THEORETICAL ANALYSIS OF AMMONIUM REMOVAL IN AN IMMOBILIZED UREASE AND ZEOLITE REACTOR-SEPARATOR

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Abstract
We have developed a theoretical model for predicting the column profiles and effluent histories of a fixed-bed reactor-separator which consists of an immobilized enzyme and an ion exchanger for removing a reaction product. The enzymatic reaction follows the Michaelis-Menten kinetics. The ion exchange process is at local equilibrium and can be described by a linear or a convex isotherm. We have examined the case of urea hydrolysis in a reactor-separator of immobilized urease and zeolite. A step input of urea produces a NH+ 4 concentration wave that increases in amplitude as it propagates through the bed. For a convex isotherm, the peak of the wave propagates through the reactor at an increasing speed. At high reaction rates, a shock (discontinuity of concentration) can develop al t he peak. After the shock formation, the speed of propagation then decreases to almost a constant value. Both the ion exchange parameters and the kinetic parameters affect the shape, the amplitude, and the speed of the wave. A slow speed of propagation and a low leakage of NH+ 4 in the effluent are favored by high ion exchange capacity and selectivity for NH+ 4 high reaction rate, slow flow rate, and reaction kinetics approaching first-order. For a system with a linear ion exchange isotherm, the peak of the NH+ 4 concentration wave propagates through the bed at the same speed as that for an ion exchange bed without reaction. The speed is only affected by the ion exchange parameters. The reaction kinetic parameters only affect the amplitude and the shape of the concentration wave. For systems with a linear ion exchange isotherm, shock does not occur under any reaction conditions.

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