Airlift reactors can be divided into two main types of reactors on the basis of their structure (Fig. 1): (1) external-loop vessels, in which circulation takes place through separate and distinct conduits; and (2) baffled (or internal-loop) vessels, in which baffles placed strategically in a single vessel 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.
Internal-loop concentric tube reactor
Gas input Gas input Gas input
All ALRs, regardless of the basic configuration (external loop or baffled vessel), comprise four distinct sections with different flow characteristics:
• Riser. The gas is injected at the bottom of this section, and the flow of gas and liquid is predominantly upward.
• Downcomer. This section, which is parallel to the riser, is connected to the riser at the bottom and at the top. The flow of gas and liquid is predominantly downward. The driving force for recirculation is the difference in mean density between the downcomer and the riser; this difference generates the pressure gradient necessary for liquid recirculation.
• Base. In the vast majority of airlift designs, the bottom connection zone between the riser and down-comer is very simple. It is usually believed that the base does not significantly affect the overall behavior of the reactor, but the design of this section can influence gas holdup, liquid velocity, and solid phase flow (1,2).
• Gas separator. This section at the top of the reactor connects the riser to the downcomer, facilitating liquid recirculation and gas disengagement. Designs that allow for a gas residence time in the separator that is substantially longer than the time required for the bubbles to disengage will minimize the fraction of gas recirculating through the downcomer
Gas separator configurations of internal-loop ALRs
Gas separator configurations of external-loop ALRs
Momentum, mass transfer, and heat transfer will be different in each section, but the design of each section may influence the performance and characteristics of each of the other sections, since the four regions are interconnected.
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