11 replies to this topic

[dnrakesh2006]

Hello,

We are designing a new Condensate stabilization Column to get stabilized condensate ( Spec : TVP at 130F 8 psia)

Condensate flow rate 8000 bpd.

we are using steam (at 15barg, saturated) for Re-boiler in coil to heat up the condensate, and we had typical control like measuring the process fluid temperature and feed it to control valve on steam side to maintain the process side temperature.

But one of my senior advised to use by-pass type control as in attached file.

I have doubt will this type of control works for re-boiler since we will not be have any control on boil up rates.

Any body has experience of using this type of control for re-boilers??

Rakesh

Attached Files

•  New Microsoft Word Document.doc   33KB   466 downloads

Edited by dnrakesh2006, 19 February 2013 - 04:49 AM.

[kabtik]

Guess the two control valves are working in split range? We have a similar control for our stripper column and debutanizer reboilers and it has always worked as far as I know. However there are a few differences.

1. Instead of steam we use hot product from a bigger column as the heating medium.

2. A three way control valve is used instead of a by-pass control in your case.

Edited by kabtik, 19 February 2013 - 12:05 PM.

[thorium90]

Is there a particular reason why your senior feels the split range control design is better?

 

The design implies that at 0%, the bypass is fully open and the other valve is fully closed. There will be no flow through the reboiler and the hydrocarbons will remain stagnant while getting heated continuously by 15barg steam. I assume this wouldnt happen very often..

 

I assume the 50-50 split is random too and not based on any real potential issues? The split range controller would likely work, but the controllability would likely be poorer. You might end up simply putting the bypass on manual closed and just using the other valve since the only way to get all the fluid to go through the exchanger is when the bypass is closed which only happens at output >50%

 

(Edit: kabtik just read my mind! I always press save change in case my browser crashes on me..)

Edited by thorium90, 19 February 2013 - 01:18 PM.

[kabtik]

I guess Thorium mean to say you control the amount of condensate leaving the reboiler. Earlier posts have clarified that for such cases it is better to control the heat transfer surface by control the level of condensate in the reboiler and I agree with that.  

 Dnrakesh2006 can use search and you will discover that there are perhaps lots of similar discussions on this forum relating to his question.

Check this link http://www.cheresour...boiler-control/

[SSWBoy]

This would never work for a reboiler. The sketch you have provided is only applicable for sensible heating (ie nophase change) of a fluid with steam.

With reboilers you manipulate the duty of the exchanger by changing the heat input, with the sketch you have provided you are trying to control the heat absorbed on the process side. Not to mention there is the practical problem of having to increase skirt height so that there is pressure drop for the control valves. I believe this is what they say 'a non starter'

An example of where you might use bypass control is if you had a hot water/oil stream as a utility heating a cooler stream. If the temperature of the cooler stream that you desired dropped a little bit you would simply increase the bypass rate on the utility side.

[dnrakesh2006]

Thanks to all for the input,

Reason why he proposed this type of scheme is he wants to utilize the steam sensible heat also along with latent heat. (Putting steam on shell side allows us to immerse some tubes in condensate so that we can use sensible heat)


i am finding very difficult time to convince him why this system doesnt work, even I showed him one of article published ( please see the attached file ).

I gave theoretical reasons why this will not work as mentioned below (since I dont have field exp. of operating this type of system)

1. There is no control on boil up quantity.
2. we should not try to utilize the sensible heat of steam since area required will be more for this and also the amount of heat you get / unit area is also less compared to latent heat.
3. In re-boilers we should avoid film boiling, because of by-pass type control there will be more chances to shifting to film boiling region since we are not controlling heating media.
4. there will be increased scaling problem in tubes ( this I am not sure )
5. Since we are not using steam side this indirectly increase the steam consumption.
6. Re-boiler hydraulics will get affect when the bypass opens since outlet of re-boiler will be two phase flow.


All above reasons are based on theory only, it can be different when it comes to operation some of them can be wrong also,

I wanted to be sure before saying by-pass type control will not work for re-boilers,

If any body using/seen By-pass type of control for Re-boilers in reality please share their experience.

Attached Files

•  Controlling Steam Heaters.pdf   224.81KB   705 downloads

Edited by dnrakesh2006, 19 February 2013 - 09:22 PM.

[Bobby Strain]

If it is not pumped, you will use the first of its kind. Not pioneering; dumb. Use what always works. Connect the TIC controller output to the condensate valve. Use the level control only for override on low level to prevent steam flow into the condensate header. Don't try to use level control in the pot; it won't work. And, the temperature control should probably be on one of the stabilizer lower trays to provide the best control.

But that takes a bit of knowledge to get it right.

 

Bobby

[thorium90]

Perhaps you can quote the following from page 3 of your attachment.

"There are many situations where bypass control cannot be used. Note that the process fluid that passes through the exchanger will experience the full temperature of the steam. If the fluid is liable to coking, polymerization or other damage at the maximum steam temperature, some other form of control must be employed","If the process fluid is being boiled, as in a reboiler on a distillation column, the bypass simply will not work at all."

Edited by thorium90, 19 February 2013 - 11:29 PM.

[dnrakesh2006]

 

Guess the two control valves are working in split range? We have a similar control for our stripper column and debutanizer reboilers and it has always worked as far as I know. However there are a few differences.
1. Instead of steam we use hot product from a bigger column as the heating medium.
2. A three way control valve is used instead of a by-pass control in your case.

 

Kabtik,

Thanks for the input, can please share sketch of the existing system.
How the flow rate of hot fluid is controlled?

Rakesh

[kabtik]

 Debutanizer reboiler.xlsx   15.89KB   486 downloadsOops, I think there is another point that makes my control different from yours. Take a look at my sketch. Thanks

 

[thorium90]

Ahh, so the control is also on the heating fluid. When the three way valve fail close, all the heavy cycle oil will bypass the reboiler. Its definitely alot more feasible than the control scheme from the OP

[dnrakesh2006]

it looks like by-pass type control ( on process fluid ) will not work in case of reboiler. thank you all for sharing the information.