Point efficiency of distillation sieve trays at elevated liquid viscosities

Abstract

Liquid viscosity is considered to be a major factor that influences distillation efficiency through its contribution to the liquid-phase mass transfer resistance. Nearly all of the distillation efficiency correlations in the literature were developed or validated using efficiency data at liquid viscosities less than 1.5 mPa.s. However, distillation at liquid viscosities greater than 1.5 mPa.s is encountered in several cases in the chemical process industry. Therefore, distillation efficiency data at liquid viscosities greater than 1.5 mPa.s are required to accurately quantify the effect of liquid viscosity on distillation efficiency and to develop improved efficiency correlations.

Point efficiency defines the most fundamental mass transfer efficiency for distillation trays upon which Murphree tray and overall column efficiencies are calculated. Oldershaw columns are proven to be effective in representing the point efficiency of commercial columns. In this dissertation, the effect of liquid viscosity on point efficiency of distillation sieve trays is reported with the data collected using a newly constructed Oldershaw distillation column for four different test systems, including a new viscous test system that was developed in this work, at liquid viscosities ranging from 0.2 to 4.2 mPa.s. It was found that the point efficiency decreases with an increase in liquid viscosity. However, the effect of liquid viscosity on point efficiency decreases as liquid viscosity is increased. In other words, the rate of change of point efficiency with liquid viscosity ((dE_OG)/(du_L )) is inversely related with liquid viscosity. This work expands the upper limit of liquid viscosity for the distillation point efficiency database in the open literature from 1.5 mPa.s to 4.2 mPa.s.

In addition, the evaluation of the O'Connell, modified O'Connell, and Duss and Taylor correlations with the point efficiency data from this work at liquid viscosities from 0.2 to 4.2 mPa.s is presented. An improved efficiency correlation with a modified liquid viscosity exponent for the liquid-phase mass transfer coefficient in the Duss and Taylor correlation is also proposed. The proposed correlation is shown to have significantly less errors in efficiency predictions than the Duss and Taylor correlation and will help design distillation columns cost-effectively, especially at elevated liquid viscosities.