Tray efficiency factors

Generally, the mass transfer between two liquid phases in liquid-liquid extraction is substantially slower than that for rectification processes. Small values of the tray efficiency factors therefore follow. With average extraction, tray efficiency factors range from 0.3-0.7 with sieve trays, and 0.3-0.6 with Koch cascade trays. Only in mixer-settler cascades and with centrifugal extractors is the actual tray efficiency almost the theoretical tray efficiency. 

The area of the holes amounts to about 5-16% of the total area of the perforated plate. The distance between neighboured trays lies between 80 and 300 mm. It increases with the column s diameter and load range. The tray efficiency factor, i.e. the separation efficiency referred to one theoretical tray, ranges between 60 and 90%. Sieve tray columns are manufactured with diameters of up to about 6 m. 

The inside-out method provides quick convergence and wide flexibility in specifications. It is relatively easy to converge a column with a variety of specifications, but it remains difficult to produce a robust and predictive fractionation model. Many real-world fractionation systems do not operate with the ideal stage assumption used in standard distillation algorithms. A popular method to deal with the non-ideal tray behavior is the Murphree tray efficiency factor [43] ... 

In distillation towers, entrainment lowers the tray efficiency, and 1 pound of entrainment per 10 pounds of liquid is sometimes taken as the hmit for acceptable performance. However, the impact of entrainment on distiUation 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 smaU, 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 lowvolatiUty, as in an absorber). 

The actual number of trays needed for a particular separation duty depends on the efficiency of the plate, and the packings if they are used. Thus, any factors that cause a decrease in tray efficiency will also change the performance of the colunm. Tray efficiencies are affected by such factors as fouling, wear and tear and corrosion, and the rates at which these occur depends on the properties of the liquids being processed. Thus the proper materials of construction must be selected for tray construction. 

When chemical equilibrium is achieved quickly throughout the liquid phase, the problem becomes one of properly defining the physical and chemical equilibria for the system. It is sometimes possible to design a tray-type absorber by assuming chemical equilibrium relationships in conjunction with a stage efficiency factor, as is done in distillation calculations. Rivas and Prausnitz [AIChE 25, 975 (1979)] have presented an excellent discussion and example of the correct procedures to be followed for systems involving chemical equilibria. 

Factors Affecting Tray Efficiency Below is a summaiy based on the industry s experience. A detailed discussion of the fundamentals is found in Lockett s book (Distillation Tray Fundamentals, Cambridge University Press, Cambridge, England, 1986). A detailed discussion of the reported experience, and the basis of statements made in this section are in Kister s book (Distillation Design, McGraw-Hill, New York, 1992). 

Experience Factors These are tabulations of efficiencies previously measured for various systems. Tray efficiency is insensitive to tray geometry (above), so in the absence of hydraulic anomalies and issues with VLE data, efficiencies measured in one tower are extensible to others distilling the same system. A small allowance to variations in tray geometry as discussed above is in order. Caution is required with mixed aqueous-organic systems, where concentration may have a marked effect on physical properties, relative volatility, and efficiency. Table 14-12 shows typical tray efficiencies reported in the literature. 

Select the method of calculation for tray efficiency. Two methods are presented the O Connell method and the two-film method. In the programs accompanying this book, you may select the O Connell method by entering either an F for fractionator or an A for absorbers. In 1946, O Connell [4] published curves on log-log plots showing both absorber and fractionator efficiencies vs. equilibrium-viscosity-density factored equations. Separate curves for absorbers and fractionators were given. Such data have been curve-fit using a modified least-squares method in conjunction with a log scale setup. The fit is found to be reasonably close to the O Connell published curves. 

Input the liquid viscosity in centipoise. The liquid here is that on the subject tray. This is an important factor in determining tray efficiency. The value should be entered accurately and derived from reliable data sources, such as a reliable computer program for tray-to-tray fractionation calculations. Values greater than the normal range may occur, causing lower tray efficiencies. 

Enter an alpha value if you have chosen F or T for the method. Enter a K value for a light key component if you chose A. Input the factor alpha or K. Alpha is defined as simply the light key K divided by the heavy key K component. The K factor is simply the particular component s vapor phase mole fraction divided by its liquid mole fraction. The alpha value is therefore a ratio of the chosen two key components. These key components should be those that readily point to how well the fractionator is doing its job of separation. For example, for a depropanizer tower, choose propane as the light key component and butane as the heavy key, since you wish to separate the propane from the butane to make a propane product specification. For a multicomponent system, you may try several components to determine a controlling alpha and/or to factor an average tray efficiency. 

Important Note Bubble cap HHD factor is equivalent to the DPntAYi dry pressure drop of valve trays. The bubble cap tray total pressure drop factor DPTRay is equivalent to the HDC2 factor of valve-type trays. You may therefore substitute these bubble cap values in the ETF efficiency equations as given for valve trays to determine bubble cap tray efficiency. 

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. 

Vapor-liquid loads. A higher vapor load reduces the vapor contact time but also increases the interfacial area (136,137). These two factors have counteracting effects on tray efficiency. Usually, the contact time dominates, and efficiency decreases with higher vapor rates (185). A higher liquid load increases tray efficiency (185) because it increases tray liquid holdup, and therefore vapor contact time. 

Figure T.10 Some factors affecting sieve tray efficiency. FRI data, total reflux, DT = 4 It, S = 24 in, hu, = 2 in, dH = 0.5 in. Both parts show a small efficiency rise with pressure. Both parts show little effect of vapor and liquid loads above about 40 percent of flood, (a) Showing efficiency reduction when fractional hole area is increased from 8 to 14 per-cent of the bubbling area (6) emphasizing little effect of vapor and liquid loads, and an efficiency increase with pressure. Af 0.14 (Both parts repeated with permission from T. Yanagi and If. Sakata, lad. Eng. Chan. Proc. Use. Dev. 21, p. 712, copyright 19S2, American Chemical Society.)...

T Factor used in the Chan and Fair tray efficiency correlation, de-... 

Ponchon-Savarit diagram. 26 Ponter underwetting theory, 516, 517 Porter rivulet model. 542 Porter and Jenkins packing HETP, 532-534 regime transition. 332 Poynting factor. 7 Prado and Fair tray efficiancy, 375 PRO/II. 169, 170,180... 

Determine the effects of the physical properties of the system on column efficiency. Tray efficiency is a function of (1) physical properties of the system, such as viscosity, surface tension, relative volatility, and diffusivity (2) tray hydraulics, such as liquid height, hole size, fraction of tray area open, length of liquid flow path, and weir configuration and (3) degree of separation of the liquid and vapor streams leaving the tray. Overall column efficiency is based on the same factors, but will ordinarily be less than individual-tray efficiency. 

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