Sorption and pervaporation of methanol/water mixtures with poly(3-hydroxybutyrate) membranes

Abstract

In this contribution, poly(3-hydroxybutyrate) (PHB) membranes were evaluated in sorption and pervaporation experiments for methanol/water separation. Single component sorption tests showed that PHB membranes have higher affinity towards methanol than water: at 40ºC the sorption degree was 18.4 and 1.1% respectively. This was in agreement with contact angle measurements, performed with both liquids, and Hansen solubility parameter. PHB sorption degree in the whole methanol/water composition range and three temperature levels were determined, showing an anomalous behavior (sigmoid like curves). A simple technique, named by us ?Desorption by Dissolution?, was developed to determine sorbed mixture composition. It basically consists on dissolving the polymer after sorption equilibrium in the mixture has been reached, to completely extract the mixture. Then, chromatographic analysis is used to determine solution composition. It does not require adapting equipment or using vacuum devices neither cold traps. When both components were present, methanol grams sorbed in the membrane were always lower than the mixture in which they were immersed, while water sorption was enhanced due to methanol presence. Very low standard deviation values were registered (< 0.84) confirming the technique reliability. Single component fluxes and mass flux ratio in pervaporation increased with temperature: methanol flux was 3.5x10-5 kg m-2 s-1 at 25°C, whereas at 50°C exceeded 7x10-5 kg m-2 s-1; water flux increased from 1.8x10-5 kg m-2 s-1 at 30ºC to 2.2x10-5 kg m-2 s-1 at 50ºC. Since permeances decreased, the increase in fluxes with temperature can be attributed to the increase in vapor pressure driving forces. Mixture pervaporation results showed similar behavior, highlighting the fact that separation factor increased with temperature as well: 3.5 at 30ºC and 3.8 at 50ºC. Selectivities were higher than 12 for 10%wt water/methanol+water mixtures. Results were discussed in terms of polymer crystallinity, solubility parameters, component size, activation energies and cluster formation.