(BAT) Reference Document for the Production of Chlor-alkali ...
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(BAT) Reference Document for the Production of Chlor-alkali ...

(BAT) Reference Document for the Production of Chlor-alkali ... (BAT) Reference Document for the Production of Chlor-alkali ...

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Chapter 6 As in usual membrane cell operation, weak caustic soda is introduced into the cathode chamber where it is enriched. However, differing from conventional membrane cells, the feed caustic is introduced at the top into the ‘percolator’ rather than at the bottom of the element to prevent hydrostatic pressure from getting too high. The ODC is installed as the next layer and is made out of PTFE, noble metals such as silver, and other active elements. The ODC is in contact with the caustic on one side while oxygen enters the porous structure from the other. The next layers are the ‘elastic element’ and the ‘current collector’. The current is flowing via this welded current collector through the elastic element to the ODC. A proper electric contact and even current distribution is reached by gently pressing all pieces together maintained by the elastic element [ 49, Euro Chlor 2010 ]. The main disadvantage of finite-gap electrolysis cells is that the porous electrodes suffer from the permeation of gas or liquid if the pressure difference between both sides exceeds relatively low values. Industrial electrodes are usually higher than 1 m resulting in a hydraulic pressure difference between bottom and top of approximately 0.1 bar while the gas pressure remains constant. This normally limits the active height to some 20 – 30 cm. Three different concepts have been described to overcome this problem [ 52, Moussallem et al. 2008 ]. Due to its porous structure the ODC can only withstand limited differential pressure between caustic and oxygen, which normally limits the active height to some 20 – 30 cm. This problem has been solved in a joint collaboration between DeNora and Bayer with the development of a pressure compensation system which supplies the oxygen to the ODC via gas pockets. The principle is shown in Figure 6.2. Figure 6.2: Principle of the gas pocket electrode for pressure compensation [Gestermann, 1998] {The figure was deleted.} The first concept is the use of the 'falling film electrolysis cell' as shown in Figure 6.1. In these falling film cells, the hydrostatic pressure is compensated by an equally high counteracted hydrodynamic pressure drop. As a result, the pressure difference between electrolyte and gas on the other side of the ODC remains constant over the whole height of the vertical electrode. The falling film concept allows for a very small electrolyte gap of less than 1 mm between the membrane and the ODC. In some cases, a porous material made of metals, metal oxides or polymers is included in the cathodic compartment of a falling film electrolyser [ 52, Moussallem et al. 2008 ]. The second concept uses compartments on the oxygen side, the pressure of which can be independently adjusted. The height of these compartments is sufficiently low to prevent electrode flooding or gas permeation. This concept was successfully applied by Bayer and Uhde/Uhdenora on a pilot scale in an electrolyser with a 2.5 m 2 electrode area and 16 bipolar elements but was later discontinued. The third concept relies on the use of electrolysis cells with horizontally disposed electrodes which has not yet been tested on a technical scale [ 52, Moussallem et al. 2008 ]. Main achievements WORKING DRAFT IN PROGRESS In laboratory experiments at Bayer with oxygen depleted cathodes supplied by the DeNora group it was shown that under normal conditions, i.e. 32% caustic, 90 ºC, the operation voltage could be reduced by about 1 volt. An energy gain of about 500-600 kWh/t chlorine produced was achieved. A four element bipolar pilot electrolyser incorporating the pressure compensation design was constructed by DeNora and was tested at the Bayer endurance test facility. It had an element size of 0.3 m 2 with a full industrial-size height of 1.3 m. The experimental results demonstrated that at 3 kA/m 2 an operation around 2 volts was possible as well as a continuous operation with 6 kA/m 2 slightly above 2.4 volts. 292 December 2011 TB/EIPPCB/CAK_Draft_1

Chapter 6 In December 1998 a test with 16 cells, each with 2.5 m 2 active area, was put into operation at the Bayer test-site in Leverkusen with good results. The results were comparable to the pilot plant operating experience; the standardised power consumption at 3 kA/m 2 was

Chapter 6<br />

In December 1998 a test with 16 cells, each with 2.5 m 2 active area, was put into operation at<br />

<strong>the</strong> Bayer test-site in Leverkusen with good results. The results were comparable to <strong>the</strong> pilot<br />

plant operating experience; <strong>the</strong> standardised power consumption at 3 kA/m 2 was

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