Tray Efficiency Scaleup

Prior to discussing scaleup, the factors that affect tray efficiency need to be addressed. These factors are addressed in Secs. 7.3.1 to 7.3.3. Considerations relevant to the effect of flow regime were previously discussed in Secs. 6.4.4 and 6.4.5. 

1 Effect of errors In Vapor-Liquid Equilibrium (VLE) on efficiency 

Errors in relative volatility are the most underrated factor that affects both tray and packing efficiency. The effects are direct when VLE errors affect separation stage requirement at a constant reflux ratio, and indirect when VLE errors affect the reflux ratio requirement (which in turn affects the stage requirement). Since higher relative volatility lowers both stage and reflux requirements (and vice versa), the direct and indirect effects complement each other and do not counteract each other. The discussion below applies to hoth tray and packed towers. 

Most efficiency data reported in the literature are obtained at total reflux. At total reflux, there are no indirect effects, and Fig. 7.6 shows the overall effect of VLE errors on column efficiency. For measurements at finite reflux ratios, the indirect effects below add to those in Fig. 7.6. 

Indirect effects. Consider a case where apparent 04 , and test data at a finite reflux are analyzed to calculate tray efficiency. Due to the volatility difference J min.apparent 7 mIn,tnie. Since the test was conducted at a fixed reflux flow rate, (H/ minUpparenc (i2/Rmin)true. A calculation based on the apparent R/Rmi will give more theoretical stages than a calculation based on the true This means a 

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Rigorous testing of a plant column is generally the most reliable method of obtaining tray efficiency. Test procedures are outside the scope of this book and are addressed in a companion book(l) and elsewhere (130). Alternative methods of obtaining tray efficiency are calculation and scaleup (or scale-down). Calculation is addressed in this section scaleup in Sec. 7.3. 

In the above procedure, errors in VLE are compensated by equivalent errors in tray efficiency. If the relative volatility calculated by the simulation is too high, fewer stages will be needed to match the measured test compositions, i.e., efficiency will be lower. Scaleup will be good as long as the VLE and efficiency errors continue to equally offset each other. This requires that process conditions (feed composition,... 

Engineers will find detailed information on key fractionation concepts. .. column process design and shortcut calculations. .. tray design and operation. .. tray efficiency. .. packing design and operation. .. packing efficiency and scaleup. .. packing capacity and pressure drop. .. and much more. 

Scaleup from Oldershaw column. One laboratory-scale device that found wide application in supplying efficiency data is the Oldershaw column (Fig. 8.13 Ref. 207). This column is available from a number of laboratory supply houses and can be constructed from glass for atmospheric operation or from metal for superatmospheric separations. Small hole diameters and small tray spacings are used. Typical column diameters are 1 to 3 in. 

Gas absorption can be carried out in a column equipped with sieve trays or other types of plates normally used for distillation. A column with trays is sometimes chosen instead of a packed column to avoid the problem of liquid distribution in a large diameter tower and to decrease the uncertainty in scaleup. The number of theoretical stages is determined by stepping off plates on a y-x diagram, and the number of actual stages is then calculated using an average plate efficiency. The plate and local efficiencies are defined in the same way as for distillation [Eqs. 

Methods for predicting efficiency also parallel those for tray columns comparison against a similar installation, use of empirical methods, direct scaleup from laboratory or pilot plant, and use of theoretically derived models. Approaches by vendors of packing usually center on comparisons with similar installations (the so-called vendor experience ) and empirical approximations. Direct scaleup from small column studies is difficult with packed columns because of the unknown effects of geometrical factors and the variations of liquid distribution that are required for practical reasons. Theoretical or semitheoretical models are difficult to validate because of the flow effects on interfacial area. It may be concluded that there is no veiy good way to predict packed column efficiency, at least for the random type packings. 

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