Implications of the Spray Regime for Design and Operation

While the classical hydraulic model provides a reasonable approximation for the froth and emulsion regimes, different mechanisms determine the hydraulics and mass transfer in the spray regime. The transition from froth to spray is gradual, and so is the change in the hydraulic and mass transfer behavior (110,111,113,114). 

Flow across the tray. The classical hydraulic model implies that liquid flows across the tray by building up a liquid head on the tray, and when this head exceeds the weir height it overflows it. This mechanism is valid in principle when liquid is the continuous phase, but in the spray regime, vapor is the continuous phase and the liquid is present as drops in the vapor space (Figs. 6.25d, 6.26c, and 6.276). In 

Close to the outlet weir, droplets that have been unsuccessful in crossing the weir form a liquid pool (113). This pool sends liquid back along the tray floor to the orifices close to the outlet weir for reatom-ization. When the pool is too shallow (e.g., about zero-in weirs), atomization may grow fiercer, producing more entrainment (36,40). 

Pressure drop. Wet pressure drop on the tray is determined by the resistance of the aerated mass to vapor flow (S. 6.3.3). As the nature the dispersion is different, one would expect a different mechanism to cause pressure drop in the spray regime. This has been confirmed by experiments (88,114). To date, not enough is known about the nature of this mechanism. 

Mass transfer. According to the classical model, mass transfer takes place by vapor-liquid contact in the froth. In the spray regime, mass transfer takes place on the surface of the drops (116). In order to aj -preciate the differences in mass transfer between the regimes, distillation systems have been classified into three types  

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