Photochemical Desulfurization and Denitrogenation Process for

Ind. Eng. Chem. Res. 2001, 40, 293-303

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Photochemical Desulfurization and Denitrogenation Process for Vacuum Gas Oil Using an Organic Two-Phase Extraction System Yasuhiro Shiraishi,† Takayuki Hirai,*,† and Isao Komasawa†,‡ Department of Chemical Science and Engineering, Graduate School of Engineering Science, and Research Center for Photoenergetics of Organic Materials, Osaka University, Toyonaka 560-8531, Japan

The simultaneous removal of sulfur- and nitrogen-containing compounds from vacuum gas oil (VGO), based on the combination of UV irradiation and liquid-liquid extraction using an oil/ acetonitrile two-phase system, has been investigated. Detailed desulfurization and denitrogenation reactivities for the various compounds in VGO have also been studied by means of field ionization mass spectrometry (FI-MS) and gas chromatography with atomic emission detection (GC-AED), respectively. When VGO and acetonitrile are mixed and photoirradiated, the sulfur and nitrogen compounds in the VGO are distributed into the acetonitrile phase and photodecomposed there to form highly polarized compounds, which do not distribute into the lowerpolarity VGO. The successive removal of these compounds from VGO to acetonitrile can therefore be carried out under moderate conditions, such that the sulfur and nitrogen contents for the VGO are decreased simultaneously to less than 1% of the feed values. FI-MS and GC-AED analyses reveal that dinaphthothiophenes, tetrahydrodinaphthothiophenes, tetrahydrodibenzothiophenes, and carbazoles, having large carbon numbers of the alkyl substituents, are the most difficult compounds to remove by the present photochemical organic two-phase extraction process. The removal efficiencies for sulfur and nitrogen compounds obtained from VGO were compared with the results obtained for light oil feedstocks, and the applicability of the present process to the refining of VGO was examined. From a consideration of the above results, a fully organized overall refining process for petroleum-derived middle and heavy feedstocks has been developed. Introduction Because of the continuous depletion of petroleum resources and the consequent increase in oil prices, heavy petroleum feedstocks are becoming increasingly more attractive as precursors for the lighter feedstocks such as gasoline and light oil. Vacuum gas oil (VGO), one of the heavier feedstocks, is produced by the vacuum distillation of atmospheric residue and is utilized as a main source for the production of catalytic-cracked gasoline and light cycle oil.1,2 Because VGO contains large amounts of sulfur- and nitrogen-containing compounds, further sequential desulfurization and denitrogenation treatment processes for the resulting lighter feedstocks are inevitably required in order to protect the environment against contamination by sulfur- and nitrogen-oxide emissions (SOx and NOx). The removal of sulfur and nitrogen species from VGO is presently carried out via catalytic hydrodesulfurization (HDS) and simultaneous hydrodenitrogenation (HDN) processes. The above species consist of compounds of higher molecular weight than those in the lighter feedstocks, and their removal is more difficult.3-5 The further refining of the VGO, therefore, requires rather severe conditions and specially active catalysts. An alternative process, able to remove the sulfur and nitrogen compounds simultaneously from the VGO under moderate * To whom correspondence should be addressed. E-mail: [email protected]. Fax: +81-6-6850-6273. Tel.: +816-6850-6272. † Department of Chemical Science and Engineering. ‡ Research Center for Photoenergetics of Organic Materials.

conditions and without requirements for hydrogen and a catalyst, is therefore required. A novel desulfurization and simultaneous denitrogenation process for light oil feedstocks, based on the combination of UV irradiation by high-pressure mercury lamp and liquid-liquid extraction using an oil/acetonitrile two-phase system, has been investigated in previous studies.6-9 In this system, the sulfur- and nitrogencontaining compounds in light oils, having relatively large polarities, are distributed into the polar acetonitrile phase. There, they are photodecomposed to form highly polarized compounds, which do not distribute into the nonpolar light oil phase. In this way, the simultaneous and successive removal of sulfur and nitrogen compounds from light oil to acetonitrile can thus be carried out under moderate conditions of operation. In the present work, the above photochemical process has now been applied to the desulfurization and simultaneous denitrogenation of VGO. To clarify the applicability of the present process, the desulfurization and denitrogenation efficiencies for VGO have been compared with previous results obtained for light oil feedstocks. The relative reactivities for the individual sulfur- and nitrogen-containing compounds in VGO were examined in detail by means of FI-MS and GCAED analytical instruments, respectively. Finally, a possible overall refining scheme for petroleum-derived middle and heavy feedstocks has been fully organized, based on a combination of the novel refining processes proposed in our present series of works, together with current petroleum refining processes employed in refinery practice.

10.1021/ie000707u CCC: $20.00 © 2001 American Chemical Society Published on Web 12/08/2000

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Ind. Eng. Chem. Res., Vol. 40, No. 1, 2001

Table 1. Properties and Composition of the VGO density at 288 Ka viscosity at 323 Kb hydrogen carbon sulfur nitrogen vanadium nickel

(g/mL) (mm2/s) (wt %) (wt %) (wt %) (ppm) (ppm) (ppm)

0.9194 31.8 13.15 84.48 1.9899 823.0 0.11 two-ring distillation IBP 10 vol % 20 vol % 30 vol % 50 vol % 70 vol % 80 vol %

(vol %) (vol %)

63.1 20.9 11.7

(K) 480 652 678 704 719 781 801

a

Analysis carried out according to the method JIS K 2249. Analysis carried out according to the method JIS K 2283. c Analysis carried out according to the method JPI-5S-49-97. b

Experimental Section 1. Materials and Procedure. Vacuum gas oil (VGO), produced by vacuum distillation of atmospheric residue, was supplied by Cosmo Petroleum Institute. The properties of the VGO are summarized in Table 1, with a boiling point range distribution from 480 K to higher than 801 K and sulfur and nitrogen concentrations of 1.9890 wt % and 823 ppm (0.0823 wt %), respectively. The VGO also contains small amounts of vanadium and nickel compounds. Acetonitrile, n-decane, n-hexane, dichloromethane, and methanol solvents were supplied by Wako Pure Chemical Industries, Ltd., and were used as received. The photoreaction experiments were performed in the same manner as previously described for light oil feedstocks.6,8,9 Because the VGO is a highly viscous liquid at room temperature, it was diluted with an n-decane solvent at various volume ratios varying from 1/2 to 1/9 and the resultant solutions used for the photoreaction experiments in order to ensure fluidity of the VGO. The diluted VGO/decane solutions were mixed vigorously by a magnetic stirrer with acetonitrile or an acetonitrile/water solution at volume ratios varying from 200/200 to 80/320 to give a total solution volume of 400 mL. The solutions were then photoirradiated by immersion of a high-pressure mercury lamp (300 W, Eikohsha Co., Ltd., Osaka, Japan) combined with air bubbling (500 mL/min) under atmospheric pressure. The temperature of the reaction mixture during photoirradiation was about 323 K. 2. Analysis. The total sulfur concentration of the VGO was analyzed by an inductively coupled argon plasma atomic emission spectrophotometer (Nippon Jarrell-Ash ICAP-575 Mark II), using tetralin (tetrahydronaphthalene) as the carrier solvent. The concentration of each individual sulfur-containing compound in the VGO was analyzed quantitatively by a field ionization mass spectrometer (FI-MS, JEOL JMSAX505H), in accordance with the procedure of Aoyagi et al.,10,11 following fractionation from the VGO. The analytical conditions used for the FI-MS determinations were as follows: ion source temperature, 323 K; ion multi voltage, 1.5 kV; probe temperature, 323-723 K (32 K/min, 10 min); scan speed, 7 min/scan; scan region, 0-2000 m/z; acceleration voltage, 3.0 kV; opposite-side

Figure 1. Schematic diagram for the fractionation and analytical procedure employed for VGO.

voltage, -5.5 kV. The total nitrogen concentration of the VGO was measured by a Total Nitrogen Analyzer (Mitsubishi Chemical Industry, TN-05), following dissolution in toluene.9 The GC-MS analyses for the nitrogen-containing compounds were carried out with a JEOL JMS-DX303HF mass spectrometer, using the electron impact (EI) ionization (70 eV) method previously described.7 The concentration of each individual nitrogen-containing compound in VGO was analyzed by gas chromatography with atomic emission detection (GC-AED, Hewlett-Packard 6890, equipped with AED G2350A), using an HP-1 column (i.d., 0.32 mm; film thickness, 0.17 µm; length, 25 m), according to the previously described procedure,9 following fractionation from the VGO. 3. Fractionation of Sulfur- and Nitrogen-Containing Compounds from VGO. The sulfur- and nitrogen-containing compounds in the VGO were fractionated and concentrated, prior to the FI-MS and GCAED analyses with slight modifications, according to several procedures from the literature.12-16 The fractionation diagram for the VGO is shown schematically in Figure 1, where the separation sequence is as follows: The VGO (ca. 10 g) sample was first adsorbed onto 80 g of activated neutral aluminum oxide packed in a glass column (i.d., 20 mm; length, 450 mm). The sample was then eluted by n-hexane (100 mL) for the saturated hydrocarbon fraction and by n-hexane/dichloromethane (60/40 v/v, 100 mL) for the aromatic and sulfur compound fraction. The nitrogen compounds and polar compounds were then eluted using dichloromethane (200 mL) and methanol (200 mL), respectively. Ninetynine percent of the sulfur and 97% of the nitrogen compounds in VGO were contained in the second and third fractions, respectively, and almost all of the vanadium and nickel compounds were contained in the final polar fraction. The third nitrogen fraction was diluted in dichloromethane and analyzed by GC-AED. The sulfur compounds in the second fraction were further separated from the aromatic compounds and were concentrated by means of ligand-exchange chromatography (Hewlett-Packard HP-1100, equipped with a spectrophotometric detector), with monitoring at a wavelength of 350 nm, in accordance with the methods

Ind. Eng. Chem. Res., Vol. 40, No. 1, 2001 295

Figure 2. LC chromatogram (detection ) 350 nm) for aromatic and sulfur compounds fraction obtained following fractionation from the VGO.

of Aoyagi et al.10,11 and Nishioka et al.15 The separations were performed on a Pd-impregnated silica gel column (GL Science, UnisilQ; i.d., 7.6 mm; length, 250 mm), using chloroform as the carrier solvent, with a flow rate of 1.0 mL/min and a sample injection volume of 100 µL. A typical LC chromatogram for the second fraction is shown in Figure 2. The sulfur compound fraction, eluted at 11-100 min, was collected and concentrated by evaporating the dichrolomethane. The part of the resulting fraction thus obtained was then analyzed by FI-MS. Results and Discussion 1. Desulfurization of VGO. 1.1. Desulfurization Behavior of VGO. The VGO/decane solution was mixed vigorously with acetonitrile and photoirradiated with air bubbling through the solution. The resulting timecourse variation in the total sulfur content of the VGO/ decane solution is shown in Figure 3 as a function of differing VGO/decane and VGO solution/acetonitrile volume ratios. The data points indicated for zero irradiation time are those obtained by simply mixing the two phases, and they thus show the equilibrium distribution concentrations for the sulfur contents of the two phases. The sulfur content of the VGO solution is shown to decrease slightly by contact with acetonitrile but then to decrease dramatically with photoirradiation time. This behavior indicates that the sulfur compounds in the VGO distribute into the polar acetonitrile phase and that, there, they are photodecomposed into highly polarized compounds, such as sulfoxide, sulfone, osulfobenzoic acid, and dicarboxylic acid. These compounds do not distribute into the nonpolar VGO phase, such that they remain in the polar acetonitrile phase, as was also found for the desulfurization of light oils.6-8 These results suggest that the present photochemical process is therefore also effective for the desulfurization of VGO as well as for that of light oils. As shown in Figure 3a-c, the desulfurization yield for VGO is seen to increase with decreasing VGO solution/acetonitrile volume ratio. The photodecomposition of the sulfur compounds proceeds mainly in the polar acetonitrile phase, and thus, when the immiscible oil/acetonitrile two phases are mixed and photoirradiated, photoscattering and light exclusion inevitably occur. The increase in acetonitrile volume suppresses these negative effects, and as a result, the desulfurization efficiency for the VGO is increased substantially.

Figure 3. Time-course variation in the total sulfur content of the VGO/decane solution during the photoirradiation of an oil/acetonitrile two-phase system. The VGO/decane volume ratios are (a) 1/2, (b) 1/4, and (c) 1/9.

Thus, at a VGO solution/acetonitrile volume ratio of 1/5 and 10 h of photoirradiation, the sulfur content of the VGO was decreased successfully to less than 22%, 13%, and 1% of the feed concentration, for the respective conditions of VGO/decane volume ratios of 1/2, 1/4, and 1/9. Compared to these values, the current HDS process is reported to decrease the sulfur content of VGO to 14% of the feed value, for a reaction temperature of 633 K and a hydrogen pressure of 8 MPa.10 The present results therefore indicate that the photochemical process is comparatively more energy-efficient and also more effective for the desulfurization of the VGO than the HDS process. As shown in Figure 3a-c, the desulfurization yield for the VGO solution is increased by decreasing the VGO/decane volume ratio. The VGO solution strongly absorbs UV light at wavelengths of 200-300 nm, which is essential for the photodecomposition of the sulfur-containing compounds.6,7 The increase in the desulfurization yield for VGO that was

296


Figure 4. Variation in the sulfur distribution ratio, DS, for VGO, commercial light oil (CLO), straight-run light gas oil (LGO), and light cycle oil (LCO), with respect to the acetonitrile/oil volume ratio, following both simple extraction (VGO/decane volume ratio ) 1/9) and also 2 h of photoirradiation. The values for DS representing the ratio of the total sulfur content of acetonitrile and of oils are as defined by eq 1. The data for CLO, LGO, and LCO are cited from previous papers.6,8

obtained thus probably results because the increase in decane volume decreases the amount of photoscattering and light exclusion, as found also for a decreasing VGO solution/acetonitrile volume ratio. To clarify the desulfurization efficiency for VGO, changes in the sulfur distribution ratio, DS, obtained during simple extraction with 2 h of photoirradiation (VGO/decane volume ratio ) 1/9), were compared with the results obtained for light oils, such as commercial light oil (CLO), straight-run light gas oil (LGO), and light cycle oil (LCO), used for previous studies.6,8,9 CLO and LGO contain mainly saturated hydrocarbons (70%), and LCO, produced by the fluid catalytic-cracking of VGO, contains as much as 75% aromatic hydrocarbons. When LCO is put into contact with acetonitrile, the resulting volume of the LCO is significantly less than that obtained for both CLO and LGO, which is caused by the distribution of a higher proportion of aromatic hydrocarbons from the LCO into the acetonitrile phase, as a result of their high polarity. The LCO, after making contact with acetonitrile, was therefore almost completely recovered by the addition of a volume of water equal to that of the feed acetonitrile at 303 K.8 The distribution ratio results are summarized in Figure 4 as a function of acetonitrile/oil volume ratio, where the data for light oils are from previous papers6,8 and the values for DS are defined as

DS )

[sulfur]in acetonitrile [sulfur]in oil

(1)

For simple extraction without photoirradiation, the DS value for VGO is lower than those obtained for the lowaromatic-content CLO and LGO but higher than that for the high-aromatic-content LCO, with a ranking order of CLO > LGO > VGO > LCO. The light oils contain rather lower-molecular-weight sulfur compounds, such as benzothiophenes (BTs) and dibenzothiophenes (DBTs), than those present in VGO.10,11 The higher extraction yields for CLO and LGO thus result because the sulfur compounds, of lower molecular weight, are distributed

into the acetonitrile more easily than those of the higher-molecular-weight compounds in VGO because of their higher solubility.6 The DS value for LCO is also rather less than that for VGO. This is caused by the addition of water, at the end of the photoirradiation, which decreases the degree of distribution of the sulfur compounds into the acetonitrile. Under conditions of photoirradiation, the DS values for all of the oils are increased by increasing the acetonitrile/oil volume ratio, with the ranking order for DS remaining the same as that obtained by simple extraction. The results , therefore, suggest that the present photochemical process is less effective for the desulfurization of VGO than for the desulfurization of low-aromatic-content light oils but is more effective than for the desulfurization of higharomatic-content light oils. 1.2. Desulfurization Reactivity of Sulfur-Containing Compounds. The desulfurization reactivity of individual sulfur-containing compounds in VGO was then studied in detail by means of FI-MS. Based on the molecular weight data thus obtained, the sulfur compounds in VGO were classified into seven compound fractions. This was done in accordance with the methods of Ueda et al.17 and Aoyagi et al.10,11 as follows:

MW (molecular weight) ) 14n + U (n ) natural number; U ) even number, -10 e U e 2) (2) where sulfur compounds, having the same number of naphthenic and aromatic rings, are involved in each U value. The relationship between the U value and the compound structure is shown in Table 2, according to Aoyagi et al.10,11 The distributions of the sulfur compound fraction, for each value of U, in the feed and in the treated VGO are summarized in Figure 5. The results, as shown in Figure 5, demonstrate that the U ) +2 and -4 sulfur compounds are the main constituents of the feed VGO. Under conditions of simple extraction (VGO/decane and VGO solution/acetonitrile volume ratios ) 1/9 and 1/1, respectively), the sulfur compounds for U ) -4, -6, and -8 decreased in percentage terms, with a ranking order of U ) -4 > -6 > -8, but the other compounds increase to some extent. This behavior occurs because, when VGO is put into contact with acetonitrile, the other constituents of the VGO, such as aromatic hydrocarbons and nitrogencontaining compounds, are also distributed into the acetonitrile phase, and the total solution volume of the VGO is thereby reduced substantially.8,16 The above results show that the sulfur-containing compounds for U ) -4, -6, and -8, which have different numbers of aromatic ring present on the C2-C3 and C4-C5 positions of the thiophene skeleton, are extracted more easily into the acetonitrile than other compounds, because of their high polarity. Following 10 h of photoirradiation, the desulfurization efficiency for the various sulfur compound fractions is seen to lie in the ranking order of U ) -4 > -2 > 0 > +2 > -10 > -8 > -6, thus revealing that the sulfur compounds with U ) -10, -8, and -6 are the most difficult compounds to desulfurize by the present process. Figure 6a-g shows the molecular weight distribution of the individual sulfur-containing compounds for respective values of U both in the feed and in the treated VGO. As shown in Figure 6a, the U ) +2 sulfur compounds consist of two major compound fractions (n

Ind. Eng. Chem. Res., Vol. 40, No. 1, 2001 297 Table 2. Representative Structures of the Sulfur-Containing Compounds in VGO

Figure 5. Distribution in sulfur compound fractions, as defined by the U value, in the feed, and in the treated VGO (VGO/decane and VGO solution/acetonitrile volume ratio ) 1/9 and 1/1, respectively), following both simple extraction and 10 h of photoirradiation. The sum of the intensity ratio for all U values for the feed VGO (total sulfur concentration of the feed VGO) is set as 100%.

e 17 and n > 17). These are identified as alkylsubstituted DBTs (n e 17) and benzothiophenobenzothiophenes (BTBTs, n > 17), as indicated in Table 2.10 For simple extraction, the BTBTs are seen to be extracted into the acetonitrile more easily than the DBTs. However, following 10 h of photoirradiation, the DBTs are then removed more effectively than the BTBTs, thus indicating that the photodecomposition of the BTBTs is more difficult than that of the DBTs. The residual percentage for both compounds, following photoirradiation, is found to increase with increasing molecular weight, thus indicating that both DBTs and BTBTs, having large carbon numbers of alkyl substituents, are difficult to desulfurize according to the present process. This tendency is consistent with the

results obtained previously for the desulfurization of DBTs and BTs from light oil feedstocks.6,8 Figure 6b shows the distribution for the U ) 0 sulfur compounds, the fraction that consists of alkyl-substituted tetrahydrobenzonaphthothiophenes (THBNTs), as shown in Table 2. The residual percentage for the THBNTs, following photoirradiation, increases with increasing molecular weight, as also found for the U ) +2 compounds. Figure 6c shows the distribution for the alkyl-substituted octahydrodinaphthothiophenes (OHDNTs) for U ) -2. The residual percentage for the OHDNTs is also seen to increase with increasing molecular weight, as also for the DBTs, BTBTs, and THBNTs. Figure 6d shows the molecular weight distribution of benzonaphthothiophenes (BNTs) for U ) -4. As shown in Figure 5, the BNTs are desulfurized more easily than the other compounds. This tendency is also identical with the results obtained by the current HDS process.10,11 Although the low-molecular-weight BNTs are removed completely by photoirradiation, the residual percentage is increased with increasing molecular weight, as is also the case for the DBTs, BTBTs, THBNTs, and OHDNTs. Figure 6e shows the distribution of BTs and tetrahydrodinaphthothiophenes (THDNTs) for U ) -6. As shown in Figure 5, this compound fraction is the most difficult to desulfurize, according to the present process. As described in our previous papers,6-8 BTs in light oils are desulfurized more easily than DBTs, thus suggesting that the sulfur compounds for U ) -6 are more likely to consist mainly of THDNTs. The molecular weight distribution, as shown in Figure 6e, reveals that the highly substituted THDNTs are difficult compounds to desulfurize. Figure 6f,g shows the distribution of tetrahydrodibenzothiophenes (THDBTs) for U ) -8 and dinaphthothiophenes (DNTs) for U ) -10, respectively, which are also difficult to desulfurize. The residual

298


Figure 6. Molecular weight distribution of the sulfur-containing compounds, according to the respective U value in the feed and in the treated VGO (VGO/decane and VGO solution/acetonitrile volume ratios ) 1/9 and 1/1, respectively), following simple extraction and 10 h of photoirradiation. (a) U ) +2, (b) U ) 0, (c) U ) -2, (d) U ) -4, (e) U ) -6, (f) U ) -8, (g) U ) -10.


percentage for both THDBTs and DNTs also increase with increasing molecular weight. The above results strongly suggest that the THDNTs (U ) -6), THDBTs (U ) -8), and DNTs (U ) -10), with especially large carbon numbers of the alkyl substituents, are the most difficult compounds to desulfurize by the present process. The desulfurization reactivities for these compounds are significantly lower than for the BTs and DBTs, thus indicating that the presence of these refractory compounds is the cause of the lower desulfurization yield, obtained for VGO as compared to light oils. In the HDS process,10 the sulfur compound fractions for U ) +2, 0, and -2 are reported to be the most difficult compounds to desulfurize, a finding that differs completely from the results obtained according to the present photochemical process. 2. Denitrogenation of VGO. 2.1. Denitrogenation Behavior of VGO. The denitrogenation behavior of the VGO, when occurring simultaneously with desulfurization, was then studied. The time-course variations in the total nitrogen concentration in the VGO/decane solution, as a function of VGO/decane and VGO solution/ acetonitrile volume ratios, is shown in Figure 7. The nitrogen concentration in the VGO solution initially decreases, as a result of contact with acetonitrile, and then decreases dramatically with photoirradiation time, as was also the case for the desulfurization of the VGO and the denitrogenation of light oils.9 This indicates that the nitrogen compounds in VGO, when distributed into the acetonitrile, are photodecomposed efficiently there into highly polarized compounds such as the NO3anion, thus providing a successful means of removal of the nitrogen compounds from VGO solution to acetonitrile. The denitrogenation rate for VGO was found to increase with both decreasing VGO/decane and decreasing VGO solution/acetonitrile volume ratios. This is because photoscattering and light exclusion are reduced at decreasing volume ratio, and as a result, the photodecomposition of the nitrogen compounds is enhanced, as was also the case for the desulfurization of VGO. At a VGO solution/acetonitrile volume ratio ) 1/5 and 10 h of photoirradiation, the nitrogen content of the VGO was decreased successfully to less than 11%, 4%, and 1% of the feed concentration, for the respective conditions of VGO/decane volume ratio of 1/2, 1/4, and 1/9. These results demonstrate that the present process can remove the nitrogen compounds in the VGO simultaneously with the removal of the sulfur compounds. To clarify the denitrogenation efficiency for VGO, variations in the nitrogen distribution ratio, DN, following simple extraction (VGO/decane volume ratio ) 1/9) with 2 h of photoirradiation, were compared with the results for light oil feedstocks. The results are summarized in Figure 8, as a function of acetonitrile/oil volume ratio, where the data for light oils are from previous paper9 and the DN values are defined as

DN )

[nitrogen]in acetonitrile [nitrogen]in oil

(3)

The nitrogen content in the acetonitrile phase was determined from the quantity removed from the oil phase, because, as previously described,9 the direct analysis of the nitrogen content of acetonitrile phase includes the nitrogen of both the transferred nitrogen species from the oil and the acetonitrile itself. For simple extraction, the DN value for VGO is lower than that for

Figure 7. Time-course variation in the total nitrogen content of the VGO/decane solution during photoirradiation of an oil/acetonitrile two-phase system. The VGO/decane volume ratios are (a) 1/2, (b) 1/4, and (c) 1/9.

the low-aromatic-content light oils with a ranking order of LGO > CLO > VGO > LCO, as was found also for desulfurization. As shown by a comparison of Figures 4 and 8, the DN value for VGO is larger than the DS value. This is because the nitrogen compounds, having a higher polarity than the sulfur compounds, are distributed more easily into the acetonitrile.9 Under conditions of photoirradiation, the DN values for VGO are increased by increasing the acetonitrile/VGO solution volume ratio. These values are higher than those for both LGO and LCO, with a ranking order of VGO > LGO > CLO > LCO at an acetonitrile/oil volume ratio ) 7. Because the concentration of the nitrogen compounds in the acetonitrile phase, when put into contact with VGO, is larger than those for LGO and CLO, the quantity of nitrogen compounds photodecomposed increases accordingly. The results indicate, therefore, that the denitrogenation of VGO, at high acetonitrile/oil volume ratio, is more effective than that for light oil

300


Figure 8. Variation in the nitrogen distribution ratio, DN, for VGO (VGO/decane volume ratio ) 1/9), commercial light oil (CLO), straight-run light gas oil (LGO), and light cycle oil (LCO), with respect to acetonitrile/oil volume ratio, following simple extraction and 2 h of photoirradiation. The values for DN representing the ratio of the concentration of the nitrogen compounds in acetonitrile and in light oils are as defined by eq 3. The data for CLO, LGO, and LCO are cited from a previous paper.9

feedstocks. As shown again from Figures 4 and 8, the nitrogen distribution ratio, DN, for VGO, following photoirradiation, is greater than the corresponding sulfur distribution ratio, DS, thus indicating that the present process is more effective for denitrogenation than desulfurization. In the current HDS and HDN processes, the removal of nitrogen compounds is reported to be rather more difficult than that of sulfur compounds.3,4 These findings differ completely from those obtained according to the present photochemical process. 2.2. Denitrogenation Reactivity of Nitrogen-Containing Compounds. GC-AED and GC-MS analyses were carried out to acquire detailed information on the nitrogencontaining compounds occurring in the VGO. Figure 9a-c shows the carbon-, sulfur-, and nitrogen-specific GC-AED chromatograms, respectively, for the nitrogen fraction obtained following fractionation as indicated in Figure 1. The structure of the chromatograms for nitrogen and carbon closely resemble each other, and no peaks appear on the sulfur chromatogram. These results indicates that the nitrogen compounds are separated successfully by the fractionation procedure. The GC-MS analysis for the nitrogen fraction shows that each peak compound appearing in Figure 9 has a molecular ion at 167 + 14m (m ) natural number, 1 e m e 11). These peak compounds were therefore identified as C1-C11 alkyl-substituted carbazoles.14 The relative quantities of the individual carbazole compounds, as determined by GC-AED, are summarized in Table 3 as a function of the carbon number of the alkyl substituents, m. The change in the composition of the nitrogencontaining compounds in the VGO following extraction (VGO/decane and VGO solution/acetonitrile volume ratios ) 1/9 and 1/1, respectively) and 10 h of photoirradiation is listed in Table 3, and the data are plotted in Figure 10 as a function of the carbon number of the alkyl substituents on carbazole, m. Under conditions of simple extraction, the residual portion for the carbazoles tends to increase with an increase in the carbon number of the alkyl substituents, as was also found in the case

Figure 9. (a) Carbon- (191 nm), (b) sulfur- (181 nm), and (c) nitrogen- (388 nm) specific GC-AED chromatograms for the nitrogen compound fraction obtained following fractionation from VGO, in accordance with Figure 1. Table 3. Quantities of Carbazoles Having Different Carbon Numbers of Alkyl Substituents (a) in the Feed VGO and Treated Oils, (b) Following Simple Extraction, and (c) Following 10 h of Photoirradiationa species

(a) (ppm)

(b) (ppm)

(c) (ppm)

C1 carbazoles C2 carbazoles C3 carbazoles C4 carbazoles C5 carbazoles C6 carbazoles C7 carbazoles C8 carbazoles C9 carbazoles C10 carbazoles C11 carbazoles total nitrogen

0.3 1.1 2.3 5.3 2.2 6.2 3.2 2.9 2.8 19.1 39.6 86.0

0 0.1 0.4 1.7 1.0 3.4 1.9 2.1 2.2 14.9 34.1 61.8

0 0 0 0.2 0.1 0.1 0.2 0.4 0.5 4.1 7.5 13.1

a VGO/decane volume ratio ) 1/9, VGO solution/acetonitrile volume ratio ) 1/1.

of the denitrogenation of light oils.9 This phenomenon is due to the decrease in the distribution of the carbazole compounds into the acetonitrile phase with increasing carbon number of the alkyl substituents, which is caused by the reduction of the polarity, as described previously.9 Under conditions of photoirradiation, almost all of the C1-C4 alkyl-substituted carbazoles were denitrogenated, but the remaining portion was still


Figure 10. Remaining percentage of carbazoles in VGO (VGO/ decane and VGO solution/acetonitrile volume ratios ) 1/9 and 1/1, respectively), following both simple extraction and 10 h of photoirradiation, with respect to the carbon number of the alkyl substituents, m. The initial amount of each alkyl-substituted carbazoles in feed VGO is set as 100%.

found to increase with increasing carbon number of the substituents. This tendency was also found for the denitrogenation of light oil feedstocks.9 These results, therefore, reveal that the highly substituted carbazoles are the most difficult compounds to denitrogenate according to the present process. 3. Organization of the Overall Refining Process. 3.1. Effect of Water on the Desulfurization and Denitrogenation of VGO. In previous studies for light oil feedstocks,6,9 overall desulfurization and denitrogenation processes, involving the recovery of acetonitrile solvent, have been fully organized. In these processes, the acetonitrile dissolved into the light oils is recovered by washing with water and is then regenerated by distillation as an azeotropic mixture [acetonitrile/water, 84/16 v/v (bp ) 350 K)], which can be reused for further desulfurization and denitrogenation. To examine the reusability of the azeotropic mixture for the further refining of VGO, the effect of water on the desulfurization and denitrogenation of the VGO (VGO/decane and VGO solution/acetonitrile volume ratios ) 1/9 and 1/3, respectively) had to be investigated. The results are summarized in Figure 11. Although both the desulfurization and denitrogenation yields for VGO following 10 h of photoirradiation are decreased by an increase in the water concentration in the acetonitrile phase, 75% of the sulfur and 90% of the nitrogen are still found to be removed successfully from the VGO for water concentrations in the range 0-20%. The present process thus appears to be practicable for both the desulfurization and the denitrogenation of VGO in the presence of 0-20% of water/acetonitrile solution, and a recovered acetonitrile azeotropic mixture (containing 16% water) can be reused for further desulfurization and denitrogenation, as for the previous process proposed for light oil feedstocks.6,9 3.2. Combination with Other Refining Processes. Previous studies have considered novel refining processes for the desulfurization, denitrogenation, and demetalation of petroleum-derived middle and heavy feedstocks, based on the combination of photochemical reaction and liquid-liquid extraction.6-9,18-20 From a consideration of these results and the results obtained in the present study, a possible combination of these new processes with current established refining pro-

Figure 11. Effect of the addition of water on the (a) desulfurization and (b) denitrogenation for VGO, following simple extraction (VGO/decane and VGO solution/acetonitrile volume ratios ) 1/9 and 1/3, respectively) and also 10 h of photoirradiation.

cesses is proposed; it is shown in Figure 12. The overall process sequence is as follows: (1) The light oil and atmospheric residue are produced by atmospheric distillation of crude oil. (2) The latter residue oil is then separated by vacuum distillation into VGO and vacuum residue. (3) The vanadium and nickel compounds in these residual fractions are removed, by the previously proposed photochemical demetalation process,18 into aqueous solution, which can then be used as a source of rare metals. When the presently considered process was applied to the desulfurization and denitrogenation of residue oils, the sulfur and nitrogen concentrations were found barely decrease. This is because these elements are distributed widely within the asphaltenic molecules. These elements, therefore, must be removed sequentially by other processes (e.g., hydrotreatment). Most of the metal compounds are removed completely in advance by the photochemical demetalation process and therefore any successive hydrotreating process can be carried out effectively without the problem of catalyst poisoning. (4) The desulfurization and denitrogenation of the VGO are then carried out by the presently proposed process, prior to the fluid catalytic-cracking and hydrocracking treatments for the production of light cycle oil and catalytic-cracked gasoline. During the photochemical process, small amounts of vanadium and nickel compounds in VGO are found to be removed simultaneously with the sulfur and nitrogen compounds by the present process. The successive hydrotreating processes therefore can be carried out effectively. In the present study, n-decane was used as the diluent for the VGO. In the practical refining for the VGO, however, the light oil fractions (straight-run light gas oil and light cycle oil) can be used as diluents in place of the n-decane solution. (5) The straight-run light gas oil, produced by atmospheric crude oil distillation, is desulfurized and denitrogenized by a new photochemical process using UV light.6,8 Alternatively, a more energy-efficient process, using visible-wavelength light (λ > 400 nm) as the light source and spiking the sample with an electrontransfer photosensitizer,19 can be employed for the refining, as straight-run oil usually contains a small amount of aromatic hydrocarbons, which act to suppress the photooxidation of sulfur and nitrogen compounds. (6) The desulfurization and denitrogenation efficiencies for light cycle oil, produced by fluid catalytic cracking

302


Figure 12. Possible combination for previously proposed desulfurization, denitrogenation, and demetalation processes combined with the presently proposed process for VGO and with the conventional petroleum refining processes.

of VGO, is extremely low, when treated by the photochemical process, as the light cycle oil contains a high proportion of aromatic hydrocarbons.8 However, refining of the light cycle oil is not necessary, because the VGO is desulfurized and denitrogenized sufficiently in advance in section 4. (7) The catalytic-cracked gasoline, produced by fluid catalytic cracking of VGO, is then desulfurized by the visible-light process and mixed with straight-run gasoline.20 All of the processes, both proposed previously and in the present work, require relatively long photoirradiation times. This problem, however, can be expected to be reduced considerably for future industrial application via the development of a more efficient photoreactor, equipped with an extractor of appropriate advanced design. All of the proposed processes are carried out under moderate conditions and need only air bubbling, such that the use of hydrogen and noble metal catalysts can be eliminated from future desulfurization and denitrogenation processing applications. The proposed process is thus applicable as an alternative, intrinsically safe and energy-efficient, potential upgrading process for petroleum-derived middle and heavy feedstocks. Conclusion A novel process for the simultaneous desulfurization and denitrogenation of fuel oils, based on a combination of UV irradiation using a high-pressure mercury lamp and liquid-liquid extraction using an oil/acetonitrile two-phase system, has been applied to the refining of vacuum gas oil (VGO), and the following results have been obtained: (1) Sulfur-containing compounds in VGO are distributed into the acetonitrile and are photodecomposed there to form highly polarized compounds, that do not redistribute back into the VGO. Thus, in this way, the sulfur content of the VGO is decreased successfully to less than 1% of the feed concentration. The desulfurization efficiency for VGO is lower than that for low-

aromatic-content straight-run light oil but higher than that for high-aromatic-content light cycle oil. (2) FI-MS analysis reveals that tetrahydrodinaphthothiophene, tetrahydrodibenzothiophene, and dinaphthothiophenes are the most difficult compounds to desulfurize according to the present process. The desulfurization reactivity for these compounds is found to be significantly lower than that for benzothiophenes and dibenzothiophenes present in light oils and has a tendency to decrease with increasing carbon number of the alkyl substituents. (3) The nitrogen-containing compounds in VGO, when distributed into the acetonitrile phase, are photodecomposed there and are thus removed from the VGO. In this way, the nitrogen content of the VGO is decreased successfully to less than 1% of the feed value. The denitrogenation efficiency for VGO is higher than that for light oil feedstocks. GC-AED analysis reveals that the carbazoles having large carbon numbers of alkyl substituents constitute the most difficult compounds to denitrogenate. (4) The desulfurization and denitrogenation of VGO proceed effectively in the presence of 16% water in acetonitrile, and thus a closed process, in which the acetonitrile and water can be recirculated as an azeotropic mixture, can be formulated. An overall refining process, based on the implementation of photochemical reaction and liquid-liquid extraction is organized for the desulfurization, denitrogenation, and demetalation of petroleum-derived middle and heavy feedstocks. Acknowledgment The authors acknowledge the members of the Central Analytical Center for Japan Energy Corporation for their help in the separation using ligand-exchange chromatography and FI-MS analyses for sulfur-containing compounds. The authors are grateful to the financial supports by Grants-in-Aid for Scientific Research (09555237 and 12555215) from the Ministry of Educa-


tion, Science, Sports, and Culture, Japan, and by Showa Shell Sekiyu Foundation for Promotion of Environmental Research. Y.S. is grateful for the Research Fellowship of the Japan Society for the Promotion of Science (JSPS) for Young Scientists and the British Council Grants for JSPS fellows visiting the United Kingdom. Nomenclature and Abbreviations BT ) benzothiophene BNT ) benzonaphthothiophene BTBT ) benzothiophenobenzothiophene CLO ) commercial light oil DBT ) dibenzothiophene DNT ) dinaphthothiophene DS ) sulfur distribution ratio defined in eq 1, DN ) nitrogen distribution ratio defined in eq 3, FI-MS ) field ionization mass spectrophotometer GC-AED ) gas chromatography with atomic emission detection HDN ) hydrodenitrogenation HDS ) hydrodesulfurization LCO ) light cycle oil LGO ) straight-run light gas oil m ) carbon number of the alkyl substituents on carbazole, n ) integral number defined in eq 2, OHDNT ) octahydrodinaphthothiophene THBNT ) tetrahydrobenzonaphthothiophene THDBT ) tetrahydrodibenzothiophene THDNT ) tetrahydrodinaphthothiophene U ) even number defined in eq 2, VGO ) vacuum gas oil

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Organic Two-Phase Liquid-Liquid Extraction System. Ind. Eng. Chem. Res. 1998, 37, 203. (7) Shiraishi, Y.; Hirai, T.; Komasawa, I. Identification of Desulfurization Products in the Photochemical Desulfurization Process for Benzothiophenes and Dibenzothiophenes from Light Oil Using an Organic Two-Phase Extraction System. Ind. Eng. Chem. Res. 1999, 38, 3300. (8) Shiraishi, Y.; Hirai, T.; Komasawa, I. Photochemical Desulfurization for Light Cycle Oil Using an Organic Two-Phase Extraction System. Solvent Extr. Res. Dev., Jpn. 2000, 7, 11. (9) Shiraishi, Y. Hirai, T.; Komasawa, I. Photochemical Denitrogenation Process for Light Oils Effected by a Combination of UV Irradiation and Liquid-Liquid Extraction. Ind. Eng. Chem. Res. 2000, 39, 2826. (10) Aoyagi, I.; Imagishi, T.; Mitani, H. Relative Desulfurization Reactivity of Thiophenes in Vacuum Gas Oil (in Japanese). J. Jpn. Pet. Inst. 1996, 39, 418. (11) Aoyagi, I.; Kobayashi, M.; Tokawa, S.; Mitani, H. Molecular Transformation of Vacuum Gas Oil in Hydrodesulfurization (in Japanese). J. Jpn. Pet. Inst. 1997, 40, 213. (12) Later, D. W.; Lee, M. L.; Bartle, K. D.; Kong, R. C.; Vassilaros, D. L. Chemical Class Separation and Characterization of Organic Compounds in Synthetic Fuels. Anal. Chem. 1991, 53, 1612. (13) Ma, X.; Sakanishi, K.; Isoda, T.; Mochida, I. Determination of Sulfur Compounds in Nonpolar Fraction of Vacuum Gas Oil. Fuel 1997, 76, 329. (14) Dorbon, M.; Bernasconi, C. Nitrogen Compounds in Light Cycle Oils: Identification and Consequences of Aging. Fuel 1989, 68, 1067. (15) Nishioka, M.; Campbell, R. M.; Lee, M. L.; Castle, R. N. Isolation of Sulphur Heterocycles from Petroleum- and CoalDerived Materials by Ligand Exchange Chromatography. Fuel 1986, 65, 270. (16) Sugimoto, Y.; Tunnukij, K.; Miki, Y.; Yamada, S. Characterization of Compound-Class Fractionation of Vacuum Gas Oil Using Capillary GC and GCMS. J. Jpn. Pet. Inst. 1992, 35, 339. (17) Ueda, K.; Matsui, H.; Malhotora, R.; Nomura, M. Chemical Characterization of Heavy Oils Using HPLC/FI-MS. Analysis of a Petroleum Vacuum Residue. J. Jpn. Pet. Inst. 1991, 34, 62. (18) Shiraishi, Y.; Hirai, T.; Komasawa, I. A Novel Demetalation Process for Vanadyl- and Nickelporphyrins from Petroleum Residue by Photochemical Reaction and Liquid-Liquid Extraction. Ind. Eng. Chem. Res. 2000, 39, 1345. (19) Shiraishi, Y.; Taki, Y.; Hirai, T.; Komasawa, I. Visible Light-Induced Deep Desulfurization Process for Light Oils by Photochemical Electron-Transfer Oxidation in an Organic TwoPhase Extraction System. Ind. Eng. Chem. Res. 1999, 38, 3310. (20) Shiraishi, Y.; Taki, Y.; Hirai, T.; Komasawa, I. Visible Light-Induced Desulfurization Process for Catalytic-Cracked Gasoline Using an Organic Two-Phase Extraction System. Ind. Eng. Chem. Res. 1999, 38, 4538.

Received for review August 1, 2000 Revised manuscript received September 29, 2000 Accepted October 2, 2000 IE000707U

Photochemical Desulfurization and Denitrogenation Process for

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