Batch reactors advantages

Eig. 3. Plot of maximum yield as a % of maximum (zero conversion) efficiency to a primary intermediate x axis is ratio of oxidation rate constants ( 2 / i) for primary intermediate vs feed ( ) plug-flow or batch reactor (B) back-mixed reactor (A) plug-flow advantage, %. 

A semi-batch reactor has the same disadvantages as the batch reactor. However, it has the advantages of good temperature control and the capability of minimizing unwanted side reactions by maintaining a low concentration of one of the reactants. Semi-batch reactors are also of value when parallel reactions of different orders occur, where it may be more profitable to use semi-batch rather than batch operations. In many applications semi-batch reactors involve a substantial increase in the volume of reaction mixture during a processing cycle (i.e., emulsion polymerization). 

In a batch reactor, the relative monomer concentrations will change with time because the two monomers react at different rates. For polymerizations with a short chain life, the change in monomer concentration results in a copolymer composition distribution where polymer molecules formed early in the batch will have a different composition from molecules formed late in the batch. For living polymers, the drift in monomer composition causes a corresponding change down the growing chain. This phenomenon can be used advantageously to produce tapered block copolymers. 

In the pharmaceutical industry, and to some extent the fine chemicals industry, an important advantage of a batch reactor is traceability. The product from a particular batch will have a uniform consistency, and can be uniquely labelled and readily traced. In contrast, the product from a continuous process may change gradually over time, and it is therefore more difficult to trace a particular impurity or fault in the material. Batch reactors are, however, rarely the most efficient in terms of throughput and energy use when the reaction kinetics are fast. Batch systems are also much more labour intensive than continuous processes. 

Although batch reactors offer some advantages that make them particularly attractive in some industry sectors, most bulk chemical production utilizes some form of continuous reactor, owing to their overall greater efficiency. 

Continuous flow reactors are almost invariably preferred to batch reactors when the processing capacity required is large. Although the capital investment requirements will be higher, the operating costs per unit of product will be lower for continuous operation than for batch reaction. The advantages of continuous operation are that it ... 

Recent reports on other forms of reactors are also available. Immobilized cell bioreactors, upflow sludge blanket reactors, draft-tube airlift reactor and other have been suggested. Each of these reactors has its own pros and cons and the advantages need to be evaluated with the whole process in mind. Further work is necessary in this area. Several process schemes have been considered and evaluated with the batch reactor design as the core BDS reactor ... 

In this chapter, we first consider uses of batch reactors, and their advantages and disadvantages compared with continuous-flow reactors. After considering what the essential features of process design are, we then develop design or performance equations for both isothermal and nonisothermal operation. The latter requires the energy balance, in addition to the material balance. We continue with an example of optimal performance of a batch reactor, and conclude with a discussion of semibatch and semi-continuous operation. We restrict attention to simple systems, deferring treatment of complex systems to Chapter 18. 

We focus mainly on the advantages and disadvantages of semibatch reactors. A semicontinuous reactor may be treated in many cases as either a batch reactor or a continuous reactor, depending on the overall kinetics and/or the phase of interest. 

First of all, before we compare flow reactors, let us mention the batch reactor briefly. The batch reactor has the advantage of small instrumentation cost and flexibility of operation (may be shut down easily and quickly). It has the disadvantage of high labor and handling cost, often considerable shutdown time to empty, clean out, and refill, and poorer quality control of the product. Hence we may generalize to state that the batch reactor is well suited to produce small amounts of material and to produce many different products from one piece of equipment. On the other hand, for the chemical treatment of materials in large amounts the continuous process is nearly always found to be more economical. 

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. 

Miller and associates (M4) measured the interaction rate in a liquid-liquid two-phase batch reactor by means of a light transmission technique, which has the advantage that it can be used with the aqueous phase or the organic phase dispersed. 

In another example of differential heating, a two-phase water/chloroform system (1 1 by volume) was heated in a microwave batch reactor (MBR)65. About 40 s after commencement, the temperatures of the aqueous and organic phases were 105 and 48°C, respectively, because of the differences in the dielectric properties of each solvent. A sizeable differential could be maintained for several minutes before cooling was begun. Differential heating is particularly advantageous for Hofmann eliminations. In a typical example,... 

As with the batch reactor, the semi-batch reactor operates discontinuously. The difference with true batch operation is that for the semi-batch reactor, at least one of the reactants is added as the reaction proceeds (Figure 7.1). Consequently, the material balance as well as the heat balance will be affected by the progressive addition of one of the reactants. Also, as with the batch reactor, there is no steady state. There are essentially two advantages in using a semi-batch reactor instead of a batch reactor ... 

These two factors mean the semi-batch reactor is a commonly-used reactor type in the fine chemicals and pharmaceutical industries. It retains the advantages of flexibility and versatility of the batch reactor and compensates its weaknesses in the reaction course control by the addition of, at least, one of the reactants. 

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