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GENERAL RESEARCH Desulfurization Process for Dibenzothiophenes from Light Oil by Photochemical Reaction and Liquid-Liquid Extraction Takayuki Hirai,* Ken Ogawa, and Isao Komasawa Department of Chemical Engineering, Faculty of Engineering Science, Osaka University, Toyonaka, Osaka 560, Japan
A desulfurization process for dibenzothiophene (DBT) and its derivatives such as 4-methyldibenzothiophene (4-MDBT) and 4,6-dimethyldibenzothiophene (4,6-DMDBT) by combination of photochemical reaction and liquid-liquid extraction has been investigated. In this, the DBTs dissolved in tetradecane were quantitatively photodecomposed by the use of a high-pressure mercury lamp and were removed to the water phase as SO42- at conditions of room temperature and atmospheric pressure. The order of reactivity for the DBTs was DBT < 4-MDBT < 4,6DMDBT, thus indicating a different tendency from that reported for the hydrodesulfurization method. The desulfurization yield of commercial light oil, however, by the proposed method was only 22% following 30 h irradiation and was caused mainly by the depression of the photoreaction of DBT by the presence of aromatic compounds in the light oil. Introduction There has been much recent interest in the deep desulfurization of light oil, since sulfur-containing compounds are converted to SOx by combustion and hence one of the main sources of acid rain and air pollution. The regulation of the sulfur content in light oil has been tightened in Japan, from 0.50 wt % in 1976, 0.20 wt % in 1992, and with the object of 0.05 wt % in 1997. The hydrodesulfurization method using Co-Mo or Ni-Mo catalysts has been widely used on the industrial scale. This method requires both a high hydrogen pressure and high temperature. Polyaromatic sulfur-containing compounds such as dibenzothiophene (DBT) and its derivatives are known to be key compounds in deep desulfurization, since their desulfurization, especially 4-methyldibenzothiophene (4-MDBT) and 4,6-dimethyldibenzothiophene (4,6-DMDBT), by hydrodesulfurization is difficult and caused mainly by the steric hindrance between the above compounds and the catalyst (Houalla et al., 1980; Kabe et al., 1992). DBT photochemistry of DBTs has been investigated mainly in the field of environmental chemistry. Sunlightphotooxidized DBTs (sulfoxide or sulfone) in spilled crude oil are toxic, and Berthou and Vignier (1986) have used DBTs as organic markers for oil pollution in the marine environment. When Arabian light oil was mixed with sea water under natural irradiation, the DBTs were photooxidized to water-soluble sulfur-oxygenated compounds and were transported into the water phase. The estimated half-lives for the photooxidation of monoand dimethylated DBT were found to be longer than that of DBT. Moza et al. (1991) reported on the photodegradation of aromatic sulfur compounds which dissolved in n-hexane and spread as a thin liquid film on water. Irradiation by a medium-pressure mercury lamp brought about the oxidation of the DBT to its sulfoxide, which then moved into the water phase. If such * Author to whom correspondence is addressed. E-mail:
[email protected].
0888-5885/96/2635-0586$12.00/0
photochemical techniques can be applied to the desulfurization of fuel oils, the process should have the following advantages: (1) No catalysts are needed. (2) It is easy to operate and to control the reaction. (3) The process is energy-saving since the reaction occurs at room temperature and under atmospheric pressure. (4) The deep desulfurization of refractory sulfur compounds such as 4-MDBT and 4,6-DMDBT as well as DBT may be feasible. In this work, the desulfurization process for DBTs by photochemical reaction and liquid-liquid extraction has, therefore, been investigated. In this, the DBTs were photodecomposed by UV light in the organic phase and the resulting sulfur compounds removed into the water phase. The effects of the water phase, gas bubbling, and presence of aromatic compounds such as naphthalene have been investigated. The desulfurization of commercial light oil was also studied. Experimental Section DBT, tetradecane (n-C14H30), NaOH, and naphthalene were supplied by Wako Pure Chemical Industries, Ltd. 4-MDBT and 4,6-DMDBT were prepared as reported by Gerdil and Lucken (1965). Commercial light oil was purified by distillation (493-623 K/760 mmHg) prior to use. The DBT was dissolved in tetradecane (100 mL) and mixed vigorously with the water phase (distilled water or 1 M NaOH (M ) mol/L), 300 mL) using a magnetic stirrer. The combined solutions were UV irradiated by immersing a high-pressure mercury lamp (300 W, Eikohsha Co., Ltd., Osaka, Japan) into the solution. The experiments were carried out with or without gas bubbling (1 L/min) and at atmospheric pressure. The temperature of the solution was about 323 K under irradiation conditions. The organic and water phases were analyzed by gas chromatography (Shimadzu GC14B equipped with FID and YHP 5921A equipped with AED) and ion chromatography (Shimadzu HIC-6A), respectively. The total sulfur concentrations in the light © 1996 American Chemical Society
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Figure 3. Effect of the bubbling rate of air on the reduction in the DBT concentration in the organic phase.
Figure 1. Effect of the water phase on the reduction in DBT concentration in the organic phase.
Figure 4. Gas chromatogram of the organic phase after 6 h of irradiation.
Figure 2. Time course of the concentration of DBT in the organic phase with and without gas bubbling.
oil and water phases were measured by an ANTEK 7000S sulfur analyzer and inductively coupled argon plasma atomic emission spectrophotometer (Nippon Jarrell-Ash ICAP-575MarkII). Results and Discussion Effect of the Water Phase on Desulfurization. Figure 1 shows the effect of the water phase on the removal of DBT from tetradecane. The initial DBT concentration (2 g/L) corresponds to a sulfur content of 0.05 wt %. When the DBT/tetradecane solution (400 mL) was irradiated without the presence of a water phase, the amount of DBT decreased with time as shown in Figure 1a, but the solution turned yellow and the viscosity of the solution increased due to the accumulation of the photodecomposed DBT in the organic phase. This problem was solved by adding the water phase. When 1 M NaOH was used, the organic phase remained clear. With distilled water, a third (emulsion) phase first appeared and then disappeared during the course of irradiation. This indicated the formation of intermediates having surface activity, since no emulsion appeared when DBT was absent from the feed organic phase. The reaction was faster in the case using distilled water than in the case using a NaOH solution. Effect of Gas Bubbling on Desulfurization. Figure 2 shows the time course of the DBT concentration in the organic phase both with and without gas bub-
bling. The reaction was remarkably facilitated by the bubbling of air or O2 but was depressed by N2 bubbling. Thus, the photodecomposition of DBT is shown to occur easily under the presence of O2. The desulfurization rate is little affected by the bubbling rate of air in the range 0.3-1 L/min as shown in Figure 3, and an air bubbling of 1 L/min can thus supply the excess quantity of O2 required for reaction. Mechanism of Desulfurization. When DBT/tetradecane was supplied, the organic phase following photoirradiation was found by GC-AED analysis to contain only unreacted DBT as the sulfur-containing compound. Thus, photodecomposed DBT moves to the third or water phase immediately. The gas chromatogram for the organic phase obtained by GC-FID analysis, after 6 h irradiation, is shown in Figure 4. Here, 75% of the DBT, supplied, was removed from the organic phase. The C5-C12 olefins are the products of the photodecomposition of tetradecane. Peak “a” is also attributable to a tetradecane derivative formed by photoreaction. Approximately 0.3% of the tetradecane was photodecomposed, as judged from the peak area. Tridecane and pentadecane are the impurities contained in tetradecane. Peak “b” is attributable to an intermediate consisting of the solvent (tetradecane) and the photodesulfurized product of DBT, since the peak decreases following further photoirradiation and shifts position when another solvent such as n-decane is used. A white precipitate was formed in the resulting water phase by the addition of BaCl2 solution, indicating the presence of SO42-, which was confirmed by ion chromatography. Figure 5 shows the time course of the SO42concentration in the water phase, together with that of the DBT concentration in the organic phase. About 80% of sulfur was converted to SO42-, and the remainder consisted of the intermediate sulfur-containing com-
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Figure 5. Time course of the DBT concentration in the organic phase and the SO42- concentration in the water phase.
Figure 8. Desulfurization of commercial light oil.
Figure 6. Effect of the initial DBT concentration on the decrease rate of the DBT concentration in the organic phase.
Figure 9. Time course of the concentrations of DBT and naphthalene in the organic phase. The initial naphthalene concentration is 50 g/L.
Figure 7. Comparison of the desulfurization rate of DBT and its derivatives.
pound which was solubilized in the emulsion or water phase. This intermediate is likely to be converted to SO42- by further photoirradiation. The removal rate of DBT from the organic phase was investigated. From the results shown in Figure 6, the reaction rate is independent of the initial DBT concentration and is, therefore, of zero order with a rate constant determined as 1.37 mM/h. The reaction rate under the present conditions is dominated by the light intensity, since the quantity of O2 supplied by air bubbling is in sufficient excess. A faster rate of desulfurization is therefore expected by the use of a more powerful light to obtain a greater light intensity. Removal of DBT Derivatives. The photodecomposition of 4-MDBT and 4,6-DMDBT was then studied as for DBT. The reactivities of both 4-MDBT and 4,6DMDBT were found to be higher than that of DBT, and the compounds were removed from the tetradecane phase more rapidly, as shown in Figure 7. The rate constants for these compounds are 1.46 and 2.18 mM/ h, respectively. This order of reactivity for these DBTs, DBT < 4-MDBT < 4,6-DMDBT, is different from that reported for the hydrodesulfurization method (Houalla et al., 1980). In the present method, the expanded
electric cloud of 4-MDBT and 4,6-DMDBT by methyl substituents is likely to ease the excitation due to photoirradiation. The proposed method is, therefore, useful for the deep desulfurization of refractory sulfur compounds such as 4-MDBT and 4,6-DMDBT. The order of reactivity obtained by the present method is also different from that obtained by irradiation by sunlight as reported by Berthou and Vignier (1986). This result is probably caused by the difference in the wavelength range of the light sources. Desulfurization of Commercial Light Oil. The commercial light oil, purified by distillation, contains DBTs (about 80%) and benzothiophene and its derivatives (about 20%) as sulfur-containing compounds and a total sulfur content of 0.105 wt %. The time course variation of the sulfur concentration in the oil phase is shown in Figure 8. The broken line shows the expected value based on the rate constant obtained for DBT (1.01 mM/h) with a 1 M NaOH solution. However, the actual desulfurization yield was found to be only 22% after 30 h of irradiation. This unexpected result is attributable to the influence of aromatic compounds in the light oil, since they absorb UV radiation to a considerable extent. The effect of naphthalene on the removal of DBT in the tetradecane system was, therefore, investigated. As shown in Figure 9, naphthalene was photodecomposed by irradiation and depressed the desulfurization significantly. The effect of the amount of naphthalene on the degree of desulfurization is shown in Figure 10. Here the concentration change of the naphthalene is compared to that of DBT at the same corresponding
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which differs from that reported for the hydrodesulfurization method. 2. The desulfurization yield of commercial light oil, however, was only 22% following 30 h of irradiation. This low yield is caused mainly by the depression of photoreaction of the DBT by the presence of aromatic compounds in the light oil and indicates the necessity for their removal prior to the application of the present method. Acknowledgment The authors acknowledge members of Kinuura Research Center of JGC Corporation for their experimental help, Prof. Kazuhide Tani and Dr. Yasutaka Kataoka of Osaka University for their help in preparing 4-MDBT and 4,6-DMDBT, and a Grant-in-Aid for Encouragement of Young Scientists (Nos. 06855087 (1994) and 07855088 (1995)) for T.H. from the Ministry of Education, Science and Culture of Japan for financial support. Figure 10. Time course of the concentrations of DBT and naphthalene in the organic phase. The initial naphthalene concentrations are 2, 5, and 10 g/L.
condition, and it is shown that the removal of DBT from the organic tetradecane phase starts once the concentration of naphthalene has decreased to about 1.5 g/L. Aromatic compounds must, therefore, be removed prior to the desulfurization based on the present method. The use of other light sources, photosensitizer and photocatalyst such as n-TiO2, may also be effective for the desulfurization of light oil. Conclusion A desulfurization process for dibenzothiophene and its derivatives (DBTs) by a combination of photochemical reaction and liquid-liquid extraction was investigated, with the following results. 1. DBTs dissolved in tetradecane were successfully photodecomposed by the use of a high-pressure mercury lamp and were removed to the water phase as SO42under conditions of room temperature and atmospheric pressure. The desulfurization was facilitated by the supply of O2 by air bubbling. The reactivity of the DBTs was in the order of DBT < 4-MDBT < 4,6-DMDBT,
Literature Cited Berthou, F.; Vignier, V. Analysis and Fate of Dibenzothiophene Derivatives in the Marine Environment. Int. J. Environ. Chem. 1986, 27, 81-96. Gerdil, R.; Lucken, E. A. C. The Electron Spin Resonance Spectra of the Dibenzothiophene Radical Anion and Its Isologs and the Electronic Structure of Conjugated Sulfur-Containing Heterocycles. J. Am. Chem. Soc. 1965, 87, 213-217. Houalla, M.; Broderick, D. H.; Sapre, A. V.; Nag, N. K.; J. De Beer, V. H.; Gates, B. C.; Kwart, H. Hydrodesulfurization of MethylSubstituted Dibenzothiophenes Catalyzed by Sulfided Co-Mo/ γ-Al2O3. J. Catal. 1980, 61, 523-527. Kabe, T.; Ishihara, A.; Tajima, H. Hydrodesulfurization of SulfurContaining Polyaromatic Compounds in Light Oil. Ind. Eng. Chem. Res. 1992, 31, 1577-1580. Moza, P. N.; Hustert, K.; Leoff, S. Photochemical Transformations of Selected Organic Chemicals in Two Phase System. Toxicol. Environ. Chem. 1991, 31-32, 103-106.
Received for review June 6, 1995 Accepted November 7, 1995X IE9503407
X Abstract published in Advance ACS Abstracts, February 15, 1996.
