pCO2 in water outlet from separator

[Bain89]

Hi Guys

I think this is straight forward but wanted a second opinion.

In a three phase separator, the inlet is a mixture with a specific pCO2.

Regarding the water outlet line, the water obviously contains CO2 and is corrosive. For the purposes of modeling, is the pCO2 the same as the inlet?

Thanks
R

 

[SJones]

How does the selected corrosion model advise you to calculate pCO[sub]2[/sub]?

Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/in/drstevejones

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

 

[EdStainless]

Likely not, as the CO2 will not partition equally in the three streams.

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P.E. Metallurgy, Plymouth Tube

 

[Bain89]

Thanks guys. We are still selecting a model, but probably ECE or Norsok.

But either way, I can use Henry's law and the amount of CO2 in the water phase while in the separator. Then the water is drained off, and I assume this concentration of CO2 will not change?

Thanks again guys

 

[Chumpes]

hello
The inlet fluid is a 3 phase fluid (liq, vap and water) and we must consider each phase at equilibrium with one another.
If you are able to calculate a partial pressure CO2 (ppCO2) into the 3 phase fluid at inlet, then the water phase (which is corrosive) has a certain activity of dissolved CO2 that corresponds to the equilibrium of CO2 dissolved into water with the partial pressure CO2 into the vapour phase (and also with the CO2 dissolved in the HC liquid phase, but we won't talk about it).

which means, if you simulate a depressurization of the water containing CO2 at equilibrium with the CO2 containing liquid/vapor HC phase, (ex : minus 0.1 bar) then a bubble of CO2 will form and that bubble will have (almost) the same composition (ppCO2) as the gas phase the water was at equilibrium with in the upstream 3 phase separator drum...

so yes, you have to consider that the sour water stream is loaded with dissolved CO2 with concentration corresponding to the ppCO2 in the 3phase separator drum, even if you don't actually have a true vapor phase in full liquid sour water stream.

regards

 

[SJones]

@Bain89 - why bother with calculations when the two models that you have named will do it for you? Select a single model and study the manual for it in depth - it will save you time and grief. Don't fall into the trap of running every single model that you can lay your hands on and then wondering why they come up with vastly different results.

Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/in/drstevejones

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

 

[Chumpes]

His problem is that his model does not give any equivalent ppCO2 (parameter required for evaluation of corrosion by wet CO2) for full liquid systems =) not having a gas phase does not mean the activity of CO2 gas or wet CO2 corrosion is nil.
The same problem exists for partial pressure H2 in full liquid HC stream for evaluation of HTHA (refer to annex of API RP 941 for this).

 

[SJones]

They don't need it with those two models using gas phase data in the separator. The models will take care of the CO[sub]2[/sub] partitioning into the water and spit out a carbon steel corrosion rate. It's a good enough approach without a process engineer running expensive software on the water system to get a virtual gas phase.

Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/in/drstevejones

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

 

[cheok_619]

The CO2 content of the full liquid stream will be determined by the partial pressure of the last gas phase in contact with the fluids.

So i will take the partial pressure of the separated gas stream.

 

[Corroneer]

@SJones: but what if we want to “estimate” the partial pressure without any package … is there a way?

 

[SJones]

@Corroneer - for a liquid full system, the "estimate" is as per the discussion above and the summary by @cheok 619. Where there is a gas phase, the usual mole fraction x system pressure will apply. If there has not been a gas phase in contact with the liquid, then it's down to fancy software.

Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/in/drstevejones

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

 

[Corroneer]

@SJones: thanks for the explanation.