Bubble columns are tall gas-liquid contacting vessels and are often used in processes where gas absorption is important (for example, bioreactors to dissolve oxygen in broths) and to limit the exposure of micro-organisms to excessive shear imparted by mechanically driven mixers. There are two types of bubble columns in general use: airlift reactors that use a baffle to separate the riser and downcomer regions, and other columns that do not use a baffle.