Porous symmetric membranes structure

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). 

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... 

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]. 

Salts rejected by the membrane stay in the concentrating stream but are continuously disposed from the membrane module by fresh feed to maintain the separation. Continuous removal of the permeate product enables the production of freshwater. RO membrane-building materials are usually polymers, such as cellulose acetates, polyamides or polyimides. The membranes are semipermeable, made of thin 30-200 nanometer thick layers adhering to a thicker porous support layer. Several types exist, such as symmetric, asymmetric, and thin-film composite membranes, depending on the membrane structure. They are usually built as envelopes made of pairs of long sheets separated by spacers, and are spirally wound around the product tube. In some cases, tubular, capillary, and even hollow-fiber membranes are used. 

Many aspects of the formation of symmetric or asymmetric membranes can be rationalized by applying the basic thermodynamic and kinetic relations of phase separation. There are, however, other parameters-such as surface tension, polymer relaxation, sol and gel structures-which are not directly related to the thermodynamics of phase separation but which will have a strong effect on membrane structures and properties. A mathematical treatment of the formation of porous structures is difficult. But many aspects of membrane structures and the effect of various preparation parameters Can be qualitatively interpreted. 

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. 

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]. 

The preceding section discussed the permeability of the homogeneous films without pores (hereafter, referred to as symmetric membranes). However, such symmetric membranes exhibit significantly low permeability, and they are hardly appUed to practical uses thereby. In order to increase the apparent permeabihty of the membrane, the polyimides are usually used in the form of asymmetric membranes. Here, asymmetric membrane denotes the structure consisting of a dense skin layer and a porous support layer, as schematically depicted in Figure 22.2. 

Membrane pressure-driven processes, namely, MF, UF, NF, and RO are normally carried out in the liquid phase. Although water permeates through the membrane, other species are partially or completely rejected. According to Fane et al.. The MF-UF range can be considered as a continuum [11] both processes involve porous membranes. MF is carried out using symmetric membranes, with pore size ranging from 0.05 to 10 pm. UF, instead, requires asymmetric membranes, with pore size from 1 to 100 nm. The NF and RO spectrum is also considered as a continuum [11]. NF/RO membranes are usually thin-film composite (TFC) structures with nonpo-rous skin. The most important features of pressure-driven membrane processes are resumed in Table 1.3. 

We now turn to the membranes themselves. These are most easily subdivided into symmetric and asymmetric structures. The symmetric membranes, which are less important, may be nonporous or porous. Nonporous symmetric membranes are typically made by spreading a polymer solution on a glass plate and allowing the solvent slowly to evaporate. Typically, 10 percent polymer dissolved in a volatile solvent like chloroform works well. 

Nonporous, microporous, and porous membranes of symmetric or asymmetric structure... 

The most simple form is a single, uniformly structured wall of a certain material, the so-called symmetric, stand-alone membranes. Examples are dense metal or oxide tubes and porous hollow fibres. To obtain sufficient mechanical strength, single-walled symmetric systems usually have a considerable thickness. 

Membrane morphology (or structure) is generally classified into two groups porous and dense. Dense fibers utilize the chemical and the physical characteristics of their structure to provide separation depending on the diffusivity or solubility of the solute species. In the case of porous membranes (Fig. 1), internal structure could be symmetric, asymmetric, or composite. Asymmetric membranes feature a very thin active layer, responsible for the separation process, supported... 

Symmetric polymeric membranes possess a uniform pore structure over the entire thickness. These membranes can be porous or dense with a constant permeability from one surface to the other. Asymmetric (also sometimes referred to as anisotropic) membranes, on the other hand, typically show a dense (nonporous) structure with a thin (0.1-0.5 pm) surface layer supported on a porous substrate. The thin surface layer maximizes the flux and performs the separation. The microporous support structure provides the mechanical strengdi. 

The effectiveness of a membrane depends both on its ability to separate particles or molecules in a selective way and on the flux that can be achieved across this membrane. Although some membranes, such as glass membranes, exhibit a symmetrical structure, in most cases they are asymmetric, consisting of several layers coated on a bulk porous support and with a gradual decrease in pore size (Fig. 3). The main advantages of an asymmetric structure are high fluxes and the ability to have tailored membranes made of materials different from the... 

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