Improvement of para-xylene SMB process performance on an industrial scale

Dead volume classified as bed head, bed tail, and bed line affects the purity and recovery in industrial-scale simulated-moving-bed (SMB) processes. Considering the three types of dead volume, the operation strategies for four, seven, and eight zones are presented for an industrial-scale para-xylene (PX) SMB process, and their performances are compared in terms of purity, recovery, and dilution. The dead volume is treated by a Courant-Friedrichs-Lewy- (CFL-) number-insensitive method of characteristics (MOC). An extended node model integrating the dead volumes has been developed and solved by using the MOC. Seven-zone operation including line and secondary flushes enhances the purity, while keeping almost the same recovery as achieved with four-zone operation. The secondary flush introducing pure PX reduces the dilution of the extract. Eight-zone operation with both a secondary flush introducing pure PX and a tertiary flush-out shows high productivity with low dilution and over 99.7% purity