Energy consumption, chlor-alkali

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. 

Electric energy is the predominant cost in the manufacture of chlorine and is the driver for most of the technical progress in the chlor-alkali industry. The busiest areas of development over the past 20 or 30 years have been related to reductions in energy consumption. Approximately 60% of the papers presented in this book deal with improvements in chlor-alkali cell internals, namely the anolyte/catholyte separator (primarily membranes) and the electrodes. 

The substitution of conventional hydrogen-evolving cathodes with oxygen-consuming gas-diffusion electrodes (GDE), often referred to as oxygen-depolarised cathodes (ODC), also allows a substantial reduction in the energy consumption of the chlor-alkali process. 

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 chemical industry is a very large consumer of energy and much is needed to be done to reduce consumption (see Section 3.6 for additional energy reduction statistics). The largest consumers of energy in the chemical industry are represented by six chemical chains, namely agricultural fertilizers, ethylene, benzene/toluene/ xylene, chlor-alkali, propylene, and butadiene. Together, they consume 1646 trillion... 

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). 

The terminology employed in the chlor-alkali industry for comparing the performance characteristics of various cells is the energy consumption expressed in kilowatt hours per metric ton (kW h/M.T) of Cl2 or NaOH. This is related to the cell voltage ( ) and the current efficiency (17) as... 

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. 

Electrodialysis has the ability to concentrate salts to high levels with much less energy consumption than evaporation would require. This capability has been utilized in Japan to make edible salt by recovering NaCl from seawater and concentrating it to 20% before evaporation. The plants there are huge some have greater than 100 000 square meters of membrane. Salt recovered by electrodialysis in Kuwait is the raw material for a chlor-alkali plant there. Electrodialysis has also been used to concentrate salts in reverse osmosis brines [32]. 

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.

For small-scale electrolysis units, energy consumption will be much less important than in a chlor-alkali process ease of operation with a minimum of maintenance and replacement of components and the initial cost of the total unit will more often determine the choice of the cell. As a result, for example, while the cathode will generally be steel, a wide range of anode materials including graphite, lead dioxide and platinized titanium have been used as well as dimension-ally stable anodes. Hence the quoted energy consumptions of hypochlorite cells lie in the range 4.5—7.0 kWh kg , considerably above those for a chlor-alkali cell. 

Many chlor-alkali producers and technology licensing companies have refined and optimized membrane cell designs to realize low energy consumption and long life. The new cell designs incorporated the zero-gap concept, which eliminated the electrolyte gap between the electrodes and the membrane. 

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 dominant role of energy consumption in chlor-alkali economics makes it the most important aspect of cell performance. If one type of cell can offer substantially lower energy consumption than others, it will have a major advantage in any techno-economic comparison. This fact is the single most important reason for the ongoing conversion of the industry to membrane cells. 

FIGURE 10.2.7. Energy distribution in a membrane chlor-alkali cell (MGC-26) operating at 5kAm . (Energy consumption 2,607 kW hr ton" of CI2 Cell voltage 3.35 V Current efficiency 97%.)... 

Chlor-Alkali Industry. ) The replacement of the mercury process in the chlor-alkali industry by a nonmercury process is considered to be urgent, for the elimination of mercury pollution and for energy conservation. The membrane process has been proven as an effective alternative to the mercury process because of its important advantages, such as 1) Freedom from mercury pollution. 2) Lower electric power consumption. 3) Small steam consumption. 

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... 

Three processes of chlor-alkali electrolysis are currently used, namely, the mercury process, diaphragm process, and membrane process. With regard to energy consumption and environmental concerns, the membrane process is the most efficient. 

The almost universal replacement of carbon by metal anodes has led, by itself, to a reduction of the energy consumption by (0-15% (the cell voltage is changed by 0.45 V in —2.5 to — 5V). But, in addition, the chlor-alkali industry has benefited because anode replacement is no longer an important factor in the way the cell house is run and because the titanium to be coated may be fabricated in many forms This has helped to revolutionize cell design. 

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