Convergence problems in Phase2

[Liam]

Hello, I'm trying to simulate the speciation of FeS systems and the solubility of a number of iron sulfide phases. I have successfully plotted activity predominance diagrams using Act2, but I am struggling with Phase2. I would like to plot the solubility of individual phases (e.g. mackinawite / pyrite) on pH/pe diagrams. However, I keep getting convergence errors. I have tried following the examples in the user guides, such as that for O2 solubility on a colour plot (this is what I would like for my system), but as soon as I replace the species for those I am interested in, it falls down.

data = thermo.tdat verify
conductivity = conductivity-USGS.dat
temperature = 25 C
H2O          = 1 free kg
Na+          = 50 mmol/kg
balance on Na+
Cl-          = 50 mmol/kg
swap HS- for SO4--
HS-          = 1 mmol/kg
swap e- for O2(aq)
pe           = -10
pH           = 3
Fe++         = 10 umol/kg

scope y
   fix pH
   slide pe to 10
scope x
   slide pH to 11
   fix pe

[Jia Wang]

Hello Liam,

Thank you for posting your Phase2 input file. To troubleshoot your script, I would actually start in the React application to see if the initial system in the basis pane has any issues. You can quickly create a React simulation with your basis by right clicking and dragging the basis pane from your Phase2 input file into the basis pane of a new React instance. If you run the React simulation, you would run into a convergence issue immediately. I think the reason for this is because your starting system is too far outside the range of the stability of water.  My guess is that at such a low pe and pH, you are very far outside the range for the stability of water and thus your model is having a very difficult time converging. You can check this actually by constructing a quick activity diagram in Act2. When I increased the pe to -9, the React simulation was able to run to completion. If I plot the mass of solution vs. the H2(aq) species concentration, I see that the value for H2(aq) is extremely large, which shows that you are far outside the stability range of water. 

 

Please note that unlike the Act2 application, Phase2 is solving a full solution to a multi-component chemical system, much like React. On the other hand Act2, simply solves the equilibrium equations used in assembling an activity diagram. These equilibrium equations can be solved for at any range of pH and Eh. Because Phase2 is essential solving a series of reaction models that include mass balance and activity calculations, it is oftentimes necessary to choose a narrower range for the variable changing in the x and y axis.

 

Hope this helps,

Jia Wang