Enhanced oxygen permeation through perovskite hollow fibre membranes by methane activation

Hollow fibre membranes of mixed conducting perovskite La_0.6Sr_0.4Co_0.2Fe_0.8O₃ (LSCF) were prepared via the combined phase inversion and sintering technique. The fibres were tested for air separation with a home-made reactor under the oxygen partial pressure gradient generated by the air/He streams. Some fibres were in situ activated by introducing methane in the He sweeping gas at high temperatures. The activated membranes with new morphology were created by transforming the inner densified surface layer to a porous structure. Compared to the original membranes, the activated gave appreciable higher oxygen fluxes. At 800 °C, the oxygen fluxes were increased by a factor of 10 after activation was carried out at 1000 °C for 1 h.

Proton conductive composite membranes

In this work we investigated the synthesis of composite organic and inorganic membranes for proton conduction. Particles derived from metal alkoxides (M(OR)n) sol-gel processes (Ti, Zr, W with phosphoric acid) were embedded in polymeric matrices of poly-vinyl alcohol, (3-glycidoxypropyl)-trimethoxysilane and ethylene glycol. The structure of the composite membranes was complex as several IR peaks were convoluted, indicating the assignment of several functional groups. However, the peaks assigned to OH groups reduced in intensity in the composite membranes, indicating that cross-linking of hydroxyl groups in the organic and inorganic phases of the membrane may have occurred. The particles allowed for re-arrangement of the polymer matrix, as crystallinity was reduced compared to a polymer blank membrane. The composite membrane process resulted in homogeneous dispersion of nanoparticles into the polymer film. Proton conduction of the inorganic phase was mainly dominated by titania. Binary mixtures of titania phosphate (sample name TiP) resulted in proton conduction of 7.15 × 10−2 S.cm−1, one order of magnitude higher than zirconia phosphate (ZrP). The addition of Zr and W to TiP forming ternary or quaternary phases also led to lower proton conduction as compared to TiP. Similar trends were also observed for the composite membranes, though the TiP composite membrane proton conduction reduced after several hours of testing at 50°C, which was mainly attributed to acid leaching.

Structure effect on the oxygen permeation properties of barium bismuth iron oxide membranes

In this work, we investigated the oxygen permeation properties of barium bismuth iron oxide within the family of [Ba_2−3xBi_3x−1][Fe_2xBi_1−2x]O_2+3x/2 for x = 0.17–0.60. The structure changed progressively from cubic to tetragonal and then to hexagonal as function of x in accordance with the different relative amounts of bismuth on A-site and B-site of ABO_3−δ perovskite lattices. We found that the oxygen flux and electrical conductivity correlated strongly, and it was prevalent for the cubic structure (x = 0.33–0.40) which conferred the highest oxygen flux of 0.59 ml min⁻¹ cm⁻² at 950 °C for a disk membrane x = 0.33 with a thickness of 1.2 mm. By reducing the thickness of the disk membrane to 0.8 mm, the oxygen flux increased to 0.77 ml min⁻¹ cm⁻², suggesting both surface kinetics and ion diffusion controlled oxygen flux, though the former was more prominent at higher temperatures. For disk membranes x = 0.45–0.60, the perovskite structure changed to tetragonal and hexagonal, and the oxygen flux was insignificant below 900 °C, clearly indicating electron conduction properties only. However, for two compositions with relatively high bismuth content, e.g. x = 0.55 and 0.60, there was a sudden and significant rise of oxygen permeability above 900 °C, by more than one order of magnitude. These materials changed conduction behavior from metallic to semiconductor at around 900 °C. These results suggest the advent of mixed ionic electronic conducting properties caused by the structure transition as bismuth ions changed their valence states to compensate for the oxygen vacancies formed within the perovskite lattices.

High performance BaBiScCo hollow fibre membranes for oxygen transport

Here we report the production of novel high performance BaBi_0.05Sc_0.1Co_0.85O_3-3 (BaBiScCo) hollow fibres delivering oxygen fluxes of 11.4 ml cm^-2 min^-1 at 950 °C. The doping of bismuth, a highly ionic conductor, at the B-site of a barium based perovskite overcame oxygen ionic transport limitations even at temperatures as low as 600 °C.

Research progress and materials selection guidelines on mixed conducting perovskite-type ceramic membranes for oxygen production

Oxygen production by air separation is of great importance in both environmental and industrial processes as most large scale clean energy technologies require oxygen as feed gas. Currently the conventional cryogenic air separation unit is a major economic impediment to the deployment of these clean energy technologies with carbon capture (i.e. oxy-fuel combustion ). Dense ceramic perovskite membranes are envisaged to replace the cryogenics and reduce O2 production costs by 35% or more; which can significantly cut the energy penalty by 50% when integrated in oxy-fuel power plant for CO2 capture. This paper reviews the current progress in the development of dense ceramic membranes for oxygen production. The principles, advantages or disadvantages, and the crucial problems of all kinds of membranes are discussed. Materials development, optimisation guidelines and suggestions for future research direction are also included. Some areas already previously reviewed are treated with less attention.

Effect of coagulants on the fouling and performance of ultrafiltration (UF) membranes

The combined coagulation and ultrafiltration (UF) system (C-UF system) is an advanced technology to treat natural organic matter (NOM) present in water. Traditional coagulants — prehydrolyzed inorganic coagulants, organic coagulants and composite coagulants were chosen to treat synthetic water containing humic acid (HA) in order to find an efficient coagulant that could remove NOM from the water effectively. The fouling, removal efficiency of UF and the chlorine decay in the permeate were used to evaluate the effectiveness of the coagulants. The initial UV254 absorption of the tested water samples were from 0.208 to 0.234, and the UV254 after coagulation was from 0.05 to 0.184. The UV254 did not increase after coagulation. Since the humic acid used was soluble, the initial turbidity of the tested water samples were very close to zero. The turbidity increased after coagulation, as the coagulants react with humic acid to form micro-flocs, which cannot be removed fully by sedimentation. The results showed that polyferric chloride could not remove humic acid efficiently during coagulation process, but removed the humic acid well when used in the C-UF system. Moreover, for polyferric chloride and UF system, the concentration of organic compounds in permeates were minimal indicating very low levels of disinfection by-product formation, if chlorinated.

Effect of operating parameters and cleaning on the performance of ceramic membranes treating partially clarified sugar cane juice

The performance of ceramic membranes with pore sizes of 0.05 and 0.10 mm in purifying limed and partially clarified sugar cane juice was investigated under different operating conditions. From various operating conditions and strategies, switching off the permeate for 5 seconds for every 5 minutes (S5sT5 m) by an automated control valve provided higher flux. From the three pH experiments conducted on the 0.05 mm membrane, the best performance was observed at a pH of 7.5. Amongst the four fouling models tested, the cake filtration model fitted the performance of both membranes with higher accuracy at a transmembrane pressure of 0.5 bar. Filtering the cane juice through the membrane reduced the turbidity by 99.7%, color by 15%, and the starch concentration by 80% as well as increased the purity by 1.4%. The effective cleaning chemical composition from experimental results showed that 1% NaOH and 3000 ppm NaOCl solution performed the best but only for the experiments that were treating limed and partially clarified juice at pH 7.5.