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INFORMATION 11 The Electrolysis process and its thermodynamic limits PUBLIC INFORMATION July 2018 The production of chlorine and caustic soda (chlor-alkali) involves using an electrolytic method. Electrolysis uses an electric current to drive a chemical reaction which otherwise would not occur spontaneously. For example, when dissolving kitchen salt in water, there is no spontaneous formation of chlorine, this only happens once an electrical current is sent through the solution. An electrolysis unit consists of 2 electrodes: 1) the anode, with a positive (+) charge 2) the cathode, with a negative (-) charge The cathode attracts positive charges, whereas negative charges migrate towards the anode. Whether these migrations actually take place and how fast the movements will be, depends on the difference in voltage between both electrodes (the higher the difference, the faster the migration). In chlor-alkali electrolysis units, chlorine gas is mostly produced from common, kitchen salt: 2 NaCl + 2 H2O Kitchen salt + water → 2 NaOH + H2 + Cl2 → caustic soda + hydrogen gas + chlorine gas The principle of electrolysis starting from a brine (kitchen salt) solution To make these chemical transformations happen, each electrode needs to have a minimal voltage or ‘potential’: Anode: 2 Cl- → Cl2 + 2e- required potential: 1.36 Volt (V) Cathode: 2 H2O + 2e- → 2 OH- + H2 required potential: -0.8277 Volt (V) This means that the energy source for this electrol- ysis unit (e.g. the local electricity supply) needs to ensure that the difference between these elec- trodes (i.e. 2.1877 V) is achieved. Using electrical laws, we can calculate the required amount of en- ergy (expressed in kWh) For 1 ton of Cl2 plus 1.1282 ton of NaOH plus 28.4 kg H2, one needs ... 1653.8 kWh To achieve this, brine (a highly concentrated salt solution in water, in this case NaCl) and water are brought into the system. The salt will dissolve in the water and split into Na+ and Cl-. At the anode, pairs of negatively charged chloride (Cl-) ions transform into chlorine (Cl2) by giving off two electrons. Cl2, being a gas, will bubble from the solution and is subsequently collected. Mean- while, at the cathode, hydrogen (H2) gas forms as the water molecules scoop up the 2 electrons men- tioned above.PDF Image | Electrolysis process and its thermodynamic limits
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