Fabrication Of Asymmetric Water-permeable Membrances And The Permeation Properties Of Heat, Moisute, And VOCs Through Membranes

Total heat exchangers are the key equipments for heat and moisture recovery fromventilation air. With total heat exchanges,70-80%of energy for conditioning fresh air couldbe saved. The high moisture permeability is necessary. Meanwhile, the membrane should beimpermeable to VOCs, to prevent the fresh air from being polluted by exhaust air. However,there is little information concerning the transmission of VOCs in a total heat exchanger in theopen literature. In this dissertatation, the moisture/VOC selectivity of these membranes isevaluated. The ideal membrane should have high moisture permeability and highmoisture/VOC selectivity simultaneously. The permeability of moisture and VOCs throughmembranes is measured. The experimental results show that dense membranes play key rolesin moisture/VOC selectivity. So the permeability mechanism of small molecular gases (vaporand VOCs) in membranes is studied carefully. The main works are summarized as following:(1) PVDF porous membranes are prepared as the substrate by wet phase inversiontechnology. Then10composite membranes are formed by coating the casting solution ontothe PVDF porous substrate layers. The10dense skin layers include either hydrophilicmembranes such as polyvinyl pyrrolidone (PVP), polycinyl alcohol (PVA), polyacrylamide(PAM), solium alginate [Na(Alg)], chitosan (CS), cellulose acetic (CA), ethylene cellulose(EC), or hydrophobic membranes, such as polypropylene (PP) and polydimethylsiloxane(PDMS), PVA with the additive LiCl and without the additive LiCl are named PVA-1andPVA-2, respectively. Then the permeability of moisture and5VOCs through compositemembranes and porous PVDF membranes is measured. The five VOCs selected are aceticacid, formaldehyde, acetaldehyde, acetaldehyde, toluene and ethane. The results shows thatthe ratios of skin layer resistance to overall resistance are invariant to skin layer types,because the skin layer resistance is substantial. The heat transfer capability of all membranesis high.(2) A one-step approach is employed to prepare an asymmetric cellulose acetate (CA)membrane. A porous support layer and a dense skin layer are formed simultaneously duringthe solvent and coagulant medium exchange. The permeability tests found that the approach issuccessful in making membranes for heat and moisture recovery. The membrane has high moisture mass transfer coefficient and high moisture/VOC selectivity. Analysis found that theoptimum solution compositions for casting membrane are: CA,10wt.%, acetic to deionizedwater,70:30. The advantages are:(1) the fabrication process of membrane is simple andenvironmental-friendly;(2) The dense layer is thinner than that of composite membrane, andthe moisture permeability is high.(3) High moisture permeability and high selectivity of moisute to VOCs are two keycriteria for material selection of water-permeable membranes. The permeability mechanismfor moisture and VOCs in dense membrane is solution-diffusion mechanism. PVA-1is thebest material. It has high vapor permeability (3.7010~-8m~2s~-1) and high moisture to VOCselectivity (360,333,340,451and333for acetic acid, formaldehyde, acetaldehyde, tolueneand ethane, respectivity). Following PVA-1are PVP and PAM. These membranes canprevernt the permeation of VOCs from the exhaust air stream to the incoming fresh air stream.In cantrast, PDMS is the worse material for total heat exchangers, followed by PP and EC.(4) Molecular dynamics (MD) simulation is used in analyzing the trsnaport mechanismsin membranes. A polymer molecular model is set up by Amorphous Cell module in MaterialStudio5.0. The sorption and diffusion processes are simulated by Grand canonical MonteCarlo (GCMC) and MD methods, respectively. The solution-diffusion mechanism inpermeability process is studied to disclose guidelines for selecting materials. The results showthat:(1) solubility has more influences than diffusivity on water vapor permeability;(2) Astrong interaction force between the polymer chains and water vapor is good to sorptionprocesses;(3) suitable additives of small molecules are useful for water vapor permeabilityenhancement.