford, and Mike Barham deserve special mention. They are
responsible for many good times at the office, on the golf course/football field, or at some
other establishment. Everyone should be so lucky to have friends like these.
Many thanks go out to Bob Spruill, Marybeth Abemathy, Tammy Low, and
Kathy Womble for their unfailing support. It was a pleasure to work with such a group of
professionals who always got the job done well, and on time.
I must express my deepest thanks to my family and friends for their unconditional
love and support throughout my life. Distance, no matter how great, has never cracked
the foimdation that their love gives me. When I am reunited wdth them, it is always as if
we never parted.
Lastly, and mostly, I would like to thank my parents. Loving, selfless, and
capable parents are and have always been my greatest earthly gift. They taught by
example and never discouraged my dreams or desires. They made my education their
priority and selflessly gave of themselves to that end. Only after I became a man did I
begin to understand the sacrifices that they made for me and my sisters. I hope to be as
strong as they one day, and I will consider myself to be successful only when I have
given to my children what they have given to me.
Ill
TABLE OF CONTENTS
ACKNOWLEDGEMENTS ii
ABSTRACT viii
LIST OF TABLES x
LIST OF FIGURES xi
CHAPTER
I. INTRODUCTION 1
n. TECHNICAL BACKGROUND . . . . 6
2.1 Background of Natural Gas and Methanol 6
2.2 ABrief History of Methanol Synthesis 7
2.3 Commercial Methanol Synthesis . . . 8
2.3.1 Synthesis Gas Preparation Techniques 8
2.3.2 Methanol Synthesis from Synthesis Gas 14
2.4 Potential Advances in Methanol Use . . 2 0
2.4.1 Methanol as Transportation Fuel 20
2.4.2 Methanol to Facilitate the use of Methane 22
HI. LITERATURE REVIEW 24
3.1 Homogeneous Partial Oxidation . . . 24
3.1.1 Effect of Reactor Walls, Additives, and Promoters 2 5
3.1.2 Kinetics and Kinetic Modeling 27
IV
3.2 Heterogeneous Catalytic Partial Oxidation 29
3.3 Methane Oxidation in the Liquid Phase 32
3.4 Methane Oxidation in Plasma Reactors 34
IV. PLASMA 36
4.1 Background . . . . . . 36
4.1.1 Plasma in Nature . . . . 37
4.1.2 Potential Applications for Plasma 38
4.2 Plasma Characteristics, Generation and Uses. 39
4.2.1 Plasma States . . . . . 39
4.2.2 Plasma Generation . . . . 41
4.3 ABrief Description of the Physics of Plasmas 45
4.3.1 Criteria for Plasma Occurrence 47
V. EXPERIMENTAL SYSTEM AND PROCEDURES 50
5.1 Experimental Approach 50
5.2 Experimental Apparatus . . . . 52
5.2.1 Feed System . . . . . 52
5.2.2 Plasma Generation System . . . 56
5.2.3 Reactor System . 61
5.2.4 Product Collection System . . . 62
5.2.5 Product Analysis . . . . 63
5.3 Experimental Procedures . . . . 65
5.3.1 System Preparation and Warm-up 66
5.3.2 Experimental Run Procedures and Checklist. 68
5.3.3 Hazards and Emergency Procedures . 70
VI. RESULTS AND DISCUSSION 73
6.1 Preliminary Phase Experiments . . . 73
6.1.1 Injection Distance . . . . 77
6.1.2 Water Concentration . . . . 80
6.1.3 Methanol Selectivity Dependence on Methane Conversion . . . . 80
6.2 Phase I Experiments . . . . . 82
6.2.1 Injection Distance 83
6.2.2 Water Concentration . . . . 83
6.2.3 Oxygen Concentration . . . 85
6.2.4 Mixing Effects . . . . 88
6.2.5 Overall Performance of Phase I Experiments. 92
6.3 Phase II Experiments . . . . . 92
6.3.1 Injection Distance . . . . 94
6.3.2 OveraH Performance of Phase II Experiments 94
6.4 Phase III E x p e r i m e n t s . . . . . 97
6.4.1 Oxygen with Methane Stream 99
6.4.2 Oxygen Divided into Plasma and Methane Streams 100
6.5 Overall Performance Evaluation . 102
VI
6.5.1 Mixing and Flow Analysis 104
6.5.2 Material Balances 104
6.6 Sources of Error . 105
Vn. CONCLUSIONS AND RECOMMENDATIONS . 108
BIBLIOGRAPHY Ill
APPENDICIES
A. CALIBRATION OF EXPERIMENTAL EQUIPMENT AND INFORMATION ON SYSTEM HARDWARE 117
B. SAMPLE CALCULATIONS 123
C. RAW EXPERIMENTAL DATA 128
VII
ABSTRACT
Methanol is one of the most widely-produced chemicals in the world. It is a key
raw material in the production of many chemicals in the petrochemical industry.
Methanol also has vast potential for expanded applications as a fuel. It is currently
produced by an energy intensive and expensive two step process. An economically
feasible one step process could significantly reduce methanol production cost, saving
millions of dollars. A methane-to-methanol process, built at remotely located methane
reserves, would convert methane into a different energy form that is much easier to
transport. This would make methane a much more attractive and valuable energy source.
The purpose of this investigation was to evaluate the feasibility of producing
methanol by direct oxidation of methane using a plasma reactor. The chemistry of
methane oxidation is well understood and free radicals play a central role in methane
oxidation reactions. Low pressure experiments by other researchers indicated that
methanol can be produced by direct oxidation of methane in plasma reactors. However,
the viability of a plasma-based methanol production process depends on its ability to
convert large quantities of methane. This work was directed at plasma reactor operation
near atmospheric pressure to increase the amount of material processed.
The focus of this mvestigation was the design and construction of an experimental
apparatus which could achieve methanol synthesis in a plasma reactor by direct oxidation
of methane at atmospheric pressure. A microwave source provided the energy to
generate the plasma. The system was designed to study the effects of reactant
viii
concentration and flow configurations on methanol production. Since high levels of
methanol selectivity are the primary consideration in direct synthesis of methanol from
methane, improvements in methanol selectivity were desired. The objective of the four
experimental phases was to investigate reactor operating conditions and improve
methanol production and selectivity.
Methanol production at atmospheric pressure was demonstrated in this plasma
system and steady improvements in methanol selectivity were achieved as the
investigation proceeded. Experiments showed that high concentrations of water and low
concentrations of oxygen improved methanol selectivity. In the last experimental phase,
oxygen was divided into both reactant streams, but this approach did not improve
methanol production. It was observed that higher methanol selectivities were obtained
only at low methane conversions. As in other plasma studies, methanol production did
not approach what would be required for commercial feasibility.
IX
LIST OF TABLES
5.1 Technical Characteristics of the Microwave Generation System 57
6.1 Preliminary Phase Experimental Parameters . . . . 74
6.2 Phase 1 Experimental Parameters . . . . . 82
6.3 Phase II Experimental Parameters . . . . . 94
6.4 Phase III Experimental Parameters . . . . . 98
6.5 Reynolds Number Calculation Results (run 0213) 104
C.l Preliminary Phase and Phase I Raw Experimental Data 129
C.2 Phase II and Phase III Raw Experimental Data 133
LIST OF FIGURES
2.1 M. W. Kellogg Methanol Synthesis Loop 19
4.1 Electron Avalanche . . . . . . . 43
5.1 Schematic of General Approach 51
5.2 Schematic Diagram of Reactor Configuration and Methane Injection 53
5.3 Schematic Diagram of Experimental Apparatus . . . 54
5.4 Schematic of Waveguide and Quartz Reactor Tube Configuration . 60
6.1 Schematic of Injection Distance . . . . . 75
6.2 Preliminary Phase-Methane Conversion Versus Injection Distance . 78
6.3 Preliminary Phase-Methanol Selectivity Versus Methane Conversion 81
6.4 Phase I-Methane Conversion Versus Injection Distance 84
6.5 Phase I-Methane Conversion Versus Oxygen Concentration. 86
6.6 Phase I-Methanol Selectivity Versus Oxygen Concentration. 87
6.7 Schematic of Mixing Region . . . . . . 89
6.8 Preliminary Phase and Phase I-Methanol Selectivity Versus
Methane Conversion . . . . . . . 93
6.9 Preliminary Phase, Phase I, and Phase II-Methanol Selectivity Versus
Methane Conversion . 95
6.10 Phase II and III-Methanol Selectivity Versus Methane Conversion . 101
XI
6.11 Preliminary Phase, Phase I, II, and III-Methanol Selectivity Versus
Methane Conversion . 103
A.l Argon Rotameter Calibration Plot 119
A.2 Methane Rotameter Calibration Plot . 120
A. 3 Oxygen Rotameter Calibration Plot . 121
A.4 Methanol Calibration Plot 122
Xll
CHAPTER I
INTRODUCTION
Methanol is one of the most widely produced (21 million tons/year) chemicals in
the world. It used to be called "wood alcohol." That name refers to the destructive
distillation of wood, the first wide-spread method of producing methanol. Methanol is
produced in such large volume because it has many uses. For example, methanol is used
as a solvent, a gasoline additive, and a chemical feedstock for the production of hundreds
of other chemicals.
Methanol is widely regarded as the most promising candidate for use as an
alternate automobile fuel. Development of automobiles and other transportation vehicles
that are able to bum pure, or nearly pure, methanol would be highly beneficial. Some of
the positive effects of developing a world-wide market for methanol as a fuel are: (a)
decreased dependence on foreign energy sources, (b) conservation of existing petroleum-
based energy reserves, (c) increased competition among energy providers, resulting in
lower cost, and (d) environmental benefits from improved emissions inherent in the
combustion of methanol, as compared to petroleum-based fuels. Adoption of methanol
as a primary automobile, or transportation, fuel would necessitate a large increase in the
world's methanol production capability. In addition, a methane-to-methanol process,
built at remotely located methane reserves, would convert methane into a different energy
form that is much easier to transport. This would make methane a much more attractive
and valuable energy source. Methanol has tremendous potential to positively affect
changes in energy infra-structure, but methanol production is very expensive.
Presently, the dominant method of producing methanol is a two-step process. The
first step is conversion of methane (natural gas) into a gas mixture of carbon monoxide
and hydrogen known as synthesis gas. This step is very energy intensive and represents
the major expense in methanol production. The second step in methanol production is
reaction of the synthesis gas over a catalyst that selectively produces methanol. There is
significant interest in developing a direct method of converting methane to methanol, as
this would eliminate the need for expensive steam reforming and save millions of dollars.
In addition, methanol production by a direct method would lower the price of methanol
making it more attractive as a potential fuel source.
Currently, the most utilized feedstock for methanol production is methane. It
plays a major role in the world's energy infra-structure and is the primary constituent in
natural gas. Natural gas is an abundant, inexpensive, clean-burning fuel that is used
throughout the world. Reserves of natural gas stand at approximately 5,000 trillion cubic
feet and those reserves equal roughly 47% of the world's petroleum reserves. Despite the
benefits and abundance of natural gas, petroleum remains the world's main source of
energy. This is because natural gas is often located in isolated reserves, difficult and
expensive to transport, and inexpensive (i.e., making it difficult to gamer a profit). The
low cost, low profit margin, and hazardous characteristics of natural gas make it
unattractive and economically unfeasible to transport over long distances. For these
reasons, huge amounts of natural gas remain locked away in distant reserves, mostly
located in the former Soviet Union and the Middle East. If a simple and effective process
could be developed to convert methane directly to methanol, methanol could be produced
from methane, on-site. A direct methane-to-methanol process could make remotely
located natural gas reserves more usable and economically attractive.
The search for a successfial direct process to achieve this goal has been underway
for more than 20 years. Most of the research into this area has fallen under two main
approaches, homogeneous gas-phase partial oxidation and heterogeneous catalytic partial
oxidation. The homogeneous studies achieved methane conversions of approximately
10%, but the corresponding methanol selectivity was very low. High selectivity to
methanol occurred only at very low methane conversion. Heterogeneous catalytic
investigations yielded lower methane conversions and methanol selectivities than
homogeneous systems. Homogeneous and heterogeneous investigations into direct
oxidation of methane to methanol have failed to achieve acceptable methanol production.
Recently, several new approaches have been investigated to produce methanol
directly from methane. Liquid phase studies and studies into supercritical fluid reactors
achieved methane conversion into methanol. Liquid phase and supercritical reactor
systems also failed to produce methanol at levels required for commercialization.
It is known that free radicals play an important role in oxidation reactions.
Plasma technology is being adopted increasingly in the processing industry and plasmas
are known to generate abundant levels of free radicals. Plasma reactors allow the
generation of free radicals followed by the addition of a stream of methane downstream
of the plasma region. This isolates the methane from the high energy region in the
plasma and allows the radicals to react at less oxidizing conditions. Reaction at under
less oxidizing conditions should promote higher selectivity towards methanol. During
the last few years, plasma reactors have been investigated to produce methanol by the
direct oxidation of methane. Low-pressure studies have demonstrated the ability to
produce methanol directly from methane and water has been shown to play a role in
methanol selectivity.
Partial oxidation of methane leads to three primary products. The desired
product, methanol, is more reactive than methane. The other two important products are
carbon oxides (carbon monoxide, CO, and carbon dioxide, CO2). Of particular concem is
the oxidation of methanol in the presence of oxygen to produce carbon oxides.
Conditions that will induce oxidation of methane will certainly induce oxidation of
methanol. The oxidation of methanol to carbon oxides is favored thermodynamically and
methanol that is produced will react further to yield carbon oxides. Methanol must be
removed from reactive conditions for it to survive the reaction process. The commercial
methanol production route overcomes this problem by dividing the process into two
steps.
The goal of this research is to investigate the production of methanol by direct
oxidation of methane in a plasma reactor. Since the viability of any process depends on
its ability to convert large quantities, the plasma reactor system should operate at
atmospheric pressure. The plasma generates free radicals in the plasma stream and mixes
them with a methane-rich stream. When the streams are mixed, the free radical reactions
commence.
Water is included in this study and its effects are investigated. The plasma reactor
isolates the methane from highly oxidizing conditions and allows reactions to occur at
lower temperature (relative to the plasma region) and more selective conditions. As
noted above, high pressure is required to achieve feasible production rates. Higher
pressure creates higher temperatures in the plasma region (relative to low pressure
studies) which may be detrimental to selective conversion of methane. The high
temperature should help to speed removal of methanol product from the reactive region
because of higher stream velocities. The plasma and methane streams will be contacted
in such a way as to accelerate mixing, allowing the reactions to proceed at desirable
conditions. This is one of the first high pressure studies of its kind to be conducted and it
is hoped that the information gained from this study proves to be useful to future
researchers.
CHAPTER II
TECHNICAL BACKGROUND
2.1 Background of Natural Gas and Methanol
Natural gas is an intemational commodity that consists primarily of methane. As
noted above, current estimations of natural gas reserves stand at about 5,000 trillion cubic
feet, which is equivalent to approximately 47% of world petroleum reserves. Over 75%
of this natural gas is located in remote locations in the former USSR and in the Arabian
Gulf countries of the Middle East. It is a clean-buming fuel source, and the use of natural
gas is steadily increasing and promises continued aggressive growth in the immediate
future. Methane, along with methanol, is an integral part of the chemical industry.
Methanol is a vitally important chemical used in a variety of areas. It is a primary
CI building block in the chemical synthesis of many compounds via esterification,
addition, carbonylation, and dehydration reactions. Some of the major derivatives of
methanol are formaldehyde, acetic acid, methyl-tert-butyl-ether (MTBE, an important
fuel octane booster), dimethyl terephthalate, and methyl acetate. It is also used as a
solvent, antifreeze, reaction inhibitor, carbon-rich substrate used in the growth of
microorganisms, and has been shown to improve yield in some crop varieties. Most of
the methanol produced world wide is used as a chemical feedstock, fijel, or fiiel additive.
Methanol holds a unique position in the chemical industry, due to its highly favorable
physical and chemical properties.
2.2 A Brief History of Methanol Synthesis
Robert Boyle is believed to have discovered methanol in 1661, though no written
record of any domestic or industrial use exists before the 19th century. Dumas and
Peligot first established the chemical and molecular identity of methanol in 1834. The
original method of producing methanol was by the destmctive distillation of wood.
"Wood spirit" or "wood alcohol" was the name given to methanol for many years,
referring to this original synthesis method. French chemist Paul Sabatier isolated the first
synthetic methanol route in 1905 and BASF commercialized the first synthetic methanol
plant in 1934. The process used a zinc/chromium oxide catalyst operating at 300°C and
200 atmospheres. Methanol produced by the BASF method was of much higher purity
than wood-derived methanol and this initiated European domination in methanol
synthesis technology. High-pressure methanol synthesis produced great contributions to
chemical reaction engineering, catalyst technology, process instrumentation, and high-
pressure technology. This highly successful process was used for many years until a
more efficient low-pressure synthesis route was discovered in 1966 (Lee, 1990). •
Methanol production in the U.S. was started in 1927 by Commercial Solvents
Corporation and DuPont. In the Commercial Solvents Corporation process, CO2
produced in the company's fermentation unit was hydrogenated to methanol at 300
atmospheres over metal oxide catalysts. The DuPont process used coal to produce a
gaseous feedstock, known as synthesis gas (syngas), consisting primarily of carbon
monoxide and hydrogen. The syngas was purified and passed over a methanol synthesis
catalyst which yielded the final methanol product. The plant produced both ammonia and
methanol until the late 1940's. At that time, plentiful supplies of natural gas became
available and natural gas replaced coal as the preferred feedstock for the production of
methanol.
In 1966, Imperial Chemical Industries, Ltd. introduced copper/zinc oxide (Cu/Zn
oxide) catalysts that were far more active in the synthesis of methanol. These catalysts
spelled the end of the high pressure methanol synthesis route. This new low pressure
methanol synthesis technology operated at temperature ranges of 250-300°C and
pressures of 50-100 atmospheres. These new Cu/Zn oxide catalysts were highly
susceptible to poisoning and deactivation (from overheating) making close control of
methanol reactors very important. Advances in syngas production and purification
removed most of the catalyst poisons and made use of the new catalysts possible. These
low pressure methanol synthesis catalysts have a lifetime of up to 4 years.
2.3 Commercial Methanol Synthesis
Today, commercial synthesis of methanol is based exclusively on heterogeneous
synthesis. As presented earlier, heterogeneous methanol synthesis consists of a two step
process. The initial step is to convert a carbon rich fuel source, most often natural gas,
into syngas. The second step is to convert the syngas into methanol.
2.3.1 Synthesis Gas Preparation Techniques
Syngas can be produced in a number of ways. The method of choice depends on
the desired characteristics of the syngas product. The principal routes of syngas
production are through steam reforming of natural gas (or other feedstock), coal
gasification, partial oxidation of heavy oils, combined or oxygen-enhanced reforming,
and heat-exchange reforming (Cheng and Kung, 1994). These syngas production routes
have different operating costs and complexities. In general, there is no overall best route
and project/site specifics will determine which syngas generation route is preferred.
A measure of synthesis gas composition is given by the stoichiometric ratio R.
moles H. R = (2 1)
2 X moles CO + 3 X moles COj
Syngas which is hydrogen-rich has R values greater than 1.0. Hydrogen-lean mixtures
have R values less than 1.0. The methanol synthesis plant performance characteristics
will define the optimum R value for peak methanol production. A brief review of the
different syngas preparation techniques is presented below.
2.3.1.1 Steam Reforming of Natural Gas
This method of syngas generation has been the preferred method for many years.
As of 1990, 75% of the world's methanol production capacity was based on natural gas
feedstock. Natural gas is an appropriate feedstock for a synthesis plant containing Cu-Zn
catalyst because it commonly possesses low sulfur content.
Steam reforming is a heterogeneous process and takes place over a nickel-on-
alumina catalyst. The reformers are large process fumaces in which catalyst-filled tubes
are heated by direct firing, which induces the reactions to proceed. Steam reforming
generates syngas through the following simultaneous reactions.
CH4 + H2O <^ CO + 3H2 (2.2)
CO + H20<:::>C02+H2 (2.3)
Reaction (2.3) is known as the water-gas shift reaction. Common steam-to-methane
ratios for steam reforming are in the range of 3 to 1. It is apparent from the above
reactions that a methane-rich feed gas generates a hydrogen-rich syngas. Commonly,
syngas with an R value of 1.3-1.4 is generated using typical natural gas feedstock.
Simply stated, the natural gas feed is preheated, desulfurized, mixed with steam, reformed
over catalyst, and then cooled. Typically, the effluent stream from a reforming plant will
be at ~850°C and 20 atmospheres. The only treatment it receives before being fed to the
methanol synthesis loop is compression.
Steam reforming is highly endothermic and high temperature, low pressure, and
high steam-to-carbon ratios enhance performance. It is very energy intensive, expensive,
and represents the major expenditure in methanol production. Elimination of this step
would provide a significant economic advantage in methanol synthesis (Cheng and Kung,
1994).
2.3.1.2 Coal Gasification
In coal gasification, syngas is generated by a combination of partial oxidation and
hydrogasification of coal feedstock through the following simultaneous reactions.
1 C + - O 2 O C O (2.4)
C + H 2 O 0 C O + H2 (2.5)
10
Equilibrium of carbon monoxide and carbon dioxide is maintained through the water-gas
shift reaction (2.3), and the following reaction.
CO2 + C o 2 C 0 (2.6)
Many different types of coal gasification units exist, including moving and fixed-bed
gasifiers, fluidized-bed gasifiers, entrained-flow gasifiers, and gasifiers based on the
molten-batch process.
Gasification equipment must be selected and designed around the properties of the
coal to be processed. Important coal properties include ash content, moisture content,
caking behavior, reactivity, particle size distribution, impurities, and fixed carbon
availability (Cheng and Kung, 1994). Very low levels of some impurities (potassium,
iron, etc.) can create a catalytic environment, modifying reaction rates. Often, effluent
gas from a coal gasification unit requires additional treatment before it can be used for
methanol synthesis. Impurities, especially sulfur and sulfur derivatives, must be removed
to preclude poisoning of the catalyst. Coal gasification yields a raw gas that is very
carbon-rich (R<1.0) and composition adjustments are frequently necessary.
This coal-based method has been expected to become the preferred syngas
generation method in the U.S. for many years. Large indigenous coal reserves were
predicted to create a boom in the use of coal as a chemical feedstock. However, the
expense of coal conversion, variation in coal characteristics, and continued availability of
natural gas feedstock have prevented large-scale conversion to coal.
11
2.3.1.3 Partial Oxidation of Heavy Oils
This process is accomplished by incomplete combustion of heavy hydrocarbon
feedstocks according to the following reactions.
n m - O 2 <^ nCO + — 2 ^ 2
C„H^ + ^ ^ 2 ^ nCO + —H2 (2.7)
C„H^ + nH20 o nCO + ( y + n)H2 (2.8)
C„H, + n 0 2 « n C 0 2 + Y H 2 (2.9)
A minimum amoimt of oxygen (1/2 mole O2 per mole of carbon) is added to achieve
complete conversion of the hydrocarbon feedstock. Steam is added to control the
reaction temperature, which affects hydrogen production via reaction (2.3). The
composition of the effluent gas is govemed by reaction (2.3) and the following chemical
reactions.
CH4 + H2O <=> CO + 3H2 (2.10)
H2S + CO2 <=> H2O + COS . (2.11)
CO + - 0 2 < » C 0 2 (2.12)
CH4 + CO2 o 2 C 0 + 2H2 (2.13)
Reactions take place at temperatures of 1350-1600°C and pressures of up to 15MPa (150
atmospheres). A major advantage of this process is that h makes use of heavy feedstock
not usable in other, vapor-only processes. Disadvantages of the process include soot
12
formation, high effluent levels of sulfur and sulfur derivatives, and the requirement that
pure oxygen must be provided for the reaction, instead of air.
Several large refining companies (including Shell Oil Co. and Texaco Inc.) have
achieved successful commercialization of this type of partial oxidation process. The raw
effluent gas from this process is highly carbon-rich and not suitable for methanol
synthesis. Along with sulfur removal, the composition of the gas must be adjusted and
excess CO2 removed before the syngas can be compressed and processed in a
conventional methanol synthesis loop (Cheng and Kung, 1994). The next two methods of
syngas production do not make use of unique feedstock, but the methods under which the
conversion takes place differentiate them from other approaches.
2.3.1.4 Combined Reforming
Combined reforming (also known as combination reforming and oxygen-
enhanced reforming) makes use of two reformers in series for the production of syngas.
The primary reformer is operated similarly to the natural gas reformer above, but the
secondary (or autothermal) reformer is injected with pure (99.5%) oxygen. Pure oxygen
in the second reformer precludes the burden of compressing large amounts of nitrogen.
The oxygen also consumes excess hydrogen, making it possible to produce a nearly
stoichiometric syngas from the natural gas feedstock.
The advantage of this approach is brought about by shifting a portion of the
reformer duty from the primary reformer to the secondary reformer. The partial
combustion that occurs in the secondary reformer heats the process stream and allows
13
reduction of the fired duty in the primary reformer. In general, this approach is more
costly than the steam reforming of natural gas approach but is justified in cases where
energy costs are extremely high. Combined reforming does offer significant
environmental benefits over other approaches, including reductions in CO2 and NO^
emissions. (Cheng and Kung, 1994).
2.3.1.5 Heat-Exchange Reforming
In heat-exchange reforming, a heat-exchange reformer is operated in series with
an autothermal reformer. The central concept in this approach is that heat generated in
the secondary or autothermal reformer is used to heat the process gas reacting within the
heat-exchange reformer. This approach is simple, effective, and can provide syngas of
nearly stoichiometric composition. Heat-exchange reforming provides the following
advantages: increased operational flexibility, high reliability, reduced maintenance and
energy costs, physically compact units, and reduced hazardous emissions.
Although both combined reforming and heat-exchange reforming are more
efficient than steam reforming, steam reforming still provides the most economical means
of syngas generation for methanol production. Readers interested in more information on
syngas generation are referred to Cheng and Kung (1994).
2.3.2 Commercial Methanol Synthesis from Syngas
Methanol synthesis reactions, shown below, are exothermic and experience a
decrease in volume as the reactions proceed towards methanol production. From these
14
considerations, methanol synthesis is favored by low temperature and high pressure.
Methanol generation from syngas occurs in the gas phase over a heterogeneous catalyst.
2.3.2.1 Methanol Synthesis Equilibrium
The synthesis of methanol occurs through the following reactions.
CO + 2H2 <^CH30H (2.14)
CO2 + 3H2 <^ CH3OH + H2O (2.15)
Reaction (2.3) is also induced over the catalyst. In fact, reaction (2.15) is the sum of
(2.14) and the reverse of reaction (2.3), as can be observed below.
CO + 2H2 <: CH3OH (2.14)
CO2 + H2 <^ CO + H2O (in reverse) (2.3)
CO2 + 3H2 « CH3OH + H2O (2.15)
Reactor effluent conditions and compositions are govemed by thermodynamic
kinetics and equilibrium. Equilibrium compositions are calculated from simultaneous
solution of the equilibrium constant expressions. The equilibrium constant expressions
for a set of independent reactions (reaction 2.14 and the reverse of reaction 2.3) depend
on the component partial pressures and are given below.
[P*co][p*H:o] ^rev2 .3 ~ r « * 1 r « * 1 V^-^"/
LP COJLP HJ 1
K [P*CH,OH]
LP C O J L P H J
15
If we take into the account the non-ideal behavior of the high pressure gases, the concept
of fugacity must be considered. The governing fugacity relation is
fi = P :< /^ , (2.18)
where
f- = fugacity of the i-th component.
Pi = partial-pressure of the i-th component, and
, = fugacity coefficient of the i-th component.
Incorporating these fugacity relations into the equilibrium expressions results in the
following expressions.
^ _ L P ' C O ] L P * H , O ] J<Z^co][<Z^H,o]
''-'•' [ P ' C O J L P ' H J [<2>COJ[<^^HJ ^ •
_ L P * C H 3 0 H ] L^CH30H ]
[P COJLP H J [^CO ] [ < ^ H J
Fugacity coefficients can be calculated or approximated in a variety of ways (Smith and
Van Ness, 1959). Many expressions exist for these temperature-dependent expressions.
For Kev2 3' Bisset (1977) proposes the relationship
5639 5 49170 hi(K, 2.3) = 13.15-——-1.08*hi(T)-5.44*10-'*T-1.13*10-'*T'+—:^ (2.21)
where T is in degrees Kelvin.
Thomas and Portalski (1958) derived the following expression for K2,4.
3921 log(K2,4) = -Y-- '7.971*log(T) + 0.002499*T-2.953*10-'*T'+10.2 (2.22)
where T is in degrees Kelvin.
16
2.3.2.2 Methanol Synthesis Kinetics
Commercial methanol synthesis processes are offered under license by process
designers and catalyst providers. Most of these providers have developed their own
proprietary kinetic rate model of the process. The literature contains many different
proposed models and most are based on consideration of the rate-limiting step in the
catalytic processes. There is continuing discussion about rate model stmcture and what
factors have the most influence on methanol synthesis. Examples of some of these rate
models are given below to illustrate the variety of the expressions.
Natta proposed that methanol is formed over specific catalysts according to the
following expression (Satterfield, 1980) given in terms of fiigacities.
••....H = fcofH. ( ^ p ^ ) ( A + Bf,o + Cf„^ + Dfc„,o„ ) (2.23) eq
Terms A, B, C, and D are experimentally determined rate parameters. Notice that there is
no term involving CO2 in the above expression. To date, it is generally accepted that CO2
plays an important role in methanol synthesis. Seyfert (1984) derived the following
expression for methanol synthesis by the low pressure method.
r„.,. = fcHfH, - % ^ ( A + Bfco + Cf„_ + Efeof„, + Ffeo,)' (2.24) eq
The terms A, B, C, E, and F are experimentally determined rate parameters. Dybkjaer
(1981) claimed that water had a profound inhibiting effect on methanol synthesis and
postulated the following expression for the rate of methanol production. The expression
17
is in terms of component equilibrium constants (K ), component activites (a,), and a
derived equilibrium term (B,).
, . ^ w ^ c o . ^ c o , , . , K:H a^ ( 1 - B , ) Tmeth = 1 ^ . ( T ) ( T — ^ )*[ K 1 1 (2.25)
^ ^ ^ ( ^ - ^ - ^ ^ K , a , ) - ^
The kinetic models presented here illustrate the variety of forms that methanol
rate models can take. Models exist that account for catalyst sites in both the reduced and
oxidized state. It is evident that rate expressions for heterogeneous catalytic methanol
synthesis can take many forms and be very complicated.
2.3.2.3 Methanol Synthesis Loop Designs
Several methods and designs exist to convert syngas into methanol. These
"converter" designs are available from a few key technology providers. Figure 2.1 shows
a methanol synthesis loop from the M. W. Kellogg Company. It utilizes a series of
adiabatic, intercooled, spherical reactors. In the loop, fresh feed gas (containing H2, CO,
CO2 and inert material) is mixed with recycle gas on the discharge side of a single-stage
recycle compressor. This fresh stream is preheated to reaction temperature in a shell-and-
tube feed effluent exchanger before passing into the first reactor vessel.
Reaction proceeds over the first reactor bed adiabatically, the effluent being
cooled indirectly by an intercooler that raises intermediate-pressure steam. The second,
third, and fourth reactors operate in a similar manner. The final reactor effluent is cooled
18
Start-up Heater Feed
V) Converter Feedgas
\
Spherical Converter
V^J Intercooler
Recycle Compressor
Catchpot
>
Purge >
i
Feed/Effluent Exchanger
Crude Methanol
\
Loop Condenser
Figure 2.1. M. W. Kellogg Methanol Synthesis Loop (Cheng and Kung, 1994)
19
in the feed effluent exchanger. The effluent from reactor 4 is typically about 5%
methanol. Once the effluent is cooled, it passes to a cmde condenser and is separated
from the circulating gas. The product is then distilled where methanol is purified to
specification.
A variety of other methanol synthesis converters are used to produce methanol
commercially, including the ICI tube-cooled converter, the ICI quench converter, and the
Lurgi tubular converter. For more information on methanol synthesis loop design, the
reader is referred to Cheng and Kung (1994).
2.4 Potential Advances in Methanol Use
Many uses of methanol have been outlined above. Some mention should be made
of the potential that methanol has for increased use. Current methanol production sources
could not meet demand if potential applications of methanol were to come to fmition.
The two areas that have potential for increased methanol use are as an altemative, clean-
buming, automobile fiiel and as an intermediate form in the use and transport of methane.
2.4.1 Methanol as Transportation Fuel
The use of methanol as a neat ftiel for automobiles has been the subject of
substantial research for many years. It is currently used as a fiiel additive and in the
manufacture of other important fiiel additives. The purpose of these additives is to offer
economic advantages in the use of methanol in automobile fuel and to decrease pollutant
emissions from automobiles. Research into methanol applications as a fiiel and ftiel
20
additive can be found in the literature (Brinkman, Ecklund, and Nichols, 1990). For
practical reasons, this discussion will briefly consider world-wide impact of increased
methanol use as a neat automobile fiael.
Assuming that the use of methanol as a fuel becomes economically and
technically feasible, world-wide increases in use will cause modest but measurable
decreases in the cost of oil. OPEC suppliers have incentives to maintain a steady market,
but even modest competition for their energy product could result in price decreases
(Brinkman, Ecklund, and Nichols, 1990). OPEC suppliers have a practical monopoly on
the world's energy market and increased use of methanol would threaten the monopoly
and decrease OPEC's "markup."
If and when ftiel methanol approaches economic feasibility, OPEC suppliers will
probably be less likely to raise oil prices. Increases in oil prices would prompt methanol
proponents to speed introduction of methanol fiiel technology. This pressure would be
placed on the entire OPEC organization and coordinated OPEC actions to raise prices
would be less probable. Creation of a transportation infrastmcture capable of using
methanol or gasoline (i.e., ftiel-flexible vehicles) would have the overall effect of
stabilizing world-wide oil prices (Brinkman, Ecklund, and Nichols, 1990).
When and if fiiel methanol is introduced, the producers would probably be a
highly diverse group of countries, much more diverse than the existing group of world oil
suppliers. An increased level of diversity among energy suppliers would create a more
secure supply of energy and decrease the likelihood that world events, political or
otherwise, could adversely affect world energy supplies. More stability in world-wide
21
energy supplies lowers the chances of energy-related economic hardship throughout the
world.
Increased dependence on methanol would create utilization of relatively
inexpensive and underdeveloped natural gas resources. Use of methanol in the area of
transportation would reduce the demand for petroleum products and measurably extend
that supply into the future. Highly developed and exploited petroleum resources would
be able to share the world's energy burden far into the future and a stable, long-lasting
energy infrastmcture will result.
Methanol is very clean-buming. Significant improvements in environmental
conditions will result if methanol replaces gasoline in substantial amounts. Areas that
will improve are public health, air pollution, and visibility. These benefits could be
doubly important in developing countries, where short-term industrialization benefits can
often overpower environmental concems (Brinkman, Ecklund, and Nichols, 1990).
2.4.2 Methanol to Facilitate the Use of Methane
As outlined above, much of the 5,000 trillion cubic feet of methane in reserve
exists in remotely located gas fields. Even though methane is abundantly available and
inexpensive, its low price makes h impractical to transport very far. Compressing and
transporting (pumping) large quantities of natural gas is very expensive and rarely
economically practical.
Methane has physical properties that make it very hazardous and problematic to
handle. It exists as a gas, even at high pressure, though liquefaction can be performed.
22
Hazards exist in handling compressed natural gas and liquefied natural gas is extremely
hazardous. Compression and liquefaction significantly improve the economics associated
with transportation, but the hazards remain. Safety concems are paramount in moving
natural gas and, although immense amounts are pumped throughout the world, it is not
considered the best economic or energy management approach. As a result, huge
amounts of natural gas stay locked in remote fields.
If conversion of methane to methanol could be accomplished on-site, in a single
step, the economic and safety factors that hinder the transport and use of natural gas could
be virtually eliminated. Problems pertaining to the handling and transport of methane
would be transformed into ones conceming methanol handling and transport. Handling
methanol, while not without hazard, is much easier and safer than handling methane.
Transportation of liquid methanol is relatively simple, safe, and economical. Conversion
of natural gas to methanol, were it less involved, could make remotely located natural gas
fields a usable and highly economical energy resource.
23
CHAPTER III
LITERATURE REVIEW
For many years, efforts have been underway to discover an economical methanol
production route directly from methane. For reasons presented above, research into this
area has been steady. Several different approaches have been investigated to achieve this
goal. The two areas that have received the most attention are homogeneous and
heterogeneous gas phase partial oxidation of methane. Other approaches, including
liquid phase and plasma reactor studies, have also received significant attention,
especially during the last decade. This brief review will focus on the most significant
research that has been conducted on the subject of methanol production directly from
methane.
3.1 Homogeneous Partial Oxidation
As mentioned previously (Chapter 1), direct oxidation of methane to methanol is
attractive because it eliminates a very energy intensive step currently required in
methanol production, steam-reforming methane into synthesis gas. Helton (1991)
reviewed and summarized economic evaluations of current methanol production reported
in the literature. He concluded that methanol production via a direct, single step method
would be competitive with existing methanol production methods (steam reformation of
methane followed by oxidation of syngas) if it achieved a single pass methane conversion
of at least 5.5 %, with selectivity to methanol of 80%. This information is presented here
because it serves as a performance standard that any prospective direct methanol
24
production process must meet or exceed to be economically feasible. The low magnitude
of the conversion requirement is indicative of the high expense associated with existing
methanol production methods. Thermodynamically, the most favorable reaction products
of methane oxidation are carbon oxides (carbon monoxide and carbon dioxide). For this
reason, the dominating consideration for methane oxidation is high selectivity to
methanol.
3.1.1 Effect of Reactor Walls, Additives, and Promoters
Yarlagadda, Morton, Hunter, and Cesser (1988) studied Pyrex tubular reactors
and reported methane conversions of 8-10% and methanol selectivities of 75-80%. The
reactions occurred at 65 atmospheres and 723 °K, with residence times of about 2
minutes. Higher selectivities were favored by low (<5%) oxygen concentrations and high
pressures (>50 atmospheres). Burch, Squire, and Tsang (1989) and Helton (1991)
attempted to reproduce the experiments of Yarlagadda et al. (1988) but they were unable
to do so. These results have yet to be reproduced by researchers.
Hunter, Cesser, Morton, Yarlagadda, and Fung (1990) studied the effects of
reactor wall composition on the homogeneous reaction of methane. Pyrex, Teflon, and
metal reactor tubes were investigated as reactor wall materials. It was concluded that, for
most of the materials tested, reactor wall composition had no effect on partial oxidation
of methane to methanol. These investigators also examined the effects of 31 promoters
(mostly C2-C4 hydrocarbons) on the partial oxidation of methane to formaldehyde. High
yields of formaldehyde were produced at the expense of methanol. In general, C3
25
hydrocarbons (and higher) were more effective promoters than the C2 hydrocarbons
present in natural gas.
Three investigators studied the effects of additives (ethane and propane) on the
homogeneous reaction of methane. These hydrocarbon additives enhance reaction
initiation by generating free radicals. Burch et al. (1989) investigated a feed composition
of 5% ethane and 95% methane. This feed composition did not alter methanol selectivity
but did produce a reduction in reaction temperature by 50K. Helton (1991) also
investigated ethane as an addhive and found that the selectivities for carbon monoxide,
formaldehyde, and carbon dioxide were similarly unaffected by the presence of ethane.
Fukuoka, Omata, and Fujimoto (1989) investigated the effects of propane on the
homogeneous reaction of methane with oxygen. It was found that the reaction
temperature was reduced by 40K. Methanol yield is reported to have increased with the
addition of propane, though specific methane conversion and methanol selectivity were
not given.
Conclusions of the above studies are summarized as follows:
a. Increasing reaction pressure in a metal reactor decreases the occurrence of
complete oxidation of methane and methanol.
b. Vycor and quartz reactors do not contribute to oxidation processes.
Introduction of beads to increase surface-to-volume ratios in the reactor
inhibits primary reactions and promotes secondary reactions. The influence of
Pyrex on oxidation is an area deserving of fiiture study.
c. Feed streams that contain hydrocarbon additives react at temperatures
approximately 50K lower than feed streams consisting of pure methane.
26
For more information on homogeneous oxidation of methane to methanol, the reader is
referred to Chou and Albright (1978), Rytz and Baker (1991), Chun (1992), Chun and
Anthony (1993), Feng (1993), and Casey and Folger (1994).
3.1.2 Kinetics and Kinetic Modeling
Chou and Albright (1978) compared experimental data from methane oxidation to
methanol over a wide range of experimental conditions to a model based on 27 gas-phase
reactions and 3 surface reactions. They concluded that the data correlated sufficiently
well (i.e., to within experimental error) to the model. The experimental data for oxidation
in aluminum, copper, and packed glass reactors differed significantly from the data for
glass reactors due to the contribution of surface reactions. Their model clarified the
contributions of the gas-phase reactions that were occurring and provided information on
the extent to which surface reactions participated.
Vardanyan and Yan (1981) modeled free-radical reactions and calculated
conversions and selectivities for methane oxidation at 738K and 1 atmosphere.
Comparison of the model with experimental data revealed that the concentration profiles
of principle intermediates (CH20«, H2O2, CH3OOH, and H02») showed satisfactory
agreement.
Onsager, Soraker, and Lodeng (1989) suggested a model that consisted of 116
elementary free-radical reactions. These researchers concluded that most of the methane
conversion occurred at low temperatures, and their kinetic simulation was effective in
explaining methane oxidation phenomena at low methane conversions. Low temperature
conversion of methane is of interest since h is where highly selective oxidation occurs.
27
Helton (1991) investigated 179 free-radical reactions dealing with the
homogeneous oxidation of a mixture of methane and ethane. The model accounted for
formation of many components not considered previously. His proposed kinetic
mechanism was found to be capable of satisfactory prediction of the reaction of
molecular oxygen with methane and ethane.
Conclusions of the above investigations are as follows:
a. Homogeneous oxidation of methane can be predicted to within experimental
error by many of the proposed free-radical kinetic models.
b. Investigations into kinetic modeling of homogeneous oxidation of methane
applied to oxygen feed concentrations of less than 10%.
c. Although oxidation products can be predicted, predictions relating to reaction
temperature, residence time, and variations in oxygen feed require ftjrther
study. They should be conducted at high methanol selectivities.
For more information on kinetic modeling of homogeneous oxidation of methane
to methanol, the reader is referred to Droege, Hair, Pitz, and Westbrook (1989), Durante,
Walker, Seitzer, and Lyons (1989), Chun (1992), Gray, Griffiths, Foulds, Charleton and
Walker (1994), and Lodeng, Lindvag, Soraker, Roterund, and Onsager (1995).
While significant research has been conducted with some promising resuhs, none
of the homogeneous partial oxidation approaches has been found to be commercially
feasible for methanol production. High selectivities have been achieved, but they occur
at very low methane conversion levels. As methane conversion is increased, methanol
selectivity decreases rapidly. No single set of reaction conditions comes close to the
conversion and selectivity requirements for commercialization.
28
3.2 ' Heterogeneous Catalytic Partial Oxidation
Since the desired product, methanol, is more reactive than methane, some
researchers believe that successftil direct synthesis of methanol from methane will require
the use of a highly selective catalyst. Methanol selective catalysts are relatively well
understood in the realm of methanol synthesis from syngas. The challenge for direct
synthesis is to develop catalysts, which selectively convert methane directly to methanol.
Lunsford (1988) concluded that the formation of methyl radicals was the initial
step in the production of oxygenated products and higher hydrocarbons from methane
over metal oxide catalysts. Experimental evidence was uncovered indicating an
important role of O ions, although several types of surface oxygen were effective in
abstracting hydrogen from methane. Methyl radicals reacted to form methoxide ions or
desorbed into the gas phase. In all cases, homogeneous and heterogeneous secondary
reactions limited the yields of the desired partial oxidation products.
Chung, Miranda, and Bennett (1988) conducted low temperature studies of the
partial oxidation of methane using molybdenum oxide catalysts. Small amounts of
dimethyl ether, dimethoxymethane, and methyl formate were formed. Several
techniques were used to observe the electronic and geometric states of the catalyst. A
mechanism was proposed involving methoxy intermediates chemisorbed on vacant
oxygen sites. The mechanism explains which types of catalyst sites account for product
formation.
Sazonov and Popovskii (1968) correlated the catalytic activity of metal oxides in
relation to certain oxygen-catalyst interactions. They found that the reaction energy of
activation was proportional to the energy of the oxygen bond. The catalysts were also
29
ranked as to the bond energy of absorbed surface oxygen and activation energy of
methane oxidation.
Helton (1991) showed that closure of the oxygen balance was cmcial in
minimizing error in calculating methanol selectivities. Large deviations in product
selectivity could occur even though carbon atom and overall material balances were ±2%.
Durante et al. (1989) operated a quartz-lined reactor with iron-sodalite catalyst at
55 atmospheres and reported methanol selectivity of 44% at 5% methane conversion.
Methanol selectivity in the quartz reactor with by-pass improved to 70% at 7% methane
conversion. High space velocities, such as those required for commercial processing,
decreased methanol selectivity to 15-28%.
Walker, Lapszewicz, and Foulds (1994) tested various catalysts to see if they
performed as claimed by the manufacturers. Manufacturers had claimed that some
catalysts convert methane to methanol at selectivities in excess of 75%. The authors
tested some of these catalysts (molybdenum oxide/ uranium oxide impregnated on
aluminum silica, iron sodalite, y-alumina, stannic oxide, and palladium on magnesium
oxide) and compared their performance to published claims and to homogeneous gas
phase results. They found that none of the catalysts performed up to the levels cited in
patents and literature. None of the catalysts showed any improvement over the
performance of the homogeneous gas phase selectivities.
Chun and Anthony (1993) reported that temperatures of heterogeneous reactions
were higher than those of homogeneous reactions under similar conditions. They report
that the cause of the higher temperature may be due to variations in the residence time of
reactants in the reaction zone and inhibition effects of the oxide catalysts. It was also
30
noted that a significant amount of homogeneous oxidation occurred in the void space of
the catalyst bed and that catalyst surfaces inhibit free-radical homogeneous reaction.
Several researchers (Liu, Liu, Liew, Johnson, and Lungsford (1984), Zhen, Khan,
Mak, Lewis, and Somorjai (1985), and Durante et al. (1989)) report that molybdenum
and vanadium oxide based catalysts actively promote methane oxidation to methanol and
formaldehyde. Using oxygen feed ratios of 3-7%, methanol selectivities of 30-70% were
achieved under a wide range of reaction conditions.
The above investigations can be summarized as follows:
a. Spectroscopic studies indicate that important catalyst surface intermediates are
methoxide ions.
b. Methane is oxidized to CO2 over oxide catalysts with a high excess of oxygen
at 1 atmosphere.
c. Oxygen balance closure is essential for accurate selectivity calculations.
d. Molybdenum and vanadium oxide based catalysts are active for methane
oxidation to methanol and formaldehyde.
e. High pressure favors methanol production and low pressure (1 atmosphere)
favors formaldehyde production.
f High selectivities for methanol are possible, but only at low (-3%) methane
conversions. High methane conversion resuhs in lower methanol selectivity,
g. Heterogeneous methane oxidation reactions occur at higher temperatures (by
40-50°C) than homogeneous methane oxidation reactions.
For more information on the catalytic partial oxidation of methane to methanol,
the reader is referred to Kaliaguine, Shelimov, and Kazansky (1978), Andmshkevich,
31
Popovskii, and Boreskov (1965), Dowden, Schnell, and Walker (1968), Spencer, (1988),
Serafin and Friend (1989), and Hargreaves and Hutchings (1990).
The aforementioned research has yielded some promising results, but none of the
heterogeneous catalytic partial oxidation approaches has been found to be commercially
feasible for methanol production. High selectivities are achievable, but again, they occur
at low methane conversions. Continued research is required to develop a catalyst that is
highly selective for direct synthesis of methanol from methane.
3.3 Methane Oxidation in the Liquid Phase
Olah, Klopman, and Schlosberg (1969) studied methane reaction at room
temperature and 1 atmosphere in the presence of FSOsH-SbFs (magic acid). They
suggested that methane behaved as a super acid (CHs^) in this solution and produced
many highly reactive ionic species. These species reacted with other ions and ethylene to
give t-butyl, t-hexyl, and t-octyl cations or higher molecular weight species.
Olah, Yoneda, and Paka (1977) studied the reaction of hydrogen peroxide with
alkanes in magic acid solutions. They concluded that peroxide and magic acid reacted at
temperatures above 273K to produce methanol.
Konig (1982) used palladium catalysts in aqueous ferric sulfate solution to
oxidize methane to methanol. The reaction was allowed to proceed at 293-303K and 30-
60 atmospheres. Methane conversion was not provided, but a methanol selectivity of
92% was reported.
Geletii and Shilov (1983) studied oxidation of methane in solutions of Pt(II) and
Pt(IV) salts in Na8HPMo6V604o (HPA-6) at 393K and 60-100 atmospheres. Methanol
32
and methyl chloride were formed in roughly equal amounts and comprised the main
reaction products. It was found that methanol underwent ftirther oxidation while the
methyl chloride product did not react further.
Periana et al. (1994) reported a novel liquid phase methane to methanol synthesis
via methyl bisulfate intermediate. Mercury ions catalyze a reaction in which
concentrated sulfuric acid oxidizes methane to give methyl bisulfate, water, and sulfur
dioxide. This process is reported to produce the highest single-pass yield (-43%) of
methanol of any catalytic methane oxidation to date.
Savage, Li, and Santini (1994) used supercritical water for the homogeneous
partial oxidation of methane to methanol. Water was chosen as the solvent in this and
other studies because of its excellent supercritical properties. They reported methanol
selectivities of 4-75%, with high selectivity occurring only at low conversions (.04%).
The major products of the experiments were carbon monoxide and carbon dioxide.
Dixon and Abraham (1992) investigated conversion of methane to methanol in
supercritical water over Cr203. They concluded that high concentrations of water inhibit
methane conversion but promote the yield of methanol. A consistent set of reaction
pathways was proposed and rate constants were calculated which accurately modeled the
experimental results.
For more information of the partial oxidation of methane to methanol in the liquid
phase, the reader is referred to Kao, Houston, and Sen (1991), McHugh and
Occhiogrosso (1987), and Webley and Tester (1991).
33
3.4' Methane Oxidation in Plasma Reactors
The chemistry of methane oxidation is well-known, although various mechanistic
kinetic models are debated. It is clear that free-radicals play a very important role in the
gas phase oxidation of methane. Plasmas offer an attractive method of generating free-
radicals for chemical synthesis. The National Research Council (NRC, 1992) has
identified plasma technology as a "future promising technology" and plasma is being
adopted in process industry (Zanetti, 1983). Research relating to methanol synthesis
from methane in plasma reactors is presented below. An introduction to the principles of
plasmas is presented in the next chapter.
Huang, Badani, Suib, Harrison, and Kablaoui (1994) used a plasma generated by
microwave energy to oxidize methane to methanol. The study focused on plasma reactor
design to control free-radical reactions and maximize selective conversion of methane to
methanol. From their reactor configuration studies, they concluded that isolation of the
methane reactant from the plasma zone minimized methane dimerization and preserved
intermediates that led to the formation of the desired product, methanol. High methanol
selectivity was obtained only at low methane conversion. All of their experiments were
conducted under vacuum (approximately 0.02 atmospheres).
Oumghar, Legrand, Diamy, and Turillon (1996) studied methane conversion into
useful products by an air microwave plasma. They concluded that the location of
methane addition from the end of the plasma discharge plays an important role in the
product distribution. The air plasma produced higher levels of methane conversion than a
nitrogen plasma. The high conversion of this plasma system suggests that it is suited to
the conversion of methane for the purpose of synthesizing syngas.
34
Badani, Huang, Suib, Harrison, and Kablaoui (1995) investigated a variety of
oxygen sources for improving partial oxidation of methane to methanol in microwave
plasma reactors. The best methanol production was achieved when both H2O and O2 are
present in the plasma stream. Two different pathways for methanol synthesis are
postulated and it was concluded that -OH radicals play an important role. Methane
conversions and methanol selectivities were found to be too low for commercialization.
Reactor configurations affect conversion and selectivity performance and oxygen sources
are important because they alter available pathways to methanol.
35
CHAPTER IV
PLASMA
To aid in the understanding of this work, plasma characteristics and uses will be
reviewed briefly. Section 4.1 provides a description and background information about
plasma and gives examples of plasma occurrences in nature. Section 4.2 discusses how
plasma is generated and sustained. It also discusses plasma use in a variety of processes.
In section 4.3, a brief description of the physics of plasmas is presented. Section 4.4
gives specific information on the microwave unit that was used to produce the plasma for
this research.
4.1 Background
The term plasma is commonly used to describe a variety of electrically
conducting, but globally neutral, materials, usually gases, that contain many interacting
free electrons, ionized atoms or molecules, and neutral particles, which exhibit collective
behavior due to long-range Coulomb forces (Bittencourt, 1986). In 1928, American
physical chemist Irving Langmuir first used the word plasma to describe this fourth state
of matter (Bova, 1971). The word is derived from an old Greek root, plassein, which
means "to shape or mold."
The study of electrical discharge in gas has been ongoing for well over 100 years.
Siemens experimented with ozone generation by silent discharge as early as the I850's.
In 1879, Sir William Crookes proposed that gases with the ability to discharge electricity
36
should be considered a fourth state of matter (Hellund, 1961). Although the boundary
between gases and plasmas overlap, the plasma state warrants this classification because
plasmas can display a vast array of physical and chemical properties depending on the
level of ionization.
It is interesting to note that ancient civilizations thought all substance existed in
the form of four elements: earth, water, wind, and fire. This idea parallels today's
accepted states of matter: solid, liquid, gas, and plasma. Aristotle even placed an order
on the ancient elements. He reasoned that earth resided at the bottom of his ordering
hierarchy, due to its substance and immobility. Liquid was next in his order because it
was much more mobile than any solid but much more defined than the wind. Wind was
next because it was considered smooth and ephemeral, yet considerably more substantive
than fire. Fire was last, due to incredible mobility, and transitory behavior. Aristotle had
correctly arranged these "elements" according to energy content 2,000 years before
anyone knew anything about the physics of gases or of electromagnetic forces (Bova,
1971). Understanding in these two fields was required before scientists could really
begin to address plasma phenomena.
4.1.1 Plasma in Nature
By all accounts, plasma is uncommon on the surface of the earth under normal
circumstances. It does occur with frequent regularity if you know where to look.
Examples of plasma in nature include; lightening, aurora borealis, aurora australis, the
ionosphere, and even an ordinary fire, since it exhibits some electrical conductivity.
37
Examples of plasma produced from man-made sources are; neon signs, electrical
discharges (i.e., from a light switch), plasmas that form along the leading edge of
supersonic or hypersonic objects in air, and all types of man-made energy releases, from
heaters to thermonuclear explosions.
If the universe were examined as a whole, the cold and solid environment here on
earth is rare. The vast majority of known matter exists in the plasma state (Frank-
Kamenetskii, 1972). In stars, very high temperatures (on the order of 10,000,000°K)
produce the ionization required to maintain the plasma state. In nebulae and interstellar
gases, the ionization is produced by the radiation given off by the stars. Plasma may be
considered unusual here, but it is the mle throughout the universe.
4.1.2 Potential Applications for Plasma
Plasma has tremendous potential for applications in many areas. Foremost under
research is hamessing the power effusion in confined fusion reactors. This approach
uses a plasma confined in a magnetic field to generate power by ftision. Intricacies of
handling very high energy plasma are the subject of intense research. Many applications
for plasma exist in the area of propulsion and space flight. Plasma has been "fired" from
a "plasma gun" at velochies of 100 kilometers/sec (Frank-Kamenetskii, 1972). This
"plasma gun" could be used as an engine that uses small amounts of ftiel and produces
high levels of thmst. Potential plasma applications as electrical conductors and high-
temperature media are numerous. Plasma is many thousands (even millions) of times
38
lighter than metals, giving plasma a significant advantage in potential electrical
applications.
4.2 Plasma Characteristics, Generation, and Uses
The properties of a plasma are defined primarily by the state of the ionized gas.
As noted before, the total number of positively and negatively charged particles must be
roughly equal, if global neutrality is to be maintained. The degree of ionization is also
very important in describing a plasma. Plasmas with a low degree of ionization possess a
number density of charged particles that is much less than the number density of neutral
particles. As the degree of ionization goes up, the population of charged particles
increases relative to the population of neutral species. At the upper extreme of ionization,
a fusion plasma is completely ionized and few neutral particles exist.
4.2.1 Plasma States
Plasmas exist under one of three conditions: (i) break down, (ii) equilibrium, and
(iii) non-equilibrium. These conditions define important characteristics of the particles
that make up the plasma. They relate to how energy is distributed in plasma.
Breakdown is when the plasma forms and it occurs as the electrical conductivity
of a gas goes up sharply. This increase of electrical current, from lO"*'' amps to more than
10- amps, often causes light emissions (Razier, 1991). It is brought about by the energy
that is accumulating in the gas. The increase in electrical conductivity results from the
rapid increase in the population of charged species (i.e., electrons and ions).
39
Equilibrium conditions of the plasma relate to the energy levels of the particles.
When an equal partition of energy exists between charged and neutral species in a
plasma, it is said to be an equilibrium plasma. If a plasma exists at high pressure and
charged particles do not travel great distances between collisions, the kinetic energy of
the particles is well-distributed, or equally partitioned, throughout the system (Eliasson
and Kogelschatz, 1991). Very low electric field also contributes to evenly distributed
kinetic energy between charged and neutral species. Often, a plasma at equilibrium exists
at high temperature. For this reason, these equilibrium plasmas are known as hot
plasmas. High temperatures cause large increases in the frequency of collisions, inducing
more equal distribution of energy throughout the system. The electronic and molecular
temperatures are equal in equilibrium plasmas.
Non-equilibrium plasmas exist under conditions contrary to those of equilibrium
plasmas (i.e., energy is not equally partitioned throughout the system). Low pressure or
high electric field causes fewer collisions to occur. This lack of frequent interaction
means that electrons, and to a certain extent ions, can maintain higher energy states than
the surrounding neutral species. Infrequent collisions enable high energy particles to
maintain high energy because they cannot distribute the energy to the other particles in
the system. In non-equilibrium plasma, the electronic temperature is higher than the
molecular temperature, often by a factor of 10 or more (Baddour and Timmins, 1967).
These plasmas are known as cold plasmas. Chemical processing applications utilize, for
the most part, non-equilibrium plasmas. This type of plasma is used in this research and
40
it will be the subject of the remainder of this discussion on plasma, unless noted
otherwise.
4.2.2 P lasma Generation
Several different methods are employed to generate plasma, each generates
plasmas with specific properties that make them attractive for a specific application. In
general, these plasma production methods can be divided into three groups (LaDue,
1993): (i) constant DC potential field, (ii) pulsed DC or altemating current (AC)
potential, and (iii) high frequency radiation.
4.2.2.1 Plasma From Constant DC Potential
Plasma generation from constant DC potential is accomplished by applying DC
voltage across two electrodes. The electrodes are maintained at high enough potential
difference to ionize gas molecules that are located between the electrodes. Some
examples of this type of plasma are glow discharges, arc discharges, and corona
discharges.
Glow discharges are low-pressure (<10 mbar) processes that occur between flat
electrodes m a tube or reactor. They are popular because they operate at low current (10"
to 10'' amps) and moderate voltages. The low pressure can result in high field and high
energy electrons that produce intense glows, hence the name. These are the discharges
used in fluorescent tubes, neon signs, and new high-efficiency light bulbs. The low
41
pressures under which these discharges occur makes them inappropriate for large-scale
chemical processing.
Arc discharges operate at higher current (>1 amp) and low voltage (several to tens
of volts). The gas is highly ionized and forms an equilibrium plasma. Arc discharges
have been investigated as a means of improving combustion (Lee, 1988).
Corona discharges are operated at or near atmospheric pressure and produce
insufficient field for complete breakdown to occur. The geometry of corona discharges
often makes use of a pointed electrode and a flat electrode. This results in a cone shaped
discharge volume. Corona discharges are used in high-speed printout devices, dry-ore
separation systems, and radiation detectors (Eliasson and Kogelschatz, 1991).
4.2.2.2 Plasma From Pulsed DC or AC Potential
The plasma from a pulsed DC or AC potential occurs from application of the
electrical potential across two electrodes. One of the electrodes is coated with a dielectric
material which accumulates charge once breakdown occurs. This buildup of charge alters
the field and intermpts the current for a time. Discharge eventually occurs and current
resumes in concert with the oscillating potential. The duration of the current pulses
depend on the pressure, ionization characteristics of the plasma, and the dielectric
properties (Eliasson and Kogelschatz, 1991). These "micro-discharges" are randomly
distributed in space and time. Examples of this type of plasma generation are known as
silent-barrier, dielectric-barrier, or simply, barrier discharges. These plasmas are ideally
suited for applications in volume plasma chemistry processing and off-gas treatment.
42
4.2.2.3 Plasma From High-Frequency Radiation
Plasma is generated by high-frequency radiation through inductive coupling of the
gas to the impinging radiation. Radio frequency (RF) radiation (10^ to 10* Hz) interacts
with gas molecules within an induction coil. Microwave frequency (10^ to 10'° Hz)
radiation is applied to gas molecules within a waveguide arrangement.
High-frequency plasmas often require the introduction of "seed" electrons to
initiate breakdown. These "seed" electrons can be introduced by a high frequency spark
(i.e., from a Tesla Coil) or a conductor placed in the plasma zone. These electrons
oscillate with the high-frequency applied field and experience inelastic collisions.
Electron collisions knock them out of phase with the field, allowing them to gain energy.
The electrons gain energy until they are more energetic than the outermost electrons in
the gas molecules. Collisions of high energy electrons with gas molecules releases more
high energy electrons in an electron avalanche (Razier, 1991). This electron avalanche is
shown in Figure 4.1.
Figure 4.1: Electron Avalanche in which electrons, represented as black dots, are emitted from the atoms or molecules, represented as open circles.
43
This avalanche of electrons rapidly increases the number of charged particles in the
plasma, in other words, induces breakdown.
Creating and maintaining high energy electrons depends on the ability of the gas
molecules to absorb energy. Electron energy is dissipated through excited states of gas
molecules and elastic collisions. Molecules without rotational and vibrational excited
states have less ability to absorb energy from high energy electrons than molecules with
rotational and vibrational excited states. Monatomic gases (i.e., Noble Gases) do not
have any rotational or vibrational excited states available to them. Diatomic gas ,
molecules have 2 rotational and 1 vibrational excited states available to them. Diatomic
gases can collide with high energy electrons and absorb some energy, placing them in
excited rotational or vibrational states. The result of this is that gases possessing
rotational and vibrational excited states naturally quench high energy electrons and,
consequently, higher power input is required to maintain the plasma state. For example, a
microwave-induced argon plasma requires 150 W of power to maintain the plasma state,
whereas, a microwave-induced nitrogen plasma requires 450 W of power to maintain the
plasma state (Krause and Heh, 1993). High energy electrons are also lost by an number
of processes, including drifting out of the plasma region and contacting reactor walls.
RF induced plasmas have the advantage of being able to isolate the electrode from
the discharge region, preventing electrode erosions and contamination of the plasma with
electrode ions. The wavelength of RF radiation is usually much larger than the plasma
dimensions. This normally creates a relatively homogeneous field inside the plasma
region. RF plasmas can be generated at pressures on the order of 1 atmosphere.
44
The high frequency of microwave radiation makes it impossible for heavier ions
to follow field oscillations. High energy electrons can follow the field oscillations, but
the result is that microwave plasmas normally exist far from thermodynamic equilibrium.
Microwave plasmas can be operated from 1 mbar to several atmospheres. Operation of
microwave plasmas over this large range of pressures makes it possible to create electron
densities from 10* to 10' cm'l The ease of operation, incorporation of flow through the
plasma region, and relatively simple modification of plasma parameters makes
microwave-induced plasmas attractive for plasma reaction chemical investigations.
4.3 A Brief Description of the Physics of Plasmas
The multitude of phenomena that are exhibited by plasmas is a consequence of
charged particles. The electrically charged particles are capable of interacting with
electromagnetic fields, as well as, generating their own electromagnetic fields.
Researchers often study plasma behavior in the presence of both magnetic and electric
fields. Plasmas are generally very good electrical conductors and good thermal
conductors. The ability of plasmas to conduct electricity and heat is due to the high
mobility of electrons.
Plasmas exhibit a variety of difftision characteristics because of charge and
mobility factors. When particle density gradients exist, particles difftise from areas of
high density to areas of lower density. The much smaller particles, electrons, can difftise
much faster and generate an electrical polarization effect. This field enhances the
difftision of the ions and tends to make both electrons and ions difftise at approximately
45
the same rate. This type of diffusion is called ambipolar diffusion (Bittencourt, 1986).
Other types of difftision, classical diffusion and Bohm diffusion, describe movement of
particles across magnetic fields.
An important characteristic of plasmas is their ability to sustain many different
types of waves. Dispersion relations characterize wave propagation modes in plasmas,
examples of these modes are called Alfven waves and magnetosonic waves. Wave
propagation in plasmas is studied to provide information on plasma properties and is
useful in plasma diagnostics.
Dissipative processes dampen wave amplitudes as energy is absorbed from the
wave by plasma particles. Some of these processes have been addressed above. One
mechanism not mentioned earlier does not involve collisions and is called Landau
damping. This mechanism for energy transfer happens when particles become trapped in
the "energy well" of an electromagnetic wave. If a particle in a plasma has roughly the
same velocity as a wave propagating through the plasma, the particle can move in concert
with the wave and the net result is that energy is transferred from the wave to the particle.
Some processes cause instabilities which result in increasing wave amplitudes. These
instability phenomena play a central role in plasma dynamics, especially in hot plasmas.
Instabilities greatly complicate confinement of hot plasmas, like those being examined
for thermonuclear energy research.
The emission of radiation is another phenomena used to characterize plasma
properties. Radiation emission in plasma results from two processes: (i) radiation emitted
by atoms or molecules, and (ii) radiation from accelerated charges. As the process of
46
ionization occurs, the opposite process of recombination also proceeds. As excited
particles combine and decay to the ground state, radiation is emitted and can be observed
in the plasma line spectra. When a charged particle traverses an electric field, it will be
acted upon by forces produced by its own charge and the charge of the particle whose
field it is traversing. Classical mechanics theory states that charged particles accelerated
by interactions with either magnetic or electric fields must radiate energy. This emission
of radiation will decrease the energy of the particle that is emitting it. This type of
radiation emission occurs in many ways and is known as bremsstrahlung.
4.3.1 Criteria for Plasma Occurrence
Without external disturbances, a plasma will be maintained in a macroscopically
neutral state. Since charged particles exist as free-moving entities, there must be some
characteristic distance over which this balance is maintained. Departures from electrical
neutrality only occur over distances that allow balance to be maintained between thermal
particle energy and electrostatic particle energy. This distance is called the Debye length
and departures from neutrality do not occur over larger distances than this. This Debye
length is expressed by
n,e
where, 8 is the permittivity of free space, k is Boltzmann's constant, T is the absolute
temperature, n is electron density, and e is the ion charge. Cmdely stated, a charged
particle in a plasma interacts only with particles located at distances less than one Debye
47
length away. The charged particle has a negligible effect on any particles farther than one
Debye length away.
It is helpful to define a Debye sphere as a sphere inside of a plasma that has a
radius of X^. Electric fields that originate outside the sphere are screened by the charged
particles in the sphere and shield the electric field located at the center. If there is not
enough space for this shielding, a plasma will not exist. The obvious requirement for a
plasma is that the physical dimensions of the plasma system be large in comparison to XQ.
If L is a characteristic length in the plasma, this first criterion of a plasma is written as
L » ^ . (4.2)
Since particle shielding is a cumulative effect, it is necessary that the number of
particles be very large. This second criterion of a plasma is expressed as
He^D » 1 • (4.3)
The third criterion for a plasma is that of macroscopic neutrality, although this
criterion is not independent of the other criterion already mentioned. This aspect has
been incorporated into the analysis previously and is expressed as
He = n i , „ , . (4.4)
Consider a plasma with a charge separation or polarization induced from an
extemal means. Overall, the plasma maintains neutrality, but inside the plasma volume
excess charge has accumulated at each "end." If the extemal disturbance is removed, the
field generated by the charge separation accelerates the particles back towards an
equilibrium poshion. Inertial effects cause the electrons to continue moving beyond the
48
equilibrium position resulting in a charge separation in the opposite direction. This
oscillatory behavior will continue and the frequency of oscillation is called the electron
plasma frequency. The fourth criterion for a plasma is that the electron-neutral collision
frequency be smaller than the electron plasma frequency. It is expressed as
t^pc > t en (4.5)
This condition must be satisfied because collisions of electrons with neutral particles tend
to dampen the oscillations described above. The electrons in the plasma must be able to
act independently, frequent collision with neutrals forces the elec' -ons to be in
equilibrium with the neutrals. Readers interested in more detailed and complete plasma
information should consult Bittencourt, 1986.
49
CHAPTER V
EXPERIMENTAL APPROACH, APPARATUS, AND PROCEDURES
Details of the experimental aspects of this research will be presented below.
Section 5.1 outlines the considerations of the experimental approach and the design
objectives of the system. Section 5.2 presents a detailed description of the experimental
system and how products are analyzed. Section 5.3 describes the experimental
procedures that are executed during experimentation.
5.1 Experimental Approach
The objectives of the experimental system are:
• generate a microwave-induced plasma at atmospheric pressure;
• generate and maintain plasma in an argon, water vapor, oxygen environment;
• isolate the methane-rich stream from severely oxidizing plasma conditions;
• configure the system so reactants can be introduced both upstream and
downstream of the plasma zone;
• ensure streams are well-mixed before entering plasma or reaction zone;
• rapidly and efficiently mix the streams below the plasma zone;
• reliably separate condensable and non-condensable products;
• use available microwave energy source; and
• sample effluent streams for analysis.
50
A viable plasma-based process to convert methane directly to methanol depends
on its ability to process large amounts of gas. Thus, operation at or above atmospheric
pressure is required. Microwave-induced plasmas are one of the few types of plasmas
that can be operated simply, safely, and reliably at higher pressures (~l atmosphere). To
our knowledge, this type of high pressure plasma investigation has not been conducted.
Incorporating reactants (like HjO and Oj) in an Argon plasma operated at
atmospheric pressure could provide essential information. Argon would be used as the
plasma medium as it will easily maintain a plasma at relatively low power. In addition,
argon will not participate in the chemical reactions to the extent that air or nitrogen
would. A schematic diagram of the approach is shown in Figure 5.1.
Argon O2 > H2O
ca
Plasma Reactor
Oxidation Reactor
Products
Figure 5.1. Schematic Diagram of General Approach
Plasma reactors give us the ability to maintain more close control over reactant
mixing and reactant exposure to strongly oxidizing conditions. The reactor will generate
free radicals and then mix that free-radical rich stream with a stream containing methane.
This approach isolates methane and methanol product from the high energy plasma
enviroimient.
51
Figure 5.2 shows schematically how the methane is injected downstream of the
plasma zone and reactions leading to methanol occur at lower temperature and less
oxidizing conditions. The lower temperature of the reaction environment, as compared to
the plasma environment, allows more selective conversion of methane to methanol.
5.2 Experimental Apparatus
A schematic diagram of the experimental apparatus is shown in Figure 5.3. It
consists of four major components; (i) the feed system, (ii) the plasma generation system,
(iii) the reactor system, and (iv) the product collection system. Most of the experimental
apparatus is located inside a walk-in vent hood for safety. Detailed descriptions of each
part of the apparatus are self-explanatory and outlined in the schematic. The product
analysis system is also part of the experimental apparatus but it is not physically attached
to the apparatus and not shown on the diagram. Figure 5.3. The analysis system will be
described in more detail below.
5.2.1 Feed System
The purpose of the feed system is to control the flow of reactants, ensure reactants
are well-mixed, and prepare them for introduction into the reactor system. Gaseous
reactants flow from cylinders through flow control devices (rotameters) and are mixed
into feed streams. One reactant stream (subsequently called the plasma stream) passed
through the high energy plasma generation zone and a second reactant stream (containing
methane and subsequently called the methane stream) is added downstream of the plasma
52
Argon
Waveguide
Microwave Energy
Adjustable Short
Products
CH4 rich stream
Figure 5.2. Schematic Diagram of Reactor Configuration and Methane Injection
53
Cfl
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Ul
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2 <u G (U a
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54
zone. The methane stream may contain other reactants, but it is the only methane source.
As discussed above, methane must be isolated from the highly oxidizing conditions in the
plasma zone to prevent the occurrence of undesirable reactions.
Needle valves and pressure gauges are placed after the rotameters so that
atmospheric conditions caimot bias flowrates. Rotameters were calibrated under constant
pressure conditions. As a result, rotameter feed and exit pressure is monitored and
maintained at these conditions during experimental runs. In this way, the high and low
pressure sides of the rotameters are maintained and the pressure drop over the rotameters
is constant and at the same level as during calibration.
Water is introduced to the plasma stream via a water saturation flask through
which the stream is bubbled. The temperature of the water and the head-space above the
water is monitored. These temperatures determine the amount of water that is introduced
into the plasma stream. It was decided that 1.5 ml was the minimum acceptable amount
of liquid product to be collected during an experimental run. This would assure that
sufficient product would be available for weighing, multiple GC injections, and would fill
the GC injection vials. The maximum amount of liquid product that could be collected
was determined by the size of the collection vessel in the apparatus. When more than 5
ml was collected, the level of product rose to a height that interfered with the smooth
flow of the reactor effluent stream (gas and liquid product). This interference caused
small pressure fluctuations in the apparatus and resulted in displacement of liquid
product, which was unacceptable. Liquid product limits were set at 1.5 ml and 5 ml.
55
Product collection limitations must be weighed against requirement conceming the
duration of the experimental runs. Very low water concentrations would extend the run
times beyond manageable limits and high concentrations would shorten runs making
multiple GC product injections impossible. All of these considerations prove to be
important later, when experimental parameters are modified to improve system operation.
Immediately downstream of the water saturation flask is a manometer to measure the
pressure inside the reactor.
When water is added to the plasma stream, the tubing carrying the stream is
heated to well above 100°C to prevent condensation of water. Silicone coated heating
tapes are used to prevent premature condensation of condensable components in the
system.
Vents are placed on both reactant streams for practicality and safety. The
operation of these vents is addressed in the section describing experimental procedures
and emergency procedures.
5.2.2 Plasma Generation System
The microwave unit used in this research was originally designed to direct the fire
of anti-afrcraft artillery. This unit possesses characteristics that are unlike those of
common commercial microwave generation systems. Detailed information on the unit
can be obtained in War Department Technical Manual TMl 1-1524, (WDTM 11-1524.
1946). The characteristics of the microwave energy that this unit produces and how those
56
characteristics impact this research will be presented in this section. The technical
characteristics of the system used in this research are listed below in Table 5.1.
Table 5.1. Technical Characteristics of the Microwave Generation System
Microwave frequency Wavelength Average power input Average power output Peak power output Pulse width Pulse repetition frequency (prf)
2.700-2.900 GHz 10.3-11.1 cm 600 Watts 300 Watts 210kWatts 7 |is/pulse 1700 pulses/second
The unit is comparable to commercial microwave ovens in some areas. The
frequency of the MW radiation is only slightly higher than that of commercial microwave
ovens (2.45 GHz). The power output of commercial microwave units is now up to 900
W, three times larger than the average power output of this unit.
The product of the pulse width and pulse repetition frequency is the total time that
the unit emits energy per unit time. This unit emits microwave energy only 1.36 % of the
time. In other words, microwaves are emitted for only 0.0136 seconds during a one
second time period. The unit is OFF more than 73 times longer than h is ON. (This may
seem unusual, but an energy source designed for a RADAR system must "look" for
aircraft at extended ranges and then must "listen" for the reflected energy to return to the
source. This is the primary reason for extended off time in this and other types of
RADAR units.)
57
One characteristic of this unit that definitely is not similar to commercial
microwave ovens is the peak power output. The peak power output of this unit is
extremely high (210kW). The high peak power of this unit has important implications on
the ability of this unh to breakdown and sustain the plasma state. Because of this high
power, contacting the magnetron located atop the modulator can be fatal. Hence, the
magnetron is covered by a protective metal mesh cage.
Often, in plasma studies, there are issues with initiating the plasma and
maintaining it. Variations in concentration of reactants or unsteady flow conditions have
been known to extinguish the plasma. The plasma generation units used in many other
studies do not possess the high power of this unit. (This particular unit did not ever allow
the plasma to extinguish under any flow condition or set of reactant concentrations.)
Sometimes, the plasma self-ignited as the energy was increased to operational levels.
This unit was capable of maintaining a plasma in air reliably, even though it has been
reported that air plasmas require up to 450 W of energy. This unit overcame potential
problems associated with maintaining the plasma state during experimental runs. The
high power of this unit is probably very inefficient due to losses, but plasma operations at
atmospheric pressure will demand higher peak and average power output.
We were unable to measure forward and reflected power levels due to lack of
equipment. The requirement for liquid cooling and pressurization inside the waveguide
indicates a significant amount of energy is being consumed by processes other than the
reaction inside the quartz reactor. It is assumed that only a fraction of the microwave
58
energy generated is absorbed by the reacting species. The energy efficiency of this
system was not an area of investigation.
The microwave generation unit is physically located outside of the vent hood and
apart from the bulk of the experimental system. The magnetron produces microwaves
that are carried in the waveguide to the reactor system. Figure 5.4 shows the waveguide
and quartz reactor tube configuration. Downstream of the reactor is a tunable "short"
which reflects the energy back through the waveguide. This allows the microwave
energy to resonate on the quartz reactor in the waveguide. The microwave energy.forms
what is known as a "standing wave" which imparts the maximum amount of energy into
the plasma stream inside the quartz tubing. The "short" is manipulated to adjust the
"standing wave" and maintain the plasma in its most vigorous state (i.e., brightest and
loudest conditions).
Reflected microwave energy is prevented from re-entering the magnetron with a
waveguide coupler. (This coupler blocks energy moving back towards the unit but allows
energy moving in the other direction to pass.) The waveguide also has "sleeves" attached
to the waveguide to prevent radiation from leaking out the gaps through which the quartz
tube passes. These "sleeves" are long enough (>7 inches) to dissipate the magnitude of
microwaves that propagate down the "sleeves." The quartz reactor tube is held in place
by fittings on the sleeves.
59
"CAJON" Fittings
Quartz Reactor Tube
Brass Sleeve
Brass sleeve CAJON'
Fittings
Figure 5.4. Schematic of Waveguide and Quartz Reactor Tube Configuration
60
5.2.3 Reactor System
The reactor system is the region in which the reactant streams and high energy
microwaves are combined. The reactor system really consists of two reaction regions as
depicted in Figure 5.2. A 24 inch, 17 mm OD, 15 mm ID quartz tube is the heart of the
reactor system. The quartz tubing is connected to the metal "sleeves" via Cajon" ^
fittings. These fittings enable airtight connection of glass or quartz with metal fittings by
using tough, flexible o-rings. The o-rings also maintain an airtight seal at the high
temperatures of the quartz tubing. The connections are well separated from the
microwave radiation and the highest temperature experienced by the o-rings is
approximately 150°C.
The plasma stream enters the top of the tube at elevated temperature (~140°C). A
smaller quartz tube (24 inch, 6mm OD, 4mm ID) is placed inside the larger tube
downstream the plasma zone (i.e., from the bottom). The methane stream is introduced
from the bottom through this smaller tube. It flows vertically upward into the plasma
stream that is flowing in the opposite direction. After mixing, the combined stream flows
downward in the annulus of the two tubes. The point of methane injection is adjustable
and can be moved closer or farther from the high energy radiation area (i.e., plasma
zone). All lines downstream of the reactor system are maintained at elevated
temperatures (>120°C) to prevent premature condensation of reactants or products.
Quartz is used exclusively to confine the reaction streams inside the waveguide.
Due to its excellent thermal properties, quartz remains a solid at temperatures above
61
1500°C. Disadvantages of using quartz are that it is very brittle and glassblowers often
have difficulty fashioning quartz into shapes.
One feature of the reactor system deserves special mention at this point. The
waveguide is designed to direct energy into the quartz reactor tube but microwave energy
is present everywhere inside the waveguide. The close proximity of the outside of the
quartz tube to the inside of the waveguide wall creates a favorable environment for
breakdown to occur. This breakdown degrades both the waveguide surface and the
outside of the quartz reactor tube. To preclude this from occurring, the waveguide is
pressurized with COj (-30 psi). As discussed above, the high pressure diatomic CO2
creates an environment that the microwave radiation cannot breakdown. Water flows
continuously through cooling coils mounted on the outside of the waveguide to prevent
hazardous waveguide temperatures.
5.2.4 Product Collection System
The methane stream is mixed with the plasma stream in close proximity to the
plasma zone (<1.5 inches in all cases). The plasma stream cools very rapidly. Mixing
with the cold (i.e., 120°C) methane stream ftirther cools the final product stream. This
product stream passes through a short stainless steel tube into an unheated section of
tubing oriented vertically downward that leads to the condenser and collector section of
the apparatus. The vertical orientation or this tube is important here because the first
drops of condensation must be prevented from accumulating anywhere in the apparatus.
62
Drops that do condense in this section fall down the tube into the condenser and
collection vessel.
The condenser consists of the tube passing through a water/ice mixture. The
subsequent cooling of the product stream, inside the tube, forces condensable products
out of the vapor phase. The product stream then passes into a collection vessel that holds
all liquid products (mostly water). Condensed liquid product accumulates in the chilled
collector for the duration of the experiment. Upon completion of the experiment, the
product is weighed and placed in an airtight sample vial for subsequent analysis by gas
chromatography.
The now cool (~5°C) gas product leaves the collection vessel and passes through
Tygon' ' tubing into a flow measuring device (bubble-meter). The flow rate of the exit
gas is recorded throughout the experiment with the bubble-meter and an electronic timer.
The noxious gas product exits the top of the bubble-meter and is vented.
5.2.5 Product Analysis
Both, gas and liquid, products were analyzed using a Hewlett-Packard (HP) 5890
series II Gas Chromatograph (GC). Gas products were analyzed during experimental
runs by taking gas samples from the exit stream with a gas-tight syringe and injecting into
the GC. Gas sample analysis required approximately 30 minutes between runs and at
least 2 samples were taken during each run. Liquid samples were collected, weighed and
transferred to amber sample vials to prevent breakdown of products sensitive to UV light.
63
These samples were stored in a refrigerator until the GC was reconfigured for the liquid
analysis at a later date.
5.2.5.1 Gas Product Analysis
Since the analysis system was not arranged for on-line measurement, the GC
operation and calibration had to be coordinated with operation of the experimental
system. This coordination will be evident when the experimental procedures are
presented in section 5.3.
Before gas samples can be analyzed, the GC must be initialized and operating
satisfactorily with a steady baseline. The gas samples were analyzed using a Supelco
packed column (15ft x 1/8 inch stainless, 60/80 mesh, Carboxen™ 1000). Proper GC
operation was checked before each experimental run with two calibration runs that
analyzed two different calibration gases (see appendix A for calibration gas specifics).
Gas product was injected into a gas sampling loop in the side of the GC. An
automatic switching valve injected the V2 ml sample into the column. Data collection was
automatic via HP Chemstation Data Analysis Software. Data files were stored in digital
form and analyzed at a later time. Gas sample components were detected using an HP
Thermal Conductivity Detector (TCD). This was the most appropriate detector for the
variety of components of interest to us. The gases that were detectable included all (even
trace compounds) of the hydrocarbon reactants (methane, ethane, ethylene, acetylene) and
the permanent gas products (carbon monoxide, carbon dioxide, hydrogen). Oxygen and
argon were not reliably separated or detected by this system.
64
5.2.5.2 Liquid Product Analysis
Liquid samples were analyzed after a series of experiments had been completed so
that many samples could be automatically injected and analyzed without supervision.
Since changing columns was fairly involved and column conditioning was slow, the
ability of the GC to rapidly analyze liquid products significantly lessened the intervals
between experiments. As with gas samples, data was collected digitally by HP
Chemstation Data Analysis Software and analyzed at a later time.
Before experimentation began, liquid calibration samples were analyzed to
determine how long they could be stored before product degradation occurred. The first
signs of detectable breakdown appeared somewhere between 14 and 21 days. The
longest interval between collection and analysis for any experimental sample was 8 days.
This is well within the 14 day limit that marks the earliest possible onset of breakdown.
Liquid samples were analyzed using a Supelco Supelcowax 10' ' capillary column
(30 m X .25 mm, .25 fim film thickness). The liquid sample components were detected
using a Flame Ionization Detector (FID) that was installed on the same GC. Liquid
products that were detectable included methanol and other compounds present in the
product (e.g., trace amounts of acetic and formic acid). The product consisted mostly
(99.9+%) of water.
5.3 Experimental Procedures
This section presents and discusses all procedures used during experiments. It
also examines the operational checklists which guided system operation. Consistent
65
procedures and checklists are required in any experimental undertaking because they
increase experimental reliability, increase safety, and decrease researcher workload.
5.3.1 System Preparation and Warm-up
The first step in the preparation phase of this experiment was to calculate the
desired flowrates and temperatures for the experiment. These flows and temperatures
were converted into rotameter readings and heater settings on the apparatus. The
conversions were performed using calibration information. Since microwave power was
not changed from run to run, it was not necessary to make modifications to the plasma
generation system.
Preparation of the experimental apparatus consists of many activities. The
experimental preparation checklist is presented below for reference and involves
obtaining supplies required for the run, cleaning and connecting components, placing
valves and switches in the correct positions, and safety. The steps in the preparation
checklist are self-explanatory.
5.3.1.1 Experimental Apparatus Preparation
Before beginning experimentation, ensure the following tasks are completed.
1. Vent hood fan and light ON (door closed as far as possible, unless entering/leaving).
2. Ensure condenser and collector are prepared vdth sufficient ice.
3. Clean saturation vessel and refill with water then re-connect to system.
4. Ensure scale is available and operational (for weighing glassware and product).
66
5. Ensure sufficient supplies of reactant gases and pressurization gas is available.
6. Ensure quartz reactor and injection tubes are clean and ready for connection.
7. Ensure appropriate bubble-meter is connected and ready for use.
8. Place vent valves (VI and V2) in the VENT position.
9. Ensure that lower CAJON"^^ fitting is disconnected so plasma can be ignited.
5.3.1.2 Experimental Apparatus Warm-up Procedure
The purpose of the warm-up phase procedures is to bring the system to steady-
state operation at the specified flows, temperatures, and reaction conditions. The name,
warm-up phase, may be misleading because the system is not only "warmed-up" but also
brought to a fijlly operational state during this phase. A primary consideration in this
phase is that the condenser section must be fully "wet" with product before the actual
experimental run is started. This is important because only a few drops of product
accumulation in the system can create large errors. This phase of the experiment may last
longer than the actual experimental run as the system settles into steady operation and
"wets" appropriate sections. Most of the steps in the experimental apparatus warm-up
procedure are self-explanatory. For other steps, ftirther explanation is provided.
1. Apply power to heating elements.
2. Prepare and install condenser, collector, and preliminary collection vessel.
3. Set reactant gas flowrates to desired levels.
4. Adjust pressure of streams exhing rotameters to appropriate level using needle valves.
5. Close valve VI to supply plasma stream to reactor section.
67
6. Ensure waveguide pressurization is connected and tight.
7. Pressurize waveguide (30 psi max), check for leaks.
8. Check cooling water connections and set water flow to appropriate level.
9. Complete the Microwave Generator Operating Procedure (operations manual).
(Note: This procedure outlines the proper way to energize the MW generation unit.)
10. Ensure plasma is lit
(Note: If plasma is not lit, refer to the operations manual)
11. Connect quartz reactor outlet to product collection section with CAJON" ^ fitting.
12. Connect methane injector quartz tubing to reactor system.
13. Check reactant flows and adjust, if necessary.
14. Close valve V2 slowly.
15. Continuously monitor system for leaks, overheating, failures, or malftmctions.
16. Monitor system for adherence to desired conditions and allow it to run until
significant liquid product is collected.
5.3.2 Experimental Run Procedures and Checklist
During the preparation and warm-up phase of the experiment, the GC calibration
should be underway. Both procedures (experimental preparation/warm-up and GC
calibration) take approximately one hour. The GC must be ftilly prepared for injection
when the experiment is started. A gas sample be injected immediately, or very soon
(within 10 minutes), after the experimental run commences because 2 samples must be
injected before the end of the experimental run. Depending on the level of water vapor
68
present, a complete experimental run may require anywhere from 35 minutes to 75
minutes. Since the analysis of gas samples takes approximately 30 minutes, slight delays
in gas injection can significantly extend the time required for an experimental run.
Data that should be recorded or calculated before an experimental run includes; (i)
atmospheric temperature and pressure, (ii) temperatures of water saturator unit, and (iii)
rotameter readings. When all experimental settings are steady at desired levels, the
experimental run can begin.
5.3.2.1 Experimental Run Operational Procedure
For simplicity, the experimental run procedures contain all information through
the final disposition of the liquid product. The experimental procedures contain items
that comprise the shutdown of the apparatus. This is because removal of liquid product
occurs after some of the apparatus has been shutdown. Shutdown is not separated
because it only takes about 5 minutes and is easily incorporated with the operational
procedure.
1. Note and record time.
2. Exchange collection tube with a clean, labeled, and pre-weighed collection tube.
3. Inject sample of reactor effluent into GC for analysis.
4. Check, adjust (if necessary) and record all temperatures, pressures, rotameter flows
(at least 4 times during experiment).
5. Record gas product flow-rate (5 readings) with bubble-meter every 10-15 minutes (at
least 4 times during experiment).
69
6. Continuously monitor system for leaks, overheating, failures, or malftmctions.
7. When GC has completed first gas sample analysis, collect and inject second sample.
8. When sufficient liquid product collected, record all system data, denote time, and
place VI and V2 in VENT positions (to discontinue reactant flows to reactor).
9. Close supply valves on all reactant cylinder regulators.
10. Complete Microwave Generator Shutdown Procedure (operations manual).
11. Unplug heating elements.
12. Disconnect collector tube, weigh tube and sample, place liquid sample in amber
sample vial, seal and label vial, then refrigerate.
13. Place condenser and collector units into refrigerator (to prepare for next run).
14. Turn off supply of waveguide pressurization gas at cylinder.
15. Turn off waveguide cooling water.
16. Do not touch quartz parts for at least Vi hour.
17. Monitor GC for completion of analysis, follow GC shutdown procedures.
18. When the system is cool, disconnect the reactor quartz parts and clean all
components in preparation for next run.
5.3.3 Safety and Emergency Procedures
Specific mention of the dangers and emergency procedures is usually exiled to the
appendix in studies such as this. The safety considerations that deal with operation of
this unit are numerous. Most deal with electrical current and radiation. The reader is
referred to WDTM TMl 1-1524 for specific safety information, especially that conceming
70
hazardous components and grounding. In the case of plasma reactors, the potential for
injury or death is real and safety considerations have to be addressed.
Primary dangers associated with the microwave system have been presented
above. One consideration that has not been addressed is radiation leaks from the
waveguide. In the case of this experiment, the waveguide was constructed, assembled,
and tested for leaks by the Department of Electrical Engineering at Texas Tech
University. Leak tests were conducted before experimentation began and after it was
completed. Radiation leaks are a potential hazard in almost any radiation system.
Another area of concem with plasma systems is high energy or temperature. As
addressed above, quartz and metal parts have the potential of reaching very high
temperatures. High temperature metals often radiate significant heat when at elevated
temperatures. Quartz components will not give such indications when they are hot. A
piece of quartz at room temperature appears much like a piece of quartz at 300°C. In fact,
quartz at 800°C may only look "warm." Quartz must be respected as a hazardous item in
these systems. It is not hazardous in a chemical or biological way, but it has physical
characteristics that make it capable of collecting extremely high amounts of energy.
Operation of this system in a vent hood with the door closed is vital for safety.
This system produces significant carbon oxide products (CO and CO2). These and other
products can produce a hazardous or deadly environment if they are not handled
correctly. Entry into the hood is required but, time spent inside the hood was minimized.
71
Safe operation of this system demands the use of personal protective equipment.
Anytime this system is in operation, several components can severely bum the skin,
instantly. Protective gloves and long sleeves are required for safety.
5.3.3.1 Emergency Shutdown Procedure
An emergency can take many forms and it is always difficult to formulate a
general procedure to account for all situations. Since many components of this system
are isolated inside a vent hood, formulation is doubly difficult. The primary concem is to
stop the generation of all microwave energy. This is simple because the controls for this
system are outside of the hood. Securing the flow of reactants is a problem because the
cylinders are inside the hood. The vent valves are also inside of the hood. The final
emergency procedure must be short and this one contains only four steps. Complete as
many of the steps as allowed by the emergency and exit quickly.
1. Depress Contactor Control Trip Button on MW Rectifier (identified by red arrow).
2. Place valve VI and V2 in Vent position.
3. Tum off all gas flows at cylinder regulators.
4. Close door to vent hood.
72
CHAPTER VI
RESULTS AND DISCUSSION
The experimental portion of this work was conducted in four phases. Section 6.1
presents results from the Preliminary Phase of experimentation. Sections 6.2 through 6.4
outline the resuhs from Phases I, II, and III. Section 6.5 presents a summation of the
overall performance of the experimental system. A discussion of the sources of error
present in this system is presented in Section 6.6.
6.1 Preliminary Phase Experiments
The initial phase of experiments was conducted to hone system operation and test
process parameters that would serve as starting points in later experiments. The
experiments conducted during this phase used water as the only oxygen source for
methanol production. Earlier researchers have indicated water to be important in the
production of methanol from methane and it is a vital component in the experimental
system used in this research. Methane reaction with water is very unfavorable compared
to methane reaction with molecular oxygen. The later experimental phases of this
research (Phases I, II, and III) also include water but, molecular oxygen serves as the
primary oxygen source for methanol production. Raw experimental data for all of the
runs can be found in Appendix C.
Two vital factors conceming system operation were examined during the
Preliminary Phase. The first was the injection distance. This is the distance from the
edge of the waveguide to the end of the methane injection tube. It was important to
73
locate the appropriate injection distance for the initial experiments. The second factor
was the amount of water present in the plasma stream.
The experiments that were conducted during the Preliminary Phase
involved injection of methane at different distances. These distances ranged from -0.25
inches (0.25 inches inside the waveguide) to 1.5 inches (1.5 inches from the edge of the
waveguide). Table 6.1 shows the Preliminary Phase experimental parameters.
Table 6.1. Preliminary Phase Experimental Parameters
Experiment
0925 0930 1008 1009 1013 1014 1030 1103 1104 1106
1106-2
Injection Distance (inches)
1.50 0.75 0.25 -0.25 0.00 0.13 0.50 0.13 0.25 0.31 0.00
Ar/CH4 Ratio
6.65 6.65 6.65 6.65 6.65 6.65 10.94 10.83 11.11 10.83 10.83
CH4/H2O Ratio
2.24 1.93 2.15 2.13 1.97 2.03 1.25 1.16 1.09 1.22 1.18
Methane Conversion
(%)
0.09 0.26 1.82
24.65 10.84 6.47 1.96 13.27 7.38 5.02 14.14
Methanol Selectivity
(%)
0.0906 0.5546 0.2500 0.0046 0.0411 0.0995 0.3620 0.0741 0.2840 0.2737
. 0.0549
Figure 6.1 shows how the injection distance is defined. It was reasoned that the
distance of methane injection would affect the extent of methane conversion. Injection
too close to the plasma zone would greatly increase methane conversion but would
probably produce excessive carbon oxides and C2 hydrocarbons, as outlined by prior
researchers.
74
Waveguide
\
Mw Energy ^
Methan Injection Distance
Quartz Reactor Tube
Plasma Zone
Methane Injection Tube
Figure 6.1. Schematic of Injection Distance, depicted above, is the distance from waveguide edge to end of methane injection rod.
75
Injection too far from the plasma zone would isolate the methane from oxidizing
conditions and prevent significant conversion of methane.
The other factor vital to system operations is the amoum of water presem in the
plasma stream. Water is the primary componem in the condensed product. If only a very
small amount of water is used during the experimental run (i.e., 0.01 ml in a 1 hr
experiment), the tiny drop can easily remain in the product collection section of the
apparatus. Water must be present in sufficient quantity to obtain samples for injection
into the GC. Water concentration requirements will be addressed in more detail when the
results of this phase are presented.
At this point, it is necessary to define, explicitly, the terms conversion and
selectivity, due to the variety of accepted definitions in use. For this research, conversion
will be defined as Methane Conversion = "^^thane reacted methane fed to reactor
Methanol selectivity will be defined as
, , , , ^ , . . methanol produced Methanol Selectivity = .
methane reacted It is noted that the product of methane conversion and methanol selectivity is methanol
methanol produced yield. Methanol Yield = .
methane fed to reactor
Yield is often used as a performance measure. Yield will not be discussed in this
research because it obscures information about vital performance issues, namely methane
conversion and methanol selectivity. For example, a yield of 5% can mean that methane
conversion is 100% and methanol selectivity is 5%, or that methane conversion is 5% and
methanol selectivity is 100%, or that conversion and selectivity levels exist somewhere in
76
between. Yield obscures these vital performance parameters and is not an appropriate
measure to evaluate reactor performance in this case.
6.1.1 Injection Distance
Figure 6.2 shows the conversion of methane plotted against the injection distance.
Methane conversion increases as the methane stream is injected closer to the waveguide.
In fact, examination of the data shows that conversion increases sharply as the injection
point is moved closer to the waveguide. This result confirms the assumptions conceming
injection distance effects on conversion that were made in section 6.1.
The primary aim of the Preliminary Phase was to select a range of injection
distances that would result in satisfactory conversion for the next experimental phase
(Phase I experiments). Since high methanol selectivity is only observed at low methane
conversion and high selectivity is a primary consideration for methane conversion to
methanol, it will probably be advantageous for this research to focus on lower conversion
levels (less than 10%). Low conversion also minimizes the waste of methane because
unconverted methane can be recovered for later use.
It is important to note that the preliminary experiments used only water and
methane as reactants (in an argon carrier gas). As noted above, the reaction between
methane and water is much less thermodynamically favorable than the reaction between
methane and molecular oxygen. The next phase of experiments (Phase I experiments)
was designed to include molecular oxygen and probably exhibhs higher conversions than
the methane/water experiments, at comparable injection distances.
77
onve
rsio
n.
Met
hane
C
0.25
0.2
0.15
0.1
n ns \J.\JJ i
n -
^
1 + i
i
i :
; 1
-
—
•
•
-
- - —
^
•
1
1 1
j
i
J
k 1 • 1
-0.5 -0.25 0 0.25 0.5 0.75 1
Injection Distance (inches)
1.25 1.5 1.75
I Preliminary Phase Experiments (11 runs, no oxygen) [
Figure 6.2. Preliminary Phase-Methane Conversion Versus Injection Distance.
78
Figure 6.2 shows that conversion levels of 3-6% are obtained at an injection
distance of roughly VA inch. An even more highly reactive componem mixture, due to the
presence of molecular oxygen, was used in the next phase of experiments. Since
conversions were expected to increase in the more reactive environment, it was
reasonable to assume that injection at 'A inch would result in conversions higher than
those desired (-10%). For this reason, a lower injection distance limh of 3/8 inches (i.e.,
injection no closer than 3/8 inches) was adopted for the next phase of study. Since
detectable conversion occurred at 1.5 inches, this distance was adopted as the upper
injection limh (i.e., most distant) for the next phase of study.
Two injection distances have been selected as the injection limits for investigation
in the next phase. A third injection location, between the endpoints, was tested. This
point was selected to help cover the range of injection more completely. The conversion
obtained at these three points is expected to exhibit similar behavior as the that shown in
Figure 6.2, higher conversions at closer injection distances. Since conversion increases
as the point of injection approaches the waveguide, the intermediate point will be selected
nearer to the least distant endpoint, where the conversion should be changing at a higher
rate. The third injection point was selected to be 5/8 of an inch. So, for the next phase of
experiments (Phase I experiments), 3/8 inches, 5/8 inches, and 1.5 inches will be the
injection locations investigated. A precise analysis or calculation of the ideal injection
distance is not possible as reaction conditions will change drastically when molecular
oxygen is added.
79
6.1.2 Water Concentration
The experimental apparatus has limitations on the amount of water that can be
used in each experiment. During the preliminary phase, the water concentration was
altered by changing the methane flowrate. The methane-to-water ratio varied from -1.1
to 2.3 during the 11 preliminary phase experimental mns. The water concentrations
tested in the preliminary phase produced sufficient liquid product and resulted in
satisfactory experimental mn times. A more detailed investigation conceming water
concentration (i.e., numerous experiments with significant variation in water
concentration) was not warranted until molecular oxygen was introduced as a reactant.
At this point, it was important to discover which water concentrations would enable
satisfactory system operation and to investigate limits on water vapor concentration.
6.1.3 Methanol Selectivity Dependence on Methane Conversion
A recurring phenomenon in methanol synthesis by direct oxidation of methane is
an apparent dependence of methanol selectivity on methane conversion. An inability to
achieve high levels of methane conversion and methanol selectivity, simultaneously, is
often referred to in the literature and represents the limiting factor in direct methanol
synthesis from methane. Figure 6.3 shows methanol selectivity plotted against methane
conversion for the preliminary phase experimental runs. Although the preliminary phase
experiments were conducted over a wide range of conditions, the results from these
experimental runs illustrate the dependence of methane conversion on methanol
selectivity. The data are very poorly correlated due to the wide range of conditions. It is
evident that higher selectivhies occur only at lower conversion levels. As more methane
80
93
93
0.006 u Z 0.005 B
J3 'o
I 0.004
i.» B
>
C/3
J3
0.002
0.001
0
0 0.05 0.1 0.15 0.2 0.25 0.3
Methane Conversion (mole methane reacted/ mole methane fed)
Preliminary Phase Experiments (11 runs, no oxygen)
Figure 6.3. Preliminary Phase-Methanol Selectivity Versus Methane Conversion
81
is converted, less and less converted methane leads to methanol formation. The optimum
progression would be to obtain high methanol selectivity at high methane conversion.
Methanol selectivity dependence on methane conversion will be used to illustrate system
performance improvements in ftiture experimental phases.
6.2 Phase I Experiments
Phase I experiments were designed to investigate the impact of three different
levels of oxygen on methanol production. Table 6.2 shows the experimental parameters
from Phase I runs. These mns were the first to contain molecular oxygen (O2), which
served as the primary oxygen source for methane oxidation to methanol.
Table 6.2. Phase I Experimental Parameters
Experiment
1118 1124 1125
1125-2* 1126 1129 0124 1201
1201-2 1202
Injection Distance (inches)
3/8 3/8 3/8 5/8 5/8 5/8 5/8 1.5 1.5 1.5
Ar/CH4 Ratio
6.56 6.56 6.56 6.56 6.56 6.56 6.56 6.56 6.56 6.56
CH4/O2 Ratio
3.88 4.44 2.97 2.93 3.82 4.44 2.93 2.88 3.82 4.44
CH4/H2O Ratio
1.76 1.88 1.92 1.55 1.72 2.02 1.59 1.91 1.91 1.84
Methane Conversion
(%)
13.500 7.850 7.613 0.167 2.430 1.705 1.029 0.054 0.024 0.027
Methanol Selectivity
(%)
0.269 0.424 0.728 8.042 0.301 0.652 1.242 0.927 0.901 1.060
Experimental run 1125-2 is denoted by an asterisk because it produced highly
erroneous results. The primary reason it was discarded is that the run produced virtually
no CO or CO2 product. Run 0124 was conducted to replicate the conditions of 1125-2.
82
The results of run 0124 were significantly differem than mn 1125-2 as can be observed in
the data. Run 0124 achieved results that compared favorably with similar mns and
carbon oxide products were detected in appreciable quantities. It was surmised that a
serious experimemal error occurred during mn 1125-2 that lead to the unbelievable
results.
m
6.2.1 Injection Distance
The effect of injection distance on conversion for Phase I experiments is shown i
Figure 6.4. Increases in methane conversion are evident at closer injection distances.
Moreover, the conversion levels at the closer injection distances are in the preferred range
for this research (<10%). Since the reaction of methane and oxygen is many orders of
magnitude more favorable, thermodynamically, than the reaction of methane and water, it
is surprising that the increase in methane conversion is not more dramatic. Injection at
3/8 inches resulted in satisfactory conversion (<10%) of the reactant mixture. The 3/8
inch injection distance was, initially, chosen to be the sole location for the Phase II
experiments. It was discovered, during Phase II experiments, that injection at 3/8 inches
did not result in sufficient conversion of the reactant gases. To obtain an improved range
of conversion, a second injection distance had to be tested. Injection at V* inch was also
tested in Phase II experiments.
6.2.2 Water Concentration
Although variations in water concentration were not planned for this experimental
phase, small but measurable variations in water concentration occurred, nonetheless.
83
i I 9i I 1
I § Ui
I O U
0 14
0 12
0 1
0 08
006 -
0 04 -
0 01 \l.\3^
0 -
1 I
; 1
X
i
i !
1 1 • !
; i
\ \
X X X
i 1
1 1 1
K —
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6
Injection Distance (inches)
X Phase I Experiments (9 runs, 3 inj. locations, runs include oxygen)
Figure 6.4. Phase I-Methane Conversion Versus Injection Distance
84
These variations were caused by imprecise heating controllers used in the apparatus. The
changes in water content, although slight, were examined to identify the effects of water
content on system performance. Upon examination of the limited (10 experiments)
experimental data, it was impossible to draw a conclusion about water content influences
on performance. The data failed to indicate a trend that would guide reaction conditions
for future experimental runs. Based on this, it was decided to test two levels of water
concentration in the next phase of experiments (Phase II experiments). One level would
be at or near the maximum amount allowed by experimental constraints, another at an
intermediate level.
6.2.3 Oxygen Concentration
Figures 6.5 and 6.6 show methane conversion and methanol selectivity,
respectively, plotted against oxygen concentration. The graphs do not indicate that
higher oxygen concentrations increase conversion and decrease selectivity. In fact,
though the data are not correlated strongly, there is some indication that higher oxygen
concentration actually increases selectivity. The effects of oxygen on this chemical
system are well-known. The most highly oxygenated products of methane oxidation (CO
and CO2) are the most favored, thermodynamically, but the least desirable. The desired
partial oxidation product (methanol) is highly reactive to molecular oxygen. Any
methanol that is formed during reaction should be isolated from oxygen in reactive
conditions. Less oxygen should favor less oxygenated products and minimize
consumption of methanol through oxidation reactions. From these considerations alone,
lower oxygen concentrations should favor the production of methanol. Less oxygen has
85
0.16
T3
93
93 • • -»
S c o Ui Wi U > C o U
0.14
0.12
0.1
0.08
0.06
u 0.04
93
;s 0.02
0 0.025 0.027 0.029 0.031 0.033 0.035 0.037 0.039 0.041
Oxygen Concentration (mole fraction)
0.043
I Injection Distance at 3/8 inches •Injection Distance at 5/8 inches • Injection Distance at 1.5 inches
Figure 6.5. Phase I-Methane Conversion Versus Oxygen Concentration
86
0.014
G 0.012
I >
1 3
I
0.01
0.008
0.006
0.004
0.002
0.025
A
•
•
•
. •
4
•
•
A
1
0.03 0.035 0.04
Oxygen Concentration (mole fi-action)
0.045
I Injection Distance at 3/8 inches •Injection Distance at 5/8 inches A Injection Distance at 1.5 inches
Figure 6.6. Phase I-Methanol Selectivity Versus Oxygen Concentration
87
the added benefit of decreasing methane conversion, which has been found to improve
selectivity and will conserve unconverted methane for recovery and use. Consequently,
the experimental data was expected to show that lower oxygen concentrations improve
system performance. This obvious inconsistency indicates that other factors are
contributing to system performance.
6.2.4 Mixing Effects
The impinging flow pattem that was designed into this system (see Figure 5.2,
"head-on" mixing of counter-current flows in concentric tubes, followed by exh of the
mixture through the annulus formed by the two tubes) is unique when compared to other
plasma experimental setups. The aim of this mixing design is to completely mix the two
streams (plasma stream and methane stream) as rapidly as possible. Mixing two streams
that are flowing in opposite directions has a more dismptive effect than mixing of co-
current flows, under similar flow condhions. A co-current flow configuration in
concentric tubes allows stream mixing to occur more slowly, allowing the reaction
conditions to change, or "decay" from a favorable state to a different or less favorable
state. Rapid mixing is doubly important in plasma systems where temperature quench
rates can be extremely high (>10^ K/minute) (La Due, 1993). This conceptual evaluation
of mixing makes the assumption that condhions at the mixing point are favorable for
methanol production. If this is tme, accelerated mixing of the streams would enable more
production of methanol.
Figure 6.7 shows an expanded view of the mixing region of the plasma stream
and the methane stream. The mixing point occurs directly above the methane injection
88
Plasma Region
• T y i i >r
A A A
n j< A
Methane Injection Tube
Highly Turbulent Mixing Region
Oxidation Zone
Quartz Reactor Tube
Figure 6.7. Schematic of the Mixing Region (formed when the plasma stream and methane stream collide inside the quartz reactor mbe.)
89
tube as the methane stream encounters the hot plasma stream flowing toward it. The
methane stream must completely change direction and move toward the outer tube wall
so that it may exit down the annulus formed by the two quartz tubes. This flow
configuration was incorporated to improve and accelerate mixing, but had a more
profound impact than expected on overall system performance.
As the plasma stream passes through the plasma region, it experiences a
tremendous temperature increase. A thermocouple inserted into the plasma region from
above indicated a temperature increase from ~150°C to over 1400°C, over just a few
centimeters distance. The temperature indicated by the thermocouple rapidly fell to
approximately 400°C when the thermocouple was moved to only 1 inch below the
plasma region. The non-equilibrium nature of the plasma indicates that the energy of the
charged species (called the electronic temperature) corresponds to a temperature higher
than that indicated by the thermocouple. This rapid heating and cooling is accompanied
by turbulence and gas velocity fluctuations. These conditions are expected in species in a
highly energetic or plasma state.
It is logical for changes in the plasma stream or methane stream to affect the size
and shape of the mixing region. An increase in plasma stream flowrate necessitates a
higher velocity through the tubing, especially as the stream experiences rapid heating.
The plasma stream collides with the methane stream at a higher velocity, reducing the
vertical penetration of the mixing region into the plasma stream. Effectively, this
increases the injection distance. It has already been established that more distant
injection results in decreased conversion and, often, increased selectivity.
90
Phase I experiments involved investigation of the effects of changing the amount
of oxygen in the plasma stream. Any increase in conversion and decrease in selectivity
resulting fi-om the thermodynamics of higher oxygen concentrations is in direct
competition with the decrease in conversion and increase in selectivity resulting from the
physical effects of mixing described above. The mixing effects seem to offset the
anticipated effects of higher oxygen concentration.
For investigations conducted at low pressure, temperature increases are much
more modest and mixing effects such as the ones discussed here may have less impact.
These and other mixing effects probably apply and should be considered in other plasma
studies conducted at atmospheric pressure. Other significant mixing phenomena may
arise from the rapid heating, cooling, and mixing of plasma streams. These effects will
be completely dependent on flow pattems and reactor configuration. Again, these effects
were not considered in the design of this experiment but do explain the unusual behavior
that was observed.
It is desired to find operational conditions under which this system will produce
methanol in the most plentifiil and efficient manner. The primary aim of Phase I
experiments was to determine the best oxygen concentration (of 3 levels tested) for
optimum methanol production. The data are inconclusive as to what oxygen level tested
is best. The primary consideration in direct production of methanol is high selectivity for
methanol, for reasons presented earlier. Accepted chemical principles indicate that lower
oxygen concentrations should favor high selectivity for methanol. Experimental data are
inconclusive, possibly due to conditions created by mixing phenomena that were not
anticipated. A decision must be made regarding oxygen concentration. It was decided to
91
operate with low oxygen concentrations for the next experimental phase. This decision
was reached because it is aligned with accepted chemical principles and a potential
explanation exists for the inconclusive experimental data. If this decision is incorrect and
oxygen concentrations should be high for increased selectivity, this will be indicated by
ftiture experimentation.
62J—Overall Performance of Phase I Experiment*;
Phase I experiments included molecular oxygen and improved methanol
production was expected. Figure 6.8 shows the overall performance of Phase I data
compared to Preliminary Phase data. The trendlines are included to show how the
separate data sets compare and do not imply linear or ftmctional dependencies. The
expected improvements in methanol selectivity are apparent and methane conversions are
at acceptable levels(=10%). Performance is improved, albeit modestly. The data also
exhibh characteristic methanol selectivity dependence on methane conversion, with
higher methanol selectivity observed at lower methane conversion.
6.3 Phase II Experiments
The goal of Phase II experiments was to improve system performance by attaining
higher levels of selectivity for methanol. The injection distance and oxygen concentration
levels were adjusted, enabling the system to reach higher levels of methanol selectivity at
desired methane conversion levels. Phase II experiments involved testing 2 injection
distances, 2 levels of oxygen concentration, and two levels of water concentration. The
oxygen concentrations tested were at the lower limit of what was reliably measurable by
92
T3 93
0.014
0.012
0.01
0.008
0.006
o «
^ o
J3 a S "
2 a 0.004
a 0.002
0
A
1
1 ' 1
y. \ 1 1
1 ^ 1
I 1
"
1 i
• Tv
ij • X
1 i • ^ ^ ^ " ^ - ^ ^
r—^——
!
1
i 1
> •
0 0.05 0.1 0.15 0.2 0.25
Methane Conversion (mole reacted/mole fed)
0.3
• Preliminary Phase Experiments • Phase I Experiments (3/8 inch Injection)
A Phase I Experiements (5/8 inch Injection) X Phase I Experiments (1.5 inch Injection)
Figure 6.8. Preliminary Phase and Phase I-Methanol Selectivity Versus Methane Conversion
93
the apparatus. The upper water level tested was near the upper limit of experimental
feasibility (i.e., more water would have made h impossible to complete the experiment
effectively). Table 6.3 shows the experimental parameters from Phase II runs.
Table 6.3. Phase II Experimental Parameters
Experiment
0213 0215 0217 0218 0225 0226
0226-2 0226-3
Injection Distance (inches)
0.375 0.375 0.375 0.375 0.250 0.250 0.250 0.250
Ar/CH4 Ratio
7.22 7.02 8.02 7.98 8.02 7.98 7.78 7.82
CH4/O2 Ratio
4.75 4.75 4.75 4.44 4.75 4.44 4.44 4.75
CH4/H2O Ratio
1.04 0.89 1.90 1.93 1.67 1.96 0.96 0.95
Methane Conversion
(%)
0.453 0.756 2.035 3.161 4.769 6.062 2.450 4.505
Methanol Selectivity
(%)
3.715 1.826 1.301 0.854 0.785 0.578 0.990 0.596
6.3.1 Injection Distance
From inspection of the methane conversion attained in Phase II runs, injection
location behavior parallels the behavior observed in earlier phases. More distant
injection resulted in lower methane conversions and conversion levels were in the desired
range (<10%). The locations selected for methane injection were appropriate for this
experimental apparatus and these experimental conditions.
6.3.2 Overall Performance of Phase II Experiments
The selectivity levels obtained in Phase II experiments are clearly higher than
those of earlier phases. Figure 6.9 shows the comparison of Phase II data with both the
94
0.04
T3 0.035
93
13 C/2
§
J3
u (J 3
2
J3 • « - •
(L)
s "o B
0.03
0.025
0.02
0.015
^ 0.01
0.005
0 0.05 0.1 0.15 0.2 0.25
Methane Conversion (mole reacted/mole fed)
0.3
• Preliminary Phase Experiments • Phase I Experiments (injection at 3/8 inches)
A Phase I Experiments (injection at 5/8 inches) X Phase I Experiments (injection at 1.5 inches)
D Phase II Experiments (injection at 1/4 inches) • Phase II Experiments (injection at 3/8 inches)
Figure 6.9. Preliminary Phase, Phase I, and Phase II-Methanol Selectivity Versus Methane Conversion
95
Phase I data and Preliminary Phase data. Progression of the experimental data away from
the origin is desired. The optimum trend in data would be high conversion and high
selectivity (i.e., progress up and to the right on the Figure 6.9). The data from Phase II
experiments shows a definite increase in the methanol selectivity over Preliminary Phase
and Phase I data. The data exhibh considerable scatter, but the Phase II data confirm
system performance improvements, especially at the 3/8 inch injection distance.
The data from this research indicate that the best methane-to-oxygen ratio for methanol
production from methane for this apparatus is approximately 4.5 to 1. A higher methane-
to-oxygen ratio may result in performance improvements but this apparatus is unable to
operate at higher ratios.
The data from this research indicate that the best water-to-oxygen ratio for
improved methanol selectivity is approximately 5 to 1. This is a surprisingly high
concentration of water. Methane conversion is favored by lower water levels because
water dilutes the system and interferes with the reactions between oxygen and methane.
High water content has been shown to favor methanol selectivity, while low water
content favors methane conversion. Data from investigations into the effects of water
content also illustrate the competition between methanol selectivity and methane
conversion.
Water has been shown to participate favorably in the production of methanol from
methane by increasing the number of pathways towards methanol production (Badani et
al., 1995). This research supports the claim that high levels of water in the plasma
stream, relative to oxygen, improve methanol selectivity. Little has been written about
96
how water participates in other important reactions occurring in this type of plasma
reactor.
For Phase II experiments, oxygen and water levels are at or near the experimental
apparatus limhs. It is not possible to make significant changes in these two parameters
without system redesign and reconstmction. Further refinement of reactant levels could
continue, resulting in slight improvements in system performance. Small refinements
will not solve this synthesis problem. Large-scale (i.e., order of magnitude)
improvements must occur for this approach to become feasible and it is unlikely that
ftirther modification of these parameters will produce significant improvement unless
changes in system design incorporated. For this reason. Phase III experiments will not
attempt further refinement of reactant concentration levels but concentrate on
investigation of a novel approach aimed at improving system performance.
6.4 Phase III Experiments
It has been theorized by other researchers that water contributes to methanol
production by increasing the number of pathways that lead to its production. Apparently,
the presence of water and radicals generated from water improved the likelihood that
methanol would result from free-radical reactions. This idea of increasing the number of
methanol synthesis pathways led to a new approach for this research. For Phase I and II
experiments, oxygen (along with argon and water) passed through the plasma region and
then reacted with the methane stream. The methane stream has always consisted of pure
methane. If oxygen were present in the methane stream, it would not be "activated" by
the plasma and could react at a lower energy state than the more energetic oxygen in the
97
plasma stream. It seems plausible that oxygen introduced by way of the cooler methane
stream would exhibit different reactivity and could potemially improve the pathways to
methanol. Phase III experiments divided the oxygen into the two reactant streams to
discover if this created measurable improvemems in methanol production.
A single experimem was conducted in which all of the oxygen was placed into the
methane stream and isolated from the plasma conditions. In light of the other planned
experiments, an experiment of this type was warranted to obtain some perspective about
system reactivity when oxygen was only presem in the methane stream. This approach
was not expected to produce significant conversion of methane since oxygen would not
be energized in the plasma stream. The oxygen was well-mixed with the methane and the
low energy state of the oxygen should have prevented appreciable reaction with methane.
This particular experiment produced some interesting results. Table 6.4 shows
experimental parameters from Phase III mns.
Table 6.4. Phase III Experimental Parameters
Experiment
0330 0331 0403
0403-2 0405 0406
0406-2
Injection Distance (inches)
0.250 0.250 0.250 0.250 0.250 0.250 0.250
Ar/CH4 Ratio
7.22 6.88 6.88 6.88 7.22 6.19 6.88
CH4/O2 Ratio
4.75 4.71 4.05 4.05 3.48 5.53 5.53
CH4/H2O Ratio
1.01 1.13 1.54 1.02 0.95 1.64 1.07
Methane Conversion
(%)
1.679 1.036 2.189 1.614 1.142 2.311 2.056
Methanol Selectivity
(%)
1.349 1.534 0.345 0.744 0.766 0.198 0.225
98
6.4.1 Oxygen with Methane Stream
The first experimental mn of Phase III (mn 0330) was conducted whh all of the
oxygen placed in the methane stream. Appreciable conversion of methane was not
anticipated in this mn. This flow configuration did indeed achieve measurable methane
conversion and respectable methanol selectivity. The run replicated the experimental
conditions of a Phase II experimental mn (Phase II, mn 0213), with the noted exception
that oxygen was included with the methane stream. That particular mn (0213) had
achieved the highest methanol selectivity of any experimental mn.
Including oxygen in the methane stream reduced methane conversion, relative to
the earlier run of Phase II. This reduction in conversion was expected because a primary
reactant has been isolated from the highly reactive conditions generated in the plasma. (It
should be noted that placement of oxygen in the methane stream induces greater
penetration of the methane stream into the plasma zone because of the mixing effects
discussed above). The high conversion and selectivity of this run implies that one, or a
combination, of the following must be tme.
1. The argon and water in the plasma stream must, to a certain extent, be capable of
producing species that (a) survive long enough to contact the methane stream and (b)
are effective in oxidizing methane.
2. The methane stream, now well-mixed with oxygen and more reactive, is significantly
easier to convert.
3. The high energy of this microwave generation system "leaks" down the waveguide
"sleeve" (which houses the quartz tube) and contributes to methane conversion.
99
4. The methane stream penetrates considerably ftirther into the plasma zone than
expected. (This factor probably has limited impact since oxygen makes up less than
2.5% of the total gas flow.)
The implications of this particular experimental run were not investigated ftirther. This
experimental mn reinforces the ideas discussed earlier about extremely complicated
energy and mixing/flow considerations involved in this high pressure (~ 1 atmosphere)
plasma reaction system.
6.4.2 Oxygen Divided into Plasma and Methane Streams
There were 6 experimental runs in which oxygen was divided equally into both
the plasma stream and the methane stream. A variety of experimental conditions were
tested in the ranges that have been identified as optimum for this apparatus. The overall
performance results of this approach are shown in Figure 6.10. It is apparent that this
approach did not result in the order of magnitude performance improvements that were
desired. For reference, this figure includes the results from Phase II.
In general, Phase III experiments do not approach the performance of Phase II
experiments. As discussed above, quicker mixing should resuh from the impinging flow
approach, enabling reactions to proceed farther before conditions degrade. The theory of
improving the pathways to methanol by placing oxygen in both streams has not been
supported. Data from this research indicate that the presence of oxygen in both streams is
counterproductive to methanol production.
Several factors may contribute to the decreased performance of Phase III
experiments.
100
0.04
0 0.01 0.02 0.03 0.04 0.05 0.06
Methane Conversion (mole reacted/mole fed)
0.07
• Phase II Experiments (injection at 1/4 inches) • Phase II Experiments (injection at 3/8 inches) A Phase III Experiments (injection at 1/4 inches;pt O oxygen in methane stream)
Figure 6.10. Phase II and Phase III-Methanol Selectivity Versus Methane Conversion
101
1. The presence of oxygen with methane means that reactive species (intermediates)
have a higher probability of being consumed by oxygen at the mixing point.
2. Any methanol that is produced will contact oxygen with greater frequency because
oxygen is present throughout the system. Methanol is easily oxidized to carbon oxide
products in the presence of oxygen.
3. The presence of oxygen mixed with methane anywhere in a highly oxidizing
environment should contribute to greater amounts of CO and CO2 produced because
these products are favored by thermodynamics.
These and other factors may make it imperative that a pure methane stream be mixed
with a plasma activated stream, or combination of streams, to maximize methanol
production in this and other plasma reactors.
6.5 Overall Performance Evaluation
Figure 6.11 shows the selectivity/conversion performance of all of the
experimental phases of this research. The progression towards higher methanol
selectivity is illustrated and performance improvements are apparent from the
Preliminary Phase through Phase II. A new approach to methanol synthesis is attempted
in Phase III, but fails to result in performance improvements. The ability of this system
to produce methanol directly from methane has been demonstrated. This system
compares favorably with other more advanced, low-pressure plasma systems.
102
0 0.05 0.1 0.15 0.2 0.25
Methane Conversion (mole reacted/mole fed)
0.3
• Preliminary Phase Experiments
• Phase I Experiments (injection at 3/8 inches)
A Phase I Experiments (injection at 5/8 inches)
X Phase I Experiments (injection at 1.5 inches)
D Phase II Experiments (injection at 1/4 inches)
• Phase II Experiments (injectionat 3/8 inches)
A Phase III Experiments (injection at 1/4 inches, pt O oxygen in methane stream)
Figure 6.11. Preliminary Phase, Phase I, II, and III-Methanol Selectivity Versus Methane Conversion (all phases)
103
6.5.1 Mixing and Flow Analysis
The rationale behind the impinging flow design of this system was to improve the
mixing characteristics in the oxidation region with the goal of improving reactor
performance. A simple Reynolds Number calculation was performed to gain some
insight into typical flow conditions existing in the three streams (plasma stream, methane
stream, combined stream) during experimentation. Reynolds Number calculations for
streams during run 0213 are shown in Table 6.5. The calculation assumes the lowest
possible viscosity for the streams.
Table 6.5. Reynolds Numbers for streams during Run 0213
Reynolds Number of the Plasma Stream (0213) (assumed to be 400°C)
60
Reynolds Number of the Methane Stream (0213) (assumed to be 250°C)
35
Reynolds Number of the Combined Stream (0213) (assumed to be 250°C)
16
It is apparent that the flow conditions present in each stream are well within the
laminar regime. The benefits of the impinging flow design used in this reactor system
should be accentuated when streams in laminar flow are mixed. Although, it is not
known whether other researchers operated under laminar or turbulent flow conditions, an
impinging flow design will always improve mixing of the plasma and methane streams at
the mixing point.
6.5.2 Material Balances
The credibility of this experimental research demands that overall material
balances are presented. It is appropriate that balance closure be presented here to support
104
findings. Although significant experimental error are present, carbon and hydrogen
element balances closed to ±25% for all experimental runs. Only 1 experimental run
exceeded 20% error (20.3%) in the carbon balance, and none exceeded 20% error in the
hydrogen balance. Of the 36 mns, 7 exceeded ±10% carbon element balance error and 5
exceeded ±10% hydrogen element balance error. All other experimental mns were
within ±10% of closure for carbon and hydrogen balances. The inability to address
oxygen balance closure is a major concem in direct methanol synthesis from methane.
This point is addressed in greater detail below. This research was conducted using
existing department equipment which necessitated some important design and
experimental limitations. The major and minor sources of error will be discussed briefly
at the end of this chapter.
6.6 Sources of Error
In any experimental research project, serious consideration of the sources of error
must occur. It is not uncommon for experimental results to contain error margins of 50-
100%. Minimizing sources of error can be an expensive proposition as more accurate
and precise equipment is purchased, experiments become more numerous, and project
duration is extended. The limited budget available for this project made it impossible to
remove some considerable error sources that were present. The major sources of error in
this experiment are listed below.
1. Small errors, or non-closure, in the oxygen balance of this chemical system (methanol
production by direct oxidation of methane) have been shown to produce large errors
105
in system performance parameters, namely methanol selectivity (Helton, 1991). The
inability of this system to even measure oxygen balance closures must be considered
the major source of error in this experiment.
2. The analysis method used in this research required that columns be interchanged
between gas and liquid analysis. Removing, replacing, and re-conditioning analytical
GC columns can create major inconsistencies in performance. Optimally, two
different gas chromatographs, with the appropriate detectors, should be used in the
analysis. This would maximize the reliability of the columns and the analysis.
3. Although every effort was made to minimize the effects of transporting samples to
the analysis system, off-line analysis must be considered another major source of
error. All activities having to do with sample handling (see Chapter V) will introduce
errors. On-line analysis would minimize these sample handling sources of error.
4. Inability to maintain constant mixing and flow characteristics must be considered a
potentially major source of error. From run to run, slight changes in flow might
create significant changes in the mixing region. As discussed above, small changes in
this mixing region may impact reaction characteristics to a greater extent than the
other experimental parameters under investigation.
Other sources of error are listed below. These are not considered "minor" but
probably have less impact than those presented above.
1. The apparatus was vented to the atmosphere for simplicity. The pressure in the
reactor was approximately 0.9 atmospheres. This corresponds to the normal local
atmospheric pressure where the research was conducted. Changes in atmospheric
pressure did occur between experiments and must have affected system performance.
106
2. Accumulation of liquid product in the system. Efforts were made to minimize this
factor but, no matter how long the system "warmed-up," water balances exhibited
significant error.
3. Inability to precisely control water temperature in the water saturation unit made it
very difficult to closely control the amount of water placed into the plasma stream. It
was possible to maintain water temperature within a few degrees, but significant
changes in water content occur within a few degrees. Consequently, it was possible
to maintain water content within a certain range, but not at a precise point.
4. The errors inherent in regulators, flow controllers (rotameters), pressure gauges,
thermometers, timers, and GC calibration cannot be disregarded or ignored.
Additionally, the equipment used to constmct this apparatus was, by no means, new.
5. Concentration levels in the GC calibration gases are advertised to be ±5%.
6. Reactor/Feed gases have appreciable concentration tolerances (±5%).
107
CHAPTER VII
CONCLUSIONS AND RECOMMENDATIONS
This investigation into methanol production via an experimental plasma-based
reactor operated at atmospheric pressure leads to the following conclusions:
1. Methanol production by direct oxidation of methane in a plasma reactor operated at
high pressure (approximately I atmosphere) has been demonstrated.
2. Low oxygen concentrations, relative to the concentration of methane, contribute to
increased selectivity for methanol in this system.
3. High water concentrations, relative to the concentration of oxygen, contribute to
increased selectivity for methanol in this system.
4. High pressure plasma reactor operation creates high temperature gradients, inducing
significant velocity fluctuations in plasma streams. Complicated flow and mixing
phenomena result from the substantial temperature increases in these systems.
Reactor configuration and stream mixing techniques had significant impact on system
performance. These should be primary considerations when designing a high
pressure (~1 atmosphere) plasma reactor system.
5. The high peak power microwave system used in this research was able to ignite and
sustain a plasma in an argon environment without difficulty. Problems sustaining a
plasma experienced by other researchers could be alleviated by microwave systems
with higher peak power. This type of system is ideal for plasma studies using air.
because of the high peak power of the system and its ability to maintain the plasma.
108
6. Placing oxygen in both streams (plasma and methane) had a detrimental effect on
methanol production, relative to methanol production with oxygen present in the
plasma stream only.
It is not possible to make a conclusion about the effects of higher pressure on methanol
production. The higher temperatures associated with plasma at high pressure should
induce less selective oxidation of methane. Methanol production in this system is
somewhat less than other low pressure systems, but still comparable. This unit was not
operated at low pressure. Detailed low and high pressure investigations will have to
occur before a conclusion about pressure effects can be made.
Significant improvements in plasma reactor performance must occur before this
approach can become useful for methanol production by direct oxidation of methane.
Investigation into reactor configuration, mixing techniques, plasma generation
techniques, and high pressure operation are critical to progress in this area.
Based on the experimental and operational insight gained from this study, the
following recommendations are made:
1. High pressure plasma studies should be continue in this area. High pressure
conversion is cmcial if this type of plasma reactor technology is to operate
commercially. High Pressure plasma reactor systems should be developed that can
accurately evaluate many factors, including power input, effects of variable rates of
quench, effects of using air as a reactant, effects of impurities on system performance,
pressure effects, effects of reactant concentration levels, etc. The systems should
109
possess high reliability and minimize contributions from experimental and analysis
errors.
2. Studies into innovative reactor configurations and novel mixing techniques should be
undertaken. The physical and chemical interactions that occur in high pressure
plasma reactors of this type are very complicated. Optimizing performance in high
pressure plasma reactors may require complicated reactors and creative flow systems.
For example, combination of a plasma energized stream with another oxygen
containing stream(s), followed by subsequent mixing with the methane stream, or
incorporation of a methanol selective catalyst into the plasma system.
3. Serious consideration should be given to the energy source for the generation of
plasma. This and other approaches use pulsed systems to create the plasma. It is
recommended that a continuous source be considered for plasma generation.
Conditions created by pulsed systems are highly variable because energy is added to
the system in short, but powerful bursts, after which, the energy is completely shut off
(hence the term "pulsed"). If a set of condhions exists, under which methanol
production is favorable, then those conditions should be established and maintained.
Continuous sources will create a more "steady-state" energy environment. (Note:
Considerable difficuhies exist in igniting and maintaining a plasma, from a
continuous energy source, in any environment at atmospheric pressure.)
110
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114
Periana, R. A., D. H. Taube, E. R. Evitt, D. G. Loffler, P. R. Wentrcek, G. Voss, and T. Masuda, "A Novel, High Yield System for the Oxidation of Methane to Methanol," Natural Gas Conversion U, H. E. Curry-Hyde and R. F. Howe, Eds., Elsevier Science B. V., 533(1994).
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16
APPENDIX A
CALIBRATION OF EXPERIMENTAL
EQUIPMENT AND INFORMATION ON SYSTEM HARDWARE
The apparatus consisted of many different mechanical devices and analytical
instmments which required calibration for reliable operation. This consists primarily of
rotameters that controlled the flow rate of reactant stream and GC calibration.
Calibration of the GC for analysis of the gas product took place before each experimental
mn and details are presented in Appendix B, which consists of sample calculations.
Specific equipment and supply information is also presented in this section.
A. 1 Calibration of Gas Rotameters
Three gas rotameters were used to control the flow of reactant gases (argon,
methane, and oxygen). Methane and oxygen flows were controlled by Brooks rotameters
(Brooks R-2-15-AAA, and Brooks Flow Controller 8744A). Argon was controlled by
Brooks rotameter (Brooks 2-65). Back pressure on the rotameter was set at the regulators
on the cylinders to approximately 40 psi. Downstream of the rotameters, pressure was
maintained constant at 938.6 torr. This level was selected to me approximately 5 psi
above normal local atmospheric pressure. The temperature was not controlled in the vent
hood, but did not ever vary by more than 1.5° from 22°C. Several flow measurements
(50) were collected with a bubblemeter and stopwatch for each flow rate represented on
each calibration plot. Volumetric flows were converted to molar flows assuming the
Ideal Gas Law applies. The average of the flows was plotted and a trendline applied.
117
Calibration curves for argon, methane, and oxygen are shown in Figures A.l, A.2, and
A.3.
Calibration for methanol product was obtained by preparation of standard
solutions for multiple injection into the GC. As expected, large variations were observed.
It was decided to make a large number of injections to minimize instmment error. Four
different concentrations were tested. The calibration for methanol is shown in Figure
A.4.
As mentioned above, calibration for the gas samples occurred before every
experiment. Two different calibration gases (Scott Specialty Gases Mix 234 and Mix
216) were injected prior to experimental runs. The output from these calibration runs
was used to analyze the data for that particular experiment. Frequent changing of the GC
columns and re-conditioning necessitated this daily re-calibration. This method should
have minimized errors. The concentration of components present in the reactor effluent
gas stream was calculated using the output from the daily calibration injections. Details
of how this was accomplished are presented in the sample calculation section (Appendix
B).
118
40 1
o o o X! C
I J3
93
a
Flowmeter reading
Figure A. 1. Argon Rotameter Calibration Plot (back pressure of 40 psig, downstream pressure of 5 psig, and 22°C).
119
Flowmeter readmg
Figure A.2. Methane Rotameter Calibration Plot (back pressure of 40 psig, downstream pressure of 5 psig, and 22°C).
120
o o o X
c
I 93
30 40 50 60
Flowmeter Reading
Figure A.3. Oxygen Rotameter Calibration Plot (back pressure of 40 psig, downstream pressure of 5 psig, and 22°C).
121
10000000
1000000
y)
o
93
100000
10000
1000
100
10 100 1000
Concentration (ppm)
10000
1
1
1-i 1
- 1 h-
1
—
i i 1 ; i ! 1
i
in42^\
1 !
! u
l/r ! i' B 1—^—
I
; 1
I !
1 \ \ Jr
1 i /
1
1 i
\X\ Jr' 1
1 1—1-1 1 1
1
i !
1
-U— — h -! !
i 1 1
1 1 1
1 I ' i Mil 1 J I I I ' '
<1 T 1 I ' l l ! \^^ • M I i
1 j II
1 1 1
: ; 1 , i
1 i 1
i 1 j 1
i 1 1 j
1
1 1
i
- 1 1 1 ' 1
1 i l l 1 i : 1 1 ' ' 1
1 " 100000
Figure A.4. Methanol Calibration Plot (multiple injections at four different concentrations).
122
APPENDIX B
SAMPLE CALCULATIONS
This section describes how quantities were calculated from the measured
variables. All of the experimental data that was measured during each experimental mn
is given in Appendix C. All of the calculations that are required to obtain the methane
conversion and methanol selectivity will be outlined. These two quantities are the most
important for this research.
B. 1 Molar Reactor Feed Rates
The total molar feed rate is obtained by simply summing the molar feed rates of
all components. Streams are assumed to behave as ideal gases throughout the analysis.
total moles into reactor time
f moles oxygen 1 +
moles argon
time
moles water 1
r + S moles methane 1
time J (B.l)
[ time J 1 time J
The amount of water being evaporated into the feed stream deserves special
attention. Multiple experiments were conducted without plasma to discover the amount
of water that was being evaporated into the system. Early estimations were based on
assuming that feed gas leaving the water sattiration unit was sattirated with water the
temperature in the headspace of the unit. Experiments revealed that gas leaving the unit
was, on the average 90% saturated. It was assumed that this was due to the relatively
short exposure of the stream to the hot liquid. The molar flow rate of the water into the
123
reactor was calculated from the molar flow rate of the plasma stream that was passing
through the saturation unit and the temperature of the headspace above the unit. Since the
mole fraction of water in the stream is known (because of our assumption of 90% stream
saturation at the headspace temperature), a simple mass balance is used to calculate the
amount of water fed to the reactor. The results of that mass balance are below. Let Y be
the mole fraction of water in the stream,
moles water [ moles gas through unit
1 X S time Ume 1(1-Y) J (B.2)
B.2 Reactor Effluent
The amount of material leaving the reactor was calculated from the two effluent
streams, gas and liquid. The volumetric flowrate of gas leaving the reactor was measured
with a bubblemeter and stopwatch, as described in Chapter V. The ideal gas law was
used to obtain the molar flowrate of gas. This effluent molar flowrate is vital to the
results of this study.
moles gas out = i
time
volume of effluent
time ^ X S
atmospheric pressure R X Temperature
(B.3)
where R is the gas constant.
The liquid product flowrate is simply measured at the end of the experimem and
is applied to the entire duration of the mn. The liquid product consists of many products,
but is essentially water. All non-water components added together do not appmach 1%.
124
It is important to maintain units in moles because this is how the analysis system
measures concentration. Both detectors measure a response that is based on molar
amount (i.e., a flame ionization detector or a thermal conductivity detector will not
indicate triple the response for a compound that is three times heavier.)
moles water out f mass collected _ J s X <
time I time J [molecular weight
1 (B.4)
B.3 Calculation of Effluent Component Concentrations
The GC analysis of the liquid and gas products creates a response for each
detected component. The primary components of interest are CO, CO2, and methanol.
These three components contain most of the carbon that is converted in the reactor. Since
this system is unable to track oxygen, analysis of carbon-containing products must
provide us with the required information.
The carbon-containing components present in the gas phase (primarily CO, CO ,
and methane) produce a response from the GC. This response is compared to the
response from the known concentration of the standard calibration gas injected prior to
the experimental mn. Componem concentration (cone.) was calculated like this.
. f cone, (ppm) of standard | ^^^^ cone, (ppm) = {component area counts) x | j . ^ ^ ^rea counts of standard J
Liquid concentrations of methanol and other products were calculated in a similar
mamier. Other than methanol, there were few products in the liquid phase. Traces of
fomiic and acetic acid were detected and quantified to the highest extent possible.
125
standard solutions of these components were mixed and injected in the GC. Liquid
concentration was calculated in the following way.
liquid cone, (ppm) = {area coums} x j cone, of standard 1 [ area counts of standard J ^ ^^
Knowing the total molar flowrate of the liquid stream and the concentration of the
components in the stream, the molar flowrate of the componems is obtained.
moles of component /total molar flowrate! tJm^ ~ component cone, (ppm) x < ^""^ [ time (B.7)
This consideration does not take into account the contribution of the non-water
constituents in the liquid. These components will not significantly affect the calculation
of the total molar flows since their concentration is so small (<1000 ppm or 0.1 molar %).
B.4 Material Balances
Since all of the flowrate data is known, it is a simple matter to compare the
amounts entering and exiting the system. To compute a carbon balance, the amount
entering was calculated from the rotameter reading and the known feed composition. The
amount leaving was calculated from the effluent flowrate and the concentration. The
amount entering was compared to the amount exiting, revealing the mass balance closure.
Carbon balances were closed to within ±10% for more than 80% of the experimental
runs. Only 7 of the 36 experimental mns exceeded ±10% error in the carbon balance.
Only 5 of the 36 runs exceeded ±10% error in the hydrogen balance. One run exceeded
126
±20% in carbon balance error (20.4%) and no mns exceeded ±20% error in the hydrogen
balance error.
B.5 Calculation of the Conversion and Selectivity
These are the two most important quantities for this research. The high degree of
variation in the GC analysis made it impossible to use differences in methane
concentration as a basis for conversion. With conversion levels in the 1% regime and low
GC reproducibility, small errors in methane concentration measurement (±5%) can easily
obscure the results. It was necessary to use the observed carbon-containing products as
the basis for conversion. Methane conversion was calculated by
S all carbon - containing products not present in feed
conversion = i time
methane fed to reactor/ /time
(B.S)
The calculation for methanol selectivity was dependent of this definition of
conversion that was used out of necessity. In short, methanol selectivity is an expression
for the amount of methane that reacts to produce methanol divided by the total amount of
methane that reacts. It is defined in this research as
methanol produced
selectivity = time
I all carbon containing products not present in feed 'time J
(B.9)
These two quantities contain most of the valuable information about the
performance of this system and are the focal points of the analysis and the performance.
127
APPENDIX C
RAW EXPERIMENTAL DATA
This section contain all of the information that was recorder during the
experimental mns. All of the data used in the analysis of this research was generated
from the information in this section.
128
Table C.l. Preliminary Phase and Phase I Raw Experimental Data
Experiment Phase
Prelim
Prelim
Prelim
Prelim
Prelim
Prelim
Prelim
Prelim
Prelim
Prelim
Prelim
Experiment Name
925
930
1008 1009 1013 1014 1030 1103
1104 1106-1 1106-2
1118 1124
1125-1 1125-2 1126 1129 124
1201-1 1201-2
1202
Experiment Duration
(min)
62.75
60
77 75 63
48.5 63 87
65 67 60
50 59
60 48 60 58 60
60 64 59
Temperature Ambient
(C) 21
23 24 21 21
23.5 23.6 24.1
23.4 23.7 23.9
24 24.3 24
24.3 24
24.1 21.8 24.2 24.4
24
Argon Flowrate (reading)
1.8 1.8 1.8 1.8 1.8 1.8 1.8 1.8 1.8 1.8 1.8
1.8 1.8 1.8 1.8 1.8 1.8 1.8 1.8 1.8 1.8
Methane Flowrate (reading)
6.6 6.6 6.6 6.6 6.6 6.6 3.96
4 3.9 4 4
6.6 6.6 6.6 6.6 6.6 6.6 6.6 6.6 6.6 6.6
Oxygen Flowrate (reading)
0 0 0 0 0 0 0 0 0 0 0
19.5 15
29.5 30 20 15 30
30.65 20 15
Atmospheric Pressure
(torr)
685.7 681.9 679 684
686.9 690
678.8 685
681.4 689.1 686.5
685.6 685.3 683.7 680.7 683.3 679
679.4 680.5 677.7 677.5
129
Table c.l . (com.)
Experiment Phase
Preli
Preli
Preli
Prel
Prel
Prel
Prel
Prel
Prel
Prel
m
m
m
m
im
im
im
im
im
im
Prelim
I
I
I
I
I
I
I
I
I
I
Experiment Name
925
930
1008 1009 1013 1014 1030
1103 1104
1106-1 1106-2
1118 1124
1125-1 1125-2 1126 1129 124
1201-1 1201-2
1202
Injection Distance (inches)
1.5
0.75
0.25 -0.25
0 0.125 0.5
0.125 0.25
0.3125 0
0.375 0.375 0.375 0.625 0.625 0.625 0.625
1.5 1.5
1.5
Temperature Water Sat. Headspace
(C)
310.71
313.15 311.21
311.483 312.94 312.5 311.76 313.44
314.02 312.72 313.15
314.21 313.18 312.47 316.18 314.62 311.69 315.65 312.41 312.59
313.32
Product mass
(collected) grams
1.9791
2.3289 2.319 1.3531 1.8817 1.7141 2.0407
2.7722 2.2198 2.1068 1.9835
3.0099 3.0631 3.0522 2.425 2.9808 2.5454 2.2691 2.8375 3.1111 2.8208
Gas Prod. vol. basis
(ml)
90 90 90 90 90 90 90 90 90 90 90
90 90 90 90 90 90 90 90 90 90
Time req. to collect
basis (sec)
6.476 6.382 6.338 5.924 6.196 6.164 6.4547 6.4284 6.6355 6.648 6.4635
6.2947 6.3095 6.221 6.0453 6.164 6.1953 6.0055 6.056 6.0987 6.164
Methanol Liq. Prod.
(raw area cts.)
185 4003 20101 5829 17056 22319 17205 23415 49855 36121 17543
96590 105666 193948 38766 19571 36487 50781 962 353 491
130
Table c.l . (com.)
Experimental Phase
Preli
Preli
Preli
Prel
Prel
Prel
Prel
Prel
Prel
Prel
m
m
im
m
im
im
im
im
im
im
Prelim
I
I
I
I
I
I
I
I
I
I
Experiment Name
925
930
1008 1009 1013 1014 1030 1103 1104
1106-1 1106-2
1118 1124
1125-1 1125-2 1126
1129 124
1201-1 1201-2 1202
Hydrogen Vap. Prod,
(raw area cts.)
0
0
1400 8000 5411 3347 512.5
2607.5 1871.5 1445 3650
577 489 253 0 0
0 0 0 0 0
Carbon Monoxide
(raw area cts.)
0
700
9303 112565 52912 31834 7112
49673 31886
23546 87888
58506 23312 18354 505.5 4749
3630.5 2800
0 0 0
Methane Effluent
(raw area cts.)
599694
738742 641752 372466 588956 563546 583390 489567
601273 527686 507878
711122 631391 688873 729281 692523 660906 759315 705438 711898 664591
Carbon Dioxide
(raw area cts.)
824
2033 7185 12685 15985 14107 8771
23184 19259 14542 27693
81520 57806 49721 964.5 18164 11949 7292 422 154 171
Acetylene Effluent
(raw area cts.)
0
0 0
10202 3605 1732
0 1920.5 1117 487 3362
1572 0 0 0 0 0 0 0 0 0
Ethylene Effluent
(raw area cts.)
836 1081 1252 2814 2768 2111
^1368.5 2120 2282 1490 2691
3105 1986.5 1891.5 1773 1205 1680 2171 1441 1610 1767
131
Table c.l . (com.)
Experiment Phase
Prelim
Prelim
Prelim
Prelim
Prelim
Prelim
Prelim
Prelim
Prelim
Prelim
Prelim
I
I
I
I
I
I
I
I
I
I
Experiment Name
925
930 1008
1009
1013
1014 1030 1103 1104
1106-1 1106-2
1118 1124
1125-1 1125-2 1126 1129
124
1201-1 1201-2 1202
Ethane Effluent
(raw area cts.)
4223
6610 6048
4545 6926 6634
5457 5609.5 6839.5 5672
5635.5
8038 7012
7094.5
6270 6204 5712
6683
5863 6060
5826
Formic Acid Liq. Prod
(raw area cts.)
0 3082 2201
1975 0 0
10788 19329 20215 13169 8601
25653 23780 24542
16694 9613 10389 47800
10641 12069 12890
Acetic Acid Liq. prod
(raw area cts.)
0 0 0 0 0 0
496 4804 5344 3956 2081
11019 13372 17894
1985 864 1423 5276
0 0 0
132
Table C.2. Phase II and Phase III Raw Experimental Data
Experiment Phase
II
II
II
II
II
II
II
II
III
III
III
III
III
III
III
Experiment Name
213
215
217 218
225 2261 2262 2263
330 331
403-1 403-2 405
406-1 406-2
Experiment Duration
(min)
50
53
61 70
75 60 60 58
60 52 57 55 52 66 60
Temperature Ambient
(C)
23.7 23.9 22.9 23.4 22.9 23
23.4
22.8
23.2 23.4 23.1 23.4 22.6 23
22.6
Argon Flowrate (reading)
1.8 1.75
2 1.99
2 1.99 1.94
1.95
1.8 2 2 2
1.8 1.8 2
Methane Flowrate (reading)
6 6 6 6 6 6 6 6
6 7 7 7 6 7 7
Oxygen Flowrate (reading)
10 10 10 12 10 12 12 10
10 15 20 20 20 10 10
Atmospheric Pressure
(torr)
682.1 670.1 672.5 676
670.7 673
673.1 673.7
674.5 675.5 678.3 681.8 675
671.1 670.4
133
Table C.2. (cont.)
Experiment Phase
II
II
II
II
II
II
II
11
III
III
III
III
III
III
III
Experiment Name
213
215 217 218 225 2261 2262 2263
330 331
403-1 403-2
405
406-1 406-2
Injection Distance (inches)
0.375
0.375 0.375 0.375 0.25 0.25 0.25 0.25
0.25 0.25 0.25 0.25 0.25
0.25 0.25
Temperature Water Sat. Headspace
(C) 321.94
324.82 309.21
309.13 311.4 308.75 321.71
322.02
322.27 321.19 315.68 323.04 323.15 316.31 321.99
Product mass
collected grams
3.6714
4.1285 2.4591 2.7689 3.6509 2.5316 4.6587 4.649
3.9546 3.4089 2.859
4.3849 3.5977 3.0756 4.6427
Gas prod. vol. basis
(ml)
90 90 90 90 90 90 90 90
90 90 90 90 90 90 90
Time req. to collect
basis (sec)
6.342 6.337 5.644 5.729 5.5913 5.704 5.776 5.746
6.146 5.4413 5.543 5.5265 6.2225 6.012 5.494
Methanol Liq. Prod.
(raw area cts.)
29184 21714 97427
101706 116601 127289 41577 45068
46398 37108 21485 21517 14746 13152 7464
134
Table C.2. (com.)
Experiment Phase
II
II
II
II
II
11
II
II
III
III
III
III
III
III
III
Experiment Name
213 215 217
218
225
2261 2262 2263
330
331 403-1 403-2 405
406-1 406-2
Hydrogen Vap. Prod.
(raw area cts.)
0 0 0
0
0
0 0 0
0
0 0 0 0 0 0
Carbon Monoxide
(raw area cts.)
1350 1544
5283 8632.5 13074
16636 6368 13674
7946 4720 13127 7267 5147 14434 11366
Methane Effluent
(raw area cts.)
747385 633088 627575 574030 684214 646539 833603 692444
771093 713577
764574 803205 719941 818502 755515
Carbon Dioxide
(raw area cts.)
2999.5 5333 13743 23192 29844 40172 16791 34328
10511 6523 13266 9560 7734 14453 12153
Acetylene Effluent
(raw area cts.)
0 0 0 0 0 0 0 0
0 0
0 0 0 0 0
Ethylene Effluent
(raw area cts.)
2205 2153
2084.5 2049 2662 2738 2898 2935
2881 2567
3233 3313 2914 3760 3436
135
Table C.2. (cont.)
Experiment Phase
11
II
II
II
II
II
II
II
III
III
III
III
III
III
III
Experiment Name
213 215
217
218
225 2261 2262 2263
330
331
403-1 403-2
405 406-1 406-2
Ethane Effluent
(raw area cts.)
6674.5 5605
5886
5539
7023 6939 7961 6977
7435
6647 7485 7750 6847 8114 7540
Formic Acid Liq. prod
(raw area cts.)
957.2 783.6
287.8
1470.2 13254 15483 9798 7079
5990 6432 3121 4974 2869 2151 310
Acetic Acid Liq. prod
(raw area cts.)
0 0
728.4
1354.2 7713 11973 1986 2784
3264
1182 4264 1880 1472 4648 4789
136
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