Reverse-Osmosis Membrane Processes

Currently, the pressure-driven membrane processes, reverse osmosis (RO), nanofiltration (NF), ultrafiltration (UF), and microfiltration (MF), are widely used in water treatment, biotechnology, food industry, medicine, and other fields (Baker 2004). However, one of the main problems arising from the operation of the manbrane units is membrane fouling, which seriously hampers the applications of manbrane technologies (Scot and Hughes 1996). 

The first separation example is seawater desalination. Traditionally, desahnation was done by distillation or simple evaporation/condensation [55]. Today, thermally driven desalination has been largely replaced by the membrane process reverse osmosis. In reverse osmosis an applied pressure exceeding the osmotic pressure of the salt solution causes water to permeate through a dense membrane. Hydrated salt ions are relatively large compared to water and have a lower permeability through the membrane resulting in relatively salt-free water being collected as the reverse osmosis permeate. 

Fig. 25. Reverse osmosis, ultrafiltration, microfiltration, and conventional filtration are related processes differing principally in the average pore diameter of the membrane filter. Reverse osmosis membranes are so dense that discrete pores do not exist transport occurs via statistically distributed free volume areas. The relative size of different solutes removed by each class of membrane is illustrated in this schematic.

The semi-permeable membrane is the heart of the reverse osmosis separation process. Semi-permeable membranes for reverse osmosis are broadly divided into two types. The earhest practical membrane was of the asymmetric type [3-6]. It consisted of an osmotically active surface layer with very small pores (less than 1 nm) with a thickness of 30-100 nm. This layer was physically supported on a porous substructure, whose porosity increased with distance from the surface layer. In such a membrane, the... 

The presence of endotoxin should be monitored by LAL method in the routine operation. Endotoxin can be removed by means of distillation, reverse osmosis, and/or ultrafiltration. Incomplete separation of mist in distillation, however, and leakage in membrane of reverse osmosis or ultrafiltration cause contamination with endotoxin. After these separation processes, of course, contamination of microbes or growth of microbes causes endotoxin contamination [2,8],... 

The range of application of the three pressure-driven membrane water separation processes—reverse osmosis, ultrafiltration and microfiltration—is illustrated in Figure 1.2. Ultrafiltration (Chapter 6) and microfiltration (Chapter 7) are basically similar in that the mode of separation is molecular sieving through increasingly fine pores. Microfiltration membranes filter colloidal particles and bacteria from 0.1 to 10 pm in diameter. Ultrafiltration membranes can be used to filter dissolved macromolecules, such as proteins, from solutions. The mechanism of separation by reverse osmosis membranes is quite different. In reverse osmosis membranes (Chapter 5), the membrane pores are so small, from 3 to 5 A in diameter, that they are within the range of thermal motion of the polymer... 

Figure 3.20 Schematic of the interfacial polymerization process. The microporous film is first impregnated with an aqueous amine solution. The film is then treated with a multivalent crosslinking agent dissolved in a water-immiscible organic fluid, such as hexane or Freon-113. An extremely thin polymer film forms at the interface of the two solutions [47]. Reprinted from L.T. Rozelle, J.E. Cadotte, K.E. Cobian, and C.V. Knopp, Jr, Nonpolysaccharide Membranes for Reverse Osmosis NS-100 Membranes, in Reverse Osmosis and Synthetic Membranes, S. Sourirajan (ed.), National Research Council Canada, Ottawa, Canada (1977) by permission from NRC Research Press...

The traditional membrane separation processes (reverse osmosis, micro-, ultra- and nanofiltration, electrodialysis, perva-poration, etc.), already largely used in many different applications, are today combined with new membrane systems such as CMRs and membrane contactors. Membranes are applied not only in traditional separation processes such as seawater desalination but also in medicine, bioengineering, microelectronics, the life in the space, etc. 

Reverse osmosis membranes. The exceptionally hi moisture regain observed with polybenzimidazole fibers prompted a team at Ctelanese Research Co to investigate the utility of polybenzimidazole films as semipermeable membranes for reverse osmosis processes, sudi as sea water desalination A continuous process was devised in which films were... 

Wong, E.W. Urethane-Polyether Block Copolymer Membranes for Reverse Osmosis, Ultrafiltration and Other Membrane Processes, ORF Record of Invention No. 335, 1969. 

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. 

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... 

Membrane separations involve the selective solubility in a thin polymeric membrane of a component in a mixture and/or the selective diffusion of that component through the membrane. In reverse osmosis (3) applications, which entail recovery of a solvent from dissolved solutes such as in desalination of brackish or polluted water, pressures sufficient to overcome both osmotic pressure and pressure drop through the membrane must be applied. In permeation (4), osmotic pressure effects are negligible and the upstream side of the membrane can be a gas or liquid mixture. Sometimes a phase transition is involved as in the process for dehydration of isopropanol shown in Fig. 1.8. In addition, polymeric liquid surfactant and immobilized-solvent membranes have been used. 

Chemical processing Reverse osmosis membrane, permselective membrane, gas separation membran-lubrication, insolubilization... 

Membrane filters were first commerciafised in 1927 by the Sartorius Company in Germany using the Zigmondy process. Reverse osmosis (RO) was first observed and studied in the 1920s. However, it remained unnoticed until rediscovered by Reid and his co-workers 30 years later. The first practical phenomenon of haemodialysis was demonstrated by Kolff in the 1940s. Membrane milestones are given in Table 1.1. 

Membrane separation processes. In Chapter 13 a detailed discussion is given of the various membrane separation processes of gas separation by membranes, dialysis, reverse osmosis, and ultrafiltration. 

Types of membranes for reverse osmosis. One of the more important membranes for reverse-osmosis desalination and many other reverse-osmosis processes is the cellulose acetate membrane. The asymmetric membrane is made as a composite film in which a thin dense layer about 0.1 to 10 pm thick of extremely fine pores supported upon a much thicker (50 to 125 pm) layer of microporous sponge with little resistance to permeation. The thin, dense layer has the ability to block the passage of quite small solute molecules. In desalination the membrane rejects the salt solute and allows the solvent water to pass through. Solutes which are most effectively excluded by the cellulose acetate membrane are the salts NaCl, NaBr, CaClj, and NajSO sucrose and tetralkyl ammonium salts. The main limitations of the cellulose acetate membrane are that it can only be used mainly in aqueous solutions and that it must be used below about 60°C. 

Seawater or brackish water is used for process applications or as potable water when fresh water is scarce. Six techniques are used for desalination. Five are evaporation processes multiple-effect thermocompression mechanical vapor compression once-through multistage flash and multistage flash with brine recirculation. The sixth process, reverse osmosis, uses membrane technology for desalination. 

Sawamoto, S., Ohya, H., Yanase K., Semenova, S., Aihara, M., Takeuchi, T., and Negishi, Y. (2000). Nanotechnological method to control pore diameter of organic-inorganic composite membrane. Part 2. Molecular-wise vapor polymerization./. Membr. Sci. 174 131-139. Kumakiri, I., Yamaguchi, T., and Nakao, S. (2000). Application of a zeolite A membrane to reverse osmosis process./. Chem. Eng.Jpn. 33 333-336. 

An effluent stream of 3 m /h containing 7.6 gd of sodium sulfate at 20 C is treated in a hybrid process reverse osmosis for concentration and production of clean process water and membrane electrolysis for the conversion of sodium sulfate into sulfuric acid (15%) and caustic soda which is used for neutralisation. 

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