Bipolar electrolyser

Each plant will utilise four Chlorine Engineers Corporation BiTAC 838 bipolar electrolysers, with 38 cells per electrolyser operating at current densities up to... 

For the membrane cellroom of the same capacity there are two choices of technology type either monopolar or bipolar electrolysers. In the case of monopolar membrane electrolysers (Fig. 15.9), such as the ICI FM1500, one membrane electrolyser can replace one diaphragm cell. Since the membrane electrolyser has smaller dimensions there is an overall space saving. The monopolar membrane electrolysers may use the same pipework galleries and overhead crane from the... 

If bipolar membrane electrolysers are installed (Fig. 15.10), such as the ICI BiChlor, then even less floor area is required for the same production capacity. In both membrane cases the space available also depends on the chosen operating current density. Utilising the 150 kA available, nine bipolar electrolysers can operate up to 16.7 kA each. This would require 112 anodes per electrolyser to manufacture the 200 000 tonnes per year of caustic soda, utilising about 360 V of the 450 V available. With bipolar electrolyser centres of 6.5 m, including operator walkways, an area of around 60 m by 14 m or 840 m2 will be required. However, more extensive pipework modifications are required with bipolar arrangements. 

If the choice is to utilise the full capacity of the existing rectifiers and install more membrane electrolysers then adequate space is available. In the 200 000 tonnes per year example, utilising the voltage saved and adding 16 extra monopolar electrolysers would take less space than the original diaphragm cells. In the case of bipolar electrolysers, the length of the electrolyser could be increased as more anodes and cathodes are added to each electrolyser. The number of electrolysers, however, would stay the same. 

Fig. 15.13 Utilisation of rectifier capacity in bipolar membrane electrolysers.

The monopolar option utilised 399 V (including bus bar losses) so an extra 16 electrolysers can be added to the circuit making an extra 28 000 tonnes per year of caustic or US 7million extra sales value (at US 250 per ECU). The bipolar option utilised only 360 V on each electrolyser so an extra 28 anodes could be added, manufacturing an extra 50 000 tonnes per year (Fig. 15.13). At US 250 per ECU, this yields an additional US 12.5 million of revenue. 

Replacement of diaphragm cells with bipolar membrane electrolysers requires a different electrical layout (Fig. 15.17) since each bipolar membrane electrolyser can only take about 17 kA of the 150 kA available (for a selected current density). This means that all nine electrolysers need to be installed together. The number of anodes in each bipolar electrolyser can be set depending on the number of diaphragm cells left on load, up to the maximum voltage of the rectifiers. 

ICI ETB identified that in order to ensure that it has the technology to provide solutions across the full range of the market, a world-scale bipolar electrolyser was needed in addition to the proven and excellent FM1500. 

Each Nestpak module in the BiChlor electrolyser is sealed along its perimeter by a combination of a bolted flange and gaskets. This allows the sealing pressure to be controlled around all of the perimeter of each individual Nestpak. In a large-area bipolar filter-press electrolyser, sealing is only achieved with massive compression equipment and even then a variable sealing pressure is likely to exist around the module perimeter. 

Advanced Cell Technology with Flemion Membranes and the AZEC Bipolar Electrolyser... 

AGC has been recently focusing on the development of a new electrolyser and a new membrane for high current density operation, a facility much requested by many users. In July 1998, AGC completed the conversion of its last diaphragm process plant to the then newest Bipolar Electrolyser, the AZEC B-l (hereinafter, B-l) with Flemion F-893 (hereinafter, F-893) membrane and also the then-newest membrane Flemion Fx-8964 (hereinafter, Fx-8964). This conversion was the result of AGC s development efforts. AGC is now on the way to the next stage of its ion-exchange membrane technology, where 6 kA /m-2 operation will be the norm and 8 kA m-2 operation will be made a feasibility. 

The recent general outlook of AZEC B-1, the efficient bipolar electrolyser... 

AZEC Improved B-1 the high performance bipolar electrolyser for high current density operation... 

The core of the electrolyser is the cell block, which is made up of a large number of usually bipolar cells in a modular structure.8 Typical sizes of a cell block range from 1 to 800Nm3/h. (Most electrolysers sold today to laboratories, the semiconductor industry, etc., have a capacity less than 60 Nm3/h.) The biggest capacities realised are about 150 MW. An alkali solution, usually 20% to 40% potassium hydroxide (KOH), is used as the electrolyte that flows between the electrodes. In alkaline solutions, the electrodes must be resistant to corrosion, have good electronic... 

Another novel type of electrochemical fluorination unit, incorporation a metal electrolyser lined with teflon, capable of both bipolar and monopolar function is claimed in a Russian patent [139] to have advantages in simplicity of assembly and operation. 

Figure 6. Bipolar arrangement of electrodes in a channel electrolyser.

Because a fuel cell functions at a low voltage (/.c., well below 1 V), it is customary to build up the voltage to the desired level by electrically connecting cells in series to form a stack . This is achieved by means of a bipolar plate-and-frame arrangement similar to that employed for electrolysers see Section 4.2, Chapter 4. There are a number of different designs of fuel cell, but in each case the unit cell has certain components in common. These are as follows. 

Figure 5.3 CJB bipolar filter press type water electrolyser.

Figure 5.5 View from above of two of a bipolar stack of solid electrolyte water electrolyser cells.

As discussed in the Introduction section, three main types of electrolytic cells have been used for the large scale production of chlorine and caustic soda mercury, diaphragm and membrane cells. The main difference in these technologies lies in the manner by which the chlorine gas and the sodium hydroxide are prevented from mixing with each other to ensure generation of pure product. Alternatively, the electrolysis of hydrochloric acid solutions is also used to produce chlorine. Individual electrolysis cells can be electrically wired in parallel (monopolar electrolysers) or in series (bipolar electrolysers). 

Various attempts were made to develop pressure water electrolysers between 1920 and the second world war, but all came short of industrial recognition Noeggerath in 1927 (150 bar), Neiderreither in 1929 (bipolar electrodes, 5 kW), Lawaczek in 1928 (300 bar), Siemens and Halske in the late 30 s (5 to 10 bar). 

Electrolysis of Hydrochloric Acid. Development of this process was conducted initially at Bitterfield in the I.G. Farben Industrie Plant (26). The German patent application covering this innovation was dated October 15, 1942. It took 14 years before this work became known and taken up by De Nora in Milan. The original HCl electrolysis cell constructed on this concept had vertical bipolar electrodes. An extensive study made of possible diaphragm materials had resulted in a choice that gave satisfaction since the last electrolyser was operated by I.G. Farben continuously for fifteen months, but the details of manufacture of the diaphragm were never published. 

Fig. 3.11 The view from above of a PPG VI161 electrolyser unit to show arrangement of electrodes. The sketch shows three of the n cells in the bipolar stack.

---