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to convert anhydrous hydrochloric acid to chlorine, developed jointly by Dupont and Kvarner Chemetics, was also recently unveiled. Similar technology is also being marketed in Europe by DeNora, an Italian firm. Chlorine and sodium hydroxide are co-products that are produced in roughly equivalent amounts through electrolysis of common salt in a brine solution (about 1.1 tons of sodium hydroxide for every ton of chlorine produced). Hydrogen is also produced in equal molar amounts with chlorine and caustic. Chemical demand for hydrogen on the Gulf Coast is significant, and it is often transported by pipeline long distances to meet the needs of oil refineries. There is also an opportunity to use fuel cell technology to closely couple the hydrogen produced with electrical power units that can feed DC power to chlorine cells. Some demonstration units using this technology are in operation outside the United States (DOW 1999). During electrolysis, two electrodes are immersed in a brine solution. When a source of direct current is attached to the electrodes, sodium ions begin to move toward the negative electrode (cathode) and chlorine ions toward the positive electrode (anode) (Sittig 1977, IND CHEM 1990, EPA 1995a, Orica 1999). If the primary products from salt electrolysis remain in contact after formation, they can react with each other to form oxygenated compounds of chlorine. Three electrolytic processes are available and use different methods to keep the chlorine produced at the anode separated from the caustic soda and hydrogen produced at the cathode. In historical order, these cells include diaphragm cells, mercury cathode or "amalgam" cells, and membrane cells. Table 6-1 provides a comparison of the various aspects of the three electrolysis cells, including electrical energy consumption. Diaphragm cells use a simple and economical brine system and require less electrical energy than mercury cells. A primary disadvantage of the diaphragm cell is the low concentration of the caustic soda solution, which requires several concentrative operations to achieve the purity needed for industrial use. The caustic contains 2 to 3 percent NaCl, requiring further purification for some industrial uses. The diaphragm cell is also known to be a source of pollution from asbestos fibers, the primary material of the diaphragm. Because of these disadvantages, mercury cathode cells began to compete with diaphragm cells early in the twentieth century. Mercury cells produce a much purer and extremely concentrated caustic product that can be used without further treatment in most cases. However, mercury has extremely serious ecological impacts and when dispersed from chemical process effluents, can enter the food chain and lead to mercury poisoning in humans. Membrane cells are the most environmentally benign of all the cell technologies, and have electricity requirements similar to those of diaphragm cells. The caustic solution produced is also essentially salt-free and more concentrated than that produced from diaphragm cells. Chemical companies have been slow to adopt membrane technology because of operational problems encountered in early installations, and because existing facilities are fully depreciated but still functional (IND CHEM 1990, Ayres 1997). Diaphragm and mercury cells include an anode and cathode in contact with a brine solution. The membrane cell cathode is only in contact with 20 to 32 percent NaOH, with very low chloride content. Features that distinguish the cells from each other include the method used to keep the three major products separated and unable to mix (chlorine gas, sodium hydroxide, and hydrogen), and the resulting product concentration (see Figure 6-2). Hydrogen must be separated from the chlorine gas as mixtures of these two gases can be explosive. 178PDF Image | The Chlor-Alkali Industry
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