Refinery Processes: Distilling

Much like a simple still, in a distilling column, liquid is heated to a vapor and lifted upward to be distilled again into separate substances. This is the beginning of the refining process. Distilling exploits the characteristic of the chemicals in crude oil to boil at different temperatures, a phenomenon that engineers chart along distillation curves. Unlike a still, a distilling column contains a set of trays that allow heated vapors to rise and collect at different levels, separating out the various liquids derived from crude oil. The top of the column is cooler than the bottom, so as liquids vaporize and rise, they condense again, collecting onto their respective trays. Butane and other light products rise to the top of the column, while straight-run gasoline, naphtha, kerosene, diesel, and heavy gas oil gather on the trays, leaving straight run residue at the base of the column. Liquids are considered “heavy” or “light” based on their specific gravity, which is determined based on its weight and density compared to that of water.