Tray columns are cylindrical vessels where liquid and gas flow in a counter-current or in a cross-current configuration and can be operated under pressure. Matter exchange occurs on a series of metal trays or plates. Tray columns can be classified depending on the flow configuration of the streams in the internal plates. Mainly two different types of column can be identified such as counter-current columns and cross-current columns. In counter-current columns, gas and liquid flow through the same perforations. The tray occupies the whole section of the column. In the cross-current plate, the liquid flows on the plate and goes down through a down comer.