Setting up an Electrolyte Simulation
|IMPORTANT: The Electrolyte Subsystem in DWSIM is still in a beta testing stage. Results obtained from calculations may not reproduce real data as expected!|
This guide will help you on setting up an electrolyte simulation in DWSIM.
Create a new simulation. Add Water from the ChemSep database.
Add the desired salts and ions from the Electrolytes database to your simulation. To calculate pH you'll also need the Hydron (H+) ion added to your simulation.
Defining Dissociation Reactions
The Electrolytes Chemical Equilibrium Solver supports Equilibrium Reactions only. To setup reactions, open the Reactions Manager and add a new equilibrium reaction.
Define your salt dissociation reaction, selecting the salt as the base component. The reaction should occur in the liquid phase. You should also define the equilibrium constant calculation source (gibbs energy, T-expression or constant value).
Calculating Water pH
To calculate the pH of the water solution in your stream, import the water dissociation reaction into your simulation. If you didn't add the Hydron and Hydroxide ions to your simulations, a message will tell you that they must be added in order to activate the reaction being imported.
The imported water dissociation reaction data contains a customized equilibrium constant expression that matches experimental data with higher accuracy when compared to the one obtained through Gibbs energy values.
Selecting a Property Package
DWSIM includes two Property Packages that support calculation of chemical equilibrium for Aqueous Electrolyte solutions: LIQUAC2 and Extended UNIQUAC.
IMPORTANT: The LIQUAC2 model is currently being tested and is not matching experimental data, therefore the Extended UNIQUAC model is recommended at this stage.
In the Property Package Configuration Window, check if the Calculate Chemical Equilibrium option is enabled.
The Aqueous Electrolyte Property Packages have their own flash algorithm, so the Flash Algorithm option in the Advanced Thermo Settings panel has no effect in the calculations.
Inputting Stream Composition
A good way of inputting stream composition is by selecting the Molality or Molarity options in the Stream Composition Editor.
For the molality option, Water amount is entered in kg, while the salt and ion amounts should be entered in mols. For the molarity option, Water amount is entered in liters, while the salt and ion amounts should be entered in mols.
Viewing Stream Properties and Results
When the stream is calculated, you will see some extra properties in the Liquid Phase properties section, like activity coefficients, pH, osmotic coefficient and water freezing point depression.
If you enabled the Chemical Equilibrium calculation, the equilibrium stream composition may be different from the one that was input. Total stream mass flow will be preserved, though.
If you're having problems with chemical equilibrium calculations not converging, you can setup a new property package with this option disabled. You can then associate this new property package with the problematic streams in order to prevent the flowsheet solver from breaking the calculation. This can happen if you have a solids-only stream, or a mixture with strong electrolytes and an input composition of non-dissociated salts, etc.