Liquid-dispersed contactors countercurrent

The term three-phase fluidization requires some explanation, as it can be used to describe a variety of rather different operations. The three phases are gas, liquid and particulate solids, although other variations such as two immiscible liquids and particulate solids may exist in special applications. As in the case of a fixed-bed operation, both co-current and counter- current gas-liquid flow are permissible and, for each of these, both bubble flow, in which the liquid is the continuous phase and the gas dispersed, and trickle flow, in which the gas forms a continuous phase and the liquid is more or less dispersed, takes place. A well established device for countercurrent trickle flow, in which low-density solid spheres are fluidized by an upward current of gas and irrigated by a downward flow of liquid, is variously known as the turbulent bed, mobile bed and fluidized packing contactor, or the turbulent contact absorber when it is specifically used for gas absorption and/or dust removal. Still another variation is a three-phase spouted bed contactor. 

Holdup and Flooding. The volume fraction of the dispersed phase, commonly known as the holdup h. can be adjusted in a batch extractor by means of the relative volumes of each liquid phase added. However, in a countercurrent column contactor, the holdup of the dispersed phase is considerably less than this, because the dispersed drops travel quite fast through the continuous phase and therefore have a relatively short residence time in the equipment. The holdup is related to the superficial velocities V of each phase, defined as the flow rate per unit cross section of the contactor, and to a slip velocity Us ... 

Readers should consult the extensive study by Macias-Salinas and Fair (2002) on axial mixing effects in packed gas-liquid contactors. Various types of situations have been considered neglect of gas-phase dispersion (desorption of O2 from water) dispersion in both phases (SO2 absorption by water) neglect of liquid-phase dispersion (water cooling with air). They have employed the rigorous analytical solution of Hartland and Mecklenburgh (1966) developed for countercurrent liquid extraction and applied it to gas absorption with gas as the raffinate phase and the liquid as the extract phase by a trial-and-error algorithm. 

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