Ionic liquid design for enhanced carbon dioxide capture by computer-aided molecular design approach

Abstract

Carbon capture and storage is an emerging technology to mitigate carbon dioxide (CO₂) emissions from industrial sources such as power plants. Post-combustion capture based on aqueous amine scrubbing is one of the most promising technologies for CO₂ capture currently. This technology, however, possesses a number of shortcomings, including high regeneration energy requirement, high solvent loss, degradation of solvent, etc. To overcome these limitations, researchers suggested different solvents and alternative technologies to replace the current amine scrubbing technique. Ionic liquids (ILs) are the most potential substitute among all. This is mainly because they have negligible vapour pressure and high thermal stability, which reduce solvent loss. However, there are up to a million possible combinations of cation and anion that may make up the ILs, which makes experimental works very time consuming and costly. In this work, optimal IL solvents specifically for carbon capture purpose are designed using computer-aided molecular design approach. This approach utilises group contribution method to estimate the thermophysical properties of ILs, and UNIFAC model to predict CO₂ solubility in the ILs. Structural constraints are included to ensure that the synthesised ILs structure will satisfy the bonding requirement. This work focuses on design of ILs based on a physical absorption mechanism, and hence no chemical reaction is involved. The results show that the designed ILs are capable of capturing CO₂ and their predicted properties are in good agreement with properties as determined through experimental works.

Appendix

UNIFAC model is a well-established method to estimate activity coefficient and it is expressed as a function of composition and temperature, as shown in Eqs. (30)–(37). The model consists of a combinatorial contribution, ln $\gamma_{i}^{C}$, which is essentially due to the differences in size and shape of the molecules, and a residual contribution, ln $\gamma_{i}^{R}$, which is due to energetic interactions (Skjold-Jørgensen et al. 1979).

$$\ln \, \gamma_{i} = \ln \, \gamma_{i}^{C} + \ln \, \gamma_{i}^{R}$$

(30)

The combinatorial contribution can be determined using Eqs. (31)–(32).

$$\ln \, \gamma_{i}^{C} = 1 - V_{i} + \ln V_{i} - 5q_{i} \left[ {1 + \frac{{V_{i} }}{{F_{i} }} + \ln \left( {\frac{{V_{i} }}{{F_{i} }}} \right)} \right]$$

(31)

$$F_{i} = \frac{{q_{i} }}{{\sum\nolimits_{j} {q_{j} x_{j} } }};\;{\text{V}}_{i} = \frac{{r_{i} }}{{\sum\nolimits_{j} {r_{j} x_{j} } }} ,$$

(32)

where x _j is the mole fraction of group j.

Pure component parameters r _i and q _i are relative to molecular van der Waals volumes and to molecular surface areas, respectively. They are calculated as the total of the group volume and group area parameters, R _k and Q _k , as shown in Eq. (33).

$$r_{i} = \sum\limits_{k} {v_{k}^{(i)} R_{k} } ; \, q_{i} = \sum\limits_{k} {v_{k}^{(i)} Q_{k} }$$

(33)

Residual contribution of the component can be determined as follows:

$$\ln \gamma_{i}^{R} = \sum\limits_{k} {v_{k}^{(i)} \left[ {\ln \varGamma_{k} - \ln \varGamma_{k}^{(i)} } \right]} ,$$

(34)

where Γ _K is the residual activity coefficient of group k, and $\varGamma_{k}^{(i)}$ is the residual activity of group k in reference solution containing only molecules of type i. These can be determined using Eqs. (35) and (36).

$$\ln \varGamma_{k} = Q_{k} \left[ {1 - \ln \left( {\sum\limits_{m} {\theta_{m} \psi_{mk} } } \right) - \sum\limits_{m} {\left( {{{\theta_{m} \psi_{km} } \mathord{\left/ {\vphantom {{\theta_{m} \psi_{km} } {\sum\limits_{n} {\theta_{n} \psi_{nm} } }}} \right. \kern-0pt} {\sum\limits_{n} {\theta_{n} \psi_{nm} } }}} \right)} } \right]$$

(35)

$$\theta_{m} = \frac{{Q_{m} X_{m} }}{{\sum\limits_{n} {Q_{n} X_{n} } }};\;X_{m} = \frac{{\sum\limits_{i} {v_{m}^{(i)} x_{i} } }}{{\sum\limits_{i} {\sum\limits_{k} {v_{k}^{(i)} x_{i} } } }} ,$$

(36)

where X _m is the fraction of group m in the mixture. The group interaction parameter ψ _mk is defined by Eq. (37).

$$\psi_{nm} = \exp \left[ { - \left( {{{a_{nm} } \mathord{\left/ {\vphantom {{a_{nm} } T}} \right. \kern-0pt} T}} \right)} \right]$$

(37)

Parameter a _nm characterises the interaction between groups n and m. For each group–group interaction, there are two parameters a _nm and a _mn , which are not the same.