Experimental results are presented on the liquid back-mixing of distillation trays. The tests were conducted in the 1.22 m diameter column of the FRI distillation unit using different systems at various pressures. Both bubble-cap and sieve trays with different designs were tested. For each test, a pulse of nonvolatile dye was injected into the reflux stream and the liquid on several trays was continuously monitored to determine the effects of the trays on this pulse. The degree of liquid back-mixing on the tray was then deduced as a function of the change in the shape of the pulse. The results show that liquid back-mixing is strongly dependent on tray designs and system properties. Certain trays and systems had a high degree of plug flow; others were highly mixed. The test results indicate that the liquid back-mixing occurring in the tray deck is much larger than that in the downcomer. Mixing pool numbers and eddy diffusivity were computed from the experimental data. The results are compared to predictions from several literature models. The predictions from the Bennett et al. and Zuiderweg correlations agree reasonably well with the experimental results.

The hydraulic and mass transfer performance of a combined knitted mesh packing and sieve tray has been measured for the distillation of methanol-water mixtures. It was found that by adding a shallow bed of packing, the Murphree tray efficiency increased by 40-50% over a wide range of concentrations and flow rates. This increase in tray efficiency can be attributed to a much smaller and more uniform bubble formation on the packed tray. Hydraulic measurements have shown that the packed trays have lower weeping and entrainment, as well as slightly higher froth heights and pressure drops.La performance hydraulique et la performance du transfen de matikre d'un garnissage B mailles perfortes et d'un plateau perfort combinks, a ktk mesurke pour la distillation de mtlanges mkthanol-eau. On a trouvt qu'en ajoutant un lit de garnissage peu profond, l'efficacitt de Murphree du plateau augmentait de 40 B 50 % pour une large gamme de concentrations et de debits. Cette augmentation de l'efficacitk du plateau peut &tre attribute B une formation de bulles plus petites et plus uniformes sur le plateau garni. Des mesures hydrauliques montrent que les fuites et I'entrainement des plateaux garnis sont plus faibles et que la hauteur de mousse et les pertes de charge sont lkgbrement plus grandes.