Ionic liquids (ILs) have proved to be one of the most impressive classes of fluids, due to their properties and applications from chemistry to engineering. Now available on large scale and with some industrial applications in the stages of commercialization, they have proven to be alternatives for many applications in industry and chemical manufacturing. Being a new class of engineering fluids, capable of replacing in many applications the existing working fluids and reaction media, their success arises mainly from their thermophysical and phase-equilibria properties, the versatility of their synthesis, manageable to be tailored for a high plethora of applications [1,2]. However, due to the continuous synthesis of new cations and anions, the actual knowledge on the ILs, their nano and micro structure, and their influence on the bulk and surface properties is far from being complete.
With this purpose one very important area was selected for this Project to be developed in Centro de Química Estrutural da ULisboa (FCUL), classified for the period 2015-2020 as a research unit of Excellency, specialized in IL properties [3-5], their measurement, simulation and prediction, in addition to the development of new applications [6]. This is Ionic Liquids as absorbents for absorption refrigeration.
An enormous amount of heat energy is daily dissipated to atmosphere as low temperature waste heat by industrial and electric power plants. This energy can be used as driving heat of absorption heat pumps for refrigeration, reducing the carbon footprint and increasing energy-efficient cooling. The research of new working pairs for absorption chillers considering IL as absorbents appears promising in overcoming the drawbacks of the state-of-the-art absorbent LiBr, such as crystallization, corrosion, and instability [6-10].
Most of the IL-based working pairs suggested have not met so far the industrial requirements. Reliable property data and models to predict thermodynamic data of multicomponent systems and transport properties such as diffusion coefficient, viscosity, and thermal conductivity are needed. Recent works [7,11,12], with collaboration of one of industries more involved in the absorption refrigeration business (EVONIK) started to discuss the problem and main proposals of new IL’s, but not all the selection criteria was applied. However, this last paper [12] shows that only three criteria (vapour pressure, water content in the vapour phase and viscosity) reduced 74 possible ionic liquids to just 6, for IL+water systems. Further criteria, namely the ecotoxicity and biodegradability of the IL’s [1,13], apart from thermal properties (heat capacity, heats of mixing, solubility and thermal conductivity) might reduce even further the acceptable (technical, economic and environmental) IL’s + refrigerant pairs.
Although the field of ionic liquids is fast growing, almost no study/attention has been paid to the characterization of the purity of these fluids, except for the water content. This will also be a fundamental task of this program.
Given the still scarcity of data and the need to develop further the structure-properties relations, new correlations and estimation schemes will be develop and heat transfer coefficients for IL’s in pilot heat exchangers will be performed.