Effect of Micromixing on Conversion

Fig ure 9-8. Conversion for a zero order reaetion in a CSTR and a PFT. (Source Wen, C. Y. and L. T. Fan, Models for Flow Systems and Chemioal Reaetors, Marcel Dekker Inc., 1975.) 

Their studies also showed the importanee of the reaeting system kineties for better design operation of CSTR reaetors. 

Consider the data of Hull and von Ronsenberg in Example 8-3 for mixing in a fluidized bed. Suppose the solids in the fluidized bed were not aeting as a eatalyst, but were aetually reaeting aeeording to a first order rate law (-r) = kC, k = 1.2 min Compare the aetual eonversion with that of an ideal plug flow. 

The flow through a reaetor is 10 dm /min. A pulse test gave the following eoneentration measurements at the outlet. 

Industrial reactors generally operate adiabatically. Cholette and Blanchet [8] compared adiabatic plug flow reactor to the CSTRmm. For exothermic reactions, they inferred that the performance of a CSTRmm is better than that of a plug flow reactor at low values of conversion, and vice-versa at high values of conversion. They further showed that the design considerations for endothermic reactions are similar to those for isothermal reactions. 

If the reactor is modeled as a tank-in-series (TIS) system, how many tanks are needed to represent this reactor  

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The tank-in-series (TIS) and the dispersion plug flow (DPF) models can be adopted as reactor models once their parameters (e.g., N, Del and NPe) are known. However, these are macromixing models, which are unable to account for non-ideal mixing behavior at the microscopic level. This chapter reviews two micromixing models for evaluating the performance of a reactor— the segregrated flow model and the maximum mixedness model—and considers the effect of micromixing on conversion. 

Compare the conversion obtained for the second-order reaction 2A —> C + D in a single CSTR with that determined from the segregated fiow result of equation (5-3). The parameter ktC = 0.079 liter/g mol. Is this result consistent with the postulated effects of micromixing on reactions of order > 1 ... 

Serra and coworkers studied the outstanding effect of mixing on conversion, molecular weight and polydispersity in free-radical polymerizations of styrene by a numerical simulation using different micromixer geometries [170, 171]. 

The results of Examples 6-6 and 6-7 are for one case, but they are representative of the situation for many reactors. We saw in Secs. 6-7 and 6-8 that extremes of RTD can have large effects on the conversion, particularly at high conversion levels. However, with relatively simple models to estimate the RTD, little error need be involved. Put differently, if an engineer were to use an ideal stirred-tank reactor to simulate a nearly ideal tubular-flow unit, the pr cted conversion would be seriously in error. However, if the measured RTD or a reasonable model were employed, the result would be approximately correct. The residual error will be due to uncertainty in the extent of micromixing. 

Let us consider the effect of macromixing and micromixing on the conversion in CSTR and PFR systems. 

Suppose 0i A is a function of a alone and that neither dS Alda nor d St-Alde change sign over the range of concentrations encountered in the reactor. Then, for a system having a fixed residence time distribution. Equations (15.48) and (15.49) provide absolute bounds on the conversion of component A, the conversion in a real system necessarily falling within the bounds. If d Alda > 0, conversion is maximized by maximum mixedness and minimized by complete segregation. If cP-s A/da < 0, the converse is true. If (P A/da = 0, micromixing has no effect on conversion. 

Second-order reactions provide the simplest example of nonlinear kinetics, where micromixing limitations have significant effects on reactant conversion. We use the two-mode model to determine the same for a typical bimolecular second-order reaction of the type... 

It follows from this model that an increased mean residence time must have a significant effect on the degree of conversion. In order to realize good micromixing, one should simply create mixing conditions such that within the mean residence time of the liquid in the reactor, the striations of the reactant streams become sufficiently thin. They should preferably become so thin that the diffusion time is shorter than the reaction time. 

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