Scale formation and growth when evaporating black liquor with high carbonate to sulphate ratio
Artikel i vetenskaplig tidskrift, 2008
Scaling in black liquor evaporators has for long
been recognised as a problem. Most problems are connected to
the precipitation of sodium salts. In the black liquor evaporation,
as the black liquor gets concentrated, sodium carbonate
and sodium sulphate will saturate and precipitate from the solution.
When precipitation occurs the salts may crystallise on the
heat transfer surface or adhere to the surface as well as they can
crystallise in the bulk. If crystals remain on the heat transfer
surface an insulating layer of scales can quickly build up causing
the heat flux to drop and eventually forcing the evaporator
to be shut down for cleaning.
The research in this paper focused primarily on scaling for
black liquors with high ratio of carbonate to sulphate, a condition
shown to be important in industrial black liquor evaporators
(Frederick et al. 2004). The scaling behaviour was investigated
experimentally in a falling film evaporator. The aim was
to find favourable operating conditions decreasing or eliminating
scaling. The examined parameters were different heat flux,
circulation flow rate, internal residence time and black liquor
feed concentration. For the investigated operating conditions the
main parameters affecting scaling were the circulation flow rate
and heat flux whereas the scaling rate was seen independent of
the internal residence time.
Fouling
Black liquor
Dicarbonate
Scaling
Evaporation
Falling film