Reaction and mass transport in immobilized urease reactors: Simulation andpH control
Abstract
The hydrolysis of urea exhibits characteristics common to enzymatic reactions in general: reaction-generated pH changes, pH dependent kinetic constants, and product inhibition. Theoretical models have been developed for urea hydrolysis within a single particle of immobilized urease and in a fixed-bed reactor. The pH-dependent and product-inhibited kinetics are described by a modified Michaelis-Menten rate expression. Ionic equilibria of product and buffer species determine the extent of reaction-generated pH changes. Nernst-Planck diffusion describes the transport of charged species. Orthogonal collocation and the IMSL routine DGEAR were used to solve the single particle and fixed-bed equation sets, respectively. Simulation results indicate the combination of pH dependent kinetics and reaction-generated pH changes determines reactor performance. For urea concentrations less than Michaelis constant K$\sb{\rm M}$, the effectiveness factor approaches that for simple Michaelis-Menten kinetics. Under experimental conditions (Damkohler number Da $>$ 1), the effectiveness factor can be five times less. External diffusion is significant, even at Biot number Bi $>$ 20, because of the strong dependence of reaction rate on pH. Nernst-Planck diffusion effects are important with ionic strength less than 10 mM. Urease has been immobilized onto Amberlite IRP-64, a weakly acidic cation exchange resin which buffers near the optimum pH for urease. The bound enzyme demonstrated as much as 1000 IU of activity per gram of support, and retained over 50% of the original exchange capacity. Simulations of fixed-bed reactors were verified experimentally with different inlet concentrations of urea, ammonia, buffer, and pH. Simulation was used to estimate intrinsic enzyme activity and pH dependent kinetic constants. The optimum pH of immobilized urease was shifted from 6.5 (that of soluble enzyme) to 7.7. This significantly reduces the magnitude of the pH effects on the kinetics. The effects of three different methods of pH control have been examined: (1) control of inlet pH, (2) addition of buffers at various concentrations, and (3) use of an ion exchanger within the fixed-bed. Control of the pH by an ion exchanger has been shown to be more effective than the use of buffers, especially in reactors with long spacetimes. The feasibility of pH control by an ion exchanger has been demonstrated experimentally. The utility of the fixed-bed reactor simulation was illustrated by optimization of a dialysate regeneration cartridge. pH control and increased flowrate are the most effective methods of increasing urea removal rate.
Degree
Ph.D.
Advisors
Wang, Purdue University.
Subject Area
Chemical engineering
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