Predicting and improving sieve tray efficiencies in distillation

A reliable and accurate tray efficiency model is of primary economic importance. Based on the dispersion theory and the penetration theory, a new point efficiency model has been developed. The model was verified by experimental data obtained from commercial-scale sieve trays under distillation conditions. The numbers of individual phase mass transfer units have been accurately determined from distillation data. Based on the determined number of individual phase mass transfer units, it was found that surface tension and its gradient have great effects on the number of mass transfer units, hence on the point efficiency. Correlations have been obtained to account for these effects on the point efficiency.; To meet the new demand in distillation, a packed tray with improved performance has been developed by combining sieve trays with a bed of mesh packing. The hydraulic and mass transfer performance of this packed tray has been studied in a distillation and an air/water column. It was found that by adding a shallow bed of packing, the Murphree tray efficiency increases by 40 to 50% over a wide range of flow rates and system properties in a small column. This increase in the efficiency can be attributed to smaller and more uniform bubbles and more stable hydraulic conditions generated on the packed trays. Hydraulic tests on an air/water system have shown that the packing can effectively reduce the tray entrainment and weeping. Consequently, the packing can increase the tray vapour capacity by 50 to 80%. However, the packing also results in a 15% increase in the total tray pressure drop and a 40 to 75% reduction of the tray's lifetime in fouling situations. Correlations have been obtained for estimating the packing effects on the tray efficiency, entrainment and pressure drop.