Two-phase organic processes

When the chemical reaction is not very rapid and obeys simple second order kinetics (see section 5.3), eq. (5.20) is applicable (when the Hatta number p 0.3). In most cases the resistance to gas phase mass transfer can be neglected, either because the gas consists of almost pure A, or because the solubility of A in the liquid is limited. We can then use eqs. (5,21) or (7.1) and (7.2a). For rapid chemical reactions we use eqs. (5.44), or (7.1) and (7.2b). 

When a gas in liquid dispersion is stirred vigorously, both the gas and the liquid phase may often be considered to be well mixed (see section 4.6.13), When the reaction is first order with respect to the gaseous reactant A, it makes no difference whether the gas bubbles mix with each other or not. Let us assume that the concentration of A at the interface is constant, and known. This case was discussed in section 7.1.3, where it silently was assumed that the kinetic constants and do not change with time. 

For relatively slow reactions ( p 0.3), eq, (7.4) can then be used for interpreting experimental fmdings. When the concentration of B is measured at various moments in time, both the chemical rate constant and the volumetric mass transfer coefficient can be determined. In principle, two measurements are sufficient, but a series of data will give more accurate results. 

In a semi-batch experiment with a stirred gas liquid reactor, the conversion of the liquid phase reactant B is measured as a function of time. We wish to estimate both kinetic constants. Both phases are well mixed. It is assumed that eq. (7.4) is applicable. The solubility of the gaseous reactant A is found from literature c,. = 0.1 kmol/m. We start with c =1.0 kmol/m. We find from 

In order to check the Hatta number p, we need additional data on the diffusivity of A and the ma Uransfer coefficient. The diffusivity is found from literature 0 = 2,10 m/s. With eq. (4.60) we estimate k = 3.10 ws, so that 

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