Aqueous alkaline liquefaction of Texas lignite
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Small particles of Texas lignite, Wilcox formation, were reacted with 0.5 and 0.25N solutions of sodium hydroxide in a two-liter batch reactor. The reaction times varied from 1 to 8 hours and were carried under an autogeneous pressure. Temperature levels of 100, 130 and 160Â°C were used. Maximum conversion levels of around 75 to 90% were discovered for these temperature- time combinations. The behavior of the kinetic data for this reaction system pointed to a two-step mechanism for the thermal dissolution of the lignite. A special numerical technique was developed to retrieve the best set of kinetic parameters, for the simplified two-step reaction model, that can represent the experimental data. For the first reaction, an activation energy of around 3000 kcal/mole was found for both a first- and second-order reaction, showing some kind of diffusion-controlled reaction or the breakage of yery weak bonds. The latter hypothesis was selected because the product extracted was found to be soluble in organic solvents in which the original coal was not soluble. Asphaltene yields of up to 10% weight fraction from the soluble fraction were reported. The second step of the reaction had an activation energy of around 16,000 kcal/mole that will represent the reaction of soluble material back to insoluble. Higher extraction yields and asphaltene production was reported for the alkaline concentration, indicating a direct correlation between ionic strength and the amount of lignite reacted. The results presented in this thesis validate the concept of the use of alkaline solutions as the optimal solvent for in-situ lignite liquefaction due to their high extraction yields, low severity operating conditions, and the low cost solvent used.