Equilibrium with electrolytes

[R-DeCassia]

Hi everyone! I'm getting started with Clapeyron.jl, and I'm trying to compute the vapor–liquid equilibrium compositions for a ternary mixture: alkane + water + sodium chloride.

I understand that the tp_flash function can be used to calculate phase compositions at a given temperature and pressure, but I'm not sure how to properly exclude electrolytes (like NaCl) from the vapor phase, since they shouldn't appear there.

I've also tried using bubble_temperature and dew_temperature, but these only return compositions at the bubble or dew point — not for arbitrary T and P.

What I’m looking for is a way to:

• Provide T and P as input,
• Get the equilibrium compositions of the liquid and vapor phases, and
• Ensure salts remain only in the liquid phase.

Has anyone worked with electrolytes in tp_flash, or has any suggestions on how to handle this case?

Thanks in advance!

[pw0908]

Hi @R-DeCassia,

You can indeed solve for the VLE of an alkane + water + sodium chloride mixture using tp_flash. To exclude the ions from the vapor phase, you would just need to specify the nonvolatiles argument (although there is currently a typo in the code where you need to specify noncondensables instead). Here's an example:

 julia> model = SAFTVREMie(["water","ethane"],["sodium","chloride"]) # you'll have to define a few parameters manually
Explicit Electrolyte Model with 4 components:
 "water"
 "ethane"
 "sodium" (+1)
 "chloride" (-1)
Neutral Model: SAFTVRMie{BasicIdeal, Float64}
Ion Model: MSABorn{Schreckenberg}
RSP Model: Schreckenberg

julia> T=298.15;

julia> p=1e5;

julia> z=[0.495,0.495,0.005,0.005];

julia> (x,n,G) = tp_flash(model,p,T,z, MichelsenTPFlash(noncondensables=["sodium","chloride"],K0=[1e4,1e-3,1e6,1e6]));

julia> x
2×4 Matrix{Float64}:
 0.000291698  0.999708     0.0        0.0
 0.979863     0.000335819  0.0099005  0.0099005

where K0 is the initial guess for the partition coefficient between the vapor and the liquid (the solution will be quite sensitive to this).

Note that I assumed you meant short-chained alkanes; if you're looking at longer chain alkanes, I would have to assume you're actually interested in LLE?

[R-DeCassia]

@pw0908, thank you for your response!

I'm currently working on setting up a vapor–liquid–liquid (VLL) equilibrium for a system containing crude oil, water, and salt. For now, I’m approaching it step by step, starting with a simpler system: hexane (as a model alkane), water, and sodium chloride.

I intend to perform these simulations at high pressures and low temperatures.

Initially, I started with a vapor–liquid (VL) equilibrium model considering only a single liquid phase, to study the vapor–liquid equilibrium compositions of the ternary system: alkane + water + sodium chloride.

Once I have a better understanding of the VLL behavior with these simpler components, I plan to move forward by defining pseudocomponents for the crude oil.

[pw0908]

VLLE is going to be very challenging... this will require a multiphase flash which we don't have for electrolytes. As of now, our electrolyte flash code can handle LLE where the salt partitions between two liquid phases. In principle, one should be able to code something up for the multiphase flash, but this would be worth a paper by itself!

A simplifying assumption could be to restrict the ions to only be present in the water-rich phase, but I don't think there's something equivalent to nonvolatile in the case of multiphase flash (@longemen3000?).

Alternatively, what is the property you're trying to get out of this? Does the vapor phase matter?

[R-DeCassia]

Haha, this is the topic of my PhD!

My research focuses on the study of vapor–liquid–liquid (VLL) equilibrium in oil–brine–gas mixtures under high pressure and low temperature conditions. The main goal is to contribute to the development of a subsea water–oil separator (SSAO), by investigating its operating window and the effects of electrolytes on phase behavior.

For this reason, it is essential to determine the composition of all three phases — vapor, aqueous liquid, and organic liquid — with the vapor and organic liquid phases being the primary products of interest. We are still in the first year of the project, exploring different paths and possibilities.