Page 137 - Adsorption Technology & Design, Elsevier (1998)

Processes and cycles 127

the separation. The flowsheet of a single bed RPSA process is shown in
Figure 5.17. There are two steps of equal duration, namely (i) feed/
pressurization and (ii) exhaust/depressurization. Figure 5.17 shows that the
feed and exhaust take place at the same end of the bed and that the product is
withdrawn from the opposite end. The cycle time is short, typically less than
20 seconds, and small particles are used to create both a high pressure
gradient across the bed and a short diffusional path for the adsorbate. The
principal disadvantage of the single bed RPSA process is that while
relatively high product purities can be obtained the product recovery
(defined as the amount of component recovered in the product per amount
of the component entering in the feed) can be low. A low product recovery
in any PSA process is undesirable because, for a given production rate, the
flowrate of feed, which must be compressed, will be high and consequently
the power requirements will be high. Descriptions of recent developments in
RPSA are provided in Chapter 7.
In practice the pressure changes with time are more complex than those
shown in Figure 5.15. Pressure does not vary linearly with time during either
the pressurization or depressurization steps (Crittenden et al. 1994; 1995).
Instead, as pressurization occurs there will be a pressure distribution along
the length of the bed and at each point the pressure will tend to increase
exponentially up to the final value. Exponentially shaped pressure-time
curves will occur also for the depressurization step. To make matters even
more complicated adsorption can occur as pressurization proceeds and
desorption can occur as depressurization proceeds. This is a particularly
important consideration for RPSA processes.
The pressurization step creates a region near the closed end of the bed
where enrichment in the least adsorbable component occurs. This region is
displaced in the production step at more or less constant pressure by the
inflow of feed gas at the other end of the bed. Depressurization and
subsequent purging are normally carried out in the reverse direction. In
most commercial PSA applications the adsorbent is selected so that the most
desired product is the least adsorbed species. The gas which is discharged
during the depressurization and purge steps is normally, but not necessarily,
a waste stream.

Figure 5.16 Two-bed vacuum swing adsorption process for the production of
nitrogen with a carbon molecular sieve (redrawn from Knoblauch
1978).