Consecutive reactions batch

For consecutive reactions batch and plug flow reactors will always give a higher selectvity than well mixed continuous reactors. For competitive-consecutive reactions the best selectivity can be found in semi-batch reactors. However, this type of reactions can in some cases be carried out with an acceptable selectivity in a continuous well mixed reactor, when sufficient excess of the "other reactant (that does not cause undesired reactions) is applied. 

Selectivity A significant respect in which CSTRs may differ from batch (or PFR) reaclors is in the product distribution of complex reactions. However, each particular set of reactions must be treated individually to find the superiority. For the consecutive reactions A B C, Fig. 7-5b shows that a higher peak value of B is reached in batch reactors than in CSTRs as the number of stages increases the batch performance is approached. 

For the consecutive reactions A B C, a higher yield of intermediate B is obtained in batch reac tors or PFRs than in CSTRs. 

At a fixed temperature, a single, reversible reaction has no interior optimum with respect to reaction time. If the inlet product concentration is less than the equilibrium concentration, a very large flow reactor or a very long batch reaction is best since it will give a close approach to equilibrium. If the inlet product concentration is above the equilibrium concentration, no reaction is desired so the optimal time is zero. In contrast, there will always be an interior optimum with respect to reaction time at a fixed temperature when an intermediate product in a set of consecutive reactions is desired. (Ignore the trivial exception where the feed concentration of the desired product is already so high that any reaction would lower it.) For the normal case of bin i , a very small reactor forms no B and a very large reactor destroys whatever B is formed. Thus, there will be an interior optimum with respect to reaction time. 

Example 7.5 Suppose the consecutive reactions 2A B C are elementary. Determine the rate constants from the following experimental data obtained with an isothermal, constant-volume batch reactor ... 

Figure 2.6. The hydrogenation of substituted nitro arenes to amines in a batch reactor involves two consecutive reactions when a standard platinum catalyst is used.

Cybulski (1990) simulated the behaviour of a batch (BSTR) and semibatch (SBSTR) reactor in which consecutive reactions take place ... 

Consecutive reactions, isothermal reactor cmi < cw2, otai = asi = 0. The course of reaction is shown in Fig. 5.4-71. Regardless the mode of operation, the final product after infinite time is always the undesired product S. Maximum yields of the desired product exist for non-complete conversion. A batch reactor or a plug-flow reactor performs better than a CSTR Ysbr.wux = 0.63, Ycstriiuix = 0.445 for kt/ki = 4). If continuous operation and intense mixing are needed (e.g. because a large inteifacial surface area or a high rate of heat transfer are required) a cascade of CSTRs is recommended. 

Let us consider a batch reactor where the following consecutive reactions take place (Smith, 1981)... 

It should be noted that there are cases in which some selectivity will be lost in choosing a semi-batch mode over a simple batch reactor. If the desired product decomposes by a consecutive reaction, the yield will be higher in the batch reactor [177]. If, on the other hand, the reactants are producing by-products by a parallel reaction, the semi-batch process will give the higher yield. In any case, if the heat production rate per unit mass is very high, the reaction can then be run safely under control only in a semi-batch reactor. 

Let us consider the batch reactor modeled in Sec. 3.9 (Fig. 3.9). Steam is initially fed into the jacket to heat up the system to temperatures at which the consecutive reactions begin. Then cooling water must be used in the jacket to remove the exothermic heats of the reactions. 

For the consecutive reactions, A B C, the specific rates are equal and B0 = 0, Find the maximum value of B/Aq in (a) Batch reactor (b) Two stage CSTR. 

If consecutive reactions are conducted in a batch reactor, the optimization of the process includes finding the optimum time to stop the batch and determining the optimum temperature. To illustrate the issues, we take the simple reactions... 

Figure 4.44 Fed-batch reactor with consecutive reactions F ramped from 2 to 1 L/min.

Figure 4.57 Consecutive reactions effect of batch time on conversion and selectivity F0 = 2 L/min.

Consider the consecutive reactions, A -A B -A C, with rate constants of kj = 1015exp(— 10,000/r) and kn = 10s exp(—5000/T). Find the temperature that maximizes bout for a CSTR with 1=2 and for a batch reactor with a reaction time of 2 h. Assume constant density with = cin = 0. 

Overalkylation can lead to tertiary alcohol formation by consecutive reaction [29]. Product quality demands to keep this impurity level <0.2%. Microreactor operation yielded the overalkylated alcohol follow-up product at 0.18%, whereas level of impurity for the batch process was 1.56% [29]. The reason is probably the lower back-mixing in the microflow system, with concentration profiles being less deteriorated from ideal that is, no excess of alkylating agent is generated locally to promote the follow-up reaction. 

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