Startup of a CSTR

Conversion does not have any meaning in startup because one cannot separate the moles reacted from the moles accumulated in the CSTR. Consequently, we miisi use concentration rather than conversion as our variable in the balance equation. For liquid-phase (o = Oj) reactions with constant overflow (V = Vj), using T = Ky/oo, we can-transform Equation (4-45) to 

For a first-order reaction ( r, = Equation (4-46) then becomes 

Letting be the time necessary to reach 99% of the steady-state concentra-lion, Cas  

Time to reach steady. state In an Isothermal CSTR 

For most first-order systems, steady state is achieved in three to four space times. 

---

Consider the startup of a CSTR for the liquid-phase reaction A products. The reactor is initially filled with feed when steady flow of feed (q) is begun. Determine the time (t)... 

In this chapter we have already discussed the unsteady operation of one type of reactor, the batch reactor. In this section we discuss two other aspects of unsteady operation. First, the startup of a CSTR is examined to determine the... 

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

Startup of a CSTR (Figure S-1) and the approach to the steady state (CD-ROM). By mapping out regions of the concentration-temperature phase plane, one can view the approach to steady state and learn if the practical stability limit is exceeded. 

Study the startup of a CSTR that initially does not contain ar f reactant. Plot concentration and temperature as a function of time using the data and reactions from the following problems ... 

Example CD9-1 Startup of a CSTR Example CD9-2 Falling Off The Steady State Example CD9-3 Proportional Integral I) Control Living Example Problems... 

We divide the chapter into two parts Part 1 Mote Balances in Terms of Conversion, and Part 2 Mole Balances in Terms of Concentration, C,. and Molar Flow Rates, F,." In Pan 1, we will concentrate on batch reactors, CSTRs, and PFRs where conversion is the preferred measure of a reaction s progress for single reactions. In Part 2. we will analyze membrane reactors, the startup of a CSTR. and semibatch reactors, which are most easily analyzed using concentration and molar How rates as the variables rather than conversion. We will again use mole balances in terms of these variables (Q. f,) for multiple reactors in Chapter 6. 

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. 

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. 

Section 13.4 discusses the startup of a CSTR and how to avoid exceeding the practical stability limit. 

Example CD 1.3-1 Use of the ARSST Example CD 13-2 Startup of a CSTR Example CD13-3 Falling Off the Steady State Example CDl.3-4 Proportional-Integral (PI) Control... 

---