Ch 4 - How Trays Work- Dumping

Source: A Working Guide to Process Equipment

CHAPTER

4

How Trays Work: Dumping Weeping through Tray Decks

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 va por should bubble bubb le up even evenly ly thr through ough the perf perforat orations ions on the tra tray y dec deck. k. The purp purpose ose of the outlet weir is to accomplish both these objectives, as follows: 1. Uneven liquid flow across across the the tray tray deck is particularly 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,

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How Trays Work: Dumping

38

A Working Guide to Process Equipment Picket weir

e r m o n c w o D

u r o p n D o w a r e a

F IGURE 4.1

Picket weir promotes even liquid cross-flow at low flows.

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.

4.1

Tray Pressure Drop 4.1.1

Total Tray ∆P

Figure 4.2 shows a simple sieve tray with a single hole. Why is it that the liquid flows over the 3-in outlet weir, rather than simply draining down through the sieve hole? It is the force of the vapor (or better, the velocity of the vapor) passing through the sieve hole which prevents the liquid from draining down the sieve hole. This is true whether we are dealing with a valve cap, extruded perforation, or a sieve hole. The valve cap does not act as a check valve to keep liquid on the tray. The author’s industrial experience has proved this unpleasant fact on numerous occasions.

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Chapter 4:

H o w Tr a y s W o r k : D u m p i n g

39

F IGURE 4.2

A simplified sieve tray.

Weir 3"

Sieve hole

Downcomer

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.

4.1.2

Dry-Tray Pressure Drop

For the force of the upflowing vapor to stop the liquid from leaking through the sieve hole shown in Fig. 4.2, the pressure drop of the vapor flowing through the hole has to equal the weight of liquid on the tray deck. The weight of liquid trying to force its way through the sieve hole is proportional to the depth of liquid on the tray deck. The pressure drop of the vapor as it accelerates through the sieve hole is

∆Pdry =

K

DV 2 Vg DL

where Pdry  dry tray pressure drop, in inches of clear liquid

DV  density of vapor, lb/ft 3 DL  density of liquid, lb/ft 3 Vg  velocity of vapor or gas flowing through the sieve hole, ft/s K  an orifice coefficient, which can be as low as 0.3 for a smooth hole in a thick plate and 0.6 to 0.95 for various valve tray caps



40

A Working Guide to Process Equipment

4.1.3

Hydraulic Tray Pressure Drop

The weight of liquid on a tray is created by the weir height plus the crest height. We have defined the crest height (in inches of clear liquid) in Chap. 3, as Crest height  0.4 (GPM/inch outlet weir length) 0.67 The actual height of fluid overflowing the weir is quite a bit greater than we calculate with this formula. The reason is that the fluid overflowing the weir is not clear liquid, but aerated liquid—that is, foam. The fluid on the tray deck, below the top of the weir, is also foam. This reduces the effective weight of the liquid on the tray due to aeration. To summarize, the weight of liquid on the tray, called the hydraulic tray pressure drop, is Phyd 

AF  WH  0.4 (GPM/inch outlet weir length) 0.67

where Phyd  hydraulic tray pressure drop, in inches of clear liquid

WH  weir height, in AF  aeration factor GPM  gallons (U.S.) per minute The aeration factor AF is the relative density of the foam, to the density of the clear liquid. It is a combination of complex factors, but is typically 0.5.

4.1.4

Calculated Total Tray Pressure Drop

The sum of the dry tray pressure drop ( Pdry) plus the hydraulic tray pressure drop ( Phyd) equals the total tray pressure drop ( Ptotal): Ptotal  Pdry  Phyd

expressed in inches of clear liquid When the dry tray pressure drop is significantly less than the hydraulic tray pressure drop, the tray will start to leak or weep and tray efficiency will be adversely affected. When the dry tray pressure drop is significantly greater than the hydraulic tray pressure drop, the liquid on the tray can blow off of the tray deck and tray efficiency will be adversely affected. For a tray to function reasonably close to its best efficiency point, the dry tray pressure drop must be roughly equal ( 50 percent) to the hydraulic tray pressure drop: Pdry  Phyd

This concept is the basis for tray design for perforated tray decks. While various valve tray vendors maintain that this rule does not hold for their equipment, it is the author’s industrial experience that



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valve trays leak just as badly as do sieve trays at low vapor hole velocities. To summarize: ∆Ptotal =

4.2

D K V Vg2 D L

+

AF

×

WH

+

0.4 (GPM/inch outlett weir length) 0.67

Other Causes of Tray Inefficiency 4.2.1

Out-of-Level Trays

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

Tilted tray deck 2"

Vapor Liquid

F IGURE 4.3

Out-of-level tray causing vapor-liquid channeling.



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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).

4.2.2

Loss of Downcomer Seal

We stated in Chap. 3 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:

Tray #2

2-1/2" 2" Tray #1

F IGURE 4.4

Sagging tray ruins downcomer seal.



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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.

4.3

Bubble-Cap Trays 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 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.



44


D o w n c o m e r

Bubble cap cap

Cap

Riser

Bubble cap riser

Weir

V a p o r

Slot Vapor

Vapor flow

f l o w F IGURE 4.5

A bubble cap showing vapor pathway in operation.

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.

4.3.1

Distillation Tower Turndown

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.



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Damaged tray decks should not be reused unless they can be restored to their proper state of levelness, which is difficult, if not impossible.

4.4

New High Capacity Trays 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

F IGURE 4.6

High capacity tray with downcomer seal plate.

Downcomer seal plate

24"

4"

Downcomer seal plate Orifice velocity NLT 0.7 ft/sec



46


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.


Ch 4 - How Trays Work- Dumping

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