Glycol reboiler too cold

[Aus02]

I'm a process engineer working on a site with glycol dehydration.

The reboiler in our glycol regeneration unit is supposed to operate at 193degC, but we can't seem to raise the temperature above 185 degC or so.

The glycol is heated in a kettle by condensing steam within tubes. Steam supply is saturated at 16.1 barg (204 degC).

There is no control on the steam flow, rather elevation of the reboiler above the steam supply drum and the steam and condensate lines have been sized to allow sufficient flow for the duty required.The condensate just flows under gravity back to the steam drum.

We have considered fouling of tubes, air in lines and other engineers have checked the elevation of the vessels and insulation.

Can anyone suggest what may be happening here? I guess if the lines are undersized there is a problem. Would there also be a problem with oversized lines?

Any help would be much appreciated.

 

[TBP]

What about the glycol side? What's the design flow of the glycol vs the actual flow? How about the glycol inlet temperature?

 

[Aus02]

Glycol flow is quite steady as pumped by PD pumps and is as per design. The temperature is failry steady and any small variation does not seem to align with any variation in the reboiler temperature.

 

[DickRussell]

We need more information. What does the equipment look like? Do you have just the kettle, or is there a stripping column that the kettle is reboiling? If there is a column, does it have trays or packing? What is the bottoms composition and pressure? And which glycol(s) are we talking about? How does the bottom water content compare with what is desired?

Two things that comes to mind when you say you can't get the bottom temperature any hotter. First is that perhaps you are driving out nearly all of the water, and the bubble point of the heavy liquid remaining is approaching that of just the glycol. As the last of your volatiles gets driven out, the temperature scoots up fast. The other thing is (if there is a column above the kettle) that perhaps you aren't getting enough stripping to get the water down to what is desired. On the other hand, if water content is where it ought to be, and temperature is just used to indicate water content for control purposes, then perhaps pressure is too low for the expected temperature. Raising the operating pressure will drive up temperature (assuming the reboiler area is adequate as the approach to steam temperature decreases), but that won't necessarily drive out more water.