Reverse osmosis salt flow rate

It is the rate of separation rather than the efficiency of salt retention that is the primary practical issue in the development of reverse osmosis desalination. In addition to a variety of other factors, the rate of reverse osmotic flow depends on the excess pressure across the membrane. Therefore the problem of rapid flow is tied into the technology of developing membranes capable of withstanding high pressures. The osmotic pressure of sea water at 25 °C is about 25 atm. This means that no reverse osmosis will occur until the applied pressure exceeds this value. This corresponds to a water column about 840-ft high at this temperature. 

It is required to design a reverse osmosis unit to process 2500 mVh of seawater at 25°C containing 3.5 wt% dissolved salts, and produce purified water with 0.05 wt% dissolved salts. The pressure will be maintained at 135 atm on the residue side and 3.5 atm on the permeate side, and the temperature on both sides at 25°C. The dissolved salts may be assumed to be NaCl. With the proposed membrane, the salt permeance is 8.0 x 10 m/h and the water permeance is 0.085 kg/rn-.h.atrn. The density of the feed seawater is 1020 kg/m ( of the permeate, 997.5 kg/nv and of the residue (with an estimated salt content of 5 wt%), 1035 kg/rnc Assuming a perfect mixing model and neglecting the mass transfer resistances, determine the required membrane area and calculate the product flow rates and compositions. 

Reverse osmosis memhraaes. The exceptionally high moisture regain observed with polybenzimidazole fibers prompted a team at Celanese Research Co to investigate the utility of polybenzimidazole films as semipermeable membranes for reverse osmosis processes, such as sea water desalination66,94). A continuous process was devised in which films were cast from solution into a water precipitation bath. The films were tested for reverse osmosis performance with a saline solution (0.5% Nad) as feed stream at a pressure of 4.14 MN m-2 and a flow rate of 19.8 m min-1. Salt rejection was ca. 95% throughout. A cellulose acetate film of the type commonly used as a reverse osmosis standard was tested under the same conditions for comparison. Table 8 shows the results. 

Figure 5.16, adapted from Kurihara,79 80 shows a comparison of several types of commercial reverse osmosis membranes in terms of salt rejection and permeate flow rate under seawater test conditions (35,000 ppm, 800 psi, 25°C). This chart emphasizes the capability of PEC-1000 to provide complete single-stage seawater desalting. In a test at Toray s Ehime desalination test facility on 42,000 ppm seawater (equivalent to Red Sea salinity), PEC-1000 spiral elements operated at 35% recovery produced a permeate having an average salinity of only 220 ppm, well below WHO standards. Average salt rejection was 99.5%. 

In order to desalinate seawater in a small-sized plarrt, it is envisaged to use a hollow-fiber modttle (Figure 14.10), the sketch of which has been formd in the Techniques de Tlngenietrr. Salt water flows inside drcular cylindrical hollow fibers of inner diameter 40 rm and outer diameter 80 jrm through which the permeate (salt free) is filtered by reverse osmosis as it passes from the inside to the outside of the fibers. In this cross-flow filtration device, a substarrtial fraction of the salt feed flow rate leaves without being filtered. 

In Section 3.4.2.1, the phenomenon of reverse osmosis (RO) through a nonporous membrane was introduced. If the hydraulic pressure of a solution containing a microsolute, e.g. common salt, on one side of a nonporous membrane exceeds that of another solution on the other side of the same membrane by an amount more than the difference of the osmotic pressures of the same two solutions, then, according to the solution-diffusion model, the solvent will flow from the solution at higher pressure to the one at a lower pressure (equation (3.4.54)) at the following rate ... 

With reverse osmosis and other filtration processes, a basic problem is concentration polarization. On the feed side of the membrane the solute can be enriched as water permeates through the membrane leaving a higher concentration of solute (salt) at the membrane surface. This is a problem particularly for static systems, but can also be a problem for dynamic systems where the flow rate past the membrane does not prevent the bormdary layer from forming. A similar polarization can occur on the permeate side of the membrane, but is generally less of a concern for high-reject dynamic membrane systems. This situation is similar to boundary layer heat and mass transfer problems well covered in the literature. 

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