A tall vertical cylindrical vessel used for the process of distillation. Hot vapour rises up the column, which is brought into intimate contact with cooled liquid descending on stages or trays for a sufficient period of time so as to reach equilibrium between the vapour and the liquid. The vapour rises up from the tray below through perforations in the tray, and the liquid on the tray flows over a weir to the tray below. In this way, the more volatile component increases in concentration progressively up the column. In continuous distillation, fresh feed is admitted at the tray corresponding to the same composition. Below the feed point, the section of column is known as the stripping section, while above is known as the rectifying section. A reboiler heat exchanger is used to boil the bottom product and produce vapour for the column. A condenser is used to condense some or all of the vapour from the top of the column. A small portion of liquid is returned to the column as reflux. The height of the column is an indication of the ease or difficulty of separation. For example, an ethylene splitter in a petrochemical refinery used to separate ethylene from ethane, which have close boiling points, requires many trays and the column is very tall. The width of the column is an indication of the internal vapour and liquid rates. In batch distillation, a charge of feed for separation is fed to the still and allowed to be boiled. The vapour is rich initially in the more volatile component or components. Scotch whisky distillation uses copper stills and operates as a single equilibrium stage. Multistage rectification is used in small production units where a pure product is required.