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Conservation of Mass with REACT


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Cu Mass Balance Test.rea

I am checking the total mass in a REACT system for the total original specified mass (basis input) in comparison to the total mass of species after running REACT.  It is a simple system of dissolving CuSO4 into water.  The total original mass specified for the system (aqueous + minerals) is 1001.0000001 grams.  When checking the final mass in the system, there is a small difference of around 0.001 to 0.0001 grams (depending on which commands I use to report the masses in the final result).  I've attached the REACT file.  Could you run this file and check the original vs. final total masses in the system?  They should be the same? 

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Hello,

After running the simulation, I ran the command 'report mass current system g' to report the mass for each component in the system and summed that mass to compare to the initial basis composition plus the 1 gram of CuSO4 added. I did another check by running the commands "report soln_mass" and "report mineral_mass" and summing these masses, the results agreed up to the 7th decimal place.  Can you tell me the commands that you're using and how you are comparing the masses?

Please note that unless you specify H2O as a free constraint, the 1 kg of H2O will be distributed amongst all aqueous species and minerals. If you're assuming a full kg of solvent, then you should set H2O as a free constraint. You can refer to section 7.2 of the GWB Essentials Guide for more on free constraints.

Best regards,
Jia Wang
Aqueous Solutions LLC

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I've attached the spreadsheet where various output data from the model has been collected.  I extracted the masses of constituents in the final system using a variety of commands, and you'll see the mass balance deviations that I've calculated from each approach.  The mass balance deviations seem to all involve the H2O mass.  I've extracted the H2O mass using "report Wmass g" and "report mass H2O g".  I have also extracted the masses of all the elements for comparison to the input.  Depending on the command used for extracting the final system masses, the mass balance deviations are 0.00000000%, -0.00000080% and 0.00001426%.  Probably I just need help understanding the H2O input/output specifications.  Thanks Jia!

Cu Mass Balance Test.xlsx

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Hello,

Thank you for supplying your excel file. With the report command, you can report the mass for the component of H2O using "report mass H2O g", which would include the mass of free water molecules of the solvent and the water required to make up the secondary species and minerals. If you want just the mass of the free water molecules, you should use "report Wmass g". If you are summing dissolved species and mineral masses, then you should be using the value from report Wmass and not report mass H2O to find the system's total mass. 

After looking a bit further, the small mismatch between your values from summing the soln_mass and mineral_mass and summing of elements system mass vs your total input mass is because of the number of significant digits carried internally within the GWB. The software carries about 9 significant digits while converting kg water to mol water when it sets up the user’s configuration. When reporting the component H2O mass after the calculation, the mass is converted from moles to grams and thus resulting in a very small mismatch between the initial input (1000 g) and output (1000.00000797 g) for H2O. To see this clearly, run "go initial" instead of "go" to have the program calculate species concentration after equilibration but without adding the 1 g of CuSO4. You can then run the command "report mass H2O g". We are working on a fix for this issue and it will be included in the next maintenance release. 

Best regards,
Jia 

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  • 4 weeks later...
Hello again,
 
I want to let you know that this issue has been fixed with the releases of GWB 15.0.1 and GWB 12.0.8.  Existing GWB 15 and GWB 12 installations should automatically update if auto-update is enabled. If disabled, you can update your installation under the Help menu in any GWB app.  
 
Best regards,
Jia
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