Reactor Selection and Operating Conditions

consider two simultaneou.s reactions in which two reactants, A and B, are being consumed to produce a desired product. D, and an unwanted product, U, resulting from a side reaction. The rate laws for the reactions 

The two reactors with recycle shown in (i) and (j) can be used for highly exothermic reactions. Here the recycle stream is cooled and returned to the reactor to dilute and cool the inlet stream, thereby avoiding hot spots and runaway reactions. The PFR with recycle is used for ga.s-phase reactions, and the CSTR is used for liquid-phase reactions. Hie last two reactors, (k) and (I), are used for thermodynamically limited reactions where the equilibrium lies far to the left (reactant side) 

In making our selection of a reactor, the criteria are safety, selectivity, yield, temperature control, and cost. 

Fi re S-2 D life rent reactors and schemes for minimizing the untvunted product. 

Example 8-2 Choice of Reactor and Conditions to Minimize Unwanted Products 

FiRurc 6.3 Diflerem reactors and schemes for mininii/ing the unwanted product. 

To maximize the ratio rf/r., maintain the concentrations of both A and B as high as possible. To do this, use 

---

One dose of Tarzlon is 250 mg in liquid form Volume of body fluid = 40 dm (Reactor selection and operating conditions) For each of the following sets of reactions describe your reactor system and conditions to maximize the desired product D. Make sketche.s where necessary to support your choices. 

Reactor Selection and Operating Conditions for Parallel Reactions ... 

Obviously, rpEi can be expressed in terms of reaction temperature and concentration or partial pressure of chemicals and so on. Therefore, the PEI rate-law expression can be helpful to analyze the effect of concentrations and temperature on the PEI transformation rate, rpEi, and to study the influence of back-mixing on process environmental performance. At least, the PEI rate-law expression can reveal the factors that control the transformation rate of PEI, so as to provide guidance for the selection of reactor type and operation conditions, and the inner structure of the reactor, which produce desired products while creating minimum undesired potential environmental impact. 

Since it is impractical to fractionate the products and reformulate them into desirable ranges of molecular weights, immediate attainment of desired properties must be achieved through the correct choice of reactor type and operating conditions, notably of distributions of residence time and temperature. Reactor selection may be made on rational grounds, for historical reasons, or to obtain a proprietary position. 

The rules have implications for reactor choice and operating conditions in situations in which a desired product undergoes subsequent decay. If the desired reaction is of higher overall order than the decay, selectivity is better in a batch or tubular reactor than in a continuous stirred tank, and in batch or tube it is better at higher charge or feed concentrations. On the other hand, if the decay is of higher order than the desired reaction, the opposite is true. 

Pyrolysis oil yield and composition are functions of reactor design and operating conditions, especially temperature, and the raw pyrolysis oil composition can be modified to improve yield and selectivity for higher value chemical products in order to produce a better return than from fuel use alone. 

The key to successful heterogeneous catalytic reactor design and operation is to have a quantitative reactor model capable of predicting the efliect of reactor scale and operating conditions on volumetric productivity and selectivity of the reactor. This does not imply that an ab initio model for each scale has to be merged into a detailed, complex model for the whole reactor. It... 

In this. section, we discu.ss various means of minimizing the undesrret prtnJuct. U, through the selection of reactor type and operating conditions. We also discuss the development of eflicieni reactor schemes. 

We evaluated a number of potential catalysts and conditions using xylitol as a model compound in a batch reactor. A catalyst was selected from this initial screening and examined in a continuous trickle-bed reactor to develop operating conditions. Finally, as resources allowed, the catalyst was evaluated in a trickle bed reactor to gain a concept of potential catalyst lifetime. 

In this chapter, we develop some guidelines regarding choice of reactor and operating conditions for reaction networks of the types introduced in Chapter 5. These involve features of reversible, parallel, and series reactions. We first consider these features separately in turn, and then in some combinations. The necessary aspects of reaction kinetics for these systems are developed in Chapter 5, together with stoichiometric analysis and variables, such as yield and fractional yield or selectivity, describing product distribution. We continue to consider only ideal reactor models and homogeneous or pseudohomogeneous systems. 

The scope of this book includes several aspects of safe process design and operation, such as the choice of reactor type, safe operating conditions, and the selection of protective systems, primarily related to chemical reactivity. However, even in a process plant where these aspects have been carefully considered and thoroughly applied, there are still numerous events that can occur and can lead to hazardous incidents. Examples of such events are ... 

When more than one reactant is involved, the relative yields of reaction products will depend on a greater number of variables. Then it is not usually possible to deduce the best operating strategy by simple inspection of the reaction scheme. Under these circumstances, it is worthwhile developing a formalised procedure for choosing the best reactor and operating conditions. Reaction selectivity is discussed in more detail below. 

This report will describe the realization of these goals by proper selection of the catalyst and operating conditions, which ultimately led to a process with exceptional turnover numbers (TON>10,000) and exceptional reactor productivities (product concentrations >25 wt% in 3 hrs). Further, the process ultimately requires little or no solvent while generating a benign NaCl waste stream. 

Possibly the chemical industry does not have as much need for mathematical models in process simulation as does the petroleum refining industry. The operating conditions for most chemical plants do not seem subject to as broad a choice, nor do they seem to require frequent reappraisals. However, this is a matter which must be settled on the basis of individual circumstances. Sometimes the initial selection of operating conditions for a new plant is sufficiently complex to justify development of a mathematical model. Gee, Linton, Maire, and Raines describe a situation of this sort in which a mathematical model was developed for an industrial reactor (Gl). Beutler describes the subsequent programming of this model on the large-scale MIT Whirlwind computer (B6). These two papers seem to be the most complete technical account of model development available. However, the model should not necessarily be thought typical since it relies more on theory, and less on empiricisms, than do many other process models. 

Therefore the association in the same team of experts in organic chemistry and in catalysis on zeolites is ideal to develop zeolite catalysts for Fine Chemicals synthesis. The organic chemists bring the necessary knowledge about reaction mechanisms (with therefore the possible prediction of secondary products, etc.) and advanced methods in organic analysis and product purification the specialists in catalysis on zeolites orient the choice of the zeolite catalysts and of activation procedures as well as the selection of the reactor and operating conditions. Moreover, the researchers should have a two-fold culture with, in addition to a... 

Patents provide valuable technology information for designers. Firstly, information about process feasibility may be collected with respect to chemistry, catalyst, safety and operation conditions. Qualitative data regarding the reaction engineering, such as conversion and selectivity, as well as the productivity and residence time are useful for the selection of the chemical reactor. Even more important are data regarding the reaction-mixture composition for the assessment of separations, namely with respect to byproducts and impurities. 

Reactions occur in reactors, and in addition to the intrinsic kinetics, observed reaction rates depend on the reactor type, scale, geometry, mode of operation, and operating conditions. Similarly, understanding of the reactor system used in the kinetic experiments is required to determine the reaction mechanism and intrinsic kinetics. In this section we address the effect of reactor type on observed rates. In Sec. 19 the effect of reactor type on performance (rates, selectivity, yield) is discussed in greater detail. 

---