Separation Of Ethylene Glycol Aqueous Solution By Novel Pervaporation Membranes

Pervaporation process will find promising applications in dehydration of ethylene glycol (EG), due to its inherent green, efficient and energy-saving characteristics. The core of pervaporation process is the development of appropriate membrane material. In this dissertation, Poly(vinyl alcohol) (PVA) was selected as main membrane material. PVA-SiO2 hybrid membranes, surface crosslinked PVA membranes and surface crosslinked PVA/PES (or PS) composite membranes were prepared. The coupling effect was investigated in consideration of the hydrogen-bonding interaction among ethylene glycol, water and PVA membrane. In addition, the optimization of process conditions was carried out systematically.PVA-SiO2 hybrid membranes were prepared by sol-gel process using PVA as the host material, tetraethoxysilane (TEOS) andγ-glycidyloxypropyltrimethoxysilane (GPTMS) as mixed silica precursors based on the different hydrolysis-condensation rate of siloxane with different structures. The organic functional group of GPTMS has the function of network former, which can in situ modify the product of TEOS. Mercapto PVA-SiO2 hybrid membranes were prepared by in situ sol-gel reaction ofγ-mercaptopropyltrimethoxysilane (MPTMS) with organic functional group as network modifier and PVA based on the different hydrolysis-condensation structure of alkoxyorganosilane under acidic or basic conditions. The effects of alkoxyorganosilane content and preparation conditions on the structure, physical and chemical properties of hybrid membranes, as well as pervaporation performance were systemically investigated.Considering the strong plasticizing effect of water and EG on PVA membrane, dehydration of EG aqueous solution by pervaporation was studied by surface crosslinked PVA membrane for the first time, the homogeneous and stable crosslinking structure surpressed the swelling of PVA membrane and showed desirable stability. The surface crosslinked PVA membrane exhibited good separation performance with a separation factor of 934 and a permeation flux of 211 gm-2h-1 for 80wt% EG in feed at 70℃. The results of swelling, sorption measurments and the permeation and diffusion data indicated that there exists strong coupling effect between water and EG, which inhibited the sorption and diffusion of EG within membrane. The feed temperature is dominant factor of EG permeation because of the low saturated vapor pressure of EG.Interfacial reaction technique has been employed to fabricate PVA/PES (or PS) thin film composite membrane. The results indicated that dispersion of the interfacial crosslinking agent borax in the support layer intensified the interface interaction between the PVA layer and PES (PS) support layer. A homogeneous, defect-free thin film of PVA on the support layer produced due to a strong interaction between the thin PVA layer and the support layer. The PES support layer was treated with 0.1wt% borax aqueous solution, and then the dried support layer was immersed into 2wt% PVA aqueous solution. The PVA separating layer with an approximate thinkness of 1-1.5μm was formed through two time's dip-coating process. The permeation flux and separation factor is 427 gm-2h-1 and 438 for 80wt% EG in feed at 70℃, respectively. The surface crosslinked PVA/PES (or PS) composite membrane showed desirable stablily and separation performance for EG/water mixture during the investigation of process conditions for 60h. The structural stability of the composite membrane is satisfactory.