24 replies to this topic

[bezuid2324]

Hi all, here is my problem.

I have a distillation column with two reboilers connected to it. These two reboilers are connected to a condensate drum. Steam is used as the heating medium in the reboilers. The amount of steam flowing to the reboilers is controlled by the level in the condensate tank. The steam to each reboiler cannot be controlled individually. The temperature on 25th tray in the distillation column has to be kept steady, and it is this temperature that is used to control the amoutn of steam each reboiler gets, as the reboilers are used to get the column tray to this temperature. If there one of the reboilers has to be seviced, open steam is sent directly into the distillation column to assist in keeping the tray temperature steady. This is the total system.

Recently the tube bundles of both reboilers has been replaced, it obviously you should see an improvement in the amount of steam (thus less steam) that has to be used, since the onl reboilers had fouled tubes and blockages. But after this change, the same amount of steam has to be used. Before the replacement, the two reboilers as well as a open steam line was used to keep the temperature steady. After the replacement of the tube bundles, it should have been so that the steam intake decreases. But this did not happend. What could possibly have this affect?

Another problem is the fact that when the level in the condensate drum is increased, you are suppose to see a drop in the steam flow. But in this system this does not happen. What could be the problem?

[markymaark]

Do you have a P&ID with all the controls and system interlocks? That would help with seeing if the steam is only level controlled by the condensate drum. It would appear that is not the case. If the temperature were different, then a different concentration would be exiting the column but the same volume may be present. This would not be ideal operation for the column. Like you said, the bottom tray temperature is controlling the steam as well. This appears more ideal.

"Another problem is the fact that when the level in the condensate drum is increased, you are suppose to see a drop in the steam flow. But in this system this does not happen. What could be the problem?"

Like I said above it would not appear to be ideal in this case. There may be high level/low level alarms on the drum but I would not suspect a LIC. A P&ID would be needed.

"After the replacement of the tube bundles, it should have been so that the steam intake decreases. But this did not happend. What could possibly have this affect?"

You never said there was a decrease in performance of the tube bundles. Hypothetically there will be some fouling/scaling of the tube bundle ID which would cause less heat transfer. But you never said this was seen. I cannot see a reason an identical replacement would not increase performance of a inefficient tube bundle.

Mark

[Art Montemayor]

Bezuid2324:

Markymaark is correct. We need at least a P&ID to further comment with accuracy on what is happening. Additionally we need a sketch or drawing of the reboilers, their tube bundle locations, and the relative location of the sole steam condensate drum with respect to the tube bundles. We need to know the type of reboiler – kettle or thermosyphon? I suspect the latter, but this should not be a guessing game. The more complete is your basic data, the more accurate and correct will be the responses and comments.

Your description of your steam usage in the reboilers is confusing. You seem to relate high steam consumption with low reboiler tube bundle efficiency. Is this correct? This is erroneous thinking and does not reflect what is actually happening in the reboiler tube bundle. The more steam that is consumed in a tube bundle under the same conditions, the more efficient it is working. That is the nature of what it is supposed to do: Transfer as much heat as it was designed to do. If it condenses more steam it can only be because the heat transfer area is working better than before. Therefore, I would expect the two bundles to transfer more heat using the same surface after being cleaned. This means that more steam would be consumed. That is good news, not bad.

If you raise the condensate level in the steam drum, the heat transfer will decrease in the two tube bundles ONLY IF THE SAME CONDENSATE LEVEL INUNDATES A PORTION OF THE TUBES IN THE TUBE BUNDLES. If the condensate drum is too far below the tube bundles, this may not occur and heat transfer will proceed as normal. That is why we need the above mentioned drawings.

[bezuid2324]

I am in the process of getting the P&ID. Just to make it clear. What I meant with more steam has to be used now after the tube bundles were replaced: As I said, when one of the reboilers are taken off line to be serviced open steam is sent directly into the distillation column. this system is also used when the reboilers fail to keep the tray 25 temperature at 110C. Before the replacement, open steam was used to help the reboilers because of the reboilers being fouled and blocked. After replacement one would expect to use less or even no open steam at all, and just let the reboilers run on their own. But this is not the case. We still have to use the same amount of open steam directly into the column.

[Art Montemayor]

Bezuid2324:

You have not made things clear:

• We need a sketch or drawing of the reboilers, their tube bundle locations, and the relative location of the sole steam condensate drum with respect to the tube bundles
• We need to know the type of reboiler – kettle or thermosyphon?
• You seem to relate high steam consumption with low reboiler tube bundle efficiency. Is this correct?

[bezuid2324]

Because of the limited access I am given regarding the internet and the fact that the company i work for is very anal with intellectual propery I cannot upload the P&ID. However I can try and explain the system. The two reboilers we are using is vertical Thermosyphon reboilers. The two reboilers are elevated off the ground, and right next to each other. The concensate drum at ground level right next to the reboilers. The condensate outlet from the reboilers and the in let to the condensate drum is about 2m apart in hight and about 1.5m in length.

What has happened is the reboilers are working too good after the tube bundles were replaced. So much so that the reboilers had to be flooded to almost 70% (while both were running) in order to keep the temperature down. But obviuosly when you flood the rebloier this far you dont really get the formation of the different regions inside. So what they decided to do is to stop using one of the reboilers, and send open steam into the distillation column instead. Thus only one reboiler is running. The system as a trip when the maximum allowable steam is reached. Since using this method of using open steam and only one reboiler, they are running too close to the maximum steam.

[Bobby Strain]

Just connect the temperature controller to the condensate outlet. If you don't have a temperature controller, get one. It will work fine for you. Don't try to use cascade control. Maybe a little controller tuning will be in order.

Bobby

[bezuid2324]

What I recently discovered. Before the replacement of the tube bundels, the condensate drum level in the condensate drum was already very high (jumping between 80% and 50%). This means to me that the area in the reboilers was already decreased alot even though a great amount of tubes were fouled and blocked.

As I undersatnd it: The tray 25 temp is measured, when this temp rises, a signal is sent to a control valave at the bottom of the condensate drum, this valve closes, causing the condensate level in the drum to rise, this in turn causes the condensate level in the reboilers to rise thus decreasing the surface area.

I hav also notices that a very large change in condensate drum level has to be made in order to have a small effect on the tray 24 temperature. Since the condensate drum level was very high before the replacement, shouldn't this mean that if you replace the tube bundle, thus having much better heat transfer for the same surface area, that the condensate drum level will have to rise even more in order to accomodate the better heat transfer?

And since the condensate drum level was mostly above 60% before the replament, after the replacement this level has to rise, meaning ither the condensate drum was design too small or the reboilers were largely over desinged???

[bezuid2324]

Just one extra question...

The level gauge in this system reads a delta P between the condensate drum and one of the reboilers. This then shows a persentage value for the level. Does this value represent the condensate level in the reboiler or the condensate drum?

[Bobby Strain]

Don't worry about the level in the drum. What you need to focus on is the temperature measuring point. It's likely that it is not located on the right tray, so large changes in duty don't affect the temperature. You need to check the response using one of the process simulators. Very few designers know how to properly locate the correct control tray, and you likely are the victim of one of the engineers.

Bobby

[Art Montemayor]

Refer to the attached Schematic drawing.

If you want to control the temperature on a certain tray in the distillation column using level control on your thermosphon reboilers (both of them working), then I would install a reliable temperature transmitter indication on that tray and set my reboiler level manually when that temperature was achieved. After doing this one time, you will know the condensate level that corresponds to the desired tray temperature under the same steam conditions in the reboilers’ chests.

As your reboilers start to foul with time, your operators will manually adjust the condensate level lower and lower. I am assuming that your reboilers have sufficient excess transfer area to allow for the expected or anticipated fouling.

You should have a standard, visual armored level gage on each reboiler chest to indicate the condensate level there, as well as on the condensate drum. The condensate level detector and controller should be mounted off the condensate drum. Note the relative height of the condensate drum as shown on the sketch and take heed of the required equalization lines that must be properly sized and installed.

Had you furnished this type of simple sketch in your original post, this topic would have already been resolved.

 Thermosyphon Reboiler Level Control.xlsx   195.71KB   745 downloads

[bezuid2324]

That drawing is exactly what the present system looks like. I couldn't upload this drawing because of the company I am working for is limiting the internet access I have. I need administrative permision to upload something to the internet.

The system is old. So the eye level gauges on the condensate drum is not working anymore. That is way I am trying to find out what level they are reading using the electronic equipment.

I had to figure out the system and it's problem myself because none of the people really know what the problem is. What I've notices is that after the repalcement of the tube bundles, they are using more steam to achieve the same vapour outlet temperature. Now to me this is not correct. As I understand, after replacing the tube bundles, you will have much better heat transfer since there are no more blocked or fouled tubes. Thus meaning you will have to use a smaller heat transfer area (higher condensate level in the reboiler) and less steam aswell.. Am I wrong? This seems not to be the case with this system. Before the tube bundles were replaced the whole system worked fine.

[bezuid2324]

Just to give you an idea of what I mean, here are a few figures:

Before replacement:
Condensate level: 40%
Steam usage: 80ton/h

After replacement:
Condensate level: 70%
Steam usage: 130ton/h

All the stats above are averages take over a 3month period. These are the conditions that had to be kept in order to keep the tray 25 temp in the distillation column constant. I have checked the feed inlet temp and all of the other conditions in the distillation column aswel as the feed loads, and these conditions stayed the same before and after replacement.

[rychurek]

bezuid2324:

As Mr Art Montemayor mentioned before:
"...You seem to relate high steam consumption with low reboiler tube bundle efficiency. Is this correct? This is erroneous thinking and does not reflect what is actually happening in the reboiler tube bundle. The more steam that is consumed in a tube bundle under the same conditions, the more efficient it is working. That is the nature of what it is supposed to do: Transfer as much heat as it was designed to do. If it condenses more steam it can only be because the heat transfer area is working better than before. Therefore, I would expect the two bundles to transfer more heat using the same surface after being cleaned. This means that more steam would be consumed..."

..."As your reboilers start to foul with time, your operators will manually adjust the condensate level lower and lower. I am assuming that your reboilers have sufficient excess transfer area to allow for the expected or anticipated fouling..."


If my interpretations is right :, if column conditions (load, feed,tray temp,...) are the same after bundle replacement, steam consumption should be the same as before. Demand for heat to keep tray temp. is constant, so amount of steam is well (if steam quality is unchanged), regardless of reboiler efficiency, Only reboiler heat transfer---> transfer area--->condensate drum level will changed.

Bezuid2324 wrote:

"Before replacement:
Condensate level: 40%
Steam usage: 80ton/h

After replacement:
Condensate level: 70%
Steam usage: 130ton/h"


Conclude:

Condensate level goes up, so less bundle area to transfer the same amount of steam like we expected. Can you confirm Steam consumption by condensate outflow from drum (valve opening/flow characteristic). If steam/condensate mass balance are ok, is there possibility reboiler steam leakage cause dp increase across the column?
.

[bezuid2324]

I have checked the condensate outlet flow from the reboilers, and the flow corresponds to the valave opening and closing, and the condensate flowrate is consistent with regars to the steam flow entering. Before and after the steam and condensate flow rate averages over a time of 3 months seem to be the same. This indicates that there can not be a leakage on the reboiler.

The way this system is reacting is very wierd.

The rest of the system reacted like it should have if you have new tube bundles. It is just the steam flow that is acting abnormal.

Could a change in tube bundle material have such a big effect. According to the material safety sheets the tubes are suppose to be stainless steel 316L. There is a suspision that they may have replaced these with tubes made out of duplex stainless steel.

[bezuid2324]

Could a change in pressure inside the column have this effect on the steam usage??

[bezuid2324]

I have checked the pressure readings over the column.

Before replacement:
Middel: fluctuates between 75 and 80 KPa
Bottom: fluctates betwee 80 and 87KPa

After replacement:
Middle: More steady at 75KPa
Bottom: steady at 82KPa

[Bobby Strain]

Seems to be operating OK. Probably a problem with the steam flowmeter. If you do a heat balance, you can verify this.

Bobby

[Bobby Strain]

What are you distilling? Acetone and water?

Bobby

[Art Montemayor]

Bobby Strain is correct. Your steam flow meter was either reading wrong before the modification, or it is reading wrong now after the modification. The correct way to establish what has/is happening is to develop a detailed heat balance and calculate the expected steam supply to the reboilers under the established feed rate.

A change in tube materials will not affect the reboiler heat transfer such that you will notice the change.

You could easily have drawn the same sketch as I for your first post; your company should not have any right to stop you from doing that. Obviously you didn't (and don't) want to make the effort to define your problem with accuracy and detail. By making it harder for our members to help you, you are making it harder for yourself.

[bezuid2324]

I do want to make an effort, because I am struggeling with this problem. You don't understand, my internet is limited. Like I said I cannot upload anything to the internet without administrative permisssion. I tried. I'm still a student and they are monitoring everything I do.

Thank you so much for the help so far. Sorry for making it difficult for you guys to understand.

[bezuid2324]

The steam entering the reboilers is 62,58 ton/h, and the condensate leaving the reboilers is 64.95 ton/h. So it doesn't look like the steam flow meter is wrong.

The temperature and pressure profiles in the column is the same as before. I've basically been through the whole system, and everything looks the same as before. The only things that changed is the steam flow, and they decreased the reboiler heat transfer area.

I really dont understand why you would need almost twice as much steam with new tubes, after decreasing the heat transfer area, to achieve the same outlet temperature as before???? It doesn't make sence

[Bobby Strain]

So, probably your earlier reference steam flow was wrong. If you share more information with us, we could probably give you some guidance. In the absence of data, you are on your own.

Bobby

[SSWBoy]

Have you actually simulated the process and seen what happens to the temperature profile when you inject live steam?

What I'm getting at is that the tray temperature you're controlling your heat input on is an arbitrary number based on the original design of the unit/operating experience for.

When you inject live steam, you would have thought that this would affect the temperature profile, maybe (possibly very likely!) you don't need to control the tray temperature to the same one as before, and that to achieve the same separation as previously you can reduce the tray temperature set point. Just a thought.

[S.AHMAD]

Bezuid2324
1. Have you done the energy and material balance? Before and after the replacement?
2. If energy balance is okay then there is nothing wrong with the operation and steam consumptions before and after the new bundle.
3. Are you comparing apple with apple or orange with apple? what I am trying to say is that, previously you run reboiler together with live stream, but now only reboiler no more live steam. The energy and material balance is no more the same before and after. Then you are not comparing the same operations.
4. Probably, reboiler + live steam is more energy efficient than running reboiler alone? The only way to confirm this is by comparing energy and material balance before and after. After all, energy and material balance is the "food" of process engineer. Without energy and material balance, we are shooting in the dark Good luck.

Edited by S.AHMAD, 21 December 2012 - 04:01 AM.