Solid-Liquid-Liquid Equilibria of the System Water, Acetonitrile, and Ammonium Bicarbonate in Multiphase Reacting Systems

The salting out of acetonitrile from water in the presence of ammonium bicarbonate was studied at atmospheric pressure from 4 to 35 °C. The coexisting phases were analyzed independently with H1 nuclear magnetic resonance, ion chromatography, calorimetry, and titrations. The binodal region was described with semiempirical functions and with the UNIQUAC thermodynamic model. The critical composition at ambient temperature is given by 0.02 g/g of salt and 0.58 g/g of acetonitrile: at lower salt concentrations; this mixture is widely employed in liquid chromatography, thanks to the buffering properties of the bicarbonate. The separation of acetonitrile and water for extraction purposes, instead, is often obtained with other ammonium salts. These ternary equilibrium data can then be useful to develop extraction and analysis procedures at buffered, mildly alkaline pH. In addition, this mixture can be encountered during the acetonitrile synthesis proposed from bioethanol and bioethylene, so these multiphase equilibria govern the separation and purification of acetonitrile in specific reactors that allow the recovery of ammonium bicarbonate as a valuable byproduct. Furthermore, the solubility of this salt in alcohols and its decomposition were investigated to target its stability in multiphase reactors, extractors, and driers.