The resulting overall reaction is DC Power 2NaCl + 2H2O → 2ΝaOH + Cl2 + H2 sodium salt water chlorine hydrogen hydroxide Early production plants were located where electric power from hydroelectric or steam sources and solid salt or brine deposits were readily available. Over the years, two processes emerged. The first was the diaphragm cell, in which a porous asbestos mat separated the anode and cathode compartments. The second was the mercury cell, where the Industrial Electrolysis and cathode was actually a pool of liquid mercury. The cathodic reaction involved the formation of a sodium amalgam 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 electrodes 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 ions products included the metals aluminum, potassium, and to pass between the anode and cathode sodium; strong chemicals such as bleach, chlorine, bromine, compartments. This results in greater and sodium hydroxide. 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 chlorate, hydrogen, include catalytically coated titanium anodes, low voltage cathodes, and oxygen, aluminum, copper, magnesium, zinc, and adiponitrile, corrosion resistant polymers used for cell a raw material for the manufacture of nylon. bodies, that have been incorporated into chlor-alkali 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 electrolyte. 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 acid in between the anode and cathode range of disciplines. cells that are similar to ones described compartments to separate the anodic above. Hydrochloric acid 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 electrode 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 diffusion 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 thermodynamics, 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 acids, 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 industrial processes FIG. 1. A modern chlorine production cell installation using ion exchange membrane cells. Each have been established successfully. electrolyzer consists of planar cell elements separated from each other by a sheet of ion exchange 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 gas diffusion electrode (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 metal was A commercial process for producing extremely rare, as expensive as silver, 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- electrolytic process. high efficiency, and 2 2 2 4 development of electrode DC Power materials that permit lower cell voltages and that are not DC Power 2A12O3 + 3C → 4Al + 3CO2 aluminum carbon aluminum carbon derived from petroleum by- 2CH =CHCN + Η Ο → ½ O + NC(CH ) CN oxide dioxide products. 2 2 2 2 4 acrylonitrile water oxygen adiponitrile One electrode in the cell is made Metal Winning and Refining of carbon and is consumed in this Many other metals are Specialty organic chemicals that are high temperature process. Several obtained from their ores (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