Semibatch reactors ideal, 66

Semibatch reactors are often used to mn highly exothermic reactions isothermally, to run gas-liquid(-solid) processes isobarically, and to prevent dangerous accumulation of some reactants in the reaction mixture. Contrary to batch of)eration, temperature and pressure in semibatch reactors can be varied independently. The liquid reaction mixture can be considered as ideally mixed, while it is assumed that the introduced gas flows up like a piston (certainly this is not entirely true). Kinetic modelling of semibatch experiments is as difficult as that of batch, non-isotherma experiments. 

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. 

The semibatch reactor is one of the primary ideal reactor types since it can not be accurately described as either a continuous or a batch reactor. A semibatch reactor is usually classified as a type of transient reactor. 

There are four ideal reactors the batch reactor (real counterpart stirred tank reactor), semibatch reactor,1 continuous stirred tank reactor (CSTR), and the plug flow tubular reactor (PFTR) (real counterpart tube reactor). For production applications, there are also numerous other reactors [7-9], An overview of typical and advanced laboratory reactors was given by Kapteijn and Moulijn [6],... 

Ideal batch and semibatch reactors. v(r) is a volumetric flow rate that can vary with time. 

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 ... 

The influence of heat transfer on yield and selectivity in scaling up batch and semibatch reactors will be illustrated using a series reaction, taking place in an ideal jacketed stirred-tank reactor. This reaction is composed of two irreversible elementary steps, both exothermic and both with first order kinetics ... 

There are two basic types of ideal reactors, stirred tanks, for reactions in liquids, and tubular or packed-bed reactors, for gas or liquid reactions. Stirred-tank reactors include batch reactors, semibatch reactors, and continuous stirred-tank reactors, or CSTRs. The criterion for ideality in tank reactors is that the liquid be perfectly mixed, which means no gradients in temperature or concentration in the vessel. 

When a reactor is charged with liquid A and B is a gas that is added continuously, it becomes a semibatch reactor. The rates of reaction depend on the concentration of B in the liquid phase, which is a function of gas solubility, pressure, and agitation conditions. However, we are often concerned with the relative reaction rates and the selectivity, which do not depend on Cb if the reaction orders are the same for both reactions. The reactions are treated as pseudo-first-order, and equations are developed for an ideal batch reactor with irreversible first-order kinetics... 

Unlike batch/semibatch reactors, the mean residence time of a CSTR at steady state is defined by the ratio of volume inside the reactor to volumetric feed rate, which at equal density of feed and reactor contents is equal to the reactor space time. The advantage of the CSTR over the batch or semibatch reactor is that it is ideally suitable for long runs of continuous production of a polymer product. Once the reactor process is brought to steady state, uniform quality and consistent product is made. However, the CSTR requires several reactor turnovers (at least 3-4) before the process is at steady state and uniform product is made [10]. 

In the analysis of batch reactors, the two flow terms in equation (8.0.1) are omitted. For continuous flow reactors operating at steady state, the accumulation term is omitted. However, for the analysis of continuous flow reactors under transient conditions and for semibatch reactors, it may be necessary to retain all four terms. For ideal well-stirred reactors, the composition and temperature are uniform throughout the reactor and all volume elements are identical. Hence, the material balance may be written over the entire reactor in the analysis of an individual stirred tank. For tubular flow reactors the composition is not independent of position and the balance must be written on a differential element of reactor volume and then integrated over the entire reactor using appropriate flow conditions and concentration and temperature profiles. When non-steady-state conditions are involved, it will be necessary to integrate over time as well as over volume to determine the performance characteristics of the reactor. 

Batch Processes In an ideal batch reactor, as defi ned in Chapter 3, all of the reactants are charged at once, after which there is no flow of mass across the system boundaries. With this definition, B cannot be added as the reaction proceeds, as in the continuous examples shown above. On the other hand, we could employ a modification of the batch reactor, known as a semibatch reactor. All of the A is added initially. Compound B is fed slowly as the reaction proceeds. This idea is represented in the Figure 7-9. 

To minimize the amount of D that will be formed, an ideal, semibatch reactor will be used. The reactor will be operated isothermally at a temperature where the values of the rate constants are ki = 0.501/mol-h 2 = 0.251/mol-h. 

Batch and semibatch reactors are ideal when the production rate of the polymer needed is small. In larger-capacity plants, continuous reactors are preferred. In these, the raw materials are pumped in continuously while the products are removed at the other end. One example of these is a tubular reactor (shown in Fig. 4.1c). It is like an ordinary tube into which material is pumped at one end. Polymerization occurs in the tubular reactor, and the product stream consists of the polymer along with the unreacted monomer. Sometimes, a stirred vessel (shown in Fig. 4.Id) is employed instead of a tubular reactor. The advantage of such a reactor is that the concentration and temperature variations... 

There are three idealized flow reactors fed-batch or semibatch, continuously stirred tank, and the plug flow tubular. Each of these is pictured in Figure 1. The fed-batch and continuously stirred reactors are both taken as being well mixed. This means that there is no spatial dependence in the concentration variables for each of the components. At any point within the reactor, each component has the same concentration as it does anywhere else. The consequence... 

In order to account for both micromixing and mesomixing effects, a mixing model for precipitation based on the SFM has been developed and applied to continuous and semibatch precipitation. Establishing a network of ideally macromixed reactors if macromixing plays a dominant role can extend the model. The methodology of how to scale up a precipitation process is depicted in Figure 8.8. 

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... 

FIG. 7-1 Types of ideal reactors (a) Batch or semibatch. (b) CSTR or series of CSTRs. (c) Plug flow. 

Fig. 3-1 Ideal stirred-tank reactors classified according to method of operation (a) flow (steady-state), (b) batch, (c) semibatch (non-steady-flow)...

Semibatch Operation In semibatch operation the rates of mass flow into and out of the system are unequal (see Fig. 3-1 c). For example, benzene may be chlorinated in a stirred-tank reactor by first adding the charge of liquid benzene and catalyst and then continuously adding chlorine gas until the required ratio of chlorine to benzene has been obtained. Operation of this kind is. batch from the standpoint that the composition of the reaction mixture changes with time. However, from a process standpoint the chlorine is added continuously. The system is still an ideal stirred-tank reactor if the... 

These considerations also apply to continnons-tank reactors, batch or semibatch. However, the manner of contact between molecnles also depends on the geometry of the reactors. One should avoid dead volume and for this purpose strong stirring can be used. The higher the stirring the better the contact between molecules and the lower the chance of dead volume. The contact is instantaneous and the concentrations in the tank or in the batch should be uniform, and if possible equal to the reactor outlet. One reaches the ideal condition when the mixture is perfectly uniform. Figure 14.2 illustrates the different cases. 

The conventional ideal reactors are batch, continuous, and semibatch. The conditions established for ideal reactors were shown in the previous section, and recapping, tanks should have perfect mixture and tubular reactors should have plug flow. 

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