Corrossion in absorber

[Sina]

Hello

Our Feed gas in gas sweetening plant has 1200 PPM H2s and 2 mol% CO2.The Amin is DEA 32 wt%. the absorber has 32 trays. maximum temperature in absorber is 102 deg C. the trays are S316 and the body of absorber is carbon steel. we have a corrosion among tray 18 until tray 22, the body and the trays are corroded. in your opinion what kind of corrosion we get into? and what we should do to mitigate or eliminate this corrosion.

Thank you in advance.
Regards.

 

[0707]

It seems thst we are speaking of the feed zone of the column, am I wright?

 

[pradipgoswami]

Hi Zakarya,

This is possibly Amine corrosion.

According to API RP-571, clause 5.1.1.1.1 this general and localized corrosion occurs principally on carbon steel in amine treating processes. Corrosion is not caused by the amine itself, but caused by dissolved acid gases (CO2 and H2S), amine degradation products, Heat Stable Amine Salts (HSAS) and other contaminants.Corrosion rates increase with increasing temperature, especially in rich amine service. Temperatures above about 220°F (104°C) can result in acid gas flashing and severe localized corrosion due to 2-phase flow.

Clause 5.1.1.1.6, refers to Prevention / Mitigation methods. The common recommendation are:-
- avoid the buildup of HSAS to unacceptable levels
-AND OTHERS AS RECOMMENDED.

By and large SS 316 resists amine corrosion, however presence of salts could deteriorate the scenario.Look at figures 5.1 through to 5.6, for illustrations.A laboratory analysis of deposited products may be useful for effective for future mitigation.


Thanks.



Pradip Goswami,P.Eng.IWE
Welding & Metallurgical Specialist
Ontario,Canada.
ca.linkedin.com/pub/pradip-goswami/5/985/299

 

[SJones]

It would be more helpful if the description of the stainless steel "corrosion" were to be more descriptive. Generally, the principal suspect in this scenario would be chloride contamination. Primarily in the feed gas and then recirculated in the loop. If the carbon steel items in the system are not suffering undue metal loss, this would reinforce the suspicion. Are you sure about the absorber temperature - it's not too far removed from the regeneration temperature and seems high for efficient and effective absorption? If you are losing stainless steel in the absorber, how is the regeneration column, reboiler, and lean-rich exchanger?

NACE Corrosion 1997, Paper 340

Problems with absorber corrosion do not appear in case histories of 21 plants operating with DEA discussed in European Federation Of Corrosion, Publication 46.

Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/pub/8/83b/b04

All answers are personal opinions only and are in no way connected with any employer.

 

[0707]

I agree with SJones and his aproach.

 

[Sina]

Dear Steve Jones

Thank you for your answer.
I attached a photo although this is not cearl. feed gas Temperature is 50 deg C. lean Amine Temperature is 55 deg C. maxium Temperature in absorber is 102 deg C in tray 20. the cl ion in amine is 9-10 PPM. gas flow rate is 13 MMSCMD and amine flow rate is 355 M3/hr.
our regeneration tower has a cladding of stainless steel. and heat exchager is carbon steel. outlet rich amine temperature is 100 deg C. we have corrosion in heat exchanger but not as high as absorber. do you think we have galvanic corrosion in absorber? because acid gas disolved in amine and an electrolyte solution is made. byond this area (tray 1-17 and tray 23-32) amine not strong electrolyte. Although in galvanic corrosion one metal is corroded, as I read in some references in some cases both metals are corroded.

 

[Sina]

Dear pradipgoswami

Thank you for your answer.
would you please upload here the API RP-571?
Thank you in advance.

 

[georgeverghese]

Tray 20 temp of 102degC seems too close to max recommended 104degC quoted by PradipG from API 571. Maybe increasing circulation rate >355m3/hr would help to bring down this temp.

 

[SJones]

Apart from the missing stud, how exactly is the corrosion issue being described in the inspection reports: pitting, thinning? If it were galvanic corrosion, then it would be the carbon steel affected. If it's thinning, then with 10 ppm chloride, the only other explanations would be very low pH; in which case the carbon steel would be suffering a lot more, or an erosion problem. I was expecting the tray to look like a lunar landscape. Is the heat of solution of the acid gases in the solvent sufficient to raise the temperature by 50 deg C? I'm still struggling with that one

Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/pub/8/83b/b04

All answers are personal opinions only and are in no way connected with any employer.

 

[rscosta1976]

Hello,

I agree with Steve. The temperature of the absorber seems to high. A typical temperature would be from 75 to 85 deg C. Having more than 100 deg C in the absorber will contribute to the overall increase of amine corrosion rates. Are you able to keep the gas within spec with this temperature?


Please consider the following questions:

1) where is the corrosion located? Is tray 1 on the bottom or top? Does your absorber has a water wash section or amine wash only?

2) From your description, I'm assuming the column has no cladding on the bottom section. Is this correct?

3) What was the problem exactly? From the picture, it's not clear what type of degradation you have. The stainless steel trays seems is good conditions (as expected) and it is not possible to understand from the photos if you have any galvanic corrosion. Please make a more detailed description of the corrosion you found. Is it general thinning or it's clearly related to velocity and turbulence in localized areas? What is the corrosion rate since last inspection? Do you have galvanic corrosion on tray support rings and/or downcomer brackets?

4) The chlorides level is low and that should not be an issue. In fact it seem too low. Chloride level (measure in lean amine) in our units is usually higher.

5) What is the rich loading, pH, HSAS and level of other amine degradation products (very important to determine the actual rich loading)?

6) you mentioned the gas feed rate and amine circulation rate, but you didn't provided design values. You need to understand if you have a velocity / turbulence issue.

It is very hard to help you without understand the full picture. However and just as a comment, please consider: the gas composition may be considered sweet with the H2S/CO2 ratio being very low. In these cases you should always expect more severe corrosion when compared to systems where you have a H2S/CO2 ratio higher than 0.5. With this gas composition and temperature over 100 deg C, you should expect high corrosion rates for un-cladded carbon steel. If you add other possible issues related to excessive flow/velocity, high loading (>0.35 mol/mol), high HSAS and low pH, than, the situation will not be nice.


Regards,

 

[georgeverghese]

If you cannot increase circulation rate to reduce temp, consider adding one or more amine intercoolers at the absorber.

 

[Chumpes]

Dear all
I agree, temperature > 100°C (!) is too high in the absorber, I have in mind 50-70°C max for efficient absorption
it is a CO2 removal unit (sweet) with very low H2S (ratio<1:20), thus carbon steel will suffer high corrosion rates in rich amine.
stainless steel 316L should be a better choice for rich amine.