Symmetric porous membranes

Symmetric membranes and asymmetric membranes are two basic types of membrane based on their structure. Symmetric membranes include non-porous (dense) symmetric membranes and porous symmetric membranes, while asymmetric membranes include integrally skinned asymmetric membranes, coated asymmetric membranes, and composite membranes. A number of different methods are used to prepare these membranes. The most important techniques are sintering, stretching, track-etching, template leaching, phase inversion, and coating (13,33). 

Figure 1.1 Schematic diagrams of the principal types of membranes (a) porous symmetric membrane (b) non-porous/dense symmetric membrane (c) liquid immobilized membrane (d) asymmetric membrane with porous separation layer (e) asymmetric membrane with dense separation layer.

Terms such as symmetric and asymmetric, as well as microporous, meso-porous and macroporous materials will be introduced. Symmetric membranes are systems with a homogeneous structure throughout the membrane. Examples can be found in capillary glass membranes or anodized alumina membranes. Asymmetric membranes have a gradual change in structure throughout the membrane. In most cases these are composite membranes... 

Porous metals have long been commercially available for particulate filtration. They have been used in some cases as microfiltration membranes that can withstand harsh environments, or as porous supports for dynamic membranes. Stainless steel is by far the most widely used porous metal membrane. Other materials include silver, nickel. Monel, Hastelloy and Inconel. Their recommended maximum operating temperatures range from 200 to 650°C. Elepending on the pore diameter which varies from 0.2 to 5 microns, the water permeability of these symmetric membranes can exceed 3000 L/h-m -bar and is similar to that obtained with asymmetric ceramic microfiltration membranes. Due to the relatively high costs of these membranes, their use for microfiltration has not been widespread. 

Figure 3.1. Scanning electron micrograph of the cross-section of a porous glass membrane with symmetric microstructurc (courtesy of Asahi Glass).

An important point to consider about hollow fiber membranes is their morphology. Hollow fiber membranes can be either symmetric or asymmetric.16 Symmetric membranes have continuous pore structure throughout. Asymmetric membranes have a dense upper layer or skin layer that is then supported with a sublayer that is significantly more porous. Figure 6.2 shows SEM images of... 

Cross-section structure. An anisotropic membrane (also called asymmetric ) has a thin porous or nonporous selective barrier, supported mechanically by a much thicker porous substructure. This type of morphology reduces the effective thickness of the selective barrier, and the permeate flux can be enhanced without changes in selectivity. Isotropic ( symmetric ) membrane cross-sections can be found for self-supported nonporous membranes (mainly ion-exchange) and macroporous microfiltration (MF) membranes (also often used in membrane contactors [1]). The only example for an established isotropic porous membrane for molecular separations is the case of track-etched polymer films with pore diameters down to about 10 run. All the above-mentioned membranes can in principle be made from one material. In contrast to such an integrally anisotropic membrane (homogeneous with respect to composition), a thin-film composite (TFC) membrane consists of different materials for the thin selective barrier layer and the support structure. In composite membranes in general, a combination of two (or more) materials with different characteristics is used with the aim to achieve synergetic properties. Other examples besides thin-film are pore-filled or pore surface-coated composite membranes or mixed-matrix membranes [3]. 

Dense phase polymer membranes with no supportive substructure, so-called symmetric membranes, must have a minimum thickness of about 1 mil (1 x 10 in. or 2.54 X 10" cm) to ensure mechanical integrity and freedom from imperfections such as holes in the membrane. Possible disadvantages of these membranes are low fluxes and limited selectivity. Asymmetric membranes overcome these limitations, being very thin dense polymers with a thickness in the order of 1 x 10 m, supported on a porous layer that is about 1 x 10 m thick. 

The thickness of symmetric membranes (porous or non-porous) ranges roughly from 10 to 200 qm. The resistance to mass transfer in the membrane is inversely proportional to the membrane thickness. A decrease in membrane thickness results in an increased permeation rate. 

The symmetric membrane has a uniform composition throughout its thickness whereas the asymmetric membrane has a dense, but thin, top layer, and a strong, more porous sublayer beneath it. 

Microfil- tration Symmetric micro-porous polymer membrane. Pore size 0.05-10 ym Hydrostatic pressure 1-5 bar Sieving mechanism, pore si2e and particle diameter determine separation characteristics Sterile filtration, clarification, cell harvesting bacteria, viruses separation. 

The membrane can be a solid, a liquid, or a gel, and the bulk phases can be liquid, gas, or vapor. Membranes can be classified according to their structures. Homogeneous or symmetric membranes each have a structure that is the same across the thickness of the membrane. These membranes can be porous or have a rather dense uniform structure. Heterogeneous or asymmetric membranes can be categorized into three basic structures (1) integrally skinned asymmetric membrane with a porous skin layer, (2) integrally skinned asymmetric membrane with a dense skin layer, and (3) thin film composite membranes [13]. Porous asymmetric membranes are made by the phase inversion process [14,15] and are applied in dialysis, ultrafiltration, and microfiltration, whereas integrally skinned asymmetric membranes with a dense skin layer are applied in reverse osmosis and gas separation applications. 

A breakthrough as far as industrial membrane applications were concerned was achieved by the development of asymmetric membranes (Loeb andSouiirajan [28]). These membranes consist of a very thin dense toplayer (thickness < 0.5. urn) supported by a porous sublayer (thickness 50-200 Jim). The toplayer or skin determines the transmit rate while the porous sublayer only acts as a support. The permeation rate is inversely proportional to the thickness of the actual barrier layer and thus asymmetric membranes show a much higher permeation rate (water flux) than (homogeneous) symmetric membranes of a comparable thickness. 

Another means of classifying membranes is by morphology or structure. This is also a very illustrative route because the membrane structure determines the separation mechanism and hence the application. If we confine ourselves to solid synthetic membranes, two tyf>es of membrane may be distinguished, i.e. symmetric or asymmetric membranes. The two classes can be subdivided further as shown schematically in figure I -5. The thicknesses of symmetric membranes (porous or nonporous) range roughly from 10 to 200 )xm, the resistance to mass transfer being determined by the total membrane thickness. A decrease in membrane thickness results in an increased permeation rate. 

Depending on their structure, symmetric membranes can be catalogued as porous and non-porous or dense (polymeric swollen-network), while asymmetric membranes for desalting applications (NF and RO, basically) consist of a dense and thin active layer and a thick porous sublayer for mechanical stability (usually an UF membrane). Moreover, supported liquid membranes (SLMs), and aetivated membranes (AMs), which basically consist in the immobilization of speeifie agents (organic solvent, carrier or ionic liquid at room temperature) in the pores/structure of a support membrane, have been developed for selective separation of valuable/contaminant compoimds [8-11]. 

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