Recycle optimization in non-linear productive chromatography—I Mixing recycle with fresh feed

This paper examines the chromatographic separation of two solutes interacting non-linearly with the sorbent, when only partial separation is achieved at the column outlet, and an intermediate cut of the effluent is recycled. It is shown that, for given feed composition and column size, there is an optimum of the amount of fresh feed injected per cycle; alternately, for given feed composition and amount treated per cycle, there is an optimal column length. This optimum is such that interferences of concentration fronts are minimized in the column. The process is analysed using the “equilibrium theory”, which neglects hydrodynamic dispersion and mass-transfer resistances. Complete analytical solutions are given in the optimal situation, for the case of mass-action law equilibria with unity exponents (Langmuir type equilibrium).The theoretical predictions are compared to experimental results for the separation of Na⁺ from K⁺ by H⁺ eluent in chloride solutions, on a commercial cation-exchanger. The results are also compared to “classical” chromatography (without recycle), and to “two-way” chromatography, presented in a previous paper1]. With respect to “classical chromatography”, for a given product purity, the optimal recycle process is shown to improve product richness (i.e. concentration of product with respect to solvent) and all performance criteria (eluent consumption, sorbent immobilization, productivity). With respect to “two-way” chromatography, it improves sorbent immobilization and productivity, but yields a poorer Na⁺ product, thus uses more eluent.