Relationship between desorbent usage and the recovery of a target product in three-zone simulated moving bed processes designed under the conditions of positive and negative flow-rate-ratios of liquid to solid phases.

For a simulated moving bed (SMB) chromatographic process, it has been common knowledge that as desorbent usage increases, the level of separation capability (i.e., the purity and recovery of a target product) continues to improve and then eventually reaches almost a constant level. To check whether there are any exceptional cases concerning such a generally accepted relationship, the effect of desorbent usage on the recovery of a target product under the condition of high purity was investigated using a three-zone SMB process for separation of xylobiose (X2) from a β-xylosidase reaction output. It was found that the considered SMB represented some unusual relationship between desorbent usage and X2 recovery, which also took quite a different pattern according to the searching region for optimal SMB operation parameters. If the optimal operation parameters are determined in the region of positive flow-rate-ratios ("m approach"), the use of a larger amount of desorbent than required to make X2 recovery reach a constant level can rather lead to a little reduction in X2 recovery. If the optimal operation parameters are selected in the region of negative flow-rate-ratios ("m approach"), there exists an optimal desorbent usage, beyond which a further increase in desorbent usage can bring about a significant reduction in X2 recovery. Comparison of the two design approaches reveals that the m approach can lead to higher X2 recovery and much lower desorbent usage than the m approach. Furthermore, such merits of the m approach over the m approach became greater with increasing the SMB throughput.