CSTR and recycling

Table 4.10.2 Equations for the conversion of reactant A in ideal reactors (isothermal, first order, constant volume. Da — kr for PER, CSTR, and recycle reactor and kt for a batch reactor).

It is seen that measurements of concentration and flow rate lead to kinetic data in terms of rates for differential, CSTR, and recycle reactors. On the other... 

Continuous-Flow Stirred-Tank Reactors. The synthesis of j )-tolualdehyde from toluene and carbon monoxide has been carried out using CSTR equipment (81). -Tolualdehyde (PTAL) is an intermediate in the manufacture of terephthabc acid. Hydrogen fluoride—boron trifluoride catalyzes the carbonylation of toluene to PTAL. In the industrial process, separate stirred tanks are used for each process step. Toluene and recycle HF and BF ... 

Illustration 9.5 indicates that one may have parallel paths leading from reactants to products and that in the case of an autocatalytic reaction, one path may be preferred over a second until the product level builds up to a point where the second becomes appreciable. In this example, the magnitudes of the rate constants are such that the vast majority of the reaction occurs by the autocatalytic path. In cases such as these it is desirable to use a CSTR or recycle reactor to enhance the reaction rate by virtue of the back-mixing of product species. 

In regard dynamics and control scopes, the contributions address analysis of open and closed-loop systems, fault detection and the dynamical behavior of controlled processes. Concerning control design, the contributors have exploited fuzzy and neuro-fuzzy techniques for control design and fault detection. Moreover, robust approaches to dynamical output feedback from geometric control are also included. In addition, the contributors have also enclosed results concerning the dynamics of controlled processes, such as the study of homoclinic orbits in controlled CSTR and the experimental evidence of how feedback interconnection in a recycling bioreactor can induce unpredictable (possibly chaotic) oscillations. 

In Figure 4.1, the CSTR is connected to a recycle loop and measurement cells. If the cells and recycle loop have a volume V and the pump has a volumetric pumping speed of Vp then the characteristic residence time is With our various... 

There is of course no point in adding recycle to a CSTR because reactants and products are assumed to be already mixed instantly, and recycle would not change the performance at all except for adding unnecessary pipes and pumps. 

For any more complex flow pattern we must solve the fluid mechanics to describe the fluid flow in each phase, along with the mass balances. The cases where we can still attempt to find descriptions are the nonideal reactor models considered previously in Chapter 8, where laminar flow, a series of CSTRs, a recycle TR, and dispersion in a TR allow us to modify the ideal mass-balance equations. 

The reaction studied in Section 2.2 has two reactants and therefore offers the possibility of adjusting the compositions of the reactants in the reactor to achieve some economic or control objective. In this section we first find the cost of operating single and multiple CSTR processes to achieve a specified conversion. Then we design an alternative process consisting of a reactor and a distillation column that separates product C from the unreacted A and B in the reactor effluent and recycles them back to the reactor. 

Reactor Selection Ideal CSTR and PFR models are extreme cases of complete axial dispersion (De = oo) and no axial dispersion (De = 0), respectively. As discussed earlier, staged ideal CSTRs may be used to represent intermediate axial dispersion. Alternatively, within the context of a PFR, the dispersion (or a PFR with recycle) model may be used to represent increased dispersion. Real reactors inevitably have a level of dispersion in between that for a PFR or an ideal CSTR. The level of dispersion may depend on fluid properties (e.g., is the fluid newtonian),... 

An initial theoretical study (Gilliland et al. 1964) established that, for a simple plant model consisting of a continuous stirred-tank reactor (CSTR) and a distillation column, the material recycle stream increases the sensitivity to disturbances together with increasing the time constant of the overall plant over those of the individual units. Moreover, it was shown that in certain cases the plant can become unstable even if the reactor itself is stable. 

Figure 3.6 A reactor-separator process, consisting of a CSTR and a distillation column. The unreacted feed material is recycled to the reactor.

The results Illustrated by Figures 3 and 4 resemble those obtained in the Berty recycle reactor under similar conditions. The space-mean, time average rates for the fixed-bed reactor were only about 50% of those measured in the Berty reactor, because, of course the former reactor achieved conversions high enough for the back reaction to become important. The significance of these observations is that 1) CSTR and differential reactors, widely used for laboratory studies, seem to reflect performance improvements obtainable with fixed-bed, integral reactor which resemble commercial units, and 2) improvement from periodic operation are still observed even tfien reverse reactions become important. 

Let us consider a CSTR/separator/recycle system, where the first-order reaction A —> P takes place. Figure 4.3(a) presents the conventional control of the plant. The fresh feed flow rate is kept constant at the value F0. The reactor holdup V is controlled by the effluent. The reaction takes place at a constant temperature, which is achieved by manipulating the utility streams. Dual-composition control of the distillation column ensures the purities of the recycle and product streams. 

Let us consider one of the simplest recycle processes imaginable a continuous stirred tank reactor (CSTR) and a distillation column. As shown in Figure 2.5. a fresh reactant stream is fed into the reactor. Inside the reactor, a first-order isothermal irreversible reaction of component A to produce component B occurs A -> B. The specific reaction rate is k (h1) and the reactor holdup is VR (moles). The fresh feed flowrate is Fs (moles/h) and its composition is z0 (mole fraction component A). The system is binary with only two components reactant A and product B. The composition in the reactor is z (mole fraction A). Reactor effluent, with flowrate F (moles/h) is fed into a distillation column that separates unreacted A from product B. 

Additional details of the economic and sizing calculations can be found in Luyben (1993). Notice that the flowsheet with the smallest annual cost has four CSTRs. Now let s compare this system with a process that has one CSTR and a column whose overhead product is recycled back to the reactor. Economic studies of this system have shown that a simple stripping column is cheaper than a full column. Table 2.3 gives size and cost data over a range of reactor sizes. 

P14-1b Make up and solve an original problem. The guidelines are given in Problem P4-1. However, make up a problem in reverse by first choosing a model system such as a CSTR in parallel with a CSTR and PFR [with the PFR modeled as four small CSTRs in series Figure P14-l(a)] or a CSTR with recycle and bypass [Figure P14-l(b)]. Write tracer mass balances and use an ODE solver to predict the effluent concentrations. In fact, you could build up an arsenal cf tracer curves for different model systems to compare against real reactor RTD data. In this way you could deduce which model best describes the real reactor. 

CSTR to maintain a constant dilution rate (the feed rate). These require some means to separate the biocatalyst from the product and recycle to the reactor, such as centrifuge or microfiltration ... 

The second mode of CSTR operation is that used by Thien (17) and by Li and Shrier (10). Here, both the external phase and the LM emulsion are in a continuous flow mode. The reactor effluents are sent to gravity settlers where the exterior phase is separated from the emulsion phase. The emulsion phase is then demulsified to recover the product followed by remulsification and recycle back to the reactor. Hatton and Wardius (48) have developed the advancing front model for the analysis of such staged LM operations. Thien (17) employed this scheme to remove the amino acid L-phenylalanine from simulated fermentation broth (dilute aqueous solution). 

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. 

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