Forward osmosis (FO) is an emerging membrane technology promising low cost water recovery from high salinity effluents that involves two steps to recover water. Initially, a concentrated draw solution (CDS) recovers water from the effluent, and subsequently, the diluted draw solution (DDS) must be treated for water recovery and draw solution regeneration. Water recovery into the CDS requires almost no energy; however, energy is required to recover water from the DDS and regenerate the CDS. Aqueous carbonated trimethylamine (TMA-CO2-H2O) is an effective draw solution as it consists of highly soluble compounds, namely TMA and CO2, and results into CDSs of high osmotic pressure, capable to achieve a high water recovery yield. Further, TMA and CO2 undergo a phase transformation, from aqueous to vapor, upon mild heating of the DDS, thus facilitating water recovery from it.
A fundamental theoretical and experimental investigation of the ternary system TMA-CO2-H2O as draw solution in Forward Osmosis (FO) was conducted. A new database for the Mixed Solvent Electrolyte (MSE) model of the OLI Systems (www.olisystems.com) software was developed to determine the interaction parameters, using new experimental data and regression analysis. The experimental data included VLE, speciation, pH, density, and conductivity. The database developed was shown to accurately predict the chemical properties of aqueous trimethylamine solutions (TMA-CO2-H2O) within 15% within the temperature range 20 to 60 °C.
Further, the TMA-CO2 separation efficiency and decomposition kinetics were systematically studied and a decomposition mechanism for the draw solution was proposed. The rate determining step was found to be TMA desorption from the DDS and the draw solute separation process stoichiometric. The temperature and pressure effects on the draw solute separation efficiency were also quantified. A new methodology to obtain speciation data at high temperatures was developed and was validated experimentally. This methodology is applicable to chemical systems that share similar properties to the ternary TMA-CO2-H2O and can be used in kinetics studies.
Finally, an energy efficient draw solution separation and regeneration process was designed and operated at both laboratory and pilot-scale. The experimental results were compared to simulations carried out in the OLI Flowsheet process modeling software within 15% error. The specific equivalent work was found to range from 6.8-16.7 kWh/m3 of fresh water produced. Therefore, the niche of the TMA-CO2-H2O based FO process in the water recovery industry was identified in the treatment of high salinity effluents, as this process is more energy efficient that the conventional water recovery processes.Ph.D
