Continuous distillation is a process in which a chemical mixture is continuously fed into a distillation column where its contents are separated through selective boiling. It is used extensively in crude oil refining to produce petroleum products, such as gasoline, lubricants, fuel oil, waxes, and petrochemicals.
In this short blog, we outline the steps by which the continuous distillation process occurs.
The Flash Zone
In continuous distillation of crude oil, product is heated before being introduced to the column (about one third of the way up the tower). This is called the Flash Zone because a percentage of the hot feed entering the tower flashes off into vapor upon encountering the low-pressure conditions of the column.
From the flash zone, the vapor in the feed stream rises up the column and the liquid falls towards the bottom. The part of the column below the flash zone is called the Stripping Section, and the part above is known as the Rectifying Section.
This section of the tower is devoted to driving off vapors (i.e., light ends) from the liquid. Generally, sieve type or bubble cap trays are used to facilitate the stripping process.The liquid at the bottom of the tower is drawn off, reheated in a reboiler and circulated back into the column. On re-introduction, the vapor generated rises and bubbles through the falling liquid, stripping out the light ends as it progresses.Over time, some of the bottom product, like heavy fuel oil is drawn off to be cooled and stored.
The rectifying section of the tower also contains trays, which assist in the stripping of component petrochemicals from the liquid. The petrochemical products become lighter as the vapor rises up the column.
At pre-determined points over the height of the rectifying section, side streams of product are drawn off with the aid of collecting pans. These include diesel, kerosene and gasoline, which are purified in stripping columns, cooled in heat exchangers,and pumped to storage vessels.
The lightest components then exit the top of the tower as vapor. They pass through a cooler and into an accumulator, where they are drawn off as naptha, which is a combination of various hydrocarbon compounds.