Quality diaphragm 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. 

Three forms of caustic soda are produced to meet customer needs purified diaphragm caustic (50% Rayon grade), 73% caustic, and anhydrous caustic. Regular 50% caustic from the diaphragm cell process is suitable for most applications and accounts for about 85% of the NaOH consumed in the United States. However, it cannot be used in operations such as the manufacture of rayon, the synthesis of alkyl aryl sulfonates, or the production of anhydrous caustic because of the presence of salt, sodium chlorate, and heavy metals. Membrane and mercury cell caustic, on the other hand, is of superior quality and... 

A major differentiator among the various chlor-alkali cells is the quality of their product caustic solutions. Diaphragm cells produce a liquor containing about 11% NaOH and... 

In a balanced plant, all the chemically treated brine flows through ion exchange, not just that equivalent to the membrane-cell production. This adds a cost to the process, because in a segregated operation only that brine corresponding to the membrane-cell production would be so treated. At the same time, it improves the quality of the brine fed to the diaphragm cells. This in turn improves the current efficiency of those cells, but little information is available in the literature, and it is not possible to quantify the phenomenon here. 

Caustic soda is made by the electrolysis of brine using both mercury cells and diaphragm cells to give caustic soda of di erent qualities. 

In the first few years after the introduction of membrane technology, diaphragm cells in several plants were equipped with membranes (retrofit) to reduce the cost of steam for cell liquor concentration, to give a small reduction in electricity consumption and better quality of caustic. This procedure is economic where steam is very expensive [188], [192],... 

The process energy consumption in a membrane cell is small compared to diaphragm and mercury cell operations and the membrane cell caustic is of the same quality as mercury cell caustic. Hence, the membrane cell technology is recognized as the most economical and preferred method for producing chlorine and caustic (see Section 6 for additional details regarding membrane cells). 

Anode performance depends on the brine quality and the operating parameters such as pH, current density, NaCl concentration, and NaOH concentration (in diaphragm and membrane cells). The contribution of the anode to the cell inefficiency, as mentioned in Section 4.4, is directly related to the losses arising from the oxygen evolution reaction, and indirectly by chlorate formation. Thus, as the %02 increases, the pH at the anode-solution interface decreases, and hence, the amount of chlorate formed will decrease as the bulk pH is lowered. The amount of O2 generated at the anode is a function of the current density, pH, the composition and surface area of the anode coating, and the salt concentration. 

In any electrolytic process, where it is necessary to control the diffusion of the products of decomposition, the most vital point to life and efficiency of the cell is the diaphragm or more recently the membrane. This tough reality, which was met with crude short lived diaphragms at the turn of the century, has forced inventors to eventually produce more stabilized materia and ion specific membranes with parallel progress in cell efficiency and quality of the electrolysis products. This paper will review the various electrochemical processes using separators in 1900 and attempt to describe the improvements achieved during the following decades. 

The diaphragm may be replaced by a cation exchange membrane which is suitable for high concentrated hydrochloric acid like Nafion (DuPont, perfluorosulfonic acid polymer, PFSA, see entry Chlorine and Caustic Technology, Membrane Cell Process ). This membrane has almost no usual porosity and is nearly exclusively permeable for ions including a hydration shell of some water molecules. Thus, product quality is significantly increased, process operation can be simplified, and cell voltage is reduced by about 0.3 V [1, 6]. However, the mechanical durability... 

The electrowetting cell was set between two plastic injection molded lenses. A flat glass plate closed the cylinder on the oil side and a truncated glass sphere mounted on a thin metal diaphragm closed the side of the salt solution. The outer diameter of the cylinder was 4 mm, the inner diameter 3 mm, and the height was 2.2 mm. The flexible metal diaphragm compensated part of the mismatch in thermal expansion between the liquids and the cylinder. The achromatized lens stack had a high optical quality. The camera was able to focus faster than the refresh rate of the CMOS sensor. 

Coating Wear and Coating Lifetime. The coating lifetime is strongly dependent not only on the type of cell—membrane, diaphragm, or mercury—but also on a range of process parameters, including brine quality, current density, and membrane or... 

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. 

In the diaphragm process, a KCl-containing, 8 -10 % potassium hydroxide solution is initially formed, whose salt content can be reduced to ca. 1.0 -1.5 % KCI by evaporation to a 50% liquor. Further purification is complicated, and the quality of liquor from mercury cells cannot be achieved. 

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