Contact pattern

Three basic fluid contacting patterns describe the majority of gas-liquid mixing operations. These are (1) mixed gas/mixed liquid - a stirred tank with continuous in and out gas and liquid flow (2) mixed gas/batch mixed liquid - a stirred tank with continuous in and out gas flow only (3) concurrent plug flow of gas and liquid - an inline mixer with continuous in and out flow. For these cases the material balance/rate expressions and resulting performance equations can be formalized as ... 

The second factor pertains to the contacting patterns for multiphase systems. Many combinations of contacting patterns are possible, even for a two-phase system. Each of the many ways of contacting two phases has associated with it a specific form of performance equation which must be developed for that particular contacting pattern. 

Of the contacting patterns in Figure 7.3, countercurrent packed beds offer the largest mass transfer driving force and agitated tanks the lowest. 

Figure 7.4 Contacting patterns for liquid-liquid reactors.

In the design of an industrial scale reactor for a new process, or an old one that employs a new catalyst, it is common practice to carry out both bench and pilot plant studies before finalizing the design of the commercial scale reactor. The bench scale studies yield the best information about the intrinsic chemical kinetics and the associated rate expression. However, when taken alone, they force the chemical engineer to rely on standard empirical correlations and prediction methods in order to determine the possible influence of heat and mass transfer processes on the rates that will be observed in industrial scale equipment. The pilot scale studies can provide a test of the applicability of the correlations and an indication of potential limitations that physical processes may place on conversion rates. These pilot plant studies can provide extremely useful information on the temperature distribution in the reactor and on contacting patterns when... 

Contacting patterns for various combinations of high and low concentration of reactants in noncontinuous operations (batch reactors). (Adapted from Chemical Reaction Engineering, Second Edition, by O. Levenspiel. Copyright 1972. Reprinted by permission of John Wiley and Sons, Inc.)... 

In order to determine the product distribution quantitatively, it is necessary to combine material balance and reaction rate expressions for a given reactor type and contacting pattern. On the other hand, if the reactor size is desired, alternative design equations reflecting the material balances must be employed. For these purposes it is appropriate to work in terms of the fractional yield. This is the ratio of the amount of a product formed to the amount of reactant consumed. The instantaneous fractional yield of a product V (denoted by the symbol y) is defined... 

This reaction set may be regarded as parallel reactions with respect to consumption of species B and as a series reaction with respect to species A, V, and W. Common examples include the nitration and halogenation of benzene and other organic compounds to form polysubstituted compounds. To characterize the qualitative behavior of such systems, it is useful to consider reactions 9.3.3 and 9.3.4 as mechanistic equations and to analyze the effects of different contacting patterns on the yield of species V. We shall follow the treatment of Levenspiel (7). 

Figure 8 1 4 Adduct of 16]aneS4(IBr)4 partial view of the intermolecular I - I and I- Br contact pattern within a ribbon...

The CSTR-recycle in Figure 4.1 has limitations and many are associated with mass transfer and time-scale issues. A CSTR contacting pattern is not perfect as input to Eq. (2). Each spectrometer associated with Figure 4.1 has its limitations, and many are associated with the class of species detectable and the sensitivity. 

To find what a reactor is able to do we need to know the kinetics, the contacting pattern and the performance equation. We show this schematically in Fig. 1.2. 

Much of this book deals with finding the expression to relate input to output for various kinetics and various contacting patterns, or... 

Ideal reactors have three ideal flow or contacting patterns. We show these in Fig. 2.1, and we very often try to make real reactors approach these ideals as closely as possible. 

In any case, the use of the proper contacting pattern is the critical factor in obtaining a favorable distribution of products for multiple reactions. 

Figure 7.1 Contacting patterns for various combinations of high and low concentration of reactants in noncontinuous operations.

Figure 7.4 The contacting pattern with the largest area produces most R a) plug flow is best, h) mixed flow is best, (c) mixed flow up to Cai followed by plug flow is best.

Using separate feeds of A and B sketch the contacting pattern and reactor conditions which would best promote the formation of product R for the following systems of elementary reactions. 

FAVORABLE CONTACTING PATTERNS FOR ANY SET OF IRREVERSIBLE REACTIONS IN SERIES, NOT JUST A R S... 

Which contacting pattern of Figs. E8.1, when properly operated, can give a higher concentration of any intermediate, the contacting pattern on the left or the one on the right ... 

This is a powerful generalization which, without needing specific values for the rate constants, can already show in many cases which are the favorable contacting patterns. It is essential, however, to have the proper representation of the stoichiometry and form of rate equation. Example 8.6 and many of the problems of Chapter 10 apply these generalizations. 

Figure 16.3 Four contacting patterns which can all give the same exponential decay RTD. Cases a and b represent the earliest possible mixing while cases c and d represent the latest possible mixing of fluid elements of different ages.

Figure 17.1. Ideal contacting patterns for two flowing fluids.

Any type of reactor with known contacting pattern may be used to explore the kinetics of catalytic reactions. Since only one fluid phase is present in these reactions, the rates can be found as with homogeneous reactions. The only special precaution to observe is to make sure that the performance equation used is dimensionally correct and that its terms are carefully and precisely defined. 

Many possible permutations of rate, equilibrium, and contacting pattern can be imagined however, only some of these are important in the sense that they are widely used on the technical scale. 

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