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

Chapter 4
Based on figure from [Denye et al., 1995]
Source: [ 207, Stitt et al. 2001 ]
Figure 4.7: Schematic Flow diagram of a catalytic decomposition reduction fixed-bed reactor
process
Catalytic fixed-bed decomposition is often used as a single end-of-pipe technique (Figure 4.7)
but it can also be combined with chemical reduction. In this case, the outflow of the fixed-bed
reactor is further treated with reducing agents. This technique is typically used for retrofitting of
plants which already use chemical reduction. Another possibility is to integrate the catalytic
fixed-bed decomposition in the liquid recirculation loop of the chlorine absorption unit. The
advantage of this process option is improved safety due to a reduction of the recirculating
hypochlorite [ 207, Stitt et al. 2001 ], [ 208, Johnson Matthey 2009 ].
In some cases, it is possible to recycle the effluent from the reactor back to the brine system
electrochemical cell. It is then necessary to control the concentration of chlorate and other
impurities, in particular for the membrane cell technique processes. Chlorate may be formed in
the chlorine absorption unit depending ion formation depends on the hypochlorite ion
concentration in the caustic scrubber recirculation liquor and the temperature in the scrubber
itself [ 75, COM 2001 ].
Environmental performance and operational data
With a catalytic fixed-bed reactor, a hypochlorite solution of 15 % by weight up to 150 g/kg can
be treated in a single pass to free oxidants levels of less than 10 mg/l 1 mg/kg in the pH range of
9 – 14 at ambient pressure and temperatures of 10 – 60 °C. with a catalytic reduction fixed bed
process. The outlet concentration essentially depends on the number of installed catalytic beds.
Since the reactor has a fixed bed configuration no emissions of metals occur, contrary to classic
WORKING DRAFT IN PROGRESS
catalytic decomposition reduction. Note that this process does not reduce the levels of bromine,
hypobromite, chlorate or bromate [Denye et al., 1995]. The lifetime of the catalyst is reported to
be several years. This technique also reduces the levels of hypobromite but not of chlorate or
bromate. Moreover, the produced oxygen (i.e. the intermediate atomic oxygen on the catalyst
surface) may react with organic compounds, thereby reducing the chemical oxygen demand
(COD) [ 75, COM 2001 ], [ 206, Denye et al. 1995 ], [ 208, Johnson Matthey 2009 ].
Cross-media effects
A potential emission of heavy metals occurs in systems using a catalyst slurry. In systems with a
fixed bed configuration, no emissions of heavy metals occur, as the catalyst is fixed in the bed.
240 December 2011 TB/EIPPCB/CAK_Draft_1