NH3(g) - water equilibrium under increasing pressures

[C3]

Hi,

I'm trying to simulate the equilibrium between NH3(g) and water under increasing pressures, from 1 atm to 10 atm at 25 degrees. However, the model fails to converge. I'm using the amm.dat database and have tried to simulate the equilibrium using SOLUTION and EQUILIBRIUM_PHASES.

Is there a way to do this?

Thanks in advance for any help.

[dlparkhurst]

I don.t think you can do it with PHREEQC. The solubility is too great and convergence fails. As a guess, physically, you may produce liquid ammonia.

You could do it with equilibrium phases; I used a very large gas phase, but the results should be the same. Solubility at 0.3 atm is 18 molal.

Code: [Select]

SOLUTION
END
GAS_PHASE
-fixed_pressure
-volume 1e6
Amm(g) 1.0
END
USE solution 1
USE gas_phase 1
REACTION_PRESSURE
1e-4  1e-3 3e-3 1e-2 3e-2 1e-1 3e-1
USER_GRAPH 1
    -headings               p TOT(Amm) Amm(aq) AmmH+
    -axis_titles            "Pressure, atm" "Amm, molality" ""
    -axis_scale x_axis      auto auto auto auto log
    -axis_scale y_axis      auto auto auto auto log
    -initial_solutions      false
    -connect_simulations    true
    -plot_concentration_vs  x
  -start
10 GRAPH_X PRESSURE
20 GRAPH_Y TOT("Amm"), MOL("Amm"), MOL("AmmH+")
  -end
    -active                 true
END


[dlparkhurst]

Actually, you should use the GAS_PHASE to ensure that the Peng-Robinson equation of state is used.

[C3]

Dear Parkhurst,

Thank you for your fast and thorough reply, that is very much appreciated. I'll work a bit more on the problem trying to get it to converge at higher pressures.