In Pursuit of the Perfect Solvent – An insight into Call 2 Project “Process-performance indexed design of task-specific ionic liquids for post-combustion CO₂ capture”

The Process-performance indexed design of task-specific ionic liquids for post-combustion CO₂ capture’ project is funded through the UKCCSRC’s Call 2 for funding. It brought together two young academics from Imperial College of London: Dr Jason Hallett, from the Chemical Engineering Department, whose research focuses in solvent design for sustainable technology, especially the development of cost-effective designer solvents for large-scale application, and Dr Niall Mac Dowell, from the Centre for Process Systems Engineering, who currently leads the Clean Fossil and Bioenergy Research Group. The project aims to design a ‘perfect’ solvent for carbon capture. This blog is written by Dr Maria Teresa Mota-Martinez who is working on the project. 

The most commonly used solvents for carbon capture are the aqueous solutions of alkanolamines, e.g. monoethanolamine (MEA). The main advantages of these solvents are that their reaction kinetics with carbon dioxide (CO₂) is relatively fast, their price is relatively low and industry already possesses the knowhow to utilise this technology. However, the volatility of MEA is relatively high resulting in substantial solvent loses. Moreover, alkanolamines form irreversible salts when they react with some of the impurities that are present in the flue gas. But the main disadvantage of aqueous alkanolamines is the high operating cost related to the highly energy demanding recovery step of the amine. Large amounts of heat needs to be provided because of i) the highly endothermic reaction enthalpy, ii) the large heat capacity of water, iii) the heat of vapour of water.

Scientists and industrialists have been working on the design of an alternative solvent for carbon capture that would overcome these inconveniences. Consequently, the new absorbent ideally should have a very low vapour pressure to prevent vaporisation, high chemical stability to avoid undesired chemical reactions, and relatively low enthalpy of sorption to reduce the energy requirements of the process.

For those reasons, we turned towards ionic liquids as promising solvents for carbon capture. Ionic liquids are organic salts that are liquids at temperatures below 100 ºC, and most of them below room temperature. They are composed of large organic cations, viz. imidazolium, pyridinium, pyrrolidinium, ammonium, etc., and mainly organic anions. Ionic liquids present extraordinary properties such as negligible vapour pressure, high thermal and chemical stability and high CO₂ loading.

The combination of all known (and to be discovered) cations and anions is so extremely large that we need a systematic methodology to discern the ionic liquids that are suitable for a certain process. Nevertheless, their vast tunability options also provides a substantial opportunity for chemical design of a ‘perfect’ solvent for carbon capture.

The design of task-specific ionic liquids has focused mainly in finding the structure that would optimise a certain property. Most of the ionic liquids that were identified to exhibit an excellent CO₂ solubility included the fluorine atoms and nitrile groups. But some of these ionic liquids were extremely viscous, making their industrial deployment unfeasible. There are hundreds of papers claiming that their particular ionic liquid is a promising solvent for carbon capture, but they base their assertion on the measurement of a limited number of properties, mainly CO₂ solubility. Nonetheless, we know that the solvent with the higher solubility is not necessary the most suitable one for a process. Other properties such as viscosity, enthalpy of solution, or heat capacity pay a significant role in the cost of the process, but these properties are usually ignored when making such claims. This imposes a priori constrain on the efficiency of the process and a gap between the estimated and the real cost of the process. What is more, the cost of ionic liquids themselves has been so prohibitive that the staggering process-related benefits of replacing traditional solvents with ionic liquids were wiped out by the price of the ionic liquids. However, Dr Jason Hallett has shown that novel protic ionic liquids have bulk costs similar to commodity organic solvents such as acetone or toluene. This opens the door to the design of novel cost-competitive ionic liquids that are more attractive for industrial processes, including carbon capture.

We are approaching the challenging task of designing a perfect ionic liquid for carbon capture from an innovative perspective. This project intends to fill the gap between the molecular design based on thermophysical properties and the economics of the carbon capture process on a more rational basis.

We are developing a comprehensive process modelling tool in gPROMS that will reflect the complex relation between the molecular structure of ionic liquids and their process performance from both monetised and non-monetised points of view. Our objective is to establish which chemical characteristics of the ionic liquids are relevant to minimise the cost of carbon capture. The ultimate goal is to use the results from the process modelling as a guide for designing structurally tailored perfect solvent for carbon capture. This is a very promising approach that will ascertain the position of ionic liquids as potential solvents in carbon capture.

The questions we are trying to answer are: Which of the main thermophysical properties of our perfect solvent have a larger effect on the process efficiency and economics? What are the boundary values of those properties? Can we find an ionic liquid that presents those optimum values? If so, what is the structure? If not, can we synthesis a cost-competitive ionic liquid with those properties? What are their advantages compared to other solvents?

Do you want to know more about our pursuit of the perfect solvent? Then keep tuned to this blog! To be continued…