320 BIOREACTORS, AIR-LIFT REACTORS Gas output Gas output Gas output Gas input Gas input Gas input Internal-loop split ALR Internal-loop concentric tube reactor External-loop ALR Figure 1. Different types of ALRs. BIOREACTORS, AIR-LIFT REACTORS J.C. MERCHUK M. GLUZ Ben-Gurion University of the Negev Beer-Sheva, Israel KEY WORDS Bubble column Fluid dynamics Gas hold-up Heat transfer Liquid flow Mass transfer Scale-up Three-phase airlift reactors OUTLINE Introduction General Airlift Reactor Morphology Advantages of Airlift Bioreactors Fluid Dynamics Flow Configuration Gas Holdup Gas Recirculation Liquid Velocity Liquid Mixing Mixing in the Gas Phase Energy Dissipation and Shear Rate in Airlift Reactors Mass Transfer Mass Transfer Rate Measurements Bubble Size and Interfacial Area Data Correlations for Mass Transfer Rate Heat Transfer Three-Phase Airlift Reactors Airlift Reactor—Selection and Design Scale-up of Airlift Bioreactors Design Improvements Summary and Conclusions Nomenclature Bibliography INTRODUCTION General The term airlift reactor (ALR) covers a wide range of gas– liquid or gas–liquid–solid pneumatic contacting devices that are characterized by fluid circulation in a defined cy- clic pattern through channels built specifically for this pur- pose. In ALRs, the content is pneumatically agitated by a stream of air or sometimes by other gases. In those cases, the name gas lift reactors has been used. In addition to agitation, the gas stream has the important function of facilitating exchange of material between the gas phase and the medium; oxygen is usually transferred to the liq- uid, and in some cases reaction products are removed through exchange with the gas phase. The main difference between ALRs and bubble columns (which are also pneumatically agitated) lies in the type of fluid flow, which depends on the geometry of the system. The bubble column is a simple vessel into which gas is injected, usually at the bottom, and random mixing is pro- duced by the ascending bubbles. In the ALR, the major patterns of fluid circulation are determined by the design of the reactor, which has a channel for gas–liquid upflow— the riser—and a separate channel for the downflow (Fig. 1). The two channels are linked at the bottom and at the top to form a closed loop. The gas is usually injected near the bottom of the riser. The extent to which the gas dis- engages at the top, in the section termed the gas separator, is determined by the design of this section and the oper- ating conditions. The fraction of the gas that does not dis- engage, but is entrapped by the descending liquid and taken into the downcomer, has a significant influence on the fluid dynamics in the reactor and hence on the overall reactor performance. Airlift Reactor Morphology Airlift reactors can be divided into two main types of re- actors on the basis of their structure (Fig. 1): (1) external- loop vessels, in which circulation takes place through sepa- rate and distinct conduits; and (2) baffled (or internal-loop) vessels, in which baffles placed strategically in a single ves- sel create the channels required for the circulation. The designs of both types can be modified further, leading to variations in the fluid dynamics, in the extent of bubble disengagement from the fluid, and in the flow rates of the various phases.