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Encyclopedia of Physical Science and Technology EN001-13 May 7, 2001 12:29

268 Adsorption (Chemical Engineering)

VIII. CHROMATOGRAPHIC PROCESSES IX. CONTINUOUS COUNTERCURRENT
PROCESSES
It is well known to the analytical chemist that efficient sep-
aration of even rather similar compounds can be achieved The possibility of operating an adsorption separation as
in a chromatographic column. The possibility of scaling a continuous countercurrent process (Fig. 8), rather than
up such a process to preparative scale is inherently attrac- in the cyclic batch mode, is theoretically attractive be-
tive, and many drugs, perfumes, and other compounds of cause countercurrent contact maximizes the driving force
high value are in fact separated in this way. However, such for mass transfer, thus providing more efficient utiliza-
processes have generally been found unsuitable for the tion of the adsorbent than is possible in either cyclic
large-scale bulk separations typical of the petrochemical batch or chromatographic systems. The main difficulty
industry, and their practical usefulness is limited to sys- is that for countercurrent contact it is necessary either to
tems with maximum throughputs of perhaps 1–2 tons/day. circulate the adsorbent or, by appropriate design of the
Themaindifficultyisthatinlarge-diameterbedstheHETP fluid flow system, to simulate adsorbent circulation. This
increases dramatically as a consequence of small nonuni- makes the design of a countercurrent system more com-
formities in the packing, thus reducing the separation ef- plex and reduces operational flexibility. For relatively easy
ficiency. Such effects can be minimized by very care- separations (high separation factor, adequate mass trans-
ful packing of the column but, even so, such processes fer rates) the balance of economic advantage generally
are generally confined to high-value products and modest lies with a cyclic batch system, but for difficult separa-
throughputs. tions in which selectivity is low or mass transfer slow
Production-scale chromatographs are generally oper- the advantage of a continuous countercurrent system in
ated under conditions somewhat different from those em- reducing the required inventory of adsorbent must even-
ployed in analytical chromatography since the objective tually outweigh the disadvantages of the more complex
is to maximize throughput rather than resolution. As a engineering.
result the column is generally operated at minimum res-
olution and under overload conditions. Feed pulses are
A. Simulated Countercurrent Systems
injected successively so that the resolution between suc-
cessive pulses is about the same as the resolution between Much of the benefit of countercurrent operation, with-
the components of each pulse. Theoretical considerations out the problems associated with circulation of the ad-
suggest that for optimal design one should run six columns sorbent, can be achieved by using a multiple-column
in parallel with feed switched in sequence to each column fixed-bed system with an appropriate sequence of col-
in such a way that the feed is injected into each column umn switching, designed to simulate a counterflow sys-
for one-sixth of the time with pure carrier flowing for five- tem. The general scheme is illustrated in Fig. 13.
sixths of the time. Such systems are widely used in wastewater treatment,

FIGURE 13 Schematic diagram showing the sequence of column interchange in a periodic countercurrent separation
process.