Energy & Fuels 2000, 14, 671-676
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Thermal Cracking of Athabasca Bitumen: Influence of Steam on Reaction Chemistry Richard P. Dutta, William C. McCaffrey,* and Murray R. Gray Department of Chemical and Materials Engineering, 536 Chemical and Materials Engineering Building, University of Alberta, Edmonton, Alberta, Canada T6G 2G6
Karlis Muehlenbachs Department of Earth and Atmospheric Sciences, 3-22 Earth Sciences Building, University of Alberta, Edmonton, Alberta, Canada T6G 2G6 Received October 27, 1999. Revised Manuscript Received February 9, 2000
Thermal cracking of Athabasca bitumen at various reaction conditions with and without the presence of steam was investigated to determine if steam has a chemical influence on coking. The reactions were done in 15 mL microautoclave reactors and a 3” diameter fluidized bed coking pilot unit over a range of reaction severity (350-530 °C, 10-60 min reaction time). The differences between reactions with and without steam were investigated by comparing elemental composition of the products and coke yield. The presence of steam decreased coke yield and decreased sulfur removal, and reduced the H/C ratio of the liquid products. Hydrogen exchange from steam to thermally cracked bitumen molecules was tested by doping water with deuterium oxide (D2O) and analyzing liquid and coke products by NMR and stable isotope mass spectrometry, respectively. Preferential deuteration of benzylic carbons was observed along with a trend of increasing deuterium transfer to liquids and coke as reaction severity increased. Free-radical, ionic, and physical mechanisms that can account for these experimental results are discussed.
Introduction Over the past decade there has been an increasing need to produce lighter hydrocarbons from heavy oil feedstocks. One of the important technologies used in upgrading of bitumen and petroleum residues is fluidcoking. This process uses a fluidized bed of hot coke particles to crack the feedstock. The bed is fluidized by a mixture of product vapors and steam, which also acts as a stripping medium to remove the distillate from the surface of the coke particles. Steam has been considered to be chemically “inert” in this process, in that it does not influence the product quality or yields. To avoid the production of sour water, the use of other fluidizing media, e.g., methane, have been considered as an alternative to steam. Therefore, the question as to the influence of steam on thermal cracking is an important one and this study was undertaken to determine this on a micro-lab scale and on a larger pilot scale. Much of the previous research into the chemical reactions of water with hydrocarbons has been limited to the lower temperature regime associated with thermal maturation of kerogen in aqueous environments ( 2° > 1°; therefore, either type of reaction intermediate could explain the NMR data. If the added steam interacted with the free radical intermediates involved in cracking and in coking reactions, then a single mechanism could potentially underlay the observed changes in coke yield and sulfur content and the NMR data for deuterium addition. If hydrogen were abstracted from water by large organic radicals, then these species would be stabilized. The resulting hydroxyl radical would then abstract a hydrogen from elsewhere in the liquid mixture. Hydrogen abstraction would account for the benzylic substitution by deuterium, and the shift in the radical population would alter the pattern of cracking and coking. The
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homogeneous abstraction of hydrogen from water is thermodynamically unfavorable in comparison to abstraction from hydrocarbon species. A free radical mechanism, therefore, could only proceed with an activation step such as adsorption of water on an active surface. The pattern of benzylic substitution of deuterium was also consistent with hydrogen exchange with water via carbocation intermediates. The formation of such ionic species would require the presence of strong acids, for example, on the surface of clay contaminants in the Athabasca bitumen. If such acidic sites were present, then they would also tend to promote hydrolysis of thioethers in the presence of water. Such reactions would tend to reduce the molecular weight of the bitumen, thereby suppressing coke formation. Hydrolysis would tend to increase the release of sulfur as hydrogen sulfide, however, opposite to the observed trend. The influence of acidic clays on aqueous reaction chemistry, particularly ionic chemistry, has been studied by Siskin and Katritzky.1,17 They have concluded that clays can enhance ionic reactions in water, and that water itself can act as an acidic catalyst. Both a free-radical and ionic mechanism could explain the nonpreferential deuteration observed at 350 °C. At this low temperature, substantial C-C bond cleavage in the bitumen for production of free radicals does not occur and acid-catalyzed cracking to produce stable carbocations would also be reduced. Pilot Plant Coking Study. The small batch reactors give a closed reaction system, which does not allow for removal of distillates as they are produced. The MCR content of Athabasca bitumen is 15.4%. Data from thermal cracking at 450 °C and 45 min reaction time has shown that coke yield exceeds 20 wt % in the closed reactor, due to further cracking of light hydrocarbons to produce gases and coke. The closed reactors also give a pressure inside the reactors that is higher than in an industrial process. Fluid-coking operates at approximately atmospheric pressure, whereas the reactors used in this study operate at approximately 1.1 MPa. The gas-phase residence time associated with fluid-coking is in the order of seconds, whereas the residence time of reactions in the microautoclaves is in the range 1060 min. These factors made it necessary to verify the deuterium exchange mechanism in a pilot plant that simulates this commercial coking process more closely. The %D data for liquids generated from the thermal cracking of bitumen in the pilot plant at 530 °C showed a decrease in deuterium transfer compared to the deuterium incorporation observed in the batch reactors (see Figure 4). This result was expected because of the decrease in gas-phase residence time and reactor pressure. The reaction severity that the liquids experienced in the pilot unit was low compared to the reactions in the microautoclave reactors (see data in Figure 5; Har for liquids produced from pilot unit is 8%). However, (17) Siskin, M.; Katritzky, A. R. Science 1991, 254, 231-237, and references therein.
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the results do show that preferential incorporation of deuterium at the benzylic position occurs at low pressures at 530 °C. This result is evidence that the underlying hydrogen-exchange mechanisms proposed from the results obtained at lower temperatures and in closed microautoclave reactors also holds under fluidcoking process conditions. The %D transferred to the coke is much lower than in the batch reactor systems. The decrease is due to decreased contact time of the steam with coke in the pilot unit. Also, the pilot unit has seed coke in the reactor prior to the start of the reaction. This coke was not produced by coking in the presence of D2O, and resulted in a low value for the %D in coke analyzed after the reaction. Conclusions Thermal cracking of Athabasca bitumen in the presence of steam showed a reduction in coke yield, a reduction in sulfur removal, and a reduction in H/C ratio, compared to thermal cracking without steam. NMR analysis of the liquids produced from thermal cracking of bitumen have shown preferential deuteration of the benzylic carbons except for coking at low reaction temperatures (350 °C). Coking reactions carried out in a pilot unit under conditions similar to commercial fluid-coking conditions, show this preferential deuteration of the benzylic carbons and suggest that even with the short residence time and low pressures of fluid-coking, water-hydrogen may be exchanged/ donated to the reacting bitumen. Stable-isotope mass spectrometry has shown deuteration of the coke produced by thermal cracking of bitumen but to a lesser extent than deuterium incorporation into the liquid fraction. Two mechanisms of hydrogen exchange between steam and hydrocarbons can explain the preferential deuteration at the benzylic position. These are a freeradical pathway and an ionic pathway involving carbocations. The overall effect of both mechanisms is hydrogen exchange, which could lead to a shifting of reactive sites away from aromatic molecules and onto aliphatic chains. This could explain the reduction in coke yield when bitumen is thermally cracked in the presence of steam. The results have shown that steam used in the fluidcoking process for bed-fluidization and stripping is not chemically “inert”. The presence of steam could influence the chemistry of the process that may alter the product quantity and quality. Acknowledgment. The authors thank G. Aarts and G. Bigam of the NMR facility, Department of Chemistry, University of Alberta, for NMR analysis. Thanks to Dr. I. Huq, S. Gillis, and M. Noble for running the reactions on the pilot plant unit at Syncrude Research Center, Edmonton. Thanks to Dr. E. Chan (Syncrude Research) for many useful discussions. The work was supported by Syncrude Canada Ltd. EF990223E
