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.

29. March 2011 by Jack
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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.

24. January 2011 by Jack
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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.

24. January 2011 by Jack
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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.

24. January 2011 by Jack
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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.

24. January 2011 by Jack
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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.

24. January 2011 by Jack
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Internal Reflux Evaporation

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.

24. January 2011 by Jack
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Conversion of Sensible Heat to Latent Heat

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.

24. January 2011 by Jack
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Effect of Feed Preheat

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?

24. January 2011 by Jack
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Heat-Balance Calculations

If you have read this far, and understood what you have read, you will readily understand the following calculation. It is simply a repetition, with numbers, of the discussion previously presented. However, you will require the following values to perform the calculations:

• Latent heat of condensation of alcohol vapors = 400 Btu/lb
• Latent heat of condensation of water vapors = 1000 Btu/lb
• Specific heat of alcohol (vapor or liquid) = 0.6 Btu/[(lb)(°F)]
• Specific heat of water = 1.0 Btu/[(lb)(°F)]

The term specific heat refers to the sensible-heat content of either vapor or liquid. The specific heat is the amount of heat needed to raise the temperature on one pound of the vapor or liquid by 1°F. The term latent heat refers to the heat of vaporization, or the heat of condensation, needed to vaporize or condense one pound of liquid or vapor at constant temperature. Note that the heat of condensation is equal to the heat of vaporization. Each is referred to as the latent heat. The sum of the sensible heat, plus the latent heat, is called the total heat content, or enthalpy.

Returning to our example in Fig. 6.1, we wish first to determine the reboiler duty. To do this, we have to supply three heat requirements:

A. Heat 9000 lb/h of water from the 100°F feed temperature to the tower-bottom temperature of 220°F.
B. Heat 1000 lb/h of alcohol from the 100°F feed temperature (where the alcohol is a liquid) to the tower overhead temperature of 160°F (where the alcohol is a vapor).
C. Vaporize 10,000 lb/h of reflux from the 150°F reflux drum temperature to the tower overhead temperature of 160°F.

Solution to step A:
9000 lb/h x 1.0 Btu/[(lb)(°F)] x (220°F – 100°F) = 1,080,000 Btu/h

Solution to step B:
1000 lb/h x 0.6 Btu/[(lb)(°F)] x (160°F – 100°F) + 1000 lb/h x 400 Btu/lb = 36,000 Btu/h + 400,000 Btu/h = 436,000 Btu/h

Solution to step C:
10,000 lb/h x 0.6 Btu/[(lb)(°F)] x (160°F – 150°F) + 10,000 lb/h x 400 Btu/lb = 60,000 Btu/h + 4,000,000 Btu/h = 4,060,000 Btu/h

The reboiler duty is then the sum of A + B + C = 5,576,000 Btu/h.

The next part of the problem is to determine the vapor flow to the bottom tray. If we assume that the vapor leaving the reboiler is essentially steam, then the latent heat of condensation of this vapor is 1000 Btu/lb. Hence the flow of vapor (all steam) to the bottom tray is

= 5,576,000 Btu/h    /  1000 Btu/lb = 5576 lb/h

How about the condenser duty? That is calculated as follows:

11,000 lb/h x 0.6 Btu/[(lb)(°F)] x (160°F – 150°F) + 11,000 lb/h x 400 BTU/lb = 66,000 BTU/h + 4,400,000 BTU/h = 4,466,000 BTU/h

We can draw the following conclusions from this example:

• The condenser duty is usually a little smaller than the reboiler duty.
• Most of the reboiler heat duty usually goes into generating reflux.
• The flow of vapor up the tower is created by the reboiler.

For other applications, these statements may be less appropriate. This is especially so when the reflux rate is much smaller than the feed rate. But if you can grasp these calculations, then you can appreciate the concept of the reboiler acting as the engine to drive the distillation column.

24. January 2011 by Jack
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