Ceramic membranes stability

The discussion so far implies that membrane materials are organic polymers, and in fact most membranes used commercially are polymer-based. However, in recent years, interest in membranes made of less conventional materials has increased. Ceramic membranes, a special class of microporous membranes, are being used in ultrafiltration and microfiltration applications for which solvent resistance and thermal stability are required. Dense, metal membranes, particularly palladium membranes, are being considered for the separation of hydrogen from gas mixtures, and supported liquid films are being developed for carrier-facilitated transport processes. 

The ceramic membrane has a great potential and market. It represents a distinct class of inorganic membrane. In particular, metallic coated membranes have many industrial applications. The potential of ceramic membranes in separation, filtration and catalytic reactions has favoured research on synthesis, characterisation and property improvement of inorganic membranes because of their unique features compared with other types of membrane. Much attention has focused on inorganic membranes, which are superior to organic ones in thermal, chemical and mechanical stability and resistance to microbial degradation. 

Less information is available about the stability of ceramic membranes. It is generally thought that ceramic membranes have excellent solvent stability. Acid conditions may be more problematic it was shown [57] that an alumina nanofiltra-tion membrane was very sensitive to corrosion effects in dynamic experiments, whereas the performance of a similar titania membrane was stable in the pH range from 1.5 to 13. 

Problems to be solved are related to membrane stability (of polymeric membranes, but also the development of hydrophobic ceramic nanofiltration membranes and pervaporation membranes resistant to extreme conditions), to a lack of fundamental knowledge on transport mechanisms and models, and to the need for simulation tools to be able to predict the performance of solvent-resistant nanofiltration and pervaporation in a process environment. This will require an investment in basic and applied research, but will generate a breakthrough in important societal issues such as energy consumption, global warming and the development of a sustainable chemical industry. 

New reactor designs and immobilisation methods have been used to extend the lifetime of lipases in scCC>2 (Lozano et al., 2004). Ceramic membranes have been coated with hydrophilic polymers and the enzyme covalently attached to these. In SCCO2, activities and selectivities were excellent and the half-life of the catalyst was enhanced. It is thought the hydrophilic layer of the membrane protected the enzyme. Operational stability of enzymes has also been increased by using ionic liquid/scC02 biphasic systems (Lozano et al., 2002 Reetz et al., 2003). 

In Chapter 10, the use of membranes for different applications are described. One of the possible membranes for hydrogen cleaning is an asymmetric membrane comprised of the dense end of a proton conduction perovskite such as BaCe0 95 Yb0 05O3 5 and a porous end to bring mechanical stability to the membrane. In this case, it is possible to take from the slurry, obtained by the acetate procedure, several drops to be released over a porous ceramic membrane, located in the spinning bar of a spin-coating machine. Thereafter, the assembly powder, thin film porous membrane is heated from room temperature up to 1573 K at a rate of 2K/min, kept at this temperature for 12 h, and then cooled at the same rate in order to get the perovskite end film over the porous membrane [50],... 

This chapter is split in two parts. The first part will briefly treat the preparation of flat ceramic membrane supports by colloidal processing. In our laboratory, these supports are used to study stability and gas separation properties of microporous silica membranes because they are easy to prepare and demand less complex testing equipment. 

J.M. Hofman-Ziiter, Chemical and Thermal Stability of (Modified) Mesoporous Ceramic Membranes , PhD Thesis, University of Twente, 1995. 

Mechanical stability against thermal cycling between ambient temperature and Top requires matching of thermal-expansion coefficients of the electrolyte and electrodes, the interconnects, and the seals. Note Ceramic strength is improved where cell design retains the ceramic membrane under a compressive stress. 

Chemical stability - Ceramic membranes are not degraded by organic solvents and can withstand exposure to chlorine. Many crystalline oxides are relatively insoluble in acidic and alkaline media hence cercunic membranes coitposed of such oxides should be relatively inert under extreme pH conditions. 

Stability to microbial degradation - Certain organic membranes are quite susceptible to microbial degradation. Ceramic membranes are not expected to undergo such degradation. 

Mechanical stability - Organic membranes contact and can undergo inelastic deformations under high pressures, leading to lower permeabilities. Ceramic membranes supported on robust materials such as stainless steel or structural ceramics can be expected to withstand very high pressures. 

Because of the unique characteristics of inorganic membranes mentioned above, the search for inorganic membranes of practical significance has been continuing for several decades. With the advent of ceramic membranes with superior stabilities coming to the separation markets, the potentials for inorganic membranes as separators and/or reactors are being explored at an accelerated rate never wimessed before. 

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