1030
Energy & Fuels 1999, 13, 1030-1036
Preparation of Stabilized Fiber from Avgamasya Asphaltite Hadi A. El Akrami, M. F. Yardim, and E. Ekinci* Chemical Engineering Department, Istanbul Technical University, 80626 Maslak, Istanbul, Turkey Received January 19, 1999. Revised Manuscript Received May 24, 1999
This paper describes different procedures applied for the preparation of stabilized fibers from Avgamasya asphaltite products. Three different methods of solvent extraction, vacuum distillation of fluidized bed pyrolysis tar, and air blowing of the vacuum distilled products were used in this study. The asphaltite pitches obtained by different methods are characterized by various techniques such as elemental analysis, thermogravimetric analysis (TGA and DTG), softening point (SP), Fourier transform infrared spectroscopy (FT-IR), and solution 1H and 13C nuclear magnetic resonance (NMR) spectroscopy. Structural characterization by elemental analysis, FTIR, and nuclear magnetic resonance spectroscopy on both vacuum-distilled and air-blown pitches showed that distillation of lighter fractions, reduction of atomic H/C ratio, and dehydrogenation raised their softening points. Structural analysis by NMR showed that asphaltite pitches consist of condensed aromatic rings combined with methylene bridges and heterocyclic groups, and substituted with alkyl and naphthenic groups. Asphaltite pitches which were prepared by solvent extraction (HI-BS) and vacuum distillation (AVD3) resulted in very good and medium spinnable green fibers, respectively. On the other hand, air-blown pitches could not be spun. The morphology of the stabilized fibers was studied by scanning electron microscopy (SEM).
1. Introduction The high cost of the present carbon fiber precursors led researchers to investigate new and cheaper sources. Hence, scientists all over the world have been endeavoring to develop materials suitable to their native countries and attempting to find new ways to make carbon fiber. As a result of this, coal tar pitch, petroleum pitch, and oil shale based pitches are studied for carbon fiber production. These alternative products have reasonable strength and stiffness and are much cheaper than polyacrylnitrile (PAN).1 To expand the possible sources of carbon fiber production, new concepts emerge depending on favorable parameters, such as suitable molecular structure and effective processing technologies. Even though asphaltites have the physical appearance of coal, they are derived from crude oil which migrated to inorganic formations. By the effect of catalytic activity of inorganic matrix, metamorphosis has taken place to produce condensed heavier hydrocarbon structures due to various chemical, physical, and biological changes. Avgamasya asphaltite is a representative deposit of the asphaltic substances of Southeastern Turkey which range from asphaltite to asphaltic pyrobitumen according to their degree of alteration. It is geochemically classified as a solid aromatic-asphaltic oil containing mineral matter derived from nearby Jurassic-Cretaceous oil deposits by alteration during migration.2 * Corresponding author. (1) Edie, D. D.; Dunham, M. G. Carbon 1989, 27, 647-655.
In previous years Turkish asphaltites were subjected to various liquefaction studies for the main purpose of synthetic fuel production.3-5 For this purpose pyrolysis, solvent extraction, supercritical solvent extraction, and moderate pressure solvent extraction processes6-8 are used, which result in high tar yields with asphaltenerich fractions. Due to the high molecular weight distribution of asphaltenes, there is a substantial thermally stable species and relatively lower volatile fraction. Therefore, asphaltite is recognized as being an encouraging starting point for carbon fiber production. 2. Experimental Section 2.1. Material. Asphaltite samples used in this study are obtained from a mine located in the Avgamasya vein near Sirnak in the Siirt province. The total asphaltite reserves of Turkey are approximately 50 million tons, and Avgamasya asphaltite constitutes the largest reserve with 14 million tons. Avgamasya asphaltite pos(2) Bartle, K. D.; Ekinci, E.; Frere, B.; Mulligan, M.; Sarac, S.; Snape, C. E. Chem. Geol. 1981, 34, 151-164. (3) Ekinci, E.; Turkay, S. Synfuels 2nd Worldwide Symposium, Belgium, 1982. (4) Yardim, M. F. Characterization of Avgamasya asphaltite pyrolysis products and structure studies. M.Sc. Thesis, Istanbul Technical University, 1984. (5) Yardim, M. F.; Tolay, M.; Ekinci, E.; Bartle, K. D. Erdo¨ l und Kohle, Petrochemie Erdgas 1986, 36, 471-472. (6) Ekinci, E.; Sarac, S.; Bartle, K. D. Fuel 1982, 61, 346-350. (7) Yurum, Y.; Ekinci, E. Composition Geochemistry and Conversion of Oil Shales; Snape, C. E., Ed.; Kluwer Academic Publication: London, 1995; pp 329-346. (8) Erdem-Senatalar, A.; Kadioglu, E.; Tolay, M.; Bartle, K. D.; Snape, C. E.; Taylor, N. Fuel 1985, 64, 1748-1753.
10.1021/ef990006d CCC: $18.00 © 1999 American Chemical Society Published on Web 07/23/1999
Stabilized Fiber from Avgamasya Asphaltite
Energy & Fuels, Vol. 13, No. 5, 1999 1031
Table 1. Proximate and Sulfur Forms Analysis of Avgamasya Asphaltite proximate analysis component
(wt %)
sulfur forms
(wt %)
moisture ash volatile matter fixed carbon calorific values
1.58 37.70 32.69 28.03 5522 kcal/kg
sulfatic combustible pyritic organic
0.15 5.80 1.46 5.14
Table 2. Characteristics of the Avgamasya Asphaltite ultimate analysis component
(wt %)
carbon hydrogen nitrogen sulfur oxygen
49.75 3.88 1.12 6.76 0.79
distribution of pyrolysis product9 component oil yield liquor gas pyrolysis residue pyrolysis losses
(wt %) 11.60 2.70 7.5 74.5 3.7
sesses the average characteristics of all the other reserves of Turkey; therefore, it was chosen as the case reserve in this study. The proximate and sulfur forms analysis of Avgamasya asphaltite and its general characteristics are given in Tables 1 and 2, respectively. 2.2. Preparation of Pitches. A pitch prepared for carbon fiber production requires a high softening point and good spinnability. If the softening point of the pitch is low, it becomes very difficult to prepare the infusible fiber without trouble in the subsequent stabilization process. For this reason, ordinary tars or pitches need to be treated by heating and/or extraction prior to spinning. Asphaltite pitches were prepared by three methods: (1) solvent extraction, (2) pyrolysis followed by vacuum distillation, and (3) air-blowing of some of the vacuum-distilled tars. For the preparation of solventextracted pitches asphaltite sample was ground to -310 µm +150 µm and then Soxhlet extracted with pyridine. An amount of 25 g of asphaltite was heated with 250 mL of pyridine at its boiling point (116 °C) and atmospheric pressure in a 500 mL flask for 24 h. The yield of solvent-free extract was 8.3% on the original basis. The product (bitumen) obtained was fractionated by different solvents such as n-hexane, benzene, and tetrahydrofuran. The resultant products were classified into hexane soluble (HS), hexane insoluble-benzene soluble (HI-BS), hexane insoluble-tetrahydrofuran soluble (HI-THFS), and tetrahydrofuran insolublepyridine soluble (THFI-PS) fractions. All solvents were removed from the extracts by rotary vacuum evaporation. Prior to vacuum distillation the asphaltite tar was first extracted with pyridine and filtered to remove pyridine insolubles, and then distilled to obtain quinoline insoluble (QI) free asphaltite tar pitch. The vacuum distillation of Avgamasya asphaltite tar was carried out at a pressure of ∼0.7 kPa and at temperatures of 320, 340, and 410 °C for 60 min. The pitches obtained at the three corresponding temperatures were coded as AVD1, AVD2, and AVD3. An amount of 15 g of vacuum-distilled sample of asphaltite (AVD1) was subjected to air blowing at 330 °C in a vertical electric furnace for 240 and 360 min. The blown air was introduced into the bed of pitch at (9) Onen, A.; Sarac, S.; Onen, A.; Ekinci, E. Fuel Process. Technol. 1992, 32, 151-158.
room temperature at a calibrated flow rate of 223 mL/ min. The pitches obtained at the two corresponding airblowing times are coded as AAB1 and AAB2, respectively. 2.3. Characterization of Pitches. The elemental and proximate analyses were performed using conventional methods. The softening points of the pitches were measured by a ring-and-ball method according to a standard procedure and thermal analysis was performed in N2 flow by using simultaneous TGA/DTG measurements in a Stanton-Redcroft STA-781 thermoanalyzer. The FT-IR and NMR spectra were used for characterization of the molecular nature of the pitch precursors. Infrared spectra of all pitches were obtained with a Nicolet 20 SXC FT-IR spectrophotometer. Solid samples were prepared as standard KBr pellets. The distilled pitches were analyzed by 200 MHz solution 1H and 13C NMR (Varian Gemini-200 NMR spectrometer), using CDCl3 and TMS as solvent and internal standard, respectively. The morphology of the produced fibers was observed by scanning electron microscopy (SEM). 3. Results and Discussion 3.1. General Properties of the Parent and Derived Pitches. Some properties of the asphaltite tar obtained from fluidized bed pyrolysis, and solvent extraction are compiled in Table 3. The extracted bitumen has lower C and H contents compared to the tar. On the other hand, the nitrogen and sulfur contents are higher for the extracted bitumen. Also, the pyrolysis tar is more aliphatic in character (H/C ) 1.35) compared to the extracted bitumen. The decrease in S/C ratio for the pyrolysis tar compared with the bitumen suggests the breakage of sulfide linkages and loss of sulfur as H2S during the pyrolysis process. The lower values of the O/C and N/C ratios for the pyrolysis tar compared with the bitumen indicate that nitrogen and oxygen structures are incorporated in the kerogen and are released on heating.6 General properties of the solvent-extracted fractions are shown in Table 4. The softening point of the solventextracted pitches increased with decreasing H/C ratio in the order of PS < HI-BS < HI-THFS. The pitch of light pre-asphaltene (obtained by THF) has a softening point as high as 220 °C compared to 132 and 93 °C for asphaltene and bitumen, respectively. The atomic H/C ratios of solvent-extracted pitches, obtained from elemental analysis, showed that HI-THFS fractions have lower values than HI-BS and PS fractions. This indicates that the HI-THFS is more aromatic than HIBS and PS fractions. The aromaticity (H/C atomic ratio) and the softening points of the solvent-extracted pitches were found to be dependent on the solvent effectiveness. The vacuum-distilled pitches were characterized by determining the softening point, yield, and elemental analysis. The characterization results are compiled in Table 5. Vacuum distillation increased the softening point from 25 to 171 °C which concurrently resulted in a decrease of H/C from 1.35 to 0.84. As mentioned above, the H/C ratio is a worthy evidence of the degree of the aromaticity of the pitch. The atomic H/C ratios of the vacuum-distilled pitches, however, were in the same order of magnitude compared with the solvent-extracted pitches (0.84 for AVD3 versus 0.85 for HI-THFS).
1032 Energy & Fuels, Vol. 13, No. 5, 1999
El Akrami et al.
Table 3. General Properties of Liquid Tar and Extracted Bitumen code PS TA
SP (°C)
yield (wt %)
C (wt %)
H (wt %)
N (wt %)
S (wt %)
S/C ×100
N/C ×100
O/C ×100
H/C
93
