Vapor fraction and H2S partial pressure 1

[CoCoE]

I would like to seek the opinions on below things.

In a slug catcher,

Top: Gas line (with H2S partial pressure approx 15 psi, CO2 approx 25 psi)
Bottom: Sour water (Zero vapor fraction, 99.8% water, 0.02% H2S+CO2)

In such a condition, for selecting the material for the bottom piping, how to consider H2S partial pressure? I feel that I must consider the Top gas piping process parameters as equilibrium gas phase. But some people conflict this by saying as there is no vapor fraction, pH2S is very very low, and this is completely in aqueous phase.

What is the correct method?

Please advise.

 

[georgeverghese]

Think this is a misinterpretation - if H2S partial pressure of equilibrium vapor is 15psi, then materials for both vapor and liquid side should be good for H2S SSCC.

Moreover, NACE 0103 says if pH of aqueous phase is less than 4, then sour service materials should be used.

 

[CoCoE]

Service is sour service, but the problem is, to what extent it is sour and which partial pressure we need to take into consideration

 

[EmmanuelTop]

Partial pressure of H[sub]2[/sub]S refers to vapor phase (NACE MR0175 Annex C). The same Annex (C2) shows how to calculate the effective vapor pressure of H[sub]2[/sub]S for liquid-only systems containing Hydrogen Sulfide. Annex D shows how to determine pH of aqueous solution based on H[sub]2[/sub]S and CO[sub]2[/sub] partial pressures.

Dejan IVANOVIC
Process Engineer, MSChE

 

[CoCoE]

Part C2 is applicable "If no equilibrium gas composition is available". My question is, can we consider the top connection (sour gas ) line as the equilibrium gas composition for the sour water (bottom connection), even though no vapor phase is availbale in the sour water piping? (99.8 Mol% aq.).

 

[EmmanuelTop]

Separator vapor composition is the equilibrium composition.

Dejan IVANOVIC
Process Engineer, MSChE

 

[CoCoE]

So I hope even though there is no vapor phase, we need to consider the equilibrium gas composition?

 

[SurajTinani]

Yes CoCoCa, you need to consider the equilibrium gas composition. Your H2S/CO2 concentrations in vapor phase of the liquid line will be same as the gas outlet of the slug catcher. You can verify this using Hysys or Unisim softwares.

 

[CoCoE]

How to calculate the bubble point pressure for 100% liquide phase lines? WHat is the widely adopted formula for finding it out?

 

[SurajTinani]

You can get it from the simulation softwares Hysys or Unisim. ISO 15156 annex C says that a good approximation for bubble point pressure would be the pressure of last gas separation, which if you verify using the above softwares would tend to be very close.

Regards,

Suraj Tinani

 

[CoCoE]

Thanks Suraj ji for your inputs.