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

Chapter 4
membrane lifetimes range between three and five years [ 26, Euro Chlor 2010 ], [ 136, Asahi
Kasei 2008 ], [ 189, Nitini 2011 ]. In general, the cell voltage increases slightly with current
density and electrode distance. Membranes for monopolar electrolysers show higher voltage
drops than those for bipolar electrolysers due to higher requirements for mechanical stability
[ 26, Euro Chlor 2010 ], [ 188, DuPont 2006 ], [ 204, Asahi Glass 2011 ].
The replacement of a membrane at the end of its service life by the best proven one which is
compatible with the type of electrolyser used, typically leads to electricity savings of
Q 20 AC kWh/t Cl2 produced [ 3, Euro Chlor 2011 ].
Cell voltage is reported for new perfluorinated Nafion® membranes to be 2.88 V at a current
efficiency of >95% (conditions: 0 mm gap, DSA anode, activated cathode, 32% NaOH, 200 g/l
anolyte, 90 °C, 3.0 kA/m2). The lifetime of the membranes ranges between 3 and 5 years.
[DuPont]
Energy saving by developing a membrane having low ohmic drop has also been realised with
Flemion membranes. They show a small increase in cell voltage while current density is
reported up to 6 kA/m2. [Asahi Glass]
Cross-media effects
Some raw materials and energy are consumed for the manufacture of the membranes. The use of
high-performance membranes may require additional brine treatment which in turn may lead to
additional consumption of energy and ancillary materials and to generation of additional waste.
Technical considerations relevant to applicability
The use of high-performance membranes is restricted to membrane cell plants. Generally, there
are no technical restrictions to their use in new electrolysis units. Existing membrane cells can
often be retrofitted depending on the availability of improved membranes from the respective
equipment provider under the given conditions [ 63, Euro Chlor 2010 ].
Economics
No specific economic information for the high-performance type membranes is known to the
author at the time of writing.
The average cost for membranes is approximately EUR 500/m 2 . For a bipolar electrolyser
operating at a current density of 6 kA/m 2 and a current efficiency of 95 %, the chlorine
production rate is 64 t/(m 2 ·yr). The average lifetime of a membrane is approximately four years.
This results in a total chlorine production of 256 t/m 2 and specific membrane costs of
EUR 2.0/t Cl2 produced which represents about 1 % of the total variable costs for an
Electrochemical Unit. If the membrane lifetime decreases to three years, this specific cost
becomes EUR 2.6/t Cl2 produced. For a monopolar membrane electrolyser operated at a current
density of 3.5 kA/m 2 , the specific costs are much higher (EUR 3.4/t Cl2 produced). Practically,
the aforementioned costs double because the gaskets have to be simultaneously replaced with
the membranes [ 26, Euro Chlor 2010 ].
When the costs for steam are higher than the costs for electricity, it is economically viable not to
significantly reduce the cell voltage because this would require compensation by additional
heating of the brine with steam to maintain the operating temperature [ 26, Euro Chlor 2010 ].
WORKING DRAFT IN PROGRESS
Driving force for implementation
The driving forces for implementation of this technique include the following:
replacement of membranes at the end of their lifetime;
reduction of costs related to energy consumption;
increased production rate;
improvement of product quality;
reduction of costs related to equipment and maintenance.
TB/EIPPCB/CAK_Draft_1 December 2011 201