Efficiency, tray relative volatility

Distillation towers— Tray efficiency and relative volatility... 

I had arbitrarily manipulated tray efficiency and relative volatility to force my computer model to match the observed plant data. It might seem that by arbitrarily selecting both the relative volatility and tray efficiency for my computer model, my calculations would be little better than a guess. 

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 combined Fenske-Underwood-Gillilland method developed by Frank [100] is shown in Figure 8-47. This relates product purity, actual reflux ratio, and relative volatility (average) for the column to the number of equilibrium stages required. Note that this does not consider tray efficiency, as discussed elsewhere. It is perhaps more convenient for designing new columns than reworking existing columns, and should be used only on at acent-key systems. 

The net effect of reducing the stripper pressure was to greatly reduce the amount of isobutane in the heavier normal butane bottoms product. Undoubtedly, most of the improvement in fractionation was due to enhanced tray efficiency, which resulted from suppressing tray deck leaking, or dumping. But there was a secondary benefit of reducing tower pressure increased relative volatility. 

Figure 14-42 shows that errors in relative volatility are a problem only at low relative volatilities for a > 1.5 to 2.0, VLE errors have negligible direct impact on tray efficiency. 

FIG. 14-42 Direct effect of errors in relative volatility on error in tray efficiency. (From H. Z. Kister, Distillation Design, copyright 1992 by McGraw-Hill reprinted by permission.)... 

Pressure Tray efficiency slightly increases with pressure (Fig. 14-43), reflecting the rise of efficiency with a reduction in liquid viscosity and in relative volatility, which generally accompany a distillation pressure increase. 

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. 

Empirical Efficiency Prediction Two empirical correlations which have been the standard of the industry for distillation tray efficiency prediction are the Drickamer and Bradford, in Fig. 14-46 [Trans. Am. Inst. Chem. Eng. 39, 319 (1943)] and a modification of it by O Connell [Trans. Am. Inst. Chem. Eng. 42, 741 (1946)], in Fig. 14-47. The Drickamer-Bradford plot correlates efficiency as a function of liquid viscosity only, which makes it useful for petroleum cuts. O Connell added the relative volatility to the x axis. 

Example 12 Estimating Tray Efficiency For the column in Example 9, estimate the tray efficiency, given that at the relative volatility near the feed point is 1.3 and the viscosity is 0.25 cP. 

Operating the column at the minimum pressure minimizes the energy cost of separation. Towering this pressure increases the relative volatility of distillation components and thereby increases the capacity of the reboiler by reducing operating temperature, which also results in reduced fouling. Reducing pressure also affects other parameters, such as tray efficiencies and latent heats of vaporization. 

Errors in relative volatility are the most underrated factor that affects both tray and packing efficiency. The effects are direct when VLE errors affect separation stage requirement at a constant reflux ratio, and indirect when VLE errors affect the reflux ratio requirement (which in turn affects the stage requirement). Since higher relative volatility lowers both stage and reflux requirements (and vice versa), the direct and indirect effects complement each other and do not counteract each other. The discussion below applies to hoth tray and packed towers. 

In the above procedure, errors in VLE are compensated by equivalent errors in tray efficiency. If the relative volatility calculated by the simulation is too high, fewer stages will be needed to match the measured test compositions, i.e., efficiency will be lower. Scaleup will be good as long as the VLE and efficiency errors continue to equally offset each other. This requires that process conditions (feed composition,... 

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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