Murphree vapor efficiency columns

This rate-based analysis of trayed columns gives an estimate of the number of overall gas phase mass transfer units corresponding to a given tray. The practical application of this analysis is to provide a means for estimating the tray Murphree vapor efficiency discussed in Section 14.3. 

The column operates with a reflux ratio L/D of 4 to 1. The Murphree vapor efficiency is 80%. 

The design procedure sizes the column to prevent flooding caused by excessive entrainment. Flooding can also occur in the downcomers, and this case is discussed later. Excessive entrainment can also cause a large drop in stage efficiency because liquid that has not been separated is mixed with vapor. The effect of entrainment on the Murphree vapor efficiency can be estimated from... 

A small distillation column separating benzene and toluene gives a Murphree vapor efficiency of 0.65 in the rectifying section where 17V = 0.8 and x z = The tray is perfectly mixed and has a liquid... 

D15. The large-scale column in Example 16-4 has a feed that is a saturated liquid with a feed mole frac z = 0.5, and separation is essentially conplete (x 1 and 0). The Murphree vapor efficiency is often approximately constant in columns. Assume the value calculated in Example 16-4, E]y[Y = 0.97, is constant in the large-scale column (plug flow trays). Calculate Ep. and Kya in the stripping section at x = 0.10 and x = 0.30, and in the enriching section at x = 0.9. Repeat the enriching section calculation at x = 0.7 (shown in Example 16-41 as a check on your procedure. 

The number of actual trays required in a tower to attain the performance of a calculated number of theoretical stages is determined by the tray efficiency. Several types of tray efficiency have been proposed however, the two most widely used are the Murphree vapor efficiency mv> defined in Section 6.2 [Eq. (6.2-1)]. which refers to individual trays, and the overall column efficiency Eo, which is defined simply as... 

FIG. 13-40 Application of a 50 percent Murphree vapor-phase efficiency to each stage (excluding the rehoiler) in the column. Each step in the diagram corresponds to an actual stage. 

Data from bubble cap and perforated tray columns for the Murphree vapor plate efficiencies are correlated [86] ... 

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 ... 

Example 8 Calculation of Rate-Based Distillation The separation of 655 Ib 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 carried 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 will 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 will be set at 167 Ib moPh 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 will be used. Plug flow of vapor and complete mixing of liquid will 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-calculate from the computed tray compositions, the component vapor-phase Murphree-tray efficiencies. 

Conversion to Murphree Tray Efficiency. For complete liquid mixing on the tray, as found in small columns (e.g., Oldershaws) and in some vacuum columns where the volumetric flow of vapor is much higher than that of the liquid, Eog = Emv- For the opposite case of plug flow of liquid across the tray. 

The AIChE method and that of Van Winkle predict the dry Murphree plate efficiency. In operation some liquid droplets will be entrained and carried up the column by the vapor flow, and this will reduce the actual, operating efficiency. 

The column solution methods described in this chapter are based on an equilibrium stage model. Normally, the vapor and liquid leaving a tray are not at equilibrium due to imperfect mixing or insufficient residence time on the tray. Tray efficiencies, discussed in Chapter 14, are parameters that relate actual performance to equilibrium stage performance. By replacing the equilibrium relationship in the model with an equation based on the Murphree tray efficiency, the actual column performance can be calculated. The accuracy of the model obviously depends on the reliability of the tray efficiencies used in the calculations. 

The required number of actual plates, Np, is larger than the number of theoretical plates, because it would take an infinite contacting time at each stage to establish equilibrium. The ratio N p. N is called the overall column efficiency. This parameter is difficult to predict from theoretical considerations, however, or to correct for new systems and operating conditions. It is therefore customary to characterize the single plate by the so-called Murphree vapor plate efficiency, (98) ... 

This presentation is limited to two models, the Murphree plate efficiency and the modified Murphree plate efficiency, and their applications to columns in the service of separating both binary and multicomponent mixtures. For convenience of application, these efficiencies are restated in terms of the vaporization plate efficiency. Definitions of the vaporization point and plate efficiency follow immediately and the Murphree plate efficiencies are defined in a subsequent section as they arise in the development of the perfectly mixed liquid phase model. 

The boiling point-equilibrium data for the system acetone-methanol at 760 mm Hg are given in Table 18.7. A column is to be designed to separate a feed analyzing 25 mole percent acetone and 75 mole percent methanol into an overhead product containing 78 mole percent acetone and a bottom product containing 1.0 mole percent acetone. The feed enters as an equilibrium mixture of 30 percent liquid and 70 percent vapor. A reflux ratio equal to twice the minimum is to be used. An external reboiler is to be used. Bottom product is removed from the reboiler. The condensate (reflux and overhead product) leaves the condenser at 25°C, and the reflux enters the column at this temperature. The molal latent heats of both components are 7700 g cai/g mol. The Murphree plate efficiency is 70 percent. Calculate (a) the number of plates required above and below the feed (b) the heat required at the reboiler, in Btu per pound mole of overhead product (c) the heat removed in the condenser, in Btu per pound mole of overhead product. 

Figure 6.25 Column profiles for Example 6.16 (a) pressure (b) Murphree vapor tray efficiencies.

The column has 30 sieve trays, with a total condenser and a partial reboiler. The solvent enters tray 5 and the feed enters tray 15, from the top. The pressure in the condenser is 1.1 atm the pressure at the top tray is 1.2 atm, and the pressure at the bottom is 1.4 atm. The reflux ratio is 5 and the bottoms rate is 960 kmol/h. Use the nonequilibrium model of the ChemSep program to estimate the separation achieved. Assume that the vapor and the liquid are both well mixed and that the trays operate at 75% of flooding. In addition, determine from the tray-by-tray results the average Murphree tray efficiency for each component. 

A vaiue for the Murphree vapor-phase efficiency E v is then calculated for the overall tray by adjusting the point efficiency Eqc to take into account the effects of liquid mixing and entrainment. A summary of the AIChE design procedure for bubble-cap trays and an example of its application are given by King. A very approximate estimate of the overall efficiency of bubble-cap columns used for absorption can be obtained from Fig. 6.4-13. This simple correlation was proposed by O Connell. ... 

Gadwa (Ref. 12) has studied the plate efficiency in the fractionation of mixtures of (1) benzene-carbon tetrachloride, (2) methanol-isobutanol, (3) methanol-ri-propanol, (4) isobutanol-water, (5) n-pro-panol-water, and (6) methanol-water. A small four-plate column containing one bubble cap per plate was employed. The bubble caps were ii - in diameter and 2 in. high containing 38 slots % in. wide by % in. high per cap. A vapor space of 5 by 5 in. was partitioned off from the overflow pipes, giving a ratio of slot area to superficial area of 0.12. The plates were spaced 11 in. apart, and overflow weirs were employed. Plate samples were taken so that the Murphree plate efficiencies could be calculated. Some of these results are given in Table 17-3. The efficiencies in this table were calculated for the vapor phase. 

Atkins and Franklin (Ref. 1) found an over-all column efficiency of 18 per cent for a natural gasoline absorber using gas oil as the absorbing liquid. Walter (Ref. 30) obtained Murphree vapor plate efficiencies from 80 to 95 per cent in a 2-in. laboratory column for air humidification. Data taken in the same unit on the absorption of propylene and isobutylene in gas oil, heavy naphtha, and mixtures of gas and lube oil, gave plate efficiencies on the vapor basis of 5 to 36 per cent. 

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