Experimental Study On The Hydrogenation Of Low Temperature Coal Tar And System Simulation

Coal pyrolysis and poly-generation technology is a promissing technology, which could improve the coal utilization efficiency and reduce the pollutant emissions. Low temperature coal tar, one of the products of coal pyrolysis, could be transformed into clean fuel oil by hydroprocessing. Though the research on coal tar hydrogenation began several decades ago, it was interrupted a few times, leading to many questions to be answered in this field. The knowledge about the relationship of reactions during hydrogenation is little. The screening of catalysts special for coal tar hydrogenation is lacked. The work about the model and simulation of coal tar hydrogenation is just at the beginning. This thesis focuses on these problems and carries out corresponding researches, aimed to a reference to the following work.Firstly, considering high content of heteroatoms in coal tar, the experimental investigation on quinoline hydrodenitrogenation (HDN) and dibenzothiophene hydrodesulfurization (HDS) over metal phosphides and commercial catalysts was first conducted. Metal phosphide catalyst showed about100%HDS conversion and90%HDN conversion, respectively, higher than the commercial one. The content of sulfur and nitrogen adsorbed on catalyst during reaction was linear with the catalyst activity decrease. The Ni2P with a small amount of Mo exhibited the highest HDN and HDS activity, due to the more acitve sites by electron transfer between Mo and Ni, and suitable acid concentration. The nitrogen-containing compounds in the feed could inhibited the HDS conversion. Prior HDN or increasing reaction temperature to higher than360℃should be carried out to achieve high HDS conversion.Then, eight typical compounds with high content in low temperature coal tar were chosen as the model compounds. The hydrotreatment of these model compounds was executed to screen eight NiW catalysts. The effect of catalyst properties on catalytic performance was also examined. The results showed that the conversions of heteroatom removal and triaromatics hydrogenation were the highest in all, above90%, while that of monoaromatics the lowest, less than20%. Among four NiW catalysts, the one with the Ni/W ratio of0.3showed highest activity of aromatics hydrogenation. Higher activity was due to the thicker and shorter slabs on catalyst. Adding phosphorus could adjust the acid properties on the surface of catalyst. However, excess phosphorus would block the pores on catalyst, resulting in the difficulty of metal reduction and dramatic activity decrease. The medium strength acid site was the responsible for the hydrogenation of aromatics. The catalyst with the best hydrotreating performance contained6wt%nickel,20wt%tungsten, and0.5wt%phosphorus. The activities of Ni2P, commercial catalyst and NiWP-0.8were compared. Ni2P showed better HDN and HDS acitivity, but poorer HDO activity than commercial catalyst. NiWP-0.8performed the best amoung the three catalyst on heteroatom removal.Based on the model compounds hydrogenation research, a series of experiments on the hydrogenation of low temperature coal tar distillate were implemented, investigating the effect of reaction temperature, pressure, and liquid hourly space velocity (LHSV) on product composition. The results revealed that cycloalkane and partially saturated aromatics were the main product compounds after hydrogenation. High temperature, high pressure, and low LHSV favored the generation of light component. By hydrogenation, heteroatom and aromatics content decreased significantly. H/C molar ratio increased from1.2to1.6～1.7. Initial boiling point dropped to80℃. The distillate was upgraded via hydrogenation. On the basis of product composition, a reaction network including phenol and alkylnaphthalene was proposed. Slightly higher H/C molar ratio and much lower oxygen content in hydrotreating product were observed over NiWP-0.8catalyst than the commercial one.Finally, besed on the previous experimental data of coal tar distillate hydrogenation, a simulation of low temperature coal tar hydrotreatment-hydrocracking technology process was developed. The effect of reaction temperature and pressure on the yield of gasoline and diesel oil distillate and the system energy consumption was investigated. The results showed that under375℃,7MPa, the yields of gasoline and diesel oil distillate were27.03and68.69%, respectively. Heating for the feed and hydrogen compressor were the main source of system energy consumption, constitute58.7%and24.3, respectively, of the total energy consumption. Elevating reaction temperature or pressure could improve the yield and quality of product, accompanying with the increase of system energy consumption. The parameters of high pressure gas-liquid separator and cyclic hydrogen desulfurizing tower were optimized as well.