Page 135 - Adsorption Technology & Design, Elsevier (1998)

Processes and cycles 125

product. The production step may occur at a pressure greater than
atmospheric with the desorption pressure being atmospheric. This practice
is normal for the production of oxygen from air using a 5A zeolite.
Alternatively, the production step could be carried out at atmospheric
pressure and the desorption step under vacuum. This is normal practice for
kinetically based separations such as the production of nitrogen from air
using a carbon molecular sieve. Vacuum regeneration is used in this case to
remove most of the oxygen which has been adsorbed in order to avoid slow
uptake of nitrogen which would saturate the adsorbent and reduce its
effectiveness for taking up oxygen during the subsequent adsorption step.
Figure 5.16 shows the simplest practical hardware layout for a PSA process
shown in schematic form in Figure 5.15. The example is the production of
nitrogen using a pressure swing between atmospheric pressure and a
vacuum.
The purge step is very important for efficient operation because the use
of a purified component flowing in the reverse direction causes the strongly
adsorbed species to be pushed back towards the bed inlet for the
subsequent adsorption step. The more strongly adsorbed components
therefore are unlikely to contaminate the product on the next adsorption
step. The volume of the purge gas, measured at the low pressure, should
normally not be less than 1-2 times the volume of the feed measured at the
high pressure. The product purity increases as the fraction of the product
used for the purge is increased but after a certain value this gain becomes
marginal. Because the increase in volume from high to low pressure
becomes greater the higher the ratio of high to low pressure in the process,
it can be seen that the actual fraction of the product stream required for the
purge could be quite small if the pressure ratio were high. The disadvan-
tage of a high pressure ratio, however, is the increased energy demand.
Clearly the design of a PSA gas separation process requires engineering
compromises to be made.
Much effort has been devoted to reducing the overall energy demands of
PSA separation processes. One of the simplest techniques for multibed
processes is to use pressure equalization between beds, i.e. to release the
pressure of a bed which is at the end of its production step to a bed which has
come to the end of its desorption step. In this way some of the energy
associated with compression can be conserved within the process. Further
details on the development of PSA cycles and their applications are
provided in Sections 7.2 and 7.3.
It is possible to operate PSA with a single packed bed. The rapid pressure
swing adsorption process (RPSA) can be considered to be a hybrid between
the four step cycle of a more conventional PSA process and parametric
pumping in which pressure is the thermodynamic parameter which causes