COUNTERCURRENT ABSORPTION HARRY E. OSBORN, JR., AND C. W. SIMlMONS Lehigh University, Bethlehem, Penna
Among the objections to the use of existing data on absorption is their source from small-scale equipment. Further, there is a lack of data on systems whose components cannot be readily analyzed. The purpose of this article is to describe the design of a large-scale testingunit, and toshow its general analytical application by a carbon tetrachloride-kerosene absorption system.
T
HE first part of the paper will describe the development
of equipment of general application to the study of
absorption systems. Previous investigation based on readily analyaahle systems led to the selection of a 12-inch column, 10 feet high, with 1-inch packing. The rest of the development resolved itself into the design of auxiliary equip ment to permit satisfactory analysis and control. In all studies of absorption systems it is necessary to he able to make a rapid analysis of the inlet and outlet gases. Since the equipment to be set up for a study of absorption should he of such a nature as to permit testing of various systems, a method of analysis had to be designed to handle a variety of gases to he absorbed. This method is based on a process which permits sufficient time, turbulenee, and liquid head to produce saturation of the inert carrier with solute. In order to produce this effect, a combination bubble and packed column was used. One such unit, known as the saturator, was placed in the system ahead of the absorber and another, the analyzer, was located after the absorber. A measure of the quantity of solute necessary for resaturation of the partially denuded gas as it passes through the analyzer irom the absorber provides a means of calculating the ahsorption.
Apparatus The absorber was 10 feet of standard 12-inch pipe with blind flange plates fitted with and I-inch standard pipe for the gas and extractor circuits, respectively. The gas inlet extended 13 inches above the flange with a tee and inverted elbows for distribution. The saturator was 8 feet and the analyzer 5 ieet of standard 6-inch flanged pipe. Both were fitted for J/6-inch vapor lines and '/riucli inlet for liquid solute addition. The vapor from the analyzer was discharged out of doors, so that the unit may be applicable to toxic systems. The absorber was equipped with a 10-inch shower-type (flat drilled face) distributing head for the extractor, packed
with 5.62 cubic feet of 1 X 1 X 1/2 inch carbon Xaschig rings on a '/,8-inch copper plate drilled with %-inch holes and supported 16 inches above the bottom flange for a height of 7 feet 2 inches, and a sight glass for liquid level observation. The free volume of the packed section was 3.97 cubic feet. The other two units were similarly equipped but with X X $/a inch Raschig rings, inverted 5-inch distributing heads, and practically full-length sight glasses. Although liquid levels were to he maintained at about the halfway mark, the tovers were packed to the top in order to eliminate entrainment. In order to compensate for the cooling effect of the latent heat o i vaporization in both saturator and analyzer, they were wound mith Nichrome wire with electrical input control by rheostats. All gas and liquid inlets and outlets on the three units were equipped with Weston thermometers, and all vapor spaces with Hays draft gages. Necessary auxiliary equipment included a 54gallon reservoir, pump, and by-pass with rotameter in the circulating extractor line, and a 200-cubic-foot gas meter in the inert carrier line. The dimensional and constructional details are shown in Figure 1.
Saturation Tests I n order to establish the general application of the apparatus, tests were made to determine whether saturation of the carrier was accomplished in both saturator and analyzer by means of materials giving a rather wide range in vapor pressure, and with variable air rates up to 20 cubic feet per minute. At low rates, wet test meters were used for measurement of volumes oi air used. The sight glasses were calibrated to give direct vaiues of solute consumption. From these values and volumes, temperatures, and pressures of carrier, gas composition may be readily calculated. It was necessary to prepare a vapor pressuretemperature curve by determination of the vapor pressure of piire solute by the Rainsay and Young method.
INDUSTRIAL AND ENGINEERING CHEMISTRY
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From these curves theoretical saturations were calculated and degree of saturation of the experimental unit was established. Table I gives the pertinent relationship with respect to carbon tetrachloride, xylene, and toluene. The results of these saturation tests established definitely that complete saturation is accomplished in both saturator and analyzer under operating absorption conditions.
Carbon Tetrachloride-Kerosene Absorption System
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'TABLE I. SATURATION DATA Air at Run Saturator No. Pressure
1
2 3 4 5 6 7 8 9 10
The extractor reservoir was filled with 50 1 gallons of kerosene, and the saturator and ana2 3 lyzer were approximately half filled with carbon 4 tetrachloride. The circulating pump was started 5 6 and the by-pass valve adjusted to produce the 7 8 correct extractor rate, indicated by the calibrated 9 rotameter. After the absorber packing had be10 11 come wetted, the extractor outlet was adjusted 12 13 to provide a liquid seal and indicate absence 14 of flooding by the level in the sight glass, ap15 16 proximately 4 inches above the flange. Carrier 17 18 (air) was bubbled through the saturator, and the 19 vapor was passed through the absorber until 20 21 equilibrium was reached. The pressure gage 22 23 a t the carrier meter had been standardized so 24 that a reading would indicate a desired carrier rate. After vapor was bubbled through the 1 analyzer to wet the packing above the liquid 2 level, the carbon tetrachloride was allowed to 3 4 come to an equilibrium level and a reading was 6 6 taken on the sight glass. The exit vapors from 7 the absorber were again passed into the analyzer, 8 9 the run was started and carried on for 30 min10 utes. Throughout the run it was necessary to adjust both carrier and extractor valves in order to maintain constant rates, and to manipulate the heater rheostats so that no appreciable change in temperature occurred in passing through the saturator and analyzer. As the carbon tetrachloride vaporized from both the saturator and analyzer, it was expected that there would be some effect on the carrier pressure gage due to change in head. A careful check indicated that, even after three runs involving extractor and carrier rates which were known to produce high absorption and thus vaporize a large amount of carbon tetrachloride in the analyzer as well as the saturator, no appreciable change in reading was noticed. However, a fresh supply
150 150 150 150 150 150 150 150 150 150
Absolute Pressure Temp.
VaDor Pressure
Liquid Vaporized
% by Vol.
Exptl.
Theoretical
Xylene Saturation (Mol. R't., 106.08; Density 0.871 G./Cc.) 30.1 80 1.93 1.27 30.6 78 1.17 1.77 31.7 67 1.16 0.767 31.5 74 1.50 '0.990 32.7 78 1.65 1.09 78 33.7 1.60 1.05 71 35.3 1.20 0.744 35.0 80 1.60 1.06 32.0 80 1.80 1.18 33.0 79 1.68 1.11
1.27 1.17 0.768 0.987 1.09 1.06 0.790 1.09 1.19 1.11
CC14 Saturation (Mol. Wt.. 153.85: Densits, 1.59 G./Cc.)
71 .O 70.1 68.9 68.9 67.7 67.4 71.1 70.1 68.8 68.9 67.8 67.4 84.1 82.1 81.1 79.9 78.2 76.9 77.6 76.3 74.4 67.4 66.4 65.2 20 20 20 20 20 20 20 20 20 20
Toluene Saturation (Mol. 85 30.9 30.9 85 31.4 84.5 31.4 84.5 84 31.9 84 31.9 84 32.4 32.4 83 85 32.9 85 32.9
3.86 3.75 3.65 3.65 3.54 3.53 3.86 3.75 3.65 3.65 3.65 3.53 5.68 5.22 5.08 4.88 4.62 4.44 4.57 4.41 4.19 3.52 3.44 3.34
1.05 1.00 1.05 1.01 1.05 1.03 1.24 1.21 1.20 1.23 1.20 1.20 1.72 1,68 1.66 1.66 1.66 1.70 1.82 1.88 1.79 1.79 1.78 1.73
10.70 10.44 10.15 10.16 9.8 9.82 11.04 10.73 10.44 10.43 10.19 10.09 16.41 16.38 14.96 14.36 13.61 13.08 13.88 13.38 13.10 11.01 10.95 10.64
10.8 10.4 10.2 10.1 9.8 9.8 11.1 10.7 10.5 10.5 10.3 10.1 16.5 15.3 15.1 14.5 13.7 13.0 13.8 13.5 13.2 11.1 11.1 11.0
Wt., 92.06: Density. 0.862 G./Co.) 1.43 109 1,013 4.82 1.43 110 1.005 4.78 1.41 108 0.987 4.71 1.41 108 4.71 0.987 106 1.39 0,946 4.51 106 1.39 0.937 4.47 102 1.39 0.907 4.34 102 1.35 0.896 4.29 1.43 105 0.902 4.38 105 1.43 0.902 4.38
'
4.77 4.77 4.62 4.62 4.48 4.48 4.41 4.28 4.47 4.47
of carbon tetrachloride was put in both saturator and analyzer after every three runs to ensure adherence to selected carrier rates. Careful attention was given to the liquid levels of the carbon tetrachloride so that saturation would be obtained a t all times. The level was maintained above a point which was known to be the lowest to produce saturation a t high carrier rates. At the end of each run the analyzer was allowed to drain for 5 minutes to ensure accurate readings since previous tests had shown 3.5 minutes to be sufficient for the level to reach equilibrium.
FIGURE2. ABSORPTION us. EXTRACTOR RATEAT VARIOUS CARRIER RATES
10
20
30
40 50 70 AB50RPTION
60
70
80
90
SEPTEMBER, 1939
INDUSTRIAL AND ENGINEERING CHEMISTRY
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INDUSTRIAL AND ENGINEERING CHEMISTRY
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After every few runs the kerosene was allowed to circulate through the system, and warm air was blown through the tower to remove absorbed carbon tetrachloride although the quantity of carbon tetrachloride present exerted a negligible vapor pressure. The data obtained, together with calculated results, are given in Table 11. Figure 2 shows the effect of extractor rate and of variation in carrier rate on percentage absorption. Although all of the runs were made with a saturated inlet vapor, it is possible to use any inlet composition and determine its vapor content by means of a n analyzer in exactly
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the same manner as the one employed for determining outlet composition.
Conclusions A resaturation method provides a n accurate and rapid means of studying absorption systems. Such a method has been used to provide data for the calculation of coeficients. Furthermore, the equipment was of sufficient size to permit reliable calculations of design and operation. A log-log plot of KQuagainst G yields a straight line with a slope of 0.88. In the relationship KQUequals yG0.88,y has a value of 0.0107.
Stability of Sulfur Suspensions J
E. J. VAN LOON, H. G. TENNENT, R. C. QUICK,l AND L. A. HANSEN Rensselaer Polytechnic Institute, Troy, N. Y.
A
METHOD has been developed (3) to remove hydrogen
sulfide from a gas, which produces sulfur in the form of a paste. It has been found that certain thioarsenates, such as sodium or ammonium thioarsenates, have the ability to react with the oxygen of air and thus liberate sulfur. After part of its sulfur is replaced with oxygen the thioarsenate molecule can again react with hydrogen sulfide and absorb more sulfur. The followingmechanism has been suggested for this reaction (3):
+ + +
+ + +
2NasHAsOa 5H2S + Na4As2S6 6H20 NarAs& -k 0 2 + Na&&Oz Na4As2SaOz H2S + Na4AszS60 Ha0 Na&s&O l/zOz + Na4AsZS~02 S
(1) (2) (3) (4)
The sulfur is obtained in the form of a froth which is filtered and washed. The resulting product is a paste containing approximately 50 per cent sulfur and 50 per cent water with small amounts of electrolytes. The sulfur may be sold in the paste form to be used as an agricultural spray or it may be melted and sold as brimstone. Sauchelli (6) pointed out that flotation sulfur in either paste form or dried powder is superior to ordinary finely divided sulfur as an insecticide. This superiority is due to the more finely divided condition of the flotation sulfur which allows better adherence on foliage and also an increased rate of sublimation or vaporization of the sulfur. The size of the smallest sulfur particles as determined microscopically is 1 micron in diameter, and clusters up to 10-15 microns were observed. The individual particles appear to be spherical. This is considerably smaller than most other forms of powdered sulfur (6). The sulfur exists in the paste in a more or less flocculated condition which can be mixed with water under proper conditions to give a fine dispersion; the latter is adapted to use as an agricultural spray. The paste as ordinarily produced is rather difficult to disperse properly in water, so the authors are reporting a study of various dispersing agents used in an 1
Praeent address, Hudson Valley Fuel Corporation, Troy, N. Y.
attempt to prepare a more wettable sulfur. A suitable sulfur paste should have the following properties: (a) A sediment, difficult to remix, should not form. ( b ) The sulfur must form a rather stable suspension when mixed with water. It was difficult to realize the first requirement while obtaining the second. The incorporation of some of the commercial wetting or dispersing agents produced a paste which was easily dispersed in water, but the resulting product was unstable and formed a clayey deposit which was difficult to remix. T o determine the effect of various wetting or dispersing agents on the dispersing properties of the sulfur suspensions, a modified Wiegner method was used as described by Bartell (1). The apparatus is shown in Figure 1. A good dispersing agent should deflocculate the sulfur particles and produce a dispersion from which the sulfur will settle out slowly. The principle of the method employed is dependent upon Stokes’ law (7) which is expressed as W = 6nqrv
(1)
where W is the resistance of a l i q u i d , w h o s e v i s c o s i t y i s v, towards the movement of a sphere of radius r and velocity v. When a constant velocity of fall is attained under the force of gravity, we have ( 5 ) FIGURE1.
SEDI-
MENTATION APPARATUS
Reproduoed by permisaion of Bartell ( 1 ) .
6 n ) = ~ 4/3rrs(d1-dz)8
(2)
where g is t h e gravitational constant_and dl and dz are the
