(117b) Surface Tension and Distillation Design

Surface tension is one of the key physical properties commonly used in design correlations for mass transfer and capacity of separation equipment, because of its effects on droplet size, film behavior and vapor-liquid disengagement. Specifically, surface tension is used in predicting mass transfer efficiency of trays and packings, entrainment behavior, downcomer performance as well as system limit of distillation columns. Mass transfer and design correlations typically use surface tension measured by observing the behavior of a liquid droplet in air at atmospheric pressure, as a surrogate for the interfacial tension of a liquid in contact with condensable vapor. Correlations used in distillation design and rating have used this approach for decades, simply because surface tension data measured in air at ambient conditions is more readily available. This presentation will discuss the differences between surface tension measured in air versus a condensable vapor and potential ramifications on the physical understanding of distillation processes.