(234f) Ionic Liquids as Absorption Media for Pure and Mixed Gas Capture | AIChE

(234f) Ionic Liquids as Absorption Media for Pure and Mixed Gas Capture


Anderson, J. L. - Presenter, University of Notre Dame
Dixon, J. K. - Presenter, University of Notre Dame
Shi, W. - Presenter, University of Notre Dame
Wilbanks, K. E. - Presenter, University of Notre Dame

Ionic liquids are being investigated for a variety of applications including reaction media, separation solvents, non-volatile electrolytes, heat transfer fluids, and gas capture. Our research interest focuses specifically on how ionic liquids (ILs) can be designed for specific and selective gas separations. One potential application for this research is the separation of industrial flue gases for removal of environmentally hazardous gases. The ILs' negligible vapor pressure and tunable properties make them ideal to replace volatile and/or corrosive solvents currently being used for these processes. To this end, gases of interest to our study include carbon dioxide, sulfur dioxide, oxygen, and nitrogen, as well as mixtures of these gases. Selection of different combinations of anions and cations influences the physical properties and functionality of an IL. Thus, one focus of this work is to understand the structure property relationship needed for enhanced gas solubility.

We are also interested in how the presence of CO2 affects the solubility of other gases in ILs. CO2 is quite soluble in ILs, while reactive gases such as O2 are only sparingly soluble. The presence of CO2 may enhance the solubility of the sparingly soluble gases like O2 in ILs but mixed gas solubility data are scarce. Consequently, another goal of this work is to measure the solubility of lower solubility gases in CO2/IL mixtures.

Gases dissolve in ILs via a physical absorption mechanism, a chemical complexation mechanism, or a combination of the two. The measurements of the solubility of pure gases in ILs are carried out using accurate gravimetric microbalances; an Intelligent Gravimetric Analyzer (Hiden Analytical Limited, England) and a Magnetic Suspension Balance (Rubotherm, Germany). The gas absorption is determined by measuring the equilibrium mass uptake by the liquid for pressures up to 10 bar and temperatures from 10 to 60 oC. In all the cases studied, solubility increases with an increase in pressure and decrease in temperature. Results show that even common ILs like 1-hexyl-3-methyl-imidazolium bis(trifluoromethylsulfonyl)imide ([hmim][Tf2N]) have excellent selectivity for CO2 relative to N2 and other components in flue gas based on pure physical absorption. In addition, ILs exhibit high absorption capacity for SO2 without degradation of the IL. This may be beneficial as both SO2 and CO2 could be removed in a single separation step, thereby eliminating one step from current industrial processes.

The physical absorption of gas increased with increased IL fluorination, though chemical absorption by ILs containing amine, acetate and other functional groups was found to yield much higher capacities than physical absorption. These chemically absorbing compounds form weak chemical complexes that have high carrying capacity and keep the energy requirements to regenerate the solvent low. To date, chemically complexing ILs are capable of achieving a ten fold increase in the carrying capacity of ILs for CO2 while maintaining good selectivity relative to other flue gas components. Desorption of CO2 was much slower for the chemically absorbed gas than for the physically absorbed gas. This is a significant disadvantage even if the energy requirements for regeneration are less than conventional amines. Therefore, we will show that we are able to measure the gas solubility in various ILs using both physical and chemical absorption.

The mixed gas solubility of CO2/O2 was measured in [hmim][Tf2N]. The solubility of O2 increased substantially compared to its respective pure gas solubility at all pressures. Thus, we show that CO2 does, indeed, enhance the solubility of O2 even at low pressures in [hmim][Tf2N]. We will also report preliminary simulation results of this mixed gas system.