Weak Black Liquor (WBL) from the brown stock washers is typically at 13-18%TS. Most of this water content must be evaporated to produce a material with high enough solids to support effective combustion in the recovery boiler, typically between 65% and 80%TS.
During evaporation to this level of solids, various volatile components (sulfur compounds, methanol, etc.) are released from the liquor and must be separated from the condensate to allow reuse in the fiberline and recausticizing. From this point of view, the evaporation plant actually serves as a"water factory"within the mill. Black liquor also contains a substantial fraction of inorganic compounds which, during the evaporation process, reach their solubility limit and can deposit as scale on the evaporator heat transfer surfaces greatly limiting the operating capacity of the evaporation plant and of the entire recovery island.
The inherent complex composition of black liquor translates into several interdependent design requirements for the evaporators:
The evaporation plant must efficiently transfer heat for the evaporation of the black liquor.
It must do so while avoiding scale formation on the heat transfer surfaces.
The evaporation plant must also produce sufficiently cleancondensate fractions to satisfy the needs of the pulp mill and recausticizing area, thus greatly reducing the fresh water intake of the mill.
Volatile components and NCGs must be removed and conditioned for safe disposal via incineration.
Application of MVR Technology:
When the concentration of evaporated liquid is raised and its boiling point is raised within a certain limit, for example, when the concentration of evaporated black liquid is below 20%, the boiling point is raised no more than 1.5 ℃.
It is used in the areas or enterprises where the electricity price is cheap and the heat energy is expensive.
2. Application of technology in black liquor evaporation of pulp
Taking the evaporation of wheat straw pulp black liquor as an example, the combination evaporation process of MVR technology and multi effect evaporation was compared with the multi effect evaporation process. The details are as follows.
2.1 main parameters and evaporation scheme
The results show that the flow rate of dilute black liquor is 128.6t/h, the temperature of dilute black liquor is 70 ℃, the concentration of dilute black liquor is 10%, the concentration of concentrated black liquor is 45%, and the evaporation water is 100t / h. The multi effect evaporation process is 5-effect evaporation station, and the combined evaporation process is MVR pre concentration + 5-effect evaporation station.
|item||5 effect station||combined evaporation process|
|MVR pre concentration||5-effect evaporation station|
|Evaporation water (t/h)||100||64.28||35.72|
|RMB/T water evaporation capacity||45||25.93|