Development and applications of two -zone SMB processes for binary separations

In this study, a two-zone SMB with a storage tank is developed using detailed simulations with Aspen Chromatography. Due to dual zone functions and mixing in the tank in a two-zone SMB, it gives slightly lower purities than a four-zone SMB with same productivity and desorbent-to-feed ratio ( D/F) for both linear and nonlinear isotherms. For dextran T6/raffinose separation (linear isotherms), the average purities are 96.3% and 96.4% for two and four-zone SMBs, respectively. However, two-zone SMBs are less complex than four-zone SMBs. Scaling rules for four-zone SMBs are extended to two-zone SMBs, and used in pressure utilization procedures by adding pumps between columns to increase productivity. Two-zone SMBs can achieve higher increases in productivity due to lower initial pressure drop than four-zone SMBs. For example, for one column per zone systems for binaphthol enantiomer separation, the productivity of a parallel two-zone SMB with four pumps is 35% higher than the corresponding four-zone SMB with constant purities and D/F. A parallel two-zone and four-zone SMB hybrid process is developed for p-xylene purification to utilize the advantages of four-zone (high purity and recovery) and parallel two-zone (high productivity) SMBs. With overall recovery of the hybrid process matching the stand-alone four-zone SMB, the feed rate was doubled and the overall productivity was 50% higher than the four-zone SMB. Thermal operation using heat exchangers to produce and control traveling thermal waves by heating or cooling the fluid before it enters a column can improve the performance of two and four-zone SMBs. For one column per zone systems with four heat exchangers for toluene/p-xylene separation (linear isotherms), the average purity increased from 84.8% to 99.2% and from 80.9% to 98.9% for four and two-zone SMBs, respectively. For one column per zone systems without heat exchangers, the average purities increased from 84.8% to 93.2% and from 80.9% to 90.1% for four and two-zone SMBs, respectively. For nonlinear isotherms for p-xylene purification in four-zone SMBs, thermal effects are complicated and the separation can be improved to satisfy industrial standard using fewer columns/sections than an industrial unit, with p-xylene purity of 99.9% and recovery of 98.6%