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The purpose of this report is to describe the electrochemical manufacture of chlorine and sodium hydroxide. Chlorine today is used globally to keep the environment free from pollution. It can clean and disinfect the water from pathogens. NaOH is also a useful product in many aspects such as purification of bauxite for the extraction of aluminum as far as a cleansing agent and in washing powder for machines and metal sheets. There are many methods of processing chlorine and sodium hydroxide as products. This report is focusing on the aspect of electrochemical design and the equipments that holds with it. It propose also on the size and the number of cells used; moreover, the anode and cathode size to achieve the required products. In addition, it shows the power that is needed to supply the cell in order to achieve the desired product and the economical potential of the process.
Chlorine (Cl) is a chemical element which has a 17 atomic number. The chlorine name came from ancient Greek. In the periodic table, it is categorized in a halogen group as number 17. At standard condition, the chlorine element is in the forms of diatomic molecular. In addition, after fluorine, it comes as the second lightest halogen, the third highest electro-negativity of all elements and has the electron affinity. Sodium chloride is the most common compound of chlorine.
Chlorine can be also found in natural as chlorine ions which are deposited in earth or in ocean. Dead Sea has a high concentration of chlorine ions and 1.9% of chlorine mass in seawater. Chloride salts are usually soluble in water; therefore, minerals with chloride contaminant can also be found in deep underground or in abundance dry climate.
Pale yellow-green gas
171.6 K,â€‚-101.5 °C,â€‚-150.7 °F
239.11 K,â€‚-34.04 °C,â€‚-29.27 °F
Liquid density at b.p.
(0 °C, 101.325 kPa) 3.2 g/L
416.9 K, 7.991 MPa
Heat of fusion
(Cl2) 6.406 kJ·molâˆ’1
Heat of vaporization
(Cl2) 20.41 kJ·molâˆ’1
Molar heat capacity
(Cl2) 33.949 J·molâˆ’1·Kâˆ’1
The chlorine has a very strong distinctive odor. The diatomic molecular of chlorine Cl2 is highly reactive molecular because the bonding between the two atoms are weak. The chlorine can be liquefied at room temperature and pressure of 7.4 bar.
[Ne] 3s2 3p5
2, 8, 7 Electron shells of chlorine (2, 8, 7)
Chloride compounds come from the chlorine that forms compound with most elements. Because the chlorine at standard condition appear in a diatomic molecular, it is mostly react with organic compounds and forms combustion of hydrocarbons.
In modern society, the electrochemical technology is reveal an increasingly significant aspect and mainly in the chemical industry. Considerable reasons for the change towards electrochemical technology which they are:
The ability in selecting chemical change without the use of toxic reagents or hazardous conditions.
Ability for recycling chemicals, metals and process streams.
Clean and efficient energy sources (especially for transport systems)
In the industrial sector, chlorine gas is manufactured by the electrolysis of sodium chloride which dissolved in water, which also yield hydrogen gas and sodium hydroxide. Electrolysis is described as the path of an electric current through an ion-containing solution which it produces chemical changes at the electrodes. The chemical equation of the process is as follow:
2 NaCl + 2 H2O (l) â†’ Cl2 (g) + H2 (g) + 2 NaOH (aq)
The following equations show a simple process for the electrolysis of chloride solutions:
2 H+ (aq) + 2 eâˆ’ â†’ H2 (g) â€¦â€¦â€¦â€¦â€¦â€¦â€¦â€¦â€¦.. Cathode
2 Clâˆ’ (aq) â†’ Cl2 (g) + 2 eâˆ’ â€¦â€¦â€¦â€¦â€¦â€¦â€¦â€¦â€¦..Anode
Therefore, the overall process is:
2 NaCl + 2 H2O â†’ Cl2 + H2 + 2 NaOH
Carl Wilhelm ScheeleThe Belgian chemist and physician Jan Baptist and Van Helmont had recognized that chlorine is in the gas phase in the year around 1630. Moreover, the Swedish chemist Carl Wilhelm Scheele had prepared elemental chlorine for the first time and studied it in 1774 which was done after long trail experiments; even though he failed. After him, a lot of scientist came to study chlorine element. For the first time Michael Faraday liquefied chlorine in 1823. On April 22, 1915, the German Army had used Chlorine gas for the first time as a weapon World War I in the Second Battle of Ypreshttp://upload.wikimedia.org/wikipedia/commons/thumb/4/4d/PSM_V31_D740_Carl_Wilhelm_Scheele.jpg/140px-PSM_V31_D740_Carl_Wilhelm_Scheele.jpg
Essential applications for the product chlorine are in a large domain of production of consumer products and industrial. For examples:
Solvents for dry cleaning
Household cleaning products
Used for bleaches and disinfection products
Used in the production of inorganic and organic chlorine compounds
Used for purifying water because of its powerful oxidizing characteristics, mainly potable water supplies and water that is used in swimming pools.
Chlorine is a toxic gas which affects the respiratory system. It attempts to assemble at the bottom of inadequately ventilated spaces because it is heavier than air. Moreover, it may react with flammable materials because the chlorine gas is a strong oxidizer.
Sodium hydroxide is an alkali in which a hydroxyl ion (OH-) is attached to a sodium ion (Na+). The approximate value of its pH is 13, for each 1 M, and it is used as a base. The use of NaOH depends on the reaction. In addition, it generally used to maintain the pH of the solution, and in some cases it is used as reducing agent. It has the molecular formula NaOH and is a highly caustic metallic base. Moreover, Sodium hydroxide is soluble in methanol, water and ethanol. It has a capacity to absorb carbon dioxide and moisture in air. There are huge uses of NaOH for example; it is used in different industries as a strong chemical base in the manufacture soap in saponification process also used to produce paper and pulp in addition NaOH is used in drinking water, textile, and detergents and as a drain cleaner. In general, sodium hydroxide is produced using electrolytic process which is also called (Chloe-alkali process). In this process, hydrogen gas (H2) is given of at the anode and chlorine gas (Cl2) at the cathode. Sodium hydroxide solution is collected near to the cathode. It has many properties such as; its melting point is at about 319°C (591 K). It is very soluble in water and considerable amount of heat is evolved due to the formation of a number of hydrates. It is also soluble in alcohol. Aqueous solution of NaOH is strongly alkaline because of its complete dissociation in water into Na+ and OH- by the equation given below:
NaOH + H2O ƒ Na+ (aq) + OH- (aq)
The uses of sodium hydroxide are as follow, in the manufacture of paper soap, viscose rayon, and many different chemicals .Moreover, it is used in the purification of bauxite for the extraction of aluminum as far as a cleansing agent and in washing powder for machines and metal sheets. In the refining of petroleum and vegetable oils sodium hydroxide is used for cotton mercerizing. It is too caustic to be used in washing clothes or hands. In laboratory, it is used as a reagent also in reclaiming rubber. In addition the soda lime is purified using NaOH.
According to the report on electrochemical production of free chlorine is that a study was held in 2009 to evaluate the suitability of using titanium based electrodes for the generation of free available chlorine (FAC) for water or wastewater disinfection. A series of laboratory experiments were conducted at room temperature in a 250 mL electrochemical cell. The results show performance of Ti based anode to be far better than that of graphite anode.
This report showed also how the advantages of producing chlorine such as the ability to provide chlorine residuals that would stay long enough to keep water disinfected, easy availability of the chemicals, simple techniques and low costs. The present research studies the effectiveness of electrochemical production of free available chlorine (FAC) using titanium (Ti) based electrodes. The advantages of Ti-based electrodes include corrosion resistance, power saving, reduced maintenance cost and avoidance of product contamination. A comparison was made with graphite anode as it is commonly used for electrolysis studies. The effects of operating parameters, concentration of electrolyte, reaction time and current density on FAC generation were determined using a statistical design of experiments.
Design Expert 6.0.7 software was used for experiment design and data analysis. The results came out that the graphite anode has the maximum FAC concentration obtained as 7.81 mg/l Cl2 at NaCl concentration 3g/L and reaction time 60 minutes. While, Ti-based anode has the maximum FAC concentration obtained at 76.07 mg/L Cl2 at NaCl concentration 3g/L and reaction time 60 minutes. Therefore, the amount of FAC generated using Ti-based anode was much higher than that produced by the graphite anode.
article which is called Electrolysis Cell, Especially for Electrochemical Production of Chlorine that the invention by Bulan, A. Gestemann, F. Marre, M. Grobholz & Hansen was focused on the electrochemical production of chlorine from aqueous solution hydrogen chloride. The objective of this invention was to provide an electrolysis cell that can produce chlorine in a reliable process and in easy handling.
It is familiar that the electrolysis of hydrochloric acid can be done in an electrolysis cell. In the cathode space, there will be pure oxygen or oxygen containing gas and in anode space, there will be a noble metal coated anode which is filled with the hydrochloric acid. Both electrodes spaces are separated from each other by a cation exchange membrane which is resting on a gas diffusion electrode (GDE) on the current collector. It expensive invention because both membrane and GDE would be replaced in case of one is damaged.
Hostin, S, Benedikovic, P. and Michalkova, A. (2009) conducted a study on a possibility of electrochemical production of chlorine for water disinfection, by using photovoltaic panels from solar energy. An experimental device was performed on a simple way of chloride production using a photovoltaic panel. Using this technology led an output of 50 W and solar irrigation of 380 to 550 W/m2 which produces chlorine about 0.3 mg/min, which is sufficient for disinfections of approximately 4000 liter water per day.
According to a journal on Applied Electrochemistry (Dec, 2005, Vol. 35, Issue 12, p1311) which was done by V. Barmashenko and J. Jorissen stated that it is possible for chlorine to recover from dilute hydrochloric acid by using a membrane called a chlorine resistant anion exchange membrane. It implements innovative possibilities to recovery of chlorine from hydrochloric acid electrolysis. In the anode compartment, electrolysis process hydrochloric acid is fed inside and needs an adequate HCl concentration for satisfying the anode with chloride ions. The input feed flows into the cathode chamber where HCl at low concentration is familiar by using an anion exchange membrane which is like cell separator. The reason is at the anode the Cl- ions can be provided by including a substance that is not consumed such as a salt. The transport of high water through the membrane for the new process could be a problem. Consequently, experiments for two processes were carried out. Using CaCl2 as high concentrated solution and for diluted HCl gas streams as absorption medium can prevent the disadvantages of water transport. In addition, in an empty anode compartment, the anode is attached to the membrane while a cell design was examined.
B. G. Hunt (1948) recognized a new type of electrolytic cell had been developed for the production of magnesium and chlorine by using molten magnesium chloride electrolyte. The design features were elimination of the usual refractory partition between anode and cathode, close spacing of electrodes and a separate metal wall. The advantage of this design was simple, low power consumption, and direct casting of metal from the cell.
According to PPG Industries (2010) that caustic soda is an important compound in different plants and in commercial applications. The manufacturing process of (NaOH) is done through the electrolysis of sodium chloride (NaCl). In PPG Industries sodium hydroxide is produced as liquid caustic soda 50% and 73% solutions in water. PPG is one of the largest global producers of caustic soda. In PPG industry NaOH is manufactures along with chlorine using the chlor alkali electrolysis process. It is an electrochemical reaction using a direct current to drive the decomposition reaction of an aqueous solution of sodium chloride (NaCl) into sodium hydroxide (NaOH) and chlorine (Cl2) and hydrogen (H2) gas by the following reaction:
2NaCl + 2H2O ƒ Cl2 + H2 + 2NaOH
Sodium hydroxide, caustic soda
Caustic soda solutions are colorless and strongly alkaline. They are not combustible and do not support combustion
Shakhashiri, (October, 2010) proposed that on the basis of mass produced, chlorine and sodium hydroxide are considered as the top ten products in chemical industry of the United State and other places in the world. Moreover, there were more than 7.3 billion kilograms of sodium hydroxide and 9.6 billion kilograms of chlorine produced in 2008. We are speaking about chlorine and sodium hydroxide because these tow chemicals industrially produced simultaneously by the same process which is the electrolysis of aqueous NaCl.
2 Na+ (aq) + 2 Cl-(aq) + 2 H2O (l) â†’ Cl2 (g) + H2(g) + 2 Na+(aq) + 2 OH-(aq)
During electrolysis process, hydrogen and hydroxide ions are formed at the cathode and chlorine is formed at the anode. The oxidation and reduction half cell reactions are expressed as showing below:
Anode reaction: 2 Cl- â†’ Cl2 + 2 e-
Cathode reaction: 2 H2O + 2 e- â†’ H2 + 2 OH-
As it is known previously, the Chlorine formed at the anode and the hydrogen formed at the cathode so they can react explosively, because of that they must be kept away from each other. Furthermore, the OH- ions formed at the cathode and it can react with chlorine that dissolved in the brine. To prevent side reactions, the products formed at the two electrodes keeps away from each other, using a porous diaphragm which is placed between the two electrodes in the electrolysis apparatus.