Liquid-Phase Resistance to Mass Transfer on Distillation Trays

Abstract

The liquid-phase resistance to mass transfer was determined for the distillation of six binary mixtures. It was found that the liquid-phase resistance to mass transfer is significant, ranging from 20 to 55% of the total mass-transfer resistance. The results also indicated that the traditional assumption of & = OL is not valid for distillation systems. If this assumption is applied, the fraction of liquid-phase resistance to mass transfer will be significantly underestimated. Tray efficiency models developed from data obtained from absorption and stripping systems (e.g., the AIChE method) were suggested to greatly overpredict the number of liquidphase mass-transfer units for distillation systems. Thus, careful examination of the use of various efficiency models for tray design is required.

Efficiency Estimation of Tray Columns Based on Flow Profiles and Vapor–Liquid Equilibrium Characteristics of Binary Mixtures

“…First, the vapor- and liquid-phase transfer units (referred to as N V and N L , respectively) are estimated based on the two-film resistance theory. The phase resistances are then added up to get the overall vapor-phase transfer units ( N OV ), from which the Murphree vapor-side point efficiency ( E OV ) is estimated using an appropriate literature model. , Based on AIChE’s bubble-tray design manual, several studies have considered distillation to be a vapor-phase-controlled (i.e., negligible liquid-phase resistance) operation . However, experimental data in the literature ,− indicate that the liquid-phase resistance can also be significant .…”

“…The phase resistances are then added up to get the overall vapor-phase transfer units ( N OV ), from which the Murphree vapor-side point efficiency ( E OV ) is estimated using an appropriate literature model. , Based on AIChE’s bubble-tray design manual, several studies have considered distillation to be a vapor-phase-controlled (i.e., negligible liquid-phase resistance) operation . However, experimental data in the literature ,− indicate that the liquid-phase resistance can also be significant . To acknowledge these facts, the transfer units of the individual phases on the tray are considered as equal, i.e., N V = N L .…”

“…1,32 Based on AIChE's bubble-tray design manual, 13 several studies have considered distillation to be a vapor-phase-controlled (i.e., negligible liquid-phase resistance) operation. 33 However, experimental data in the literature 1,34−37 indicate that the liquid-phase resistance can also be significant. 33 To acknowledge these facts, the transfer units of the individual phases on the tray are considered as equal, i.e., N V = N L .…”

“…33 However, experimental data in the literature 1,34−37 indicate that the liquid-phase resistance can also be significant. 33 To acknowledge these facts, the transfer units of the individual phases on the tray are considered as equal, i.e., N V = N L . This is applied to all trays in the column.…”

A new crossflow rotating bed, Part 3: Stage efficiency model

“…The arbitrary assumption of equality between θ L and θ G was not generally agreed and has not been verified by the experimental results. Chen and Chuang (1995) has deduced that for common distillation systems, the exposure time of the liquid elements is several or even more than 20 times that of the gas elements, which results from the high liquid viscosity and the less violent mixing in the liquid phase. Based on Higbie's assumption, in the stage i, the exposure time of the liquid elements θ L could be similarly estimated as the time taken by a droplet to travel a distance equals to its diameter:…”