Packed Bed Adsorption of Biomolecules: A New Technique for the Retrieval of Film Mass-Transfer Coefficients

ABSTRACT

Packed bed columns are often used to capture biomolecules from clarified feed-stocks. During the design and operation of packed bed columns, reliable forecasting of bed outlet concentration is important, and within this context, mass transfer coefficient from the bulk solution to the particle external surface, i.e. film mass transfer coefficient k_f is one of the main parameter to be determined.

In a stirred tank adsorption of biomolecules, Hunter and Carta [1] described an interesting technique for the experimental determination. Nevertheless k_f values calculated by this technique are limited to stirred tank adsorption processes. It is obvious that stirred tank processes do not duplicate the hydrodynamic conditions encountered in packed bed adsorption of biomolecules. Conventional determinations of k_f values in packed bed adsorption columns require the measurement of column effluent concentrations for highly favorable adsorption isotherms when column loading is low. This might impose a serious limitation when column residence time is low or the biomolecule adsorption kinetic is slow. In these conditions, the difference between the column inlet and effluent concentrations becomes small. Therefore, a small error in the concentration measurement might be reflected as a large error on the determined k_f value.  Özdural [2] showed that at a recycle adsorption system, which is composed of a reservoir and an adsorption column, neglecting axial dispersion Eq. (1) gives the relationship between column inlet and effluent concentrations, for the case of strongly favorable adsorption isotherms.

                            (1)

Where c_out(t) is the column outlet concentration, c_in(t) is the column inlet concentration, c₀ is the reservoir initial concentration, L is bed height, v is interstitial velocity, R is adsorbent particle radius, t is time, e is bed voidage. A recycle adsorption system approaches to a single pass column system when t ⋍ 0, where c₀  becomes equal to c_in(t), and Eq. (1) reduces to Eq. (2)

                                             (2)

or

                                                              (3)

In order for this method to work, and to retrieve the k_f value via Eq. (3), the experimental conditions should be so planned that when  t ⋍ 0  the obtained [c_out(t)/ c_in(t)]  value must be non-zero. In this work this new and simple technique, i.e. Eq. (3)  is employed for chromatographic columns (I.D. = 4.6 mm) having three different bed lengths (L = 2.5 cm, 5 cm and 10 cm) packed with Q Sepharose™ XL resin. Q Sepharose™ XL is ion exchange adsorbent specially designed for use in packed bed colums to capture biomolecules from clarified feed-stocks. As a biomolecule, BSA solution at different concentrations is fed to the column at various flow rates by a chromatographic pump. Column outlet is monitored by a UV/VIS chromatography detector which is linked to a computer through a data acquisition unit. It was concluded that the film mass transfer coefficients in packed bed adsorption columns could easily be acquired with this new technique by the proper adjustment of the experimental conditions that leads to non-zero values of [c_out(t)/ c_in(t)]  when t ⋍ 0.

References:

[1] Hunter A.K., Carta G. (2000) Journal of Chromatography A, 897: 81.

[2] Özdural A.R. (1994) Chemical Engineering and Technology, 17: 285.