Jump to content



Featured Articles

Check out the latest featured articles.

File Library

Check out the latest downloads available in the File Library.

New Article

Product Viscosity vs. Shear

Featured File

Vertical Tank Selection

New Blog Entry

Low Flow in Pipes- posted in Ankur's blog

0
- - - - -

Reboiler Performance Issues


26 replies to this topic
Share this topic:
| More

#1 JKM

JKM

    Veteran Member

  • Members
  • 49 posts

Posted 25 March 2018 - 09:41 PM

Dear All,

 

Our client is facing problem in Reboiler for distillation tower. Column bottom design temperature is 90 degC and operating temperature is 70 degC. Reflux ratio is quite low and hence the spec is not matched.

 

We tried to increase steam flow rate by:

- FV full open in steam inlet to reboiler

- Steam Trap bypass valve is 100% open

 

With trial & error, highest temperature attained was 70 degC.

 

Hence I requested to close Steam Trap bypass Valve to check the temperature. It was 65 0C.

 

I suspected steam trap capacity is small so I requested client to open steam trap bypass slightly.

If it is small, the duty of reboiler will be increased.

If it is adequate, the duty of the reboiler will decrease.

 

The temperature decreased to 56 deg C. Hence the steam trap capacity is OK.

It was checked the steam is flow to Steam Condensate header thru test.

 

But I can not understand why the temperature is down to 65 deg C from 70 degC while Steam Trap bypass valve closed. In my opinion, if the surface area is same, steam trap bypass close is more efficient for more heat exchange, even though flow rate is reduced to more than half.

 

I suspect flow rate blockage in steam line but flow rate is high enough while steam trap bypass open.

Thermosyphon reboiler circulation is continued, since heat is exchanged continuously.

 

I would be grateful if somebody can shed light on the reason why temperature reduced from 70 degC to 65 degC in case of Steam Trap Bypass Closed.

 

JKM

 



#2 Nikolai T

Nikolai T

    Gold Member

  • Members
  • 119 posts

Posted 26 March 2018 - 01:45 AM

Hello,

 

What are steam parameters?

 

Regards



#3 JKM

JKM

    Veteran Member

  • Members
  • 49 posts

Posted 26 March 2018 - 01:54 AM

Dear Nikolai,

 

Thanks a lot for your response.

 

The steam condition is 5.5 barg @ 165 degC.

 

Regards

JKM



#4 Nikolai T

Nikolai T

    Gold Member

  • Members
  • 119 posts

Posted 26 March 2018 - 02:34 AM

First I thought the steam is superheated, but your steam is saturated. Also it may be due to poor condensate removal. Could you please show us p&id? What kind of steam trap do you have?



#5 Art Montemayor

Art Montemayor

    Gold Member

  • Admin
  • 5,721 posts

Posted 26 March 2018 - 10:37 AM

This type of steam bundle failure can be solved more readily when one has an ISOMETRIC sketch of all the steam piping - going into the reboiler steam bundle as well as the condensate coming out of the bundle, into the steam trap and out to recovery.  Also show any condensate drum employed and the steam equalizer line, size and non-condensibles' purge.   No piping lengths are needed - just an accurate isometric sketch.  The P&ID is only useful to indicate what is installed, not how.

 

I've solved this very type of problem more than once and the problem(s) have been condensate hang-up, non-condensable accumulation, or lack of positive condensate gravity drop.

 

You tell us a lot of temperature readings - but you fail to positively tell us WHAT flow or fluid you are reporting on - and where in the system.



#6 Zauberberg

Zauberberg

    Gold Member

  • ChE Plus Subscriber
  • 2,702 posts

Posted 26 March 2018 - 11:16 AM

Another phenomenon that may occur is vapor blanketing, if excessive quantities of hot fluid are introduced in the reboiler. The excessive hot fluid flow creates rapid vaporization of cold fluid which, in turn, creates vapor film that impedes heat transfer. This normally does not occur in steam service but, as Art pointed out, we don't know how your system physically looks like.

 

Try closing all bypasses and set steam control valve at 50-70% open. See what happens.



#7 Bobby Strain

Bobby Strain

    Gold Member

  • Members
  • 2,772 posts

Posted 26 March 2018 - 12:58 PM

Your approach is not a good troubleshooting technique. You need to collect all the data for the system and compare it to design values. What you are looking for as a result is the heat transfer coefficient, and anything unusual. And when you provide information that Art suggested, we might even learn whether the exchanger is vertical or horizontal.

 

Bobby



#8 JKM

JKM

    Veteran Member

  • Members
  • 49 posts

Posted 26 March 2018 - 07:09 PM

Dear Nikolai, Art, Zauberg, Bobby,

 

Thanks a lot for your valuable detailed feedback.

 

Kindly find enclosed the P&ID and the isometric drawing of the system.

 

Below are the requested information:

System: Thermosyphon reboiler of a column

Heat Exchanger Orientation: Vertical

 

Fluid Tube Side: mixture of Water / monomer / C3

                           Inlet temperature(0C)/Pressure(barg): 84.8 / 21.3

 

Fluid Shell Side: LP Steam

                           Inlet temperature (0C) / Pressure(barg): 165 / 5.5

 

Steam Trap Type: Ball Float

 

Even though the tube side fluid is monomer but there is no possibility of polymerization.

 

Regards

JKM

 

 

Attached Thumbnails

  • LS Inlet 3D Snapshot.png
  • LS Inlet-2.png

Edited by JKM, 08 April 2018 - 09:35 PM.


#9 Nikolai T

Nikolai T

    Gold Member

  • Members
  • 119 posts

Posted 26 March 2018 - 08:58 PM

Hello,

 

I can't open attached pdf file. Also condensate line 3D is desirable.

 

Regards



#10 JKM

JKM

    Veteran Member

  • Members
  • 49 posts

Posted 26 March 2018 - 09:08 PM

Hi Nikolai,

 

Thanks for pointing out problem with the PDF file. I replaced it with new one. Kindly recheck whether you can download or not.

 

Steam Trap downstream pipeline is removed. LP condensate is discharging to the drain even though above situation is continued. So i think 3D for the LP condensate is of no use.

 

JKM


Edited by JKM, 26 March 2018 - 11:00 PM.


#11 Nikolai T

Nikolai T

    Gold Member

  • Members
  • 119 posts

Posted 27 March 2018 - 01:30 AM

Which steam flow do you have? Did you analyse heat exchanged? I think column heat balance calculation is needed.



#12 Nikolai T

Nikolai T

    Gold Member

  • Members
  • 119 posts

Posted 27 March 2018 - 03:05 AM

Column sump doesn't have baffle. Why do you exclude possibility of polymerization?



#13 JKM

JKM

    Veteran Member

  • Members
  • 49 posts

Posted 27 March 2018 - 06:18 PM

Dear Nikolai,

 

Other issues such as polymerization possibility and other issues are checked in consultation with LICENSOR. So I requested review of only steam side.

 

JKM



#14 Art Montemayor

Art Montemayor

    Gold Member

  • Admin
  • 5,721 posts

Posted 27 March 2018 - 09:32 PM

JKM:

 

One immediate item I don't spot in your isometric and P&ID is the customary non-condensable vent valve usually installed at the top of the thermosyphon shell with an extended internal nozzle that curves upwards to within a 1/2" of the top tubesheet so as to purge all accumulated non-condensables.  All I can spot are two 3/4" shell valves located relatively low on the reboiler shell marked V (vapor?) and D (drain?).

You need to vent non-condensables (at the high point) - especially in a vertical thermosyphon that inherently traps them at the top of the tube bundle and if not purged, will accumulate and slowly blanket the top heat transfer area and reduce capacity.



#15 Bobby Strain

Bobby Strain

    Gold Member

  • Members
  • 2,772 posts

Posted 27 March 2018 - 10:26 PM

Art brings up a point that has been discussed for decades, maybe a century. Most non-condensibles are heavier than water vapor. Hence they will collect at the lower end of the shell side. I always provide vents both at the top and near the bottom. With your design, the shell side is expected to not accumulate condensate. Hence the heat transfer is controlled by varying the steam condensing temperature. However, if condensate flow is restricted, the shell will pressure, providing driving force to remove the condensate. While also increasing the condensing temperature to compensate for any flooded area.  You don't have any data for anyone to reach a conclusion. You need heat exchanger performance data to even begin your investigation. Now, checking the trap is a good place to start since it can be done quickly. But, if you are not careful, what you actually conclude with this simple test may well be wrong. So, start with the basics, and maybe pursue the issue of the steam trap at the same time. All plant superintendents want lots of action. They acquaint this with solving the issue. So, have a plan, and execute it using good engineering principles, and discipline. Your licensor does not appear to be UOP. The column controls are too sophisticated for UOP.

 

Good Luck.

Bobby



#16 JKM

JKM

    Veteran Member

  • Members
  • 49 posts

Posted 27 March 2018 - 11:00 PM

Dear Art,

 

Vent near the top of the heat exchanger is already installed even though in the P&ID it looks like it is being installed in the middle of the heat exchanger. This reboiler worked well only for one day and after that capacity deteriorated. So we are our wits end for the reason for deterioration.

 

JK



#17 Nikolai T

Nikolai T

    Gold Member

  • Members
  • 119 posts

Posted 28 March 2018 - 02:04 AM

JKM,

 

Which results do you have at this moment?

 

Regards

Nikolai


Edited by Nikolai T, 28 March 2018 - 03:25 AM.


#18 JKM

JKM

    Veteran Member

  • Members
  • 49 posts

Posted 28 March 2018 - 02:29 AM

Bobby, Nikolai

 

Thanks for suggestion. Your comments are useful in our fact finding activity.



#19 Bobby Strain

Bobby Strain

    Gold Member

  • Members
  • 2,772 posts

Posted 28 March 2018 - 06:49 PM

There is the possibility that your contractor did not properly clean the system prior to startup. So you may have debris plugging the exchanger tubes. It's always best to tell us that this is a startup when you first post your query. And, after our repeated requests, you still have given no performance data.

 

Bobby



#20 JKM

JKM

    Veteran Member

  • Members
  • 49 posts

Posted 28 March 2018 - 08:53 PM

Dear Bobby, Nikolai,

 

Sorry for the inconvenience caused for the lack of information. Yes it is a start-up process. Plant is commissioned recently and operation is yet to be stabilized.

Regarding the possibility of debris, the heat exchanger was opened and found to be clean.

 

We also varied the process composition and checked the possibility of polymerization and other issues. It was also discussed with LICENSOR and found to be not the case.

 

Reboiler duty reduced significantly after one-day operation. Due to which(expectedly) the light component loss has been increased.

 

Other possible solution which we thought was to reduce the Thermosyphon Reboiler circulation rate, however the reduction in duty is quite significant. It was also discussed with LICENSOR.

 

So after the troubleshooting and deliberation we think problem is with the Steam Side. Let me know if any other input is required from my side.



#21 Bobby Strain

Bobby Strain

    Gold Member

  • Members
  • 2,772 posts

Posted 28 March 2018 - 11:06 PM

Still, it is not possible for us to help without performance data. And I presume that you didn't inspect the shell side. And, as I pointed out before, most non-condensibles will collect  at the lower portion of the steam side. It sounds like you didn't inspect the bottom of the tower, either. When you get desperate, you can build another exchanger to piggy-back on the one you have. But this would be a gamble if there is some problem with circulation on the tube side. So, you need to be rigorous in your analysis. Which means you need good, complete, and accurate performance data. And a simulation to verify the observed behaviour.

 

Bobby



#22 JKM

JKM

    Veteran Member

  • Members
  • 49 posts

Posted 28 March 2018 - 11:43 PM

Hi Bobby,

 

Could you please let me know what exact performance data you required. Accordingly I will ask to OWNER. We inspected shell side, however we did not inspect column bottom. We will check the column bottom as well.

 

JK



#23 Pilesar

Pilesar

    Gold Member

  • Members
  • 547 posts

Posted 29 March 2018 - 10:10 AM

  Reduce feed until you can get the column products in spec. Can you never get hotter column bottoms no matter what the feed rate? This points to process-side problems, not steam problems.

  I suspect the problem is on the process side. Probably your process side circulation is too low. How did you reduce circulation? Have the exchanger designer recheck the design using the measured characteristics of your sump fluid (sump level, density, viscosity, composition, etc.) If the original assumptions of process-side hydraulics were wrong, this may just be a poor design. The fact is, the exchanger is being fed a much different material than it was designed for. If the designer did not rate the exchanger for these conditions, the column may not be able to get out of its hole without rate reduction. Some thermosiphons rapidly lose performance if the process conditions are significantly different from design. It is good practice to rate thermosiphon exchanger designs at alternate operating conditions to improve robustness. If you have short tubes (10 ft or so) then you may not have a large operating hydraulic range.

  If you are lucky enough to have a butterfly valve in the reboiler suction line, try throttling it to increase inlet pressure drop.

  One clue is that the performance dropped after a day in service. Perhaps heavy components began to build in the sump. You might try purging the sump of collected material to see how column operation changes.

  Adjust the level in the sump up and then down to see how performance is affected. Confirm the level is not so high that it interferes with the reboiler return.

  Cool material in the sump is a symptom of too many light components. Consider whether you are somehow bypassing the column internals through excessive weeping or downcomer leaking or damaged trays or open internal manways or inadequate packing height or poor liquid distributors or missing wall wipers in a structured packed column.

  Have you checked all external lines to see if there is any way you can be sending light materials directly to the sump? This may happen where you have a common spare pump for reflux and bottoms product.

  Scan the column to find possible problems there.



#24 Bobby Strain

Bobby Strain

    Gold Member

  • Members
  • 2,772 posts

Posted 29 March 2018 - 02:17 PM

You need to measure what is on the process datasheet, both tube side and shell side. And while you are inspecting the column sump use a boroscope to inspect the pipe from the column to the exchanger. And while you are shutdown, add a connection between the exchanger and trap so that you can fully empty the exchanger. Make sure you don't have any leaks that can deliver process fluid into the shell side. And we could be more help if we knew the exchanger tube length, diameter, count, and shell diameter, and baffle details. If we have the composition of the tower bottom we might have more to offer. With the little information you gave, it would appear that the tower bottom is two liquid phases, one water rich, the other hydrocarbon rich. Is this the licensors first commercial unit?

 

Bobby


Edited by Bobby Strain, 29 March 2018 - 03:17 PM.


#25 JKM

JKM

    Veteran Member

  • Members
  • 49 posts

Posted 30 March 2018 - 03:46 AM

Bobby,Pilesar

 

I will ask the OWNER to carry out the test you suggested. 

 

This is not the first Commercial Unit. There are many units operating worldwide.

 

Following are design data you requested:

Tube Length: 2440 mm

No. of Tubes: 365

Tube type: Plain

OD/Thk : 1" / 14BWG

Tube Pitch: 31.75

Layout Angle: 30

Baffle Type: Single Segmental

Baffle % Cut: 30.4

Baffle Spacings: 340 mm

Shell ID: 700 mm

 

Regarding damage in the Tray, we do not suspect as the Column temperature (except the sump) is maintained by proportional reduction in the reflux flow. The problem is only the sump. The sump temperature is lower than the FEED Temperature.

 

Regards

JK






Reply to this topic



  

Similar Topics