(601d) Understanding Dextran Retention Data for Hollow Fiber Ultrafiltration Membranes

Although dextran
retention tests have become a standard method for characterizing ultrafiltration
(UF) membranes, most quantitative analyses of dextran transport have been
performed with flat sheet membranes using simple stirred ultrafiltration cells
in which the transmembrane pressure (and thus the filtrate flux) is nearly
uniform across the membrane surface. 
The objective of this work was to
develop a more fundamental understanding of the factors governing dextran
retention tests for large pore size hollow fiber ultrafiltration membranes
suitable for use in bioprocessing applications, e.g., the purification of
vaccines.  Experiments were performed using a series of hollow fiber membranes
with molecular weight cutoffs around 500 kDa provided
by GE Healthcare.  Data were
analyzed using a theoretical model that accounts for:  (1) the axial variation in transmembrane pressure, and in
turn the filtrate flux, due to the significant pressure drop associated with
flow through the lumens of the hollow fiber membranes, (2) the axial variation
in the bulk mass transfer coefficient due to the continued growth in the
concentration polarization boundary layer along the length of the membrane, and
(3) the effects of dextran polarization on the local filtrate flux associated
with the high permeability of these large pore size membranes.  The intrinsic sieving coefficients of
the membrane were evaluated using available hydrodynamic models assuming a
log-normal pore size distribution, allowing us to describe the entire dextran
retention curve using only a single adjustable parameter. 

The measured dextran retention coefficients were a very
strong function of the permeate and feed flow rates.  For example, the retention coefficient
of a 1000 kDa dextran fraction decreased from R =
0.41 to less than 0.02 as the permeate flow rate was increased from 3 to 18
mL/min at a feed flow rate of 120 mL/min.  This reduction in dextran retention was
a direct result of concentration polarization in the hollow
fiber module at the high permeate flow rates.  The experimental data were in excellent agreement with model
calculations over the entire range of dextran molecular weights and flow
conditions.  The results provide
important insights into the proper design and interpretation of dextran
retention tests for hollow fiber ultrafiltration membranes.