Semibatch and Semicontinuous Reactors

A semibatch reactor is a variation of a batch reactor in which one reactant may be added intermittently or continuously to another contained as a batch in a vessel, or a product may be removed intermittently or continuously from the vessel as reaction proceeds. The reaction may be single-phase or multiphase. As in a batch reactor, the operation is inherently unsteady-state and usually characterized by a cycle of operation, although in a more complex manner. 

A semicontinuous reactor is a reactor for a multiphase reaction in which one phase flows continuously through a vessel containing a batch of another phase. The operation is thus unsteady-state with respect to the batch phase, and may be steady-state or unsteady-state with respect to the flowing phase, as in a fixed-bed catalytic reactor (Chapter 21) or a fixed-bed gas-solid reactor (Chapter 22), respectively. 

In this section, we consider various modes of operation of these types of reactors, their advantages and disadvantages, and some design aspects. Since there are many variations of these reactors, it is difficult to generalize their design or analysis, and consequently we use an example for illustration. 

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In a batch reactor, the first two terms in equation 12.2-1 are absent. In a semibatch reactor, one of these two terms is usually absent. In a semicontinuous reactor for a multiphase system, both flow terms may be absent for one phase and present for another. In a continuous reactor, the two terms are required to account for the continuous inflow to and outflow from the reactor, whether the system is single-phase or multiphase. 

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. 

Batch and Semibatch Polymerization. The reactor is normally operated in a semicontinuous mode by delaying vinyl acetate, solvent, and initiator. The same reactor can be used for stripping the poly(vinyl acetate) solution, provided that careful addition of methanol is used in order to prevent the viscosity in the reactor from becoming excessive (249). The disadvantages of batch polymerization are lack of product consistency and unsatisfactory economics in large scale production (250,251). The true batch reaction, where all the reactants are added to the reactor at time zero, yields a product having a very broad molecular weight distribution of limited commercial value. 

The CSTR operator, Rc, has an identical term to describe accumulation under transient operation. The algebraic sum of the two other terms indicates the difference of in-flow and out-flow of that species. This operator also describes semibatch or semicontinuous operation in cases where the volume can be assumed to be essentially constant. In the more general case of variable volume, V must be included within the differential accumulation term. At steady state, it is a difference equation of the same form as the differential equation for a batch reactor. 

ProCGdures. From a technological point of view heterophase polymerizations can be carried out either batchwise, semicontinuous (or semibatch), or continuous. In the batchwise case the reactor is filled with all ingredients before the polymerization is started and the reactor content is removed at the end of the polymerization. In a semibatch procedure, at the start of the poljunerization the reactor is filled only partially and a stream of either neat monomers or monomer emulsion with constant or deliberately changed composition is fed continuously until the reactor is filled. After a final post-feeding batch reaction period, the reactor is emptied. A continuous procedure means that all necessary ingredients are fed and final latex is removed continuously. In all three cases the poljunerization can be carried out in the absence or presence of preformed particles (so-called seed particles). Reactions in the absence of seed particles are frequently called ab initio polymerizations and require that particle nucleation takes place. Table 7 is an... 

Batch-, stirred-tank-, extractive semibatch-, recirculating batch-, semicontinuous flow-, continuous packed-bed-, and continuous-membrane reactors have been used as enzyme reactors, with dense gases used as solvents. 

Emulsion polymerization reactors are made of stainless steel and are normally equipped with top-entry stirrers and ports for addition of reactants. Control of the reaction exotherm and particle size distribution of the polymer latex is achieved most readily by semibatch (also called semicontinuous) processes, in which some or all of the reactants are fed into the reactor during the course of the polymerization. Examples are given in Chapter 8. In vinyl acetate copolymerizations, a convenient monomer addition rate is such that keeps the vinyl acetate/water azeotrope retluxing. at about 70°C. 

In continuous processes the reactants are fed to the reactor and the products withdrawn continuously the reactor operates under steady-state conditions. Continuous production will normally give lower production costs than batch production, but it lacks the flexibility of batch production. Continuous reactors will usually be selected for large-scale production. Processes that do not fit the definition of batch or continuous are often referred to as semicontinuous or semibatch. In a semibatch reactor, some of the reactants may be added or some of the products withdrawn as the reaction proceeds. A semicontinuous process can be one which is interrupted periodically for some purpose, for instance, for the regeneration of catalyst. 

Batch, recirculating batch, extractive semibatch, semicontinuous flow, continuously stirred tank (CSTR) and continuous packed bed reactors have alt been succesfully tested as enzyme reactors for SCFs (Figure 4.9-1). References to helpful descriptions for designing small-scale reactors for enzymatic studies are collected in Table 4.9-1. 

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. 

A BR is sometimes operated in a semicontinuous (semibatch) mode one or several of the reactants are fed into the reactor during the course of the reaction. This mode of operation is typical in the case of strongly exothermic reactions, thus avoiding excessively high temperatures in the reactor. By the semicontinuous operation mode, the product distribution can also be optimized for certain types of mixed reactions. For instance, in a mixed reaction of the types A + B R and R + B S, the yield of the intermediate, R, can be maximized by adding B in a batch containing an excess of A. 

Results for the static mixer in both laboratory scale 0.008 m (0.8 cm) and plant scale 0.0254 m (2.54 cm) operation were excellent. No change in selectivity or product distribution occurred over this scale-up. When there are compelling reasons to use a semibatch reactor instead of a semicontinuous system, the reactor... 

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