Theory and Application of Membrane Separation

JUAN HONG, Purdue University

Abstract

Ultrafiltration and reverse osmosis have been employed for concentrating macromolecules and for purifying solvents. The fractional membrane separation between two solutes can be achieved by taking advantage of the difference in intrinsic rejection coefficients. The flux equations for two solute system are derived based upon irrevers- ible thermodynamics and shown to embrace all the previously reported theories when applied to a one solute system. An experimental study of stirred cell ultrafiltration is performed with glucose and cellobiose solutions. The rejection coefficients for both glucose and cellobiose de- crease with decreasing pressure. The separation factor, which is the ratio of rejection coefficient of cellobiose to that for glucose, changes from 2 to 4 when flux is decreased 3 fold.The successful use of membranes, coupled with reaction, has resulted in the development of the concept of membrane reactor. Cellobiase enzyme reaction on cellobiose is chosen for membrane reactor analysis. A pressurized stirred vessel provided with membrane which rejects enzyme completely is designated as the membrane reactor. The theoretical analysis of membrane reactor for cellobiose hydrolysis is carried out in terms of the steady state con- version and the flow rate through membrane, which is mainly monitored by applied pressure. When the flow rate exceeds the critical point, a significant fraction of enzyme loaded. initially inside membrane reactor is localized in the con- centration polarization layer where shear from stirring is. high. Subsequently, the deactivation of enzyme inside the reactor is accelerated and gives low conversion due to the exchange of enzyme in bulk with deactivated enzyme in the polarization layer via convection and back diffusion. Con- sequently, the successful operation can be obtained by con-trolling the polarization of the enzyme. A method for determining diffusivity by taking advantage of the concentration polarization phenomena above the membrane surface in an ultrafiltration cell is another application of membranes to practical problems. A comparison with existing methods is made and the theoretical back- ground for the new method is described. The diffusion co- efficient of glucose in aqueous solution is shown to be measured within 20 min and determined as (7.09 ± 0.64)x10-6 cm²/sec. Theoretically, the time involved in determining low diffusivity components such as bovine serum albumin is shown to be less than that for high diffusivity components.

Degree

Ph.D.

Subject Area

Chemical engineering

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