Tray columns, flooding

Percentage of Flood for Existing Ballast Tray Columns... 

In the physical arrangement, make certain that the pressure balance level, plus an allowance for froth, establishes a height that is below the bottom tray of the column to avoid flooding the column. In addition, the estimated froth height on top of the liquid should still be below the level of the vapor return from the reboiler. 

The method of calculation introduced in this chapter not only allows an exact determination of the column diameter for nonpulsed sieve tray columns, but also allows a good estimation of the diameters of pulsed and stirred extractors. For the latter, however, more exact specific equations exist for the flooding point, see for example [1,4]. 

Flooding is by far the most common upper capacity limit of a distillation tray. Column diameter is set to ensure the column can achieve the required throughput without flooding. Towers are usually designed to operate at 80 to 90 percent of the flood limit. 

As the liquid holdup increases, the effective orifice diameter may become so small that the liquid surface becomes continuous across the cross section of the column. Column instability occurs concomitantly with a rising continuous-phase liquid body in the column. Pressure drop shoots up with only a slight change in gas rate (condition C or C ). The phenomenon is called flooding and is analogous to entrainment flooding in a tray column. 

The maximum capacity of a tray column is usually limited by the onset of flooding, which occurs when liquid excessively accumulates inside the column. Flooding is... 

Figure 6.7 Common flooding mechanisms in tray columns, (a) Spray entrainment flood (ft) froth ontrainment flood (c) downcomer backup flood Id) downcomer choke flood. (Parle a and ft reproduced from Dr. D. C. Hausch, Discussion of Paper Presented In the Fifth Session, Proceedings of the International Symposium on Distiliation, the Institution of Chemical Engineers (London), I960, reprinted courtesy of the Institution of Chemical Engineers, UK- Parts c and d from H. Z. Kister. Distillation Operation. Copyright C 1990 6y McGraw-Hill, Inc. reprinted by permission.)...

A downcomer must be sufficiently large to transport all of the liquid downflow without choking. If the friction losses in the downcomer and or downcomer entrance are excessive, liquid will back up onto the tray and eventually flood the column. This is termed downcomer choke. The prime design parameter is the downcomer top area, where friction losses are highest- Further down the downcomer, vapor disengages and the aerated liquid downflow is greatly reduced. With sloped downcomers, the downcomer bottom area is normally set at about 1.7 to 2 times less than the area at the top of the downcomer (1,8,9,10,48). This taper is small enough to keep the downcomer top area the prime downcomer choke variable. 

Effect of column diameter (at constant UV and percent of flood). As column diameter increases, both the liquid and vapor flow rates increase as the square of the diameter. The area for vapor flow also increases as the square of the diameter, so the vapor load remains unaffected. On the other hand, the area available for liquid flow only increases in proportion to the diameter. Therefore, the liquid rate per unit of weir length increases, the increase being proportional to the column diameter. The operating point on Fig. 6.29 will therefore shift horizontally to the right, toward the emulsion regime. Increasing the number of liquid passes on the tray reverses the above action, and shifts the operating point back to the left. 

The flood-point and the maximum pressure drop criteria gave comparable tower diameters. The more conservative of the two criteria gives diameters of 5.50 and 6.12 for the top and bottom section of the tower, respectively. As the diameters for the top and bottom sections are not much different, it is attractive to use uniform column diameter. The decision of whether to make the top and bottom section diameters the same is based on the same criterion as for tray columns (Sec. 6.5.3). The preliminary column diameter is the larger for the two column sections, i.e., 6.12 ft. This diameter is normally rounded to the next nearest half foot, but in this example it is rounded only to the next nearest quarter foot. A diameter of 6.12 is far closer to 6 ft than to 6.5 ft. Column diameter is relatively small, and three excessive inches substantially increase the costs. The column is operated at high pressure, and high-pressure shells are expensive. Therefore, the preliminary column diameter is 6 ft 3 in. 

Similar to tray columns, packed columns operated at high gas velocities causes backmixing, and low gas velocities reduce the mass transfer rate. If the gas velocity is too high, the column will flood. In addition, at low liquid flow rates the packing will not wet completely, resulting in a reduction in mass-transfer. Another problem is the tendency for the liquid to channel. To minimize this effect, redistributors have to be installed every 5 to 10 m (16.4 to 30.5 ft) [23] to even out the liquid flow. 

Entrainment Corrections. Above about 80% of flood, the recirculation of liquid as entrainment between trays undermines the countercurrent action of the tray column, and efficiency therefore suffers. This is a particular problem in vacuum distillation where it may be optimum to allow a certain amount of liquid entrainment in initial design. Figure 13.41 shows an approximate method for entrainment correction to column efficiency or Murphree efficiency. The abscissa scale is the same parameter used for flooding prediction (Figure 13.32(b)). The ordinate value is used to correct from a dry to a wet efficiency (with entrainment) ... 

The internal flow of liquid and vapor must be re-evaluated from the standpoint of column capacity, both in the design and performance studies of columns. The physical dimensions of a column can handle only limited ranges of vapor and liquid flow rates. The objective of this chapter is to evaluate the hydraulic aspects of fluid flow in trayed columns. The column performance is examined with regard to factors such as flooding, entrainment, pressure drop, mass transfer, and tray efficiency. 

The column cross-sectional area, and hence the tray diameter, is sized such as to prevent flooding. Tray sizing is initially based on preventing entrainment flooding, but is subsequently checked for downcomer flooding and other considerations. Above a certain vapor velocity, the vapor flood velocity, liquid droplets are entrained with the vapor, causing flooding. The tray should be sized such that the actual vapor velocity is below the vapor flood velocity. 

FIGURE 12.43 Flooding capacity of baffle tray columns. [J. R. Fair, 1993. Hydrocarbon Proc. 72 (5) 75.]... 

The upper limit of the velocity in a sieve-tray column is determined by the flooding point or by the velocity at which entrainment becomes excessive. Flooding occurs when the liquid in the downcomer backs up to the next plate, and this is determined mainly by the pressure drop across the plate and the plate spacing. Near the flooding point, most of the pressure drop comes from the term in Eq. 

A sieve-tray column with 15 plates is used to prepare 99 percent methanol from a feed containing 40 percent methanol and 60 percent water (mole percent). The plates have 8 percent open area, in. holes, and 2-in. weirs with segmental downcomers, (a) If the column is operated at atmospheric pressure, estimate the flooding limit based on conditions at the top of the column. What is the F factor and the pressure drop per plate at this limit (Z>) For the flow rate calculated in part (a) determine the F factor and the pressure drop per plate near the bottom of the column. Which section of the column will flood first as the vapor rate is increased ... 

Design a sieve-tray column for the ethanol absorber of Example 4.4. For alcohol absorbers, Kister (1992) recommends a foaming factor Ff = 0.9. The liquid surface tension is estimated as a = 70 dyn/cm. Take do- 5 mm on an equilateral-triangular pitch 15 mm between hole centers, punched in stainless steel sheet metal 2 mm thick. Design for an 80% approach to the flood velocity. 

A depropanizer is a distillation operation encountered in almost all oil refineries. Our task here is to design a sieve-tray column to separate 1000 mol/s of a mixture containing 100 mol/s of ethane, 300 mol/s of propane, 500 mol/s of n-butane, and 100 mol/s of n-pentane at 298 K and 15 atm. The distillate should contain no more than 3.5 mol/s of n-butane, and the bottoms should contain no more than 3.5 mol/s of propane. The trays should operate at about 70% of flooding (Taylor and Krishna, 1993). 

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