Gas-liquid CSTR

Strictly gas-phase CSTRs are rare. Two-phase, gas-liquid CSTRs are common and are treated in Chapter 11. Two-phase, gas-solid CSTRs are fairly common. When the solid is a catalyst, the use of pseudohomogeneous kinetics allows these two-phase systems to be treated as though only the fluid phase were present. All concentration measurements are made in the gas phase, and the rate expression is fitted to the gas-phase concentrations. This section outlines the method for fitting pseudo-homogeneous kinetics using measurements made in a CSTR. A more general treatment is given in Chapter 10. 

Example 11.5 Suppose that the liquid phase in a gas-liquid CSTR contains a catalyst for the hrst-order reaction of a compound supplied from the gas phase. The reaction is... 

Some specific aspects in the modeling of gas-liquid continuous-stirred tank reactors are considered. The influence of volatility of the liquid reactant on the enhancement of gas absorption is analyzed for irreversible second-order reactions. The impact of liquid evaporation on the behavior of a nonadiabatic gas-liquid CSTR where steady-state multiplicity occurs is also examined. 

The second part of this paper is devoted to assessing the influence of liquid evaporation on the steady-state behavior of gas-liquid CSTRs. The reaction factor expression developed in the first part is utilized for this purpose. 

The occurrence of steady-state multiplicity in gas-liquid CSTRs has been demonstrated in experimental (9) and theoretical investigations (cf., 10). The irreversible second-order reaction system, in particular, has been treated extensively in several theoretical studies (10-15). These studies are however based on neglecting energy and material losses which result from evaporation of the liquid. 

In this section we develop a model to simulate the behavior of a nonadiabatic gas-liquid CSTR taking into account volatility of the liquid reactant. The model is then tested for its capability to predict some experimental results. 

Model vs. Experimental Results. The model described by the preceding equations was used to simulate the behavior of an experimental gas-liquid CSTR (chlorination of n-decane ( 9)), and its predictions were compared to those of a model in which the liquid volatility is neglected. The physicochemical parameters used in the simulation are given elsewhere (10, 16), and the computational procedure adopted to solve for 0 is also reported elsewhere (16). 

It is demonstrated that volatility of the liquid reactant has a detrimental effect on the enhancement of gas absorption. It is also shown that failing to account for effects due to liquid evaporation in the modeling of gas-liquid CSTRs can lead to significantly different predictions of the number and region of multiple steady states. 

Gas-liquid CSTR with jacketed cooling/heating Fixed interlace CSTR Wetted wall Laminar jet... 

We apply the concepts discussed above to design a CSTR that operates at 55 °C for the chlorination of benzene in the hquid phase. It is necessary to account for all three chlorination reactions. Chlorine gas is bubbled through the liquid mixture in the CSTR and it must diffuse across the gas-liquid interface before any of the reactions can occur. For this particular problem, it is reasonable to assume that chlorine is present as a solubilized liquid-phase component, and its molar density in the inlet liquid stream is given as a fraction e of the inlet molar density of pure liquid benzene. In a subsequent example discussed in Chapter 24, a two-phase gas-liquid CSTR analysis is presented which accounts for the realistic fact that benzene enters the reactor in an undiluted liquid stream, and chlorine is actually bubbled through as a gas. It is sufficient to consider that the fraction e = 0.25 remains constant for all simulations. In the first chlorination step, benzene reacts irreversibly with dissolved chlorine to produce monochlorobenzene and hydrogen chloride ... 

DESIGNING A MULTICOMPONENT ISOTHERMAL GAS-LIQUID CSTR FOR THE CHLORINATION OF BENZENE TO PRODUCE MONOCHLOROBENZENE... 

Design a two-phase gas-liquid CSTR that operates at 55°C to accomplish the liquid-phase chlorination of benzene. Benzene enters as a liquid, possibly diluted by an inert solvent, and chlorine gas is bubbled through the liquid mixture. It is only necessary to consider the first chlorination reaction because the kinetic rate constant for the second reaction is a factor of 8 smaller than the kinetic rate constant for the first reaction at 55°C. Furthermore, the kinetic rate constant for the third reaction is a factor of 243 smaller than the kinetic rate constant for the first reaction at 55°C. The extents of reaction for the second and third chlorination steps ( 2 and 3) are much smaller than the value of for any simulation (i.e., see Section 1-2.2). Chlorine gas must diffuse across the gas-liquid interface before the reaction can occur. The total gas-phase volume within the CSTR depends directly on the inlet flow rate ratio of gaseous chlorine to hquid benzene, and the impeller speed-gas sparger combination produces gas bubbles that are 2 mm in diameter. Hence, interphase mass transfer must be considered via mass transfer coefficients. The chemical reaction occurs predominantly in the liquid phase. In this respect, it is necessary to introduce a chemical reaction enhancement factor to correct liquid-phase mass transfer coefficients, as given by equation (13-18). This is accomplished via the dimensionless correlation for one-dimensional diffusion and pseudo-first-order irreversible chemical reaction ... 

The sequence of equations presented below is required to solve the isothermal gas-liquid CSTR problem for the chlorination of benzene in the liquid phase at 55°C. After some simplifying assumptions, the problem reduces to the solution of nine equations with nine unknowns. Some of the equations are nonlinear because the chemical kinetics are second-order in the liquid phase and involve the molar densities of the two reactants, benzene and chlorine. The problem is solved in dimensionless form with the aid of five time constant ratios that are generated by six mass transfer rate processes (1) convective mass transfer through the reactor, (2) molecular transport in the liquid phase across the gas-liquid interface for each of the four components, and (3) second-order chemical reaction in the liquid phase. 

Performance curves for this gas-liquid CSTR, based on the preceding system of equations and parameters, are illustrated in Figure 24-1. A reasonable design corresponds to 10 < r/X < 10, where the total outlet flow rate of chlorobenzene is between 60 and 93% of the inlet flow rate of liquid benzene, and 45% of the total chlorobenzene product exits the CSTR as a liquid. 

Further manipulation of the system of equations in Section 24-4.7 that describe the performance of the gas-liquid CSTR ... 

Design a two-phase gas-liquid CSTR for the chlorination of benzene at 55°C by calculating the total volume that corresponds to an operating point where r/X = 500 on the horizontal axis of the CSTR performance curve in Figure 24-1. The time constant for convective mass transfer in the liquid phase is r. The time constant for second-order irreversible chemical reaction in the liquid phase is If the liquid benzene feed stream is diluted with an inert, then 7 increases. The liquid-phase volumetric flow rate is 5 gal/min. The inlet molar flow rate ratio of chlorine gas to liquid benzene... 

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