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

Chapter 4
was the solution at that time to a specific situation; today it might not have been built for such
large quantities.
3. ATOCHEM in Jarrie (Isère, France), loading area:
Containment of a separated loading area (2 x 58 tonnes railway tanks), chlorine destruction unit
and water spray equipment. The cost was approximately 1.5 million euros (September 1998).
The cost can vary, depending on adaptations needed to existing installations.
References
[Gest 87/130, 1996], [Gest 92/175, 1993], [J. Loss Prev. Proc. Ind., March/94], [Mason, 1995]
4.3.5.1.2 The chlorine absorption unit
Description
This technique consists in reducing channelled emissions of chlorine to air by using a chlorine
absorption unit containing a caustic soda solution and using either packed towers or ejector
systems.
Technical description
The purpose of the chlorine destruction unit is to avoid large emissions of chlorine gas to the
environment during irregular plant operation and/or emergencies, and to take care of all
chlorine-containing waste gases during normal operation. The most common way to destroy the
chlorine is to absorb it in weak caustic soda to produce sodium hypochlorite. The chemical
reactions taking place are described in Section 2.6.12.2.
The absorption system can make use of packed towers or venturi ejectors. However, packed
columns, even though more complex, are better in case of emergency if electricity supply fails.
They can continue to absorb chlorine from a pressure relief system using caustic soda stored in a
gravity-fed head tank. The concentration of caustic soda should not exceed 22% NaOH because
of the risk of salt deposition, causing blockages in the absorption plant, and freezing.
The absorption system can make use of batch or continuous systems as well as of packed
columns or ejectors [ 192, Euro Chlor 2011 ]:
Packed columns are usually based on a counter-current flow with the chlorine gas
entering the bottom of the column and the scrubbing solution entering the top. They
have the advantage of larger caustic retention volumes and good mass transfer. In case
of emergency, they can continue to absorb chlorine from a pressure relief system using
caustic soda stored in a gravity-fed head tank even if electricity supply fails. However,
packed columns require an additional fan for sucking the chlorine vent and are more
susceptible to plugging.
Ejectors based on the Venturi principle use a high-pressure liquid stream to create a
vacuum which sucks the chlorine into the eductor where it reacts with the scrubbing
solution. They have the advantage of being simple as only one pump is needed for the
caustic transport and gas suction. In addition, they remain effective at low and high
chlorine concentrations in the vent gases and are less sensitive to salt deposition.
However, ejectors show high power consumption and the mass transfer is limited to a
single stage per unit so that multiple stages or a combination with packed columns are
usually required to provide adequate chlorine removal.
WORKING DRAFT IN PROGRESS
The design of an absorption system requires clear specifications of:
the maximum quantity of chlorine;
the composition of the gas stream;
the maximum instantaneous flow.
TB/EIPPCB/CAK_Draft_1 December 2011 223