A type of chemical reactor that has an external loop from the main body of the reactor through which a portion of the reacting material flows. Used for liquid-phase reactions, it operates as a form of gas lift in which gas is admitted into the bottom and rises as bubbles. After disengagement of the bubbles, the reacting materials descend into the bottom of the reactor via the external loop. This type of reactor does not require mechanical mixing to achieve good gas–liquid dispersions. There are, however, difficulties in controlling the rate of mass transfer and the consumption of energy.