Electrolysis membranes

Platinum Platinum-coated titanium is the most important anode material for impressed-current cathodic protection in seawater. In electrolysis cells, platinum is attacked if the current waveform varies, if oxygen and chlorine are evolved simultaneously, or if some organic substances are present Nevertheless, platinised titanium is employed in tinplate production in Japan s. Although ruthenium dioxide is the most usual coating for dimensionally stable anodes, platinum/iridium, also deposited by thermal decomposition of a metallo-organic paint, is used in sodium chlorate manufacture. Platinum/ruthenium, applied by an immersion process, is recommended for the cathodes of membrane electrolysis cells. ... 

Thanks to its high stability and permselectivity, Nation has been used as a Na+ conductor in membrane electrolysis of brine in the chlor-alkali industry. This application was introduced in the early 1980s and is by far the most important use of ionomer membranes. 

In ion-exchange membrane electrolysis, it is important to control impurities contained in the brine. 

Easily controlled. Membrane electrolysis processes require brine with sulphate content of no more than approximately 7.0 g l-1 (as Na2S04). The RNDS achieves this level quite easily through automated control. 

Figure 12.1 shows a scheme of the brine system for the membrane electrolysis process. The RNDS is installed at the point of depleted brine flow. Figure 12.2 illustrates the principle of the RNDS operation. The required area for the RNDS set-up in a chlor-alkali plant having a capacity of 135 000 tonnes of NaOH per annum is 54 m2. 

As was mentioned previously, an effective system, RNDS , has been developed to remove particular impurities from brine used in membrane electrolysis procedures. The basic concept of RNDS is to bring the feed brine into contact with an ion-exchange resin containing zirconium hydroxide for the adsorptive removal of impurities. For the removal of the sulphate ion from brine, commercial plants utilising RNDS are already in service. For the elimination of iodide and silica, pilot-scale testing is being planned. 

Advances during the past 20 years in membrane, electrolyser, electrode, and brine purification technologies have substantially raised the performance levels and efficiency of chlor-alkali production by ion-exchange membrane electrolysis, bringing commercial operations with a unit power consumption of 2000-2050 kWh per ton of NaOH or lower at 4 kA m-2 current density with a membrane life of four years or longer. 

Figure 10.19 Schematic of a single bipolar membrane (not to scale) showing generation of hydroxyl and hydrogen ions by water splitting in the interior of the membrane. Electrolysis takes place in the thin interfacial region between the anodic and cathodic membranes...

Ion-exchange membranes are currently used not only for more or less conventional separation processes like membrane electrolysis (mainly the chlor-alkali process), electrodialysis, dialysis or electro-ultrafiltration (cf. Table 2.1), but also in various... 

Membrel cell — (membrane electrolysis) Electrochemical cell developed by BBC Brown Boveri Ltd, now joined with ASEA AB, to ABB Asea Brown Boveri Ltd) for water electrolysis. A polymeric cation exchange membrane acting as -> solid electrolyte is placed between a catalyst-coated porous graphite plate acting as cathode and a catalyst-coated porous titanium plate acting as anode. 

Figure 1.7. Schematic layout of proton exchange membrane electrolysis cell.

Membranes may be hastily classified according to the driving force at the origin of the transport process (1) a pressure differential leads to micro-, ultra-, nanofiltration, and reverse osmosis (2) a difference of concentration across the membrane leads to diffusion of a species between two solutions (dialysis) and (3) an electric potential difference applied to an ion-exchange membrane (lEM) leads to migration of ions through the membrane (electrodialysis, membrane electrolysis, and... 

Figure 8.3 The Chlor-Alkali Membrane Electrolysis cell...

Utilization of evaporated salt is gaining in importance due to the progressive introduction of chloralkali membrane electrolysis technology, which places high demands on the purity of the sodium chlorine-brine utilized. 

But membrane electrolysis suffers a severe drawback the high costs for the solid polymer electrolyte. If membranes being... 

Fig. 23 Scheme for H2O membrane electrolysis (not to scale). The contact elements are also the current collectors. The principle displayed originates from fuel cell technology, compare Chapter 8 of this volume. 

The process itself and environmental aspects of this have been analyzed for efficiency of operation and air and water pollution control measures [61]. Membrane electrolysis coupled to electrodialysis has been studied for waste-water treatment [62]. 

E. Gain, S. Laborie, P. Viers et al.. Ammonium nitrate wastewater treatment by... membrane electrolysis and electrodialysis, J. Appl. Electrochem. 32(9), 969—975, Sept. (2002). 

Using the treated membrane electrolysis of sodium chloride solution was carried out under the same electrolysis conditions as 1). The treated surface of the membrane was faced to the cathode side in the electrolyzer. When 6.5 N sodium hydroxide solution was obtained as catholyte, the current efficiency was 93% and the cell voltage was 3,85v. On the other hand, the ion exchange membrane not treated by phosphorous pentachloride and triethylamine showed the current efficiency of 52% and the cell voltage of 3.68v when 6.5 N sodium hydroxide was obtained as catholyte. 

It is obvious that membrane electrolysis is much more efficient because most of the current was consumed by bromine formation reaction. The current efficiency value in EL-2 tests that was run for 60 minutes was 20% higher values compared to the tests in undivided cells. 

T. Shimohira, Y. Saito, K. Saito and H. Miyake, Design and future of Fx-50 membrane A membrane for production of 50% caustic soda, Rep. Res. Lab. Asahi Glass Co., Ltd, 1993, 43, 119-128 J.D. Powers, Membrane electrolysis process for producing concentrated caustic. USPat. 4,900,408, 1990 T. Hiyoshi and A. Kashiwada, Fluorinated cation exchange membrane, Jpn. Pat. JP 5-222220 (unexamined application). 

Figure 6.23 Example flow diagram of ion exchange membrane electrolysis of sodium chloride solution.

Figure 6.26 Principle of production of highly pure tetramethylammonium hydroxide by membrane electrolysis. A anion exchange membrane C cation exchange membrane TMA+ tetramethylammonium ions.

Long-term stability and contamination is a problem with both alkaline and polymer membrane electrolysis electrolytes. Alkaline electrolytes adsorb carbon dioxide readily and form carbonates. Polymer membrane systems must use very pure de-ionized water or they will accumulate cations that displace protons and increase the cell resistance over time. 

Membrane electrolysis technology is well established and appropriate for smaller scale facilities. One application is its use as an oxygen generator in submarines, where the hydrogen is considered only a byproduct. Drawback is the high-cost membrane production [14]. 

PEM (proton exchange membrane) electrolysis the electrodes are separated by a proton-conducting polymeric solid electrolyte (membrane)... 

Membrane electrolysis cells have many applications in the food industry (dairy, wine, fruit juice, etc.), water softening, purification or recovering effluents from electroplating and other chemical processes. Possibly the best known processes are desalinating brackish water with a moderate salt content (other processes such as reverse osmosis are used upstream) and demineralising whey in the dairy industry. 

To find an example of an industrial process using membrane electrolysis cells, one can refer to the illustrated board entitled Electrodialysis... 

Electric membrane processes, including electroosmosis, electrodialysis, and membrane electrolysis, are studied for different applications along the power generation cycle (Andalaft et al. 1997 Hobbs 1999 Hegazy et al. 1999). The novel anion exchange membrane was applied to separate and C1 ions by electrodialysis (Inoue et al. 2004). The membrane exhibited high selectivity for iodine ions over chlorine ions, and the ratio of electroconductive membrane permeabilities of 1 and C1 was 6.2, while the diffusion membrane permeabilities of the two components were almost the same. 

According to their temperature and the nature of the electrolyte membrane, electrolysis cells can be classified in to two families, as for fuel cells (see Section 15.2.2). Here only the main viable processes are presented. Low-temperature electrolysis cell ... 

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