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15ИННОВАЦИОННЫЕ РЕШЕНИЯ
INNOVATIVE SOLUTIONS
The article has entered in publishing office 10.01.10. Ed. reg. No. 695 Статья поступила в редакцию 10.01.10. Ред. рег. № 695
TOWARDS MORE ENERGY-EFFICIENT CHLOR-ALKALI ELECTROLYSIS
S. Pinnow, I. Moussallem, T. Turek
Institute of Chemical Process Engineering Clausthal University of Technology, Leibnizstrasse 17, 38678 Clausthal-Zellerfeld, Germany E-mail: turek@icvt.tu-clausthal.de
Referred: 25.01.10 Expertise: 30.01.10 Accepted: 05.02.10
The energy efficiency of chlor-alkali electrolysis can be improved by utilization of the co-product hydrogen. Combustion in boilers saves costs for the fuel and reduces the overall energy demand. Production of electric energy from hydrogen in fuel cells is not economic if the high investment costs are considered. The most promising solution is avoidance of hydrogen formation through application of oxygen depolarized cathodes reducing the electric energy demand by 30%. Economic analysis shows that this new technology is profitable. After intensive worldwide research and development over more than 30 years, the first industrial-scale facility is scheduled to go on stream in Germany by 2011.
Keywords: electrolysis, chlor-alkali electrolysis, oxygen depolarized cathodes, energy efficiency.
НА ПУТИ К БОЛЕЕ ЭНЕРГОЭФФЕКТИВНОМУ ХЛОР-ЩЕЛОЧНОМУ ЭЛЕКТРОЛИЗУ
С. Пинноу, И. Муссалем, Т. Турек
Заключение совета рецензентов: 25.01.10 Заключение совета экспертов: 30.01.10 Принято к публикации: 05.02.10
Энергетическую эффективность хлор-щелочного электролиза можно повысить за счет использования водорода, который является побочным продуктом электролиза. Сгорание в котлах экономит расходы на топливо и снижает общее энергопотребление. С учетом высоких капитальных затрат получать электроэнергию из водорода в топливных элементах неэкономично. Наиболее перспективным решением является предотвращение образования водорода за счет использования деполяризованных кислородных катодов, благодаря чему потребление электроэнергии снижается на 30%. Экономический анализ свидетельствует о прибыльности этой новой технологии. После интенсивных научных исследований и разработок, проводимых по всему миру на протяжении более 30 лет, к 2011 году в Германии планируется запуск первой промышленной установки.
Ключевые слова: электролиз, хлор-щелочной электролиз, деполяризованные кислородные катоды, энергетическая эффективность.
Stefan Pinnow is a scientific coworker of Prof. Turek in the Chemical Process Engineering institute at the Clausthal University of Technology, Clausthal-Zellerfeld, Germany, scientific coworker at the Aachen University of Applied Sciences, Department of Applied Sciences and Technology (2001), scientific coworker at the Clausthal University of Technology, Institute of Chemical Process Engineering (since 2007). His research interests include electrochemistry, especially chlor-alkali electrolysis with oxygen depolarized cathodes. He is author of 2 articles.
Stefan Pinnow
International Scientific Journal for Alternative Energy and Ecology № 4 (84) 2010
© Scientific Technical Centre «TATA», 2010
Imad Moussallem
Imad Moussallem is a scientific researcher at Evonik Degussa GmbH, Process Technology and Engneering, Germany. Education: Chemistry at the Lebanese University in Lebanon (1994-1998); Chemical Engineering at the University of Erlangen Nürnberg (2000-2004); Doctoral studies at the Clausthal University of Technology, Institute of Chemical Process Engineering (2005-2008). Experience: Internship and Diploma work at Merck KGaA in Germany, Central Process Engineering (2003-2008); Scientific co-worker at the Clausthal University of Technology, Institute of Chemical Process Engineering (2005-2008); Scientific researcher at Evonik Degussa GmbH, Process Technology and Engineering-New Processes (Since 2008). His scientific interests: inorganic electrochemistry, chlor-alkali electrolysis with oxygen depolarized cathodes, organic electrochemistry, electrocarboxylation, building pilot chemical plants for bringing new technologies to market. He is author of 2 articles.
Thomas Turek is professor of Chemical Process Engineering at Clausthal University of Technology, Germany. His research interests include the development of heterogeneous catalysts, reaction engineering of multiphase systems, and electrochemical reaction engineering. Selected reaction engineering topics are catalysts and reactors for Fischer-Tropsch synthesis, catalytic removal of nitrogen oxides from nitric acid plants, and hydrogenations reactions over structured catalysts. Current projects in electrochemical reaction engineering deal with the manufacture of gas-diffusion electrodes and membrane-electrode assemblies and their use in PEM fuel cells or as oxygen depolarized cathodes in electrolysis processes. He is author of 70 technical papers and book chapters and holder of several patents.
Thomas Turek
Introduction
Chlorine is an important base chemical indispensable for the manufacture of polymeric materials. However, chlorine production is also one of the most energy intensive industrial processes in the world as Cl2 must be obtained, together with sodium hydroxide and hydrogen, through electrolytic splitting of sodium chloride solutions. Given the large worldwide chlorine production capacity of more than 60 Mt-a-1 the present overall energy demand amounts to ca. 21011 kW h per year equivalent to about 1% of the global electricity generation. It is obvious that a significant improvement of the energy efficiency in the chlor-alkali industry would be highly desirable.
A key factor for the energetic improvement of chlorine production is the co-product hydrogen as it contains a considerable amount of the energy used during the electrolysis process in chemical form. If hydrogen cannot be used for chemical synthesis reactions such as hydrogenations, it must be utilized to enhance the overall efficiency of the chlor-alkali electrolysis. The traditional way is combustion of excess hydrogen in boilers allowing for savings in natural gas or other fuels. In recent years it has been suggested to use fuel cells that are able to convert about 50% of the chemical energy stored in hydrogen with air to electric energy. This long-known method for hydrogen utilization has become quite popular and is currently being evaluated in several demonstration
projects in chlor-alkali plants. The most radical solution for the hydrogen energy issue is the direct integration of fuel cell technology in the electrolysis process through application of so-called oxygen depolarized cathodes. In this way the formation of hydrogen as a co-product is avoided and the electric energy demand for the modified chlorine production process can be significantly reduced [1].
The present contribution gives an overview about the different possibilities for energetic improvement of chloralkali electrolysis. After a technical and economic comparison of the concepts, the use of oxygen depolarized cathodes as most promising technology is described in more detail.
Industrial chlor-alkali electrolysis processes
At present, three different electrolytic processes are employed industrially for the production of chlorine and sodium hydroxide solution. Older plants using the diaphragm cell process (Griesheim cell) and the mercury cell process (Castner-Kellner cell), both developed in the late 19th century, are still operated with a global market share of around 50%. The modern membrane cell process was developed in the early seventies of the last century and is now state of the art [2, 3]. These three processes employ different methods to separate the products formed at anode and cathode. In all processes, chloride ions are oxidized at the anode yielding chlorine gas:
2Cl- ^ Cl2 + 2e-. (1)
At the cathode of the diaphragm and membrane processes, water and electrons react to hydrogen gas and hydroxy! ions:
2H2O + 2e- ^ H2 + 2OH-
(2)
In these processes, the undesirable migration or diffusion of hydroxyl ions to the anode is prevented by a separator (diaphragm or membrane) suppressing the side reaction between chlorine and hydroxyl ions to hypochlorite ions. In the mercury process, the cathodic formation of hydroxyl ions is avoided through formation of a sodium/mercury amalgam in a first reaction step. The amalgam is then transferred to a second reactor where hydrogen and sodium hydroxide are formed by reaction of the sodium with water while mercury is recycled to the electrolysis cell.
2NaHgx + 2H2O ^ 2Hgx + H2 + 2Na+ + 2OH-. (3)
The mercury process has the advantage of producing high-quality caustic soda with simple brine purification. However, it is the most energy intensive of all processes and consumes up to 3600 kWh of electric energy per ton of chlorine. The diaphragm process produces a lower-quality caustic soda at higher energy demand than required for the membrane process. Thus the capacity shares for the older processes have been declining over the last decades and all new plants are based on the favourable membrane cell process because of lower
capital investment and operating costs. This latest chloralkali technology has been constantly improved to such an extent that no substantial reduction of the energy demand can be expected from further process modifications.
Possibilities of energetic improvement through hydrogen utilization or prevention
Fig. 1 depicts the possibilities for an overall energetic improvement for chlor-alkali electrolysis. We assume that the co-product hydrogen is not required as chemical reactant at the given production site.
The common use in that case is the replacement of fuels such as natural gas by hydrogen in boilers.
This way of hydrogen utilization saves costs for the boiler fuel and reduces the overall demand for electric energy. The second possibility considered is the production of electric energy from excess hydrogen with air in fuel cells. For economic comparison of these possibilities it must be taken into account that fuel cells require considerable investment costs. The third concept consists of a shift from classical hydrogen-evolving to oxygen depolarized cathodes (ODC). By consumption of oxygen at the cathode, the following stoichiometry of the electrolysis is obtained:
2NaCl + H2O + ViO2 ^ 2NaOH + Cl2
(4)
Fig. 1. Possibilities for energetic improvement of chlor-alkali electrolysis
Fig. 2. Energy demand for a chlor-alkali plant with 100 kt-a Ch (@ 2000 kWh/t NaOH) and possible energy savings by hydrogen utilization or avoidance
International Scientific Journal for Alternative Energy and Ecology № 4 (84) 2010
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Thus the energy demand for the electrolysis process is quite strongly reduced while on the other hand investment costs for new electrodes and cell modifications as well as the fact that chlor-alkali electrolysis with ODC requires pure oxygen rather than air must be considered.
In the following we briefly summarize an energetic and economic analysis of the different concepts that has been presented elsewhere [4]. It is assumed that the chlorine plant has a production rate of 100 kt-a-1 Cl2 and an energy demand of 2000 kWh/t NaOH (for new plants, existing ones have a higher energy consumption). At 100% current efficiency this corresponds to a required electric power of 25.8 MW for the electrolysis. With the lower heating value of hydrogen (-241.8 kJ/mol) one can calculate that the chemical energy content of this co-product is equivalent to 10.8 MW or 42% of the electric energy demand of the electrolysis plant. For the conversion of chemical to electric energy in boilers an efficiency factor of 0.4 was assumed while fuel cell systems may reach a higher efficiency factor of 0.5. On the other hand, the cell voltage in chlor-alkali cells can be reduced by at least 0.9 V through application of oxygen depolarized cathodes [1]. Assuming a typical cell voltage of 3.0 V for a conventional membrane cell operated at a current
density of 4 kA-m- , the efficiency of the ODC concept amounts to a quite high value of 0.72. Fig. 2 summarizes these calculations and reveals that between 17% (H2 combustion) and 30% (ODC electrolysis) of the total energy demand for classical chlor-alkali electrolysis may be recovered.
A more detailed analysis comparing the utilization of hydrogen in fuel cells or avoidance by ODC technology with the standard combustion of H2 in boilers must take into account the hydrogen value (based on heating value equivalence and natural gas price), investment costs and, in case of ODC, also the costs for the required pure oxygen. Fig. 3 shows selected results of these economic calculations.
It can be seen that fuel cells are not economic at the assumed values for electricity price (60 €/kW-h) and investment costs for the fuel cell system (1500 €/kWei).
Given the relatively low efficiency gain of fuel cells compared to electricity production in thermal boilers (50 vs. 40%) very low investment costs for fuel cells would be required for an economic operation. This might be possible in the future if a market with mass production of automobiles with hydrogen fuel cells evolves [6]. In that case however, hydrogen as co-product from chlor-alkali plant would be a premium fuel easily saleable on the hydrogen market.
Fig. 3. Economics of hydrogen avoidance through ODC technology (left) and hydrogen utilization in fuel cells (right) compared to standard hydrogen combustion (electricity price 60 €/MWh, conversion costs for ODC 0.7 Mio. €/a, investment costs fuel cell
1500 €/kWel)
In contrast to electric energy production in fuel cells, hydrogen avoidance by conversion to ODC technology is already economic at today's electricity prices and investment costs even taking into account the additional costs for oxygen (Fig. 3, right). This can be explained by the very high energetic efficiency of the ODC principle. The calculated net profit is around one quarter of the original savings due to cell voltage reduction. In the future, the economics of the ODC technology might be improved by two factors. Firstly, advanced electrodes could allow for an even higher cell voltage reduction and secondly, possible rising electricity prices would strongly increase the net profit of converting conventional membrane electrolysis to ODC technology.
Chlor-alkali electrolysis with ODC
Economic analysis has revealed that ODC technology is energetically preferred to hydrogen utilization in fuel cells or through combustion in boilers. Operating principle, design of electrodes and electrolyzers as well as current status and future prospects of this technology will be described in the following chapter.
Operating principle Fig. 4 shows that the difference between classical membrane electrolysis (left) and the one with ODC technology (right) is the use of a special cathode.
While hydrogen evolves at traditional cathodes, hydrogen formation is avoided by reaction with oxygen at oxygen depolarized cathodes.
O2 + 2H2O + 4e- ^ 4OH-.
(5)
Given that the formation of chlorine at the anode remains unchanged, the following overall stoichiometric equation for the formation of chlorine and sodium hydroxide results.
2NaCl + H2O + /O2 ^ 2NaOH + Cl2. (6)
The main advantage of the novel electrolysis with ODC is explained in Fig. 5, where the electrode
potentials are depicted as a function of current density. The thermodynamic potential difference for the classical electrolysis with hydrogen-evolving cathode is about 2.2 V. In practice the overall total cell voltage at typical current densities of 4-6 kA-m-2 including anode and cathode electrode over-potentials and ohmic resistances in membrane, electrodes and electrolyte is approximately 3.0 V. Theoretically, the cell voltage of chlor-alkali electrolysis can be reduced by 1.23 V through application of an ODC. However, due to higher overpotentials at the oxygen depolarized electrode, one can expect a gain of approximately 1.0 V at current densities of industrial relevance.
Fig. 4. Comparison of chlor-alkali electrolysis with hydrogen-evolving (left) and oxygen depolarized cathodes (right)
current density I kA rrf*
Fig. 5. Electrode potentials in chlor-alkali electrolysis as a function of current density
Catalysts, electrodes and electrolysers
As oxygen depolarized cathodes must allow for the intimate contact of liquid electrolyte, a catalyst and oxygen, they are porous fuel cell-type gas diffusion electrodes. The electrochemical reaction can only take place where the three reactants (oxygen from the gas phase, water from the liquid phase and electrons from the solid phase) meet. Direct contact of all reactants is only possible at the three-phase boundary that has a limited extension. However, the contact between the reactants is improved by dissolution of oxygen in the liquid electrolyte and diffusion to the active sites of the catalyst. It is evident that a gas diffusion electrode used as oxygen depolarized cathode has to meet several requirements for successful operation such as
- chemical stability in concentrated NaOH solution at temperatures of up to 90 °C;
- mechanical stability in large-scale electrolyzers with geometric areas of several m2;
- high electronic conductivity and low thickness;
- high surface area;
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- active electrocatalyst;
- hydrophobic/hydrophilic pore structure for good contact between oxygen and electrolyte;
- no breakthrough of gas and flooding by liquid, even at height-dependent pressure differences between gas and liquid;
- long-term stability and
- acceptable costs.
In principle, there are different ways to manufacture porous gas diffusion electrodes. Important components are the solid electrocatalyst, a polymeric component (usually PTFE) and a metallic current collector. These components can be uniformly distributed around the metallic current collector, while in other cases separate electrode layers with different functionality (reaction, diffusion) are employed (Fig. 4, right). The most important catalyst systems for oxygen reduction in alkaline solution are platinum, typically deposited on carbon carriers, and silver either as pure metal or also carbon-supported [1]. While the catalytic activity of both materials under chlor-alkali conditions is relatively similar [6], electrodes based on pure silver have the great advantage that no oxidation of carbon occurs. It has been shown in several investigations that carbon-based electrodes exhibit initially good performance but deteriorate considerably after several weeks or months (e.g. [7, 8]). Since an economically viable process for chlor-alkali electrolysis with ODC requires an electrode lifetime of several years, carbon-free silver systems with higher durability appear to provide the best prospects for the development of commercial electrodes.
The preparation of electrodes can be accomplished by "dry" or "wet" processes. During dry preparations a mixture of the catalyst and PTFE is ground to form particles that are distributed on the metallic current collector by calendaring or pressing. In wet preparation processes a paste or a suspension of the catalyst and PTFE, the stability of which can be improved by adding surface active agents, is employed. Pastes are then fixed on the current collector either by screen printing or by calendering, while less viscous suspensions are usually sprayed. In contrast to most gas-diffusion electrodes used for fuel cells, oxygen depolarized cathodes for chlor-alkali electrolysis are much larger with areas of up to several m2.
Standard electrolyzers for membrane chlor-alkali processes cannot be equipped with ODCs without modification. The reason is the relatively large hydrostatic pressure difference between oxygen and the thin electrolyte column between membrane and ODC with a height of 1 m or more. This might lead to a breakthrough of oxygen in the upper region of the cells while at the bottom NaOH solution could permeate through the porous cathode. Several solutions have been developed to overcome these difficulties. The most promising is the use of compartments on the oxygen side with independent adjustment of the pressure. A second concept is the use of so-called falling film electrolysis
cells, where the electrolyte is allowed to flow downward from the top end of the cell by gravity. Thus the hydrostatic pressure is compensated by an equally high counteracted hydrodynamic pressure drop.
Previous developments and current status
The idea of chlor-alkali electrolysis with oxygen depolarized cathodes is more than 50 years old. After the membrane chlor-akali process was commercialized in the 1970s first research and development programs for chlor-alkali electrolysis with ODC were initiated by many companies (see summary in [1]). Two research programs carried out by Eltech Systems in the USA and Hoechst in Germany lead to relatively advanced processes already during the 1980s. Eltech Systems used carbon-based fuel cell electrodes and carried out tests in laboratory scale and commercial size cells. Hoechst developed electrodes consisting of silver/PTFE mixtures which were successfully operated in a pilot cell at a cell voltage of 2.17 V under a current density of 3 kA/m2. Despite these relatively promising developments, the achieved savings in cell voltage of around 0.8-0.9 V were judged insufficient for an economic change to the new ODC technology at the then relatively low electric energy prices.
In the 1990s a second phase of research and development was started by the Japanese Ministry of Trade and Industry (MITI) in cooperation with the Japan Soda Industry Association (JSIA). The partners of this program developed electrodes and conducted electrolyzer tests at several companies such as Toagosei, Mitsui Chemical, and Kaneka. Very recently, Kaneka and Toagosei have reported promising results from their new chlor-alkali process. Demonstration tests started in 2007 have shown that the electricity consumption could be decreased by one third compared to the membrane process as current industry standard [9].
In Germany, a further program was started by Bayer in cooperation with Uhde and DeNora. Already in 1998, large-scale industrial electrolysis cells with elements of 2.7 m2 were equipped with ODCs and successfully operated. A first breakthrough for electrolysis processes with ODC was achieved when Bayer commercialized a novel HCl electrolysis process with ODC in cooperation with Uhde and DeNora in 2004. After commissioning of a smaller installation with 20 kt/a Cl2 in Brunsbuettel, Germany, a world-scale HCl-ODC electrolysis plant with a chlorine capacity of 215 kt/a went operational in Caojing, China, in 2008. Regarding chlor-alkali electrolysis with ODC, Bayer started a joint development project with several partners from industry and universities in 2006 that is funded by the German Ministry of Education and Research (BMBF) (www.klimazwei.de). The success of the project is highlighted by the recent announcement that a first industrial-scale facility with a chlorine capacity of 20 kt/a is scheduled to go on stream in Germany by 2011 (Bayer press release 11.11.2009).
Outlook
After intensive research and development over more than 30 years chlor-alkali electrolysis with oxygen depolarized cathodes is on its way to commercialization. This technology is a viable option for energy conservation in the chlor-alkali industry as it reduces the electric energy demand by 30%. Since rising electric energy prices are generally expected for the future, the economic benefit of this new technology will become even more pronounced. Moreover, reduced electricity consumption also decreases the corresponding carbon dioxide emissions making chlor-alkali electrolysis an environmentally more benign process.
Conclusions
Hydrogen utilization is the key factor for the development of more energy-efficient chlor-alkali electrolysis processes. The common strategy is hydrogen combustion in boilers that saves costs for the boiler fuel and reduces the overall energy demand for chlorine production. The use of hydrogen for production of electric energy in fuel cells could energetically improve the process. However, fuel cells are far from economic operation given the currently high investment costs for this technology. The most radical and promising solution for the hydrogen energy issue is the direct integration of fuel cell principles in the electrolysis process through application of so-called oxygen depolarized cathodes. In this way the formation of hydrogen as a co-product is avoided and the electric energy demand for the modified chlorine production process can be reduced by about 30%. Economic analysis reveals that ODC technology is profitable even at present electricity prices and the economic benefit may rise in case of higher electricity prices in the future. ODC technology requires the development of suitable gas-diffusion electrodes and adjusted electrolyzer designs. After intensive research and development in several countries over more than 30 years chlor-alkali electrolysis with oxygen depolarized cathodes is on its way to commercialization as the first industrial-scale facility is scheduled to go on stream in Germany by 2011.
Acknowledgement
Financial support by the German Ministry of Education and Research (BMBF) within the project "CO2 Reduction During the Production of Basic Chemicals" under the klimazwei research program is gratefully acknowledged.
References
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© Scientific Technical Centre «TATA», 2010
