J.S. Henderson
Magdalen College
Thesis submitted for the degree of D. Phil., Trinity term, 1999
The results of an experimental investigation of novel methods for separating cellular material from liquids are reported here. The primary objective was to attempt for the first time to optimise the design of screw-threaded inserts that are a loose fit within tubular membranes, for efficient and economical operation.
Pressure drop was minimised by the use of a twin-start helical insert and by increasing the clearance between insert and membrane. A simple model was developed that produced upper- and lower-bound estimates for measured pressure drop. Heated thin film gauges measured wall shear stress at two positions, and also provided an estimate of vortex strength and frequency.
Taken together, axial mixing, shear stress and turbulence measurements identified the flow rate for each insert at which operation was most efficient. It is argued that vortices generated by both helical and axial flow components were present at this point, and owing to a laminar instability, the proportion of the flow in each path was not constant. Averaged over the length of a filter, this ensures good cross-stream mixing which, combined with a high wall shear stress, explains the excellent filtration flux obtained with helical flow promoters.
The second half of the investigation concerned affinity separations. A model system of
an ion-exchange membrane and a representative protein, bovine serum albumin, was studied.
Langmuir kinetics were shown to hold under static conditions, but under dynamic conditions, the
adsorption was controlled by a surface diffusion mechanism. Membrane sheets were stacked in
holders of two sizes, and breakthrough curves were plotted. Consideration of breakthrough
efficiency led to the identification of preferred operating conditions. Capacity and
efficiency were maximised by loading a short stack of large-diameter membrane discs at a low
flow rate and low feed concentration. The methods described will assist in the development of
an innovative new technique for antibody removal, when the attachment of synthetic ligands to a
membrane is perfected.