Douglas R. Lloyd

Microporous membrane formation via thermally-induced phase separation. II. Liquid—liquid phase separation

Abstract Microporous membranes have been prepared via thermally-induced liquid—liquid phase separation of isotactic polypropylene—n,n bis (2-hydroxyethyl) tallowamine mixtures. The thermally-induced phase separation process is discussed in terms of the thermodynamics of the binary mixture and possible phase separation mechanisms. It is demonstrated that membranes can be produced by liquid—liquid phase separation followed by solidification of the polymer or by solid—liquid phase separation.

Microporous membrane formation via thermally-induced phase separation. III: Effect of thermodynamic interactions on the structure of isotactic polypropylene membranes

Abstract Isotactic polypropylene membranes were prepared via thermally-induced phase separation using three isotactic polypropylene—diluent systems, at three concentrations, and two thermal histories. The three diluents had similar structure and molar volume, but differed in end-group structure and therefore differed in interactions with the isotactic polypropylene (iPP). The two thermal histories represented isothermal and non-isothermal thermally-induced phase separation processes. Membrane structure as determined by scanning electron microscopy depended on the phase separation mechanism (liquid—liquid versus solid—liquid), which depended on the concentration-dependent isotactic polypropylene—diluent interactions and the thermal history.

Membrane distillation. II. Direct contact MD

Abstract Pure water direct contact membrane distillation (DCMD) experiments were used to measure the permeability parameter associated with the molecular diffusion in membrane distillation (MD). The fluxes given by a recently reported MD model, which is based on the dusty-gas model of gas transport through porous media, showed good agreement with the experimental results over the entire range of feed temperatures studied. The model was also capable of predicting flux as a function of the difference between bulk feed and permeate temperatures for the limiting case in which only molecular diffusion contributes to flow. The DCMD experiments were performed in this work with a new laboratory-scale module that does not require a support for flat-sheet membranes. The resulting DCMD fluxes were two to three times higher than those reported in the literature for either DCMD or reverse osmosis. The MD model was also used to predict the performance of DCMD desalination, and the results were compared to those of reverse osmosis, in terms of both water production rates and NaCl rejection.

Membrane distillation. I. Module design and performance evaluation using vacuum membrane distillation

Pure water vacuum membrane distillation (VMD) experiments were performed to evaluate the heat and mass transfer boundary layer resistances in a new laboratory-scale membrane module. The membrane module designed for this work is unique in that it can use flat-sheet membranes without a support. Additionally, the membrane module and associated apparatus were designed to achieve relatively high feed and permeate Reynolds numbers within the module. These two factors led to a dramatic reduction in boundary layer resistances, which resulted in improved VMD fluxes. This paper also examines a new complete VMD model based on the dusty-gas model, which accounts for both Knudsen and viscous mass transport across the membrane. The new model was used to predict the performance of VMD with pure water and ethanol-water solutions.

Formation of anisotropic membranes via thermally induced phase separation

Abstract The applicability of the thermally induced phase separation (TIPS) process to the production of anisotropic membranes was investigated. To induce an anisotropic structure, diluent was evaporated from one side of the polymer–diluent melt-blended, thereby creating a concentration gradient in the nascent membrane prior to inducing phase separation. The system used to prepare these membranes was isotactic polypropylene (iPP) in diphenyl ether. The resulting membrane structures showed that this evaporation process was useful in producing anisotropic structures. The effects of evaporation time and initial polymer concentration on the anisotropic membrane structure were investigated. The evaporation process was analysed by solving appropriate mass transfer and heat transfer equations. The agreement between the calculated results and the experimental data on the membrane weight loss and the membrane thickness was satisfactory. The membrane structures are discussed in detail based on the calculated polymer volume fraction profiles in the membranes.

Microporous membrane formation via thermally-induced phase separation. V. Effect of diluent mobility and crystallization on the structure of isotactic polypropylene membranes

Abstract The role of diluent in thermally-induced phase separation membranes was examined for solid—liquid phase separation systems in terms of the diluent mobility and crystallization temperature. Inter- and intra-spherulitic voids were formed in isotactic polypropylene/n-alkane and isotactic polypropylene/n-fatty acid systems. The diluent mobility played an important role in determining those structure. The diffusivities of some diluents through the isotactic polypropylene melt were measured by microdensitometry to support the explanation of the structure in terms of diluent mobility. Diluents with high crystallization temperatures affected the structure due to the diluent crystallizing prior to polymer crystallization.

Microporous membrane formation via thermally-induced phase separation. IV. Effect of isotactic polypropylene crystallization kinetics on membrane structure

Abstract Non-nucleated and nucleated isotactic polypropylene microporous membranes have been prepared via thermally-induced solid—liquid phase separation of polymer-diluent mixtures. Three linear alkanes and mineral oil were used as the diluents and adipic acid was used as the nucleating agent. The membranes were formed under isothermal and non-isothermal crystallizing conditions. Membrane structures as determined by scanning electron microscope depended on polymer concentration, cooling rate, and the addition of nucleating agent.

Microporous membrane formation via thermally-induced phase separation. VI. Effect of diluent morphology and relative crystallization kinetics on polypropylene membrane structure

Abstract Microporous membranes were prepared via thermally-induced phase separation of isotactic polypropylene blended with hexamethylbenzene. The effects of melt composition and isothermal crystallization temperature on the microstructure of the resulting membranes were determined via scanning electron microscopy. The structures formed were interpreted in terms of the sequence and mechanism of phase transformations in the supercooled melt.

Microporous membrane formation via thermally-induced phase separation. VII. Effect of dilution, cooling rate, and nucleating agent addition on morphology

Abstract Microporous polypropylene films were produced from nucleated and non-nucleated melt-blends of isotactic polypropylene (iPP) and dotriacontane (C32H66) using non-isothermal thermally-induced phase separation. Adipic acid was used as the nucleating agent. The morphology of these microporous iPP films as determined by thermal optical microscopy and scanning electron microscopy is reported. The effect on film structure of diluent crystallization is also shown.

Compaction of microporous membranes used in membrane distillation. I: Effect on gas permeability

Abstract Membrane compaction effects are well documented in RO, UF, and gas separation, but little attention is paid to compaction effects in microporous membrane processes. This paper examines the effects of compaction on the gas permeability of microporous membranes to be used in MD. Contrary to what is observed in RO and UF systems where membrane permeability is reduced by compaction, limited compaction of microporous membranes can result in enhanced permeability. This enhancement is the result of decreased membrane thickness, which increases the pressure gradient across the membrane. An experimental method is devised to determine the effects of membrane compaction on permeability, and a theory for modifying flux equations to include a parameter for membrane compaction is introduced.