Froth structure and mass transfer on distillation trays

The knowledge of froth structure on a distillation tray is crucial to estimate the mass transfer efficiency in the froth regime. The physical properties of the gas/liquid system are among the factors whose effect on distillation performance is rather unclear; especially the effect of surface tension gradient on froth structure is one aspect that has not been addressed adequately in the past. Many experimental studies have shown that surface tension gradient developed in surface tension positive systems gives rise to stabilized froth and hence raises tray efficiency. Thus the knowledge of the extent to which the surface tension gradient can cause the froth stabilization is very important to the designers.; In this thesis, a gas hold-up correlation has been proposed, which includes the effect of surface tension gradient on froth structure in binary mixtures. The correlation is validated by the experimental gas hold-up data obtained from a 90 mm diameter shallow bubble column using four binary mixtures. The correlation is able to predict the often-reported enhancement of gas hold-up of surface-active binary mixtures, which has not been accounted for by any other correlation. This correlation is further adopted in developing a point efficiency model based on the efficiency data obtained from an Oldershaw column with six binary mixtures. This is by far the first successful attempt to include surface tension gradient effect in a point efficiency model.; A froth model describing the dispersion structure in froth regime on a large-scale industrial tray has been proposed. The model characterizes the froth as a mixed regime where both bubbling and continuous jetting occur simultaneously. The turbulent break-up theory is applied to determine the bubble size distribution in froth. A fundamental model, based on the proposed froth structure, has been developed to predict the tray efficiency in both froth and spray regimes. This efficiency model includes the volume fraction of gas, bypassing the bubbles in the form of continuous jet, as the determining factor of the regimes and explains the smooth transition between froth and spray regimes. The model has been verified by published experimental efficiency data obtained from large-scale distillation trays.