Casting osmosis membranes

Membranes and Osmosis. Membranes based on PEI can be used for the dehydration of organic solvents such as 2-propanol, methyl ethyl ketone, and toluene (451), and for concentrating seawater (452—454). On exposure to ultrasound waves, aqueous PEI salt solutions and brominated poly(2,6-dimethylphenylene oxide) form stable emulsions from which it is possible to cast membranes in which submicrometer capsules of the salt solution ate embedded (455). The rate of release of the salt solution can be altered by surface—active substances. In membranes, PEI can act as a proton source in the generation of a photocurrent (456). The formation of a PEI coating on ion-exchange membranes modifies the transport properties and results in permanent selectivity of the membrane (457). The electrochemical testing of salts (458) is another possible appHcation of PEI. 

The Effect of Phosphoric Acid as a Casting Dope Ingredient on Reverse-Osmosis Membrane Properties... 

Relationship Between Nodular and Rejecting Layers. Nodular formation was conceived by Maler and Scheuerman (14) and was shown to exist in the skin structure of anisotropic cellulose acetate membranes by Schultz and Asunmaa ( ), who ion etched the skin to discover an assembly of close-packed, 188 A in diameter spheres. Resting (15) has identified this kind of micellar structure in dry cellulose ester reverse osmosis membranes, and Panar, et al. (16) has identified their existence in the polyamide derivatives. Our work has shown that nodules exist in most polymeric membranes cast into a nonsolvent bath, where gelation at the interface is caused by initial depletion of solvent, as shown in Case B, which follows restricted Inward contraction of the interfacial zone. This leads to a dispersed phase of micelles within a continuous phase (designated as "polymer-poor phase") composed of a mixture of solvents, coagulant, and a dissolved fraction of the polymer. The formation of such a skin is delineated in the scheme shown in Figure 11. 

The patents 9 47) filed by Richter and Hoebn in du Pont include many aromatic and heterocyclic condensed polymers for asymmetric reverse osmosis membranes, for example, the condensed polymers 4 from 3-arninobcnzhydrazide, 4-amino-benz-hydrazide (5), isophthaloyl chloride, and terephthaloyl chloride (6). The membranes were cast from polyamide solutions containing 0 3 % LiCl in dimethylacetamide,... 

Reverse osmosis membranes were also prepared from polyamides with pendant carboxamide groups 90). For example, 4,4 -diaminodiphenylmethane-3,3 -dicarbox-amide-isophthaloyl chloride copolymer 33 was dissolved in DMF containing LiCl, cast to 250 p thickness, dried at 100 °C for 15 min, and gelled in ice water to give a membrane with the water flux permeability of 900 1/m2 day and salt rejection of 80% (0.5% NaCl aqueous solution, 30 kg/cm2). After heating the membrane in... 

The temperature of the water used to precipitate the casting solution is important this temperature is controlled in commercial membrane plants. Generally low-temperature precipitation produces lower flux, more retentive membranes. For this reason chilled water is frequently used to prepare cellulose acetate reverse osmosis membranes. 

The first composite reverse osmosis membrane reported in the technical literature was developed by Peter Francis of North Star Research Institute in 1964 (4). This membrane was formed by float-casting an ultrathin film of cellulose acetate (CA) upon a water surface, removing the membrane from the water surface by lamination onto a pre-formed microporous support film and drying to bond the membrane to the support. This float-casting procedure has since been described in the technical literature for both flat sheet and tubular membranes ( 5, 6, T). 

Fabrication of a thin film composite membrane is typically a more expensive route to reverse osmosis membranes because it involves a two-step process versus the one-step nature of the phase inversion film casting method. However, it offers the possibility of each individual layer being tailor-made for maximum performance. The semipermeable coating can be optimized for water flux and solute rejection characteristics. The microporous sublayer can be optimized for porosity, compression resistance and strength. Both layers can be optimized for chemical resistance. In nearly all thin film composite reverse osmosis membranes, the chemical composition of the surface barrier layer is radically different from the chemical composition of the microporous sublayer. This is a common result of the thin film composite approach. 

The first composite reverse osmosis membrane to be developed and described consisted of an ultrathin film of secondary cellulose acetate deposited onto a porous Loeb-Sourirajan membrane.3 The ultrathin film of cellulose acetate was fabricated by a water surface float-casting technique. This has been described to some extent in the published technical literature,4 5 and in considerable detail in several reports on government-funded research projects.3 6 Figure 5.2 illustrates this process schematically. 

Figure 5.5 Cross section and surface of a microporous polysulfone sheet used in composite reverse osmosis membranes (a) total cross section of a polysulfone sheet cast on a nonwoven polyester fabric, then delaminated prior to freeze-fracture for SEM (note fiber trecks on backside of the sheet) (b) backside of sheet showing cellular structure, which extends through 85% of the sheet thickness (c) transition region from cellular to nodular structure near film surface (d) dense nodular structure at the surface (e) high magnification of the extreme top surface cross section (f) high magnification view of the surface structure showing tha texture of the top surface.

Strathmann prepared an all-polyimide composite membrane-both bottom and top layers.97 A microporous asymmetric film of the polyamic acid intermediate was cast by quenching in acetone, then dried and thermally cyclized to the polyimide at 300°C. The microporous polyimide sheet was then overcoated with a dilute solution of the same polymer, which was allowed to evaporate to give a 300-angstrom-thick coating. This was also cyclized to the polyimide to generate a fully solvent resistant reverse osmosis membrane. 

In 1960, a kind of flat sheet asymmetric cellulose acetate semipermeable membrane was firstly casted by Loeb and Srinivasa with competitive flux property, which led to a rapid progress in developments of cotmnercially available osmosis membranes in the following 1960s and early... 

Cellulose acetate membranes developed by Loeb and Sourirajan for the purpose of seawater desalination continue to be useful in various membrane applications, despite the development of new membrane materials and new membrane preparation techniques. Because of its historical importance, the casting method of the first successful reverse osmosis membrane is described below in detail. 

Usually, the casting solution for aromatic polyamide reverse osmosis membranes includes the electrolyte additive, and the composition is near the critical concentration. Then the radius of macromolecular spheres should be nearly equal to 52 X, as indicated by Table 2, and their packing fa.shion is close to the cubic packing. Approximating the square-shaped interstitial void area generated between four neighboring spheres (see Figure 4.11) by a circle of equal area, the effective radius of such interstitial void spaces is 27 A. 

P. Neogi, Mechanism of pore formation in reverse-osmosis membranes during the casting process, AIChE Journal 29 (1983) 402-410. 

Nolle, M.C.M. et ah. Cellulose acetate reverse osmosis membranes made by phase inversion method Effects of a shear treatment applied to the casting solntion on the membrane structure and performance. Separation Science and Technology, 2011.46(3) 395 03. 

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