inlet temperature to the compressor is given by
W dCv- - (1 - C,Tc)T,,:(Cc dTi
+ C,)
Literature Cited
(AT)
The unit cost of the refrigeration produced in the cycle,
C,, will be decreased as the inlet temperature to the compressor rises only if the value of the term (A - CpTc)in eq A7 is negative, which is an equivalent statement of eq 4. Since this result is independent of TI itself, if eq 4 is satisfied and a vapor-liquid exchanger is used, Ti should take on the largest value allowed by the second law of thermodynamics. If the assumption of the ideal gas law and constant specific heat are not satisfied, a similar result is obtained (Barn&, 1973), except that Cp must be calculated at TI and instead of X the value X + AHc must be used, where AH, represents the difference between low-pressure and high-pressure vapor enthalpy at T,.
Acknowledgments The authors are grateful to the Consejo Nacional de Ciencia y Tecnologia of Mexico for their support of one of them (F. J. B.) and to the Computer Center of The University of California for use of their facility.
AlChE Student Contest Problem, American Institute of Chemical Engineers, New York, N. Y., 1959. Barnes, F. J.. Ph.D. Dissertation, Department of Chemical Engineering, University of California, Berkeley. Calif.. 1973. Barnes, F. J., King, C. J.. Process Technol. I n t . . 17, 933 (1972). Bourauet. J. M . . Hydrocarbon Process., 49 ( 4 ) .9 3 (1970). Bressler. S. A,, Chem. Eng., 73, ( l o ) , 124 (1966) Charlesworth. P. L.. Brit. Chem. Eng., 10, 834 (1965). Dreyfus, S.E., Oper. Res., 17, 395 (1969). Hendry, J. E., Rudd, D. F.. Seader, J. D., AIChEJ., 19, 1 (1973). Kaufman. A., "Graphs, Dynamic Programming and Finite Games," Academic Press, New York, N. Y., 1967. King, C. C., Trans. lnst. Chem. Eng., 36, 162 (1958). King, C. J., 25th Annual Institute Lecture, American Institute of Chemical Engineers, Annual Meeting, Philadelphia, Pa., Nov 1973. King, C. J., Gantz, D. W., Barnes, F. J., Ind. Eng. Chem.. Process Des. Develop., 11, 271 (1972). Ludwig, E. E., "Applied Process Design for Chemical and Petrochemical Plants," Vol. 3, Gulf Publishing Co., Houston, Texas, 1965. Menzies. M. A., Johnson. A. I., Can. J. Chem. Eng.. 5 0 , 290 (1972). Perry, J. H.. Ed., "Chemical Engineers' Handbook, 4th ed. McGraw-Hill. New York, N. Y., 1963. Peters, M . S.. Timmerhaus. K. D., "Plant Design and Economics for Chemical Engineers,"2nd ed, McGraw-Hill, New York. N. Y., 1968. Potts, R. B., Oliver, R. M., "Flows in Transportation Networks," Academic Press, New York, N. y.. 1972. Soave, G.. Chem. Eng. Sci.. 27, 1197 (1972). Steen-Johnsen, H.. Hydrocarbon Process., 46 ( l o ) , 126 (1967). Stettenberg. L. M., Chem. Eng., 79 ( 1 ) . 95 (1972). Swearingen. J. S., Hydrocarbon Process.. 49 ( 4 ) .93 (1970)
Receicedfor revieu: December 18, 1973 A c c e p t e d June 3, 1974 Presented a t the National Meeting of the American Institute of Chemical Engineers, Tulsa, Okla., March 1974.
Reaction of Sodium with Dibenzothiophene. A Method for Desulfurization of Residua Heinz W. Sternberg,* Charles L. Delle Donne, Raymond E. Markby, and Sidney Friedman Pittsburgh Energy Research Center. U. S.Department of the Interior, Bureau of Mines, Pittsburgh. Pennsylvania 75273
Treatment of a petroleum residuum containing 1.65% S with excess metallic sodium (3.1 mol of Na/ mole of S) in t h e presence of hydrogen at 350°C decreases t h e s u l f u r content to 0.3-0.4%. To lower the s u l f u r content below 0.1% requires 6 mol of Na/mole of S. Parallel experiments with dibenzothiop h e n e dissolved in decahydronaphthalene resulted in >99% removal of s u l f u r to form primarily diphenyl. T h e rate of desulfurization is inversely proportional to hydrogen pressure and proceeds even in its absence. In t h e absence of hydrogen, however, char formation is favored.
Introduction Current technology of residuum desulfurization is based on the Co-Mo catalyzed reaction of hydrogen with sulfur compounds in the charge stock to produce hydrogen sulfide which is readily separated from the oil (Allund, 1972; Arey, et al., 1967; Enke, 1972; McKinley, 1957; Schuman and Shalit, 1970). The purpose of the present work was to explore an alternative method for the removal of sulfur based on the specific interaction of metallic sodium with organically bound sulfur. Methods for the removal of sulfur from petroleum distillates with metallic sodium or sodium hydride in the presence or absence of hydrogen have been reported in the literature (Bashilov and Kupriyanov, 1960; Weinberger, et al., 1970) and patents (Aschenbach, et al., 1963; Deutsche Gold-Und-Silber Scheideanstalt, 1963). A portion of organically bound sulfur in residua is associated with the asphaltene fraction and is believed to be present
in the form of more difficultly removable multiring thiophenes (Bestougeff, 1959; Hunt and O'Neal, 1965; Schuman and Shalit, 1970) such as dibenzothiophene (DBT) and a considerable amount of work has been published on the catalyzed hydrodesulfurization of model compounds (Ahuja, et al., 1970; Schuman and Shalit, 1970). In contrast, little is known about desulfurization of such compounds with alkali metals, except for the work by Gilman and his coworkers who showed that DBT can be desulfurized by treatment with metallic lithium in refluxing dioxane (Gilman and Dietrich, 1957; Gilman and Esmay, 1953), and the partial hydrodesulfurization of DBT reported by Friedman and coworkers, who used NaRb and hydrogen at 250°C (Friedman, et al., 1971). To learn more about the mechanism of sulfur removal by alkali metals and to establish guidelines for optimization procedures, we investigated the removal of sulfur from DBT and from a residuum using metallic sodium as a reagent. I n d . Eng. C h e m . . Process D e s . D e v e l o p . , Vol. 13, No. 4 , 1 9 7 4
433
Results and Discussion Two series of experiments were carried out. In one series, a solution of DBT in decahydronaphthalene containing 1.65% sulfur was treated in a magnetically stirred autoclave at 350°C with metallic sodium, either in the presence or absence of hydrogen. In a second series, a residual oil (atmospheric residuum from a California crude oil) containing 1.65% sulfur was treated in a similar way. The experimental results are summarized in Tables I and 11. DBT in decahydronaphthalene solution treated with sodium metal at 350°C under nitrogen (run l, Table I) is converted to diphenyl, and an insoluble char plus sodium sulfide. The reaction is strongly exothermic. A possible reaction scheme, based on analogous reactions between Li and DBT (Eisch, 1963; Gilman and Dietrich, 1957; Gilman and Esmay, 1953) is as follows.
Na I
I
+ 2Na
-+
(7-7) \
1 1 +
h2S
(2)
Na
Ka
I1
The intermediate adduct I1 can react further either by accepting hydrogen from decahydronaphthalene or DBT to produce diphenyl and sodium or sodium hydride, or by splitting off sodium or sodium hydride and polymerizing (char formation). The formation of diphenyl from DBT in the absence of hydrogen gas (run 1, Table I) requires an explanation. Two possible sources of hydrogen must be considered, i.e., the solvent (decalin) and its impurities (tetralin and octalin) on the one hand and DBT itself on the other. A comparison of mass spectrometric analyses of the solvent before and after run 1 (Table I) showed that 16.8 mmol of Ha was given off by the solvent. Since 36.8 mmol of diphenyl was produced in this run, the hydrogen furnished by the solvent accounts only for 46% of the amount of diphenyl formed. On the other hand, the conversion of DBT with an H/C ratio of 8:12 to 4.2 g char with an H/C ratio of 5:i2 furnishes 42.5 mmol of H2, more than enough to account for the 36.8 mmol of diphenyl formed. This result indicates that a substantial portion of the hydrogen required to convert the radical .C6H4-C6H4- to diphenyl must have come from the conversion of DBT to char. (The 4.2 g of char produced may be viewed as consisting of a monomer with an H/C ratio of 5:12 and a molecular weight of 5H + 12C = 149. Hence, 4.2 g of char correspond to 4200/149, Le., 28.2 mmol of (C12H5). Since these 28.2 mmol of C12H5 were produced from 28.2 mmol of DBT whose H/C ratio is 8:12, it follows that this conversion liberated 3 x 28.2 X 0.5, i.e., 42.5 mmol of H2.) In the presence of hydrogen, sodium hydride, which is formed rapidly from sodium and hydrogen at 300°C (Hurd, 1952; Kantak and Sen, 1968), might replace sodium as an attacking reagent. If sodium hydride does not react as an attacking reagent, or if it reacts more slowly than sodium, then hydrogen would be expected to have an inhibitory effect on the removal of sulfur. That hydrogen retards the desulfurization of DBT by 434
Ind. Eng. Chem., Process Des. Develop., Vol. 13, No. 4. 1974
sodium metal may be seen by comparing the amount of sulfur removal a t 1200 and at 200 psi hydrogen pressure. At 1200 psi and 1 hr reaction time (run 4), only 51% and a t 2 hr (run 3), only 61% sulfur was removed, whereas at 200 psi at the same reaction times (runs 6 and 7) 99% of the sulfur was removed. At long reaction time (6 hr), desulfurization is completed even at high hydrogen pressure (runs 2 and 5). Hydrogen also decreases char formation. At high hydrogen pressures (runs 2, 3, and 4), only traces of char are formed. Hydrogen at high pressure also retards desulfurization of residual oil by metallic sodium as may be seen by comparing runs 1 and 2 (Table 11).At 1200 psi, the sulfur content decreased by 32% (from 1.65% to 1.12%), while at 200 psi it decreased by 57% (from 1.65% to 0.71%) during the same length of time. The rate of sulfur removal from residual oil is much slower than that from DBT as may be seen by comparing run 4 (Table II) with run 7 (Table I). Under identical experimental conditions (Na/S ratio, temperature, pressure, and initial sulfur concentration of 1.65% S), the sulfur content of the residual oil decreased by 77% in 6 hr, whereas that of the decahydronaphthalene solution of DBT decreased by 99% in 1 hr. Increasing the reaction time from 6 to 18 hr (runs 4 and 5, Table 11) did not lead to any further decrease in sulfur content, whereas increasing the Na/S ratio did (runs 6 and 7, Table 11).These results show that lowering the sulfur content of residual oil below the 0.3 to 0.4 level requires an amount of sodium far in excess of that required for lowering the sulfur content from 1.65% to the 0.3 to 0.4% level. Apparently, when the reaction has reached a point where most of the reactive sulfur compounds have been desulfurized, other constituents of the residual oil, e.g., weakly acidic hydrocarbons, compete with less reactive sulfur compounds for the remaining sodium. In this connection, it is of interest that, unlike the case of DBT in decahydronaphthalene solution, desulfurization of residual oil at longer reaction times is more effective in the presence than in the absence of hydrogen (runs 4 and 3, Table 11). This may be due to the fact that residual oil, having less available hydrogen than decahydronaphthalene, cannot provide the hydrogen required for hydrogenolysis of adduct I1 (eq 3) as readily as decalin. The reduced Solvent
4-1 (Decal i n )
7-7
Na Na
n
R-R
+
ZNa(orNaH) (3)
Hbipinyl ( * R-Re)
+
ZNa(or NaH)
Polymer (Char)
rate of hydrogenolysis of adduct I1 in the absence of hydrogen may play an important role at longer reaction times, i.e., at the point where reactive sulfur-free constituents of the residual oil are beginning to compete with the less reactive sulfur containing compounds for the remaining sodium metal. To avoid the use of large amounts of sodium, we explored the possibility of reducing the sulfur content of the residual oil by pretreatment with hydrogen (run 8),hydrogen in the presence of CaO and Ca(OH)2 (run 9), and Co-Mo catalyzed hydrogenation (run ll), prior to treatment with metallic sodium. Pretreatment with hydrogen and CaO-Ca(OH)n (run 9) was not any more effective than hydrogen alone (run 8) and did not substantially reduce the amount of sodium required for lowering the sulfur content to the 0.4 level as may be seen from a compar-
Table I. Desulfurization of Dibenzothiophene (DBT) with Metallic Sodium in Decalin at 350°C
Run' no. 1 2 3 4 5 6 7
Gas
Pressure, cold, psi
Time, hr
100 1200 1200
1 6
N2 H2 H2
1200
HZ HZ H2 HZ
200 200
2 00
Pr oducts recovered, Mole p e r cent of DBT charged
Sulfuc removed,
% C99.91 99 61 51 99 99 99
2
1 6 2 1
DBT
Diphenyl
Charb
Total
0.1
52.0 93.2 59.3 45.6 81.3 91.0 89.1
39.8 0.1 Trace Trace 14.3 1.0 0.7
91.9 94.0 98.7 94.8 96.2 93.0 90.2
0.7 39.4 49.2 0.6 1.0
0.4
,
All runs were carried out with a solution containing 1.65% S (13 g of DBT in 140 ml of decahydronaphthalene) and 5 g of metallic sodium, corresponding to a n N a : S weight ratio of 2.2: 1.0 (mole ratio 3.1: 1.0). See text. Q
Table 11. Desulfurization of Residual Oild Containing 1.65% S ' ~ _ _ _ _
No. 1 2 3 4 5 6 7 8 9 10 11 12 13 14
Charge Residual Residual Residual Residual Residual Residual Residual Residual Residual
oil oil oil oil oil
oil oil oil oil
Product f r o m Run 9 Residual oil Product from Run 11 Residual oil Product from Run 13
Additive p e r 100 g of charge, g 3.7 Na 3.7 Na 3.7 Na 3.7 Na 3.7 Na 5.7 Na 7 . 1 Na None 9.7 g Ca(OH), 7.4 g CaO 3.2 Na 6.8 g CO-MO 2 . 0 Na
Gas pressureb (cold), psi
Time, hr
3.1 3.1 3.1 3.1 3.1 4.8 6.0
1200 2 00 200b 2 00 2 00 200 200 2 00 200
3.1
6.1
Na/S mole r a t i o
6.7 CO-MO 6.7 CO-MO
Sulfur in product,
~
B
Recovery, wt %
0.5 0.5 6 6 18 2 6 6 6
1.12 0.71 0.54 0.38 0.37 0.15 0.08 1.43 1.45
100 99 93 86 86 100 74 99 96
2 00
6
0.46
94
1500 200
6 6
0.46 0.17
89 96
1500 1500
6 6
0.47 0.20
86 82
All runs with Na were carried out at 350°C, those with Co-Mo at 375°C. * All runs were carried out under hydrogen except run 3 , which was carried out under nitrogen. Product soluble in toluene and free of material volatile below 90" at 25 mm Hg. About half of the unrecovered material consisted of char and half of material volatile below 90" at 25 mm Hg. d Atmospheric residuum from California crude with an .4PI at 60°F gravity of 11.5.
ison of runs 4 and 10. The Co-Mo hydrogenated oil containing 0.46% sulfur reacted with metallic sodium to give a product with 0.17% sulfur (run 12). Although desulfurization by sodium and by Co-Mo undoubtedly proceed by different reaction mechanisms, the rate of sulfur removal in both cases drops sharply after a level of 0.3 to 0.4% sulfur is reached, as may be seen by comparing runs 4 and 5 with 13 and 14. Experimental Section Reagents. Dibenzothiophene, decahydronaphthalene, and metallic sodium were of the highest purity available commercially and were used without further purification. The residual oil, supplied by the Union Oil Co. of California (Feedstock Number F-2505), was an atmospheric residuum from California crude with an API at 60°F gravity of 11.5. It contained 1.65% S as determined in our laboratory by the Parr-Oxygen method. The catalyst was Har-
shaw Chemical Corp. No. 0402T Co-Mo/Si02-A1203 catalyst in Ih-in. diameter pellets. Apparatus. All experiments were carried out in a magnetically stirred, 1000-ml stainless steel autoclave provided with a metal liner to prevent contact of autoclave walls with the sodium metal. The stirrer was operated at a speed of 600-650 rpm. To ensure efficient stirring, the distance between stirrer blade and bottom of liner was kept to a minimum in.). Efficient stirring was found to be essential for optimum reaction. Procedures (a) Desulfurization of Dibenzothiophene (DBT). The autoclave was charged with a solution of DBT in decahydronaphthalene made up to contain 1.65% sulfur (13 g of DBT in 140 ml of decalin) and 5 g of sodium metal and pressurized to the desired nitrogen or hydrogen pressure. The contents of the autoclave were heated with stirring to Ind. Eng. Chem., Process Des. Develop., Vol. 13, No. 4, 1974
435
350" (heat-up time 45 min), and after a specified length of time, the contents were rapidly cooled to room temperature by means of an internal cooling coil. The reaction product consisting of decalin solution and solids (sodium, sodium hydride, sodium sulfide, and char) was worked up as follows. A sample of the solution was withdrawn, filtered, and analyzed by glc using a 118 in. x 12-ft stainless steel column packed with 3% Apiezon L on ChromosorbG-AW-DMCS. The rest of the solution was decanted from the solids, and the residue was treated with ethanol to decompose unreacted sodium and sodium hydride. The ethanol solution was acidified and separated from the char by filtration. The char was washed with water until free of acid and dried a t 90" in vacuo to constant weight. An ultimate analysis of the char obtained in run 1 (Table I) showed that it contained 94.8% carbon, 3.26% hydrogen and 0.52% sulfur, i.e., hydrogen and carbon in a ratio of 5:12. (A polymer of the type suggested in eq 3 would have an H/C ratio of 8:12.) Recovery of starting material (DBT) and products (diphenyl, char) ranged from 92 to 99%. For the purpose of calculating per cent of recovery, it was assumed that the char was formed from DBT by desulfurization and dehydrogenation to give a C12H5 polymer according to Based on this assumption, 149 mg of char (C12Ho) corresponds to 1 mmol of DBT or diphenyl, The recoveries of DBT, diphenyl, and char for the runs listed in Table I are shown in columns 5, 6,7, and 8 of Table I. (b) Desulfurization of Residuum. The autoclave was charged with 165 g of residuum and either sodium or Co-Mo catalyst and pressurized to the desired hydrogen or nitrogen pressure. The autoclave was heated and the stirrer was started when the temperature had reached 280". Desulfurization with sodium was carried out a t 350" and with Co-Mo a t 375". Total heat-up time was 45 min to reach 350" and 65 min to reach 375". After a specified length of time, the heating was stopped and the contents of the autoclave were rapidly cooled by an internal cooling coil. The pressure was released, the autoclave was opened, and the contents removed with 600 to 700 ml of toluene. The toluene solution was centrifuged to remove solids and the centrifuged solution was filtered to remove any suspended fines. The filtered solution was stripped of toluene a t 90" and 25 mm Hg and analyzed for sulfur (Parr-Oxygen method) and for ash. The ash content of sodium treated residuum samples ranged from 1 to 2%. By washing the toluene solution of the sodium treated residuum with a 50% aqueous alcohol solution, the ash could be
436
ind. Eng. Chem., Process Des. Develop.. Vol. 13,No. 4,1974
quantitatively removed. This ash did not contain any appreciable amounts of sulfur, since the sulfur content of the residuum remained the same after the aqueous alcohol wash. Conclusion Sodium, a t 350°C and with relatively low hydrogen pressure (200 psi cold), can reduce the sulfur content of a 1.65%sulfur petroleum residuum to 0.3 to 0.4% a t a sodium-sulfur mole ratio of 3.1:l. At higher Na/S ratios (6:1), sulfur content can be reduced to below 0.1%. Dibenzothiophene dissolved in decahydronaphthalene can be almost quantitatively desulfurized by sodium a t 3W0,even in the absence of hydrogen. Hydrogen, which decreases the rate of desulfurization, also inhibits char formation. Sulfur removal involves direct interaction of sodium with the organosulfur compound. Retardation of sulfur removal a t elevated hydrogen pressure is probably due to the formation of sodium hydride. Literature Cited Ahuja, S. P., Derrien, M. L.. LePage, J. F., lnd. Eng. Chem., Prod. Res. Develop., 9,272 (1970). Allund, L.. Oil Gas J . , 70, 79 (1972). Arey, W. F.. Blackwell, N. E., Reichle. Seventh World Petroleum Congress, Mexico City, Panel 20,Paper l a , 1967. Aschenbach, K., Osterloh, K . , Rothe, W.. Belg. Patent 623,975 (Feb 14, 1963);Chem. Abstr., 59,3697e (1963). Bashilov, A. A,. Kupriyanov, V. A,, Tr. Groznensk. Neft. lnst.. No. 24, 8 (1960);Chem. Abstr., 57,1147a (1962). Bestougeff, M., World Petroleum Congress, 5th Annual Meeting, Proceedings, Section V, Paper 12,1959. Deutsche Gold-Und-Silber Scheideanstalt Vorm. Roessler and Otto C. & Co. GmbH, Belg. Patent 625,739 (Mar 29, 1963; Chem. Abstr., 59,
251 Oc (1963). Enke. C. G.. ErdoelKohle, Erdgas, Petrochem.. 25, 671 (1972). Eisch, J. J . , J. Org. Chem., 28, 707 (1963). Friedman, S.,Kaufman, M. L., Wender. I . , J. Org. Chem.. 36, 694
(1971). Gilman. H., Dietrich, J. J., J. Org. Chem.. 22, 851 (1957). Gilman, H., Esmay, D. L., J. Amer. Chem. Soc., 75,2947 (1953). Hung, R. H.. O'Neal, M. J . , Jr., in "Advances in Petroleum Chemistry and Refining." Vol. 10, p 25, J. J. McKeeta. Jr., Ed., Interscience. New York, N. Y.. 1965. Hurd. L. T.. "Chemistry of the Hydrides," p 31, Wiley, New York. N. Y.,
1952. Kantak, W. N.. Sen, D. M., Res. lnd., 13 (2),63 (1968):Chem. Abstr.,
70,59351 (1969). McKinley, J. B., in "Catalysis," P. H. Emmett, Ed., Vol. V , p 405, Reinhold, New York. N. Y., 1957. Schuman, S.C.. Shalit, H., Catal. Rev., 4 (2),245 (1970). Weinberger, S. M., Navarro, L. J.. Bonilla, C. F., Rev. SOC. Ouim. Mex., 1 4 (l), 13 (1970); Chem. Abstr., 73,27189n (1970).
Received for review F e b r u a r y 25, 1974 A c c e p t e d July 8,1974 Reference t o a c o m p a n y or p r o d u c t n a m e i s m a d e t o f a c i l i t a t e und e r s t a n d i n g a n d does n o t i m p l y endorsement by t h e B u r e a u o f Mines.
