9 replies to this topic

[mwaqaskhanbhatti]

Dear members my critical and urgent query is as follows,

 

I am working as a process engineer on a Gas sweetening Plant using MDEA as sweetening agent.The problem we are facing is excess TDS in our lean MDEA .i.e. more than 3000 ppm as the measuring range of our Instrument is not more than 3000 so it can be more than this.This problem occured due to carrying over of sour water in MDEA Absorber.

 

Why this problem occured;

Our wells natural gas composition is

72 % methane,

21 % Nitrogen,

4 mole % water

H2S; 400 PPM

 

Gas relative density is .6780

Now the case is our well is depleting and amount of water is increasing day by day currently our plant running on 2MMSCFD with 2000 bbl sour water having following specs

Formation /sour water

TDS= 33000 ppm

Chlorides=26000 ppm

 

Now the issue is what should be done?Either all the volume of MDEA should be drained in order to save SS Heat ex-changer and Coil of MDEA cattle type reboiler. Please brief any recovery method available to lessen this affect and also discuss the future problems which can occur if we continue using this high TDS MDEA in our Amine Unit which includes , Plate type ex-changer,SS heat exchangers,MDEA cattle type reboiler,Booster pump,Charge pump and off-course bubble cap trays of amine absorber.Please suggest the best way out.

[Art Montemayor]

In order to operate a gas sweetening operation it is IMPERATIVE that the unit have and employ an efficient separation of entrained liquids and solids PRIOR to the raw gas entering the amine absorber.  This was true 60 years ago and is still true today.  To employ an inefficient liquids and solids separation prior to the absorber is to propagate trouble and eventual shutdown in the sweetening unit due to contamination and poisoning of the MDEA solution.  That is why I can bet my years salary that your unit's operation and maintenance manual states that you must employ PURE water as the diluent in the amine solution and that you must protect it with efficient pre-separation.  I am certain that you are not allowed to employ dirty, contaminated, or entrained gas entering your absorber - or the warranty on the process and license that you have will be voided.

 

You simply cannot allow any free liquids or solids to enter the MDEA absorber - period.  Therefore, the simple and direct answer to your query is to follow that simple rule:  employ an efficient separation device that will ensure that requirement.  If you don't have one installed at present, then procure and install one.  Additionally, you should be employing absolutely pure water to prepare or add to your MDEA solution.  re-distilled water is one source that you can use to guard and preserve your solution in a clean and required state.

[mwaqaskhanbhatti]

Art Montemayor,

 

Sir i appreciate your suggestion and let me clear that we have installed filter media before gas stream entering the absorber i.e three phase production separator as well as inlet filter separator. Actually this happened due to sudden flow diversion from well site to plant site .The velocity  and amount of the water  was so much that entertained liquid carried over passing both the units three phase production separator as well as Inlet filter separator and residing at the bottom of the absorber.Here its important to mention that when MDEA unit was started up for sweetening purpose the circulation of MDEA started.As I already mentioned that the resided water after passing the Flash tank and Particulate and Charcoal filter entered the stripper from where it mixed with MDEA reboiler already present Mdea Solution.

Before this ,the above mentioned problem never came as entrained water never carried over as it did this time. and also we already use Deminerlized water for MDEA solution to make up level.

My concern is about knowing the affect of this high TDS & Chlorides containing water in All the MDEA unit.How much its impact would be on any of the equipment like excahngers,absorber SS bubble cap trays or Reboiler Coil.

 

Before this incident our TDS in lean MDEA were used to be in the range of 900-1000.Also please tell the allowable TDS limit for leam MDEA.

[Pronab]

If possible maximize the flow through the charcoal filter, since already MDEA solvent has been contaminated. There are on-line reclaim system which can be a purification process of MDEA solvent.

[Zauberberg]

Since you experienced this slug in the inlet separation system, the equipment could have been damaged and making things worse ever since the incident took place. It is probably a good idea to perform inspection of the separation system.

 

33,000 ppm of TDS is a huge number, indicating contamination with salts/formation water. You either need to reclaim MDEA or to fill in a new batch of amine. You are at significant risk of high corrosion rates.

 

Attached are technical papers on amine contamination. You can perform lab analysis of your amine and see where you are. I would say that 1,000 ppm of TDS is generally a good number but it very much depends on what are these contaminants. All amine suppliers recommend maximum 100 ppm of TDS for the makeup water.

 

 

Attached Files

•  contaminant-reporting-in-amine-gas-treating-service.pdf   219.77KB   128 downloads
•  Hc Processing Mar 96-Apr 97.pdf   133.45KB   72 downloads
•  solvent-quality-guidelines-tech-bulletin.pdf   27.34KB   68 downloads
•  TP-194-Effectiveness-of-Continuous-Removal-of-HSS-from-Contaminated-amine-scrubbing-solutions.pdf   596.67KB   85 downloads
•  hc processing apr-may 1993.pdf   104.69KB   66 downloads

[mwaqaskhanbhatti]

As this solution of MDEA having strength 10 % is residing in reboil-er and circulating in all the MDEA regenration Unit.What kind of test procedures should be done in order to decide whether to drain this solution or keep it as such.The only thing I can see is the TDS above 3000.Morever can anybody tell that what is the value of conductivity of MDEA solution ,which shows its in safe range or not?

[Zauberberg]

See attached reports from my previous post, they show an extensive list of amine contamination parameters and their influence on amine system.

 

It would be good to engage the lab in determining the most critical parameters, such as Chlorides or Cyanates. If the plant is designed and built from 300 series stainless steels, there is a limit on maximum Chloride concentration. Exposing austenitic stainless steels to Chloride concentrations which are above the maximum recommended limits can lead to a very unpredictable and furious metal cracking with very bad consequences.

[Bobby Strain]

There are a few companies that will bring equipment to your facility and clean your solvent. This might save you some money.

 

Bobby

[Zauberberg]

I asked our corrosion expert with regards to allowable Chloride levels. Stress Corrosion Cracking (SCC) can occur if Chloride levels exceed 500ppm in the amine solution. Typical areas of attack are reboiler tubes, rich/lean exchangers, and some regenerator sections. To minimize SCC, exchanger U-bends of the reboiler are usually solution annealed and no mechanical straightening is permitted after the stress-relief treatment.

 

You can do a quick check on Chloride content and proceed with corrective actions discussed in previous posts. SCC is very unpredictable and may catch you off-guard, so we recommend a prompt action.

[Irfan Ahmed]

Regularly drain reflux accumulator also provide an option for adding low TDS water in Reflux Accumulator. Raise reflux temp upto 150F. Hope this will work.