Thin dense layer

The mere preparation of porous membranes is accompanied with a noticeable decrease of permselectivity 11, which is undesirable for reverse osmosis and ultrafiltration, A thin dense layer should be adopted to attain a high permeability with — out the decrease of permselectivity, but this necessarily decreases the mechanical strength. This conflict is largely resolved by the construction of asymmetric or composite membranes as described also in the present review. 

Most polymers that have been of interest as membrane materials for gas or vapor separations are amorphous and have a single phase structure. Such polymers are converted into membranes that have a very thin dense layer or skin since pores or defects severely compromise selectivity. Permeation through this dense layer, which ideally is defect free, occurs by a solution-diffusion mechanism, which can lead to useful levels of selectivity. Each component in the gas or vapor feed dissolves in the membrane polymer at its upstream surface, much like gases dissolve in liquids, then diffuse through the polymer layer along a concentration gradient to the opposite surface where they evaporate into the downstream gas phase. In ideal cases, the sorption and diffusion process of one gas component does not alter that of another component, that is, the species permeate independently. 

Similarly, impervious yttria-stabilized zirconia membranes doped with titania have been prepared by the electrochemical vapor deposition method [Hazbun, 1988]. Zirconium, yttrium and titanium chlorides in vapor form react with oxygen on the heated surface of a porous support tube in a reaction chamber at 1,100 to 1,300 C under controlled conditions. Membranes with a thickness of 2 to 60 pm have been made this way. The dopant, titania, is added to increase electron How of the resultant membrane and can be tailored to achieve the desired balance between ionic and electronic conductivity. Brinkman and Burggraaf [1995] also used electrochemical vapor deposition to grow thin, dense layers of zirconia/yttria/terbia membranes on porous ceramic supports. Depending on the deposition temperature, the growth of the membrane layer is limited by the bulk electrochemical transport or pore diffusion. 

In EVD, a modified form of chemical vapor deposition (CVD), an electrochemical potential gradient is used to grow a thin, dense layer of the ionic conducting oxide (e.g., yttria-stabilized zirconia) on a porous substrate. EVD is either a single-step or a two-step process depending on the nature of the substrate. For a porous substrate, the first step involves pore closure by CVD (i.e., deposition from the vapor of an oxide layer by reaction of a chloride gas precursor compound with water vapor or oxygen) ... 

One-dimensional diffusion through a flat membrane will be treated in the following discussion. The effects of membrane asymmetry will be neglected since the process of permselection occurs in the thin dense layer of effective thickness, Z, at the membrane surface. In such a case, the expression for the local flux of a penetrant at any point in the dense layer can be written as shown in Equation 1 C14) ... 

One can actually consider the trapped solution morphology as a functional definition of the asymmetric membranes. It should be emphasized that this viewpoint clearly differentiates asymmetric membranes that have shown the highest reverse osmosis fluxes from membranes with a thin dense layer of normal solid morphology. 

Reverse osmosis was not commercially practical until techniques for increasing productivity were developed. The principal discovery (32) involved a casting procedure that results in asymmetric membranes having a thin dense layer of polymer, approximately 0.2)a thick, supported on a porous sublayer as Illustrated in Figure 8. These membranes are called Loeb membranes (33). Current commercial membranes of this type are made of cellulose acetate, aromatic polyamides, and certain composites that achieve water fluxes of the order of 1.0 m /m day with NaCl rejections of 99% or more (27). As seen in Equation 25, rejection increases with applied pressure. 

The cellulose acetate membranes are asymmetric and fabricated from a single polymer. The use of electron microscopy in the 1960s demonstrated that the cellulose acetate membranes consisted of a relatively thin dense layer and a thicker porous layer of the same material. The membrane thickness is usually about 100 micrometers with the dense layer accounting for about 0.2% of the thickness and the remainder being an open cell porous matrix (see Figure 4.5). 

The term asymmetric refers to membranes comprised of a porous spongy wall supporting a very thin dense layer. The thin skin layer, approximately 0.5... 

Membrane materials are available in various shapes, such as flat sheets, tubular, hollow fiber, and monolithic. Flat sheets have typical dimensions of 1 m by 1 m by 200 pm thickness. Tubular membranes are typically 0.5 to 5.0 cm in diameter and up to 6 m in length. The thin, dense layer is on either the inside or the outside of the tube. Very small-diameter hollow fibers are typically 42 pm i.d. by 85 pm o.d. by 1.2 m long. They provide a very large surface area per unit volume. Honeycomb, monolithic elements of inorganic oxide membranes are available in hexagonal or circular cross section. The circular flow channels are typically 0.3 to 0.6 cm in diameter (Seader and Henley, 2006). 

The porous layer is required to ensure the thermo-mechanical integrity of the thin dense layer. This support has (i) to allow gas species diffusion, (ii) to present good chemical and thermal compatibility with the dense layer, (iii) to have mixed-conducting properties in order to shift oxygen exchanges at gas/solid interfaces inside its whole thickness, and (iv) to be of low cost for industrial applications. 

Improved performances were obtained with a multilayer reactor composed of a thick porous support, a thin dense layer and a thin porous catalytic layer. Perovskite materials were used for all three layers. Reactor development includes the use of a fugitive material to perform controlled-porosity in the support layer and co-sintering to avoid cracks or deformations in the sintered membranes. 

Kong, C., Kanezashi, M., Yammmoto, T., Shintani, T., and Tsuru, T. 2010. Controlled synthesis of high performance polyamide membrane with thin dense layer for water desalination. Journal of Membrane Science 362 76-80. 

On this substructure a thin dense layer (in the range of 0.5 to 10 pm thick) is coated that has a very high separation capability. Different coating techniques are in use, most commonly a solution of the respective polymer in an appropriate solvent is spread onto the porous substructure. The solvent is evaporated, followed by further treatment to effect crosslinking of the polymer. Photosensitive, solvent-free prepolymers may be used for coatings that are later crosslinked by irradiation, e.g. with UV-light or electrons. 

Generally, the palladium-based membranes can be subdivided into supported and laminated ones. In the supported membranes, a thin dense layer of a palladium alloy is deposited onto a porous support such as porous Vycor glass (silica gel). Nevertheless, using this kind of support, the palladium layer is easily stripped off owing to the loss of an anchor effect [53]. 

Most membranes used in industries have an asymmetric structure. Figure 2.1 shows schematically a typical cross-sectional view of an asymmetric membrane [3]. It consists of two layers the top one is a very thin dense layer (also called the top skin layer), and the bottom one is a porous sublayer. The top dense layer governs the performance (permeation properties) of the membrane the porous sublayer only provides mechanical strength to the membrane. The membranes of symmetric structures do not possess a top dense layer. In the asymmetric membrane, when the material of the top... 

Types of membranes for reverse osmosis. One of the more important membranes for reverse-osmosis desalination and many other reverse-osmosis processes is the cellulose acetate membrane. The asymmetric membrane is made as a composite film in which a thin dense layer about 0.1 to 10 pm thick of extremely fine pores supported upon a much thicker (50 to 125 pm) layer of microporous sponge with little resistance to permeation. The thin, dense layer has the ability to block the passage of quite small solute molecules. In desalination the membrane rejects the salt solute and allows the solvent water to pass through. Solutes which are most effectively excluded by the cellulose acetate membrane are the salts NaCl, NaBr, CaClj, and NajSO sucrose and tetralkyl ammonium salts. The main limitations of the cellulose acetate membrane are that it can only be used mainly in aqueous solutions and that it must be used below about 60°C. 

The fracture of the layered composite originated in the dense layer on the tensile surface owing to lower strain to failure, compared to the porous layer. Actually, a fracture origin was always confirmed in the dense layer. The strength behavior strongly depended on the thickness of the dense layer. The material with the thin dense layer presented high strength, over 1 GPa. 

It is also possible to employ composite membranes which are skiimed asymmetric membranes. However, in conqx>site membranes, the toplayer and sublayer originate from different polymeric materials each layer can be optimised independently. Generally die supportlayer is already an asymmetric membrane on which a thin dense layer is deposited. Several methods have been developed to achieve this such as dip-coating, interfadal polymerisation, in-situ polymerisation and plasma polymerisation. 

Composite membranes constitute the second type of structure frequently used in reverse osmosis while most of the nanofiltration membranes are in fact composite membranes. In such membranes the toplayer and sublayer are composed of different polymeric materials so that each layer can be optimised separately. The first stage in manufacturing a composite membrane is the preparation of the porous sublayer. Important criteria for this sublayer are surface porosity and pore size distribution and asymmetric ultrafiltration membranes are often used. Different methods have been employed for placing a thin dense layer on top of this sublayer ... 

These membrane consists of a very dense top layer or skin with a thickness of 0.1 to 0.5 pm supported by a porous sublayer with a thickness of about 50 to 150 pm. These membranes combine the high selectivity of a dense membrane with the high permeation rate of a very thin membrane [9]. Composite membranes are, in fact, skinned asymmetric membranes. In composite membranes, the top layer and sublayer originate from different materials (polymeric or inorganic). Each layer can be optimized independently. In general, the support layer is already an asymmetric membrane on which a thin dense layer is deposited. 

Microcapsules are non-dense spherical shaped volumes. They are formed by a shell (polymeric wall in this case) with an empty volume inside that can be used to encapsulate compounds. The structure of the shell (a thin, dense layer, or a thick, porous layer, etc.), together with its nature (chemistry, material), determines the release rate of the encapsulated compound. There are several ways to obtain microcapsules. Some of these methods are based only on physical phenomena, certain are based on polymerization reactions and others combine both physical and chemical procedures. Most authors agree in classifying them all in two different groups chemical processes (like in-situ polymerization or desolvation in liquid media) and mechanical processes (e.g. spray drying or electrostatic deposition) [44]. A novel technology for microcapsule production, based on the employment of microdevices in continuous mode, has been presented [45]. Immersion precipitation is used in this case, in a way similar to that explained for flat membranes. 

It should be pointed out that, for simplicity, a two-layer model has been assumed for the HR95 reverse osmosis membrane but a more complex structure (indudtng an intermediate layer with gradual changes in pore radii/porosity from one to another layer or three-layer model) could be more realistic (64, 65]. In this context, the partial inclusion of that intermediate layer in the thick porous sublayer could in some way affect the estimated values, but clearly it would modify those associated with a thin dense layer (the layer thickness, mainly). 

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