Diglycolamine Plant Design Question

[jensen41455]

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.

 

[moltenmetal]

If it's a centrifugal pump actually designed for 10 gpm, you should be able to simply throttle the flow to 5 gpm without undue wear and tear on the pump.

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.

 

[EmmanuelTop]

What kind of reboiler you have? Kettle, thermosyphon, forced circulation? And where exactly do you recycle pump discharge?

http://antwrp.gsfc.nasa.gov/apod/astropix.html

 

[jensen41455]

Ok well to begin with it is an electrically driven gear pump. We only get air bubbles in the system when we break the piping apart for repairs (something that happens fairly frequently). The pump is designed to output 10 gpm, but we reduce the flow via bypass to prevent sweeping the DGA out of the contactor and in to our TEG system.

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.

 

[moltenmetal]

Slowing down the SPEED of your gear pump to reduce its output will increase its life. Install a VFD and close the bypass.

 

[EmmanuelTop]

If you have rich solvent flash drum in the plant (it should be there), the simplest and the most efficient would be to route the pump recycle stream to the flash drum.

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

 

[jensen41455]

Thanks for the help guys. I'll look in to a VFD (the company is pretty cheap) and I'll see about running it in to the flash drum.

 

[jensen41455]

Well I checked on the VFD and cannot find a manufacturer that makes a VFD for a Class 1 Div. 2 electric motor (which we use). This has something to do with not being able to effectively and accurately monitor the internal temperature on a sealed motor and Div. 2 motors are not fully labeled for use with a VFD. So, I guess that option is out.

 

[EmmanuelTop]

Examine all potential places where you can recycle fresh regenerated DGA. As said, in my plant we use regenerator tower bottoms as the minimum flow receiver. Perhaps in your case the flash drum would not be 100% suitable - particularly if the recycle flow is significant % of total DGA flow going to the Regenerator. You may affect heat input (reboiler) requirements in a substantial amount.

Other possible routes are: amine surge tank, if it's there in the plant.

http://antwrp.gsfc.nasa.gov/apod/astropix.html

 

[moltenmetal]

We've used VFDs on HUNDREDS of TEFC 3 phase motors in Div 2 locations, so I don't know where you're getting your information from. All you need is a motor which is rated for Div 2 and inverter duty- the VFD itself needs no rating other than just the usual UL/CSA etc. for general purpose electrical safety. You do not need to certify motors and VFDs as pairs unless the VFD is mounted as part of the motor.

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.

 

[jensen41455]

The information on the variable drives I recieved was from Reliance. I'm only going by what I have found.

http://www.reliance.com/prodserv/standriv/appnotes/d7736.pdf

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.

 

[moltenmetal]

What Reliance is saying is presumptuous, as there are PLENTY of vendors who DO supply motors stamped as suitable for use in Div 2 under inverter duty.

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.

 

[moltenmetal]

Another idea: if it's a 3450 rpm motor, simply switching to a 1725 rpm motor will get you there too...no VFD needed! You'd need to check your gear pump's performance curve to be sure, as they all have some "slip".

 

[atayto]

I think both ideas are ideal you need to choose one of them. You can contact ABB company they have enourmous chose of VSDs. Changing 2 pole motor to 4 pole motor is another good idea as it was recomended by moltenmetal. The major think which I recomend is to get rid of operation with open bypass valve. It is unsafe. what will hapen if high pressure gas will go to lower rating pump suction side when pump stops. Gas blow by, explosion, fire. I suspect it already happens if you find gas in pump suction. My preference to use VSD or change motor with different speed and use bypass only during start up. I would suggest lock this valve closed after start up

 

[dcasto]

by throwing 5 gpm of cold amine into the reboiler, you just increased the duty SIGNIFICANTLY, you can not do that without the the promblems you have seen.

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.

 

[maddocks]

The only effective way is to reduce pump throughput. Recycling the excess DGA back to the reboiler will load the stripping system resulting in a huge reboiler demand, increased tower traffic and now, a huge load on the condenser. If you cycle it back to the rich flash tank, the effect is the same. You simply have to reduce pump flow. The VFD option is really the best way to deal with this. Alternately, you could recycle the pump back to a small flash vessel, degas the DGA to the flare header and let the pump take the now de-gassed liquid. Seems expensive and troublesome.