Mass-transfer coefficients alternate forms

Godfrey (1994) recommends alternative forms. 16.7.4 Mixer Mass-Transfer Coefficients... 

Possibilities for a single resistance include a linear rate expression with a lumped parameter mass transfer coefficient based either on the external fluid film or on a hypothetical solid film, depending on which film is controlling the rate of uptake of adsorbate. A quadratic driving force expression, again with a lumped parameter mass transfer coefficient, may be used instead. Alternatively, intraparticle diffusion, if the dominant form of mass transfer, may be described by the general diffusion equation (Pick s second law) with its appropriate boundary conditions, as described in Chapter 4. 

Thus, the mass transfer coefficient k is analogous to the reciprocal of the resistance. An alternative form of Ohm s law is... 

However, in some other cases, alternative forms of the mass transfer coefficients lead to simpler final equations. This is especially true for gas adsorption, distillation, and extraction described in Chapters 10-14. There, we will frequently use kx, the third form in Table 8.2-2, which expresses concentrations in mole fractions. In some cases of gas absorption, we will find it convenient to respect seventy years of tradition and use kp, with concentrations expressed as partial pressures. In the membrane separations in Chapter 18, we will mention forms like kx but will carry out our discussion in terms of forms equivalent to k. 

Van Deemter Equation the first theoretical expression of the dependence of H on u H = A-pB/u-pC.u, where the coefficients A, B and C were explicitly derived in terms of physico-chemical parameters. An alternative and quite commonly used formulation is that of Knox h = A.vJ -l-B/v-l-C.v, where h = H/dp and v = u.(dp/D ) are the reducedpbdn height and linear velocity. The Knox equation is a convenient functional form for fitting experimental data (it accounts for a minor coupling observed between multipath dispersion and resistance to mass transfer) but does not provide expressions for the coefficients in terms of more fundamental parameters. 

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