Vacuum swing adsorption (VSA) constitutes a variant of PSA technology where
desorption phase takes place at subatmospheric pressure. Vacuum is obtained by
means of a blower, and specific zeolite adsorbents are employed. The main advantages of VSA systems (compared to PSA) are lower specific separation energy
consumptions and the simplification of the system obtained by the reduction of
some system components. This fact results in compact design layouts and more
automated systems that can be delivered as fully packaged pre-tested skids. Nevertheless, the capital investment costs are higher than those associated with PSA, and
the choice between both methodologies is usually driven by the projected production
amounts. As a general rule, VSA systems are preferred to PSA systems for oxygen
productions beyond 20 tons/day. Starting from this value, the higher energetic
efficiencies of VSA systems (competitive with those offered by cryogenic distillation) compensate the associated increase in capital investment. Hybrid systems
called VPSA (vacuum pressure swing adsorption) are also available in the market.
4.3 Hydrogen Production
Pure hydrogen is primarily obtained by steam reforming of natural gas. In this
process, high temperatures (>700
C) are used combined with medium pressures
(20–30 bar) in order to catalytically obtain a gas mixture containing H 2 (70–80%),
CO 2 (15–25%), CO (3–6%), and CH 4 (1–3%) [13]. PSA is then used to separate
hydrogen at molar purities higher than 99.99% from the impurities (CO 2 , CO, and
CH 4 , among others, at traces level). In order to optimize H 2 purity and recovery, the
PSA schemes (Fig. 4) have been considerably sophisticated over the years [14–
16]. At least four to ten columns are used in parallel.
Figure 5 shows an example illustrating the different steps of a typical H 2 PSA
cycle [17]. The key steps are I, II and V, VI. Steps I and II are the adsorption
steps. The impurities are captured in the column and pure H 2 is produced. After
depressurization, the blowdown steps V and VI desorb the impurities CO 2 , CH 4 , and
CO. The intermediate steps are pressure equalization steps, aimed at minimizing the
energy consumption and maximizing H 2 recovery, as well as keeping the top of
column clean to assure an extremely high H 2 purity.
In contrast to what was said about oxygen production, where PSA processes were
more adapted for moderate purity O 2 , PSA is the only purification process that can
produce H 2 of 99.999% purity. In order to adapt to the multicomponent nature of the
feed, different adsorbent layers are used in series. In the first bed layer, desiccants
(alumina or silica gel) are used in order to remove H 2 O (the synthesis gas is saturated
with water). In a second layer, low polar adsorbents such as activated carbons are
used to remove the second most polar constituent of the synthesis gas, i.e., CO 2 .
Once the polar compounds are largely removed, the remaining impurities, i.e., CH 4
and CO, are treated by means of zeolitic adsorbents (generally 5A or NaX). The
stacking of adsorbents allows achieving the best compromise between high retention
of the impurities (i.e., high H 2 purity) and good regenerability for all the components
Industrial Zeolite Applications for Gas Adsorption and Separation Processes
205
desorption phase takes place at subatmospheric pressure. Vacuum is obtained by
means of a blower, and specific zeolite adsorbents are employed. The main advantages of VSA systems (compared to PSA) are lower specific separation energy
consumptions and the simplification of the system obtained by the reduction of
some system components. This fact results in compact design layouts and more
automated systems that can be delivered as fully packaged pre-tested skids. Nevertheless, the capital investment costs are higher than those associated with PSA, and
the choice between both methodologies is usually driven by the projected production
amounts. As a general rule, VSA systems are preferred to PSA systems for oxygen
productions beyond 20 tons/day. Starting from this value, the higher energetic
efficiencies of VSA systems (competitive with those offered by cryogenic distillation) compensate the associated increase in capital investment. Hybrid systems
called VPSA (vacuum pressure swing adsorption) are also available in the market.
4.3 Hydrogen Production
Pure hydrogen is primarily obtained by steam reforming of natural gas. In this
process, high temperatures (>700
C) are used combined with medium pressures
(20–30 bar) in order to catalytically obtain a gas mixture containing H 2 (70–80%),
CO 2 (15–25%), CO (3–6%), and CH 4 (1–3%) [13]. PSA is then used to separate
hydrogen at molar purities higher than 99.99% from the impurities (CO 2 , CO, and
CH 4 , among others, at traces level). In order to optimize H 2 purity and recovery, the
PSA schemes (Fig. 4) have been considerably sophisticated over the years [14–
16]. At least four to ten columns are used in parallel.
Figure 5 shows an example illustrating the different steps of a typical H 2 PSA
cycle [17]. The key steps are I, II and V, VI. Steps I and II are the adsorption
steps. The impurities are captured in the column and pure H 2 is produced. After
depressurization, the blowdown steps V and VI desorb the impurities CO 2 , CH 4 , and
CO. The intermediate steps are pressure equalization steps, aimed at minimizing the
energy consumption and maximizing H 2 recovery, as well as keeping the top of
column clean to assure an extremely high H 2 purity.
In contrast to what was said about oxygen production, where PSA processes were
more adapted for moderate purity O 2 , PSA is the only purification process that can
produce H 2 of 99.999% purity. In order to adapt to the multicomponent nature of the
feed, different adsorbent layers are used in series. In the first bed layer, desiccants
(alumina or silica gel) are used in order to remove H 2 O (the synthesis gas is saturated
with water). In a second layer, low polar adsorbents such as activated carbons are
used to remove the second most polar constituent of the synthesis gas, i.e., CO 2 .
Once the polar compounds are largely removed, the remaining impurities, i.e., CH 4
and CO, are treated by means of zeolitic adsorbents (generally 5A or NaX). The
stacking of adsorbents allows achieving the best compromise between high retention
of the impurities (i.e., high H 2 purity) and good regenerability for all the components
Industrial Zeolite Applications for Gas Adsorption and Separation Processes
205