Parallel reactions CSTRs

Figure 5.4-69. Parallel reactions of different reaction order I SBR or PFR, 2 CSTR.

Parallel reactions, oai = om2, a i = am = 0, Ei > E2. The. selectivity to the desired product increases with temperature. The highest allowable temperature and the highest reactant concentrations should be applied. A batch reactor, a tubular reactor, or a cascade of CSTRs is the best choice. 

Figure 4-8 Ca, Cb, andCc versus (fci+/t2)t for parallel reactions AB, A C in a CSTR. Plots shown are for 2/ = 0-1/ 1.0, and 10. The selectivity to B and yield of B are independent of conversion for equal-order kinetics. The selectivities and yields are the same for both PFTR and CSTR, but the CSTR requires a longer r for a given conversion.

Example 4-4 Calculate the selectivities of forming B in CSTR and PFTR reactors at 90% conversion of A starting with Cao = 4 moles/hter in the parallel reactions... 

For parallel reactions both reactor types give equal selectivities if the orders of both reactions are the same, and the CSTR can be preferred even though its X would be longer because it may be less expensive. The CSTR is preferred for parallel reactions whenever the desired reaction has a lower order than the undesired reaction, while the PFTR is preferred (for both yield and r) whenever the desired reaction has a higher order. 

The following parallel reactions take place in a CSTR ... 

For the two parallel reactions described in Example 8-11, the CSTR energy balance is... 

In biocatalysis, selectivity is also of great importance. However, in biocatalysis competition often takes place between parallel reactions A —> P and A —> Q (which is also important in homogeneous enantioselective catalysis), or A P and B Q (kinetic resolution, A and B are enantiomers and so are P and Q). With the latter type of competition, the selectivity in a CSTR is reduced as compared to a PFR, just as in the consecutive reactions of the example. On the other hand, for the parallel reactions of A, the selectivity in a CSTR is the same as in a PFR. 

A series parallel reaction takes place in a CSTR (Bequette, 1998).[12] The governing equations are ... 

Batch, semibatch, and continuous stirred tank reactors residence time 600 to 15,000 s (10 min to 4 h) heat of reaction primarily exothermic reaction rate slow to moderate. High-pressure autoclaves <100 L. Unique to semibatch phases liquid, gas-liquid, liquid-liquid, gas-liquid catalytic solid. Use where a batch operation is appropriate (Section 16.11.6.24), but one reactant (e.g., gas) needs to be added continuously or if the initial reaction rate is very high. Selectivity is best for parallel reactions. For more details, see CSTR, Section 16.11.6.26. 

Example H-5 Piwluetion of Ac etic Anhydride Example 8-9 CSTR with CooUna Coil Example H-IO Parallel Reaction in a PER with Heat Effects Example H-H Multiple Reactions in a CSTR... 

P8-28- This problem concerns series reaction with heat effects in a CSTR. The instruction could extend the problem statement in tlie text to ask the students to carry out a parameter sensitivity. E.g. increase/decrease/ j and 2- This problem could be alternated with P8-31, which involves parallel reactions. 

PS-31. Parallel reactions with heal effects in a CSTR. Quite straight forward using the POLYMATH non-linear equation solver. 

Butadiene is the main reactant in the system of parallel reactions, in which the dimer (R) is the main product. The CSTR yield is given by Equation 16.34 ... 

A simple model for a pair of exothermic parallel reactions, A - C, in a CSTR containing a cooling coil might be written as... 

Fig. 3. Consecutive-parallel reactions in CSTR / Separator / Recycle a) Control structure b) Bifurcation diagram showing multiple steady states.

Undesirable parallel reactions can thus force us to select a CSTR, although the low production capacity requires a considerably larger reactor volume than in the case of a PFR or a BR. 

Mr. P (MSc) suggests that a CSTR should be selected, since backmixing in parallel reactions always favors the reaction of the lowest order, that is, A B. Mr. Q (MSc), however, claims that when dealing with reactions of the type A B C, one should always select a PFR, because a PFR favors the formation of the intermediate product, B. Mr. Q backs up his argument with the tables below, which illustrate cb/cqa as a function of the conversion level of A. The tables (calculated by Mr. Q) are, however, valid for PFRs only. To resolve the conflict, the boss Mr. H (PhD) delegated to Miss S (a student) the task of finding out the possibilities for utilizing a CSTR. Let us assume that you are Miss S ... 

Cyclic Runs. Having established the inlet ratio of H2 Buta-diene for which the steady-state yield is maximised for a given residence time, the cyclic runs were carried out such that the mean value of the feed compositions were as near as possible to their optimum steady-state values. The mole fractions of H2 and Butadiene were cycled out of phase in a symmetrical square wave fashion. Such symmetrical wave forms need not be the optimum periodic operation. Indeed, Farhad Pour et al. (7 ) demonstrated theoretically that it was possible to obtain further improvement in the selectivity of series-parallel reactions in a CSTR when asymmetrical rather than symmetrical square waves are considered. Theoretically the search for the optimum wave modulation, or the number of switches over one cycle time, can be computed by search methods or optimisation routines. However, in this work we arbitrarily limited ourselves to symmetrical square waves which are 180 out of phase accepting that such a configuration may, indeed, be quite far from the optimum periodic mode. 

The reapportionment of residence time in r u ial flow is analogous to variable volume CSTRs in series. As such, radial flow would be expected to affect product selectivity in series and parallel reactions. For two parallel first order, exothermic reactions, the particle equations [3, 4] become... 

Suppose there are two parallel, first-order reactions in a steady-state CSTR. Show that neither the fed-batch nor fast-fill-and-hold strategies can achieve a bumpless startup if the reactions have different rate... 

Illustration 9.5 indicates that one may have parallel paths leading from reactants to products and that in the case of an autocatalytic reaction, one path may be preferred over a second until the product level builds up to a point where the second becomes appreciable. In this example, the magnitudes of the rate constants are such that the vast majority of the reaction occurs by the autocatalytic path. In cases such as these it is desirable to use a CSTR or recycle reactor to enhance the reaction rate by virtue of the back-mixing of product species. 

A liquid-phase reaction takes place in two CSTRs operating (at steady-state) in parallel at... 

Consider two CSTRs of unequal sizes, Vx and V2, operating in parallel, as shown in Figure 17.2. The liquid-phase reaction, A - products, is first-order and both tanks operate at the same T. How should the total feed rate, q0, be split (at S) so as to maximize the rate of production in the combined exit streams (at M) ... 

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