Effect of Addition of K to Ni/SiO2 and Ni/Al2O3 Catalysts on Hydrothermal Cracking of Bitumen

注意：本論文已在《Sekiyu Gakkaishi (J. Japan Petrol. Inst.) 》43 (5), pp.357. 發表
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Jie CHANG, Noritatsu TSUBAKI*, and Kaoru FUJIMOTO
Department of Applied Chemistry, School of Engineering, The University of Tokyo,
Hongo 7-3-1, Bunkyo-ku, Tokyo, 113-8656, Japan

Abstract:Hydrothermal cracking of Canadian Athabasca bitumen was performed over Ni/SiO2 and Ni/Al2O3 catalysts under 703 K and 5.0 MPa of hydrogen pressure in a batch reactor. Comparing with thermal cracking under the same reaction conditions, hydrothermal cracking process obviously suppressed the formation of coke from 5.5wt% to 3.5wt% (Ni/SiO2) and to 3.0wt% (Ni/Al2O3), and the formation of gaseous hydrocarbon products. To decrease coke formation further, a small amount of potassium was impregnated in the catalysts. The spectroscopy of NH3-TPD showed that the amount of acidic sites in both catalysts, Ni/Al2O3 and Ni/SiO2, was dramatically decreased by K2O modification. The acid-catalyzed polymerization of residuum induced by acidic sites in the catalyst, that might result in the formation of coke, was suppressed by neutralization of the acidic sites. Adding 3% of potassium onto Ni/SiO2 decreased the yield of coke from 3.5wt% to 2.1wt%.
Keywords:Hydrothermal process, Catalyst modification, Acidity, TPD

1. Introduction
Hydrothermal cracking of heavy oil is a tailor-made combined process of thermal cracking and catalytic hydrogenating that has been developed to produce high quality transportation fuels and middle distillate products from residual oils1). This process can suppress the formation of gas and coke , and it can as well promote the yield of middle distillate (kerosene + gas oil) compared with thermal and hydrocracking at the same levels of conversion2), 3). Higher reaction temperatures can increase residuum conversion, but the selectivity of gas and coke will be increased simultaneously.
Coke is an unavoidable by-product in any heavy oil upgrading processes. But the mechanism of coke formation during petroleum refining is only now beginning to be understood4). It is closely related to the mechanism of thermal decomposition of petroleum constituents and changes in the character of the liquid medium. When catalytic hydroprocess is applied to a residuum, some molecular species within the asphaltene fraction, which contain nitrogen and other heteroatoms, or complex molecules formed during reaction, are absorbed readily by the catalyst. These species may condense and degrade to coke, and eventually may deactivate the active sites of the catalyst. The authors modified hydrothermal cracking catalysts by adding potassium and these catalysts obviously suppressed the formation of coke in the upgrading of bitumen.

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