Cellulose Tri Acetate (CTA) Membrane in sheet form.
Dr. K. C Thomas
Qualification:
Masters in Science.
Ph.D in Chemistry from Mumbai University, India in 1988, on 'Semi-Permeable membranes for Osmotic/Reverse Osmotic Separations'.
Work:
Worked more than 25 years as a research scientist for the development and characterization of CTA membranes for sea water desalination by Reverse Osmosis. I also developed CTA/CA blend membranes for nanofiltration and ultrafiltration for use in various separation/ concentration applications.
Contact e-mail:
[email protected]
Links
http://www.sciencemag.org
http://en.wikipedia.org
Desalination of seawater and brackish water for drinking as well as for industrial applications is being used extensively for the past three decades. Multi stage Flash distillation and Reverse Osmosis are the two main technologies used for this purpose.
Reverse Osmosis (RO) employs semi-permeable membranes which allow only pure water to pass through, leaving all other impurities, including dissolved salts to concentrate at one side of the membrane. The pore-diameter of these membranes is in Angstrom unit range (5-10). High pressures, depending on the salinity of the feed water, are required (40-70 Kg/cm2), to force pure water through these membranes. Many types of RO modules are in use to house the membranes, viz. spiral, hollow fiber and plate and frame.
Background
Sea water contains 3.5% salt (35000 parts per million).To make this potable water, more than 99% of this salt is to be removed from the seawater feed. That means a good membrane should have > 99% salt rejection and at the same time adequate water flow expressed as litres/m2/hour, in the range 45-50 at 60-70Kg/cm2 pressure.
Commercial RO modules produced by membrane manufacturers, use polyamide polymer as membrane material. Membranes are made as thin-film-composites (TFC). TFC membranes in sheet form are suitable for making spiral module. Million gallons/day plants are being operated in many countries using both of these module types.
Polyamide membranes are destroyed by chlorine which should be added in feed water in 0.1 to 0.5 ppm range to control bacterial growth in modules and other systems during RO plant operation. Currently, in all plants using polyamide membrane, chlorine is removed before the feed enters the RO modules. Cellulose tri acetate (CTA) membrane can tolerate the chlorine dose added to the feed water to control the bacterial growth in RO systems. CTA membranes are so far used as hollow fiber only because CTA sheet membranes with adequate water flux could not be made.
Product Proposal
A breakthrough has been achieved in preparing sheet membrane using CTA, thus enabling spiral module fabrication for seawater desalination. This new CTA membrane in sheet form is made using a casting solution with new additive and by employing new casting variables. Performance evaluation of these membranes, shows > 99% salt rejection and 40-45 l/ m2/hr water flux at 800-1000 psig pressure with seawater feed. Membrane is suitable for feed pH 4-9.
CTA, unlike polyamide is a natural polymer, is cheap and readily available. RO spiral modules made with CTA membrane can be used for domestic RO units in cities where feed water is chlorinated. CTA membranes have very good compaction and bacterial resistance similar to polyamide membranes.
For details, contact [email protected]