Page 47 - Adsorbents fundamentals and applications

32 SORBENT SELECTION: CRITERIA

O 2

A B

N 2

Air
Figure 3.1. The Skarstrom cycle. Two steps are involved in the cycle for each bed:
(1) repressurization followed by feed in bed A, while O 2 is withdrawn as the product, and
(2) blowdown and purge with O 2 (both downward) in bed A while N 2 is withdrawn. The
functions of A and B are reversed in the next cycle. Note that this is not used commercially for
air separation, but is used for drying.

The Skarstrom cycle has been widely used for air-drying. For oxygen produc-
tion from air and other bulk separations, however, the product recovery was too
low, and hence the energy requirement too high, to be economical.

Co-Current Depressurization. The first major process improvement after the
invention of the Skarstrom and Guerin–Domine cycles was the introduction of
the co-current depressurization step. To incorporate this step into the Skarstrom
cycle, the adsorption step is cut short well before the breakthrough point, that is,
the concentration front is far from reaching the outlet of the bed. The adsorption
step is immediately followed by co-current depressurization before the bed is
desorbed by further blowdown and purge, as required in the Skarstrom cycle.
The large void space in the adsorber plays a limiting role in the recovery of the
strong adsorptive. The void fraction ranges from approximately 67% for silica and