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

ments of resuspension rates are particularly difficult, because the resuspension event
may be triggered periodically by a storm or flood or by an especially energetic fish
chasing prey at the bottom of the lake. Regrettably, the sediment-water interface is
not easily accessible, thus measurements are few, difficult, and expensive.
Measurement of diffusion D values usually involves setting up a system in which
there is a known fugacity driving force (f 1 – f 2) and the capacity to measure N,
leaving the overall transport D value as the only unknown in the flux equation. A
difficulty arises because, for a two-resistance in series system, it is impossible to
measure the concentrations or fugacity at the interface; therefore, it is not possible
to deduce the individual D values that combine to give the overall D value. The
subterfuge adopted is to select systems in which one of the resistances dominates
and that resistance can be equated to the total resistance. Guidance on chemical
selection can be obtained from the location of the substance on Figure 7.7.
Air phase mass transfer coefficients (MTCs) can be determined directly by
measuring the evaporation rate of a pool of pure liquid, or even the sublimation rate
of a volatile solid. The interfacial partial pressure, fugacity, or concentration of the
solute can be found from vapor pressure tables. The concentration some distance
from the surface can be zero if an adequate air circulation is arranged, thus DC or
Df is known. The pool can be weighed periodically to determine N, and area A can
be measured directly, thus the MTC or evaporation D value is the only unknown.
Worked Example 7.3
A tray (50 ¥ 30 cm in area) contains benzene at 25°C (vapor pressure 12,700 Pa).
The benzene is observed to evaporate into a brisk air stream at a rate of 585 g/h.
What are D and k M, the mass transfer coefficient?
Since the molecular mass is 78 g/mol, N is 585/78 or 7.5 mol/h.
©2001 CRC Press LLC
Df = (12700 – 0) Pa
D = 7.5/12700 = 5.9 ¥ 10 –4 mol/Pa h
A is 0.5 ¥ 0.3 or 0.15 m 2 . Z A is 1/RT or 4.04 ¥ 10 –4 . Since D is k MAZ, k M is 9.7 m/h.
In conventional units, DC is 12,700/RT or 5.13 mol/m 3 .
N = k MADC
Thus, k M = 9.7 as before.
Obviously, the two approaches are algebraically equivalent. Using an experimental
system of this type, the dependence of k M on wind speed can be measured.
Measurement of overall intermedia D values or MTCs is similar in principle, Df
applying between two bulk phases. A convenient method of measuring water-to-air
transfer is to dissolve the solute in a tank of water, blow air across the surface to
simulate wind, and measure the evaporation rate indirectly by following the decrease
in concentration in the water with time. If the water volume is V m 3 , area is A m 2 ,
and depth is Y m, then