Energy & Fuels 1994,8, 557-560
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Kinetic Effects in Resid Hydrocracking R. H. Heck* and F. T. DiGuiseppi Central Research Laboratory, Mobil Research and Development Corporation, P.O. Box 1025, Princeton, New Jersey 08534- 1025 Received October 4, 1993. Revised Manuscript Received February 4, 1994"
Results from autoclave experiments show that vacuum resid from Maya crude can be hydrocracked to a high yield of light liquid product. However, in order to achieve this high conversion (>-go%) without also generating a high yield of coke, the reaction rate must be low during the initial rapid conversion reactions. This allows the hydrogenation rate of the thermally generated fragments to outpace their rate of condensation to coke. After the initial rapid conversion reactions are complete (> 50%conversion),the temperature can be raised and the reaction rate increased without generating significant amounts of coke. Analysis of the feed and products supports a conversion mechanism involving the cracking of largely aliphatic fragments away from a largely aromatic core, with this core becoming increasingly refractory as conversion proceeds.
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Introduction It has been shown previously14 that resid conversion to lighter products, whether or not it is carried out in the presence of hydrogen and/or a metal catalyst, is largely thermally driven. Hydrogen, especially in the presence of a suitable catalyst to activate it, is effective for suppressing coke formation during resid conversion. But hydrogen, even in the presence of a catalyst, has very little effect on the conversion of petroleum resid to lighter products. The maximum attainable conversion before the onset of significant coking for a 525 O C + Maya resid was shown in autoclave experiments3 to be 25, 33, 50, and 60 wt % respectively when operating in the following four modes: thermal, thermal with gaseous hydrogen, thermal with liquid H-donor addition, and thermal with dispersed molybdenum plus gaseous hydrogen. These results were for a particular set of operating conditions: 13.8 MPa of hydrogen pressure, a relatively narrow range of temperatures (438-454 "C), and reaction times of less than 60 min. The present work explores the effects of temperature, reaction time, and hydrogen pressure on resid conversion. The objective is to explore routes for achieving high resid conversion to lighter products in minimal reaction time without the generation of excessive coke.
Experimental Section A 525 O C + vacuum resid from Maya crude was processed in a stirred autoclave. All experiments were done at 13.8 MPa of hydrogen pressure at temperatures ranging from 393 to 454 "C with reaction times from 24 to 1058 min. Approximately 75 g of resid was weighed into a 300-cm3autoclave, and hydrogen was fed continuously to the autoclave by bubbling into the resid at a rate of 240 Llh. Pressure in the autoclave was held at 13.8 MPa, allowing excess hydrogen and light gaseous reaction a Abstract published in Advance ACS Abstracts, April 1, 1994. (1) Miki,Y.; Yamadaya, S.; Oba, M.; Sugimoto,Y.J . Catalysis 1983, 83, 371-383. (2) Le Page, J. F.; Davidson, M. Reu. IFP 1986,41, 131. (3) Heck,R. H.:Rankel.L. A.; DiGui8eppi.F. T.FuelProcess.Technol. 1992,30,60-81. (4) Gray, M. R.; Khorasheh, F.; Wanke, S. E.; Achia, U.; Krzywicki, A.; Sanford, E. C.; Sy, 0. K. Y.; Ternan, M. Energy Fuels 1992,6,478485.
0887-0624/94/2508-0557$04.50/0
products to escape through a back pressure controller into an evacuated collection bomb. Molybdenum hexacarbonyl was added to the autoclave along with the resid to achieve a Mo concentration of 650 ppm. Molybdenum hexacarbonyl decomposes well below reaction temperatures to yield CO and atomic molybdenum. In the presence of H2S this molybdenum is easily sulfided. Previous studies have shown the formation of clusters of metal sulfide at similar conditions.6 The minimal amount of CO formed from the decomposition of the carbonyl was continuously swept from the autoclave along with the excess hydrogen feed. The autoclave was heated rapidly to reaction temperature ( - 2 5 6 0 min), at which time conversion of the resid to lighter products (e525 "C)ranged between 25 and 35% with minimal formation of either light gas (
