PVAm-PVA Composite Membranes Incorporated with Carbon Nanotubes and Molecular Amines for Gas Separation and Pervaporation
This study deals with polyvinylamine (PVAm)-poly(vinyl alcohol) (PVA) based composite membranes incorporated with carbon nanotubes (CNTs) and molecular amines (e.g., piperazine (PZ), triethanolamine (TEA), N-methyldiethanolamine (MDEA), PZ/TEA and PZ/MDEA blends, diethylenetriamine (DETA) and triethylenetetramine (TETA)) for CO2 separation, solvent dehydration by pervaporation, and hydrogen purification. The effects of the parameters involved in the procedure of membrane formation and operating conditions on the membrane performance were investigated. Composite membranes comprising of a skin layer of PVAm-PVA incorporated with CNTs and a microporous polysulfone substrate were developed for CO2 separation from flue gas and dehydration of ethylene glycol by pervaporation. The membranes were characterized with Fourier transform infrared (FTIR), Raman spectroscopy, X-ray diffraction (XRD), contact angle measurement and water sorption uptake, using dense films of PVAm-PVA/CNTs, to determine the effects of CNTs on the intermolecular interactions, degree of crystallinity, surface hydrophilicity, and degrees of swelling of the membranes. For CO2/N2 separation, adding CNTs in the membrane was shown to enhance CO2 permeance while retaining a similar CO2/N2 selectivity; a CO2 permeance of 18.5 GPU and a CO2/N2 ideal selectivity of 64 were obtained at 0.6 MPa feed pressure. For pervaporative dehydration of ethylene glycol, the incorporation of CNTs into the membrane was shown to increase both the permeation flux and separation factor, and at 70℃ a permeation flux of 146 g/(m2.h) and a separation factor of 1160 were achieved at 1 wt% water in feed using a PVAm-PVA/CNT composite membrane containing 2 wt% MWNTs. Novel facilitated transport membranes containing both PVAm as fixed carriers and various molecular amines as mobile carriers were fabricated and used for CO2 separation from N2 and H2, as well as CO2 separation from ethanol fermentation off gas. For membranes containing a single amine (i.e., PZ, DETA or TETA), the CO2 permeance increased with an increase in the amine content in the membrane until the amine content is sufficiently high, beyond which a further increase in the amine content would decrease the membrane performance. The facilitation in CO2 transport was more significant with membranes containing mixed amines (e.g., PZ/TEA and PZ/MDEA). Among all the molecular amines tested, TETA was shown to be most effective in facilitating CO2 transport in terms of CO2/N2 permselectivity. Using a PVAm-PVA/TETA composite membrane with a TETA to polymer (i.e., PVAm plus PVA) mass ratio of 150/100, a CO2 permeance of 22.6 GPU and a CO2/N2 selectivity of 86.5 were obtained at 0.6 MPa feed pressure for the removal of CO2 from flue gas, a CO2 permeance of 23.3 GPU and a CO2/H2 selectivity of 28.5 were obtained at 0.6 MPa feed pressure for CO2 separation from H2, a water vapor permeance of 16700 GPU was obtained at 25℃ and 2.5 mol% water vapor concentration in the feed for dehydration of ethanol fermentation off gas.