| Description |
The total, domestic tar sand resource is estimated to be about 60 billion barrels and is geographically scattered and geologically diverse. The domestic resource is consolidated and oil wet, that is, the sand grains are cemented together and the oil sticks directly to the sand grains. Tar sand bitumen is solid at room temperature, with average molecular weights in excess of 500 and specific gravities of about 1.0. Typically, about 70 percent of the bitumen is soluble in a light, aliphatic solvent (e.g., hexane). This fraction is called maltenes. The remaining fraction is soluble in carbon disulfide and benzene and is called asphaltenes. Asphaltenes can lead to coke formation when heating bitumen to temperatures above 316 °C (600 °F) . Extracting synfuels from tar sand resources offers a significant contribution to the nation's liquid fuels needs. Domestic liquid fuels production has declined at several hundred thousand barrels per day per year for the last several years. The U.S. liquid fuels supply is now at 52 percent imports. Since 1985, imports have increased by 3.4 million barrels per day (MMbbl/d) and domestic production has declined by 1.5 MMbbl/d. Proven crude oil reserves are down to 26.8 billion barrels. Undiscovered crude is an estimated 34.8 billion barrels. No mechanisms or activities are currently in place to reverse the trends of declining production and increasing imports. Possible extraction methodologies include both in situ and surface extraction schemes. In situ recovery includes steam drive, steam cycling, and combustion. Less than 10 percent of the U.S. domestic resource is amenable to surface processing, which includes solvent-, thermal-, or water-assisted extraction schemes. If asphalt is a desired by-product, solvent- or waterassisted extraction is preferred since high recovery rates are needed for economics. Pyrolytic recovery upgrades the synfuel, which is good for refinery feedstock but not for asphalt production. Thermal extraction results in reduced average molecular weight, density, and viscosity by breaking down the larger molecules. Solvent recovery economics are sensitive to solvent losses in product or on tailings. The research program supported by the U.S. Department of Energy (DOE) includes a variety of surface extraction schemes. The University of Utah has process development units (PDU) employing fluidized bed, hot, water-assisted, and fluidizedbed/ heat-pipe, coupled combustor technology. Considerable process variable test data have been gathered on these systems: (1) a rotary kiln unit has been built recently; (2) solvent extraction processing is being examined; and (3) an advanced hydrogenation upgrading scheme (hydropyrolysis) has been developed. The University of Arkansas, in collaboration with Diversified Petroleum, Inc., has been working on a fatty acid, solvent extraction process. Oleic acid is the solvent/surfactant. Solvent is recovered by adjusting processing fluid concentrations to separate without expensive operations. Western Research Institute has a PDU-scale scheme called the Recycle Oil Pyrolysis and Extraction (ROPE) process, which combines solvent (hot recycle bitumen) and pyrolytic extraction. As this variety of research shows, liquid fuels recovery is sensitive to process/resource interactions. And, because the domestic resource is so varied, it is necessary to develop several approaches to tar sand recovery. |
