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

Chapter 2
The cooling water is generally should be free of traces of ammonium salts to avoid the
formation of nitrogen trichloride. This method has the advantage of better mass-transfer
characteristics and higher thermal efficienciesy.
Closed-circuit direct cooling of chlorine combines the advantages of the two methods. The
chlorine-laden water from the cooling tower is cooled in titanium plate coolers and is recycled.
The surplus condensate is treated exactly like the condensate from indirect cooling
[ 1, Ullmann's 2006 ].
2.6.4 Cleaning of wet chlorine
Following primary cooling, chlorine gas is demisted of water droplets and impurities such as
brine mist impurities. Impurities are removed mechanically by using special filters with glass
wool fillings or porous quartz granules, or by means of an electrostatic precipitator. Chlorine is
then passed to the drying towers [ 1, Ullmann's 2006 ].
2.6.5 Drying
Chlorine from the cooling system is more or less saturated with water vapour. The water content
is typically 1 – 3 vol-%. This must be reduced in order to avoid downstream corrosion and
minimise the formation of hydrates [ 38, O'Brien and White 1995 ] [Brien-White, 1995].
The drying of chlorine is carried out almost exclusively with concentrated sulphuric acid
(96 – 98 wt-%) [Ullmann’s, 1996]. Drying is accomplished in countercurrent sulphuric acid
contact towers in two to six stages, which reduce the moisture content to less than
20 mg/m 3 ppm [ 54, Euro Chlor 2010 ]. [Stenhammar] The remaining moisture content depends
on the temperature and concentration of the sulphuric acid in the last drying stage. For lowtemperature
liquefaction (see Section 2.6.8), a lower moisture content is required, which can be
achieved by adding more equilibrium stages to the drying towers or by using molecular sieves
to levels of 3 – 9 mg/m 3 [ 3, Euro Chlor 2011 ], [ 54, Euro Chlor 2010 ].
The number of stages is usually increased to lower the final strength of the spent sulphuric acid.
For example, three stages are needed to reach a spent acid concentration of 50 – 65 wt-% while
six stages are needed for a final concentration of 30 – 40 wt-%. The columns contain plastic
packing resistant to chlorine and sulphuric acid to improve fluids distribution, increase
efficiency and lower pressure drops, and thus reduce energy consumption. The heat liberated
during dilution of the circulating acid is removed by titanium heat exchangers, and the spent
acid is dechlorinated chemically or by stripping. The concentration of the spent acid depends on
the number of drying stages and the further potential use or method of disposal. In some cases,
the acid is reconcentrated to 96 wt-% by heating it under vacuum and is subsequently
recirculated. Sometimes the acid is sold or used in waste water treatment, otherwise it becomes
waste [ 3, Euro Chlor 2011 ], [ 54, Euro Chlor 2010 ].
Dry chlorine leaving the top of the drying tower passes through high efficiency demisters to
prevent the entrainment of sulphuric acid droplets. The spent acid usually becomes a waste
product or requires reprocessing if it is reused. For example, it has to be dechlorinated by air
blowing and may be reconcentrated before being sold or used for effluent treatment.
WORKING DRAFT IN PROGRESS
2.6.6 Cleaning of dry chlorine
When leaving the top of the drying tower, dry chlorine passes through high efficiency demisters
or a packed bed to prevent the entrainment of sulphuric acid droplets.
TB/EIPPCB/CAK_Draft_1 December 2011 49