Series reactions batch reactors

The concept of a well-stirred segregated reactor which also has an exponential residence time distribution function was introduced by Dankwerts (16, 17) and was elaborated upon by Zweitering (18). In a totally segregated, stirred tank reactor, the feed stream is envisioned to enter the reactor in the form of macro-molecular capsules which do not exchange their contents with other capsules in the feed stream or in the reactor volume. The capsules act as batch reactors with reaction times equal to their residence time in the reactor. The reactor product is thus found by calculating the weighted sum of a series of batch reactor products with reaction times from zero to infinity. The weighting factor is determined by the residence time distribution function of the constant flow stirred tank reactor. 

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... 

Even today s organic syntheses are still mainly governed by a step-by-step approach bond cleavage and bond making are done one by one. Lack of selectivity and/or incompatible reaction conditions are the underlying causes. The high selectivity that enzymes show under comparable conditions, that is in aqueous systems, allows in principle the use of several biocatalysts in one reactor system. This could be a batch reactor, series of columns or any other system. A promis-... 

Solution We wish to avoid as much as possible the production of di- and triethanolamine, which are formed by series reactions with respect to monoethanolamine. In a continuous well-mixed reactor, part of the monoethanolamine formed in the primary reaction could stay for extended periods, thus increasing its chances of being converted to di- and triethanolamine. The ideal batch or plug-flow arrangement is preferred, to carefully control the residence time in the reactor. 

A continuous flow stirred tank reactor (CFSTR) differs from the batch reactor in that the feed mixture continuously enters and the outlet mixture is continuously withdrawn. There is intense mixing in the reactor to destroy any concentration and temperature differences. Heat transfer must be extremely efficient to keep the temperature of the reaction mixture equal to the temperature of the heat transfer medium. The CFSTR can either be used alone or as part of a series of battery CFSTRs as shown in Figure 4-5. If several vessels are used in series, the net effect is partial backmixing. 

Two additional results were noted during this series of experiments. It was found that plugging of the reactor occurred when the conversion reached about 60%. No satisfactory explanation or cause for the plugging was determined. It was also noted that, regardless of the rate of polymerization, no further reaction occurred after a period of about 60 to 75 minutes. This is in contrast to reaction times of up to three hours for the same recipe used in a batch reactor. 

If a model is available for the reaction chemistry and kinetics, then a temporal superstructure can be developed to represent a batch reactor in the time dimension with a series of reactor compartments that connect to each other sequentially in the time dimension3. This temporal superstructure network, representing a batch reactor, is created... 

For the case where all of the series reactions obey first-order irreversible kinetics, equations 5.3.4, 5.3.6, 5.3.9, and 5.3.10 describe the variations of the species concentrations with time in an isothermal well-mixed batch reactor. For series reactions where the kinetics do not obey simple first-order or pseudo first-order kinetics, the rate expressions can seldom be solved in closed form, and it is necessary to resort to numerical methods to determine the time dependence of various species concentrations. Irrespective of the particular reaction rate expressions involved, there will be a specific time... 

ILLUSTRATION 9.2 QUANTITATIVE DEVELOPMENT OF SERIES REACTION RELATIONSHIPS FOR BATCH AND PLUG FLOW REACTORS... 

Show that, for the bimolecular reaction A + B - products, kscr is given by equation 6.4-17. 6-4 Some of the results obtained by Hinshelwood and Askey (1927) for the decomposition of dimethyl ether, (CH3)20 (A), to CH4, CO and H2 at 777.2 K in a series of experiments in a constant-volume batch reactor are as follows ... 

These look similar to series reactions, but the solution is quite different. In a PFTR or batch reactor the mass-balance equations are... 

The first-order series reactions A B, B C have activation energies of 8 and 10 kcal/mole, respeetively. In a 1 liter batch reactor at 100°C the selectivity to B is 50% and the conversion is 50% in a reaetion time of 10 min with = 1 mole/liter. The solvent is water and the reactor can be pressurized as needed to maintain liquids at any temperature. 

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. 

A comparison of the various types of reactor concepts, in a general sense, is actually only possible between the batch, the CSTR and the PFR. The cascade of CSTRs, depending on the number of vessels n in the series, more or less behaves as an ideal mixer for n->l or an ideal plug flow for n- - . The fed-batch reactor is more difficult to situate. Although the concentration of compounds important for the rate of reaction can be controlled optimally during the whole fed period, the reactor volume is only partially utilized, especially in the beginning. Nevertheless, this reactor concept certainly has decisive advantages in many cases, as shown by the fact that it is one of the most widely used. 

In these equations it is understood that CA may be (a) the concentration of A at a particular time in a batch reactor, (b) the local concentration in a tubular reactor operating in a steady state, or (c) the concentration in a stirred-tank reactor, possibly one of a series, also in a steady state. Let St be an interval of time which is sufficiently short for the concentration of A not to change appreciably in the case of the batch reactor the length of the time interval is not important for the flow reactors because they are each in a steady state. Per unit volume of reaction mixture, the moles of A transformed into P is thus 9LAP6t, and the total amount reacted (9lAP + 3tAQ)St. The relative yield under the circumstances may be called the instantaneous or point yield will change (a) with time in the batch reactor, or (b) with position in the tubular reactor. 

The same general conclusions apply since backmixing of products with reactants should be avoided, a tubular plug-flow reactor or a batch reactor is preferred. However, there is one respect in which a series reaction involving a second reactant B does differ from simple series reaction with one reactant, even when the orders are the same. This is in the stoichiometry of the reaction the reaction cannot proceed completely to the product Q, even in infinite time, if less than two moles... 

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

Fig. 2.5 Time histories of Ca (continuous line), Ci (dotted line), and Cp (dashed line) in a batch reactor for irreversible series reactions. Initial conditions are ...

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