Retrofitting technology to achieve zero gap membrane electrolyzers in chlor-alkali indusrties

The chlor-alkali industry is the industry that produces Chlorine (Cl2) and alkali, Sodium hydroxide (NaOH) or Potassium hydroxide (KOH), by electrolysis of a salt solution. The main technologies applied for chlor-alkali production are mercury, diaphragm and membrane cell electrolysis, mainly using sodium chloride (NaCl) as feed or to a lesser extent using potassium chloride (KCl) for the production of potassium hydroxide. The diaphragm cell process (Griesheim cell, 1880s) and the mercury cell process (Castner-Kellner cell, 1890s) were both introduced in the late 1800s. The membrane cell process was developed much more recently (1970). Each of these processes represents a different method of keeping the chlorine produced at the anode separate from the caustic soda and hydrogen produced, directly or indirectly, at the cathode. Currently, 95% of world chlorine production is obtained by the chlor-alkali process. Chlorine is largely used in the synthesis of chlorinated organic compounds. VCM for the synthesis of PVC still remains the driver of chlor-alkali production in most European countries. The Mercury cell process, being phased out worldwide because of the toxic character of Mercury and the diaphragm cell process. Best Available Techniques for the production of chlor-alkali is considered to be membrane technology [1]. Membrane electrolytes are advantageous to others, including; environmental friendly, less power consumption, high efficiency and easy operation. There are several electrolyzer design licensors, each of which has its own technology, such as ThyssenKrupp, Ineos. AKC, AGC. Uhdenora (one of ThyssenKrupp s subsidiaries) is one of the most famous licensor which established BM-2.72 single element (one anode, one cathode and one membrane in between). Version I/II of this type was introduced in early 1990s and was developed until the last version (ver. 6) in 2012. The main criteria of new version is reducing of power consumption. For example, in version 3 (which has been used in Iran Chlorine plants like Arvand Petrochemical Complex), the distance between anode and cathode is 1.2 mm (called 13th Annual Electrochemistry Seminar of Iran Materials and Energy Research Center (MERC), 22- 23 Nov, 2017 222 Narrow Gap Cells) and power consumption is about 2170 kWh/1 ton NaOH 100% at current density of 6 kA/m2 . By reducing the gap between the electrodes, Uhdenora started to decrease energy consumption and finally found version 6 (called Zero Gap) and much less electricity consumption (about 2020-2035 kWh/1 ton NaOH 100% at current density of 6 kA/m2). Uhdenora is trying to invent version 7 with the lower energy consumption which is still unknown for clients. One interesting technology which is going to be common in the world is direct modification of single electrolyzer version 3 or 4 to almost zero gap [2, 3, 4, 5]. This change in the distance of the electrodes is carried out with specific knowledge and instruments and really is worth the research.