Hydrocarbon Compression

Archive for the ‘Tray’ Category

High Capacity Trays

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All vendors now market a high capacity tray. These trays have a 5 to 15 percent capacity advantage over conventional trays. Basically, the idea behind these high capacity trays is the same. The area underneath the downcomer is converted to bubble area. This increase in area devoted to vapor flow reduces the percent of jet flood.

But what keeps vapor from blowing up the downcomer? What prevents loss of the downcomer seal? If the downcomer seal is lost, surely the downcomer will back up and flood the upper trays of the column.

The design I’m most familiar with is the NorPro high capacity tray shown in Fig. 4.6. The head loss through the orifice holes in the downcomer seal plate shown is sufficiently high to prevent loss of the downcomer seal. These trays flood rather easily when their design downcomer liquid rates are exceeded. However, when operated at design downcomer liquid rates they perform very well indeed, and have shown quite a high vapor-handling capacity as compared to conventional trays.

The downcomer seal plate shown in Fig. 4.6 is an example of a dynamic downcomer seal. The Koch-Glitsch “Nye” tray also uses a dynamic downcomer seal to increase vapor-handling capacity. All trays with a dynamic downcomer seal suffer from two disadvantages:

• Loss of flexibility in that the liquid rates cannot be varied over too great a range without either flooding or unsealing the downcomers.
• Tray installation complexity is always increased, sometimes with terrible consequences.

For these reasons, high capacity trays using dynamic downcomer seals are best avoided on new columns. They should be reserved for use on retrofit tower expansion projects.

high capacity trays High Capacity Trays

Written by Jack

January 24th, 2011 at 8:44 am

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Distillation Tower Turndown

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The problem we have been discussing—loss of tray efficiency due to low vapor velocity—is commonly called turndown. It is the opposite of flooding, which is indicated by loss of tray efficiency at high vapor velocity. To discriminate between flooding and weeping trays, we measure the tower pressure drop. If the pressure drop per tray, expressed in inches of liquid, is more than three times the weir height, then the poor fractionation is due to flooding. If the pressure drop per tray is less than the height of the weir, then poor fractionation is due to weeping or dumping.

One way to stop trays from leaking or weeping is to increase the reflux rate. Assuming that the reboiler is on automatic temperature control, increasing the reflux flow must result in increased reboiler duty. This will increase the vapor flow through the trays and the dry tray pressure drop. The higher dry tray pressure drop may then stop tray deck leakage. The net effect is that the higher reflux rate restores the tray efficiency.

However, the largest operating cost for many process units is the energy supplied to the reboilers. We should therefore avoid high reflux rates, and try to achieve the best efficiency point for distillation tower trays at a minimum vapor flow. This is best done by designing and installing the tray decks and outlet weirs as level as possible. Damaged tray decks should not be reused unless they can be restored to their proper state of levelness, which is difficult, if not impossible.

Written by Jack

January 24th, 2011 at 8:39 am

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Bubble-Cap Trays

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The first continuous distillation tower built was the “patent still” used in Britain to produce Scotch whiskey, in 1835. The patent still is to this day employed to make apple brandy in southern England. The original still, and the one I saw in England in 1992, had ordinary bubble-cap trays (except downpipes instead of downcomers were used). The major advantage of a bubble-cap tray is that the tray deck is leakproof. As shown in Fig. 4.5, the riser inside the cap is above the top of the outlet weir. This creates a mechanical seal on the tray deck, which prevents liquid weeping, regardless of the vapor flow.

bubble cap tray Bubble Cap Trays

Bubble-cap trays may be operated over a far wider range of vapor flows, without loss of tray efficiency. It is the author’s experience that bubble-cap trays fractionate better in commercial service than do perforated (valve or sieve) trays. Why, then, are bubble-cap trays rarely used in a modern distillation?

There really is no proper answer to this question. It is quite likely that the archaic, massively thick, bolted-up, cast-iron bubble-cap or tunnel-cap tray was the best tray ever built. However, compared to a modern valve tray, bubble-cap trays

• Were difficult to install, because of their weight.
• Have about 15 percent less capacity because when vapor escapes from the slots on the bubble cap, it is moving in a horizontal direction. The vapor flow must turn 90°. This change of direction promotes entrainment and, hence, jet flooding.
• Are more expensive to purchase.

But in the natural-gas fields, where modern design techniques have been slow to penetrate, bubble-cap trays are still widely employed, to dehydrate and sweeten natural gas in remote locations.

Written by Jack

January 24th, 2011 at 8:37 am

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Loss of Downcomer Seal

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We stated that the top edge of the outlet weir is maintained about 0.5 in above the bottom edge of the inlet downcomer to prevent vapor from flowing up the downcomer. This is called a 0.5-in positive downcomer seal. But for this seal to be effective, the liquid must overflow the weir. If all the liquid is weeping through the tray deck, there will be no flow over the weir, and the height of the weir will become irrelevant. Figure 4.4 shows the result of severe tray deck leakage:

1. The downcomer seal is lost on tray deck 1.
2. Vapor flows up the downcomer between tray decks 1 and 2.
3. Liquid flow is backed up onto the tray above, i.e., onto tray deck 2.
4. The dry tray pressure drop through tray 2 decreases due to low vapor flow through the tray deck.
5. The hydraulic tray pressure drop on tray 2 increases due to increased liquid level.
6. Tray 2 will now start to weep, with the weeping concentrated on the low area of the tray.
7. 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 is leaking through its tray deck.

The net result of this unpleasant scenario is loss of both vapor-liquid contacting and tray efficiency. Note how the mechanical problems (i.e., levelness) of tray 1 ruins the tray efficiency of both trays 1 and 2.

downcomer seal Loss of Downcomer Seal

Written by Jack

January 24th, 2011 at 8:32 am

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Causes of Tray Inefficiency : Out-of-Level Trays

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When trays weep, efficiency may not be significantly reduced. After all, the dripping liquid will still come into good contact with the upflowing vapor. But this statement would be valid only if the tray decks were absolutely level. And in the real world, especially in large (>6-ft)-diameter columns, there is no such thing as a “level” tray. Figure 4.3 shows the edge view of a tray that is 2 in out-of-level.

As illustrated, liquid accumulates on the low side of this tray. Vapor, taking the path of least resistance, preferentially bubbles up through the high side of the tray deck. To prevent liquid from leaking through the low side of the tray, the dry tray pressure drop must equal or exceed the sum of the weight of the aerated liquid retained on the tray by the weir plus the crest height of liquid over the weir plus the 2-in out-of-levelness of the tray deck.

Once the weight of liquid on one portion—the lowest area—of a tray deck exceeds the dry tray pressure drop, the hydraulic balance of the entire tray is ruined. Vapor flow through the low area of the tray deck ceases. The aeration of the liquid retained by the weir on the low area of the tray deck stops, and hence the hydraulic tray pressure drop increases even more. As shown in Fig. 4.3, the liquid now drains largely through the low area of the tray. The vapor flow bubbles mainly through the higher area of the tray deck. This phenomenon is termed vapor-liquid channeling. Channeling is the primary reason for reduced distillation tray efficiency, because the vapor and liquid no longer come into good, intimate contact.

The common reason for out-of-levelness of trays is sagging of the tray decks. Sags are caused by pressure surges and sloppy installation. Sometimes the tray support rings might not be installed level, or the tower itself might be out of plumb (meaning the tower itself may not be truly vertical).

tray out of level Causes of Tray Inefficiency : Out of Level Trays

Written by Jack

January 24th, 2011 at 8:29 am

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How Trays Work

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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:

1. Uneven liquid flow across the tray deck is particularly detrimental to good vapor-liquid mixing. For example, if half
of the tray deck has stagnant liquid, then the vapor bubbling through the stagnant liquid cannot alter its composition.

Let me explain. A tray deck is a flat plate with holes. Liquid runs across the plate. Vapor bubbles up through the holes. If liquid only runs across part of this plate, vapor will still bubble up through the holes in the whole plate.

The vapor bubbling up through that portion of the tray deck where the liquid flow is active will mix with the flowing liquid. The flowing liquid will wash out the heavier components from the rising vapors.

On the other hand, the vapor bubbling up through that portion of the tray deck where the liquid flow is zero will also mix with the stagnant liquid. But it’s like trying to wash dirty clothes in dirty water. The stagnant liquid cannot wash out the heavier components from the vapors, because it is already saturated with these heavier components.

Uneven liquid flow is promoted by the outlet weir being out of level. Liquid will tend to flow across that portion of
the tray with a lower than average weir height. The portion of the tray upstream of the high part of the outlet weir will contain stagnant liquid. However, if the crest height (i.e., the height of liquid over the weir) is large, compared to the out-of-levelness of the tray, then an even liquid flow across the tray will result. To achieve a reasonable crest height above the outlet weir, a weir loading of at least 2 GPM per inch of weir length is needed. When liquid flows are small, the tray designer employs a picket weir, as shown in Fig. 4.1.

2. 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.

picket weir How Trays Work

Written by Jack

January 24th, 2011 at 8:25 am

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Carbon Steel Trays

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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.

Of course, any factor (dirt, polymers, gums, salts) that causes a reduction in the open area of the tray deck will also promote jet flooding. Indeed, most trays flood below their calculated flood point, because of these sorts of problems. Trays, like people, rarely perform quite up to expectations.

The use of movable valve caps in any service where deposits can accumulate on the tray decks will cause the caps to stick to the tray deck. It’s best to avoid this potential problem. Use of grid trays with fixed cap assemblies is preferred for most services.

Written by Jack

January 24th, 2011 at 8:20 am

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Trays Jet Flood

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Figure 3.8 is a realistic picture of 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:

• The foam height on the tray
• The vapor velocity through the tray

High vapor velocities, combined with high foam levels, will cause the spray height to hit the underside of the tray above. This causes mixing of the liquid from a lower tray with the liquid on the upper tray. This backmixing of liquid reduces the separation, or tray efficiency, of a distillation tower.

When the vapor flow through a tray increases, the height of froth in the downcomer draining the tray will also increase. This does not affect the foam height on the tray deck until the downcomer fills with foam. Then a further increase in vapor flow causes a noticeable increase in the foam height of the tray deck, which then increases the spray height.

When the spray height from the tray below hits the tray above, this is called the incipient flood point, or the initiation of jet flooding. Note, though, that jet flood may be caused by excessive downcomer backup. It is simple to see in a glass column separating colored water from clear methanol how tray separation efficiency is reduced as soon as the spray height equals the tray spacing. And while this observation of the onset of incipient flood is straightforward in a transparent tower, how do we observe the incipient flooding point in a commercial distillation tower?

jet flood Trays Jet Flood

Written by Jack

January 24th, 2011 at 8:19 am

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Total Height of Liquid in Downcomer

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To summarize, the total height of clear liquid in the downcomer is the sum of four factors:

• Liquid escape velocity from the downcomer onto the tray below.
• Weir height.
• Crest height of liquid overflowing the outlet weir.
• The pressure drop of the vapor flowing through the tray above the downcomer.

Unfortunately, we do not have clear liquid, either in the downcomer, on the tray itself, or overflowing the weir. We actually have a froth or foam called aerated liquid. While the effect of this aeration on the specific gravity of the liquid is largely unknown and is a function of many complex factors (surface tension, dirt, tray design, etc.), an aeration factor of 50 percent is often used for many hydrocarbon services.

This means that if we calculated a clear liquid level of 12 in in our downcomer, then we would actually have a foam level in the downcomer of 12 in/0.50 = 24 in of foam.

If the height of the downcomer plus the height of the weir were 24 in, then a downcomer foam height of 24 in would correspond to downcomer flooding. This is sometimes called liquid flood.

This discussion assumes that the cross-sectional area of the downcomer is adequate for reasonable vapor-liquid separation. If the downcomer loading (GPM/ft2 of downcomer top area) is less than 150, this assumption is okay, at least for most clean services. For dirty, foamy services a downcomer loading of 100 GPM/ft2 would be safer.

Written by Jack

January 24th, 2011 at 8:06 am

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Tray Vapor-Flow Pressure Drop

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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.

Let us assume that vapor flowing through a tray deck undergoes a pressure drop of 1 psig (lb/in2 gauge). Figure 3.7 shows that the pressure below tray deck 2 is 10 psig and the pressure above tray deck 2 is 9 psig. How can the liquid in downcomer B flow from an area of low pressure (9 psig) to an area of high pressure (10 psig)? The answer is gravity, or liquid head pressure.

The height of water needed to exert a liquid head pressure of 1 psig is equal to 28 in of water. If we were working with gasoline, which has a specific gravity of 0.70, then the height of gasoline needed to exert a liquid head pressure of 1 psig would be 28 in/0.70 = 40 in of clear liquid.

vapor downcomer backup Tray Vapor Flow Pressure Drop

Written by Jack

January 24th, 2011 at 8:04 am

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