2. Techniques for Hydrogen Production
A.Treatment of certain gas mixtures (side products)
1. Catalytic Reforming of Naphtha
2. 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 coal
ii. From Wood/Biomass
iii. From Flash Pyrolysis
2. Steam Treatment
• Steam reforming
C.Decomposition of Water
1. Electrolysis
2. Thermochemical cycles
What is synthetic gas (syngas) ?
Synthesis gas (syngas) is a mixture of hydrogen, carbon monoxide and carbon dioxide in
various proportions
4. 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
5. Steam Treatment
(Steam Reforming Process)
1. Thermodynamic & Kinetics of Reaction
2. Catalyst and Process Conditions
3. 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 a
year 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
6. Thermodynamic and Kinetic of Reactions
Definition:
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)CO
CH4 + 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
8. Thermodynamic and Kinetic of Reactions
Raising 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)
9. Thermodynamic and Kinetic of Reactions
Product 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 Equilibrium
to prevent rxn (6), then add excess H2O
CO + H2O CO2 + H2 (7)
It is critical to keep catalyst surface free from carbon to prevent
deactivation
Build up of carbon due to :
Cracking polymerization / dehydrogenation rxns
Can be minimised by :
1. Use excess steam to reverse rxn (3)
2. Choice of catalyst support
3. Presence of Alkali to promote rxn (3)
10. Steam Reforming
Feed Gas
(C2 – C6 Hydrocarbons)
Intermediates
CH4, Alkenes, H2
Oxygenated species
End Product
CH4, CO, CO2, H2
H2O
Product Gas
CH4, CO, CO2, H2
CARBON
Build – up of carbon
Equilibration
Steam Reforming
Thermal and Catalytic
Cracking
Polymerization
Dehydrogenation
Cracking
C + H2O CO + H2
to remove C
11. The catalyst and its conditions of use
Catalyst
For primary steam reforming
Ni/Al2O3
Ni/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 Conditions
Steam : HC = 2 to 4 (2-3 X higher than the stoichiometry)
T = 850 – 940 oC
P =1.5-2.5 x106 to 4 x 106 Pa absolute
Feed= 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
13. Steam Reforming
B. Reactor used for steam reforming (steam reformer):
1. Dimension of reactor
Type : Tubular reactor
100 – 1000 tubes
Internal diameter : 10cm
External diameter : 12cm
Length : 50m
Width : >10m ,
Height : >20m
2. Operating condition
Temperature : 950oC
Pressure : 15 – 40 bar
3. Catalyst employed
Nickel on alumina support
14. 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 = 950oC
P = 15 – 40bars
15. Partial Oxidation Processes
A. 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 Wastes
a. Gasification of biomass (wood)
b. Flash pyrolysis
17. Thermodynamics & Reactions Kinetic
Transformation 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
injection
CO + 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
18. Enthalphy and Entrophy Variations in Reactions
Associated with the Partial Oxidation of Methane
Reactions ∆Ho
298 (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 + 4H2
Reaction (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 + H2
Reactions (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
19. Thermodynamics & Reactions Kinetic
To 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 2CO
C + H2O CO + H2
Production of Carbon increases with decrease in “HC” ratio in feed
Requires presence of steam because not sufficiently formed during
combustion
Increase 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
20. 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 HC
b) Heat recovery section
c) 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
sludge
Extract is homogenized with feed then sent to POX reactor
Texaco
Stripping the fuel/crude oil (in presence of heavier HC) then
separate and recycle the naphtha
21. Hydrogen Production – Partial Oxidation
Partial
Oxidation
Unit
Cooler Condenser
Absorber
Flash
Natural gas
H2O
Oxygen
Steam
Recycled
CO2
MDEA
C. Process flow diagram of POX H2, CO
22. Process Technology
A. 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.
25. Technological aspects : POX of
petroleum cuts
B. Reactor used for POX:
Type: Fluidized bed reactor
Operating Condition :
Temperature = 1000oC – 1500oC
Pressure = 150atm
Catalyst employed :
• Composition of the synthesis gas produced using POX (molar
basis):
30 – 50% hydrogen
20 – 45% carbon monoxide
about 2 – 20% methane
about 0.5 – 2% carbon dioxide
less than about 0.5% higher hydrocarbons
27. History
Ancient technique of producing hydrogen ~ since early 19th
century
1940s – growing availability of low-cost natural gas slowly
subsituted coal gasification process.
Recently, diminishing sources of natural gas creates interest in
production of gases from coal.
However, operation cost is double the cost of producing
hydrogen from natural gas
Coal is heated up to 900oC with a catalyst and without air
2 methods of coal gasification
i. 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 coal
a) Coal is continuously reacted in a vertical retort with air and
steam
b) Product is called producer gas
28. Technological aspects : Coal Gasification
Initial 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 flow
Hydrocarbon 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, Texaco
Methane content is very low therefore does not require
specific fractionation
Removal of ash and soot it vital where coal gasification technique is
applied
29. 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
31. Technological aspects : Conversion of
lignocellulose’s wastes
A) The gasification of wood.
Comprises of three stages
i. Drying between 100 and 300oC
ii. Pyrolysis between 200 and 500oC or higher
iii. Reduction and oxidation which occur between O2, moisture,
CO2, CO and C at temperature below 1000oC
Three types of gasifiers
Fixed Bed
Bed actually moving, with the fuel flowing by gravity
Ash removed at the bottom of reactor by mobile grid
system or in batches
Gasses flow in parallel, co- or countercurrent contact
or also perpendicular to each other
Entrained bed
Fluid bed
32. Technological aspects : Conversion of
lignocellulose’s wastes
b) Flash Pyrolysis
Developed by Garret Energy Research and Engineering,
subsidiary of Occidental Petroleum and by Battelle
Columbus
Wood drying using equipment with isolated gas transfers
Pyrolysis at 800 to 900oC using flue gas obtained by
combustion of residues formed – produced effluents with
higher heating value
Heat 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 tars
Cleaned by combustion and recycled
33. 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
34. 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
