April, 1926
INDUSTRIAL A N D ENGINEERIA-G CHEMISTRY
357
The Origin of Carbon Disulfide in the Carbonization of Coal’ By Wilbert J. Huff DEPARTMENZ’ O F G A SENGINEERING, THE JOHNSHOPKIXSUNIVERSITY, BALTIMORE, 3ID.
and allowed to gasify freely. When it was heated so rapidly that a dense, coherent coke resulted from a high-volatile coal, carbon disulfide was found in the product9 of gasification, although the quantity of the coal carbonized was small and the tarry and gaseous products had only a small amount of space and time contact with carbon. A possible explanation for this formation is advanced, and the experimental findings are shown to be in accord with large-scale tests and technical practice. It is suggested that the rate of change of temperature be included as an important variable in further related studies.
The part played by carbon disulfide in determining the use of high-sulfur coals for gas-making purposes has been indicated, and a laboratory study of carbonizing conditions which produce this compound made. It is shown that carbon disulfide apparently obtained by interaction between the constituents of the gas and the coke was really derived from the reheated coke alone, and no evidence in support of the widely accepted secondary interaction theory was obtained. Coal was carbonized without producing detectable quantities of carbon disulfide when it was heated slowly
.. HE sulfur content of coals and oils is a n important criterion for determining their suitability for gasmaking purposes, because much of this sulfur appears in the crude works gas as hydrogen sulfide and “organic sulfur.” Both of these forms are very undesirable and their concentrations in gases purified for public distribution are rigorously limited by statute. Hydrogen sulfide may be completely eliminated by the iron oxide process, although this becomes costly when gases high in this impurity are treated. Apparently, no satisfactory process for the removal of carbon disulfide is yet available. This is unfortunate, because the supplies of conveniently situated lowsulfur gas coals are dwindling and such coals command a good premium, while rising oil prices have already compelled many gas-makers, notably those on our west coast, to give serious consideration to the use of high-sulfur oils. Some progress is being made toward lowering the cost of treating gases high in hydrogen sulfide, but the use of high-sulfur coals and oils will be retarded until either some process for the removal of the organic sulfur meets the technical and economic demands of the industry or the gas-making processes are so modified that much of the sulfur which now appears as organic sulfur is converted into some less troublesome form. Recognizing the importance of further information in this field, Hutton and Thomas2 a few years ago in the gas engineering laboratories of this university made an extended study of sulfur distribution during the carbonization of a high-sulfur gas coal. This study showed that the organic sulfur content of the gas could be reduced by various expedients, such as low flue temperatures, full charges, and by liming the charges. While making some reference to current theories, Hutton and Thomas did not attempt to develop a n adequate explanation for these reductions in organic sulfur. Some recent experiments by the writer have thrown additional light upon this subject and an explanation for some of the findings of Hutton and Thomas will be advanced in the present paper.
T
Formation and Removal of Carbon Disulfide
It is impossible to review adequately here the many experimental findings and derived theories dealing with the mode of formation of organic sulfur, and the decomposition 1 Received January 7, 1926. To be presented before the Division of Gas and Fuel Chemistry at the 71st Meeting of the American Chemical Society, Tulsa, Okla., April 5 to 9, 1926.
.. or removal of this impurity. It will suffice to state that the greater portion of this impurity is carbon disulfide, and that the sulfur in this form may amount to as much as 80 per cent of the total organic s u l f ~ r . This ~ ~ ~carbon disulfide is generally believed to be the result of secondary reactions between carbon and hydrogen sulfide a t high temperatures‘ and not a primary product from the decomposition of the coal or oil. The carbon disulfide may be decomposed by passing the gas over refractory surfaces a t 1200’ F. or higherI5 or by the action of gas or steam on catalytic surfaces at lower temperatures.6 Physical solution in oil, absorption in charcoal, or chemical combination with sulfided lime or alkinated cellulose have also been suggested. (17 None of these processes have received extensive use. The desirability of minimizing the production of carbon disulfide has led to some modifications in the design and operation of water-gas sets.* It has, however, had very little influence upon coal-carbonizing processes. This may be due in part to the rather universally accepted theory that the carbon disulfide is the product of a definite equilibrium between the carbon-sulfur compounds and hydrogen sulfide,g and that in consequence carbon disulfide is an impurity unavoidable in bulk carbonization. The evidence cited in support of any such an equilibrium is meagerlo and it is questionable whether or not it applies to gas-making conditions. The experiments of Powell,” in a recent study in t h e laboratories of the U. S. Bureau of Mines, bear more directly upon the origin of carbon disulfide in coking processes and seem to confirm the generally accepted theory that carbon disulfide is derived ’from a secondary reaction between t h e Cos Age, 47, 88 (1921). Griffiths, Gas J., 169, 611 (1925). 4 Meade, “Modern Gas Works Practice,” 1921, p. 395; Powell, Report of the Purification Committee, Am. Gas Assoc., 1923. Pabst, A m . Gas Light J . , 94, 407 (1911); J . Gas Lighting, 113, 906 (1911); Gas World, 68, 210 (1913). 6 Carpenter, J . Gas Lighting, 122, 1010 (1913); 123, 30 (1913); 116. 928 (1914); Gas World, 60, 39, 863 (1914); Evans, J . Soc. Chem. Ind., 34. 9 (1915); Rideal and Taylor, British Patent 130,654 (March 2, 1918); Fulweiler, A m . Gas Assoc. Monthly, l,184 (1919). 7 A further discussion of these and other processes will be found in the report by Powell, loc. cit., and also in Meade, loc. cit., p. 610. 8 Klein, A m . Gas Assoc. Monthly, 6 , 183 (1923). 9 Powell, loc. c i t . ; Fischer (English edition by Lessing), “Conversion of Coal into Oils,” 1926, p. 232. 10 Fischer cites Witzeck, J . Gasbel, 46, 67 (1903). 11 Tins JOURNAL, 12, 1069 (1920). f
8
1ND USTRIAL ANDrENGINEERING CHEMIXTRY
358
hot carbon and hydrogen sulfide. I n these experiments 5 grams of coal were slowly carbonized in a silica tube. This small quantity was chosen because "the primary coking action was desired without the secondary effects produced by the travel of the gases through the hot coking mass." The gases obtained were tested for carbon disulfide and for organic sulfur. None were found. Powell therefore concluded that carbon disulfide was not formed when the products of distillation passed out of the retort without undergoing reactions due to travel over extensive hot coke surfaces. This explanation was generally accepted.'*
Vol. 18, No. 4
indicating that its absence was not due to secondary decomposition on the asbestos after formation. Secondary Interaction
The next experiments were designed to determine whether the carbon disulfide could be formed by secondary interaction between the products of the distillation and red hot coke. A second electric furnace was built, similar to the one first used. When placed end to end on line around the silica tube (of translucent quartz, 30 inches long, 7/8 inch inside diameter, wall '/8 inch) used as a carbonizing chamber.
FUSED JllltA TUBE
Figure 1-Apparatus
Used in S o m e of the Secondary Interaction Experiments
Other explanations may be advanced, however. Among these are the possibilities that (1) tbe analytical methods employed were not sufficiently precise to permit the detection of traces of carbon disulfide in the small total gas volume (less than 2 liters) ; (2) the carbon disulfide was destroyed as i t passed out of the distillation zone over the hot asbestos plug used by Powell; (3) some other coking condition, such as the rate of change of temperature, differed from that which exists in ordinary carbonizing practice and was the cause of the absence of the carbon disulfide. These possibilities were accordingly investigated. The more important results of the analytical study are reported in a previous paper by the ~ 7 r i t e r . l ~This study showed that the presence or absence of traces of carbon disulfide could not be predicted with certainty by the methods previously employed. It was therefore necessary to develop a suitable method, as described in the paper cited. With this available, experiments similar to those of Powell were performed. The coal used had the following proximate anaIysis:'* Moisture Volatile matter Fixed carbon Ash
Sulfur B. t. u. (as received)
Per cent 1.5 36.8 56.0 5.7 1.5
14,030
No carbon disulfide was found, although the experiments were repeated a number of times. Since the gas passed over hot asbestos plug, after leaving the coke, it was necessary to establish whether or not carbon disulfide was being formed and decomposed thereon. Certain preliminary experiments had shown that the carbon disulfide ordinarily contained in Baltimore city gas could not be detected after this gas had passed over acid-washed asbestos under certain conditions a t furnace temperatures as low as 600' C. Accordingly, a 5-gram charge of coal was carbonized with no asbestos plug between the coal charge and the tar filter. No carbon disulfide was detected, 13
Porter, "Coal Carbonization," p. 102; Wibaut, Brennsfoff-Chem.,
3, 273 (1922). 11
J . A m . Chem. SOC.,48, 81 (1926).
The writer is indebted to A. C. Fieldner, U. S. Bureau of Mines, for the data presented in this analysis. 14
the two furnaces permitted about 4 inches of the tube to project a t each end. A 5-gram charge of coal was placed within the tube so that it covered a length of about 4 inches in the center of the first furnace, which will hereafter be designated as the coal furnace. This coal was held in place by plugs of asbestos a t each end of the charge. Within the second furnace was placed a tight cartridge of by-product coke, broken to pass through 4 and be held upon 20 mesh. This cartridge was about 7.5 inches long and weighed 31.5 grams. One end of the cartridge extended to the terminal end of the second furnace, while the other rested against a n asbestos plug which was placed 3.25 inches from the end of the:coal furnace. The cartridge was rammed tightly against the asbestos plug after everything was in place. A filter consisting of a long plug of cotton held in a glass tube 1 inch in diameter and 8 inches long was attached to the exit end of the silica tube to remove the tar from the gas. A bubbler containing a 10 per cent solution of cadmium chloride was attached to remove hydrogen sulfide. From this the gas was Ijassed into a gasometer bottle. At the end of the carbonizing period the gas was taken from the bottle for the determination of its carbon disulfide content. The apparatus is indicated in Figure 1. The system was swept free from air by the use of carbon dioxide, and the coke furnace brought to a predetermined temperature as indicated on a chromel-alumel thermocouple placed a t the center of this furnace between the heating unit and the silica tube. Throughout the experiment this temperature was maintained as nearly constant as possible by the aid of a rheostat. The current was turned on in the heating furnace and the coal charge distilled as before. The distillation products, both gas and tar, thus passed out over hot coke surfaces, and gave a n opportunity for the production of carbon disulfide by decomposition of the complex organic sulfur compounds of the tar or gas, or by interaction between the carbon of the coke and the hydrogen sulfide of the gas. After each experiment a fresh cartridge of by-product coke was used, in order to obviate any secondary effect due to any unusually heavy deposition of tarry matter on the coke surface. A number of experiments were made, using coke furnace temperatures of 600", 700", and 800" C. I n every case carbon disulfide was found in the gas, the greatest amount
April, 1926
INDUSTRIAL AND ENGINEERING CHEMISTRY
being observed when the coke furnace temperature was 600' C. These experiments seemed to confirm the secondary reaction theory, and were therefore extended. Carbon Disulfide Derived OnIy from Coke
To determine whether the carbon disulfide was being produced by the decomposition of rich gas or tarry compounds which occur in the first part of the destructive distillation, or by interaction with hydrogen sulfide which is found in all stages of gas production, the gas from a n experiment a t a coke-furnace temperature of 600" C. was cut into two fractions, so that about equal volumes of gas were contained in each. The carbon disulfide was found in the first fraction, thus favoring the theory that the carbon disulfide was produced by decomposition of rich gas or tar. To investigate this further, two charges were distilled through the same coke cartridge, and the gas from each was cut into two fractions as before. The four samples were then examined for carbon disulfide. This compound was found only in the first rich gas sample, and was absent from the three following samples, thereby indicating that the carbon disulfide was derived from the coke, and when once swept out was not subsequently produced by interaction with the rich gas and tar. S o carbon disulfide was found when a cartridge of sugar charcoal was substituted for the coke. Finally, air which had previously been passed over red-hot copper turnings to reduce its oxygen content was led over a fresh coke cartridge in a furnace maintained at 600" C. Carbon disulfide was found in the exit gas. Since the air contained neither sulfur nor carbon, one must conclude that the broken by-product coke a t its first reheating gives off carbon disulfide and that it quickly loses this property in a hot gas stream. These experiments therefore offered no confirmation of the theory that the carbon disulfide was produced by secondary reactions involving constituents of the gas after this is formed. Coke-Oven Gas over Hot Coke
I n order to investigate further the production or decomposition of carbon disulfide in a gas as it passed over a hot coke surface, a coke cartridge alone was placed in the clean tube and a coke-oven gas containing both carbon disulfide and hydrogen sulfide was passed over it. Only one furnace was employed. The cartridge was of the previously adopted length and weight and occupied the same position with reference to the furnace as did the previous cartridges. No asbestos plug was employed. No tar filter was necessary, but the usual cadmium chloride bubbler was retained. From this, the gas was led directly into the apparatus used for determining the carbon disulfide. I n each experiment 1970 cc. of gas were passed over the cartridge. The gas for all the experiments was derived from a n 18-liter sample of coke-oven gas taken immediately after leaving the ammonia saturators a t the Sparrows Point coke plant of the Bethlehem Steel Company. This gas had, of course, been substantially freed from tar and ammonia, but contained the normal amount of light oils, carbon disulfide, and hydrogen sulfide. Analyses showed that 1970 cc. contained 0.0152 gram of hydrogen sulfide and 0.0010 gram of carbon disulfide-or, expressed in the customary units of gas engineering practice, the hydrogen sulfide concentration of this gas was 337 grains per 100 cubic feet and the organic sulfur due to carbon disulfide was 18.6 grains per 100 cubic feet. The gas was passed over the cartridge at a rate of about 2 liters per hour.
359
The coke was swept out and a furnace temperature of 600" C. maintained for the first experiment. The exit gas showed only 0.00046 gram of carbon disulfide, or a loss of at least 54 per cent due to decomposition. (The exit gas contained not only the undecomposed carbon disulfide from the gas, but also any carbon disulfide which may have been given up by the coke.) With the same coke cartridge and a furnace temperature of 700' C. only a trace of carbon disulfide, too small to be determined quantitatively, was found in the exit gas. A third experiment a t 800" C. using the same coke cartridge gave no carbon disulfide whatsoever. This temperature range covers those usually maintained in the carburetor and superheater during the run in watergas manufacture, and also those of much of the gas in horizontal retort practice.I5 The experiments fail, however, to show any production of carbon disulfide by interaction between hydrogen sulfide and coke, and do show that increasing temperatures on the coke and other surfaces over the temperature range studied accelerate the decomposition of the carbon disulfide. This is in direct contradiction to the theory in question. That the carbon disulfide which did pass over the coke was dependent upon its presence in the entering gas rather than upon its synthesis on the coke surface was shown by again placing the silica tube within both furnaces. Within the tube a t the middle of the first furnace was placed a plug of acid-washed asbestos about 1 inch long to aid in decomposing the carbon disulfide in the entering-gas, while the coke cartridge was maintained in the second furnace as usual. Both furnaces were maintained at 700" C. and a sample of the by-product coke-oven gas described above passed through. No carbon disulfide could be detected in the exit gas. Conditions Not Hitherto Considered
Since no detectable quantity of carbon disulfide was produced either as a primary product from the carbonization of the coal under the conditions described or by secondary interaction between the distillation products and coke, yet is invariably produced by the carbonization of coal under ordinary high-temperature retort conditions, it must result from some condition or conditions not hitherto considered, which prevail in retort practice and were not observed in the tube experiments. The ascertainment of these might do much towards establishing the origin of carbon disulfide in the coking process. One such condition is the rate of change of temperature of the coal which is undergoing decomposition. I n commercial practice the retort is charged while hot and flue wall temperatures of perhaps 1OOO" C. or more transmit the heat to the charge a t a rate which is limited only by the heat-transmitting properties of the coking charge and the retort walls. Within the charge a very steep temperature gradient occurs at the plastic zone of fusion. On the coal side close to this plastic zone temperatures only slightly above that of boiling water may prevail, while on the coke side not far removed from the zone temperatures of 700' and 800" C. may be encountered. On the coal side very little gasification occurs, while the impermeable conditions prevailing in the plastic zone prevent much gasification in this stage of the process. The rapid advance of this plastic zone with a minimum of gasification during this stage is essential for the production of a dense coherent coke from a gas coal of high volatile content. Since the coking charge is heterogeneous and since in consequence many irregularities must exist within the process, certain portions Lewes, "The Carbonization of Coal
I'
1912, especially p. 122.
,
vi tlii, ixking 0111~rgc tir:' p r o l ~ l > lsuli.jiv:teil y to a t,eiiijx:ratiire tlistc. sliowiiig important quantitiw of carbou disulfide, eliange arid delayed gnsifiralion iiincli inore inarkcvl than Tlic
_-
-
I
I _-
~
April, 1926
I,\-DL’STRIAL
A S D ENGINEERLVG CHEMISTRY
361
to Lewes,” “If the humus be slowly heated, a large proportion of the oxygen is given off in combination with hydrogen as water vapor, but if the temperature be raised quickly a larger proportion combines with the carbon to form carbon monoxide and dioxide.”
retorts, chamber ovens, and by-product coke ovens as compared with horizontal gas retorts no doubt have much to do with the greater concentrations of carbon disulfide generally observed in horizontal retort practice. I n these comparisons a probable contributing factor is the variation in secondary decomposition of the carbon disulfide on the Variation in Large-Scale Carbonizing Processes coke surface after the gas is formed. To coke high-sulfur coals with the minimum of carbon hIoreoi-er, the experimental findings of Hutton and Thomas2 disulfide resulting, our present information indicates that are in accord with the experimental observations just given. the operator should choose that carbonizing process which It will be recalled that these experimenters found an increase offers a slow rate of heating, and also, if possible, an extenin carbon disulfide concentration both when higher tem- sive travel of the formed gas over heated surfaces. peratures were employed and when smaller coal charges Further Investigations were burned off. I n both cases the increase in carbon disulfide attends an increase in the rate of temperature Limitations upon the passage of the gas over heated coke change, for, as Hutton and Thomas observed, “with the surfaces may, however, be revealed by a study of the hydrogen smaller charge, the temperature rise through the mass is sulfide, carbon, and carbon disulfide reactions a t tempermore rapid.” atures higher than those covered by the preceding experiA variation in the production of carbon disulfide is noted mental work. as the charge in a horizontal retort is carbonized. This An extension of this study may indicate the part played yariation during the distillation is probably related to pro- by the pyritic and organic coal sulfur. gressive variations in the rate of temperature change in The importance of the rate of heating in determining different parts of the retort. However, these temperature the chemical form of the sulfur in the gas indicates that changes are governed by conditions which are very complex this rate must have an important influence upon the mode and imperfectly understood. The fracturing of the coke of decomposition of the sulfur in the coal, the so-called and the effect of this upon the path of the travel of the gas re-absorption of the sulfur during carbonization, and the is one complicating factor. It is therefore difficult to relate amount and distribution of the sulfur in the coke. Previous this variation in carbon disulfide production with these studies have developed such points chiefly through labtemperature changes. oratory investigations in which the coal was heated slowly. The low rates of heat transmission encountered in low- A further investigation to include the rate of heating as temperature carbonization explain the absence or lowered an important variable may give information which more production of carbon disulfide in some such processes. The closely parallels commercial conditions. lowered rates of heat transmission prevailing in vertical Further consideration of the origin and decomposition of carbon disulfide within the water-gas machine is suggested. 17 Loc. c i t . p. 1%
A Foam Meter‘ By H. Earnest Williams 961 MAPLEAvE., MORSEMERE, N. J.
HERE are few referencesin theliterature to methods for measuring the foaming tendencies of liquids and the foam-reducing power of oils used for removing froth. The methods to which reference may be found involve agitation of the mixture to be tested in a graduated tube, and the height to which the froth rises above the surface of the liquid Ts the basis on which conclusions are drawn. Kryz2 uses a device consisting of a wooden float carrying a n upright rod to which is attached a strip of paper, whose color is changed by the moisture in the foam. I n the method of Trotman and Hackford3 the mixture is mechanically shaken in a graduated tube and the height to which the foam rises is measured. -4method used in testing the foaming tendency of beer4is based on the same principle. Recently it was necessary to test the foaming tendency of certain paint-like liquids and the efficiency of diverse foamreducing greases and oils. Owing to the opacity of these liquids it was impossible to note between which marks on a graduated tube the foam on an agitated mixture was contained. Furthermore, there was froth on the surface of the liquid as well as air interspersed throughout the mass. Con-
T
1
Received October 23, 1928. 70 (1924). Farben-Zlg., 16, 2291 (1921). A m Brewers’ Reu., 26, 270, 6 , 3119 (1911).
* Oesferr. Chern.-Zfg., 27, 3 4
sequently, since a volume measurement of the foam could not be made, an attempt was made to base measurements on weight. I n operating the apparatus to be described no attempt is made to weigh foam or froth itself. Apparatus and Method
The top of an acid bottle was removed and inverted to form a bowl. At the bottom of the bowl, in the neck of the bottle, was placed a rubber stopper ,bearing a glass tube. The upper end of the tube did not project beyond the stopper into the bowl, and to the lower end of the tube was attached a rubber tube which could be closed with a pinchcock. Around the top of the bowl was placed a wooden collar on which was mounted an egg beater, to the driving wheel of which was wired a pulley so that the beater could be beltdriven by an electric motor. The cold-water paint to be tested was poured into the bowl, the pinchcock being closed, until the bowl was two-thirds full, the mixing part of the beater blades being submerged in the mixture. After stirring the material for 3 minutes the motor was stopped and the pinchcock was a t once opened wide to allow the beaten liquid to run into a weighed flask placed under the outlet of the rubber tubing. The total volume of the flask is called the “foam meter volume”
