Alkali production

Table 12. Nafion 90209 Performance in Chlor-Alkali Production...

Removal of brine contaminants accounts for a significant portion of overall chlor—alkali production cost, especially for the membrane process. Moreover, part or all of the depleted brine from mercury and membrane cells must first be dechlorinated to recover the dissolved chlorine and to prevent corrosion during further processing. In a typical membrane plant, HCl is added to Hberate chlorine, then a vacuum is appHed to recover it. A reducing agent such as sodium sulfite is added to remove the final traces because chlorine would adversely react with the ion-exchange resins used later in the process. Dechlorinated brine is then resaturated with soHd salt for further use. 

Electrolytic Preparation of Chlorine and Caustic Soda. The preparation of chlorine [7782-50-5] and caustic soda [1310-73-2] is an important use for mercury metal. Since 1989, chlor—alkali production has been responsible for the largest use for mercury in the United States. In this process, mercury is used as a flowing cathode in an electrolytic cell into which a sodium chloride [7647-14-5] solution (brine) is introduced. This brine is then subjected to an electric current, and the aqueous solution of sodium chloride flows between the anode and the mercury, releasing chlorine gas at the anode. The sodium ions form an amalgam with the mercury cathode. Water is added to the amalgam to remove the sodium [7440-23-5] forming hydrogen [1333-74-0] and sodium hydroxide and relatively pure mercury metal, which is recycled into the cell (see Alkali and chlorine products). 

Fig. 1. Flow diagram for chlor-alkali production by a membrane cell process.

H. Y. Castner (US/UK) and C. Kellner (Vienna) independently developed commercial mercury-cathode cell for chlor-alkali production... 

These are rapidly replacing emulsion and alkali products for use in industrial washing machines. They are generally used at pH 7 5-9, considerably lower than corresponding alkali products. 

Slow, except in presence of alkalis products formed are complex polymeric quaternary ammonium salts dimerizes fairly rapidly in water. 

A bipolar membrane is a sandwich of a cation and an anion exchange membrane which splits H20 to H+ and OH- under a potential of about 0.9 V. The reactor consists of a stack, of bipolar membranes, cation exchange membranes and anion exchange membranes arranged between a single anode and a single cathode with parallel hydraulytic circuits for the salt, acid product and alkali product, Fig. 32. 

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. 

There will be no increase in mercury chlor-alkali production capacity. This is an unequivocal reiteration of a commitment made in 1995. It represents a de facto commitment to phase-out as mercury cellrooms reach the end of their working life. 

Lindley, A.A. (1997) An Economic and Environmental Analysis of the Chlor-Alkali Production Process Mercury Cells and Alternative Technologies. Prepared for the European Commission (DG III C-4, Chemicals, Plastics, Rubber). See also OSPAR Document WOCAI 99/5/8 (Madrid, 1999). 

Precise electrical data acquisition within the industrial electrolytic plant typical of chlorate and chlor-alkali production facilities represents a significant challenge as the precision of the data obtained is usually degraded in an environment characterised by electrical noise induced by rectifiers and by strong electromagnetic fields. In some cases, rectifier-induced noise such as harmonics and switching peaks in the order of... 

In chlor-alkali production, EMOS should be able to determine problems with both anode coatings and membranes. The literature is replete with examples of the effect of different impurities on membranes [2] and of the analysis of different problems using polarisation curves to determine their cause [3, 4]. These analysis techniques have been incorporated into the expert system in the form of approximations of the polarisation curves. Use is made of the familiar k-factor (see Equation 8.2) or the more accurate logarithmic form of this factor (Equation 8.3) ... 

EMOS has to date been mostly used in chlorate manufacture, but R2 in Montreal, Canada has recently installed its system on an FM-21 1500-type cell chlor-alkali production facility. This is presently a pilot installation, with only six cells currently being monitored. This installation has led to the monitoring of cell currents rather than cell voltages owing to the monopolar design of these electrolysers. It is too soon to make detailed conclusions about this installation as it has only been fully operational since January 2000. 

Sajima, Y., Nakao, M., Shimohira, T. Miyake, H. (1992) Advances in flemion membranes for chlor-alkali production. In Modern Chlor-alkali Technology (ed. T.C. Wellington), Vol. 5, pp. 159-175. Hays Chemical Distribution Ltd., Sandbach, UK. 

Advances during the past 20 years in membrane, electrolyser, electrode, and brine purification technologies have substantially raised the performance levels and efficiency of chlor-alkali production by ion-exchange membrane electrolysis, bringing commercial operations with a unit power consumption of 2000-2050 kWh per ton of NaOH or lower at 4 kA m-2 current density with a membrane life of four years or longer. 

Chlor-alkali production Electrochemical synthesis Water-organic liquid separation Organic liquid mixture separaion Fermentation products recovery and purification Cell harvesting, virus and antibody concentration Protein desalting, concentration and fractionation Blood processing, including artificial kidney Isolation, concentration, and identification of solutes and particulates... 

Chlor-alkali production — With a 63% production volume of the total world chlorine capacity of about 43.4 million tons (in 1998), the chlor-alkali (or chlorine-caustic) industry is one of the largest electrochemical technologies in the world. Chlorine, Cl2, with its main co-product sodium hydroxide, NaOH, has been produced on industrial scale for more than a century by -> electrolysis of brine, a saturated solution of sodium chloride (-> alkali chloride electrolysis). Today, they are among the top ten chemicals produced in the world. Sodium chlorate (NaC103) and sodium hypochlorite (NaOCl, bleach ) are important side products of the... 

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