Manufacture and optimization of tubular ceramic membrane supports
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Inorganic membranes can be considered an alternative to organic membranes, due to their thermal, chemical and mechanical stability under harsh conditions. Ceramic membranes are used as support structures to increase permeability through composite inorganic membranes in separation processes. Tubular α-alumina membrane supports with smooth inner surfaces can be manufactured by means of the centrifugal casting technique. In this study, the effect of three different AKP powder sizes (0.25, 0.31 and 0.6μm) and sintering temperatures (1000 to 1400°C) on the properties of the α-alumina supports were investigated in order to determine the optimum particle size and sintering temperature which would yield a porous support with an optimized permeability, while still retaining the smooth inner surface and adequate mechanical strength. A study concerning the possible replacement of the expensive AKP powder range with the less expensive Alcoa CT 3000 SG powder was also undertaken. The supports manufactured by centrifugal casting were characterized in terms of dimensions and by mercury porosimetry, water permeability and SEM. A novel strength testing apparatus was developed in order to determine the mechanical strength from the inside of the tubular structures. The effect of the polymer concentration, which is added to stabilize the colloidal suspension used in the centrifugal casting technique, as well as the influence of the sintering rate during polymer burn-off, was also investigated. A larger particle size resulted in an increased porosity, pore size and permeability, while a decrease in linear shrinkage and mechanical strength was observed. There was a decrease in porosity and permeability with increasing sintering temperature while the linear shrinkage and mechanical strength increased. The AKP-30 (0.31 μm) and AKP-15(O.61 μm) had a different particle packing than the AKP-50(0.25μm) supports and consequently a decrease in pore size with increasing sintering temperature was observed for both the AKP-30 and AKP-15 supports, while the pore size remained constant for the AKP-50 supports. Increased polymer concentration resulted in an increase in permeability, pore size and porosity, while the mechanical strength of the support decreased. This was due to the evolution of "cracks" during sintering. The sintering rate had no profound influence on the properties of the membrane supports. The powder with the widest particle size distribution (AKP-15 powder) resulted in support structures with the widest pore size distribution. The aim of the study, i.e. the optimization of the supports, was attained when comparing the results in this study to our previous work as well as the available literature. Compared to our own previous study, the permeability increased from 28 to 41 L.h.-1bar.-1m.-2, the porosity from 36 to 37% and the pore radius from 99 to 167nm for the AKP-15 supports sintered at 1200°C. Structural cracking and warping during sintering of the powder compacts made from untreated Alcoa CT 3000 SG powder indicated that the particle size distribution (PSD) of the powder was bimodal and hence too wide for centrifugal casted membrane support manufacture. Removal of impurities and powder fractionation by means of both an acid and a column treatment was attempted. The removal of a large amount of fine particles by means of the acid treatment resulted in an improved inner surface of the ceramic supports. Although the size range variation of fractions obtained by the column treatment could not be detected by particle size analysis (Malvern Mastersizer), the characterization of three supports, manufactured by combining different fractions, indicated that some degree of fractionation did occur. Characterization of these defect-free supports showed that fractions from the upper section of the column consisted of smaller particles with a narrower PSD compared to fractions from the lower sections of the column. Numerous attempts to repeat these results were, however, unsuccessful, suggesting that the fractionation by means of the acid and column treatments were unpredictable with a low repeatability. Further work is required to obtain a repeatable fractionation, which would be essential in order to prepare centrifugal casted membrane supports using the Alcoa CT 3000 SG powder.