Abstract
With the recent escalation in the cost of crude oil the ability to produce low molecular weight ash-free products in high yield from coal has become increasingly important. An investigation has therefore been carried out to determine the potential of an extraction technique, gas extraction, within the field of coal processing. The technique utilises a supercritical gas, compressed to a density approaching that of a light organic solvent, which, when contacted with a substrate, increases the vapour pressure of its components and can be used to effect a fractionation. Two extraction units, a two litre capacity unit operating up to 6000 p.s.i. and 100 C and a 0.36 litre capacity unit working up to 2000 p.s.i. and 400 C, have been used to determine equilibrium gas phase concentrations of a number of low molecular weight coal derived materials in a series of compressed gas solvents. Gas phase concentrations approaching 10% w/w were recorded during an extraction in which 50 % w/w of a high temperature coke oven tar was effectively distilled at extraction temperatures up to 100 C in an atmosphere of compressed ethylene, A simple relationship has been postulated between gas phase concentration and gas density, A third extraction unit, a 10-litre capacity stirred autoclave, has been used to contact batches of coal and depolymerised coal material with a range of solvents at a pressure of 1500 psig and temperatures in excess of. 250 C, Gas extraction of thermally depolymerised coal produced an extract yield approaching 20% w/w which is far in excess of the tar yield obtained by existing carbonisation techniques. The degree of extraction is limited by the molecular weight of the substrate and it was shown that higher yields, up to 40% w/w, of essentially ash free material could be obtained if a more severe depolymerisation reaction was employed. In this investigation hydrogenation using a hydrogen donor solvent, tetralin, was used. It was concluded that, although the technique did not offer any particular advantages over other fractionation techniques for low molecular weight coal material, it certainly possesses the ability to produce low ash content extract without the necessity for a filtration stage, and in conjunction with a more powerful depolymerisation reaction, possibly hydrogenation, and improved solid/gas contacting the extract yield could approach that obtained by solvent extraction using heavy coal derived oils.