Simulation of Stirred Reactors with Highly Exothermic Reactions

9 Simulation of Stirred Reactors with Highly Exothermic Reactions 

Of great importance because of the prevalence of semibatch reactors are the effects of temperature rise in semibatch reactors with exothermic chemical reactions. Several semibatch cases with exothermic reaction were simulated by Randick (2000) using Fluent with Pairin. The method used for the simulations was the same as that described in Example 13-12. The simulations frequently required 2 or more days on a fast computer. The reactions were A - - B C (Tri) and A -t- C D (kR2). The conditions for the semibatch simulations were Tri = 36 L/kmol s, kR2 = 3.6 L/kmol s, AHri = 0, AHr2 = —1.0 x 10 J/kmol, Eri = 0, Er2 = 1.0 X 10 to 3.0 X 10 J/kmol, where kR, are rate constants, AHri are heats of reaction, and Eri are activation energies. Eigure 13-39 shows the results of simulations that were done in the semibatch mode. 

This preliminary research shows that prediction of yield on scale-up of a highly exothermic chemical reaction series can be particularly difficult. The heats of reaction and activation energies of each reaction must be well known. The 

It must be emphasized that even though these results seem reasonable and that extensive validation of stirred-tank simulations has been done for various continuous flow and semibatch cases, only isothermal validation has been done, and no large ratio scale-up validations have been done. The feed time of 50 s for a 3.8 m vessel is very short and exaggerates the heat effects. These aspects of the problem need to be studied before complete confidence may be placed on the CFD simulation of highly exothermic reactions. 

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