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

Chapter 3
current) and the efficiency of the electrolytic reaction itself (efficiency with which the electrons
are effectively used to produce chlorine) [ 63, Euro Chlor 2010 ], [ 65, Millet 2008 ].
U0 (in V) is a constant term that depends on the cell characteristics. U0 is composed of the
difference in electrode potentials and the activation overpotentials at zero current. The minimum
value of U0 is imposed by thermodynamics and, for a given cell technique, mostly depends on
the material and coating of the electrodes (~ 2.35 V for diaphragm and membrane cells; ~ 3.15
V for mercury cells) [ 63, Euro Chlor 2010 ], [ 65, Millet 2008 ].
The term K · j represents the overpotential during electrolysis (j > 0) which is composed of the
activation and concentration overpotentials at the two electrodes, the resistance overpotential of
the anolyte and catholyte including the contribution from gas bubbles, the resistance
overpotential of the separator (diaphragm or membrane) and the resistance overpotential of the
electrical conductors. The factor K (in V·m 2 /kA) therefore depends on the geometry of and the
distance between the electrodes, the nature of the separator between the electrodes (i.e.
diaphragm or membrane), the temperature and electrolyte concentrations of the liquids in both
the anolyte and the catholyte compartment as well as the internal equipment pressure. K is
essentially determined by the technique of the electrolysers and is influenced by the operating
conditions [ 63, Euro Chlor 2010 ], [ 65, Millet 2008 ].
As an example, the normal range of specific electrical energy consumption w versus current
density j for the mercury cell technique is shown in Figure 3.1 [ 63, Euro Chlor 2010 ].
Cell voltage in V
Source: [ 1, Ullmann's 2006 ]
Current density in kA/m 2
Specific energy consumption
In AC kWh/t Cl2 produced
Figure 3.1: Cell voltage and specific electrical energy consumption versus cell current density
for the mercury cell technique
For all electrolysis cells, lower current densities mean lower energy consumption resulting in
lower operating costs. However, this leads to larger or an increased number of electrolysers
resulting in higher investment and maintenance costs if the same overall production rate is to be
achieved. In general, increasing the production rate of a cell by increasing the current density to
values higher than the normal range leads to a disproportionate increase in the electric resistance
losses, and hence to a disproportionately higher specific energy consumption [ 63, Euro Chlor
2010 ].
WORKING DRAFT IN PROGRESS
Chlor-alkali installations often operate at varying current densities depending on the demand for
the products and on the fluctuations in electricity prices. Operating conditions and electricity
consumption of the electrolysis cells of chlor-alkali installations in EU-27 and EFTA countries
are shown in Table 3.5.
TB/EIPPCB/CAK_Draft_1 December 2011 73