Quantitative analysis of bulk water in water/lithium bromide mixtures with ionic liquids as a working fluid in absorption heat pumps and influence on the thermophysical properties

Identifier:  TDX:4525

Handle: https://hdl.handle.net/20.500.11797/TDX4525

Authors:  Latorre Arca, David

Abstract:
This doctoral thesis addresses the improvement of absorption refrigeration and heat pump systems using water and lithium bromide as working fluid by incorporating the following ionic liquids (ILs) as additives: ethylammonium nitrate, propylammonium nitrate, ethylammonium chloride, 1-butyl-3-methylimidazolium bromide and 1,3-dimethylimidazolium chloride. These absorption systems, which offer a more sustainable alternative to the vapour compression technologies used in HVAC (heating, ventilation and air conditioning) systems, have the major disadvantage of lithium bromide crystallisation. The research focuses on quantifying the free water content in H2O/LiBr mixtures with ionic liquids as additives and analysing their influence on thermophysical properties, using near-infrared spectroscopy and multivariate curve resolution. The results indicate that ILs increase the amount of water that maintains their structural properties due to interactions between lithium bromide and ionic liquids. The crystallisation of lithium bromide is effectively delayed by using the selected ionic liquids as additives, extending the operation range of the systems using this working fluid. In addition, the results show that ILs can both increase and decrease the vapour pressure, depending on composition and temperature. In general, the results show that density, sound velocity, dilution enthalpy, electrical conductivity and thermal conductivity decrease in the presence of these ionic liquids, while dynamic viscosity increases under typical operating conditions of the application. The solute-solvent interactions are also analyzed comparatively from a thermodynamic point of view by means of properties derived from the apparent molar volume and isentropic compressibility.