Plate and column efficiencies of continuous rectifying columns for binary mixtures

Abstract

Plate EfficienciesIn the design of the distillation column, the plate efficiency plays an important role. The plate efficiency has been defined as Murphree's plate efficiency with the vapor composition in equilibrium with the liquid leaving the plate. This efficiency is reasonable only when the liquid is well mixed (as described in the later sections) and there is no composition gradient of the liquid on the plate. In practice, however, the liquid composition will vary as the liquid flows on the plate. In the case of larger columns and systems of large relative volatilities, Murphree's plate efficiencies have been found to be more than 100%.In our previous paper ( I ) , t it was pointed out that at a fixed point efficiency E o~t Murphree's plate efficiency ( E M V ) 1, based on the vapor in equilibrium with the liquid leaving the plate, decreases with the increase of reflux ratio, and plate efficiency ( E M V )~, based on the vapor composition in equilibrium with the liquid entering the plate, increases with the increase of reflux ratio. A modified plate efficiency was therefore defined as where (9. " ) i S the vapor composition in equilibrium with the liquid at the center of the plate.If the liquid flows unidirectionally on the plate as a plug flow, the relation between the plate efficiencies and the point efficiency is shown in Figure 1. For a fixed point efficiency, Murphree's plate efficiency (EMv) 1, which is shown with the broken line, increases with A where A = mV/L, and EM")^ decreases with A as shown with the dotted line. The modified plate efficiency ( EMV) 112, however, does not vary so much with the change of A as shown with the solid line.Taking into account the mixing of the liquid on a plate, Figure 2 shows the relation between the plate efficiency and the point efficiency. For a fixed Peclet's number N p e , the variation of ( E M v )~/~/ E o G for A is much smaller than that of (EMv)l/Eoc. In the case of AEOG < 1 (this condition holds for almost all the practical columns), the value of ( E M~)~/~/ E o G is less than 1.04 for any value of N p e ,

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

“…It is possible for the Murphree tray efficiency to exceed unity especially for large columns and systems of high relative volatilities. 32,51 In that case, the resulting tray step (as illustrated in Figure 4) would actually transcend the VLE curve. Thus, the maximum tray efficiency is limited to unity here.…”

“…When the higher values of m combine with low axial liquid mixing and high point efficiencies, then the corresponding tray efficiencies become greater than unity (not shown here). It is possible for the Murphree tray efficiency to exceed unity especially for large columns and systems of high relative volatilities. , In that case, the resulting tray step (as illustrated in Figure ) would actually transcend the VLE curve. Thus, the maximum tray efficiency is limited to unity here.…”

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

“…Some empirical models for the stage efficiency, e.g. English and Van Winkle (1963), Onda, et al (1971) andMacFarland, et al (1972), expressed efficiency in terms of all the possibly relevant dimensionless groups which also include some characteristic dimensions of the specific tray. The other empirical models generally related the overall efficiency to a few key variables, the most important of which was liquid viscosity.…”

Model verification and analysis of the CO2-MEA absorber–desorber system

“…The theory of the phase equilibrium model in the case of a packed column means that there is a certain height of packing, the so-called height of equivalent theoretical plate (HETP), where the vapor and liquid streams that leave this height of packing are in phase equilibrium with each other. Any effect causing the deviation of mass transfer from ideality is taken into account by introducing the Murphree or column efficiencies (Onda et al, 1971). The different efficiencies take the non-idealities into consideration and can describe the column composition profile in agreement with the real trays or stages.…”