Chlorine Membrane cells

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. 

The electrolysis of potassium chloride [7447-40-7] KCl, to produce chlorine and potassium hydroxide in membrane cells requires similar but unique membranes. Commercial membranes currendy employed in high performance membrane electroly2ers include Du Pont s Nafion 900 series and Asahi Glass s Plemion 700 series. 

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. 

Because of limited commercial experience with anode coatings in membrane cells, commercial lifetimes have yet to be defined. Expected lifetime is 5—8 years. In some cases as of this writing (ca 1995), 10-years performance has already been achieved. Actual lifetime is dictated by the membrane replacement schedule, cell design, the level of oxygen in the chlorine gas, and by the current density at which the anode is operated. 

Mild steel cathodes are used extensively in chlor-alkah and chlorate cells. Newer activated cathode materials have been developed that decrease cell voltages about 0.2 V below that for cells having mild steel cathodes. Some activated cathodes have operated in production membrane cells for three years with only minor increases in voltage (17). Activated cathodes can decrease the energy consumption for chlorine—caustic production by 5 to 6.5%. 

Membrane cells generally produce high quaUty chlorine. Higher than normal H2 concentrations in CI2 indicate that holes exist in the membrane. 

There have been a number of cell designs tested for this reaction. Undivided cells using sodium bromide electrolyte have been tried (see, for example. Ref. 29). These have had electrode shapes for in-ceU propylene absorption into the electrolyte. The chief advantages of the electrochemical route to propylene oxide are elimination of the need for chlorine and lime, as well as avoidance of calcium chloride disposal (see Calcium compounds, calcium CHLORIDE Lime and limestone). An indirect electrochemical approach meeting these same objectives employs the chlorine produced at the anode of a membrane cell for preparing the propylene chlorohydrin external to the electrolysis system. The caustic made at the cathode is used to convert the chlorohydrin to propylene oxide, reforming a NaCl solution which is recycled. Attractive economics are claimed for this combined chlor-alkali electrolysis and propylene oxide manufacture (135). 

In the membrane process, the chlorine (at the anode) and the hydrogen (at the cathode) are kept apart by a selective polymer membrane that allows the sodium ions to pass into the cathodic compartment and react with the hydroxyl ions to form caustic soda. The depleted brine is dechlorinated and recycled to the input stage. As noted already, the membrane cell process is the preferred process for new plants. Diaphragm processes may be acceptable, in some circumstances, but only if nonasbestos diaphragms are used. The energy consumption in a membrane cell process is of the order of 2,200 to 2,500 kilowatt-hours per... 

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. 

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). 

The wastewater generated in the membrane cell and other process wastewaters in the cell are generally treated by neutralization.28 Other pollutants similar to those in mercury and diaphragm cells are treated in the same process stated above. Ion exchange and xanthate precipitation methods can be applied in this process to remove the metal pollutants, while incineration can be applied to eliminate some of the hydrocarbons. The use of modified diaphragms that resist corrosion and degradation will help in reducing the amount of lead, asbestos, and chlorinated hydrocarbon in the wastewater stream from the chlor-alkali industry.28... 

Fig. 1.8 Chlorine values from membrane cells as at the second quarter of 1999 based on exported caustic soda (to Australia).

For over a hundred years the chlor-alkali industry has used the mercury cell as one of the three main technologies for the production of chlorine and caustic soda. For historical reasons, this process came to dominate the European industry - while in the United States the asbestos diaphragm cell took the premier position. Over the last two decades developments in membrane cells have brought these to the forefront, and membrane cells of one kind or another now represent the technology of choice worldwide. 

Next, ways to augment liquefaction and bring chlorine recovery to more than 99% are considered. To eliminate some of the many variables, only the off-gas from a two-stage liquefier, as in Fig. 7.2, condensing 97.7% of the chlorine from a membrane-cell installation is utilised. The gas is available at 800 kPa. 

Orica is the largest producer of chlorine in Australia and currently operates three chlor-alkali plants on the east coast. Two of these plants (in Melbourne and Sydney) are mercury cell plants dating back over 50 years while the third plant is a small, modern membrane cell plant in the central Queensland town of Gladstone. The mercury cell plants have both reached the end of their useful economic lives. 

Gladstone 9000 Liquid chlorine, hypo, HCI ICI FM21 SP membrane cells... 

Nufarm (Melbourne) 6000 Gaseous chlorine to integrated herbicide manufacture Lurgi membrane cells... 

Elite (Brisbane) 7000 Gaseous chlorine to hypo and HCI manufacture Subsidiary of Ionics Inc. Ionics membrane cells... 

Kiwi Brands (Melbourne) 1500 Gaseous chlorine to hypo for domestic bleach Part of Sara Lee. Ionics membrane cell... 

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