Change in Gibbs free energy due to mixing

[KameronC]

As the title implies, I am interested in calculating the free energy of mixing for two solutions. I would like to use this calculation to compute the minimum work of separation for a desalination process. I have searched through the documentation and this forum but have not found any way to output the Gibbs energy, enthalpy, or entropy of a solution (relative to some standard state). Below is a very basic example of what I would like to do.

Code: [Select]

SOLUTION 1
    temp      25
    pH        7
    pe        4
    redox     pe
    units     mol/kgw
    density   1
    Na        0.001
    Cl        0.001
    -water    1 # kg

SOLUTION 2
    temp      25
    pH        7
    pe        4
    redox     pe
    units     mol/kgw
    density   1
    Cl        5
    Na        5
    -water    1 # kg

MIX 1
    1    1
    2    1

# SOME CALCULATION TO REPORT THE GIBBS ENERGY OF MIXING

My understanding is that the mixing module itself does not consider energy changes due to mixing (which is why the mixture temperature is always just a weighted average of the two starting solution temperatures). However, I am hoping that I can access the total Gibbs free energy (again relative to some reference state) for all three solutions and calculate the change myself. Any help is appreciated!

[dlparkhurst]

I am sorry, I replied a month ago, but apparently the reply never got posted.

I am sure you are aware of the work of Yvana Ahdab that addresses this subject.

I did some related work with PHREEQC. PHREEQC does not directly calculate the free energy of solution, but based on the work "Effect of composition and nonideal solution behavior on desalination calculations for mixed electrolyte solutions with comparison to seawater", by Karan H. Mistry, Harrison A. Hunter, John H. Lienhard V, equation 38 provides a way that PHREEQC can calculate the least work of separation. PHREEQC will calculate activities of all species (although there may be a question of the activity scale for the Pitzer calculations), and thus, all the necessary terms of equation 38 are available.

Here is a PHREEQC script that generates figure 10 from the Mistry paper.

[John Mahoney]

I also wrote a response last month but I do not see it.
 
MY comment was to consider the GEMS software package from the Paul Scherrer Institute in Switzerland .

GEMS is Gibbs Energy Minimization Software (I think).   I have not used this program so I cannot comment further.

http://gems.web.psi.ch/

Good Luck