Separators chlor-alkali

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. 

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 first practical example of electrodes able to satisfy many of the characteristics required in the application of chlor-alkali electrolysis is a particular family of doublesided gas-diffusion electrodes introduced some years ago under the trade name of ESNS , by E-TEK Inc. (now a Division of DeNora North America). The dual function (electrode and separator) of this electrode structure was achieved with an accurate choice of the basic components. 

Because of the wide availability of chlor-alkali technology, it was decided to tender for fixed-price contracts for engineering, procurement and construction of the two chlor-alkali plants. The other areas - namely Outside Battery Limits for both plants and the chlor-paraffin plant in Melbourne - were managed by Orica under separate contracts. The chlor-paraffin plant process design (batch chlorination of paraffin oil and wax) was carried out in-house by Orica. 

Having recently won the 1999 Product Achievement Award for Filter Applications from Filtration + Separation magazine, the SRS has become the state-of-the-art technology for sulphate removal. Development of the SRS enhances Kvaerner Chemetics record as a provider of cost-effective and environmentally friendly solutions for the chlor-alkali and sodium chlorate industries. 

Physical separation of the chlor-alkali/EDC plant from the cracking/oxyhydro-chlorination plant does create some complications regarding the necessary duplication of feedstocks and services. In particular, there will be the need for two ethylene supplies and for two independent thermal oxidation systems. Approximately half of the total ethylene must be provided as the feed to the EDC unit with the remainder fed to the oxyhydrochlorination unit. The EDC unit as well as the cracking and oxyhydrochlorination units will generate off-gases that require emission control,... 

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. 

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. 

Ion-exchange membranes are currently used not only for more or less conventional separation processes like membrane electrolysis (mainly the chlor-alkali process), electrodialysis, dialysis or electro-ultrafiltration (cf. Table 2.1), but also in various... 

Chlor-alkali production Electrochemical synthesis Water-organic liquid separation Organic liquid mixture separaion Fermentation products recovery and purification Cell harvesting, virus and antibody concentration Protein desalting, concentration and fractionation Blood processing, including artificial kidney Isolation, concentration, and identification of solutes and particulates... 

The use of polyperfluorosulfonic acid membranes as the cell separator was first demonstrated about three decades ago. Yet it was not until the mid-1980s when the economic advantages of membrane cells over the traditional mercury- and diaphragm-cell technology were fully demonstrated—consequent to better membrane performance, higher caustic product concentrations, and lower power consumption. Retrofitting chlor-alkali facilities with membrane cells accounted for much of the growth and sustenance of this industry over the past two decades. 

Ion-exchange membrane is designed to allow the transport of primarily sodium ions and water from the anolyte to the catholyte compartment, whereas the diaphragm allows the percolation of all the anolyte through the separator. The cation-conducting, ion-exchange membrane is structured to reject anions, as indicated in Fig. 26.9. The chlor-alkali membranes in use today consist of one or more perfluorinated ion-exchange polymeric mate-... 

In the last twenty-five years a new process has been developed in the chlor-alkali industry that uses a membrane to separate the anode and cathode compartments in brine electrolysis cells. The membrane is superior to the diaphragm used in diaphragm cells because the membrane is impermeable to anions. Only cations can flow through the membrane. Because neither Cl- nor OH- ions can... 

The chemical reduction process is also being used on a limited basis to remove mercury and lead from wastewater. Sodium borohydride is currently used in some chlor-alkali plants (inorganic chemicals manufacturing) to reduce the soluble mercury ion to metallic mercury, which is then removed from solution by granular activated carbon adsorption, or solid-water separation process. 

Cell separators (or dividers) are generally required in an electrochemical cell in order to prevent both intermixing of anolyte and catholyte, and, possibly, shorting between anode and cathode.In many cases, without a separator, the cell either does not work at all or works at a much lower efficiency and with a shorter cell life. This is particularly true for the chlor-alkali celP and the Fe-Cr redox cell," both of which require membranous separators. 

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... 

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. 

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. 

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. 

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. 

Much of the interest in Nafion materials stems from its use as a membrane separator for chlor-alkali production and other electrochemical applications (6). Hence, it is relevant to study the material in the presence of water of swelling, with or without sorbed electrolytes. The presence of water and counterions gives us an NMR handle with which to study Nafion. Solvent and ionic mobility is rapid enough in the aqueous regions of swollen Nafion to yield high resolution NMR signals. 

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... 

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