Recovery of Sulfwr From Sour Acid

Recovery of Sulfwr from Sour Acid Gas: A Review of the Technology John S. Eow Department of Chemical Engineering, University of Leeds, Leeds, LS2 9JT, United Kingdom

i%e modified Clausprocess is the major technology for the recovery of elemental sulfur from H S and SO2 A number of commercial technologies for the recovery of sulfur from acid gases are also highlighted here. A Claus tail-gas clean-up treatment is essential to give high sulfur recovery efficiency from sour acid gases. Generally, the existing tail-gas clean-up technologies can be classified into two groups: those that attain 99% overall sulfur recove ry efliciency, and those that achieve 99.9% efficiency, including the sulfur recovered in the Claus unit. Theseprocesses are the Amoco’s Cold Bed Adsolption (CBA), the SNPMLurgi Sulfreen, the IF8 the SCOT, the Beavon, and the Wellman-Lordprocesses. The SCOT process is generally the most reliable and flexible technology. Process comparisons are also summarized in terms of the sulfur recove y efficiency, hazards and disadvantages, reliability and advantages, plant capacity and ecological impacts. Several changes and new trends are also highlighted here, such as the introduction of non-permselective catalytic membrane reactors for the Claus reaction, and the in situ adsop- tion of water inside the Clam catalytic reactor. ?be successful utilization of H S by converting it to sulfur and H2 attains the triple objectives of waste minimization, resource utilization, and environmental pollution reduction. Photochemical and plasmochemical methods are still in the development stage. Application of electrochemical technology to H2S requires further development. Research for an optimum porous catalyst structure is ongoing for obtaining a relation of micropores and macropores which would provide effective conversion of H S and SO2

INTRODUCTION Sulfur is often considered one of the four basic raw

materials in the chemical industry. It can be produced from various sources using many different methods, such as conventional mining methods, o r it can be recovered as a byproduct from sulfur removal and recovery processes [ll.

However, changes worldwide have affected sulfur sources and the amounts consumed in the last 30 years [ l l . Recovered sulfur production has become more significant as sour feedstocks are increasingly uti-

lized, and environmental laws on emissions and waste streams have continued to tighten worldwide [2 , 31. For example, voluntary sulfur from the Frasch mining process supplied only 25% in 1995, compared to about 53% in 1980. Recovered sulfur increased from 5% of the total production in 1950, to 67% in 1996 111. Discov- ery and development of large sour natural gas fields in many countries have also been important factors in this rapid growth. Increased processing of sour crude oil and tighter pollution control has caused most refineries to recover the sulfur content of its crude oil.

Historically, sulfur recovery processes focus on the removal and conversion of hydrogen sulfide (H2S) and sulfur dioxide (S02) to elemental sulfur [4 , 51, as these species represent the largest source of potential sulfur emission [61. H2S occurs naturally in many natural gas wells, and is produced in large quantities in the desulfurization of petroleum stocks [7-91. It has been considered a liability, which only occasionally can be an asset, depending on the international sulfur price [51. It has a high heating value, but its use as a fuel is not possible because one of its combustion products is S o l , which is not environmentally acceptable. There- fore, one of the immediate alternative routes for the utilization of H2S is to break it down to its constituent elements of hydrogen and sulfur [lo, 111.

Various processes for the removal of SO, in the combustion gases have been reviewed [121. The majority of the processes are based on a throw-away process, in which alkali or alkali earth metal reacts with SO, to form metal sulfate 113-171. However, this approach results in the disposal of large quantities of sulfate waste materials. Direct catalytic oxidation of SO2 to SOg, and subsequent absorption of SO3 in water to produce sulfuric acid, is an alternative method [17, 181. This approach applies to process or combustion gases containing moderate to high concentrations of SO2. Copper smelters are the primary example.

Environmental Progress (V01.21, No.3) October 2002 143

Currently the modified Claus technology is widely used, compared to other processes, to produce elemental sulfur from H2S present in gases from oil refineries, natural gas, coal gasification and other industries [9, 19-23. It was developed by C.F. Claus, in 1883 1241 and was significantly modified in the late 1930s by I.G. Farbenindustrie AG [251. Major improvements were not made in the technology itself or its application until such a process was required in the United States in 1950s. This technology has now advanced to a stage where overall recovery has increased from 9042% to the level of 98-99% of inlet sulfur [3, 23, 261.

STUDIES AND RESEARCH ON CIAUS PROCESS MECHANISMS AND TECHNOLOGY Figure 1 shows a simplified process diagram of a

Claus plant. Acid gas contains HzS, C02 and H20 as major components, and N 2 and hydrocarbons as minor components. Ammonia (NH3) is also present in sour water stripper gas [27, 281. The Claus furnace and the waste heat boiler are normally treated as a single unit [29, 301. Monnery, et al. [311 and Nasato, et al. [301 identified the reaction furnace as one of the most important, yet least understood, parts of the modified Claus process. The initial sulfur conversion occurs there, the SO2 required by downstream catalytic reactors is produced there, and contaminant destruction is supposed to take place there [22, 321. However, many side reactions also occur, reducing sulfur recovery and producing unwanted substances [201. According to Nasato, et al. [301, Kaloidas and Papayannakos [331 and Dowling, et al. [34], the disassociation and re-formation of H2S in the furnace is important as it provides a portion of the sulfur and a majority of the H2 for other reactions and consumes H2S that could be used in the Claus reactions. At temperatures below 1,000" C and residence times below 0.5 second, the H2.S cracking rate is insignificant [321. Below 950" C, the overall conversion of H2S is low even at a long residence time. Therefore, the main purpose of the reaction furnace is to provide optimum temperature and residence time so that the exiting ratio of H2S to SO2 is 2: 1, maximizing catalytic conversion downstream [221.

The waste heat boiler, usually a shell and tube heat exchanger, cools the furnace exit gases from 1,188" C to 154" C in one or two tube passes, generating low-pressure steam [301. This is to condense the sulfur products (mostly Sg and S6, and a small amount of S2> [4, 21, 221. Moreover, at 154" C, the sulfur products are at their lowest viscosities [35, 361. Hence, the products would easily flow through the pipes into the sulfur pit. To prevent the pipes from becoming blocked, a low pressure jack- eted steam generated in the waste heat boiler is introduced around the pipes. Two reactions are believed to occur in the waste heat boiler tubes:

The principal reaction of the Claus process are as follows [4, 9, 22, 37, 381:

(4) 3 n

2H,S + SO, & 2H20 + - S, + beat

where n is the average molecular species of the sulfur vapor product, with n = 2 to 8 and possibly more. In Reaction (3), about a third of the H2S is combusted in the reaction furnace to form a stoichiometric amount of S02 , which is then reacted with the remaining H2S in Reaction 4 to yield elemental sulfur and water [23, 391. Reaction 3 is carried out in the furnace at 1,188" C, usually under partial oxidation [40, 411.

Reaction 4 is an equilibrium reaction favored at low temperature in the presence of a catalyst [21, 37, 421. In order to increase conversion, Bonsu and Meisen 1431 proposed using fluidized bed reactors, rather than conventional fixed-bed reactors, so that the last reactor could be continuously operated below the sulfur dew point. According to Puchyr, et al. [211 and Bonsu and Meisen [431, if equilibrium conversion could be achieved in each reactor, the use of fluidized-bed reactors could result in an overall H2S conversion of 99.5 Yo.

The most widely used Claus catalyst is non-promoted spherical activated alumina [23, 441. However, Paik and Chung [17] reported that Co-Mo/AlzOg, which is usually used for hydrodesulfurization of a petroleum feed stock, can convert SO2 with H2S selectively to elemental sulfur at lower temperature than that com- monly used. However, the hydrogenation of SO2 to H2S occurring on metal sulfide sites was found to be much slower than the Clam reaction on alumina 1181. The active sites for the SO2 hydrogenation was believed to be sulfur vacancies in metal sulfide, and the most effective catalyst had an ability to form and regenerate sulfur vacancies most easily.

In the Claus process, other sulfur compounds will form, such as carbon disulfide (CS2) and carbon oxy- sulfide (COS), and these compounds can often con- tribute from 20 to 50% of the pollutants in the tail-gas [44, 451. COS and CS2 are usually hydrolyzed in the catalytic converter [21, 381, as shown below:

CS2 + 2H20 + 2H2S+ C02 (6)

Studies carried out by Laperdrix, et al. [461 also reveal that the presence of 0 2 traces in the CSZ-H~O mixture caused a decrease in the activity of alumina and titania catalysts due to sulfate formation. IR studies show that sulfate species are reduced by H2S at 320° C on titania, in contrast to the sulfate species on alumina, implying that titania is much more effective than alumina when the CS2 i H20 feed also contains H2S and 0 2 traces [461.

The temperature of the first catalytic reactor is maintained at about 350" C to hydrolyze COS a n d CSz, while that of the subsequent r e a c t o r s is just above the sulfur vapor dew point [421. Transition metal oxides can be used to modify gamma-alumina

144 October 2002 Environmental Progress (V01.23, N0.3)

~~ ~

Recycle from Tail-gas Unit

I I ~

Preheater Preheater

I I

feed

I I

I Steam G-l t I

furnace boiler

1 Boilkr feed

water

Figure 1. Simplified two-stage Claus process flow diagram.

p, Reactor

I

Condenser 0

TO Tail-gas Unit

1 Reactor 0

Condenser 0 w Sulphur Pit

to form a catalyst that is effective at temperatures higher than the dew point of sulfur [47-491. However, thermodynamics provide a strong incentive to operate the catalytic converters at low temperature [3, 501 as a lower temperature should increase the exothermic reaction efficiency. Under these conditions, the produced sulfur would be deposited, thus deactivating the catalyst by fouling [3, 511 and/or decreasing the specific surface area and pore volume [23, 52-541. Uncondensed gas, mainly H2S, S02, COS, CS,, N2, unburned hydrocarbon and NH3, are reacted in the lower temperature catalytic reactors [381. Alvarez, et al. [531 and Pineda and Palacios [541 showed that H2S conversions higher than 90% can be achieved using concentrations in the range of 1-5% with a relatively slow catalyst deactivation, especially if the operation conditions and catalyst properties are optimized.

The adverse effect of water on alumina catalyst, especially at low temperatures, has been recognized as being responsible for low activity in the COS hydrolysis [55, 561 and a decrease in H2S conversion [571. Conversion with low water content, such as 5% water vapor, was found to be 2 to 2.5 times higher than that obtained with 35% water content, apparently due to a competition with SO2 and H2S for adsorption sites. The results by Laperdrix, et al. [261 and Steijns and Mars [571 also indicate that, in the presence of Sn

and H20, H2S and SO2 can be produced. However, according to Ledoux, et al. [31, the use of a new type of support, such as Sic, and a nickel-based active phase provide an active, extremely selective and stable catalyst for the oxidation of H2S into elemental sulfur by 0 2 at relatively low temperature. The catalyst exhibited a high and stable H2S conversion even at a sulfur loading of more than 60%. While in the feed without water, a rapid deactivation was observed. Water assists in the mechanical removal and transport of the sulfur formed by the particles of the active phase on the hydrophilic part of the support (i.e., oxycarbide or oxide of Si) to the hydrophobic part (i.e., Sic), leaving free access to the active particles even at high sulfur loading [31.

From thermodynamic calculations, Laengrich and Cameron [21, Ledoux, et al. [31, Anon [191, Opekar and Goar 1271, Grancher 1581, and Pearson 1591 recom- mended three or four catalytic converters operating under steady state conditions at low temperature. Thermodynamic calculations indicate the possibility of reaching efficiencies > 99% [501. Unfortunately, these results cannot be obtained with current technology due to reaction kinetic limitations and, particularly, because of sulfur deposition in the catalyst pores [37, 52, 54, 601. As a catalyst is being covered by sulfur, a change in the process kinetics should be expected,


with activity declining. The life and performance of a catalyst, such as Sepiolite with a structural formula Si12Mg~030(OH)4(H20>4.8H20 [51, 61, 62,1, are thus closely related to its ability to preserve activity through fouling. Dowling, et al. [341, Alvarez, et al. [511 Alvarez, et al. (531, Guijarro, et a1.[631, Steijns, et al. [641, and Dudzik and George [651 suggested that the sulfur radicals formed during the reaction also act as a catalyst, yielding water and new sulfur radicals. As the exposed surface decreases and sulfur radicals stabilize and condense, depending on the temperature and the vicinity of the chains, so does the accessible reactive sulfur, and the reaction decays. Therefore, according to Alvarez, et al. [511 and Pineda and Palacios [52, 541, the deactivation process under the same reaction conditions and temperature should be strictly related to the texture and macroporosity of the catalyst.

Under the reaction conditions, micropores in catalysts are useless as they are normally lost with the first gram of sulfur produced [621. Mesopores and small macropores, by contrast, are critical because the reactants can diffuse less and less towards the solid active surface as sulfur deposits are produced in voids and throats of the porous system, deactivating the catalysts [23, 661. There seems to be no diffusional limitation to the reactants in the macroporous system of the catalyst up to the studied level of Guijarro, et al. [621. The open structure should allow a better reactant flow through the voids, which would result in greater activity and longer life of the catalyst [661. From the investi- gation of the performance of treated alumina as catalyst at low temperature, Pineda and Palacios [521 con- cluded that by applying either acidic or basic treatments to optimize the textural properties, the activity of the catalyst was greatly enhanced.

So far not enough information is available on reaction kinetics in the presence of catalyst [23, 42, 59, 671. In designing the reactors, the reaction rate expression must be known. Factors possibly effecting the rate expression are: (1) the so-called kinetics of the Claus reactions and of the reverse reactions, (2) the influence of the mass and heat transfer inside the catalyst, (3) the effect of catalyst granulometry, (4) the effect of sulfur deposition as a function of the operating conditions, and (5) the influence of reversible and irreversible catalyst aging [23, 41, 60, 671. Reaction is limited by the rate of internal diffusion through the porous particle [231. It may also be limited by the rate of external mass transfer if the linear velocity of the gases is too low. Conse- quently, it is preferable to use catalyst of small particle size and large macroporosity.

To reach the highest possible conversion of COS and CS2, the outlet temperature of the first converter must exceed 350" C. Under this condition, H2S and SO2 conversion at equilibrium are lower [371. The Claus reaction rate is reduced in the second and third catalytic converters because of lower temperatures imposed by thermodynamics, lower H2S and SO2 partial pressure, and in order to maintain higher conversion of H2S [71. The second and third reactors operate at their lowest possible temperature, which is the temperature of the sulfur dew point. Sulfur formed in each stage is condensed and recovered in the sulfur pit to achieve maximum conver-

sion in the reactors. Unrecovered sulfur, in elemental or combined form (HzS, COS, and CS2) is treated in the amine system [5, 681.

There are two principal process variations [6, 21, 58, 691. The first is the straight-through process, in which all of the air and the acid gas pass through the combustion zone. In this process, the primary purpose of the reaction furnace and the waste heat boiler is to oxidize approximately 1/3 of the H2S feed to SO2 so that stoichiometric proportions of these two main reactants are present for the reactors [211. The second is the split-flow process, in which all the air and at least 1/3 of the acid gas pass through the combustion zone, with the remaining acid gas sent to the first catalytic reactor.

ALTERNATIVE TECHNOLOGIES FOR SULFUR RECOVERY Beside the Claus technology, many commercial

processes based on adsorption, absorption, and wet oxidation have been used for removing H2S from gases [14,70, 711. Dry catalytic processes based on the selective catalytic oxidation of H2S to elemental sulfur have recently been developed 1721. Examples of commercial catalysts developed for the processes are the titanium-based catalysts for the MODOP process [73, 741 and the iron-based catalysts for the Superclaus process [75, 761. Titanium based catalysts require a stoichiometric amount of 0 2 , but are easily poisoned by water vapor, while iron-based catalysts are resistant against water, but requires an excess amount of O2 [721. Vanadium oxide catalysts have also been reported to be active with a stoichiometric amount of O2 [77, 781. However, the catalysts have not yet been widely used, due to uncertainty about water poisoning effect and long-term stability [721.

Paik and Chung [171 reported a new catalytic system, cobalt molybdenum on alumina, for the reduction of SO2 with H2. The catalyst is active and selective for the production of sulfur at significantly lower temperatures (around 300" C). When the feed ratio of H2 to SO2 is optimized to 3.0, about 80% yield of sulfur could be obtained. The selective reduction of SO2 to elemental sulfur occurs in two reaction steps; SO2 is first totally hydrogenated to H2S on the metal sulfide phase, followed by the Claus reaction to produce elemental sulfur over acidic alumina support [171.

Many factors affect the selection of the gas-treating process including the volume, temperature, and pressure of gas to be processed, the selectivity required, the desirability of sulfur recovery, the types and concentrations of impurities present, air pollution laws and specifications to be met, as well as capital and operating costs [2, 281. Table 1 shows a comparison among the major types of gas-treating processes, which can generally be categorized as adsorption, absorption, and conversion [681.

From Table 1, it is clear that, of the three process types, the conversion process, such as the Claus process, has the highest efficiency at 94% to 97%, as well as the least number of hazards and disadvantages. Moreover, its problems can be easily solved by using the correct stoichiometric amount of air, fuel gas, and acid feed gas [61. The adsorption process is limited to a low volumetric

146 October 2002 Environmental Progress (V01.21, No.3)

~~ ~~ ~~~~

Table 1. Comparison of the alternative processes for sulfur recovery. Part one of two.

Adsorption Process

- Overtime, all of the iron oxide becomes sulphided and its adsorptive capacity becomes exhausted.

- The zinc sulphide formed cannot be oxidised back to zinc oxide. - The sulphur removed via this process is usually not recovered. The sulphur and sorbent both undergo disposal.

- Limited capacity of sorbent bed.

- Limited to gas streams of limited volumetric rate having low concentration

- Safety is most important of H2S.

because H2S is extremely toxic and quickly paralyses the sense of smell.

- Molecular sieves developed to extend the operating range. - Molecular sieves can be controlled to target the removal of certain components selectively. - Molecular sieves can be regenerated.

- The advancement of integrated gasification combined cycle (IGCC) power plants develops the fluidised bed adsorption bed processes which are able to withstand severe operating condition.

- The sorbent bed has a

- Limited volumetric rate limited capacity.

having low concentration of H2S.

Absorption Process

- Other components in the feed gas may react with and degrade the amine solution. - Solution must be purged and fiesh amine added periodically.

higher solvent circulation rates and higher regeneration energy. - MEA process has shown a higher tendency towards corrosion and foaming.

- Safety is most important as H2S is extremely toxic.

- MEA process require

- Solvent can be regenerated for reuse.

- Many absorption processes also removed COz and to a lesser extent COS, So2 and mercaptans. - MEA removes both H2S and COz nonselectively. - MEA lowest solvent cost

and lowest hydrocarbon co- absorption relative to other mine process.

- Amine absorption processes can be applied when H2S Concentration is relatively low (e.g. <5-25 mol %).

- Can achieved high purity in treated gas.

Conversion Process

94 % to 97 Yo

- Processing streams having lesser amounts of H2S may result in combustion flame instability.

- The kinetics are incompletely understood.

sulphur vapour condensation.

- Water vapour af fec t adversely the equilibrium of the reaction of H2S and SO2 over the catalyst.

importaut as H2S is extremely toxic.

- Fog formation during

- Safety is most

- The most widely used to convert H2S to sulphur. - Operate at nearly atmospheric pressure.

- 0 2 enrichment of the combustion air can be used to increase the processing capacity of an existing unit and to extend operation to low concentration H2S feed.

improves sulphur recovery, increase contaminant destruction and more reliable operation.

- The Claus process has proven very reliable and is considered a mature technology. - H2S concentration can be reduced fiom 0.85% in Claus off-gas to 4Oppm in incinerated

- SO2 concentration can be reduced fiom 0.42Yo in Claus off-gas to 0.02% in incinerated

- 0 2 enrichment

SCOT Off-gS.

SCOT Off-gas.

Environmental Progress (Vo1.21, No.3) October 2002 147

Table 1. Comparison of the alternative processes for sulfur recovery. Part two of two.

Ecological Impact

costs

c - The sulphur removed is

generally not recovered, therefore it affects the environment.

- The disposal of sorbent material also affects the environment.

- High operating costs due to sorbent materials disposal.

- 2 adsorption towers.

- Chemical and physical solvents leakage can affect the surrounding ecology.

health. - Also affect workers'

- High solvent costs. - 1 absorber, 1 regenerator, 1

cooler, 1 heat exchanger, 1 reboiler, 1 condenser

I

of feed gas with high

~ ecological impact with a tail-gas treatment.

- Suitable to employ various Claus tail-gas treatment processes.

- Consists of 1 combustion furnace, 1 waste heat boiler, 1 condenser and a series of catalytic stages, each employing 1 reheat, 1 catalyst bed and 1 sulphur condenser.

feed rate containing low concentrations of H2S. The Claus process has also proven to be very reliable and mature [691. Only the Claus process, among the three, can treat large amounts of feed gas with high H2S concentration, and produce minimum ecological impact with a tail-gas treatment unit [40, 501. At the same time that the capabilities of the conversion process have dra- matically improved, innovations and process optimiza- tion have reduced its capital and operating costs [ll.

CLAUS TAIL-GAS TREATMENT TECHNOLOGIES In the early Claus sulfur recovery plants, the tail-

gases were usually exhausted to atmosphere through a stack without any treatment. Sometimes the gases were incinerated after leaving the last converter, and the SO2- containing tail-gas was passed through a tall stack [581. As the need to reduce SO2 emissions receives greater emphasis, Claus technology has to be improved to obtain higher recovery rates. At the present time, most Claus plants are unable to meet existing or proposed air pollution regulations in developed countries without additional methods of reducing or eliminating the sulfur content of the exhaust gas [5, 661. Adding a tail-gas cleanup process should be the last resort, as it is expen- sive in terms of investment and energy consumption, depending on the process selected [51.

Several processes have been studied for application as a Claus plant tail-gas cleanup service [2, 791. Many commercial processes are based on low temperature Claus reactions or on the removal of H S from tail- gases by absorption and adsorption 16 8 I . However, these processes require batch or periodic operation, and, sometimes, heavy installation costs [91. A Claus tail-gas desulfurization process should preferably be: (1) easy to operate and flexible; (2) based on familiar technology and easily adapt to existing Claus units; (3) generate no secondary air/water pollution or waste; and (4) deliver a high degree of desulfurization over a wide range of operating conditions.

Generally, there are two broad classes of tail-gas cleanup treatment [21, as illustrated in Figure 2. The former consists of processes which allow the Claus reactions to take place under more favorable conditions. These processes claim an overall sulfur-recovery efficiency of approximately 93%, including sulfur recovered in the Claus main unit. Three processes under this group are Amoco's Cold Bed Adsorption (CBA), the SNPMLurgi Sulfreen, and the IFP processes [71].

In the adsorption process, gas from the main Claus plant last condenser is fed to an adsorption reactor, operating between 130" C and 150" C, and containing conventional Claus catalyst. The low temperature favors equilibrium conversion. Sulfur vapor condenses on the bed and is removed, shifting the equilibrium towards higher conversion. Gas from the reactor is then incinerated. While one reactor is on adsorption cycle, a second reactor is being regenerated. Howev- er, regeneration for removing sulfur deposit from catalyst surface leads to a decrease in sulfur storage capacity and in initial desulfurization activity [31. Hot gas from the first Claus reactor vaporizes the condensed sulfur and reactivates the catalyst. The gas is then cooled and the sulfur vapor condenses. Gas is returned to the Claus cycle just downstream of the first Claus sulfur condenser.

The Sulfreen process, developed by the Societe Nationale des Petroles d'Aquitaine (SNPA) and Lurgi Gesellschaft GmbH, uses a vapor-phase extended Claw reaction carried out below the sulfur dew point. The process operates in the same temperature range as the CBA method, with the produced sulfur being deposited on a alumina catalyst bed. In the two-reactor case, one reactor is in service while the other is being regenerated. The Sulfreen design uses a closed regeneration loop containing a sulfur condenser and a regeneration gas heater, usually an indirect-fred unit with stainless steel tubes.

The Institute Francais de Petrole (IFP) developed a treating process used for Claus plant tail-gas cleanup,


Tail-gas Cleanup Treatment rn 99% sulfur

(1) Amoco’s Cold Bed Adsorption

(2) SNPALurgi Sulfreen Process (3) IFP Process

(CBA) Process

Figure 2. Major tail-gas cleanup treatment processes.

recovery efficiency + (1) Shell Claus Off-Gas Treating

(SCOT) Process (2) Beavon Process (3) Wellman-Lord Process

Stack

Reducing L

gas steam r

i - Sorbent Bed A -)+.+ (Sorption)

sulfur - *,orbent Bed B*

Regeneration off-gas (Regeneration)

Figure 3. Simplified MOST process flow diagram [391.

in which the tail-gas from the Claus unit is contacted by an IFP solvent, a high boiling point glycol. Both HZS and SO2 are thus absorbed. The Claus reaction then converts these compounds to sulfur. This entire process occurs above the sulfur melting point. As sulfur has low solubility in the IFP solvent, liquid sulfur accumulates at the bottom of a packed contacter and is withdrawn. Treated gas leaves the top of the tower and is incinerated. Solvent is circulated back to the

top of the tower. In normal operation, no fuel or steam is required except the condensate for make-up.

The Mobil Oil SO, Treatment (MOST) process consists of combusting the Claus tail-gas with air, converting all sulfur species to SOz/SOg [801. The SOx is then sorbed onto a solid sorbent, and the sulfur is reduc- tively desorbed as a concentrated stream of mainly SO2 and HlS, which can then be recycled to the Claus plant for further processing. Catalyst screening for this


Table 2. Comparison among three-stage Claus, 1 CBA, and 2 CBA processes for Claus plant. Part one of two.

Tail-Gas Cleanup Unit

Conversion efficiency (Overall recovery capacity) Hazards &

Disadvantages

3-Stage Claus with indirect reheat

95 % - 97.8 %

- The kinetics of the Claus process are incompletely understood.

adversely the equilibrium of the reaction of HIS and SOz over the catalyst.

- Fog formation can also be a problem during condensation of the sulphur vapour.

and quickly paralyzes the sense of smell.

- One major problem can occur with the operation of the main burner (on reaction furnace).

- Water vapour affect

- HzS gas extremely toxic

2-Stage Claus (96% recovery) with 1 CBA stage in tail gas cleanup unit Up to approximately 99 %

- Mechanical and maintenance problems associated with the gas switching valves and the regeneration gas blower and heater.

- Requires good control of stoichiometric HzS/SOZ ratio.

gradually declines as sulphur condenses on the bed.

- Catalyst activity

- The process requires multiple reactors.

2-Stage Claus (96% recovery) with 2 CBA stages in tail gas cleanup unit

Up to approximately 99.5%

- Mechanical and maintenance problems associated with the gas- switching valves and the regeneration gas blower and heater.

- Requires good control of stoichiometric H 2 S / S 0 2 ratio.

gradually declines as sulphur condenses on the bed.

multiple reactor.

- Catalyst activity

- The process requires

application focuses on examining alumina and mag- nesium aluminates, with oxidation promoters such as ceria, vanadia, and platinum, where effective SO2 oxidation promoters are required. The materials with the highest SOx uptake are a commercial FCC SOx transfer additive, and a vanadia/ceria-promoted, magne- sium aluminate (V/Ce/Mg2A1205) spinel, with 54% and 46% SO, uptake, respectively. During most of the adsorption period, the SO2 level in the effluent from the sorbent bed is below 1 ppmV [801.

According to Stern, et al. [391, the MOST process, which can combust sulfur containing species and selectively capture SO2 produced, offers operational advantages over other wet scrubbing processes. A simplified process flow diagram of the MOST process is shown in Figure 3. The tail-gas is sent to a burner which oxidizes the remaining H2S to SO2 and SO3. The burner effluent, which contains 1 to 4% 0 2 , goes to sorbent Bed A, where adsorption of the SOx takes place. The tail-gas is then sent to the stack. Reducing gas flows through Bed B to desorb the sulfur as a concentrated stream of H2S and S 0 2 , which is then diverted to the Claus unit. At the end of the cycle, Bed A is loaded with sulfur, while Bed B had its sulfur removed. At this moment, the valve positions are changed, causing the regeneration gas to flow through Bed A and the tail-gas to flow through Bed B. The process is described in detail by Stern, et al. [391.

The second class includes processes capable of achieving overall sulfur recoveries in the range of 99.5% to 99.9%. This level corresponds to about 300 ppmV or less total sulfur in the exhaust gas. Three commercial processes of this type are the Shell Claus Off-Gas Treating (SCOT), the Beavon, and the Well- man-Lord processes. The SCOT process consists of a reduction stage, followed by a concentration stage that provides a H2S-rich stream to be recycled to the Claus plant. A simplified flow diagram of the process is shown in Figure 4. The concentration process is similar to the amine gas sweetening process common- ly used in gas processing. In the reduction section, all sulfur compounds and any free sulfur in the Claus tail- gas are completely converted into H2S with H2 or a mixture of Ha and CO over a cobalt/molybdenum on alumina catalyst at a temperature of about 300" C [44, 811. The tail-gas contains some H2 and CO. The hot gas is then cooled, and water is condensed in a cooling tower. The cooled gas, which normally contains up to 3 vol. % H2S and 20 vol. % C 0 2 , is then coun- tercurrently scrubbed by an alkanolamine solution in an absorption column [ 51. A conventional stripper can be used to strip the acid gases from the solvent. These gases are recycled to the Claus plant inlet. The remaining tail-gas, normally containing 200 to 300 ppmV HzS, is then incinerated. The SCOT process has been designed for minimum pressure drop so that it can be easily added to an existing Claus unit.


Table 2. Comparison among three-stage Claus, 1 CBA, and 2 CBA processes for Claus plant. Part two of two.

Reliability & Advantages

Plant capacities

Ecological impact

costs

- Has proven very reliable and is considered as mature technology.

- Use of modem, high- intensity, efficient mixing main burners can result in a more stable flame, especially with leaner feeds.

destruction for compounds such as hydrocarbons, NH3, mercaptons, etc.

- Reduced or nil oxygen breakthrough.

- Improved Claus thermal conversion, and much wider turndown or turnup operations.

- Improvements are being made in better Claus catalysts and improved process control. - In most U.S. states, a sulphur recovery unit of 20 Itd or larger will require some form of tail gas cleanup.

recovery units of 50 ltd or larger normally require a tail gas cleanup unit.

- Safety is very important in plants handling and processing hydrogen sulphide gas. - Poisoning by H2S. - Nowadays, units without tail gas treatment cannot meet the regulations.

- Much better contaminant

- In Canada, Sulphur

- Has the lowest cost because no tail-gas treatment unit. However it is the least efficient and unable to meet the specifications.

- Low energy consumption.

- 260°F to 300°F for operation; favours equilibrium conversion.

- Can reduce SO2 content to about 1500 ppmv. - Uses the same construction and materials proven in the Claus plant.

- Requires little plot space and only minor modification to an existing plant.

- Example, Amoco built a 1500 ltd sulphur plant with CBA near

- Requires 2 reactors, 1 condenser and 1 blower for addition.

Calgary.

- Level of COS and CS2 is not reduced, therefore affecting the surrounding air quality. - Problems have occurred with H2S spikes during the regeneration procedure which have resulted in occasional environment violations.

. A new sulphur recovery costs 1.5 times more than a standard 3-stage Claus unit.

- Capital cost to convert is about % that of the Claus plant.

~ ~~

- 260°F to 300°F for operation; favours equilibrium conversion. - Low energy consumption.

- Can reduce SO2 content to less than 1000 ppmv.

- Uses the same construction and materials proven in the Claus plant.

- Produce more but need more equipment and energy.

costs.

2 blowers.

- More efficient at higher

- 4 reactors, 2 condensers,

- Levels of COS and CS2 are not reduced very much.

- Two new sulphur recovery units cost 3 times more than a standard 3-stage Claus unit.

- Capital cost to convert is about that of the Claus Dlant.


Fuel Air

Furnace el Uni

Reactor 9 t Emuent

Fuel Air Absorber

I

Reactor Effluent

Air SWS Stripper

Figure 4. Simplified process flow diagram of the SCOT process.

Recycle to front end Claus Unit I

~~

Regenerator

The Beavon process, developed by Ralph M. Par- sons Company and Union Oil of California, has been used in several Claus tail-gas cleanup units [821. The first industrial unit, at Wintershall AG, Linden, Ger- many, started in January 1978, and has performed very satisfactorily, aimed at 98.5 to 99.5% overall sulfur recovery. In this process, tail-gas from the Claus unit is first treated by a reducing gas over a cobalt- molybdenum catalyst to convert all sulfur-containing species to H2S. The Claus. gas usually contains a significant portion of the required reducing agents. Addi- tional reducing gas is supplied by an auxiliary burner, which is also used to maintain a temperature between 315" C and 370" C [821.

Residual concentrations of COS, CS2 and CH SH

cooled in a condenser to about 150' C to 190" C, and contacted by a sodium carbonate-bicarbonate solution at a pH > 7 to scrub out any SO2 that might have passed through the catalyst bed without being reduced. The cooled gas then goes to a Stretford absorber where it is contacted by a sodium carbonate- sodium bicarbonate solution containing sodium vana- date, and an oxidation catalyst, where the H2S in the feed gas is absorbed and oxidized to sulfur. Additional holding time for this reaction is provided by a reaction tank. Air is then used to oxidize the vanadium back to the pentavalent state. The recovered sulfur forms a

for the reactor are low. The reduced gases are t i en

froth at the top of the oxidizer which is skimmed off, filtered, washed and dried, and melted [821. The Beav- on/Stretford process can reduce sulfur emissions to several ppm, but is less effective than the SCOT process in removing CS2 or COS, or mitigating any CO which may pass through the Claus plant [391.

In the Wellman-Lord process, the Claus tail-gas is incinerated, then cooled to about boo C and fed to an absorber, where it is contacted by a sodium sulfide solution. The solution reacts with SO2 to form bisulfide. Steam is used to drive off the SO2 and much of the aqueous solution in the evaporator/crystallizer (831. Sodium sulfide crystals precipitate here, forming a slurry. Gas from the evaporator/crystallizer is cooled to recover most of the vaporized water, which is used to dissolve the crystals. The SO2 gas is recycled to the front end of the Claus unit. To complete the regeneration process, the solvent is also treated with sodium hydroxide, reacting with any remaining bisulfide to form sodium sulfide and water. The H2S/S02 ratio control is not critical, as the Wellman-Lord process removes sulfur after the tail-gas is incinerated. Tank- age can be added to allow operation for up to three days while the regeneration cycle is down.

Table 2 provides a comparison among a three-stage Claus with indirect reheat unit, a two-stage Claus with one CBA unit and a two-stage Claus with two CBA units. Comparison among the Sulfreen, the IFP, and


~ ~~

Table 3. Comparison among Sulfreen, IFP, and SCOT processes for Claus plant. Part one of two

Tail-Gas Cleanup Unit

Conversion efficiency (Overall recovery capacity)

2-Stage Claus (95.5% recovery) with Sulfieen process tail gas cleanup unit Upt to approximately 99%

- Mechanical and maintenance problems associated with the gas- switching valves and the regeneration gas blower and heater.

- Limited by equilibrium conversion and sulphur vapour pressure losses.

- Require careful operation of the parent sulphur plant to achieve maximum recovery.

- More complicated than CBA process.

- Not very energy efficient. - operating temperature is quite low at 2609: to 300OF.

- Facility is compact. - Some H2S is oxidised by injection of a small quantity of air, monitored by an analyser, in order to provide an optimal H2S/S02 ratio at the Sulfieen reactor inlet.

operation makes operator familiarisation more simple.

- The simplicity of

2-Stage Claus (95% recovery) with IFP process tail gas cleanup unit

98.1 %to 99.4 %

- Air control needed for correct H2S/S02 ratio. - Proper operation of the Claus plant is required to miniiise COS, CS2 in the Claus tail gas for optimum performance.

- Operation of an IFF' installation is quite different fiom the parent Claw unit, presenting a new set of operating problems.

- Some difficulties present in the CBA and Sulhen processes. - IFP solvent has good thermal and chemical stability, and low volatility reducing solvent losses.

- Recovered sulphur is high quality. - No fuel or steam is required other than condensate for makeup. - Low operating ternperatwe at 125OC. - Retrofit is not complicated as installation requires little plot space and does not recycle any gas to the Claus feed.

Clam plant (94% recovery) with SCOT process tail gas

99.9 + % cleanup unit

- High temperature needed for catalyst at 575°F.

- Not a good selection for direct treating of the tail gas from Claus plant that processes a feed gas with a high C02, low H2S content.

- The concentration process is similar to the mine gas-sweetening processes, making SCOT process easier to operate.

- Flexibility and overall process reliability are good. - Catalyst life is good.

- Presulphiding of the

- Controlled bum-off catalyst is not critical.

followed by resulphidmg is said to restore catalyst to its original activity.

- The reduction step converts essentially all sulphur-containiig compounds to H2S.

- Absorption column is aimed at achieving essentially complete removal of H2S while coabsorbing only a fraction of the C02 present. - Can be designed to operate from 20% of design fedrate up to full rate.

- Changes in the feed have only a small effect on

Environmental Progress (V01.21, N0.3) October 2002 153

Table 3. Comparison among Sulfreen, IFP, and SCOT processes for Claw plant. Part two of two.

- 2 reactors using conventional Claus catalyst beds, 1 condenser, 1 regeneration gas heater.

operation (in 1992) after Claus units &om about 50 to 2200 tpd of sulphur and 5 presently under construction.

- 40 Sulfreen units in

- Proper operation of the Claus plant to minimise COS and CS2 in the Claus tail gas. COS and CS2 can cause air pollution.

amounts to 30% to 45% of the Claus unit cost for the conventional version and 40% to 55% for the improved version.

- Utility requirements per ton of sulphur: electricity 300kW, catalyst about 4 lb for conventional version and 5 Ib for the improved version.

- Operating costs are much lower than solvent-based Drocesses.

- Sulfreen investment

- Capable of reducing the SO2 content of the incinerated tail gas to as low as 1000 ppmV.

- Require only a contactor, 1 pump, 1 solvent heater for start up.

- Tail gas contains some H2 and CO which is toxic in significant quantity.

- Does not affect CS2 and cos. - The IFP solvent is relatively inexpensive, keeping initial and operating costs down.

overall sulphur recovery. - Reduces the sensitivity of the overall sulphur recovery facility to variations in the air supply rate.

- Can be designed for minimum pressure drop to make it more suitable for add-on installation. - Proven and familiar equipment is used in each step of this process.

- Produces no secondary waste streams.

- From 10 todstream day (sulphur intake) to 2100 todstream day equivalent Claus capacity.

virtually complete conversion of elemental

is obtained (i.e. residual S02contents -= 10 ppm).

- With an excess of H2,

Sulphur and SO2 into H2S

- Most widely used. - 130 units (in 1992) are committed, with capacity from 3 to 2100 tpd of fresh sulphur feed. - The tail gas contains some H2 and CO. CO in significant quantity can cause health problems.

- Capital costs equal to the costs of the Claus plant.

- For a new facility with SCOT design optimised for the best possible fit, the cost can be as low as 75-85% of the Claus unit.


Table 4. Comparison between Beavon and Wellman-Lord processes for a Claus plant. -

Tail-GS Cleanup Unit

Conversion e ffciency (Overall recovery capacity)

Hazards& Disadvantages

Reliability & Advantages

Plant capacities

Ecological impact

costs

Claus plant with Beavon process tail-gas cleanup unit

99.4 Yo

- At high temperature around 600 to

- Big space is needed. - Absorber has to be clean-out about

- Absorber plugging. - Plugging in the sulphur fioath lines. - The reducing catalyst life is only about 2 years. - Concerns over vanadium used in the process have limited its application.

- Residual concentration of COSY CSz and CH3SH fiom the reactor are low.

- Some sections of the unit are coated with plastic to avoid corrosion by deposited sulphur.

holding time for reaction.

during nonnal operations.

type of sulphur recovery plant if adequate plot space is needed. - All pressures are near atmospheric.

- To be able to achieve 100 ppmV or

- The clean tail gas containing less

700°F.

every 6 months.

- A reaction tank can provide additional

- No tail gas incinerator is required

- Suitable for add-on installations to any

less total sulphur in tail gas.

10 ppm HzS when using the newer solvents, which are highly selective mine type solvents. - There are more than 15 Beavon MDEA plants in the U.S. and Japan. 2 Beavon- Selectox plants are in the U.S. and Germany.

- After cooling in the reactor, HzS, CSZ, COS gases are treated by the Stretford process. The exit gas is discharged with no further processing.

- Total investment is approximately

- High operating costs for sour gas equal to that of the parent Claw plant.

disposal during absorber clean outs every 6 months, reduction of catalyst changeouts and any mechanical failure.

Claus plant with Wellman Lord process tail-gas cleanup unit

99.9 + Yo

- Process chemistry and equipment are not familiar to many plant personnel, thus compounding operating and training difficulties.

- HzS/SOz ratio control is not critical to

- Tankage can also be designed into the design tail gas treating.

facility to allow design operation for up to 3 days while the regeneration cycle is down. - This process is well-suited for high COz streams as it does not recycle C02 with the SO2.

- To achieve SOt emission level at 200 ppmV or less.

- A bleed stream must be treated to take out sodium sulphate.

- High capital cost because it requires exotic metallurgy.

- Capital cost about 130-150% of the parent Claus unit for a 100 ltd unit.

Environmental Progress (V01.21, N0.3) October 2002 155

b’

the SCOT processes are given in Table 3, while Table 4 illustrates the comparison between the Beavon and the Wellman-Lord processes for a Claus plant. By con- sidering the advantages and disadvantages of all the six methods in Tables 2 to 4 , it is clear that the SCOT process is generally the most suitable for a tail-gas treatment unit. A Claus plant with a SCOT unit can achieve conversion of 99.9% or more. If the feed gas to the Claus plant contain a low CO2 concentration and a high H2S content, then the SCOT process is a good selection for direct treating of the tail-gas. More- over, the amine system in the SCOT process is much easier to operate compared to other processes. The SCOT process, with good catalysts, is very reliable and flexible to disturbances.

One of the most important features of the SCOT process is that it can operate from 20% of design feed rate to full rate. Therefore, its ability to cope with changes in the feed conditions minimizes any effect on overall sulfur recovery. The SCOT process can also be designed for minimum pressure drop, thus making it more suitable for add-on installations. It is also quite environmentally-friendly since it produces no secondary waste streams. Using excessive H2, the SCOT process can achieve a residual SO2 contents of less than 10 ppm. The capital cost can be as low as 75% to 85% of the main Claus unit if the design is optimized [441.

MODOP and Superclaus processes seem to be very attractive as they can convert H2S directly to elemental sulfur by selective catalytic oxidation and do not require periodic operation [91. Superclaus seems to be superior to MODOP since the catalyst for the former can tolerate the presence of water. However, Super- claus uses ten times more 0 2 than the stoichiometric amount for converting H2S to sulfur, and cannot be applied to treat H2S higher than 5%. Recently, it is claimed that Fe-Cr/Si02 catalyst can give sulfur yields of more than 90% at the Superclaus condition [841. The catalyst is known to show little decrease in the sulfur yield, even in the presence of 30 vol. % water vapor in the feed. Vanadium/silica (V/SiO2) catalyst shows a decrease in the yield when excess water is introduced in the feed. The use of a stoichiometric amount of 0 2 with V/SiO2 is possible to treat highly concentrated H2S I31, whereas the Superclaus catalyst is limited to H2S concentration of less than 5 vol. Yo.

The following is a simplified guide for selecting a sulfur-recovery process configuration: (1) try a “best

. : c:ata&st.3: wat<r: sprbept.(zpt)l&+) - : Sulphur- . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

current technology” Claus sulfur unit first; and (2) consider a Claus sulfur-recovery unit plus a tail-gas unit as a last resort. Tables 1 to 4 summarize the recovery capability of several sulfur recovery systems, ranging from the simple to the complex. This should help in a preliminary determination of the system for a particular requirement.

POSSIBLE IMPROVEMENTS IN TAIL-GAS TREATMENT PROCESSES Claus sulfur recovery units at existing installations

will need to process significantly more sour acid gas in the future. Therefore, current technology might not be able to cope with increasing capacity and concentration of H2S in the acid gas. Several changes and new trends are currently being studied, and other technologies are under further development. More- over, the more stringent environmental regulations in the future might force some plants to shut down.

Conventional technology for carrying out the Claus reaction involves a series of fixed bed catalytic reactors, with interstage removal of sulfur by condensing the product vapor stream, as shown in Figure 1. An alternative strategy that is much more thermodynami- cally efficient is the in situ adsorption of water inside the reactor using a packed bed with a mixture of catalyst and zeolite adsorbent [85l, as illustrated in Figure 5. This allows high conversions in a single-stage Claus process. The sulfur formed can be separated from the almost completely converted gas emerging from the adsorptive reactor in a single stage of condensation. The sulfur-free gas can then be reheated and used for the regeneration of the second adsorptive reactor [851.

Several companies are investing large amounts of money to improve the sulfur recovery process. Several new technologies are being studied or developed. For example, BOC is investing in a development facility to improve the SURE oxygen-based Claus sulfur recovery process at Courtaulds Chemicals’ carbon disulfide production plant at Stretford, Manchester, UK. The Claus system can be made more efficient by enriching o r replacing the combustion air with 0 2 . The development unit at Courtaulds’ is part of BOC Gases’ program to develop the next generation of oxygen-based Claus processes and burners.

Courtaulds already has a large Claus unit at its carbon disulfide plant. Part of the feed stream to this unit is diverted to the development unit, and recovered sulfur plus the residual gas stream are both returned

156 October 2002 Environmental Progress (v01.21, No.3)

4 SO2 + S + H20

Figure 6. A non-permselective ceramic catalytic membrane reactor for the Claus reaction [861.

to the main Courtaulds plant. The composition of the feed to the BOC development facility can be modified to represent virtually any commercial installation. The research program will also develop computational fluid dynamics models for detailed kinetic studies of the Claus process.

In another development, Kenneth Klabunde and Shawn Decker, both of Kansas State University, are developing a sulfur content removal technique. They have produced calcium oxide crystals coated with iron oxide, offering a greater surface area and a coat- ing that helps increase the reactivity of calcium oxide with the acid gases. The 7 nm crystals are twice as efficient at removing SO2 as the current method. Klabunde also perceives many other possible applica- tions for his research, including an alternative to incin- eration of industrial waste, protection of soldiers from chemical agents, and the removal of chlorinated compounds.

RESEARCH ON NEW CONCEPTS FOR REMOVAL OF H2S FROM TAIL-GASES Thermal cracking of H2S at temperatures between

1,370" C and 1,650" C is being studied by the Alberta Sulfur Research Laboratory (ASRL), Calgary. ASRL has built a semi-works unit and installed it at Petro-Canada's Wildcat Hills plant (near Cochrane, Alberta). The company plans to use a special ceramic membrane to separate the produced H2 from the elemental sulfur. The laboratory is also working on a further development stage of the thermal cracker in which the ceramic material will also serve as a semipermeable membrane to allow the removal of H2 formed in the cracker.

A new configuration of catalytic membrane reactor, introduced by Sloot, et al. B61, consists of two cham- bers separated by a non-permselective ceramic membrane, as shown in Figure 6. The active components of the catalyst can be easily incorporated within the membrane. The membrane functions as a physical barrier between the reactants which are fed to the

opposite sides of the membrane. Figure 6 also shows the arrangement of the ceramic membrane reactor for carrying out the Claus reaction 1861. This reactor type has specific advantages for reactions requiring strict stoichiometric feed of reactants. Any variation in the molar fluxes of the reactants will result in a shift of the reaction zone without affecting the reaction stoi- chiometry [861. This allows greater flexibility of the reactor to feed rates of H2S and S02 . It is also often desired that all the products of a reaction be directed to one side of the membrane. In this case, the produced sulfur should be directed to the SO2 side [871. This can be achieved by applying an overpressure at one side of the membrane to generate the combined effect of convective and diffusive flows [87-891. Further- more, a homogeneously active membrane can be produced by using sintered stainless steel as the membrane for the concept of separated feed of reactants [891.

Veldsink, et al. I901 suggest that the membrane reactor shown in Figure 6 can be used for kinetically fast exothermic heterogeneous reactions. By feeding the reactants on both sides of the membrane, premix- ing of the reactants is avoided. Therefore, thermal problems, such as the formation of explosive mixtures and the occurrence of thermal runaways, will not take place "90, 911. However, accurate controlling of heat balances of the membrane reactor will be a major task for any large-scale industrial unit. Therefore, efficient means to supply or remove heat from any large-scale membrane reactor will have to be developed. Accord- ing to Adris and Grace [921 and Mlezko, et al. 1931, the combination of membranes and fluidized-bed reactors are advantageous because fluidized beds provide good temperature control.

Much attention is paid to the search for an optimum porous catalyst structure, i.e., the relationship between micropores and macropores, which would provide effective conversion of H2S and SO2 during the entire period of adsorption until the reactor


switches into the catalyst regeneration mode [661. The larger the volume of micropores, and the smaller their size, the greater the amount of sulfur that can be extracted by the catalyst, and the smaller the sulfur losses in the vapor phase. The catalyst efficiency in tail-gas treatment processes is determined not only by the relation of micropore and macropore volume, but also by the number of micropores that the reactant molecules must go through to get from one macropore to the next [661.

Raymont [LO, 111 came up with an alternative route for the utilization of H2S by breaking it down to its constituents. The interest in utilization of H2S as a source of H2 and sulfur has intensified in recent years due to: (1) global prospect for hydrogen energy and waste minimization; (2) the unavoidable production of H2S from gas plants, refineries and metallurgical processes; and (3) the cost of a tail-gas clean-up process for Claus plants that can exceed the value of the recovered sulfur if the environmental regulations are made more stringent [81. A suitable technology for the production of H2 and sulfur must meet the triple objectives of waste minimization, resource utilization, and environmental pollution reduction.

Photochemical 194-971 and plasmochemical [98-1031 technologies are still in the development stages and are not mature enough to be applied to large-scale chemical processing. Electrochemical technology [104- 1101 is established in certain areas, such as biochemi- cal and biomedical separation processes, but its application to H2S requires further development in the area of storage and disposal techniques, proper equipment materials, and knowledge of possible side reactions. In addition, it is unlikely that electrochemical processes can be competitive at today's electricity costs. Of the thermal methods, membrane, thermal diffusion, and solar technologies have not yet developed very far [81. In fact, membrane technology, which appears very attractive, is essentially a technology for the future. For chemicals as difficult as H2S, the application has to wait until the technology matures to less demanding processes.

As an alternative to the physico-chemical processes, Basu, et al. [201 demonstrated that the anaerobic, photo- synthetic bacterium, Chlorobium thiosulfatophilum, could convert H2S to elemental sulfur in a single step at atmospheric conditions. The autotrophic bacterium uti- lizes C 0 2 as carbon source, while energy for cell metab- olism is provided by incandescent light and H2S oxidation [201. Almost all the H2S could be converted in a residence time of a few minutes. Moreover, high concentrations of H2S or organics did not seem to affect the conversion efficiency.

CONCLUSIONS The modified Claus process is the major technolo-

gy currently used to recover elemental sulfur from H2S and SO2. Studies and research on the Claus process mechanisms and technology have been described. A number of current commercial technologies for the recovery of sulfur from sour acid gas have also been described and compared. Under modern

environmental regulations in developed countries, a Claus tail-gas cleanup treatment is essential to achieve very high sulfur recovery efficiency. Established tail- gas cleanup processes are Amoco's Cold Bed Adsorp- tion, the Sulfreen, the IFP, the SCOT, the Beavon, and the Wellman-Lord processes. The SCOT process is the most reliable and flexible to disturbances.

Several changes and new trends in the conversion of H2S and SO2 to elemental sulfur have also been highlighted in this review paper. Two examples of the recent improvement in the Claus tail-gas treatment process are the introduction of the non-permselective catalytic membrane reactors and in situ water separation by zeolite adsorbent. The success in the utilization of H2S by breaking it down to elemental sulfur will signlfy the attainment of the three objectives of waste minimization, resource utilization, and environmental pollution reduction.

Based on the considerations in this review, the following two processes for the conversion of H2S and/or SO2 merit further analysis to act as the basis for a prospective commercial technology: (1) catalytic thermal decomposition at reduced pressure between 1,000" C and 1,200" C in a fixed bed reactor; and (2) two-step sulfide processes in the temperature range of 500" C to 650" C in fluidized bed reactors as reactor- regenerator systems. However, there may, in fact, not be a universal approach for the selection of a desulfurization process. The economics of a process may be influenced by diverse factors, making different processes desirable based on plant size, the source, temperature and concentration of H2S, the local energy situation and/or the environmental regulations. Therefore, it is important to explore new ways of H2S and SO2 elimination which will lead to high conversions at minimum cost, to increase sulfur recovery.

Photochemical and plasmochemical methods are still in the development stage, while the electrochemical technology is established in certain areas, but its application to H2S requires further development. Research for an optimum porous catalyst structure is ongoing in order to obtain a relation of micropores and macropores which would provide effective conversion of H2S and SO2 during the entire period of adsorption.

ACKNOWLEDGMENTS The author would like to acknowledge the assis-

tance given by Mohd. Rusydan Abdul Naim and Ikhsan Masadi, currently with the National Petroleum Company of Malaysia (Petronas). Appreciation is also due to Yasser Hussain, Ron Towers and Dr. John Lamb of University of Surrey for their helpful com- ments and suggestions. Special appreciation is due to Professor Mojtaba Ghadiri for his various supports.

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Recovery of Sulfwr From Sour Acid

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