plication of this technology is shown in Figure 7. Polymerization is carried out in a 53 000-gal. reactor with a condenser mounted on top for evaporation cooling. A large amount of residual butadiene is recovered via the condenser immediately after polymerization. Latex is pumped into a further 53 000-gal., agitated vessel, containing the PEO solution. The further process steps, residual hydrocarbon recovery and concentration, are carried out continuously. The production plant is shown on the photograph in Figure 8, where the 53 000-gal. reactors can be seen on the left, in the middle the hydrocarbon recovery and concentration units, and on the right the storage tanks. The plant shown has played an important part in ensuring that the West European Tufted Carpet Industry is supplied with sufficient amounts of good-quality highly concentrated butadienehtyrene latex.
Acknowledgments The author thanks H. 0. Dopp and H. Huelsiek for their conscientious execution of the experiments and viscosity measurements. The permission of Chemische Werke Huels AG to present this paper is also appreciated. Literature Cited Basiev. I. M., Shainskii, Ya. B., Sov. Rubber Technol., 23, 25 (1964). Beckmann, G. Chem. Technol., 3,304 (1973).
Blackley, D. C.. "High Polymer Latices: Their Science and Technology", Vol. I., Applied Science Publishers Ltd., London, 1966, Chapter V, p 298. Burnett, G. M., Cameron, G. G., Thorat, P. L , J. Polym, Sci., Polym. Chem. Ed., 8, 3435 (1970). Davies, A. A,, paper presented at Latices and Dispersions Symposium, Hastings, May 14-15 (1974). ECN (Eur. Chem. News), 27, 19 (1975), market report. Heinz, W., Chem. lng. Tech., 45, 739 (1973). Howland, L. H.. Aieksa, E. J., Brown, R. W., Borg, E. L., Rubber Plast. Age, 42, 868 (1961). Johnson, P. H., Kelsey, R. H.. Rubber World, 139, 227 (1958). Jones, R. D., Proc. Rubber Technol. Conf., 4th London, 485 (1962). Kolaczewski, M. S.,Hobson, R. W.. Hamill, J. J. (to Goodyear Tire and Rubber Co.), US. Patent 857 272 (Dec 4, 1959). Maron, S. H., Elder, M. E., Ulevitch. J. N., J. Colloid Sci., 9, 89 (1954). Rushton, J. H., Chem. Eng. Prog., 47 (9), 485 (1951). Schlueter, H., Kraenzlein, P. (to Chemische Werke Huels AG), German Patent 1 208 879 (Feb 17, 1961). Schlueter. H. (to Chemische Werke Huels AG), German Patent 1 213 984 (Dec 6, 1963). Schlueter, H., Adv. Chem. Ser., 142, 99 (1975). Talalay, L., Proc. Rubber Technol. Conf., 4th London, 442 (1962). Von Brachel, H., Schuemmer, P.. Chem. lng. Tech., 45, 693 (1973). Waterman, J. H., Aeijelts Averink. J. W.. Boerma. J., LaHey. G. E., J. lnst. Rubber hd., 1(3), 168 (1967). '
Receiued for review April 29,1976 Accepted February 7,1977 Paper presented at the Centennial Meeting of the American Chemical Society, Division of Industrial and Engineering Chemistry, Symposium, "New Processes and Technology of the Overseas Chemical Industries", New York, N.Y., April 8, 1976.
Sulfur Compounds in Coal Amlr Attar'' and William H. Corcoran Jet Propulsion Laboratory, Pasadena, California
The literature on the chemical structure of the organic sulfur compounds (or functional groups) in coal is reviewed. Four methods were applied in the literature to study the sulfur compounds in coal: direct spectrometric and chemical analysis, depolymerization in drastic conditions, depolymerization in mild conditions, and studies on simulated coal. The data suggest that most of the organic sulfur in coal is in the form of thiophenic structures and aromatic and aliphatic sulfides. The relative abundance of the sulfur groups in bituminous coal is estimated as 50:30:20%, respectively. The ratio changes during processing and during the chemical analysis. The main effects are the transformation during processing of sulfides to the more stable thiophenic compounds and the elimination of hydrogen sulfide.
The total ratio of sulfur in coal varies in the range of 0.2 to 10 wt % (Deurbrouck, 1972). In most samples, it is in the range of 1.0 to 4.0 w t %. Traditionally, the sulfur compounds have been classified into two groups, inorganic and organic. Organic sulfur is that which is bound to the hydrocarbon structure of the coal. Inorganic sulfur is the remainder. Inorganic sulfur appears mainly in two forms, as the disulfides pyrite and marcasite and as sulfate. Most of the inorganic disulfides appear as pyrite; so the name pyrite is often used for all the disulfides. Very limited amount of organic sulfur compounds can be isolated from coal without changing the organo structure. Organic sulfur is categorized according 'Address correspondence to this author a t the Department of Chemical Engineering, University of Houston, Houston, Texas 77004. 168
Ind. Eng. Chem., Prod. Res. Dev., Vol. 16, No. 2, 1977
to the type of functional group in which it appears, namely sulfidic, thiophenic, mercaptanic, etc. The accepted working definition of the classes of sulfur compounds in coal is according to the following general guidelines (ASTM part 26D 2492, 1574; British Standards Institution, BS 1016, 1975; International Organization for Standardization, 1960). (1)Sulfate sulfur: the sulfur equivalent of the sulfate which dissolves in boiling 5 N HCl in 30 min. (2) Pyritic sulfur: the sulfur equivalent of the iron which dissolves in boiling 2 N "03 in 30 min, assuming that it was in a compound with the formula FeS2. (3) Organic sulfur: the difference between the total sulfur and the inorganic sulfur. Standard techniques to determine the classes of sulfur in coal have recently been reviewed by Shimp et al. (1975). The ratio between the two general classes of sulfur may vary in the range from 4:1 to 1:3 inorganic/organic sulfur; however, it is usually close to 2:1 (Gluskoter and Simon, 1968).
The sulfate sulfur is present in the form of iron, calcium, and barium sulfates, and its ratio increases with the length of exposure of the coal to air (Gluskoter, 1975). Small amounts of free sulfur, galena (PbS), chalcopyrite (CuFeSe), arsenopyrite (FeAsS), and sphalerite (ZnS) have also been found (Gluskoter, 1975). Iron pyrite has the formula FeS2 and is present in two crystalline forms, pyrite and marcasite. The pyrite has a cubic crystal structure, a density of 5.0 g/cm3, and decomposes rapidly above 700 “C. The marcasite is rhombic with a density of 4.87 and decomposes to sulfide and sulfur in the range of 450-500 “C (Schwab and Philinis, 1947). Whelan (1954) studied ground British coals, 0.5-40 p , and found that the marcasite crystals are intimately coated with coal, while the pyrite is present mainly as loose particles. Whelan’s conclusion is supported by Gluskoter (1975) studies. The forms of distribution of the sulfur suggest three points relative to the separation technology: (1)density based separation of pyrite (density 5.0 g/cm3) from coal (density -1.2-1.6 g/cm3) is possible; (2) crushing the coal will allow more complete separation of the iron disulfide because a larger part of the marcasite will be removed; and (3) thermal processing of coal may result in reaction between the disulfide and the organic matrix. Indeed, the finer the coal is crushed, the more effective is the density--separation technique. The amount of organic sulfur in coal sample is usually obtained routinely, but without development of any detailed data. Basically, three kinds of information on the sulfur compounds are of value: (1)the distribution of the organic sulfur between the possible organic functional groups; (2) structural and chemical relations between the sulfur-containing functional groups, including the pyrites and the organic matrix; and (3) kinetic parameters for reactions of the sulfur groups with other reagents, e.g., hydrogen. Three approaches have been used in the examination of the organic sulfur compounds in coal: (1)direct examination of the coal; (2) decomposition of the coal, or “depolymerizing” the organic matrix, followed by examination of the fragments; and (3) chemical incorporation of sulfur compounds into an organic matrix (“simulated coal”) and examination of the properties of the products. The most straightforward method is the first; however, very little information on the sulfur compounds was derived by direct examination of the coal samples. Some information was obtained from examination of the products of destruction of coal, but the nature of the products depends on the method of destruction used. Very little relevant knowledge resulted from the work on simulated coal. The basic difficulty seemed to be the lack of a method which would unambiguously determine if the simulated coal indeed was an analogue. Direct spectroscopic examination of coal was carried out using IR, Raman, and UV techniques. The spectroscopic approach has been reviewed by Speight (1971). The main useful method for analysis of functional group distribution is by infrared, but coal cannot be totally solubilized so t,hat examination in solution or suspension is of little use. Pellets and mulls of finely divided coal have been studied, but no data relevant to the functional groups of sulfur has been mentioned. The IR bands of sulfur are not very strong and are rarely well resolved. Because Raman spectrometry is relatively more sensitive to sulfur-containing groups, it has some potential for sulfur groups analysis even though the dark color of coal samples, degradation of the coal under the laser beam, and chemiluminescence may be severe problems. Although Raman spectra of coal have been obtained (e.g., by Friedel and Carlson, 19721, no specific work on sulfur compounds was mentioned. An investigation by IR of the mineral matter in coal, notably FeS2, was made by Estep et al. (1968). The reaction of sulfur functional groups with selective reagents was used
with moderate success in solid coal analysis. Methylation by methyl iodide was used by Postovski and Harlampovich (1936) to estimate the amount of sulfdic sulfur in accord with the reaction:
R
S‘ ’R
+
CHJ
-
Determination of the amount of I- left in the sample is a measure of the concentration of organic sulfides. Mercaptans and thiophenols release HI when reacted with CH3I. RSH
+ CH3I
+
RSCH3
+ HI
The HI released thus can be used to estimate the amount of -SH groups in the sample. More recently similar ideas were used by Bogdanova and Boranski (1961) and by Prilezhaeva et al. (1963) to evaluate the organic sulfur functional groups distribution in coal. The data suggest that, in bituminous coals, the organic sulfides constitute some 5-20% of the organic sulfur while the rest is assumed to be thiophenic. About 70% of the sulfidic sulfur is in an unstable form. Thiophenic, condensed thiophenic, and aryl sulfides were determined by difference because they do not react with Me1 (Prilezhaeva et al., 1963). Nondestructive extraction of coal, followed by examination of the sulfur compounds, showed that the ratio of the total C to the organic sulfur does not change as a result of the extraction (Van Krevelen, 1961). That does not necessarily imply that the sulfur-functional group distribution is identical in the solid and in the extract. However, because sulfur is a minority element, it is unlikely that it will induce selective extraction. Attempts to determine the sulfur-functional group distribution in some bituminous coal extracts have been reported by Minra and Yanagi (1963). Tetrahydrofuran, dimethylformamide, and benzene were used. The following distribution was found for Mieke coal: thiol (-SH), 3-9%; disulfide (-S-S-), 6-13%; aliphatic sulfide (R-S-R), 28-37%; thiophenic and aryl sulfide (Ar-S-),7-19%; and undetermined -30%. We believe that all of the undetermined S is in a condensed thiophenic structure. Moreover, it is unlikely that unstable groups like thiolic or disulfidic will survive the caolification process. Therefore, more work should be done on the analysis of sulfur functional groups. Note should be made that thiophenic sulfur compounds are the major constituents of the sulfur compounds in shale oil (Veretennekova and Petrov, 1964, and Paulson, 19753. Depolymerization of coal was accomplished by hydrogenation, pyrolysis, oxidation, and reactive solvents. Only the most stable sulfur compounds will survive catalytic hydrogenation a t high temperatures and pressures. Hydrodesulfurization of coal at a hydrogen pressure of 2000-4000 psi and a temperature of 450 “C followed by examination of the product by mass spectroscopy allowed the identification of fourteen different organosulfur compounds. Thirteen contained thiophene or condensed thiophene rings (Akhtar et al, 1974). The thiophenic compounds which were not hydrodesulfurized accounted for at least 20-40??of the total organic sulfur; however, no data were given which would allow a quantitative estimate. Another decomposing technique which has bees used extensively is coking. Coked coal releases hydrogen sulfide as well as other volatile sulfur compounds. Most of the conclusions of the classic work of Powell (1920, 1923) are still valid. The important ones are (1) iron pyrite decomposes when heated and releases half of its sulfur (some of the pyrite sulfur reacts with the coal and forms H2S and other compounds as well, e.g., CS2); (2) some of the pyritic sulfur reacts with the coal and forms very stable compounds that do not readily decompose; (3) sulfate sulfur remains in the coal, probably as Ind. Eng. Chem., Prod. Res. Dev., Vol. 16, No. 2, 1977
169
a calcium sulfide; and (4) the H2S which is released may react with the hot coke to form CS2. Interaction between the pyritic sulfur and the organic matrix during carbonization has been discussed broadly in the literature (Georgiodis and Gaillord, 1954, Howard, 1963, Cerni-Simic, 1962, Peet et al., 1969, El-Kodda and Ezz, 1973, Given and Jones, 1966, Maa et al. 1975).Several conclusions can be drawn, while acknowledging that contradictions exist. (1) Carbonization a t temperatures up to 1400 "C will not desulfurize the coal completely. Sulfur will be retained both in organic and inorganic forms. Coking at 1600 "C desulfurizes the coal (95%), but substantial loss of material occurs (ElKodda and Ezz, 1973). (2) Approximately 66% of the inorganic sulfur and 73% of the organic sulfur are retained in the coke. About 23% of the inorganic and 26% of the organic sulfur goes with the gas (Eaton e t al., 1949). Slightly contradicting numbers are quoted by Fuchs (1959). He found that about 50% of the total sulfur is retained in the coal; 3% goes to the tar; and 45% goes out in the gas. The actual distribution depends strongly on the rank of the coal. (3) Larger ash content increases the amount of sulfur that is retained in the coke. (4) A larger amount of sulfur goes to the gas and tar when the volatile percentage is larger. ( 5 ) The overall desulfurization that is achievable reaches a plateau a t 800 "C. Further increase of the temperature does not substantially contribute to the desulfurization. In addition to H2S and CS2, thiophene and its derivatives are the major components of the tar oils and gases that evolve (Muder, 1963 p 634). Some sulfides and disulfides can be found in the light oil; mercaptans were recognized in the gases (Peet et al., 1969). We believe that the mercaptans and disulfides are reaction products of the pyrite or the H2S with the organic matrix. Moreover, it is very likely that the disulfide is a secondary product of mercaptans oxidation, because the -S-S- bond is too weak to withstand the coalification andlor the coking processes. The organic sulfur that is retained in the coke is probably in the form of condensed thiophenic rings or aryl sulfides. No other C-S bond, namely, a bond without resonating electrons, can stand the high coking temperatures. Because dehydrocyclization and aromatization can occur as a result of the carbonization process, the distribution of sulfur functional groups in the tars does not necessarily represent the distribution in the original coal. The dehydrocyclization and aromatization support the well-accepted assumption that most of the organic sulfur in coal, and probably all the sulfur in coke, is in the form of thiophenic structures or of resonating aryl sulfides. Other techniques have been used in an attempt to learn about the sulfur-containing compounds in coal, notably, oxidation by nitric acid, chlorine in aqueous and nonaqueous media, air in basic media, and potassium permanganate; no significant ideas relevant to the chemical nature of the sulfur, however, have been obtained. A more promising approach is via chemical depolymerization with a solvent and a strong Lewis acid, (e.g., BF3 or p-toluenesulfonic acid (Dorlage, et al., 1972). Such depolymerization is mild so that only aliphatic bridges between aromatic lamellae are broken. Smaller molecules are produced which can potentially be separated and characterized more easily. Depolymerization of some Russian bituminous coals, using the same method, led Rodionova and Brauskii (1970) to the following conclusions: (1)the ratios of sulfides and disulfides in coal do not vary much (variation in the total organic sulfur is due to the major sulfur constituents, the thiophenic compounds); and (2) thiolic groups do not exist in coal in a detectable amount. However, depolymerization is still new, but it has a large potential for determination of functional groups. Many investigators have studied simulated coals and cokes. Basically, sulfur was incorporated into an organic matrix 170
Ind. Eng. Chem., Prod. Res. Dev., Vol. 16, No. 2, 1977
which was then coked a t a high temperature and pressure. Not much was learned from these studies on the actual existence of sulfur in coal. Some inference on the relative stability of sulfur-containing groups in coking may be made, however. The data of Steedman and coworkers (1966, 1968, 1972) as well as those of Blayden and Patrick (1970) support the following conclusions: (1)thiol groups are unstable and tend to be eliminated as HzS; (2) thiophenic structures are very stable and tend to condense with the organic matrix as the temperature is increased; (3) complete desulfurization of thiophenic sulfur compounds by coking is impossible (larger sulfurcontaining structures are formed on carbonization). These conclusions support the information that was derived by coking. They do not, however, add new insights on coal. The major problem in the work on simulated coal seems to stem from the lack of a method to determine unambiguously in what ways the simulated coal does indeed simulate coal. Acknowledgment The support of the Directors Discretionary Fund of the Jet Propulsion Laboratory, Pasadena, Calif., is gratefully acknowledged.
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1974. ASTM D 2492-68(1974).See also ASTM 0271. Blayden, H. E., and Partick, J. W., Fuel, 49, 257 (1970). Bogdanova. V. A,. and Boranski, A. O., Krat. Soobshch. Nauchno-lssled. Rabotakh., 2a, sb 68 (1961);Chem. Abstr., 63, 17741e (1965). British Standards Institution, BS 1016,Part II, 507 (1975). Cerni-Simic, S.,Fuel, 41, 141 (1962). Deurbrouck, A. W., U.S. Bureau of Mines, RI 7633,''Sulfur Reduction Potential of the Coals of the United States", Washington, D.C.. 1972. Dorlage, L. J., Werdner, J. P., and Block, S. S.,Fuel, 53, 10 (1972). Eaton, S.E., Hyde, R. W., and Rood, N. H., Anal. Chem., 21, 1062 (1949). El-Koddah, N., and Ezz, S. Y., Fuel, 52, 128 (1973). Estep, P. A., Kovach, J. J., and Karr. C., Jr., Anal. Chem., 40(2), 358 (1968). Friedel, R. H.. and Carlson, G. L.. Fuel, 51, 194 (1972). Fuchs, W., &ensst.-Chem., 32, 274 (1951). Georgiodis, C., and Gaillord, G., C. R. Acad. Sci., 236, 255 (1954). Given, P. H.. and Jones, J. R., Fuel, 45, (1966). Given, P. H., and Wyss, W. F., BCURA Mon. Bull., 25(5), 165 (1961). Gluskoter, H. J., and Simon, J. A,, 111. State Geol. Surv., circ. 432 (1968). Gluskoter, H. J., Am. Chem. SOC.,Div. Fuel Chem., Prepr., 20 (2),94 (1975). Howard, H. C., in "Chemistry of Coal Utilization", Supplementary Volume, Lowry. H. H., ed.. Wiley, New York, N.Y., 1963,p 340. International Organization for Standardization, 13157 (1960). Maa, P. S.,Lewis, R. C., and Harmran. C. E., Jr., Fuel, 54, 62 (1975). Minra, Y., and Yanagi, Y., Nenlyo Kyokai-Shi, 42, 21 (1963);Chem. Abstr., 61, 10501 h (1964). Muder, R. E., in "Chemistry of Coal Utilization", Supplementary Volume, Lowry, H. H.. Ed., Wiley, New York, N.Y., 1963,p 629. Nikvdlna, S. E., Baranskii, A. O., Kudashov, F. I., and Serova, N. A,, Nauchnolssled. lnst. Nefte-Vgklebim., Sin. lrkutsh., Univ., ll(l),88 (1969);Chem. Abstr., 78, 6349c (1973). Pauison, R. F., Am. Chem. SOC.,Div. f u e l Chem., Prepr., 20(2), 183 (1975). Peet, N. J., Simeon, S. R., and Stott, J. B., Fuel, 46, 259 (1969). Porter, H. C., "Coal Carbonization", ACS monograph series, The Chemical Catalog Co., Inc., New York, N.Y., 1924,p 102. Postovski, J. J., and Harlampovich, A. B., Fuel, 15, 229 (1936). Powell, A. R., lnd. Eng. Chem., 12(11),1069 (1920). Powell, A. R . , J. Am. Chem. SOC.,45, l(1923). Prilezhaeva, E. N.,Fedorovskaya, N. P., Miesserova, L. V., Domanina, O., and Khaskina, I. M., Tr. lnst. Goryuch. lskop., Akad. Nank. SSSR, 21, 202(1963); Chem. Abstr., 60,6215h (1964). Robertson, H. W., and Steedman. W.. Fuel, 45, 375 (1966). Rodionova, L. E.. and Barauskii, A. O., lzv. Nauchho-lssled, lnst. NeRe-Vgelchim., Sen. lrkufsk., Univ., 12, 93 (1970);Chem. Abstr., 75, 5122 (1971). Schwab, G. M., and Philinis. J., J. Am. Chem. SOC.,69, 2588 (1947). Scott, C. L., and Steedman, W., Fuel, 51, 10 (1972). Shimp, N. F., Helfurntine, R. J., and Kuhn, J. K., Am. Chem. SOC.,Div. Fuel Chem., Prepr., 20(2), 99 (1975). Speight, J. G., Appl. Spectrosc. Rev., 5(2),211-264 (1971). Van Krevelen, 0. W., "Coal", Eisevier, Amsterdam, 1961,p 171. Veretennekova, I. V., Petrov, A. A., p 133 in "Chemistry of Organic Sulfur Compounds in Petroleum and Petroleum Products", VoI. VI, R. 0. Obolentsev, Ed., 1964.Israel Program Scientific Translations, Jerusalem, 1967. Whelan, P. F., J. lnst. Fuel, 27, 255-458,464 (1954).
Received for reuieul M a y 10, 1976 Accepted December 17, 1976
