Reverse osmosis (RO)
Cellulose acetates (CA) asymmetric single polymer membranes have been largely replaced by thin film composite (TFC) membranes (most commonly polysulfones (PS) or polyether sulfones (PES) combined with a polyamide rejection layer). In the composite systems each layer can be individually optimized and the top layer reduced in thickness, allowing higher flux. CAs are still used in hollow fiber reverse osmosis (RO) membranes and some waste water and food and beverage RO applications, where fouling problems due to high suspended solids are more common.
Forward osmosis (FO)
The cellulosic membrane materials from Eastman are also used in forward osmosis (FO) membranes, both in industrial applications and for food and beverage concentration. The family of cellulose acetates produced by Eastman for membrane applications, are high-purity with a very consistent viscosity. These versatile materials are used in a wide range of forward osmosis applications, from personal hydration devices, to concentrating juices and nutritional supplements, and recovering fresh water from highly polluted effluents. In these applications, the anit-fouling properties of cellulose acetate are very important.
Eastman cellulose esters have been used in various FO membrane formats, including spiral wound modules, plate and frame, hydration pouches and hollow fibers. Image shows an example of an osmosis module, courtesy of Fluid Technology Solutions (FTS),
When used on liquid streams with high levels of organics or scaling minerals, water removal membranes such as reverse osmosis (RO) can foul easily. Also, when concentrating foods or beverages, thermal evaporation can negatively affect key flavors. In these cases, FO is an excellent method to dewater such sensitive or high fouling streams. The draw solutions which drive the FO process are often regenerated with RO in tandem with the FO process.
Reduction of fouling in ultra- and micro-filtration
Membrane fouling is a common problem in ultra-(UF) and micro-filtration (MF), causing poor flow and performance and reducing membrane service life. Cellulose acetates have been used to modify the hydrophilicity of hydrophobic polymers such as polyvinylidene fluoride (PVDF) membranes used in UF and MF, making the membrane surface more hydrophilic. This allows for better contact with the aqueous feed stream and a reduction in fouling. Table 2 shows that water uptake due to hydroxyl content creates relatively high surface tension, thereby reducing fouling.