Bed-line flushing and optimization in simulated moving-bed recovery of para-xylene

Eight-zone para-xylene (PX) simulated moving-bed (SMB) involves three flushing sequences to prevent purity deterioration caused by SMB bed-lines connecting a rotary valve to beds. Beside the recycle flow from the last zone to the first, the extract fluid trapped in bed-line (BL) is fed back to one bed ahead of the extract bed in accord with the line flushing (LF) and secondary flushing (SFi), forming an internal recycle flow. Both LF and SFi form the closed loop to increase purity and recovery, while the tertiary flushing (TFo) works somewhat independently of the other flushing streams for recovery enhancement. Thus, a two-level optimization strategy was proposed to identify optimum bed-configuration and flushing flow rates of LF, SFi and TFo. This study also examined how axial dispersion inside BL influences recovery and purity. The paper shows that the annual profit of the 8-zone SMB increases by 50 tons of PX/year over that of 7-zone SMB. (C) 2012 Elsevier B.V. All rights reserved.