Performance analysis of multilayered ultrathin film membranes: A computational study
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Industrial membranes are often thin film composites comprised of a thin, dense skin layer (100 nm) performing molecular separation and a porous bulk of the membrane (150-200 μm) providing mechanical strength. As the selective layer is produced thinner to enhance the permeance and thus reduce the capital cost, the porosity and pore size of the microporous support may restrict the concentration profile of the permeant in the selective layer. The initial goal of this work is to elucidate the effect of porous support on the membrane permeance using an integrated experimental and simulation approach. Track-etched polycarbonate (PC) Nano-filtration membranes with well-defined pores were used as model porous supports. Thin film composite membranes were prepared comprised of a selective layer of perfluoropolymer (Teflon AF ® 1600) with a good film formation and good stability. The effect of the geometric restriction introduced by the porous support on the gas permeance is thoroughly evaluated as a function of membrane parameters including the thickness of the selective layer and the pore size and porosity of the PC supports. The experimental results are also consistent with simulation results obtained using a 3-dimensional computational model. Both experiments and simulations show that gas permeance decreases with increasing pore size and decreasing support porosity. For example, thin film composite membranes comprised of a 50 nm thick selective layer on a support with a porosity of 3.1% and pore radius of 25 nm show 10 times lower permeance than that without the porous support. The surface morphology of the support, geometric restrictions because of pore penetration is identified. To mitigate the aforementioned drawbacks, application of commercial tri-layer membranes that consist of two juxtaposed ultrathin layers, a selective layer and highly permeable "gutter" layer that are supported on a porous substrate is considered. The selective layer provides molecular separation, the gutter layer enhances the permeance of the penetrant and the porous support provides mechanical strength with negligible resistance to mass transport. The model, validated using published data for two-layer membranes is used here for the first time to systematically study the performance of a three-layer membrane as a function of key variables including layer thicknesses, porosity and pore size. A key finding is that the introduction of a gutter layer between the selective layer and support can enhance the overall permeance of the penetrant by up to an order of magnitude, but this gain is accompanied by an undesired decrease in selectivity. The analysis also shows for the first time that optimum membrane performance (i.e., a maximum increase in permeance with negligible decrease in selectivity) is realized when the thickness of the gutter layer is in the range of 1 2 times the pore radius. The modeling approach provides clear and practical guidelines for designing ultrathin composite membranes to achieve high permeance and selectivity for low cost and energy efficient molecular separations taking into account constituent membrane nano-features and material properties.