The Perfectly Mixed Flow Reactor

The stirred flow reactor is frequently chosen when temperature control is a critical aspect, as in the nitration of aromatic hydrocarbons or glycerine (Biazzi-process). The stirred flow reactor is also chosen when the conversion must take place at a constant composition, as in the copol3rmerization of butadiene and styrene, or when a reaction between two phases has to be carried out, or when a catalyst must be kept in suspension as in the polymerization of ethylene with Ziegler catalyst, the hydrogenation of a-methylstyrene to cumene, and the air oxidation of cumene to acetone and phenol (Hercules-Distillers process). 

Finally, several alternate names have been used for what here is called the perfectly mixed flow reactor. One of the earliest was continuous stirred tank-reactor, or CSTR, which some have modified to continuous flow stirred tank reactor, or CFSTR. Other names are backmix reactor, mixed flow reactor, and ideal stirred tank reactor. All of these terms appear in the literature, and must be recognized. 

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Flush The flush reaction path model is analogous to the perfectly mixed-flow reactor or the continuously stirred tank reactor in chemical engineering (Figure 2.5). Conceptually, the model tracks the chemical evolution of a solid mass through which fresh, unreacted fluid passes through incrementally. In a flush model, the initial conditions include a set of minerals and a fluid that is at equilibrium with the minerals. At each step of reaction progress, an increment of unreacted fluid is added into the system. An equal amount of water mass and the solutes it contains is displaced out of the system. Environmental applications of the flush model can be found in simulations of sequential batch tests. In the experiments, a volume of rock reacts each time with a packet of fresh, unreacted fluids. Additionally, this type of model can also be used to simulate mineral carbonation experiments. 

Figure 2.5. Schematic representation of the perfectly mixed-flow reactor model. A refers to the component of interest C denotes the concentration and stands for the feed rate or reaction progress variable.

These conclusions can be readily quantitatively visualized as shown in Fig. 10.2.b-3, which is based on the geometric nature of the plug flow or batch reactor design equation versus that for the perfectly mixed flow reactor. 

For the perfectly mixed flow reactor, the mass balances Eq. 10.2.b-2 lead to — -<0 ... 

This reactor type is the opposite extreme of the plug flow reactor dealt with in Chapter 9. The essential feature is the assumption of complete uniformity of concentration and temperature throughout the reactor, as contrasted with the assumption of no intermixing of successive fluid elements entering a plug flow vessel. Therefore, in the perfectly mixed flow reactor, the conversion takes place at a unique concentration (and temperature) level which, of course, is also the concentration of the effluent. To approach this ideal mixing pattern, it is necessary that the feed be intimately mixed with the contents of the reactor in a... 

Comparison of (10.3.1-2) and (10.3.1-3) shows that again there are differences between the yields obtained in the reactor types discussed here. A specific example is shown in Fig. 10.3.1-2, where it is seen that the batch or plug flow reactor has greater selectivity for Q relative to the perfectly mixed flow reactor. For sets of first-order reactions, Wei [1966] has shown that the convexity of reaction paths is decreased from plug flow to mixed reactors because of the intermingling of fluid elements with different extents of reaction, and so the... 

CHAPTER 10 THE PERFECTLY MIXED FLOW REACTOR TABLE 10.3.2-1... 

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