Mixed Matrix Carbon Molecular Sieve and Alumina (CMS-Al₂O₃) Membranes

This work shows mixed matrix inorganic membranes prepared by the vacuum-assisted impregnation method, where phenolic resin precursors filled the pore of a-alumina substrates. Upon carbonisation, the phenolic resin decomposed into several fragments derived from the backbone of the resin matrix. The final stages of decomposition (>650 degrees C) led to a formation of carbon molecular sieve (CMS) structures, reaching the lowest average pore sizes of similar to 5 angstrom at carbonisation temperatures of 700 degrees C. The combination of vacuum-assisted impregnation and carbonisation led to the formation of mixed matrix of CMS and a-alumina particles (CMS-Al2O3) in a single membrane. These membranes were tested for pervaporative desalination and gave very high water fluxes of up to 25 kg m(-2) h(-1) for seawater (NaCl 3.5 wt%) at 75 degrees C. Salt rejection was also very high varying between 93-99% depending on temperature and feed salt concentration. Interestingly, the water fluxes remained almost constant and were not affected as feed salt concentration increased from 0.3, 1 and 3.5 wt%

Inter-layer free cobalt-doped silica membranes for pervaporation of ammonia solutions

This study demonstrated the application of a new type of interlayer-free cobalt-doped silica membrane in treating ammonia solutions by pervaporation applied towards wastewater treatment. For enhanced hydrothermal stability, cobalt-doped silica (CoSi) membranes with increasing cobalt concentrations from 1 to 35 mol% were prepared and evaluated, namely CoSi-1, 5, 20 and 35. These membranes exhibited high water fluxes of 66 L m h for CoSi-1 and 15.5 L m h for CoSi-35 at 45 °C. The fluxes of the membranes decreased with increasing cobalt concentration; while the rejection to total nitrogen (TN, ammonia nitrogen) increased and hence allowed selective passage of water molecules. Enhanced thermostability was observed for the membranes, particularly CoSi-35 that exhibited TN rejection up to 99% at high temperature of 65 °C and highly alkaline environment (pH > 10). Also, the CoSi-35 membrane showed stable performance in treating ammonia present in industry wastewater by achieving stable TN and mineral rejections of 97% and 99%, respectively. Fouling was observed and confirmed by SEM morphological analysis and EDX elemental inspection. The results indicated the deposition of low solubility salts such as CaSO

High performance perovskite hollow fibres for oxygen separation

Ba(0.5)Sr(0.5)Co(0.8)Fe(0.2)O(3-delta) (BSCF) hollow fibres are very attractive to deliver high oxygen fluxes at low manufacturing costs. These membranes efficiently separate oxygen from air via ionic diffusion, and have been exclusively prepared with sulphur containing binders, a vital ingredient to aggregate ceramic particles to shape hollow fibre geometries. Here we show that the choice of binder influences the stoichiometry of the crystal structure with the formation of non-ionic conduction domains, ultimately affecting oxygen ionic diffusion. Our synthesis method used sulphur free binders for the preparation of BSCF hollow fibres, resulting in an oxygen flux increase of 105% to 9.5 ml min(-1) cm(-2) at 950 degrees C. In addition, catalytic surface modification of the BSCF hollow fibres further increased oxygen fluxes to 14.5 ml min(-1) cm(-2). More importantly, oxygen fluxes significantly increased by a factor of 10 below 700 degrees C. thus reducing operating temperature requirements by 250 degrees C. (C) 2010 Elsevier By. All rights reserved

Modelling gas purification systems employing molecular sieve silica membranes

In this work, we simulate the transient behaviour of gas purification systems employing silica derived membranes and formulate a model that is capable of predicting this transient behaviour in the presence of a CO2 and H-2, both as single gases and as a binary gas mixture. The model is based on continuity and assumes that the membrane properties do not change between single gas and mixed gas cases. Based on these properties, the complete characterization of transient membrane performance is obtained thereby providing an explanation based upon driving forces for very significant differences between single and mixed gas measurements. The simulated results are validated with independent mixed gas experiments and are then extended to analyse the difference in performance for a gas separation system with mixed gas feed using membranes with at least one order of magnitude difference in selectivity based on single gas permeations. The model also allows simulation for membranes complying with temperature dependency mechanism. Our results show that the increase in performance of the system for mixed gas feed is not commensurate with the increase in the selectivity based on single gas permeations

Exposing the molecular sieving architecture of amorphous silica using positron annihilation spectroscopy

Despite extensive research into silica gels, little exists that describes the porous properties of the fundamental molecular sieving framework due to limitations of current characterization techniques. This paper describes the novel use of positron annihilation lifetime spectroscopy (PALS) for rapid quantitative measurement of sub-nanometer pores in amorphous molecular sieving silicas. The well-established N2 adsorption technique was also used, but most materials appeared nonporous as adsorption cannot detect pores less than the size of the N2 molecule. PALS, on the other hand, detected hierarchical trimodal porosity in all silicas, peaking at around 3, 8, and 12A ° . Gas permeation probing through membranes made using the same silicas verified the 3A° size cutoff and provided strong evidence that the intermediate and larger pores are not continuously connected for large molecule diffusion. For these ‘‘sponge-like’’ silicas, all molecules must pass through the fundamental silica framework that provides the selectivity. The quantity and size of intermediate and large pores was in turn found to contribute to the permeation performance. A hydrostabilized templated silica had more intermediate and larger pore interconnection evident by high N2 adsorption and slightly lower selectivity than the same non-templated material. A material tailored for improved H2/CO2 separation had a reduced size of the smaller pores, which gave it better selectivity. Studies of temperature evolution of silica gels from 200–600 8C using PALS conclusively showed that temperature increases the pore size of the fundamental silica framework. This observation fits well with widely reported progressive network assembling via formation of siloxane groups causing the silica to ‘‘inflate’’, which now describes in better detail the porous features of the fundamental silica network

Ultra-microporous membrane separation using toluene to simulate tar-containing gases

This study investigates the performance of ultra-microporous cobalt oxide silica membranes for processing simulated gas streams containing toluene as a model tar compound in gasification. The performance of the membranes was initially investigated for He (simulating H 2 ), CO 2 , N 2 and Ar in a range of temperatures. Subsequently, toluene was added to a gas mixture containing He and tested to simulate the effect of toluene as a tar compound in gasification. The membranes delivered molecular sieving features, showing activated transport as the permeation of the smaller molecular gas He increased with temperature whilst the permeation decreased for the other larger molecular gases. Prior to toluene exposure, He permeance increased by almost twofold from 3.6 × 10 − 8 to 7.1 × 10 − 8  mol m − 2  s − 1  Pa − 1 as the temperature was raised from 50 to 200 °C. Under a feed gas containing 0.24 mol% toluene, He permeance decreased by an average value of 17%. Upon regeneration of the membrane by heat, He permeance was not fully recovered, a clear indication of tar fouling. A toluene balance calculation showed toluene being retained by the membrane