Perforated plates used in distillation columns that permit vapour to rise up through a layer of liquid as bubbles and designed in such a way that the liquid is unable to flow down at the same time. There are various designs such as sieve trays, which are located both above and below the feed tray. Stripping trays are located below the feed tray in which the concentration of the less volatile component in the liquid increases as the liquid flows down the column. The purity of the bottom product is increased by increasing the number of trays. Rectifying trays are located above the feed tray in which the concentration of the more volatile component is enriched as the vapour rises from tray to tray up the column. Again, the purity is increased by increasing the number of trays.