 Evolved from bubble column concept  Recirculation of liquid through a downcomer connecting the top and bottom of the main bubbling section  Efficient mixing by high circulation flow rate  No mechanical agitators – no large shear forces  Can satisfy a high oxygen demand  Eg. production of secondary metabolites by plant cells

 Two types: ALR with internal loop and ALR with external loop  Continuous injection of air at the bottom of the riser creates a density difference with the downcomer as the air escapes at the top of the reactor.  Because of the density difference, a liquid flow from the bottom to the top exists in the riser and from the top to the bottom in the downcomer, resulting in circulation of continuous liq phase.

 The absence of mechanically moving parts  A low shear rate  A low defect rate  Adequate gas-phase disengagement at the top  Unique combination of controlled flow and good mixing properties  Well-defined residence time  Easily operated under sterile conditions

 Internal loop ALR (AILR)  External loop ALR (AELR)

 A well-defined residence time in the individual sections of AELR  An adjustable gas-phase disengagement at the top  Accessability for measurement and control in both riser and downcomer  Excellent heat exchange and temp control  Optimal hydraulic diameter for both riser and downcomer and therefore low friction rate