Thermal Operation of Four-Zone Simulated Moving Beds

Simulation results with Aspen Chromatography 2004 show that thermal four-zone simulated moving beds (SMBs) operated in the traveling wave mode (fluid is heated or cooled) can significantly improve the separation without radial heat transfer limitations. For toluene/ p -xylene separation (linear isotherms) in a four-zone SMB, the average purity increased from 84.8% to 99.2% and from 92.0% to 99.99% with four heat exchangers for one and two columns per zone, respectively. The average purities increased from 84.8% to 93.2% and from 92.0% to 99.0% when cold feed and hot desorbent were used with no heat exchangers for one and two columns per zone, respectively. The additional dead volumes introduced by heat exchangers had an insignificant effect. For separation of p -xylene from other C 8 isomers (nonlinear isotherms), a p -xylene purity of 99.9% and recovery of 98.6% were obtained with a thermal SMB with significantly fewer columns/sections than an isothermal SMB.