A new multicolumn, open-loop process for center-cut separation by solvent-gradient chromatography.

A comprehensive description of a new process--the GSSR (Gradient with Steady State Recycle) process--for center-cut separation by solvent-gradient chromatography is provided, highlighting its versatility, flexibility, and ease of operation. The GSSR process is particularly suited for ternary separation of bioproducts: it provides three main fractions or cuts, with a target product contained in the intermediate fraction. The process comprises a multicolumn, open-loop system, with cyclic steady state operation, that simulates a solvent gradient moving countercurrently with respect to the solid phase. However, the feed is always injected into the same column and the product always collected from the same column as in a batch process; moreover, both steps occur only once per cycle. The GSSR process was experimentally validated in a pilot unit, using the purification of a crude peptide mixture by reversed phase as a proof of concept; the crude mixture is roughly 50% pure and some of its impurities have isocratic retention times very close to that of the target peptide. Experimental results are reported in terms of cyclic steady-state profiles and process performance indicators, which include product purity and yield. A simplified model-based approach, which uses only a few key components of the crude mixture, is employed to assist in the explanation of the process operation. By dynamically adjusting the switching interval while the process is running, to correctly position the composition profile with respect to the outlet ports, pure product satisfying the target specifications--98% purity and 95% recovery--was obtained under stable operation in the pilot unit.