GB775273A - Processes for distilling liquid mixtures - Google Patents

Abstract

In a process of distillation of liquids in a fractionating column, the entire quantity of liquid collecting on one of the intermediate trays is withdrawn from the column and led to a side-stripper, the liquid bottom product of the side-stripper being discharged and the vaporous top product of the side-stripper being returned to the main column at a point above and near to the withdrawal tray, and reflux is produced for the lower sections of the column by withdrawing liquid from a tray below the total draw-off tray, cooling this liquid, and then re-introducing it into the column at a higher level but below the total draw-off tray. The process is applicable to the distillation of crude petroleum oil, particularly when it is desired to separate a fraction of comparatively wide boiling range which is suitable as a feedstock for catalytic crackng plant. Preheated crude oil is introduced through line 1 into column 2, which is provided with bubble-cap or spray trays, where it is fractionated at a pressure of about 1.0-1.5 atm. Gasoline vapours are withdrawn from the top of the column. The entire quantity of liquid <PICT:0775273/III/1> <PICT:0775273/III/2> which collects on tray 3 and on tray 4 is withdrawn, and passed to side strippers 5 and 6, respectively, each of which is provided with a reboiler. The top products from strippers 5 and 6 are returned to the main column, and the bottom products (e.g. gas oil and kerosine) are discharged to storage tanks (not shown). The reflux for the part of the main column below tray 4 is obtained by taking off a quantity of liquid from tray 7, cooling part of it in cooler 8, and returning the cool liquid to a higher tray (but situated under tray 4). The remainder of the liquid withdrawn from tray 7, which is still hot, is pumped to the part of column 2 above tray 4. The residual oil from the base of column 2 is passed via line 11 and heater 22 to column 23, which is operated under vacuum. Steam is introduced through line 24, into the base of column 23. The lower part of this column is empty and serves as a flash column; while higher up there are bubble cap trays and above them a number of spray trays. Asphaltic bitumen is withdrawn from column 23 through line 25; and side-streams are withdrawn from trays 26 and 27 at temperatures of about 280 DEG C. and 170 DEG C. respectively. The greater part of the liquid withdrawn from tray 26 is cooled in cooler 29, and returned to the column at a point immediately below tray 27. Part of the liquid from tray 27 is returned via cooler 33 to column 23 as reflux; and the remainder is passed to stripper 31, which is supplied with steam through line 32. The vapours obtained at the top of the stripper are returned to column 23. The part of the oil from tray 26 which is not returned to column 23 is cooled in cooler 28 and then mixed with the bottoms material from stripper 31 after it has been cooled in cooler 34; this mixture forms the feedstock for catalytic cracking. High-pressure steam can be raised in coolers 28 and 29; and low-pressure steam can be raised in coolers 33 and 34. The gasoline obtained at the head of column 2 may be stabilized by the process claimed in Specification 775,274.