In practice, batch adsorption generally requires large amounts of adsorbent because of relatively low product adsorption. Thus, a continuous or semicontinuous operation would be more efficient, but until recently, this has been severely constrained by the difficulties in processing viscous or debris-laden streams through fixed beds that act as depth filters and quickly clog. The use of a fluidized bed for batch adsorption of biomolecules is not entirely new, having been demonstrated in hybrid form for antibiotic recovery as early as 1958 (22). Because of the limitation of a well-mixed fluidized system to one theoretical plate, there was little inherent advantage of fluidized bed contactors over stirred tank systems. However, the introduction of segmented beds, magnetic stabilization, and, most recently, Pharmacia's expanded bed chromatography system has allowed multistage contacting in fluidized systems that exhibit comparable dispersion to packed beds of similarly sized particles (38).
Expanded-Bed Chromatography. In process-scale chro-matography of proteins, most separations rely more on selectivity than on a high number of theoretical plates because achieving the latter necessarily requires high pressures (small diameter particles) and costly equipment. Given the accepted preference for low-pressure operations, a step from conventional packed-bed systems to recently developed expanded bed systems is often appropriate for unclarified feeds. These stable fluidized beds show axial dispersion and dynamic capacities comparable to those measured in packed bed.
An analogous technique for stabilizing fluidized beds incorporating the application of uniform magnetic fields to beds of magnetically susceptible adsorbent particles was extensively investigated by Rosensweig (39) and subsequently applied to biological separations by Burns and Graves (40). Although these systems have not been commercialized, they exhibit similar hydrodynamic characteristics to expanded-bed adsorption, facilitating processing of debris-laden streams while maintaining dispersion characteristics not dramatically greater than corresponding fixed beds. In this case, a uniform magnetic field prevents tumbling of the fluidized bed, creating a multistage contactor for potentially improved separation efficiency.
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