# Why does the partial pressure of carbon dioxide change so much when SG is greater than zero？

I use eos2 to simulate phase transition (phase change)

it is a very simple model to study the Pco2 change.

but it is a little bit hard to explain.

I found SG>0, the PCO2 will change dramatically and weird.

can anyone help me to explain this phenomenon?

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• Very interesting problem, and nice illustrations.  I have no experience with EOS2, but I think what is happening is that for the T=99 case when you have single-phase liquid conditions for all times, Pco2 is telling you the amount of CO2 that is dissolved in the liquid phase, via Henry's Law.  In Cell 2, Pco2 gradually increases as the P difference between cells 1 and 3 drives flow with more dissolved CO2 into cell 2.  There is no gas phase because the combined partial pressures of water vapor (Psat) and CO2 (Pco2) is less than the total pressure.  For the T=100 case, Psat is higher so a gas phase forms, but in cell 2 when T<100, there is not that much water vapor, more of the gas phase is CO2, and thus Pco2 is bigger.  As T increases to 100, Psat increases and Pco2 decreases.  Could you post your input file?  I'd like to try a few things with it, and maybe compare EOS2 to ECO2N, which I have used a lot.  Christine

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• Christine Doughty

Thanks for you explanation.

what should I provide except for flow.inp if I want to share with you my input file?

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• Christine Doughty

Hi Christine,  Thanks for your impatience.

In T=100 C case:

I just wanna know that the PCO2 increases to 0.6 bar and then decreases to 750Pa, it is hard to explain the reasons because the maximum PCO2 at the boundary is 1200Pa.

In my mind, it should be 615 Pa because the left cell is 1200 Pa, and the right cell is 35Pa. but it is 750Pa finally when it is stable.

In a with a gas-phase situation, PCO2 means a percentage of the total gas phase, Ptotal = PCO2 + Psteam; so, it can reach 0.6 bar, correct?

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• Liheng Wang It looks like everything I need is in flow.inp.  Thanks.

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• Liheng Wang  You need to run your simulation longer to get to steady state.  At steady state, Pco2 in the middle element is the same as Pco2 in the high-pressure boundary, for both the T=99 and T=100 cases.  At steady state, the total pressure in the middle element is the average of the high-pressure boundary and the low-pressure boundary, as you intuitively expect.  But remember that you have a steady flow from high pressure to low pressure that is carrying the Pco2 of the high-pressure boundary, so eventually, the middle element has that Pco2 also.

In the T=100 case, when a gas phase first forms there are interesting transient behaviors, which I described qualitatively in my previous post.  Note that for this case, both boundary elements have gas saturation of 1.

Here are quick plots I made of the conditions in element 2 as a function of time.  Note that time is plotted on a log scale.  I used the FOFT option in TOUGH3 to make these plots.  They show mass fraction of CO2 instead of partial pressure, but these are just two different ways of representing how much CO2 is present.

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• Christine Doughty

Great job!