Effect of salt gradients on the separation of dilute mixtures of proteins by ion-exchange in simulated moving beds

Salt gradients can improve the efficiency during fractionation of proteins by ion-exchange in simulated moving beds (SMBs). The gradients are formed using feed and desorbent solutions of different salt concentrations. The thus introduced regions of high and low affinity may reduce eluent consumption and resin inventory compared to isocratic SMB systems. This paper describes a procedure for the selection of the flow-rate ratios that enables successful fractionation of a dilute binary mixture of proteins in a salt gradient. The procedure is based on the so-called "triangle theory" and can be used both for upward gradients (where salt is predominantly transported by the liquid) and downward gradients (where salt is predominantly transported by the sorbent). The procedure is verified by experiments.