Binary Ionic Liquid Mixtures for CO₂ Absorption

COSMO-RS Ionic Liquids Gas solubility VLE

Ionic liquids have been investigated extensively for CO₂ absorption on account of their unusually high selectivity compared to traditional absorbents. Binary ionic liquid mixtures provide an appealing route for process-tunable CO₂ selectivity without requiring the design of new synthesis pathways. In this example, we will show how the COSMO-RS-IL method can be used to predict the separation efficiency of these binary ionic liquid mixtures, facilitating fast computational pre-screening and optimization.

Blending the Ionic Liquids

For this example, we will look at a blend of [OMIM][BF₄] and [OMIM][Tf₂N]. The individual ions are included in the CRS database.

1. Start AMScrs: SCM → COSMO-RS

2. Choose Search compound in list of added compounds or database to access the database

3. Type OMIM in the search bar. Select the compound from the list and click the OK button to add it to the workspace

4. Repeat this step for BF₄ and Tf₂N

Once the ions have been added to the workspace, we can make the ionic liquids.

1. Choose Compounds → Compound with multiple forms

2. In the Dissociated section, select IL_cation_1-octyl-3-methyl-imidazolium and IL_anion_tetrafluoroborate as compounds by using the dropdown menus. This will define a dissociated cation-anion pair

3. Click the Save button to create a new parameter file for the ionic liquid. In the save dialog, set the name to OMIM_BF4 and choose Save

4. Replace the anion with IL_anion_bis(trifluoromethylsulfonyl)amide

5. Click the Save button to create a new parameter file for the second ionic liquid. In the save dialog, set the name to OMIM_Tf2N and choose Save

Both ionic liquids will now be available as compounds in the AMScrs workspace. This allows us to select them in the property calculations.

Note

When predicting properties for ionic liquids that are not included in the database, you will need to first run a compound calculation in order to obtain the required COSMO-RS parameters. Check out the COSMO-RS tutorials to learn more.

Calculating CO₂ Solubility

AMScrs supports calculating the solubility in mixed solvents. In order to calculate the solubility of CO₂, we first add it as a compound from the database.

1. Go back to Compounds → List of Added Compounds

2. Choose Search compound in list of added compounds or database to access the database

3. Type CO2 in the search bar. Select the compound from the list and click the OK button to add it to the workspace

We can now define an ionic liquid mixture in order to generate the solubility data. The low-temperature, medium-pressure regime will be considered here, in order to compare with experimental reference data.

1. Go to Properties → Solubility in Mixture

2. Under Solvent, make sure that the setting for 2 components is selected

3. Select OMIM_BF4 and OMIM_Tf2N as the solvent compounds by using the dropdown menus

4. Set the Mole fraction of both ionic liquids to 0.5 to create an equimolar mixture

5. Set Temperature: number of steps to 2

6. Set Temperature: from to 243.15 K and Temperature: from to 273.15 K

7. Set Pressure for solubility gas: number of steps to 10

8. Set Pressure: from to 3.45 bar and Pressure: to to 25.21 bar

9. Under Solutes, make sure that the phase is set to Gas

10. Use the button to bring up the list of compounds. Select Carbon dioxide from the list and click Add

11. Press Run to perform the COSMO-RS calculation

Both a graph and table containing the calculated solubility data will be generated.

Compressibility Factor

A comparison with experimental solubility data is given in the figure below. A slight underestimation of the predicted CO₂ solubility is noted, although the general trends and slope of the solubility curves are reproduced quite well. Increased deviations at lower temperatures / higher pressures suggest that compressibility may be responsible.

Fig. 77 Comparison between experimental solubilities[1] and predicted values obtained from COSMO-RS for an ideal gas

AMScrs calculations assume ideal gas behavior by default. We can account for non-ideal compressibility by using experimental fugacity data.[2] (Where fugacity coefficients depend on both temperature and pressure.) After this correction, results are seen to align closer with experiment.

Fig. 78 Comparison between experimental solubilities[1] and predicted values obtained from COSMO-RS accounting for non-ideal compressibility[2]

For this example, we have focused on reproducing solubility data for a prescribed mixture. The fast calculations offered by COSMO-RS also allow us to easily evaluate other blend ratios or mixtures of different ionic liquids in order to pre-optimize high-solubility absorbents.

Tip

Automated workflows can help speed up large-scale screenings:

• An example is available for the automated screening of ionic liquids

• Check out the PyCRS page for more info on setting up custom workflows

References

[1] (1,2)

Z. Lei, C. Dai, W. Wang, B. Chen, UNIFAC Model for Ionic Liquid CO2 Systems. Thermodynamics and Molecular-Scale Phenomena, 60, 716-729 (2013)

[2] (1,2)

S. Angus, B. Armstrong, K.M. de Reuck, International Thermodynamic Tables of the Fluid State 3: Carbon Dioxide, ISBN: 0-08-020924-6. Pergamon Press, 385 (1976)