Page 259 - Adsorption Technology & Design, Elsevier (1998)

234 Selected adsorption processes

(a) simultaneous pressurization with air when nitrogen is selectively
adsorbed by the zeolite adsorbent thus producing a stream enriched
in oxygen, part of which is delivered to supply or storage as product,
and the remaining portion used as purge gas to the adjacent twin
adsorbent layer followed by;
(b) simultaneous countercurrent depressurization and countercurrent
purge by the oxygen-enriched gas produced in the other twin
adsorbent layer so as to desorb the nitrogen produced in step (a).

The sequence of operations is such that layer A undergoes step (a) for a fixed
time between 3 and 10 seconds, while layer B is subjected to step (b) for the
same length of time. The roles played by layers A and B are then switched to
complete the cycle of total fixed time between 6 and 20 seconds. This allows
the continuous supply of a stream of oxygen-enriched product. A sublayer of
desiccant can be incorporated before each adsorbent layer to ensure that the
feed to each layer is dry prior to effecting separation. Multiple stacks of pairs
of adsorbent layers increase the productivity of the adsorber unit. The rate
of production of oxygen as a function of oxygen purity obtained from such a
unit is shown in Figure 7.23. These production rates are a ten to twenty fold
increase over those shown by a conventional PSA process and the oxygen
recovery is superior to that in a Skarstrom cycle (usually of a cycle duration
of the order of minutes) when the oxygen purity does not exceed 60%.
Finally, the plant is simpler having only a single vessel rather than the two or
more vessels required in the usual PSA process.
Another novel development, discussed by Sircar (1993a), includes
fractional vacuum swing adsorption using a calcium X zeolite for the
simultaneous production of c. 80-90% pure oxygen and 98% pure nitrogen.
Four steps are involved in the cycle:

(a) air at atmospheric pressure is passed through the adsorbent bed which
has been presaturated with oxygen-rich air and which is the effluent;
the step is continued until N2 just commences to break through the
bed when the effluent is vented until the bed is saturated with air;
(b) the column containing the adsorbent is evacuated to relatively low
pressure (approximately 0.5 bar) and the effluent gas wasted;
(c) the bed is then evacuated further and the effluent collected as 98%
pure N2;
(d) the bed is then repressurized to just below atmospheric pressure with
a part of the O2-enriched gas produced during step (a). A new cycle of
events is then commenced.