Types of Trays in a Column

Distillation columns are thermal separation units. These centuries-old methods of separating a liquid or gas into its basic components work by making use of the different boiling points of liquids and by recycling gas-liquid flow. The columns change temperature as they rise up, resulting in different mixtures of the components at different levels on the column. The two main types of distillation columns are batch and continuous columns. Tray columns are one kind of continuous column, and there are several types of trays, or plates, that are used in tray columns.

Bubble Cap Trays

• These once commonly used trays feature little chimneys, or risers, which are fitted over each of the holes in the tray. A cap is fitted over each riser, with a space left between to allow vapors to pass. The vapors come up through the risers, and the caps filter them downward. The vapors escape through slots in the caps and pass through the liquid on the trays, causing bubbling.

Sieve Trays

• Sieve trays, along with another type called valve trays, have replaced bubble cap trays in many distillation columns. This is because, in part, they are easier to maintain than bubble cap trays and are also more efficient. Sieve trays are constructed very simply; they consist of metal plates with holes of varying numbers, sizes and patterns. The vapor is directed straight upward through the liquid on these trays.

Valve Trays

• These trays are similar to bubble cap trays in that they, too, are equipped with caps over the holes in the plates. Instead of having a space between the holes and the caps, though, valve trays have caps that are lifted by the force of the vapor coming through the holes. The caps allow vapors to come through and then direct them horizontally through the liquid on the trays. Valve trays mix the components more thoroughly than do sieve trays, according to the Newcastle University School of Chemical Engineering and Advanced Materials.