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Photochemical Denitrogenation Processes for Light Oils Effected by a Combination of UV Irradiation and Liquid-Liquid Extraction 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
Denitrogenation processes for light oils, based on a combination of UV irradiation and liquidliquid extraction, have been investigated. Two extraction systems, one oil/water and the other oil/acetonitrile, were used for the denitrogenation of three separate light oils, of differing nitrogen content and hydrocarbon composition. The denitrogenation results, obtained for three model nitrogen compounds (aniline, indole, and carbazole), were compared with those obtained for the actual light oils. In the oil/water system, the photodecomposition of carbazole was found to be strongly suppressed by the presence of double-ring aromatic hydrocarbons. This adverse effect, however, was reduced by the addition of hydrogen peroxide to the water phase, and in the presence of 30% hydrogen peroxide and 36 h of photoirradiation, the nitrogen content of the light oils was decreased successfully to indoles > carbazoles. 3.2. Photoreaction of Nitrogen-Containing Compounds in Acetonitrile. The three model nitrogen compounds were each dissolved in acetonitrile and then
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Figure 11. Time-course variation in the concentration of the nitrogen-containing compounds in acetonitrile, during photoirradiation, both with and without the addition of naphthalene (80 mM). Table 8. Quantities of Anilines in (a) Feed LCO and in Treated Oils, (b) Following Simple Extraction (LCO/ Acetonitrile Volume Ratio ) 1/10) and (c) Following 10 h of Photoirradiation LCO (ppm) species
(a)
(b)
(c)
aniline C1-aniline C2-aniline C3-aniline C4-aniline
0.9 8.1 19.6 7.5 18.6
0.1 1.2 5.9 3.5 9.5
0 0.4 2.5 1.8 7.7
total anilines
54.7
20.2
12.4
photoirradiated. Figure 11 shows the time-course variation in the concentrations of the nitrogen-containing compounds in the acetonitrile. The concentrations of all the nitrogen compounds decreased with photoirradiation time, with carbazole being the most difficult compound to be photodecomposed, as was found also in the xylene/ water two-phase system. The photodecomposition rate of indole obtained in acetonitrile is observed to be rather slower than that observed for indole in xylene solution, as shown in Figure 3. This is because the indoles are stabilized in the polar solvent by the formation of an indole-solvent excited-state complex.22 The photodecomposed products for all the model nitrogen compounds in acetonitrile were identified by ion chromatography analysis as NO3- ion, as was the case also for the xylene/ water two-phase system, thus suggesting that these are highly polarized compounds that do not distribute into the nonpolar light oil. In previous desulfurization studies,10 it was found that a large quantity of the aromatic hydrocarbons also distribute into the acetonitrile phase. Thus, the photoreactivities of the nitrogen compounds, in the presence of naphthalene, were also studied. These results are shown in Figure 11 and show that no decrease in the photodecomposition rate for carbazole in acetonitrile was observed, even though the photodecomposition of carbazole in xylene was suppressed significantly by naphthalene, owing to the transfer of triplet energy from the excited-state carbazole to the ground-state naphthalene. This probably does not occur in acetonitrile because the highly polarized intermediates, produced during the photodecomposition of carbazole, are stabilized in the polar acetonitrile. Photodecomposition of the other nitrogen compounds, aniline and indole, in acetonitrile was also found to proceed in the presence of
Figure 12. Time-course variation in the total nitrogen content for (a) CLO, (b) LGO, and (c) LCO, during photoirradiation of the oil/acetonitrile two-phase system, as a function of acetonitrile/oil volume ratio.
naphthalene. Thus, the nitrogen compounds, when distributed into the acetonitrile, may be photodecomposed effectively as well as for the sulfur compounds.10 3.3. Denitrogenation of Light Oils. Mixtures of each light oil and acetonitrile were also photoirradiated. The time-course variation in the total nitrogen content for the light oils, as a function of feed acetonitrile/oil volume ratio, during photoirradiation is shown in Figure 12. The data points corresponding to an irradiation time of zero are those obtained by mixing of the two phases and indicate the resulting equilibrium distribution concentration for the nitrogen compounds in the light oil phase. The nitrogen concentrations for all the light oils decreased slowly with respect to irradiation time. For example, taking a volume ratio of 7 following 10 h of photoirradiation, the nitrogen contents for CLO were
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decreased from 80 to 2 ppm and for LGO from 160 to 1 ppm. The denitrogenation rate for LCO, under the above conditions, was rather slower than those for CLO and LGO. This effect was caused in this instance by the addition of water, following the end of the photoirradiation, which decreased the distribution of the nitrogen compounds into the acetonitrile. The denitrogenation of the LCO was, however, substantially enhanced by further increases in the acetonitrile volume ratio. Thus, at a volume ratio of 15, the nitrogen content for LCO was decreased successfully from 243 to
