Unsteady state reactors Semibatch

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. 

Remark 1. Eq.(9) can be used on a CSTR in unsteady state operation or in a semibatch reactor when there is no phase change. 

Equipment in which homogeneous reactions are effected can be one of three general types the batch, the steady-state flow, and the unsteady-state flow or semibatch reactor. The last classification includes all reactors that do not fall into the first two categories. These types are shown in Fig. 4.1. 

Up to now we have focused on the steady-state operation of nonisothermal reactors. In this section the unsteady-state energy balance will be developed and then applied to CSTRs, plug-flow reactors, and well-mixed batch and semibatch reactors. 

Equation (9-9) applies to a semibatch reactor as well as unsteady-state operation of a CSTR. 

Closure. After completing this chapter, the reader should be able to appi the unsteady-state energy balance to CSTRs, semibatch and batch reactor The reader should be able to discuss reactor safety using two examples on a case study of an explosion and the other the use of the ARSST to hel prevent explosions. Included in the reader s discussion should be how t start up a reactor so as not to exceed the practical stability liniit. After reac ing these examples, the reader should be able to describe how to operat reactors in a safe maimer for both single and multiple reactions. 

Consider the reaction Ag -I- B C taking place in a semibatch reactor with a concentration Cg and an active volume V. Assume the reaction is first order to A and to B and that the gas-phase resistance is negligible. The unsteady-state material balance for A is... 

Case 5 Mixed gas, batch liquid, and unsteady state In this semibatch operation, a batch of liquid is taken in a reactor and a stream of gas is passed through it (Figure 11.4d). The problem here is usually one of calculating the time needed for a given conversion in a reactor of known volume. 

Equation (13-9) applies to a semibatch reactor, as well as for the unsteady-state operation of a CSTR and is also shown in Table I l-I as Equation (Tl l-I.I). 

In Chapter 5 we discussed the unsteady operation of one type of reactor, the batch reactor. In this section, we discuss two other aspects of unsteady operation startup of a CSTR and of semibatch reactors. First, the startup of a CSTR is examined to determine the time necessary to reach steady-state operation [see Figure 6-4(a)]. and then semibatch reactors are discussed. In each of these cases, we are interested in predicting the concentration and conversion as a function of time. Closed-form analytical solutions to the differential equations arising from the mole balance of these reaction types can be obtained only for zero- and hrst-order reactions. ODE solvers must be used for other reaction orders. 

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