Continuous stirred reactor parallel reactions

A water-cooled, continuous stirred-tank reactor is used to carry out the following exothermic parallel reactions... 

Both batch and continuous stirred tank reactors are suitable for reactions that exhibit pseudo-zero-order kinetics with respect to the substrate concentration. In other words, under operating conditions the rate is more or less independent of the concentration of the substrate. However, for reactions where pseudo-first-order kinetics with respect to the concentrations of the substrates prevail, a batch tank reactor is preferred. Batch tank reactors are also ideally suited when there is a likelihood of the reactant slowly deactivating the catalyst or if there is a possibility of side product formation through a parallel reaction pathway. 

A typical chemical plant flowsheet has a mixture of multiple units connected both in series and in parallel. As noted in the previous chapter, the common topology consists of reaction sections and separation sections. Streams of fresh reactants enter the plant by being fed into the reaction section (or sometimes into the separation section) through a heat exchanger network. Here the chemical transformations occur to produce the desired species in one or more of a potentially wide array of reactor types continuous stirred tank, tubular, packed bed, fluidized bed, sparged, slurry, trickle bed, etc. 

Most of the step combinations examined in this chapter are covered in advanced texts on kinetics and reaction engineering [G1-G10], with the exception of coupled parallel steps and reactions with fast pre-dissociation (Sections 5.3 and 5.6, respectively) and the equations for continuous stirred-tank reactors. The material is reviewed here for the user s convenience and for ease of reference. 

In reactions with parallel steps of different reaction orders or with sequential steps, selectivities depend on the reactor type. Batch and plug-flow tubular reactors give higher selectivities to the product formed by the parallel step of higher order, or to the first product in a step sequence, than do continuous stirred-tank reactors. 

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. 

In the previous examples the membranes have been considered generally as semiperineable barriers for the separation of small molecules from bigger ones. When in parallel to the separation a chemical reaction takes place in the bulk solution or in the membrane itself, the system may be identified as a true membrane reactor. A classical example is a stirred-tank enzymatic reactor connected by a continuous recirculation loop to an ultrafiltration or dialysis unit. Such a system, when well designed, permits the continuous removal of the reaction products from the bulk solution without loss of enzyme (or the insoluble or macromolecular substrate ). 

Pfeil and coworkers presented a model for the synthetic pathway of formose, shown in Scheme 5. A similar, but more detailed, model was given by Mizuno and coworkers, who investigated the intermediates in the reaction by chromatographic fractionation of alditol acetate derivatives by g.l.c. (see Table IV). Weiss and coworkers conceptualized the formose reaction as a consecutive-parallel scheme (see Scheme 6) proceeding to the C level, and reported a series of experiments in the continuously stirred tank-reactor previously mentioned to determine the effect of various concentrations of formaldehyde and calcium hydroxide on the reaction rate. The advantage of the tank reactor is that conversions in the autocatalytic system can be controlled, and reaction rates can be measured directly. When the formaldehyde feed-rate was kept constant, and the feed rate for calcium hydroxide varied, products were obtained... 

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