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

Chapter 4
4.3.5.3 Carbon tetrachloride-free chlorine purification and recovery
liquefaction and purification
Description
This technique consists in preventing emissions of carbon tetrachloride to air by using carbon
tetrachloride-free techniques for the elimination of nitrogen trichloride and for the recovery of
chlorine from the tail gas of the liquefaction unit.
Technical description
Carbon tetrachloride is still used at some locations for removal of nitrogen trichloride (NCl3)
and for absorption of tail gas. In 2009, four chlor-alkali plants in the EU-27 were using carbon
tetrachloride for the extraction of nitrogen trichloride (NCl3) from chlorine and/or for the
recovery of diluted chlorine from waste gas (see Section 3.4.3.2.4). Carbon tetrachloride is
classified as toxic and has a high ozone depletion potential (ODP = 0.73) [ 76, Regulation
EC/1272/2008 2008 ], [ 198, WMO 2006 ]. Although its use is generally prohibited, an
exemption is granted for its use as a process agent for the aforementioned applications in
installations already existing on 1 September 1997 [ 78, Regulation EC/1005/2009 2009 ].
However, other alternatives which do not use CCl4 are available and applicable to existing
plants.
First of all, if chlorine can be used directly without liquefaction it may not be necessary to
remove the NCl3. A preventative technique measure to avoid the accumulation of NCl3 is to
ensure low concentrations of ammonium (and other nitrogen-containing compounds which may
lead to the formation of NCl3) in the raw materials, for example by using vacuum salt without
ferrocyanides (see Section 4.3.2.3.6). Potential nitrogen sources are described in
Section 2.6.11.4. specify low ammonium ion concentration in purchased salt (for example
vacuum salt without addition of ferrocyanides to avoid caking, Another technique is to purify
the brine to remove by removing ammonium ions (for example by the stripping of ammonia
with air under alkaline conditions or by chlorination at a pH higher than 8.5 or hypochlorite
treatment of the brine (breakpoint chlorination)) [ 36, Euro Chlor 2010 ].
Available techniques for NCl3 destruction, not using CCl4, include [ 201, Piersma 2001 ]:
elimination from gaseous chlorine by reaction with sodium hydroxide (2 NCl3 +
6 NaOH V N2 + 3 NaCl + 3 NaOCl + 3 H2O) or hydrochloric acid (NCl3 + 4 HCl V
NH4Cl + 3 Cl2)
destruction in gaseous chlorine (2 NCl3 V N2 + 3 Cl2) by using:
UV radiation (dry chlorine; wavelength range 360 – 479 nm);
thermal treatment at temperatures of 95 – 100 °C;
activated carbon filters (filters also remove other impurities);
destruction in liquid chlorine (2 NCl3 V N2 + 3 Cl2) by thermal treatment at 60 – 70 °C.
Adsorption with activated carbon filters. This technique also removes other impurities,
such as organic compounds; NCl3 is decomposed into nitrogen and chlorine.
ultra-violet light; and
High metal temperatures, particularly of copper base alloys at temperatures of 80-100
°C, to decompose NCl3.
Elimination of NCl3 by reaction in a number of chemical processes, for example
absorption of chlorine containing NCl3 in caustic soda.
WORKING DRAFT IN PROGRESS
Several methods are available for handling the residual gas (non-condensables such as CO2, O2,
N2 and H2 saturated with chlorine) leaving the liquefaction unit. The most common is absorption
in caustic soda to produce sodium hypochlorite (see Section 4.3.5.1). The product, depending on
the market, is often saleable. If not, it is destroyed using the techniques described in
Section 4.3.6.3. Other methods include the manufacture of HCl, FeCl3 or ethylene dichloride.
228 December 2011 TB/EIPPCB/CAK_Draft_1