CSTR train

Figure 11.7 PFR-CSTR train sized to produce 400000kg/year of celecoxib (Basis Operating 350 days/year).

As an alternative to a cascade of CSTR trains, a novel continuous reactor with a Couette-Taylor vortex flow (CTVF) has been proposed, which can realize any flow pattern between plug and perfectly mixed flows [361-366]. A continuous Couette-Taylor vortex flow reactor (CCTVFR) consists of two concentric cylinders with the inner cylinder rotating and with the outer cylinder at rest. Figure 29 shows a typical flow pattern caused by the rotation of the inner cylinder. 

Macro- and miniemulsion polymerization in a PFR/CSTR train was modeled by Samer and Schork [64]. Since particle nucleation and growth are coupled for macroemulsion polymerization in a CSTR, the number of particles formed in a CSTR only is a fraction of the number of particles generated in a batch reactor. For this reason, their results showed that a PFR upstream of a CSTR has a dramatic effect on the number of particles and the rate of polymerization in the CSTR. In fact, the CSTR was found to produce only 20% of the number of particles generated in a PFR/CSTR train with the same total residence time as the CSTR alone. By contrast, since miniemulsions are dominated by droplet nucleation, the use of a PFR prereactor had a negligible effect on the rate of polymerization in the CSTR. The number of particles generated in the CSTR was 100% of the number of particles generated in a PFR/CSTR train with the same total residence time as the CSTR alone. 

P4-29, (CSTR train) The elementary liqiud-phase reaction... 

Another reason for using different reactor sizes along the CSTR train is the variation of polymerization rate with monomer conversion. This factor is not a major consideration if the final conversion is modest as in the case of styrene-butadiene rubber (SBR) processes. Normal exit conversions are 55 to 65% in such systems, and the residual monomer is recovered and recycled. If a very high conversion is desired one must deal with the problem that the polymerization rate is low at high conversions. The final reactor in the series needs to be very large if the desired conversion approaches 100%. Likewise, batch reaction cycle times become large if high conversions are desired. 

Continuous reactors comprised of a CSTR train are often operated with a single feed location in the first reactor. The use of intermediate feed locations can be advantageous for several reasons. First, if the conversion in the first reactor is low or... 

MULTIPLE CHEMICAL REACTIONS IN A CSTR TRAIN 1-3.1 Generalized Steady-State Analysis... 

Consider the following generic complex multiple reaction scheme that occurs isothermally in a liquid-phase CSTR train. Both reactors operate at the same temperature. In the first elementary step, 1 mol of reactant A and 2 mol of reactant B reversibly produce intermediate product D, which is the desired product ... 

Helpful hints. Use the conjugate gradient method of optimization with 2 degrees of freedom. In other words, you should develop a set of n equations in terms of n + 2 variables that describe the steady-state operation of three independent chemical reactions in a train of two chemical reactors. Maximization algorithms implicitly use two additional equations to determine optimum performance of the CSTR train ... 

The same reactant conversion can be achieved in the exit stream of the last reactor in series when the total volume of a CSTR train is less than the volume of the one-CSTR setup. 

Design the CSTR train by specifying the residence time t in minutes and the temperature T in Kelvin for each reactor that maximize the conversion of reactant A in the exit stream of the second CSTR. The gas constant R is 1.987 cal/mol K. 

Figure 2-1 Multiple chemical reactions in a CSTR train transient molar density response in the exit stream of the first reactor. Approximately 4ti or St] is required to achieve steady-state behavior in the first reactor, where is 15 min.

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. 

Typical examples of polymer production in CSTRs include styrene-butadiene rubber (SBR), acrylonitrile-butadiene rubber (NBR) and polychloroprene in CSTR trains polystyrene and its copolymers PVC and PVAC (and their copolymers) and LDPE. 

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. 

No other reactor configuration (batch, semibatch, plug flow) exhibits the range of dynamic behavior of the CSTR. However, over a finite time interval, other configurations can sometimes exhibit a narrower range of dynamic behavior. If a semibatch reactor is operated such that the rate of reaction is just balanced by the rate of dilution from the feed, a pseudo-steady state may exist. In this case, the concentration of reactants and products in the reactor will remain constant over the time interval necessary to fill the reactor [27]. This may be exploited to provide constant polymer properties during the filling and start-up of a CSTR or CSTR train. 

The composition of copol3mier produced in a steady-state CSTR will not, except for small stochastic variations, change with time. However, the copolymer produced in different reactors of a CSTR train will usually be different. The polymer formed in the first reactor will contain a higher fraction of the more reactive monomer than that formed in downstream reactors. Copol mier composition can be controlled by feeding monomer at various points along the reactor system. The relative flow rates of these intermediate feed streams can be controlled to achieve a variety of composition profiles within the reactor train. 

Several techniques have been employed to try to prevent and/or control undersirable reactor transients. These include (i) the use of a small stirred pre-reactor in a CSTR train, (ii) the application of careful start-up procedures, (iii) the use of a tubular pre-reactor, (iv) the use of a previously manufactured seed latex in the feed stream, (v) recipe variations and (vi) feedback control. Each of these techniques will be discussed in this section... 

Because the MWD polydispersity goes to infinity in a CSTR, step-growth copolymerization is rarely done in a CSTR or CSTR train. Since the reactivity ratios for step-growth monomers vary only slightly, there is little compositional drift in batch step-growth copolymerization, so there is little advantage, in terms of CCD, to step-growth CSTR polymerization. 

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