A MECHANISTIC ANALYSIS OF SIEVE TRAY FROTH HEIGHT AND ENTRAINMENT

A phenomenologically based model for froth height was developed using the assumption of both vapor- and liquid-continuous zones within the total froth height The model demonstrates the importance of liquid and vapor rates and determines that the Weber number (and therefore droplet size and surface tension) has little effect on froth height. A composite air/water database including entrainment data for both the spray and froth regimes is used to develop an entrainment correlation as a function of the ratio of tray spacing to froth height (T-s/h(2 phi))and the average froth density (phi(2 phi) = h(L)/h(2 phi)). For conditions resulting in the ratio of liquid inventory to the perforation diameter (h(L)/d(H)) < 4, the froth height equation requires an empirical correction for froth height, and entrainment depends on phi(2 phi). This results from the predominantly vapor-continuous nature of the froth. For large values of h(L)/d(H), the theoretical value for h(2 phi) appears adequate and entrainment is essentially independent of phi(2 phi), since most of the liquid is contained in the liquid-continuous region located far away from the underside of the tray above. In such cases, entrainment is from a light spray on top of the liquid-continuous region, and therefore entrainment depends much less on phi(2 phi). The different dependency of entrainment on phi(2 phi) can be used as a method to identify the spray-froth transition. For h(L)/d(H) < 2 a spraylike condition exists; for h(L)/d(H) > 2, a frothlike condition exists. The reported dependency of air/water system entrainment on major geometry and operating conditions is described well by the new correlation. Alternate correlations for the non-air/water entrainment data, however, are required. New methods for estimating the impact of entrainment on efficiency are given.