Hydrocarbon Compression

Seal Pans Downcomer

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The downcomer from the bottom tray is submerged in a seal pan (see Fig. 8.3), to preserve its downcomer seal. I always set the horizontal dimension between the over-flow lip of the seal pan, (dimension y) the downcomer at four inches, so I never have to worry about restricting liquid flow from the bottom tray. This horizontal dimension should be equal to or greater than the vertical clearance between the downcomer and the seal pan floor (dimension ? which is typically, two to three inches). If a deformation of the downcomer reduces the horizontal clearance between the seal pan overflow lip, and the downcomer, the resulting restriction can cause the bottom tray to flood due to downcomer back-up. If the bottom tray floods, flooding will progress up the column. With time, the entire column will flood due to the small restriction in the seal pan. That’s why a detailed trayby-tray inspection is important.

seal pan Seal Pans Downcomer

Written by Jack

April 30th, 2011 at 12:44 pm

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Inlet Weirs

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Most trays have outlet weirs devoted to maintaining the downcomer seal. But some trays have inlet weirs too, or inlet weirs, but no outlet weirs. A sketch of an inlet weir is shown in Fig. 8.2. Note the horizontal distance between the downcomer and the inlet weir (dimension x). This distance ought to be equal to or greater than the downcomer clearance—that is, the vertical space between the tray floor and the bottom edge of the downcomer. Unfortunately, a small deformation of the downcomer may push the downcomer quite close to the inlet weir. The resulting reduction in the horizontal clearance between the inlet weir and the downcomer will restrict the liquid flow. This will cause downcomer backup and tray flooding of the trays above.

inlet weirs Inlet Weirs

Often, there is no process reason for the use of inlet weirs, especially at higher liquid rates. Then, the inlet weirs may be removed. But some tray types, such as “Exxon Jet Tab,” trays or total trap-out chimney trays with no outlet weir, absolutely require the use of inlet weirs.

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March 29th, 2011 at 9:22 am

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Improper Downcomer Clearance

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The bottom edge of the downcomer from the tray above should be 0.25 to 0.5 in below the top edge of the weir of the tray below. This is called a positive downcomer seal. Without a positive downcomer seal, vapor will flow up the downcomer and displace the downflowing liquid. This will cause flooding due to excessive downcomer backup.

On the other hand, if the bottom of the downcomer is too close to the tray below, then the “head loss under the downcomer” will be excessive. Typically, a minimum downcomer clearance is 1.5 to 2 in. Too small a downcomer clearance will result in restricting the liquid flow from the downcomer. This will also cause excessive downcomer backup and flooding. Check the correct downcomer clearance on the vender tray drawings prior to the tower inspection.

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March 29th, 2011 at 9:19 am

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Loss of Downcomer Seal Due to Leaks

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The vertical edges to the downcomers are bolted to bars welded to the vessel wall. These are called, “downcomer bolting bars.” Gaskets are often used to tightly seal the edge of the tray downcomer to these bars. If the bolts are loose or if the gaskets are missing, vapor will blow into the downcomer and displace the descending liquid. Downcomer backup and flooding may result.

The area underneath the downcomer is called the downpour area. If a tray deck corrodes, it often first holes through in the downpour area. This will cause flooding due to downcomer back-up.

The bottom edge of the downcomer will be somewhat flexible in larger diameter towers. If the width of the tower is less than 5 ft, then the downcomer bolting bars prevent flexing of the bottom edge of the downcomer. However, if the width of the downcomer is over 5 or 6 ft, then downcomer bracing brackets (see Fig. 8.1) are required. The bottom edge of the downcomer should be immobilized by attachment to the bolting bar or bracing bracket every 4 to 5 ft, of downcomer width.

downcomer bracing Loss of Downcomer Seal Due to Leaks

Recall that the pressure outside the downcomer is slightly greater than the pressure inside the downcomer. Therefore, a force will push the downcomer toward the vessel wall and reduce the open area of the downcomer. This restriction promotes downcomer backup and flooding. Don’t expect to see this deformation of the downcomer during your inspection. Once the vapor flow through the tray stops, the downcomer will spring back to its design position.

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March 29th, 2011 at 9:15 am

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Tray Deck Levelness

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For smaller diameter towers a visual check of tray deck levelness is sufficient. For two-pass trays, a small diameter tower is less than 8 ft.

For single-pass trays, a diameter of less than 6 ft is small.For towers of 10 ft or more in diameter, check for out-of-levelness of a tray check using a carpenter’s laser level, available in hardware stores for about $40. Purchase a level that has short tripod legs. Use the bubble to level up the legs. Set the level on one end of the tower, and check the height of the red beam at the other end and at the center of the tray for out of levelness. As it is often dim and dusty in the tower, the trace of the red laser may be clearly visible. Low points and areas of the tray deck which are out of level can now be easily identified.

The more level the tray, the better the mixing efficiency between vapor and the liquid. Certainly, if the tray out-of-levelness is greater than the height of the weir, tray efficiency will be badly degraded.

Checking for weir out-of-levelness is easy. Set the laser level on the edge of the weir. Using the bubble glass level indicator, adjust the laser level to a true horizontal position. The line of red light compared to the top of the weir will indicate how much of the weir is out of level. A weir that is more than 0.5 in out of level should be re-adjusted. If it is not, stagnant liquid pools behind the higher section of the weir, as described in the prior chapter, will result and ruin the tray‘s efficiency.

Written by Jack

March 29th, 2011 at 9:10 am

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Excessive Thermosyphon Circulation

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In a once-through reboiler, the liquid flow coming out of the reboiler is limited to the bottoms product. In a circulating reboiler, the liquid flow coming out of the reboiler can be extremely high. If the reboiler return nozzle is located too close to the bottom tray of the tower, the greater volume of liquid leaving the nozzle can splash against the bottom tray. This alone can cause the entire column to flood. The best way to stop this flooding is to lower the tower bottom level.

Sometimes higher rates of thermosyphon circulation are good. They help prevent fouling and plugging of the reboiler due to low velocity and dirt in the bottoms product and especially high vaporization rates. If the percentage of vaporization in a once-through reboiler is above 60 percent and dirt in the bottoms product is expected, then a circulating reboiler would be the better choice.

Written by Jack

March 29th, 2011 at 5:17 am

How Reboilers Work

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Four types of reboilers are :

• Once-through thermosyphon reboilers
• Circulating thermosyphon reboilers
• Forced-circulation reboilers
• Kettle or gravity-fed reboilers

There are dozens of other types of reboilers, but these four represent the majority of applications. Regardless of the type of reboiler used, the following statement is correct: Almost as many towers flood because of reboiler problems as because of tray problems.

The theory of thermosyphon, or natural circulation, can be illustrated by the airlift pump shown in Fig. 7.1. This system is being used to recover gold bearing gravel from the Magdalena River in Colombia, South America. Compressed air is forced to the bottom of the river through the air line. The air is injected into the bottom of the riser tube. The aerated water in the riser tube is less dense than the water in the river. This creates a pressure imbalance between points A and B. Since the pressure at point B is less than that at point A, water (as well as the gold and gravel) is sucked off the bottom of the river and up into the riser tube. We can calculate the pressure difference between points A and B as follows:

cal 11 How Reboilers Work

where

HRW = height of water above the bottom of the riser, ft
DRW = specific gravity of fluid in the riser; in this case 1.0
HRT = height of the aerated water in the riser tube, ft
DRT = specific gravity of aerated water in the riser tube (this number can be obtained only by a trial-anderror calculation procedure)
AP = differential pressure between points A and B, psi

In a thermosyphon or natural-circulation reboiler, there is, of course, no source of air. The aerated liquid is a froth or foam produced by the vaporization of the reboiler feed. Without a source of heat, there can be no vaporization. And without vaporization, there will be no circulation. So we can say that the source of energy that drives the circulation in a thermosyphon reboiler is the heating medium to the reboiler.

airlift pump How Reboilers Work

Written by Jack

January 24th, 2011 at 10:42 am

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Internal Reflux Evaporation

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The tray temperatures in our preflash tower, shown in Fig. 6.4, drop as the gas flows up the tower. Most of the reduced sensible-heat content of the flowing gas is converted to latent heat of evaporation of the downflowing reflux. This means that the liquid flow, or internal reflux rate, decreases as the liquid flows down the column. The greater the temperature drop per tray, the greater the evaporation of internal reflux. It is not unusual for 80 to 90 percent of the reflux to evaporate between the top and bottom trays in the absorption section of many towers. We say that the lower trays in the absorption section of such a tower are “drying out.” The separation efficiency of trays operating with extremely low liquid flows over their weirs will be very low. This problem is commonly encountered for towers with low reflux ratios and a multi component overhead product composition.

crude preflash Internal Reflux Evaporation

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January 24th, 2011 at 10:36 am

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Conversion of Sensible Heat to Latent Heat

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When we raise the top reflux rate to our preflash tower, the tower-top temperature goes down. This is a sign that we are washing out from the upflowing vapors more of the heavier or higher-molecular-weight components in the overhead product. Of course, that is why we raised the reflux rate. So the reduction in tower-top temperature is good.

But what happened to the sensible-heat content (the heat represented by the temperature) of the vapors leaving the tower? As the vapor is cooler, the sensible-heat content decreased. Where did this heat go?

A small part of the heat was picked up by the extra liquid draining from the top tray. This extra liquid comes from the extra reflux. But the liquid flow through the tower is too small to carry away much heat. The main reason why the vapors leaving the top tray are cooler is vaporization; in other words, the sensible-heat content of the flowing vapors is converted to latent heat of vaporization.

But what is vaporizing? The reflux, of course. The sensible-heat content of the vapors, which is reduced when the reflux rate is increased, is converted to latent heat as the vapors partially vaporize the incremental reflux flow.

As the reflux rate is raised, the weight flow of vapor through the top tray, and to a lesser extent through all the trays below (except for the bottom tray), increases. This increase in the weight flow of vapor occurs even though the external heat input to the preflash tower is constant. The weight flow of vapor to the bottom tray is presumed to be solely a function of the pounds of vapor in the feed.

Written by Jack

January 24th, 2011 at 10:33 am

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Effect of Feed Preheat

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Up to this point, we have suggested that the weight flow of vapor up the tower is a function of the reboiler duty only. Certainly, this cannot be completely true. If we look at Fig. 6.2, it certainly seems that increasing the heat duty on the feed preheater will reduce the reboiler duty.

Let us assume that both the reflux rate and the overhead propane product rate are constant. This means that the total heat flow into the tower is constant. Or the sum of the reboiler duty plus the feed preheater duty is constant. If the steam flow to the feed preheater is increased, then it follows that the reboiler duty will fall. How does this increase in feed preheat affect the flow of vapor through the trays and the fractionation efficiency of the trays?

The bottom part of the tower in Fig. 6.2—that is, the portion below the feed inlet—is called the stripping section. The upper part of the tower—that is, the portion above the feed inlet—is called the absorption section.

Since both the reflux flow and the overhead product flow are constant in this problem, it follows that the weight flow of vapor leaving the top tray is also constant, regardless of the feed preheater duty. Actually, this statement is approximately true for all the trays in the top or absorption part of the tower. Another way of saying this is that the heat input to the tower above the feed tray is a constant.

But for the bottom stripping section trays, a reduction in reboiler duty will directly reduce the vapor flow from the reboiler to the bottom tray. This statement is approximately valid for all the trays in the stripping section of the tower.

As the flow of vapor through the absorption section trays is unaffected by feed preheat, the fractionation efficiency of the trays in the upper part of the tower will not change as feed preheat is increased. On the other hand, the reduced vapor flow through the stripping section may increase or decrease fractionation efficiency—but why?

feed preheat Effect of Feed Preheat

Written by Jack

January 24th, 2011 at 10:13 am

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