Series-parallel reactions, batch reactor

Figure 8.13 Distribution of materials in a batch or plug flow reactor for the elementary series-parallel reactions...

Figure 9.22 Example of series parallel reaction in a batch reactor. Temperature on left scale, selectivity on right scale.

Ranade (1993) considered the case of a semi-batch stirred reactor to carry out diazotization reactions. The underlying chemistry can be represented by classical series-parallel reactions ... 

At the same time, as a chemist I was disappointed at the lack of serious chemistry and kinetics in reaction engineering texts. AU beat A B o death without much mention that irreversible isomerization reactions are very uncommon and never very interesting. Levenspiel and its progeny do not handle the series reactions A B C or parallel reactions A B, A —y C sufficiently to show students that these are really the prototypes of aU multiple reaction systems. It is typical to introduce rates and kinetics in a reaction engineering course with a section on analysis of data in which log-log and Anlienius plots are emphasized with the only purpose being the determination of rate expressions for single reactions from batch reactor data. It is typically assumed that ary chemistry and most kinetics come from previous physical chemistry courses. 

If the size of the production unit requires higher radiant power than can be provided, for technical reasons, by one lamp, clusters of light sources may be installed, which, consequently will alter the diameter or the height of the inner core of, for example, an annular photochemical reactor. However, following the check list of concepts (vide supra), optimal reaction conditions will in most cases limit the size of the photochemical reactor, and the planned rate of production may require several reactor units installed in a parallel mode (batch process) or in series (continuous process). 

In a recent example Kappe and co-workers112 scaled up a series of reactions from a single mode to a multimode parallel batch reactor. Typically reactions were scaled from 1 to 100 mmol. The transformations included... 

Figure 1.7 shows typical composition profiles. Notice that there is a peak in the CB at some point in time. The higher the value of feB relative to fee, the higher the peak in CB and the earlier in the batch the peak occurs. The batch should be stopped when the peak occurs if we wish to maximize selectivity. Thus batch time is an important operating parameter for series reactions. This is not the case for parallel reactions. Reactor temperature should be adjusted to favor feB. 

The major disadvantage of batch reaction now is the hold-up time between batches. Although the actual reaction time necessary to process a given amount of feed may be substantially less than for a time-averaged reactor such as a CSTR, when the hold-up time is added, the total process time may be greater. Other disadvantages of the batch reactor are dependent on the particular type of reaction being considered, such as whether the reaction is in parallel or series. 

Equation (19-22) indicates that, for a nominal 90 percent conversion, an ideal CSTR will need nearly 4 times the residence time (or volume) of a PFR. This result is also worth bearing in mind when batch reactor experiments are converted to a battery of ideal CSTRs in series in the field. The performance of a completely mixed batch reactor and a steady-state PFR having the same residence time is the same [Eqs. (19-5) and (19-19)]. At a given residence time, if a batch reactor provides a nominal 90 percent conversion for a first-order reaction, a single ideal CSTR will only provide a conversion of 70 percent. The above discussion addresses conversion. Product selectivity in complex reaction networks may be profoundly affected by dispersion. This aspect has been addressed from the standpoint of parallel and consecutive reaction networks in Sec. 7. 

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. 

Load Division 5. Labs 3 and 4 of The Reactor Lab for batch reactor which parallel and series reactions, respectively, can be carried Investigate how dilution with solvent affects the selectivity for diffe reaction orders, and write a memo describing your findings. 

Chapter 2 covers the basic principles of chemical kinetics and catalysis and gives a brief introduction on classification and types of chemical reactors. Differential and integral methods of analysis of rate equations for different types of reactions—irreversible and reversible reactions, autocatalytic reactions, elementary and non-elementary reactions, and series and parallel reactions are discussed in detail. Development of rate equations for solid catalysed reactions and enzyme catalysed biochemical reactions are presented. Methods for estimation of kinetic parameters from batch reactor data are explained with a number of illustrative examples and solved problems. 

This case study is an example of how a common reaction can provide the basis for modeling a novel reaction system a gas-liquid-solid reaction performed in the batch mode the solid in this case is first dissolved followed by chemical reaction with a product of the reactive absorption of the solute gas. Unlike Case Study 11.9, where all steps were in series, here some steps occur in parallel. Moreover, the rate-controlling mechanisms often change with time and process conditions. These facets of the problem are dealt with to determine the maximum production capacity of a reactor, which can often be a cost-determining issue. The lesson here is that maximizing the use of an existing reactor is sometimes preferable to designing a new one. 

For cases in which product distribution is important and reactor size is secondary, equations have been worked out for the batch, longitudinal, and backmixing reactors. These equations will be presented for the three major types of complex reactions, namely, parallel, series, and complex series. 

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