Top-tray flooding

Another purpose of the pumparound is to suppress top-tray flooding. If tray 1 in Fig. 12.2 floods, the operator would observe the following ... 

Top-tray flooding Tower flooding caused only by flooding on the top tray. 

As the weir height of the trays is 3 in, it is a safe assumption that the low tray efficiency is due to tray deck dumping, rather than flooding. As shown in Fig. 3.3, this column has no reflux. This is a typical design for strippers when feed is introduced on the top tray, there is no need for reflux. 

It is normal to assume that the vapor leaving the top of a tower is at its dew point. That is, it is at equilibrium with the liquid on the top tray of the tower. Unfortunately, this assumption falls apart if the tower is flooding and liquid is being entrained overhead from the column, with the vapor. However, assuming a normal, nonflooded condition, we will guess that the tower-top temperature is 140°F. Using the vapor-pressure curves provided in Fig. 9.1, we would calculate as follows ... 

From the designer s point of view, the top tray of the stripper must have a several times greater number of sieve holes or valve caps on its tray deck than the bottom tray. If, however, all the trays in the stripper are identical, then either the bottom tray will leak (see Chap. 2), or the top tray will flood. Either way, stripping efficiency will suffer. 

Well, we have already discussed this phenomenon in Chap. 1, in the section on tray flooding. Water carryover from the top of tray 1, as shown in Fig. 15.1, is simply a clear signal that our little four-tray distillation tower is flooding. But what is causing the trays to flood ... 

Liquid inlets. Liquid enters the top tray via a hole in the column shell, often discharging against a vertical baffle or weir, or via a short, down-bending pipe (Fig. 17), or via a distributor. Restriction, excessive liquid velocities, and interference with tray action must be avoided, as these may lead to excessive entrainment, premature flooding, and even structural damage. Disperser units (e.g., perforations, values) must be absent in the liquid entrance area (Fig. 17) or excessive weeping may result. 

Sieve trays with 10% hole area and 0.5 cm diameter holes will be used. Trays are available in standard diameters of 0.25 m increments (0.25,0.50, 0.75, 1, 1.25, 1.50,. .., m). Based on the top tray conditions, determine the required tray diameter rounded up to the nearest larger standard size. Assume a tray spacing of 0.5 m, a foaming factor of 0.80, and a fraction of flood of 0.80. The liquid density is given as 730 kg/m and the vapor density may be estimated based on the ideal gas equation. The liquid surface tension is 27 dynes/cm. 

The column has 3 m diameter sieve trays with 0.5 cm diameter holes and 10% hole area. The tray spacing is 45 cm. Assuming a foaming factor of 0.85, calculate the vapor flood velocity at the top tray. Check if the column diameter is acceptable. The fraction of flood velocity should be within a 60-85% range. 

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. 

Conditions for the top tray of a distillation column are as shown below. Determine the column diameter corresponding to 85% of flooding if a valve tray is used. Make whatever assumptions are necessary. 

Premature flooding occurred after several months in service. Iron and other metal carbonates formed deposits 1 in thick on some of the top trays. The solids originated in the natural gas stream. Problem was solved by cleaning, and recurrence prevented by annual add wash-out... 

In this case only the top tray will flood, and a tray AP survey will not reveal the problem. To recognize this difficulty, one need only observe what happens when the reflux rate is increased. 

The plugged top tray will prevent the reflux from cascading down to the lower trays. The liquid reflux will just overflow into the condensers and circulate back to the reflux drum. The tip-off to this problem is that neither the reboiler duty nor the bottoms temperature is affected in the normal way by raising reflux. The tower s heat balance appears as if the reflux rate had never been increased. This is not much different from the signs of normal tower flooding, except that the AP on all but the top tray is not excessive. 

Reduced ASTM gap Flooding Upset tray decks Pumparound rate too high Top trays corroded Water in reflux Downcomer Installation... 

When I inspected the tower, I found an absolutely positive indication of floodings a vent line on the tower overhead line emitted liquid when cracked open. This observation, coupled with the low measured pressure drop per tray, indicated that only the top tray was flooding. When we opened the tower for inspection, vve found the deck of the top tray encrusted with corrosion products. These deposits had caused a high pressure drop on only the lop tray and had caused the downcomer on only the top tray to back-up. 

Normally, vapor expanding from about 200 psig through the bypass would cool only about 20°F. But there was a temperature drop of more than 80 F, indicating that liquid propane was expanding through the bypass and that liquid propane was present in the overhead vapor line, and that at least the top tray was flooding. 

If a tower does become flooded in the bottom section, a common operator error is to try to pump the level out too quickly. This can easily damage trays by imposing a downward acting differential pressure produced by a large weight of liquid on top of the tray and a vapor space immediately below the tray. To eliminate the flooding, it is better to lower feed rate and heat to the reboiler. It is important to be patient and avoid sudden changes. 

Here, we refer to small amounts of water rather than large slugs that could damage the trays. Often the water will boil overhead and be drawn off in the overhead accumulator bootleg (water drawoff pot). However, if the column top temperature is too low, the water is prevented from coming overhead. This plus too hot a bottom temperature for water to remain a liquid will trap and accumulate water within the column. The water can often make the tower appear to be in flood. 

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. 

---