Dose rate of Triethanolamine for TEG gas dehydration on platform?

[QCorrChemical]

To maintain the pH (6.5-7.5) of the Gas Dehydration system, pH balancer Triethanolamine (TEA)was recommended.

Can anyone advise on how the treatment dosages can be
determined? Can we take a sample and run a titration to see at what volume the pH is best maintained?

Please let me know.

TEA is weak base with pKa of 7.8 @25c.

Thanks

 

[Zoobie]

Could you not use a pH probe? I would think that this could be installed as an on-line instrument. Your dosing pump could work off of this.

Just a thought.

 

[QCorrChemical]

what type of on-line instrument (pH???)can be used since concentrated TEG/water mixture might interfere traditional pH measurement?

Is 0.2g-TEA/liter-absorbed-water a good starting estimation?

Thanks.

 

[Zoobie]

I don't know the answer to your question. You say 'might interfere (with) traditional pH measurement'. Have you confirmed this? Have you spoken with a vendor/manufacturer of probes/analyzers? Its been a while since I have dealt with pH instrumentation (I have changed industries) but I know that not all are the same in terms of quality of measurement and robustness/tolerance of unusual/harsh environments.

pH measeurement might very well not work in this application. I just threw it out there as a suggestion. Good luck!

 

[Chemeng1122]

I dont think a pH probe would work in this service because to accurately measure pH of TEG, it has to be diluted with water (usually ~50% water).

Andrew Lechelt
Technical Support Engineer
Quadra Chemicals

http://www.quadrachemicals.com

 

[Zoobie]

Thanks alechelt. I have never tried to measure the pH of TEG. I wonder if an on-line sample could be withdrawn and diluted, then measured. I have seen on-line auto-titrators that used some dilution in sample preparation. Could something like that work? Not like I need to be asking these questions....its not my TEG unit that is having the trouble.

 

[QCorrChemical]

Thanks for the discussion.

First, it is kind of difficult to really simulate or calculate the pH of glycol/water mixture in the TEG gas dehydration system since pH is effected by many factors:

1) CO2 or other acidic gases in the gas phase from upstream (TEG is a very weak acid - 50ppm as HAc) [particularly, CO2 in gas phase serves as a H+ reservoir in the liquid phase]
2) The amount of Water absorbed into TEG

Second, TEG is probably so concentrated (i.e. not aqueous) that triditional pH measurement would be interfered by the absence of water or little water. But I think we can still use titration method to determine the initial dose rate if proper pH measurement is adopted and water content is probably estimated/measured.

The problem is: it is hard to simulate the real scenario in the contractor. The pH of TEG/water will be different if the titration is operated at atmospheric pressure/temp.

I am still looking for solutions. Please share your experiences. Thanks.

Regards,

Victor

 

[Chemeng1122]

Maybe lets try to understand the issue at the heart of the matter.
Why do you require ongoing pH monitoring and adjustment? Is it a sour gas that you are treating? Is there oxygen in your gathering system that is degrading the glycol?

We have dealt with many sour dehys that have ongoing pH control issues. The recommendation to them is to a have a weak caustic solution on hand to maintain the pH >7 on a regular basis (shift change). The recommended treat rate is about 0.5 vol% for them, provide adequate time for mixing, check the pH and re-treat if necessary. This is hopefully done on a regular basis, but I cant say for certain (we are the glycol supplier, we dont operate the plants).

Of course this acid neutralization process produces salts in the system (acid-base) reaction, depending on the acid in question, these salts can cause probolems in the long run. We have found that because of this, sour dehys or other plants with pH control issues require glycol replacement and plant flushes to prevent significant fouling issues and associated corrosion problems.

The risk of using an amine for pH adjustment is that they are generally weak bases which can be an issue, particularly if you are processing a sour gas. In the reboiler, you can "regenerate" the amine, break the amine-acid bond and emit sour gasses in the overheads, which is not only dangerous, but very corrosive to the TEG regen system, which is generally carbon steel. It is for this reason that we generally recommend a weak caustic solution (stronger base - will stay bonded with the acid at the higher temperatures).

Andrew Lechelt
Technical Support Engineer
Quadra Chemicals

http://www.quadrachemicals.com

 

[QCorrChemical]

Thanks for the information, Andrew.

This TEG system does not contain sour gas (H2S) but 1 to 2 vol% of CO2.

High pH will case foaming in the glycol contactor resulting poor dehydration. It is requested to maintain the operation at optimum pH 6.5 to 7.5.

I am still not clear where/how you get the liquid sample and how you check pH which is strongly affected by the CO2 content in the wet gas?

Are you suggesting a batch treatment of a weak caustic solution to TEG? or 0.5 vol% of TEG recirculation rate?

Thanks.

 

[monaco8774]

QCorr

I work in operations presently supporting 19 TEG systems in different parts of the world, here is what we do if its any use.

We regularly measure pH we take a sample of the TEG, dilute 50% with pH neutral distilled water, let the temperature go to ambient and use a pH probe.

Very rarely if ever does any of the pH drop below 6.5, (nearly all of them have a tendency to go alkaline, despite everything I've ever read telling me the opposite?) we have CO2 and H2S on many of these facilities maximum 600ppm H2S and 8% CO2 even on these pH remains fine mainly because of the TEG losses and replacement with fresh TEG. However it does sometimes happen. When it does we also use TEA or another alkyl primary amine but the main reason for that, I think is its convenient for most of the suppliers as we use it onboard as a gas side corrosion inhibitor ( that may be another reason why the TEG rarely goes acidic, who knows?.)

Interesting about the preference for weak caustic acid (never stop learning in this game), but I think that since we may only add so infrequently it would be logistically impractical to try and store the additional chemical offshore.

 

[Chemeng1122]

QCorr,

Have you experienced foaming with high pH or is this something you haev heard/read/etc..? Pure TEG had a pH in the 7-8 range, it is my experience that operating with a higher pH does not mean for certain that you will experience foaming, rather it is an indication that you have some sort of alkaline contamination in the system - which may or may not increase the foaming tendnecy. If the contaminant is a corrosion inhibitors (which are surfactants, often filming amines) then it is likely that you will see foaming.

Taking a lean sample off the pump discharge or a rich sample before passing through any heat exchangers is recommended, mainly for safety reasons. Taking a lean sample off the accumulator can be dangerous. As you have CO2 in your gas, it is likely that your rich pH would be slightly lower than the lean because of CO2 solubility in the glycol.

Yes, we recommend 0.5vol% batch treatments for pH adjustment. I should mention as well that we have also recommended amines for pH adjustments, but you have to be careful in TEG service as reboiler temperatures are typically well above the degradation temperature for most amines.



Andrew Lechelt
Technical Support Engineer
Quadra Chemicals

http://www.quadrachemicals.com