Relay simulated moving bed chromatography: concept and design criteria.

We present a new class of multicolumn chromatographic processes that change the classical way of handling the product outlets of simulated moving-bed (SMB) chromatography to avoid the use of flow controllers or an extra pump-the objective is to have just two- or three-way valves at a column outlet-while maintaining the analogy with the SMB in terms of displaced volumes of fluid per switch interval. In this class of processes the flow through a zone (or column) is always in one of the three states: (i) frozen, (ii) completely directed to the next zone (or column), or (iii) entirely diverted to a product line. We use the term relayed stream to refer to this particular type of manipulation of the outflow from a column. For this class of processes we derive a SMB analog-the R-SMB process-and demonstrate, under the framework of the equilibrium theory, that this process has the same separation region as the classical SMB for linear adsorption systems. In addition, the results from the equilibrium theory show that the R-SMB process consists of two distinct cycles that differ only in their intermediate sub-step: one cycle for selectivities α smaller than (3+√5)/2 and another cycle for larger values of α; in the former case no product stream is collected during the intermediate sub-step, whereas in the latter case both product streams are collected. We also examine the R-SMB process under conditions of finite column efficiency and compare its performance against those of the classical open- and closed-loop SMBs. Our simulation results show that the R-SMB process requires less desorbent and is more productive than the standard SMB processes under conditions of finite column efficiency and that the comparison increasingly favors the R-SMB as the column efficiency decreases.