University of Ghana http://ugspace.ug.edu.gh UNIVERSITY OF GHANA Optimization of Some Influence Factors in the Electrolytic Production of Sodium Hydroxide A thesis presented to the: Department of NUCLEAR ENGINEERING UNIVERSITY OF GHANA, LEGON By AGYEMANG WIREDU MARK (10301889) BSc Chemistry, 2014 In partial fulfilment of the requirements for the degree of MPHIL NUCLEAR TECHNOLOGY APPLICATIONS IN PETROLEUM AND MINING INDUSTRIES JULY, 2018 University of Ghana http://ugspace.ug.edu.gh DECLARATION I, AGYEMANG WIREDU MARK, the under signed, hereby declare that this thesis is my own work and to the best of my knowledge it has never been submitted for the award of a degree in any university. All sources of information used in this work have been duly cited and acknowledged. ............................................................................. AGYEMANG WIREDU MARK DATE: …………………………. …………………………………. …………………………………. DR. ANDREW NYAMFUL DR. K.A. DANSO (PRINCIPAL SUPERVISOR) (CO-SUPERVISOR) DATE: …………………………. DATE: ………………………….. ii University of Ghana http://ugspace.ug.edu.gh DEDICATION Nothing is impossible with God and to my dearest brother Dr. Michael Agyemang-Wiredu, you are God sent and a lovely brother. Finally, I would like to thank my wonderful sisters and brothers Ewurama Darkoa, Marian Wiredu, Isaac Wiredu and my lovely twin brothers Sampson and Samuel Asante for their support and endless encouragement throughout this process. God bless you all. iii University of Ghana http://ugspace.ug.edu.gh ACKNOWLEDGEMENTS First and foremost, my sincere thanks go to Almighty God who provided the life, support and academic enthusiasm needed to carry out this research. I also want to express my heartfelt gratitude to my project supervisors Dr. Andrew Nyamful and Dr. K.A. Danso and other lecturers at the Nuclear Engineering Department of University of Ghana especially Dr. Seth Debrah and Dr. Vincent Agbodomegbe whose technical expertise and tireless contributions ensured the success of my work. I also like to take this opportunity to show my gratitude to my father Mr. Paul Badu Wiredu and my dear mother Mrs. Mary Kwarteng for their support and understanding during the entire period that this noble academic exercise was undertaken. Last but not the least, I thank the staff of Ghana Atomic Energy Commission for the help they offered me throughout this academic exercise. In addition, I thank all my colleagues in the Department of Nuclear Engineering for all the constructive advice and criticisms. iv University of Ghana http://ugspace.ug.edu.gh ABSTRACT Sodium hydroxide is one of the most important chemicals in the global chemical trade. It is the main feedstock used in the manufacture of many products used in everyday life. The production of sodium hydroxide in a country is a measure of the development of its chemical industry. In this study, some factors that influence production of NaOH from NaCl using electrolytic cell were optimized. The optimum conditions for the production of NaOH were found to be 500 SAL brine strength, 6.0cm electrode gap, 5 electrodes in both anode and cathode compartments and 80V. The pH of the catholyte obtained at the optimum conditions was 13.88 at 360.3K, 2 hours and 5.40A. v University of Ghana http://ugspace.ug.edu.gh TABLE OF CONTENTS DECLARATION .................................................................................................................. ii DEDICATION .................................................................................................................... iii ACKNOWLEDGEMENTS ................................................................................................. iv ABSTRACT .......................................................................................................................... v TABLE OF CONTENTS ..................................................................................................... vi LIST OF TABLES ............................................................................................................ viii LIST OF FIGURES ............................................................................................................. xi LIST OF ACRONYMS ...................................................................................................... xii CHAPTER ONE ................................................................................................................... 1 INTRODUCTION ................................................................................................................ 1 1.1 Background of the study .............................................................................................. 1 1.2 Statement of the problem ............................................................................................ 3 1.3 Justification ................................................................................................................. 3 1.4 Main objective ............................................................................................................. 4 CHAPTER TWO .................................................................................................................. 5 LITERATURE REVIEW...................................................................................................... 5 2.1 Introduction ................................................................................................................. 5 2.2 The Membrane Cell ..................................................................................................... 7 2.2.1 Recent Advances in the Membrane Cell Process ................................................. 8 2.2.2 Comparison of the Membrane Cell Process, the Diaphragm Cell and Mercury Cell ............................................................................................................................... 10 2.3 The Diaphragm Cell .................................................................................................. 10 2.4 Mercury Cell .............................................................................................................. 12 CHAPTER THREE ............................................................................................................. 15 MATERIALS AND METHODS ........................................................................................ 15 3.1 Descriptive Features of the Salt Refining Plant. ....................................................... 15 3.2 Electrolytic Cell Setup ............................................................................................... 18 3.3 Purification of Seawater ............................................................................................ 20 2 3.3.1 Removal of Mg2+, K+ and SO4 ions from Seawater by Precipitation Method . 20 3.3.2 Removal of Mg2+and K+ ions ............................................................................. 20 2 3.3.3 Removal of SO4 ions ........................................................................................ 20 3.3.4 Filtration of the Precipitated Salts from the Seawater ........................................ 20 vi University of Ghana http://ugspace.ug.edu.gh 3.3.5 Percentage of Mg2+, K+ and SO 2 4 ions in the Precipitated Salts by EDAX Analysis ....................................................................................................................... 21 3.3.6 Production of High Grade Sodium Chloride Salt ............................................... 21 3.3.7 Determination of the Moiture Content of the High Grade Sodium Chloride Salt ..................................................................................................................................... 22 3.3.8 Recrystallization of the High Grade Sodium Chloride Salt ................................ 22 3.3.9 Determination of Percentage Purity of the High Grade Sodium Chloride Salt .. 23 3.4 Preparation of the Saltbrine ....................................................................................... 23 3.5 Experimental Protocol of Electrolysis ....................................................................... 24 3.6 Test for the Presence of Sodium Hydroxide.............................................................. 25 3.7 Quantitative Determination of Sodium Hydroxide. .................................................. 25 3.8 Percentage Yield of Sodium Hydroxide .................................................................... 26 CHAPTER FOUR ............................................................................................................... 28 RESULTS AND DISCUSSION ......................................................................................... 28 4.1 Effect of Brine Strength and Applied Voltage on the pH of NaOH. ......................... 28 4.2 Effect of Electrode Number on the pH of NaOH. ..................................................... 43 4.3 Effect of Gaps between the Electrodes on the pH of NaOH. .................................... 46 4.4 Effects of the Optimum Parameters on the pH of NaOH. ............................................. 49 4.4.1 Effect of Brine Strength ......................................................................................... 49 4.4.2 Effect of Gaps between the Electrodes ................................................................... 50 4.4.3 Effect of Electrode Number ................................................................................... 51 4.4.4 Effect of Voltage .................................................................................................... 52 CHAPTER FIVE ................................................................................................................. 53 CONCLUSION AND RECOMMENDATION .................................................................. 53 5.1 Conclusion ................................................................................................................. 53 5.2 Recommendation ....................................................................................................... 53 REFERENCES .................................................................................................................... 54 vii University of Ghana http://ugspace.ug.edu.gh LIST OF TABLES Table Page 2 Table 3. 1: Results showing Percentage Ionic Composition of Mg2+, K+ and SO4 . ......... 21 Table 3.4. 1: Sodium Chloride Brine Table. ....................................................................... 23 Table 4.1.1: Results of pH, Time, Temperature, Specific Gravity and Current at 300 SAL and at 80V. .......................................................................................................... 28 Table 4.1.2: Results of pH, Time, Temperature, Specific Gravity and Current at 400 SAL and at 80V. .......................................................................................................... 29 Table 4.1.3: Results of pH, Time, Temperature, Specific Gravity and Current at 500 SAL and at 80V. .......................................................................................................... 30 Table 4.1.4: Results of pH, Time, Temperature, Specific Gravity and Current at 600 SAL and at 80V. .......................................................................................................... 31 Table 4.1.5: Results of pH, Time, Temperature, Specific Gravity and Current at 700 SAL at 80V. ................................................................................................................ 32 Table 4.1. 6: Results of pH, Time, Temperature, Specific Gravity and Current at 300 SAL at 70V. ................................................................................................................ 32 Table 4.1. 7: Results of pH, Time, Temperature, Specific gravity and Current at 400 SAL at 70V. ................................................................................................................ 34 Table 4.1.8: Results of pH with Time, Temperature, Specific Gravity and Current at fixed Brine strength 500 SAL at 70V. .......................................................................... 34 Table 4.1.9: Results of pH, Time, Temperature, Specific Gravity and Current at 600 SAL at 70V. ................................................................................................................ 35 Table 4.1.10: Results of pH, Time, Temperature, Specific Gravity and Current at 700 SAL at 70V. ................................................................................................................ 35 viii University of Ghana http://ugspace.ug.edu.gh Table 4.1.11: Results of pH, Time, Temperature, Specific Gravity and Current at 30 0SAL at 60V. ................................................................................................................ 36 Table 4.1.12: Results of pH, Time, Temperature, Specific Gravity, Current and at 40 0SAL at 60V. ................................................................................................................ 36 Table 4.1.13: Results of pH, Time, Temperature, Specific Gravity and Current at 50 0SAL and at 60V. .......................................................................................................... 37 Table 4.1.14: Results of pH, Time, Temperature, Specific Gravity and Current at 60 0SAL and at 60 V. ......................................................................................................... 37 Table 4.1.15: Results of pH, Time, Temperature, Specific Gravity and Current at 70 0SAL and at 60V. .......................................................................................................... 38 Table 4.1.16: Results of pH, Time, Temperature, Specific Gravity and Current at 300 SAL and at 50V. .......................................................................................................... 38 Table 4.1.17: Results of pH, Time, Temperature, Specific Gravity and Current at 400 SAL and at 50V. .......................................................................................................... 39 Table 4.1.18: Results of pH, Time, Temperature, Specific Gravity and Current at 500 SAL and at 50V. .......................................................................................................... 39 Table 4.1.19: Results of pH, Time, Temperature, Specific Gravity and Current at 600 SAL and at 50V. .......................................................................................................... 40 Table 4.1.20: Results of pH, Time, Temperature, Specific Gravity and Current at 700 SAL and at 50V. .......................................................................................................... 40 Table 4.1.21: Results of pH, Time, Temperature, Specific Gravity and Current at 300 SAL and at 40V. .......................................................................................................... 41 Table 4.1.22: Results of pH, Time, Temperature, Specific Gravity and Current at 400 SAL and at 40V. .......................................................................................................... 41 ix University of Ghana http://ugspace.ug.edu.gh Table 4.1.23: Results of pH, Time, Temperature, Specific Gravity and Current at 500 SAL and at 40V. .......................................................................................................... 42 Table 4.1.24: Results of pH, Time, Temperature, Specific Gravity and Current at 600 SAL and at 40V. .......................................................................................................... 42 Table 4.1.25 Results of pH, Time, Temperature, Specific Gravity and Current at 700 SAL and at 40V. .......................................................................................................... 43 Table 4.2.1: Results showing variation of pH with Time at (500 SAL, 80V, 6.0 cm Electrode Gap, 2 electrodes in Anode and Cathode Compartments). ................ 43 Table 4.2.2: Results Showing Variation of pH with Time at (500 SAL, 80V, 6.0 cm Electrode Gap, 3 Electrodes in Anode and Cathode Compartments)................. 44 Table 4.2.3: Results Showing Variation of pH with Time at (500 SAL, 80V, 6.0 cm Electrode Gap, 4 Electrodes in Anode and Cathode Compartments)................. 45 Table 4.2.4 Results Showing Variation of pH with Time at (500 SAL, 80V, 6.0 cm Electrode Gap, 5 Electrodes in Anode and Cathode Compartments)................. 45 Table 4.3.1: Results Showing Variation of pH with Time at (500 SAL, 80V, 7.0 cm Electrode Gap, 5 Electrodes in Anode and Cathode Compartments)................. 46 Table 4.3.2: Results Showing Variation of pH with Time at (500 SAL, 80V, 8.0 cm Electrode Gap, 5 Electrodes in Anode and Cathode Compartments)................. 47 Table 4.3.3: Results Showing Variation of pH with Time at (500 SAL, 80V, 9.0 cm Electrode Gap, 5 Electrodes in Anode and Cathode Compartments)................. 48 Table 4.3.4: Results showing variation of pH with Time at (500 SAL, 80V, 10.0 cm Electrode Gap, 5 Electrodes in Anode and Cathode Compartments)................. 48 x University of Ghana http://ugspace.ug.edu.gh LIST OF FIGURES Figure Page Figure 2. 1: Diagram of an electrolytic cell. ......................................................................... 5 Figure 3.1. 1: A feed hopper for salt collection. ................................................................. 15 Figure 3.1. 2: A stainless steel container for receiving salt from feed hopper. ................... 16 Figure 3.1. 3: A screw conveyor for transporting salt into the washer. .............................. 16 Figure 3.1. 4: A filtrating pump for transferring separated salt from the centrifuge cell into a classifier....................................................................................................... 17 Figure 3.1. 5: A centrifuge cell for separating salt in the slurry from unwanted particles. 17 Figure 3.1. 6: A dryer for drying the refined salt (it has heat exchange column). .............. 18 Figure 3.1. 7: Silo for storing NaCl..................................................................................... 18 Figure 3.2. 1: Graphite electrodes electroplated with copper. ............................................ 19 Figure 4. 4.1: A graph of pH against brine strength............................................................ 49 Figure 4. 4.2: A graph of pH against gaps between the electrodes. .................................... 50 Figure 4. 4.3: A graph showing the trend in pH with increasing number of electrodes. .... 51 Figure 4. 4.4: A graph showing the trend in pH as voltage increases. ................................ 52 xi University of Ghana http://ugspace.ug.edu.gh LIST OF ACRONYMS DSA Dimensionally Stable Anodes ODC Oxygen Depolarised Cathode WEO World Energy Outlook xii University of Ghana http://ugspace.ug.edu.gh CHAPTER ONE INTRODUCTION This chapter outlines the role of sodium hydroxide in the chemical and alkaline fuel cell industry. It also delineates the objectives and justification of the research. 1.1 Background of the study Due to rapid industrial and population growth, the energy demand worldwide is increasing daily. According to IEA World Energy Outlook, the global population is expected to increase by 30% the next 25 years, where 80-90% of the increase is expected to be in developing countries (Singh et al., 2012). Fossil fuels provide 81% of the world's commercial energy supply (OECD, 2009). Although fossil fuels play a crucial role in the world energy market (Goldemberg, 2006), they are finite and non-renewable on a human scale and are limited physically and more stringently economically (Perez et al., 2014). The reserves of these conventional energy sources particularly oil, natural gas and coal are not only non-renewable but they are also depleting rapidly (Rabbani et al., 2014). Moreover, the main environmental challenge in our time is to avoid or reduce the impacts of global warming. Carbon dioxide is the main greenhouse gas and 70-75% of all carbon dioxide emissions is due to combustion of fossil fuels (Halvorsen et al., 1989). The total carbon dioxide produced as a result of combustion of fossil fuels since the beginning of the industrial revolution actually exceeds the observed increase in the atmosphere (Boden et al., 2009). There is a narrow window of time left to begin the long process of stabilizing greenhouse gas concentrations at a level that can avert devastating and irreversible impacts from climate change (O'Niell et al., 2002). According to world energy outlook (WEO) 2007, there is worldwide research into other reliable energy resources to replace fossil fuels and this is mainly due to the uncertainty surrounding the future supply of fossil fuels as well as 1 University of Ghana http://ugspace.ug.edu.gh the carbon dioxide emissions resulting from combustion of fossil fuels. For example US Department of Energy has proposed a transition plan from an oil based economy to a hydrogen economy by 2040. Germany has planned to obtain all required energy from hydro and renewables. Canada is heavily investing in renewables and offering different incentives to domestic and industrial users who are partially or fully meeting their energy demands by renewable energy (Rabbani et al., 2014). Fuel cell can provide clean energy and reduce the energy supply demand gap (Daghe et al., 2017). The use of fuel cells in both stationary and mobile power applications can offer significant advantages for sustainable energy (Alhassan et al., 2006). Also, by integrating the application of fuel cells in connection with renewable energy storage and production methods, sustainable energy requirement may be achieved (Melean, 2001). The fuel cell (for example oxy-hydrogen generator) technology has also proven its technical viability in several domains of application such as backup power generation and vehicle propulsion (Garche et al., 2015). Oxy-hydrogen is an enriched mixture of hydrogen and oxygen bonded together molecularly and magnetically (Brown, 1978). Since the oxy-hydrogen generator (alkaline fuel cell) uses sodium hydroxide solution as catalyst in its operation, the influencing factors affecting the yield of sodium hydroxide need to be investigated. Sodium hydroxide is among the top 10 chemicals manufactured in the world, and is involved in the production of several products that are used in everyday life (O’Brien et al., 2005). Sodium hydroxide is among two primary elements of 60 % of Europe’s chemical industry production (Siracusano et al., 2010). It is used in chemical industries and for manufacture of products such as glass, textiles, soap, detergents, paints, and ceramics (Saksono et al., 2013) and most importantly, as fuel supplement in alkaline fuel cells. It has also found applications in ovonic renewable hydrogen process for reformation of organic matter (Reichman et al., 2010) and also in hydrothermal gasification of glucose and other biomass 2 University of Ghana http://ugspace.ug.edu.gh to produce hydrogen gas (Onwudili et al., 2009). It is also used together with steam to pyrolyze dechlorinated PVC and activated carbon to generate sodium carbonate (Kamo et al., 2006). Knowledge of the influencing factors in electrolysis of brine is vital for improving the performance of electrolytic cells (Domga et al., 2016) for sodium hydroxide production. In this present work, electrolytic cell was designed and used to study the most important factors such as brine strength, gap between electrodes, number of electrodes and voltage and to investigate their effect on maximization of sodium hydroxide production. 1.2 Statement of the problem The consumption of fossil fuels spawns environmental considerations in addition to issues of energy demand and resource availability (Perez et al., 2014). At the global level, scientists warn that the combustion of fossil fuels is significantly changing the world’s climate system. In order to overcome the draw backs of the fossil fuels, it is the time to replace fossil fuels with electrolyte such as sodium hydroxide and be used in HHO generator to run engines etc. The performance of the HHO generator is based solely on the catalytic efficiency of the electrolyte material used; it has become more vital to investigate design parameters which could best be suitable in producing quality electrolyte from cheap alternative sources. 1.3 Justification With the increasing demand for energy worldwide, an appropriate technology which could give an unlimited supply of energy and at the same time not undermining the safety of the environment must be sort after. The brown gas (oxy hydrogen) has by far proved to be suitable in replacing the diminishing fossil fuels. The performance of the oxy hydrogen 3 University of Ghana http://ugspace.ug.edu.gh generator rests solely on the catalytic efficiency of the electrolyte material (sodium hydroxide) used in its operation; it has therefore become more imperative that the engineering parameters influencing the yield of sodium hydroxide be investigated. 1.4 Main objective To construct an electrolytic cell and then using it to produce NaOH from purified saltbrine. The specific objectives are 1. To construct a rectifier and cathode and anode compartments. 2. To produce high quality NaCl from purified seawater. 3. To optimize parameters such as brine strength, gap between electrodes, number of electrodes and voltage and to study their influence on the electrolytic production of NaOH from NaCl. The next chapter presents some pertinent literature on chlor-alkali technologies. 4 University of Ghana http://ugspace.ug.edu.gh CHAPTER TWO LITERATURE REVIEW This chapter describes the various technologies used in chlor-alkali production. It also outlines some recent improvements in the chlor-alkali industry. 2.1 Introduction Sodium hydroxide is produced by electrolysis process. Electrolysis is a process whereby the bonds in a compound are separated by passing an electric current through it (Saksono et al., 2013). Sodium hydroxide is produced in an electrolytic cell (Domga et al., 2016). The electrolytic cell (Figure 2.1) is a vessel filled with electrolytes (which are separated by an ion exchange membrane) in which the electrodes are immersed and connected electrically to a power supply (Cardarelli et al., 2008). Figure 2. 1: Diagram of an electrolytic cell. During electrolysis of brine, NaCl is dissociated in the solvent and forms ions (Na+ and Cl- ions) in solution. The positive electrode is called anode and negative electrode is called cathode. Each electrode attracts ions which have opposite charges so that ions with negative charges are attracted to the anode compartment and ions with positive charges are attracted to the cathode compartment (Saksono et al., 2013). Chloride ions (Cl-) which are discharged 5 University of Ghana http://ugspace.ug.edu.gh at the anode compartment, are oxidised to chlorine gas at the anode compartment. Hydrogen ions (H+) produced at the cathode compartment, are reduced to hydrogen gas at the cathode compartment. Sodium ions (Na+) discharged at the anode compartment, migrate to the cathode compartment and combine with hydroxyl ions (OH-) to form sodium hydroxide in the cathode compartment (Domga et al., 2013). The overall reaction taking place in the electrolytic cell proceeds through the following reactions: At the anode: Cl-→Cl -2 + 2e . At the cathode: 2H2O + 2e - → 2OH- + H2. Overall reaction: 2H2O +2NaCl → Cl2 + 2NaOH + H2. One challenge in the overall reaction is the evolution of oxygen gas (Hansen et al., 2010) which occurs due to direct anodic oxidation, chlorate formation and lastly decomposition of hypochlorite which compete with the reaction leading to the formation of chlorine gas and this eventually results in lowering the current efficiency from 1-5% (Domga et al., 2016). 2H2O + 2e - → O2 + 4H + + 4e-. 6ClO- + 3H2O → ClO3 +4Cl - + 6H+ + 1.5O2 + 6e -. 2HOCl → 2HCl + O2. Usually selective electrodes and anolyte acidification are measures used to reduce the oxygen side reaction to allow a current efficiency of about 99% (Karlsson et al., 2015). In order to increase the current efficiency (by reducing the oxygen side reactions) in the electrolytic production of sodium hydroxide, all modern cells since the 1970's use Dimensionally Stable Anodes (DSA). The DSA is an industrial electrode which is made of RuO2 as an active component (Chandler et al., 1997), inert oxide such as TiO2 as stabilizer and coated onto titanium substrate (Malpas et al., 2006). The main processes used for 6 University of Ghana http://ugspace.ug.edu.gh production of sodium hydroxide are diaphragm, mercury and membrane cell electrolysis. The difference in these cells lies in the manner in which the sodium hydroxide and chlorine gas are prevented from missing with each other in order to obtain pure products (Domga et al., 2016). The metal used as cathode is steel in the case of a diaphragm cell, nickel in membrane cell and mercury in mercury cell (Siracusano et al., 2010). In the membrane cell, there are two types of electrolyzers namely monopolar and bipolar. In the monopolar type, the anode and cathode are arranged electrically in parallel whereas in the bipolar the cathode of a cell is connected to the anode of an adjoining cell, so that the cells are in series and each electrolyzer has its merits and demerits (Zeng et al., 2010). Despite being an old technology, the membrane process is influenced by membrane suppliers (Ana, 2005). The main problem associated with these three processes is the high consumption of electrical energy which usually represents a substantial part of the production cost (Stantorelli et al., 2009). Also, there are only five major suppliers of cell technology and they are: Asahi Kasei Corporation, Chlorine Engineers, Eltech Systems Corporation, INEOS Chlor and Uhde (O'Brien et al., 2005). 2.2 The Membrane Cell Due to incidents of mercury poisoning in Minamata and Niigata in 1972, many countries for example Japan started switching to membrane cells and today has no more mercury cells in operation (O'Brien et al., 2005). The power consumption in the membrane cell is the lowest compared to the three technologies (Wiberg, 2001). However, certain industries such as the rayon industry consider the sodium hydroxide quality from membrane cells to be slightly inferior to that produced via the mercury cell method due to the higher chloride and chlorate content. The key principle of the membrane process is the selective permeability of the membrane. The membrane allows only specific components to permeate through it 7 University of Ghana http://ugspace.ug.edu.gh (Lakshmanan et al., 2013). The actual transport processes occurring within the membrane are yet to be understood fully (Grotheer et al., 2006). Research is on-going in an effort to better understand how these membranes work so as to tailor make membranes with highly selective permeability and exhibiting low electrical resistance (Grotheer et al., 2006). The ion exchange membrane separates the cathode side from the anode side. The membrane currently used in industry is comprised of two layers to be able to withstand the significantly different conditions on either side of it. The two layers are made of per fluoro sulphonic acid and per fluoro carboxylic acid films (Lakshmanan et al., 2013). A saturated brine solution that has been purified is channeled to the anode compartment while demineralized water is fed to the cathode compartment. During the electrolysis, sodium ions and some water permeates through the membrane and enter the cathode compartment. Chlorine gas is formed in the anode compartment. In the cathode compartment, water molecules are electrolyzed and release hydrogen gas. The hydroxyl ions combine with the sodium ions that permeate through the membrane to form sodium hydroxide. The depleted brine that leaves the anode compartment is sent for processing and re-concentration before it is fed back to the anode compartment. In the cathode compartment, only 32% of the sodium hydroxide concentration is taken out for further processing, while demineralized water is continuously fed to the cathode compartment to maintain the sodium hydroxide concentration in the compartment at 30%. Based on current technology, the highest sodium hydroxide concentration that can be produced in the membrane electrolyser is 35% (Moorhouse 2001). 2.2.1 Recent Advances in the Membrane Cell Process Several developments have come into play to enhance the membrane cell process. For example Dupont developed N2030/NE2040 membranes which exhibit lower voltage drop. 8 University of Ghana http://ugspace.ug.edu.gh Akzo Nobel developed a nano-filtration method for removing sulphates from brine prior to electrolysis. Lanxess developed new smaller resins with greater adsorption capacity, extending resin on-line time by 50-100%. Uhdenora reduced power consumption in the electrolyser from 2130 kWhr/t in 2004 to 2035 kWhr/t in 2012 via version 6 BM2.7 electrolyser. Uhdenora again teamed up with Bayer to develop and test the NaCl-ODC with significant power reduction of 25% (Lakshmanan et al., 2013). One of the significant developments in the chlor-alkali industry is the oxygen-depolarized cathode (ODC) process in which oxygen is introduced into the electrolysis chamber via a porous cathode. This process yields a significant reduction in electrical power consumption of 30%, with hydrogen no longer evolved from the cathode. Instead of hydrogen, oxygen is released from the cathode of ODC cells (Kiros et al., 2008). Initial studies indicate that the existing membrane plants cannot be modified to the ODC process due to differences in operating conditions and use of the special cathodes (Chlistunoff, 2005). A demonstration electolyzer with a capacity of 20,000 tons per annum of chlorine has been operational for Bayer at its site in Uerdingen since May 2011. Research work is ongoing to determine types of electrode coatings to be used (Moussallem et al., 2012). The ODC process has also been put to commercial use for electrolysis of HCl to produce chlorine, with significant savings in electrical costs. Here too, oxygen is introduced at the cathode and no hydrogen is evolved. This is a significant development given that in most synthesis reactions using chlorine, one major by-product is hydrochloric acid. The use of the ODC process helps recover chlorine from HCl. 9 University of Ghana http://ugspace.ug.edu.gh 2.2.2 Comparison of the Membrane Cell Process, the Diaphragm Cell and Mercury Cell There are distinct advantages in using the membrane cell technology. The polluting nature and environmental concerns associated with the other technologies, has resulted in all new chlor-alkali plants being built to be of the membrane cell process (Lakshmanan et al., 2013). . The membrane cell does not use harmful substances such as mercury or asbestos and hence there can be no potential release of these materials into the product or the environment. However, higher quality of salt and brine are required in the membrane process which results in higher cost. The membrane cell consumes less electrical power compared to mercury and diaphragm cells. The sodium hydroxide produced in the membrane cell may contain higher chlorate levels than in the mercury and diaphragm cell process. Also, the production of per fluoro carboxylic acid and per fluoro sulphonic acid in the membranes do result in the release of pollutants (Karrman et al., 2010). 2.3 The Diaphragm Cell It was in 1851 that the combined processing of sodium hydroxide and chlorine came about in what is today known as the diaphragm cell process. The Griesham diaphragm cell went into operation in Germany in 1890 (Schneiders et al., 2001). The diaphragm cell underwent continuous improvements until 1920 when asbestos became the major material used for the diaphragm and remained the major material used for the following 80 years (O'Brien et al., 2005). In the diaphragm cell process, asbestos were used to separate the sodium hydroxide from the chlorine (Euro Chlor, 2007). The diaphragm cell produced sodium hydroxide solution of around 12%, but its major drawback was the high salt content (Lakshmanan et al., 2013) in its sodium hydroxide. Platinum or magnetite were the early material used for the anodes of the diaphragm cells. However due to cost considerations as well as limitation 10 University of Ghana http://ugspace.ug.edu.gh on the current density when using magnetite, graphite became the major anode material until the 1970’s. It was in the 1970’s that a titanium base coated with a catalytic layer comprising of mixed oxides was developed (Lakshmanan et al., 2013). They were successfully used with long life and low cell voltage. These anodes became known as Dimensionally Stable Anodes and were used by all the technology suppliers (Lakshmanan et al., 2013). There were developments made to improve the asbestos diaphragms such as mixing asbestos with polytetrafluoroethylene which produced a dimensionally stable diaphragm (O'Brien et al., 2005). Then came a stabilized diaphragm, which had a polymer added into the asbestos slurry, and produced a diaphragm that did not swell (IPPC, 2001). This was followed by the development of bipolar diaphragm cells with one side as the anode and the other side as the cathode (Lakshmanan et al., 2013). They were assembled together into a cell and tightened in place with tie rods which made them appear to look like plate and frame filter presses. The reactions occurring in the diaphragm cell are as follows: At the anode: 2Cl- → Cl2 + 2e - 2NaCl → 2Na+ + 2Cl- At the cathode: 2H+ + 2e- → H2 2Na+ + 2OH- → 2NaOH As the sodium hydroxide concentration increases at the anode, there is a greater likelihood for it to be discharged into oxygen and water (Wiberg, 2001). For this reason, diaphragm grade sodium hydroxide is only produced around 12-15% concentration in the electrolytic cell. The height of brine in the anode compartment is maintained above the level in the cathode side to enable brine to slowly permeate through the diaphragm (Varjian, 2003). Saturated brine is fed to the anode compartment, where electrolysis of the brine takes place. Chlorine is formed at the anode. While sodium ions travel through the diaphragm, some sodium chloride are also transported along with it. At the cathode, hydrogen gas is formed 11 University of Ghana http://ugspace.ug.edu.gh and the weak sodium hydroxide containing around 14% sodium chloride is sent for further processing. The solution is then channeled through three or four evaporation processes to achieve a concentration of 50% (Lakshmanan et al., 2013). The salt crystallizes out on saturation of the sodium hydroxide and it is separated from it via crystallization and filtration and reused in the process. The quality of the sodium hydroxide from diaphragm cells is considered to be low due to its high salt content of up to 1.3% and high sodium chlorate content of 0.3% with other impurities also at levels exceeding that from the other electrochemical processes (Lakshmanan et al., 2013). The sodium hydroxide produced from the diaphragm cell is also known as diaphragm cell grade and finds application in industries without stringent quality requirements. 2.4 Mercury Cell The Castner-Kellner mercury cell went into operation in 1892 (IPPC, 2011). The process within a mercury cell differs somewhat from that in a diaphragm cell. The mercury cell is composed of an electrolyzer and a denuder or sometimes called a decomposer (O'Brien et al., 2005). The reactions in the electrolyzer are as follows: Anode: 2Cl‾ → Cl2 + 2e‾ Cathode: 2Na+ + 2Hg + 2e‾ → 2Na-Hg Overall Reaction: 2NaCl + 2Hg → 2Na-Hg + Cl2 Mercury acts as the cathode in this reaction and also forms an amalgam with sodium. Mercury is not consumed in the process. This amalgam is reacted with water in the denuder to produce sodium hydroxide at the desired concentration. That sodium hydroxide of 50% concentration can be produced directly from the denuder, without any need for further concentrating it (Euro Chlor, 1998). In the denuder the following reactions occur: 12 University of Ghana http://ugspace.ug.edu.gh 2Na-Hg → 2Na+ + 2Hg + 2e‾ 2H2O + 2e ‾ → 2OH‾ + H2 Overall reaction: 2Na-Hg + 2H2O → 2NaOH + H2 + 2Hg At the outlet of the decomposer, the sodium hydroxide which is at 50% strength does not require further processing. The mercury cell process is known to produce the highest quality caustic soda, but its major drawback is that its products has some mercury present (Wiberg 2001). Mercury levels are typically below 1ppm while sodium chloride content is typically below 10ppm, and sodium chlorate content is below 1ppm. The sodium hydroxide produced from this process is referred to as Mercury Cell Grade or Rayon Grade. This is because the production of rayon prefers sodium hydroxide such as that produced via the mercury cell process (O'Brien et al., 2005). Due to its polluting nature and the move away from mercury, there are no more new mercury cells being installed in the world. There are still a few old mercury cell units operating, but due to calls for greener technologies, their days are numbered. Diaphragm technology made up 15% of the capacity, with 39% coming from membrane technology. As at the end of 2011, mercury process has dropped to 31%, diaphragm process to 13% and the membrane process has increased to 52% (IPPC, 2011). Globally, the number of mercury process plants have reduced from 92 plants in 2002 with a capacity of 9million mt/yr down to 57 plants in 2010 with 5.5million mt/yr capacity (Euro Chlor, 2011). There is a voluntary commitment to phase out the use of the mercury cell process by 2020 by European manufacturers (Euro Chlor, 2012). The chlor-alkali industry claims that the levels of mercury from sodium hydroxide manufacture have reduced substantially over the years, and that the industry is now a minor contributor to environmental mercury levels (Singh, 2010). One of the points of concern from mercury cells is the residual levels of mercury (typically 0.5ppm and below) present in products from the cell (Dufault et al., 2009) and the use of mercury containing chemicals for potential 13 University of Ghana http://ugspace.ug.edu.gh microbial action that converts mercury to methyl mercury which is highly toxic (Gochfeld, 2003). Other disadvantages of the mercury cell process are the potential hazards from exposure to mercury and mercury vapours which are a known neurotoxin. They also require larger floor space than diaphragm cells and membrane cells and consume more energy for the production of chlor alkali products (Lakshmanan et al., 2013). Some former mercury process chlor-alkali plant sites show signs of soil contamination with mercury. A lot of work are required to remove the mercury from such soils. One such site is in Botany, Australia which operated a mercury process chlor-alkali plant on that site from 1944 to 2002 before switching to the membrane cell process. The soil at the plant and surrounding sites have been found to be contaminated with some elemental and some dissolved mercury leaving a bad legacy for chemical industry and requiring costly remedial measures (Golder Associates Pty Ltd 2012). Some former mercury based chlor-alkali sites in Europe have shown signs of contamination and require clean up (Bernaus et al., 2006). Some former mercury based chlor-alkali plants such as Saltville in Virginia, and Lavaca Bay in Texas have been classified as Superfund sites (Rule et al., 1998). The next chapter describes the materials and methods used in the research. 14 University of Ghana http://ugspace.ug.edu.gh CHAPTER THREE MATERIALS AND METHODS This chapter presents materials and methods used in this work. 3.1 Descriptive Features of the Salt Refining Plant. The salt refining plant (Model number GYSR-2000, made by Shanghai Genyond Technology Co., Ltd in China) consists of a feed hopper (Fig.3.1.1) in which salt of volume of about 3m3 is fed manually using a shovel. Figure 3.1. 1: A feed hopper for salt collection. The feed hopper has a rectangular shape and made of stainless steel with length 65.0cm, breadth 40.0 cm and height 18.0cm and contains a grate that has a rectangular shape of length 20.0cm, breadth 15.0cm and height 10.0cm that separates (through agitation of a vibrator) the smaller salt crystals (0.1 - 0.2mm) from the bigger ones (0.25 – 0.40mm) into a rectangular stainless steel container of length 250.0cm, breadth 125.0cm and height 85.0cm (Fig.3.1.2). 15 University of Ghana http://ugspace.ug.edu.gh Figure 3.1. 2: A stainless steel container for receiving salt from feed hopper. The salt collected in the stainless steel container (upgraded salt) is transferred through a screw conveyor (Fig.3.1.3) into a washer and classifier equipment. Figure 3.1. 3: A screw conveyor for transporting salt into the washer. The washer and classifier equipment are made of stainless steel. The washer has valves and nozzles for adjusting the brine flow rate as well as the direction of the salt slurry. The classifier equipment is rectangular in shape and has length 15.0 cm, breadth 9.5 cm and height 6.5 cm. The classifier has a sieve embedded in it and it separates the salt particles according to their sizes by agitation. From the washer and the classifier, the salt slurry goes into a centrifuge cell for the salt and other unwanted solid substances to be separated. A 16 University of Ghana http://ugspace.ug.edu.gh filtrating pump (Fig.3.1.4) transfers the separated salt obtained from the centrifuge cell (Fig.3.1.5) into another classifier made of the same material and same dimensions as described earlier. Figure 3.1. 4: A filtrating pump for transferring separated salt from the centrifuge cell into a classifier. Figure 3.1. 5: A centrifuge cell for separating salt in the slurry from unwanted particles. In the classifier, there are sets of valve for adjusting the brine flow rate. From the classifier, the salt moves into a salt dryer for drying through heating; and salt crystals formed, are reduced to desired particle sizes in a grinding mill. 17 University of Ghana http://ugspace.ug.edu.gh Figure 3.1. 6: A dryer for drying the refined salt (it has heat exchange column). Figure 3.1. 7: Silo for storing NaCl. 3.2 Electrolytic Cell Setup The anode compartment comprised a brine solution in a cylindrical plastic container of height 18.5cm and diameter 7.0cm in which stood a cylindrical graphite rod (as electrode) of length 30.0cm and diameter 1.0 cm held vertically through a 1.1cm diameter hole made in a circular aluminium plate of diameter 12.5cm and thickness 1.0cm placed on top of the cylindrical plastic container. The cathode compartment comprised distilled water in a cylindrical plastic container in which stood a cylindrical graphite rod (as electrode) of length 30.0cm and diameter 1.0cm held vertically through a 1.1cm diameter hole made in a circular 18 University of Ghana http://ugspace.ug.edu.gh aluminium plate of dimensions as described in the case of the anode. The cylindrical plastic containers of the two compartments was placed on a horizontal plane surface and were connected by a salt bridge. The salt bridge was placed horizontally at a distance 4.5cm vertically down from the top of any of the cylinders. The salt bridge was made of Perspex glass of bore diameter 1.0cm and length 6.0cm and had been impregnated with an absorbent cotton wool that had been dipped in the brine solution. The electrode in the anode compartment was connected to the positive terminal of a rectifier. Similarly, the electrode in the cathode compartment also connected to the negative terminal of the rectifier using 16.0mm flexible copper wires. The voltage range of the rectifier was between 0 and 400V inclusive. Figure 3.2. 1: Graphite electrodes electroplated with copper. The graphite electrodes were manufactured by Qingdao Tennry Carbon Co., Ltd, professional manufacturer of graphite products in Shandong Province in China. 19 University of Ghana http://ugspace.ug.edu.gh 3.3 Purification of Seawater 2 3.3.1 Removal of Mg2+, K+ and SO4 ions from Seawater by Precipitation Method 3.3.2 Removal of Mg2+and K+ ions 8 cm3of 15 %w Na2HPO4 solution were added to 1000 cm 3 of seawater of pH 7.0 (obtained from Pambros Salt Industry at Weija, Accra) being stirred at 100rpm. The pH of the mixture was adjusted to 9.0 using 2N NaOH solution. After 4 minutes, 2 cm3 of the 15 %w Na2HPO4 solution were added gradually to the mixture and the pH of the mixture adjusted to 9.0 as described earlier. This process was repeated every 4 minutes till total volume of 15 %w Na2HPO4 solution added was 16 cm 3 maintaining the pH of the mixture at 9.0. MgCl2(aq) + KCl(aq) + Na2HPO4(aq) + NaOH(aq) → MgKPO4↓(s) + 3NaCl(aq) +H2O(l). 2 3.3.3 Removal of SO4 ions 10 cm3 of 15% w CaCl2 solution were added to the mixture obtained in 3.3.2 above and the pH of the mixture adjusted to 9.0 using 2N NaOH solution. After 4 minutes, 5 cm3 of 15% w CaCl2 solution were added steadily to the mixture and the pH of the mixture adjusted to 9.0. This process was repeated every 4 minutes till total volume of 15% w CaCl2 solution added was 30 cm3 maintaining the pH of the mixture at 9.0. MgSO4(aq) + CaCl2(aq) → CaSO4↓(s) + MgCl2(aq). 3.3.4 Filtration of the Precipitated Salts from the Seawater The mixture obtained in 3.3.3 above was filtered and the residue was air dried to constant weight. The dried residue was subjected to EDAX analysis to determine percentage of Mg2+, 20 University of Ghana http://ugspace.ug.edu.gh 2 K+ and SO4 ions removed using modification of the method described by (Pujiastuti et al., 2015). 2 3.3.5 Percentage of Mg2+, K+ and SO4 ions in the Precipitated Salts by EDAX Analysis The EDAX analysis showed that the precipitated salts consisted of 13.9% Mg2+ ions, 2.1% + 2K ions and 28.4% SO4 ions similar to the results obtained by (Pujiastuti et al., 2015). 2 Table 3.1 Results showing Percentage Ionic Composition of Mg2+, K+ and SO4 . Mg2+ K+ SO24 13.9% 2.1% 28.4% 3.3.6 Production of High Grade Sodium Chloride Salt Sea water was concentrated through evaporation in a stainless steel tank of volume of about 1000 cm3. The concentrated sea water was then heated to a temperature of 1500C to obtain impure sodium chloride crystals (unrefined sodium chloride). The unrefined sodium chloride was ground to a desired solid particle size by a grinding mill. The ground unrefined sodium chloride salt was loaded into the feed hopper. The feed hopper was agitated by a vibrator which caused separation of the fine solid unrefined salt crystals from the bigger salt crystals through the grate into the stainless steel container. The fine solid unrefined salt crystals were transported by a conveyor belt into a washer compartment for a refined sodium chloride salt to be produced. In the washer, fine and loose particles of impurities such as gypsum and fine sand were separated from the impure salt by counter current effects with upward flowing brine. The quality of the refined sodium chloride salt produced depends on 21 University of Ghana http://ugspace.ug.edu.gh the efficiency of the brine flow. From the washer, the refined sodium chloride salt was pumped into a solid-liquid classifier for a pure solid-liquid sodium chloride to be produced. The mass of the pure solid-liquid sodium chloride produced was measured before drying. The solid-liquid sodium chloride was placed in a stainless steel container and heated by an electric heater to a temperature of about 150 0C for pure sodium chloride crystals to be obtained. The mass of the pure sodium chloride crystals was measured using a mass balance; and the produced sodium chloride salt was analysed for moisture content. 3.3.7 Determination of the Moiture Content of the High Grade Sodium Chloride Salt Mass of high grade NaCl produced before drying (M0) ꞊ 250kg. Mass of high grade NaCl produced after drying (M1) ꞊ 249.25kg. 250  249.25 Hence % moisture content of the high grade NaCl salt ꞊  0.3% 250 3.3.8 Recrystallization of the High Grade Sodium Chloride Salt 400g of the produced sodium chloride salt was placed in a beaker mounted on a heating mantle. 150 ml of distilled water was added to the salt just to melt it whilst being stirred and allowed to boil (100 0C) for 15 minutes. The beaker containing the boiled salt solution was covered and then placed undisturbed in a water bath containing ice cubes for 30 minutes. The settled upgraded solid sodium chloride salt was separated from the sodium chloride salt solution through filtration. The mass of the solid sodium chloride salt measured was 399.3g. The recrystallized salt produced (396.2g) was recrystallized again by the method described above and the mass of recrystallized salt (second recrystallization) was 395.1g. The process was repeated three times and the total mass of recrystallized NaCl salt obtained was 1186.1g. 22 University of Ghana http://ugspace.ug.edu.gh Total mass of salt used in the recrystallization process was 1200g. The total mass of sodium chloride produced after recrytallization was 1186.1g. 3.3.9 Determination of Percentage Purity of the High Grade Sodium Chloride Salt The percentage purity was calculated as mass of dreid NaCl to the mass of the NaCl used in 1186.1g the recrystallization process. Percentage Purity of the high grade NaCl = 100 = 1200g 98.84%. 3.4 Preparation of the Saltbrine Thirty degrees salometer (300 SAL) brine strength was prepared by dissolving 85.7g of sodium chloride salt of purity 99.9% in a pure distilled water; and stirred to dissolve the salt completely and the total volume made to 1000ml by modifying the method described by (Hilderbrand et al., 2013). A salometer was used to check the value of the brine strength prepared. The procedure was repeated for 400, 500, 600 and 700 SAL by dissolving sodium chloride salt of masses 117.7, 151.6, 187.8 and 226.1g respectively in distilled water. Table 3.4. 1: Sodium Chloride Brine Table. Salometer Degrees (⁰SAL) % NaCl by weight g/L NaCl Specific Gravity 30 7.919 85.7 1.058 40 10.558 117.7 1.078 50 13.198 151.6 1.098 60 15.837 187.8 1.118 70 18.477 266.1 1.139 23 University of Ghana http://ugspace.ug.edu.gh 3.5 Experimental Protocol of Electrolysis A pure distilled water of volume 500ml whose temperature 301.3K and pH=7.0 had been measured using a Bench-top pH meter was placed in the cylindrical plastic container of the cathode compartment of the electrolytic cell described earlier. An equal volume of 300 SAL brine strength whose specific gravity 1.078 had been measured using a hydrometer was placed in the cylindrical plastic container of the anode compartment. The length of the salt bridge (electrode gap) connecting the two plastic containers was 6.0cm. Each compartment of the electrolytic cell carried one graphite electrode (total surface area of each electrode was 95.77cm2). The electrodes were connected to the rectifier as described earlier. The voltage of the rectifier was set to 80V; corresponding current 0.15A registered was recorded at time 0 min. Temperature, pH of the distilled water and the specific gravity of the saltbrine as well as current were recorded every 30 mins till a constant pH value was obtained. The process was repeated for 400, 500, 600 and 700 SAL brine strengths at 80V and 6.0 cm electrode gap; 300, 400, 500, 600 and 700 SAL at 70V and 6.0cm electrode gap; 300, 400, 500, 600 and 700 SAL at 60V and 6.0cm electrode gap; 300, 400, 500, 600 and 700 SAL at 50V and 6.0cm electrode gap; 300, 400, 500, 600 and 700 SAL at 40V and 6.0cm electrode gap. The highest pH = 13.84 of the catholyte at 348K was obtained in 4hours and this corresponded to 500 SAL, specific gravity 1.028 of the anolyte at 80V and the registered current was 3.01A. The optimization process continued maintaining 500 SAL, 80V and 6.0 cm electrode gap. The electrode number of each compartment was increased by one rod (electrode) after each experiment till each compartment had five electrodes. The highest pH = 13.88 of the catholyte at 360.2K was obtained at 5 electrodes in 2 hours and this corresponded to specific gravity 1.086 of the anolyte and the current registered was 5.39A. The process continued maintaining 500 SAL, 80V, 5 electrodes in each compartment and the electrode gap was 24 University of Ghana http://ugspace.ug.edu.gh increased from 6.0 cm to 10.0 cm by an increment of 1.0 cm after each experiment. The highest pH = 13.88 of the catholyte at 360.3K was obtained at 6.0cm electrode gap in 2 hours and this corresponded to specific gravity 1.089 of the anolyte and the current registered was 5.40A. 3.6 Test for the Presence of Sodium Hydroxide 5 ml of 1.5M ammonium nitrate solution were mixed with 5 ml of 0.5M silver nitrate solution. 10 ml of 5% dextrose solution were poured into a Florence flask that had been rinsed with acetone and had been allowed to air dry completely. To the dextrose solution in the Florence flask, 10ml of the combined mixture of ammonium nitrate solution and silver nitrate solution prepared above were added. 10 ml of 10% catholyte solution were added to the solution in the Florence flask and the flask stoppered. The stoppered flask was gently swirled to mix the contents and the final solution formed was made to come into contact with the inner surface of the flask including the neck by rotating the flask continuously whilst tilted. Bright and shiny silver ‘’mirror’’ formed within two minutes in the inner surface of the flask indicating the presence of sodium hydroxide in the catholyte solution following the method described by (Batavia, 2004). 3.7 Quantitative Determination of Sodium Hydroxide. 20 ml of the catholyte solution of pH 13.88 were titrated against 0.952M HCl solution using methyl orange as indicator. The average volume of HCl solution used was 9.67 ml. Mass of sodium hydroxide produced was 0.368g. 25 University of Ghana http://ugspace.ug.edu.gh 3.8 Percentage Yield of Sodium Hydroxide Actual Yield of NaOH Total volume of the catholyte solution was 500ml. Average volume of 9.67ml of HCl(aq) solution of 0.952M were titrated against 20ml of the catholyte solution using methyl orange as an indicator. Mass of NaOH(aq) obtained in the 20ml of the catholyte solution was 0.368g. Therefore total mass (actual yield) of NaOH(aq) in the 500ml of the catholyte solution was 9.200g. Theoretical Yield of NaOH According to Faraday’s first law of electrolysis, m ꞊ ZIt, where Z ꞊ electrochemical equivalent of sodium (2.38 × 10-4 g ̸ C). I ꞊ Current (5.40A). t ꞊ time in secs (7200s) . m ꞊ mass of Na produced at the cathode compartment ꞊ 2.38 ×10-4 × 5.40 × 7200 ꞊ 9.253g. Dissociation of NaOH NaOH(aq) → Na + − (aq)+ OH(aq) 9.253 Moles of Na+(aq) ꞊ ꞊ 0.402 moles. 23 Hence, moles of NaOH(aq) in the catholyte solution ꞊ 0.402 moles. Therefore mass of NaOH(aq) (theoretical yield) in the catholyte solution ꞊ 0.402 × 40 ꞊ 16.08g. Actual yield Percentage yield of NaOH(aq) ꞊ × 100. Theoretical yield 9.2 Percentage yield of NaOH(aq) ꞊ × 100 ꞊ 57.22%. 16.08 26 University of Ghana http://ugspace.ug.edu.gh The next chapter shows results obtained from the experimental work and sets out to explain the conditions under which the desired product is maximum. 27 University of Ghana http://ugspace.ug.edu.gh CHAPTER FOUR RESULTS AND DISCUSSION 4.1 Effect of Brine Strength and Applied Voltage on the pH of NaOH. As set out in the objective of the study, the quality of NaCl produced was determined by measuring the pH of the NaOH obtained. Table 4.1.1 to 4.1.5 show results obtained for varying brine strengths at 80V, 6.0 cm electrode gap and 1 electrode in both anode and cathode compartment. Table 4. 1.1: Results of pH, Time, Temperature, Specific Gravity and Current at 300 SAL and at 80V. pH Current % Weight Time (s) Temperature Specific Current density (A) of NaCl (K) gravity (A/cm2) 7.05 0.15 7.919 0 301.3 1.078 0.188 7.10 0.24 1800 301.7 1.022 0.300 7.66 0.28 3600 301.8 1.018 0.350 8.16 0.35 5400 301.9 1.013 0.440 9.67 0.42 7200 303.4 1.011 0.523 10.23 0.56 9000 307.1 1.009 0.700 10.43 0.62 10800 308.0 1.005 0.775 12.55 0.68 12600 308.6 1.002 0.850 12.72 0.74 14400 314.6 1.001 0.925 From Table 4.1.1, the pH of NaOH increases with increase in electric current and reaction time. This trend could be explained by the fact that high currents as a result of the high applied voltage (80V) favoured the feasibility of the reaction by facilitating the dissociation of more NaCl into Na+ and Cl- ions as well as H2O into H + and OH- ions in both the anode and cathode compartments respectively. The high currents also increased the mobility of the ions in solution by causing the Na+ ions to migrate faster to combine with the hydroxyl ions (OH-) in the cathode compartment to form more NaOH as indicated by the magnitude of the 28 University of Ghana http://ugspace.ug.edu.gh pH of the catholyte. The rise in temperature of the catholyte with increase in reaction time as observed in Table 4.1.1 is also due to increase in electric current and the observed rise in temperature did not offset the amount of NaOH produced during the course of the reaction. The specific gravity of the saltbrine decreases as the reaction proceeded showing that more of the NaCl were converted to NaOH as the reaction time increased. In Table 4.1.1, the experiment could not be carried out beyond a pH of 12.72 because the design of the experiment was within a specific time of 14400 seconds. For subsequent results from Table 4.2.1 to 4.3.3, although 14400 seconds has not been attained, the experiment was limited to a certain pH at the end of the reaction so as to preserve the integrity of the anode and cathode compartments by preventing melting and also avoiding decomposition of the desired product. Table 4.1.2 depicts results obtained using 400 SAL brine strength under the conditions of 80V, 1 electrode in both anode and cathode compartment and 6.0 cm electrode gap. Table 4.1. 2: Results of pH, Time, Temperature, Specific Gravity and Current at 400 SAL and at 80V. pH Current % Weight Time (s) Temperature Specific Current density (A) of NaCl (K) gravity (A/cm2) 7.11 0.15 10.558 0 302.7 1.078 0.188 9.64 0.31 1800 307.4 1.061 0.388 10.43 0.35 3600 309.1 1.042 0.440 12.07 0.40 5400 314.7 1.035 0.500 12.17 0.62 7200 319.9 1.021 0.775 12.54 1.08 9000 326.7 1.013 1.350 13.34 2.50 10800 337.8 1.009 3.125 13.82 2.64 12600 347.6 1.004 3.300 13.83 2.88 14400 349.3 1.001 3.600 29 University of Ghana http://ugspace.ug.edu.gh The trend in the pH of the NaOH observed in Table 4.1.2 is similar to that observed in Table 4.1.1 However the rise in pH of the NaOH in Table 4.1.2 is more appreciable than that observed in Table 4.1.1 A maximum pH 13.83 of NaOH obtained in Table 4.1.2 as compared to pH 12.72 obtained in Table 4.1.1 under the same conditions of experiment shows that the brine strength 400 SAL favoured the formation of NaOH better than 300 SAL. Table 4.1.3 shows the trend in pH of the NaOH measured after each reaction time. Table 4.1. 3: Results of pH, Time, Temperature, Specific Gravity and Current at 500 SAL and at 80V. pH Current % Weight Time (s) Temperature Specific Current density (A) of NaCl (K) gravity (A/cm2) 7.09 0.15 10.558 0 301.9 1.098 0.188 9.87 0.36 1800 309.2 1.072 0.450 10.56 0.42 3600 311.1 1.069 0.525 12.12 0.46 5400 314.9 1.066 0.575 12.48 0.66 7200 321.2 1.054 0.825 12.84 1.12 9000 328.4 1.048 1.400 13.44 2.63 10800 340.8 1.036 3.288 13.60 2.96 12600 348.9 1.028 3.700 13.84 3.01 14400 350.2 1.026 3.763 The trend in pH observed in Table 4.1.3 is the same as that observed in Tables 4.1.1 and 4.1.2. However a maximum pH 13.84 observed in Table 4.1.3 shows that the brine strength 500 SAL favoured the formation of the desired product (NaOH) better than 300 and 400 SAL brine strengths under the same conditions of experiment. 30 University of Ghana http://ugspace.ug.edu.gh Table 4.1. 4: Results of pH, Time, Temperature, Specific Gravity and Current at 600 SAL and at 80V. pH Current % Weight Time (s) Temperature Specific Current density (A) of NaCl (K) gravity (A/cm2) 7.09 0.15 15.12 0 301.6 1.118 0.188 10.52 0.80 1800 326.3 1.109 1.000 11.01 1.60 3600 334.4 1.106 2.000 11.24 3.20 5400 341.3 1.102 4.000 11.52 3.50 7200 356.4 1.090 4.375 11.80 4.40 9000 358.3 1.060 5.500 12.12 5.02 10800 358.9 1.021 6.275 12.26 5.12 12600 359.2 1.011 6.400 12.46 5.16 14400 360.1 1.009 6.450 The pH of the NaOH at the end of the reaction time with 600 SAL from Table 4.1.4 is 12.46 and 11.62 with 700 SAL in Table 4.1.5. The observation is inconsistent with that observed in Tables 4.1.1 to 4.1.3. The result is due to the fact that the applied voltage was not sufficient to cause complete dissociation of the NaCl and H2O molecules into their primitive states to facilitate the formation of the desired product. 31 University of Ghana http://ugspace.ug.edu.gh Table 4.1. 5: Results of pH, Time, Temperature, Specific Gravity and Current at 700 SAL at 80V. pH Current % Weight Time (s) Temperature Specific Current density (A) of NaCl (K) gravity (A/cm2) 7.04 0.15 10.558 0 301.9 1.098 0.188 10.68 0.36 1800 309.2 1.072 0.450 11.00 0.42 3600 311.1 1.069 0.525 11.12 0.46 5400 314.9 1.066 0.575 11.23 0.66 7200 321.2 1.054 0.825 11.36 1.12 9000 328.4 1.048 1.400 11.42 2.63 10800 340.8 1.036 3.288 11.53 2.96 12600 348.9 1.028 3.700 11.62 3.01 14400 350.2 1.026 3.763 Table 4.1.6 to 4.1.25 show results obtained for pH of NaOH with the same controlled parameters mentioned in the previous tables but at 70V. As delineated in the research objectives, the influence of voltage on the pH of NaOH was investigated. Table 4.1.6 to 4.1.25 depict results of pH dependence on the applied voltage. Table 4.1. 6: Results of pH, Time, Temperature, Specific Gravity and Current at 300 SAL at 70V. pH Current % Weight Time (s) Temperature Specific Current density (A) of NaCl (K) gravity (A/cm2) 7.08 0.15 7.919 0 301.2 1.038 0.188 7.12 0.22 1800 301.5 1.018 0.275 7.54 0.26 3600 301.8 1.016 0.325 8.12 0.34 5400 301.9 1.014 0.425 9.61 0.40 7200 302.9 1.011 0.500 10.18 0.53 9000 306.8 1.008 0.663 10.38 0.60 10800 307.5 1.006 0.750 12.54 0.65 12600 308.6 1.002 0.813 12.58 0.71 14400 310.2 1.001 0.888 32 University of Ghana http://ugspace.ug.edu.gh From Tables 4.1.6 to 4.1.10, the applied voltage was 70V and the remaining three parameters were kept constant. The pH values for NaOH for all the brine strengths are much lower at 70V compared with equal brine strengths electrolyzed at 80V. The highest pH of NaOH is 13.84 at 80V with 500 SAL brine strength from Table 4.1.3. Meanwhile from Table 4.1.8 with 500 SAL brine strength electrolyzed at 70V, the maximum pH for NaOH obtained from the present study is 13.72 at the same reaction time of 14400 seconds as observed from Table 4.1.3. The subsequent tables; Table 4.1.1 to 4.1.25 show similar trend when the same brine strengths were electrolyzed at 60, 50 and 40V. Although when the brine strengths were electrolyzed at 80V, 500 SAL was observed to be the optimum with pH of NaOH 13.84 at the end of the reaction time. The optimum brine strength 500 SAL when electrolyzed at 40V at 14400 seconds, the maximum pH of NaOH obtained was 13.22 indicating that the applied voltage had a significant influence on the amount of NaOH produced as observed in the pH values. The trend could be explained by the fact that the voltages were associated with high electric currents which facilitated the dissociation of NaCl and H2O molecules into their respective ions. These high currents as a result of the high voltage also increased the mobility of Na+ ions from the anode to the cathode compartment for combination with the hydroxyl ions for formation of the desired product (NaOH). 33 University of Ghana http://ugspace.ug.edu.gh Table 4.1. 7: Results of pH, Time, Temperature, Specific gravity and Current at 400 SAL at 70V. pH Current % Weight Time (s) Temperature Specific Current density (A) of NaCl (K) gravity (A/cm2) 7.05 0.15 10.558 0 302.2 1.058 0.188 9.58 0.28 1800 306.8 1.049 0.350 10.40 0.32 3600 308.5 1.040 0.400 12.10 0.36 5400 313.7 1.032 0.450 12.14 0.58 7200 320.9 1.018 0.725 12.50 1.06 9000 324.8 1.011 1.325 13.30 2.44 10800 336.9 1.009 3.05 13.56 2.58 12600 344.6 1.002 3.225 13.60 2.62 14400 345.2 1.001 3.275 Table 4.1. 8: Results of pH with Time, Temperature, Specific Gravity and Current at fixed Brine strength 500 SAL at 70V. pH Current % Weight Time (s) Temperature Specific Current density (A) of NaCl (K) gravity (A/cm2) 7.12 0.15 13.198 0 301.9 1.098 0.188 9.84 0.32 1800 309.2 1.072 0.400 10.52 0.38 3600 311.1 1.069 0.475 12.10 0.40 5400 314.9 1.066 0.500 12.44 0.62 7200 321.2 1.054 0.775 12.81 1.10 9000 328.4 1.048 1.375 13.43 2.58 10800 340.8 1.036 3.225 13.67 2.89 12600 348.9 1.028 3.613 13.72 2.93 14400 349.3 1.023 3.663 34 University of Ghana http://ugspace.ug.edu.gh Table 4.1. 9: Results of pH, Time, Temperature, Specific Gravity and Current at 600 SAL at 70V. pH Current % Weight Time (s) Temperature Specific Current density (A) of NaCl (K) gravity (A/cm2) 7.06 0.15 15.12 0 301.9 1.118 0.188 10.48 0.76 1800 324.6 1.112 0.95 10.96 1.58 3600 332.1 1.108 1.975 11.16 3.18 5400 340.8 1.104 3.975 11.34 3.46 7200 354.7 1.096 4.325 11.75 4.38 9000 355.4 1.084 5.475 12.18 5.00 10800 357.8 1.078 6.250 12.22 5.08 12600 358.9 1.066 6.350 12.26 5.12 14400 359.6 1.058 6.400 Table 4.1. 10: Results of pH, Time, Temperature, Specific Gravity and Current at 700 SAL at 70V. pH Current % Weight Time (s) Temperature Specific Current density (A) of NaCl (K) gravity (A/cm2) 7.11 0.15 18.477 0 301.4 1.139 0.188 10.58 0.92 1800 326.8 1.132 1.15 11.10 1.78 3600 335.3 1.128 2.225 11.24 3.40 5400 340.2 1.124 4.250 11.32 3.70 7200 348.6 1.119 4.625 11.42 4.60 9000 356.2 1.108 5.750 11.48 5.02 10800 363.6 1.009 6.275 11.54 5.15 12600 365.2 1.004 6.438 11.63 5.24 14400 365.4 1.001 6.450 35 University of Ghana http://ugspace.ug.edu.gh Table 4.1. 11: Results of pH, Time, Temperature, Specific Gravity and Current at 30 0SAL at 60V. pH Current (A) % Weight Time (s) Temperature Specific Current density of NaCl (K) gravity (A/cm2) 7.11 0.15 7.919 0 301.7 1.058 0.188 7.18 0.24 1800 301.8 1.016 0.300 7.48 0.28 3600 301.9 1.012 0.350 8.16 0.36 5400 302.0 1.011 0.450 9.59 0.42 7200 302.4 1.009 0.525 9.66 0.50 9000 305.3 1.004 0.625 10.28 0.58 10800 306.5 0.98 0.725 10.68 0.64 12600 307.2 0.94 0.800 11.74 0.78 14400 310.4 0.92 0.975 Table 4.1. 12: Results of pH, Time, Temperature, Specific Gravity, Current and at 40 0SAL at 60V. pH Current % Weight Time (s) Temperature Specific Current density (A) of NaCl (K) gravity (A/cm2) 7.12 0.15 10.558 0 302.1 1.078 0.188 9.69 0.22 1800 305.9 1.075 0.275 10.36 0.30 3600 307.4 1.056 0.375 12.12 0.34 5400 310.9 1.044 0.425 12.16 0.54 7200 318.9 1.038 0.675 12.48 1.04 9000 320.8 1.032 1.300 13.03 2.42 10800 332.4 1.024 3.025 13.20 2.56 12600 340.9 1.016 3.200 13.34 2.80 14400 341.6 1.014 3.500 36 University of Ghana http://ugspace.ug.edu.gh Table 4.1. 13: Results of pH, Time, Temperature, Specific Gravity and Current at 50 0SAL and at 60V. pH Current % Weight Time (s) Temperature Specific Current density (A) of NaCl (K) gravity (A/cm2) 7.13 0.15 13.198 0 301.9 1.098 0.188 9.80 0.30 1800 308.2 1.086 0.375 10.50 0.36 3600 310.6 1.072 0.450 12.12 0.40 5400 313.5 1.068 0.500 12.42 0.58 7200 320.2 1.050 0.725 12.78 1.08 9000 324.3 1.044 1.350 13.40 2.44 10800 338.7 1.032 3.050 13.58 2.79 12600 346.8 1.023 3.488 13.62 2.83 14400 347.3 1.018 3.538 Table 4.1. 14: Results of pH, Time, Temperature, Specific Gravity and Current at 60 0SAL and at 60 V. pH Current % Weight Time (s) Temperature Specific Current density (A) of NaCl (K) gravity (A/cm2) 7.14 0.15 15.837 0 301.4 1.118 0.188 10.46 0.72 1800 322.4 1.114 0.900 10.92 1.52 3600 330.8 1.112 1.900 11.14 3.14 5400 338.1 1.109 3.925 11.30 3.38 7200 350.6 1.098 4.225 11.68 4.30 9000 353.8 1.089 5.375 12.12 4.78 10800 354.5 1.082 5.975 12.20 5.00 12600 356.6 1.078 6.250 12.35 5.10 14400 357.4 1.072 6.375 37 University of Ghana http://ugspace.ug.edu.gh Table 4.1. 15: Results of pH, Time, Temperature, Specific Gravity and Current at 70 0SAL and at 60V. pH Current % Weight Time (s) Temperature Specific Current density (A) of NaCl (K) gravity (A/cm2) 7.18 0.15 18.477 0 301.4 1.139 0.188 10.54 0.90 1800 324.4 1.136 1.125 11.10 1.74 3600 332.6 1.130 2.175 11.20 3.38 5400 338.6 1.126 4.225 11.28 3.68 7200 344.9 1.124 4.600 11.38 4.56 9000 352.9 1.118 5.700 11.44 5.02 10800 364.8 1.109 6.275 11.52 5.08 12600 365.0 1.106 6.350 11.56 5.11 14400 365.4 1.104 6.388 Table 4.1. 16: Results of pH, Time, Temperature, Specific Gravity and Current at 300 SAL and at 50V. pH Current % Weight Time (s) Temperature Specific Current density (A) of NaCl (K) gravity (A/cm2) 7.08 0.15 7.919 0 301.3 1.058 0.188 7.20 0.30 1800 301.6 1.018 0.375 7.50 0.32 3600 301.7 1.014 0.400 8.24 0.40 5400 301.8 1.013 0.500 9.60 0.45 7200 302.1 1.011 0.563 9.74 0.60 9000 304.6 1.010 0.750 10.31 0.65 10800 305.2 1.006 0.813 10.72 0.71 12600 306.4 0.99 0.888 11.84 0.92 14400 309.7 0.96 1.15 38 University of Ghana http://ugspace.ug.edu.gh Table 4.1. 17: Results of pH, Time, Temperature, Specific Gravity and Current at 400 SAL and at 50V. pH Current % Weight Time (s) Temperature Specific Current density (A) of NaCl (K) gravity (A/cm2) 7.11 0.15 10.558 0 301.1 1.078 0.188 7.22 0.31 1800 301.8 1.077 0.388 7.53 0.33 3600 301.9 1.070 0.413 8.30 0.42 5400 302.1 1.066 0.525 9.64 0.47 7200 302.4 1.060 0.588 9.78 0.64 9000 304.8 1.052 0.800 10.40 0.68 10800 305.6 1.048 0.850 10.82 0.79 12600 306.9 1.042 0.988 12.01 0.98 14400 310.2 1.036 1.225 Table 4.1.18: Results of pH, Time, Temperature, Specific Gravity and Current at 500 SAL and at 50V. pH Current % Weight Time (s) Temperature Specific Current density (A) of NaCl (K) gravity (A/cm2) 7.09 0.15 13.198 0 301.3 1.098 0.188 7.25 0.34 1800 301.9 1.093 0.425 7.58 0.36 3600 302.2 1.090 0.450 8.36 0.44 5400 302.6 1.088 0.550 9.68 0.52 7200 302.9 1.084 0.650 10.82 0.68 9000 304.9 1.080 0.850 11.45 0.73 10800 305.8 1.076 0.913 12.88 0.84 12600 307.1 1.072 1.050 13.44 1.01 14400 310.6 1.069 1.263 39 University of Ghana http://ugspace.ug.edu.gh Table 4.1. 189: Results of pH, Time, Temperature, Specific Gravity and Current at 600 SAL and at 50V. pH Current %Weight Time (s) Temperature S pecific Current density (A) of NaCl (K) gravity (A/cm2) 7.12 0.15 15.837 0 301.1 1.118 0.188 7.30 0.36 1800 302.0 1.116 0.450 7.60 0.39 3600 302.6 1.111 0.488 8.38 0.48 5400 302.9 1.110 0.600 9.71 0.57 7200 303.2 1.109 0.713 9.86 0.74 9000 305.1 1.106 0.925 10.49 0.77 10800 305.6 1.101 0.963 10.93 0.85 12600 307.4 1.080 1.063 12.11 1.06 14400 311.2 1.065 1.325 Table 4.1. 20: Results of pH, Time, Temperature, Specific Gravity and Current at 700 SAL and at 50V. pH Current %Weight Time (s) Temperature Specific Current (A) of NaCl (K) gravity density (A/cm2) 7.09 0.15 18.477 0 301.6 1.139 0.188 7.34 0.40 1800 302.3 1.138 0.500 7.65 0.44 3600 302.8 1.135 0.550 8.44 0.54 5400 303.1 1.132 0.675 9.76 0.63 7200 303.6 1.130 0.788 9.92 0.80 9000 305.4 1.128 1.000 10.56 0.84 10800 306.0 1.124 1.050 11.10 0.90 12600 308.1 1.120 1.125 12.20 1.10 14400 312.2 1.116 1.375 40 University of Ghana http://ugspace.ug.edu.gh Table 4.1. 191: Results of pH, Time, Temperature, Specific Gravity and Current at 300 SAL and at 40V. pH Current %Weight Time (s) Temperature Specific Current density (A) of NaCl (K) gravity (A/cm2) 7.09 0.15 7.919 0 301.1 1.058 0.188 7.18 0.28 1800 301.4 1.054 0.35 7.47 0.30 3600 301.2 1.050 0.375 8.19 0.36 5400 301.6 1.044 0.450 9.46 0.41 7200 301.5 1.039 0.513 9.69 0.56 9000 302.0 1.034 0.700 10.28 0.60 10800 304.1 1.029 0.750 10.68 0.66 12600 305.9 1.024 0.825 11.70 0.88 14400 309.2 1.021 1.100 Table 4.1. 20: Results of pH, Time, Temperature, Specific Gravity and Current at 400 SAL and at 40V. pH Current %Weight Time (s) Temperature Specific Current density (A) NaCl (K) gravity (A/cm2) 7.09 0.15 10.558 0 301.0 1.078 0.188 7.20 0.29 1800 301.4 1.077 0.363 7.50 0.30 3600 301.5 1.074 0.375 8.27 0.40 5400 302.0 1.070 0.500 9.61 0.44 7200 302.2 1.066 0.550 9.72 0.60 9000 304.4 1.062 0.750 10.36 0.64 10800 305.2 1.058 0.800 10.78 0.75 12600 306.4 1.054 0.938 12.00 0.95 14400 310.0 1.050 1.188 41 University of Ghana http://ugspace.ug.edu.gh Table 4.1. 21: Results of pH, Time, Temperature, Specific Gravity and Current at 500 SAL and at 40V. pH Current %Weight Time (s) Temperature Specific Current density (A) NaCl (K) gravity (A/cm2) 7.06 0.15 13.198 0 301.1 1.098 0.188 7.21 0.32 1800 301.6 1.095 0.400 7.55 0.33 3600 302.0 1.092 0.413 8.34 0.41 5400 302.4 1.090 0.513 9.65 0.50 7200 302.6 1.088 0.625 10.79 0.64 9000 304.5 1.084 0.800 11.42 0.70 10800 305.3 1.082 0.875 12.83 0.81 12600 306.8 1.079 1.013 13.22 0.99 14400 310.2 1.075 1.238 Table 4.1. 224: Results of pH, Time, Temperature, Specific Gravity and Current at 600 SAL and at 40V. pH Current %Weight Time (s) Temperature Specific Current density (A) NaCl (K) gravity (A/cm2) 7.11 0.15 15.837 0 301.3 1.118 0.188 7.29 0.32 1800 301.9 1.117 0.400 7.58 0.36 3600 302.2 1.115 0.450 8.36 0.43 5400 302.4 1.112 0.538 9.68 0.54 7200 303.0 1.109 0.675 10.84 0.72 9000 304.8 1.104 0.900 11.45 0.74 10800 305.1 1.100 0.925 12.90 0.83 12600 306.9 1.090 1.038 13.08 1.03 14400 311.0 1.080 1.288 42 University of Ghana http://ugspace.ug.edu.gh Table 4.1. 235: Results of pH, Time, Temperature, Specific Gravity and Current at 700 SAL and at 40V. pH Current %Weight Time (s) Temperature Specific Current density (A) NaCl (K) gravity (A/cm2) 7.05 0.15 18.477 0 301.4 1.139 0.188 7.32 0.38 1800 302.0 1.138 0.475 7.61 0.40 3600 302.5 1.136 0.500 8.40 0.52 5400 303.0 1.134 0.650 9.72 0.60 7200 303.2 1.131 0.750 9.88 4.0.76 9000 305.1 1.129 0.950 10.53 0.80 10800 305.8 1.126 1.000 11.00 0.88 12600 306.9 1.124 1.100 12.18 1.10 14400 311.8 1.120 1.375 4.2 Effect of Electrode Number on the pH of NaOH. Table 4.2.1 to 4.2.4 show results with optimum brine strength and applied voltage and the dependence of the pH of NaOH with varying number of electrodes in both the anode and cathode compartments. Table 4.2.1: Results showing variation of pH with Time at (500 SAL, 80V, 6.0 cm Electrode Gap, 2 electrodes in Anode and Cathode Compartments). pH Current %Weight Time (s) Temperature Specific Current density (A) NaCl (K) gravity (A/cm2) 7.11 0.15 15.837 0 300.1 1.118 0.094 9.90 0.60 1800 318.2 1.112 0.375 9.96 1.32 3600 326.9 1.100 0.825 10.50 2.80 5400 334.8 1.091 1.750 12.30 3.10 7200 340.4 1.088 1.938 12.60 4.42 9000 348.5 1.076 2.763 12.84 4.64 10800 350.8 1.069 2.900 13.85 4.88 12600 354.6 1.048 3.050 43 University of Ghana http://ugspace.ug.edu.gh In Table 4.2.1, two electrodes were used in both anode and cathode compartments; the maximum pH of NaOH obtained was 13.85 at a reaction time of 12600 seconds. However, the pH of NaOH was observed to increase appreciably at a reduced reaction time when the number of electrodes were increased from two to five as shown in Table 4.2.1 to 4.2.4. In Table 4.2.2, three electrodes were used. The maximum pH of NaOH obtained was 13.86 at a reaction time of 10800 seconds. A similar trend was observed in Table 4.2.3 with four electrodes. The pH of NaOH was 13.87 at a reaction time of 9000 seconds. The highest pH 13.88 of NaOH was obtained at a reaction time of 7200 seconds when five electrodes were used as shown in Table 4.2.4. The trend in the results indicates that the pH of the NaOH depends on the number of electrodes. This observation is due to the fact that the higher the number of electrodes, the greater the surface area of the electrodes in contact with their respective electrolytes. The larger surface area of the electrodes increased the quantity of effective charge per unit area which facilitated the dissociation of more NaCl in the anolyte compartment leading to the formation of more NaOH in the catholyte compartment as indicated by the magnitude of the pH values. Table 4.2.2: Results Showing Variation of pH with Time at (500 SAL, 80V, 6.0 cm Electrode Gap, 3 Electrodes in Anode and Cathode Compartments). pH Current %Weight Time (s) Temperature Specific Current density (A) NaCl (K) gravity (A/cm2) 7.08 0.15 15.837 0 302.3 1.118 0.063 10.52 0.82 1800 319.4 1.110 0.342 12.42 1.52 3600 328.2 1.096 0.633 12.62 3.11 5400 336.8 1.083 1.296 12.88 3.24 7200 348.7 1.072 1.350 13.80 4.63 9000 352.9 1.066 1.929 13.86 5.00 10800 358.9 1.053 2.083 44 University of Ghana http://ugspace.ug.edu.gh Table 4.2.3: Results Showing Variation of pH with Time at (500 SAL, 80V, 6.0 cm Electrode Gap, 4 Electrodes in Anode and Cathode Compartments). pH Current %Weight Time (s) Temperature Specific Current density (A) NaCl (K) gravity (A/cm2) 7.06 0.15 15.837 0 302.1 1.118 0.047 11.26 1.76 1800 329.6 1.110 0.550 12.74 3.20 3600 338.2 1.090 1.000 12.90 3.63 5400 349.4 1.088 1.134 13.82 4.89 7200 356.2 1.076 1.528 13.87 5.29 9000 359.5 1.066 1.653 Table 4.2.4: Results Showing Variation of pH with Time at (500 SAL, 80V, 6.0 cm Electrode Gap, 5 Electrodes in Anode and Cathode Compartments). pH Current %Weight Time (s) Temperature Specific Current density (A) NaCl (K) gravity (A/cm2) 7.06 0.15 15.837 0 302.9 1.118 0.038 11.74 3.12 1800 336.5 1.100 0.780 12.74 4.16 3600 350.2 1.098 1.040 13.86 4.88 5400 356.9 1.091 1.220 13.88 5.40 7200 360.3 1.089 1.350 The subsequent results set out to investigate the effect of gaps between the electrodes on the pH of NaOH. 45 University of Ghana http://ugspace.ug.edu.gh 4.3 Effect of Gaps between the Electrodes on the pH of NaOH. As delineated in the objective of the research, Table 4.3.1 to 4.3.3 illustrate results of the effect of the gaps between the electrodes on the pH of NaOH. Three controlled parameters are constant and the gaps between the electrodes are varied to ascertain their influence on the pH of NaOH. Table 4.3.1: Results Showing Variation of pH with Time at (500 SAL, 80V, 7.0 cm Electrode Gap, 5 Electrodes in Anode and Cathode Compartments). pH Current %Weight Time (s) Temperature Specific Current density (A) NaCl (K) gravity (A/cm2) 7.12 0.15 15.837 0 302.9 1.118 0.038 11.18 3.10 1800 335.5 1.114 0.775 12.70 4.12 3600 348.4 1.101 1.030 12.88 4.84 5400 354.3 1.098 1.210 13.85 5.31 7200 358.2 1.092 1.328 13.87 5.34 9000 359.7 1.093 1.335 From the tables, the pH of NaOH is observed to decrease with increase in the gaps between the electrodes at the end of the reaction. In Table 4.3.1, the electrode gap was 7.0 cm. The pH of NaOH at the end of the reaction is 13.87 at a reaction time of 9000 seconds. The pH reduced to 13.86 at a reaction time of 10800 seconds in Table 4.3.1 when the electrode gap was increased to 8.0 cm. In Table 4.3.2, pH of NaOH is 13.85 at a reaction time of 12600 seconds when the electrode gap was 9.0 cm. A similar effect is observed in Table 4.3.3 when the electrode gap was increased to 10.0 cm. The pH of NaOH is 13.84 at a reaction time of 14400 seconds. The highest pH 13.88 of NaOH was observed in Table 4.3.1 at a reaction time of 7200 seconds when the electrode gap was 6.0 cm under the same conditions of experiment. The trend shows that the gap between the electrodes has significant influence 46 University of Ghana http://ugspace.ug.edu.gh on the pH of the NaOH. The observation could be explained by the fact that ions travel during reactions in order to cause product formation. By increasing inter electrode distance, the Na+ ions in the anode compartment had to travel relatively longer distances to combine with the hydroxyl ions (OH-) in the cathode compartment to form NaOH. A shorter inter electrode distance would therefore imply that these ions would combine more readily and comparatively at reduced reaction time. However, an electrode gap below 6.0 cm was not used in this work due to the dimensions of the anode and cathode compartments. An electrode gap below 6.0 cm could have led to short-circuiting and electric sparks. Table 4.3.2: Results Showing Variation of pH with Time at (500 SAL, 80V, 8.0 cm Electrode Gap, 5 Electrodes in Anode and Cathode Compartments). pH Current %Weight Time (s) Temperature Specific Current density (A) NaCl (K) gravity (A/cm2) 7.12 0.15 15.837 0 302.9 1.118 0.038 11.18 3.10 1800 335.5 1.114 0.775 12.70 4.12 3600 348.4 1.101 1.030 12.88 4.84 5400 354.3 1.098 1.210 13.80 5.25 7200 358.2 1.092 1.313 13.84 5.30 9000 359.7 1.093 1.325 13.86 5.33 10800 359.9 1.090 1.333 47 University of Ghana http://ugspace.ug.edu.gh Table 4.3.3: Results Showing Variation of pH with Time at (500 SAL, 80V, 9.0 cm Electrode Gap, 5 Electrodes in Anode and Cathode Compartments). pH Current %Weight Time (s) Temperature Specific Current density (A) NaCl (K) gravity (A/cm2) 7.12 0.15 15.837 0 302.9 1.118 0.038 11.18 3.10 1800 335.5 1.114 0.775 12.70 4.12 3600 348.4 1.101 1.030 12.88 4.84 5400 354.3 1.098 1.210 13.74 5.25 7200 358.2 1.092 1.313 13.78 5.28 9000 358.6 1.093 1.320 13.82 5.30 10800 359.9 1.088 1.325 13.85 5.33 12600 360.0 1.090 1.333 Table 4.3.4: Results showing variation of pH with Time at (500 SAL, 80V, 10.0 cm Electrode Gap, 5 Electrodes in Anode and Cathode Compartments). pH Current %Weight Time (s) Temperature Specific Current density (A) NaCl (K) gravity (A/cm2) 7.12 0.15 15.837 0 302.9 1.118 0.038 11.18 3.08 1800 334.1 1.117 0.770 12.70 4.00 3600 346.4 1.115 1.000 12.88 4.62 5400 352.6 1.112 1.155 13.66 5.00 7200 355.4 1.110 1.250 13.73 5.14 9000 355.1 1.100 1.285 13.76 5.23 10800 356.4 1.099 1.308 13.80 5.28 12600 357.8 1.095 1.320 13.84 5.30 14400 358.5 1.088 1.325 48 University of Ghana http://ugspace.ug.edu.gh 4.4 Effects of the Optimum Parameters on the pH of NaOH. As outlined in the research objectives, the effect of the optimum parameters were investigated to ascertain their influence on the maximization of the pH of NaOH. 4.4.1 Effect of Brine Strength Figure 4.4.1: A graph of pH against brine strength. To study the effect of brine strength on the production of sodium hydroxide (indicated by the magnitude of the pH of the catholyte), different brine strengths were used (Domga et al., 2016). Figure 4.4.1 shows that pH of the catholyte increase with increase in brine strength of the anolyte. Maximum pH of 13.84 of the catholyte occurred at 500 SAL. A Similar work conducted by (Saksono et al., 2013) confirmed the observed trend. The observed trend could be explained by the fact that as electric current passed through the electrolytic cell, NaCl in the anode compartment dissociated into Na+ ions and Cl- ions. In the same vein, H2O in the cathode compartment dissociated into H+ ions and OH- ions. The positively charged Na+ ions in the anode compartment were attracted by the negatively charged electrode (cathode). The Na+ ions on reaching the cathode compartment formed NaOH with the OH- in the cathode compartment. Hence increased the concentration of NaOH (pH of the catholyte). 49 University of Ghana http://ugspace.ug.edu.gh The H+ ions in the cathode compartment was reduced to H2 gas at the cathode compartment. Similarly, Cl- ions in the anode compartment were oxidized to Cl2 gas at the anode compartment. As brine strength increased beyond 500 SAL, pH (amount of NaOH) decreased to a minimum value of 12.30 corresponding to a brine strength of 700 SAL. The decrease in pH could be explained by the fact that high brine strength could lead to the formation of blisters and hence increased in ohmic resistance leading to a decrease in NaOH production as observed by (Domga et al., 2016). 4.4.2 Effect of Gaps between the Electrodes Figure 4.4. 2: A graph of pH against gaps between the electrodes. Figure 4.4.2 shows that the pH of the catholyte decreased as the gap between the electrodes increased. Highest pH 13.88 occurred at an electrode gap of 6.0cm. A similar work carried out by (Ana, 2005) on chlor-alkali production confirmed the observed trend. Author (Leroy et al., 1979) also did a similar work and reported a comparable trend. This observed trend could be explained by the fact that the smaller the electrode gap, the lower the ohmic resistance and hence the higher the current that dissociates H2O and NaCl into H +, OH- and Na+ and Cl- ions respectively. This high current also speeds up ions from one compartment to the other for more NaOH to be formed and hence increase in pH. However, if the gap is 50 University of Ghana http://ugspace.ug.edu.gh very small, there is entrapment of gas bubbles between the electrodes (Nagai et al., 2003). Again, a close gap results in higher frequency of short-circuiting and electric sparks (Strathmann et al., 2004). 4.4.3 Effect of Electrode Number Figure 4.4.3: A graph showing the trend in pH with increasing number of electrodes. Figure 4.4.3 indicates that the pH of the catholyte increased as the number of electrodes in each compartment increased. Similar works carried out by (Sengupta et al., 1994; Domga et al., 2016) confirmed the observed trend. The observed trend may be explained by the fact that as the number of electrodes (in the two compartments) increased, the total surface area of the electrodes in contact with their respective electrolyte also increased and hence quantity of effective charge per unit area increased which facilitated the dissociation of more NaCl in the anolyte compartment leading to the formation of more NaOH in the catholyte compartment; and hence increased the pH of the catholyte. However, when the number of electrodes in each compartment is too high, it could result in short –circuits due to reduction of inter-electrodes distance (Daneshvar et al., 2007). 51 University of Ghana http://ugspace.ug.edu.gh 4.4.4 Effect of Voltage Figure 4.4.4: A graph showing the trend in pH as voltage increases. Figure 4.4.4 shows that the pH of the catholyte increased with respect to increase in the applied voltage. Similar results were obtained by (Saksono et al., 2013; Rabbani et al., 2014). This outcome could be explained by the fact that increasing the applied voltage increase electrical charge in the electrolytic cell leading to the dissociation of more NaCl and H2O molecules and hence formation of more NaOH and so increased pH of the catholyte. However, beyond pH 13.62 which corresponded to a voltage of 60V, the pH of the catholyte began to level due to overheating of the electrolytic cell which caused the temperature of the electrolytes to increase. The increase in temperature increased the electrical resistance and hence reduced the amount of electric charge that passed through the electrolytes. The reduction in electric charge accounted for the drop in the pH of the catholyte due to reduction in the dissociation of NaCl and H2O molecules in both anolyte and catholyte compartments respectively. 52 University of Ghana http://ugspace.ug.edu.gh CHAPTER FIVE CONCLUSION AND RECOMMENDATION 5.1 Conclusion Some factors that influence quantity of NaOH produced from NaCl through electrolysis were studied and optimized. Quantity of NaOH produced increased with increase in brine strength. Optimum brine strength obtained was 500 SAL. Beyond 500 SAL, the amount of NaOH produced decreased as a result of formation of blisters in the salt bridge which caused electric resistance to increase. The quantity of NaOH produced also decreased with increased in the gap between the electrodes. This is because the charged species had to travel longer distance from one compartment to the other for product formation. The optimum gap between the electrodes was 6.0cm. NaOH production was observed to increase with increased in the number of electrodes. The highest electrodes used in this experiment was 5 in each compartment. The greater the number of electrodes, the greater the number of effective charge per unit area. NaOH production increased with increased in effective voltage (which is proportional to effective current). The optimum voltage was 80V. The highest pH which corresponded to the quantity of NaOH produced was 13.88 at 360.3K and 5.40A. 5.2 Recommendation It is recommended that further studies should focus on the optimization of temperature and current to ascertain their influence on maximization of NaOH production. 53 University of Ghana http://ugspace.ug.edu.gh REFERENCES Alhassan, M., & Garba, U.M. (2006). Design of an Alkaline Fuel Cell. Leonardo Electronic Journal of Practices and Technologies. (pp.99-106). Ana, C.B. (2005). Chlor-Alkali Membrane Cell Process: Study and characterization. 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