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
• Inlet line
• Outlet line
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.
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.
At a Gulf Coast refinery, the reboiler thermosyphon circulation could not be reestablished after a turnaround. The tower was reopened and a lessthan-alive contract employee was found stuck in the reboiler draw-off nozzle. At the Good Hope Refinery (when I was the technical manager), we once left a complete scaffold (poles, boards, everything) in the bottom of a debutanizer tower. Rags, hard hats, plywood, and especially plastic bags left in packed columns should be removed from inside draw sumps and downcomers. I know it’s rough on the knees, but crawl across every tray and look into each downcomer. One lost flashlight in a small downcomer may flood every tray in the tower. A rag caught on a vortex breaker in a jet fuel draw box has caused a complete refinery shutdown.
Check the tray clips, tray panels, and downcomer bolting bars. At least the nuts and bolts should be finger-tight. If you find a single loose nut, insist that every nut on that tray be retightened. I will check the tray clips and 10 percent of the downcomer bolting bar nuts for finger-tight.
This is perhaps an idea you remember from high school, but never quite understood. The phase rule corresponds to determining how many independent variables we can fix in a process before all the other variables become dependent variables. In a reflux drum, we can fix the temperature and composition of the liquid in the drum. The temperature and composition are called independent variables. The pressure in the drum could now be calculated from the chart in Fig. 5.4. The pressure is a dependent variable. The phase rule for the reflux drum system states that we can select any two variables arbitrarily (temperature, pressure, or composition), but then the remaining variable is fixed.
A simple distillation tower, like that shown in Fig. 5.2, also must obey its own phase rule. Here, because the distillation tower is a more complex system than the reflux drum, there are three independent variables that must be specified. The operator can choose from a large number of variables, but must select no more than three from the following list:
• Tower pressure
• Reflux rate, or reflux ratio
• Reboiler duty
• Tower-top temperature
• Tower-bottom temperature
• Overhead product rate
• Bottoms product rate
• Overhead product composition
• Bottoms product composition
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
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.
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.
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.
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.
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.
Contract maintenance workers often will not replace the tray manways unless the tray manway is adjacent to a tower external manway. They reason that once the tray manways that are visible from the tower manway are closed, there is no way for someone to inspect the other trays. This problem is not just common—it is universal. The maintenance force at the Good Hope Refinery pulled this nasty trick on me at the coker fractionator. Equipped with my crescent wrench, I opened the tray internal manway below the side tower manway. I discovered that the 12 trays below this point had their manways stacked in their downcomers. In 1990, I worked on a project to improve fractionation at the Chevron Refinery crude distillation unit in El Segundo, California. When the tower was opened to implement my design, the tray manways were found lying on the tray decks below the diesel draw tray. The lesson is, inspect each tray and then witness the closure of each tray manway, separately.