Optimal Design of Chemostats Connected in Series with Microbial Wall Growth

I. M. Abu-Reesh

Abstract
The optimum design of chemostats, or continuous stirred tank reactors (CSTRs) connected in series with microbial wall growth is described. The optimum design of chemostats in series is based on the minimum overall reactor volume required to achieve a certain degree of substrate conversion. Topiwala –Hamer model for wall attachment is assumed and cell growth is described by Monod equation. Realistic values of cell concentration at the reactor wall are considered. The effect of kinetic and operating parameters namely, Monod constant, inlet substrate mass concentration, cell concentration at the reactor wall, substrate conversion and number of chemostats on the optimum design were determined. The effect of wall attachment on the total optimum dimensionless residence time is more pronounced at low inlet substrate concentration. The performance of the optimum design of a series of chemostats was compared with plug flow reactor and with the more convenient design of equal size chemostats required to achieve the same inlet and outlet conditions. The degree of benefit from using chemostats in series rather than one chemostat depends on the kinetic and operating parameters. The degree reduction (%) in the total volume using the optimum design compared to equal-size chemostats, can be up to 70 % depending on the kinetic and operating parameters. Up to five chemostats connected in series were described in this study.

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Keywords
Wall growth, Monod kinetics, chemostats, CSTRs in series, PFR, optimisation