Considerable research and development effort currently is underway in an effort to develop commercially practical processes for upgrading raw coals (bituminous, subbituminous and lignite) to provide a more ecologically acceptable product, i.e., one with both a low sulfur and a low ash content.
Various proceses, including gasification, liquefaction, pyrolytic and combinations of these processes have been investigated. One promising process, solvent extraction, comprises dissolving ground coal in solvent, in the presence of hydrogen, to provide a mixture consisting of dissolved coal, in solution, as well as solid insoluble coal values and mineral matter or ash. Separation of the dissolved coal from the insoluble materials has proved to be arduous and costly when attempted by either filtration or with the use of hydroclones.
Moreover, even when such separation attempts have been successful, the ash content of the separated dissolved coal has been found to be higher then desired. An ash content of 0.16 weight percent or less in the dissolved coal is desirable since, with such a low ash level, it would be possible to burn such low ash coal in electric power plants or the like without the necessity of adding costly fly-ash removal equipment for the stack-gases.
It also has been found that when filters or hydroclones are used as separation devices that an undesirable quantity of dissolved coal remains either in the filter cake or hydroclone underflow with the ash. In this connection, while it is recognized that carbon values are required for the generation of hydrogen used in the initial coal dissolving step, such filter cake or hydroclone underflow is so high in dissolved coal that it is not practical to use such a valuable product as a feed for a hydrogen generation plant. Further, hydroclones fail to remove sufficient ash to meet the desired 0.16 weight percent ash level for the dashed coal.
Many prior art processes have been found to be impractical. Some depend upon the utilization of unusual or especially prepared coal dissolving solvents. See for example U.S. Pat. No. 3,867,275. Further, such dissolving solvents frequently are generated from various petroleum fractions rather than being derived from coal. Clearly, coal derived solvent is preferred to one derived from petroleum.
Other prior art processes have proved to be economically unsuitable because they require treatment of the entire dissolved effluent with a second liquid intended to enhance or promote separation of unsuitable materials from the dissolved coal. One disadvantage of such processes lies in the fact that large volumes of effluent have to be treated. This can be understood from the fact that two to three or more parts of dissolving solvents must be employed for each part of more virulent coal dissolved in the initial dissolution step. In one prior art process, a special kerosine compound is prepared via a controlled hydrogenation step. See U.S. Pat. Nos. 3,852,182; 3,852,183 and 3,856,675. Use of such solvents requires relatively long settling rates. Another process, U.S. Pat. No. 3,791,956, requires the use of solvents such as n-decane, cyclohexane or decalin, all of which are expensive and result in processes which do not appear to be economical.
Successful processes for the deashing of coal utilizing critical pressure solvent techniques are disclosed in U.S. Pat. Nos. 3,607,716 and 3,607,717. In such processes, following the initial dissolution of particulate coal in suitable solvents, in the presence of hydrogen, and at elevated temperatures and pressures, gaseous products and very light hydrocarbon fractions are separated from the dissolver effluent which then is advanced directly to a vacuum stripping tower. Overheads from that tower can be fractionated to yield light hydrocarbons and process solvent which may be recycled to the dissolving step. Bottoms from the vacuum tower, which still contain the mineral ash, undissolved coal and dissolved coal, then are subjected to critical pressure solvent deashing utilizing light organic solvents. Under suitable conditions of elevated temperature and pressure a heavy, ash-containing phase separates from a light substantially ash-free coal phase. The phases are separated, of course, and further treated as desired.