Chloralkali electrolysis process

The chloralkali electrolysis process is by far the most important source of electrolytically generated hydrogen because hydrogen from alkaline or membrane water electrolysis usually cannot compete with hydrogen from steam reforming followed by shift reaction and PSA purification and therefore is not performed on a large scale (62, 40). 

A more obvious method of avoiding the inevitable production of CaCl2 is the combination of the chloralkali electrolysis with the chlorohydrin process. This is also being pursued intensively 54-55>. A modification proposal by Lummus is shown in the block diagram below 56) ... 

This process has been used to make functionalized perlluorovinyl ethers [e.g., F2C = CF0CF2CF(CF3)0CF2CF2S02F] which are copolymerized to form fluorinated membrane materials (Nafion, Flemion), which arc important in chloralkali electrolysis. ... 

The same situation is true for the most popular electrochemcial process, namely, chloralkali electrolysis. The heart of the process, the cell room, is an even smaller component of the whole process [15]. 

Hydrogen is also formed in large quantities as a byproduct in petrochemical processes, refineries, coking plants (coke oven gas) and in chemical and electrochemical processes e.g. chloralkali-electrolysis. Other processes such as the photochemical production of hydrogen or thermal dissociation of water are only used in special applications and are currently industrially unimportant. 

This hydrogen is, however, mostly used in house. Hydrogen is also produced in other petrochemical and chemical processes (synthesis of olefins, ethyne, styrene, acetone). Coke oven gas contains over 50 volume % of hydrogen, from which it can be isolated. Finally hydrogen occurs as a valuable byproduct in chloralkali-electrolysis (directly with the diaphragm process or indirectly with the amalgam process and hydrochloric acid hydrolysis) see Section 1.7.3.3. The electrolysis processes account for less than 5% of the worldwide production of hydrogen. 

A couple of hydrogen accidents were associated with the chlorine production by chloralkali electrolysis, which are included in Table 8-4 under Electrolysis malfunction . Fire or explosion occurred when for some reason, for example a reversal of cell polarity, the hydrogen entered the chlorine processing equipment downstream of the electrolysis cells [104]. 

In addition to electrochemical energy conversion in fuel cells, the reaction has applications in energy storage in metal-air batteries, in several industrial processes as the chloralkali electrolysis, and it causes corrosion of metals and alloys in the presence of air. That is why the efforts have been focused on elucidating the mechanism of this reaction and developing proper catalysts. 

Production of caustic soda solution. In 1998, the worldwide production capacity was about 54 million tons per year. Ca. 96-98% of this amount is produced by chloralkali electrolysis [313). The three processes are described in detail in chapter 5 (Mercury Cell Process), chapter 6 (Diaphragm Process) and chapter 7 (Membrane process), a comparison of the relative qualities is given in chapter 9. 

A mixture of hydrogen and chlorine gas, eventually in combination with air, can be very explosive if one of the components exceeds certain limits. In chlorine production plants, based on the electrolysis of sodium chloride solutions, there is always a production of hydrogen. It is, therefore, essential to be aware of the actual hydrogen content of chlorine gas process streams at any time. There are several places in the chlorine production process where the hydrogen content in the chlorine gas can accumulate above the explosion limits. Within the chloralkali industry, mainly two types of processes are used for the production of chlorine—the mercury- and the membrane-based electrolysis of sodium chloride solutions (brine). 

Figure 7-29 Salt electrolysis to produce CI2 gas and NaOH solution in a chloralkali process. This is the process by which most alkali and chlorine are produced The reactor operates continuously with brine solution flowing into the cell and NaOH and Q2 gas flowing out.

Sodium chloride is plentiful as rock salt, but the solid does not conduct electricity, because the ions are locked into place. Sodium chloride must be molten for electrolysis to occur. The electrodes in the cell are made of inert materials like carbon, and the cell is designed to keep the sodium and chlorine produced by the electrolysis out of contact with each other and away from air. In a modification of the Downs process, the electrolyte is an aqueous solution of sodium chloride. The products of this chloralkali process are chlorine and aqueous sodium hydroxide. 

Several industrial processes use mercury in large amounts, and the resulting potential for spills and loss to the environment is great. One of the largest is the chloralkali industry, in which mercury is used as an electrode for the electrolysis of brine to form chlorine gas and sodium hydroxide ... 

To the extent that it is discernible in the products and processes, appropriate aspects have been incorporated in the revision, for example see membrane technology in the chloralkali and hydrochloric acid electrolysis. 

Any of the products of brine electrolysis, chlorine, sodium hydroxide, and hydrogen can be hazardous if released. When releases do occur, it is usually from process upsets or breakdowns, which may be minimized by the construction of fail-safe plants, proper maintenance, and by safe transport and storage practices. Probably of greater long-term concern is the mercury loss experienced through the process streams of a mercury cell chloralkali operation. These losses can also carry over to the products of the diaphragm cell, even though this does not use mercury, if a common brine well or common salt dissolver is used for both sets of cells. 

Chlorine and sodium hydroxide production by the electrolysis of brine solutions necessarily locks the ratio of the two products to the theoretical ratio of the process [45, 46]. When the market for sodium hydroxide exceeds the market for chlorine, the causticization of sodium carbonate to sodium hydroxide (Section 7.3) may be used by some suppliers and consumers to supplement the available sodium hydroxide without producing large amounts of excess chlorine. Another expedient for large-scale chloralkali producers faced with this situation is to stimulate the chlorinated solvent or hydrochloric acid markets in an attempt to increase the consumption of chlorine to restore the balance. These measures are not usually rapid enough to respond over the short term unless the solvent plant is also operated by the chloralkali producer. 

Mercury compounds continue to have numerous commercial uses. Besides its use as a preservative, mercury is used in the manufacture of many technical and medical instruments including blood pressure measurement devices, manometers, thermometers, and barometers. Mercury is also used in production of certain types of fluorescent lamps and in the chloralkali industry, where chlorine and caustic soda are produced using brine electrolysis in mercury cells. Metallic mercury is used in the production of precious metals such as gold and silver. As part of the production process, metallic mercury can be used to concentrate gold from... 

The hydroxides are formed by electrolysis of aqueous brines as part of the chloralkali process. The metal formed at the cathode immediately reacts with water to produce the hydroxide and hydrogen gas. 

The chloralkali water electrolysis is the only large-scale technological method to be commercialized, where the H2 is actually a byproduct of the chlorine production and mostly used as the thermal energy source and substitute of natural gas. A solution of salt in water is electrolytically decomposed into hydrogen and soda lye (cathode) and chlorine (anode) as shown in Fig. 5-9 for the mercury process ... 

The chloralkali process, which involves the electrolysis of brine, is widely used for the production of sodium hydroxide and chlorine gas. During electrolysis it is necessary to keep the sodium hydroxide separate from the chlorine, to prevent the formation of sodium hypochlorite, NaOCl, and this determines cell design. In older processes, the cathode used was flowing mercury. At this electrode, sodium is formed, and this dissolves in the mercury to form a sodium amalgam. The sodium amalgam is removed continually from the cell and reacted with water to produce hydrogen gas and... 

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