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

Chapter 2
The membranes used in the chlor-alkali industry are commonly made of perfluorinated
polymers. The membranes may have from one up to three layers, but generally consist of two
layers (Figure 2.12). One of these layers consists of perfluorinated polymer with substituted
carboxylic groups and is adjacent to the cathodic side. The other layer consists of perfluorinated
polymer with substituted sulphonic groups and is adjacent to the anodic side. The carboxylate
layer exhibits a high selectivity for the transport of sodium and potassium ions and largely
prevents the transport of hydroxide, chloride, hypochlorite, and chlorate ions, while the
sulphonate layer ensures good mechanical strength and a high electrical conductivity. To give
the membrane additional mechanical strength, the membrane it is generally reinforced with
PTFE fibres. The membranes must remain stable while being exposed to chlorine on one side
and a strong caustic solution on the other. Commercially available membranes are optimised for
use in a specific strength of caustic. Depending on the particular design, membrane sizes range
from 0.2 to 5 m 2 . The general economic lifetime of chlor-alkali membranes is approximately
three years, but ranges between 2-5 years [Euro Chlor report, 1997]. three to five years [ 1,
Ullmann's 2006 ], [ 26, Euro Chlor 2010 ].
Anodic element
Sulphonate layer
mechanical strength
high electrical
conductivity
PTFE grid
tear strength
high mechanical
resistance
stiffness
Hydrophilic coating
Cl2 gas release
reduced cell voltage
Source: [ 23, BASF 2010 ] [ 24, Asahi Kasei 2008 ]
S C
Figure 2.12: Sectional drawing of a membrane
Cathodic element
Carboxylate layer
high selective ion permeability
prevention of OH - migration
(high current efficiency)
prevention of Cl- diffusion
(low content of NaCl in NaOH)
Sacrificial thread (warp/weft)
consistently good brine
distribution
Hydrophilic coating
H2 gas release
reduced cell voltage
In the design of a membrane cell, minimisation of the voltage drop across the electrolyte is
accomplished by bringing the electrodes close together. However, when the gap is very small,
the voltage increases because of the entrapment of gas bubbles between the electrodes and the
hydrophobic membrane. This effect is avoided by coating both sides of the membrane with a
thin layer of a porous inorganic material to enhance the membrane’s ability to release the
gaseous products from its surface. These improved membranes have allowed for the
development of modern cells with zero-gap or finite-gap cathode structures [ 10, Kirk-Othmer
2002 ], [ 26, Euro Chlor 2010 ].
WORKING DRAFT IN PROGRESS
2.4.3 Monopolar and bipolar electrolysers
Membranes cells can be configured either as monopolar or bipolar. As in the case of the
diaphragm cell process, the bipolar cells have less voltage loss between the cells than the
monopolar cells. 4 However, the number of cells connected together in the same circuit is
limited.
4
It has been estimated that approximately 30 kWh are lost each year per tonne chlorine produced. Energy
losses from monopolar electrolysers are about 150 kWh per tonne chlorine produced compared to bipolar ones.
TB/EIPPCB/CAK_Draft_1 December 2011 35