Energy & Fuels 2003, 17, 255-256
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Communications Difference in Extraction Yields between CS2/NMP and NMP for Upper Freeport Coal Toshimasa Takanohashi,* Fengjuan Xiao, Takahiro Yoshida, and Ikuo Saito Institute for Energy Utilization, National Institute of Advanced Industrial Science and Technology, Tsukuba 305-8569, Japan Received June 24, 2002. Revised Manuscript Received October 17, 2002 Iino et al. found1 that a carbon disulfide/N-methyl-2pyrrolidinone (CS2/NMP) mixed solvent gives high extraction yields (40-65%, daf) at room temperature for several bituminous coals. They reported that no significant chemical reaction occurred during the extraction from the results of ultimate analysis and FT-IR. Dyrkacz investigated2 the solvent properties of the CS2/NMP mixed solvent using density, viscosity, solvatochromic, and FTIR measurements, and suggested that some type of NMP chain oligomer may be an important factor in understanding the synergistic extraction of coal in the mixed solvent. Dyrkacz et al. also reported3 that in the mixed solvent extraction, the shape of solvents, i.e., flatness, is an important factor when their basicities are similar. The mechanism of extraction, however, why the mixed solvent gives the high extraction yields, is not understood. It is also known4 that NMP alone also gives high extraction yields when the extraction is carried out at 202 °C (boiling point of NMP)s 66 wt % (daf) for bituminous coal, although the extraction yield with NMP also at room temperature was extremely low, less than 10 wt %.1 We also reported5,6 that for high-rank raw coals such as Upper Freeport coal, sorption rate and swelling rate of solvent was relatively low, compared to lower-rank coals, and they did not appear to attain the equilibrium even after two weeks.7 These results suggest that the diffusion of solvent into the high-rank raw coals might be retarded by intermolecular interactions in the coals, such as stacking among aromatic rings. In the present study, extractions under several mild conditionss changing time, temperature, and type of solventswere carried out. Here, we report a consideration on the role of CS2 in the CS2/NMP mixed solvent extraction, from the comparisons of data of NMP and CS2/NMP extractions. Upper Freeport coal (Argonne Premium Coal Sample, APCS-1) was used as a coal sample. In room-temperature extraction, 1 g of the sample and 60 mL of CS2, NMP, or CS2/NMP (1:1 by volume) were put into a 100 mL * Corresponding author. Phone: +81-298-61-8441. Fax: +81-298-618408. E-mail:
[email protected]. (1) Iino, M.; Takanohashi, T.; Ohsuga, H.; Toda, K. Fuel 1988, 67, 1639. (2) Dyrkacz, G. Energy Fuels 2001, 15, 918. (3) Dyrkacz, G. R.; Bloomquist, C. A. A. Energy Fuels 2001, 15, 1409. (4) Cai, M. F.; Smart, R. B. Energy Fuels 1993, 7, 52. (5) Shimizu, K.; Takanohashi, T.; Iino, M. Energy Fuels 1998, 12, 891. (6) Takanohashi, T.; Iino, M.; Nishioka, M. Energy Fuels 1995, 9, 788. (7) Hsieh, S. T.; Duda, J. L. Fuel 1987, 66, 170.
extraction tube, and then it was put into a water bath with ultrasonic irradiation. The extraction was performed under ultrasonic irradiation (38 kHz) for 30 min. After centrifugation at 15000 r/m for 60 min, the extract and residue obtained were separated by filtration with a membrane filter (0.8 µm). Fresh mixed solvent was added to the residue, which was again extracted in the same procedure above. These procedures were repeated until the supernatant became almost colorless (5-7 times). To see the effect of NMP diffusibility into the coal, before the above extraction the coal sample was treated by soaking in NMP for 2-8 weeks at room temperature, and then the same extraction was carried out. In addition, treatments in NMP at 100, 200, and 300 °C were performed; 1 g of sample was mixed with 5 g of NMP in a stainless tube and then heated under nitrogen of 0.4 MPa for 60 min using an oven. After the treatment, the extract and residue were separated by centrifugation and decantation in the same way. All extract fractions and residues were dried in vacuo at 80 °C for 24 h. The extraction yield was calculated from the weight of the residue (eq 1):
extraction yield (wt%, daf) ) [1 - (residue (g)/coal (g))] × 100 (1) [1 - (ash (wt %, db))/100] The extraction yields at room temperature are summarized in Table 1. When the soaking treatment was not done before the extraction, the yields with CS2, NMP, and CS2/NMP were 3, 18, and 59 wt % (daf). It is noted that the amount of NMP-soluble material obtained from fractionation of whole CS2/NMP extract (59% of raw coal) was 91% on the extract basis and 54% on the whole coal basis. These results indicate that considerable amounts of solvent-soluble components remain in the raw coal when NMP alone is used for extraction, and almost (91%) of CS2/NMP extract is practically soluble in NMP. It gives us a notion that diffusibility of NMP into the coal inside is inadequate in the case of extraction with NMP alone. Upper Freeport coal was reported8,9 to have more micropores into which ordinary reagents including NMP (8) Takanohashi, T.; Nakano, K.; Yamada, O.; Kaiho, M.; Ishizuka, A.; Mashimo, K. Energy Fuels 2000, 14, 720. (9) Takanohashi, T.; Terao, Y.; Yoshida, T.; Iino, M. Energy Fuels 2000, 14, 915.
10.1021/ef020141h CCC: $25.00 © 2003 American Chemical Society Published on Web 11/14/2002
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Table 1. Extraction Yields of Upper Freeport Coal with CS2, NMP, or CS2/NMP at Room Temperature and the Quantity of Solvent Solubles in the CS2/NMP Extract
a
sample
solvent
soaking
extraction yield (wt %, daf)
recovery (wt %, db)
raw coal raw coal raw coal raw coal raw coal raw coal CS2/NMP extract CS2/NMP extract
CS2 NMP CS2/NMP NMP NMP NMP NMP CS2
none none none 2 weeks, NMP 4 weeks, NMP 8 weeks, NMP none none
3 18 59 29 27 28 91 (54)a 15 (9)a
99.8 99.6 98.9 99.6 99.8 101.2 99.7 99.9
Based on the whole coal.
Table 2. NMP Extraction Yields of Upper Freeport Coal after Heat Treatments in NMP treatment solvent NMP NMP NMP
treatment temp. extraction yield recovery (°C) (wt %, daf) (wt %, db) 25 100 200 300
18 48 56 58
99.6 103.4 102.7 104.5
could diffuse only marginally, while for the extraction residue after removing extract components, any solvents could easily diffuse.6,10 Table 1 also shows the extraction yields when soaking in NMP was done before the NMP extraction. After two weeks soaking, the yield increased to 29% from 18% without soaking. However, longer soaking of four and eight weeks did not have an increased effect. The increase in yield of about 10% by soaking can be the diffusibility effect of NMP into the coal inside. The yield of 29% shows that the diffusibility is still incomplete because 54% of whole coal should be soluble in NMP at room temperature as described above. The sites of intermolecular interactions that NMP can dissociate at room temperature may be limited. Table 2 shows the extraction yields with NMP at 100300 °C. By treatment at 100 °C, the yield greatly increased to 48 wt % (daf). The effect of treatment at 100 °C can be an increased diffusibility of NMP, since there are few covalent bonds that are cleaved at 100 °C. Therefore, NMP would diffuse into the inside of coal by dissociating noncovalent interactions in the coal. Furthermore, the treatments at 200 and 300 °C increased the yields to 56 and 58%, respectively. Thus, the difference between 200 and 300 °C was not observed, showing (10) Takanohashi, T.; Terao, Y.; Iino, M. Fuel 2000, 79, 349.
that soaking at 200 °C might be sufficient for NMP to diffuse into the coal. It should be noted here that the value of extraction yield (56%) at 200 °C is close to 54%, that is the yield of NMP solubles (on the whole coal basis) obtained from fractionation of whole CS2/NMP extract, as shown in Table 1. Thus, the NMP soluble yield after 200 °C treatment was similar to the NMP-soluble yield after CS2/ NMP extraction at room temperature. This result suggests that the strength of CS2/NMP at room temperature can be almost similar to that of NMP at 200 °C. From the results described above, we may not conclude the effect of CS2 in the CS2/NMP extraction at room temperature. However, it should be noted here that the diffusibility of NMP into the coal inside has a significant effect on the extraction yield with NMP alone. We reported8 that CS2 can interact relatively strongly with higher rank coals such as Upper Freeport coal. Thus, CS2 may be able to diffuse in the micropores of Upper Freeport coal since it has a straight shape and a high affinity with higher rank coals. In addition, CS2 was reported1 to show a high solubility to n-paraffins (aliphatic compounds). It is known that CS2 alone gives low swelling ratios and extraction yields to any coals.1,11 Investigating how CS2 serves to enhance the diffusibility of NMP into the coal micropores will be further needed. Acknowledgment. This work has been carried out as one of the “Research for the Future” projects of the Japan Society for the Promotion of Science (JSPS) through the 148th committee on coal utilization technology of JSPS. EF020141H (11) Takanohashi, T.; Terao, Y.; Iino, M.; Yun, Y.; Suuberg, E. M. Energy Fuels 1999, 13, 506.
