PERFORMANCE IMPROVEMENT OF A CHEMICAL REACTOR BY NATURAL OSCILLATION

Abstract

The thesis describes a novel operation strategy for improvement in the performance of (a) wastewater treatment by activated sludge process and (b) ethanol production from glucose. The dynamic behaviour of two coupled continuous stirred-tank reactors with recycle is studied when one of the reactor is being operated under limit cycle regimes producing self-sustained natural oscillations. The periodic output from one reactor is then used as forced input into the other reactor. For activated sludge system it has been observed and demonstrated through computation that the overall performance of the activated sludge system can be enhanced in terms of time-averaged conversion by employing the above mentioned operation strategy for some particular combination of reactor volume and recycle ratio. For the Activated Sludge system, the best improvement occurs when sewage bacteria concentration is around 30 mg/l, and the total dilution rate was around 0.17 h-1• When the recycle ratio was 0.35, the substrate concentration in effluent decreased from 2.05 mg/I to 1.95 mg/l. For this system, however, the improvement achieved was not very much. For ethanol production system, it was found that using the above-mentioned procedure the performance of the system can also be improved. The improvement obtained was much greater compared to the activated sludge system for some combination of operating parameters. For the case without recycle, it has been found that significant improvement occurred when total dilution rate was 0.03 h-1• The product concentration increased from 59.9 g/l to 87.8 g/l when larger reactor was placed first in the sequence while keeping the total reactor volume constant and operating the overall system under oscillatory state instead of steady state. For the case with recycle, the most significant improvement occurred when the total dilution rate was 0.03 h-1 initial glucose concentration was 180 g/l and the concentration factor was 1.4. The product concentration increased from 89.8 g/l to 93.6 g/l. For this system, the operating region was much different than that of the activated sludge system. Unlike the activated sludge system, oscillatory operation was not observed beyond certain dilution rate. The most significant criterion of the operation strategy applied in this work lies in the fact that the performance enhancement of the overall system was achieved through natural oscillation rather than forced oscillation. In experimental studies, and in eventual application to industrial processes this new concept of coupling free and forced oscillation is advantageous, as it does not require any additional external energy.