Introduction General

The term airlift reactor (ALR) covers a wide range of gasliquid or gas-liquid-solid pneumatic contacting devices that are characterized by fluid circulation in a defined cyclic pattern through channels built specifically for this purpose. 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 liquid, 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 produced 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 disengages at the top, in the section termed the gas separator, is determined by the design of this section and the operating conditions. The fraction of the gas that does not disengage, 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.

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