Ceramic membranes dense membrane structure

In this chapter membrane preparation techniques are organized by membrane structure isotropic membranes, anisotropic membranes, ceramic and metal membranes, and liquid membranes. Isotropic membranes have a uniform composition and structure throughout such membranes can be porous or dense. Anisotropic (or asymmetric) membranes, on the other hand, consist of a number of layers each with different structures and permeabilities. A typical anisotropic membrane has a relatively dense, thin surface layer supported on an open, much thicker micro-porous substrate. The surface layer performs the separation and is the principal barrier to flow through the membrane. The open support layer provides mechanical strength. Ceramic and metal membranes can be either isotropic or anisotropic. 

The methods of preparing inorganic membranes with tortuous pores vary enormously. Some use rigid dense solids as the templates for creating porous structures while many others involve the deposition of one or more layers of smaller pores on a premanufactured microporous support with larger pores. Since ceramic membranes have been studied, produced and commercialized more extensively than any other inorganic membrane materials, more references will be made to the ceramic systems. 

Dense ceramic ion-conducting membranes (CICMs) are emerging as an important class of inorganic membranes based on fluorite- or perovskite-derived crystalline structures [18]. Most of the ion-conducting ceramics discovered to date exhibit a selective ionic oxygen transport at high temperatures >700°C. Ionic transport in these membranes is based on the following successive mechanisms [25] ... 

This chapter focuses on the chemical processing of ceramic membranes, which has to date constituted the major part of inorganic membrane development. Before going further into the ceramic aspect, it is important to understand the requirements for ceramic membrane materials in terms of porous structure, chemical composition, and shape. In separation technologies based on permselective membranes, the difference in filtered species ranges from micrometer-sized particles to nanometer-sized species, such as molecular solutes or gas molecules. One can see that the connected porosity of the membrane must be adapted to the class of products to be separated. For this reason, ceramic membrane manufacture is concerned with macropores above 0.1 pm in diameter for microfiltration, mesopores ranging from 0.1 pm to 2 nm for ultrafiltration, and nanopores less than 2 nm in diameter for nanofiltration, per-vaporation, or gas separation. Dense membranes are also of interest for gas... 

Retention of ionic species modifies ionic concentrations in the feed and permeate liquids in such a way that osmotic pressure or electroosmotic phenomena cannot be neglected in mass transfer mechanisms. The reflection coefficient, o, in Equations 9.4 and 9.5 represents, respectively, the part of osmotic pressure force in the solvent flux and the diffusive part in solute transport through the membrane. One can see that when o is close or equal to zero, the convective flux in the pores is dominant and mostly participates to solute transport in the membrane. On the contrary, when o is close or equal to 1, mainly diffusion phenomena are involved in species transport through the membrane, which means that the transmembrane pressure is exerted across an almost dense structure. Low UF and NF ceramic membranes stand in the former case due to their relatively high porous volume and pore sizes in the nanometer range. Relevant results have been published concerning the use of a computer simulation program able to predict solute retention and flux for ceramic and polymer NF membranes [28]. 

The macrostructure of the ceramic hollow fiber membranes can be controlled by modulating the suspension compositions as well as the spinning conditions, as shown in Figure 18.4." " When water is used as both internal and external coagulants, a sandwich-like structure, i.e. a central dense layer integrated with fingers on both sides is formed (Figure 18.4(a)). 

Shao, Z., Xiong, G., Tong, J. et al. (2001) Ba effect in doped Sr(Coo.8Feo.2)03 g on the phase structure and oxygen permeation properties of the dense ceramic membranes. Separation and Purification Technology, 25,419-429. 

A detailed discussion of the mathematical models of oxygen flow in ceramic membranes is given elsewhere. Typical materials employed in dense ceramic membranes have a brownmillerite or perovskite structure. The most commonly studied application for this kind of membrane is the catalytic partial oxidation of methane (POM) to obtain synthesis gas,... 

In fact, all the challenges faced by dense ceramic oxygen-permeable membrane reactors as described in Chapter 5 have to be faced by PCMRs. Searching for better membrane materials, developing effective membrane synthesis methods, and improving chemical and structural stability of the current membrane materials will be the focus of active... 

Abstract Dense ceramic membrane reactors are made from composite oxides, usually having perovskite or fluorite structure with appreciable mixed ionic (oxygen ion and/or proton) and electronic conductivity. They combine the oxygen or hydrogen separation process with the catalytic reactions into a single step at elevated temperatures (>700°C), leading to significantly improved yields, simplified production processes and reduced capital costs. This chapter mainly describes the principles of various types of dense ceramic membrane reactors, and the fabrication of the membranes and membrane reactors. 

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