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Pressure drop due to CO2 diffusion in water
Posted Aug 16, 2021, 11:36 a.m. EDT Heat Transfer & Phase Change, Chemical Reaction Engineering Version 5.6 3 Replies
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I am going to model the diffusion of CO2 into water. Initially, the gas phase includes 100% CO2 while the liquid phase, water, has no dissolved CO2 inside. I need to track the pressure drop in gas phase due to CO2 diffusion in water until the equilibrium is reached. I used Transport of Diluted Species (tds) module for both gas and liquid phases in 1 D. However, considering a diffusion coefficient (D) for gas phase which consists of 100% CO2 does not have any meaning as there is just CO2 in this phase. Any idea how to model the gas phase without using the Transport of Diluted Species (tds) module?
P.S. The flux at the interface is defined based on two film theory: K(CCO2,g - (CCO2, water*H)), So I need the CO2 concentration in gas phase ( CCO2,g) as a variable at each time.
Thank you in advance.
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Henry's law+diffusion in water.
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Thank you Lasse.
The problem is that for Henry's law implementation, I use the partition condition node as a boundary condition. Since this node prescribes the ratio between the concentration of CO2 in two different phases, I need to model the gas phase, too, besides the water phase. However, By considering a geometry for the gas phase I need to use a module for modeling its physics. So, I used again Transport of Diluted Species (tds) module for the gas phase which requires identification of diffusion coefficient (D). But considering a diffusion coefficient for the gas phase which consists of %100 CO2 does not have any physical meaning.
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There is no mass transfer in gas phase. Use mass balance and ideal gas law to estimate the pressure drop:
n° = n_gas + n_w = p·V_g/RT + V_w·c_w p = Kc·c_w at the interface; Kc = Henry's law constant on the conc. scale. n° = p°·V_g/RT; p° = initial pressure
How to implement the gas phase with Comsol, I have not checked.
But I calculated that assuming that the volume of water phase is much larger than that of the gas phase. This is what I got:
p/p° = exp(K²t)·erfc(K√t)
K = RT√D/[Kc·(V_g/A)] where A is the interfacial area of the gas-water interface, and D is the diffusion coefficient of CO2 in water.
Lasse