Cathode membrane cell

Chloiine is pioduced at the anode in each of the three types of electrolytic cells. The cathodic reaction in diaphragm and membrane cells is the electrolysis of water to generate as indicated, whereas the cathodic reaction in mercury cells is the discharge of sodium ion, Na, to form dilute sodium amalgam. 

Separation of the anode and cathode products in diaphragm cells is achieved by using asbestos [1332-21 -4] or polymer-modified asbestos composite, or Polyramix deposited on a foraminous cathode. In membrane cells, on the other hand, an ion-exchange membrane is used as a separator. Anolyte—catholyte separation is realized in the diaphragm and membrane cells using separators and ion-exchange membranes, respectively. The mercury cells contain no diaphragm the mercury [7439-97-6] itself acts as a separator. 

Catalytic cathodes in membrane cell operations exhibit a voltage savings of 100—200 mV and a life of about 2 + yr using ultrapure brine. However, trace impurities such as iron from the caustic recirculation loop can deposit on the cathode and poison the coating, thereby reducing its economic life. 

Mild steel cathodes are used extensively in chlor-alkah and chlorate cells. Newer activated cathode materials have been developed that decrease cell voltages about 0.2 V below that for cells having mild steel cathodes. Some activated cathodes have operated in production membrane cells for three years with only minor increases in voltage (17). Activated cathodes can decrease the energy consumption for chlorine—caustic production by 5 to 6.5%. 

There have been a number of cell designs tested for this reaction. Undivided cells using sodium bromide electrolyte have been tried (see, for example. Ref. 29). These have had electrode shapes for in-ceU propylene absorption into the electrolyte. The chief advantages of the electrochemical route to propylene oxide are elimination of the need for chlorine and lime, as well as avoidance of calcium chloride disposal (see Calcium compounds, calcium CHLORIDE Lime and limestone). An indirect electrochemical approach meeting these same objectives employs the chlorine produced at the anode of a membrane cell for preparing the propylene chlorohydrin external to the electrolysis system. The caustic made at the cathode is used to convert the chlorohydrin to propylene oxide, reforming a NaCl solution which is recycled. Attractive economics are claimed for this combined chlor-alkali electrolysis and propylene oxide manufacture (135). 

In the membrane process, the chlorine (at the anode) and the hydrogen (at the cathode) are kept apart by a selective polymer membrane that allows the sodium ions to pass into the cathodic compartment and react with the hydroxyl ions to form caustic soda. The depleted brine is dechlorinated and recycled to the input stage. As noted already, the membrane cell process is the preferred process for new plants. Diaphragm processes may be acceptable, in some circumstances, but only if nonasbestos diaphragms are used. The energy consumption in a membrane cell process is of the order of 2,200 to 2,500 kilowatt-hours per... 

It is perhaps obvious that oxygen would be the second gas to be purified electro-chemically. Langer noted in 1964 that the same sort of apparatus used for H2 could be used for 02 [16], with the impure feed admitted to the cathode chamber of the same type of membrane cell, and the pure product obtained at the anode. In alkaline... 

The electrolysis in aqueous sulfuric acid with methanol as a cosolvent was perfomed in a filterpress membrane cell stack developed at Reilly and Tar Chemicals. Because of the low current density of the process, a cathode based on a bed of lead shot was used. A planar PbOa anode was used. The organic yield was 93% with approximately 1% of a dimer. The costs of the electrochemical conversion were estimated as one-half of the catalytic hydrogenation on a similar scale. 

A typical 5 kA Eco cell has a cathode drum with a radius of 0.37 m, a height of 0.74 m and a cathode-membrane gap of about 1 cm. The cathode is rotated at 100-200 rev.min-1. In rotating-cylinder electrode cells, high fractional conversion can be obtained by employing an Eco cascade cell. 

Of the chlorine production capacity installed in Germany, which totalled 4.4 million tonnes in 2003, 50% were from the membrane cell process, 27% from the mercury cell process and 23% from the diaphragm cell process. The mercury cell process has been the subject of environmental policy criticism for years because of its use of mercury cathodes and resulting pollutant emissions. Hence, no new mercury plants will be... 

Figure 10. COD evolution as a function of electrolysis time for electrolysis of solutions containing 50 mMNa2S04 + 1 mMFe + dye (a) 0.082 mMDR23, (b) 0.25 mM A07, (c) 0.33 mM Am, (d) 0.1 mM AG25, (e) 0.1 mM Am, (f) 0.17 mM BB9. Electrolyses were carried out in a membrane cell with a reticulated vitreous carbon cathode (5 cm x 5 cm x 1 cm).

Chlorine is produced industrially by electrolysis of brine using either mercury cathode cells or, preferably, various commercially available membrane cells. Chlorine gas is hberated at the anode while sodium hydroxide and hydrogen are liberated at the cathode ... 

Since the products of the electrolysis of aqueous NaCl will react if they come in contact with each other, an essential feature of any chloralkali cell is separation of the anode reaction (where chloride ion is oxidized to chlorine) from the cathode reaction (in which OH- and H2 are the end products). The principal types of chloralkali cells currently in use are the diaphragm (or membrane) cell and the mercury cell. 

Because the cations are readily exchangeable, the membranes allow rather free passage of Na+ from anode to cathode compartments to match current flow in the external circuit. Since OH- or Cl penetration is negligible, substantially pure NaOH solution can be made in a membrane cell. 

So-called zero-gap membrane cells in which cathode and chlorine-evolving anodes are touching the cation exchange membrane, which separates the anode from the cathode compartment, are also state of the art (35). Bipolar chlorine electrolyzers have also been developed (36), for instance, at ICI, an achievement that could only be envisaged due to the introduction of RuCL-coated titanium anodes. 

The dimensionally slahle characteristic of the metal anode made the development of the membrane chlorine cell possible. These cells arc typically arranged in ail electrolyzer assembly which docs not allow for anodc-ro-cathode gap adjustment alter assembly. Also, very close tolerances are required. The latitude that titanium affords the cell designer has made a wide variety of monopolar and bipolar membrane cell designs possible. 

FIGURE 18.17 A membrane cell for electrolytic production of CI2 and NaOH. Chloride ion is oxidized to CI2 gas at the anode, and water is converted to H2 gas and OH-ions at the cathode. Sodium ions move from the anode compartment to the cathode compartment through a cation-permeable membrane. Reactants (brine and water) enter the cell, and products (CI2 gas, H2 gas, aqueous NaOH, and depleted brine) leave through appropriately placed pipes. 

In laboratory experiments, selective membranes were already applied years ago for the pH-control during the electro-dialysis of pH-sensitive colloids. In the three-compartment cells used, the electrode chambers were rinsed with distilled water. On account of the high mobility of the H+ ions, the desalting cell was inclined to become acid. To oppose this effect, anode- and cathode membranes with different polarity were sought for. At the beginning the influence of these membranes on the current efficiency — i.e. the amount of salt removed per unit of charge flown through — was mentioned only sporadically (5,23). 

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