Reboiler steam pressure 1

[Waleed217]

Hi,
I phase one problem in my plant with column reboiler steam pressure it was 46 to.50 kpa but it start increased day by day until reach 380 kpa.
We try to drain condensate drum to bring pressure down but still with low condensate showing high pressure.
So, we redusing our reflax flow and steam flow but we did't get any improvment.

It will be my pleasure to heard from you any solution.

Best regards.

 

[Waleed217]

For your info we use mix C4 as feed and when pressure reach 380 kpa steam control valve output increased from 39% to 100%.

 

[TD2K]

A reboiler operation is governed by Q = UAdT. Any change in one variable has to be made up by a change elsewhere.

If your steam side pressure is coming up, the reboiler is trying to compensate for some change by increasing the steam pressure and therefore its condensing temperature to get the required heat transfer.

You said you have tried reducing the reflux so it doesn't sound like you've changed operating conditions to require more duty out of the reboiler.

Things I would initially check are:

1. Rule out any change in process feed composition or yield quality. That can change the bottoms temperature and therefore the dT, the required duty, etc. You may not have a problem but rather pushing up against equipment design limits.
2. Check that non-condensibles haven't built up on the steam side. They can significantly reduce the temperature the steam condenses at. You likely want to look at the exchanger drawings to see where non-condensibles could build up and hopefully you have a vent valve in that area.
3. If you can check the temperature over the length of the steam side, that might help you rule out any condensate backing up in the exchanger.
4. Double check measurements, you don't want to be troubleshooting only to find out that a temperature you are using is incorrect.

Finally, has the reboiler had a history of fouling? A increase in fouling on either side is going to require a higher steam pressure to compensate for it.

 

[sshep]

Dear Waleed217,

I have had similar problems several times in my career, such as thread124-66970 (2003, this forum), which had a variable area reboiler equiped with both a steam condensate level valve and a steam control valve- allowing for the control of any two of steam flow, chest pressure, and condensate level. The symptom is steam valve 100% open, chest pressure at the steam supply, and no heat going into the process. After diagnoses of the steam and condensate side, you are left with the conclusion that there is a problem in the heat transfer on the process side. This is straight from the Q=U*A*MTD relation, but leaves the question of why U has dropped.

Fouling is one possibility, but if you can restart this thing, then film boiling may be considered. In the film boiling condition, the tube wall temperature takes on the condensing temperature of the supply steam where as the design tube wall temperature is something lower. This effectively means that there are two stable but very different operating conditions: design and film boiling. Film boiling leads to a poor U value which the controller tries to compenstae for by higher MTD, achieved by higher steam condensing temperature (you see as chest pressure). Fluxuations in the steam system can kick you into film boiling and it is unrecoverable without intervention due to controller wind-up.

I am suprised that Lieberman and others have not discussed how to get out of the condition. The operating fix is to what the chest pressure outside while either: 1) putting the steam control valve in manual and closeing it, or 2) reducing the steam flow setpoint as required below the actual flow (which also closes the valve via control). Keep doing this until the chest pressure gets below the expected normal value- i.e. when it was running normally. The tower will slump during this period due to reduced heat input, but the heat input will shoot up when you have crossed from film boiling to design boiling, then you can raise the steam rate again.

Maybe this isn't your problem, but I have seen it so often that I think it could be, especially given the chest pressures you cite and the light nature of your process side.

best wishes,
Sean Shepherd