Membrane cation-exchanger

A bipolar membrane is a sandwich of a cation and an anion exchange membrane which splits H20 to H+ and OH- under a potential of about 0.9 V. The reactor consists of a stack, of bipolar membranes, cation exchange membranes and anion exchange membranes arranged between a single anode and a single cathode with parallel hydraulytic circuits for the salt, acid product and alkali product, Fig. 32. 

A cation-selective membrane (cation-exchange membrane) contains negatively charged fixed ions, mostly sulfonic acid groups, and cations as counterions, for example, H+ or Na+. In an anion-selective... 

The performance of the cell depends greatly on the life of the ion exchange membranes. Cation-exchange membranes from Ionics and Tokuyama Soda (e.g.. Ionics 61 AZG, Tokuyama CLE-E) were most stable, having lives of more than half a year. 

Figure 6.8 Current-voltage relation during electrodialysis of a sodium chloride solution using an ion exchange membrane (concentration polarization). Ion exchange membrane cation exchange membrane (NEOSEPTA CL-25T) measured under natural convection of a 0.104 N NaCl solution at 25.0 °C.

Electrodialysis is a process in which ions contained in the solution are separated by membranes in the presence of an external electric field. The membranes employed in this process are called ion-exchange membranes. There are two types of membranes cation-exchange membranes, which are penetrable only for cations, and anion-exchange membranes, penetrable for anions only. 

The most important components in a fuel cell are the Membrane Electrolyte Assembly (MEA) and the bipolar plates. The MEA usually consists of an electrolyte membrane, which is coated with catalytically active platinum-electrodes and a gas diffusion layer of hydrophobic graphite. As the electrolyte membrane cation exchange polymers are used. A crucial break-through was reached here by the employment of fluoridated polymers. The market leader here is Nation developed by the company Dupont. 

A membrane cation-exchanger has been used to permit transport of cadmium from an aqueous system into a 2% sodium-chloride solution on the other side of the membrane-separated cell (437). This type of technique should be very useful if the transport is reasonably rapid. 

Anion exchange membrane Cation exchange membrane Figure 14 iiiustration of the eiectrodiaiysis process applied to desaiination. 

The fourth fully developed membrane process is electrodialysis, in which charged membranes are used to separate ions from aqueous solutions under the driving force of an electrical potential difference. The process utilizes an electrodialysis stack, built on the plate-and-frame principle, containing several hundred individual cells formed by a pair of anion- and cation-exchange membranes. The principal current appHcation of electrodialysis is the desalting of brackish groundwater. However, industrial use of the process in the food industry, for example to deionize cheese whey, is growing, as is its use in poUution-control appHcations. 

Sodium nitrite has been synthesized by a number of chemical reactions involving the reduction of sodium nitrate [7631-99-4] NaNO. These include exposure to heat, light, and ionizing radiation (2), addition of lead metal to fused sodium nitrate at 400—450°C (2), reaction of the nitrate in the presence of sodium ferrate and nitric oxide at - 400° C (2), contacting molten sodium nitrate with hydrogen (7), and electrolytic reduction of sodium nitrate in a cell having a cation-exchange membrane, rhodium-plated titanium anode, and lead cathode (8). 

Soluble Sta.nna.tes, Many metal staimates of formula M Sn(OH) are known. The two main commercial products are the soluble sodium and potassium salts, which are usually obtained by recovery from the alkaline detinning process. They are also produced by the fusion of stannic oxide with sodium hydroxide or potassium carbonate, respectively, followed by leaching and by direct electrolysis of tin metal in the respective caustic solutions in cells using cation-exchange membranes (27). Another route is the recovery from plating sludges. 

A.sahi Chemical EHD Processes. In the late 1960s, Asahi Chemical Industries in Japan developed an alternative electrolyte system for the electroreductive coupling of acrylonitrile. The catholyte in the Asahi divided cell process consisted of an emulsion of acrylonitrile and electrolysis products in a 10% aqueous solution of tetraethyl ammonium sulfate. The concentration of acrylonitrile in the aqueous phase for the original Monsanto process was 15—20 wt %, but the Asahi process uses only about 2 wt %. Asahi claims simpler separation and purification of the adiponitrile from the catholyte. A cation-exchange membrane is employed with dilute sulfuric acid in the anode compartment. The cathode is lead containing 6% antimony, and the anode is the same alloy but also contains 0.7% silver (45). The current efficiency is of 88—89%, with an adiponitrile selectivity of 91%. This process, started by Asahi in 1971, at Nobeoka City, Japan, is also operated by the RhcJ)ne Poulenc subsidiary, Rhodia, in Bra2il under Hcense from Asahi. 

The feed solution containing ions enters a compartment whose walls are a cation-exchange and an anion-exchange membrane. If the anion-exchange membrane is in the direction of the anode, as shown for the middle feed compartment, anions may pass through that membrane in response to an electrical potential. Tne cations can likewise... 

Membranes Ion-exchange membranes are highly swollen gels containing polymers with a fixed ionic charge. In the interstices of the polymer are mobile counterions. A schematic diagram of a cation-exchange membrane is depicted in Fig. 22-57. 

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