Batch and Semibatch Reactors

The material balance can also be written in terms of the fractional conversion and it is  

CHAPTEH 3 Rear.tnrs for Msasuring Rsantinn Rates 

Ideal batch and semibatch reactors. v(r) is a volumetric flow rate that can vary with time. 

An important class of carbon-carbon bond coupling reactions is the Diels-Alder reactions. An example of a Diels-Alder reaction is shown below  

If this reaction is performed in a well-mixed isothermal batch reactor, determine the time necessary to achieve 95 percent conversion of the limiting reactant (from C. Hill, An Introduction to Chemical Engineering Kinetics and Reactor Design, Wiley, 1977, p. 259). 

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The temperature rise due to this exothermic reaction then approaches the adiabatic temperature rise. The final steady state is always characterized by conditions T = T, and c = 0. A batch reactor, in which a zero order reaction is carried out, always has a unique and stable mode of operation. This is also true for any batch and semibatch reactor with any order or combination of reactions. 

Figure 14.1 Various modes of operation for batch and semibatch reactors.

Remark 3 Batch and semibatch reactors can also be studied by considering their space equivalent PFRs. The space equivalent of a single batch reactor can be regarded as a PFR with no side streams. The optimal holding time for the batch operation can be determined by the optimal PFR length and the assumed linear velocity of the fluid in the PFR. 

Westerterp, K.R. (2006) Safety and runaway prevention in batch and semibatch reactors-a review. Chemical Engineering Research and Design, 84 (A7), 543-52. 

Bosch, J., Strozzi, F., Zbilut, J.P. and Zaldivar, J.M. (2004) On-line runaway detection in isoperibolic batch and semibatch reactors using the divergence criterion. Computers and Chemical Engineering, 28 (4), 527-44. 

Up to now we have focused on the steady-state operation of nonisothermal reactors. In this section the unsteady-state energy balance will be developed and then applied to CSTRs, plug-flow reactors, and well-mixed batch and semibatch reactors. 

Transient CSTR, Batch, and Semibatch Reactors with Heat Exchanger—Ambient Temperature Not Spatially Uniform... 

A number of solved problems for batch and semibatch reactors can be found in... 

The designs of batch and semibatch reactors, covered in Chapters 6 and 9, addressed only the operating time needed to obtain a given conversion (or extent). The size of the batch was not considered. The latter is determined by the required production rate of the unit. The operation of batch reactors usually consists of four steps ... 

The small reactor volumes and the flexible arrangement of microstructured devices can be applied to design multipurpose plants, and traditional batch and semibatch reactors can be replaced. A considerable process intensification and enhanced product selectivity and yield have been shown [20]. Furthermore, continuous reactor operation may help in providing consistent product quality. 

In a batch reactor, the reactants are initially charged and, after a certain reaction time, the product(s) are recovered batchwise. In the semi-batch (or fed-batch) reactor, the reactants are fed continuously, and the product(s) are recovered batch-wise. In these batch and semibatch reactors, the concentrations of reactants and products change with time. 

The influence of heat transfer on yield and selectivity in scaling up batch and semibatch reactors will be illustrated using a series reaction, taking place in an ideal jacketed stirred-tank reactor. This reaction is composed of two irreversible elementary steps, both exothermic and both with first order kinetics ... 

Table 4. Batch and semibatch reactor results (after 5400 s).

We can envision a mechanism of one or more steps for each of these unit operations and we can write a rate equation for each step. We can then relate each of these individual rate equations to an overall rate constant. For a mechanism with two or more steps in series, one step will be slower than the other steps we say this slow step is the rate controlling step. For example, a gas—liquid reaction in a laboratory-sized reactor is either heat transfer controlled or reaction rate controlled. If we cannot supply heat fast enough to maintain the reaction or if we cannot remove heat fast enough to control the reaction, we say the reaction is heat transfer controlled. If, on the other hand, we can supply or remove heat faster than required by the reaction, then we say the reaction is reaction rate controlled. In general, laboratory-sized batch and semibatch reactors have large heat transfer surface area to reaction volume ratios therefore, transferring heat to... 

Batch and semibatch reactors are most often used for low production capacities, where the cost of labor and dead time are only a small fraction of the unit cost of the product. They are generally encountered in the area of specialty chemicals and polymers and in pharmaceuticals, in particular, in plants with a wide variety of products. Large batch reactors are used in fermentations for antibiotic production, an example of which is given in Fig. 8-1. 

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