| Tray efficiency is a crucial factor in the analysis of sieve tray distillation columns. However, the prediction of point efficiency and its relation to tray efficiency has remained as a largely unresolved problem. A mechanistic model based on vapor-liquid mass transfer fundamentals, developed for air-water by Prado and Fair (1990), was extended to apply to the prediction of tray efficiency for distillation systems. Correlations were obtained to calculate the parameters that characterize each individual mass transfer zone of the vapor-liquid contacting dispersion in a tray, i.e., jet height, bubble formation, fraction of small bubbles in the froth, and bubble size distribution. The hydrodynamic fundamentals for aerated vessels were used to represent a sieve tray. In a similar fashion, the maximum bubble size was obtained from turbulent flow isotropic theory as a function of turbulence and system physical properties. Flow visualizations in bubble columns and the study of bubble stability were the basis for predicting bubble size distributions. The fraction of small bubbles in the froth was found to depend upon the bubble stability, the degree of turbulence, and the liquid viscosity.; Murphree tray efficiencies from the open literature were used to develop the model. For comparison purposes, Murphree efficiency was converted to point efficiency by means of a mechanistic eddy diffusion model. Point efficiency data gathered in a semi-industrial column with conventional and high viscosity liquid systems completed an extensive data base. A total of 233 experimental points were used to validate the model. The average deviation for the entire database was -10.8%. The mean absolute deviation was 21.4%. These deviations were significantly less than those exhibited by the Chan-Fair model when compared with the same data base. In summary, the model properly accounted for the tray geometry (i.e. hole area, hole size and weir height), different liquid/vapor ratios, and the change in physical properties within a wide range.; The extension of the model to the prediction of efficiency in new tray devices with similar hydrodynamics characteristics, was successfully investigated. These encouraging results indicate that the model takes into account the most important hydraulic and mass transfer mechanisms, and is able to describe the phenomena existing in a liquid-vapor mixture on a distillation tray. The model should be generally applicable to the prediction of tray efficiency for hydrocarbon and aqueous systems, applications that exist in the chemical and petrochemical refining industries. |
