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Home , Chlorine production, Electrolysis, Electrolytic process

The resulting overall reaction is

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2NaCl + 2H2O → 2ΝaOH + Cl2 + H2 Early production plants were located where electric power from hydroelectric or sources and solid salt or deposits were readily available. Over the years, two processes emerged. The first was the diaphragm cell, in which a porous mat separated the and compartments. The second was the cell, where the Industrial and cathode was actually a pool of liquid mercury. The cathodic reaction involved the formation of a sodium which was separated in a second cell Electrochemical Engineering (the stripper cell). The mercury cell used by Morris Grotheer, Richard Alkire, more electrical energy but produced a product of higher purity than the and Richard Varjian diaphragm cell. Factors, including environmental concerns about mercury with modifi cations by Venkat Srinivasan and John Weidner and asbestos, product purity of the diaphragm cell, and the availability of The introduction of the electrical dynamo in the early 1870s new and materials, currently made large scale, relatively inexpensive electric power available favor a third system, the membrane cell. The membrane is an impervious for commercial scale chemical production. The early electrolytic separator that allows only sodium products included the aluminum, , and to pass between the anode and cathode sodium; strong chemicals such as bleach, chlorine, bromine, compartments. This results in greater and . Over the years, a wide variety of product purity than the diaphragm cell and lower energy consumption than materials, primarily metals and strong oxidizing agents, have the mercury cell. The membrane is the been produced electrolytically. Among those produced today latest in a series of developments, which are chlorine, sodium hydroxide, sodium , hydrogen, include catalytically coated , low , and , aluminum, , , , and adiponitrile, corrosion resistant used for cell a raw material for the manufacture of nylon. bodies, that have been incorporated into chlor- cells in the past 30 years. Experience has shown that cell designs Electrochemical reactors are called manner. Improved understanding of evolve constantly and it is expected that electrolysis cells. The cells consist scientific principles and the application new technology will be incorporated into of a container, the cell body; two of new materials can lead to more membrane cells in the years ahead. electrodes, the anode and cathode, efficient cell designs and processes. One example of the constant where the electrochemical reactions The constant evolution of technology evolution in design is the development occur; and an . Some cells provides challenging and rewarding of a process that produces chlorine have a diaphragm or membrane careers to engineers and scientists in a by electrolyzing hydrochloric in between the anode and cathode range of disciplines. cells that are similar to ones described compartments to separate the anodic above. is produced and cathodic products. While general Examples of Industrial as a by-product in various organic purpose electrolysis cells are available, Electrolytic Processes processes and its handling and disposal cells are usually custom designed for While a summary of industrial pose serious environmental problems. a particular process. The electrolysis activity follows, a more complete Electrolyzing the acid to supplement cells used to produce the various discussion of industrial activity is the chlorine produced from chlor- chemicals and metals cited above published annually in the Journal of The alkali cells allows for a viable means to differ significantly from one another. Electrochemical Society. Some specific dispose the chemical. Figure 1 describes Electrolytic processes consume references for additional reading are a new development in this area where more than 6% of the total electrical given at the end of this section. a significant reduction in the energy use was achieved by using an generating capacity of the United Chlorine and Sodium Hydroxide States. It is the responsibility of material that allows for the use of oxygen the electrochemical engineer in Two common chemicals produced by as the feed (an oxygen depolarized industry to simultaneously manage electrolysis of salt solutions are chlorine cathode), along with a design to feed the electrical consumption and chemical and sodium hydroxide. The principal oxygen into the system (a gas production. He or she must apply electrode reactions that occur in the electrode). The concept described here is relevant scientific and engineering electrolysis of salt solutions are expected to be implemented in the next- principles to design, construct, and generation electrochemical technologies (Anode) 2Na+ + 2Cl- Cl + 2Na+ + 2e- operate a process in an economical, → 2 to enhance energy savings. - - safe, and environmentally conscious (Cathode) 2H2O + 2e → H2 + 2 OH

52 The Electrochemical Society Interface • Spring 2006 control of cell conditions, impurities are left behind, either as undissolved solids or as dissolved species that do not plate out. The scientific fundamentals that underlie copper electrorefining include , kinetics, mass transport, and current and potential distribution phenomena. However, fundamental concepts must be transformed into engineering designs to achieve economical operation and high quality product. Membrane Processes Membranes make possible an enormous variety of separation processes. Examples of membrane separations include desalting of brackish water and of seawater, demineralization of food products, separation of amino , and recovery of resources from wastewater streams. In these processes, the key component is a membrane that permits some chemicals to go through but not others, thus separating them. Understanding of transport processes across membranes is far from complete, Photo reproduced by permission of Bayer MaterialScience AG. although many FIG. 1. A modern cell installation using exchange membrane cells. Each have been established successfully. electrolyzer consists of planar cell elements separated from each other by a sheet of Extensive activity is currently directed membrane. The cell uses hydrochloric acid and oxygen to produce chlorine gas and water. The cell shown toward obtaining better insight into how above is the first plant built with a new electrode termed an oxygen depolarized cathode (ODC), and a (GDE) to feed the oxygen. Developed by a joint cooperation of De Nora Tecnologie membranes work and toward achieving Elettrochimiche, Bayer Material Science, and Uhde, these changes reduce the energy demand from 1700 new membrane recipes that exhibit kWh per ton of chlorine to 1000 kWh per ton of chlorine. The companies involved received the New high selectivity, low ohmic resistance, Electrochemical Technology Award of the ECS Industrial Electrolysis and Electrochemical Engineering and robust strength over wide ranges Division in 2005 for this advance. The ODC electrolysis of HCl may be seen as the forerunner of a new of pH and chemical environments. family of electrochemical processes all based on the GDE technology and all characterized by lower energy consumption compared to conventional plants. (Picture courtsey of the three companies.) New industrial membrane processes are constantly emerging as this technology Aluminum for understanding these effects and matures. for achieving optimum production Prior to its manufacture by conditions. Electro-organic Synthesis electrolysis, aluminum was A commercial process for producing extremely rare, as expensive as , In the aluminum industry, intensive organic chemicals that is currently and just as prized. Today aluminum is research efforts are currently directed practiced on a scale comparable to the an inexpensive and widely available toward increasing cell efficiency and inorganic chemicals and metals cited material valued for its corrosion toward conserving energy by reducing above is the electrohydrodimerization of resistant properties. The large demand cell voltage. Among the major research acrylonitrile to adiponitrile for aluminum products is reflected in concepts are development of process that aluminum production consumes sensors and control schemes + - Anode H2O → 2H + ½ O2 + 2e more electrical power than any other for maintaining consistently Cathode 2CH = CHCN + 2H O + 2e- →NC(CH ) CN + 2OH- . high efficiency, and 2 2 2 4 development of electrode DC Power materials that permit lower cell and that are not DC Power 2A12O3 + 3C → 4Al + 3CO2 aluminum carbon aluminum carbon derived from by- 2CH =CHCN + Η Ο → ½ O + NC(CH ) CN dioxide products. 2 2 2 2 4 acrylonitrile water oxygen adiponitrile

One electrode in the cell is made Metal Winning and of carbon and is consumed in this Many other metals are Specialty organic chemicals that are high temperature process. Several obtained from their (winning) or produced electrochemically include

unwanted reactions also occur, so purified from impure stock (refining) perfluorooctanoic acid [CF3(CF2)6COOH] the production efficiency based on by electrochemical processes. Among and perfluorooctanesulfonic acid

consumption is less than numerous examples are copper, , [CF3(CF2)7SO3H]. Several electro-organic l00%. Among the considerations that zinc, magnesium, and titanium. In processes have been practiced on a influence cell efficiency and lower the copper industry, for instance, semicommercial scale. Examples are the consumption are temperature, spacing electrorefining is carried out by placing oxidation of benzene to benzoquinone between electrodes, electrode material, impure copper sheets in a cell, dissolving and the epoxidation of propylene to electrolyte consumption, cell size, source them by electrolysis in a bath of sulfuric propylene oxide. Many electro-organic of raw material, and production rate. acid, and pure copper reactions are facile, and scientific Clearly, engineering skill is required at the other electrode. By judicious (continued on next page)

The Electrochemical Society Interface • Spring 2006 53 Industrial Electrolysis... led to evolution of wholly new cells 7. Advances in Mathematical Modeling and (continued from previous page) and systems. These trends, which have Simulation of Electrochemical Processes - and - Oxygen Depolarized Cathodes and Activated been accompanied by a maturing and understanding is constantly improving. Cathodes for Chlor-Alkali and Chlorate The reasons for the relative lack of deepening of fundamental principles of Processes, J. W. Van Zee, T. F. Fuller, large scale electro-organic processes electrochemical engineering and science, P. C. Foller, and F. Hine, Editors, PV 98-10, are primarily economic. As feedstock have generated an enormous number of The Electrochemical Society Proceedings and energy conditions change and new process options and technologies. Series, Pennington, NJ (1998). technology progresses, additional For example, the introduction of high electro-organic processes may achieve surface area porous electrodes into About the Authors economic viability. electrolytic cells significantly improves MORRIS GROTHEER, a member of ECS since 1963, the recovery of metals from very dilute is retired from the Kerr McGee Corp. Electrochemical Engineering solutions such as occur in the rinse RICHARD C. ALKIRE holds the Charles J. While each of the processes above use streams of electroplating operations. and Dorothy G. Prizer Endowed Chair in the The opportunities for the future are Department of Chemical and Biomolecular cells designed for a specific purpose, they Engineering at the University of Illinois and is a are all bound together by fundamental exciting.  Fellow and Honorary Member of ECS. He received principles that govern the operation. three of the Society’s most prestigious awards: Additional Reading Known collectively as the principles the Carl Wagner Award in 1985, the Edward G. of electrochemical engineering, these 1. V. Srinivasan, P. Arora, and P. Ramadass, Acheson Award in 1996, and the Vittorio de Nora Award in 2004. He served as ECS President concepts include transport processes, J. Electrochem. Soc., 153, K1 (2006); R. C. Alkire and T. W. Chapman, for the 1985-86 term. He may be reached at current and potential distribution Electrochem. Soc. Interface, 12(4), 46 (2003). [email protected]. phenomena, thermodynamics, 2. in Mineral and Metal RICHARD VARJIAN is a Technical Leader in the kinetics, scale-up, sensing, control, and Processing VI, F. M. Doyle, G. H. Kelsall, New Ventures Business of The Dow Chemical optimization. With use of quantitative and R. Woods, Editors, PV 2003-18, The Company in Midland, Michigan, where his role is methods, many salient features of cell Electrochemical Society Proceedings Series, developing new process technologies. He is a past Pennington, NJ (2003). operations can be modeled in concise chair of the ECS IEEE Division. He may be reached 3. Energy and Electrochemical Processes for a at [email protected]. mathematical form. Thus, it has been Cleaner Environment, E. W. Brooman, increasingly possible to predict cell C. Cominellis, C. M. Doyle, and J. Winnick, VENKAT SRINIVASAN is a scientist at the Lawrence behavior without the cost of an extensive Editors, PV 2001-23, The Electrochemical Berkeley National Lab in Berkeley, California, where he works with a team of researchers solving empirical (trial and error) program of Society Proceedings Series, Pennington, NJ (2002). the multitude of problems that prevent Li-ion preliminary study. These principles cut 4. Tutorials in Electrochemical Engineering- batteries from being used in hybrid-electric across all electrochemical industries Mathematical Modeling, R. F. Savinell, vehicles. Venkat is a regular contributor to Tech and can be applied with equal success J. M. Fenton, A. C. West, S. L. Scanlon, Highlights (featured in Interface) and the author whether they are used to design a plant and J. Weidner, Editors, PV 99-14, The of the report of the Electrolytic Industries for the years 2001, 2002, and 2004. He may be reached to produce chemicals or to design a Electrochemical Society Proceedings Series, at [email protected]. battery or a cell for use in an electric Pennington, NJ (1999). 5. Chlor-Alkali and Chlorate Technology: OHN EIDNER car. While these concepts are well J W. W is a professor of chemical R. B. MacMullin Memorial Symposium, engineering at the University of South Carolina. He established in the electrolytic industry, H. S. Burney, N. Furuya, F. Hine, and has published 53 refereed papers on the synthesis their use in other areas has blossomed in K.-I. Ota, Editors, PV 99-21, The and characterization of electrochemically active the last few years. Concepts of current Electrochemical Society Proceedings Series, materials, and the design and optimization and potential distribution within cells, Pennington, NJ (1999). of electrochemical systems. Professor Weidner originally conceived by electroplaters, are 6. Environmental Aspects of Electrochemical is currently Secretary/Treasurer of the ECS Technology, E. J. Rudd and C. W. Walton, Industrial Electrolysis and Electrochemical now being applied throughout the field. Editors, PV 99-39, The Electrochemical Engineering (IEEE) Division. He may be reached at The impact on industry of these Society Proceedings Series, Pennington, NJ [email protected]. (2000). developments has been impressive. Beneficial cross fertilization of ideas has occurred recently at an explosive rate. For example, ion exchange membranes, originally developed for fuel cells in space capsules, have revolutionized chlor-alkali production. These same membrane materials have been readapted for use in redesigned fuel cells for terrestrial applications. Similarly, the oxygen depolarized cathode and the gas diffusion electrode, described in Fig. 1, are concepts borrowed from fuel cells and used for chlorine production. Future of Electrochemical Technology The development, design, and operation of electrochemical processes have seen enormous advances within the last few decades with profound changes in the recent past. The consequences of energy feedstock and pollution constraints have led to the need for dramatic process changes and reoptimizations. The introduction of new materials in electrolytic cells has

54 The Electrochemical Society Interface • Spring 2006