Mass transfer rate volatile component concentration

At a particular location in a distillation column, where the temperature is 350 K and the pressure 500 m Hg, the mol fraction of the more volatile component in the vapour is 0.7 at the interface with the liquid and 0.5 in the bulk of the vapour. The molar latent heat of the more volatile component is 1.5 times that of the less volatile. Calculate the mass transfer rates (kmol m 2s 1) of the two components. The resistance to mass transfer in the vapour may be considered to lie in a stagnant film of thickness 0.5 mm at the interface. The diffusivity in the vapour mixture is 2 x 10 5 m2s Calculate the mol fractions and concentration gradients of the two components at the mid-point of the film. Assume that the ideal gas law is applicable and that the Universal Gas Constant R = 8314 J/kmol K. 

For a semi-batch operation for the first stages, optimal variations of pressure and temperature can be calculated based on the above relationships plus the assumption of phase equilibrium, or on a simple relationship for the mass transfer of each volatile component Y (Eq. (55), with the mass transfer rates per unit volume Ji of component Y , mass transfer coefficient of component i kfi, interface area per unit volume a , and equilibrium concentration [Yj at the interface). 

In processing, it is frequently necessary to separate a mixture into its components and, in a physical process, differences in a particular property are exploited as the basis for the separation process. Thus, fractional distillation depends on differences in volatility. gas absorption on differences in solubility of the gases in a selective absorbent and, similarly, liquid-liquid extraction is based on on the selectivity of an immiscible liquid solvent for one of the constituents. The rate at which the process takes place is dependent both on the driving force (concentration difference) and on the mass transfer resistance. In most of these applications, mass transfer takes place across a phase boundary where the concentrations on either side of the interface are related by the phase equilibrium relationship. Where a chemical reaction takes place during the course of the mass transfer process, the overall transfer rate depends on both the chemical kinetics of the reaction and on the mass transfer resistance, and it is important to understand the relative significance of these two factors in any practical application. 

In this case it is assumed that a pure gas A is being absorbed in a solvent eontaining a chemically inert component B. Both the solvent and B are not volatile and the fraction of A in the liquid bulk equals zero. The binary mass transfer coefficient Kij between A and the solvent in eq. (4) is given a typical value of 1 X lO" m/s, whereas the total concentration of the liquid Cr is set to 1 x 10 mol/m, also a typical value. Parameters to be chosen are the solubility of A, x i, the fraction of B in the solvent Xg, the mass transfer coefficient between A and B, K/ g and the mass transfer coefficient between B and the solvent, Kg. The results of the calculations are presented in Table 1. Since both the solvent and component B possess a zero flux. Kgs has no influence on the mass transfer process and has therefore been omitted. The computed absorption rate has been compared with the absorption rate obtained from analytical solutions for the following cases. 

Rate Measures for Interfacial Processes Terminology used for reporting rate data can be confusing. Normally rate data are reported on a volumetric basis with transfer rate and effective area combined. For example, kLa denotes mass-transfer data per unit volume. The L subscript means it is referenced to the molar concentration difference between the interface and the bulk liquid. This is commonly used on data involving a sparingly soluble (high relative volatility) component. Note that the lowercase k means the data deal only with the resistance in the liquid phase. 

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