8 - Counter-current systems

Summary

Introduction

Partition chromatography resulted from the marrying of two techniques, that of chromatography and that of counter-current solvent extraction. All of the ideas are simple, and had peoples’ minds been directed that way, the method would have flourished perhaps a century earlier. In fact the minds of laboratory workers seem to have been closed to counter-current procedures … In industry the use of the counter-current principle led to great economies in heat and solvents, and its value was obvious.

A.J.P. Martin, Nobel Lecture, December 12, 1952

Even though chromatography is often considered, and used, as a discontinuous process, chemical engineers have conceived and implemented efficient continuous chromatographic systems.

The basic principle of continuous chromatographic processes is that they promote a counter-current contact between the fluid phase and the chromatographic solid-phase medium. In such a system, the solid moves downwards, let us say thanks to gravity, whereas the fluid phase is pumped upwards. The feed mixture containing two solutes (or more generally two fractions to be separated) is injected continuously somewhere halfway along the system. Depending on their respective affinities for the chromatographic medium and on the choice of flow rates, the solutes may move upwards or downwards. This directional difference allows a continuous separation of one stream into two fractions. The counter-current contact is maintained thanks to the motion of the solid phase (the bed), so these systems are often called true moving bed (TMB) systems. The qualification true refers to the fact that the chromatographic medium is truly moving. We will use the expressions true moving bed, moving bed and counter-current systems equivalently for systems in which the solid and fluid phases are in contact counter-currently in a continuous manner.

Although these systems are attractive in principle, difficulties arising from solid handling have limited the applications of TMB, among them waste water treatment and uranium enrichment (Wesselingh and Van Der Meer, 1986; Streat, 1986a,b; Wankat, 1986; Eldridge, 1993).