Cellulosic membranes water desalination

Low viscosity cellulose propionate butyrate esters containing 3—5% butyryl, 40—50% propionyl, and 2—3% hydroxyl groups have excellent compatibihty with oil-modified alkyd resins (qv) and are used in wood furniture coatings (155). Acetate butyrate esters have been used in such varied apphcations as hot-melt adhesive formulations (156), electrostatically spray-coated powders for fusible, non-cratering coatings on metal surfaces (157—159), contact lenses (qv) with improved oxygen permeabiUty and excellent wear characteristics (160—162), and as reverse-osmosis membranes for desalination of water (163). 

The successful development of asymmetric cellulose acetate membranes by Loeb and Sourirajan in the early sixties, at the University of California, Los Angeles, has been primarily responsible for the rapid development of Reverse Osmosis (RO) technology for brack sh/sea water desalination. Reverse Osmosis approaches a reversible process when the pressure barely exceeds the osmotic pressure and hence the energy costs are quite low. Theenergy requirement to purify one litre of water by RO is only O.OO3 KW as against 0,7 KV required just to supply the vaporisation energy to change the phase of one litre of water from liquid to vapour by evaporation. Thus RO has an inherent capability to convert brackish water to potable water at economic cost and thus contribute effectively to the health and prosperity of all humanity. 

Reverse osmosis is used as a method of desalting seawater, recovering wastewater from paper mill operations, pollution control, industrial water treatment, chemical separations, and food processing. This method involves application of pressure to the surface of a saline solution, thus forcing pure water to pass from the solution through a membrane that is too dense to permit passage of sodium and chlorine ions. Hollow fibers of cellulose acetate or nylon are used as membranes, since their large surface area offers more efficient separation. See dialysis membrane diffusion desalination. 

Phosgene has been employed in the modification of the surfaces of cellulose-acetate membranes used for water desalination and waste water treatment [1450]. Similarly, phosgene has been used to surface-modify porous diaphragms for electrolytic cells [324]. Aluminium and aluminium-based alloys can be etched at a high rate when COClj is used in a mixed gas plasma [1004], as can semiconductors (see Section 9.12). 

Hon-celluloslc Membranes. Despite an Intensive search for more favorable membrane polymers, cellulose acetate remained the best material for reverse osmosis until 1969 when the first B-9 permeator for brackish water desalination was Introduced by Du Font. Richter and Hoehn ( ) Invented aromatic polyamide asymmetric hollow-fiber... 

Cellulosic Membranes. The first asymmetric membrane for gas separation appeared in 1970 (Table II), and It was not surprising that this membrane was a modified CA membrane of the Loeb-Sourirajan type (17). Gelled CA membranes for water desalination must be stored wet In order to maintain their permeation performance. However, In gas permeation, wet, plasticized membranes tend to lose their properties with time due to plastic creep of the soft material under pressure and due to slow drying during which the microporous sublayer may collapse and thus increase the thickness of the dense skin-layer. Gantzel and Merten (17) dried CA membranes with an acetyl-content of 39.4% by quick-freezing and vacuum sublimation at... 

While the cellulose acetate membranes are compacted at the moderate pressures required for brackish water desalination, the thin film composite membranes exhibit bo compaction at these pressures and a very low compaction rate at pressures of 1,000 psig. 

With the development of the integral asymmetric cellulose acetate membranes for desalination of saline water a few researchers turned again to perva-... 

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]. 

Badawi and Ashraf (2013) obtained carbon nanotubes-cellulose acetate nanocomposites membranes for water desalination. To this end, cellulose acetate (CA) (Mw = 52,000 Da) membranes were prepared by phase inversion (PI) using acetone... 

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. 

To overcome the problems of cellulose acetate membranes, many synthetic polymeric materials for reverse osmosis were proposed, but except for one material, none of them proved successful. The only one material, which could remain on the market, was the linear aromatic polyamide with pendant sulfonic acid groups, as shown in Figure 1.2. This material was proposed by DuPont, which fabricated very fine hollow fiber membranes the modules of this membrane were designated B-9 and B-10. They have a high rejection performance, which can be used for single-stage seawater desalination. They were widely used for mainly seawater or brackish water desalination and recovery of valuable materials such as electric deposition paints, until DuPont withdrew them from the market in 2001. 

The separation of water from soluble dissolved species in water using membrane processes is often referred to as reverse osmosis, as pressiue must be applied to overcome the osmotic pressiue. This is particularly relevant for water desalination but is also important for itrilk and whey concentration, fruit juice concentration, maple syrup concentration, and suaose and dextrose concentration. The membranes employed for these processes are similar, and the membrane module con-straction is usually in the spiral woimd configuration. Early desalination membranes were based on cellulose acetate as an asymmetric membrane produced by the phase inversion process. Another early reverse osmosis membrane involved extraded thin hollow fibers of polyamides (both aliphatic and aromatic). The rntetfadal polymerization of polyamides on a pororrs polysirlfone support developed in the 1970s (FihnTec) became one of the preferred membranes and... 

I wish to take this opportunity to express my deep appreciation of the work of the authors involved, and 1 particularly appreciate the fact that the Editors planned to bring out this book to commemorate the announcement of the first successful cellulose acetate reverse osmosis membrane for sea water desalination, in 1960. 

The first reverse osmosis modules made from cellulose diacetate had a salt rejection of approximately 97—98%. This was enough to produce potable water (ie, water containing less than 500 ppm salt) from brackish water sources, but was not enough to desalinate seawater efficiently. In the 1970s, interfacial composite membranes with salt rejections greater than 99.5% were developed, making seawater desalination possible (29,30) a number of large plants are in operation worldwide. 

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