Energy consumption, chlor-alkali production

The development of the membrane cell cut the energy consumption in chlor-alkali production. A good cell will produce a ton of caustic for around 2400 kWh. Membrane caustic can only be produced up to around 35%. Several cell designers have tried to develop a cell and membrane combination that would allow 50% caustic to be made, but this has proved to be commercially elusive so far. Membrane cells have probably reached the theoretical limit on energy consumption for a commercial plant. In Japan, power consumption has been cut by 30% over the last 20 years as the conversion from mercury cell progressed. 

Currently, major research and developmental emphasis is toward achieving an energy consumption of less than 2000 kWh/M.TNaOH. The membrane technology is so advanced that a reduction of —100 kW h/M.T NaOH will reach the practical minimum value. Thus, the membrane cell process promises to be the main technology for chlor-alkali production in the near future. 

Figure 3.14 Comparison of the energy consumption in the three cell technologies for chlor-alkali production. Full lines represent electrolysis only broken lines represent total energy consumption including evaporation and heating the electrolyte.

Mild steel cathodes are used extensively in chlor-alkali 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%. 

The units of the energy consumption figures calculated using Eq. (5) are in DC kWhr/unit product. However, some chlor-alkali plants require data on the energy consumption expressed in AC kW hr/unit product, in which case the rectifier efficiency, rectifien has to be taken into account ... 

The chlor-alkali industry [16,17] has been one of the great drivers for innovation in electrochemical technology. The reason for this is clear worldwide, chlorine is manufactured on a scale of some 50 million tons per year at approximately 700 sites, and uses some 15 GW of electrical energy (1-2% of world production). Only a marginal improvement in energy consumption, a more convenient cell operation (less component replacement and/or cell down time), or exit streams closer to the traded forms of the products... 

World capacity for chlorine exceeds 45 x 10 t/a. With an annual energy consumption of about 1.5 X 10 kWh, the chlor-alkali process is one of the largest industrial consumers of electrical energy. The chlorine worldwide production of a country is an indicator of the state of development of its chemical industry. 

The last few pages have very much stressed the importance of energy consumption in the chlor-alkali industry. This is appropriate but it is perhaps important to remind ourselves that other factors, particularly purity of the products, environmental factors and ease of management of the cell house, are probably equally important. 

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