When Blood Pressure Drop

Distillation Trays
FIG. 14-41 Sequence of steps for theoretical prediction of tray efficiency. (From H. Z. Kister, Distillation Design, copyright © 1992 by McGraw-Hill; reprinted by permission. )

Factors Affecting Tray Efficiency Below is a summary 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).

Errors in Vapor-Liquid Equilibrium (VLE) Errors in relative volatility are the most underrated factor affecting tray efficiency. Figure 14-42 shows the direct effect of the errors [Deibele and Brandt, Chem. Ing. Tech. 57(5), p. 439 (1985); Roy P. and G. K. Hobson, I. Chem. E. Symp. Ser. 104, p. A273 (1987)]. At very low relative volatilities (a < 1.2), small errors in VLE have a huge impact on tray efficiency. For instance, at a = 1.1, a —3 percent error gives a tray efficiency 40 to 50 percent higher than its true value (Fig. 14-42). Since VLE errors are seldom lower than ±2 to 3 percent, tray efficiencies of low-volatility systems become meaningless unless accompanied by VLE data. Likewise, comparing efficiencies derived for a low-volatility system by different sources is misleading unless one is using identical VLE.

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.

Most efficiency data reported in the literature are obtained at total reflux, and there are no indirect VLE effects. For measurements at finite reflux ratios, the indirect effects below compound the direct effect of Fig. 14-42. Consider a case where aapparent < atrue and test data at a finite reflux are analyzed to calculate tray efficiency. Due to the volatility difference Rmin,appalent > Rmin,trae. Since the test was conducted at a fixed reflux flow rate, (R/Rmin)appalent < (R/Rmm)™. A calculation based on the apparent R/Rmin will give more theoretical stages than a calculation based on the true R/Rmin. This means a higher apparent efficiency than the true value.

The indirect effects add to those of Fig. 14-42, widening the gap between true and apparent efficiency. The indirect effects exponentially escalate as minimum reflux is approached. Small errors in VLE or reflux ratio measurement (this includes column material balance as well as reflux rate) alter R/Rmin. Near minimum reflux, even small R/Rmin errors induce huge errors in the number of stages, and therefore in tray efficiency. Efficiency data obtained near minimum reflux are therefore meaningless and potentially misleading.

Liquid Flow Patterns on Large Trays The most popular theoretical models (below) postulate that liquid crosses the tray in plug flow with superimposed backmixing, and that the vapor is perfectly mixed. Increasing tray diameter promotes liquid plug flow and suppresses backmixing.

The presence ofstagnant zones on large-diameter distillation trays is well established, but the associated efficiency loss is poorly understood; in some cases, significant efficiency losses, presumably due to stagnant zones, were reported [Weiler, Kirkpatrick, and Lockett, Chem. Eng. Progr. 77(1), 63 (1981)], while in other cases, no efficiency difference was observed [Yanagi and Scott, Chem. Eng. Progr., 69(10), 75 (1973)]. Several techniques are available for eliminating stagnant regions (see Kister, Distillation Design, McGraw-Hill, New York, 1992, for some), but their effectiveness for improving tray efficiency is uncertain.

Weir Height Taller weirs raise the liquid level on the tray in the froth and emulsion regimes. This increases interfacial area and vapor contact time, which should theoretically enhance efficiency. In the spray regime, weir height affects neither liquid level nor efficiency. In distillation systems, the improvement of tray efficiency due to taller weirs is small, often marginal.

Length of Liquid Flow Path Longer liquid flow paths enhance the liquid-vapor contact time, the significance of liquid plug flow, and therefore raise efficiency. Typically, doubling the flow path length

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.)

(such as going from two-pass to one-pass trays at a constant tower diameter) raises tray efficiency by 5 to 15 percent.

Fractional Hole Area Efficiency increases with a reduction in fractional hole area. Yanagi and Sakata [Ind. Eng. Chem. Proc. Des. Dev. 21, 712 (1982)] tests in commercial-scale towers show a 5 to 15 percent increase in tray efficiency when fractional hole area was lowered from 14 to 8 percent (Fig. 14-43).

Tray Efficiency Versus Vapour Rate

FIG. 14-43 Efficiency reduction when fractional hole area is increased, also showing little effect of vapor and liquid loads on efficiency in the normal operating range (between excessive weeping and excessive entrainment). Also shown is the small increase in efficiency with pressure. FRI data, total reflux, DT = 1.2 m, S = 610 mm, hw = 50.8 mm, dH = 12.7 mm. (Reprinted with permission from T. Yanagi and M. Sakata, Ind. Eng. Chem. Proc. Des. Dev. 21, 712; copyright © 1982, American Chemical Society.)

FIG. 14-43 Efficiency reduction when fractional hole area is increased, also showing little effect of vapor and liquid loads on efficiency in the normal operating range (between excessive weeping and excessive entrainment). Also shown is the small increase in efficiency with pressure. FRI data, total reflux, DT = 1.2 m, S = 610 mm, hw = 50.8 mm, dH = 12.7 mm. (Reprinted with permission from T. Yanagi and M. Sakata, Ind. Eng. Chem. Proc. Des. Dev. 21, 712; copyright © 1982, American Chemical Society.)

Hole Diameter The jury is out on the effect of hole diameter on tray efficiency. There is, however, a consensus that the effect of hole diameter on efficiency is small, often negligible.

Vapor-Liquid Loads and Reflux Ratio Vapor and liquid loads, as well as the reflux ratio, have a small effect on tray efficiency (Fig. 14-43) as long as no capacity or hydraulic limits (flood, weep, channeling, etc.) are violated.

Viscosity, Relative Volatility Efficiency increases as liquid viscosity and relative volatility diminish. These effects are reflected in the O'Connell correlation (below).

Surface Tension There is uncertainty regarding the effect of surface tension on tray efficiency. Often, it is difficult to divorce the surface tension effects from those of other physical properties.

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.

At pressures exceeding 10 to 20 bar (150 to 300 psia), and especially at high liquid rates, vapor entrainment into the downcomer liquid becomes important, and tray efficiency decreases with further increases in pressure [Zuiderweg, Int. Chem. Eng. 26(1), 1 (1986)].

Maldistribution Maldistribution can cause major efficiency reduction in multipass trays (>two passes). Further discussion is given under "Number of Passes."

600 Chocolate Recipes

600 Chocolate Recipes

Within this in cookbook full of chocolate recipes you will find over 600 Chocolate Recipes For Chocolate Lovers.

Get My Free Ebook


Responses

  • kiros
    When blood pressure drop?
    7 years ago

Post a comment