Distillation efficiencies

The ultimate goal of a distillation tower is separation of products, and naturally distillation efficiency becomes the key performance metric. A tower, if properly designed, can achieve 10% higher distillation efficiency. In operation, the tower operated with better distillation efficiency requires less energy use. 

Many different measures of efficiency have been developed. Let us look at the two commonly used measures, which are the stage-based Murphree tray efficiency (Murphree, 1925) and overall efficiency. 

The overall tower efficiency is defined as the ratio between the number of theoretical stages and the actual number of stages required for the 

The overall efficiency lumps everything that happens in the column into one value. Based on the assumptions of constant molar overflow and constant value of ijm for trays in each distinctive section in a tower, Lewis (1936) developed a relationship between the Murphree tray efficiency and overall efficiency, which is expressed as  

Equation (12.3) applies separately to rectifying and stripping sections as the V/L ratio is different between these two sections. In the rectifying section, vapor rate is higher while liquid rate is lower compared with those in the stripping section. However, in each section, equation (12.3) is based on the assumptions of straight operating and equilibrium lines and constant VIL ratio and from tray to tray. 

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When this is combined with the definition of minimum separation work, an approximation for distillation efficiency for an ideal binary can be obtained ... 

MacFarland, S.A., Sigmund, P.M., Van Winkle, M. Predict Distillation Efficiency, Hydrocarbon... 

In distillation towers, entrainment lowers the tray efficiency, and 1 pound of entrainment per 10 pounds of liquid is sometimes taken as the limit for acceptable performance. However, the impact of entrainment on distillation efficiency depends on the relative volatility of the component being considered. Entrainment has a minor impact on close separations when the difference between vapor and liquid concentration is small, but this factor can be dominant for systems where the liquid concentration is much higher than the vapor in equilibrium with it (i.e., when a component of the liquid has a very low volatility, as in an absorber). 

Energy X X X Less heat loss via integration of units (e.g., reactive distillation) Efficient heat transfer possible (high heat transfer rates)... 

Direct Scale-Up of iMboratory Distillation Efficiency Measurements It has been found by Fair, Null, and Bolles [Ind. Eng. Chem. Process Des. Dev., 22,53 (1983)] that efficiency measurements in 25- and 50-mm- (1- and 2-in-) diameter laboratory Oldershaw columns closely approach the point efficiencies [Eq. (14-129)] measured in large sieve-plate columns. A representative comparison of scales of operation is shown in Fig. 14-37. Note that in order to achieve agreement between efficiencies it is necessary to ensure that (1) the systems being distilled are the same, (2) comparison is made at the same relative approach to the flood point, (3) operation is at total reflux, and (4) a standard Oldershaw device (a small perforated-plate column with downcomers) is used in the laboratory experimentation. Fair et al. made careful comparisons for several systems, utilizing as large-scale information the published efficiency studies of Fractionation Research, Inc. 

Aittamaa (1981) simulated a number of experiments with the systems ethanol-benzene-n-heptane, and chloroform-benzene-n-heptane (data were obtained at Hoffmann-La Roche in a fair size pilot scale column) and 1-butanol-ethanol-water in a 12 sieve-tray column. The Hoffman-La Roche data were taken in a column having 24 sieve trays and 30-cm inside diameter. Unlike many studies of distillation efficiency, these experiments were not carried out at total reflux. The measured flow rates and compositions of the feed, distillate, and... 

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