Semibatch Reactor SBR

This assessment criterion shows very clearly the fundamental difference between BR and SBR. Especially for the isothermally operated BR the example discussed has shown, that the ratio of Damkoehler to Stanton number should be significantly smaller than 1 to be sure of safe operating conditions. Exactly the opposite is required for the SBR. Semibatch reactors must be operated in an ignited state in order to be able to utilize the degree of heat production rate. Batch reactors have to be operated in the extinguished state with the consequence already outlined in Section 4.3.1.3, that they are suitable for slow and moderately exothermic processes only. 

BR = batch reactor SBBC = semibatch bubble column SBBPR = semibatch bubble photoreactor CBPR = continuous bubble photoreactor system SBPR = semibatch photoreactor BPR = batch photoreactor SBBT = semibatch bubble tank CFPR = continuous flow photoreactor CFCB = continuous flow bubble column CST = continuous flow stirred tank SBR = semibatch stirred reactor SFC = stopped flow cell. 

Ideal batch reactors (BRs) including semibatch reactors (SBRs)... 

Semibatch Reactor (SBR) In semibatch operation, a gas of limited solubility or a liquid reactant may be fed in gradually as it is used up. An ideal isothermal single-phase semibatch reactor in which a general reaction network takes place has the following general material balance equation ... 

Different types of reactors are applied in practice (Figure 1.14). Stirred tank reactors (STR), very often applied for homogeneous, enzymatic and multiphase heterogeneous catalytic reactions, can be operated batchwise (batch reactor, BR), semi-batchwise (semibatch reactor, SBR) or continuously (continuous strirred tank reactor, CSTR)... 

Semibatch reactors (SBRs) are very common in industrial organic synthesis in general. The basic principle is that a reactant is placed in the reactor and the same or a second reactant, usually the latter, is added continuously. The product... 

Four types of reactors will be considered here the batch (BR), the semibatch (SBR), the plug flow (PFR), and the continuous stirred tank (CSTR). The concept of a batch reactor will be well-known to the reader. A semibatch reactor is one in which at least one reaction component is added over time. This may be a very small volume of material such as a catalyst or initiator, or the feed may comprise a large portion of the final volume of the reactor contents, as in the case of the semibatch addition of a comonomer. 

Polymerization of reactants is a common occurrence in many reactions. Although this is also a parallel scheme, it will be noticed that high concentrations of A combined with low concentrations of B will favor the desired product R. Thus, a semibatch reactor (SBR) would be the preferred candidate since the above condition is met in this reactor. We will see the design equations and principles of operation of SBRs later in this chapter. On the other hand, the common BR, PFR, and MFR would all give lower selectivities because they all allow the second reaction to proceed without hindrance. 

At this point the remark made in Section 4.1.3.1 about an optimized start-up strategy for the cooled CSTR shall be explained. The safety technical assessment procedure for the cooled isoperibolic SBR has demonstrated that in the case of correct design a prediction of the maximum reaction temperature is easily possible. This can be utilized for the optimization of the start up of the CSTR. The later steady state operating temperature of the CSTR is defined as the set value for the maximum SBR process temperature. In a next step one of the two reactants of the CSTR process is charged initially. Then the reactor is started as a semibatch process by feeding the second reactant. When the maximum temperature is reached, the feed of the initially charged reactant is started, and the feed streams are adjusted in such a way that the Stanton number of the CSTR is established. This way the initial oscillations are elegantly avoided. 

Figure 19.10 A matrix of membrane reactor configurations (al) semibatch tank reactor with ESU (SBR or BR-ESU) (a2 and a3) batch reactor with flat and tubular ISU (BR-ISU) (hi and b2) continuously stirred reactor with flat and tubular ISU (CSTR-ISU) (c) plug-flow reactor with ESU (PFR-ESU) and (d) PER with ISU (PFR-ISU).

Continuous processes will generally not be used for acrylic surface coating latexes, adhesives, and other low tonnage products. Semibatch processes are frequently used in which not all the ingredients are added initially. Continuous systems are favored for large-volume polymers like SBR in order to increase the reactor output and reduce fluctuations in product properties. Such continuous reactor trains usually consist of a series of batch reactors each connected with bottom inlets and top outlets and operated with continuous overflow. 

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