Liquefaction complex, hydrogen

Direct Liquefaction Kinetics Hydrogenation of coal in a slurry is a complex process, the mechanism of which is not fully understood. It is generaly believed that coal first decomposes in the solvent to form free raclicals which are then stabilized by extraction of hydrogen from hydroaromatic solvent molecules, such as tetralin. If the solvent does not possess sufficient hydrogen transfer capability, the free radicals can recombine (undergo retrograde reactions) to form heavy, nonliquid molecules. A greatly simplified model of the liquefaction process is shown below. 

Figure 10. Block flow diagram for hydrogen purification-liquefaction complex ( ll)...

A more complex reaction model was proposed from the results of a kinetic study of thermal liquefaction of subbituminous coal. Data were obtained over a temperature range of 673 to 743 K (752 to 878°F) at 13.8 MPa (2000 psia) by using two solvents, hydrogenated anthracene oil (HAO), and hydrogenated phenanthrene oil (HPO), at a coal-solvent ratio of 1 15. Results were correlated with the following model ... 

Sternberg et al. (7) showed that the presence of asphaltenes in coal-derived oils caused a marked increase in the viscosity. This group also showed that these asphaltenes were acid-base complexes and that hydrogen bonding occurs between the acidic and basic components of asphaltenes (9, 10). Recent work (J3, 11) on coal liquefaction bottoms has shown the importance of hydrogen bonding on the viscosity of coal liquids. 

The second, catalytic liquefaction process is similar to the first except that there is a catalyst in direct contact with the coal. ZnCl2 and other Friedel-Crafts catalysts, including AICI3, as well as BFj-phenol and other complexes catalyze the depolymerization-hydrogenation of coals, but usually forceful conditions (375 t25°C, 100-200 atm) are needed. Superacidic HF-BF3-induced liquefaction of coals8 involves depolymerization-ionic hydrogenation at relatively modest 150-170°C. 

Study of the mechanism of this complex reduction-liquefaction suggests that part of the mechanism involves formate production from carbonate, dehydration of the vicinal hydroxyl groups in the cellulosic feed to carbonyl compounds via enols, reduction of the carbonyl group to an alcohol by formate and water, and regeneration of formate (46). In view of the complex nature of the reactants and products, it is likely that a complete understanding of all of the chemical reactions that occur will not be developed. However, the liquefaction mechanism probably involves catalytic hydrogenation because carbon monoxide would be expected to form at least some hydrogen by the water-gas shift reaction. 

Coal is a complex macromolecule of predominantly aromatic character that lends itself to alkylation reactions in which an alkyl group is substituted into an aromatic ring for a hydrogen atom. Indeed, the alkylation of coal has received considerable attention as a means of increasing the solubility of coal in various organic solvents thereby enhancing the reactivity to liquefaction (Larsen and Kuemmerle, 1976 Davidson, 1982 Given, 1984 Stock, 1987 Baldwin et al., 1991 Sharma and Mishra, 1992). 

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