# Hydrocarbon Compression

## Once-Through Thermosyphon Reboilers

Figure 7.2 shows a once-through thermosyphon reboiler. The driving force to promote flow through this reboiler is the density difference between the reboiler feed line and the froth filled reboiler return line. For example:

• The specific gravity of the liquid in the reboiler feed line is 0.600.
• The height of liquid above the reboiler inlet is 20 ft.
• The mixed-phase specific gravity of the froth leaving the reboiler is 0.061.
• The height of the return line is 15 ft.
• Feet of water per psi =2.31.

The differential pressure driving force is then

What happens to this differential pressure of 4.7 psig? It is consumed in overcoming the frictional losses, due to the flow in the

• Reboiler
• Inlet line
• Outlet line
• Nozzles

If these frictional losses are less than the 4.7 psig given above, then the inlet line does not run liquid full. If the frictional losses are more than the 4.7 psig, the reboiler draw-off pan overflows, and flow to the reboiler is reduced until such time as the frictional losses drop to the available thermosyphon driving force.

The once-through thermosyphon reboiler, shown in Fig. 7.2,
operates as follows:

• All the liquid from the bottom tray flows to the reboiler.
• None of the liquid from the bottom of the tower flows to the reboiler.
• All the bottoms product comes from the liquid portion of the reboiler effluent.
• None of the liquid from the bottom tray flows to the bottom of the tower.

This means that when the once-through thermosyphon reboiler is working correctly, the reboiler outlet temperature and the towerbottom temperature are identical. If the tower-bottom temperature is cooler than the reboiler outlet temperature, something has gone wrong with the thermosyphon circulation.

Written by Jack

October 8th, 2019 at 10:46 am

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## Loss of Once-Through Thermosyphon Circulation

There are several common causes of loss of circulation. The common symptoms of this problem are

• Inability to achieve normal reboiler duty.
• Low reflux drum level, accompanied by low tower pressure, even at a low reflux rate.
• Bottoms product too light.
• Reboiler outlet temperature hotter than the tower-bottom temperature.
• Opening the steam or hot-oil inlet heat supply valve does not seem to get more heat into the tower.

The typical causes of this problem are

• Bottom tray in tower leaking due to a low dry tray pressure drop
• Bottom tray, seal pan, or draw-off pan damaged
• Reboiler partially plugged
• Reboiler feed line restricted
• Reboiler design pressure drop excessive
• Tower-bottom liquid level covering the reboiler vapor return
nozzle

If the loss of circulation is due to damage or leakage inside the tower, one can restore flow by opening the start-up line (valve A shown in Fig. 7.2), and raising the liquid level. But if the reboiler is fouled, this will not help.

Figure 7.3 shows a once-through thermosyphon reboiler with a vertical baffle. This looks quite a bit different from Fig. 7.2, but processwise, it is the same. Note that the reboiler return liquid goes only to the hot side of the tower bottoms. Putting the reboiler return liquid to the colder side of the tower bottoms represents poor design practice. While most designers do it this way, it is still wrong.

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October 5th, 2019 at 10:54 am

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## Circulating vs. Once-Through Thermosyphon Reboilers

We said before that it was wrong to return the effluent from a oncethrough reboiler with a vertical baffle to the cold side of the tower’s bottom. Doing so would actually make the once-through thermosyphon reboiler work more like a circulating reboiler. But if this is bad, then the once-through reboiler must be better than the circulating reboiler. But why?

• The once-through reboiler functions as the bottom theoretical separation stage of the tower. The circulating reboiler does not, because a portion of its effluent back mixes to its feed inlet. This back mixing ruins the separation that can otherwise be achieved in reboilers.

• Regardless of the type of reboiler used, the tower-bottom product temperature has to be the same, so as to make product specifications. This is shown in Fig. 7.5. However, the reboiler outlet temperature must always be higher in the circulating reboiler than in the once-through reboiler. This means that it is more difficult to transfer heat in the former than in the latter.

• Because the liquid from the bottom tray of a tower with a circulating thermosyphon reboiler is of a composition similar to that of the bottoms product, we can say that the circulating thermosyphon reboiler does not act as a theoretical separation stage. However, the liquid from the bottom tray of a tower with a once-through thermosyphon reboiler can be quite a bit lighter in composition (and hence cooler) than the bottoms product composition, and thus we say that the once-through thermosyphon reboiler does act as a theoretical separation stage. The cooler the liquid flow from the bottom tray of a tower, the less the vapor flow through that tray. This is because the hot vapor flowing up through a tray heats up the downflowing liquid. This means that there is a greater vapor flow through the bottom tray of a tower with a circulating thermosyphon reboiler than there would be through the bottom tray of a tower with a once-through thermosyphon reboiler. Everything else being equal, then, the tower served by the circulating reboiler is going to flood before the tower served by the once-through reboiler.

Written by Jack

October 4th, 2019 at 11:12 am

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## Kettle Reboilers Process

Reboilers are sometimes inserted into the bottom of a tower. These are called “stab-in” reboilers. It is not a terribly good idea, because it makes it more difficult to fix a leaking or fouled reboiler without opening the tower itself. However, the “kettle” reboiler, shown in Fig. 7.7, has essentially the same process performance characteristics as the stab-in reboiler, but is entirely external to the tower.

Note that in a kettle reboiler the bottoms product level control valve does not control the level in the tower; it controls the level on the product side of the reboiler only. The liquid level on the boiling or heat-exchanger side of the kettle is controlled by the internal overflow baffle. But what controls the tower-bottom liquid level?

To answer this, let us see how such a gravity-fed or kettle reboiler works:

1. Liquid flows out of the tower into the bottom of the reboiler’s shell.
2. The liquid is partially vaporized.
3. The domed top section of the reboiler separates the vapor and the liquid.
4. The vapor flows back to the tower through the riser line. This is the column’s stripping vapor or heat source.
5. The liquid overflows the baffle. The baffle is set high enough to keep the tubes submerged. This liquid is the bottoms product.

The liquid level in the bottom of the tower is the sum of the following factors:

• The nozzle exit loss of the liquid leaving the bottom of the tower
• The liquid feed-line pressure drop
• The shell-side exchanger pressure drop, which includes the effect of the baffle height
• The vapor-line riser pressure drop, including the vapor outlet nozzle loss

Note that it is the elevation, or the static head pressure, in the tower that drives the kettle reboiler. That is why we call it a gravityfed reboiler. Also, the pressure in the kettle will always be higher than the pressure in the tower. This means that an increase in the reboiler heat duty results in an increase of liquid level in the bottom of the tower.

Should the liquid level in the bottom of the tower rise to the reboiler vapor return nozzle, the tower will certainly flood, but the reboiler heat duty will continue. Unfortunately, reboiler shellside fouling may also lead to tray flooding. This happens because the fouling can cause a pressure-drop buildup on the shell side of the reboiler.

Remember, though, that the increased tower-bottom liquid level will not be reflected on the indicated bottom level seen in the control room, which is actually the level at the end of the kettle reboiler. This is a constant source of confusion to many operators who have towers that flood as a result of high liquid levels, yet their indicated liquid level remains normal.

Written by Jack

October 2nd, 2019 at 9:07 am

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## Circulating Thermosyphon Reboilers

The important differences between a once-through thermosyphon reboiler and a circulating thermosyphon reboiler is critical. Figure 7.4 shows a circulating reboiler. In this reboiler

• The reboiler outlet temperature is always higher than the tower-bottom temperature.
• Some of the liquid from the reboiler outlet will always recirculate back into the reboiler feed.
• Some of the liquid from the bottom tray drops into the bottoms product.
• The tower-bottom product temperature and composition are the same as the temperature and composition of the feed to the reboiler.

The liquid feed rate to the once-through thermosyphon reboiler is limited to the amount of liquid overflowing the bottom tray. The liquid feed rate to the circulating thermosyphon reboiler can be quite high—limited only by the available liquid head thermosyphon driving force. However, we should note that the liquid head thermosyphon driving force for a circulating thermosyphon reboiler is proportional to the height of the liquid level in the bottom of the tower above the reboiler inlet nozzle, whereas with a once-through thermosyphon reboiler, as described previously, the corresponding height is the elevation of the floor of the draw-off pan sump above the reboiler inlet nozzle.

For a circulating thermosyphon reboiler, the rate of circulation can be increased by

• Increasing the steam or hot-oil flow through the reboiler. This reduces the specific gravity or density of the froth or foam in the reboiler effluent line.
• Increasing the tower bottoms liquid level. However, should this level reach the reboiler return nozzle, thermosyphon flow will be restricted or even stop. Then the reboiler heat duty will be reduced, and the tower pressure will drop. Sometimes this may cause the tower to flood.

Written by Jack

September 30th, 2019 at 10:59 am

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## Forced-Circulation Reboilers Process

Figure 7.6 shows a once-through forced-circulation reboiler. Such a reboiler differs from a thermosyphon reboiler in that it has a pump to force circulation, rather than relying on natural or thermosyphon circulation. This extra pump seems rather wasteful—and it is.

The great advantage of forced circulation is that careful calculation of the pressure drop through the reboiler and associated piping is not critical. But as we can see in Fig. 7.6, the operator now has two tower bottom levels to control. Further, if the hot-side liquid level rises above the reboiler return nozzle, the force of the vapor and liquid rushing back into the column will cause the trays to flood, but the reboiler heat input will not be affected.

Most often, forced circulation is used with fired reboilers. If flow is lost to such a reboiler, furnace tube damage is likely to result. Hopefully, this is less likely to occur with a forced-circulation reboiler. Also, the higher pressure drop of a furnace may force the designer to use a pump. Sometimes we also see a forced-circulation reboiler system if the reboiler heat is to be recovered from a number of dispersed heat sources that are far away from the tower and hence a lot of pressure drop has to be overcome.

Written by Jack

September 29th, 2019 at 9:01 am

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## Excessive Thermosyphon Circulation

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

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:

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.

Written by Jack

January 24th, 2011 at 10:42 am

Posted in Reboiler