CHEMICAL ENGINEERING

MASS TRANSFER 227

The Lewis numberDh/Cp+hDDPr/ScDDCp+/k.

D/DHDDCp+/kD 90

CAiCAo D 1 .54 kmol/m^3

HSD28 kJ/kmol

∴ TD 1. 54 ð 28

p

90 

/ 1000 ð 4. 186

 D 0 .1degK

PROBLEM 10.10

In a packed column, operating at approximately atmospheric pressure and 295 K, a 10%
ammonia-air mixture is scrubbed with water and the concentration is reduced to 0.1%.
If the whole of the resistance to mass transfer may be regarded as lying within a thin
laminar film on the gas side of the gas-liquid interface, derive from first principles an
expression for the rate of absorption at any position in the column. At some intermediate
point where the ammonia concentration in the gas phase has been reduced to 5%, the
partial pressure of ammonia in equilibrium with the aqueous solution is 660 N/m^2 and
the transfer rate is 10^3 kmol/m^2 s. What is the thickness of the hypothetical gas film if
the diffusivity of ammonia in air is 0.24 cm^2 /s?

Solution

The equation for the rate of absorption is derived in Section 10.2.2 as:

NADD/RTL

PA 2 PA 1

 (equation 10.23)

If subscripts 1 and 2 refer to the water and air side of the stagnant film and subscriptsA
andBrefer to ammonia and air, then:

PA 1 D 66 .0kN/m^2 andPA 2 D 0. 05 ð 101. 3

 D 5 .065 kN/m^2

DD 0. 24 ð 10 ^4 m^2 /s, RD 8 .314 kJ/kmol K,

TD295 K andNAD 1 ð 10 ^3 kmol/m^2 s

∴ LDD/NART
PA 2 PA 1

D 0. 24 ð 10 ^4 / 10 ^3 ð 8. 314 ð 295 

 66. 0  5. 065

 D 0 .000043 m

The negative sign indicates that the diffusion is taking place in the opposite direction
and the thickness of the gas film is 0.043 mm.

PROBLEM 10.11

An open bowl, 0.3 m in diameter, contains water at 350 K evaporating into the atmo-
sphere. If the air currents are sufficiently strong to remove the water vapour as it is formed