Unformatted text preview: on was first proposed,
analyzed, and substantiated by experiments in two articles by R. L. Pigford, B. Baker, and D.E.
Blum, I&EC Fundamentals, 8, 848 -851 (1969) and I&EC Fundamentals, 8, 144 (1969). As with
parametric pumping, the technique uses fixed beds and is based on differences in the extent of
equilibrium adsorption of the two components (A and B) in the feed mixture, and a significant
decrease in equilibrium adsorption as the temperature is increased. However, while parametric
pumping involves periodic reversals of flow direction and temperature, cycling zone adsorption
involves only reversals in temperature. Unlike other fixed-bed schemes, batch-type regeneration
of the adsorbent is not required.
The first sketch on the following page shows the basic idea of cycling zone adsorption.
Feed fluid of a fixed composition enters a jacketed fixed bed, maintained at low temperature, TC,
causing component A to be adsorbed to a greater extent than B. At breakthrough, the
composition of the exiting fluid approaches that of the feed and the bed temperature is raised to
TH , which causes the adsorbate, rich in component A, to be expelled, raising the concentration of
A in the exiting fluid. After a time, the effluent composition again approaches that of the feed.
To continuously produce two products, one richer in A, yh, than the feed, and the other leaner in
A than the feed, yl, two fixed beds are employed, as shown in the second sketch on the following
page. One bed is exactly one-half cycle out of phase with the other. The output of each column
is periodically switched, but the degree of separation is limited.
The degree of separation is significantly improved by employing two or more beds, called
zones, in series, as in the third sketch on the following page. Compared to parametric pumping,
theoretical analysis predicts that n cycling zones is equivalent to n parametric pumping cycles.
Thus, much more equipment is required for cycling zone adsorption than for parametric
pumping. However, with the former, products are produced continuously, instead of in batches.
A further simplification in operation is achieved by replacing the jacketed beds by heat
exchangers in the manner of the two recuperative mode in parametric pumping, as shown in the
In the review article by N. H. Sweed, AIChE Symp. Series, 80, No. 233, 44-53 (1984),
parametric pumping and cycling zone adsorption, after more than 15 years following their
invention, are found to have remained laboratory curiosities, despite their early promise. This is
probably due to difficulties in swinging the temperature, and the popularity of PSA for gas. Exercise 15.33 (continued)
Analysis: (continued) Exercise 15.34
Subject: Separation of propylene from propane by continuous, countercurrent adsorption.
Given: Feed gas mixture of 55 mol% propane (C3) and 45 mol% propylene (C3=).
Continuous, countercurrent adsorption at 25oC and 1 atm with silica gel. Equilibrium data in
Find: Using the McCabe-Thiele method...
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