Unformatted text preview: 6), and R. H. Wilhelm, A. W. Rice, R. W.
Rolke, and N. H. Sweed, I&EC Fundamentals, 7, 337-349 (1968). The technique is based on
differences in the extent of equilibrium adsorption of the two components in the feed mixture,
and a significant decrease in equilibrium adsorption as the temperature is increased. Two
different basic equipment arrangements and operations have been proposed, both in a closedsystem operation, as shown in the sketches on the next page. Both consist of a fixed-bed column
packed with adsorbent particles, a bottom piston-reservoir, and a top piston-reservoir. In a direct
mode, the column is jacketed so that the column may be alternately heated or cooled. In a
recuperative mode, heat exchangers (one for heating and one for cooling) are provided at
opposite ends of an adiabatically operated column.
Consider the direct mode, in the context of the example of the experiment reported in the
second literature reference above. The column, at ambient temperature, T1, and packed with
silica gel particles, is charged with a liquid feed mixture of 20 vol% toluene and 80 vol% nheptane and brought to equilibrium. The bottom reservoir is also filled with the same feed
mixture, but the top reservoir is initially empty. The toluene is more strongly adsorbed than the
n-heptane. The column is heated by the jacket to temperature T2 and the fluid in the bottom
reservoir is pushed into the column by upword movement of the driving piston. At the same
time, fluid is drawn into the upper reservoir as the driven piston is forced upward. Because the
capacity of the adsorbent is less at the higher temperature and because the adsorbent is more
selective for the toluene, the fluid entering the upper reservoir is more concentrated in toluene.
This constitutes the first heating half cycle. The direction of fluid flow is now reversed for the
first cooling half cycle. The column is cooled to T1 by the jacket and the top piston becomes the
driving piston, pushing the fluid from the top reservoir down into the column. At the lower
temperature, the toluene is more selectively adsorbed so that the fluid entering the bottom
reservoir is richer in n-heptane. Thus, after one cycle, a partial separation is achieved. Further
separation occurs by conducting additional cycles, perhaps 50, after which the reservoirs are
emptied into product receivers. Experimental results indicate that although the toluene is almost
completely removed from the n-heptane, a nearly pure toluene is not achieved (i.e. high recovery,
but not a high purity).
In the recuperative mode, the top of the column is maintained at a hot temperature, T2,
and the bottom is maintained at a cooler temperature, T1. Again the fluid is pushed first to the top and then to the bottom for a number of cycles to achieve a separation. Exercise 15.33 (continued)
Analysis: (continued) Exercise 15.33 (continued)
Cycling Zone Adsorption:
The separation of binary liquid or gas mixtures by cycling zone adsorpti...
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