This article is intended as a complement to a recent article in this journal, "Comparatiue Study of Flow Schemes for a Simulated Countercurrent Adsorption Separation
Process"by Ching et al. (1992). These authors carried out experiments on continuous separation of three carbohydrate mixtures with both the usual four-section scheme and a three-section scheme: fructose dextran (MW 5 9,400), rafinose dextran (MW = 6,000), and ftuctose-rafinose. These two schemes were modeled as equiualent true countercurrent systems. From the experiments and model results they conclude that the equiualent true countercurrent model is fully adapted to represent the four-section scheme, but a major discrepancy arises with the three-section scheme: the rafinate concentration is not correctly predicted.
From these results it may be concluded that when a physically meaning@ modeling of the equivalent true countercurrent is realized, the apparent discrepancy disappears. From another standpoint, such a mismatch between the experiments and model predictions would not have been observed with a model that considers the periodic port motion. Such a model foresees a cyclic steady state where extract and rafinate compositions vary throughout the period T; they can be considered as constant only when aueraged ouer the period. It is worth comparing the results of these two models, especially when the equivalent countercurrent representation is not very close to the physical phenomenon it should represent.
We agree with their major finding that a three-zone scheme represents necessarib a higher desorbent demand than a four-zone scheme.