CSTR’s in series

Excessive backmixing can be very difficult to prevent at low reactant viscosities, so an LFR may not be practicable where dissolved polymer solids are low, either due to low conversions or high solvent levels. Under these conditions, LFR behavior can be approached by incorporating a sufficient number of CSTR s in series. 

The patent suggests that the first reaction zone can be replicated by three CSTR s in series with the first operating at 100°C and about 35% solids, the second at 115-120°C and 65% solids, and the third at 140°C and about 85% solids. This... 

CSTR Designs and Use. A patent granted to Mitsui Toatsu Chemicals, Inc. (32 ) describes a styrene polymerization process involving 3 to 5 CSTR s in series. 

It is readily apparent that equation 8.3.21 reduces to the basic design equation (equation 8.3.7) when steady-state conditions prevail. Under the presumptions that CA in undergoes a step change at time zero and that the system is isothermal, equation 8.3.21 has been solved for various reaction rate expressions. In the case of first-order reactions, solutions are available for both multiple identical CSTR s in series and individual CSTR s (12). In the case of a first-order irreversible reaction in a single CSTR, equation 8.3.21 becomes... 

In order to reduce the disparities in volume or space time requirements between an individual CSTR and a plug flow reactor, batteries or cascades of stirred tank reactors ard employed. These reactor networks consist of a number of stirred tank reactors confiected in series with the effluent from one reactor serving as the input to the next. Although the concentration is uniform within any one reactor, there is a progressive decrease in reactant concentration as ohe moves from the initial tank to the final tank in the cascade. In effect one has stepwise variations in composition as he moves from onfe CSTR to another. Figure 8.9 illustrates the stepwise variations typical of reactor cascades for different numbers of CSTR s in series. In the general nonisothermal case one will also en-... 

Maximization of rectangles applied to find the optimum intermediate conversion and optimum sizes of two CSTR s in series. (Adapted from... 

The total volume of the cascade is then 3 (1.60) or 4.8 m3, which is again a significant reduction in the total volume requirement but not nearly as great as that brought about in going from one to two CSTR s in series. 

If two identical ideal CSTR s in series are employed (each with a volume equal to that determined in part 2), by how large a factor can the flow rate of the feedstream be increased while maintaining the conversion level constant at the value used in parts 1 and 2 ... 

If one employs these same two ideal CSTR s in series and maintains a constant feed rate, what conversion is achieved ... 

The values of the group kCBOz that correspond to 95% conversion and one or two CSTR s in series may be determined from Figure 8.16. They are approximately 350 and 70, respectively. Thus,... 

Analysis of CSTR Cascades under Nonsteady-State Conditions. In Section 8.3.1.4 the equations relevant to the analysis of the transient behavior of an individual CSTR were developed and discussed. It is relatively simple to extend the most general of these relations to the case of multiple CSTR s in series. For example, equations 8.3.15 to 8.3.21 may all be applied to any individual reactor in the cascade of stirred tank reactors, and these relations may be used to analyze the cascade in stepwise fashion. The difference in the analysis for the cascade, however, arises from the fact that more of the terms in the basic relations are likely to be time variant when applied to reactors beyond the first. For example, even though the feed to the first reactor may be time invariant during a period of nonsteady-state behavior in the cascade, the feed to the second reactor will vary with time as the first reactor strives to reach its steady-state condition. Similar considerations apply further downstream. However, since there is no effect of variations downstream on the performance of upstream CSTR s, one may start at the reactor where the disturbance is introduced and work downstream from that point. In our generalized notation, equation 8.3.20 becomes... 

If we were to extend the analysis developed in equations 8.3.22 to 8.3.25 to the case of just two CSTR s in series, the equation we would have to solve to determine the composition of the effluent from the second reactor would be the following form of equation 8.3.61. 

Kem Engineer has been asked to scale up an existing process to obtain an increased production capacity for compound B. At present the process is carried out in two CSTR s in series. The reaction involved has the following stoichiometry. 

A combination of two identical CSTR s in series is to be used, to prepare a mixture of polysulfonated aromatic compounds. The reaction will occur isothermally in the liquid phase and may be represented as... 

An exothermic reaction with the stoichiometry A 2B takes place in organic solution. It is to be carried out in a cascade of two CSTR s in series. In order to equalize the heat load on each of the reactors it will be necessary to operate them at different temperatures. The reaction rates in each reactor will be the same, however. In order to minimize solvent losses by evaporation it will be necessary to operate the second reactor at 120 °C where the reaction rate constant is equal to 1.5 m3/kmole-ksec. If the effluent from the second reactor corresponds to 90% conversion and if the molal feed rate to the cascade is equal to 28 moles/ksec when the feed concentration is equal to 1.0 kmole/m3, how large must the reactors be If the activation energy for the reaction is 84 kJ/mole, at what temperature should the first reactor be operated ... 

A V - W where V is the desired product. These liquid phase reactions are to be carried out in a cascade of two equal volume CSTR s in series. If the reactors are to be sized so as to maximize the concentration of species V in the effluent from the second reactor, determine the reactor volumes necessary to process 500 gal/hr of feed containing 6 moles/gal of species A. No V or W is present in the feed. What fraction of the A ends up as V The rate constants kx and k2 are both equal to 0.5 hr - L... 

Use the F(t) curve for two identical CSTR s in series and the segregated flow model to predict the conversion achieved for a first-order reaction with k = 0.4 ksec-1. The space time for an individual reactor is 0.9 ksec. Check your results using an analysis for two CSTR s in series. 

Using our measured rate data and equilibria from Pressman and Lucas, the estimated reactor residence times for 85% conversion with 1 M H2SO4 at 105°C are 14 hours for a batch or plug flow reactor, 85 hours for a single completely stirred tank reactor (CSTR), or 33 hours for two CSTR s in series. If the reaction was carried out at the scrubber site, no additional purification should be required, but there would be a makeup requirement for sulfuric acid. 

A "cascade" comprises a number of CSTR s in series in the idealized model perfect mixing is assum for each reactor. 

It follows from calculations in the proceeding section that the necessa reactor volume of a continuous stirred tank reactor (CSTR) needed to obtain a high degree of conversion is relatively large. A so-called "cascade of CSTR s (a number of CSTR s in series) can be a practical alternative. Let us assume that we replace one CSTR with volume V by a series of n equal CSTR s that have the same total volume. The mean residence time in each reactor is then x/n. We can calculate the relative degree of conversion in each consecutive reactor, for any reaction order, with eq. (3.49), where X is replaced by x/n. We find then for... 

Assume that a certain first order reaction (constant density) requires a mean residence time in one CSTR of 3 hours (10,800 s) to reach a degree of conversion of 0.99. If we would use three CSTR s in series, we find from equations (3.38) and (3.51) that the total mean residence time of the cascade to reach 0.99 conversion is only 1193 s, or approximately 20 min. This means that the total reactor volume needed to obtain the desired conversion is in this case 9 times smaller This ratio increases as the desired degree of conversion is higher. 

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