Mass overall coefficients

The relationship of the overall gas-phase mass transfer coefficient to the individual film coefficients maybe found from equations 4 and 5, assuming a straight equiHbrium line ... 

Equation 28 and its liquid-phase equivalent are very general and valid in all situations. Similarly, the overall mass transfer coefficients may be made independent of the effect of bulk fiux through the films and thus nearly concentration independent for straight equilibrium lines ... 

The main conclusion to be drawn from these studies is that for most practical purposes the linear rate model provides an adequate approximation and the use of the more cumbersome and computationally time consuming diffusing models is generally not necessary. The Glueckauf approximation provides the required estimate of the effective mass transfer coefficient for a diffusion controlled system. More detailed analysis shows that when more than one mass transfer resistance is significant the overall rate coefficient may be estimated simply from the sum of the resistances (7) ... 

Overall Coefficients. Often overall coefficients of heat and mass transfer are available, rather than the film coefficients used eadier. In that case equation 35 can be rewritten as... 

K a overall mass-transfer coefficient per volume of contacting column ... 

Kc Overall gas-phase mass-transfer coefficient for dilute systems kmoP[(s-m )(mole fraction)] (lbmol)/[(h-fF)(mole fraction)]... 

Overall enthalpy mass-transfer coefficient kmoP[(s-m )(mole fraction)] ... 

Comparison of Eqs. (5-255) and (5-257) shows that for systems in which the eqmlibrium line is straight, the overall mass transfer coefficients are related to each other by the equation... 

The overall gas-phase and hquid-phase mass-transfer coefficients for concentrated systems are computed according to the following equations ... 

When it is known that Hqg varies appreciably within the tower, this term must be placed inside the integr in Eqs. (5-277) and (5-278) for accurate calculations of hf. For example, the packed-tower design equation in terms of the overall gas-phase mass-transfer coefficient for absorption would be expressed as follows ... 

Volumetric Mass-Transfer Coefficients and Kia Experimental determinations of the individual mass-transfer coefficients /cg and /cl and of the effective interfacial area a involve the use of extremely difficult techniques, and therefore such data are not plentiful. More often, column experimental data are reported in terms of overall volumetric coefficients, which normally are defined as follows ... 

Extrapolation of KgO data for absorption and stripping to conditions other than those for which the origin measurements were made can be extremely risky, especially in systems involving chemical reactions in the liquid phase. One therefore would be wise to restrict the use of overall volumetric mass-transfer-coefficient data to conditions not too far removed from those employed in the actual tests. The most reh-able data for this purpose would be those obtained from an operating commercial unit of similar design. 

Traditional Design Method The traditionally employed conventional procedure for designing packed-tower gas-absorption systems involving chemical reactions makes use of overall volumetric mass-transfer coefficients as defined by the equation... 

Interfacial Area This consideration in agitated vessels has been reviewed and summarized by Tatterson (op. cit.). Predictive methods for interfacial area are not presented here because correlations are given for the overall volumetric mass transfer coefficient liquid phase controlhng mass transfer. 

Overall Mass-Transfer Coejficient Tatterson (op. cit.) and Zlokarnik (op. cit.) have summarized the hterature covering overall mass-transfer coefficients. There is much scatter in the experimental data because the presence of surface-ac tive agents and electrolytes have a significant effect on the mass transfer. The correlation of Van t Riet [Ind. Eng Chem. Process Des. Dev., 18(3), 357 (1979)] is recommended ... 

Measurements simply of the extent of extrac tion in an agitated vessel lead to the overall Volumetric mass-transfer coefficients, Kca, or... 

HTU = Height of a transfer unit, ft Kga = Overall gas mass-transfer coefficient, lb moles/(hr) (fF) (atm)... 

Kqa = Gas film overall mass transfer coefficient, lb mols/hr (fP) (ATM)... 

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