Stirred-tank reactor semibatch mode

Figure 3.1 Batch, continuous, and semibatch stirred tank reactors. The mode of operation is schematically indicated.

Therefore, many traditional designs, such as stirred tank reactors, incorporate heat transfer in the process (jacket, external or internal coil, etc.). However, in these devices, there is a significant distance between the heat transfer site and the site of the chemical reaction where heat is released. As a consequence semibatch mode is implemented while batch mode and/or systems are diluted. 

Factors re.sponsible for the occurrence of scale-up effects can be either material factors or size/shape factors. In addition, differences in the mode of operation (batch or semibatch reactor in the laboratory and continuous reactor on the full scale), or the type of equipment (e.g. stirred-tank reactor in the laboratory and packed- or plate- column reactor in commercial unit) can be causes of unexpected scale-up effects. A simple misuse of available tools and information also can lead to wrong effects. 

The ideal tank reactor is one in which stirring is so efficient that the contents are always uniform in composition and temperature throughout. The simple tank reactor may be operated in a variety of modes batch, semibatch, and continuous flow. These modes are illustrated schematically in Figure 8.1. In the simple batch reactor the fluid elements will all have the same composition, but the composition will be time dependent. The stirred tank reactor may also be... 

A condensation reaction is to be performed in a stirred tank reactor in the semibatch mode. The solvent is acetone, the industrial charge (final reaction mass) is 2500 kg, and the reaction temperature is 40 °C. The second reactant is added in a stoichiometric amount at a constant rate over two hours. Under these conditions, the maximum accumulation is 30%. The reaction does not produce any gas and its heat release rate is 20 Wkg h The reactor is equipped with a condenser with a cooling power of 250kW and the vapor tube has a diameter of 250mm. The reactor can be considered open. 

The semibatch reactor is a cross between an ordinary batch reactor and a continuous-stirred tank reactor. The reactor has continuous input of reactant through the course of the batch run with no output stream. Another possibility for semibatch operation is continuous withdrawal of product with no addition of reactant. Due to the crossover between the other ideal reactor types, the semibatch uses all of the terms in the general energy and material balances. This results in more complex mathematical expressions. Since the single continuous stream may be either an input or an output, the form of the equations depends upon the particular mode of operation. 

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. 

Semibatch and continuous stirred-tank reactors (CSTRs) are much more commonly found in polyolefin production. Semibatch reactors are the standard choice for laboratory-scale polymerizations, while CSTRs dominate industrial production, as will be seen in Section 2.5. The equations derived above are easily translated into semibatch and CSTR operation mode by simply adding terms for the inflow and outflow streams in the reactor. For instance, consider Equation 2.49 for the zeroth moment of dead chains. The molar flow rate [mol s ] leaving the reactor is given by... 

In addition to processes involving gas-liquid reactions, stirred-tank reactors can also be used for single (liquid)-phase reactions. Moreover, their operation is not limited to the continuous mode, and they can be easily adapted for use in semibatch and batch modes. The absence of a gas phase does not pose important structural and operational differences from those stated earlier for multiphase systems. However, in the case of single-phase operation, the aspect ratio is usually kept lower ( 1) to ensure well mixing of the reactive liquid. Regardless of the number of phases involved, stirred-tank reactors can approach their ideal states if perfect mixing is established. Under such conditions, it is assumed that reaction takes place immediately just... 

The stirred-tank reactor is the reactor most commonly used in emulsion polymerization. This reactor may operate in batch, semibatch, or continuous mode. 

Independently of the operation mode (batch, semibatch, or continuous), in well-mixed stirred-tank reactors the properties do not vary significantly with the position in the reactor, and time is the only independent variable. Therefore, the necessary balances for the reactor design may be made at macroscopic level - that is, for the reactor as a whole. 

The reactor is operated in the semibatch mode with component A being sparged into the stirred tank. Unreacted A and the reaction products leave through the gas phase so that the mass of liquid remains constant. To the extent that these assumptions are true and the catalyst does not deactivate, a pseudo-steady-state can be achieved. Find (flg)o j. Assume that Henry s law is valid throughout the composition range and ignore any changes in the gas density. 

Semibatch or Semiflow Reactors. Semibatch or semiflow operations usually take place in a single stirred tank using equipment extremely similar to that described for batch operations. Figure 8.1 indicates some of the many modes in which semibatch reactors may be operated. 

There are a variety of limiting forms of equation 8.0.3 that are appropriate for use with different types of reactors and different modes of operation. For stirred tanks the reactor contents are uniform in temperature and composition throughout, and it is possible to write the energy balance over the entire reactor. In the case of a batch reactor, only the first two terms need be retained. For continuous flow systems operating at steady state, the accumulation term disappears. For adiabatic operation in the absence of shaft work effects the energy transfer term is omitted. For the case of semibatch operation it may be necessary to retain all four terms. For tubular flow reactors neither the composition nor the temperature need be independent of position, and the energy balance must be written on a differential element of reactor volume. The resultant differential equation must then be solved in conjunction with the differential equation describing the material balance on the differential element. 

There are five primary reactor designs based in theory batch, semibatch, continuous-stirred tank, plug flow, and fluidized bed. The operating expressions for these reactors are derived from material and energy balances, and each represents a specific mode of operation. Selected reactor configurations are presented in Fig. 1. 

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