Product quality membrane cell caustic

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... 

The quality of caustic from membrane cells depend on the relative rates of transport of various species across the membrane. The user should be aware that the rates of transport of anions are less sensitive to the current density than is the rate of transport of Na+ or K" ". When production is curtailed or stopped, the concentrations of anions such as Cl and CIOJ increase. This becomes an important effect when there are tight product specifications. This situation is more likely in the case of KOH production. Not all membranes are equal when it comes to rejecting Cl and CIOJ. 

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... 

An ideal cation exchange membrane allows sodium cation to pass, but no hydroxy and chlor anions. The cell liquor contains only a small amount of sodium chloride so that it can be used as product caustic soda without any further treatment. In some cases the cell liquor is further concentrated according to the consumer s requirements. The quality of caustic soda produced from the membrane process is similar to that from the mercury process. 

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. 

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