Supercritical extraction of coal

Batch reactors, use in supercritical extraction of coal, 251-252 Benzylphenylamine... 

The maximum conversion of coal to liquid products has been the primary objective of most work on supercritical extraction of coal reported in the literature. A 1975 article by Whitehead (1), one of the first references to supercritical coal extraction, presented data on supercritical extraction of coal by coal tar or petroleum... 

Some work has been reported utilizing alcohols for supercritical extraction of coal. Makabe et al.(5) reported extraction of coal with ethanol-sodium hydroxide mixtures with the objective of maximizing extraction yield no sulfur data were reported. Methyl alcohol reaction with a low volatile bituminous West Virginia coal at higher temperatures (460-600 °C) was reported by Garner et al.(J>). Promotion of coal gasification was the objective of that study sulfur content of the resultant char was not reported. 

It may be noted that if the solvent is liquid under ambient conditions, the mechanical energy requirements of the pressure reduction method of solute recovery may be considerably reduced since repressurisation of the solvent in liquid form may be used rather than repressurisation of gaseous solvent. The pilot plant used by the UK National Coal Board (now British Coal) for the supercritical extraction of coal was of this type [17]. However, the decrease in mechanical energy requirements is balanced by increased thermal requirements. 

Yu X, Wang X, Bartha R, et al. 1990. Supercritical fluid extraction of coal tar contaminated soil. Environ Sci Technol 24 1732-1738. 

The extraction of coals with supercritical fluids is a promising route for the production of liquid fuels from coal. Generally, hydrocarbon solvents, notably toluene, have been used as the supercritical fluid. Supercritical water extraction has not received the same attention and only recently the first detailed study was reported. In that work, Holder et al. ( l) obtained high conversions for extraction of a German brown coal (70-75%) and a Bruceton bituminous coal (ca. 58%) with supercritical water at ca. 375°C and 23 MPa. These high yields, however, contrast with the much lower conversions briefly reported (.2—5) for other coals and there appears to be considerable variation in the extractive power of supercritical water with different coals, even allowing for the differences in the extraction procedure used. None of the above reports discussed in any detail the chemical nature of the products, nor how the products compare with those obtained from more conventional solvents. 

Whitehead, J. C., "Development of a Process for the Supercritical Gas Extraction of Coal", paper presented at the 88th AIChE National Meeting, June 1980. 

In contrast to the previously reported work utilizing supercritical solvent extraction of coal, the major objective of our research effort is to develop a desulfurization process that will result in a solid product suitable for combustion in existing coal fired utility boilers. 

Other reactions studied by these authors include the thermal dimerization of methyl acrylate and the reaction of benzene with certain alkynes in a Diels-Alder reaction. Supercritical extraction in coal processing may also be classified under this scheme. This subject is reviewed by Williams (1981) it is not included in this chapter. 

Compressed gases (such as carbon dioxide) can also be used effectively as solvents for extraction of coal. Toluene, dodecane, p-cresol, etc. can be applied under supercritical conditions. Some advantages of using supercritical fluid as solvents are (1) an extract with low molecular weight (approximately 500) and higher hydrogen content can be obtained, (2) solvent recovery is easy, and (3) the reduction in the operating pressure or temperature precipitates the extract. 

Application of supercritical gas extraction (Schneider et al., 1980 Bright and McNally, 1992 Kiran and Brennecke, 1993) has lately received considerable attention when applied to coal. For example, studies have been reported relating to the chemical nature of extracts of coal (volatile matter content in excess of 36%) obtained using toluene (under pressure) at 350°C (660°F). The extracts contained aromatics (benzene derivatives, naphthalene derivatives, and phenanthrene derivatives) as well as n-paraffins, sterane, and materials such as phytane, pristane, and farnesane (Table 10.2) (Bartle et al., 1975 Smith and Smoot, 1990). There was a predominance of n-alkanes. 

Low-Molecular-Weight Constituents in a Supercritical Gas Extract of Coal... 

FIGURE 10.2 Average molecular structures of supercritical gas extracts of coal. (From Bartle, K.D. et al.. Fuel, 54, 226, 1975.)... 

While the yields of extract may be lower than that can be obtained with some liquid solvents and the use of high pressures might appear to be disadvantageous, there are nevertheless two positive features related to supercritical gas/fluid extraction of coal ... 

Supercritical fluid solvents have been tested for reactive extractions of liquid and gaseous fuels from heavy oils, coal, oil shale, and biomass. In some cases the solvent participates in the reactions, as in the hydrolysis of coal and heavy oils with water. Related applications include conversion of cellulose to glucose in water, dehgnincation of wood with ammonia, and liquefaction of lignin in water. 

Supercritical extraction has been used increasingly in recent years for specialized processes. These processes include separation of drugs from plants, oils from vegetable seeds, impurities from labile materials, and chemical feedstocks from coal and petroleum residual. The utility of supercritical extraction processes stems principally from the enhanced solubility characteristics of CO2 near its critical point and the ease with which the solvent can be recovered for recycle. 

The technique of extracting virtually nonvolatile substances is particularly useful for materials that decompose before reaching boiling point and is therefore well suited to the extraction of the liquids formed when coal is heated to about 400°C (750°F). Thus, supercritical gas or fluid extraction affords a means of recovering the liquid products when they are first formed, avoiding undesirable secondary reactions (such as coke formation), and yields of extract up to 25 or 30% have been recorded. 

The main conclusion to be drawn from the experimental data presented here is that fractionation of residuum through the use of a supercritical fluid system incorporating internal reflux produced by retrograde condensation results in sharper fractions than those obtained by ordinary supercritical extraction. The capability of the FDU to process coal-derived residuum in the... 

Extraction of Australian Coals with Supercritical Aqueous Solvents... 

Conversions between 42-68% were obtained for supercritical water extraction of Victorian brown coals at 380°C and 22MPa, considerably higher than using toluene under the same conditions. The conversions obtained with a bituminous and a sub-bituminous coal were much lower. Pressure had a marked effect on both the conversion and the extract composition, whereas temperature had only a slight effect. Considerably higher conversions were achieved using dilute sodium hydroxide rather than water. The composition of the products is discussed. 

The potential use of supercritical water appears especially attractive for the extraction of brown coals with their high water content, 5Q-70% for Victorian brown coals, thus removing the need for a coal-drying stage. The drying and extraction of these low rank coals would occur in a single process. The purpose of the present study was to investigate the feasibility of the extraction of Australian black and brown coals with supercritical water. The... 

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