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

Chapter 2
water (used for brine preparation or direct cooling): ammonia in distilled water or
steam condensate, ammonium (from fertilisers) and humic acids in surface water or
groundwater;
ancillary materials: caustic soda which was purified by using liquid ammonia;
sulphuric acid contaminated with ammonium.
1 ppm of NH3 in brine is enough to give >50 ppm NCl3 in liquid chlorine. In plants which use
direct contact water cooling of the chlorine gas before drying and compression, NCl3 may also
be formed if the water is polluted with nitrogen compounds [Gest 76/55, 1990].
Nitrogen trichloride is characterised by its utmost instability. Experimental results show that a
concentration of NCl3 greater than 3 wt-% by weight at ambient temperature is capable of
accelerated decomposition which is strongly exothermic. Experimental results show that
chemical decomposition of NCl3 in liquid chlorine at concentrations > 3 wt-% is rapid and
strongly exothermic [ 35, Euro Chlor 1990 ].
NCl3 has a higher boiling point (71 °C) than chlorine (-34 °C) and any NCl3 present in the
chlorine gas will thus concentrates in the liquid phase in a chlorine liquefaction process. 1 ppm
of NH3 in brine is enough to produce > 50 ppm NCl3 in liquid chlorine. Any evaporative
handling of liquid chlorine in subsequent processes is potentially dangerous due to the further
selective concentration of NCl3 in the liquid phase [ 35, Euro Chlor 1990 ].
The formation of NCl3 can be reduced by appropriate selection and control of the raw materials,
by stripping ammonia with air under alkaline conditions, or by oxidation of
ammonium/ammonia to molecular nitrogen (breakpoint chlorination) using chlorine at a pH
higher than 8.5 or hypochlorite. Methods to remove NCl3 from chlorine after it is formed
include extraction with carbon tetrachloride as well as catalytic destruction using, for example,
ultraviolet radiation or activated carbon [ 36, Euro Chlor 2010 ].
Methods to remove NCl3 from chlorine are described in Section 4.1.6.
2.6.11.5 Bromine
The quantity of bromine present depends on the quality of the salt used. Its concentration is
generally higher if chlorine is obtained by electrolysing potassium chloride to obtain potassium
hydroxide (see Section 2.9). Bromine, like water, can accelerate the corrosion of the materials.
In addition to the reduction of the bromide (Br - ) levels via the raw material specifications,
bromide can be removed from the brine by oxidation and stripping of bromine which is then
absorbed in a caustic solution. Another technique is to distil the liquefied chlorine to enrich the
heavy end with bromine, which can then be further extracted for sale or the mixture can be
destroyed [ 42, Euro Chlor 2010 ].
Non-condensable gases (CO2, O2, N2, H2)
WORKING DRAFT IN PROGRESS
There are several ways to deal with the non-condensable gases, depending on the layout of the
chlorine liquefaction unit. Some are described below.
Dilution with air and production of weak NaOCl
During chlorine gas compression and cooling, most of the chlorine gas is condensed. However,
the non-condensable gases (H2, CO2, O2, N2) increase in concentration. By diluting the
remaining chlorine gas with air, the concentration of hydrogen can be kept below the explosion
limit. This allows additional liquefaction of chlorine gas. The remaining gases after liquefaction
(so-called ‘tail gas’) have to be purged from the system. The tail gas still contains a significant
54 December 2011 TB/EIPPCB/CAK_Draft_1