Adsorption in fluidized beds

Up to now, there have been no data available in the literature describing solids mixing in fluidized beds of particles suited for protein adsorption. Therefore we will be restricted to a general discussion on particle movement and have to extrapolate some findings to fluidized bed adsorption. In fluidized beds of... 

Due to the fact that protein adsorption in fluidized beds is accomplished by binding of macromolecules to the internal surface of porous particles, the primary mass transport limitations found in packed beds of porous matrices remain valid. Protein transport takes place from the bulk fluid to the outer adsorbent surface commonly described by a film diffusion model, and within the pores to the internal surface known as pore diffusion. The diffusion coefficient D of proteins may be estimated by the semi-empirical correlation of Poison [65] from the absolute temperature T, the solution viscosity rj, and the molecular weight of the protein MA as denoted in Eq. (16). 

Summarizing this short discussion it has to be stated, that up to now experiments providing absolute numbers of kf during protein adsorption in fluidized beds are not available, the interpretations are based on correlations derived for small ions. As ion exchange with fluidized resins is performed at much higher Reynolds numbers and mostly is not limited by particle side transport, the validity of the correlations for proteins has to be proven. Nevertheless, the influence of bed expansion at increased linear flow rate cannot be neglected and fluid side mass transport should be considered as a system parameter governing the sorption process in a fluidized bed under certain conditions. 

Morton PH, Lyddiatt A (1992) Direct recovery of protein products from whole fermentation broths A role for ion exchange adsorption in fluidized beds. In Slater MJ (ed) Ion exchange advances. Elsevier, London... 

This set of equations is sufficient to characterize a particulate matrix which should be used in fluidized bed adsorption regarding its fluidization behavior. It has to be noted, however, that the correlations have been developed for the fluidization of spherical particles of uniform diameter. In reality, most adsorbents are provided with a certain distribution of particle diameter. In this case, classified fluidization occurs and a modified equation should be used to describe the hydrodynamics of bed expansion [21]. For an estimation of the suitability of a certain matrix for fluidized bed adsorption the correlations shown above are convenient to use and provide sufficient information. The minimum fluidization velocity may be calculated using an average particle diameter as recommended by Couderc [22], In the next section, conventional as well as new matrices shall be described under this respect. 

As in fluidized bed adsorption, proteins are bound to porous particles as well, these parameters will remain important and must be considered when describing protein adsorption to fluidized beds. As mentioned above, fluidizing the adsorbent allows free movement of the particles within the adsorbent bed, so dispersion in the solid phase is another component determining process performance. 

In Fig. 5, the liquid phase axial dispersion coefficient Daxi and the Bodenstein number Bo calculated from this relationship according to Eqs. (9), (10) and (13) are plotted for a range of linear velocities used in fluidized bed adsorption. The physical parameters of the commercial Streamline SP adsorbents (average particle size 247 pm, average particle density 1143 kg/m3, terminal settling velocity 0.0044 m/s, n = 4.7 as described by Chang and Chase [37]) were... 

For controlled pore glass as it has been used for the adsorption of BSA and monoclonal antibodies in fluidized beds [59] (mean dp 200 pm, pp 1240 kg/m3) we find for 5 cm/min linear flow rate Daxp to be 1.2-10 5 m2/s according to... 

Obviously liquid residence time is not an appropriate parameter to describe pore diffusion effects in fluidized bed adsorption. This may be elucidated by assessing particle side transport by a dimensionless analysis. Hall et al. [73] described pore diffusion during adsorption by a dimensionless transport number Np according to Eq. (17), De denoting the effective pore diffusion coefficient in case of hindered transport in the adsorbent pores and Ue the... 

For the fluidized bed process the bed expansion as a consequence of an increase in linear flow rate has to be considered. In a simplified picture diffusive transport takes place in a boundary layer around the matrix particle which is frequently renewed, this frequency being dependent on velocity and voidage, as long as convective effects, e.g. the movement of particles are neglected. Rowe [74] has included these considerations into his correlation for kf in fluidized beds, which is applicable for a wide range of Reynolds numbers, including the laminar flow regime where fluidized bed adsorption of proteins takes place (Eq. 19). The exponent m is set to 1 for a liquid fluidized bed, a represents the proportionality factor in the correlation for packed beds (Eq. 18) and is assumed as 1.45. 

Finally, in a concluding paper Dr. Jorg Thommes considers enzyme recovery in Fluidized Bed Adsorption as a Primary Recovery Step in Protein Purification . As important as it is to track down new enzymes and selectively modify them, it remains equally important to actually make them available in the flask on the bench in adequate quantities at low cost with sufficient purity. Recovery is of central significance in this respect. Fluidized bed adsorption combines the process steps of cell separation, concentration and primary cleaning in recovery work. The procedure can also be excellently transferred from the laboratory to the pilot scale. 

Modification of silica gel with volatile or gaseous compounds is performed in the vapour phase. Industrial-scale reactors and laboratory scale gas adsorption apparatus have been used. In the industrial field, fluidized bed and fluid mill reactors are of main importance. In fluidized bed reactors,82 the particles undergo constant agitation due to a turbulent gas stream. Therefore, temperatures are uniform and easy to control. Reagents are introduced in the system as gases. Mass transport in the gas phase is much faster than in solution. Furthermore, gaseous phase separations require fewer procedural steps than solution phase procedures, and may also be more cost-effective, due to independence from the use and disposal of non-aqueous solvents. All these advantages make the fluidized bed reactors preferential for controlled-process industrial modifications. 

Although in a number of cases chromatography is used in packed-bed mode, there are an increasing number of examples of the use of solid phases in fluidized-bed mode. This does not change the adsorption phenomenon based on the complementarity of the solute for the solid phase. Fluidized-bed columns are essentially used to resolve specific problems related to the feedstock, as detailed later. 

Frequency response methods have been found useful in both theoretical and experimental analysis of gas mixing in fluidized beds. Experiments in a fluidized-bed reactor related to mixing theory were made by Bamstone and Harriott 24). Testin and Stuart have measured diffusion coefficients in gas-solid adsorption studies 25). 

QQS) discuss practical aspects of PSA design such as pressure drops, the velocity limit to prevent fluidization of the bed, retaining the heat of adsorption in the bed and start-up. 

A later version of the process (RNDS) is based on the use of a resin containing zirconium hydroxide in fluidized bed reactors. It removes the need for filters and filter cake handling. The sulfate-containing brine, after acidification with HCl, passes through the adsorption bed. The adsorption reactions are... 

Nguyen HV, Potter OE. Adsorption effects in fluidized beds. In Keairns DL, ed. Fluidization Technology, Vol 2. Washington DC Hemisphere, 1976, pp 193-200. Palchonok GI, Tamarin AI. Mass transfer at a moving particle in a fluidized bed of coarse material. J Eng Phys 47 916-922, 1984. 

Immobilization of jS-D-galactosidase either by entrapment in cellulose acetate or by adsorption with cross-linking onto nylon yielded materials that are unsuitable for use in fluidized bed reactors, since their densities are close to unityHeavier particles were obtained by incorporating tungsten or stainless steel. 

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