Design and Optimization of Different Operational Modes of 7-Zone SMB Model of p-Xylene Separation Process

Design and Optimization of Different Operational Modes of 7-Zone SMB Model of p-Xylene Separation Process

Yueying Yu, Kevin Wood and Y. A. Liu, SINOPEC/AspenTech Center of Excellence in Process System Engineering, Department of Chemical Engineering, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061

We have designed and analyzed Aspen Chromatography simulations of the industrial-scale adsorptive separation of p-xylene from a mixture of C8 aromatics in a 7-zone simulated moving bed (SMB) unit. The typical 7-Zone SMB model has two inlet streams (feed and desorbent), and two outlet streams (extract and raffinate), and also two flushing sequences (a line flush stream and a secondary flush stream). In this study, we first validate our 7-zone SMB model with industrial data, and then consider three different operational modes of our validated model, including a 8-zone SMB operation, the VARICOL mode and the PowerFeed mode. The 8-Zone SMB operation adds another feed out-flush stream into the 7-Zone SMB model, in order to increase both the purity and recovery of the product. The VARICOL mode is based on a nonsynchronous shifting of the inlet and outlet valves in a 24-bed SMB system. We also analyze the changing feed flow rate during the switching time in the PowerFeed mode of the SMB operation. We investigate the effect of dead volumes from the rotary valve to the inlet and outlet ports, as well as within each adsorbent bed. We present the resulting SMB performance for each model in terms of purity and recovery of the p-xylene, desorbent consumption and adsorbent productivity.  We also compare the results from different operational modes, and give quantitative recommendations on how to improve the performance of the standard 7-zone SMB operation.