Thermodynamic Properties and Transport Behavior of Glycol Ether - Water Systems for Forward Osmosis

Abstract

The rising use of desalination in recent years has created a need to improve desalination techniques. The current leading method of desalination is Reverse Osmosis (RO). Forward Osmosis (FO) is emerging as a lower energy alternative technique for desalination. FO uses a draw solution with a higher osmotic pressure than sea water to draw water through a membrane. The diluted draw solution is then separated into a concentrated draw solution stream and a water-rich stream preferably by using low energy separation methods. The concentrated draw solution is then recycled back to the FO process and the water-rich stream is then purified further using other methods. The draw agent plays an important role in the effectiveness of the FO process. The draw solution must have the following characteristics: high osmotic pressure, minimum reverse draw solute flux to the feed side, no toxicity, low cost, and most importantly easy recovery. One of the most promising draw agents to use in FO are a class of organic chemicals that, when in aqueous solutions, manifest either a lower critical solution temperature (LCST) or an upper critical solution temperature (UCST) or both. The optimal FO draw solution with a lower critical solution temperature will have a high osmotic pressure in comparison to sea water, undergo rapid phase separation at temperatures slightly above their LCST yielding an essentially draw agent-free water-rich phase, and suffer from minimal concentration polarization during the FO process. The primary objective of the thesis was to characterize the thermodynamic driving forces and transport behavior of LCST exhibiting draw solutions in the context of their viability as a FO draw solution. The goals of the research were to: (1) demonstrate that by mixing glycol ethers (GEs) with different LCSTs the LCST of the solution can be shifted, (2) measure how osmotic pressure changes with composition at different temperatures for glycol ether solutions exhibiting different LCSTs, and (3) determine kinematic viscosities and self-diffusion coefficients under different conditions. A mixture of tripropylene glycol mono-n-butyl ether (TPGBE) and tripropylene glycol methyl ether (TPGME) were chosen as model draw solutions. The model system of TPGBE and TPGME demonstrated the capacity to tune the LCST. Both the vapor pressure osmometry data and water activity data determined that osmotic pressure increases as temperature decreases, osmotic pressure increases as draw solution concentration increases and osmotic pressures were high enough for FO. The kinematic viscosities of the various compositions of draw solutions decreased with increasing temperature and increased with increasing GE concentration. The NMR diffusion experiments revealed that the TPGBE and TPGME molecules diffuse at similar rates and their self-diffusion coefficients decreased with increasing GE concentration. The ability to tune the LCST to the desired temperature and high osmotic pressures make TPGBE+TPGME draw solutions viable for forward osmosis. However, these chemicals do not exhibit ideal phase separation for draw solution recovery.