Semibatch reactor advantages

OA2 > CMI, am > am Concentration of B should be minimized. Dosing of B into the reaction mixture containing A is advantageous. A semibatch reactor or a cross-flow reactor is recommended. 

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. 

The temperature-controlling features of this reaction scheme dominate selection and use of the reactor. However, the semibatch reactor does have some of the advantages of batch reactors temperature programming with time and variable reaction time control. 

Bubble columns where a gas is dispersed through a deep pool of liquid are commonly used in industry as absorbers, strippers, or reactors when a large liquid holdup, large liquid residence time, or large heat transfer is needed. They may be operated either countercurrently, cocurrently, or semibatch. Other advantages of bubble columns are the absence of moving parts, minimum maintenance, small floor space, ability to handle sol-... 

Liquid-Phase Reactions. Semibateh reactors and CSTRs are used primarily for liquid-phase reactions. A semibatch reactor (Figure 1-15) has essentially Units the same disadvantages as the batch reactor. However, it has the advantages of... 

Unlike batch/semibatch reactors, the mean residence time of a CSTR at steady state is defined by the ratio of volume inside the reactor to volumetric feed rate, which at equal density of feed and reactor contents is equal to the reactor space time. The advantage of the CSTR over the batch or semibatch reactor is that it is ideally suitable for long runs of continuous production of a polymer product. Once the reactor process is brought to steady state, uniform quality and consistent product is made. However, the CSTR requires several reactor turnovers (at least 3-4) before the process is at steady state and uniform product is made [10]. 

Batch and semibatch reactors are ideal when the production rate of the polymer needed is small. In larger-capacity plants, continuous reactors are preferred. In these, the raw materials are pumped in continuously while the products are removed at the other end. One example of these is a tubular reactor (shown in Fig. 4.1c). It is like an ordinary tube into which material is pumped at one end. Polymerization occurs in the tubular reactor, and the product stream consists of the polymer along with the unreacted monomer. Sometimes, a stirred vessel (shown in Fig. 4.Id) is employed instead of a tubular reactor. The advantage of such a reactor is that the concentration and temperature variations... 

Batch operation requires a larger inventory than the corresponding continuous reactor. Thus, there may be a safety incentive to change from batch to continuous operation. Alternatively, the batch operation can be changed to semibatch in which one (or more) of the reactants is added over a period. The advantage of semibatch operation is that the feed can be switched off in the event of a temperature (or pressure) excursion. This minimizes the chemical energy stored up for a subsequent exotherm. 

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. 

The differences between a single CSTR and a batch reactor are similar to those between semibatch and batch reactors, except that they are usually more pronounced. The addition of more reactors to a series system tends to reduce some of the observed performance differences. A typical example of different behavior is the heat release profile. An advantage often cited for continuous reactor systems is a constant heat load with fully used reactor volume. Batch reactors are not usually operated full, and the heat load is nonuniform. In addition, portions of the batch reaction cycle are devoted to charging and emptying the reactor and sometimes for heating the reagents to polymerization temperature. Thus, the production rate per unit volume can be higher in a continuous system. 

Free-radical polymerizations are exothermic, and so the heat produced during polymerization must be removed. This is not a significant problem in a laboratory scale however, heat transfer problems constitute a restriction for batch processes in an industrial scale. In the case of semibatch and CSTR, the cold monomer and water feed are beneficial for heat removal so that much higher production rates are feasible than for a batch reactor of the same volume. For tubular reactors, their large heat transfer area is advantageous for the strongly exothermic polymerizations. 

These equations remain valid for bioreactors provided that one employs a suitable mathematical representation of the rate of disappearance of the substrate that is the limiting reagent. In Illustration 13.3 we employ an alternative form of the design equation to determine the holding time necessary to achieve a specified degree of conversion in a strictly batch bioreactor. This illustrative example also indicates how overall yield coefficients are employed as a vehicle for taking the stoichiometry of the reaction into account. Illustration 13.4 describes how one type of semibatch operation (the fed-batch mode) can be exploited to combine the potential advantages of batch and continuous flow operation of a stirred-tank reactor. 

In principle the use of a well-stirred bioreactor in a continuous flow mode offers significant advantages over operation in a batch or semibatch mode, but the majority of bioreactors in industrial use are operated in the latter modes. However, the actual performance of single CSTR or a cascade of such reactors often fails to meet the expectations... 

Operation of various types of bioreactors in a perfusion mode (21, 22) enables the design engineer to combine several advantages of traditional modes of operation of well-stirred bioreactors (e.g., semibatch operation in a fed batch mode or use of recycle with a chemostat). Perfusion consists of operation in a mode in which a fresh growth medium (possibly together with a recycled growth medium) is fed to a bioreactor containing viable cells that are retained within the bioreactor by permselective membranes, microfllters, immobilization, or by partial separation and recovery from the reactor effluent followed by recycle to the entrance of the reactor. 

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