| Description |
Batch thermal solution of Hell's Hole Canyon oil shale in hydrocarbon solvents has been studied at temperatures of from 274° to 410°C. Three solvents were used: cyclohexane; 1,2,3,4-tetrahydronaph-thalene; and creosote oil. Five hundred mi 11iliters of solvent and 70 grams of -8 +28 mesh shale particles made up the charge to the 1-liter autoclave reactor. In 24-hour, constant-temperature runs under autogenous pressure, each solvent was found to be capable of removing over 90 percent of the original organic material at extraction temperatures of from 360°-375°C. Cyclohexane (in the supercritical state at all extraction temperatures) extraction resulted in somewhat greater yields than tetralin extraction in runs below 365°C but approximately the same yields as tetralin at higher extraction temperatures. With one oil shale sample, virtually identical yields (which were from 5 to 30 percent lower than those noted for all other samples at comparable temperatures) were observed in both cyclohexane and tetralin extractions. This finding suggests that mass-transfer restrictions within some oil shales may restrict the yield regardless of the solvent used. Carbon-13 nmr analyses of the spent shales showed that the aromaticity of the residual organic material in tetralin-extracted shale increased slightly from the raw shale value of 0.27 and remained essentially constant at a value of 0.31 as extraction temperature increased. Uniform removal of both aromatic and aliphatic organic carbon from the shale is thereby indicated. Pyrolytic processing, in contrast, removes only aliphatic carbons. In constant temperature runs of 24-hour duration, the aromaticity of an oil shale sample increased from the raw oil shale value of 0.17 to 0.28 after cyclohexane extraction at 326°C, while a tetralin-extracted sample of the same oil shale had an aromaticity of only 0.20 after extraction at 321°C. This finding suggests that tetralin is slightly more effective than cyclohexane at removing aromatic portions of kerogen. The nitrogen content of the cyclohexane extracts, which ranged from 1.58 to 2.24 percent, was comparable to that of values reported in the literature for shale oils. The tetralin extracts had somewhat lower nitrogen content, ranging from 0.58 to 1.95 percent. Extract nitrogen content generally increased with extraction temperature. Simulated distillation by gas chromatography shows that the boiling range of the extracts shifted to lower temperatures as extraction temperature increased. An exception was observed for a very rich oil shale, for which a maximum in production of low-boilers with cyclohexane was observed at an extraction temperature of 365°C; either lower or higher extraction temperatures produced a higher boiling extract. Cyclohexane extracts were generally lower boiling and of wider boiling range than tetralin extracts: 48.5% of a 320°C cyclohexane extract of a 17 gal/ton shale boiled below 375°C and 19.8% boiled between 375° and 425°C; for a 326°C tetralin extract of the same shale, the corresponding numbers were 23.2% and 54.8%. Liquid-phase extraction with tetralin may therefore be considered to be a more gentle process in that less alteration of the original organic structure occurs. Kinetic parameters for tetralin extraction were determined by integral, differential, and difference-differential analysis of yield data taken by exposing the sample charge to a temperature ramp (0.2°C/min) then quenching after the desired maximum temperature had been reached. A competitive first-order reaction mechanism for the formation of the soluble bitumen was suggested by the results of each of the analyses: Kerogen --------> Bitumen T<350°C Bitumen T>35O°C. No evidence was found to indicate a difference between the low and high temperature reaction products, the proposed mechanism merely states that two parallel pathways for bitumen formation exist. Mathematically fitting the conversion data to this model resulted in the calculation of the following Arrhenius activation energies and pre-exponential factors: Ai = 2.2 x lOVmin T < 350°C Ej = 20.8 kcal/mole A2 = 1.7 x 1013/min T > 350°C . E2 = 45.7 kcal/mole Good agreement with literature values for low-temperature kerogen conversion was noted. |
