Reverse osmosis flux seawater desalination

The pressure to be used for reverse osmosis depends on the salinity of the feedwater, the type of membrane, and the desired product purity. It ranges from about 1.5 MPa for low feed concentrations or high flux membranes, through 2.5—4 MPa for brackish waters, and to 6—8.4 MPa for seawater desalination. In desalination of brackish or sea water, typical product water fluxes through spiral-wound membranes are about 600—800 kg/m /d at a recovery ratio RR of 15% and an average salt rejection of 99.5%, where... 

Albany International Research Co. has developed an advanced hollow fiber composite reverse osmosis membrane and module under the name of Quantro II . This composite membrane is comprised of a porous hollow fiber substrate on which has been deposited a rejection barrier capable of fluxes of commercial importance at high rejection of dissolved salts at elevated temperatures. Resistance to active chlorine has been demonstrated. Proprietary processes have been developed for spinning of the fiber, establishment of the rejection barrier and processing of the fiber to prepare modules of commercial size. Prototype modules are currently in field trials against brackish and seawater feed solutions. Applications under consideration for this membrane include brackish and seawater desalination as well as selected industrial concentration processes. 

As Figure 5.12 shows, Toray s PEC-1000 crosslinked furfuryl alcohol membrane has by far the best sodium chloride rejection combined with good fluxes. This explains the sustained interest in this membrane despite its extreme sensitivity to dissolved chlorine and oxygen in the feed water. Hollow fine fiber membranes made from cellulose triacetate by Toyobo or aromatic polyamides by Permasep (Du Pont) are also comfortably in the one-stage seawater desalination performance range, but the water fluxes of these membranes are low. However, because large-surface-area, hollow fine fiber reverse osmosis modules can be... 

Sablani S.S., Goosen M.F.A., Al-Belushi R., and Gerardos V., Influence of spacer thickness on permeate flux in spiral-wound seawater reverse osmosis systems. Desalination 146 2002 225-230. 

As of the end of 1984, the desalination of brackish water accounted for 82% of capacity. This is due to the fact that early reverse osmosis membranes were incapable of single stage seawater desalination and, thus, they were limited to brackish water desalination. Within the last 10 years, significant advances have been made in both the flux and rejection capability of membranes and reverse osmosis is technically able to desalt seawater in a single stage. In the recent past, it has been an effective competitor to the distillation process in seawater desalination. In fact, reverse osmosis is now beginning to replace existing distillation capacity in the Middle East.4... 

It is noted that the osmotic pressure of seawater is about 400 psig and an operating pressure of between 800 and 1,000 psig is required to obtain optimum flux and rejection. If a recovery of only 50% were used, the reject osmotic pressure would be in excess of 700 psig and the average osmotic pressure would be about 550 psig. This would present problems in obtaining a potable product of 500 mg/C or less. Consequently, most seawater desalination reverse osmosis units are operated at a recovery of from 25 to 35%. 

The properties of FT-30 membranes have been reviewed in several publications, including reverse osmosis performance under seawater and brackish water test conditions.60"62 In commercially produced spiral-wound elements, the FT-30 membrane typically gives 99.1 to 99.3% salt rejection at 24 gfd flux in seawater desalination at 800 psi and 25°C. In brackish water applications, FT-30 spiral elements can be operated at system pressures of as low as 225 psi while producing water at 22 to 24 gfd. Similar flux levels are possible with the TFC-202 and LP-300 membranes, as mentioned earlier. But it is notable that those membranes achieve such high fluxes through use of extremely thin surface barrier layers about only one-tenth the thickness of the FT-30 barrier layer. 

In order to filter a solution containing very fine particles, as is the case when seawater is to be desalinated, it is therefore necessary to apply, between the compartment containing the particles (upstream) and the downstream compartmerrt, a pressure difference AP = P+-P- greater than the osmotic pressrrre. As long as AP - Posm < 0, the flux of solvent is an osmosis flitx. Filtration occrrrs for AP - Posm > 0. This is termed reverse osmosis, since the flux of solvent is in the... 

Example 3.4.2 Earlier literature on reverse osmosis desalination of seawater at a high AP = 102 atm provided the following information for a cellulose acetate membrane A = 8.03 X 10 gmol H20/cm -s-atm it(C,y) = bCif = 45.7 atm DimKim/Sm) for Salt = 1.774 X10 cm/s Xif= 17.7 X 10 Xip = 0.4 X 10 . The membrane has a high rejection for the solute i, which is NaCl. Calculate the values of water flux and the salt flux through the membrane. Calculate the salt concentration on two sides of the membrane, assumed to be of the solution-diffusion type. Calculate the salt rejection R. [Noter. Current membranes are more productive. Therefore AP is around 54.4-68 atm (800-1000 psi). One should calculate A based on the final concentration of seawater desired.)... 

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