| Dissolved oxygen (DO) is the oxygen molecule from ambient air dissolved in water. The dissolved oxygen has strong oxidative property under high temperature and high pressure, which is readily to accelerate the metal oxidation and corrosion, and has adversely effects in many industrial applications, such as semiconductor, power plant, food, biotechnology, and pharmaceutical productions. There are several traditional methods used for DO removal, such as thermal degassing, vacuum degassing, purging with nitrogen, and oxidation-reduction reaction by adding sodium sulphite or hydrazine. The conventional physical methods often have bulky constructions and high operating costs, and the chemical methods are not very desirable because of the problems of changing the water quality affected by the reducing agents (such as toxicity or the ionic content of water). These methods have some disadvantages which require the development of new technology to replace.Membrane technology for deoxygenation is a new type of Gas-Liquid membrane separation technology. In recent years, membrane process has attracted more and more attention due to its great advantages over conventional deoxygenation processes, including faster mass transfer efficiency, larger surface area per unit volume, tighter modular construction, environmental protection, higher efficiency and lower energy costs. Over the past several years, research in membrane technology for DO removal from water has mainly focused on the improvement of design membrane module and devices, the feasibility of fabricating high performance membranes for deoxygenation process is less studied. The object of this study is to provide highly permeable and easily industrial polymer matrix membranes for removal of the dissolved oxygen.An ideal membrane used for deoxygenation process is often required for both high flux and selectivity to improve the degassing efficiency. The polymer membrane for removal DO from water can be divided into two basic types:non-porous and porous membrane. In this study, two polymr maerials, polydimethylsiloxane (PDMS)and polyvinylidene fluoride (PVDF), are coniderd to explore the deoxygenated effect and mass transfer process from two different separation mehanisms. The PDMS membrane with, excellent permeability is the separation principle of solution-diffusion mechanism, and the hydrophobic surface of porous PVDF membrane is the separation principle of micro-diffusion mechanism. This study focuses on the modification and characterization of the membrane, the membrane properties of deoxygenation process, and oxygen mass transfer analysis.Since the separation performance of non-porous homogeneous membrane depends on the intrinsic properties of the polymer material, the selectivity of membrane material largely determines the separation effect and permeation flux; therefore, the membrane material needed large diffusion coefficient for oxygen. A cross-linked matrix composed of PDMS membrane with incorporated the silica networks was manufactured. The membrane properties and morphology structure were characterized by FT-IR, SEM and cross-linking density measurements. The PDMS hybrid membranes on the deoxygenation experiment by vacuum degassing process were investigated. Results showed that the PDMS hybrid membranes improved in the oxygen removal efficiency effectively at different TEOS content, and the best performance was obtained when the weight ratio of PDMS/TEOS concentrations was10:5. With the TEOS/PDMS ratio rising from0.1to0.5, the oxygen flux increased from1275to2182mg-m-2?h-1while the selectivity increased from109.8to139.7. The effects of operating parameters were also investigated, results indicated that the degassing performances are related to the operating temperature, vacuum level and feed flow rate. When the membrane was nonporous, the mass transfer process is complianced with the solution-diffusion model, the temperature dependence of the permeation flux could be expressed by Arrhenius relationship.Porous membranes used for degassing from water usually are characteristic of microporous structure and hydrophobicity in order to promote mass transfer between phases. A high-performanced PVDF membrane is prepared with phase inversion method, in order to improve the membrane deoxygenation performance effectively, the influence of PVDF and LiCl concentrations in casting solution was studied, membranes were examined by SEM, contract angle, porosity and pore size and FT-IR, and the membranes on the deoxygenation experiment by vacuum degassing process were investigated. Results showed that with increase the PVDF concentration, the permeation flux of oxygen was decreased. And the best concentration of casting solution was15wt.%. The membrane morphologic was strongly influenced by the adding of LiCl.The presence of LiCl makes membrane a higher porosity and mean pore size, the deoxygenation performance was improved. The hydrophobic of membrane was maintained, which is very suitable for deoxygenation process. PVDF membrane is an ideal hydrophobic material for removal DO from water. A hydrophobic organic-inorganic composite membrane was prepared by the incorporation of nano SiO2to the PVDF solution via phase inversion method, which was intended for DO removal application. In order to maintain the hydrophobicity of PVDF MMMs, SiO2surface was grafted with n-dodecyl chains by a silane coupling agent dodecyltrimethoxysilane (DTMS). The structures and properties of PVDF composite membranes were characterized, and the degassing performance in a laboratory scale membrane degassing system for DO removal from water under vacuum condition had been studied. The effect of operating temperature, downstream vacuum level and water flow rate were investigated. The long-term experiment was also investigated to evaluate the membrane performance. Results indicated that the introducing of SiO2particles affected the morphologies and properties significantly. As compared with the original PVDF membranes, the hybrid membranes improved the oxygen removal efficiency effectively, and the maximum permeation flux was obtained when the SiO2loading was2.5wt.%. The final dissolved oxygen concentrations of PVDFmembranes were kept the same under the long-term treatment with a decrease to the same DO concentration of15ppb.A matrix composed of PVDF membrane with dispersed Fe3O4particles was prepared with phase inversion method and applied to removal of dissolved oxygen from water. Fe3O4particles were modified by a silane coupling agent n-dodecyltrimethoxysilane to preserve the hydrophobicity of Fe3O4/PVDF hybrid membranes. The FTIR, contact angle, porosity, pore size and morphology were also examined. Compared with the original PVDF membranes, the Fe3O4/PVDF hybrid membranes improved in the oxygen removal efficiency effectively at different Fe3O4loadings. It had the maximum oxygen removal efficiency and oxygen permeation flux when Fe3O4concentration was10wt.%. Oxygen permeation flux of10wt.%was2997.47mg/m2h, which was about2times higher than that of pure PVDF membrane (1488.61mg/m2h). The modified Fe3O4/PVDF membrane had much enhancement of productivity and efficiency in degassing process. With the introducing of magnetic field, the permeation flux of oxygen was improved, which suggest that the paramagnetic oxygen molecular was attracting by the magnetic field. Compared to the modified membrane of oxygen separation properties, Fe3O4/PVDF hybrid membrane improved the separation performance significantly. |
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