Thermal Stability of High-Sulfur Crude Oils H. J. COLEMAN, C. J. THOMPSON, H. T. RALL, AND H. 31. SMITH P e t r o l e u m Experiment Station: Bureau of Mines, Bartlesville, Okla.
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PROCEDURE. The apparatus is assembled as indicated and charged with approximately 1000 grams of crude oil. A slow stream of inert gas is bubbled through the crude oil for about 30 minutes a t room temperature to remove dissolved hydrogen sulfide. The Aask is then heated and all condensable vapors are returned to the flask, while materials not condensed under these conditions are swept into the collecting train. The temperature of the still pot can be raised and maintained a t any desired temperature level by removal of an appropriate quantity of reflux. I n the experiments reported here the temperature levels used, as indicated below, were obtained by successive withdrawal of distillate and appropriate control of heat input.
NE of the first problems faced by API Project 48A in its efforts to identify the sulfur compounds occurring naturally in crude oil was to devise a simple test t o indicate the maximum temperature a t which a given sample could be processed, as by distillation, without decomposing the sulfur compounds. Obviously, to make this test simple enough for routine use it was assumed that any decomposition of sulfur compounds would be accompanied by evolution of hydrogen sulfide, low-boiling thiols, or both. Cognizant of this assumption, the writers developed a relatively simple test that has possibilities of modification t o accommodate several purposes. This test has been used for determining the thermal stability of the sulfur compounds naturally present in several crude oils, and in some distillates, and for studying the effect of adding elemental sulfur and tri- and polysulfides to crude oils. As a result of these experiments, several interesting and possibly important reactions have been noted, but as continuation of these investigations is outside the present scope of the work of the project in identifying the sulfur compounds in crude oils, this work is reported here with the thought that other investigators may become sufficiently interested to pursue further the problems the data suggest.
Pot Temperature F. c. Room temperature
210 310 410 510 610
99 154 210 266
321
Time on Plateau, Min.
30 180 180
180
180 180
ANALYSIS.Throughout each temperature plateau the quantities of evolved hydrogen sulfide and volatile thiols are determined by analysis of the absorber solutions a t the end of each 30-minute or shorter interval. As the reflux condenser is cooled with water a t about 10" C., presumably the greater proportion of thiol trapped in the absorbers is methanethiol, and possibly some ethanethiol. However, a t later stages of the heating the odor of thiols of higher molecular weight can be detected in the absorbers. These were undoubtedly present in trace quantities. Thiols may also be present that do not react with the silver nitrate as used and escape undetected.
APPARATUS 4YD PROCEDURE
APPARATUS.The apparatus (Figure 1) consists of a 2-liter, round-bottom flask equipped with a 29/42 standard-taper joint. T o this joint is attached a glass-bead-packed column about 12 inches long, having a thermowell in the top for measuring the vapor temperature, and a side arm connected to a condenser cooled to 10" C. By means of a stopcock, sealed a t the bottom
TYPICAL CURVES
Figure 1. Apparatus for Determining Thermal Stability of the condenser, any desired amount of reflux may be withdrawn. The flask has a sealed-in thermowell and a side arm, the latter to allow introduction of a small amount of inert gas below the surface of the charge to sweep uncondensable gases through the system. I n later experiments this opening was also adapted for withdrawing samples from the flask. At the top of the condenser a capillary line leads into an absorption train consisting of absorbers (sometimes three are necessary) filled with acidified cadmium sulfate solution for the collection of hydrogen sulfide and two absorbers containing silver nitrate solution for retention of thiols. A Y-stopcock in the line from the top of the column permits use of a second train of absorbers while the solut,ions in the first are being analyzed and the flasks prepared for re-use.
Figure 2 illustrates typical data obtained with Wilmington, Calif., and Wasson, Slaughter, and Goldsmith, Tex., crude oils. In these graphs the quantity of hydrogen sulfide evolved is shown on the left ordinate, in terms of grams of sulfur per minute times 1000, and indicated on the graph by the crosshatched area. Thiol evolution is covered by the same scale and is indicated by the black areas. The abscissa is a time scale in minutes. The right ordinate is a temperature scale in degrees Fahrenheit, and the curve with the several plateaus is the time-temperature curve with actual temperature readings indicated by circles. DISSOLVED HYDROGEN SULFIDE AND VOLATILE THIOLS.The hydrogen sulfide and thiols evolved a t room temperature are gases that were dissolved in the crude oil and are not the result of thermal decomposition. In the examples illustrated only Goldsmith and Wasson crude oils had appreciable quantities of these compounds in solution. Of the crude oils investigated Yates, Tex. (not shown), had by far the largest amount of hydrogen sulfide in solution, equivalent to approximately 0.5% sulfur as dissolved hydrogen sulfide. The Wasson crude oil shows small quantities of thiols, and other work has shown that both methanethiol and ethanethiol are present in this oil. FIYDROGEN SULFIDE A N D THIOLS FROM T H E R X A L DECOJIPOSITION OR OTHERREACTIOSS.Little decomposition, as indicated by hydrogen sulfide and thiol evolution, occurs a t 200" F. In general, the first significant signs of thermal instability are shown between 200' and 300" F., as for the Slaughter and Goldsmith crude oils (Figure 2). 4 t 400" F. the Goldsmith oil evolves hy-
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Figure 2. Thermal Stability Test of High-Sulfur Crude Oils
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drogen sulfide copiously, appreciable quantities are yielded by the Slaughter oil, and the Wasson oil shows some-evidenceof a similar reaction. At this temperature the Wilmington oil has not reacted a t all, and first shows evidence of reaction a t about 500" F. One unexpected feature is typical of all the curves-that is, an alternating "peak-and-valley" system. This results from maximum evolution of hydrogen sulfide near the start of a temperature plateau where a reaction occurs, and then diminution of the evolution on each plateau as though the compounds decomposing a t that temperature were being exhausted. An increase in pot temperature brings other compounds, stable a t the lower temperature, into a reaction environment with subsequent increase in the hydrogen sulfide evolution. In all cases, the generation of hydrogen sulfide and low-boiling thiols, principally methanethiol, is used as a criterion of decomposition of the sulfur compounds. Any thermal conversion of the sulfur compounds to other forms has not been determined in these tests but probably such reactions do occur. There appear to be three general causes for the type patterns found: (1) reaction of elemental sulfur a t about 300' F. with some component of the crude oil, whereby hydrogen sulfide is released, illustrated by Goldsmith crude oil; (2) decomposition of naturally occurring thermally labile sulfur compounds, illustrated by Wasson and especially Wilmington crude oils; and (3) decomposition of thermally labile sulfur compounds formed by the reaction of hydrocarbon material with elemental sulfur (discussed below) or by some synthetic process from naturally occurring sulfur compounds. In some crude oils, such as Goldsmith, it seems probable that all three reactions occur.
sulfide a t about 400' F.can be attributed to the reaction of elemental sulfur with hydrocarbons in the crude oil. Data confirming this are presented in Figure 3, which shows stability curves for Bradford, Pa., crude oil, which contains little sulfur, and for the same crude oil with 0.5% elemental sulfur added. The pronounced hydrogen sulfide peak a t 400" F. for Bradford crude oil with added sulfur, and the successive peaks a t 500" and
EFFECT OF SULFUR
HUNDREDS OF MINUTES Figure 3. Thermal Stability of Bradford Crude Oil
E L m f E N T A L SULFUR. Goldsmith crude oil contains about 1% of elemental sulfur (.I.), and the copious evolution of hydrogen
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generation a t this temperature, in the quantities obtained, is peculiar to these experiments and is not observed except in trace quantities (see Figure 3) with natural crude oil, even with elemental sulfur added. This, however, does not prove the absence of polysulfides in (#rude oil, as small amounts might be undetected
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content of the residue during the course of the espriiment. Figure 4 presents data for Fuhrman-llascho, Tex , crudr oil, and, in addition to the type of data presented in the preceding figures, a curve indicating the content of elemental sulfui in the still pot is shown by the solid line, with a supplementary scale on the left ordinate. Elemental sulfur content remains almost constant until the pot temperature is elevated to 400" F , a t which temperature hydrogen sulfide is evolved in large quantities. At this point the elemental sulfur content drops rapidly to 7er0, indicating iapid conversion to hvdrogen iulfide and other sulfur compounds. REACTIOSSTTITHOUT ELE\IEKTA4L S I - L F V R . Sulfur colllpOundS normally present in the crude oil also are sources of unstable compounds and hence of the hydrogen sulfide produced b v their depornposition. This is apparent from inspection of the curves for Wasson and Wilnlington crude oils (Figure 2); the first contains little elemental wlfur (0.001%) and the second none detectable bv the polaroyraph. The "stability" curve of JVasson crude oil indicates little hrdrogen sulfide a t 400" F. and that for Wilmington none, showing that in these examples the hydrogen sulfide evolved a t 500' and 600" F. results from reactive sulfur compounds present in the oil and not from elemental sulfur POLYSULFIDES. The opinion is frequently expressed t h a t polysulfides are present naturally in crude oils. This, so far as the authors are aware, has not been positively established. Early chemical literature reports the formation of di-, tri-, tetra-, and pentasulfides from the reaction of 3-thiapentane (ethyl sulfide) with elemental sulfur. Disulfides are reported to react even more easily n i t h elemental sulfur t o form polysulfides. Thus
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tial to add another scrubber to remove all the thiols from the gas stream. This scrubber contained amperometric titrating solvent (silver nitrate, acetone, ammonium nitrate, and ammonium hydroxide), and the thiols thus collected were determined by an amperometric procedure. I t is believed that one of the thiols generated in the trisulfide experiment was the 2-methyl-2-propanethiol (tert-butylmercaptan). This thiol can be accounted for by simple breakage of the S-S linkage, a known weak bond, t o yield elemental sulfur and a GHgS- radical which subsequently acquires hydrogen to form the mercaptan. Table I gives the sulfur recovered in the gas phase from the trisulfide stability test.
TABLEI. SULFURRECOVERY I S GAS PHASEO F STABILITY TEST
TRISL~LFIDE
Sulfur Recovered5 Temperature Thiols Hydrogen Sulfide ~ ~ t ~ Plateau, F. G. % G. % % 200-300 0.006 0.098 0.104 0.0003 0.0049 400 0.2293 4.59 0.9174 18.35 22.94 500 0.1059 2.12 0.9512 19.02 21.11 600 0.0116 0.23 0.2525 5.05 6.28 a Based on a 1-kg.charge of crude oil containing 0.49 wt. % added sulfur a6 trisulfide plus 0.01 wt. % elemental sulfur naturally present. _ _ _ _ _ ~
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INDUSTRIAL AND ENGINEEBING CHEMISTRY
temperature reached 400' F. Instead, this evolution is slower than is generally observed if elemental sulfur is present at the beginning of the 400" F. heating period (compare with Figure 3). I n other words, t h e hydrogen sulfide evolution pattern suggests slow release of elemental sulfur a n d i t s subsequent prompt reaction with the constituents of With added di-tert-butyl trisulfide a n d di-tert-butyl polysulfide t h e c r u d e oil. At the end of the components precludes its presence above 400' F., and none was 400" F. plateau, 18 % of the total sulfur originally present is acfound. counted for by the hydrogen sulfide evolved. For Bradford Figure 5 (right) presents data analogous t o those shown at the crude oil to which elemental sulfur has been added, about 50% left, but for the polysulfide rather than the trisulfide. The overof the sulfur is released as hydrogen sulfide by the end of the 400' all picture is very similar, but in keeping with the probable F. temperature plateau. If it can be assumed that the trisulfide presence of elemental sulfur, the hydrogen sulfide evolution is releases one third of its sulfur as elemental sulfur and 50% of this more rapid neay the start of the 400' F. plateau. Also, there is reacts with the oil to form hydrogen sulfide on the 400" F. greater hydrogen sulfide evolution on the subsequent temperature plateau (as in the case of elemental sulfur added to crude oil), the plateaus, similar to that noted with crude oils. Thiol formation, 16.6% sulfur as hydrogen sulfide required would be in substantial in both the vapor and liquid phase, is similar. These observations agreement with the 18% actually recovered. on thiols agree with the data of Farmer and Shipley (6),who found T o obtain the data pertaining to the thiols shown in Figure 5 thiols as the result of the decomposition of polysulfides derived (left), an amperometric titration for determining thiols was used from the reaction of elemental sulfur with diolefins. for following the thiol content of the oil in the still pot. Although ASPHALTIC MATERIALS.Evidence t h a t asphaltic material is the accuracy of this procedure has not been proved for this applithe source of some of the sulfur compounds undergoing thermal cation, it appears to be accurate enough for following trends. It decomposition is indicated in Figure 6, where "stability" experiis seen that the thiol concentration in the charge increased simulments on Santa Maria Valley crude oil and on the same crude oil taneously with the increase in rate of hydrogen sulfide evoludeasphaltized are shown. No large peak is indicated for the sultion, and reached a maximum at the end of the period of fur-hydrocarbon reaction, and polarographic analysis of the crude large hydrogen sulfide producoil indicates no elemental sultion on the 540' F. temperfur. The large hydrogen sulature plateau. This indicates 800 fide peak on the 600" F. plateau DE AS PH ALT IZ ED formation of small quantities for the crude oil and its reducS. M.V. I 1 of h i g h-mol e c u l a r - w e i g h t tion in the case of the deasthiols. If polysulfides are presphalted material to one third ent in a crude oil, they would its original magnitude are inbe expected t o react similarly terpreted as evidence that asand form thiols. Unfortuphalts contribute heavily to the nately, comparative data on thermally labile sulfur coma crude oil are not availpounds present in some crude a able. oils. The amount of elemental DISCUSSIOR sulfur originally present in 6 Becaugeof the limited knowlsmall percentage in the crude edge of the reactions described, oil drops to near zero as the it does not seem feasible t o attemperature approaches 4 800 tempt any discussion of their 400' F., a fact previously essignificance, but rather to sumtablished for other sulfurmarize what is known and to 2 400 bearing crude oils. No evistate some questions that arise dence of the presence of elefrom these facts. A solution t o mental sulfur as a result of some of these problems, or even 0 decomposition of the trisulpartial solutions, would aid '0 2 4 6 8 1 0 fide is found in early stages greatly in understanding the HUNDREDS OF MINUTES of the experiment; its rapid over-all problems of sulfur in Figure 6. Thermal Stability of Original and reaction with the crude-oil Deasphaltized Santa Maria Valley Crude Oil petroleum.
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On the basis of the work reported in this paper and data in the literature, the present knowledge concerning the reactions under consideration may be summarized as follows: Some crude oils contain elemental sulfur. This elemental sulfur begins to react with some components of t h e crude oil a t about 200" to 300" F., with evolution of hydrogen sulfide. The reaction reaches its maximum observable rate at about 400' F., accompanied by a maximum evolution of hydrogen sulfide and rapid depletion of the elemental sulfur content. Approximately 50% of the elemental sulfur can be accounted for by the hydrogen sulfide evolved at 400" F. The remaining sulfur forms thermally labile compounds that decompose a t temperatures above 400' F., releasing hydrogen sulfide. Sulfur reacts with n-tetradecane and tetrahydronaphthalene (Tetralin) a t 176" F. (80" C . ) , and with paraffin wax (melting point 160" to 170' F.) a t 240" F. (110' C.), withincipient evolution of hydrogen sulfide. There is some knowledge of the reaction of sulfur with hydrocarbons (1-3, 6, 8),but most of the data pertain to temperatures above 395" F. (200" C.) and pressures above atmospheric. Tri- and pol sulfides react with components of crude oil in the same way as ecmental sulfur in that hydrogen sulfide is formed but they also form thiols in much greater quantity than does the equivalent quantity of elemental sulfur. Crude oils containing sulfur compounds but no elemental sulfur react like Bradford crude, with sulfur or polysulfide added after the initial release of hydrogen sulfide at 400 F. Asphaltic constituents contain thermally labile sulfur compounds t h a t decompose t o yield hydrogen sulfide a t temperatures of 500" F. and above.
Vol. 45, No, 12
Some implications from this work relate to the refining of crude oil. Obviously, if a crude oil contains polysulfides and even though no thiols are originally present, upon distillation a t a temperature of 400" or 500' F. thiols will be produced and a sour distillate obtained. The reaction of crude-oil constituents with sulfur to yield hydrogen sulfide suggests the possibility of field treatment for certain crude oils in which the elemental sulfur content is high. If it were economical t o heat these crude oils in the field in a closed system to 400' F., a considerable portion of the sulfur would be released as hydrogen sulfide, and this reaction would not occur in costly refinery equipment where it might cause more serious replacement problems. Possible removal of the elemental sulfur as such from the crude oil before any heat treatment is worthy of thought. It is obvious that the answer to some of the questions posed would be helpful in refinery operations. It is hoped that means will be found to study further the reaction of sulfur, polysulfide, and hydrocarbons a t atmospheric pressure. ACKNOWLEDGMENT
The authors are indebted to the Phillips Petroleum Co. for the tri- and polysulfides used in this work, and to the Union Oil Co. of California for the Santa Maria Valley California crude-oil samples.
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A review of the foregoing statements raises many questions, of which the following are typical. With what hydrocarbons in the crude oil does elemental sulfur react? Is there any preferential selection based on type, structure, or molecular weight, and if so, what? Do the threshold temperatures of the reactions differ with hydrocarbon type, structure, or molecular weight? What are the threshold temperatures for these reactions? What type or types of sulfur compounds are formed in addition to hydrogen sulfide? What hydrocarbons, if any, are formed by the reaction of sulfur with hydrocarbons? What are the kinetics of these reactions? What is the effect of pressure on these various reactions? If the reaction can proceed slowly a t low temperature, what ia the significance of the presence or absence of elemental sulfur in crude oil as regards its origin? Will a disulfide plus elemental sulfur act like a trisulfide? Are polysulfides present in crude oils? Can all the sulfur compounds present in a crude oil be attributed to the results of previous reaction with elemental sulfur? An example of the significance of the answers to these questions is the question of the origin of elemental sulfur and hydrogen sulfide in crude oils and the relationship of the content of these compounds to the temperature of the producing formation. Certainly, if the sulfur has been indigenous to the oil, the oil must have had a very low temperature history. On the other hand, if the sulfur dissolved in crude oil results from contact between sulfur and oil after the formation of the oil, there would be no proof of a low temperature thermal history before solution of the sulfur in the oil, but a low temperature environment after the solution of the sulfur in the crude oil would be indicated. Finally, it seems probable that, although the reaction does not appear to proceed rapidly a t temperatures below 300' F., it could be progressing slowly and, over a relatively short geological period, could well account for the hydrogen sulfide found in some crude oils, and thus the difference between the hydrogen sulfide, elemental sulfur, and sulfur compound content of crude oils might be accounted for by the temperature differences of the producing horizons. The authors have been unable t o obtain bottom-holetemperature data for the wells from which samples were obtained, but this information would also be an interesting field of study from a geological as well as a chemical point of view.
REFERENCES Baker, R. B., and Reid, E. E., J . Am. Chem. SOC.,51, 1566 (1929). Friedmann, Walter, Be?. 49, 1344 (1916). Friedmann, Walter, Petroleum, 11, 978 (1916). Eccleston, B. H., Morrison, Marilyn, and Smith, H. M., Anal. Chem., 24,1745 (1952). Farmer, H. E., and Shipley, F. W., J . Polymer Sci., 1, 293 (1946). Hoffert, W. H., and Wendtner, K., J . Inst. Petroleum, 35, 171 (1949). Schuhe,' W. A., Short, G. H., and Crouch, W. W., IND.ENG. CHEX.,42,916 (1950). Shepard, A. F., Henne, A. L., and Midgley, Thos., Jr., J . Am. Chem. SOC.,56,1355 (1934). RECEIVED for review June 8, 1953. ACCEPTEDAugust 17, 1953. Presented before the Division of Petroleum Chemistry at the 123rd Meeting of the AMERICAN CHEMICAL SOCIETY,Los hgeles. Calif. Part of the work of American Petroleum Institute Research Project 48A on "Production, Isolation, and Purification of Sulfur Compounds and Measurement of Their Properties," which the Bureau of Mines conducts at Bartlesville, Okla., and Laramie, Wyo.
Corrections Influence of Hydrogen Sulfide on Flame Speed of Propane-Air Mixtures I n the article on "Influence of Hydrogen Sulfide on the Flame Speed of Propane-Air Mixtures" [ I N D . ENG.CHEM., 45, 2361-6 (1953)l on page 2364 in the paragraph below Figure 9 the fourth sentence should read: For rich mixtures Zc,/c*, < 1 and for lean mixtures 2 c,,/c*. > 1, indicating inhibition in both instances. PHILIP F. KURZ
Chemigum SL-An Elastomeric Polyester-Urethane I n the article on "Chemigum SL-An Elastomeric PolyesterUrethane" [Seeger, K. V., et al., IND.EXG.CHEW., 45, 2538 (1953)l an error was made in Table VI. I n the last column the entries under Chemigum SL should have been: 0, 0.32, 0.48, 0.56, 0.64, 0.88, 1.04, 1.36, and 2.24. N. V. SEEGER
