Vapour pressure measurements of ammonia/ionic liquids mixtures as suitable alternative working fluids for absorption refrigeration technology

Abstract

In the present paper, we report new data on the vapour pressure of ammonia with 1-(2-hydroxyethyl)-3-methylimidazolium tetrafluoroborate [EtOHmim][BF4], 1-(2-hydroxyethyl)-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [EtOHmim][Tf2N], choline bis(trifluoromethylsulfonyl)imide [N111(2OH)][Tf2N], and trimethyl-N-propylammonium bis(trifluoromethylsulfonyl)imide [N1113][Tf2N]. The vapour pressure of the ionic liquid (IL) mixtures with ammonia was measured from 293.15 to 373.15 K at five compositions (from ∼0.219 to 0.954 mol of ammonia) using a static method. The presence of a hydroxyl group in the cation structure of the ILs enhances the ammonia solubility. Experimental measurements of vapour pressure for ammonia/IL mixtures were correlated using an Antoine type equation and the Non-Random Two-Liquids (NRTL) model. Estimated values using both models presented root mean square deviation values lower than 10%. Excess Gibbs energy was calculated from the activity coefficients and the results were analysed as criteria for selecting the most suitable IL as absorbent for ammonia. Also, to evaluate the suitability of the studied binary mixtures as a working fluid, their vapour pressure (calculate using the Antoine equation) was compared with the most common salts used for this application to observe their deviation from Raoult's law. The data obtained from the analysis of the gE–x relationship and the deviation from Raoult's law for each system suggests that the most suitable absorbent for ammonia is [EtOHmim][BF4].