Chlorine production from membrane cells

Figure 10.10 Chlorine production from membrane cells.

Using a polymer electrolyte membrane cell in which flowed through the anode chamber. The major intermediate chlorinated products from tetrachloroethene or tet-rachloromethane were trichloroethene or trichloromethane, and these were finally reduced to a mixture of ethane and ethene, or methane (Liu et al. 2001). 

In 1988 diaphragm cells accounted for 76% of all U.S. chlorine production, mercury cells for 17%, membrane cells for 5%, and all other production methods for 2%. Corresponding statistics for Canadian production are diaphragm cells, 81% mercury cells, 15% and membrane cells, 4% (5). for a number of reasons, including concerns over mercury pollution, recent trends are away from mercury cell production toward the more environmentally acceptable membrane cells, which also produce higher quality product and have favorable economics. 

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. 

The term chlor-alkali refers to those products obtained from the commercial electrolysis of aqueous sodium chloride. These are chlorine, sodium hydroxide, and sodium carbonate. The first two are produced simultaneously during the electrolysis while the latter is included because it is also produced in small quantities and shares many of the end uses of sodium hydroxide. Perfluorinated ionomer membranes are permeable to sodium ions but not the chloride ions, and hence they are useful for these electrolytic cells. The arrangement of a typical membrane cell is shown in Figure 10.2. 

Of the chlorine production capacity installed in Germany, which totalled 4.4 million tonnes in 2003, 50% were from the membrane cell process, 27% from the mercury cell process and 23% from the diaphragm cell process. The mercury cell process has been the subject of environmental policy criticism for years because of its use of mercury cathodes and resulting pollutant emissions. Hence, no new mercury plants will be... 

Since the products of the electrolysis of aqueous NaCl will react if they come in contact with each other, an essential feature of any chloralkali cell is separation of the anode reaction (where chloride ion is oxidized to chlorine) from the cathode reaction (in which OH- and H2 are the end products). The principal types of chloralkali cells currently in use are the diaphragm (or membrane) cell and the mercury cell. 

Membrane cell An electrolytic cell used for the production of sodium hydroxide, hydrogen and chlorine from brine in which the anode and cathode are separated by a membrane. 

A small, but important, variation of NaCl electrolysis substitutes KC1 as the feed. Both mercury cells and membrane cells are used for producing chlorine and KOH (caustic potash). The KOH is concentrated for sale as a 45 percent solution or as a solid containing 88 to 92 percent KOH. A big use for KOH is in the manufacture of liquid soaps and detergents. The analogous sodium soaps and detergents are generally solids. Approximately 1.5 percent of chlorine production results from the electrolysis of KC1.36... 

Chlorine from Potassium Hydroxide Manufacture. One of the coproducts during the electrolytic production of potassium hydroxide employing mercury and membrane cells is chlorine. The combined name plate capacity for caustic potash during 1988 totaled 325,000 t/yr and growth of U.S. demand was expected to be steady at 2% through 1990 (68). 

Production of CI2 and NaOH by electrolysis of NaCl is a huge industry with annual production capacity in excess of 50 million tons of NaOH per year. Membrane cells are the state-of-the-art technology, but mercury and diaphragm cells are still used because the capital cost for their replacement is substantial. The mercury cell technology is more than a century old and stiU accounts for nearly half of the world s production capacity. Chlorine evolves from a DSA (dimensionally stable anode) situated above a pool of mercury with NaCl brine in between. Mercury reacts with sodium to form sodium amalgam, which is removed and hydrolyzed in a separate reactor. 

Chlorine is sold as a gas or a liquid, and caustic soda is sold as 50% or 73% solution, or as anhydrous beads or flakes. There is also a significant market for caustic soda in the concentration range of 10-25%. The caustic soda product is designated by the manufacturing technology used, namely mercury, diaphragm, or membrane cells. More than 95% of the chlorine and 99.5% of the caustic in the world are produced by these cell technologies. About 0.5% of caustic soda is made chemically from soda ash, and KOH accounts for 2.5% of the caustic produced in the world. 

Analysis of the chlorine flows shows that the anode current efficiency is different from the cathode current efficiency. The current efficiency commonly used in discussing membrane cells is the caustic current efficiency, determined by the amount of hydroxide ion lost by leakage through the membranes into the anolyte. The hydrogen current efficiency, on the other hand, is nearly 100%. In other words, the electrode process is nearly quantitative, but the membranes allow some of the product of electrolysis to escape. 

For added security, vents from the tails tower and the product storage tank can be scrubbed with a caustic liquor. The acid solution itself will contain a small amount of dissolved chlorine. In most plant applications, as for example in the acidification of depleted brine, this is not a problem. Other uses, such as the regeneration of the brine softening resin in a membrane-cell plant, may require that this chlorine be removed. Adsorption on activated carbon is probably the simplest technique for this small-scale process, which is similar to that described in Section 7.5.9.3B. 

Energy consumption is measured in kilowatt-hours per ton of product, the product being either chlorine or caustic. Most operators and technology suppliers choose caustic as the basis for measurement. This choice reflects the practical difficulties of measuring chlorine production accurately and taking into account system losses that end up principally as hypochlorite or HCl. Another comphcation is the dependence of anolyte current efficiency on the amount of acid or alkali present in the feed brine (Section 7.5.6.1). The caustic current efficiency, for all practical purposes, depends only on the membrane efficiency. It becomes more convenient and usually more accurate to measure the production of caustic. One need only measure the amount of solution produced and analyze its caustic content. Again for convenience and accuracy, and assuming the use of membrane cells, it is best to measure the output of cell liquor. This separates the electrolyzer and evaporator test runs. These measurements make it possible to calculate the anode current efficiency from analytical data and hence, to calculate chlorine production and specific power consumption. 

Chlorine is produced not only by the electrolysis of sodium chloride solutions but also from HCl, KCl, and other metal chlorides, by both chemical and electrochemical methods. The amount of chlorine from alternative processes is about 5.9% of the total world production. In the United States, it was about 4.0% of the total in 2002 [1]. Most of this chlorine was from the electrolysis of KCl in mercury or membrane cells (Table 15.1) and from HCl. Only small amounts are produced by the electrolysis of other metal chlorides. 

As discussed in the Introduction section, three main types of electrolytic cells have been used for the large scale production of chlorine and caustic soda mercury, diaphragm and membrane cells. The main difference in these technologies lies in the manner by which the chlorine gas and the sodium hydroxide are prevented from mixing with each other to ensure generation of pure product. Alternatively, the electrolysis of hydrochloric acid solutions is also used to produce chlorine. Individual electrolysis cells can be electrically wired in parallel (monopolar electrolysers) or in series (bipolar electrolysers). 

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