Gas exchange and particle size questions

[jakatch]

Hi GWB team,

 

I'm running a flow-through reaction with water permeating a parcel of rock. The incoming fluid is in equilibrium with CO2 in the atmosphere (pCO2 = 300 uatmos), but I do not fix CO2 fugacity. 

Question 1) If CO2 fugacity is not fixed, does that mean that there is no "head space" in the reactor to exchange with the fluid as it moves through the rock parcel? If CO2 fugacity is fixed, does that mean air-fluid exchange is happening? 

Question 2) I was wondering how is air-fluid gas exchange dealt with in GWB? What is the gas transfer velocity set as? Are there ways to edit this?

Question 3) When reactant particles dissolve or precipitate, how does GWB deal with how particle size changes? Does GWB use a shrinking core model? Are there ways to edit this?

 

Thanks for the help!

Jo

[Jia Wang]

Hello Jo,

If you do not fix fugacity, then the program will use the constraints provided initial input file to calculate the fugacity in equilibrium to the fluid. The fugacity will change accordingly as reactions occur through your simulation. It's important to note that the GWB does not keep track of the physical volume of gas even if you designate pore space. The program itself does not model headspace strictly.

If the gas fugacity is fixed, then the gas is kept at a constant and the fluid-gas exchange occurs instantaneously. You can use a sliding fugacity path (see section 3.6 Sliding activity and fugacity path in the GWB Reaction Modeling User Guide) set your fugacity to vary linearly or logarithmically from its initial to a final value the user set. If you are setting a kinetic rate for your gas reactions between a fluid and external gas reservoir, you can either use a built in or custom rate law in section 4.5 Gas Transfer Rate in the GWB Reaction Modeling User Guide. 

The program does not keep track of particle size of minerals but it does keep track of mineral volume as precipitation and dissolution occurs. For a kinetic mineral, you can include a constant specific surface area for a built-in rate law. This would imply that particle size does not change for kinetic minerals but more will accumulate as precipitation occurs. However with a custom rate law, you can set up a relationship for which the specific surface area may vary over the course of the simulation. For more information on reaction kinetics and custom rate laws, please see sections 4 and 5 in the GWB Reaction Modeling User Guide. 

Hope this helps,
Jia Wang
Aqueous Solutions LLC