Diglycolamine Plant Design Question
Diglycolamine Plant Design Question
(OP)
We run a DGA/TEG pretreatment system at our LNG plant and I've been trying to convince my boss to make a few changes to it to make things better. It is only a 5 GPM system, but the pump we use puts out 10 GPM. We use a bypass to regulate the flow in to the contactor. The bypass used to be routed right back to the suction of the pump, but that caused very short pump life because any air bubbles in the system would just get circulated and never burped out. I convinced him to move the bypass back to the reboiler and while it extended pump life, it caused a multitude of problems including low reboiler temps, little to no reflux, and I believe it is overloading the TEG with water because we are passing quite a bit downstream in to our gas compressor scrubbers.
Would there be a more suitable place to put the bypass? I was thinking downstream of the flash drum just before our lean/rich DGA exchanger on the rich DGA side. Any help would be greatly appreciated.
Would there be a more suitable place to put the bypass? I was thinking downstream of the flash drum just before our lean/rich DGA exchanger on the rich DGA side. Any help would be greatly appreciated.





RE: Diglycolamine Plant Design Question
If it's a pump actually designed for 50 gpm, being operated on the left side of the curve already at 10 gpm, spilling 5 (or more) gpm back to somewhere on the suction side is likely necessary for decent pump life and stable performance.
If it's a positive displacement pump, you need a VFD or a stroke adjustment.
If the concern is vapor buildup in the pump, the only places to fix that are at the pump suction or the pump discharge piping. The recycle per se has nothing to do with it. You need to alter the piping so that gas bubbles leave the system via the discharge or don't enter at the suction.
RE: Diglycolamine Plant Design Question
http://antwrp.gsfc.nasa.gov/apod/astropix.html
RE: Diglycolamine Plant Design Question
The reboiler is a kettle reboiler, and the bypassed DGA from the discharge of the pump is going right back in to the top of the reboiler. My idea was to re-route that to just upstream of the rich/lean DGA exchanger on the rich DGA side. I haven't experimented with it yet, but I'm wondering what kind of results I can expect from that. Good thing or bad thing?
I appreciate any help. I'm working with what I have, but it's not much.
RE: Diglycolamine Plant Design Question
RE: Diglycolamine Plant Design Question
I don't know what is the reboiler/tower configuration in your case - perhaps you are initiating operation instabilities by invoking some kind of additional interaction between the reboiler and the column. In my plant, booster pump minimum flow is directed to the tower bottoms, not to the reboiler. It doesn't sound like a good idea to me.
If there is a rich DGA flash drum, you have solved your problem.
http://antwrp.gsfc.nasa.gov/apod/astropix.html
RE: Diglycolamine Plant Design Question
RE: Diglycolamine Plant Design Question
RE: Diglycolamine Plant Design Question
Other possible routes are: amine surge tank, if it's there in the plant.
http://antwrp.gsfc.nasa.gov/apod/astropix.html
RE: Diglycolamine Plant Design Question
WEG and numerous other motor manufacturers make suitable motors, which come fully stamped for the area classification and for inverter duty. Most will state a turn-down ratio covered by the stamp: our normal turndown standard is 10:1 for a PD pump (i.e. at constant torque). In your case, you will be turning the motor down by 50% or less, so it's not a major issue.
The VFD itself would normally go in your MCC room rather than in the field. If it needs to be field-mounted in the Div 2 area you will probably need to put it in an NFPA 496 Z-purge box.
RE: Diglycolamine Plant Design Question
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We don't have an amine surge tank, but I believe I'm going to build some piping to try plumbing it in to the flash drum and if it doesn't work I'll switch some valving around and go back the other way.
RE: Diglycolamine Plant Design Question
Give WEG a call. Changing a motor and adding a VFD is going to make your pump last longer AND solve your process problem rather than merely moving it elsewhere.
RE: Diglycolamine Plant Design Question
RE: Diglycolamine Plant Design Question
RE: Diglycolamine Plant Design Question
NOW for the real lesson, reboiler temperature is meaningless on an amine unit. What is the boiling point of amine-water? Does that temperature change? can you tell you temperature controller to raise the temp on a pot of water boiling to 250F? NOPE, the temperature is fixed. Your amine reboiler (stripper) needs a defined BTU/hr input. Replace the temperature controller with a fixed heat input control. If it is gas fired, set the burner pressure to the pressure required to get you your duty.
RE: Diglycolamine Plant Design Question