Ind. Eng. Chem. Res. 1996, 35, 335-337
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Capture and Recovery of Indole from Methylnaphthalene Oil in a Continuous Supercritical CO2 Extraction Apparatus over a Fixed Bed of Anion-Exchange Resin Kinya Sakanishi,* Hiroaki Obata, Isao Mochida, and Tsuyoshi Sakaki† Institute of Advanced Material Study, Kyushu University, Kasuga, Fukuoka 816, Japan
Capture and recovery of nonbasic aromatic nitrogen compounds such as indole from crude methylnaphthalene oil in coal tar fractions were examined using a continuous supercritical CO2 extraction apparatus with the fixed bed of an anion-exchange resin. A model feed consisting of 10 wt % indole with 10 wt % quinoline in 1-methylnaphthalene and crude methylnaphthalene oil (CMNO) was used for the supercritical CO2 separation procedures. Indole was selectively adsorbed on an anion-exchange resin (Amberlite IRA-904) for the first 120 min, with quinoline and 1-methylnaphthalene being eluted under the supercritical CO2 (323 K, 80 atm) conditions. After the resin was saturated with indole, the feed was stopped and the adsorbed indole was recovered. Preextraction of the resin with supercritical CO2 before the recovery of indole from the adsorbent improved the concentration of recovered indole through the selective elution of quinoline and 1-methylnaphthalene remaining in the dead space in the flow line and fixed bed. Indole in CMNO was also effectively separated by the same procedures as described above. The adsorption and desorption mechanisms of indole are briefly discussed based on the above results. Introduction Coal tar contains a number of valuable nitrogen compounds for use in the preparations of pesticides, drugs, dyes, and pigments. On the other hand, the nitrogen compounds are undesirable impurities in hydrocarbon oils because they contribute to air pollution, have unpleasant odors, and deteriorate upgrading catalysts. The present authors have proposed application of metal sulfates, which are fairly acidic when dehydrated while neutral when hydrated (Mochida et al., 1990, 1991, 1992; Sakanishi et al., 1992). Nitrogen compounds in crude methylnaphthalene oil diluted in nonpolar solvents are captured on the adsorbent and recovered with polar solvents such as methanol and THF, regenerating the acidity of the adsorbent for its repeated use. Supercritical fluids have been reputed to be a unique solvent to allow the selective and energy-saving extraction and separation procedures (Ekart et al., 1993; Campbell and Lee, 1986; Wilhelm and Hedden, 1986). In addition to such advantages of supercritical extraction, the dissolving ability of supercritical CO2 is easily controlled by changing its temperature and pressure, thus the adsorption and desorption of the substrates being designed by the supercritical solvent. In previous papers (Sakanishi et al., 1994, 1995a,b), the authors reported that basic quinolines in methylnaphthalene oil can be selectively separated using a batch-type or a continuous-feeding supercritical CO2 extraction apparatus with the adsorbent of 10 wt % alminum sulfate supported on silica gel, and quinolines can be effectively recovered by adding a small amount of THF as an entrainer to supercritical CO2. The adsorbent can be repeatedly used through the above adsorption and desorption cycle. In the present study, the continuous-feeding supercritical CO2 extraction apparatus was applied to the separation and recovery of indole, which is one of the †
Kyushu National Industrial Research Institute, Tosu, Saga 841, Japan.
0888-5885/96/2635-0335$12.00/0
most valuable nonbasic nitrogen-containing aromatic compounds contained in the crude methylnaphthalene oil obtained from coal tar. Several papers have reported on the separation of indole using cyclodextrins as the host for inclusion complexation (Orstan et al., 1987; Uemasu and Nakayama, 1987), high-pressure crystallization (Yamamoto et al., 1991, 1993), or an anionexchange resin as adsorbent for column chromatography separation (Hara et al., 1990). However, cyclodextrins are soluble in water but insoluble in most organic solvents, making it difficult to contact efficiently with indole which has a low solubility in water. Dissociation of cyclodextrin inclusion compounds in hot water is another problem because of the low concentration of recovered indole and difficulty in water removal. Highpressure crystallization also has problems in the high installation cost and the formation of complex cocrystallization products among the heterocyclic nitrogen compounds. The chromatographic procedure using the anion-exchange resin of Amberlite IRA-904, which has been reputed to have selective affinity to indole, is one of the most promising methods. However, elution using organic solvents such as hexane and alcohols needs to distill the solvent to concentrate indole. Hence, in the present study, the supercritical CO2 extraction apparatus equipped with the fixed bed of the anionexchange resin was used for the energy-saving separation and effective recovery of indole from methylnaphthalene oil. Experimental Section A model methylnaphthalene oil (indole (ID), 10 wt %; quinoline (Q), 10 wt %; 1-methylnaphthalene (1-MN), 80 wt %: their guaranteed grade used) was prepared as the model feed. A commercial crude methylnaphthalene oil (CMNO) was also used as a feed for comparison. One of the anion-exchange resins, Amberlite IRA-904 (abbreviated as IRA-904), was used as an adsorbent. Some properties of the anion-exchange resin are summarized in Table 1. It was dried at 323 K for 10 h before use. © 1996 American Chemical Society
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Ind. Eng. Chem. Res., Vol. 35, No. 1, 1996
Figure 1. Continuous supercritical CO2 separation apparatus. Table 1. Properties of Anion-Exchange Resin exchange capacity resin
structure
H2O content: (H2O %) (style)
relative surface area (m2/g of dry R)
porosity (mL/mL of R)
average pore size (nm)
exchange group
(mequiv/mL of R)
(mequiv/g of dry R)
Amberlite IRA-904
MR
56-62 (c1)
63
0.51
64.5
-N+(CH3)3X-
0.7
2.6
Figure 1 shows the continuous supercritical CO2 extraction apparatus used in the present study. The model CMNO was fed at the flow rate of 0.045 mL/min to the fixed bed of 2.0 g of IRA-904 with the supercritical CO2 (323 K, 80 atm; flow rate of 6 L/min). At the extraction time of 300 min, the adsorbent was saturated with indole, and then supercritical CO2 at 323 K and 80 atm was flowed for a couple of hours to wash out methylnaphthalene and quinoline remaining in the dead space of the flow line and fixed bed. The concentrated indole on the adsorbent was recovered by washing with methanol after the depressurization. The eluted and recovered compounds, which were sampled in 20 min intervals, were qualitatively and quantitatively analyzed by GC-FID (50 m capillary OV-101 column, 383 K). Results Figure 2 shows the elution profile of the model methylnaphthalene oil. Indole was not eluted until the feeding time of 100 min, with only methylnaphthalene and quinoline being recovered in the separation vessel. Indole started to elute at 120 min. After 140 min, the indole concentration of the eluate gradually increased to reach that of the model feed. When the feed was stopped at 300 min, 2.37 g of the concentrated indole was recovered by washing out with methanol. Table 2 shows the effects of supercritical CO2 preextraction of the saturated adsorbent on the concentration of indole. The concentration of indole in the adsorbate was about 3 times larger by the preextraction with supercritical CO2 at 80 atm and 323 K for 1 h than that of the original feed, indicating that 1-methylnaphtha-
Figure 2. Extraction profile of the model feed using the continuous extraction apparatus under supercritical CO2 conditions. Conditions: pressure, 80 atm; temperature, 323 K; extraction time, 300 min; adsorbent IRA-904, 2.0 g; CO2 flow rate, 6 L/min; feed flow rate, 0.045 mL/min; feed composition, 1-MN 80 wt %; Q 10 wt %; ID 10 wt %.
lene and quinoline remaining in the dead space of the fixed bed have been effectively eluted by the preextraction. Supercritical CO2 at 323 K washed out such remaining 1-methylnaphthalene and quinoline, leaving the indole on the adsorbent. Supercritical CO2 (323 K, 80 atm; preextraction time, 2 h) concentrated indole to 96 wt % in the adsorbate, although only 0.33 g was recovered by this treatment. Recycle use of IRA-904 was achieved by washing the used resin with methanol and drying at 323 K for 10 h under vacuum before the next run. As is shown in run 4 of Table 2, 0.28 g of the concentrated indole was recovered at the content of 98 wt % in the second run. CMNO principally consisted of 1- and 2-methylnaph-
Ind. Eng. Chem. Res., Vol. 35, No. 1, 1996 337 Table 2. Effects of CO2 Preextraction of Saturated Adsorbent on the Concentration of Recovered Indolea preextraction conditions
run no.
pressure (atm)
1 2 3 4c
80 80 80 80
time (h)
residual wt on adsorbent (g)
indole content (%)b
0 1 2 2
2.37 0.82 0.33 0.28
28.3 66.4 95.8 98.0
a Adsorption conditions: 323 K, 80 atm, 300 min; CO flow rate, 2 6 L/min; original indole content 10%. b Desorption conditions: indole was recovered from ion-exchange resin with MeOH. c Recycle use of adsorbent: ion-exchange resin was washed with MeOH and dried at 323 K for 10 h.
thalenes and other polyaromatic hydrocarbons with nitrogen compounds of quinoline, isoquinoline, methylquinoline, and indole. The concentration of indole in CMNO is ca. 3%. Indole was completely and exclusively removed from crude methylnaphthalene oil, and highly concentrated indole was recovered by the above supercritical CO2 separation procedures. Discussions The present study revealed that nonbasic nitrogen compounds such as indole can be separated from methylnaphthalene oil obtained from coal tar using the supercritical CO2 extraction procedure with the anionexchange resin. Indole has a slightly acidic NH group which can be exchanged by its anion form with the corresponding anion of the resin. The present study did not show any evidence to the ion exchange of indole with IRA-904, although indole was exclusively adsorbed on IRA-904 by their interactions through the acid-base or Coulombic attraction. The adsorbed indole on IRA-904 was effectively recovered by methanol, indicating that the interaction between indole and IRA-904 may be weak enough to desorb indole from IRA-904 by methanol, while indole was selectively adsorbed on IRA-904 with the aid of the antisolvent effect of supercritical CO2 alone. The adsorbed indole can be recovered by the supercritical CO2 with a small amount of methanol as an entrainer as previously reported (Sakanishi et al., 1995a). The acidity of indole may be enhanced in nonpolar supercritical solvent without leveling effects. Slightly acidic indole and basic quinoline bases can be separatively recovered by the continuous-feeding supercritical CO2 extraction apparatus with the two fixed beds of IRA-904 and aluminum sulfate supported on silica gel, respectively. Such a two-step adsorption process enables the selective recovery of indole and quinolines using entrainers of methanol and THF, respectively, as well as the purification of methylnaphthalene oil through the removal of nitrogen compounds.
Literature Cited Campbell, R. M.; Lee, M. L. Supercritical Fluid Fractionation of Petroleum- and Coal-Derived Mixtures. Anal. Chem. 1986, 58, 2247. Ekart, M. P.; Bennett, K. L.; Ekart, S. M.; Gurdial, G. S.; Liotta, C. L.; Eckert, C. A. Cosolvent Interactions in Supercritical Fluid Solutions. AIChE J. 1993, 39 (2), 235. Hara, T.; Horii, M.; Ishikawa, T. Utilization of Coal-Derived Middle and Heavy Distillates as Chemicals. Proc. Coal Sci., Jpn. 1990, p 230. Mochida, I.; Fei, Y. Q.; Sakanishi, K.; Usuba, H.; Miura, K. Capture and Recovery of Basic Nitrogen Species in Coal Tar Pitch Using Nickel Sulfate as Adsorbent. Chem. Lett. 1990, 515. Mochida, I.; Sakanishi, K.; Usuba, H.; Miura, K. Removal of Basic Nitrogen Species in Coal-Tar Pitch by Metal Sulphates Supported on Silica Gel. Fuel 1991, 70, 761. Mochida, I.; Fei, Y. Q.; Sakanishi, K.; Korai, Y.; Usuba, H.; Miura, K. Carbonization of Coal Tar Pitch Denitrogenated by Metal Sulfates. Carbon 1992, 30, 241. Orstan, A.; Ross, J. B. A. Investigation of the β -CyclodextrinIndole Inclusion Complex by Absorption and Fluorescence Spectroscopies. J. Phys. Chem. 1987, 91, 2739. Sakanishi, K.; Sun, Y. N.; Mochida, I.; Usuba, H. Removal and Recovery of Nitrogen Compounds in Crude Methylnaphthalene Oil Using Aluminium Sulfate Supported on Silica Gel. Fuel Process. Technol. 1992, 32, 143. Sakanishi, K.; Obata, H.; Mochida, I.; Sakaki, T. Removal and Recovery of Nitrogen and Sulfur Compounds from Coal Tar Using Supported Aluminium Sulfate under Supercritical CO2 Conditions. Prepr. Div. Fuel ACS 1994, 39 (3), 761. Sakanishi, K.; Obata, H.; Mochida, I.; Sakaki, T. Capture and Recovery of Nitrogen Compounds in Methylnaphthalene Oil Using Aluminium Sulfate Supported on Silica Gel Under Supercritical CO2 Conditions. J. Energy Inst., Jpn. 1995a, 74, 109. Sakanishi, K.; Obata, H.; Mochida, I.; Sakaki, T. Removal and Recovery of Quinoline Bases from Methylnaphthalene Oil in a Continuous Supercritical CO2 Separation Apparatus with a Fixed Bed of Supported Aluminium Sulfate. Ind. Eng. Chem. Res. 1956, in press. Uemasu, I.; Nakayama, T. Concentration of Indole in Coal Tar Using R-cyclodextrin as the Host for Inclusion Complexation. J. Inclusion Phenom. Mol. Recognit. Chem. 1987, 7, 327. Wilhelm, A.; Hedden, K. A Non-isothermal Experimental Technique to Study Coal Extraction with Solvents in Liquid and Supercritical State. Fuel 1986, 65, 1209. Yamamono, Y.; Sato, Y.; Ebina, T.; Yokoyama, C.; Takahashi, S.; Nishiguchi, N.; Tanabe, H. Separation of Heterocyclic Compounds by High Pressure (I), J. Fuel Soc., Jpn. 1991, 70, 533. Yamamoto, Y.; Sato, Y.;Ebina, T.; Yokoyama, C.; Takahashi, S.; Nishiguchi, N.; Tanabe, H. Separation of Heterocyclic Compounds by High Pressure (II)sSeparation from QuinolineIsoquinoline System. J. Fuel Soc., Jpn. 1993, 72, 263.
Received for review May 9, 1995 Revised manuscript received August 24, 1995 Accepted September 19, 1995X IE950284+
X Abstract published in Advance ACS Abstracts, December 1, 1995.
