Expanded bed adsorption is an integrated technology that allows the introduction of a particle containing feedstock without the risk of blocking the bed. Provided a perfectly classified fluidized bed (termed expanded bed) is formed in the crude feed, a sorption performance comparable to packed beds is found. During the application of biomass containing samples to stable expanded beds an increase in bed expansion due to the higher density and viscosity of the feed is encountered. In this article it is investigated whether the expanded bed condition is also fulfilled during the transition in bed expansion from lower to higher density (i.e., from an equilibration buffer to a biomass containing feedstock). Residence time distribution analyses were performed by using model systems and a yeast suspension during this transition phase. It is shown that in systems in which the biomass does not interact with the fluidized stationary phase, the perfectly classified fluidization is maintained also during this transition phase regardless of the type of feedstock. Additional bed expansion takes place in an "ordered" manner without compromising bed stability. In case of biomass/adsorbent interactions, a deterioration in bed stability is found directly when the crude feed is loaded.

Buoyancy-induced mixing occurs during expanded bed adsorption processes when the feed stream entering the bottom of the system has a lower density than that of the fluid above it. In the absence of a headspace, mixing in the expanded bed can be modeled as a single, well-mixed vessel, with first-order dynamics. In the presence of a headspace, the system exhibits second-order dynamics for the densities typically encountered in protein chromatography, and can be modeled as two well-mixed vessels (the expanded bed and the headspace) arranged in series. In this paper, the mixing dynamics of the expanded bed are described and a mathematical model of the system is presented. Experimental measurements of density changes during the dilution of sucrose and salt solutions in a STREAMLINE 25 column are presented. These show excellent agreement with predictions using the model. A number of strategies for wash and elution in expanded mode, both in the presence and absence of headspace, are discussed.