Rarely published industrial-scale distillation efficiency data are presented. The Murphree tray efficiencies are determined from the i-butane/n-butane fractionator performance data. Point efficiencies, numbers of overall vapor phase transfer units, numbers of vapor and liquid phase transfer units, and liquid phase resistances of mass transfer are backcalculated from the Murphree tray efficiencies. Various efficiency prediction and scale-up methods have been tested against experimental results. A new model for the prediction of the numbers of vapor and liquid phase transfer units has been developed. The model can be applied to hydrocarbon systems at high pressure. The influence of the mass-transfer coefficients, the interfacial area, and the vapor and liquid residence times on mass transfer has been analyzed separately, and as a result the NTU correlations for vapor and liquid phases are obtained. The constants of the model can be obtained by fitting the model to experimental efficiency data from a similar system.