Examples estimating distillation tray efficiency

Example 8-13 Estimating Distillation Tray Efficiency by Equations 8-7OA and 8-7OB (used by permission of McFarland et al. [86])... 

Analysts must recognize the above sensitivity when identifying which measurements are required. For example, atypical use of plant data is to estimate the tray efficiency or HTU of a distillation tower. Certain tray compositions are more important than others in providing an estimate of the efficiency. Unfortunately, sensor placement or sample port location are usually not optimal and, consequently, available measurements are, all too often, of less than optimal use. Uncertainty in the resultant model is not minimized. 

Estimate the tray efficiency in the distillation of the ternary system methanol(l)-l-pro-panol(2)-water(3) considered in Examples 12.2.1 and 13.3.1. 

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. 

Example 13.4-1 Finding mass transfer coefficients from stage efficiencies On one tray of an acetone-water distillation we find that equals 0.84, x equals 0.70, and y +1 equals 0.76. The stage is at 59 °C and 1 atm it has a vapor flow of 0.14 kg mol/sec and a froth volume of 0.04 m. Assuming that both vapor and liquid are well mixed, estimate the Murphree efficiency and the mass transfer coefficient on this stage. 

Calculate the effect of the change in slope of the equilibrium line on the local efficiency in a sieve-tray distillation column. Use the benzene-toluene system as an example, and predict / for plates where the mixture is mostly toluene and for those where it is mostly benzene, starting with an estimated value of for the middle of the column. 

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