Effect of two‐pass trays on distillation efficiencies

Abstract

AbstractTwo important structural factors that affect distillation efficiencies, the outlet weir heigh and the liquid flowpath length, are investigated. Performance and efficiency data of an industrial scale i‐butane/n‐butane distillation column equipped with two‐pass trays are used as a basis for the calculations. A mathematical development for a new method for predicting the numbers of vapor and liquid phase transfer units is given. This method together with some other NTU calculation methods is used to assess the effect of the outlet weir height on efficiencies. The effect of outlet weir height on the Murphree tray efficiencies is investigated using the observed point efficiencies and different point efficiency to the Murphree tray efficiency relation methods. The effect of varying liquid flowpath lengths on efficiencies is studied by calculating the Murphree tray efficiencies for one‐pass and two‐pass trays. The results obtained using the NTU calculation method presented in this study show that a certain outlet weir height point efficiency reaches its maximum. Most of the other methods give opposite results giving a minimum for point efficiency at a certain outlet weir length. The results also show that the Murphree tray efficiencies for one‐pass trays are higher than for two‐pass trays. Obviously, this is caused by the longer liquid flowpath length of one‐pass trays. The Murphree tray efficiencies are also calculated for an industrial‐scale MTBE purification column. The column is equipped with two‐pass trays in the stripping section and with one‐pass trays in the rectifying section. The Murphree tray efficiencies of one‐pass trays are considerably higher than the two‐pass tray Murphree tray efficiencies.