Fabrication of Novel Membranes for Desalination in Membrane Distillation (MD) Process

• Main Author: SAIKAT SINHA RAY
• Other Authors: SHIAO-SHING CHEN
• Format: Others
• Language: en_US
• Published: 2018
• Online Access: http://ndltd.ncl.edu.tw/handle/5p7n5g

博士 === 國立臺北科技大學 === 環境工程與管理研究所 === 106 === Currently, Membrane distillation (MD) is one of the emerging processes for sustainable desalination. The membranes materials commonly utilized in MD process are made of highly hydrophobic materials, such as polyvinylidene fluoride (PVDF), polypropylene (PP), and polytetrafluoroethylene (PTFE). Typically, these membranes are commercially available casted by utilizing different techniques such as phase inversion, stretching, thermally induced phase separation. In this study, five types of novel membranes were investigated and compared with the commercially available membranes. Electrospinning technique has been utilized in the first three chapters whereas, knife casting device was used for next two chapters for fabricating effective membrane for MD process. The objectives of this study were focused on (1) study the effect of operational conditions of different techniques (electrospinning device, knife casting device) for membrane fabrication of MD membranes; (2) fabrication of hydrophobic and superhydrophobic membranes; (3) to analyse membrane characteristics; (4) using the proposed membrane for specific MD applications (i.e. desalination); and (5) investigate the long-term performance of fabricated membrane. A novel category of improved performance membrane consisting of a hydrophobic mat and a hydrophilic electrospun layer for MD application has been presented. The nanofibrous nonwoven layer was fabricated by electrospinning of polyvinylalcohol (PVA) incorporated with Triton X-100 directly onto the polypropylene (PP) mat. To render the MD process effective in terms of high permeate flux and salt rejection %, the concept of dual layer membrane is utilized with hydrophobic PP mat on top and hydrophilic PVA layer on bottom. In this study, PVA nanofibrous layer has been fabricated by incorporating non-ionic surfactant Triton X-100 for uniformity and homogeneity in fiber diameter. Additionally, the PP mat acts as a top support layer and PVA-TX nanofiber acts as base layer which absorb the water molecules (condensed vapor), that enhances the vapor flux across the membrane. In the next study, a novel class of enhanced performance membranes consisting of a highly hydrophobic nanofibrous layer and hydrophilic cellulose filter paper (CFP) for membrane MD was thoroughly examined. The nanofibrous layer was produced through electrospinning of polysulfone (PSF) doped with a sodium dodecyl sulfate (SDS) surfactant for uniformity and homogeneity in fiber diameter. The SDS surfactant was found to be the ideal additive for increasing the conductivity of the PSF polymeric solution, which helps in lowering the critical voltage required to initiate the electrospinning process, resulting in greater elongation of the nanofibers because of the increase in charge density. In the fabricated membranes, the PSF– SDS nanofibrous layer acts as a top active layer, whereas the CFP acts as the bottom supportive layer. Cellulose paper can absorb water molecules, which enhances vapor flux across the membrane in MD. Superhydrophobic membranes are necessary for effective membrane-based seawater desalination. This paper presents the successful fabrication of a novel electrospun nanofibrous membrane composed of polysulfone and Cera flava, which represents a novel class of enhanced performance membranes consisting of a superhydrophobic nanofibrous layer and hydrophobic polypropylene (PP). Cera flava, which helps lower the surface energy, was found to be the ideal additive for increasing the hydrophobicity of the polysulfone (PSF) polymeric solution because of its components such as long-chain hydrocarbons, free acids, esters, and internal chain methylene carbons. In the fabricated membrane, consisting of 10 v/v% Cera flava, the top PSF– CF nanofibrous layer is active and the lower PP layer is supportive. The hybrid membrane possesses superhydrophobicity, with an average contact angle of approximately 162°, and showed high performance in terms of rejection and water flux. This work also examined the surface area, pore size distribution, fiber diameter, surface roughness, mechanical strength, water flux, and rejection percentage of the membrane. The salt rejection was above 99.8%, and a high permeate flux of approximately 6.4 LMH was maintained for 16 h of operation. This study presents the successful casting of a three-layered membrane composed of a top superhydrophobic coating onto a polypropylene (PP) mat through simple sol–gel processing of Octadecyltrimethoxysilane (OTMS), and the bottom layer was casted with hydrophilic poly(vinyl alcohol) (PVA) by using a knife casting technique; this membrane represents a novel class of improved-performance membranes consisting of a top superhydrophobic coating onto a hydrophobic PP mat and hydrophilic layer (PVA) at the bottom. OTMSs are well known lowsurface-energy materials that enhance superhydrophobicity, and they were observed to be the ideal chemical group for increasing the hydrophobicity of the PP mat. The PVA layer acted as base layer absorbing the condensed vapor and thus enhancing the vapor flux across the membrane. The hybrid three-layered membrane exhibited superhydrophobicity, with an average contact angle of more than 160°, and demonstrated high performance in terms of rejection and water flux. A novel superhydrophobic octadecyltrimethoxysilane (OTMS)-modified composite membrane was fabricated by incorporating carbon black (CB) into a polyvinylidene fluoride (PVDF) membrane. CB has been treated with different methods, including extraction and hightemperature calcination, to modify the nature of the surface. The incorporation of CB into a PVDF solution enhanced the mechanical and tensile properties of the fabricated membrane. OTMSs are known for their low surface energy, which improves hydrophobicity. Moreover, OTMSs were found to be the ideal agents for increasing the hydrophobicity of PVDF membranes. The results of surface treatment with OTMS, CB loading, particle size, and membrane properties and their effect on desalination performance were thoroughly studied. The composite membrane was observed to be superhydrophobic with a contact angle of more than 150° and exhibited high desalination performance in terms of permeate water flux and salt rejection.