PDF Publication Title:
Text from PDF Page: 002
This means that a minimum energy demand of 1654 kWh / tonne Cl2 is required to bring about the reac- tions, forming the required chemicals which store this energy in any product where this chlor-alkali ends up. This means that, when one uses those products in other processes, the 1653.8 kWh potentially can (partly) be released again, depending on the effi- ciency of the reactions taking place. For example, when H2 and Cl2 react to produce gaseous HCl, a significant amount of energy (720 kWh) is released as heat, the reaction temperature being around 2500oC. This high temperature HCl gas still contains a lot of energy. It should be noted that simply putting the thermo- dynamic minimum energy demand of 1654 kWh/ton Cl2 into the system, will not lead to efficient chlo- rine production though. There are several reasons for this: • The mentioned voltage of 2.1877 Volt may be suffcient to allow the reactions to occur, but at much too slowly a rate. Usually be- tween 2.5 and 3.5 Volt is needed to cause the electrons to move towards the elec- trodes at an acceptable speed and to give an efficient reaction rate at the electrodes. • Once the reaction has occurred, the reaction products need to remain separated. Indeed, H2 and Cl2 can form an explosive gas mixture as well as forming HCl, whilst Cl2 and NaOH will form NaClO (hypochlorite or ‘bleach’). To separate them, a special membrane (or a ‘diaphragm’) is installed between both elec- trodes. Having this essential separation brings about a a (small) drop in the voltage. • Finally, whenever electrons travel through an electrical conducting system, there are slight losses (as heat). This will also be the case for the current taking care of the ener- gy supply. No system is 100% efficient at car- rying these electrons without small losses. It is evident that more energy will be required than the amount of energy that is eventually stored in any chlor-alkali product. The following table gives an idea of the additional voltages: Over-potential at the electrodes 0.10 – 0.27 Volt Membrane/diaphragm 0.40 – 0.50 Volt Losses in the conducting systems 0.05 – 0.10 Volt TOTAL 0.55 – 0.87 Volt Euro Chlor Communications Rue Belliard 40 (box 15) B—1040 Brussels Tel. +32.2.436.9502 eurochlor@cefic.be In terms of energy consumption, this results in an additional 416 – 658 kWh per tonne of chlorine. It is self-evident that, when building more modern electrolysis units, engineers carefully weigh the reduction in energy losses versus the required reac- tion speed and continuously look into new technical developments to reduce the losses. Next to the electric equipment aspects leading to voltage losses, there is one additional physical and one additional chemical aspect that need to be taken into account. The separation wall (membrane or diaphragm) is not 100% perfect, meaning that some OH- molecules will manage to get through. Some competing reactions may also occur on the anode: 1) Water may be transformed into oxygen, ‘polluting’ the chlorine gas: 2H2O → O2 +4H+ +4e- 2) OH- that back-migrated from the cathode side via an imperfection in the membrane, may: - form hypochlorite or chlorate by reacting with chlorine - form water and oxygen as follows: 4 OH- → O2 + 2 H2O + 4e- When these reactions occur, they are considered as a loss of reaction efficiency. Typical reaction effi- ciencies are in the range of 92% to 99%. Taking everything together, it can be concluded that the total energy consumption when producing chlorine through the electrolysis process described earlier, consists of the following components: Thermodynamic minimum 1654 kWh / tonne Cl2 Voltage losses 416 - 658 kWh / tonne Cl2 Efficiency losses 21 - 201 kWh / tonne Cl2 TOTAL 2091 – 2513 kWh / tonne Cl2 (> 70%) ( 23%) (5%) The improvements for energy saving will focus on developing better membranes (higher efficiency) and membranes with a lower voltage (lower energy consumption). Finally, the economic aspect needs consideration, meaning that any investment shall be weighed up against operational costs. Much more about chlorine at www.eurochlor.org Chlorine chemistry applications: www.chlorinethings.eu A Sector Group ofPDF Image | Electrolysis process and its thermodynamic limits
PDF Search Title:
Electrolysis process and its thermodynamic limitsOriginal File Name Searched:
11-Electrolysis-thermodynamics.pdfDIY PDF Search: Google It | Yahoo | Bing
Salgenx Redox Flow Battery Technology: Power up your energy storage game with Salgenx Salt Water Battery. With its advanced technology, the flow battery provides reliable, scalable, and sustainable energy storage for utility-scale projects. Upgrade to a Salgenx flow battery today and take control of your energy future.
| CONTACT TEL: 608-238-6001 Email: greg@salgenx.com | RSS | AMP |