Abstract
Controlling vapour pressure is necessary for the viability of aqueous ammonia solutions in commercial applications such as refrigeration. In this study, Gibbs ensemble Monte Carlo (GEMC) simulations were used to calculate the vapour–liquid equilibrium (VLE) of ammonia–water–MCl mixtures, M = Na or Cs, within the isobaric–isothermal- (NpT-) ensemble. The results indicate that in the presence of alkali metal additives, there is a non-negligible ‘salting-in’ effect for ammonia in the liquid phase. Experimental measurements of the liquid phase concentration of ammonia confirm the GEMC results i.e. the vapour loss rates in systems containing ionic additives is slightly lower. Gibbs ensemble Monte Carlo simulations also indicate that ammonia prefers to solvate aqueous cations as a result of electrostatic interactions. Ab-initio calculations show that the M+–ammonia complex is energetically more stable than the M+–water complex. The difference in the binding free energy Δ(ΔG bind(M+–NH3)−ΔG bind(M+–H2O)) depends on the size of the cation and is highest for the smallest tight cations (e.g. Li+) and lowest for the most polarisable cations (Cs+).
Highlights
IntroductionIts high volatility is a major concern when aqueous ammonia liquors are used in many industrial process settings e.g. ammonia based air-conditioning [1] and CO2 scrubbing from gas streams [2]
Ammonia is a low-cost chemical, which is widely available commercially
Suppressing ammonia vapourisation from aqueous ammonia solutions is important for many processes e.g. refrigeration or ammonia based CO2 capture processes
Summary
Its high volatility is a major concern when aqueous ammonia liquors are used in many industrial process settings e.g. ammonia based air-conditioning [1] and CO2 scrubbing from gas streams [2]. ? OH- is 1.774 9 10-5 at 25 °C i.e. ammonia is a weak base, pKb 4.76 [3]. Due to this small value, it can be assumed that [NH4?(aq)] ( [NH3(aq)] and ammonia vapour losses will be significant except at the lowest pH values. Typical engineering solutions to suppress ammonia vapourisation at high pH involve cooling or pressurisation. This often has an unsatisfactory effect on operating costs, especially if refrigeration is needed in warmer climatic zones
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