4 replies to this topic

[MaverickJK]

I am reading & learning here for past 2 year, but this is the my first direct interaction in foroum. I am Jitendra, Process Engineer with 3.5 years of experience.

I am designing condensate pot for vertical thermosyphon reboiler. Steam is coming to reboiler at 3.6 kg/cm2g & condensing inside the reboiler. In reboiler, I am ensuring that outlet nozzle will always be liquid flood so that steam will not come out & I utilized it fully. To control the reboiler duty, I have control valve downstream of condensate pot. I calculate the volume of condensat pot based on turndown steam flowrate as this is the case when my condensate pot will be used mainly for buildup level in reboiler & will fill with condensat. My main concern is relative elevation of condensate pot with reboiler. I calculated this elevation difference between reboiler & pot by calculating pressure drop appyling bernouli's equation. I calculated the pressure drop as sum of that can occur at outlet nozzle of reboiler and pipe from nozzle to condensate pot nozzle at condensate flowrate corresponding to design flowrate of steam. I have done this design based on attached paper "Reboilers & Condensers operating problem" case study-1.
still I have doublts, whether my understanding is correct or not ?

Attached Files

•  reboilerand condenser operating problems.pdf   159.01KB   918 downloads

[ankur2061]

Jitendra,

Try to get hold of the following book. It gives very detailed description along with a sketch for arrangement of condensate pots for reboilers:

Section 17.1.3: Condensate Pots
Chapter 17: Reboilers, Condensers, & Pressure Controls
Distillation Operations by Henry Z. Kister

Additionally have a look at the following link for reboiler steam flow control. Our beloved Art Montemayor has shared some of his valuable operating experience:

http://www.eng-tips....d=87417&page=34

Hope this helps.

Regards,
Ankur.

Edited by ankur2061, 17 April 2010 - 01:32 PM.

[Art Montemayor]

Jitendra:

I have done what you are describing; the first one I designed and installed was in 1970. It was done on a Furfural Stripper and worked very well.

I believe you either do not have a clear understanding of the reboiling process (heat exchange-wise) or you have not explained completely how you intend to design your thermosyphon system. Please refer to the attached Excel workbook and look at the sketch I have supplied.

Note that I show how the steam supply is to be supplied in order to accomplish the reboiling turndown ratio you desire.

You state: “In reboiler, I am ensuring that outlet nozzle will always be liquid flood so that steam will not come out & I utilized it fully.” This is wrong. You must design for the maximum heat transfer rate in the reboiler (at 100% capacity) and thus, should have 100% full gravity drain of the condensate formed into the condensate pot.

You also state: “I calculate the volume of condensate pot based on turndown steam flowrate as this is the case when my condensate pot will be used mainly for buildup level in reboiler & will fill with condensate.” This is also wrong. You must not fill the condensate pot full. You must always leave a vapor space in order to fully control the condensate level. You also do not design the pot for volume. You primarily design it for LEVEL. The liquid volume in the pot should be just enough for proper level control. Note that this system does not require a conventional steam trap - the pot's level control the perfect steam trap.

You are correct in your concern for the relative elevation of condensate pot with the reboiler and calculating the elevation difference by applying Bernoulli’s equation. The article you cite is a good guide line for you. However, I think you need to focus more on the specific mechanisms taking place during the actual heat transfer in the thermosyphon reboiler.

One item that the theory does not cover or mention is the possibility of reboiler tube and shell corrosion due to condensate contamination with CO₂ or other corrosives. This proposed system works. However, the tradeoff is that the steam and subsequent condensate must be kept very pure. Otherwise, you will have to apply stainless steel materials to the reboiler to fight off any corrosive effects.

Good luck.

Attached Files

•  Condensate Control on Thermosyphon Reboiler.xls   38.5KB   931 downloads

[Art Montemayor]

Jitendra:

My comments to your latest posting are as follows –

• From what I understand of your description (since your schematic is not complete in showing all instrumentation), you propose to try to control the DYNAMIC steam condensate level in the reboiler’s steam chest (shell side). I don’t understand how you intend to do this. I have never seen this done. All that is required is a level controller that works on a manually set point related to the turndown capacity that you desire. I don’t see any thing other than that required.
• I agree that you don’t need a steam supply control valve if you can depend on the steam main remaining constant at the pressure you desire in the reboiler’s steam chest. However, my experience is that I can’t trust that to happen and therefore always install a pressure controller and a valve to ensure that happens.
• You state, “Condensate which is coming out of reboiler is subcooled in maximum duty requirement case” and this is something I don’t agree with. I fail to see how you can have subcooled steam condensate in a gravity drained system – especially in the maximum capacity case. By nature, the majority of all steam heating produces a saturated condensate.
• I don’t agree with your reason for ensuring that only condensate will come out from reboiler by making the condensate level in the design case above the outlet nozzle itself. When the reboiler’s duty requirement is less than its design requirement, the condensate level should be increased to cover more tube surface by throttling the level control valve. This is done automatically through the level controller.
• I don't recommend you equalize your pot with the incoming steam line. As the author of the article you cited states and I show in my schematic, you should equalize the condensate pot's vapor space with that of the steam chest on the reboiler.

Basically, you should stick to what the article you cited says and with what I have sketched out in my workbook. The method of opertion is to manually select a turndown ratio by setting a condensate level height on the reboiler's steam chest.

I hope these comments are of some help.
Good Luck.

[Zauberberg]

Jitendra,

Have a look into the attached article, and also keep an eye on Art's advices. The difference between control schemes presented in the first article and the other scheme by you, is conceptual: the first one relies on manipulating steam supply (condensing) pressure while having condensate pot just to ensure liquid seal (similarly to steam traps), while your scheme involves manipulation of liquid level inside the pot (and therefore in the reboiler).

Manipulating liquid level affects heat transfer in a quite non-linear and time-delayed manner, and probably isn't the best way to do it. Your way of thinking is correct, but the big questions are:

- How much can you rely on calculated pressure drops, especially condensing dP?
- What if something partially plugs the condensate line during operation?
- How flexible is this control system?
- And, as Walt explains in his article, against what kind of disturbances this system is effective?

It's an interesting discussion in any case. Hope to hear some more opinions on the subject.

Attached Files

•  CONTROLLING STEAM HEATERS.pdf   224.81KB   723 downloads