ea HARRISON
ABSORPTION AND HUMIDIFICATION CARLSON,
C. E. I. DU PONT DE NEMOURS a COMPANY, INC., WILMINGTON, DEL.
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water and wet-bulb temperatures for mechanical draft cooling towers by several examples intended for the operator rather than the designer. Atelin and Sharov (1) presented some welcome data on absorp tion coefficients for drying air with sulfuric acid in towers packed mainly with 2-inch Raschig rings. I n a tower 14 feet in diameter and with 16 feet of packing, absorption coefficients for water vapor at a liquor rate of 2300 lb./(hr.) (sq. ft.) and an air velocity of 1.6 ft./sec. were found to increase 50% aa the acid strength waa raised from 80 to 95%. The over-all gas film H.T.U.values were between 3.1 and 4.3 feet. Absorption coefficients given for a 7-foot-diameter tower packed with 8.7 fee€ of 2-inch rings for an acid rate of 5100 lb./(hr.) (sq. ft.) and an air velocity of 1Pt./sec. were lower than in the larger column because of dirty packing and poor liquid distribution. Kuzminykh (16) revealed graphs of the absorption coefficient for nitrogen trioxide in sulfuric acid in towers packed with 2-inch rings or 2- and 3-inch quartz, showing its variation with the gas and liquid rates. An article by Oldershaw, Simenson, Brown, and Radcliffe (18) gave considerable practical data on the absorption of hydrogen * chlorjde resulting from the chlorination of methane to produce 32-35% hydrochloric acid. Experimental data on columns between 3 and 12 inches in diameter packed with 8-mm. or 2inch Raschig rings showed that 33% hydrochloric acid can be made in an uncooled absorber from feed gases containiag from 10 to 100% hydrogen chloride. This is mainly possible because the inlet gas is dry. Absorption performance data were also given on 10- and 18-inch columns packed with 1-, 11/2-, or %inch rings and cooled by a film of water on the outside when producing 32-35oJo hydrochloric acid from gas containing 4 0 4 5 % hydrogen chloride. The absorption of hydrogen chloride in such towers results in the vaporization of considerable water and the formation of a fog in the exit gas, which can be removed by passing the gas through glass wool. Removing chlorine from the hydrochloric acid by air stripping or boiling the acid was discussed. Deed, Schutz, and Drew ( 6 ) measured rates of desorption of oxygen and carbon dioxide from water by air and rectification of isopropanol-water mixtures on ljl-inch rings in a &inch column. At liquid rates of 100-400 lb./(hr.) (sq. ft.) the liquid film H.T.U. values for carbon dioxide desorption were somewhat higher than ones extrapolated by an exponential function of the H.T.U. with liquid rate from the values at liquid rates between 1000 and 5000 lb,/(hr,) (sa. ft.). At low liquid rates the H.T.U. values were erratic and depended on the liquid distributor. Liquid film H. T. I T . values from the rectification data at liquid rates between 120 and 400 lb./(sq. ft.) were scattered and usually 50% higher than predicted from carbon dioxide desorption data. Deed, Schutz, and Drew concluded that the present lack of gas film data makes it impossible to demonstrate that the individual gas and liquid film resistances may be added to give an over-all H.T.U. The difficulties encountered by Deed, Schuta, and Drew in obtaining reproducible liquid distribution at low rates indicates that a study of the design of liquid distributors and their effect on transfer rates would be worth while. Sprays and Spray Towers. A cyclone spray tower is a vertical cylindrical chamber with the absorbent sprayed from nozzles
HE number of articles on absorption and humidification is returning to normal after the readjustments during and following the war. Considering first a group of theoretical papers, Edmister (10)reviewed the mathematical methods of calculating the number of theoretical plates in absorbers for petroleum gases that have been proposed by Kremser, Horton and Franklin, and himself. In a review of the thermodynamic efficiency of multistage separation processes, Benedict (3) derived equations for the minimum work and oil flow necessary to separate two gaseous components by extractive distillation. The equations afie limited to the isothermal absorption of slightly soluble components, following Henry's law in the oil used as solvent. Emphasis was placed on the reversibility of the process, compared to the separations effected by distillation, extraction, thermal diffusion, etc. In their new book Marshall and Pigford (16)presented mathematical solutions to problems of simultaneous absorption and slow chemical reaction in the liquid flowing in packed or plate towers, and the rate of approach to equilibrium of a blah absorp tion column. York (99)gave a clear exposition of the selection of the coil surface in an air conditioning unit for cooling and dehumidifying air. Illustrative examples of the design of units cooled with an evaporating refrigerant or brine with the somewhat empirical method used by the manufacturers to rate their equipment were given. Considerable attention was paid to the practical limitations of the sizes commercially available, economic temperature differences and flows, and methods of control. Performance of Packed Towers. In a comprehensive paper on the performance of small mechanical draft towers for coolifig water with countercurrent air flow, Simpson and Sherwood (22) reviewed the theory of the enth'alpy driving force and the data available in the literature on such towers. Simpson and Sherwood presented new data for cooling water in towers about 2 by 4 feet in cross section with a total height of 7 feet when packed with l/+-inch mesh wire cloth, wooden slats, or Maaonite sheets. The performance of the combination of the packing and the spray water distributor was calculated as coefficients of total heat transfer with an enthalpy driving force over the range of water rates from 900 to 1500 Ib./ (hr.) (sq. ft.) and air rates from 700 to 1500 lb./ (hr.) (sq. ft.). The heat transfer coefficient was found to decrease 25% as the inlet water temperature increased from 85" to 115" F. Although 2-inch rings gave the highest heat transfer coefficients of any packing, for the same pressure drop 2-inch rings have lower transfer coefficients than slat packings. This study led to the conclusion that cooling towers for 3- to 10-ton air conditioning units could be installed in the limited height of a basement to give 90" F. water with an 85" F. inlet wet-bulb temperature of the air and a low fan horsepower. DeFlon (7) presented approximate performance curves for an atmospheric water cooling tower, showing the permisable water flow aa a function of the temperature change and the approach to the webbulb temperature, with three other graphs to correct for wind velocity, tower height, or wet-bulb temperature. QeFlon illustrated the relation of the permissible water flow to the 5
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I N D U S T R I A L A N D ENQINEtERfN
&longthe axis and with the gas entering tangentially at the bottom and leaving a t the top. The nearly cross flow of gas arid liquid limit its application to absorbents with no equilibrium partial pressure of the solute or to cascs in which the solvent does not need to be saturated. Johnstone and Silcox (13) mcasured the number of transfer units in a cyclone spray tower 2 feet in diameter and 14 feet high for the absorption of sulfur dioxide in sodium carbonate solutions and for the humidification of air. The number of transfer units in the spray zone and on the wall was measured separately and correlated with the gas velocity, number of sprays, and the dimensions of the chamber. The number of transfer units in the spray zope, which is much greater than on the wall, is inversely proportional to the 0.8 power of the gas velocity. The cyclone spray tower can give two transfer units with a pressure drop of less than 0.2 inch of water. Bradley, Evans, and Whytlaw-Gray (6) measured the rate of evaporation of 0.5-mm. drops of butyl phthalate at 20" C. into still air at pressures between 0.1 and 10 mm. of mercury. The rate of evaporation did not increase to infinity as the total pressure approached the vapor pressurc of butyl phthalate, but approached the rate of arrival of molecules at the surface, which could be predicted from the kinetic theory. These experimenters reviewed the theory of Fuchs, which indicated that the evaporation rate per unit area of drops smaller than 0.001 cm. at atmospheric pressure is the same as for large drops at pressures below 10 rnm. of mercury. Both cases are limited by the mean speed of the gas molecules rather than normal diffusion. The breakup of a liquid jet may take place by surface tension at low velocities, air friction, or inertia and viscous effects at high velocities. Merrington and Richardson (17)measured the drop size distribution of eleven liquids from jets into air and found that the drop size varied inversely with the jet speed in the range ' of 25 to 110 meters per second and with the 0.2 power of the kinematic viscosity. At these high rates the drop size was independent of the nozzle diameter and the surface tension of the liquid. Below a limiting velbcity of about 5 meters per second the drops are of a uniform size related to the nozzle diameter and independent of the velocity. The length of the continuous jet had previously been found proportional to the velocity; Merrington and Richardson showed that the stable length increases considerably with the viscosity. Oil Absorption. I n trying to correIate the data from forty natural gasoline absorbers at pressures between 35 and 1800 lb./ (sq. in.), Ragatz and Richardson (19) employed a plot of the percentage absorbed of propane or the butanes on a scale to give a linear relation with the absorption factor (the ratio of the slope of the operating line to that of the equilibrium curve) on an arithmetical scale. It is difficult to assess the value of this contribution without the test data, which have not been published. The correlation does not predict how the percentage adsorbed varies with the number of plates in the column. Additional propane for liquefied petroleum gas or cracking to chemical raw materials can be recovered in existing plants, Thornton ($4) pointed out, by increasing the absorber pressurc, raising the oil ratc, or lowering its tcmpcraturc. In absorbing propane, butanos, and heavier from natural gas, some methanc. and ethane arc present in thc rich oil leaving the absorber. Whisller (26) described two methods of removing tho undesired methane and ethane, gave plant operating data on one system, and compared equipment and operating costs in relation to the market for the propanc recovered by the two systems. Kniel and Slager (14) described a plant for the purification of ethylene from the pyrolysis of propane which used absorption in a low-molecular-weight and partially aromatic oil to separate the methane from the cracked gases. Flowsheets, operating temperatures and pressures, reflux ratios, and some estimate of the power required for refrigeration and compression were presented, but sufficient data were not given for comparison with the alternatives of low temperature distillation or carbon adsorption.
G CHEMISTRY
Vol. 40, No. 1
Oil for absorbing benzene from coke oven gas gradually increases in density and viscosity, and the absorption and stripping of benzene become less efficient. Wilson (28) studied thc suhstances present in coke oven gas and aromatic absorption oil which cause or accelerate the thickening. Tar, hydrogen sulfide, oxygen, hydrogen cyanide, hydrogen chloride, styrene, and indene in the coke oven gas, or phenols, water, and corrosion products of iron in the oil are the usual agents. Phenol with hydrogen cyanide and ferric chloride may form a phenol-aldehyde resin. Methods of operating the gas plant and the purification system can alleviate some of the thickening. The new oil should be extracted with caustic to remove the phenols, and part of the oil in circulation should be distilled to remove the tar. I n search of an explanation for the foaming of lubricating oils containing additives, Robinson ($0)found that the rate of rise of 0.5- to 2-mm. air bubbles in plain oil with a viscosity of about 10 poises followed Stokes' law. With a similar oil containing an additive, bubbles started at a speed expected from Stokes' law and slowed down to half the initial velocity after a travel of a few centimeters. Absorption with Chemical Reaction. Denbigh and Prince (8) measured the rate of absorption of nitrogen dioxide in 10 to 60% nitric acid in a wetted-wall tower 1 1 / 4 by 5 inches at atmospheric pressure, varying the temperature from 25" to 40 " C. and the gas rate threefold. The ratio of nitrogen dioxide to nitric oxide varied fivefold during the recirculation of a batch of gas diluted with nitrogen. Calculation of the absorption rates as the mass transfer coefficient proposed by Chambers and Sherwood resulted in the absorption coefficient varying with the ratio of nitrogen dioxide to nitric oxide. Denbigh and Prince obtained a better correlation by using [(N~O~)-C(N~O~)~/~(NO) as the driving force, wit11 c a function of the acid concentration and temperature. This results from considering the reactions as NgO4 HzO = HNOz "03 followed by 2 HNOz = NO NO2 HzO. This mechanism was not successful in explaining the reverse reaction for absorbing nitric oxide in nitric acid. Denbigh and Prince observed no formation of visible nitric acid
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mist. Deringer (9) discovered that aqueous solutions or slurries of magnesium and cuprous chlorides are good absorbents for carbon monoxide. The solubility of carbon monoxide, which reached 60 cc. per cc. of solution, and the rate of solution in a rotating vessel were measured for several ternary mixtures. The rate of absorption decreased from 0.18 to 0.05 in arbitrary units as the temperature increased from 10" t o 30" C. This unusual effect is thought to be the result of the transfer of carbon monoxide to the solid phase. Carbon monoxide may be desorbed a t room temperature by lowering the pressure. The solution has been tried on a pilot plant scale for removing carbon monoxide from city gas, absorbs oxygen very slowly, and does not deposit metallic copper. Shneerson and Leibush (21) made an extensive study of the effect of temperature, gas and liquid rates, percentage conversion to carbonate, and concentration on the rate of absorption of carbon dioxide in aqueous solutions of mono-, di-, and triethanolamines, using a 25-mm. tower packed with 60 em. of 5-mm. glass rings. Monoethanolamine absorbs carbon dioxide much faster than di- or triethanolamine. At 0% conversion to carbonate the over-all gas film coefficients in 3.5 N solutions of mono-, di-, and triethanolamine are 3800, 1400, and 125 cu. m./(hr.) (cu. m.) (atm.), respectively. The absorption coefficient decreases nearly linearly with the percentage converted to carbonate; the absorption coefficient in 2 N monoethanolamine a t 50' C. decreases from 2900 to 660 as the conversion increases from 0 to 90%. Shneerson and Leibush found a similar decrease as the carbon dioxide content of the gas increased. The absorption coefficient is independent of the gas rate and increases with about the 0.3 power of the liquid rate. As the concentration of amine is increased, the absorption coefficient passes through a
January 1948
INDUSTRIAL AND ENGINEERING CHEMISTRY
maximum between 2 and 5 N , depending upon the amine. Maximum coefficients were obtained between 50” and 60’ C. These coefficients on 5-mm. rings are 50% greater than Cryder and Maloney found with 3/4-inch rings under the same conditions. Spector and Dodge (23) measured the rate of absorption of the carbon dioxide in atmospheric air in 2.5 N sodium or potassium hydroxides in a 12-inch tower packed with 8/d-inch Rmchig rings or 1-inch Berl saddles. The over-all gas film absorption coefficient increased with about the 0.2 power of the liquor rate in the range of 700 to 15,000 lb./(hr.) (sq. ft.). The absorption coefficient increased with the 0.35 power of the gas rate below 500 1b.j (hr.) (sq.ft.) and with a lower and decreasing power at high rates. Tepe and Dodge had previously found no effect of gas rate on the absorption of 2% carbon dioxide from air. The absorption coefficient with the driving force expressed in pressure units (not mole fraction) decreased with the 0.5 power of the absolute pressure in the range of 15 to 115 lb./(sq.in.). Potassidm hydroltide solutions gave rates of absorption about 25% greater than sodium hydroxide. The absorption coefficients in sodium hydroxide solutions are of the same size as Shneerson and Leibush found with monoethanolamine. BelopolskiI (2) derived equations for absorption with chemical reaction at a moderate velocity in the liquid which are similar to Hatta’s, except that the rate of disappearance of the absorbed inaterial by reaction was taken as the reaction rate constant limes the difference in concentration at any point in the film and in the main body of liquid. Hatta took its rate of disappearance as equal to the reaction rate constant times only the concentration in the film. The difference is apparently to allow for a reversible reaction, but the reasoning is not clear. The paper by Van Krevelen and Hoftijzer (26) on absorption arid reaction in packed towers holds some hope of clarifying this confused field. They started by recorrelating Sherwood and IIolloway’s liquid film absorption data on rings and saddles, using an equation with three dimensionless groups. The firm thickness, which is employed as the characteristic length, is estimated from the viscous flow of films on smooth vertical walls. The Ilatta theory of absorption with a moderate reaction velocity in the liquid is modified with this same fictive film thickness. The dimensionless groups for physical absorption are multiplied by the Hatta group containing the reaction rate constant to obtain the absorption coefficient with simultaneous reaction. Data from the literature and their own work, which are not tabulated, illustrate the correlation of the rates of absorption of carbon dioxide in carbonate and caustic solutions. Cryder and Maloney’s data on carbon dioxide absorption in diethanolamine solutions with varied concentrations, temperatures, and liquid rates seem to be adequately represented by a single equation. Van Krevelen and Hoftijzer have made a bold advance and are not troubled if some points are threefold off. The paper is expected to provoke controversy, but it is hoped that the ensuing discussion will advance this field. Miscellaneous. Guyer and Pfister (f2) made an extensivc study of the rate of absorption of carbon dioxide from single bubbles rising through pools of water or aqueous solutions a t 20” C., varying the bubble size from 0.003 to 0.05 cc., the liquid depth from 5 to 240 cm., and the carbon dioxide content of the bubble from 14 to 100%. The rate of absorption in cc./(sq. cm.) (sec.) was nearly independcnt of tho bubble size. Thc rate of absorption was slower in solutions of sodium carbonate, bicarbonate, or sulfate than in pure water, but it was faster in triethanolamine or sodium hydroxide. Absorption of carbon dioxide from bubbles containing air was slower than calculated from measurements on pure carbon dioxide, by allowing for the partial pressure. For 10-em. depths the absorption during bubble formation and disengagement was about equal to that on rising through the liquid. Wiley, Parkinson, Gehm, Wisniewski, and Bartsch (27’) reported on alleviating the low dissolved oxygen content of a river
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receiving sulfite paper mill waste with a high biochemical oxygen demand by aerating with compressed air through porous platc diffusers. Processes for absorbing sulfur dioxide from the combustion gases of high sulfur coals were discussed by Francis (If) with emphasis on the power stations in Great Britain using lime as a n absorbent. Bliss (4) gave methods of estimating costs of steel bubble-cap columns, stoneware absorption towers, and the common tower packings. Simpson and Sherwood (82) tabulated estimated costs and power requirements for large forced-draft water cooling towers. LITERATURE CITED
Atelin, A. G., and Sharov, M. M., Khim. Prom., 1945, No. 2 , 7-9.
BelopolskiI, A. P., J. Applied Chem. (U.S.S.R.), 19, 1181-8 (1946).
Benedict, M., Chem. Eng. Progress, Trans. Am. Inst. Chem. Engrs., 43, 41-60 (1947).
Bliss, H., Chem. Eng., 54, 126-38 (1947). Bradley, R. S., Evans, M. G., and Whytlaw-Gray, It. W., Proc. Roy. SOC.(London), 186, 368-90 (1946). Deed, D. W., Schuts, P. W., and Drew, T. B., IND. ENG. CHEM.,39, 766-74 (1947).
DeFlon, J. G., Petroleum Processing, 2, 249-52, 433, 437, 439 (1947).
Denbigh, K. G., and Prince, A. J., J . Chem. SOC.,1947, 790-801. Deringer, H., Chimia (Switz.), 1, 125-31 (1947). Edmister, W. C., Petroleum Engr., 18, No. 13, 130, 132, 134, 136, 139, 142, 144 (1947).
Francis, W., Power & Works Engr., 41, 17-21, 25,37-40, 75-7, 102-6 (1946).
Guyer, A., and Pfister, X., Helv. Chim. Acta, 29, 1173-83, 140012 (1946). Johnstone, H. F., and Silcox, H. E., IND.ENG.CHEM., 39,808-17 (1947). Kniel, L., and Slager, W. H., Chem. Eng. Progress, 43, 335-42 (1947). Kusminykh, I. N., J . Applied Chem. (U.S.S.R.), 20, 309-18 (1947). Marshall, W. R., Jr., and Pigford, R. L., “Application of Differ-
ential Equations to Chemical Engineering Problems,” Univ. of Delaware, Newark, Del., 1947. Merrington, A. C., and Richardson, E. G , Proc. Phus. Soe. (London), 59, I, 1-13 (1947). Oldershaw, C. F., Simenson, L., Brown, T., and Radcliffo, I’ , Chem. Eng. Progress, 43, 371-8 (1947).
Ragata, E. G., and Richardson, J. A., PetroEeum Refiner, 25, 582-6 (1946).
Robinson, J. V., J . Phys. & Colloid Chem., 51, 431-7 (1947). Shneerson, A. L., and Leibush, A. G., J . Applied Chem. (U.S.S.R.), 19, 869-80 (1946). Simpson, W. M., and Sherwood, T. K., Refrig. Eng., 52, 535-43, 574-6 (1946).
Spector, N. A,, and Dodge, B. F., Trans. A m . Inst. Chem. Engrs., 42, 827-48 (1946).
Thornton, D. P., Jr., Petroleum Processing, 2, 583-4, 586, 589, 591 (1947).
Van Krevelen, D. W., and Hoftijzer, P. J., Rec. truv. chim., 6 6 , 49-70 (1947).
Whistler, A. M., Petroleum Refiner, 25, 645-8 (1946). Wilev. A. J.. Parkinson. L.. Gehm. H. W.. Wisniewski, T. F., and Bartsch. A. F.. Pawer Trade J . . 124. No. 12, 59-64 (1947). Wilson, 0.B.; Coke’Sm&?less-FuelAge, 7, 231-5 (1945); 8 , 12-4, 18, 38-41, 64-6 (1946).
York, 5. E., Heating and Ventilating, 44, 66-75 (1947). R E C B I V November ~D 3, 1947.
