Chemical Reaction in a Stirred Tank

The blending of chemical reactants is a common operation in the chemical process industries. When a competitive side reaction is present, the final product distribution is often unknown until the reactor is built. This is partly because the effects of the position of the feed stream on the reaction by-products are difficult to predict. In this example from Bakker and Fasano (1993), the product distribution for a pair of competing chemical reactions is calculated with CFD and compared with experimental data from the literature. The model used here is a slightly modified version of the standard Magnussen model discussed in Section 5-2.2.1. 

The following competitive-consecutive reaction system was studied  

This is the reaction system used by Bourne et al. (1981) and Middleton et al. (1986). The first reaction is much faster than the second reaction Ki = 7300 m / mol s versus K2 = 3.5 m /mol s. The experimental data published by Middle-ton et al. were used to determine the Magnussen model constants. Two reactors were studied, a 30 L reactor equipped with a D/T = 5 Rushton turbine and a 600 L reactor with a D/T = Rushton turbine. In the CFD analysis, a converged flow field was computed first for each reactor, using experimental data for the impeller boundary conditions. The reactants A and B were then introduced to the tank on an equimolar basis. The reactant A was assigned a weak but uniform concentration throughout the vessel. The reactant B was added in a high concentration in a small region. The calculation of the flow fleld variables was disabled after the addition of the reactants, and the species calculations alone 

In the reaction model used here it was assumed that small scale mixing affected only the first reaction and that once this reaction had occurred, the species were locally well mixed. As a result, small scale turbulent mixing did not affect the second reaction. This was achieved by using different values of the Magnussen model constants for the two reactions. 

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