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

Chapter 4
4. The power supply
In most cases, the transformers and rectifiers are replaced, either because they have reached the
end of their life (approximately 30 years) or because their technical characteristics are
incompatible with the requirements of new membrane cell electrolysers. Existing mercury
electrolysers are, with a few exceptions, monopolar (high current, low voltage), while new
membrane electrolysers are bipolar (low current, high voltage) [ 211, Dibble and White 1988 ],
[ 232, Euro Chlor 2010 ].
Reuse of the rectifiers depends on the type of membrane cell configuration and power
requirements. The decision to renew the power supply or not will depend, among other things,
on the balance between investment and operating costs. A monopolar conversion should be
possible using existing rectifiers and transformers, as, with few exceptions, existing plants are
monopolar. Bipolar conversions can also be carried out keeping the existing power supply, one
example being the conversion of Donau Chemie in Brückl (Austria) in 1999 [Schindler]. A
careful review of voltage is required to ensure that adequate control is achievable for the
membrane system.
5. Gas treatment facilities
Chlorine and hydrogen collection and treatment are not essentially different in membrane and
mercury cell plants compared to the mercury process. Process steps to remove mercury are no
longer needed. The main concern for the new cell room is the method of pressure control of the
gases. The membrane cell technique technology requires a steady differential pressure of
hydrogen over chlorine. This means that a differential pressure controller must be added to the
existing control system [ 211, Dibble and White 1988 ].
The chlorine and hydrogen gases leave the cell at higher temperatures than with the mercury
cell technique technology. The gases will be are saturated with water vapour and so therefore
the loading on gas coolers will be higher, as will the volume of resulting condensate. In
addition, the primary cooling system for hydrogen is usually part of the mercury cells. New
cooling equipment may be needed is therefore needed in most cases [ 232, Euro Chlor 2010 ].
Chlorine from membrane cells contains more oxygen, which for some applications needs to be
removed prior to downstream use of the chlorine. For this purpose, total liquefaction followed
by subsequent evaporation of liquid chlorine is necessary. Larger liquefaction units than those
normally in operation are therefore required for these applications.
6. Caustic treatment
The mercury cell technique technology produces 50 wt-% caustic soda. Membrane cells require
a recirculating system with associated heat exchange and dilution and to produce 33 wt-%
caustic soda. If more concentrated caustic soda is needed, an evaporator system is also
necessary [ 211, Dibble and White 1988 ]. These changes usually require an extension of the
cooling water circuit and steam production [ 232, Euro Chlor 2010 ].
WORKING DRAFT IN PROGRESS
7. Process piping
The reuse of existing process piping is not appropriate when converting to the a membrane cell
technique process, as the physical location is often very different from that required by
membrane electrolysers. The existing piping may also be made of material unsuitable for use
with membrane electrolysers [ 211, Dibble and White 1988 ].
An example of a conversion is the Borregaard chlor-alkali plant in Sarpsborg (Norway). Data
are summarised in Table 4.2.
150 December 2011 TB/EIPPCB/CAK_Draft_1