Reactor nonisothermal adiabatic

Nonisothermal reactors with adiabatic beds. Optimization of the temperature profile described above assumes that heat can be added or removed wherever required and at whatever rate required so that the optimal temperature profile can be achieved. A superstructure can be set up to examine design options involving adiabatic reaction sections. Figure 7.12 shows a superstructure for a reactor with adiabatic sections912 that allows heat to be transferred indirectly or directly through intermediate feed injection. 

Figure 7.12 Superstructure for a Nonisothermal reactor with adiabatic sections.

Choice between Nonisothermal Nonadiabatic Multitubular Reactors and Adiabatic Reactors... 

To illustrate the above points in a simple manner, let us consider a nonisothermal adiabatic fixed-bed reactor packed with nonporous catalyst pellets for a simple exothermic reaction... 

A perfectly mixed nonisothermal adiabatic reactor carries out a simple first-order exothermic reaction in the liquid phase ... 

Compare and comment on the results of previous two parts. What will be the effect if the reactors are nonisothermal, adiabatic, and both the reactions are exothermic. 

In order to size or analyze a nonisothermal reactor, the material balance(s) and the energy balance must be solved simultaneously. This solution is relatively straightforward if the reactor is adiabatic, so that s where our discussion will begin. For an adiabatic reactor, with a single reaction taking place, the temperature is directly related to the fractional conversion, as shown in Section 8.4.3. Let s consider a situation where the energy balance reduces to a linear relationship between temperature and fractional conversion, i.e., where Eqn. (8-20) is valid. 

Previous chapters have discussed how isothermal or adiabatic reactors can be scaled up. Nonisothermal reactors are more difficult. They can be scaled by maintaining the same tube diameter or by the modeling approach. The challenge is to increase tube diameter upon scaleup. This is rarely possible and when it is possible, scaleup must be based on the modeling approach. If the predictions are satisfactory, and if you have confidence in the model, proceed with scaleup. 

In order to assess the design of both the reactor and the heat exchanger required to control T, it is necessary to use the material balance and the energy balance, together with information on rate of reaction and rate of heat transfer, since there is an interaction between T and /A. In this section, we consider two cases of nonisothermal operation adiabatic (Q = 0) and nonadiabatic (Q = 0). 

In adiabatic operation, there is no attempt to cool or heat the contents of the reactor (that is, there is no heat exchanger). As a result, T rises in an exothermic reaction and falls in an endothermic reaction. This case may be used as a limiting case for nonisothermal behavior, to determine if T changes sufficiently to require the additional expense of a heat exchanger and T controller. 

Let us return to the graphical construction we developed in earlier chapters for isothermal reactors, because for nonisothermal reactors T is stiU the area under curves of plots of 1/r versus Cao — CA. For the first-order irreversible reaction in an adiabatic reactor 1/fad is given by... 

The reactor system may consist of a number of reactors which can be continuous stirred tank reactors, plug flow reactors, or any representation between the two above extremes, and they may operate isothermally, adiabatically or nonisothermally. The separation system depending on the reactor system effluent may involve only liquid separation, only vapor separation or both liquid and vapor separation schemes. The liquid separation scheme may include flash units, distillation columns or trains of distillation columns, extraction units, or crystallization units. If distillation is employed, then we may have simple sharp columns, nonsharp columns, or even single complex distillation columns and complex column sequences. Also, depending on the reactor effluent characteristics, extractive distillation, azeotropic distillation, or reactive distillation may be employed. The vapor separation scheme may involve absorption columns, adsorption units,... 

One of the simplest practical examples is the homogeneous nonisothermal and adiabatic continuous stirred tank reactor (CSTR), whose steady state is described by nonlinear transcendental equations and whose unsteady state is described by nonlinear ordinary differential equations. 

Write the steady-state mass and heat balance equations for this system, assuming constant physical properties and constant heat of reaction. (Note Concentrate your modeling effort on the adiabatic nonisothermal reactor, and for the rest of the units, carry through a simple mass and heat balance in order to define the feed conditions for the reactor.)... 

An industrial system consists of a nonisothermal CSTR (with a cooling jacket) and a tubular adiabatic reactor in series. The reaction is a first-order irreversible reaction ... 

It is the purpose of this chapter to discuss presently known methods for predicting the performance of nonisothermal continuous catalytic reactors, and to point out some of the problems that remain to be solved before a complete description of such reactors can be worked out. Most attention will be given to packed catalytic reactors of the heat-exchanger type, in which a major requirement is that enough heat be transferred to control the temperature within permissible limits. This choice is justified by the observation that adiabatic catalytic reactors can be treated almost as special cases of packed tubular reactors. There will be no discussion of reactors in which velocities are high enough to make kinetic energy important, or in which the flow pattern is determined critically by acceleration effects. 

If a reactor is operated at nonisothermal or adiabatic conditions then the material balance equation must be written with the temperature, T, as a variable. For example with the PFR, the material balance becomes ... 

Plot the fractional conversion and temperature as a function of time for the batch reactor system described in Example 9.3.3 if the reactor is now adiabatic (U = 0). Compare your results to those for the nonisothermal situation given in Figure 9.3.3. How much energy is removed from the reactor when it is operated nonisothermally ... 

To identify the additional information neeessary to design nonisothermal reaetors, we eonsider the following example, in which a highly exothermic reaction is carried out adiabatically in a plug-flow reactor. 

We shall recapitulate the governing equations in the next section and discuss the economic operation in the one following. The results on optimal control are essentially a reinterpretation of the optimal design for the tubular reactor. We shall not attempt a full derivation but hope that the qualitative description will be sufficiently convincing. The isothermal operation of a batch reactor is completely covered by the discussion in Chap. 5 of the integration of the rate equations at constant temperature. The simplest form of nonisothermal operation occurs when the reactor is insulated and the reaction follows an adiabatic path the behavior of the reactor is then entirely similar to that discussed in Chap. 8. 

In summary, the operation of commercial reactors falls into three categories isothermal, adiabatic, and the broad division of nonadiabatic, where attempts are made to approach isothermal conditions, but the magnitude of the heat of reaction or the temperature level prevents attaining this objective. Quantitative calculations for isothermal and nonisothermal homogeneous reactors are given in Chaps. 4 and 5. 

We have been discussing adiabatic reactors in Fig. 5-lheat-transfer rates are usually insufficient to change the form of the curves. Hence the same conclusions apply to almost all nonisothermal conditions. 

We begin a discussion of scaleup relationships and strategies for tubular reactors. Results are restricted to tubes with a constant cross-sectional area. Chapter 3 discusses only isothermal or adiabatic reactors, but the relationships in Tables 3.1-3.3 include scaleup factors for the nonisothermal reactors that are discussed in Chapter 5. These results assume constant density, but Tables 3.4 and 3.5 give some specialized results for ideal gases when the pressure drop down the tube is significant. 

The method of operating the reactor, such as adiabatic, isothermal or nonadiabatic-nonisothermal. 

Use of Rate Equations in Reactor Design. The method of using the rate equations for catalytic reactions to calculate the reactor size and amount of catalyst needed for a specified conversion and feed rate is very similar to the method used for noncatalytic reactions. The calculations may be divided into three types, namely, those for isothermal reactors, adiabatic reactors, and nonisothermal nonadiabatic reactors. In all three cases where the feed rate F and the desired conversion x are specified, the weight of catalyst needed can be calculated from the expression... 

There are three general methods of operation for chemical reactors, namely, isothermal, adiabatic, and nonisothermal nonadiabatic. 

In the isothermal case, just enough heat is added or removed to keep the temperature constant throughout. In the adiabatic reactor, no heat is added or, removed during the course of the reaction. In the noniaothemud nonadiabatic reactor, some heat is either added or removed during the reaction but the temperature does not remain constant. Almost all commercial reactors are operated as nonisothermal nonadiabatic reactions. 

---