H2S equilibria

[sevy]

Hello,

I am working on modelling a neutralization tank where a strong acid is dosed. This will of course lower pH and shift the sulfide equilibrium. I want to specifically know how much H2S (g) will be produced in the headspace of my tank.

I have started by modelling the reaction by titrating my caustic solution and letting it interract with a GAS phase with a fixed volume. This is true in practice but what happens is the pressure increases in the headspace, which in reality does not happen because in my case there is an offgas vent to prevent pressure build-up beyond 1,5 bars. I noticed that pressure build-up and constant volume affects my H2S gas-liquid equilibrium. Does anyone have any suggestions or comments to help me generate reliable results?

Any response would be much appreciated!

PS: I tried to equilibriate with a constant pressure GAS phase but the method won't converge. Besides PHREEQC defined this option for gas bubble which form and stay dissolved so I do not think it is the right way to go for me anyways...