Coal donor solvent

W. R. Eppedy and J. W. Taunton, "Exxon Donor Solvent Coal Liquefaction Process Development," paper presented at Coal Dilemma II ACS Meeting, Colorado Spriags, Colo., Feb. 12, 1979. 

Direct Hydrogenation. Direct hydrogenation of lignitic and other coals has been studied by many investigators. Lignite can be slurried with an anthracene-oil solvent, heated to a temperature of 460—500°C with 1 1 CO H2 synthesis gas at pressures to 28 MPa (280 atm) in a 2 kg/h reactor. The product hquids are separated, and in a commercial process, a suitable hydrogen-donor solvent would be recycled (54). 

W. N. Mitchell, K. L. Trachte, md S. Zaczepinski, "Performance of Low-Rank Coals in the Exxon Donor Solvent Process," paper presented at / 0th Biennial Eignite Symposium, Grmd Porks, N.D., May 1979. 

Fig. 17. Schematic diagram of the coal hquefaction section of the Exxon donor solvent (EDS) process. To convert MPa to psi, multiply by 145.

Several processes progressed to demonstration scales but have not been commercialized, primarily because of economic inabiHty to compete with available petroleum products. The H-Coal process developed by Hydrocarbon Research, Inc. was demonstrated at Catiettsburg, Kentucky using a 545 t/d plant and DOE support. The Exxon donor solvent Hquefaction process was not commercialized either. 

Depending on its rank, coal can be dissolved in as little as one minute in the temperature range of 623 to 723 K (662 to S42°F) in suitable solvents, which are assumed to promote thermal cracking of the coal into smaller, more readily dissolved fragments. These fragments may be stabilized through reactions with one another or with hydrogen supplied either by a donor solvent or from a gas phase. 

Magnitudes of /cg, /cp, /c, and indicate the importance of direct reactions with coal, where and are for hydrocracking reactions in the conversion process. Data for and from the experiments with HPO indicate that oil production from coal is increased by the use of a good hydrogen donor solvent. 

Selection of Solvents. The extraction yield of a low rank coal (Annesley) has been determined after digestion using a selection of solvents (Table II). The results show large variations in solvent power and, in particular, the high extraction yields obtained with hydrogen donor solvents. It is important to differentiate between the ability of a solvent to prevent polymerisation of the dissolved coal by hydrogen transfer, and its ability to retain the dissolved coal in solution. For example, Tetralin is frequently quoted as an... 

Conversion of coal to benzene or hexane soluble form has been shown to consist of a series of very fast reactions followed by slower reactions (2 3). The fast initial reactions have been proposed to involve only the thermal disruption of the coal structure to produce free radical fragments. Solvents which are present interact with these fragments to stabilize them through hydrogen donation. In fact, Wiser showed that there exists a strong similarity between coal pyrolysis and liquefaction (5). Recent studies by Petrakis have shown that suspensions of coals in various solvents when heated to 450°C produce large quantities of free radicals (. 1 molar solutions ) even when subsequently measured at room temperature. The radical concentration was significantly lower in H-donor solvents (Tetralin) then in non-donor solvents (naphthalene) (6). 

If the initial reactions of coal are purely thermal, one might expect that the H-donor level will be of minor importance if times are kept short. In fact, all coals contain a certain portion of material that is extractable by pyridine. On heating coals to liquefaction temperatures, some additional material also becomes soluble in even non-donor solvents. Thus, there is a portion of all coals which can be solubilized with little dependence on the nature of the solvent. 

Solvent additives to the melt (Table 3) fall into two categories extractive and reactive. The extractive solvents (decane, perchloroethane, o-dichlorobenzene, and pyrrolidine) had negligible effect on solubility, possibly due to the preferential wetting of the coal by the solvent and exclusion of the ZnCl2 melt. Reactive solvents (anthracene oil, indoline, cyclohexanol, and tetralin) all incorporated strongly. Donor solvents, tetralin and indoline, increase the "corrected solubility, whereas anthracene oil and cyclohexanol have negligible effect. 

To describe in fundamental terms the dissolution of coal in a hydrogen-donor solvent requires an experimental approach that allows the chemical changes that occur within the coal during dissolution to be discussed. This, in turn, requires a direct method of determining the structural features in coal before it is reacted. 

Hooper, R.J., "The Dissolution of a Victorian Brown Coal in a Hydrogen-Donor Solvent", PhD Thesis, University of Melbourne, Australia, April 1978... 

The conversion of coal to liquid fuels is usually carried out in the presence of an H-donor solvent (H-don) such as Tetralin. 

Alcoholic KOH. We have reported on the use of isopropyl alcohol as an H-donor solvent in coal conversion, and specifically on the effects of the addition of strong bases such as KOH to the system (la). We found that i-PrOH brought about a conversion of Illinois No. 6 coal very similar to the conversion level obtained by Tetralin under the same conditions. These results are listed in Table IV in text, along with the results of more recent experiments using methanol as the solvent and adding KOH to the system (lb). 

REACTIVITY OF VARIOUS H-DONOR SOLVENTS WITH CC13 AND COAL... 

However, as pointed out above, the commonly proposed free radical mechanism is not entirely consistent with the observed behavior of H-donor solvents and coal. Further, a thermally promoted C-C or C-0 bond-scission is inconsistent with our observations in the -PrOH work at 335°C. As also mentioned, a major fraction of the coal was converted in this system to a product with a number-average molecular weight of less than 500. If we consider that the rate constant for the unimolecular scission of the central bond in bibenzyl is expressed (5) as... 

The dialin donor solvents were also used directly in coal liquefaction studies. Inasmuch as details of coal structure are unknown, the present theory can only be tested in a qualitative way, as follows. First, if the liquefaction of coal occurs under kinetic control with hydrogen-transfer from the donor solvent involved in the rate-determining step, then we should expect the dialin donors to be more effective than the control solvent T.et-ralin (and also Dfecalin). This is suggested by the theory because the dialins possess higher energy HOMOs than Tetralin and... 

The dialins were also found to be more effective donor solvents than Tetralin in the liquefaction of an Illinois No. 6 HVA bituminous coal. For example, at 400 C and 0.5 hr reaction time, A2-dialin yielded 16% more pyridine-solubles and 37% more hex-... 

The preceding experiments offer preliminary support to our notion that pericyclic pathways might be intimately involved in the mechanism of coal liquefaction. More specifically, the results indicate that pericyclic group transfer reactions constitute a plausible pathway for the transfer of hydrogen from donor solvents to coal during liquefaction. 

An Isotopic Study of the Role of a Donor Solvent in Coal Liquefaction... 

The research involved the use of deuterium gas as a tracer to follow the incorporation of hydrogen into coal. Neither donor solvent nor catalyst was used in those experiments. The liquefaction product was solvent fractionated, and the fractions were examined for deuterium incorporation in each structural position. 

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