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

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