CSTRs, two-phase

The inlet molar flow rate of chlorine gas is k times the inlet molar flow rate of liquid benzene, and /c = 2 is a parameter that remains constant for each simulation. The overall objective of this problem is to design a two-phase CSTR that will maximize the rate of production of monochlorobenzene. Economics should be considered from a quahtative viewpoint. Generate graphs of ... 

There are a total of 17 variables that describe the performance of this two-phase CSTR. If the condition of perfect backmixing is achieved, then these unknowns in the outlet liquid and gas streams are the same as the corresponding variables... 

The next task is to generate 17 equations that relate the 17 unknowns defined above. If this can be done, then the performance of the two-phase CSTR is unique to each set of parameters. Furthermore, if one solves the system of equations described below for a wide range of liquid-phase residence times, then CSTR performance curves can be generated for the outlet flow rate of monochlorobenzene, which is the desired product. For clarity, the 17 coupled algebraic equation are numbered below. 

Steady-state mass balances at the gas-liquid interface for each component provide four more equations that describe the performance of the two-phase CSTR. The interface is the control volume, in which there is no accumulation of mass. Hence, the rate at which component j diffuses toward the interface from the bulk gas mixture must be balanced by the rate at which it diffuses away from the interface into the bulk liquid phase. Using the notation above,... 

The last restriction defines the interfacial temperature 7). The performance of the two-phase CSTR is completely defined in terms of 18 equations and 18 unknowns. 

The next objective is to identify a time constant for each important mass transfer rate process and solve the system of equations for the two-phase CSTR in terms of these time constants. This approach allows one to develop generic solutions in dimensionless form. For example, six time constants can be defined for (1) convection in the liquid phase (r), (2) chemical reaction in the liquid phase (X), and (3-6) interphase mass transfer for each component (0y, j = B,C1,M,H). Obviously, these six time constants produce five dimensionless ratios. Remember that time constants represent order-of-magnitude estimates of the time scales of mass transfer rate processes. The time constant for convective mass transfer in the liquid phase is equivalent to the liquid s residence time ... 

Summary of Equations That Describe the Two-Phase CSTR Performance... 

The two-phase CSTR problem has been reduced to the solution of the following nine nonlinear algebraic equations for S, xj, and yj, where j = B,C1,M,H,... 

Rigorous TVial-and-Error Solution of the Two-Phase CSTR without Assistance from a Nonlinear Algebraic Equation Soiver... 

Qualitatively sketch the outlet gas-phase mole fraction of chlorine (i.e., CI2) and HCl versus log(r/A.) in the two-phase CSTR at a constant value of the inlet molar flow rate ratio of chlorine gas to liquid benzene. Put both curves on the same set of axes. 

Now consider a two phase CSTR consisting of an isobaric flash separator with a simultaneous chemical reaction in the liquid phase, as shown in Fig. 6.1. 

Good improvement in selectivity is possible by using a two-phase CSTR over a conventional reactor. Since the desired product, B is the lightest boiling component in the mixture, vaporization improves yield and selectivity to B. It is therefore logical to string together a series of CSTRs where the hquid and vapor streams move in a counter-current fashion (Fig. 6.4). 

Fig. 6.4 Schematic of a counter-current cascade of two-phase CSTR s...

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