Simulated Moving Bed (SMB) Separators

Key Insight

Simulated Moving Bed (SMB) separators are a type of countercurrent adsorption process fundamentally different from classical chromatography, despite their discussion alongside it due to their role in bioprocessing. Such systems involve the countercurrent flow of stationary and mobile phases, which can be achieved either with a 'true moving bed' system or, more practically, with a 'simulated countercurrent' system. The latter employs an arrangement of packed columns with periodic switching of beds in a direction opposite to the fluid flow using a system of valves. This setup efficiently simulates countercurrent flow without the mechanical complexities of moving particles, maximizing mass transfer efficiency and adsorbent utilization.

In general, components that are more strongly adsorbed will travel in the direction of the adsorbent flow, while more weakly adsorbed components travel in the opposite direction. At steady state, SMB systems split feed components into two fractions: 'heavies' (more strongly adsorbed species), which leave with the adsorbent at the fluid feed end, and 'lights' (more weakly adsorbed species), which leave with the fluid phase at the opposite end. Recovery of the 'heavies' usually requires a subsequent desorption step. SMB systems are primarily limited to binary splits; multi-component separations necessitate multiple units. The SMB concept originated from UOP's 'Sorbex' process for hydrocarbon separations and has expanded to industrial applications for sugars, amino acids, fine chemicals (especially chiral separations), and increasingly for biopolymers.

Conceptually, an SMB system can be understood as an equivalent True Moving Bed (TMB) system comprising four countercurrent adsorption zones. The feed, containing components A and B, is supplied between zones II and III. The least strongly adsorbed species, A, is recovered in the raffinate stream between zones III and IV, while the more strongly adsorbed species, B, is recovered in the extract stream between zones I and II. The adsorbent is recirculated, and a desorbent is added. Zone III adsorbs B while letting A pass, Zone II desorbs A while adsorbing B, Zone I desorbs B for adsorbent recycling, and Zone IV adsorbs A. The advantages of SMB over conventional isocratic elution chromatography include greater productivity, lower eluent consumption, and less product dilution, achieved through countercurrent action and internal recycling of partially resolved products. Disadvantages include equipment and operational complexity, limited robustness requiring delicate control, and applicability to binary splits only. Despite these, SMB is advantageous for very difficult separations or when adsorbent and eluent costs are high. Hybrid systems like gradient SMB and multicolumn countercurrent solvent gradient purification (MSGP) are being developed for bioseparations, offering further productivity gains and enabling three-fraction separations.