Reverse osmosis, comparison with

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. 

Nurlaila prepared SPPO TFC membranes by coating Desal E-500 substrate membranes with 0.5 or 1.0 wt % of SPPO (intrinsic viscosity of the base PPO polymer, 1.58 dL/g in chloroform lEC value, 1.87 meq/g). e membranes were then heat treated in a hot water bath, before being subjected to reverse osmosis experiments with NaCl or MgS04 solution. The results of RO experiments are given in Table 11. Comparison of the results from several series of experiments such as 1-4, 6-9, and 10-14, leads to a conclusion that the flux increases with heat treatment with little sacrifice in solute rejection. 

The technology that competes with ion exchange in wastewater application is reverse osmosis (RO), therefor it is appropriate to make some comparisons. Direct cost comparisons are not straightforward, and requires comparison of some of the hidden cost parameters. Since there appear to be few detailed comparisons in the open literature, there exists the general impression that RO is more economical than ion exchange. Whereas this may be true in a number of applications, as a general rule this is not the case. 

The same nanofiltration experiments were performed with a 50-A ultrafiltration membrane (available from US Filter/Membralox, Warrendale, PA,USA), this time with a monodentate phosphite ligand (24) used for comparison and toluene as the solvent (Table V). Both higher retentions and flux rates for the dendrimers were obtained relative to what was observed with the reverse osmosis membranes. Dendrophite G4 was used in three subsequent reactions carried out with this procedure. 

Table 1 shows treatment costs for the technology (based on a processing rate of 20 gpm) in comparison to other groundwater treatment technologies (i.e., chemical reduction and precipitation, chemical precipitation with sedimentation or filtration, activated carbon adsorption, ion exchange, reverse osmosis, and electrodialysis) (D168869, Table 13). 

The effect of concentration polarization on specific membrane processes is discussed in the individual application chapters. However, a brief comparison of the magnitude of concentration polarization is given in Table 4.1 for processes involving liquid feed solutions. The key simplifying assumption is that the boundary layer thickness is 20 p.m for all processes. This boundary layer thickness is typical of values calculated for separation of solutions with spiral-wound modules in reverse osmosis, pervaporation, and ultrafiltration. Tubular, plate-and-ffame, and bore-side feed hollow fiber modules, because of their better flow velocities, generally have lower calculated boundary layer thicknesses. Hollow fiber modules with shell-side feed generally have larger calculated boundary layer thicknesses because of their poor fluid flow patterns. 

S. Jain, S.K. Gupta, Analysis of modified surface pore flow model with concentration polarization and comparison with Spiegler-Kedem model in reverse osmosis system, J. Membr. Sci. 232 (2004) 45-61. 

Rautenbach R and Albrecht R. The separation potential of pervaporation I Discussion of transport equations and comparison with reverse osmosis. J. Memb. Sci. 1985 25 1-23. 

Figure 4.11. Comparison between different wastewater treatment processes. MF microfiltration unit, UF ultrafiltration unit, RO reverse osmosis unit. From Cote et al. [4.102], with permission from Elsevier Science.

During the 1960 s, reverse osmosis was compared with other methods of demineralization. It was indicated in these comparisons that reverse osmosis could not compete favorably with ion exchange at dissolved solids concentrations below 700 mg/fi and that its most favorable area of use would be from about 1,200 to 5,000 mg/6 dissolved solids. This idea has been totally refuted because some of the most successful applications of reverse osmosis, particularly as part of the process to produce high purity water, have been in treating low dissolved solids water. Water containing 200 mg/ dissolved solids or less has been treated at costs equal to or lower than those of ion exchange alone. 

Eq. (7.37) with the conditions (7.35), (7.39), and (7.40) is the same as in the problem of concentration profile development in reverse osmosis (see Section 6.4). A comparison with this problem shows that the condition (7.39) takes place in the reverse osmosis problem if we replace i/2z+F by VwC . The density of current is i = Fz+j+, where j+ is the flux of ions. It is now possible to use the solution derived in Section 6.4. At small distances from the entrance (x/h 0), diffusion layer thickness is insignificant, and it is possible to introduce the selfsimilar coordinate (6.76)... 

Based on the comparison performed, reverse osmosis appears to be the most attractive seawater desalination process for desalination co-located with a MASLWR plant because of its lowest capital cost and the highest MASLWR net power supplied to the electrical grid. 

In a related study, Khedr (2013) assessed the removal of radium, U, as uranyl cation, or carbonate complexes, and radon by reverse osmosis (RO) and nanofiltration (NF) in comparison with the conventional methods of ion exchange resins (lERs), chemical precipitation/softening, coagulation, and adsorption on surface active media. lERs and chemical softening achieved radionuclide rejection from 32 to 95%, but with loss of process efficiency due to undesired... 

Use of reverse osmosis is not widespread in the mining sector due to high operating costs in comparison to other options. Its use in plating systems for recovery of Cd, Ni and Cr is common where both the metals and the clean water are recycled. Feed streams with up to 10 000 mg 1 total dissolved solids can be treated in modern systems where the membrane is usually in fine tubular form with water flowing from inside the tube to the outside. 

A comparison of typical design conditions for the two alternatives, including reverse osmosis is presented in Table 7.4. It was assumed the pretreatment system would have to treat (at least occasionally) degraded water, and that two stages of media filtration with coagulant addition were required. 

Coker, S. D., Beardsley, S. S., and Whipple, S. S. (1994). An economical comparison of demineralization with reverse osmosis and ion exchange technology. Power-Gen Americas. Las Vegas, Nevada. 

B. Hofs, R. Schurer, D.J.H. Harmsen, C. Ceccarelli, E.F. Beerendonk, E.R. Cornelissen. Characterization and performance of a commercial thin film nanocomposite seawater reverse osmosis membrane and comparison with a thin film composite. Journal of Membrane Science 446 (2013) 68-78. 

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