Experimental determination and modelling of thermophysical properties of ammonia/ionic liquid mixtures for absorption refrigeration systems

Identifier:  TDX:2539

Handle: https://hdl.handle.net/20.500.11797/TDX2539

Authors:  Cera Manjarres, Andry

Abstract:
Recently, the use of Ionic Liquids (ILs) as new absorbents has been proposed in refrigeration by absorption to overcome some handicaps of conventional fluids, given that this kind of substances have a wide window of opportunities to tailor easily their thermophysical properties according their application. Nevertheless, the lack of information about the thermophysical properties of pure ILs and its mixtures with natural refrigerants makes difficult the evaluation of their potential. Therefore, the main objective in this thesis is the experimental determine the most relevant thermophysical properties of pure ILs and binary mixtures with ammonia. It is necessary to highlight that the ILs used herein have been selected based on the literature taking into account their low viscosity, high capacity to absorb ammonia and high thermal stability. Four of six ILs studied herein has been reported as suitable for this application. Two of them ILs containing the common choline cation with two different anions; bis(trifluoromethylsulfonyl)imide and trifluoromethanesulfonate were designed for this application. The others are 1-(2-Hydroxyethyl)-3-methylimidazoliumtetrafluoroborate and choline (trifluoromethylsulfonyl)imide are available commercially. In addition, it was selected ILs 1-(2-Hydroxyethyl)-3-methylimidazolium [EtOHmim] bis(trifluoromethylsulfonyl)imide and finally N-Trimethyl-N-propylammonium[N1113] bis(trifluoromethylsulfonyl)imide, which does not have and hydroxyl group in its structure in order to know the effect of this functional group on the thermophysical properties. Due to there is the hypothesis that the presence of this group increase promotes the interactions between the ammonia and the IL. The first step before measuring thermophysical properties is necessary to confirm the structure of the ILs and quantify their impurities mostly water. In the first task were used several common techniques used such as Resonance Magnetic Nuclear (NMR) and FTIR. The second task the water content and halide content were determined by means and Coulometer and Atomic absorption. Then, thermophysical properties such as density, viscosity, heat capacity and thermal conductivity of ILs were measure, and in the case of mixtures the properties measured were vapor liquid equilibrium, density and viscosity. Furthermore, to accomplish the measurements of mixtures some modifications to experimental techniques, and new units to manage these substances are required. Among the most relevant are: design and implementation of: 1) a sample injection-preparation cell to measure density-viscosity, 2) a system to measure vapor pressure of the mixtures. The experimental data is correlated and/or modeled to create a database required to do a thermodynamic analysis about performance of refrigeration absorption cycle. Results show that ILs studied herein with a hydroxyl group in the cation structure present a better coefficient performance than conventional working fluids (ammonia-water and ammonia-lithium nitrate) and different ILs used for this application. On the other hand, the mass flow circulation ratio was quite high when compare with conventional working fluids. Nevertheless, the 1-(2-Hydroxyethyl)-3-methylimidazolium tetrafluoroborate presented the lowest value of this parameter among the IL studied herein and also when was compare with different ILs.