Reactor temperature multiple reactions

Below, we describe tbe design formulation of isothermal batch reactors with multiple reactions for various types of chemical reactions (reversible, series, parallel, etc.). In most cases, we solve the equations numerically by applying a numerical technique such as the Runge-Kutta method, but, in some simple cases, analytical solutions are obtained. Note that, for isothermal operations, we do not have to consider the effect of temperature variation, and we use the energy balance equation to determine tbe dimensionless heat-transfer number, HTN, required to maintain the reactor isothermal. 

The choice of reactor temperature depends on many factors. Generally, the higher the rate of reaction, the smaller the reactor volume. Practical upper limits are set by safety considerations, materials-of-construction limitations, or maximum operating temperature for the catalyst. Whether the reaction system involves single or multiple reactions, and whether the reactions are reversible, also affects the choice of reactor temperature, as we shall now discuss. 

Because the characteristic of tubular reactors approximates plug-flow, they are used if careful control of residence time is important, as in the case where there are multiple reactions in series. High surface area to volume ratios are possible, which is an advantage if high rates of heat transfer are required. It is sometimes possible to approach isothermal conditions or a predetermined temperature profile by careful design of the heat transfer arrangements. 

It should be emphasized that these recommendations for the initial settings of the reactor conversion will almost certainly change at a later stage, since reactor conversion is an extremely important optimization variable. When dealing with multiple reactions, selectivity is maximized for the chosen conversion. Thus a reactor type, temperature, pressure, and catalyst are chosen to this end. Figure 2.10 summarizes the basic decisions which must be made to maximize selectivity. ... 

As illustrated by the examples above, the possibility of removing the generated heat from the reaction zone decreases with an increase in reactor size. As proven above, it can happen that the temperature of the reaction mixture in a full-scale reactor becomes higher than in the laboratory flask reactor. If multiple chemical reactions of distinctly different temperature sensitivities take place, differences in yields and selectivities between small and large reactors will be observed. This has a large influence on safety also. The laboratory reactor might still show satisfactory performance, while the industrial reactor might even explode. 

For reaction systems involving multiple reactions producing by products, selectivity and reactor yield can also be enhanced by appropriate changes to the reactor temperature, pressure and concentration. The appropriate choice of catalyst can also influence selectivity and reactor yield. The arguments are summarized in Figure 6.912. 

When multiple reactions are possible, certain of the products have greater economic value than others, and one must select the type of reactor and the operating conditions so as to optimize the product distribution and yield. In this subsection we examine how the temperature can be manipulated with these ends in mind. In our treatment we will ignore the effect of concentration levels on the product distribution by assuming that the concentration dependence of the rate expressions for the competing reactions is the same in all cases. The concentration effects were treated in detail in Chapter 9. 

As pointed out in the introduction to Chapter 7, in multiple reactions both reactor size and product distribution are influenced by the processing conditions. Since the problems of reactor size are no different in principle than those for single reactions and are usually less important than the problems connected with obtaining the desired product material, let us concentrate on the latter problem. Thus, we examine how to manipulate the temperature so as to obtain, first, a desirable product distribution, and second, the maximum production of desired product in a reactor with given space-time. 

In a variable-density reactor the residence time depends on the conversion (and on the selectivity in a multiple-reaction system). Also, in ary reactor involving gases, the density is also a function of reactor pressure and temperature, even if there is no change in number of moles in the reaction. Therefore, we frequently base reactor performance on the number of moles or mass of reactants processed per unit time, based on the molar or mass flow rates of the feed into the reactor. These feed variables can be kept constant as reactor parameters such as conversion, T, and P are varied. 

For multiple reactions we are not only interested in the conversion but also the selectivity to form a desired product and the yield of that product. In fact, selectivity is fiequently much more important than conversion because we can always increase the conversion by using a larger reactor, a lower flow rate, or a higher temperature, but poor selectivity necessarily requires consumption (loss) of more reactant for a given amount of desired product, and separation of reactants and products and disposal costs increase markedly as the amount of undesired product increases. 

These considerations are only valid for isothermal reactors, and we shall see in the next two chapters that the possibility of temperature variations in the reactor can lead to much more interesting behavior. We will also see in Chapter 7 that with catalytic reactors the situation becomes even more complicated. However, these simple ideas are useful guides in the choice of a chemical reactor type to carry out multiple-reaction systems. We will stiU use these principles as the chemical reactors become more complicated and additional factors need to be included. 

Reaction control Critical control only through reactor temperature Safe, excellent with fast reactions, accumulation with slow reactions Excellent if multiplicity is avoided. Possible hysteresis at startup and shut down. Excellent, due to cooling capacity and thermal dilution... 

Emulsion Polymerization in a CSTR. Emulsion polymerization is usually carried out isothermally in batch or continuous stirred tank reactors. Temperature control is much easier than for bulk or solution polymerization because the small (. 5 Jim) polymer particles, which are the locus of reaction, are suspended in a continuous aqueous medium as shown in Figure 5. This complex, multiphase reactor also shows multiple steady states under isothermal conditions. Gerrens and coworkers at BASF seem to be the first to report these phenomena both computationally and experimentally. Figure 6 (taken from ref. (253)) plots the autocatalytic behavior of the reaction rate for styrene polymerization vs. monomer conversion in the reactor. The intersection... 

The behaviour of the trajectories - the relation between the reactor temperature and the conversion XA of the reactant - has been analysed extensively by us [16,17,18,19] for multiple reactions. 

Hence, this temperature depends on the composition of the reaction mixture, Therefore, for exothermic equilibrium reactions the temperature should be lowered as more reactant is converted, to keep the rate high. Thus this type of reaction is often carried out in multiple stages (e.g. multiple reactors or multiple zones within a reactor), each with its own temperature, adapted to the reaction mixture composition. 

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