A Desulfurization Process for Light Oils Based on the Formation and

whereas benzothiophenes (BTs) gave rise to the corresponding chlorine-adduct products, on their unsaturated bond. The sulfimides of alkyl-substituted ...
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Ind. Eng. Chem. Res. 2002, 41, 4376-4382

A Desulfurization Process for Light Oils Based on the Formation and Subsequent Adsorption of N-Tosylsulfimides Yasuhiro Shiraishi, Tomoko Naito, Takayuki Hirai,* and Isao Komasawa Department of Chemical Science and Engineering, Graduate School of Engineering Science, and Research Center for Solar Energy Chemistry, Osaka University, Toyonaka 560-8531, Japan

A desulfurization process for light oils has been investigated, based on the formation and subsequent adsorption of N-tosylsulfimides, produced by the reaction of the sulfur compounds in the light oils, with chloramine T (sodium N-chloro-p-toluenesulfonamide). Dibenzothiophenes (DBTs) in light oils are converted, by the reaction with chloramine T dissolved in methanol, to form the corresponding sulfimides and are removed into the methanol under moderate conditions, whereas benzothiophenes (BTs) gave rise to the corresponding chlorine-adduct products, on their unsaturated bond. The sulfimides of alkyl-substituted DBTs, formed during the reaction, remained in the resulting light oil, such that the deep desulfurization (0.05 wt %) failed to achieve. These compounds however were removed successfully by the addition of aluminum oxide adsorbent and the sulfur concentration of the light oil was decreased to less than 0.05 wt %. The desulfurization of high-aromatic-content light oil is relatively more difficult, because the aromatic hydrocarbons are chlorinated by reaction with chloramine T, proceeding competitively with chlorination of the sulfur compounds. Introduction A novel desulfurization process for light oils was studied in previous work, based on removal of sulfimides, produced by reaction of sulfur compounds with chloramine T.1 Dibenzothiophenes (DBTs), present in light oils, are converted under moderate conditions, by the addition of chloramine T dissolved in methanol (MeOH) in the presence of a small quantity of acetic acid (AcOH), to form the corresponding sulfimides and are removed into the MeOH. In this process, however, the sulfur concentration of light oils was hardly decreased to a target level (0.05 wt %), because the sulfimides of DBTs, having a high carbon number of alkyl substituents, remain in the resulting light oil, as a result of their high hydrophobicity. The sulfimides however could be removed by subsequent adsorption using aluminum oxide or silica gel, such that the deep desulfurization was achieved successfully. The present work is an extensive study of the above new deep desulfurization technology for light oils based on chloramine T.1 Detailed studies on the desulfurization reactivity of sulfur compounds, such as DBTs and benzothiophene (BT), have been investigated, using n-tetradecane solution, containing the pure sulfur compounds as a model light oil. The reactivity for each compound obtained was correlated with the net electron density value on the sulfur atom, as estimated by semiempirical MO calculations. The effect of the addition of aromatic hydrocarbons on the desulfurization was examined, and the products and reaction pathways for the aromatic compounds were also identified. Three light oils, of differing sulfur and aromatic concentration, were employed to examine the feasibility of the present process. The individual sulfur compounds, remaining in the light oils, following desulfurization, were determined * To whom correspondence should be addressed. E-mail: [email protected]. Fax: +81-6-6850-6273. Tel.: +816-6850-6272.

quantitatively by gas chromatography using an atomic emission detector (GC-AED) analysis, and their reactivities were compared with those obtained by MO calculation. Experimental Section 1. Materials and Analysis. AcOH (acetic acid), BT, chloramine T (trihydrate), DBT, MeOH, naphthalene, tetralin, and n-tetradecane were purchased from Wako Pure Chemical Industry, Ltd., and were used as received. Methyl-substituted DBTs were synthesized according to standard procedures.2 Three light oils, consisting of straight-run light gas oil (LGO, sulfur content 1.38 wt %), commercial light oil (CLO, 0.179 wt %), and light cycle oil (LCO, 0.132 wt %), were used in the investigations. The properties of the oils are identical to those used in a previous study.3 Concentrations of total sulfur and of the individual sulfur compounds in the light oils and of the sulfur and aromatic compounds in tetradecane were analyzed by inductively coupled plasma atomic emission spectroscopy, GC-AED, and gas chromatography with flame ionization detector, respectively, as described previously.3 The reaction products for BT, tetralin, and naphthalene and for the chlorine concentrations in the light oils were determined by gas chromatography/mass spectrometry (GC/MS) and GCAED, respectively.2 The electron density values on the sulfur atom for DBTs are cited from the previous paper.3 2. Procedure. The full desulfurization procedure was as described in the previous paper,1 and only a brief description is presented here. Each light oil (50 mL) was heated, with stirring, to a designated temperature, and a MeOH solution (50 mL), containing a required amount of chloramine T, was then added. AcOH was then added to the resulting heterogeneous solution. The resulting mixture was cooled to room temperature, and the light oil was then recovered from the MeOH solution by decantation with several water washings. To clarify the relative desulfurization reactivity of the sulfur com-

10.1021/ie010620o CCC: $22.00 © 2002 American Chemical Society Published on Web 07/17/2002

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Figure 2. Correlation between the desulfurization rate constant, k, for DBTs from tetradecane and the net electron density on the sulfur atom, as estimated by MO calculation.3 The net electron density is the sum of all the occupied orbitals on the sulfur atom. Key: 1, DBT; 2, 4-methyl-DBT; 3, 4,6-dimethyl-DBT. Desulfurization conditions: temperature, 323 K; tetradecane volume, 50 mL; MeOH volume, 10 mL; AcOH volume, 1 mL; [DBTs]initial in tetradecane ) 11 mM (0.55 mmol); [chloramine T] ) 1.1 mmol.

that the desulfurization rate of DBTs from tetradecane may be expressed as

[DBT] ) [DBT]initial exp(-kt) Figure 1. Reaction pathways for (a) DBT and (b) BT with chloramine T.

pounds, n-tetradecane solutions, each containing an individual sulfur compound (11 mM), corresponding to a sulfur content of 0.05 wt %, were also employed as model light oils. Recovery experiments of the resulting sulfimides, from the light oils following reaction with chloramine T, were conducted by adsorption using Al2O3 (particle size 45-150 µm, surface area 107.6 m2/g) at 298 K for 30 min with stirring. Results and Discussion 1. Desulfurization of Sulfur Compounds from Tetradecane. 1.1. Desulfurization Reactivity of DBT and BT. The S-imidation process, occurring between DBT and chloramine T, is described in detail in previous work,1 and only a brief description is made here. DBT dissolved in tetradecane is converted, by the reaction with chloramine T dissolved in MeOH in the presence of catalytic amount of AcOH, to form the corresponding sulfimide, which is removed into the MeOH solution. Both MeOH and chloramine T are insoluble in the nonpolar tetradecane, and the resulting tetradecane is recovered easily by simple decantation. The sulfimide of the DBT, removed into MeOH solution, is recovered successfully as a precipitate, formed by the addition of water to the resulting MeOH solution. As shown in Figure 1a,4,5 the S-imidation of DBT is initiated by chlorination of the nucleophilic sulfur atom for DBT 3 by the free state chloramine T 2, produced by the hydrolysis of the chloramine T 1. The intermediate 4, of which the sulfur atom for 3 is chlorinated, is then converted to sulfimides 5. The present reaction is accelerated with increasing concentration of AcOH and chloramine T. The S-imidation rate for DBTs exhibits a first-order dependence on the initial DBT concentration,1,4,5 such

(1)

where k is the first-order rate constant for DBTs and t is the reaction time. The rate constant for the desulfurization of DBT, in the presence of a 2-fold molar excess of chloramine T, based on the initial concentration of DBT, was estimated by a least-squares fitting to be 0.445 h-1. An Arrhenius plot for the desulfurization of DBT gave a straight line relationship, from which the activation energy for the desulfurization of DBT was estimated as 42.3 kJ/mol. As shown in Figure 1a, the present reaction is initiated by the chlorination of the DBT by free state chloramine T, such that the reactivity for the DBTs should depend on the net electron density on the sulfur atom for the DBTs. As shown in Figure 2, a linear relationship is obtained between the desulfurization rate constant, k, and the net electron density on the sulfur atom for DBTs, as obtained by semiempirical MO calculation.3 This indicates that the DBTs, of high net electron density on their sulfur atom, are desulfurized more effectively according to the present process. The MO calculation data, obtained in the previous study,3 demonstrated that the electron density values for DBTs increase with increasing carbon number of the alkyl substituents. It is thus to be expected that when the present process is applied to the desulfurization of actual light oils, the DBTs, having alkyl substituents of high carbon number, will be desulfurized more easily than those with the alkyl substituents of low carbon number. The desulfurization yield for BT from tetradecane was found to be only 13.5%, following 8 h of reaction, in the presence of 2-fold molar excess of chloramine T, based on the initial sulfur concentration of tetradecane, whereas almost all the DBT was removed by the same desulfurization conditions. When water was added to the resulting MeOH solution, the precipitate formed was analyzed by 1H and 13C NMR, following recrystallization in MeOH/hexane. Zero formation of sulfimide of the BT was detected, thus suggesting that the reaction of BT

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with chloramine T does not produce the corresponding sulfimide. Svoronous has also reported the synthesis of the sulfimides of BT using chloramine T is unsuccessful.6 To identify the products for BT, the precipitate was dissolved in dichloromethane and analyzed by GC/MS. The analysis demonstrated two peaks, having molecular ions at m/z 170 and 171, which were identified respectively from their fragmentation patterns as 2-chloro2,3-dihydro-BT, 8, and p-toluenesulfoneamide, 9, according to the mechanism as shown in Figure 1b. Damin et al.7 have reported that the reaction of olefins with chloramine T, in the presence of AcOH and MeOH, produced the corresponding chlorinated paraffins and p-toluenesulfoneamide, via the formation of chlorineadduct intermediates. As described previously,8 the BT has an unsaturated bond on the C2-C3 position of the molecule, which has a high electron density, such that halogen addition occurs easily. Thus, as shown in Figure 1b, in the reaction of BT 6 with chloramine T 1, BT does not form the S-chlorinated intermediate but instead forms the chlorine-adduct intermediate 7 on its unsaturated bond by reaction with the free state chloramine T 2. The intermediate 7 is then hydrolyzed to give rise to the compounds 8 and 9. The GC analysis for the tetradecane demonstrated no peaks other than those for BT, thus indicating that the products of BT are removed into the MeOH solution, by virtue of their high polarity. 1.2. Effect of Aromatic Hydrocarbons on Desulfurization. The reaction products of the aromatic hydrocarbons, formed during the present process, were then identified. Tetralin and naphthalene were used as model components3,8 and were treated with chloramine T dissolved in MeOH, in the presence of AcOH at 323 K. The resulting materials were extracted with dichloromethane, washed with water, and concentrated by evaporation. GC/MS analysis of the products for naphthalene demonstrated the formation of p-toluenesulfoneamide, 9 (Figure 3a), as was also the case for BT, and also detected a peak, having a molecular ion at m/z 162. This compound was confirmed from its fragmentation pattern to be 1-chloronaphthalene, 11. It is reported that aromatic compounds, such as aniline and 1-methoxynaphthalene, react with chloramine T in aqueous AcOH solution to give rise to their corresponding chlorosubstituted compounds on the aromatic ring, as the major products, accompanying the production of ptoluenesulfoneamide.9,10 Thus, the reaction of naphthalene 10 proceeds via the direct substitution of hydrogen atom on naphthalene by the chlorine atom on the free state chloramine T 2, as shown in Figure 3a. Astonishingly, as shown in Figure 1b, BT gave rise to a chlorineadduct product, while the naphthalene gave rise to a chlorine-substituted product. Since the unsaturated bond on the thiophene ring for BT has both aromatic and olefinic properties, the chlorine-adduct product is therefore probably formed from the BT, in the same manner as for olefins.7 GC/MS analysis of the products for tetralin also demonstrated the production of p-toluenesulfoneamide, 9, and detected the presence of various kinds of chlorinesubstituted or -nonsubstituted compounds, having molecular ions at m/z 161, 148, 146, 166, 160, 196, 192, 178, and 180, respectively, depending on the retention time order. These compounds were identified from their fragmentation patterns as shown in Figure 3b. From the distribution of the products, it is thought the

Figure 3. Reaction pathways for (a) naphthalene and (b) tetralin with chloramine T.

reaction of tetralin 12 is probably initiated by the direct substitution of the hydrogen atom on tetralin by the free state chloramine T 2, which then produces the 1-chloro1,2,3,4-tetrahydronaphthalene, 13, to accompanying the formation of p-toluenesulfoneamide, 9. The chlorine atom on compound 13 is then substituted by solvolysis with MeOH or water to give rise to the compounds, 1-methoxy-1,2,3,4-tetrahydronaphthalene, 14, 1,2,3,4tetrahydro-1-naphthol, 15, and 1,2,3,4-tetrahydronaphthalene-1-one, 16. Compound 14 is then chlorinated by 2 to form 1-chloro-4-methoxy-1,2,3,4-tetrahydronaphthalene, 17, which is then converted by solvolysis to 1-methoxy-1,2,3,4-tetrahydronaphtlanene-4-ol, 18, and 1,4-dimethoxy-1,2,3,4-tetrahydronaphthalene, 19. Compound 16 is converted, as in the same manner as compound 14, to produce 1-chloro-1,2,3,4-tetrahydronaphthalene-4-one, 20, and 1,2,3,4-tetrahydronaphthalene1,4-dione, 21. The major product of the tetralin, in this reaction, was found as compound 14. No reaction products from compound 15 were detected in this analysis. The results obtained suggest that the reaction of one-ring aromatics with chloramine T does not occur on the aromatic ring but rather occurs on the naphthenic ring of the molecule. The effect of aromatic hydrocarbons on the desulfurization of DBT and BT from tetradecane was then studied. Tetralin and naphthalene were added to the tetradecane together with DBT or BT and used for the experimental studies. The results are summarized in Figure 4. Here, a reactivity ratio, η, the ratio of the desulfurization yield of DBTs and BTs in the presence

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Figure 4. Variation in the reactivity ratio, η, for DBT (circle symbols) and BT (square symbols), in tetradecane and in the presence of naphthalene (closed symbols) or tetralin (open symbols). The value for η is defined as the ratio of the obtained desulfurization yield for the sulfur compounds, obtained in the presence and in the absence of aromatics and defined in eq 2. Reaction conditions: time, 8 h; temperature, 323 K; tetradecane volume, 50 mL; MeOH volume, 10 mL; AcOH volume ) 1 mL; [sulfur compounds]initial in tetradecane ) 11 mM (0.55 mmol); [chloramine T] ) 1.1 mmol.

Figure 5. Time-course variation in the residual percentage of sulfur in light oils, during reaction with chloramine T. Reaction conditions: temperature, 323 K; light oil volume, 50 mL; MeOH volume, 50 mL; AcOH volume, 2 mL; [chloramine T] ) 2-fold molar excess based on the initial sulfur concentration of the feed light oils.

of aromatics to that obtained in the absence of aromatics,3,8 is defined as

η ) [desulfurization yield of DBT or BT in the presence of aromatics]/ [desulfurization yield of DBT or BT in the absence of aromatics] (2) The reactivity ratios, η, for both DBT and BT are seen to decrease with an increasing concentration of the added aromatics. This is because these aromatics are chlorinated by the reaction with chloramine T, in a competitive manner to the chlorination of the sulfur compounds. The decrease in the reactivity ratio is greater for BT than that for DBT. This is because the chlorination of the unsaturated bond for BT proceeds much more slowly than that for the sulfur atom for the DBT. The addition of naphthalene was found to prevent the desulfurization for both DBT and BT much more significantly than with tetralin. Thus, when the present process is applied to the desulfurization of actual light oils, the DBTs and BTs in high-aromatic-content light oils may be expected to be much more difficult to desulfurize than for low-aromatic-content oils. 2. Desulfurization of Light Oils. 2.1. Reactivity of Different Light Oils with Chloramine T. The desulfurization reactivity of actual light oils, such as LGO, CLO, and LCO, with differing sulfur and aromatic concentrations was then studied. Time-course variations in the remaining percentage of sulfur in the light oils; the reaction being carried out in the presence of 2-fold molar excess of chloramine T, based on the initial sulfur concentration of the feed light oils, are shown in Figure 5. The residual percentage of sulfur for CLO is seen to decrease at first as the reaction time increases but is then almost saturated at times greater than 10 h. The desulfurization for LGO also proceeds with time at first, but the residual percentage then increases for times of 10-30 h. The residual percentage for LCO was found to be significantly greater than that for LGO and CLO, and the percentage also increased for times of 10-

Figure 6. Variation in the residual percentage of sulfur in light oils, during reaction in the presence of differing quantities of chloramine T. Reaction conditions: temperature, 323 K; time, 10 h; light oil volume, 50 mL; MeOH volume, 50 mL; AcOH volume, 2 mL.

30 h, as also the case for LGO. Figure 6 shows the variation in the residual percentage of sulfur in the light oils, following 10 h of reaction in the presence of differing quantities of chloramine T. The residual percentages for all the light oils are thus shown not to be decreased effectively, even by the addition of large quantities of chloramine T. The desulfurization efficiency for light oils lies in the ranking order LGO > CLO > LCO. The desulfurization efficiencies obtained are thus significantly lower than would be expected from the model experiments. As described previously,1 the sulfimides of DBTs, having a large carbon number of hydrophobic alkyl substituents, formed by the reaction with chloramine T, have a low polarity, such that these are not removed into the MeOH solution but remain in the resulting light oil. The low desulfurization efficiency for light oils, obtained in Figures 5 and 6, thus results owing to the accumulation of the sulfimides of DBTs in the resulting light oils. In the present process, NaCl is formed during the course of reaction, as shown in Figure 1a, and dissolved in MeOH solution. The solubility of the sulfimides in MeOH thus decreases with time. The increase in sulfur concentration of CLO and LGO, as shown in Figure 5, probably results because of the

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Table 1. Quantities of Alkyl-Substituted BTs and DBTs in (i) Feed and (ii) Treated Light Oils following the Reaction with Chloramine Ta straight-run light gas oil (LGO)

commercial light oil (CLO)

light cycle oil (LCO)

species

(i) (wt %)

(ii) (wt %)

(i) (wt %)

(ii) (wt %)

(i) (wt %)

(ii) (wt %)

C6 BT BTs (total) DBT C1 DBT C2 DBT C3 DBT C4 DBT C5 DBT C6 DBT DBTs (total) total sulfur

0.02424 0.03230 0.11545 0.13036 0.08182 0.17938 0.56355 0.03258 0.09325 0.14333 0.14400 0.09050 0.02664 0.28614 0.81644 1.37999

0.01304 0.01713 0.05434 0.06996 0.03657 0.08357 0.27461 0.00063 0.02059 0.02850 0.02788 0.01234 0.00316 0.02449 0.11769 0.39230

0.00721 0.00464 0.00965 0.01022 0.00405 0.01319 0.04896 0.00301 0.01475 0.01840 0.01777 0.01223 0.00361 0.06058 0.13035 0.17900

0.00483 0.00294 0.00515 0.00565 0.00193 0.00615 0.02665 0.00123 0.00470 0.00764 0.00704 0.00417 0.00132 0.02135 0.04745 0.07410

0.01454 0.01069 0.01124 0.00783 0.00248 0.00696 0.05374 0.00501 0.01706 0.01521 0.00550 0.00189 0.00051 0.03308 0.07826 0.13200

0.00233 0.00844 0.00878 0.00831 0.00233 0.00447 0.03467 0.00284 0.00893 0.00699 0.00290 0.00105 0.00020 0.01222 0.03513 0.06980

a Desulfurization conditions: time, 10 h; temperature, 323 K; [chloramine T] ) 2-fold molar excesses based on initial sulfur concentration of the feed light oils; light oil volume, 50 mL; MeOH volume, 50 mL; AcOH, 2 mL. b CLO > LGO. This order is the same as that for the aromatic concentration of light oils, as described previously.3 The polarity of the light oil increases with increasing aromatic concentration,3 and the solubility of sulfimides in the light oil increases accordingly, such that the high-aromatic-content light oil contains a large quantity of the sulfimides of the DBTs. The net reaction yield for the sulfides [{1 - (sulfides in light oils following reaction/total sulfur in feed light oils)} × 100] was estimated as 71.6% for LGO, 58.6% for CLO, and 47.1% for LCO, respectively. This suggests that the sulfides in high-aromatic-content light oil are more difficult to desulfurize than those in low-aromaticcontent oil. This is because the aromatics are chlorinated competitively by reaction with chloramine T, together with the chlorination of the sulfides, as shown in Figure 4. The reaction yields for DBTs and BTs [{1 - (DBTs or BTs in light oils following reaction/DBTs or BTs in feed light oils)} × 100] were estimated as 85.6% and 51.3% for LGO, 63.6% and 45.6% for CLO, and 55.1% and 35.5% for LCO. This indicates that the

Figure 7. Residual percentage of alkyl-substituted (a) DBTs and (b) BTs in light oils, following reaction with chloramine T, as a function of the carbon number of the alkyl substituents, n for DBTs and m for BTs. The initial amount of each alkyl-substituted DBTs and BTs in the feed light oils, as shown in Table 1.i, is set as 100%. Reaction conditions: temperature, 323 K; time, 10 h; light oil volume, 50 mL; MeOH volume, 50 mL; AcOH volume, 2 mL; [chloramine T] ) 2-fold molar excess based on the initial concentration of the feed light oils.

reaction yield obtained for DBTs is greater than that for BTs for all the light oils and that the yields of DBTs and BTs obtained from high-aromatic-content light oils are lower than those from low-aromatic-content oil. The

Ind. Eng. Chem. Res., Vol. 41, No. 17, 2002 4381 Table 2. Variation in Concentration of Chlorine in Light Oils, following (a) Reaction with Chloramine Ta and (b) Subsequent Adsorption by Aluminum Oxideb (a)

feedstocks LGO CLO LCO

b

Figure 8. Variations in the residual percentage of sulfur in light oils, following adsorption using Al2O3, as a function of the quantity of the Al2O3 added. The resulting sulfur concentrations of the light oils, following adsorption, are also shown. Adsorption conditions: temperature, 298 K; time, 30 min. The light oils used for the experiments were obtained following 10 h of reaction with a 2-fold molar excess of chloramine T, as also employed for the results shown in Figure 5. The sulfur concentrations for each light oil were 0.4987 wt % for LGO, 0.0983 wt % for CLO, and 0.1169 wt % for LCO, respectively.

residual percentage for the individual sulfides in the light oil following reaction with chloramine T is shown in Figure 7. It is seen that these have a tendency to decrease with increasing carbon number of the alkyl substituents, n for DBT and m for BT. These results thus reveal that the present process desulfurizes the highly alkyl-substituted BTs and DBTs more effectively. This observation for DBTs also agrees reasonably well with that predicted by MO calculation results, as described previously.3 2.2. Adsorption of Sulfimides Remaining in Light Oils and the Property of the Product Oils. It is obviously important to remove the sulfimides of the DBTs, remaining in light oils following reaction with chloramine T, to achieve deep desulfurization. As reported previously,1 sulfimides can be removed successfully by the addition of solid adsorbents, such as silica gel and Al2O3, whereas an extraction technique, based on water-soluble polar solvents, such as DMSO, DMF, and acetonitrile, was found to be ineffective. The removal efficiency of sulfimides from three light oils based on adsorption was therefore chosen for further study. Light oils, obtained following 10 h of reaction with 2-fold molar excess of chloramine T, based on the initial sulfur concentration of the feed light oils, were used in the experiments. Al2O3 was added to the light oils and stirred for 30 min at 298 K. The variations in the residual percentage of sulfur, in the light oils, as a function of the quantity of the Al2O3 added, are summarized in Figure 8. The residual percentage of sulfur in all the light oils decreased with increasing quantity of Al2O3. The desulfurization yield obtained was found to lie in the ranking order CLO > LGO > LCO, with the amount of sulfur, adsorbed per unit weight of Al2O3, being 32.7 g/kg for LGO, 16.1 g/kg for CLO, and 7.60 g/kg for LCO, respectively. This shows that the removal efficiency of sulfur by Al2O3 increases with a decreasing aromatic concentration in the light oils, following a ranking order LGO > CLO > LCO3 and thus suggesting that the removal of sulfimides from high-aromaticcontent (high polarity) light oils is relatively difficult to achieve.

chloramine T (mol/mol-sulfur in feed light oils)

chlorine concn (ppm)

(b) chlorine concn (ppm)

2 10 2 10 2 10

1230 5431 0 3110 3132 6019

1166 5340 0 2871 3078 5954

a Desulfurization conditions are identical to those in Figure 5. Adsorption conditions are identical to those in Figure 8.

GC-AED analysis for the light oils, following adsorption using 0.3 g/mL oil of Al2O3, gave removal yields of sulfides [1 - (sulfides in light oil following adsorption/ sulfides in light oil before adsorption) × 100] as 8% for LGO, 11% for CLO, and 17% for LCO, thus indicating that almost all the sulfur, removed by the adsorption, is attributed to the sulfimides of the DBTs. As indicated in Figure 8, the sulfur concentration for CLO however was decreased successfully to below the target level (0.05 wt %), following reaction with chloramine T and subsequent adsorption of the resulting sulfimides. The sulfur concentrations for LGO and LCO however failed to reach target level. To achieve deep desulfurization, further reaction, using a larger amount of chloramine T and subsequent adsorption with a larger amount of adsorbent, is therefore necessary. The proposed process comprises only very simple stages, carried out under moderate conditions at atmospheric pressure, and is thus applicable as an effective, safe, and energy-efficient desulfurization process for light oils. As shown in Figure 3, the aromatic hydrocarbons present in light oils are chlorinated by reaction with chloramine T during the desulfurization. Also as shown in Table 2a, although the chlorine concentration in CLO was nearly zero, during reaction in the presence of a 2-fold molar excess of chloramine T, based on the initial sulfur concentration of CLO, the concentration was increased much larger when using 10-fold molar excess of chloramine T. The chlorine concentrations in LGO and LCO, following reaction in the presence of both 2and 10-fold chloramine T, are higher than those of CLO. LGO contains a larger quantity of the sulfur compounds than CLO, and a considerable amount of chloramine T is required for desulfurization, thus causing the chlorination of aromatics inevitably. The sulfur concentration of LCO is, however, comparable to that of CLO but contains a larger amount aromatics, and as a result, the chlorination of aromatics proceeds more significantly for LCO than for CLO. Since these chlorinated compounds have a higher polarity than the other constituents of light oils, the concentrations of chlorine are decreased slightly by the addition of Al2O3, as shown in Table 2b. However, almost all of the chlorine remains in the resulting light oils. The remaining chlorine has a detrimental effect on the properties of light oils, such that these must be removed by sequential refining processes (e.g., hydrotreating). These findings suggest that the addition of a large amount of chloramine T causes inevitably the chlorination of aromatic hydrocarbons, such that the present process should be applied to the desulfurization of low-sulfur- and low-aromaticcontent light oils.

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Conclusion

Nomenclature

A desulfurization process for light oils, based on the removal of sulfimides produced by the reaction of sulfurcontaining compounds with chloramine T, has been investigated, and the following results have been obtained. (1) DBTs, when dissolved in tetradecane, are converted by reaction with chloramine T dissolved in MeOH, to form the corresponding sulfimides, which are removed into the MeOH solution. The desulfurization reactivity for DBTs depends on the net electron density on the sulfur atom. The reaction of BTs with chloramine T, on the contrary, gave rise to the corresponding chlorine-adduct product. (2) The aromatic hydrocarbons in light oils were chlorinated during the present desulfurization process to form the corresponding chlorine-substituted and -adduct products. The latter were then converted by solvolysis into several products having methoxyl, hydroxyl, and carbonyl groups. The desulfurization of sulfur compounds was suppressed by an increasing the concentration of the aromatics. (3) The desulfurization of light oils, by reaction with chloramine T, is unsuccessful, owing to an accumulation of the sulfimides, produced during reaction, in the resulting light oil. The desulfurization of high-aromaticcontent light oil is relatively difficult, with the sulfur compounds, having a low carbon number of alkyl substituents, being the most difficult compounds to desulfurize. (4) The sulfimides, remaining in resulting light oil, are removed successfully by subsequent adsorption, using solid adsorbents such as Al2O3 and silica gel. The removal of sulfimides remaining in high-aromaticcontent light oil was however relatively difficult. GCAED analysis showed that a considerable quantity of chlorinated compounds remains in the resulting light oils.

k ) desulfurization rate constant for DBTs, h-1 n ) carbon number of alkyl substituent on DBT molecule m ) carbon number of alkyl substituent on BT molecule η ) reactivity ratio of sulfur-containing compounds defined in eq 2

Acknowledgment The authors are grateful for the financial support of Grants-in-Aid for Scientific Research (No. 12555215) from the Ministry of Education, Culture, Sports, Science and Technology and to the Division of Chemical Engineering for the Lend-Lease Laboratory System.

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Resubmitted for review December 30, 2001 Revised manuscript received May 24, 2002 Accepted June 2, 2002 IE010620O