Page 151 - Separation process principles 2

116 Chapter 3 Mass Transfer and Diffusion

Temperature of inlet water, 25.15"C, Temperature of outlet water, The operation was countercurrent, with the gas entering at the bot-
25.35OC tom of the vertical tower and the acid passing down in a thin film
Partial pressure of water in inlet air, 6.27 ton, and in outlet air, on the inner wall. The change in acid strength was inappreciable,
20.1 torr and the vapor pressure of ammonia over the liquid may be as-
The value for the diffusivity of water vapor in air is 0.22 cm2/s at sumed to have been negligible because of the use of a strong
0°C and 1 atm. The mass velocity of air is taken relative to the pipe acid for absorption. Calculate the mass-transfer coefficient, kp,
wall. Calculate: from the data.
(a) Rate of mass transfer of water into the air 3.40 Anew type of cooling-tower packing is being tested in a lab-
oratory column. At two points in the column, 0.7 ft apart, the fol-
(b) KG for the wetted-wall column
lowing data have been taken. Calculate the overall volumetric
3.39 The following data were obtained by Chamber and
mass-transfer coefficient K,a that can be used to design a large,
Shenvood [Ind. Eng. Chem., 29, 14 15 (1937)l on the absorption of
packed-bed cooling tower, where a is the mass-transfer area, A, per
ammonia from an ammonia-air system by a strong acid in a wetted-
unit volume, V, of tower.
wall column 0.575 in. in diameter and 32.5 in. long:
Inlet acid (2-N H2SO4) temperature, OF 76
Bottom
81
Outlet acid temperature, OF
Water temperature, OF 120
Inlet air temperature, OF 77
Water vapor pressure, psia 1.69
Outlet air temperature, OF 84
Mole fraction H20 in air 0.001609
Total pressure, atm 1 .OO
Total pressure, psia 14.1
Partial pressure NH3 in inlet gas, atm 0.0807 Air rate, lbmolth 0.401
Partial pressure NH3 in outlet gas, atm 0.0205 Column area, ft2 0.5
Air rate, lbmolh 0.260 Water rate, lbmolk (approximate) 20