Hybrid simulated moving bed processes for the purification of p-xylene

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.