Single-phase semi-batch reactors

The use of this type of reactor is widespread as a liquid phase reactor in the manufacture of fine chemicals, including many polymers, pharmaceuticals, etc.. Mostly stirred tanks are used, equipped with cooling (or heating) jackets for temperature control. Usually one reactant is introduced first, the liquid is heated to a desired temperature, and a second reactant is introduced at a controlled feed rate. Sometimes other reactants are introduced, either at the same time, or one after the other. It may be practical to introduce first only a solvent, and 

When compared to a CSTR, the semi-batch reactor has some particular advantages the feeding rate of the reactant(s) can be controlled independently of the residence time, so that a more complete conversion may be obtained. By choosing various feeding programs a flexibility is obtained that is difficult to reali in a continuous reactor. Also the temperature can be changed during the process. And finally, this type of reactor is often preferred when more than one product has to be made in the same reactor. 

In many real processes there are two additional problems other chemical reactions occur at the same time, leading to mostly undesired byproducts, and the mixing cannot be considered perfect when at least one (intermediate) reaction is very rapid. It is the combination of these two phenomena that can make the scaling-up of semi-batch reactors rather difficult. 

The selectivity of rapid reactions may be sensitive to mixing conditions, particularly in the case of competitive and competitive-consecutive reactions. A detailed description of all the relevant phenomena may be quite complicated. However, we can see a priori that good mixing and well controlled feed rates will generally be favourable for the selectivity of semi-batch processes, hi section 3.4 the relation between the selectivity and the degree of conversion was calculated for a few types of reaction pairs, for batch (or plug-flow) reactors, and for CSTR s. In sections 5.2.2 and 5.2.3 the influence of incomplete micro-mixing on selectivity was briefly discussed, for turbulent and laminar flow, respectively. 

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It is good practice to enumerate the unknowns and applicable equations when setting up a model for the noni so thermal semi-batch reactor. As discussed in Chapter 4, for a single-phase system, the molar concentrations of the components, and the temperature and the pressure specify all Intensive variables of the reactor. If we use the reactor volume as the single extensive variable, then we have + 3 unknowns. As in Chapter 4, we have n.s equations from the material balances and one equation of state. The energy balance of this chapter provides an additional equation. Finally, we must specify the reactor pressure or some other/System. coBStraint. that. determines the reactor pressure. 

Batch or semi-batch reactors can be simulated as stand-alone or coupled with a continuous process. Specification may include cycle operation with buffer tanks, as well as reactions in single and multi-phases. 

In this chapter the most important operation modes of reactors are considered. Models are developed by combining simple reaction kinetics for single-phase reactions with mass balances for five ideal model reactors the ideal batch reactor the semi-batch reactor the plug flow reactor the perfectly mixed continuous reactor and the cascade of perfectly mixed reactors. For isothermal conditions, conversions can be calculated on the basis of chemical kinetics only. 

By process, we mean what occurs inside the reactor. If the material in the reactor is single phase and homogeneous, then the process is a reaction. Such a reaction can occur in a batch, a semi-batch, or a continuous reactor, depending upon our design. However, if the material in the reactor is multiphase, e.g., gas—liquid or two immiscible liquids, then it is a process. In other words, conversion of reactant to product involves more than chemical reaction it involves multiple steps, some of which are physical, such as diffusion across a phase boundary. If diffusion across a phase boundary or diffusion through one of the phases in the reactor is slower than the chemical reaction, then we define the process as diffusion rate limited. If physical diffusion occurs at a much higher rate than chemical reaction, then we define the process as reaction rate limited. ... 

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