Tank-type chemical reactor

Chapter 3 concerns the dynamic characteristics of stagewise types of equipment, based on the concept of the well-stirred tank. In this, the various types of stirred-tank chemical reactor operation are considered, together with allowance for heat effects, non-ideal flow, control and safety. Also included is the modelling of stagewise mass transfer applications, based on liquid-liquid extraction, gas absorption and distillation. 

Chemical Kinetics, Tank and Tubular Reactor Fundamentals, Residence Time Distributions, Multiphase Reaction Systems, Basic Reactor Types, Batch Reactor Dynamics, Semi-batch Reactors, Control and Stability of Nonisotheimal Reactors. Complex Reactions with Feeding Strategies, Liquid Phase Tubular Reactors, Gas Phase Tubular Reactors, Axial Dispersion, Unsteady State Tubular Reactor Models... 

A useful classification of types of chemical reactors is in terms of their concentration patterns. Certain limiting or ideal types are represented by Figure 4.1 which illustrates batch reactors, continuous stirred tanks and tubular flow reactors. This chapter is concerned with the sizes, performances and heat effects of these ideal types. They afford standards of comparison and are often as close enough to the truth as available information allows. 

Simpler optimization problems exist in which the process models represent flow through a single pipe, flow in parallel pipes, compressors, heat exchangers, and so on. Other flow optimization problems occur in chemical reactors, for which various types of process models have been proposed for the flow behavior, including well-mixed tanks, tanks with dead space and bypassing, plug flow vessels, dispersion models, and so on. This subject is treated in Chapter 14. 

The CRE approach for modeling chemical reactors is based on mole and energy balances, chemical rate laws, and idealized flow models.2 The latter are usually constructed (Wen and Fan 1975) using some combination of plug-flow reactors (PFRs) and continuous-stirred-tank reactors (CSTRs). (We review both types of reactors below.) The CRE approach thus avoids solving a detailed flow model based on the momentum balance equation. However, this simplification comes at the cost of introducing unknown model parameters to describe the flow rates between various sub-regions inside the reactor. The choice of a particular model is far from unique,3 but can result in very different predictions for product yields with complex chemistry. 

There are several control problems in chemical reactors. One of the most commonly studied is the temperature stabilization in exothermic monomolec-ular irreversible reaction A B in a cooled continuous-stirred tank reactor, CSTR. Main theoretical questions in control of chemical reactors address the design of control functions such that, for instance (i) feedback compensates the nonlinear nature of the chemical process to induce linear stable behavior (ii) stabilization is attained in spite of constrains in input control (e.g., bounded control or anti-reset windup) (iii) temperature is regulated in spite of uncertain kinetic model (parametric or kinetics type) or (iv) stabilization is achieved in presence of recycle streams. In addition, reactor stabilization should be achieved for set of physically realizable initial conditions, (i.e., global... 

A CHEMICAL REACTOR is any type of vessel used in transforming raw materials to desired products. The vessels themselves can be simple mixing tanks or complex flow reactors. In all cases, a reactor must provide enough time for chemical reaction to take place. 

Fig. 8.1 Types of ideal chemical reactors (Levenspiel, 1996). The batch recirculation reactor system consists of a vessel of total volume V and a reactor of a specified volume Vr, CSTR continuously stirred tank reactor.

Three major types of chemical reactor systems are used to produce emulsion polymers batch, semicontinuous, and continuous. Batch reactors usually consist of stirred tanks with various forms of heat removal... 

Reactor designs for AOP depend on the mode of operation (a) homogeneous or heterogeneous operation, (b) radiation source used, and (c) addition of chemical. AOP reactors are operated in either batch or flow-through mode with or without recycle. For homogeneous AOP, tank type batch reactors are often used. The flow-through mode is used in radiation-based AOP for water with low contaminant concentration (less than 10 ppm). 

It is possible to express Eq. (3-10) for isothermal operation in simpler forms when assumptions such as constant density are permissible. These will be considered in Chap. 4. The constant-density form of Eq. (3-10) was used in Chap. 2 to calculate rate constants from measured conversions or concentrations as a function of time (see, for example. Sec. 2-7). It is important to recall that we could determine the rate equation for the chemical step from a form of Eq. (3-10) because the reactor is assumed to be an ideal stirred-tank type, with no physical resistances involved. 

Be able to use the energy balance for a tank-type chemical reactor (Sec. 14.1)... 

The Balance Equations for a Tank-Type Chemical Reactor, 779... 

THE BALANCE EQUATIONS FOR A TANK-TYPE CHEMICAL REACTOR... 

In this section we are concerned with the analysis of tank-type reactors used for liquid-phase reactions. A schematic diagram of a tank reactor is given in Fig. 14.1-1. To develop a quantitative description of such reactors, we will assume that its contents are well mixed, as is the case with many industrial reactors, so that the species concentrations and the temperature are uniform throughout the reactor. We do not assume that the reactor exit stream is in chemical equilibrium, since industrial reactors generally do not operate in such a manner. Using the entries of Table 8.4-1, the species mass and total energy balances for a reactor with one inlet and one outlet stream are, respectively, ... 

The three main reactor types developed thus far — batch, continuous-stirred-tank, and plug-flow reactors — are useful for modeling many complex chemical reactors, and to this point we have neglected a careful treatment of the fluid flow pattern within the reactor. In this chapter we explore some of the limits of this approach and develop methods to address and overcome some of the more obvious limitations. 

As the main responsible for the changes in the material balance, the chemical reactor must be modelled accurately from this point of view. Basic flowsheeting reactors are the plug flow reactor (PFR) and continuous stirred tank reactor (CSTR), as shown in Fig. 3.17. The ideal models are not sufficient to describe the complexity of industrial reactors. A practical alternative is the combination of ideal flow models with stoichiometric reactors, or with some user programming. In this way the flow reactors can take into account the influence of recycles on conversion, while the stoichiometric types can serve to describe realistically selectivity effects, namely the formation of impurities, important for separations. Some standard models are described below. 

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