P3 Techniques for Hydrogen

(Synthesis) Production

• Treatment of Gas Mixtures• Decomposition of Hydrocarbons• Decomposition of Water

Techniques for Hydrogen ProductionA.Treatment of certain gas mixtures (side products)

1. Catalytic Reforming of Naphtha2. Dehydrogenation reaction / process of alkanes (C1, C2, C4)3. Chloroalkali process

B.Decomposition of hydrocarbons and other organic raw materials (coal, lignite, wood)1. Partial Oxidation

• Partial Oxidation of Hydrocarbon(POX)• Gasification

i. From coalii. From Wood/Biomassiii. From Flash Pyrolysis

2. Steam Treatment• Steam reforming

C.Decomposition of Water1. Electrolysis2. Thermochemical cycles

What is synthetic gas (syngas) ?Synthesis gas (syngas) is a mixture of hydrogen, carbon monoxide and carbon dioxide in various proportions

Methane

LPG

Naphta

Fuel Oil

Vacuum residue

Asphalts

Coal

Biomass

Desulfurization Steam reforming

H2O

Partial Oxidation(autothermal)

Distillation H2O

H2S absorption

Shift ConversionH2O

CO2

Drying

Final Purification

Hydrogen

CO2 (and H2S)Absorption

Air

Sulfur Unit

Sulfur

Main Scheme for Hydrogen Production (Method 2)

Main Scheme for Hydrogen Production (Method 2)

Operations (side)a) Conversion of CO with steam (shift conversion)b) Extraction of acid gases CO2 and H2S, supplemented

(supplemented in the case of S-containing effluents by a Claus unit designed to prevent pollutant releases into the atmosphere)

c) Final Purification designed to eliminate the last traces of CO

Pretreatment Processes :a) For steam reforming: desulphurization (to protect

catalyst)b) For partial oxidation with oxygen: air distillation

Steam Treatment (Steam Reforming Process)

1. Thermodynamic & Kinetics of Reaction2. Catalyst and Process Conditions3. Process Technology

Steam reforming of natural gas is currently the least expensive method of producing hydrogen

A large steam reformer which produces 100,000 tons of hydrogen ayear can supply roughly one million fuel cell cars with an annual average driving range of 16,000 km

New processes are constantly being developed to increase the rate of production

Thermodynamic and Kinetic of ReactionsDefinition:Steam reforming is a process to reform hydrocarbons in the presence of H20 to produce synthesis gas (SYNGAS) using catalyst (supported Ni-based) at a prescribed reaction conditions:

“HC”(CH4, LPG, Naphtha) + H20 CO + 3H2

Steam reforming is based essentially on the controlled oxidation of methane, by water, or more generally, hydrocarbons. Main reactions are:CnHm + ¼ (4n – m)H2O 1/8 (4n + m)H2 + 1/8 (4n – m)COCH4 + H2 0 CO + 3H2 (steam reforming) ____(1) CO + H2 0 CO2 + H2 (water-gas shift reaction) ____(2)

Reaction (1) is exothermic and complete between 400 & 600oC.Reaction (2) is endothermic and exentropic hence favored by low temperatures. However limited by equilibrium as shown in table.

Nickel catalyst

Thermodynamic and Kinetic of Reactions

(oC) CH4(mole %)

H2O(mole %)

CO(mole %)

H2(mole %)

427 42.6 42.6 3.7 11.1

527 30.0 30.0 10.0 30.0

627 14.5 14.5 17.5 52.5

727 5.55 5.55 22.2 66.7

827 1.80 1.8 24.1 72.3

927 0.20 0.5 24.5 74.5

Equilibrium Concentrations CH4 + H2O CO + 3H2

Thermodynamic and Kinetic of ReactionsRaising the proportion of steam in the reaction mixture cannot make possibly complete conversion

Can only be done by secondary reforming or post combustion (resembles POX in presence of catalyst) – mostly used for ammonia synthesis

High T makes CO conversion to H2 difficult therefore requires a separate operation to convert the CO by low T steam

Steam is needed not only for reaction, but also to prevent the conversion of :2CO CO2 + C (Boudouard’s equilibrium rxn)Which is replaced by the action of steam on CO :CO + H2O CO2 + H2 (water-gas shift reaction)

Thermodynamic and Kinetic of ReactionsProduct distribution are determined by:

1. Thermodynamics of reaction (1) and (2); steam reforming & water-gas shift reaction

2. Activity of the catalyst used

Reactions (3-6) leading to carbon formation (undesirable reactions)CO + H2 C + H2O (3)2CO C + CO2 (4)CH4 C + 2H2 (5)2CO CO2 + C (6) Boudouard’s Equilibriumto prevent rxn (6), then add excess H2OCO + H2O CO2 + H2 (7)

It is critical to keep catalyst surface free from carbon to preventdeactivation

Build up of carbon due to : Cracking polymerization / dehydrogenation rxnsCan be minimised by :

1. Use excess steam to reverse rxn (3)2. Choice of catalyst support3. Presence of Alkali to promote rxn (3)

Steam ReformingFeed Gas

(C2 – C6 Hydrocarbons)

Intermediates CH4, Alkenes, H2

Oxygenated species

End ProductCH4, CO, CO2, H2

H2O

Product GasCH4, CO, CO2, H2

CARBONBuild – up of carbon

Equilibration

Steam Reforming

Thermal and Catalytic Cracking

PolymerizationDehydrogenation

Cracking

C + H2O CO + H2to remove C

The catalyst and its conditions of useCatalyst

For primary steam reformingNi/Al2O3Ni/Al2O3-K (to slow down carbon formation, K is added to help action of steam on CO)Ni/Al2O3-Ca (use for naphtha feedstock)Mg/SiO2-Al2O3-K (use for naphtha feedstock)Ni/Al2O3-U (use for naphtha feedstock)

Typical Operating ConditionsSteam : HC = 2 to 4 (2-3 X higher than the stoichiometry)T = 850 – 940 oCP =1.5-2.5 x106 to 4 x 106 Pa absoluteFeed= CH4, Ethane, Naphtha (free from S) to prevent deactivation of catalyst due to poisoning

Although thermodynamically, steam reforming reactions are favored at low pressure, but to obtain high H2 purity and save cost on compression, the process is normally carried out at high pressure

Steam reforming furnace section

Steam Reforming B. Reactor used for steam reforming (steam reformer):

1. Dimension of reactorType : Tubular reactor 100 – 1000 tubesInternal diameter : 10cm External diameter : 12cmLength : 50m Width : >10m , Height : >20m

2. Operating conditionTemperature : 950oCPressure : 15 – 40 bar

3. Catalyst employedNickel on alumina support

Steam Reforming

C. Process flow diagram of steam reforming

Desulphurizer

Steam Generator

Steam Reformer

Natural Gas Steam:NG = 1.6 – 4 H2:CO = 3 – 4

T = 950oCP = 15 – 40bars

Partial Oxidation ProcessesA. Thermodynamic and Reaction Kinetics

B. Technological Aspects – three groups depending on raw material :

1. Partial Oxidation of Petroleum Cuts

2. Coal Gasification

3. Conversion of Lignocellulose Wastesa. Gasification of biomass (wood)b. Flash pyrolysis

Partial Oxidation

Hydrocarbon Fractions

Thermodynamics & Reactions KineticTransformation Considered : a. Combustion reaction

CH4 + 3/2 O2 CO + 2H2O ___(1)

b. Carbon monoxide equilibrium reaction due to presence of water formed during combustion, or added by steam injectionCO + H2O CO2 + H2 (water-gas shift reaction) __(2)

c. Hydrocarbon decomposition reaction CH4 C + 2H2 (side reaction) ___(3)

Reaction (1) is exothermic & exentropic and takes place adiabatically

Enthalphy and Entrophy Variations in Reactions Associated with the Partial Oxidation of Methane

Reactions ∆Ho298 (kJ / mol) ∆So

298 u.e.

1. CH4 + H2O CO + 3H2 206.225 214.83

2. CO + H2O CO2 + H2 – 41.178 42.42

3. CH4 + 2H2-O CO2 + 4H2Reaction (1 + 2)

165.047 172.41

4. CH4 C(g) + 2H2 74.874 75.01

5. 2CO C + CO2 Reactions (4 + 2 – 1)

– 172.528 –176.54

6. C(g) + H2O CO + H2Reactions (1 – 4)

131.350 134.10

7. CH4 + CO2 2CO + 2H2 247.402 257.25

8. CH4 + 3/2 O2 CO + 2H2O – 519.515 81.62

Thermodynamics & Reactions KineticTo shift the equilibrium of reaction (2) to form the most H2

Use excess water, low reaction temperature

Presence of CO2 and water helps to eliminate side reactions (rxn(3)) which occurs at high temperature by means of :

CO2 + C 2COC + H2O CO + H2

Production of Carbon increases with decrease in “HC” ratio in feedRequires presence of steam because not sufficiently formed during combustionIncrease in pressure at fixed temperature would result in:

larger water requirement decrease in O2 requirement increase in residual methane content

Can be offset by raising the temperature

Technological aspects : POX of petroleum cuts

Generally thermal and use burners e.g. Texaco & Shell Some use contact masses but not favorable (high temperatures employed and danger of carbon deposit on the contact masses)Flow sheet comprises

a) A burner in which O2 and preheated steam are injected with HCb) Heat recovery sectionc) Carbon black removal section (by washing or filtration)

Next two figures show Texaco and Shell POX unit, whose special features are :

Shell : Recover carbon by washing with water then extract the sludgeExtract is homogenized with feed then sent to POX reactor

TexacoStripping the fuel/crude oil (in presence of heavier HC) then separate and recycle the naphtha

Hydrogen Production – Partial Oxidation

PartialOxidation

Unit Cooler CondenserAbsorber

Flash

Natural gas

H2O

OxygenSteam

Recycled

CO2

MDEA

C. Process flow diagram of POX H2, CO

Process TechnologyA. Steps in hydrogen production via partial oxidation (POX):

1. Natural gas, oxidant (such as O2) and moderating agent (steam) enter the POX unit to be combusted and reacted

2. Reaction that takes place CH4 + 1.5O2 = CO + 2H20 (POX) (3)CO + H2 0 = CO2 + H2 (water-gas shift reaction) (4)

3. Synthesis gas leaves the POX unit and enters a cooler. Some of the water vapour in the gas is condensed and removed.

4. Cooled synthesis gas is now conveyed to an absorber to separate CO2 from the mixture. Absorbent normally used is an amine solvent. In this case methyl diethanolamine solution (MDEA ) is used.

5. Absorbed CO2 in the amine solution is later passed to a flash, where CO2 is removed from the stream and MDEA is regenerated.

Gas

Scr

ubbi

ng

Strip

ping

SteamGeneration

Fuel

oil

Strip

ping

Par

tial O

xida

tion

Carbon Separation

Boiler Feed water

Fuel oil

Oxygen or air

Fuel oil and carbon

Naphtha

NaphthaHP Steam

Water and carbon

Water

Product Gases

Steam

Hydrogen manufacture by partial oxidation. Texaco Process

Preheating

Water recycleC

arbo

n Fi

ltrat

ion

Gas

Scr

ubbi

ng

Par

tial O

xida

tion

Carbonrecovery

Oxygen or air

Boiler Feed water

Fuel Oil Ste

am G

ener

atio

n

Boiler Feed water

HP Steam

Naphtha

Product Gases

Make-up water

Waste waterHydrogen manufacture by partial oxidation. Shell Process

Technological aspects : POX of petroleum cuts

B. Reactor used for POX:Type: Fluidized bed reactor Operating Condition : Temperature = 1000oC – 1500oCPressure = 150atm Catalyst employed :

• Composition of the synthesis gas produced using POX (molar basis):

30 – 50% hydrogen20 – 45% carbon monoxideabout 2 – 20% methaneabout 0.5 – 2% carbon dioxideless than about 0.5% higher hydrocarbons

Coal Gasification

HistoryAncient technique of producing hydrogen ~ since early 19th

century1940s – growing availability of low-cost natural gas slowly subsituted coal gasification process.Recently, diminishing sources of natural gas creates interest inproduction of gases from coal.However, operation cost is double the cost of producing hydrogen from natural gasCoal is heated up to 900oC with a catalyst and without air

2 methods of coal gasificationi. Simple method

a) Coal heated in a retort in the absence of air b) Coal partially converted to gas with a residue of coke c) Technique introduced by a Scottish engineer ~ William

Murdock d) Pioneer to the commercial gasification of coal in 1792.

ii. Complete conversion of coala) Coal is continuously reacted in a vertical retort with air and

steam b) Product is called producer gas

Technological aspects : Coal GasificationInitial activity : Crushing, drying and grinding of feed

Three types of coal gasification installation :1. Moving (incorrectly called fixed) bed reactors – Lurgi

Operated in counter current flowHydrocarbon content (CH4, C2H6) high - require their separation from the gas produced and supplementary steam reforming

2. Fluidized bed reactor – Winkler Hydrocarbons other than methane are not formed

3. Entrained-bed (dual flow) reactor – Koppers, TexacoMethane content is very low therefore does not require specific fractionation

Removal of ash and soot it vital where coal gasification technique is applied

Typical Composition of a Dry Crude Gas Produced by Partial Oxidation (% vol)

Feedstock Fuel Oil Coal

Reactor Type Burner Entrained Bed Moving Bed Fluidized Bed

Components :

H2 47.3 34.7 38.1 40.0

CO 46.7 52.4 21.0 35.0

N2 + A 0.2 0.9 0.8 1.6

CO2 4.4 10.3 29.0 21.0

CH4, C2H6 0.6 0.1 9.0 2.0

H2S + COS 0.8 1.6 1.4 0.4

NH3 – – 0.7

Biomass/Wood

Technological aspects : Conversion of lignocellulose’s wastes

A) The gasification of wood.Comprises of three stagesi. Drying between 100 and 300oCii. Pyrolysis between 200 and 500oC or higheriii. Reduction and oxidation which occur between O2, moisture,

CO2, CO and C at temperature below 1000oC

Three types of gasifiersFixed Bed

Bed actually moving, with the fuel flowing by gravityAsh removed at the bottom of reactor by mobile grid system or in batchesGasses flow in parallel, co- or countercurrent contact or also perpendicular to each other

Entrained bedFluid bed

Technological aspects : Conversion of lignocellulose’s wastes

b) Flash PyrolysisDeveloped by Garret Energy Research and Engineering, subsidiary of Occidental Petroleum and by Battelle ColumbusWood drying using equipment with isolated gas transfersPyrolysis at 800 to 900oC using flue gas obtained by combustion of residues formed – produced effluents with higher heating valueHeat exchanger occurs on the biomass itself which advances by gravity from one section to the next or by means of solid heat transfer medium which retains tarsCleaned by combustion and recycled

Typical Compositions of Dry Gases Produced by Wood Gasification (%vol)

Process Partial Oxidation Flash Pyrolysis

N2 0.3 –

H-2 28.4 15.5

CO 47.5 32.5

CO2 17.2 38.0

CH4 11.5

Heavy 2.5

Total 100.0 100.0

6.6

Biomass Gasification• The conversion of lignocellulose wastes, or dry biomass (wood) can be

achieved after reducing feedstock to suitable particle size distribution• Proceed to

– partial oxidation process, similar to coal– Flash pyrolysis

• Gasification of Biomass (wood) Process– Drying @ 100-300 oC– Pyrolysis between 200-500 oC or higher– Reduction and oxidation, which occur between oxygen, mositure, carbon dioxide,

carbon monoxide and carbon @ 1000 oC for wood– Licensors (technology owner)

• Union Carbide• Flash Pyrolysis Process

– Licensors (Garrett Energy Research and Engineering- Occidental Petroleum, Batelle Columbus

– Pyrolysis between 800-900 oC