This invention relates to processes for converting solid carbonaceous material to a carbonaceous liquid, and more particularly, to a pretreatment procedure for increasing liquefaction yield in such processes.
Solid carbonaceous material such as coal, coke, peat, shale, wood, etc., can be converted to a carbonaceous liquid by contact with a hydrocarbonaceous liquid solvent in the presence of a source of hydrogen at a temperature in excess of 350.degree. C. but not more than about 475.degree. C. Often the liquefaction product or an intermediate liquid stream in the process is recycled for use as the liquid solvent. A slurry of the solid carbonaceous material, such as coal, and the solvent is rapidly preheated to liquefaction temperatures and maintained at such temperature in the presence of hydrogen for periods of varying from 5 minutes to several hours.
It is known that temperatures above 350.degree. C. are required before any significant breakage of coal compound bonds and liquefaction of solid coal material occurs. Thus, it has long been believed that temperatures of at least 350.degree. C. were required to promote the liquefaction of coal. Such temperatures permit cleavage of carbon to carbon bonds for hydrogenation and the formation of lower molecular weight material with increased H/C ratio. Consequently, it has been common practice in prior processes to heat the coal, or other carbonaceous material, rapidly to a temperature of 350.degree. C. or higher on commencement of heating.
Various approaches have been taken in an attempt to increase liquefaction yield. Some prior processes have used two stage liquefaction, but with both stages carried out at temperatures above 350 .degree. C. However, the high temperatures rapidly obtained permit pyrolysis reaction within the coal before the solvent penetrates the fine pore structure. In other prior art processes, a slurry of the coal in powdered form and the solvent was heated in a storage or process feed tank to a temperature of 150.degree. C. or lower, prior to being subjected to liquefaction conditions. Tests have shown that temperatures of 150.degree. C. or lower are ineffective in increasing liquefaction of carbonaceous material in any reasonable period of time.