Inorganic membranes symmetric

Membranes are classified as organic or inorganic, taking into account the material used for their syntheses porous or dense, based on the porosity of the material applied and symmetric and asymmetric for a membrane made of a single porous or dense material or for a membrane made of a porous support and a dense end, respectively [16,64], We are fundamentally interested here in asymmetric inorganic membranes made of a porous end to bring mechanical stability to the membrane and made of alumina, silica, carbon, zeolites, and other materials, and a dense end to give selectivity to the membrane (see Chapter 10). However, we also analyze the performance of porous polymers. 

Opposiny-reactants mode. When immobilized with a catalyst or enzyme, the interconnected tortuous pores or the nearly straight pores of a symmetric inorganic membrane provides a relatively well controlled catalytic zone or path for the reactants in comparison with the pellets or beads in a fixed or fluidized bed of catalyst particles. This unique characteristic of a symmetric membrane, in principle, allows a novel reactor to be realized provided the reaction is sufficiently fast. The concept applies to both equilibrium and irreversible reactions and does not utilize the membrane as a separator. Consider a reaction involving two reactants, A and B ... 

Membrane and Membrane Design Most membranes are polymers in nature, but some inorganic membranes have become available. The most common membranes are based on polysulfone, cellulose acetate, polyamide, fluoropolymers, and other compounds. Formation of a symmetric membrane structure is an important element in the success of UF/NF membrane separation (16). The other considerations for membrane separation are as follows (1) separation capabilities (retention or selectivity), (2) separation rate (flux), (3) chemical and mechanical stabilities, and (4) membrane material cost. 

First of all, inorganic membranes can be divided into two main categories unsupported and supported (see Fig. 1). The former are also called symmetric, the latter asynunetric. 

Thermoporometry is a method which measures cavity sizes and not inlet sizes. It has been mainly used for the characterisation of organic mesoporous membrane texture [70-73] but has been also applied to inorganic alumina symmetric membranes [73] with a good reliability. However the solidification of water in small pores may sometimes damage the membrane structure due to the expansion of the condensate and consequently different results can be obtained after several runs [74]. 

As pointed out by Nunes and Peinemann [108], inorganic membranes are usually preferred because many processes at the industrial level are carried out at high temperature. However, polymeric membranes can be used for H2/hydrocarbon separation in the platformer off gases from refineries and for CO2 separation in coal plants. Polymeric manbranes for GS can be symmetric or asymmetric, but should have a dense selective layer. Three types of membrane structures can be employed (1) homogeneous dense manbranes (symmetric) (2) integrally skinned asymmetric membranes and (3) composite membranes. 

Membranes are used for a wide variety of separations. A membrane serves as a barrier to some particles while allowing others to selectively pass through. The pore size, shape, and electrostatic surface charge are fundamental to particle removal. Synthetic polymers (cellulose acetate, polyamides, etc.) and inorganic materials (ceramics, metals) are generally the principal materials of construction. Membranes may be formed with symmetric or asymmetric pores, or formed as composites of ultra thin layers attached to coarser support material. Reverse osmosis, nanofiltration, ultrafiltration, and microfiltration relate to separation of ions, macromolecules, and particles in the 0.001 to 10 pm range (Rushton et al. 1996). 

However, membranes can be symmetric or asymmetric. Many porous or dense membranes are asymmetric and have one or several more porous supporting layers and a thin skin layer which, in fact, gives selectivity. If these two layers are made of different materials, the membrane is a composite one. On some occasions, dense membranes have inclusions of other materials these are, of course, also composite membranes. In the case of gas separation membranes it has became usual to include inorganic charges in a polymeric membrane to get what is called a mixed matrix composite membrane. 

MF is used to remove suspended solids, algae, and bacteria. Membranes can be prepared from either polymeric or inorganic materials. The structure is usually symmetric. UF also removes viruses, colloids, and macromolecules. The separation ability of a UF membrane is usually expressed in terms of molecular weight cut off (MWCO). This is defined as the molecular weight of the solute that is 90% retained by the membrane. Typical MWCO values for UF membranes ranges from 1 to 300 kDa [11]. Although for MF membranes permeability is usually affected by the entire membrane thickness, performance of the UF membranes mostly depends on the skin-layer properties. 

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