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are available for heat transfer for a given rate of mass transfer in this
type of equipment because of the low mass-transfer rate inherent in
wetted-wall equipment. In addition, this type of equipment lends
itself to annular-type cooling devices.
Gilliland and Sherwood [Ind. Eng. Chem., 26, 516 (1934)] found
that, for vaporization of pure liquids in air streams for streamline flow,
kgDtube
� Dg
OTHER TOPICS FOR DISTILLATION AND GAS ABSORPTION EQUIPMENT 14-83
FIG. 14-77 Mass-transfer rates in wetted-wall columns having turbulence promoters. To convert
pound-moles per hour-square foot-atmosphere to kilogram-moles per second-square meter-atmosphere,
multiply by 0.00136; to convert pounds per hour-square foot to kilograms per second-square meter, multiply
by 0.00136; and to convert inches to millimeters, multiply by 25.4. (Data of Greenewalt and Cogan
and Cogan, Sherwood, and Pigford, Absorption and Extraction, 2d ed., McGraw-Hill, New York, 1952.)
PBM
� P
0.83 0.44 = 0.023NRe NSc (14-171)
where Dg = diffusion coefficient, ft2 /h
Dtube = inside diameter of tube, ft
kg = mass-transfer coefficient, gas phase,
lb⋅mol/(h⋅ft2 ) (lb⋅mol/ft3 )
PBM = logarithmic mean partial pressure of inert gas, atm
P = total pressure, atm
NRe = Reynolds number, gas phase
NSc = Schmidt number, gas phase
Note that the group on the left side of Eq. (14-171) is dimensionless.
When turbulence promoters are used at the inlet-gas section,
an improvement in gas mass-transfer coefficient for absorption of
water vapor by sulfuric acid was observed by Greenewalt [Ind. Eng.
Chem., 18, 1291 (1926)]. A falling off of the rate of mass transfer
below that indicated in Eq. (14-171) was observed by Cogan and
Cogan (thesis, Massachusetts Institute of Technology, 1932) when a
calming zone preceded the gas inlet in ammonia absorption (Fig.
14-77).
In work with the hydrogen chloride-air-water system, Dobratz,
Moore, Barnard, and Meyer [Chem. Eng. Prog., 49, 611 (1953)]
using a cocurrent-flow system found that Kg�G 1.8 (Fig. 14-78)
instead of the 0.8 power as indicated by the Gilliland equation.
Heat-transfer coefficients were also determined in this study. The
radical increase in heat-transfer rate in the range of G = 30
kg/(s⋅m2 ) [20,000 lb/(h⋅ft2 )] was similar to that observed by Tepe
and Mueller [Chem. Eng. Prog., 43, 267 (1947)] in condensation
inside tubes.
FIG. 14-78 Mass-transfer coefficients versus average gas velocity—HCl
absorption, wetted-wall column. To convert pound-moles per hour-square footatmosphere
to kilogram-moles per second-square meter-atmosphere, multiply
by 0.00136; to convert pounds per hour-square foot to kilograms per secondsquare
meter, multiply by 0.00136; to convert feet to meters, multiply by 0.305;
and to convert inches to millimeters, multiply by 25.4. [Dobratz et al., Chem.
Eng. Prog., 49, 611 (1953).]