McKay, Donald. "Front matter" Multimedia Environmental Models ...

significant resistance to transfer at the interface. It appears that if it does exist, it is
small and unmeasurable. In any event, we do not know how to estimate it, so it is
convenient to ignore it and assume that equilibrium applies. We thus argue that there
is no interfacial resistance, and C WI and C AI are in equilibrium.
and
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C AI/C WI = K AW = Z A/Z W = H/RT
C AI/K AW = C WI
The solute then diffuses in the air from C AI to C A in the bulk air with a mass
transfer coefficient k A. We can write the flux equations for each phase, noting that
the fluxes N must be equal, otherwise there would be net accumulation or loss at
the interface.
or more conveniently,
or
which is
N = k WA(C W – C WI) = k AA(C AI – C A) mol/h
C W – C WI = N/k WA
C AI – C A = N/k AA
C AI/K AW – C A/K AW = N/(k AAK AW)
C WI – C A/K AW = N/(k AAK AW)
Adding the first and last equations to eliminate C WI gives
or
where
C W – C A/K AW = N(1/k WA + 1/k AAK AW) = N/k OWA
N = k OWA(C W – C A/K AW) = k OWA(C W – P/H)
1/k OW = 1/k W + 1/k AK AW = 1/k W + RT/Hk A
The term k OW is an “overall” mass transfer coefficient that contains the individual
k W and k A terms and K AW. It should not be confused with K OW, the octanol-water