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 The total operation is the traditional SMB operation, where feed or product are added or withdrawn continuously at a constant rate.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.