Fundamentals of 'Gas-Assisted' Separations for Dilute Aqueous Systems

The separation of dilute aqueous systems is a reoccurring challenge within industry, especially for processes with biomass or bio-renewable based feedstocks.  Many processes involve energy-intensive separations such as distillation, liquid-liquid extraction followed by distillation, etc. which significantly affects costs and sustainability.  As a potential alternative to thermal-based approaches, we introduce a method called “Gas-Assisted” Separations.  Here, a gas is dissolved into the aqueous solution to induce the target solute from the aqueous phase into a new phase (liquid or solid) rich in that target solute.  The systems can be decanted/filtered to recover the solute and the gas can be recovered with depressurization.  These processes differ from supercritical fluid technology where the compressed gas/fluid is the solvent itself and require much higher pressures.  This “salting-out” with an easily recoverable dissolved gas “antisolvent” (versus a solid salt or liquid, etc.), may have very different molecular mechanisms for each gas and solute combination.  For instance, the use of gaseous CO2 would lead to both a decrease in pH and increase in the ionic strength which may have specific interactions with a given solute.  In this presentation, the fundamental phase equilibrium will be overviewed for this vapor-liquid-liquid equilibrium (VLLE) behavior with temperature, pressure, and initial loading.  Examples from literature will be used to illustrate the process.