1. Field of the Invention
This invention relates to a method and apparatus for physically separating hydrocarbon oil by fractional distillation into components having different boiling ranges, and more particularly, to method and apparatus for fractional distillation subject to coking threshold temperature limits.
2. Description of the Prior Art
In certain types of distillation towers, the inlet temperature of the material to be fractionated is limited to a maximum allowable temperature due to coking tendency of the hydrocarbon feed at high temperatures or due to other factors. In general, the higher the heat input to such distillation towers, the higher the desired product yield will be. Hereinafter, the terms "fractionator", "distillation tower", "vacuum tower" or the like refer to the above-described type of tower, where the hydrocarbon feed and products are subject to coking threshold temperatures. The above-noted coking threshold temperature depends upon the particular type of system involved, i.e., whether it is an atmospheric distillation tower, a vacuum tower, etc. In such systems, as a result of the pressure drop which occurs through the transfer line from the preheater outlet to the feed inlet to the distillation tower, in practice the temperature of the feed at the tower inlet is somewhat lower than the coking threshold temperature. This temperature drop results in less vapor and also more light ends remaining in the liquid portion of the inlet line to the tower. This in turn results in less vapor flashing upwardly in the tower and more liquid, which has more light ends content, descending in the tower. Further, the hydrocarbon partial pressure of the light ends causes more light ends to be absorbed in the liquid, with the same result as above. Hence, it is generally desirable to add heat to the above-noted type of distillation tower, provided the feed temperature coking threshold is not exceeded.