Hydroaromatic solvents

Most hydroaromatic solvents have the capability of becoming irreversibly adducted by acceptor free radicals which could arise from the coal. (On-going work indicates that the presence of oxygen and sulfur functions on the free radicals will enhance adduction.)... 

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. 

TABLE 10.1 Extraction of Coal Using Aromatic-Hydroaromatic Solvents... 

Influence of Coal Rank on the Degree of Extraction by Aromatic and Hydroaromatic Solvents... 

Different types of other coal liquefaction processes have been also developed to convert coals to liqnid hydrocarbon fnels. These include high-temperature solvent extraction processes in which no catalyst is added. The solvent is usually a hydroaromatic hydrogen donor, whereas molecnlar hydrogen is added as a secondary source of hydrogen. Similar but catalytic liquefaction processes use zinc chloride and other catalysts, usually under forceful conditions (375-425°C, 100-200 atm). In our own research, superacidic HF-BFo-induced hydroliquefaction of coals, which involves depolymerization-ionic hydrogenation, was found to be highly effective at relatively modest temperatnres (150-170°C). 

Hydrogen donors are, however, not the only important components of solvents in short contact time reactions. We have shown (4,7,16) that condensed aromatic hydrocarbons also promote coal conversion. Figure 18 shows the results of a series of conversions of West Kentucky 9,14 coal in a variety of process-derived solvents, all of which contained only small amounts of hydroaromatic hydrocarbons. The concentration of di- and polyaromatic ring structures were obtained by a liquid chromatographic technique (4c). It is interesting to note that a number of these process-derived solvents were as effective or were more effective than a synthetic solvent which contained 40% tetralin. The balance between the concentration of H-donors and condensed aromatic hydrocarbons may be an important criterion in adjusting solvent effectiveness at short times. 

The first is a high-temperature solvent extraction process in which no catalyst is added. The liquids produced are those that are dissolved in the solvent, or solvent mixture. The solvent usually is a hydroaromatic hydrogen donor, while molecular hydrogen is added as a secondary source of hydrogen. 

The variation in thermal decomposition yields, which depend on bed geometry, pressure, solvents, etc, are in agreement with the suggested role of internal aliphatic or hydroaromatic hydrogen in stabilizing free radicals in the competitive evolution of light species and tar. 

To examine the distribution of acidic C-H sites in PSOC 1197, derived in part from polynuclear hydroaromatic compounds, the pH 12 O-methyl coal was treated for 68 h at 0 °C, in separate experiments, with an excess of the conjugate bases of 9-phenylfluorene (pKa 18.5), fluorene (pKa 22), and triphenylmethane (pKa 31) as their lithium salts (BLi) THF was used as the solvent ... 

A majority of the proposed models suggest coals to consist of several ring aromatic and hydroaromatic structural units, cross-linked through aliphatic and ether bridges to form the three dimensional structure and in the pores and cavities of this structure reside weakly linked smaller molecules which are easily extracted by solvents. The model proposed by Solomon and Shinn are representative examples of this macromolecular model. 

The extractive chemical disintegration process can be called direct coal liquefaction. Here, solvents rich in hydroaromatic components are especially suited in extracting nearly all of the reactive coal macerals. These types of solvents actively participate chemically in bond breakage and stabilization, are consumed or structurally changed, and are normally used at temperatures considerably in excess of 300°C (570°F). On the other hand, because of the heterogeneous nature of coal it is a distinct possibility there may be/could be no clear operational or mechanistic distinction between extractive disintegration and extractive chemical disintegration processes. 

In the higher-rank bituminous coals, a- and P-naphthols are both effective solvents, but, for subbituminous coal, P-naphthol may produce even less extract than phenanthrene but a-naphthol may extract as much as 83% w/w of the coal. Ring compounds, such as phenanthrene, appear to be superior for extracting bituminous coals of medium rank. A study of extraction of Dutch bituminous coals at 200°C-400°C (390°F-750°F) illustrated the importance of simultaneous hydrogenation via the hydroaromatic portion of the solvents diphenylamine has been found to be an effective catalyst for transfer of hydrogen in this manner because it increases the yield of extract. 

The flexibility of coal structure and mobility of loosened molecules are responsible for the successive extraction of coal in solvents having different chemical characteristics. In fact, coal has a heterogeneous structure having different structural units, that is, polyaromatic, hydroaromatic, and paraffinic units linked through C-C, C-N-C, C-O-C, and C-S-C linkages. 

Table 3.8, which shows the solvent power of polynuclear aromatics and hydroaromatics for coal, illustrates that hydroaromatics, which also occur in anthracene oil, display particularly high solvent powers. 

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