Tray deck vapor flow

Valve trays. Figure 6.216 illustrates the dry pressure drop of a typical valve tray as a function of vapor velocity. At low vapor velocities, all valves are closed (i.e., seated on the tray deck). Vapor rises through the crevices between the valves and the tray deck, and friction losses through these crevices constitute the dry pressure drop. Once the closed balance point (CBP) is reached, there is sufficient force in the rising vapor to open some valves. A further increase in vapor velocity opens more valves. Since vapor flow area increases as valves open, pressure drop remains constant until all valves open. This occurs at the open balance point (OBP). Further increases of vapor velocity cause the dry pressure drop to escalate in a similar manner to a sieve tray. When two weights of valves are used in alternate rows on the tray, a similar behavior applies to each valve type. The result is the pressure drop-vapor velocity relationship in Fig. 619e. 

Possibly 90 percent of the trays seen in the plant are of these types. Perforated tray decks all have one feature in common they depend on the flow of vapor through the tray deck perforations, to prevent liquid from leaking through the tray deck. As we will see later, if liquid bypasses the outlet weir, and leaks through the tray deck onto the tray below, tray separation efficiency will suffer. 

We have yet to discuss the most important factor in determining the height of liquid in the downcomer. This is the pressure drop of the vapor flowing through the tray deck. Typically, 50 percent of the level in the downcomer is due to the flow of vapor through the trays. 

When vapor flows through a tray deck, the vapor velocity increases as the vapor flows through the small openings provided by the valve caps, or sieve holes. The energy to increase the vapor velocity comes from the pressure of the flowing vapor. A common example of this is the pressure drop we measure across an orifice plate. If we have a pipeline velocity of 2 ft/s and an orifice plate hole velocity of 40 ft/s, then the energy needed to accelerate the vapor as it flows through the orifice plate comes from the pressure drop of the vapor itself. 

Figure 1.8 is a realistic picture as to what we would see if our towers were made of glass. In addition to the downcomers and tray decks containing froth or foam, there is a quantity of spray, or entrained liquid, lifted above the froth level on the tray deck. The force that generates this entrainment is the flow of vapor through the tower. The spray height of this entrained liquid is a function of two factors ... 

One of the most frequent causes of flooding is the use of carbon steel trays. Especially when the valve caps are also carbon steel, the valves have a tendency to stick in a partially closed position. This raises the pressure drop of the vapor flowing through the valves, which, in turn, pushes up the liquid level in the downcomer draining the tray. The liquid can then back up onto the tray deck, and promote jet flood, due to entrainment. 

A distillation tray works efficiently, when the vapor and liquid come into intimate contact on the tray deck. To this end, the liquid should flow evenly across the tray deck. The vapor should bubble up evenly through the perforations on the tray deck. The purpose of the outlet weir is to accomplish both these objectives, as follows ... 

Uneven vapor flow bubbling-up through the tray deck will promote vapor-liquid channeling. This sort of channeling accounts for many trays that fail to fractionate up to expectations. To understand the cause of this channeling, we will have to quantify total tray pressure drop. 

On the other hand, bubble caps (or even the more ancient tunnel cap trays) are different, in that they do not depend on the vapor flow to retain the liquid level on the tray deck. More on this later. For now, just recall that we are dealing only with perforated tray decks. 

Vapor flows up the downcomer, between tray decks 1 and 2. 

The dry tray pressure drop through tray 2 decreases, due to low vapor flow through the tray deck. 

Tray 2 now has most of its vapor feed flowing up through its outlet downcomer, rather than the tray deck and most of its liquid flow, leaking through its tray deck. 

The liquid on the tray deck was at its bubble, or boiling, point. A sudden decrease in the tower pressure caused the liquid to boil violently. The resulting surge in vapor flow promoted jet entrainment, or flooding. 

The vapor flowing between trays was at its dew point. A sudden increase in tower pressure caused a rapid condensation of this vapor and a loss in vapor velocity through the tray deck holes. The resulting loss in vapor flow caused the tray decks to dump. 

Unfortunately, packing does not redistribute liquid, or internal reflux. Unless the initial reflux distribution is good, the liquid flow distribution through the entire packed bed will be poor. Figure 7.1 shows a common orifice plate liquid distributor. Vapor flows up through the large chimneys, and liquid drains through the smaller distribution holes in the tray deck. 

Let s further assume that the orifice plate distributor is 1 in out-of-level. This could easily happen in a 14-ft 0-in-ID tower. Figure 7.2 shows the results. The flow of internal reflux or liquid through the higher portion of the tray deck falls to zero. Worse yet, vapor starts to... 

And when the volume of a vapor flowing through a tray increases, so does its velocity. Any increase in vapor velocity through a tray results in higher tray pressure drop. And what is it that causes trays to flood Why, it is high tray deck pressure drop. 

Recently there has been an increasing trend to replace the conventional trays depicted in Fig. 5 by trays having receiving pans that terminate some 15 cm above the tray deck. This provides more column cross-sectional area for vapor flow and allows increased vapor capacity. Even greater vapor capacity can be obtained from trays that utilize localized, upward co-current flow of vapor and liquid. But, as each tray then requires a vapor-liquid separation device, they are more expensive and are used only in specialized applications. 

Low-velocity vapor-only feeds often enter via bare nozzles or V baffles (above). At higher velocities, perforated vapor spargers are used. At high velocities, vapor horns and Schoepentoeters are often preferred. Alternatively or additionally, a vapor distributor may be mounted above the feed. The vapor distributor is a chimney tray (Fig. 14-72) where liquid is collected on the deck and flows via downcom-... 

Picket fence weirs are used in low-liquid-rate applications (Fig. 8). Picket fence weirs can serve two purposes at low liquid rates. First, they reduce the effective length of the weir for liquid flow increases the liquid height over the weir. This makes tray operation less sensitive to out-of-level installation. Second, pickets can prevent liquid loss (blowing) into the downcomer by spraying. This occurs at low liquid rates when the vapor is the continuous phase on the tray deck. Picket fence weirs should be considered if the liquid load is less than 1 gpm per inch of weir (0.0267 ft /sec/ft, 0.00248 m /sec/m). At liquid rates lower than 0.25 gpm per inch of weir (0.00668 ft / sec/ft, 0.000620 m /sec/m) even picket fence weirs and splash baffles have a mixed record in improving tray efficiency. Operation at liquid rates this low strongly favors the selection of structured packing. 

Because this problem occurred in December, it was possible to operate the isostripper well below the design pressure. To increase the volumetric vapor flow (but not the mass flow) through the tray decks, we suggested that the tower pressure be lowered from 120 psig to 65 psig. The objective was to increase the dry tray pressure drop by about 1 in. of liquid per tray. 

Although trays can corrode through, a more common cause of damage is unit upsets. A high liquid level, above the flash zone, will cause the trays to be bumped by the up-flowing vapors. Slugs of water can dislodge tray decks when the water suddenly flashes. 

Many towers equipped with valve or sieve trays do not operate efficiently at low feed rates. This is due to tray-deck leakage. As the pressure drop of the vapor flowing through the sieve holes falls below the weight of the liquid on the tray deck (as determined by the height of the weir), the tray will start to leak. 

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