ABSORPTION AND HUMIDIFICATION This review covers last year’s papers on gas absorption and the mechanics of packed tower operation. Wherever possible and desirable, literature coverage has been extended to earlier related work in the field. Theoretical papers on mass transfer and diffusion theory have been omitted, since these will presumably be the subject of review of another series of this journal. Although gas absorption does not extend into the field of liquid-liquid contacting, papers on the mechanics of liquid-liquid flow have nevertheless been covered. The reason for this is the trend of recent years to lean on existing gas-liquid correlations and possibly develop universal relationship, valid for both types of flow. New processes and equipment have only then been covered ifthere has been an interest in a topic or development for a number of years, or if basic design data have been made available.
Behavior of a large number of liquid-liquid systems in ring and saddle beds was investigated by Gayler and Pratt (13). The object was to study liquid holdup and pressure drop. The relation between pressure drop and the velocity of the dispersed phase seemed to be of interest. The family of curves, in which the velocity of the continuous phase was the parameter, bore a striking resemblance to liquid-gas data, when the latter are presented in the form of Ap versus L,with G as the parameter. The holdup investigation was continued by Gayler, Roberts, and Pratt (14). They proposed the following equation for predicting fractional holdup, x
T
HE field of gas absorption and its practical applications particularly in operation of packed towers are discussed in this review. PACKED TOWERS
This section deals with a discussion of pressure drop, loading, and flooding phenomena in packed towers.
Ud
5 +F X u, =
€50
s(l
- 5)
Quantity io was termed the “characteristic velocity” and presented the limiting drop velocity a t zero throughput for both phases. As would be expected, Go depended on the nature of the liquid pair, column diameter, and packing size. Typical values of characteristic velocity were given for a range of operating conditions, New data of Sat0 and r\’ishida (3,5),observed in a 6-cm. diameter tower, packed with l-cm. diameter rings s u p ported essentially the earlier work of Rreckenfield and Wike (6). The same problem was also studied by Fujita, Kikuzawa, and Watanabe (21). Their correlation is of the form
GAS-LIQUID SYSTEMS AND LIQUID-LIQUID SYSTEMS
l h s s u r e drop, loading, and flooding Phenomena in ga51iquid SYstiems have been d k m e d by the miter (266). The major b r a t U r e data pertaining to Operation Of large towers (> 8 inches) were supplemented by new data observed in a 16-inch diameter tower, and empirical Pressure drop working equations were proposed. The equations were of the form (72
Ap = a X lOSL-
Y = 0.60e-2X
P
x
where = u,/ud and Y is a complex expression incorporating U. and the usual packing and fluid properties. Kafarov and Planovskaya (21,dd) also investigated liquid-liquid flow in a 42mm. diameter column, packed with small rings. The system considered were benzene-water, acetic acid-water, and chlorobenzene-water. From the results of extraction studies, they concluded that optimum conditions (as far as mass transfer was concerned) resulted when thorough phase emulsification existed.
They are valid up to the loading range. For the important loading-flooding region, the generalized correlation method of Sherwood, Shipley, and Halloway (36) yielded an isobaric presentation of pressure drop data. An analysis of the Lubin data (29), observed in a 24-inch diameter tower, led to a further improvement, by evaluating the ratio of the density of water to that of the irrigating liquid, and include it in the commonly accepted flooding correlation (36). A generalized loading velocity-pressure drop correlation for gas-liquid systems was also proposed by Otake and Kimiura (32). It included the usual fluid and packing properties as well as packing size. Pressure drops attending rectification in columns packed with small rings were correlated in an empirical way by Kafarov and Blyakham (80). A friction factor-Reynolds number plot was relatively satisfactory for Reynolds numbers in excess of 100. Below this value the data spread was considerable. Similar to single phase flow through paokings, their two-phase flow f versus Re curve indicated a region of gradually varying slope. The proposed formulations do not account for all major variables and must therefore be considered specific. Gas-liquid pressure drop, loading, and flooding were also discussed by Zenz (44). No new data were given, and the paper was primarily an exposition of work published earlier by the author (45).
EQUIPMENT
The new Intalox saddle packing material was described in the patent literature (26). Hayter (16) evaluated distillation performance of a new corrugated gauze packing, fabricated from double layers of crimped wire gauze, stacked a t right angles. The test columns were of 1- and 2-inch diameter. With benzenecarbon tetrachloride as test mixture preflooding was believed unnecessary. Boil-up rate seemed to have only little effect on fractionation. Within the limits of the tests no efficiency loss was apparent as packing height was increased. For distillation with Stedman packing Bragg (4)proposed that
61
INDUSTRIAL AND ENGINEERING CHEMISTRY
62
where, P = the number of theoretical plates per foot of column, and R = reflux rate in gallons per hour. His data indicated no dependence between P and column diameter for operation well below loading. It was stressed, however-though no data are given in the paper-that a limiting lower reflux rate is likely, below which the formula would cease to be valid. A packing s u p port for glass columns was described by Pantages and Feldnian (33). To be used in small scale distillation columns, it will permit easy packing removal without the need of column inversion. Sieve tray operation mechanics was investigated by Arnold, Plank, and Schoenborn ( I ) . Trays of various designs were exposed to countercurrent air and water flow. The experimental pressure drops and their dependence on gas and liquid rates were in many respects similar to flow through layers of packing.
ABSORPTION BASIC STUDIES
'
New data for the system ammonia-water were reported by Wen (41). The tower was a 76/8-in~hcarbatc unit and as packings 1-inch carbon rings, I-inch Berl, and 1-inch Intalox saddles were used. Irrigation rates ranged from about 2000 to 10,000 pounds per square foot per hour. Gas mas8 velocities extended up to 600 pounds per square foot per hour. Wen's data for Raschig rings and Berl saddles were in fairly good agreement with the results obtained by Fellinger in 1941 a t Massachusetts Institute of Technology, and by Molstad and coworkers as reported in Trans. Am. Inst. Chem. Engrs., 39, 605 (1943). Observed HTU values of Berl saddles were on the avcrage 25% lower than ring data for a similar flow range. Comparison between the two types of saddles indicated that Intalox saddles had appreciably lower HTU values than the Berl saddles. The improvement w&s considerably in excess over what could be expected from a consideration of total surface area. It was therefore concluded that in the Intalox saddle bed the area availability may have been higher, which may have yielded to more complete wetting of the packing and resulting higher coefficients. New carbon dioxide absorption data were obtained by Blum, Stutznian, and Dodds (3). As absorbents sodium and potassium hydroxides as well as carbonates were used. The towers, of 2.8and 4-inch diameter, had been charged with I/d-, 3/g- and l/2-inch Raschig rings. &a values were independent of gas rate, but increased with increasing liquid rate; the increase was more marked a t low than at high irrigation rates. KGUincreased also with the normality of the absorbing solution, reaching a maximum value near a 1.5 normality. Packing size had no noticeable effect on the capacity data. By assuming that the amount of carbon dioxide transferred per unit area of tower cross section is proportional to the concentration of ions and nonions that are operative in the removal of carbon dioxide from the solution, they proposed the following empirical equation:
Na
=
K'
= =
4'
OH- = Cots = h =
pound moles per hour multiplied by the tower cross section function of liquid rate ionic strength average top and bottom normalities of hydroxide average top and bottom normalities of carbonate packing height
Although the equation presents the data reasonably well, there is at present no evidence that would indicate to what extent, if a t all, this data interpretation may be extended to other systems. Partial pressures of ammonia and carbon dioxide in the system ammonium nitrate-ammonia-carbon dioxide-water were measured by Levi and Vasilenko (2'7). The data extend over a considerable range of total pressures and temperatures. Whitney, Han, and Davis (42) reported on absorption of sulfyr ?oxide in water, sodium carbonate, and sodium sulfite, media o! interest to
Vol. 46, No. 1
pulp and paper technologists. The tower of kinch diameter was packed to a height of 2 feet with 1-inch Raschig rings. As expected, both film resistances were important with water as absorbent. With strongly alkaline sodium carbonate solutions, the resistance was primarily in the gas film, whereas with bicarbonate and sulfite liquors it reverted to the liquid film side. The new data, koa, h a , were some 25% higher than the Whitney and Vivian (43)data for the same system and packing. The difference was attributed to the fact that in their unit, the ratio of packing to column diameter was much larger than in the Whitney and Vivian apparatus. This would have led to a more pronounced wall effect, and it may be asked whether this should not have resulted in generally lower, rather than higher capacity coefficients. With water as absorbent and a temperature of 90' F. individual film coefficients were given by
*
kca = 0.026G0.6L0.s7 kLa
=
0.095L0~"
Other absorption data of carbon dioxide in distilled water were reported by Garcia (18). The same source also gives desorption data of chlorine and bromine from water solution by contacting with air in a packed tower. Earlier gas-film absorption coefficients reported by Surosky and Dodge (59) and Houston and Walker (18) were re-examined by Niwa and Fujita @ I ) , and used as hasis for a working correlation. The resulting gas-film coefficient equation was then applied to the Whitney and Vivian (43) sulfur dioxide-water data. The liquid-film coefficients thus obtained served as basis for proposing a dimensional equation of the Sherwood-Holloway type. Values predicted by this equation were reported to be 30 to 35% lower than corresponding data proposed by the Shermood-Hollowayequation Absorption of chlorine in 2-ethyl-1-hexene and carbon tetrachloride yields definite compounds of known properties, Specific reaction rates may be altered by addition of a homogeneous iodine catalyst. Roper (34) recognized this, and the fractional increase of the liquid-film coefficient could be reported in terms of catalyst quantity present, hence rate of specific reaction rate. The work was done in a disk column, reported earlier by Stephens and Morris (38). For the calculation of individual film coefficients, over-all coefficients were obtained first for the physical absorption of chlorine in carbon tetrachloride. Liquid-film coefficients were then calculated by making allowance for the gas-film component. The resulting liquid-film data, valid for reaction conditions, were then reported in the form of the correlation proposed earlier by Stephens and Morris for absorption of chlorine in ferrous chloride solution, An analysis of the problem of temperature rise in the surface of a liquid absorbent and its effect upon the absorption of the gas was made by Danckwerts (8). Basis of the analysis was that the temperature effect will be too small to influence the solubility or diffusivity of the solute gas or the reaction velocity constant. The developed relationships, when applied to the case of dissolving carbon dioxide in solution, containing carbonate ions to form the bicarbonate ions, indicated that for periods of contact of about l/* second the temperature rise was indeed too small to influence absorption rate appreciably. A limestone-packed tower was used by Gilbert, Hobbs, and Sandberg (16)to absorb hydrogen fluoride from a gas stream containing also air, carbon dioxide, and water vapor. Penetration studies with different kinds of limestone indicated that the limestone structure wa8 important. With oolitic limestones, the envelope seemed softer than with nonoolitic limestone. Data analysis yielded an empirical relationship for evaluating bed depth, to absorb a desired fraction of the entering hydrogen fluoride. Absorption coefficients were proportional to the 0.8 power of gas mass velocity, indicating largely gasfilm control, Gas temperature was also a significant variable as well as particle size and state of penetration of the reaction zone into the limestone body. The latter effects were strikingIy indicated by a plot showing absorption coefficients in relation to
A
INDUSTRIAL AND ENGINEERING CHEMISTRY
January 1954
s/t,where S is specific surface area of the particles and t is the thickness of the fluoride envelope.
L
E
= = = = = = =
p
=
UO bd X
ABSORPTION PROCESSES
Processes that have been described during the year, incorporab ing in one phase or another a gas absorption are so numerous that it will be impossible to refer to them in this paper. Instead attention has been focused on a small number of processes where the emphasis has been for years to achieve improvements in a specific absorption operation. Development work has been in progress for several years to improve the selective removal of hydrogen sulfide and carbon dioxide from combustion and similar gases. The recent work of Wainwright and coworkers (40) is of interest in this respect. It reports on the use of di- and triethanolamine for removal of carbon dioxide and hydrogen sulfide from synthesis gas. The work extends to a pressure of 300 pounds per square inch gage add investigates the merits of amines of various strengths. A patent by Hutchinson (19) deals with the same subject. A report by the Fluor Corp. (0) discusses the same problem, when using methyldiethanolamine. Operating data of a commercial installation, where a 20% aqueous methyldiethanolamine solution is used, arc reported in a paper by Miller and Kohl (30)and reference is made to earlier studies (10, 25). AIR-WATER COOLING
The air-water system is widely distributed. It forms the basis of all water cooling towers that utilize the heat exchange between water and air, to bring about evaporative cooling. I n most water cooling towers, the fluid quantities handled are so large that pressure drops and hence power considerations become very important. For this reason special towers of wood slat or similar constructions have been developed in preference to packed towers, that would probably give a better volumetric capacity coefficient, though at the expense of a considerably higher power expenditure. The perforinance of cooling towers of various internal element design has been described by Chilton (7). Test data covering a considerable flow range led to the definition of a performance coefficient
where a’ = G = L = (X) =
Merkels cooling factor gas mass velocity water mass velocity resistance to air flow in velocity heads
The performance coefficient was shown to be related to the a p proach to the wet bulb temperature. Obviously, the lower the performance coefficient, the better the performance of the tower. Cooling tower performance has been considered also by Baker and Hart (9). Mass transfer coefficients per unit tower volume, divided by gas mass velocity, were reported in relation to water rate. Spray tower data were analped in a similar manner. Another cooling tower study reported by Spurlock (97) had as its object the evaluation of the effect of slat type, spacing, and angle of arrangement on over-all performance. From the material of construction point of view, the paper of Browning and Bublitz (6) describing wood deterioration in cooling towers should be of interesl. Water cooling in spray ponds was discussed by Langhaar ( 2 4 ) . Also related to this topic is a paper by Hendrickson (17)on air washer performance, and another paper by Lewis and White (28), who discussed simplified humidification calculation for airwater and other systems.
a =
C =
f = G =
NOMENCLATURE specific packing surface area water tower performance coefficient friction factor in Fanning equation gas mass velocity
a, p
Ap
63
liquid mass velocity flow rate of the continuous phase in a liquid-liquid system flow rate of the dispersed phase in a liquid-liquid systeni fractional holdup in liquid-liquid system constants pressure drop fraction voids in packed tower gas density
LITERATURE CITED (1) Arnold, D. S.,Plank, C. 8., and Schoenborn, E. M., Chem. Eng. Progr., 48,633-42 (1952). (2) Baker, D. R., and Hart, L. T., Petroleum Refiner, 31, 97-103 (October 1952). (3) Blum, H. rl., Stutsman, L. F., and Dodds, W. S., IND. ENG. CHEM., 44, 2969-74 (1952). (4) Bragg, L. B., Ibid., 45, 1676 (1953). (5) Breckenfield, R. R., and Wilke, C. R., Chem. Eng. Progr., 46, 187 (1950). (6) Browning, B. L., and Bublits, L. O., IND.ENG.CHEM,, 45, 1516 (1953). (7) Chilton, H., Proc. Inst. Elec. Engrs. (London),99, Pt. 11, 440-52 (1 952). (8) Danckwerts, P. V., Appl. Sci. Research, A3, No. 5 , 386 (1952). (9) Fluor Corp., Ltd., Petroleum Refiner. 32, No. 9, 124 (1953). (10) Frazier, H. D., and Kohl, A. L., IND.EXG.CHEM.,42, 2288-92 (1950). (11) Fujita, S.,Kikuzawa, T., and Watanabe, S.’, Chem. Eng. ( J a p a n ) , 17, 230-5 (1953). (12) Garcia, L. A. R., Rev. fuc. cieiic. pl~im.(C’niv. nacl. La, Plafa),24. 41-51 (1949). (13) Gayler, R., and Pratt, H. R. C., Trans. Inst. Ch,em,. Engrs. (London),29, 110 (1951). (14) Gayler, R . , Roberts, N. W., and Pratt, IT. It. C., ihi?.? 31, 5 7 (1953). (15) Gilbert, S . , Hobbs, I. A., and Sandherg, W. D., Ghem. Eng. Progr., 49, 120-7 (1953). (16) Hayter, A. J., Ind. Chemist, 28, 50-64 (1952). (17) Hendrickson, H. M., Heating und Venlilating, 50, 91-103 (July 1953). (18) Houston, R. W., and Walker, C. ti., IND. ENG.CHEM., 42, 1105 (1950). (19) Hutchinson, A. a,, U. S.Patent 2,613,132 (Oct. 7 , 1952). (20) Kafarov, V. V., and Blyakham, L. I . , J . Appl. Chem. (U.S.S.R.7, 24, 1441-57 (1951). (21) Kafarov. V. V.. and P1anovskas.a. 31.A , . Ibid.. 24. 699 (1951). i22j a d . , P. 1397. (23) Kohl, A. L., Petroleum Processing, 6, No. 1, 26 (1951). (24) Langhaar, J. W., Chem. Eng., 60, 194 (August 1953) (25) Leva, Max, Am. Inst. Chem. Engr8, Collected Researrh Papers Symposium 9, 1953. (26) Leva, Max, U. S. Patent 2,639,909 (:\fay 26, 1953). (27) Levi, B. I., and Vasilenko, K. A., J . A p p l . C I i m . (U.S.S.R.), 24. 795 (1951). (28) Lewis, J. G., and White, R. R., IND.ENG. CHEM.,45, 486-8 (1953). (29) Lubin, R.,Ph.D. thesis, Unir. of Missouri, 1949. (30) Miller, F. E., and Kohl, A. L.., Oil and Gas J., 51, 175-6 (April 27, 1953). (31) IViwa, Y., and Fuji&, S.,Chem. Eng. ( J a p a n ) , 16, 3 9 9 4 0 4 (1962). (32) Otake, T., and Kimiura, M., Ibid., 17, 261-8 (1953). (33) Pantages, R., and Feldman, J., IND. ENG. CHEM.,44, 2783 (1952). (34) Roper, G. H., C h m . Eng. Sci., 2, No. 1, 18 (1953). (35) Sato, T., and Nishida, T., Chem. Eng. ( J a p a n ) , 16, 366-71 (1952). (36) Shkrwood, T. K., Shipley, G. H., and Halloway, F. A. L,IND. ENG.CHEM.,30,768 (1938). (37) Spurlook, B. H., Heating, Piping Air Conditioning, 25, 115-21 (June 1953). (38) Stephens, E. J., and Morris, G. €I.,Chem. Eng. Progr., 47, 232 (1951). (39) Surosky, A. E., and Dodge, B. F., IND.ENG.CHEW, 42, 1 1 1 2 (1950). (40) Wainwright, H. W., Egleson, G. C., Brock, C. M.,Fisher, J., and Sands, A. E., U . S. Bur. Mines, Rept. Invest. 4891 (October 1952). (41) Wen, C. Y., M.S. thesis, Univ. of West Virginia, 1952. (42) Khitney, R. P., Han, S.T., and Davis, J. L., T a p p i , 36, 172-5 (April 1953). (43) Whitney, R. P., and Vivian, J. E . , Chem. Eng. Progr., 45, No. 5, 323 (1949). (44) Zenz, F. A,, Ibid., 43, No. 8 , 415 (1947). (45) Zenz, F. A,, Chem. Eng., 60,176 (August 1953).
