(634e) 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.