Thermodynamic Analysis of 1-Cyclhexylpiperidine for SPS FO Water Treatment Processes

Tertiary Amines have been found to switch polarity upon reaction with CO₂ to form highly concentrated solutions that can be utilized to draw fresh water from high osmotic pressure mixtures such as seawater. By studying the ‘switch’ (gas contacting and degassing reactions) of these switchable polarity solvents we can aid in the optimization and scale-up of SPS FO water treatment processes. 1-cyclohexylpiperidine (CHP) has been found to produce up to 70 wt% solutions with an osmotic pressure over 500 atm. Gas contacting experiments on this species were performed at variable temperatures and pressures of CO₂, and concentrations were measured through freezing point osmometry. Results show that 1-cyclohexylpiperidine produces higher concentration solutions at lower temperatures and higher partial pressures of CO₂. The rate of draw solution formation decreases as temperatures and partial pressures of CO₂ go down. It was determined that optimal temperatures for these reactions were greater than around 77 °C for degassing and less than 51 °C for gas contacting operations. By deriving an equilibrium equation that models this data and performing a van’t Hoff analysis, the values of ΔH° and ΔS° were determined to be -68.9 kJ/mol and -212 J/mol/K respectively. Some general kinetic implications can be garnered from this data, but further experiments are suggested to verify these kinetic models and obtain accurate rate constants.