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N THE course of a general investigation on the efficiency of packed towers, experiments were undertaken to de-
termine the comparative efficiency of packing materials for the absorption of gases, using towers of several sizes. Results are reported in this paper for the absorption of ammonia from air using 3/4-, I/*-, and l/d-inch crushed stone, and 1-, 3/4-, and 1/2-inch clay spheres in towers 3, 6, and 11.3 inches in diameter, with packed depths of about 4 and 8 feet. Water was used as the scrubbing liquid. The ammonia concentration in the inlet gas was approximately 5 per cent by volume. The air rate was about 2 feet per second in most of the tests, and the water rate was equivalent to 1 gallon per (minute) (square foot).
The Absorption of THOMAS H. CHILTON, HOMER R. DUFFEY, AND HARCOURT C. VERNON E. I. d u Pont de Nemours & Company, Inc., Wilmington, Del.
Apparatus and Methods The apparatus used in this work is shown in Figure 1: The towers, distributors, and grids were the same as those described in connection with work on liquid distribution (1). The three towers were 3- and 6-inch iron pipe size (3.07 and 6.07 inches i. d., respectively), and 11.3inches i. d. The 3- and 6-inch towers were 15 feet high, and the 11.3-inch tower was only 4 feet high. Air was expanded from about 100 pounds per square inch and metered through a sharp-edged orifice. Ammonia, supplied from a cylinder, was vaporized, metered through an orifice, and added to the air stream. The air-ammonia mixture was sampled before it reached the absorption tower. Water was admitted through distributors such as that shown in Figure 2. All distributors were provided with centering devices and were so placed that water was discharged about 1 inch above the top of the packing. The water left the distributors through nozzles of such size that they imparted a moderate velocity to the water, corresponding to a few inches of head. The exit-gas sampling line was connected to the tower at a point above the distributor. The saturator used (in certain runs) to humidify the air-ammonia mixture was a bubble-cap column with a single active plate. The overflow for the bottom plate was blanked off. The device, C , shown in Figure 1 maintained a constant liquor level on the plate. The capillary tube, H , indicated the rate of flow of water to the plate. The upper plate was used as an entrainment separator. Before starting a run in which the saturator wa8 used, the air-ammonia mixture was bubbled through the water in the saturator until equilibrium had been established.
KG values ranging from 0.11 to 0.29 pound mole per (hour) (square foot) (atmosphere) were obtained for the absorption of ammonia from an air-ammonia mixture containing 5 per cent by volume of ammonia, using 4- and 8-foot depths of several sizes of two solid packings, one regular (spheres) and one irregular (crushed stone), in towers 3, 6, and 11.3 inches in diameter at an air rate of approximately 2 feet per second and a water rate of 1 gallon per (minute) (square foot).
weight as the average of the particles. For the nominal and 3/rinch sizes these values were 0.29, 0.485, and 0.775 inch. The clay spheres were toy marbles obtained from the J. E. dlbright Company, Ravenna, Ohio. The painted coating on some lots was removed by soaking
Data on the packing characteristics are shown in Table I. The crushed stone was obtained locally and was carefully screened before using, The equivalent diameter referred to in this paper is the diameter of a sphere having the same AIR BLEED
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ABSORPTIONTOWERS FIGURE 1. THREE-,SIX-,AND TWELVE-INCH A. B. C.
Compressed-air tank Saturator Constant-level tank
D.
Gas-sampling taps
F.
Vaporizer
E. Orifices
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Q. Exit liquor H. Capillary tube J. Pressure-drop manometer
Gases in
Packed Towers
Experiments on Solid Packing Material
the exit liquor rate, and the ammonia flow was obtained from the inlet gas analysis. The partial pressure of ammonia in equilibrium with the exit liquor was determined from the equation (3): log (p/m)
4.699
-
(1922/T)
The log mean of the inlet and exit driving forces was used in calculating the “height of a transfer unit” (H. T. U.) and in calculating Koa ( 2 ) . A material balance was calculated for each run (Table 11). Runs in which the ammonia lost from the gas differed by more than 10 per cent from the ammonia absorbed by the water were discarded. From other experiments on several packings, the values of KGawere found to increase with the 0.G power of the gas rate and with the 0.6 power of the liquor rate over the range covered h e r e . I n o r d e r t o eliminate the effect of variations in gas and liquor rates, all values of H . T . U . , Koa and Kff were adjusted to an air rate of 540 pounds per (hour) (square foot), equivalent to an air velocity of 2 feet per second, and a water rate of 500 pounds per (hour) (square foot). I n calculating the p r e s s u r e drop a t the standard gas rate, 540 pounds per (hour) (square foot), it was assumed that AP increased with the square of the gas velocity. The differences b e t w e e n t h e measured and corrected values of A P , H. T. U., F I G U R E 2 . FOUR-POINT and Kffa are small (Table 11). DISTRIBUTOR FOR 11.3In three runs it was necessary to INCHCOLUMN correct for a 40 per cent change in gas rate, but in all other runs the change in gas rate was less than 10 per cent, while the maximum deviation of the liquor rate from the standard was 12 per cent. Data on the liquor distribution for the various charges of packing are given in Table 11. An explanation of the method used in recording the results was given in a previous paper (I).
It was found that there is little difference between the efficiency of equivalent sizes of spheres and stone but, in general, the pressure drop is higher for the crushed stone than for the spheres. For a given packing, KGa increases with the 0.5 to 0.6 power of the surface per unit volume. Tower diameter is without effect on the absorption efficiency provided the initial distribution is satisfactory and the ratio of tower diameter to packing diameter is 8 to 1 or greater.
the marbles in ammonia solution. The actual average diameters of the marbles were 0.46, 0.74, and 0.99 inch. The method of operation was as follows: The water flow was adjusted to approximately 500 pounds per (hour) (square foot). Air was then admitted at the desired rate. Ammonia was added to the air stream and the flow adjusted to give approximately 5 per cent by volume. All of these flows were kept constant by control of the pressure drops through the orifice meters. After the temperatures of the inlet gas and exit water had reached constant values, samples of inlet and exit gas and of exit liquor were analyzed for ammonia. When equilibrium had been reached (indicated by check analyses), the liquor rate was checked and the final readings were made. The inlet gas was analyzed by drawing the mixture into a weighed, evacuated flask containing a few cubic centimeters of water, reweighing, and titrating the solution with 0.1 N hydrochloric acid. The percentage of ammonia in the exit gas was determined in the following way: A measured amount of standard hydrochloric acid was run into a test tube and diluted to 20 cc. with distilled water. One drop of methyl orange was added. The exit gas mixture was then drawn through the acid by siphoning water from a 5-gallon bottle. The volume of water siphoned from the bottle when the indicator color changed, the temperature of the water, and the height of the water level above the delivery tube were recorded. Preliminary tests showed that the ammonia was completely absorbed when the rate of passage of the gas was less than 100 cc. per minute. The percentage of ammonia in the exit liquor was determined by titration of an aliquot portion with 0.1 N hydrochloric acid.
Results That it was possible to obtain a good degree of reproducibility of results is shown by comparing the following sets of duplicate runs : Run No. 85 59 4
Calculations
0
Packing 8/4-in. stone
3/4-in. spheres 6 Packing removed and replaced.
Packed Depth, Ft. 4.05 4 . 50a 8.33 8.33
H. T. U., Ft. 1.61 1.68 1.51 1.63
The reproducibility was not always so good. I n one case where inconsistent results were obtained, measurements of the liquor distribution showed that the charge of packing happened to exhibit a marked channeling tendency.
The air flow was calculated from the orifice reading, using
a constant of 0.61. The flows calculated from the orifice readings for the water and ammonia were not sufficiently reliable. Hence the water flow rate was obtained by measuring 299
INDUSTRIAL AND ENGINEERING CHEMISTRY
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TABLEI. CHARACTERISTICS OF Nomi- Apparnal ent Size Density
Kind of Packing
In. Broken stone Broken stone Broken stone Clay spheres Clay spheres Clay spheres CI
‘/a 1/z
Lb./ cu. ft. 96 91
a/k
88
y 4
85 81 77
Water No. of to Pieces per Wet Cu. Ft. % of total vol. 6 69,700 5 14,750 3 3,560 4 20,200 3 4,700 2 1,950
The loading velocity was determined for air (density
NO.
59 70 87 17 18 90 6 80 20
Packing 8/4-in. stone 1-in. spheres */4-in. spheres l/Z-in, spheres
Packed Depth, Ft. 4.50 8.33 4.13 8.75 8.75 4.0 8.33 4.0 8.33
PACKINQS
Loading Mean Dimensions
Surface
2.$k!
In. 0 5 X 0.3 X 0.15
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0.87 X 0.43 X 0.25 1.25 X 0.67 X 0.55 0.46 0.74 0.99
Ve-
locity Voids,
Ft./sec.a 1.5 2.8 3.6
150 94 52 92 57 41
2.3 3.2 4.7
Yo
41 43 54 46 51 50
Remarks Passed 0.40411. soreen; held on 0.24-in. Passed 0.58411. screen’ held on 0.40-in. Held on 0.5s-in. sorein Uniform Uniform Uniform
0.075 lb./ou. ft.) at a standard water rate of 600 lb./(hr.)(sq. ft.).
The gas mixture before entering the absorption column had a low relative humidity. This resulted in cooling of the liquor in the lower part of the tower by evaporation, whereas by absorption alone it would have been heated. The result was that the true equilibrium partial pressure of ammonia above the liquor throughout the tower could not readily be estimated from the terminal temperatures. In order to determine the error involved, the gas mixture was passed through a saturator before entering the absorption tower (Runs marked d , Table 11). Run 53 on the 3/4-in~hstone in the 6-inch tower with the saturator gave an H. T. U. of 1.64 feet; run 59 without the saturator gave an H. T. U. of 1.58 feet. It is concluded that the use of the saturator on the gas mixture had no appreciable effect. It must be noted, however, that the saturator was adiabatic, and consequently the effect in increasing the humidity of the air was relatively small. It was concluded that a single central stream is satisfactory for distributing the water in a 6-inch tower (compare runs 53 and 56), but a single stream on the side wall is not satisfactory (run 57). A single central stream is not satisfactory for distributing the water in the 11.3-inch tower (compare runs 62 and 64). Other results (not reported here) indicated that a four-point distributor is adequate for a 12-inch tower. These conclusions are in agreement with these reached in a previously reported study on liquor distribution (1). The effect of depth of packing (in the 6-inch column) is shown in the following table: Run
VOL. 29, NO. 3
H. T. U., Ft. 1.58 1.97 2.12 2.21 1.90 1.90 1.98 1.22 1.45
The apparent trend toward greater H. T. U. with greater depth of packing is not believed to be particularly significant, but to be due, in part a t least, to greater uncertainty in the measurements at the greater depth of packing. Figure 3 shows a plot of Kciavs. a for the packings tested. The absorption coefficient is evidently proportional to some power of the packing surface less than unity. If the point for the ‘/d-inch stone is given less weight on account of the fact that it has been obtained by extrapolation to a velocity actually above the loading point, the relation appears to follow a power between 0.5 and 0.6. At the water rates used, the packing surface is probably not completely “wetted.” Thus the percentage of the surface actually active for absorption (3) decreases as the packing surface is increased (at the same liquor rate) and only a part of the increased surface is effective.
Figure 3 also shows the relative absorption coefficients of equivalent sizes of stone and spheres. While the 3/4-in~h stone appears to be somewhat better than the 3/4-inoh spheres, the l/rinch stone has a slightly better coefficient than the l/Z-inch spheres. The pressure drop ( A P ) ,as shown by Table 11, for the 3/4-inch stone is 1.7 times that for the 8/4-inch spheres a t the same gas rate; that for the l/Anch stone is the same as for the f/z-inch spheres. With a ratio of tower diameter to packing diameter of 8 to 1 or greater and adequate initial liquor distribution, there is no substantial effect of tower diameter on final liquor distribution or absorption efficiency. With ratios of tower diameter to packing diameter less than 8 to 1, there is poor final distribution (1); this is reflected in low values of KG (compare runs 19 and 87), but the increased wall area supplied by the tower provides for additional absorption so that the KGavalues are practically the same.
Comparison with Previous Data Sherwood and Kilgore (4) give data for absorption of ammonia on l/S-inch coke in a 4-inch tower a t gas rates varying from 160 to 510 pounds per (hour) (square foot) and a liquor rate of 323 pounds per (hour) (square foot). Since the conditions are close to those used in the present work, the experimental values of Kaa have been adjusted to a gas rate of 540 pounds per (hour) (square foot) and a liquor rate of 500 pounds per (hour) (square foot), making the same assumption with regard to the variation of Koa with gas and liquor rates as was used above in adjusting the present data. The values of KGa varying from 13.9 to 18.5, with an average value of 16, are in satisfactory agreement with the value of 14.6 for KGa on l/%-inchstone in the 6-inch tower obtained during the present work. 20
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d0 JO
a, sq.
lk
0
ft./cu. ft.
ABSORPTION COEFFICIENT Kaa vs. SURFACE UNITVOLUME FOR SOLIDPACKING MATERIALS IN 6INCHTOWER
FIQURE 3. PER
CRUSHED STONE
MARCH, 1937 - t - b i ~ m r-her- w m O ~ O O O N O W O dlo "N" 3 m 3 m 33 00000
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INDUSTRIAL AND ENGINEERING CHEMISTRY O N N o ~ ~ m m0 3 m m i m e~u3r-a comw 34313 3 3 3
3 3 m 4
0000 0 0 0 0000 0000 000
301
Summary 1. Adequate initial water distribution is necessary to secure maximum absorption efficiency. 2. There is no definite effect of depth in the range of eight to sixteen tower diameters. 3. These results are in agreement with the findings in the work on liquor distribution. 4. There is no effect of tower diameter on the absorption efficiency of the packings tested when the ratio of tower diameter to packing diameter is 8 to 1 or greater. 5. For a given type of packing, the increase in absorption coefficient is proportional to some fractional power of the increase in surface. 6. There is little difference between the absorption efficiencies of equivalent sizes of clay spheres and crushed stone.
Nomenclature surface per unit volume, sq. ft./cu. f t . H. T. U. = height of a transfer unit, ft. KQU= lb. moles/(hr.)(cu. ft.)(atm.) K G = Ib. moles/(hr.) (sq. ft.) (atm.) p = partial pressure, atm. AP = pressure drop, in. water/ft. packing m = gram moles/1000 grams water T = temp., O C. abs. a
=
Literature Cited (1) Baker, T., Chilton, T. H., and Vernon, H. C., Trans. A m . Inst. Chem. Engrs., 31, 296-315 (1935). (2) Chilton, T. H., and Colburn, A. P., IND.ENQ.CHEM.,27, 255-60 (1935). (3) Kowalke,. 0. L., Hougen, 0. A., and Watson, K. M., Bull. Univ. Wis., Eno. Expt. Sta. Series, N o . 68 (1925); Chem. & Met. Eng., 32, 443-6, 506-10 (1925). (4) Sherwood, T. K., and Kilgore. A. J., IND. ENG. CHEM.,18. 744-6 (1926). RECEIVBD October 31, 1936.
Courtesy, E . I . du Pont de Nemours S Co.
GIANTCOMPRESSOR IN THE DU PONT W.VA. PLANTAT BELLE,
