Triacetate Membranes for Reverse Osmosis

Desalination Membrane Operating Department, Toyobo Co., Ltd., Osaka, Japan 

The commercialized RO modules consist of cellulose triacetate hollow-fiber type and polyamide spiral-wound type. The cellulose triacetate hollow-fiber RO modules are used around the world for seawater desalination mainly because of excellent features such as a chlorine tolerance and fouling resistance. 

This summary describes the history of cellulose triacetate RO membrane, description of cellulose triacetate hollow-fiber RO modules of Toyobo for seawater desalination, actual operation results, and recent RO modules of cellulose triacetate including most recently developed advanced modules. 

2 HISTORY OF CELLULOSE ACETATE MEMBRANE 2.2.1 Development of Loeb-Sourirajan Membrane 

The Saline Water Act was enacted in the United States on July 3, 1952. Then, the Office of Saline Water (OSW) was installed in the Department of the Interior in order to study the method of obtaining freshwater from seawater and brackish water economically. This effort accelerated development of RO membranes. The RO process was proposed as one 

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Polymer Plasticizer. Nylon, cellulose, and cellulose esters can be plasticized using sulfolane to improve flexibiUty and to increase elongation of the polymer (130,131). More importantly, sulfolane is a preferred plasticizer for the synthesis of cellulose hoUow fibers, which are used as permeabiUty membranes in reverse osmosis (qv) cells (131—133) (see Hollow-FIBERMEMBRANEs). In the preparation of the hoUow fibers, a molten mixture of sulfolane and cellulose triacetate is extmded through a die to form the hoUow fiber. The sulfolane is subsequently extracted from the fiber with water to give a permeable, plasticizer-free, hoUow fiber. 

R/0 unit Reverse Osmosis Unit for water purification in small aquariums and miniature yard-ponds, utilizes a membrane under pressure to filter dissolved solids and pollutants from the water. Two different filter membranes can be used the CTA (cellulose triacetate) membrane is less expensive, but only works with chlorinated water and removes 50-70% of nitrates, and the TFC membrane, which is more expensive, removes 95% of nitrates, but is ruined by chlorine. R/0 wastes water and a system that cleans 100 gallons a day will cost ft-om 400 to 600 with membrane replacement adding to the cost. A unit that handles 140 gallons a day will cost above 700,00. 

Seawater or brackish water can be purified by reverse osmosis. To maximise the flow of water through a polymer membrane, the polymer must have a high water permeability, yet a low permeability for the salts. To maximise efficiency, the membrane area must be large and its thickness as small as possible consistent with a lack of pinholes. A high pressure is applied to the salt water side of the membrane. Because it is thin, a cellulose triacetate membrane is supported on a porous cellulose nitrate-cellulose acetate support structure to resist the pressure. To make the unit compact the composite membrane is spirally wound on to an inner cylinder, and the edges glued together. When a pressure of 70 bar is applied to the seawater side, NaCl rejection levels in excess of 99.7% can be achieved. 

Reverse osmosis membranes were first developed from cellulose acetate (CA) and a cellulose triacetate (CTA). These CA and CTA membranes have been in commercial use for many years. The CTA membrane, especially, is continuing to be used all over the world today. This wide adoption demonstrates that the CTA membrane is excellent as a RO... 

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