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.

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