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

Chapter 3
hydrogen at low pressure and 50 wt-% caustic soda or potash, starting from salt, water,
electricity and steam. Neither the energy required to extract, purify and transport the raw
materials is included in this comparison nor is the energy required for liquefaction and
vaporisation of chlorine. In the case of the diaphragm and membrane cell techniques,
liquefaction and evaporation are often necessary to obtain chlorine with a purity similar to the
one obtained by using the mercury cell technique.
If global energy consumption figures are to be determined for each chlor-alkali manufacturing
technique, steam and electricity have to be expressed in the same units. The most logical way is
to refer to the primary energy necessary to produce both steam and electricity. For this purpose,
a power generation efficiency of 40 % and a steam production efficiency of 90 % was assumed
as was done in a 2009 publication of the International Energy Agency [ 64, IEA 2009 ].
This leads to a primary energy consumption of 9.0 GJ per MWh of electricity consumed and,
considering an exergy of 2.5 GJ/t steam (at 10 bars and with condensate return at 90 °C),
approximately 2.8 GJ per tonne of steam consumed [ 63, Euro Chlor 2010 ]. Furthermore, steam
consumption based on caustic produced was converted to steam consumption based on chlorine
produced by multiplying it by the stoichiometric factor of 1.128. A comparison of the total
energy consumption of the three cell techniques is shown in Table 3.9.
Table 3.9: Total energy consumption of chlor-alkali installations in EU-27 and EFTA
countries
Electrolysis
Process equipment
cells ( 1 Other
electrical
) equipment
( 1 ) ( 2 Caustic soda
concentration
(
)
1 ) ( 3 Total
)
Electricity AC kWh/t Cl2 3400 200 NA 3600
Mercury
Steam t/t NaOH NA NA 0 0
cell
Primary
technique
energy ( 4 GJ/t Cl2 30.6 1.8 0 32.4
)
Electricity AC kWh/t Cl2 2800 200 NA 3000
Diaphragm
Steam t/t NaOH NA NA 2.6 2.6
cell
Primary
technique
energy ( 4 GJ/t Cl2 25.2 1.8 8.1 35.1
)
Electricity AC kWh/t Cl2 2600 200 NA 2800
Membrane
Steam t/t NaOH NA NA 0.70 0.70
cell
Primary
technique
energy ( 4 GJ/t Cl2 23.4 1.8 2.2 27.4
)
( 1 ) Median values of chlor-alkali installations in EU-27 and EFTA countries. The values may vary considerably
from one plant to another depending on the current density and other plant-specific factors.
( 2 ) Energy consumption for chlorine liquefaction/vaporisation is not included.
( 3 ) Caustic concentration may not be necessary.
( 4 ) Assuming an exergy of 2.5 GJ/t steam (at 10 bars and with condensate return at 90 °C), a power generation
efficiency of 40 % and a steam generation efficiency of 90 %.
NB: NA = not applicable.
WORKING DRAFT IN PROGRESS
The mercury cell technique is characterised by the highest electrical energy consumption.
However, no steam is required to concentrate the caustic solution. The consumption of electrical
energy with the diaphragm cell technique is lower, but the total energy consumption is higher
because of the steam required to concentrate the caustic. The consumption of electrical energy
of the membrane cell technique is the lowest and the amount of steam needed for concentration
of the caustic solution is moderate resulting in the lowest total energy consumption. While these
general conclusions are widely accepted, it is necessary to go into more details when it comes to
evaluating the energy consumption of a specific plant.
TB/EIPPCB/CAK_Draft_1 December 2011 79