1. Field of the Invention
This invention relates to coal liquefaction and is particularly concerned with multiple-stage hydrogen-donor coal liquefaction.
2. Description of the Prior Art
A number of different processes are being developed for the production of liquid hydrocarbons from coal. Among the most promising of these are processes in which the feed coal is first contacted with a hydrogen-containing gas and a hydrogen-donor solvent at elevated temperature and pressure in a liquefaction reactor and a portion of the liquid product is then catalytically hydrogenated in a solvent hydrogenation reactor to generate additional liquid products and a hydrogen-donor solvent for recycle to the liquefaction step. Within the liquefaction zone, the high molecular weight constituents of the coal are cracked and hydrogenated to form lower molecular weight vapor and liquid products. The effluent from the liquefaction reactor is then separated into gases, low molecular weight liquids, and a bottoms stream containing high molecular weight liquids and unconverted mineral matter. The separation of the liquefaction reactor effluent is normally made in such a manner as to produce a bottoms stream consisting of liquids that boil above about 1000.degree. F. The bottoms stream is composed primarily of high molecular weight hydrocarbons formed when the original high molecular weight coal constituents are only partially converted in the liquefaction reactor. Depending on the liquefaction conditions, the bottoms stream will normally contain from about 40 to about 60 weight percent of these high molecular weight hydrocarbons based on the weight of the original dry coal feed.
Although the process outlined above has numerous, advantages over other liquefaction processes, the limited amount of low molecular weight liquids that can be produced, the excessive quantity of high molecular weight bottoms formed and the high consumption of hydrogen, which results from the production of undesirably large quantities of gases and saturated liquids, renders the process somewhat inefficient. To make the process economically more attractive, it is desirable to further convert the bottoms into lower molecular weight liquids and to decrease the hydrogen consumption.