Diaphragm process

Industrial diaphragm cells consist of a box in which the anode plates are mounted vertically parallel to one another. The cathodes are flat hollow steel mesh structures covered with asbestos fibers, optionally impregnated with fluoro-organic resins, and fit between the anodes (see Fig. 1.7-8, 1.7-9 and 1.7-10). 

The salt solution fed into the anode chamber passes through the diaphragm into the cathode chamber. The chlorine produced at the anode is drawn off upwards and hydrogen and sodium hydroxide mixed with residual salt are produced at the cathode. 

Upon electrolysis, the sodium chloride content of an initially saturated solution falls to ca. 170 g/L. The reactions at the anode are the same as in the mercury process. However, hydrogen is produced at the steel cathode  

The cell alkali leaving the cathode chamber contains ca. 

Recovery of sodium hydroxide The alkali solution is evaporated to 50% by weight of sodium hydroxide, whereupon the salt, except for a residual 1%, precipitates out. This salt is very pure and can be further utilized for concentrating depleted brine or, in the case of combined plants, in the mercury process. 

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Early demand for chlorine centered on textile bleaching, and chlorine generated through the electrolytic decomposition of salt (NaCl) sufficed. Sodium hydroxide was produced by the lime—soda reaction, using sodium carbonate readily available from the Solvay process. Increased demand for chlorine for PVC manufacture led to the production of chlorine and sodium hydroxide as coproducts. Solution mining of salt and the avadabiHty of asbestos resulted in the dominance of the diaphragm process in North America, whereas soHd salt and mercury avadabiHty led to the dominance of the mercury process in Europe. Japan imported its salt in soHd form and, until the development of the membrane process, also favored the mercury ceU for production. 

Diaphragm process Fig. 28. Flow diagrams of the Mercury and Diaphragm chlor—alkah processes. 

Alkaline Chloride Electrolysis by the Diaphragm Process System Hooker, Uhde GmbH, Dortmund, Germany, 1985. 

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... 

The membrane cell produces a very pure caustic soda solution and consumes less energy unlike the mercury and diaphragm processes.24 Also, it poses less pollution risk to the environment unlike... 

Size The molecular enlargement achieved by dendrylation can be variously utilized. For example, the separation of the dendrylated component from smaller molecular species by means of ultrafiltration makes use of a size difference This procedure is particularly advantageous with respect to catalytically active focal functionalities, because it simplifies the recovery of the catalyst and allows even continuous diaphragm processes.14,10 14 17-221... 

Chlorine-alkali electrolysis is the largest application of such materials as these are the only materials that can be used successfully in this process. As this process provides alkali of better quality than the conventional diaphragm process, and is much more attractive environmentally than the mercury process, its part in industrial world manufacturing of alkali is expanding rapidly. 

AGC has been recently focusing on the development of a new electrolyser and a new membrane for high current density operation, a facility much requested by many users. In July 1998, AGC completed the conversion of its last diaphragm process plant to the then newest Bipolar Electrolyser, the AZEC B-l (hereinafter, B-l) with Flemion F-893 (hereinafter, F-893) membrane and also the then-newest membrane Flemion Fx-8964 (hereinafter, Fx-8964). This conversion was the result of AGC s development efforts. AGC is now on the way to the next stage of its ion-exchange membrane technology, where 6 kA /m-2 operation will be the norm and 8 kA m-2 operation will be made a feasibility. 

The so-called hell process gives a better yield than the diaphragm process. The anode is contained in a bell-shaped vessel open at the bottom, the cathode being outside. The sodium-hydroxide solution formed at the cathode floats on the surface of the sodium-chloride solution, and is thus kept from contact with the chlorine evolved at the anode. The gas is removed at the top of the vessel.1... 

This hydrogen is, however, mostly used in house. Hydrogen is also produced in other petrochemical and chemical processes (synthesis of olefins, ethyne, styrene, acetone). Coke oven gas contains over 50 volume % of hydrogen, from which it can be isolated. Finally hydrogen occurs as a valuable byproduct in chloralkali-electrolysis (directly with the diaphragm process or indirectly with the amalgam process and hydrochloric acid hydrolysis) see Section 1.7.3.3. The electrolysis processes account for less than 5% of the worldwide production of hydrogen. 

Purification for the diaphragm process precipitation with sodium carbonate... 

The diaphragm process generally utilizes brine. Multivalent ions such as Ca , Mg-+, Fe +, and silica, which block the diaphragm, are precipitated out by the addition of sodium hydroxide and sodium carbonate. 

Fig. 1.7-8. Schematic representation of the electrolysis of aqueous salt solutions by the diaphragm process.

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