649
Hydrogen Production
of a purification method. These are (1) hydrogen recovery, (2) product purity, (3) pressure profile,
(4) reliability, and (5) cost, an equally important parameter is not considered here since the emphasis
is on the technical aspects of the purification process.
23.7.1 Wet sCruBBIng
Wet scrubbing systems, particularly amine or potassium carbonate systems, are used for
removal of acid gases such as hydrogen sulfide or carbon dioxide (Dalrymple et al., 1994).
Most systems depend on chemical reaction and can be designed for a wide range of pressures
and capacities. They were once widely used to remove carbon dioxide in steam reforming
plants but have generally been replaced by PSA units except where carbon monoxide is to be
recovered. Wet scrubbing is still used to remove hydrogen sulfide and carbon dioxide in partial
oxidation plants.
Wet scrubbing systems remove only acid gases or heavy hydrocarbons but they do not remove
methane or other hydrocarbon gases, and hence have little influence on product purity. Therefore,
wet scrubbing systems are most often used as a pretreatment step, or where a hydrogen-rich stream
is to be desulfurized for use as fuel gas.
23.7.2 Pressure sWIng AdsorPtIon unIts
PSA units use beds of solid adsorbent to separate impurities from hydrogen streams leading to
high-purity high-pressure hydrogen and a low-pressure tail gas stream containing the impurities
and some of the hydrogen. The beds are then regenerated by depressuring and purging. Part of the
hydrogen (up to 20%) may be lost in the tail gas.
PSA is generally the purification method of choice for steam reforming units because of its
production of high-purity hydrogen and is also used for purification of refinery off-gases, where it
competes with membrane systems.
Many hydrogen plants that formerly used a wet scrubbing process (Figure 23.6) for hydrogen
purification are now using PSA (Figure 23.7) for purification. The PSA process is a cyclic process
that uses beds of solid adsorbent to remove impurities from the gas and generally produces higher
purity hydrogen (99.9 vol. % purity compared to less than 97 vol. % purity). The purified hydrogen
Hydrogenator
PSA unit
Membrane unit
High-pressure
off-gas
Medium-pressure
off-gas
Low-pressure
off-gas
Compressor
Reformal
Recovered
hydrogen
To shift
converter
Sulfur removal
FIGURE 23.5 Hydrogen purification.
Hydrogen Production
of a purification method. These are (1) hydrogen recovery, (2) product purity, (3) pressure profile,
(4) reliability, and (5) cost, an equally important parameter is not considered here since the emphasis
is on the technical aspects of the purification process.
23.7.1 Wet sCruBBIng
Wet scrubbing systems, particularly amine or potassium carbonate systems, are used for
removal of acid gases such as hydrogen sulfide or carbon dioxide (Dalrymple et al., 1994).
Most systems depend on chemical reaction and can be designed for a wide range of pressures
and capacities. They were once widely used to remove carbon dioxide in steam reforming
plants but have generally been replaced by PSA units except where carbon monoxide is to be
recovered. Wet scrubbing is still used to remove hydrogen sulfide and carbon dioxide in partial
oxidation plants.
Wet scrubbing systems remove only acid gases or heavy hydrocarbons but they do not remove
methane or other hydrocarbon gases, and hence have little influence on product purity. Therefore,
wet scrubbing systems are most often used as a pretreatment step, or where a hydrogen-rich stream
is to be desulfurized for use as fuel gas.
23.7.2 Pressure sWIng AdsorPtIon unIts
PSA units use beds of solid adsorbent to separate impurities from hydrogen streams leading to
high-purity high-pressure hydrogen and a low-pressure tail gas stream containing the impurities
and some of the hydrogen. The beds are then regenerated by depressuring and purging. Part of the
hydrogen (up to 20%) may be lost in the tail gas.
PSA is generally the purification method of choice for steam reforming units because of its
production of high-purity hydrogen and is also used for purification of refinery off-gases, where it
competes with membrane systems.
Many hydrogen plants that formerly used a wet scrubbing process (Figure 23.6) for hydrogen
purification are now using PSA (Figure 23.7) for purification. The PSA process is a cyclic process
that uses beds of solid adsorbent to remove impurities from the gas and generally produces higher
purity hydrogen (99.9 vol. % purity compared to less than 97 vol. % purity). The purified hydrogen
Hydrogenator
PSA unit
Membrane unit
High-pressure
off-gas
Medium-pressure
off-gas
Low-pressure
off-gas
Compressor
Reformal
Recovered
hydrogen
To shift
converter
Sulfur removal
FIGURE 23.5 Hydrogen purification.