Integral asymmetric polymeric membranes

Some of the most common commercially available Pis used for forming integral asymmetric polymeric OSN membranes are shown in Table 16.3. There are several published examples of OSN membranes based on Pis Strathmann (1978) developed solvent-stable membranes based on PDMA-ODA (4,4 -oxydianiline) (by reacting benzidine with pyromeUitic anhydride). For increased solvent/thermal resistance these membranes were cyclized with iV,lV-dicyclohexylcarbodiimide. After all processing steps the membranes were able to withstand 50 days exposure to dichloromethane and cyclohexane without loss of mechanical stability. Alegranti (1978), developed OSN membranes by a similar procedure differing only in the cyclization step. At 5.5 MI these membranes could perform hexane/ethanol (50%/50%) mixture separation (75% ethanol in the permeate). 

In integral asymmetric membranes, both toplayer and the sublayer consist of the same material. These membranes are prepared by phase inversion techniques. For this reason it is essential that the polymeric material firom which the membrane it to be prepared is soluble in a solvent or a solvent mixture. Because most polymers are soluble in one or more solvents, asymmetric membranes can be prepared from almost any material. 

The majority of the commereial gas separation membranes are made by wet phase inversion method whieh results in an integrally skinned asymmetrie membrane. This method was first used by Loeb and Sourirajan to produee cellulose acetate membranes for desalination of sea water. An alternative method for making gas separation membranes uses an ultra-porous skinned asymmetric membrane over which a thin polymer film is deposited by either coating or by interfacial polymerization. This method was developed by Cadotte for the creation of in situ dense skin thin film composite membranes for water desalination. These membrane fabrication techniques were made commercially successful for gas separation membranes by a brilliant empirical discovery for in situ sealing of the tiny pinhole defects on the skin of the membrane. 

RO is the most relevant membrane-based technique for seawater desalination [98]. Similar to NF, RO is carried out using asymmetric membranes with a nonporous skin layer. Membranes can be integrally skinned or TFC. The most important technique for the preparation of such membranes is IP, which has been already described in Section 1.6.3 devoted to NF membranes. As reported by Lee et al. [99], the studies about the preparation of polymeric membranes for RO application, from 1950 to 1980, focused on the search for optimum membrane materials. Subsequently, the performance of RO membranes was improved by controlling membrane formation reactions and using catalysts and additives. 

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. 

A typical polymeric composite membrane is composed of a porous film with a dense polymer barrier layer of a different material formed over the top. Composite membranes have the advantage over integral-asymmetric structures that different polymers may be used for the different layers, depending on their properties (e.g. a polymer with the correct selectivity for a separation problem can be used over a support structure of a different porous material). 

In Sec. 3 our presentation is focused on the most important results obtained by different authors in the framework of the rephca Ornstein-Zernike (ROZ) integral equations and by simulations of simple fluids in microporous matrices. For illustrative purposes, we discuss some original results obtained recently in our laboratory. Those allow us to show the application of the ROZ equations to the structure and thermodynamics of fluids adsorbed in disordered porous media. In particular, we present a solution of the ROZ equations for a hard sphere mixture that is highly asymmetric by size, adsorbed in a matrix of hard spheres. This example is relevant in describing the structure of colloidal dispersions in a disordered microporous medium. On the other hand, we present some of the results for the adsorption of a hard sphere fluid in a disordered medium of spherical permeable membranes. The theory developed for the description of this model agrees well with computer simulation data. Finally, in this section we demonstrate the applications of the ROZ theory and present simulation data for adsorption of a hard sphere fluid in a matrix of short chain molecules. This example serves to show the relevance of the theory of Wertheim to chemical association for a set of problems focused on adsorption of fluids and mixtures in disordered microporous matrices prepared by polymerization of species. 

Most polymeric OSN membranes have an asymmetric structure and are porous with a dense top layer. This asymmetry can be divided into two major types the integral type, where the whole membrane is composed of the same material, and the thin-film composite (TFC), where the membrane separating layer is made of a different material. 

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