Semibatch reactors, temperature control

The batch process is similar to the semibatch process except that most or all of the ingredients are added at the beginning of the reaction. Heat generation during a pure batch process makes reactor temperature control difficult, especially for high soHds latices. Seed, usually at 5—10% soHds, is routinely made via a batch process to produce a uniform particle-size distribution. Most kinetic studies and models are based on batch processes (69). 

In a semibatch emulsion homopolymer reactor, monomer is converted into polymer via a strongly exothermic reaction [2, 42]. First, water, surfactant, and initiator are loaded into the reactor. Then, monomer and heat are added to start the polymerization reaction, and after some period of time (in which the reaction sites have been formed), heat generated by the polymerization is removed in order to have an adequate reactor temperature control. Once the entire monomer load has been added, the temperature is increased to exhaust the monomer down to a small prespecified value. The monomer addition and heat exchange rates must be coordinated so that the batch is finished as soon as possible with adequate safety margin and... 

With batch reactors, it may be possible to add all reactants in their proper quantities initially if the reaction rate can be controlled by injection of initiator or acqustment of temperature. In semibatch operation, one key ingredient is flow-controlled into the batch at a rate that sets the production. This ingredient should not be manipiilated for temperature control of an exothermic reactor, as the loop includes two dominant lags—concentration of the reactant and heat capacity of the reaction mass—and can easily go unstable. 

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. 

Figure 18. Degree of polymerization with time in response to step change in reaction temperature in controlled semibatch reactor. Key ------, WADP -------,...

A semibatch reactor is a type of batch configuration used particularly for processes, which employ very reactive starting material. Only one reactant, plus solvent if required, is present in the reactor at the start of the reaction. The other reactant(s) is then added gradually to the first, with continued stirring and control of the temperature. Through control of the rate of addition of one reactant, the temperature of the reacting mixture may be kept uniform as the reaction proceeds. 

When operating under semibatch Policy II, common practice is to maintain the reactor contents at low or starved monomer concentrations. This provides for relatively straightforward temperature control and overall reactor operation. However, when such low monomer concentrations are used over the duration of the polymerization, the potential for significant long-chain branching and crosslinking exists. The molecular weight profile would, therefore, be radically different from a batch process. 

An industrial batch reactor has neither an inflow nor an outflow of reactants or products while the reaction is being carried out. Batch reactions can be carried out in droplet microreactors, where nanoliters of fluid are individually manipulated using techniques such as electrowetting on dielectric (EWOD) and surface tension control. Semibatch reactors are used in cases where a by-product needs to be removed continuously and to cany out exothermic batch reactions where a reactant has to be added slowly. Microfluidics allows precise control of concentration and temperature, which allows batch and semibatch reactions to be carried out in a continuous manner. Figure 1 shows the general components of a simple industrial-reactor semp, compared with a laboratory-scale setup to carry out a reaction with microfluidic chips. 

Cyclization of pseudoionone to P-ionone is an important reaction used in the synthesis of vitamin A. Conventionally, pseudoionone is slowly dosed to a stirred tank reactor containing a biphasic mixture of concentrated sulfuric acid and an organic solvent to control the temperature of the highly exothermic reaction [88]. The reaction takes place in the acid phase, where by-products are formed very quickly. The by-product formation is observed to increase with increasing temperature. The product yield obtained in conventional semibatch reactors is in the range of 70%. 

Optimal Temperature Control of Semibatch Polymerization Reactors... 

In many cases, the temperature in a batchwise operated reactor changes during the course of reaction and the evolution of the temperature can only be controlled by an appropriate cooling or heating. In the semicontinuous operation mode (semibatch), some reactants are supplied batchwise while others are supplied continuously. Thus, beside cooling or heating, we then also have to consider an appropriate strategy of reactant addition as a second parameter to control the course of the reactor temperature and reactant concentration. 

In any case, heat-transfer requirements are largely determined by the operation mode. Batch is the most critical operation because high polymerization rates are achieved due to the high monomer concentration. In semibatch mode, the heat generated can be easily controlled by the monomer feed rate. In these reactors, extra cooling is provided by the cold feed. In continuous mode, the continuous cold feed facilitates the control of the reactor temperature, particularly when the reactor temperature is high. 

Emulsion polymerization studies reported in the scientific literature are usually based on experiments with batch or semibatch reactor systems. Since most workers in the field are familiar with such reactors, the thrust of this discussion will be to compare continuous reactors with batch and semi-batch operations. The particular areas to be reviewed include (i) inhibitor effects, (ii) particle age distributions, (iii) particle nucleation, (iv) copolymerization, (v) particle morphology, (vi) temperature control and heat removal and (vii) polymerization kinetic models. 

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