Tray columns point efficiency

M.J. Lockett and I.S. Ahmed, Tray and point efficiencies from a 0.6 meter diameter distillation column, Chem. Engng. Res. Des., 61 (1983) 110-118. 

Example 8 Calculation of Rate-Based Distillation The separation of 655 lb mol/h of a bubble-point mixture of 16 mol % toluene, 9.5 mol % methanol, 53.3 mol % styrene, and 21.2 mol % ethylbenzene is to be earned out in a 9.84-ft diameter sieve-tray column having 40 sieve trays with 2-inch high weirs and on 24-inch tray spacing. The column is equipped with a total condenser and a partial reboiler. The feed wiU enter the column on the 21st tray from the top, where the column pressure will be 93 kPa, The bottom-tray pressure is 101 kPa and the top-tray pressure is 86 kPa. The distillate rate wiU be set at 167 lb mol/h in an attempt to obtain a sharp separation between toluene-methanol, which will tend to accumulate in the distillate, and styrene and ethylbenzene. A reflux ratio of 4.8 wiU be used. Plug flow of vapor and complete mixing of liquid wiU be assumed on each tray. K values will be computed from the UNIFAC activity-coefficient method and the Chan-Fair correlation will be used to estimate mass-transfer coefficients. Predict, with a rate-based model, the separation that will be achieved and back-calciilate from the computed tray compositions, the component vapor-phase Miirphree-tray efficiencies. 

Tbe best-established theoretical method for predicting E is that of tbe AlCbE [Buhhle-Tray Design Manual, American Institute of Chemical Engineers, New York, 1958). It is based on tbe sequential prediction of point efficiency, Murpbree efficiency, and overall column efficiency ... 

Biddulph [90] emphasizes the importance of using point efficiencies rather than tray efficiencies or overall column efficiencies, due to the wide fluctuations that often exist. 

Mols of distillate or overhead product, lb mols/hr or batch distillation, mols Mols component, i, in distillate Vaporization efficiency of steam distillation Overall column efficiency Overall tray efficiency Eqg = Murphree point efficiency, fraction Murphree plate/tray efficiency, = E ... 

The calculated point efficiency must be converted to overall column efficiency, which will lower its value and make it closer to the O Connell prediction. The calculated value of Eog is slightly higher than obtained experimentally (Eog = 0.83-0.92) at the University of Delaware for bubblecap trays (Annual Progress Report of Research Committee, Tray Efficiencies in Distillation Columns, AIChE, New York, 1955). 

Plate efficiencies for tray columns, 661-667 Plate stabilities for tray columns, 686 Platforms, cost of, 711 Plug-flow reactor design, 727-728 Point efficiencies for tray columns, 661 Poiseuille s law, 481n. 

A horizontal concentration gradient will develop in the liquid due to mass transfer into and from the liquid as the liquid flows across the tray. Thus, the composition of the vapor above the froth will change as we traverse the tray even if the composition of the vapor just below the tray is uniform. The point efficiency defined in the preceding section models the mass transfer processes at a particular point on the tray but does not take into account the fact that the liquid may have a significant concentration change as it crosses the tray. Thus, the point efficiency must be related to the tray efficiency before it can be used in column design calculations. 

If the liquid is completely mixed in the horizontal direction (a reasonable approximation for small diameter columns) then the tray efficiency and the point efficiency are one and the same (all points being equal as it were) and = E y... 

The distillation of a mixture of ethanol(l)-Lbutyl alcohol(2)-water(3) in a sieve tray column was considered in Example 12.2.2. Here, we continue that problem by determining the point efficiencies for this system under the conditions specified in the prior example. For the record the composition of the vapor entering the tray is... 

The point efficiency is a fundamental criterion of mass transfer, whereas the overall tray efficiency is the value that can be measured, especially for larger distillation columns. Large-column... 

Large-scale tray column efficiencies can be predicted from measurements in laboratory columns as small as 25 mm (1.0 in) diameter. Comparative studies (laboratory vs. commercial) have shown that the use of a special laboratory sieve tray column, the Oldershaw column, produces equivalent separations. A glass Oldershaw column is shown in Figure 12.61. Each tray has a center downcomer that discharges to one side of the tray below. Comparisons between laboratory and commercial scale efficiencies are shown in Figure 12.62 ° and Table 12.11. The Oldershaw value of overall efficiency is equivalent to the point efficiency of the larger column. 

FIGURE 12.62 Comparison of tray point efficiencies for different column sizes. Cyclohexane/ -heptane system, 0.20-0.27 atm pressure, total reflux. (Separations Research Program, Univ. of Texas at Austin.)... 

Notice that even though the point efficiency for the conditions prevailing at the bottom of the column is significantly smaller than the point efficiency just above the feed tray, the entrainment-corrected Murphree tray efficiencies at both points are very similar. 

---