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Thermosyphon reboiler operation

Affect of liquid levels on heat duty in a recirculating system without a baffle

Petroleum refinery naphtha splitter

Operation of a recirculating reboiler on a naphtha splitter

Subject: Thermosyphon Reboiler
Date: Tue, 28 Nov 2000 13:06:57 -0600

I hope you don't mind if I ask you a question. You seem very knowledgeable in the refining industry. I am a Top Operator in the Reforming Division of a refinery.

We have a tower that receives a cracked gasoline and takes a cut of light gasoline off the top that we don't want in the reformer. The bottoms go to the reformer platformer. We do this because reforming light ends actually lowers the octane and thus your reformer has to work harder.

On the bottom of this tower we have a thermosyphon reboiler that uses 500 degree F (260 degree C) diesel as its heat medium. I was told by our engineer that if I raised the level in the tower, I could get more heat transfer to the tower product. We were at max diesel flow on the other side of the reboiler and had a good level in the tower. Is this true?
The tower runs at about 12 pounds and about 300 degrees F (149 degrees C) on the bottom of the tower.
Thank you for taking the time to answer this question.

T., US Gulf Coast Refinery

Subject: Thermosyphon Reboiler
Date: Tue, 28 Nov 2000

T.,

One question, there are three major different reboiler configuration types 1) once through, 2) recirculating with a baffle on the thermosyphon side, and 3) recirculating without a baffle. Your question implies that you have configuration 3. This is the least common configuration in a refinery.

Could you check on what baffle arrangement you have in the bottom of the tower? It does make a difference to the answer. Additionally, there are other, less common configurations as well. It would be helpful also to know the temperature into the reboiler, out of the reboiler, and of the product (if you have them on the control system). This tells a lot about how the reboiler is working.

Andew Sloley.

Subject: RE: Thermosyphon Reboiler
Date: Tue, 28 Nov 2000 14:58:28 -0600

Thank you so much.

The reboiler is a U tube exchanger with the hot oil (595 degrees F, 313 degrees C) going into the tube side and out of the tube side at 310 degrees F (154 degrees C). On the tower side, the shell inlet is 295 degrees F (146 degrees C) and the shell outlet is 305 degrees F (152 degrees C). We have a 60 percent level in the tower.

The inlet of the shell side comes from very close to the bottom of the tower and splits at the shell inlet. At the outlet of the shell the split joins back together and enters the tower just below Tray 1. There are 50 trays in the tower and Tray 1 is probably 40 feet from the bottom. There are no baffles in the tower bottom. I am attaching some drawings. I hope they open for you. They are from our data system.

<Tif files attached>

T., US Gulf Coast Refinery

Subject: Thermosyphon Reboiler
Date: Wed, 29 Nov 2000

T.,

Summary

I'll start out with the summary so you can get the results fast, then a detailed background will follow that you can look at in any level of detail you want.

The only effective operating changes that get more duty into the exchanger are:

Systems can always behave unpredictably, however, the risk of operating upset for a small level change is low for this system. Changing the level and looking at the result (is the diesel temperature out of the reboiler lower, this equals more heat into the tower) is straightforward and cheap. If you want, try it. Just don't be surprised if the duty change is impossible to find.

I'd be interested in hearing what happens if you do try the higher liquid level.

Details

The Basic System

What your figures show is a recirculating reboiler without a baffle, exactly as you described. The major reason to use this configuration is that the tower requires a relatively large percentage of the liquid traffic entering the boot be vaporized for tower boilup.

The major question you have is about the affect of higher liquid level in the boot. Will it increase, decrease, or have no affect on the heat you can get into the system?

To answer your question, we'll look at some general characteristics of this type of system as well as specific factors for your system. Figure 1 shows your unit, approximately to scale.

Figure 1

Reboiler system, approximately to scale

Liquid descends the tower, overflows from tray 1's downcomers and enters the sump. Liquid from the sump goes to the reboiler, partially vaporizes, and returns as a two-phase mixture into the sump. The returning liquid from the reboiler mixes with the liquid that enters the sump from tray 1. The sump liquid going to product and reboiler feed is a mix of the tray 1 liquid and the liquid returned from the reboiler.

The mixed liquid in the sump must have the product composition required. Liquid from tray 1 has a lighter composition that the product. Since the tray 1 liquid mixes with the reboiler return liquid to generate the product, the reboiler return liquid must be heavier than the product composition.

Temperature, pressure, and composition are related. If you fix two of them, the other is fixed as well. Tower operating pressure is fixed by the control system against the need to get the heat out of the overhead condenser and by tower capacity required. Since the mixed sump composition is set by downstream requirements, the temperature required in the sump is now set as well. Since the sump liquid is lighter than the reboiler return liquid, the reboiler return must be hotter than the sump liquid.

Looking at changes in recirculation rate for your system requires looking at two different factors: first, how much can a level change affect recirculation rate: second, how does a change in recirculation rate affect heat transfer and duty in the reboiler.

How Level Changes Recirculation Rate

Recirculation in these systems is driven by the density difference between the outlet line and the inlet line. Figure 2 shows your system with the important heights noted.

Figure 2

Reboiler system, thermosyphon driving forces shown

The system pressure balances based on the total driving force for flow must equal the total resistance to flow:

or

Increasing the height of high-density liquid (level in the boot), increases the driving force for flow. This increases circulation rate. However, increased circulation rate, increases pressure drops as well. For most systems, the pressure drops in the inlet piping and the reboiler are small compared to the driving force in the inlet pipe and the pressure drop plus static head in the outlet pipe.

For your system and the pipe diameters shown, increasing the height within the boot will have only a minor impact on circulation rates. The total height from the liquid level to the exchanger inlet flange is around 33 feet (10 meters). Increasing the height by an extra one or two feet (less than one meter) will have only a minor affect on recirculation rate.

How Recirculation Affects Heat Transfer

Heat duty (Q) from an exchanger is defined by its surface area (A), corrected log mean temperature difference (LMTDc), and overall heat transfer coefficient (U).

To get more duty from an exchanger you must increase either U, A, or LMTDc. Obviously, you don't change the exchanger area. This leaves U and LMTDc.

Product composition sets the sump temperature. The sump temperature is the result of mixing colder liquid from tray 1 with hotter liquid from the reboiler. Assuming the composition changes are small, the higher the recirculation rate, the lower the reboiler outlet temperature you need to get the mixed temperature to the product conditions. Increased circulation rate improves the LMTDc.

However, to get more duty into the tower, you must get more duty out of the diesel (hot oil) heat source. Since you state that the diesel flow is fixed, getting more heat out of the diesel requires cooling it more. Currently, the exchanger reboiler is pinched on the diesel outlet. The diesel outlet is only 10 degrees F (5 degrees C) hotter than the inlet of the reboiler feed. The maximum duty that you could ever get out of the diesel would be to get it down to the temperature of the shell inlet. This would only increase the tower duty by (10)/(595-305)=3.4 percent. Even achieving this would require an exchanger with infinite surface area and no internal temperature pinches in the exchanger. Very little benefit is available from increasing the recirculation rate.

Given that the exchanger is pinched, changes in U also have only a small impact on exchanger performance. Normally, we think that increasing the velocity through an exchanger increases U. This is not always true for reboilers.

This is a very complex problem that really doesn't merit discussing in detail for your unit, because the pinch on the heat source overrides all the other factors. Results depend upon the flow rate, percent vaporized, two-phase flow regimes, heat flux present, physical properties, and geometry of the system. The Handbook of Heat Transfer [1] has a very good discussion of the problem on pages 15.75 to 15.84. The examples shown include a kettle reboiler and a horizontal thermosyphon reboiler just like yours. Careful review of the text shows that heat transfer coefficients can decrease in some circumstances with increasing recirculation. I have observed this behavior in a few units and completely agree that it is real and not just a theoretical problem.

[1] Rohsenow, W.M., Hartnett, J.P., and Cho, Y.I. Handbook of Heat Transfer. The McGraw-Hill Companies, Inc., New York, 1998.

Andrew Sloley.

 

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This page updated 28 November 2000.
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