CSTR unsteady-state operation

As in the case of a batch reactor for commercial operation, a CSTR is normally used for a liquid-phase reaction. In the laboratory, it may also be used for a gas-phase reaction for experimental measurements, particularly for a solid-catalyzed reaction, as in Figure 1.2. The operation is normally one of steady-state, except for startup, shutdown, and operational disturbances or upsets, in which cases unsteady-state operation has to be taken into account. 

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

Remark 1. Eq.(9) can be used on a CSTR in unsteady state operation or in a semibatch reactor when there is no phase change. 

Design equations for unsteady-state operation are needed for start-up of CSTRs or for semibatch operation. These equations must have the ability to predict accurately the concentration or conversion changes before steady-state flow is obtained. Starting with the general design equation, and assuming perfect mixing, we obtain... 

Sec. d.7 Unsteady-State Operation of Reactors 4.7.1 Startup of a CSTR... 

In many cases this can prove to be a very restrictive assumption, so that in general the evaluation of transients for CSTR sequences with non-first-order kinetics is a numerical problem. Various other cases of unsteady-state operation are summarized in the papers of Piret and coworkers cited in Chapter 4, and in the text by Cooper and Jeffries [A.R. Cooper and G.V. Jeffries, Chemical Kinetics and Reactor Design, Prentice-Hall, Inc., Englewood Cliffs, NJ, (1973)]. 

Equation (13-9) applies to a semibatch reactor, as well as for the unsteady-state operation of a CSTR and is also shown in Table I l-I as Equation (Tl l-I.I). 

Up to now we have focused on the steady-state operation of nonisothermal reactors. In this section the unsteady-state energy balance wtU be developed and then applied to CSTRs, plug-flow reactors, and well-mixed batch and semibateh reactors. 

In this chapter, we have already discussed the unsteady operation of one type of reactor, the batch reactor. In this section, w C discuss two other aspects of unsteady operation startup of a CSTR and seniibatch reactors. First, the startup of a CSTR is examined to determine the time necessary to reach steady-state operation [see Figure 4-14(a)], and then semibaich reactors are discussed, in each of these cases, we are interested in predicting the concentration and conversion as a function of lime. Closed-form analytical solutions to the differential equations arising from the mole balance of the.se reaction types can be obtained only for zero- and first-order reactions. ODE solvers must be used for other reaction orders. 

The relationship between conversion and conductivity developed above was used to interpret data from the CSTR As described in Chapter 5, in interpreting data from a CSTR we must take into account the unsteady-state reaction conditions that prevail as temperature is ramped. Even for the case of zero volume expansion, the design equation pertinent to TS-CSTR operation has to account for the constantly changing operating conditions ... 

Figure 6.1 illustrates the fact that for various ranges of kinetic and reactor parameters it is possible for the mass and energy conservation relations for a CSTR to be in stable balance at more than one condition. This may imply that there are other balance conditions that are unstable the point needs to be examined. Which of the stable balances is attained in actual operation may be dependent on the details of startup procedure, for example, which are not subject to the control of the designer. Thus, it is important to investigate reactor stability using unsteady-state rather than steady-state models. 

Consider answer (C). CSTRs are not normally operated in the unsteady-state, perfectly mixed mode. 

Closure. After completing this chapter, the reader should be able to af ly the unsteady-state energy balance to CSTRs, semibatch and batch reactors. The reader should be able to discuss reactor safety using the ONCB and the T2 Laboratories case studies of explosions to help prevent future accidents. Included in the reader s discussion should be how to start up a reactor so as not to exceed the practical stability limit. After studying these examples, the reader should be able to describe how to operate reactors in a safe manner for both single and multiple reactions. 

Equation (3-15) describes the unsteady-state behavior of a CSTR. This is the equation that must he solved to explore strategies for starting up the reactor, or shutting it down, or switching from one set of operating conditions to another. 

The major purpose of this paper is to present experimental results for the emulsion polymerization of vinyl acetate (VA) and methyl methacrylate (MMA) in a single CSTR. Both steady state and transient results will be presented and discussed. Possible causes for prolonged unsteady behavior will be outlined and several techniques for achieving steady operation with a CSTR will be described. 

Startup of a CSTR In reactor startup it is often very important /ttm lenriperature and concentrations approach their steady-state values. For example, a significant overshoot in temperature may cause a reactant or product to degrade, or the overshoot may be unacceptable for safe operation. If cither case were to occur, we would say that the system exceeded its practical stability limit. Although we can. solve the unsteady temperature-time and concentration-time equations numerically to see if such a limit is exceeded, it is often more insightful to study the approach to steady state by using the temperature-concentration phase plane. To illustrate these concepts we shall confine our analysis to a liquid-phase reaction carried out in a CSTR. 

The safely technical assessment of the unsteady operating behaviour of a cooled CSTR is an important element of the overall evaluation of the normal operating conditions. In the case of the CSTR, a non-steady state behaviour always exists during startup and shut-down. As these phases turn out to be critical their discussion and interpretation shall be supported graphically. 

An important consideration in reactor operation is the time dependence of MSS, that is, the transients that develop during start-up or shutdown. Qualitative curves of versus time for an adiabatic CSTR are shown in Figure 13.11 for a simple first-order irreversible reaction. Note that the system converges to the upper or lower steady state (A a,ss,u or A a ssj)-Any apparent approach to the middle unsteady steady state is deceptive because it quickly turns to either of the extreme steady states. The behavior is identical with respect to temperature. 

---