In Chlor-Alkali Operations

MetaUic ions are precipitated as their hydroxides from aqueous caustic solutions. The reactions of importance in chlor—alkali operations are removal of magnesium as Mg(OH)2 during primary purification and of other impurities for pollution control. Organic acids react with NaOH to form soluble salts. Saponification of esters to form the organic acid salt and an alcohol and internal coupling reactions involve NaOH, as exemplified by reaction with triglycerides to form soap and glycerol,... 

Chlor-alkali producers have been actively pursuing the development of oxygen electrodes for some time and it is unlikely that the full 1.23 V saving can be achieved at current densities typically used in chlor-alkali operations. Many problems remain to be solved before the oxygen cathode is ready for commercial... 

Membranes used in chlor-alkali operations are typically hydrolyzed copolymers of tetrafluoroethylene and perfluoro vinyl ether monomer containing an ion-exchange group, represented by the following general formula93 ... 

Volume V addresses corrosion, environmental issues, and future developments. Chapter 14 covers the principles of corrosion and how to minimize corrosion in chlor-alkali operations, exemplified by case studies. Alternate processes for producing chlorine and caustic are outlined in Chapter 15. 

Physicochemical data related to CI2, NaOH, NaCl, and other materials encountered in chlor-alkali operations are presented in the Appendix in Volume V. 

Engineering handbooks and vendors literature contain descriptions and design details for most types of conveyor. This section gives equations for the capacity of certain types and for the amount of power that they consume. These are empirical and derived from handbook and catalog data. They should not be used in place of specific information given by equipment suppliers. They are supplied here once again to give the reader a practical feel for the systems used in chlor-alkali operation. 

Direct removal of iodide by ion exchange is possible. However, standard resins have capacities too low to be of practical value in chlor-alkali operations. Therefore, the approach has been first to oxidize iodide to iodine as by Eq. (79), and then to allow it to form a complex ion with chloride ... 

CORROSION IN CHLOR-ALKALI OPERATIONS 14.5.1. General Aspects... 

Note that the metal oxidation reaction releases electrons by the reaction MM+ -be. Iron is susceptible to oxidation by reaction (35) or (36), while Ni and stainless steels will corrode by reactions (37) to (40). Another reaction of interest in chlor-alkali operations is oxidation of Fe or Ni in alkaline media ... 

The extent of oxidation of transition metals in NaOH solution is a function of NaOH concentration and temperature, as will be shown later. Most of the corrosion problems observed in chlor-alkali operations are traceable to the above reactions. By properly counteracting the reactants, many commonly reported corrosion problems can be avoided. 

D. L. Peet and J. H. Austin, Nafion Pefluorinated Membranes, Operation in Chlor—Alkali Plants, Chlorine Institute Plant Managers Seminar, Tampa, The Chlorine Institute, Feb. 1986. 

Mercury accounting is a non-trivial exercise as anyone who has ever been involved in its details will be well aware. It is intrinsic to the nature of chlor-alkali operations and... 

Peet, D.L. Austin, J.H. (1986) Nafion perfluorinated membranes operation in chlor-alkali plants. 

Mild steel cathodes are used extensively in chlor-alkali 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%. 

This is evidenced by the amount of literature on ionomers and by the appearance of two monographs devoted to the subject (J, ). Most of the research effort on the ionomers has focused on only a small number of materials, notably ethylenes (3-9 ), styrenes (10,11), rubbers (12-16) and recently aromatic (17) and fluorocarbon-based ionomers (18). The last material is known for its high water permeability and cation permselectivity. Because of its unique properties, it has been employed as an ion-exchange membrane in chlor-alkali cell operations in electrochemical industries. Perfluorinated ion-exchange membranes are the subject of the present chapter. 

Electroosmotic effects also influence current efficiency, not only in terms of coupling effects on the fluxes of various species but also in terms of their impact on steady-state membrane water levels and polymer structure. The effects of electroosmosis on membrane permselectivity have recently been treated through the classical Nernst-Planck flux equations, and water transport numbers in chlor-alkali cell environments have been reported by several workers.Even with classical approaches, the relationship between electroosmosis and permselectivity is seen to be quite complicated. Treatments which include molecular transport of water can also affect membrane permselectivity, as seen in Fig. 17. The different results for the two types of experiments here can be attributed largely to the effects of osmosis. A slight improvement in current efficiency results when osmosis occurs from anolyte to catholyte. Another frequently observed consequence of water transport is higher membrane conductance, " " which is an important factor in the overall energy efficiency of an operating cell. 

J.T. Keating, Effect of brine purity on performance of commercial electrolyzer, Soda Enso (Soda Chlorine), 1994, 45, 366-375 J.H. Austin, Operation in chlor-alkali plants, 3rd London International Chlorine Symposium, June, 5-7 1985. 

Perfluorinated membranes used in chlor-alkali cells normally have a thin layer of carboxylate on the cathode-facing surface of a sulfonate membrane. Nafion 901 was introduced as such a membrane [38]. It achieved 33% NaOH concentration with 95% current efficiency in cells operating at 3 kA/m and 3.3 to 3.9 V. The carboxylate layer can be prepared by lamination, but the layer can be... 

Equation (128) was proposed as one of the causes of the (aesence of O2 in electrolytic sodium chlorate operations using graphite anodes. However, there-is no evidence for its participation in chlor-alkali cells because (1) the anolyte pH in these cells is ca. 3-5 whoB hypochlorite is not a major species and (2) the anode potentials are much lower dian that required for reaction (128) to take place at any measurable rate. 

J.T. Keating and H.M.B. Gemer, High Current Density Operation—The Behavior of Ion Exchange Membranes in Chlor-Alkali Electrolyzers. In S. Sealy (ed.). Modem Chlor-Alkali Technology, Vol. 7, Royal. Society of Chemistry, Cambridge (1995), p. 135. 

J.H. Austin, Nafion Perfluorinated Membranes Operation in Chlor-Alkali Plants. In K. Wall (ed.) Modem Chlor-Alkali Technology, Vol. 3, Ellis Horwood, Chichester (1986), p. 131. 

Many different methods are used to transfer the unloaded salt to the process. Most coarse salt handles well, and most of the conunon types of conveyor are used where appropriate. This section reviews some of the characteristics of the different salts and describes the types of conveying equipment found most often in chlor-alkali plants. First, we consider some of the properties that are important to the design and operation of conveying systems. 

While inclined-plate separators are put together from a number of sub-assemblies, they are supplied as package units from one source. The sub-assemblies tend to be constructed in the vendors standard materials. A designer or operator must review these for use in chlor-alkali brine at the process temperature. 

Many types of filter are supplied for this sort of operation. In chlor-alkali brine treatment, the two most frequently encountered are the pressure leaf filter and the candle filter. Both types are capable of removing submicron particles and producing a filtrate with less than 1 ppm of suspended solids. A leaf filter, as the name implies, contains a number of thin, flat elements that are active on both sides. In chlor-alkali brine plants, the leaves normally are suspended vertically in a tank. The tank may be horizontal, in which case the leaves are circles or rounded squares, or vertical, in which case the leaves are approximately rectangular and of different widths. 

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