Morphological, mechanical and gas transport properties of stainless steel and composite hollow fibres

Abstract

This work focuses on the development of stainless steel (SS) and SS composite hollow fibres (with carbon (CSS) and alumina (CASS)) as novel, robust inorganic membranes prepared via dry-wet phase inversion from a spinning dope containing a mixture of SS and/or alumina particles, polymeric binders and solvents. The morphological features and mechanical properties of the hollow fibres were evaluated by varying the spinning dope composition, including: different binders; SS-to-alumina volume, binder-to-solvent, and binder-to-SS particle ratios; the spinning dope viscosity; and the SS particle size (6-45m); as well as sintering parameters such as temperature, atmosphere and time.It was found that the addition of larger particles to the spinning dope favoured the creation of large macrovoids in both the inner and outer shells of the hollow fibre. In contrast, small particles delayed de-mixing, forming sponge-like regions at the outer shell, although the finger-like macrovoids were retained at the lumen. Polyvinylpyrrolidone (PVP) was used as a viscosity enhancer, altering the kinetics of the phase inversion process, leading to an increase in finger-like macrovoids with increasing PVP addition. Polyetherimide (PEI) was preferred as the polymeric binder due to its favourable phase inversion kinetics. Conversely, polyethersulfone promoted faster de-mixing resulting in finger-like macrovoids at both surfaces.The morphological structure of the sintered SS hollow fibres did mimic the morphology of the green fibres. Sintering was controlled by SS mass diffusion limitations, lower densification was achieved at 950°C with necks forming between particles in close contact. At 1000°C, surface diffusion became important to densification. At 1050 and 1100°C, smaller pores in the sponge-like region started closing whilst larger finger-like pores and macrovoids remained open as the interparticle space was too wide to be filled by surface diffusion. Densification, as function of mass diffusion, was accelerated for smaller particles and retarded for larger particles, showing an inverse impact in the total surface area available for diffusion. Fibres with SS particle loadings below 50wt% showed irregular geometries. However at 70wt% particle loading the required particle packing condition was achieved to form inter-particle necks during sintering.The mechanical resistance of the hollow fibres was closely related to morphology and densification.Samples with high porosity showed low mechanical strength, especially for porosity related to finger-like macrovoids, whereas smaller pores (sponge-like region) resulted in higher mechanical iii strengths. Higher densification led to stronger hollow fibres, due to bold necks that could withstand higher loads.CSS hollow fibres were created by pyrolysing the polymeric binder during the sintering process, resulting in inter-particle pore filling with the degraded and retained carbon. The morphology of the CSS hollow fibres resembles the green fibres after the sintering/pyrolysis process, tho...

Recovery of Lignosulfonates from Spent Sulfite Liquor Using Ceramic Hollow-Fiber Membranes

“…In contrast, tubular ceramic membranes are produced by dip-coating and have a consistent sponge-like pore structure. The inner surface area is generally higher for membranes with a sponge-like pore structure, resulting in more pronounced adsorption effects and a greater likelihood of fouling [58,59,60,61,62,63,64]. Our data show that the wall shear stress is an important parameter for the evaluation of membrane performance in terms of the permeate flux, especially when comparing different membrane geometries.…”