(330j) On the Use of Recycle in Reverse Osmosis Method for Desalination of Sea Water

The osmotic pressure is the primary contributor of the high pressure needed to desalinate sea water at a 3.6 % by weight concentration during the reverse osmosis operations using semi-permeable membranes with a good balance of salt rejection rates and water flux rates. Examples of such membranes are aromatic polyamide and thin film composite membranes. Compared with solutions of starch, sugar for the salt solutions the solute molecular size is small. On account of this, the osmotic pressure calculated by equating the fugacities of the two phases assumed to be in equilibrium on either side of the membrane increases with decrease in solute size exponentially. i.e.,

Pi = (Pw ? Ps) = -RT/Vw ln(Xw)

Thus for NaCl the osmotic pressure at the high pressure pump needed was found to be 16.0 atm compared with starch or sugar where it is a lot lower. The use of recycle was evaluated as shown in Figure 1.0. For example, 63 lakh litres of water at a salt concentration of 100ppm was generated from a feed with a salt concentration of 3.6 wt % NaCl. The size of the pump needed was 16 atm. Inorder to reduce the load of the high pressure pump recycling the treated water with the feed water was considered. The actual feed into the RO module will be lower in solute concentration, which will decrease the osmotic pressure need to be overcome (Figure 1). For a recycle ratio of 3.3 the pressure at the pump was found to be 3.7 atm. The residue to product ratio is 0.5. 82% reduction in pump size needed can be achieved in this example