Pervaporation of ethanol–water mixtures through poly(1-trimethylsilyl-1-propyne) (PTMSP) membranes

The separation of alcohol–water mixtures by pervaporation has been preferentially carried out through hydrophobic membranes because it is more practical to permeate alcohol through the membrane. Nevertheless, most polymeric membranes are water permselective as the molecular size of water is smaller than that of ethanol. Ethanol-permselective membranes have been limited to silicon- and fluorine-containing polymers, for example, polydimethylsiloxane (PDMS), poly(1-trimethylsilyl-1-propyne) (PTMSP) and poly(tetrafluoroethylene) derivatives. PTMSP is known to show preferential permeation of ethanol in the pervaporation of ethanol–water mixtures. The permselectivity of this material is related to the existence of free volume and to the hydrophobicity of the membrane surface. The EtOH separation factor ( α H O ) reached a value of 10.7 at 10 wt% of ethanol in the feed, and after 450 h it decreased to 2 an average value of 8. The initial value is similar to that of PDMS, a well-known etanol-permselective membrane. Although PTMSP presented good characteristics for the separation of ethanol–water mixtures, the separation factor and the specific permeation rate (R) decrease with operation time due to a swelling process; however, the latter reached a value similar to the initial one after this period.

The performance of the membrane has been studied over a period of 450 h, and the separation factor decreased with time at low rate, so that the study of the membrane degradation process would require long time experimental runs. On the other hand, the specific permeation rate could be considered constant with operation time, although a slight decrease could be observed at about 200 h of operation. As there was no evidence that the temperature affected this variable the decrease of the specific permeation rate was ascribed to a swelling process.