An operation strategy, partial feed, is developed to increase SMB separation efficiency. With the partial-feed mode of operation, the feed and raffinate flow rates vary with time. The separation of Dextran T6 and raffinose, which have linear isotherms, is modeled with the Aspen Chromatography simulator in a four-zone SMB for both normal-feed and partial-feed operations. Partial-feed operation significantly improved the separation with one column per zone. Either the purity and the recovery or the productivity could be increased. Partial feed introduces two additional degrees of freedom, namely, feed length and feed time. The optimum values for these parameters were determined. The improved separation is explained using local equilibrium theory.

Single-cascade simulated moving bed (SMB) systems with a side-stream withdrawal are developed for separating ternary systems. Several single-cascade SMB systems are studied for the separation of ternary mixtures within the linear range. Design parameters, operating conditions, minimum desorbent-to-feed ratios, and productivities are determined with the local equilibrium theory, and purities are determined through detailed Aspen Chromatography simulations. A series of initial feed compositions are simulated to determine the optimal regions. The single-cascade SMB is particularly applicable when there is little of the most strongly adsorbed component, a significant amount of the middle component, and the separation between these components is easy. A surprising finding is that the purity of the middle-component product is usually higher in an SMB than in a true moving bed. A few quaternary separations with a side-stream SMB followed by a binary SMB are also studied.

Three-zone simulated moving bed (SMB) systems are studied for binary separation within the linear range. Simulations were done by modifying the well-known Lapidus and Amundson dispersion model to include changing velocities and variable feed concentrations. Several different operations, total feed and withdrawal and partial feed and withdrawal, are tested and compared. The total operation is the traditional SMB operation, where feed or product are added or withdrawn continuously at a constant rate. The partial operations have variable port flow rates. For both, the three-zone SMB with one column per zone improves its productivity, desorbent efficiency, product purities, and recoveries. With the same total amount of adsorbent, the separation achieved in a three-zone SMB with 1 column/zone and partial feed and partial withdrawal was slightly better than the separation in a traditional four-zone SMB with 2 columns/zone.

Variable flow rate operation is developed to increase simulated moving bed (SMB) separation. With this mode of operation, the average flow rates of all external and internal flows can vary with time. The separations of dextran T6 and raffinose and of 1,1′-bi-2-naphthol enantiomers, representing linear and nonlinear equilibrium systems, respectively, are tested. They are modeled with the Aspen Chromatography simulator for a four-zone SMB with one and two columns per zone. A simplex double-layer optimization method with a pulse flow design strategy is developed to maximize the SMB separation. Variable flow rate operation improves the separation for both SMBs, but improvement is more significant for the one column per zone SMB. For the SMB with one column per zone, for the linear system, the extract purity increased from 85.3 to 91.4%, and the raffinate purity increased from 89.2 to 94.9%. For the nonlinear system, the extract product purity increased from 85.0 to 94.1%, and the raffinate purity increased from 84.9 to 90.3%. The improved separation is explained using the differences between the true moving bed and the SMB.