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December, 1924
INDUSTRIAL A N D ENGINEERING CHEMISTRY
of 1 sq. cm. in 1 hour when the concentration difference is 1 gram of oxygen per cubic centimeter of solution across a fdm 1 cm. thick. .The “effective film thickness” in centimeters is therefore 0.067 divided by the value of k ~ .I n these experiments, under normal operating conditions the value of k~ is about 3.3. The effective liquid fdm thickness is therefore 0.067 -= 0.02 cm.
Absorption of Hydrogen Sulfide By F. W. Sperr, Jr. THE KOPPERS CO., MELLON INSTITUTE, PITTSBURGH, PA.
HE absorption of hydrogen sulfide is of great importance in the artificial gas industry, and is characterized by the unusually high degree of efficiency which is required. Coal gas or water gas entering the purification apparatus contains from 100 to 800 or more grains of hydrogen sulfide per 100 cubic feet while the gas leaving the purification apparatus must not stain the lead acetate paper after exposure for 1minute. The efficiency of purification must, therefore, be practically 100 per cent. Until recently the absorption of hydrogen sulfide was accomplished by dry absorbent material. Up to about forty years ago lime was generally used. This, however, was entirely displaced by hydrated iron oxide, which is the standard dry purifying material today. The chemistry of the iron oxide purifying process is very interesting, and although it has been made the subject of many investigations it has not yet CONCLUSIONS been thoroughly worked out and still presents some interesting and important problems. However, a discussion of dry The results of this study of comparative absorption rates purification would not be pertinent to this symposium, which for four gases can be adequately explained by the two-fdm is dealing primarily with absorption of gases in liquids. theory. On the assumptions that oxygen absorption is conAlthough the absorption of hydrogen sulfide by hydrated trolled by liquid film diffusion and that ammonia absorption iron oxide is a very efficient process, it has a number of pracin hydrochloric acid solution is controlled by gas film diffu- tical disadvantages, such as large amount of ground area sion, the absorption rates for sulfur dioxide, ammonia, and required, variation in quality of oxide, labor and inconvenhydrochloric acid can be predicted with an accuracy of 15 ience of emptying and refilling boxes and revivifying oxide, per cent or better. danger of explosions, difficulty of purifying gases containing BIBLIOGRAPHY high percentages of hydrogen sulfide, and impossibility of purifying gas under high pressure. For many years consider1-Lewis and Whitman, I n d . Eng. Chem., p. 1215, current issue. able effort has been made to develop processes in which the 2-Doiman and Masson, J . SOC.Chem. Ind., 89,236 (1920). 3-Lewis, J. Ind. Eng. Chem., 8, 825 (1916). hydrogen sulfide could be removed by treatment with a liquid. 4-Bohr, A n n . Physik Chem., 68, 500 (1899). Many inventors have endeavored to imitate the prdcess of b B e c k e r , Phil. Mag., 46, 581 (1923). dry purification by employing various iron compounds in G H i i f n e r , Z . physik. Chem., 27, 227 (1898). solution or suspension, while others have endeavored to make 7--Winkleman, Landolt-Bornstein Tabellen, Vol. V, 1928, p. 249. use of the ammonia recovered in gas works practice. Various processes have been proposed employing metallic salts, DISCUSSION such as those of copper or lead, various alkaline solutions, Dr. Donnan remarked that the absorption theory as pre- sulfur dioxide, or sulfites, hypochlorite solutions, etc. Howsented in the first paper and confirmed in the paper just ever, until within the last few years no liquid purification procread appeared to be a sound development of the subject. ess has proved a practical success and with two or three His earlier work had been based on studies of the less soluble exceptions none has passed beyond the laboratory scale. A few years ago this company developed a liquid process gases, and there was no question but what the concept of diffusion through two films must be considered in any com- for the absorption of hydrogen sulfide which has proved very prehensive treatment covering gases of widely varying successful. About twenty-five plants in America are now employing this process and several new installations are solubilities. being made. The process is in operation under a variety of conditions and treats different kinds of gas, including Foreign Commerce Handbook coke oven gas, coal gas from retorts, water gas, and oil gas, Sources of information of value to exporters and importers in while one plant is being built to purify natural gas. The the solution of many of the problems involved in foreign trade plant capacities range from 2 to 30 million cubic feet of gas are indicated in the Foreign Commerce Handbook for 19241925, issued by the Foreign Commerce Department of the per 24 hours. Chamber of Commerce of the United States. PROCESS The pamphlet is designed to contribute to a wider appreciation of the facilities available to American business men in the conduct The process uses as an absorbing medium a dilute solution of overseas trade. Sources of information and service are listed of sodium carbonate, the ordinary strength of which is equivaunder ninety separate headings. The handbook also contains a review of the constructive action lent to from 1to 3 per cent sodium carbonate. The apparatus taken by the Chamber of Commerce of the United States with usually employed consists of a single tower divided in two reference t o foreign trade matters. Among the more important topics acted upon are: support of the Foreign Service of the sections each of which is filled with suitable packing. The upper section is known as the absorber, and the lower section Departments of State and Commerce, passports and visas, tariff principles, taxation of Americans abroad, standards of quality, as the actifier. The gas passes through the absorber, where China Trade Act, foreign chambers of commerce in the United it is washed with the sodium carbonate solution. This soluStates, American chambers of commerce abroad, foreign student tion flows down through the actifier, through which air is exchange, Merchant Marine Act, investments abroad, treaty blown. The air removes the hydrogen sulfide and the solurelations, the Webb-Pomerene Act, and commercial arbitration. 3 .3
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The effective gas film thickness may be estimated in a similar manner, using Winkleman’s’ data on diffusion through gases. From his figures an approximate coefficient may be calculated, showing that 0.25 gram of ammonia would diffuse through a film of air: 1 sq. em. in area and 1 em. thick in 1 hour when the difference in ammonia concentration a t the two ends of the column was 1 atmosphere. These exact conditions are, of course, purely fictitious and merely express the units in which the specific diffusion coefficient is expressed. The effective film thickness is then 0.25 divided by k , for ammonia (2.4) or 0.1 em. Although the accuracy of this estimate IS poor, it shows that the gas film has a considerably greater efrective thickness than the liquid film under the conditions of operation in these experiments.
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INDUSTRIAL A N D ENGINEERING CHEiWISTRY
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tion thus regenerated is pumped back to the absorber. The operations of circulation, purification, and actification are continuous and automatic and require very little attention. The process involves the two reversible reactions : HzS COz
++ Na2COs = NaHS + NaHC03 HzO + Na2C03 = 2NaHC03
(1) (2)
In the absorber these reactions proceed*from left to right, while in the actifier, with air acting to carry out the carbon
LIQUIDPURIFICATION
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GAS & E L ~ C T RCOMPANY IC
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As a matter of fact, secondary reactions do occur, but they result in the formation of only two compounds and only small amounts of these are produced. These two compounds are sodium thiosulfate and sodium thiocyanate. Sodium thiosulfate appears to be the end product of oxidation in this process. No sodium sulfate or any other oxysulfur compound has been found to be present. Now it is an interesting fact that sodium thiosulfate is not formed when pure air is blown through a pure solution of sodium hydrosulfide with or without the presence of sodium carbonate. It is believed that the sodium thiosulfate is produced through oxidation of higher sulfides of sodium which result from the action of free sulfur on the sodium hydrosulfide. Free sulfur might be found through the agency of small amounts of iron compounds in the system. Sodium thiocyanate appears to result from the action of hydrocyanic acid on the sodium sulfides and is only present in the systems purifying coal gas. The formation of sodium thiosulfate and khiocyanate is relatively slow and these substances accumulate in the circulation system until the rate of their formation is balanced by the rate of mechanical loss. Both salts are very soluble and the maximum amounts present in any system are far from the crystallization point. The usual consumption of sodium carbonate in the application of the process to the purification of coal gas is about 1 pound for every 8 pounds of hydrogen sulfide removed, and of this, roughly, one-third is due to mechanical losses, one-third to formation of sodium thiosulfate, and one-third to the formation of sodium thiocyanate. How-
dioxide and hydrogen sulfide, conditions are made favorable for the reactions to proceed from right to left. In addition there may be the following reactions: C02 COz
+ H 2 0 + NaHS = NaHC03 f HzS + HzO + 2NaHS = NazCOs + 2HzS
(3) (4)
It will be seen that an excess of sodium bicarbonate is essential for proper actification. This excess is produced by the carbon dioxide which occurs in practically all artificial gas. If there is any deficiency of carbon dioxide, as for exarrlple, in the case of natural gas, it may readily be supplied artificially. The amount required is very small and there have been indications that even the slight excess of sodium bicarbonate resulting from secondary reactions, as in the formation of sodium thiosulfate, may be sufficient to make the process effective. Besides removing hydrogen sulfide, the process has been found valuable in extracting hydrocyanic acid, which is an objectionable constituent of coal gas and is not satisfactorily removed by oxide purification. The reactions involved are similar to those occurring in the case of hydrogen sulfide and take place simultaneously with the latter. SECONDARY REACTIONS Reference has been made to secondary reactions. It will be noted that the oxygen of the air takes no reactive part in the process proper. If there were no secondary reactions, there would, theoretically, be no consumption of sodium carbonate and no products other than gaseous hydrogen sulfide, hydrocyanic acid, and carbon dioxide. In the early stages of the development of this process there was much concern as to what might occur with respect to secondary reactions, since there was a possibility of forming all sorts of oxysulfur compounds as well as cyanogen derivatives. It was a question whether large quantities of sodium carbonate might not be lost on account of the formation of such compounds and whether the oxidation of sodium hydrosulfide might not go as far as sodium sulfate, as the literature had indicated.
CONSOLIDATED G A S , ELECTRIC LIGIIT & POWER COMPANY
LIQUID PURIFICATION P L A N T ,
ever, some plants show much lower soda consumption figures. In one instance, the average for several months’ operation was 0.03 pound per pound of hydrogen sulfide removed.
TOWER PACKING I n this connection, not only were the ordinary problems of obtaining efficient contact between gas and liquid to be con-
Decembcr, 1924
INDUSTRIAL A N D ENGILVEERINGCHEMISTRY
sidered, but the apparatus had to be arranged so that it would deal efficiently with gas containing tarry matter and naphthalene, which would clog many types of packing. I n fact, a good deal has come to be expected of this liquid purification process in addition to the simple removal of hydrogen sulfide, and it is usually demanded that the apparatus be a gas cooler, a naphthalene washer, a tar extractor, a cyanide washer, and a sulfur remover all in one. Three principal types of packing have been ued-viz., coke, wooden hurdles, and spiral tile. Coke (0.5 to 1 inch size), if carefully screened, gives good results with gas free from suspended matter and is the cheapest of the three. However, it is much more subject to trouble on account of stoppage with dirty gas. Both wooden hurdles and spiral tile are very satisfactory from the standpoint of efficiency and have given excellent results under a variety of conditions. One plant, recently put into operation, has steel turnings as packing material and is giving very satisfactory results.
EFFICIESCY OF PROCESS In all the liquid purification plants thus far installed, the process operates to remove 85 to 90 per cent of the hydrogen sulfide, the remainder being taken out in the oxide purifiers. In this way the latter are used as catch boxes and the investment in them is not lobt. The insta1lat)ion of a liquid purification plant has the effect of increasing the capacity of the oxide boxes two and a half times and makes it unnecessary to renew the oxide except after intervals of many months. It often happens that the amount of purifying capacity is the limiting factor in the growth of a plant on any particular site. In such cases, with dry purification an increase of capacity can only be obtained by building a new plant on another site, while with liquid purification the capacity of a plant on a given sito can be more than doubled. With such an installation the system is able to deal with great fluctuations in the hydrogen sulfide content of the gas. The liquid purification process absorbs such fluctuations, since it is able to treat gas of 1000 grains hydrogen sulfide per 100 cubic feet just as readily and even more efficiently than gas of 100 grains per 100 cubic feet. Thus, the catch boxes are only required to deal with comparatively small amounts and minor fluctuations. The process can also be adapted for complete removal of hydrogen sulfide, and developments are now under way which are expected to make possible the recovery of the sulfur in the hydrogen sulfide.
Bureau of Standards Annual Report Scientific investigations and tests resulting in large savings to the Government and to American industry through improvement in processes and the fixing of uniform standards are featured in the annual report of George K. Burgess, director of the Bureau of Standards. Investigations made with orifice meters for measuring gas, corrosion of underground pipes, and tests conducted covering impact stresses in highway bridges, braking systems for automobiles, and other studies, have resulted in the application of improved methods in engineering practices that are of direct and substantial savings to the industrial public. Other contributions to the public interest are the successful development of methods of reducing the loss in the baking of Japan ware, the assistance rendered the optical glass industry in the United States, the progress made in the better utilization of cotton linl ers and other cotton wastes, and the development of a method for reclamation of gasoline from dry-cleaning processes. An increase of more than one hundred and twenty five times its initial volume has taken place in the testing work of the Bureau of Standards during the twenty-three years of its existence. During the year just closed 135,852 tests were conducted by all divisions of the Bureau, as compared with 115,729 in 1923. Most of the tests were executed for other branches of the Government, but a great deal of testing was also done for commercial firms and individuals. Unfortunately, some of this latter work had to be refused because the demand exceeded the facilities. The inability to meet this demand is unfortunate.
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Quantitative Analysis of Mists and Fogs, Especially Acid Mists' By Harold C. Weber MASSACHUSETTS INSTITUTE OF TSCHNOLOGY, CAMBRIDGE, MASS.
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HIS paper is presented, not because it contains any new
methods of analysis, but rather to describe in detail the methods that the author has found successful for the determination of mists, especially sulfuric acid mists, in the plant. Such a paper has a place in an absorption symposium, for much of the published work on absorption is unreliable, because the methods of gas analysis involved completely ignore the necessity of analyzing for any mist that may be present. There can be no doubt that many of the irregularities in absorption data, especially where these data concern the absorption of acid gases, can be satisfactorily explained by the assumption that a mist was present during the analytical absorption. A mist or fog is made up of small particles of varying sizes. These particles may be either liquid or solid. One concept is that each individual particle is surrounded by an air film which effectively insulates it from its neighbors. According to this idea, then, any method of analysis to be effective must present some means of breaking through this air film. It is possible, however, to explain practically all the peculiarities associated with mist analysis and absorption without assuming the existence of' this protective air film. If we assume that the mist particles are so small that they do not settle out readily by gravity and that they are so large that their rate of diffusion is extremely small compared with the diffusion rate for gases, it is possible to see that extreme difficulty would be experienced in dissolving them. From this discussion it is evident that any method of analysis which proposes to analyze for mist by the mere bubbling of the gas through an absorbing medium should be looked upon with distrust. After a careful search of the whole field two fundamentally different methods of analysis were selected as possibilities. The first of these consists of passing the gas containing the mist through some type of porous membrane in which the pores are sufficiently small so that the mist particles are effectively trapped. Whether this trapping occurs by an actual scraping aside of the protecting gas film, whether it is due to the throwing out of particles as they pass through the torturous passages of the porous medium, or whether it is due to some other cause will not be discussed a t this point. The second method which presented itself was an adaptation of the principle involved in the Cottrell precipitator to a small portable apparatus for use in the plant.
POROUS MEMBRANE METHOD Several types of porous membrane apparatus were tried out, but only three of them gave satisfactory service; and of the three only two were finally retained. The first method tried was that involving the use of an asbestos mat. This mat was formed by catching a suspension of long-fibered asbestos on an ordinary 4-inch porcelain Buchner funnel. The preparation of these mats is rather difficult and the subsequent drying which must be done before the mat can be used is especially difficult. Cracks are very liable to form slightly below the asbestos surface. These cracks may be entirely invisible to the eye, but nevertheless they allow mists to pass through. The funnels themselves are clumsy to handle, and owing to Presented under the title "The Quantitative Determination of Mists and Smokes wlth Special Reference to the Determination of Sulfur Trioxide Mists."
