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

Chapter 4
Economics
{Please TWG provide information.}
Driving force for implementation
The driving forces for implementation of this technique include the following:
reduction of costs related to energy consumption for brine reconcentration;
reduction of costs related to additional equipment necessary for brine recirculation.
Example plants
Solvay in Póvoa de Santa Iria (Portugal), chlorine capacity 26 kt/yr;
Ineos ChlorVinyls in Runcorn (United Kingdom), chlorine capacity of membrane cell
unit 400 kt/yr.
Reference literature
[ 3, Euro Chlor 2011 ], [ 37, Euro Chlor 2010 ], [ 57, EIPPCB 2011 ], [ 71, UK Environment
Agency 2009 ]
4.3.2.3.8 Plate evaporators
Description
This technique consists in using plate evaporators for the caustic concentration in membrane
cell plants instead of the common falling film evaporators with a shell-and-tube design. The
plate evaporator uses metal plates to transfer heat from steam to dilute caustic in order to
evaporate water from the latter.
Technical description
The caustic solution leaving the membrane cells (~ 32 wt-%) needs to be concentrated in many
cases to 50 wt-% in order to be traded as commodity. This is commonly achieved by using
falling film evaporators with a shell-and-tube design [ 46, Ullmann's 2006 ].
A plate evaporator consists of a series of thin, corrugated plates which are gasketed, welded or
brazed together depending on the application. The plates are compressed together in a rigid
frame to form an arrangement of parallel channels where one fluid flows in the odd-numbered
channels, while the other flows in the even-numbered channels. The advantage of plate
evaporators is that the fluids are exposed to a much larger surface area because they are spread
out over the plates. This facilitates the transfer of heat and greatly increases the speed of the
temperature change.
Commercially available plate evaporators for caustic concentration are rising film evaporators
with countercurrent steam flow. For the condensing step, plate heat exchangers are used [ 144,
Alfa Laval 2011 ].
Plate evaporators are more compact than shell-and-tube evaporators and therefore require less
space and construction materials. Compared to the shell-and-tube design, approximately one
WORKING DRAFT IN PROGRESS
third of the material is required for the evaporators and approximately one sixth for the
condensers [ 144, Alfa Laval 2011 ]. Generally, caustic evaporators are designed with two or
three effects depending on the balance between investment and operating costs.
Achieved environmental benefits
The achieved environmental benefits of this technique include the following:
reduction of energy consumption;
reduction of raw material consumption.
208 December 2011 TB/EIPPCB/CAK_Draft_1