Current density, chlor-alkali process

As membranes employed in the chlor-alkali industry are generally of the non-crosslinked type, their properties are influenced significantly by the conditions in which they are utilized. The extreme temperature, concentration and current density to which they are subjected in the chlor-alkali process are not encountered in other applications such as electrodialysis. Clarification of the membrane properties is therefore both practically necessary and theoretically interesting, and application of ion cluster theory has been attempted (42, 43). 

Energy Consumption. Electric power consumption of electrolysis is the major part of the energy consumption in a chlor-alkali process. The power consumption of the membrane process has recently been greatly reduced by various improvements. The latest performance of Asahi Chemical s membrane process realized at a commercial plant and also in an industrial scale cell is shown in relation to current density in Figure 13 (82). 

This means that the electrical energy required for electrolysis is reduced by about 56%. It has been reported that the voltage drop between electrodes attains 1.93-1.95 V at a current density of 30 A dm-2 in a semi-commercial electrolyzer.126 Further energy saving is possible in the chlor-alkali process. 

High chlorine production yield and suitable lifetime are achieved with mixed metal oxide electrodes (MMO), initially developed for the chlor-alkali process [4]. They consist of a base metal, a titanium plate, or grid for saltwater chlorination coated with a combination of at least two metal oxides including titanium, ruthenium, iridium, or tantalum [5]. Elaborated proprietary production processes are used to achieve reliable electrodes. The chlorine faradaic yield under typical pool working conditions (3 g/L NaCl, current density 50 mA/cm ) is around 60 % [6]. Good quality electrodes achieve several years of service time (several thousands of hours of continuous operation) with a 3 h reversal time and at 60 % of the nominal maximum power. Lifetime is reduced by high current densities and by frequent polarity reversal. Operation at... 

The development of high current density electrolysis technology is a continuing effort. Asahi Chemical s focus is currently on the confirmation of stable long-term performance and reliability, in preparation for the supply of this process equipment and technology to chlor-alkali producers. 

The effect of the cost of the ion exchange membrane on the total cost of electrodialysis or electrolysis is large because the membrane is relatively expensive. The lifetime of the membrane depends on the purpose and conditions of electrodialysis or electrolysis. A membrane for the electrodialytic concentration of seawater to produce sodium chloride has a lifetime of over 10 years, and that in the chlor-alkali membrane process, which is operated at ten times or more higher current density than that of seawater concentration, is over 5 years. However, in applications for food industries, the lifetime of the membrane is relatively short due to periodical sanitary cleaning of the electrodialyzer by acid or alkali solution, and sometimes oxidizing agents. 

Many industrial electrolytic processes (e,g, electroplating (section 8.1), chlor-alkali (Chapter 3), water electrolysis (section 5,2) and fluorine evolution (section 5J )) operate primarily under charge-transfer control. Any increase in the average current density is expected to cause a less uniform current distribution. 

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