Entrainment and Maximum Vapour Flow Rate of Trays

Abstract

This is a report on free entrainment measurements in a small (0.20 m × 0.20 m) air-water column. An adjustable weir controlled the liquid height on a test tray. Several sieve and valve trays were studied. The results were interpreted with a two- or three-layer model of the two-phase mixture on the tray. The top or spray layer is gas continuous: in the other layers, the liquid is continuous and contains small bubbles. Large bubbles erupt from the liquid-continuous phase, ejecting drops into the top layer. The distribution of the ejection velocities is taken to be Gaussian. The model allows prediction of the entrainment at different heights and for different gas velocities. As a result, a new equation for the maximum allowable vapour velocity can be presented. Two regimes were recognized: a ’low-liquid-height’ and a ‘high-liquid-height’regime. In the low-liquid-height regime, there are only two layers. Here the entrainment decreases with increasing liquid height and depends strongly on the type and geometry of the tray deck. At the transition, an intermediate liquid continuous layer develops. In the high-liquid-height regime, there are three layers. Here a further increase in liquid height causes the entrainment to increase. Moreover the entrainment is independent of the type and geometry of the tray deck. The results are compared with measurements from Fractionation Research Inc. on a large tray column. Similar conclusions can be drawn from these measurements, although there are differences owing to the difference in scale of the equipment. The model suggests that potentially large gains in the maximum allowable vapour rate of trays should be possible. A way of optimizing the vapour capacity of existing trays by an appropriate choice of the weir dimensions is presented.