Yield CSTR, ideal

The CRE approach for modeling chemical reactors is based on mole and energy balances, chemical rate laws, and idealized flow models.2 The latter are usually constructed (Wen and Fan 1975) using some combination of plug-flow reactors (PFRs) and continuous-stirred-tank reactors (CSTRs). (We review both types of reactors below.) The CRE approach thus avoids solving a detailed flow model based on the momentum balance equation. However, this simplification comes at the cost of introducing unknown model parameters to describe the flow rates between various sub-regions inside the reactor. The choice of a particular model is far from unique,3 but can result in very different predictions for product yields with complex chemistry. 

In an ideal CSTR the concentration of any substance in the effluent stream is identical to the concentration throughout the reactor. Consequently, it is possible to obtain the RTD from conceptual considerations in a fairly straightforward manner. A material balance on an inert tracer that has been injected as a pulse at time t = 0 into a CSTR yields for f > 0... 

In addition to the one-parameter models of tanks-in-series and dispersion, many other one-parameter models exist when a combination of ideal reactors is to model the real reactor. For example, if the real reactor were modeled as a PFR and CSTR in series, the parameter would be the fi action,/, of the total reactor volume that behaves as a CSTR Another one-parameter model would be the fi action of fluid that bypasses the ideal reactor. We can dream up many other situations which would alter the behavior of ideal reactors in a way that adequately describes a real reactor. However, it m be that one parameter is not sufficient to yield an adequate comparison between theoiy... 

In CSTRs the source of non-kinetic influences is often the recirculation rate. In principle this can be varied in a given reactor. All too often, however, the reactor is simply operated at its maximum recirculation rate and it is assumed that this yields ideal CSTR behaviour. Many procedures exist for calculating when diffusion or heat transfer effects are expected to cause distortions. None of these are better than the experimental test, and most are not as good. This is particularly true in the case of pore diffusion in catalysts. However, calculational methods can give an idea of whether trouble of this kind is to be expected, and encourage one to perform the experimental tests. 

Conversion of reactant for single, ideal CSTR, and as a function of internal flowrate In a 2-CSTR mixing model. . Yield of desired product C for single, ideal CSTR, and as a function of internal flowrate, p = QrIQ.z, in a 2-CSTR... 

Overall the analysis here should convey the message that generalizations concerning selectivity or yield performance in nonideal reactors with reference to an ideal model are slippery conversion, however, is perhaps somewhat more predictable. We may normally expect modest taxes on conversion as the result of nonideal exit-age distributions if the reaction system involves selectivity/yield functions these will also be influenced by the exit-age distribution, but the direction is not certain. Normally nonideality is reflected in a decrease in yield and selectivity, but there are possible interactions between the reactor exit-age distribution and the reaction kinetic parameters that can force the deviation in the opposite direction. Keep in mind that the comparisons being offered here are not analogous to those for PFR-CSTR Type III selectivities given in Chapter 4, which were based on the premise of equal conversion in the two reactor types. 

Thus, for Michaelis-Menten kinetics, a PFR type reactor, predominantly a packed-bed reactor (PBR, Figure 9.1b) is preferred to the continuous stirred-tank reactor (CSTR, Figure 9.1a), since it requires less biocatalyst to reach the same level of conversion. In this case, ideal reactors are those with high space time/yield to increase the efficiency of the transformation. PBRs with immobilized catalyst have a clear advantage in that voidage is low 34% compared to over 80-90% for CSTR [35]. However, if pH control is required, the use of a PFR is not advised. In case of substrate inhibition, a CSTR (Figure 9.1a) operated at high conversion is to be preferred. On the other hand, when product inhibition is pronounced, a... 

The continuous production of aliphatic chiral alcohols was demonstrated by Leuchs et al. as shown in Scheme 6.20 by using continuous stirred tank reactor (CSTR). The CRED from Lactobacillus brevis was used in a biphasic system with MTBE as cosolvent to reduce aliphatic ketones with the general structure 53. It was shown that inaeasing the chain length resulted in a decreased yield due to less available substrate in the aqueous phase. The continuous process was run with a ketone concentration of 100 mmol/L and IPA was used as the reductant (1 mol/L). It was also demonstrated that 200 rpm was ideal for longevity of the enzyme. Doubling the residence time did not lead to a sigttificant increase in the space-time yield [30]. 

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