Temperature and Pressure Effects on the Separation Efficiency and Desorption Kinetics in the NH₃–CO₂–H₂O System

The present work is a continuation of a previous study on the separation efficiency and desorption kinetics of thermolytic draw solutions in forward osmosis (FO). The aim of the present work was to study the combined effects of temperature (30, 40, and 50 °C) and pressure (0.75, 0.55, and 0.35 bar) on the separation efficiency of the draw solutes from an aqueous carbonated ammonia (NH₃–CO₂–H₂O) draw solution and compare them to aqueous carbonated trimethylamine (TMA–CO₂–H₂O). In the present work, CO₂ proved to desorb faster from solution compared to NH₃ with an observed NH₃:CO₂ average molar ratio in solution of 1.71, ranging from 1.07 to 2.64. The separation process resulted in almost 90% draw solute removal after 90 min at 50 °C, 0.35 bar. The draw solute desorption kinetics were studied based on the overall concentration of total dissolved NH₃ and CO₂ in the draw solution. According to the proposed mechanism, the decomposition of NH₄[NH₂CO₂] in the aqueous phase proved to be the rate-limiting step of the separation process.