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

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Chapter 4<br />

evaporation unit integrated in <strong>the</strong> brine circuit <strong>of</strong> <strong>the</strong> electrolysis plant. When <strong>the</strong> once-through<br />

brine process is operated, this energy consumption can be avoided [ 37, Euro <strong>Chlor</strong> 2010 ].<br />

Achieved environmental benefits<br />

The achieved environmental benefit <strong>of</strong> this technique is a reduction <strong>of</strong> energy consumption.<br />

Environmental per<strong>for</strong>mance and operational data<br />

In a membrane cell plant, <strong>the</strong> water inputs to <strong>the</strong> brine circuit (via ancillary materials to <strong>the</strong> feed<br />

brine) are considerably lower than <strong>the</strong> water outputs (electro-osmosis, water in chlorine, brine<br />

purge). Indeed a considerable quantity <strong>of</strong> water (about two tonnes per tonne <strong>of</strong> chlorine) passes<br />

through <strong>the</strong> membrane from <strong>the</strong> anodic compartment to <strong>the</strong> cathodic compartment<br />

[ 37, Euro <strong>Chlor</strong> 2010 ]. Assuming a salt consumption <strong>of</strong> 1.7 tonnes per tonne <strong>of</strong> chlorine<br />

produced, this would result in about 3.2 tonnes <strong>of</strong> water to be evaporated per tonne <strong>of</strong> chlorine<br />

produced.<br />

Evaporation could be done using steam in a multiple effect evaporation unit or using electricity<br />

via mechanical vapour recompression [ 37, Euro <strong>Chlor</strong> 2010 ]. Table 4.15 shows <strong>the</strong> related<br />

energy consumption figures.<br />

Table 4.15: Energy consumption <strong>for</strong> brine reconcentration in membrane cell plants using<br />

solution-mined brine<br />

Unit<br />

Multiple effect evaporation<br />

Number <strong>of</strong> effects<br />

3 4 5<br />

t steam/t water evaporated 0.38 0.29 0.23<br />

t steam/t Cl2 produced 1.25 0.93 0.75<br />

kWh/t water evaporated<br />

Mechanical vapour recompression<br />

45<br />

kWh/t Cl2 produced<br />

Source: [ 37, Euro <strong>Chlor</strong> 2010 ]<br />

149<br />

When <strong>the</strong> once-through brine process is operated, this energy consumption <strong>for</strong> brine<br />

reconcentration can be avoided [ 37, Euro <strong>Chlor</strong> 2010 ]. However, this amount is reduced due to<br />

<strong>the</strong> fact that <strong>the</strong> full volume <strong>of</strong> fresh brine is preheated prior to electrolysis, while in brine<br />

recirculation plants only <strong>the</strong> make-up volume requires preheating [ 71, UK Environment<br />

Agency 2009 ]. In addition, <strong>the</strong> flow rate through <strong>the</strong> cells in plants with once-through brine<br />

system is lower compared to plants with brine recirculation, in order to reduce raw material<br />

consumption. As a consequence, <strong>the</strong> temperature <strong>of</strong> <strong>the</strong> inflowing brine needs to be lower so<br />

that <strong>the</strong> cells are operated within <strong>the</strong>ir temperature limits. This effect leads to an increase in<br />

electricity consumption <strong>of</strong> approximately 3 % compared to plants with brine recirculation [ 3,<br />

Euro <strong>Chlor</strong> 2011 ].<br />

Cross-media effects<br />

The disadvantage <strong>of</strong> <strong>the</strong> once-through brine process is that it uses about twice as much salt as a<br />

brine recirculation system because half <strong>of</strong> it is discharged as depleted brine. Reported<br />

consumption values <strong>for</strong> once-through membrane cell plants amounted to 3.1 and 3.8 tonnes <strong>of</strong><br />

salt per tonne <strong>of</strong> chlorine produced as compared to a median <strong>of</strong> 1.7 tonnes <strong>of</strong> salt per tonne <strong>of</strong><br />

chlorine produced <strong>for</strong> plants with brine recirculation systems (see also Section 3.3.1) [ 57,<br />

EIPPCB 2011 ]. At <strong>the</strong> same time, 9 – 11 tonnes <strong>of</strong> water per tonne <strong>of</strong> chlorine produced are<br />

discharged which contain high concentrations <strong>of</strong> chloride.<br />

WORKING DRAFT IN PROGRESS<br />

Technical considerations relevant to applicability<br />

Generally, <strong>the</strong>re are no technical restrictions to <strong>the</strong> applicability <strong>of</strong> this technique.<br />

TB/EIPPCB/CAK_Draft_1 December 2011 207

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