Abstract
Ceramic foams are a promising alternative to conventional catalyst supports due to their macro-porosity, which should enhance mass transport properties during reactions. Whilst direct foaming is a straightforward production method, the use of kitchen mixers commonly reported in the literature to initially froth the ceramic slurry limits understanding of scale-up. This study reports a systematic experimental investigation of the impact of mixing parameters on the properties of the foams produced in an agitated baffled vessel of diameter, T=175mm, equipped with an up-pumping pitch blade turbine with diameter of either D=0.23T or 0.51T and a bottom round sparger with a diameter of 45mm. The flow conditions in the present study were in the low to mid transitional regime (50<Re<1000). Design of Experiments (DoE) was employed to generate a series of screening experiments by variation of sparging time, air flow rate, impeller speed and impeller diameter. The mixing behaviour was described as a function of relevant dimensionless groups (Re, Fr, Flg, etc.) whilst the gas–liquid flow regime was estimated by examination of a ceramic particles free system. The properties of the foams obtained were correlated with key dimensionless numbers, though the exponents obtained deviated from values in the published literature. In addition, the rheology of the foam was correlated to the bubble size distribution showing that rheology measurements have potential for at-line measurement to control the structure of the produced material.
Highlights
The direct foaming of a ceramic slurry is a simple and versatile method to generate macroporous ceramic materials
The effects of impeller speed, impeller diameter, gas flow rate, sparging time and sparging disk mean pore size have been investigated through design of experiment (DoE) in which foam density, foam porosity, foam stability and bubble size distribution were selected as responses
It was established that foam stability is not dependent on its manufacturing process but is only affected by the initial ceramic slurry formulation
Summary
The direct foaming of a ceramic slurry is a simple and versatile method to generate macroporous ceramic materials. These can be used as refractory insulators, filters for molten metals filtration and catalyst supports (Gauckler et al, 1985; Colombo and Sheffled, 2005). The percentage of solid in the initial slurry influences both the foam porosity and bubble size. Gonzenbach et al (2007) observed a reduction in bubble size and porosity as the solid percentage was increased; they suggested that the decrease in porosity was due to the higher viscosity of the suspension that in turn hindered the air entrainment within the suspension. PH affects both the particle stabilization in the initial slurry and the dissociation of the functional group of the amphiphile, with the ideal pH reported to be equal to the pKa of the functional group (Gonzenbach et al, 2007; Studart et al, 2006)
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