Dry-wet phase separation

Pesek, S.C. and W.J.Koros, Aqueous Quenched Asymmetric Polysulfone Hollow Fibers Prepared by Dry/Wet Phase Separation , J. Membr. Sci., 88, 1-19, (1994). Scott, J., Hollow Fibers - Manufacture and Applications , Edited by J. Scott, Noyes Data Corporation, New Jersey, U.S.A., (1981). 

Pinnau, I., and Koros, W. (1991), Structures and gas separation property asymmetric polysulfone membranes made by dry, wet, and dry/wet phase-inversion, J. Appl. Polym. Sci., 43,1491-1502. 

Phase inversion is a process in which a polymer is transformed from a liquid to a solid state. There are a number of methods to achieve phase inversion. Among others, the dry-wet phase inversion technique and the temperature induced phase separation (TIPS) are most commonly used in the industrial membrane manufacturing. The dry-wet phase inversion technique was applied by Loeb and Sourirajan in their development... 

Cellulose acetate is the material for the first-generation reverse osmosis (RO) membranes. The announcement of cellulose acetate membranes for seawater desalination by Loeb and Sourirajan in 1960 triggered the applications of membrane separation processes in many industrial sectors. Cellulose acetate membranes are prepared by the dry-wet phase inversion technique. 

Membrane structures can also be formed by a microphase separation process in which the outermost region of the cast membrane undergoes phase separation induced by solvent evaporation, while the bulk of the structure is formed by sol vent/non-sol vent exchange during a quench step. This type of structure formation is defined as a dry/wet phase inversion process [Pinneau et al., 1990]. 

PEEK-WC is a modified PEEK, as shown in Figure 6.2. Ultrathin asymmetric gas separation membranes of modified PEEK can be prepared by a dry/wet phase inversion technique. Under optimized conditions, membranes with an open cellular morphology and an ultrathin dense skin... 

J. C. Jansen, M. G. Buonomenna, A. Figoli, and E. Drioli. Ultra-thin asymmetric gas separation membranes of modified PEEK prepared by the dry-wet phase inversion techrrique. Desalination, 193(l-3) 58-65, May 2006. 

The fastest growing desalination process is a membrane separation process called reverse osmosis (RO). The most remarkable advantage of RO is that it consumes little energy since no phase change is involved in the process. RO employs hydraulic pressure to overcome the osmotic pressure of the salt solution, causing water-selective permeation from the saline side of a membrane to the freshwater side as the membrane barrier rejects salts [1-4], Polymeric membranes are usually fabricated from materials such as cellulose acetate (CA), cellulose triacetate (CTA), and polyamide (PA) by the dry-wet phase inversion technique or by coating aromatic PA via interfacial polymerization (IFP) [5]. 

An RO membrane acts as a barrier to flow, allowing selective passage of a particular species (solvent) while other species (solutes) are retained partially or completely. Solute separation and permeate solvent (water in most cases) flux depend on the material selection, the preparation procedures, and the structure of the membrane barrier layer [5,15]. Cellulose acetate (CA) is the material for the first generation reverse osmosis membrane. The announcement of CA membranes for sea water desalination by Loeb and Sourirajan in 1960 triggered the applications of membrane separation processes in many industrial sectors. CA membranes are prepared by the dry-wet phase inversion technique. Another polymeric material for RO is aromatic polyamide [16]. 

Hydrophilic MF membranes can be made by the dry-wet phase inversion technique, which can also be used to make PVDF membranes. On the other hand, other hydrophobic microflltration membranes are made by the thermally induced phase separation technique. In particular, semicrystalline PE, PP, and PTFE are stretched parallel to the direction of film extrusion so that the crystalline regions are aligned in the direction of stretch, while the noncrystalline region is ruptured, forming long and narrow pores. Hydrophobic membranes do not allow penetration of water into the pore until the transmembrane pressure drop reaches a threshold called the liquid entry pressure of water (LEPw). These membranes can therefore be used for membrane distillation. The track-etching method is applied to make microfiltration membranes from PC. 

The polymer solution is brought into contact with the glycerol. During thi.s time no phase separation occurs but due to the outdiffusion of NMP the polymer concentration at the topside of the solution increases. The second coagulation bath contains water and the demixing occurs immediately. In this way a thin dense toplayer is obtained supported by a porous sublayer. Table III. 11 summarizes some results of integrally skinned membranes prepared from different polymers with both the wet-dry and wet phase separation techniques. All membranes intrinsic selectivities indicating that no defects are present. 

Pinnau, I. (1991). Skin formation of integral-asymmetric gas separation membranes made by dry/wet phase inversion (gas separation). Ph.D. Dissertation, University of Texas, Austin, TX. 

Normal-phase separations are very sensitive to water in the solvent. To speed the equilibration of the stationary phase with changing eluents, organic solvents for normal-phase chromatography should be 50% saturated with water. This can be done by adding a few milliliters of water to dry solvent and stirring. Then separate the wet solvent from the excess water and mix the wet solvent with an equal volume of dry solvent. 

In an alternative approach, MIP membranes can be obtained by generating molec-ularly imprinted sites in a non-specific matrix of a synthetic or natural polymer material during polymer solidification. The recognition cavities are formed by the fixation of a polymer conformation adopted upon interaction with the template molecule. Phase inversion methods have used either the evaporation of polymer solvent (dry phase separation) or the precipitation of the pre-synthesised polymer (wet phase inversion process). The major difficulties of this method lay both in the appropriate process conditions allowing the formation of porous materials and recognition sites and in the stability of these sites after template removal due to the lack of chemical cross-linking. 

Asymmetric membranes are usually produced by phase inversion techniques. In these techniques, an initially homogeneous polymer solution becomes thermodynamically unstable due to different external effects and the phase separates into polymer-lean and polymer-rich phases. The polymer-rich phase forms the matrix of the membrane, while the polymer-lean phase, rich in solvents and nonsolvents, fills the pores. Four main techniques exist to induce phase inversion and thus to prepare asymmetric porous membranes [85] (a) thermally induced phase separation (TIPS), (b) immersion precipitation (wet casting), (c) vapor-induced phase separation (VIPS), and (d) dry (air) casting. 

Following cultivation of the plant material, the leaves from which the cocaine will be prepared are harvested and dried in the sun. From these, coca paste and, subsequently, cocaine is produced. In general, coca paste is prepared by one of two methods. The first involves wetting the leaves and macerating them with dilute sulfuric acid, thus forming the water-soluble sulfate salts of the alkaloids. The mixture is then extracted with kerosene. After phase separation, the aqueous layer is basified with ammonia, lime or sodium carbonate, and the alkaloids precipitated. They are then recovered by filtration. 

Figure 29.2 Schematic representation of an aqueous LBL coating layer on the phase separated silicone hydrogels surface in wet and dry conditions.

Sol 2 is present either when one phase separates into two phases or when two phases are prevented from recombining into a single phase. It is expedient to entitle this factor inoompatibilityt and to discuss the various phase inversion processes in terms of the reasons for incompatibility. In the sections to follow four phase inversion processes are discussed a dry process, a wet process, a thermal process and a polymer assisted phase inversion process. 

For many years polymeric membranes have been utilized widely for material separation without detailed characterization of the pore size and the pore size distribution. Most of the commercially available membranes are prepared by either a dry or a wet phase-inversion process. These membranes are formed by the phase separation of multicomponent polymer-solvent systems, the underlying principle being phase separation of the polymer solution. 

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