Mercury energy consumption

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. 

Table 2.2 A comparison of energy consumption mercury versus membrane. ... 

Fig. 17.2 Unit energy consumption of membrane, diaphragm, and mercury processes.

The mercury cell thus gives very pure NaOH and, in terms of energy consumption, is also more economical than the diaphragm cell. However, the inevitable leakage of some mercury into local rivers or lakes can have (and has had) serious consequences because of the bacterial conversion of Hg to methylmercury ion, CHaHg4-, which then becomes concentrated in successive steps of the food chain ... 

Because of a higher theoretical decomposition voltage mercury cathode electrolyzers have also a higher theoretical energy consumption. If for the preparation of 100 kg of sodium hydroxide 67 kA-hr. are required according to Faraday s law, then the theoretical energy consumption equals... 

The difference between theoretical and actual energy consumption is lower with mercury cathode electrolyzers than with diaphragm or bell-jar electrolyzers as there are less secondary voltage losses (there is no diaphragm and the distance between electrodes is three to four times shorter). 

Both properties contributed to reducing the energy consumption in operating the cells. With mercury cells, there is the added advantage that the dimensional stability of the new anode eliminated the continual adjustment of electrode spacing that is necessary with graphite electrodes. 

The electrical energy consumption is ca. 20% less than that in the mercury process. 

The process energy consumption in a membrane cell is small compared to diaphragm and mercury cell operations and the membrane cell caustic is of the same quality as mercury cell caustic. Hence, the membrane cell technology is recognized as the most economical and preferred method for producing chlorine and caustic (see Section 6 for additional details regarding membrane cells). 

The reconstituted metaUic mercury is returned to the cell. The major advantage of the mercury cell is that it produces very pure NaOH at 50% concentration. But electrical energy consumption is high, and the threat of mercury pollution is a major concern. Traces of mercury appear in the NaOH and in air emissions from the cells. Because of their design it is impractical to modify mercury cells into membrane cells. 

In seeking the conditions for operating the cell, both the absolute energy consumption and the slope of the lines, essentially the cell resistance, are important since there will be a trade-off between energy consumption and the rate of production (i.e. current density) when the total cost is taken into account the slopes indicate the additional energy which must be consumed to permit a faster production rate. It is the low resistance of the mercury cell which allows the use of high current densities without an unreasonable voltage penalty. 

Several modifications have been made in recent years to achieve low energy consumption and to minimize mercury losses to the environment. The inlet and outlet end boxes were redesigned (Figs. 5.9 and 5.10) to be air-tight, and the use of wash water was eliminated or reduced. The iidet end box has been equipped with a cell bottom cleaning... 

Grube s horizontal mercury cell more closely resembled the standard salt electrolyzers and had an asbestos diaphragm between the electrodes in order to isolate the amalgam cathode from sulfuric acid. This cell produced pure caustic soda free of chloride ions from its amalgam decomposer. However, voltage and energy consumption still was high ( 4,000 kWhr t NaOH). 

Electrolysis has been used for the production of chlorine and sodium hydroxide for over a hundred years. Even so, the technology has developed very rapidly during the last decade and the trends Indicate well the application of the principles of electrochemical engineering. Of course, the driving force for these developments Is the massive scale of the process, approximately 3.3 x 10 ton Cl2/year and the consequent energy consumption, some 10 kWh/year. In addition, concern over mercury pollution has led to a need to develop cells which do not use the amalgam system. 

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. 

Dimming controls are available for fluorescent as well as mercury vapor lamps. They reduce light output to what is required for the tasks involved. The percentage of light reduction is close to the percentage of energy consumption reduction. In no case should lighting levels be reduced to less than what is required. Worker productivity will suffer safety and security hazards may be created. 

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