Ionic Liquids: Part 2

It is not surprising that ionic liquids have attracted the attention of the scientific community in numerous fields in the chemical technology for separations in general, and in the CO2 capture field in particular. In our previous blog we described the extraordinary properties, such as their high stability and negligible vaporisation, which make ionic liquids promising replacement of conventional solvents. If you search in Google Scholar for the words “Ionic Liquids” and “CO2 capture”, it lists the stunning figure of more 4,000 papers, which increases to the staggering number of 32,000 when searching for “Ionic Liquids” and CO2.

But why are ionic liquids so interesting for CO2 capture? First of all, the solubility of CO2 in some task-specific ionic liquids is higher than in amine solutions. For example, the solubility of CO2 in the 1-hexyl-3-methylimidazolium tetracyanoborate ionic liquid ([hmim][B(CN)4]) is 41 mol% at 20 bar and 303 K, or 130 g CO2 per litre of solvent. The maximum loading capacity of 30% aqueous solution of monoethanolamine (MEA) is 0.5 moles of CO2 per mole of amine, or 115 g CO2 per litre of solvent.

Secondly, most ionic liquids do not form a chemical bond with the CO2 and in most cases they do not contain significant amounts of water, in contrast to aqueous solutions of amines. This means that the heat required for their regeneration (~2 GJ per ton of CO2 for some ionic liquids) is considerably lower than the one for amines (~4 GJ per ton of CO2 for MEA). But is this enough to claim that ionic liquids are promising for carbon capture?

To answer that question, we have developed a process-performance indexed evaluation tool for ionic liquids as solvents for carbon capture. Our tool assesses the process performance and the economics of the capture plant based on the phyisco-chemical properties and phase equilibrium data of CO2 and other gases in ionic liquids, together with general cost models and financial parameters. With the assistance of some wonderful students at Imperial College London, we developed a database which compiled the physic-chemical and phase equilibrium data of ionic liquids available in the literature. To our dismay, we found that only a limited number of ionic liquids had been fully characterised in the literature, in spite of the fact that over hundred ionic liquids have been labelled as promising for CO2 capture.

Our results show that the cost of carbon capture using ionic liquids is in the order of USD 100 per ton of CO2 captured (without CO2 compression) for the most promising ionic liquids evaluated. This is a very encouraging result, as cost is in the same order of magnitude than amine-based solvents (~70 USD per ton of CO2).

This opens a promising door for further development in the field of ionic liquids for carbon capture. Ionic liquids are “designer solvents”, and therefore, we are convinced that we will be able to obtain the right combination of cation and anion to optimise the properties of the liquid to further minimise the costs. For that, we have shed some light on the direction to be taken according to our screening results: promising ionic liquids should have relatively small molecular weight, low viscosity and enhanced CO2 solubility at low pressures. 

Read the related blog Ionic Liquids: Part 1

Author(s): 
M. T. Mota-Martinez