Some chemical reactions are conducted in a batchwise mode [6]: raw material is charged initially into the reactor and the reaction allowed to proceed until such a time that the desired quantity of product is obtained. This is an unsteady state process wherein the conditions within the reactor with respect to the number of moles of various species vary with respect to time. Other conditions, such as the temperature and pressure, may vary depending on the mode of operation. Some reactors are operated isothermally, that is, at constant temperature, by either removing the heat from or supplying it to the reactor as needed. Other reactors are operated adiabatically, that is, with no heat/energy exchange with the surroundings. The reactor temperature will vary with respect to time in this case. Similarly, particularly for gaseous reactions, pressure may vary with respect to time if the reaction is conducted under constant volume conditions. For gaseous reactions, the pressure variation with the progress of the reaction must to be taken into account when the reaction stoichiometry involves a change in the total number of moles going from the reactants to the products. The component balance shown in equation 9.1 is simplified for a batch reactor by setting the molar flow rate terms to 0, leading to equation 9.4.
The reactor may also be operated in a continuous mode, with the raw material fed continuously to the reactor and the product withdrawn continuously. These reactors typically operate under steady-state conditions wherein the reactor conditions remain invariant with respect to time. Equation 9.1 simplifies to equation 9.5 for continuous steady-state reactors.
Continuous reactors may also be operated isothermally or adiabatically. Figure 9.2 shows the schematics of a batch reactor and three types of continuous reactors [5]. The batch reactor does not have any influent or effluent streams, whereas all the continuous reactors have both the influent and effluent streams. The stirrer/agitator in the batch reactor indicates that contents are well mixed; that is, there are no spatial variations in conditions with location within the reactor. Similarly, the contents of the first of the continuous reactors—the mixed flow reactor (MFR) or the continuous stirred tank reactor (CSTR)—are well mixed, and uniform conditions exist throughout the reaction vessel. The other two reactors are tubular reactors having the configuration of a long pipe. Obviously, the contents of these types of reactors are not well mixed, and the conditions (concentrations and possibly temperature and pressure) vary as a function of position or location within the reactor. The first of these reactors is essentially an empty pipe or tube through which the reacting fluid flows, typically in what is termed the plug flow pattern. This plug flow reactor (PFR) is used for conducting homogeneous gas- or liquid-phase reactions. It is also possible to operate the reactor such that the flow within the reactor is laminar (refer to Chapter 5, “Computations in Fluid Flow”); however, such laminar flow reactors (LFRs) are not as common as the PFRs. As mentioned earlier, many chemical reactions require the use of solid catalysts. The fourth configuration shown in the figure is a packed bed reactor (PBR), a tubular reactor filled with solid catalyst particles operated in plug flow mode for conducting heterogeneous reactions.
Figure 9.2 Batch and continuous reactors: (a) batch reactor, (b) mixed flow reactor/continuous stirred tank reactor, (c) plug flow reactor, (d) packed bed reactor.
Adapted from: Fogler, H. S., Elements of Chemical Reaction Engineering, Fourth Edition, Prentice Hall, Upper Saddle River, New Jersey, 2004.
It can be seen that the molar flow rates in equation 9.5, or the rate of change of moles in equation 9.4, are determined by the specified production rate. Knowing intrinsic reaction rate ri allows us to determine the reactor volume or perform other calculations for reactor design based on equation 9.4 for a batch reactor or equation 9.5 for continuous reactors, as described shortly. Note that sometimes reactions are conducted in a semibatch mode wherein one of the reactants is added to another reactant that is already present in the reactor and no product is withdrawn from the reactor until the reaction is completed. Other semibatch modes may include no inflow of reactants but a continuous removal of one or more of the product streams, and many other arrangements involving complex feed/product withdrawal and heating/cooling cycles. Such complex operations require the use of equation 9.1 for design and are not dealt with in this text.
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