Reactor choice parallel reactions

Figure 2.3 Choice of reactor type for mixed parallel and series reactions when the parallel reaction has a higher order than the primary reaction.

Parallel reactions, oai = om2, a i = am = 0, Ei > E2. The. selectivity to the desired product increases with temperature. The highest allowable temperature and the highest reactant concentrations should be applied. A batch reactor, a tubular reactor, or a cascade of CSTRs is the best choice. 

Figure 5.6 Reactor choice for parallel reaction systems. (From Smith R and PetelaEA, 1991, The Chemical Engineer, No. 509/510 12, reproduced by permission of the Institution of Chemical Engineers).

The possibility of a species reacting by parallel paths to yield geometric isomers or entirely different products is often responsible for low yields of a desired product. If circumstances are such that the orders of the desired and unwanted reactions are different with respect to one or more species, it is possible to promote the desired reaction by an appropriate choice of reactor type and reaction conditions. 

The results of Sections VI,B and C for multiple reactions still hold for flow reactors. The selectivity function [Eqs. (67), (69), or (71)] apply exactly to an ideal MER, within the reactor and at its exit. For an ideal CER, the same equations give the local selectivity along the reactor (60-62). The choice of suitable electrochemical reactors for parallel steps depends then on the reaction order of the desirable path with respect to the reactant. Although the surface and volume requirements of a MER are larger than those of a CER, the former would favor a low-order path. An economic trade-off exists, therefore, between reactor costs, subsequent separations of unwanted products, and waste of raw reactants. 

Now for kjCjio kj, or o, > 02, it seems reasonable to expect that the parallel reaction is more critical than the consecutive step in decreasing the yield of Q, and based on the above paragraphs the optimum choice would be a perfectly mixed reactor rather than a plug flow reactor—this will be verified by calculations. Also, for kjCjio < k2< or o, <02, the consecutive reaction should dominate, and the plug flow reactor should be best However, for a, 02, it is not so clear which is (he optimum reactor type. 

Table I0.3.a-I Optimum reactor choice for consecutive and parallel reactions. 

Figure 11.5 Reactor choice for different forms of yp [A] curves for the parallel reaction A- R,A- S.

For a certain type of production kinetics, a stirred tank reactor is in any case the best choice from the kinetic point of view. A backmixed reactor always favors the reaction with the lowest reaction order among parallel reactions of different orders for instance, in the... 

But what is the correct choice a byproduct reaction calls for a continuous well-mixed reactor. On the other hand, the byproduct series reaction calls for a plug-flow reactor. It would seem that, given this situation, some level of mixing between a plug-flow and a continuous well-mixed reactor will give the best... 

The secondary reactions are parallel with respect to ethylene oxide but series with respect to monoethanolamine. Monoethanolamine is more valuable than both the di- and triethanolamine. As a first step in the flowsheet synthesis, make an initial choice of reactor which will maximize the production of monoethanolamine relative to di- and triethanolamine. 

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 Trambouze Reactions—Reactions in parallel. Given the set of elementary reactions with a feed of AO = 1 mol/liter and v = 100 liters/min we wish to maximize the fractional yield, not the production of S, in a reactor arrangement of your choice. 

In continuous emulsion polymerization of styrene in a series of CSTR s, it was clarified that almost all the particles formed in the first reactor (.2/2) Since the rate of polymerization is, under normal reaction conditions, proportional to the number of polymer particles present, the number of succeeding reactors after the first can be decreased if the number of polymer particles produced in the first stage reactor is increased. This can be realized by increasing emulsifier and initiator concentrations in the feed stream and by lowering the temperature of the first reactor where particle formation is taking place (2) The former choice is not desirable because production cost and impurities which may be involved in the polymers will increase. The latter practice could be employed in parallel with the technique given in this paper. 

The choice of the appropriate reactor applied for kinetic measurements is determined by the type of reaction (simple, parallel, or consecutive), the reaction heat and the phase state of the reaction mixture. In general, reactors with simple, almost ideal mixing behavior are preferred in order to obtain simple material balances. 

The production of gasoline from methanol is a parallel process to the Fischer-Tropsch synthesis of hydrocarbons from syngas (Section 4.7.2). A shape-selective zeolite (ZSM-5) was the catalyst of choice in the process put on stream in 1987 by Mobil in New Zealand however the plant was later closed. The zeolite was used at ca. 400°C in a fluid catalyst reactor, which allows prompt removal of the heat of reaction. 

We shall develop next a single-channel model that captures the key features of a catalytic combustor. The catalytic materials are deposited on the walls of a monolithic structure comprising a bundle of identical parallel tubes. The combustor includes a fuel distributor providing a uniform fuel/air composition and temperature over the cross section of the combustor. Natural gas, typically >98% methane, is the fuel of choice for gas turbines. Therefore, we will neglect reactions of minor components and treat the system as a methane combustion reactor. The fuel/air mixture is lean, typically 1/25 molar, which corresponds to an adiabatic temperature rise of about 950°C and to a maximum outlet temperature of 1300°C for typical compressor discharge temperatures ( 350°C). Oxygen is present in large stoichiometric excess and thus only methane mass balances are needed to solve this problem. 

In this chapter, we discuss reactor selection and general mole balances for multiple reactions. First, we describe the four baste types of multiple reactions series, parallel, independent, and complex. Next, we define the selectivity parameter and discuss how it can be used to minimize unwanted side reactions by proper choice of operating conditions and reactor selection. We then develop the algorithm that can be used to solve reaction engineering problems when multiple reactions are involved. Finally, a number of examples are given that show how the algorithm is applied to a number of real reactions. 

Undesirable, parallel side reactions are a common complication in industrial processes. The desire to avoid side reactions can justify the choice of another reactor type or different... 

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