Uses of a CSTR

The use of a single-stage CSTR for HF alkylation of hydrocarbons in a special forced-circulation shell-and-tube arrangement (for heat transfer) is illustrated by Perry et al. (1984, p. 21-6). The emulsion copolymerization of styrene and butadiene to form the synthetic rubber SBR is carried out in a multistage CSTR. 

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As we noted earlier, there may be heat transfer considerations or other factors that dictate the use of a CSTR even when the yield considerations are unfavorable. In such cases the yield... 

To determine the form of the rate law, values of (-rA) as a function of cA may be obtained from a series of such experiments operated at various conditions. For a given reactor (V) operated at a given % conditions are changed by varying either cAo or q. For a rate law given by (—rA) = kAcA, the parameter-estimation procedure is die same as that in the differential method for a BR in the use of equation 3.4-2 (linearized form of the rate law) to determine kA and n. The use of a CSTR generates point ( -rA) data directly without the need to differentiate cA data (unlike the differential method with aBR). 

The experimental investigation of the form of the rate law, including determination of the rate constants kf and kr, can be done using various types of reactors and methods, as discussed in Chapters 3 and 4 for a simple system. Use of a batch reactor is illustrated here and in Example 5-4, and use of a CSTR in problem 5-2. 

High-volume products such as styrene-butadiene rubber (SBR) often are produced by continuous emulsion polymerization. This is most often done in a train of 5-15 CSTRs in series. CSTR polymerization will result in a broader PSD than batch polymerization due to the wide residence time distribution in a CSTR, though the use of a CSTR train will tend to mitigate this effect. 

As we noted earlier, there may be temperature con-trol/heat transfer considerations or other factors that dictate the use of a CSTR even when the yield considerations are unfavorable. In such cases the yield of the desired product may be significantly increased by using a battery of CSTRs in series. If desired, one has the additional flexibility granted by using tanks with different volumes or tanks... 

The widest spectrum of dynamic behavior is observed in the CSTR. As we have seen, the use of a CSTR or CSTR train for polymerization reactions may be justified in some cases by kinetic considerations. However, before implementing CSTR polymerization, the engineer should be aware of the unique dynamics associated reactions in a CSTR which are exothermic and/or autocatalytic, or involve nucleation phenomena. 

An example of the use of a CSTR is given below. The kinetics of the reaction A -> P is studied in a CSTR. The space time, t, and possibly the inlet concentration level, Coa> are varied and ca, fA-data are obtained. A rate law is assumed, for example, the reaction rate for component A is ta = —kcj. A plot of the concentration of component A, c, versus —rA gives the rate constant, k, as the slope of the curve, as illustrated in Figure A9.4. 

Consider the use of a CSTR to produce a copolymer of uniform composition. Figure 11.4b. Here,// is the composition of the gross feed (fresh feed-I-recycle) to the reactor and Fj and// are the steady-state compositions in the reactor and its effluent stream. 

D>au If u > otD, we want to keep Ca as low as possible. Following the line of reasoning above, otu > d will favor a dilute feed. It also favors the use of a CSTR rather than a PFR for continuous processes. 

One of the most promising ways of dealing with conversion oscillations is the use of a small-particle latex seed in a feed stream so that particle nucleation does not occur in the CSTRs. Berens (3) used a seed produced in another reactor to achieve stable operation of a continuous PVC reactor. Gonzalez used a continuous tubular pre-reactor to generate the seed for a CSTR producing PMMA latex. 

Use Scalable Heat Transfer. The feed flow rate scales as S and a cold feed stream removes heat from the reaction in direct proportion to the flow rate. If the energy needed to heat the feed from to Tout can absorb the reaction exotherm, the heat balance for the reactor can be scaled indefinitely. Cooling costs may be an issue, but there are large-volume industrial processes that have Tin —40°C and Tout 200°C. Obviously, cold feed to a PFR will not work since the reaction will not start at low temperatures. Injection of cold reactants at intermediate points along the reactor is a possibility. In the limiting case of many injections, this will degrade reactor performance toward that of a CSTR. See Section 3.3 on transpired-wall reactors. 

The performance of the laminar flow reactor is appreciably worse than that of a PFR, but remains better than that of a CSTR (which gives T=0.5 for kt= 1). The computed value of 0.4432 may be useful in validating more complicated codes that include diffusion. 

Solution The reactions are the same as in Example 12.5. The steady-state performance of a CSTR is governed by algebraic equations, but time derivatives can be useful for finding the steady-state solution by the method of false transients. The governing equations are... 

D. Increase in Processing Rate Arising from the Use of a Cascade of Two CSTR s at a Specified Degree of Conversion... 

For unsteady-state operation of a CSTR, the full form of the material balance, equation 14.3-2 or its equivalent, must be used. 

In a chemical process, the use of recycle, that is, the return of a portion of an outlet stream to an inlet to join with fresh feed, may have the following purposes (1) to conserve feedstock when it is not completely converted to desired products, and/or (2) to improve the performance of a piece of equipment such as a reactor. It is the latter purpose that we consider here for a PFR (the former purpose usually involves a separation process downstream from a reactor). For a CSTR, solution of problem 14-26 shows that recycling alone has no effect on its performance, and hence is not used. However, it provides a clue as to the anticipated effect for a PFR. The recycle serves to back-mix the product stream with the feed stream. The effect of backmixing is to make the performance of a PFR become closer to that of a CSTR. The degree of backmixing, and... 

In Table 18.1, values of RcBmax are given for K - 0, K = 1 and For this type of network, with normal kinetics, the performance of a CSTR cannot surpass that of a PFR that is, use of a PFR leads to a greater cB max at a smaller However, if the CSTR is staged, its performance can be closer to that of a PFR. 

The specific rate is k = 5.2 1 iter/gmol-hr at 82 C. Equal molal quantities the reactants are to be used. They are supplied as aqueous solutions, bicarbonate as 15 wt% and the chlorhydrin as 30 wt%. Production of glycol to be 20 kg/hr at 95% conversion. Specific gravity of the feed mixture 1.02. Find the required reactor volumes of a PFR and of a CSTR. 

Prom the results presented in this chapter, it has been shown that the first step in the control problem of a CSTR should be the use of an appropriate mathematical model of the reactor. The analysis of the stability condition at the steady states is a previous consideration to obtain a linearised model for control purposes. The analysis of a CSTR linear model is carried out trough a scaling up reactor s volume in order to investigate the difference between the reactor and jacket equilibrium temperatures as the volume of the reactor changes from small to high value. 

In this section an innovating approach based on the use of a battery of interval observers functioning in parallel is presented. Such an approach allows us to detect a violation of the assumptions related to the unknown inputs (substrates concentrations at the input of the process). In order to illustrate the principle of this approach, consider again a mono-biomass, mono-substrate bioprocess within a CSTR, described by equations (3) to which the dynamics of one of the products, represented by P, has been added ... 

In addition to using different catalyst flow patterns in packed and slurry reactors, the flow can be varied to attain different catalyst contacting patterns. As shown in Figure 7-27, many flow patterns such as radial flow and fluid recirculation can be used. These allow variation of the flow velocity u for a given reactor size and residence time x. These recirculation flow patterns approach the flow of recycle reactors so the reactor performance approaches that of a CSTR at high recirculation. 

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