I,ON AND lcCATI O N SeE UNIVERSITY
ROBERT L. PIGFORD OF DELAWARE, NE
P
ROGRESS in absorption and humidification during the past year has been noteworthy in that reliable data have been published for mass transfer rates in packed towers and for fluidized beds, several interesting industrial absorption procesaes have been described, and equipment design methods and diffusion theory have been advanced. DIFFUSION THEORY
. L1
Wilke'(69) presented a method for estimating diffusion coefficients in liquids based on the Eyring theory of absolute reaction rates and the Stokes-Einstein equation. Data o various solutes in water, methanol, benzene, and were correlated with an average d of 10% between calculated and observed values. The coefficient is shown to depend on the molal volume of the solute and on the viscosity and other properties of the solvent. The effect of tempe?ature is expressed by its effect on the viscosity. McMurtrie and Keyes (81) measured the diffusivity of hydrogen peroxide in air. Furry (7') gave an elementary explanation of diffusion phenomena in gases. Pollard (96)discussed the diffusive separation they flow through capillary tubes into an evacuated s Mass transfer from plane surfaces into turbulent air streams waa analyzed theoretically by Calder (2), following the concepts of eddy diffusivity previously introduced by Sutton (61) and Pasquill (24). Observed rates of evaporation of liquids from smooth planes as-well as from grassy ground agreed with the theory. When the velocity distribution lows the l/,power law, the evaporation r proportional to the 0.78 power of the air velocity and the 0.89 power of the length of the surface. For rough surf nent in the velocity becomes equal to u mathematical problem were also given for source. Klinkenberg and Mooy (17) classified the various groups of physical constants useful in diffusion Frank and Henry (6)solved the mathematical problem of diffusion into a slab, using various arbitrarily diffusion coefficient with concentration. article (6) on evaporation of small liquid been published by the National Advisory nautics. PERFORMANCE OF PACKED TOWERS
Thus, the concentration of dissolved solute at the interface is lower than would be the case if both chemical and physical equilibrium existed at this point, and the concentration difference causing mms transfer into the liquid is therefore abnormally small, By assuming that the sulfur dioxide molecules do not react at all until they penetrate into the liquid, Whitney and Vivian computed pseudo liquid-film coefficients which agree excdlently with predictions based on oxygen stripping experiments. This paper and the previous one (67)by the same authors appear to be the most promising advances in the field of simultaneous absorption and reaction. Hutchings, Stutsmavn, and Koch (16) determined rat- of absorption of acetone in water in towers filled with 0.375- t o 1.251gas-film heights of trans€er units for 0.375-inch rings at G 256 r hour per square foot to 4.2 feet for 1.25= 483 pound per hour per square ments using 1.25-inch rings, the e inlet gm,whereas the temperaosite direction at the top of the tower. The liquid temperature must have gone through a maximum value in the tower, because of the heat of solution of acetone. are approximately equal t o those reported r and Scheibel (93) for the m e system. Koch, Stutzman, Blum, and Hutchings (18) report data using the same packings in 6- and 10-inch towers t o absorb carbon dioxide in water from air. The H.T.U. values lie between those of She:wood and Holloway (30)and those of Sherwood, Draemel, and Ruckman (29) and are independent of packing size and of 3 to 1.4 feet for liquid hour per square foot. ported a careful study of rates of vaporization of water into air in a tower filled with 1inch carbon rings. Except at liquid rates above 2000 pounds per hour per square foot, the interfacial area for heat transfer ww greater than that for maas transfer, because of incomplete wetting exit water temperature waa 1 to 2 O F . above diabatic saturation in adiabatic experimenta in which the water flowed through the apparatus without a temperature change. The mass transfer coe5cients, after correction for end effects, pproximately with similar mass transfer data for other published by Sherwood and Holloway. The over-all enthalpy driving force is not entirely correct for use in oling towers, owing t o the heat transfer resist-
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Whitney and Vivian ($8) reported extensive data on rates of absorption of sulfur dioxide in water for an &inch diameter tower filled to a depth of approximately 2 feet with 1-inch ceramic Raschig rings. The over-all resistance to mass transfer showed all the characteristics of experiments in which e e to mass transfer in both phases, but when the e separate phases were calculated from the data liquid phase were abnormally high. The explanation apparently h the same $8 that proposed previously by the same authors ($7) from their work on chlorine absorption in water. oxide molecules which dissolve at the interface do diately with t h e water; they may diffuse sonie d liquid before nonvolatile sulfite or bisulfite ions are produeed.
) describe a process for recovering hydrogen fluoride from stack gases by passing them through e tower Calcium fluoride is formed by reon t h e solid surface, and this may phosphate rock by tumbling and ined 80 to 95% calcium fluoride. Recovery of hydrogen fluoride waa 96% complete in a bed 4 feet deep. H.T.U. values ranged from 1.2 to 2.3 feet, depending on the freshness of the solid surfaoe. Hougen ($8) aa well as Hobson and Thodos (IS) of unsteady-state mass transfer from packed be&. The latter article extended the previous work of Hougen, Gam-
17
18
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INDUSTRIAL AND ENGINEERING CHEMISTRY
son, and Thodos (la) and Hougen and Wilke (14) to liquids, showing that the transfer coefficient is proportional to the 2/* power of the Schmidt group, even for the extreme range from gmes t o liquids. Taeker and Hougen ( 3 2 ) give data on evaporation of water from porous Raschig rings and Berl saddles. The j factors (or the former are 19% lower than for spheres and cylinders at the same modified Reynolds number, because of the relative inaccessability of the inside surface of the rings. Saddles gave lower j factors than rings a t the same Reynoldb number, These data are not directly useful for designing packed absorbers bemuw an unknown fraction of the packing surface is wet when liquid flows through the packing. Grixnley (8) observed the flow of liquid layers down vertical surfaces, noting the velocity at which ripples first appeared. He also measured the average horizontal cross sectional area of the liiquid stream in a packed tower by an electrical method, finding that this quantity increased with increasing liquid rate. Hr found that the H.T.U. for carbon dioxide absorption in a film oi water running down a vertical surface had a minimum value oT 3.7 feet a t a water Reynolds number of 100. The higher values at lower Reynolds numbers are possibly due to lack of uniformit:, h the liquid flow rate. MASS TRANSFER IN FLUIDIZED BEDS
Two papers on mass transfer in consolidated and fluidized beda allow comparisons to be made of mass transfer and friction for the fluidized and the nonfluid conditions. Resnick and White ( 9 7 ) evaporated ground naphthalene into air, carbon dioxide, and hydrogen in chambers varying from 22 to 41 mm. in diarnetw. The specific surface areas of the irregular particles were determined by the permeability method. Per unit of particle surface, the mass transfer rate \$as smaller for bhe smaller particles at a constant value of the Reynolds number based on the particle size. The diffusivity of naphthalene in the fluidizing gas had a surprisingly small effect. In the region of “slugging” or streaming fluidization (as contrasted with the bubbling action which takes place at lower gas velocities) t h e j factors for the various particle5 were satisfactorily correlated with a quantity proportional to the ratio of the gas velocity to the terminal. velocity of the average particle. McCune and Wilhelm ( 2 0 )used water to fluidize almost spnerioal pellets or flakes (0.25 inch to 18 mesh) of 2-naphthol. BJ using the measured diffusivity of this substance in water, the J factors for the consolidated beds were found to agree exceIlently with the data of Hougen et al. (IS,1 4 )on evaporation of water into air. For fluidized beds the j factor was less at a given Reynolds number than for a stationary bed, though the ratio of mass trankfer coefficicrit to pressure drop was lower for the fixed bed. When compared on the basis of equal water velocities past the particles, by plotting j e and A-K, /e, data for single particles, fixed beds, and fluidized beds n-ere brought into agreement except a t high watei velocities, where a correction to the relative velocity was necessari because of lack of uniformity in the velocity distribution wii h i n the apparatus. ABSORPTION AND CHEMICAL REACTIONS
Pozin (16)has continued his work in this field, investigating the absorption of ammonia from air in water and dilute acids. Overall gas-film absorption coefficients were substantially the same for sulfuric, nitric, and hydrochloric acids, but liquid-film resistance caused the coefficients to be lower for organic acids such as formic, acetic, oxalic, and malonic. Turkhan (55) measured rates of absorption of carbon dioxide from air batchwise in sodium hydroxide. He found that the normality of the alkali did not affect the absorption rate after the normality exceeded a critical value which depended on the partial pressure in the gas. Thus, he was able to interpret his data in t e r m of the Hatta theory of rapid irreversihIe reaction within the liquid film.
Vol. 42, No I
DESIGN METHODS
Edinister ( 4 ) refined his method of allowing for variations in liquid and vapor fiom rates and temperature changes in naturah gasoline absorbers. When the absorption factor, A, defined as the vaporization equilibrium constant times the ratio of the gas to liquid flow rates for a tray, varies through an absorber, an effective average A may be calculated for use in the familiar Kremser formula, which gives the required number of theoretical plates. For a two-plate absorber, the effective A may be calculated exactly from t,he values corresponding to the two plates by using LL simple formula. Edmister nom presents correction factors which may be applied Lo the formula to allow for plate-to-plate variations of the absorption factor in coiumns having more than two theoretical trays. The correction factors are derived by assuming arbitrary but plausible variations of A from plate-toplate. This method of designing absorbers i~ shorter than thr vigorous method of stepwise heat and material balances and seemr to give good results. Tiller (33) shows iiory the difference equations which arise in the theory of absorbers may be solved graphically. €€ealso calk attention (3.4)t o the well known principle that the exit gas from an absorber cannot reach equilibrium wit,h the inlet scrubbing liquid--that is, the degree of removal of solute is limited if the ratio of molar liquid flow to molar gas flow is smaller than the rr. ciprocal slope of the straight equilibrium line. Tiller suggests that the degree of absorption in a finite column be cornpared with that in an infinite column in considering the absorption efficiency and recommends than an absorption factor slightly less t’hanunity be used in design calcuiations. Klinkenberg (16) presents a nomogram from which concentruLion changes can be calculated when a solute is being removed from a gas which passes through a stationary bed of adsorbent. The method described is limited to cases in which the equilibrium gas composition is proportional to the concentration of adsorbed sol11t,r OPERATION AND PLANT EQUIPMENT
A new process for removing sulfur dioxide and sulfur tiioxidr from flue gases has been discussed by Woollam and Jackson (40)
The process involves absorption in aqueous ammonia solution kept a t a pH value between 5 and 7. This hydrogen ion concentration is low enough to prevent loss of gaseous ammonia yet high enough for negligible corrosion of steel equipment. The strong absorption liquor is heated in an autoclave to convert a part of the bisulfite to elemental sulfur vhile the remainder goea to ammonium sulfate. From 95 to 98yo of the inlet sulfur dioxide was removed from flue gas, using a pilot plant tower 3 feet in diameter and 18 feet tall and packed with 2.5- to I-inch rings. Ileitz, Oldershaw, BroRn, and Barnard (10) described a pilot plant used for recovering ethane from natural gas. It was necessary to rcmove carbon dioxide from the feed gas before separating the hydrocarbons, and this was accomplished by scrubbing with 40% potassium hJdroxide solution in an absorber having six conventional bubble trays and thirteen trays perforated with 9.045-inch diameter holes, The perforated trays were more effective than the bubble trays, owing to the larger liquid holdup on each. Foaming was reduced by adding a small quantity of soap solution to the alkali, Production of essentially anhydrous hydrogen rhloride by ail absorption process was discussed by Brumbaugh, Tillman, and Sutter (I). Impure hydrogen chloride gas can be fed to the absorber a t atmospheric pressure. The concentrated aqueous hydrochloric acid is sent to a stripping column which operates under pressure. The almost pure hydrogen chloride gas leaving this unit is dried by cooling wiLh refrigerated brine in a heat exchanger and the stripped acid is returned to the absorber. The dried hydrogen chloride contains less than 0.005% by weight of mater vapor. Coidl, Bishop, and Gaylord (3) report results of a
January 1950
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P,
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
pilot plant stu chloride cooler the 0.875-inch pounds of hydrogen chloride per hour per square foot of cooled surface when the feed gas contained 91% hydrogen chloride and the cooling water was a t 78” F. The maximum absorption rate was roughly proportional to the percentage of hydrogen chloride in the feed gas. Stepwise calculations were made to show how the gas and liquid compositions and temperatures changed along the cooled tube. The water content of the gas went through a maximum as water was evaporated from the weak arid and was condensed farther down the tube in the strong acid. Valentine (36) discussed the use of agitated vessels for stagewise countercurrent absorption, pointing out that these devices are preferred over packed or tray-type units when suspended solids are present. The maximum gas-handling capacity of the largest impeller used (60-inch diameter) is about 500 cubic feet per minute, however. The gas may be fed to the top surface of the liquid, being sucked into the liquid by an impeller installed just under the surface. Such installations have proved useful for production of calcium carbonate from lime water and flue gas, and for catalytic hydrogenation of vegetable oils. MISCELLANEOUS
Munter, Aepli, and Kossatz ($9)measured the partial pressures of hydrogen fluoride and water over aqueous hydrofluoric acid. Hadden (9) discussed the estimation of equilibrium vaporization constants for hydrocarbon solutions. He prevents a new nomogram which correlates available data and discusses the correction factors which must be applied to the K factor for methane when it is dissolved in a high molecular weight oil. The curves of the correction factor, C, against molecular weight a maximum value of C = 1.1 to 1.4,a t a molecular the value of C increases with increasing pressure. are smaller a t higher molecular weights. NOMENCLATURE
.4
c D, D, GM j ka
*
NR* P
v
HL e 8
c L P
= absorption factor, defined as ratio of equilibrium vspor-
ization constant to the quotient of the molar liquid and gas flow rates for a plate, dimensionless = correction factor to the equilibrium vaporization corlstant to allow for chemical dissimilarity between solut,e and solvent = particle diameter, feet = diffusivity, square foot/hour = molar mass velogity of fluid, pound-mole/(hour) (square foot) = ( k a P / G ~ ) ( p / p D v ‘)8* ’ = mass transfer coefficient, pound-mole/(hour)(squarr foot)(atm.) = Reynolds number, DpVp/p = total pressure, atm. = fluid velocity, feet/hour = height of a liquid-film transfer unit, Feet = porosity or fraction free space in a fluidized or packed be dimensionless = viscosity, pound-mass/(foot) (hour) = density, pound-mass/cubic foot
19
(11) Hignett, T. P.,and Siegel, M. R., Ibid., 41, 2493-98 (1949). (12) Hobson, M., and Thodos, G., C h m . Eng. Progress, 45, 517-24 (1949). (13) Hougen,’O. A., Gamson, B. W., and Thodos, G., Trans. Am. Znst. Chem. Engrs., 39, 1 (1943). (14) Hougen, 0.A., and Wilke, C. R., Ibid., 41,445 (1945). (15) Hutchings, L.E., Stutzman, L. F., and Korh, H. A., Jr., Chem. Eng. Progress, 45,253-68 (1949). (16) Klinkenberg, A., IND. ENQ. & E M . , 40,1992-4 (1948). (17) Klinkenberg, A., and Mooy, H. H., Chrm. Eng. Progress, 44, 17-36 (1948). (18) Koch, H. A., Stutzman, L. F., Blum, H. A., and Hutchings, L.E.,Ibid., 45,677-82 (1949). (19) McAdams, W. H., Pohlenz, J. B., and St. John, R. C., Ibid., 45. 241-52 (1949’1. (20) McCune, L. K., and Wilhelm, R. H., IND.ENQ. CHBM.,41, 1124-34 (1949). (21) McMurtrie, R. L.,and Keyes, F. G., J . Am. Chem. SOC.,70, 3756-8 (1948). (22) Munter, P. A., Aepli, 0. T., and Kossats, R. A,, IND. ENQ., CHEM.,41, 1504-6 (1949). (23) Othmer, D. F.,and Scheibel, E. G., Trans. Am. Znst. Chem, Engrs.. 37, 211 (1941). (24) Pasquill, F.,Proc. Roy. Soc. ( h d o n ) , 182A, 76 (1943). (25) Pollard, W.G., Phys. &v., 78,762-74 (1948). (26) Pozin, M.E.,and Smirnova, M. A., J . Applied Chem. (U.S. S.R.),20,754-61 (1947). (27) Resnick, W., and White, R. R., Chern. Eng. Progress, 45, 37790 (1949). (28) Sherwood, T. K.,“Absorption and Extraction,” p. 194, New York, McCrraw-Hill Book Co., 1937. (29) Sherwood, T.K.,Draemel, F. C., and Ruckman, N. E., IND. ENG.CHEM.,29,2.82 (1937). (30) Sherwood, T. K.,and Holloway, F. A. L., Trans. Am. Znst. Chem. E n g r ~ .36, , 39 (1940). (31) Sutton, 0.G.,PTOC.Roy. SOC.(Lon&), 135A, 143 (1932); 146A, 701 (1934); Quart. J. Roy. Met. SOC.,73, 257 (1947). (32) Taeker, R.G., and Hougen, 0. A., Chem. Eng. Progress, 45,18893 (1949). (33) Tiller, F. M.,Ibid., 44,299-306 (1948). (34)Zbid., 45, 391-401 (I94Q). (35) Turkhan, E. Ya., J . Applied Chem. (U.S.S.R.), 21, 927-36 (1948). (36) Valentine, K.S.,Chem. Eng., 55, 117-8 (December 1948). (37) Vivian, J. E., and Whitney, R. P., Chem. Eng. Progress, 43, 691-702 (1947). (38) Whitney, R. P.,and Vivian, J. E., Zbid., 45, 823-337 (1949). (39) Wilke, C. R.,Zbid., 45, 218-24 (1949). (40) woollam, J. P. V., and Jackson, A., TTan8. Inst. Chem. Engre. (London),23,43-51 (1945). REOEIVED November 22, 1949.
CJ r---
CHARGING HOPPER
AIR EJIX.lOR] GAS TOSTACK
USION ILOT- PLANT FURNACE
6-IN.STEEL PIPE
BIBLIOGRAPHY
(1) Brumbaugh, C. C.,Tillman, A. B., and Sutter, R. C., IND.ENQ. CHEM.,41,2165-67 (1949). (2) Calder, K. L.,Quart. J . Mech. Applied Math., 2,153-76 (1949). (3) Coull, J., Bishop, C. A., and Gaylord, W. M., Chem. Eng. Progress, 45, 525-31 (1949). (4) Edmister, W. C.,Ibid., 44, 615-18 (1948). (5) Frank, J., and Henry, M. E., Trans. Faraday SOC.,45, 636-50 (1949). (6) Fuchs, N.,Physik. 2. Sowjetunion, 6, 224 (1934) trans. Nail. Advisory Comm. Aeronaut., Tech. Mem. 1160 (August 1947). (7) Furry, W. H., Am. J. Phys., 16,63-78 (1948). (8) Grimley, S. 8.,Trans. Znst. Chem. Engrs. (London), 23, 228-35 (1945). (9) Hadden, 9. T., Chem. Eng. Progress, 44,37-54 (1948). (10) Heitz, R. G., Oldershaw, C. F., Brown, W. E., and Barnard, R. D.,IND.ENQ.CHEM.,41, 1540-43(1949).
NICAL BAFFLE *DlA X 9”HIGHJ
OISCHARGE VALVE
Tower for Absorption
of .Fluorine 4mLamp.Limestone (39
