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Solution Modify implementation
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Topic: Solution Modify implementation (Read 1216 times)
Pak
Top Contributor
Posts: 68
Solution Modify implementation
«
on:
May 06, 2015, 03:27:35 PM »
I have a doubt about implementation of total number of moles using Solution_Modify. (I have multicomponent diffusion [MCD])
1) I run Phreeqc and I get the total concentration of the master species (including O and H) (mol/kgw) and the charge balance (eq/kgw).
2) I transport these concentrations using the same units. (I am using a constant diffusion (D=1e-12) and constant advection (v=0 m/s).)
I have read that Phast just Transport the "excess" of H and O, I wonder if that excess can be as well negative.
What I understand from "excess" is that once you get the total of H (mol/kgw) you subtract the H in 1 kgw of pure water (for the corresponding temperature and pressure) and after the transport you add the subtracted value. Is it right?
(In my case where diffusion is constant , is it important to use just the "excess" of H and O or can I transport the total O and H concentrations?)
3) Using solution modify, the new (after transport) total moles of master species, H, O and charge balance (mol) must be inputted.
From transport I will have (mol/kgw or eq/kgw) so I need to multiply my new values by the mass of water.
This mass of water has changed since there has been a transport of H and O. Would it be a good approximation to use the older values of mass water or should I run a Phreeqc calculation with just O and H to get the new value of mass water and then multiply the new concentration of the master species by this new calculated mass of water.
Thank you for your help
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dlparkhurst
Top Contributor
Posts: 1134
Re: Solution Modify implementation
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Reply #1 on:
May 06, 2015, 04:51:20 PM »
PHREEQC solution definitions are in terms of moles (not concentration). The transport calculations are probably done in terms of concentration (often mg/L or mol/L), so there is a conversion that is necessary from reaction to transport and transport to reaction.
PHREEQC (using phreeqc.dat or pitzer.dat) calculates the density of the solution (RHO), the kilograms of water (TOT("water"), not necessarily 1.0), and the solution volume (SOLN_VOL), so it is possible to make conversions from reaction to transport. However, for transport you probably expect a liter to go into reaction and a liter to come out; this is not necessarily the case.
You can use fancier transport that allows the volume to change, and possibly iterate to use the PHREEQC volumes in the transport equations, or you can make some simplification.
PHAST removed one kilogram of water from total H and total O to get excess H and O (so a negative quantity is possible). PHAST used mass fraction in its transport calculations, and then 1 kg of water was added to get total H and total O for the next reaction calculation. Alternatively, one could remove the exact mass of water (TOT("water")) to get excess H and O, but then the question is how much water to add when you go from transport to reaction. Again these quantities are in moles. Perhaps you could transport the mass of water, so the amount would be known.
Alternatively, you could transport total H and total O (converted to transport units), and then consider how to convert results from concentration back to total moles for PHREEQC.
There is no simple way to be exact, so you will have to make your own choices on what simplifications you want to make.
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Pak
Top Contributor
Posts: 68
Re: Solution Modify implementation
«
Reply #2 on:
May 13, 2015, 02:23:35 PM »
A difference of 10e-5 between my results of total O and H from Phreeqc and other software that uses Phreeqc for chemistry but another one for transport. Can lead to different values?
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dlparkhurst
Top Contributor
Posts: 1134
Re: Solution Modify implementation
«
Reply #3 on:
May 13, 2015, 03:08:04 PM »
If the difference is in the mass of water, then it should not make much difference.
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