Reaction characteristics from the transient response of reactors

In this work, a method of utilising the transient response characteristics of tank flow reactors in kinetic studies is presented. By following those changes in the effluent concentration which result from a step change in the inlet concentration of reactants, it is possible to determine whether the reaction is first, second, or some other order. Rate constants can also be found if the reaction is first or second order. For a reactor with a first order reaction, the differential equation describing the rate of change of the effluent concentration of a reactant was shown to be: [formula could not be replicated]. For a reactor with a second order decomposition reaction, or a second order reaction with equal concentrations of reactants, the corresponding differential equation was shown to be: [formula could not be replicated]. These solutions were verified by comparison with similar equations found in the literature. The first case shows a linear response, while in the second case, the response is non-linear, and depends on the inlet concentration. Thus if the system shows a linear response, then the reaction must be first order. For this case, the rate constant can be found by comparing the results with those which would be obtained if there were no reaction. If the reaction is not first order, a computer programme using iteration methods can be employed to obtain values of the rate constant at various times during the step change. If these values are constant, or show random scatter, then the reaction is second order, and the rate constant has the value found. If the "rate constant" shows consistent trends, the reaction is not second order. The practical feasibility of this method was tested by building a tank flow reactor, flow system and effluent monitoring system, and applying the method to a well-studied second order reaction (the alkaline hydrolysis of ethyl acetate). The euqipment was first tested to ensure its behaviour approximated closely to the assumptions made in deriving the differential equations. Then step changes in inlet reactant concentrations were imposed on the reactor, which was initially at steady state. An analysis of the response curves was made in accordance with the theory, and this analysis indicated that the reaction indicated that the reaction was not strictly second order in the range of concentrations used. The "rate constant" was found to vary systematically from about 0.052 litre/mole-sec at inlet concentrations of 0.35 mole/litre, to about 0.075 at 0.08 mole/litre. An investigation into the possible causes of this behaviour indicated that it probably was due to the reaction not being second order, and not a result of faults in the equipment or method. On the basis of the theoretical development and the experimental work, it was concluded that this method of kinetic analysis was valid, and was a significant addition to the widely accepted steady state tank flow reactor technique.