Hybrid Simulated Moving Bed Processes for the Purification of p‐Xylene

Abstract

A parallel two‐zone and four‐zone SMB hybrid process was developed for p‐xylene purification. By placing a high‐productivity parallel two‐zone SMB in front of an existing four‐zone SMB, feed was enriched in the parallel two‐zone SMB before it was fed to the four‐zone SMB, which allows a higher feed rate to the four‐zone SMB. A general design procedure for hybrid SMB‐SMB process was developed. Simulations were done with Aspen Chromatography and Aspen Plus. Overall adsorbent productivity was increased by more than 100%, but with 3–5% lower overall p‐xylene recovery. With overall recovery of the hybrid process matching the base case, the feed rate was doubled and the overall adsorbent productivity was 50% higher than the base case. The heat duties of distillation columns per unit product were similar for these hybrid processes compared to the base case. If a two‐feed raffinate distillation column was used, the heat duty of distillation columns per unit product in the hybrid process was 2–4% lower than the base case. Both parallel two‐zone and four‐zone SMBs were also developed for use in a hybrid SMB‐crystallization process for p‐xylene purification. SMB productivity can be increased from 31% to 75% by adding additional pumps between columns to operate columns at their pressure limits. Although additional pumps are useful for both systems, the productivities of the parallel two‐zone SMBs were 9% and 16% higher than the corresponding four‐zone SMBs with the same number of recycle pumps. For a four‐zone SMB with two columns per zone and eight recycle pumps, a 114% increase of productivity was achieved, which is 22% higher than a parallel two‐zone with one column per zone and four recycle pumps.