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

Chapter 4
Information on the composition of the gas stream includes temperature and pressure ranges as
well as concentration ranges of other substances which may affect the process by, for example,
reacting with the caustic soda (e.g. CO2), potentially forming explosive gas mixtures (e.g. H2,
NCl3) or altering the heat balance of the system (e.g. due to condensation of water vapour from
the gas stream, due to water evaporation from the scrubbing solution and due to caustic dilution
with water) [ 192, Euro Chlor 2011 ].
It is very important to ensure that in case of a major accident, a massive release of chlorine
could be absorbed. The size of the unit is of great importance and should be calculated to cope
with emergency conditions. Caustic soda supply and the storage capacity for caustic soda and
hypochlorite solution should be correctly sized as well. It is essential that faults are detected
quickly and adequate instrumentation with alarms should be applied at the vent absorption plant
to ensure that warning is given if equipment fails. In particular, temperature control of the
absorption solution is essential, as is measurement to prevent over-chlorination. The emergency
power supply should be tested periodically.
The proper design, maintenance and operation of the chlorine absorption unit are based on a risk
assessment study to avoid discharge of major amounts of chlorine to the atmosphere during all
possible situations. This can be achieved by, depending on the technique [ 192, Euro Chlor
2011 ]:
sizing of the absorption unit for a worst case scenario in terms of produced chlorine
quantity and flow rate;
sizing of the caustic soda supply and storage capacity so as to ensure an excess at all
times;
sizing of the column so as to prevent flooding at all times;
preventing the ingress of liquid chlorine in the absorption unit, for example by using
knock-out pots in cases where some liquid chlorine can be present in the vent;
preventing the backflow of scrubbing liquid into the chlorine system by using devices
such as hydraulic guards and knock-out pots;
preventing the precipitation of solids in the absorption unit by choosing an appropriate
scrubbing liquid; maintaining the sodium chloride content below 22 wt-% when using
18 – 22 wt-% sodium hydroxide;
using heat exchangers in the circulating absorbent to limit the temperature below 55 °C
at all times;
supplying dilution air to prevent the formation of explosive gases after chlorine
absorption;
using construction materials which withstand the extremely corrosive conditions at all
times;
using redundant equipment such as an additional scrubber in series with the one in
operation, stand-by and spare fans, stand-by and spare pumps;
providing an independent backup power supply (e.g. diesel generators or direct current
batteries with an inverter) for electricity-dependent equipment such as the gas suction
system, the caustic circulation pumps, the key instruments for process control and the
lighting;
providing an automatic switch to the backup power supply in case of emergencies
including periodic tests on this supply and the switching system;
providing process control by monitoring the following parameters in connection with
an alarm system:
WORKING DRAFT IN PROGRESS
chlorine in the outlet of the scrubber and the surrounding area;
temperature of the scrubbing liquids;
reduction potential and alkalinity of the scrubbing liquids;
suction pressure;
flow rate of scrubbing liquids;
pressures of pumping solutions as well as chlorine process and relief headers;
224 December 2011 TB/EIPPCB/CAK_Draft_1