CFD STUDIES AND EFFECT OF RHEOLOGY ON HOLLOW-FIBER ULTRAFILTRATION OF MACROMOLECULES

Abstract

Separation of macromolecules from the protein solution is of vital importance for the biochemical industries. Therefore, many researchers have shown great interest in membrane based separation process such as ultrafiltration for the separation of macromolecules solutions and many attempts have been made to optimize the process for better design. Ultrafiltration is a most successful pressure driven membrane based separation process widely used in bioprocessing industries for the purpose of protein separation, and for the purification and concentration of macromolecular solution. Ultrafiltration is very similar to conventional filtration method except that in this case filtration takes place at molecular level. A porous membrane layer is used (pore size 1-100 nm) which acts as a barrier to the flow of certain molecular species larger than the size of membrane pores. Several widely different designs known as modules, using membranes of various shapes, size and support are available. One of the most useful modules is a hollow fiber membrane in which membrane layer is moulded inside of the polymer cylinder to provide large specific area per unit volume. The retained solute particle on the membrane surface contributes to the membrane fouling and concentration polarization which is the major backdrop associated with the membrane separation process. Concentration polarization is the deposition of solute particle on nearby membrane surface caused by convective-diffusive transport of solute. Fouling is a decisive factor for the viability of any ultrafiltration process. Several attempts have been made to predict the complex fouling behaviour. Numerical technique is an attractive method to investigate the fouling phenomena in the membrane module. For the better design of any system, a design engineer needs more detailed understanding of the process (using reliable and accurate analytical tools). Same is true in case of membrane modules also. However, due to the microscopic nature of the membrane separation processes, it becomes very difficult to practically observe all the activities near the membrane surface (or inside a micro-tube) in a greater detail. To overcome this, one has to rely upon mathematical tools to simulate possible microscopic behavior and predict the macroscopic outcome of a membrane module that can be compared to validate the model, i.e., the confirmation of the microscopic unseen activities in terms of macroscopic behavior. For such validation of membrane processes, concepts of fluid dynamics in conjunction with the concepts of separation processes have proved to be a powerful tool. ii In the present work a computer simulator for membrane processes involving hollow fibre ultrafiltration membranes has been developed using staggered grid finite volume method (SGFVM). In the present approach the entire flow chamber is divided into a large number of control volume elements (similar to the finite volume) and each element is considered to be an independent unit. All mass flux and velocity components are calculated at the boundaries instead at the centre (nodal point) of volume element leading to staggering of nodes for velocity and concentrations. It is observed that this method is very fast and computer memory requirement is very low. Considering the complexities involved in the simulation of ultrafiltration through hollow fibre membranes, present work has been divided into five sections. First section introduces membrane processes in general with an emphasis to the flow behaviour in micro-tube hollow fibre membrane. A summary of available literature related to present work is presented in the second chapter. The third section of the work is devoted to the development of the computer simulator for the concentration profile, permeation velocity, etc. In the fourth section, a comparison of the simulator results with experimental data from the literature and the effects of the solute transport properties on the profile of permeate flux and solute concentration on membrane surface is tested with single macromolecular compounds. The fifth section deals with ultrafiltration of binary mixture of Bovine Serum Albumin (BSA) and Lysozyme (LYS). Flow behavior in micro-tube hollow fiber membrane using simplified equation of motion Equation of motion was greatly simplified to represent permeation of incompressible Newtonian fluid with variable viscosity through a micro-tubular membrane. It was observed that even after huge simplification, the accuracy of the results was same as that obtained by previously reported results based on rigorous formulation. Due to concentration polarization, viscosity of the fluid is expected to increase near the membrane surface. The results of this part suggest that such increase in viscosity leads to considerable change in the flow behavior of a membrane module and the commonly believed parabolic velocity profile in a membrane module is not possible, if viscosity of the fluid is not constant throughout the tube. The algorithm developed for this section is found to be numerically robust and simple so that it can be easily integrated with other simulation runs such as the cases of permeation with diffusion. iii Permeation Characteristics of Hollow Fiber Membrane The objectives of this part is to integrate the hydrodynamics model developed in the previous section with the mass transfer and resistance in series model. Separation performance (flux and solute deposition) of aqueous macro molecular solution of Dextran T500 was investigated using hollow fiber membrane. The proposed model integrate the resistance-in-series model as boundary condition for membrane coupled with mass, momentum and continuity equations to scrutinize the effects of the feed concentration, velocity and transmembrane pressure (TMP) on the local permeate velocity or flux. Using the developed model with resistance-in-series model and solute viscosity variation taken into account, the variations of the flux and membrane surface concentration of solute along the membrane length were also examined. Based on the simulated results of the permeation flux, it was shown that the values obtained using mathematical model was in good agreement with the experimental data published in literature. Whereas, different trends of variation of the fouling and concentration polarisation resistance along the length of membrane module are observed at the three different feed velocity conditions. Effect of Variable Transport Properties on Permeation Characteristics The effect of the transport properties of the solute on the permeate flux in ultrafiltration of Dextran T500 has been investigated. The model developed in previous section can also work with constant and variable properties. The model takes into consideration the effects of the variations of properties, such as, viscosity, diffusivity and osmotic pressure throughout the hollow-fiber membrane module. Determination of these three solute transport properties was based on solute concentration on the membrane surface. Finally, the effect of concentration polarization on the diffusivity and viscosity is calculated and the change in the permeate flux for the various feed velocity conditions were studied. The results of these simulations show that, contrary to the constant property, the effects of variable diffusivity and variable viscosity are both significant and comparable in magnitude and the osmotic pressure has a negligible effect on separation of Dextran T500 molecules. Ultrafiltration of Binary Macro-Molecular Mixture The UF of binary macromolecular solutes Bovine serum albumin (BSA) and lysozyme (LYS) mixtures through a polysulphone hollow fiber membrane have been studied by computational iv method. The behaviour of each solute was analyzed individually and also in binary mixture using changes in the parameters such as TMP, velocity, concentration ratio and pH. The comparison of simulation results obtained for single solute solution and the binary solute solution showed interesting results that show the importance of solute rehology other than the solute - solute and membrane - solute interactions. The numerical technique developed for the hollow fiber ultrafiltration module is completely general and can be used for any theoretical description for key solution properties