Analysis and modelling of residence time distribution in a high speed gas reactor

Abstract

Flow patterns and the associated residence time distribution are important factors affecting the performance of a chemical reactor. This dissertation investigates the flow patterns in a scale-model of a gas reactor where opposing inlet jets are used to create mixing. The effects of different vessel geometries and gas flowrates are investigated. The residence time distribution (RTD) was measured by means of radioactive tracer techniques. The extremely short mean re^’dence time in the vessel necessitated the use of sharp tracer pulses and a computerised digital recording system. Flow visualisation experiments were also performed to aio interpretation of the results. The results were analysed using the method of frequency analysis, in which the coefficients of the Fourier Series arc used to characterise the RTD curves. It was found that the shape of the RTD curve is unaffected by change in flowrate; i.e. the flow patterns are independent of flowrate over the range investigated. The geometry of the vessel however plays a major role. The effect of reducing the length/diameter ratio of the reactor is an increase in the mixedness in the vessel. Internal recirculation patterns are well-defined at a higher length/diameter ratio. The RTD curves were also used to set up mathematical flow models. Simple multi-parameter models were used, where the vessel was viewed as consisting of interconnected flow regions such as plug flow, stirred tank, and axial dispersion. Close fits of the mathematical models to the experimental RTD curves were obtained.