Actual trays

After actual theoretical trays are determined (see Actual reflux and theoretical stages) one needs to estimate the actual physical number of trays required in the distillation column. This is usually done by dividing the actual theoretical trays by the overall average fractional tray efficiency. Then a few extra trays are normally added for offload conditions, such as a change in feed composition. 

Experience for a given service is the best guide for extra trays. 

Branan, C. R., The Process Engineer s Pocket Handbook, Vol. 2, Gulf Publishing Co., 1983, p. 4. 

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Fractional equihbrium stages have meaning. The 11.4 will be divided by a tray efficiency, and the rounding to an integral number of actual trays should be done after that division. For example, if the average tray efficiency for the process being modeled in Fig. 13-36 were 80 percent, then the number of actual trays required would be 11.4/0.8 = 14.3, which would be rounded to 15. 

The application of a 50 percent Murphree vapor-phase efficiency on a y-x magram is illustrated in Fig. 13-40. A pseudo-equilibrium cui ve is drawn halfway (on a vertical line) between the operating hnes and the true-equilibrium cui ve. The true-equilibrium cui ve is used for the first stage (the partial reboiler is assumed to be an equilibrium stage), but for 1 other stages the vapor leaving each stage is assumed to approach the equilibrium value only 50 percent of me way Consequently, the steps in Fig. 13-40 represent actual trays. 

Presaturators. A presaturator to provide lean oil/gas contact prior to feeding the lean oil into the tower can be a good way of getting more out of an older tower. Absorber tray efficiencies ran notoriously low. A presaturator that achieves equilibrium can provide the equivalent of a theoretical tray. This can easily equal 3-4 actual trays. Some modem canned computer distillation/absorp-tion programs provide a presaturator option. 

Assume 8 actual trays or 2 theoretical trays. From Figure 8-17 the glycol circulation rate is 2.8 gal TEGAb H2O. Size for 3.0 galAb. 

Distillation design is based on the theoretical consideration that heat and mass transfer from stage to stage (theoretical) are in equilibrium [225-229]. Actual columns with actual trays are designed by establishing column tray efficiencies, and applying these to the theoretical trays or stages determined by the calculation methods to be presented in later sections. 

This applies to any pair of components. My experience suggests adding +1 theoretical tray for the reboiler, thus making the total theoretical trays perhaps a bit conservative. But, they must be included when converting to actual trays using the selected or calculated tray efficiency ... 

Because a column cannot operate at total reflux and produce net product from the column, a reflux ratio of about 1.1 to 1.5 times the mmmMm reflux will usually give practical results. Be aware that as the reflux ratio comes down approaching the minimum, the number of theoretical and then corresponding actual trays must increase. 

Note that these values for theoretical trays do contain corrections in overall efficiency, and hence are not the actual trays for the binary distillation column. Efficiencies generally run 50-60% for systems of this type which will yield a column of actual trays almost twice the theoretical at the operating reflux. 

This suggests that caution must be exercised when establishing a tray efficiency for any type contacting device by (1) using actual test data if available for some similar system or (2) comparing several methods of predicting efficiency, and (3) possible use of a more conservative efficiency than calculated to avoid the possibility of ending up with a complete column with too few actual trays—a disastrous situation if not discovered prior to start-up operations. 

For a constant reflux ratio, the value can be almost any ratio however, this ratio affects the number of theoretical plates and, consequently, actual trays installed in the rectification section to achieve the desired separation. Control of batch distillation is examined in Reference 134. 

The results indicate that 25 theoretical plates are minimum then by assuming an efficiency of 50%, total actual trays of 50 should be adequate. Choose values of k (see nomenclature) and solve for b and Xj by ... 

Eduljee [107] evaluated published data and corrected relationships for determining the number of actual trays versus actual reflux with reasonably good agreement ... 

If the number of actual trays, S, calculates to be 27 or greater, then revert to the following for better accuracy Second Trial ... 

From the theoretical trays at operating reflux the actual trays for installation are determined ... 

Determining the number of theoretical and actual trays in a distillation column is only part of the design necessary to ensure system performance. The interpretation of distillation, absorption, or stripping requirements into a mechanical vessel with internal components (trays or packing, see Chapter 9) to carry out the function requires use of theoretical and empirical data. The costs of this equipment are markedly influenced by the column diameter and the intricacies of the trays, such as caps, risers, weirs, downcomers, perforations, etc. Calcvdated tray efficiencies for determination of actual trays can be lost by any unbalanced and improperly designed tray. 

Tray No. 1 = Top/Btm Section (Name/Descriptlon) Tray Numbers Included Loading at Actual Tray No. Number of Trays Required... 

Total number of actual trays in tower Number of caps per tray Number of slots per bubble cap Valve density, number of valves per ft or Number of valve units on a valve tray Depth of notches in weir, in or Exponent defined by Equations 8-288 and 327 Dry tray pressure drop for 50% cut baffles, in. liquid per baffle or... 

Actual tray pressure drop, in. liquid Prandtl number dimensionless Fractional opening in the circumference or a valve or. Pi... 

About 65% efficiency is to he expected in this service, requiring three actual trays. 

The feed tray lies between ideal trays 5 and 6, and in practice, the eighth actual tray from the top would be used. 

In general, distillation columns should be operated at a low pressure. For example, Fig. 3.3 shows an isobutane-normal butane stripper. This fractionator is performing poorly. A computer simulation of the column has been built. The column has 50 actual trays. But in order to force the computer model to match existing operating parameters (reflux rate, product compositions), 10 theoretical separation stages (i.e., 10 trays, each 100 percent efficient) must be used in the model. This means that the trays are developing an actual tray efficiency of only 20 percent. 

The calculations made thus far are of theoretical trays, that is, trays on which vapor-liquid equilibrium is attained for all components. Actual tray efficiencies vary widely with the kind of system, the flow rates, and the tray construction. The range can be from less than 10% to more than 100% and constitutes perhaps the greatest uncertainty in the design of distillation equipment. For hydrocarbon fractionation a commonly used efficiency is about 60%. Section 13.14 discusses this topic more fully. 

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