Separators in chlor-alkali cell

The principal application of "Nafion" currently is as a membrane separator in chlor-alkali cells, shown schematically in Figure 1. In this process water is decomposed in the cathode compartment to produce caustic and hydrogen, while saturated brine is fed to the anode compartment where the chloride ion is reduced to chlorine gas. The role of the membrane is to separate the two compartments, allow the facile transport of sodium ions from the anode to cathode compartments, and to restrict the flux of hydroxyl ions across the membrane. In the classical picture of ion exchange membranes (14) where the ion exchange sites are... 

Mass transfer through separators in chlor-alkali cells has been studied by several authors and the general one-dimensional model (see Fig. 16) proposed by Mukaibo82 is discussed in this section. Anolyte flows from the anode compartment through the diaphragm toward the cathode side under a hydrostatic head with a velocity of v. Hydroxyl ions generated in the cathode compartment move... 

The sensitivity to hydrolysis is a key issue in many applications. The ester bond in 4GT-PTMO copolymers is sensitive to hydrolysis however, it is fairly protected since most of the ester is contained in a crystalline structure. The addition of a small amount (1-2%) of a hindered aromatic polycarbodiimide substantially increases the lifetime of this material in the presence of hot water or steam (Brown et al., 1974). Polyurethanes are susceptible to hydrolytic attack, especially those with polyester soft segments. However, polyester soft segment polyurethanes are generally more resistant to oils, organic solvents, and thermal degradation. lonomers will swell when exposed to water in fact, a commercial hydrated perfluorosulfonic ionomer (Nation) is used as a membrane separator in chlor-alkali cells. Styrene-diene copolymers and polyolefin TPEs are insensitive to water. 

Electric energy is the predominant cost in the manufacture of chlorine and is the driver for most of the technical progress in the chlor-alkali industry. The busiest areas of development over the past 20 or 30 years have been related to reductions in energy consumption. Approximately 60% of the papers presented in this book deal with improvements in chlor-alkali cell internals, namely the anolyte/catholyte separator (primarily membranes) and the electrodes. 

The current state-of-the-art proton exchange membrane is Nafion, a DuPont product that was developed in the late 1960s primarily as a permselective separator in chlor-alkali electrolyzers. Nation s poly(perfluorosulfonic acid) structure imparts exceptional oxidative and chemical stability, which is also important in fuel cell applications. 

Membranes can be characterized by their structure and function, that is how they form and how they perform. It is essential that the cation exchange membranes used in chlor-alkali cells have very good chemical stability and good structural properties. The combination of unusual ionic conductivity, high ionic selectivity and resistance to oxidative hydrolysis, make the perfluorinated ionomer materials prime candidates for chlor-alkali membrane cell separators. 

Perfluorinated ionomer membranes have been developed for use as separators in chlor-alkali electrolysis cells. Using an automated test apparatus, the current efficiency and voltage drop of such a high performance membrane were evaluated as a function of several cell parameters. Results are plotted as three dimensional surfaces, and are discussed in terms of current theories of membrane permselectivity. 

The major use of perfluorinated membranes, at present, is as separators in chlor-alkali ceiis. The combination of low resistances, high current efficiencies at high solution concentrations, and high temperatures that can be achieved results in 20-30% lower energy requirements than those achieved with diaphragm or mercuiy cells. The long membrane lifetime (typically 2 years) results in low cost for membrane replacement. (Asbestos diaphragms usually last for only a year or less.)... 

Overvoltages for various types of chlor—alkali cells are given in Table 8. A typical example of the overvoltage effect is in the operation of a mercury cell where Hg is used as the cathode material. The overpotential of the H2 evolution reaction on Hg is high hence it is possible to form sodium amalgam without H2 generation, thereby eliminating the need for a separator in the cell. 

In chlor-alkali diaphragm cells, a diaphragm is employed to separate chlorine hberated at the anode from the sodium hydroxide and hydrogen generated at the cathode. Without a diaphragm, the sodium hydroxide formed will combine with chlorine to form sodium hypochlorite and chlorate. In many cells, asbestos diaphragms are used for such separation. Many types of diaphragm cells are available. 

The most important commercial application of perfluorinated ionomer membranes is currently in the chlor-alkali industry. These materials are used as permselective separators in brine electrolysis cells for the production of chlorine and sodium hydroxide. This... 

Only after viewing the membrane as a thin film semiconductive phase can one begin to seriously evaluate its potentialities. It is a multidimensional problem, and in the chlor-alkali cells the water transport is controlled by brine concentration while caustic strength controls the cathode efficiency. The membrane provides a low energy pathway for the phase change and separation process. 

Nafion (a registered trademark of E. I. du Pont de Nemours and Go.) and other perfluorinated ion exchange membranes have received much recent consideration as electrolytic separators in electrochemical applications, particularly chlor-alkali cell technology. The systems of current commercial interest, as well as descriptions of many physical property investigations, are reported throughout this volume. 

The separator in the chlor-alkali cell is by far the most important component. It allows the free passage of electrical current and keeps reactants and products apart by maintaining sufficient gradients between its phase boundaries. 

Detailed investigations by MacMullin et al.72 show that the value of Kx/r for many porous beds is 3.666 0.098. Since the tortuosity of a porous asbestos diaphragm is —1.5 and Kx varies in the range of 5-5.5, the model describing the porous beds is applicable for the separators used in diaphragm chlor-alkali cells. 

The perfect separator in a chlor-alkali cell (a) would pass only sodium ions without allowing the transport of chloride ion from anolyte to catholyte (leads to Cl contamination of the NaOH) or hydroxide ion from catholyte to anolyte (causes O2 contamination of the CI2), (b) would have a low resistance, and (c) would be stable to wet chlorine and 50% sodium hydroxide over a long period of time. Moreover these properties should be maintained even when the catholyte is 50% sodium hydroxide as ideally it would be when leaving the cell. 

In the case of a diaphragm or membrane chlor-alkali cell, there are three ohmic drops to consider anode to separator, cathode to separator, and across the separator. The anode to cathode distance in ELTECH s H-4 cell is 7.74 imrt, the thickness of the diaphragm is 3.05 mm, and the cathode to diaphragm distance is negligible. The anode to diaphragm ohmic drop, // a/s. niay now be estimated from the conductivity data of the electrolytes, which are presented in the appendix to this section. 

FIGURE 4.7.6. Mass transfer through the separator in a diaphragm type chlor-alkali cell. 

The chlorine evolution reaction and the hydrogen evolution reaction at the anode and the cathode, respectively, in a chlor-alkali cell are controlled by the electrochemical and/or chemical steps rather than by mass transfer. However, the transport phenomena across the separator, either a porous diaphragm or an ion-exchange membrane, are governed by the solution flow near the surface. The disproportionation reaction of hypochlorites in a chlorate cell is diffusion-controlled process. Consequently, knowledge... 

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