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Guide to Electrochemical Technology for Synthesis, Separation, and Pollution Control

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Guide to Electrochemical Technology for Synthesis, Separation, and Pollution Control Guide To Electrochemical Technology For Synthesis, Separation, and Pollution Control 72 Ward Road ! Lancaster, NY 14086-9779 Tel: 716-684-0513, 800-657-9176 ! Fax: 716-684-0511 www.electrosynthesis.com Guide to Electrochemical Technology for Synthesis, Separation and Pollution Control Prepared for Electrosynthesis Company, Inc. by Professor Derek Pletcher, University of Southampton Chemical manufacturers and users are daily faced with decisions associated with the need to improve chemical processes (e.g., increase selectivity, separate difficult mixtures, decrease energy consumption, recover the value of chemicals in waste streams, minimize the discharge of a toxic by-product, etc). Sometimes appropriate technology is available, but often it is necessary to evaluate whether an R&D program is likely to provide an economic and timely solution to the needs of the company. A number of technologies may warrant consideration and possible approaches may include electrolysis. Commonly, however, research directors, plant managers, and other technical support providers have relatively little knowledge of and/or experience with electrochemical technology. This Guide seeks to show that modern electrochemical technology can offer the preferred solution to a range of problems, and several illustrative examples are described. On the other hand, electrochemistry is not the answer for all situations; therefore the Guide also discusses those factors that should be considered to determine whether electrolysis is a viable option. © Electrosynthesis Company, Inc., 1999. All rights reserved. 72 Ward Road ! Lancaster, NY 14086-9779 ! Tel: 716-684-0513, 800-657-9176; Fax: 716-684-0511 www.electrosynthesis.com Table of Contents WHAT IS ELECTROLYSIS? .................................................................................................................. 3 APPLICATIONS OF ELECTROCHEMICAL TECHNOLOGY........................................................ 4 WHY CONSIDER ELECTROLYSIS NOW? ........................................................................................ 6 WILL ELECTROCHEMICAL TECHNOLOGY SOLVE YOUR PROBLEM? ................................. 7 EXAMPLES OF ELECTROLYTIC PROCESSES ................................................................................. 8 2 WHAT IS ELECTROLYSIS? each mole of chlorine formed, two moles of sodium hydroxide must also be produced. Furthermore, it may be seen Electrolysis is a unit process in which that the rate of chemical change is chemical change results from electron proportional to the cell current and the transfer reactions across the total amount of product may be electrode/solution interfaces. A voltage calculated from the charge passed via applied between the two electrodes in Faraday’s Law. an electrolytic cell drives these reactions. A schematic of an electrolytic cell for the Overall, the process objective (the manufacture of Cl2 and NaOH is shown conversion of sodium chloride to in Figure 1. chlorine and sodium hydroxide) is achieved through two selective electrode reactions and a selective e- transport process through the membrane. In common with all cation processes, a chlor-alkali plant will need Cl2 exchange NaOH + H2 membrane several other unit processes to prepare the feedstock for the cell and recover the + products. In a chlor-alkali plant, the - Na - e e brine for the cell will be treated to - - remove hardness and transition ions, ANODE Cl " ½Cl2 ½ H2 + OH # H2O CATHODE and then heated; the products are converted into the appropriate form for NaCl sale or downstream use. Figure 1. Schematic representation of an electrolysis cell for the The technology for the chlor-alkali manufacture of chlorine and sodium hydroxide. process, including the design of the cell and the development of high- The feedstock is aqueous NaCl brine, performance cell components and the fed to the anolyte compartment between other unit processes in a plant, has been the anode and a cation permeable described,1 while a further series of membrane. Chloride is oxidized to books cover the fundamental chlorine gas at the anode surface and electrochemistry, electrochemical water is reduced to hydrogen gas and engineering and membrane hydroxide ion at the cathode. In order to chemistry.2,3,4 maintain charge balance, migration of ions must occur, and the membrane is 1 D. Pletcher and F. C. Walsh, Industrial designed to allow the passage of only Electrochemistry, Chapman and Hall, London 1990. Na+ from anolyte to catholyte. As a 2 D. Pletcher, A First Course in Electrode Processes, The result, sodium hydroxide is formed in Electrochemical Consultancy, Romsey, 1991. 3 F. C. Walsh, A First Course in Electrochemical the catholyte compartment. Also, in Engineering, The Electrochemical Consultancy, order to preserve charge balance, the Romsey, 1993. rate of transfer of electrons at the two 4 T. A. Davis, J.D. Genders and D. Pletcher, A First Course in Ion Permeable Membranes, The electrodes must be equal – therefore for Electrochemical Consultancy, Romsey, 1997. 3 APPLICATIONS OF different sites. The chlor-alkali ELECTROCHEMICAL industry is a major user of electric TECHNOLOGY power, consuming ~ 125 x 109 kWh/year, > 1% of the total electricity generated. Not The present commercial applications of surprisingly, energy consumption electrochemical technology are many is a major consideration in process and diverse. Figure 2 is taken from a operation. Also, the scale of the recent EPRI report, Electrolytic Processes, industry warrants the 5 Present and Future Prospects, which development of highly optimized reviewed applications under the chapter cell designs and high-performance headings: anodes, cathodes and separators. A modern cell house will be fully ! The Chlor-Alkali Industry automated and will operate ! Metal Extraction continuously without maintenance ! The Manufacture of Inorganics for several years. ! The Manufacture of Organics ! The world production of ! The Recycle of Chemicals and potassium permanganate, KMnO4 , Materials is only ~ 20,000 tons/year at a few ! Separation and Purification sites. Energy consumption is a less ! Pulp and Paper important consideration in the ! Water and Effluent Treatment process economics, and the ! Atmosphere Control and electrolysis plant is more Improvement traditional, without components ! Destruction of Toxic Waste specifically developed for this ! Soil and Groundwater process. Remediation ! Electrolysis is used for the on-site manufacture of pure arsine, AsH3, for the electronics industry. It While all this technology is based on the avoids storage of highly toxic gas since the cells may be switched same fundamental principles, its on/off as the gas is required. practical manifestations may be quite Highly automated and safe units different with, for example, cell are operated on a very small scale. configurations, electrode materials and sizes, electrolytes and separators, which A similar story could be told with are each designed to meet the particular respect to the manufacture of organic demands of the application. Even within chemicals or with respect to the a particular heading, the processes may technology for process stream recycle be quite different. For example, consider and environmental protection. Thus, the manufacture of inorganic chemicals: electrolysis is used for the manufacture of the polymer intermediate, ! The world production of Cl2 is ~ 45 adiponitrile, at several sites with a total x 106 tons/year at some 700 world production of ~ 300,000 tons/year. It is also used for the 5 EPRI Report, Electrolytic Processes, Present and Future production of a number of fine Prospects, Report Number TR-107022, December 1997. 4 Manufacture of Inorganic Chemicals: e.g., Cl2/NaOH, Soil Remediation: in situ treatment of soils to remove - - - KOH, ClO3 , ClO2 , BrO3 , H2, O2, F2, O3, H2O2, MnO2, toxic materials. - 2- 2- 2- Cu2O, MnO4 , Cr2O7 , S2O8 , N2O5, NH2OH, SnO3 , 4+ Ce , and AsH3. Atmosphere Control: devices for removing toxic substances or improving the O2 content of atmospheres. Synthesis of Organic Compounds: including monomers, fine chemicals, and pharmaceutical and Metal Finishing: e.g., electroplating, anodizing, agrochemical intermediates. electropainting, and surface modification. Extraction of Metals: e.g., Al, Na, K, Mg, Li, Cu, Zn, and Manufacture of Electronic Components: e.g., printed Ga. circuit boards, deposition of metal contacts, and semiconductor layers. Recycle of Chemicals and Process Streams: e.g., metal recovery and refining, recycle of redox reagents, Metals Fabrication: e.g., electrochemical machining, salt splitting, and electrodialysis for stream concentration. electroforming, and electrogrinding. Chemicals Purification and Separation: e.g., the Corrosion Control: e.g., anodic and cathodic protection refining of metals, the separation of organics from and sacrificial anodes. electrolytes, and purification of amino acids by electrodialysis. Batteries and Fuel Cells: e.g., batteries for electronic devices, household and portable devices, and fuel cells Water and Effluent Treatment: e.g., salt removal; ClO−, for vehicles and on-site power generation. H2O2 and O3 treatment; removal of metal ions to < 1 ppm; and removal of organics, nitrate and radioactive ions. Sensors: for monitoring atmospheres, warning of toxic hazards, optimization of engine/furnace performance, Total Destruction of Toxic Materials: e.g., destruction and monitoring water quality, process streams, medical of contaminated waste from nuclear industry, and applications,
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