The membrane cell
The Membrane Cell
The membrane cell is another example of an electrochemical cell that is used to produce chlorine. In an electrochemical cell electrolysis takes place where the decomposition of a substance is caused by an electric current which passes through the electrolyte. An electrolyte is a solution that can conduct electricity due to the presence of free ions. In the membrane cell, sodium chloride in an aqueous solution is present in the form of ions (Na+ and Cl-) and is decomposed as a result of the electric current passing through the cell and produces chlorine (2Cl- ? Cl2 + 2e-) and other products, which are hydrogen (2H+ + 2e- ? H2 ) and sodium hydroxide (Na+ + OH- ? NaOH), commonly known as caustic soda.
With regard to the membrane cell, the process to produce chlorine is the most commonly used process today. This is because it is the most economically viable process and the most environmentally safe process, as it does not produce harmful waste products, containing asbestos or mercury, which the Diaphragm Cell and Mercury Cell do respectively.
The membrane cell makes use of an ion exchange membrane to separate the anode and cathode compartments (refer to figure 1). This membrane works on a principle of osmosis according to charges by attracting and allowing ions of a different charge to itself to flow through it. Therefore, this membrane in the Membrane Cell only allows the positive metal ions to flow through it, since it is negatively charged. However, some negative ions may pass through into the cathode compartment, although they do not participate in any reactions that take place in that compartment. The membrane is made out of perfluorocarboxylic and perfluorosulfonic, acid-based films which are situated between the anode and the cathode. Saturated brine, commonly known as salt water, is pumped into the anode compartment of the cell, while distilled water is pumped into the cathode compartment of the cell. When an electrical current is passed through the cell, the chlorine ion (Cl -) undergoes oxidation at the anode (2Cl- ? Cl2 + 2e- ) and thus it is transformed into chlorine gas which is extracted. However, simultaneously the sodium ion (Na+), together with some of the brine containing the negative chlorine ions, passes through the membrane and moves into the cathode compartment. In this compartment, the positive sodium ions react with the negative hydroxyl (OH-) ions to form sodium hydroxide (Na+ + OH- ? NaOH), commonly known as caustic soda, and the positive hydrogen ions (H+) undergo reduction to produce hydrogen gas (2H+ + 2e- ? H2). Therefore the overall reaction which takes place in this cell can be written as follows:
2 NaCl + 2 H2O ? Cl2 + H2 + 2 NaOH
The efficiency of the Membrane Cell
The main advantage of the Membrane Cell compared to the Diaphragm Cell and the Mercury Cell is that it is much more efficient. The Membrane cell requires the least amount of electricity to operate it and it is much more environmentally friendly, since it does not produce hazardous waste products like mercury or asbestos. In addition it is extremely safe to operate it and it produces a consistently high quality of caustic soda.
Risks associated with the operation of the Membrane Cell
One of the main risks in operating this system results from damage to the membrane which can cause a mechanical failure of the system. This can happen when the membrane is insufficiently wet by the electrolytic solutions (due to gas formation) resulting in an increased resistance across the remaining part of the membrane and therefore an increase in the voltage across the cell. This in turn can cause blistering and damage to the membrane, thus allowing mixing of the electrolytic gasses. If the right conditions prevail, the mixing of the gasses can sometimes cause an explosion. In addition the excessive voltage can cause uneven gas separation in the electrolytic compartments which may give rise to pressure pulse surges across the membrane with the eventuality of mechanical failure of the membrane. Another risk in operating the Membrane Cell is that careful precautions must be taken when extracting the chlorine and hydrogen gasses from the cell. This is because the chlorine gas is highly toxic and the hydrogen gas is extremely flammable.
One of the products that is readily produced from the plant is Sodium Hydroxide, commonly known as Caustic Soda. This product is made during the process of electrolysis when extracting chlorine from brine water.
This product is produced when the positive Sodium ions (Na+) from saturated brine water (NaCl) move across from the anode compartment, through the membrane and into the cathode compartment. In the cathode compartment the Na+ ions react and join up with the negative Hydroxide ions (OH-) form the water molecules to produce Sodium Hydroxide (NaOH) which is Caustic Soda (2Na+ + 2H2O + 2e? ? H2 + 2NaOH). The NaOH builds up at the cathode as more Na+ ions and OH- ions join together and Hydrogen gas (H2(g)) is discharged at the cathode.
Sodium hydroxide can be used as a powerful industrial cleaning agent where it is usually referred to as ?Caustic Soda?. It can dissolve grease, oils, fats and protein based deposits and thus it is very effective for cleaning drains. It is added to water and the solution is then heated and used to clean industrial apparatus such as process equipment and storage tanks. It is also commonly used as an ingredient in oven cleaners and as a solution it can be used to dissolve grease on glass stainless bake ware. Another common use of Sodium Hydroxide is to produce parts? washer detergents. These detergents are extremely strong cleaning chemicals and include surfactants, rust inhibitors and defoamers. A parts waher heats water and the detergent in a closed cabnate and then sprays the hot solution at high pressure against the dirty parts in order to de grease them.
This use of Sodium hydroxide as a cleaning agent has replaced many solvent based chemicals. These were banned in the 1990?s when the use of trichlor was declared illegal, because of its negative effects on the environment. Thus these parts? washers which use Sodium Hydroxide based detergents are a great improvement in comparison to the solvent based cleaning methods. However, Sodium Hydroxide based products must be handled with extreme caution. It can irritate the lungs if inhaled and it can become a serious irritant if it comes into contact with the skin.
In addition Sodium Hydroxide can be used to produce other products such as domestic soap.
Soap is manufactured by causing a reaction between Sodium Hydroxide with Magnesium chloride according to the following chemical equation. 2NaOH + Mg(Cl)2 ? 2NaCl + Mg(OH)2 . As a result of the Magnesium Chloride a white precipitate appears (if Iron(3) Chloride was used instead of the Magnesium Chloride, a red precipitate would have been formed.) This product is extremely beneficial and considered vital for hygiene and thus is an essential pat of human cleansing. It is also used for washing cutlery and crockery as it dissolves fats easily. In addition it is used for cleaning surfaces in order to prevent bacteria from developing on unclean exteriors and causing unhygienic conditions. Therefore, soap is used in all situations by everyone in order to ensure a clean and hygienic environment. The ingredients in soaps are all treated with modern methods to ensure that their concentration levels are so small that they do not have a significant impact on the environment.