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Energy & Fuels 1997, 11, 194-201
Modeling of Two-Stage Coal Coprocessing Process Prakash K. Ramdoss and Arthur R. Tarrer* Department of Chemical Engineering, Auburn University, Auburn, Alabama 36849 Received May 1, 1996X
Coal coprocessing can be done either in a single-stage or in a multistage process. It has been found that multistage processing offers some advantages over single-stage processing. In this study, a two-stage process was selected for coal liquefaction. In the first stage (solvent production stage), the waste materials were liquefied at milder conditions to obtain a suitable solvent for the coal liquefaction. In the second stage, the liquid product obtained from the first stage was used as solvent for coal. The two-stage liquefaction process was carried out in the absence of externally added catalysts. The reactions were carried out in a tubing bomb microreactor at temperatures of 350-450 °C and 1250 psig of cold hydrogen pressure for reaction times of 15120 min at 10% coal loading. Total conversion, and conversion to asphaltenes, oils, etc., were monitored. A model for coal liquefaction has been developed as part of this study. A rigorous parameter estimation method was used to estimate the model parameters. The predictions for the coal conversions were found to be within (3% of the observed values. Total conversion as high as 99% and oil conversion up to 90% were obtained in two-stage processing. Two-stage processing leads to higher coal conversion and oil yield compared to single-stage processing. The model parameters for the single-stage and two-stage processes were compared.
Introduction Currently, over 44 billion pounds of plastic waste materials, 280 million automotive tires, and about 1.2 billion barrels of waste oil are generated in the United States each year.1 Disposal of these waste materials poses a great environmental problem. Coprocessing these waste materials with coal is one way of utilizing these waste materials without having to dispose of them.2 This approach not only solves the growing environmental problem associated with the disposal of these waste materials but also recovers the valuable materials by converting them into useful products. In general, the difference between coal coprocessing and direct coal liquefaction is that in coprocessing there is no recycle solvent and instead fresh solvent is used continuously during the process. Unlike direct liquefaction, the solvent used for coal liquefaction is also a reactant and is upgraded simultaneously. The chemical and physical properties of this solvent change significantly during the process.3,4 Tetralin was found to be a good hydrogen donor solvent of the many organic solvents studied for coal liquefaction.5 It can significantly increase the coal liquefaction yields, but it is very expensive to use. It has been shown that good hydrogen donors such as tetralin and 9,10-dihydrophenanthrene promote the production of light products during coprocessing of Illinois No. 6 coal with Maya resid.6 In * Author to whom correspondence should be addressed. X Abstract published in Advance ACS Abstracts, November 15, 1996. (1) Huffman, G. P. Coliquefaction of Waste Material with Coal; Final Report, Contract DE-AC-22-94PC92100, Dec 1994. (2) Haggin, J. Chem. Eng. News 1986, Sept, 22, 34. (3) Whitehurst, D. D.; Mitchell, T. D.; Farcasiu, M. Coal Liquefaction; Academic Press: New York, 1980. (4) Awadalla, A. A.; Smith, B. E. Fuel 1982, 61, 631. (5) Schindler, H. D. Coal LiquefactionsA Research and Development Needs Assesment; Final Report, Contract DE-AC01-87ER30110, Vol. 2, p 4. (6) Curtis, C. W.; Tsai, K. J.; Guin, J. A. Fuel Process. Technol. 1987, 16, 71.
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liquefaction of coal, the hydrogen donor ability of the solvent is not the only criterion that influences the coal dissolution; the physical properties of the solvent also play an important role.3,4 Coal coprocessing studies with tire showed that the high hydrogen content and aromaticity of the rubber were useful in improving the conversion of coal to liquid products.7 Recent studies by Ramdoss et al.8 showed that coal conversions and oil yield increased appreciably following coprocessing with waste grease and waste oil. Yan et al.9 used inexpensive solvents such as fluid catalytic cracking bottoms and heavy oils for coal liquefaction. The free radicals formed during the dissolution of coal may be stabilized by hydrogen transfer from hydroaromatic species within the solvent. Rincon et al.10 studied coal liquefaction with mixed oils (anthracene oil and petroleum heavy oil) and showed that a positive synergistic interaction was obtained by using the mixed solvents. Taghiei et al.11 studied the feasibility of coliquefaction of waste plastics with coal. They found that total conversion level reached over 90% and oil was as high as 60-80% in the presence of HZSM-5 zeolite and iron catalysts. The oil yields for coal-plastic were higher than the oil yield for coal and plastics alone, implying synergistic effect. The recent trend in coal coprocessing is toward two-stage processing.12 In the first stage of the two-stage coal coprocessing, the waste materials were hydrocracked and thermocracked at temperatures between 350 and 450 °C. This hydrocracked material is then used as the solvent for the coal liquefaction in (7) Liu, Z.; Zondlo, J. W.; Dadyburjor, D. B. Energy Fuels 1994, 8, 607. (8) Ramdoss, P. K.; Kuo, C. H.; Tarrer, A. R. Energy Fuels 1996, 10, 996. (9) Yan, T. Y.; Espenscheid, W. Fuel Process. Technol. 1983, 7, 121. (10) Rincon, J. M.; Angulo, R. Fuel 1986, 65, 899. (11) Taghiei, M. M.; Feng, Z.; Huggins, F. E.; Huffman, G. P. Energy Fuels 1994, 8, 1228. (12) Ramdoss, P. K.; Tarrer, A. R. Modeling of Two-Stage Coal Coprocessing. Presented at the AIChE Annual Meeting, Miami, 1995.
© 1997 American Chemical Society
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Table 1. Characteristics of Sample Bank Waste Oil A specific gravity sulfur (wt %) water (wt %) ash (wt %) Al (ppm) As (ppm) Ba (ppm) Ca (ppm) Co (ppm) Cr (ppm)
0.888 1.01
