Reaction Multiple reactions, mixed

Multiple reactions in parallel producing byproducts. Consider again the system of parallel reactions from Eqs. (2.16) and (2.17). A batch or plug-flow reactor maintains higher average concentrations of feed (Cfeed) than a continuous well-mixed reactor, in which the incoming feed is instantly diluted by the PRODUCT and... 

The second use of Equations (2.36) is to eliminate some of the composition variables from rate expressions. For example, 0i-A(a,b) can be converted to i A a) if Equation (2.36) can be applied to each and every point in the reactor. Reactors for which this is possible are said to preserve local stoichiometry. This does not apply to real reactors if there are internal mixing or separation processes, such as molecular diffusion, that distinguish between types of molecules. Neither does it apply to multiple reactions, although this restriction can be relaxed through use of the reaction coordinate method described in the next section. 

Airlift loop reactor (ALR), basically a specially structured bubble column, has been widely used in chemical industry, biotechnology and environmental protection, due to its high efficiency in mixing, mass transfer, heat transfer etc [1]. In these processes, multiple reactions are commonly involved, in addition to their complicated aspects of mixing, mass transfer, and heat transfer. The interaction of all these obviously affects selectivity of the desired products [2]. It is, therefore, essential to develop efficient computational flow models to reveal more about such a complicated process and to facilitate design and scale up tasks of the reactor. However, in the past decades, most involved studies were usually carried out in air-water system and the assumed reactor constructions were oversimplified which kept itself far away from the real industrial conditions [3] [4]. 

We first explain the setting of reactors for all CFD simulations. We used Fluent 6.2 as a CFD code. Each reactant fluid is split into laminated fluid segments at the reactor inlet. The flow in reactors was assumed to be laminar flow. Thus, the reactants mix only by molecular diffusion, and reactions take place fi om the interface between each reactant fluid. The reaction formulas and the rate equations of multiple reactions proceeding in reactors were as follows A + B R, ri = A iCaCb B + R S, t2 = CbCr, where R was the desired product and S was the by-product. The other assumptions were as follows the diffusion coefficient of every component was 10" m /s the reactants reacted isothermally, that is, k was fixed at... 

When dealing with multiple reactions, selectivity or reactor yield is maximized for the chosen conversion. The choice of mixing pattern in the reactor and feed addition policy should be chosen to this end. 

From previous chapters, it is clear that the fluid mixing pattern within a reactor of a given size will affect the conversion achieved from that reactor in the case of multiple reaction schemes, the product distribution, and hence yield and selectivity, will also be dependent on mixing and... 

When multiple reactions take place on mixing two reactant fluids and when these reactions proceed to an appreciable extent before homogeneity is attained, segregation is important and can affect product distribution. 

When the rate equation is complex, the values predicted by the two models are not necessarily limiting. Complexities can arise from multiple reactions, variation of density or pressure or temperature, incomplete mixing of feed streams, minimax rate behavior as in autocatalytic processes, and possibly other behaviors. Sensitivity of the reaction to the mixing pattern can be established in such cases, but the nature of the conversion limits will not be ascertained. Some other, possibly more realistic models will have to be devised to represent the reaction behavior. The literature has many examples of models but not really any correlations (Naumann and Buffham, 1983 Wen and Fan Westerterp et al., 1984). 

The CSTR is particularly useful for reaction schemes that require low concentration, such as selectivity between multiple reactions or substrate inhibition in a chemostat (see Section IV). The reactor also has applications for heterogeneous systems where high mixing gives high contact time between phases. Liquid-liquid CSTRs are used for the saponification of fats and for suspension and emulsion polymerizations. Gas-liquid mixers are used for the oxidation of cyclohexane. Gas homogeneous CSTRs are extremely rare. 

In a recycle reactor, part of the exit stream is recycled back to the inlet of the reactor. For a stirred-tank reactor, recycle has no effect on conversion, since we are essentially just mixing a mixed reactor. For a plug flow reactor, the effect of recycle is to approach the performance of a CSTR. This is advantageous for certain applications such as autocatalytic reactions and multiple reaction situations where we have a PFR but really require a CSTR. 

The repetitive nature of oligomeric synthesis has enabled the rapid implementation of solid-phase and automated methods for DNA [20,21,85,86] and peptide combinatorial libraries. Using these systems for the synthesis of single compounds or mixtures of compounds, multiple reaction vessels numbering 8 [45], 15 [80], 20 [59], 24 [50], 25 [57,58], 36 [53-55,77-79], 48 [26,39-41], or 96 [42-44] can be manipulated. Only a few of these systems enable automated resin mixing and splitting within the instrument to generate mixtures of compounds [53,59,78,87,88],... 

Additional aggravating circumstances arise from the fact that chemical steps which are transfer limited will proceed differently in the industrial plant than on laboratory-scale. The selectivity of multiple reactions such as competing consecutive and parallel reactions depend very much on the extent of micro-mixing in the system. These facts are well known from Chemical Reaction Engineering textbooks. Conversely, these reactions are carried out to obtain details about the extent of micro- and macro-mixing in stirring. 

The Eulerian (bottom-up) approach is to start with the convective-diffusion equation and through Reynolds averaging, obtain time-smoothed transport equations that describe micromixing effectively. Several schemes have been proposed to close the two terms in the time-smoothed equations, namely, scalar turbulent flux in reactive mixing, and the mean reaction rate (Bourne and Toor, 1977 Brodkey and Lewalle, 1985 Dutta and Tarbell, 1989 Fox, 1992 Li and Toor, 1986). However, numerical solution of the three-dimensional transport equations for reacting flows using CFD codes are prohibitive in terms of the numerical effort required, especially for the case of multiple reactions with... 

I m not talidng about fun you can have at an amusement park, but CRE fun. Now that we have an understanding on how to solve for the exit concentrations of multiple reactions in a CSTR and how to plot the species concentration down the length of a PER or PER, we can address one of the most important and fun areas of chemical reaction engineering. This area, discussed in Section 6.1, is learning how to maximize the desired product and minimize the undesired product. It is this area that can make or break a chemical process financially. It is also an area that requires creativity in designing the reactor schemes and feed conditions that will maximize profits. Here you can mix and match reactors, feed steams, and side streams as well as vary the ratios of feed concentration in order to maximize or minimize the selectivity of a particular species. Problems of this type are what I call digital-age problems - because... 

Two types of precursor can be used in CVD of superconducting mixed oxides metal halides and metal-organics The use of metal-organic molecules results in some carbon in the films, which can hinder performance, so metal halides gained popularity in early work. Halide precursors call for a much higher deposition temperature. In the case of metal halide reactions, the chemistry follows that presented earlier (see 17.2.5.2.2) but with multiple reactions occurring simultaneously. The gas phase composition and temperature are controlled to obtain the desired stoichiometry. An example of such a reaction would be that used to make Bi2Sr2CaCu20s+ c(or Bi-2212) at 760-820°C ... 

Multiple Reactions With multiple reactions, the total current is the sum of the currents from the individual reactions with anodic currents positive and cathodic currents negative. This is called the mixed potential principle. For more details see Bard and Faulkner, Electrochemical Methods Fundamentals and Applications, 2d ed., Wiley, 2001. 

Mixing effects on product distribution are of importance in multiple reactions because the impact of product distribution on design and economics can be profound. In such reactions, the desired product is one of two or more possible products. Economics is directly affected by the yield of the desired product and both design and economics are affected by downstream separation requirements. 

In many catalytic systems multiple reactions occur, so that selectivity becomes important. In Sec. 2-10 point and overall selectivities were evaluated for homogeneous well-mixed systems of parallel and consecutive reactions. In Sec. 10-5 we saw that external diffusion and heat-transfer resistances affect the selectivity. Here we shall examiineHEieHnfiuence of intrapellet res ahces on selectivity. Systems with first-order kinetics at isothermal conditions are analyzed analytically in Sec. 11-12 for parallel and consecutive reactions. Results for other kinetics, or for nonisothermal conditions, can be developed in a similar way but require numerical solution. ... 

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