Separating polymer solutions using high pressure lower critical solution temperature (LCST) phenomena

Abstract

AbstractA lower critical solution temperature (LCST) phase split can be used as an alternative to steam stripping for separating polymer solutions, By adding a supercritical fluid (SCF) additive to the polymer solution, the LCST can be lowered, thus minimizing the possibility of polymer degradation and also reducing the thermal energy requirements for the process. Experimental results for the poly(ethylene‐co‐propylene)‐hexane‐SCF ethylene system are shown as an example of the type of phase behavior observed with polymer‐solvent‐SCF additive solutions. Adding 20 percent (w/w) ethylene to the polymer solution lowers the temperature of the LCST by 109°C. The addition of 30 percent (w/w) ethylene to the polymer solution lowers the temperature of the LCST curve sufficiently to merge this curve with the upper critical solution temperature (UCST) curve. When the lower critical end point (LCEP) is plotted against the critical temperature of the solvent the data for poly(ethylenie‐co‐propylene)‐solvent systems are well represented by a single curve. A more fundamental modeling approach is needed to estimate the pressure of the LCEP and the concentration of SCF additive necessary to merge the LCST and the UCST curves. Patterson's theory of corresponding states can be used for these calculations.