Rejection of sodium chloride

The results are reported as gallons per day per square foot of membrane area (membrane flux) and as rejection of sodium chloride. Rejection is calculated as follows ... 

The Donnan ion effect is also reflected in the lower rejection of sodium chloride at the higher salinity level (0.5% vs. 0.1%). The chemistry of this membrane is shown as follows ... 

Chemical modification The surface of a membrane can be modified by chemical reactions. For example, when the surface of a polyamide composite membrane is brought into contact with a strong hydrofluoric acid solution, the top polyamide layer becomes sHghtly thinner by a chemical reaction with hydrofluoric acid. As a result, the flux increases considerably while the rejection of sodium chloride is unchanged or slightly increased [8]. 

The importance of proper RO membrane selection has already been discussed. A review of commercially available RO membranes revealed five different basic membranes that could provide organic recovery. Cellulose acetate and cellulose acetate blends, aromatic polyamide, polyamide thin-film composite, cross-linked polyimine thin-film composite (FT-30), and polybenzimidazole were available when this work was performed. Only the first four types were commercially available. All membranes were available with excellent salt rejection (>97 sodium chloride). Two types of membranes, cellulose acetate and FT-30, have shown short-term (<2-months intermittent use) resistance... 

Figure 5.5 Water permeability as a function of sodium chloride permeability for membranes made from cellulose acetate of various degrees of acetylation. The expected rejection coefficients for these membranes, calculated for dilute salt solutions using Equation (5.6),...

The relative performances of membranes produced for the desalination market are shown in Figure 5.12, a plot of sodium chloride rejection as a function of... 

Continuous electrodeionization systems can achieve 95% rejection of boron and silica, and 99+% rejection of sodium and chloride. This performance is possible due to voltage-induced dissociation of water that effectively regenerates a portion of the resin thereby allowing removal of weakly ionized species such as silica and boron.19 In fact, the boron in the effluent from a CEDI system can be lower than that in the effluent from a mixed-bed ion exchange system.13... 

Unless otherwise stated in the monograph, transfer 10 g of a solid sample, or 10 mL of a liquid sample, accurately weighed, into a 100-mL flask having a standard-taper neck. Add 10 mL of acetic anhydride and 1 g of anhydrous sodium acetate, mix these materials, attach a reflux condenser to the flask, and reflux the mixture for 1 h. Cool, and through the condenser, add 50 mL of water at a temperature between 50° and 60°. Shake intermittently for 15 min, cool to room temperature, transfer the mixture completely to a separator, allow the layers to separate, and then remove and reject the lower, aqueous layer. Wash the oil layer successively with 50 mL of a saturated sodium chloride solution, 50 mL of a 10% sodium carbonate solution, and 50 mL of saturated sodium chloride solution. If the oil is still acid to moistened litmus paper, wash it with additional portions of sodium chloride solution until it is free from acid. Drain off the oil, dry it with anhydrous sodium sulfate, and then filter it. 

Roth et al. [80] proposed a method to determine the state of membrane wear by analyzing sodium chloride stimulus-response experiments. The shape of the distribution of sodium chloride in the permeate flow of the membrane revealed the solute permeation mechanisms for used membranes. For new membranes the distribution of sodium chloride collected in the permeate side as well in the rejection side was unimodal. For fouled membranes they noted the presence of several modes. The existence of a salt leakage peak, as well as an earlier detection of salt for all the fouled membranes, gave evidence of membrane stmcture modification. The intensive use of the membranes might have created an enlargement of the pore sizes. Salt and solvent permeabilities increased as well. While this is a difficult paper to follow, it may be of use to those who want to develop new methods for measuring membrane degradation. 

The influence of metal oxide derived membrane material with regard to permeability and solute rejection was first reported by Vernon Ballou et al. [42,43] in the early 70s concerning mesoporous glass membranes. Filtration of sodium chloride and urea was studied with porous glass membranes in close-end capillary form, to determine the effect of pressure, temperature and concentration variations on lifetime rejection and flux characteristics. In this work experiments were considered as hyperfiltration (reverse osmosis) due to the high pressure applied to the membranes, 40 to 120 atm. In fact, results reproduced in Table 12.3 show that these membranes do not behave as h)qjerfiltra-tion membranes but as membranes with intermediate performances between ultra- and nanofiltration in which surface charge effect of metal oxide material plays an important role in solute rejection. 

The common argument that only elements are pure substances and that sodium chloride contains sodium and chlorine should be rejected. This is because one measures constant values for the density or for the melting point of sodium chloride, and will find such constants of other compounds in well-known published tables. Having a defined density, melting and boiling point, the related substance is considered as a pure substance, e.g. water or ethanol. Pure substances are elements and compounds ... 

The solution-diffusion model seems to represent the performance of a reverse osmosis membrane. Figure 4.3 shows the salt rejection and flux of a low pressure polyamide membrane as a function of applied pressure. The membrane was operated on a 5,000 mg/ aqueous solution of sodium chloride at 25°C. As can be seen, there was no product water flow until the applied pressure exceeded the osmotic pressure (50 psi). After this, the flux increased linearly as would be predicted by the above water flux equation. Rejection is poor at lower pressures and increases rapidly until it reaches an asymptote at an applied pressure of about 150 psig. This can be attributed to a near constant flow of salt with a rapidly increasing product water flow which results in a more dilute product or in increased rejection. These data tend to substantiate the assertion of the solution-diffusion model that flow is uncoupled. 

Figure 5.8 Salt rejection and flux of IMF-40 membrane as a function of sodium chloride concentration and feedwater pressure.

This reaction normally proceeds in the opposite direction, and Berthollet suggested that the extremely high concentration of sodium chloride was responsible for the reversal. Berthollet was here suggesting that the normal affinities of substances could be altered by the masses present he was anticipating the law of mass action of Guldberg and Waage (Chapter 13). However, Berthollet also speculated that a compound could be of a variable composition determined by the masses of reactants employed in its preparation. Berthollet and Proust became engaged in a controversy on this issue (Chapter 6), and after 1808 most chemists sided with Proust and rejected Berthollet s views. An unfortunate consequence of this was that for many years most chemists dismissed any notion of mass action. 

Along similar lines, other researchers have been looking into nanocomposite membranes. Researchers at the University of Colorado at Boulder have been developing lyotropic liquid crystals (LLCs) to form what they call nanostructured polymer membranes. The LLCs can form liquid crystalline phases with regular geometries which act as conduits for water transport while rejection ions based on size exclusion. In bench-scale tests, nanostructuered polymer membranes exhibited a rejections of 95% and 99.3% of sodium chloride and calcium chloride, respectively. These membranes also exhibited greater resistance to chlorine degradation than... 

Water for the kainite conversion comes from the hydrated MgSO. This solution is saturated with K SO. Use of potassium sulfate mother Hquor as a source of water for the reaction lowers the K SO lost in the MgCl2 solution, which is rejected as a waste stream from the process. It also is a solvent for sodium chloride that enters the process as a contaminant in kainite. 

The unit operation demonstrates that membrane life over two years has been demonstrated and that the selected materials of construction are correct. The recovery of sulphate-lean brine is 85-90% during normal operation. In a trial run under extreme conditions, with the unit modified to operate in recycle mode, the concentration of sodium sulphate in the reject stream was increased to 190g l-1 a 90% sulphate rejection rate was achieved during this trial. The sodium chloride concentration decreased on the concentrate side of the membrane and increased in the... 

Another ion-exchange system has recently been developed by Nippon Rensui [8]. The Nippon Rensui system employs an amphoteric ion-exchange resin called Diaion DSR01 which can be eluted with water. In the DSR01 system, the resin takes up the sodium chloride and rejects the sodium sulphate. Purified NaCl is then recovered from the resin by water elution. This seems like a difficult approach since it is necessary to take-up huge amounts of NaCl on the resin in order to separate out a relatively small amount of sulphate impurity. Excessive dilution of the purified brine may also be an issue with this process. 

Physically, ion channels are tiny pores that stud the surface of all cells. The ion channels are important for, among other things, the function of muscles and the nervous system. These channels allow the passage of potassium, calcium, sodium, and chloride ions. Through a balance of electrical forces and chemical bonds, ion channels are specific for one ion for instance, a potassium ion channel will reject a sodium ion trying to enter its channel. An excellent visualization of the overall process is found at the website http //www. rockefeller.edu/pubinfo/howkion.html. It will be helpful to look at this website before going any further in the discussion. 

Feed solution used in all experiments contained sodium chloride at a concentration level of 5,000 ppm. Membrane salt rejection is evaluated from conductance measurements of product water and expressed as percent rejection, %R, or desalination ratio, D. . These units are defined by the following equations in which Cp and Cf are sodium chloride concentrations in feed and product respectively. Note that D. is very sensitive to concentration changes and expands rapidly as 100% rejection is approached. 

Membrane Properties. The performance range of ammonia-modified membranes in low pressure operation is indicated in Figure 6 along with the performance of the reference membrane (I, reference membrane IV, ammonia-modified membrane). The lower boundary of the performance range refers to a solvent-to-polymer ratio of 3, the upper boundary to a ratio of 4. While the salt rejection towards univalent ions of the ammonia-modified membrane is limited to below 80 %, the maximum low pressure flux is over 15 m /m d (approaching 400 gfd) at a sodium chloride rejection of the order of 10 %. This membrane thus exhibits the flux capability of an ultrafiltration membrane while retaining the features of reverse osmosis membranes, viz. asymmetry and pressure resistance. 

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