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

Chapter 2
2.6 Chlorine processing production, storage and handling
2.6.1 General description
Generally, before the chlorine can be used, it goes through a series of processes for cooling,
cleaning, drying, compression and liquefaction. In some applications, it can be used as a dry gas
without the need for liquefaction. Very occasionally it can be used directly from the
electrolysers. A general flow of chlorine from the electrolysers to storage is presented in Figure
2.1 Figure 2.14. The chlorine process usually takes hot, wet cell gas and converts it to a cold,
dry gas. Chlorine gas leaving the electrolysers has a temperature of is at approximately
80 – 90 ºC and is saturated with water vapour. It also contains brine mist, impurities such as N2,
H2, O2, CO2 and traces of chlorinated hydrocarbons. Electrolysers are operated at essentially
atmospheric pressure with only a few mbar milli-atmospheres differential pressure between the
anolyte and the catholyte.
Figure 2.14: The flow of chlorine from the electrolysers to storage
[Euro Chlor report, 1997] {The figure was removed because the information is contained in Figure 2.1.}
2.6.2 Materials
The strong oxidising nature of chlorine requires a careful choice of construction materials at all
stages of processing, depending on the operating conditions (temperature, pressure, state of
matter, moisture content). Most metals are resistant to dry chlorine at temperatures below
100 °C. Above a specific temperature for each metal, depending also on the particle size of the
metal, spontaneous ignition takes place (150 – 250 °C for iron). Carbon steel is the material
most used for dry chlorine gas (water content below 20 ppmw). Wet chlorine gas rapidly attacks
most common metallic materials with the exception of tantalum and titanium, the latter being
the preferred choice in chlor-alkali installations. However, if the system does not remain
sufficiently wet, titanium ignites spontaneously (ignition temperature ~ 20 °C). Other
construction materials such as alloys, graphite, glass, porcelain and polymers are used
depending on the conditions. Oils or greases generally react with chlorine upon contact unless
they are fully halogenated [ 1, Ullmann's 2006 ], [ 3, Euro Chlor 2011 ].
2.6.3 Cooling
In the primary cooling process, the total volume of gas to be handled is reduced and a large
amount of moisture is condensed. Cooling is accomplished in either one stage with chilled water
or in two stages, with chilled water only used in the second stage. Care is taken to avoid
excessive cooling because, at around 10 ºC, chlorine can combine with water to form a solid
material known as chlorine hydrate (Cl2 · n H2O; n = 7 – 8). Maintaining temperatures above
15 °C 10 ºC prevents blockages in process equipment [ 1, Ullmann's 2006 ], [ 54, Euro Chlor
2010 ].
WORKING DRAFT IN PROGRESS
Two methods are most frequently used to cool chlorine gas [ 38, O'Brien and White 1995 ].
One method is indirect cooling through a titanium surface (usually in a single-pass vertical
shell-and-tube heat exchanger). The resultant condensate is either fed back into the brine
system of the mercury or membrane cell technique process or is dechlorinated by
evaporation in the case of the diaphragm cell technique process. This method causes less
chlorine to be condensed or absorbed and generates less chlorine-saturated water for
disposal. [Brien-White, 1995]
Another method is direct contact with water. The chlorine gas is cooled by passing it
directly into the bottom of a tower. in which the packing is divided into two sections, for 2stage
cooling. Water is sprayed from into the top and flows countercurrent to the chlorine.
48 December 2011 TB/EIPPCB/CAK_Draft_1