Info
2000 4000 6000 10,000 20,000 Average <?, (Ib.)/(hn)(sq.ft.)
2000 4000 6000 10,000 20,000 Average <?, (Ib.)/(hn)(sq.ft.)
FIG. 1478 Masstransfer coefficients versus average gas velocity—HCl absorption, wettedwall column. To convert poundmoles per hoursquare footatmosphere to kilogrammoles per secondsquare meteratmosphere, multiply by 0.00136; to convert pounds per hoursquare 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).]
Gaylord and Miranda [Chem. Eng. Prog., 53,139M (1957)] using a multitube cocurrentflow fallingfilm hydrochloric acid absorber for hydrogen chloride absorption found
Mi75
DtubeG
where Kg = overall masstransfer coefficient, (kg mol)/(s m2 atm) Mm = mean molecular weight of gas stream at inlet to tube Dtube = diameter of tube, m
G = mass velocity of gas at inlet to tube, kg/(sm2)  = viscosity of gas, Pas
Note that the group DubeG/l is dimensionless. This relationship also satisfied the data obtained for this system, with a singletube fallingfilm unit, by Coull, Bishop, and Gaylor [Chem. Eng. Prog., 45, 506 (1949)].
The rate of mass transfer in the liquid phase in wettedwall columns is highly dependent on surface conditions. When laminarflow conditions prevail without the presence of wave formation, the laminarpenetration theory prevails. When, however, ripples form at the surface, and they may occur at a Reynolds number exceeding 4, a significant rate of surface regeneration develops, resulting in an increase in masstransfer rate.
If no wave formations are present, analysis of behavior of the liquidfilm mass transfer as developed by Hatta and Katori [J. Soc. Chem. Ind., 37, 280B (1934)] indicates that k, = 0.422 .
Dl = liquidphase diffusion coefficient, m2/s p = liquid density, kg/m3 Z = length of surface, m kl = liquidfilmtransfer coefficient,
(kgmol)/[(sm2)(kgmol)/m3] r = liquidflow rate, kg/(sm) based on wetted perimeter  = viscosity of liquid, Pa s g = gravity acceleration, 9.81 m/s2
FIG. 1479 Liquidfilm resistance in absorption of gases in wettedwall columns. Theoretical lines are calculated for oxygen absorption in water at 55°F. To convert feet to meters, multiply by 0.3048; °C = % (°F — 32). (Sherwood and Pigford, Absorption and Extraction, 2d ed., McGrawHill, New York, 1952. )
FIG. 1479 Liquidfilm resistance in absorption of gases in wettedwall columns. Theoretical lines are calculated for oxygen absorption in water at 55°F. To convert feet to meters, multiply by 0.3048; °C = % (°F — 32). (Sherwood and Pigford, Absorption and Extraction, 2d ed., McGrawHill, New York, 1952. )
When Z is large or r/pBF is so small that liquid penetration is complete, and kf = 11.800 Df/BF
A comparison of experimental data for carbon dioxide absorption obtained by Hatta and Katori (op. cit.), Grimley [Trans. Inst. Chem. Eng., 23, 228 (1945)], and Vyazov [Zh. Tekh. Fiz. (U.S.S.R.), 10, 1519 (1940)] and for absorption of oxygen and hydrogen by Hodgson (S.M. thesis, Massachusetts Institute of Technology, 1949), Henley (B.S. thesis, University of Delaware, 1949), Miller (B.S. thesis, University of Delaware, 1949), and Richards (B.S. thesis, University of Delaware, 1950) was made by Sherwood and Pigford (Absorption and Extraction, McGrawHill, New York, 1952) and is indicated in Fig. 1479.
In general, the observed masstransfer rates are greater than those predicted by theory and may be related to the development of surface rippling, a phenomenon which increases in intensity with increasing liquid path.
Vivian and Peaceman [Am. Inst. Chem. Eng. J., 2, 437 (1956)] investigated the characteristics of the CO2H2O and Cl2HCl, H2O system in a wettedwall column and found that gas rate had no effect on the liquidphase coefficient at Reynolds numbers below 2200. Beyond this rate, the effect of the resulting rippling was to increase significantly the liquidphase transfer rate. The authors proposed a behavior relationship based on a dimensional analysis but suggested caution in its application concomitant with the use of this type of relationship. Cognizance was taken by the authors of the effects of column length, one to induce rippling and increase of rate of transfer, one to increase time of exposure which via the penetration theory decreases the average rate of mass transfer in the liquid phase. The equation is kh j u,
where Dt = diffusion coefficient of solute in liquid, ft2/h g = gravityacceleration constant, 4.17 X 108 ft/h2 h = length of wetted wall, ft ke = masstransfer coefficient, liquid phase, ft/h

Post a comment