Jiangsu Hanpu Mechanical Technology Co., Ltd |
Verified Suppliers
|
|
Black Liquor Evaporation : Design and Operation
Requirement
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.
Concentrators
A special of evaporator designs specifically engineered to address
the two issues associated with the processing of black liquor at
high concentrations:
1.Precipitation of supersaturated components from the liquor
At some point, typically around 50-55%TS,water soluble sulfate and
carbonate sodium salts exceed their solubility limits and begin to
precipitate from the black liquor being evaporated. The double salt
burkeite is the first to precipitate in the concentration process
while dicarbonate, another sodium double salt, reaches its
solubility limit later on, around 60%TS.Control of this
precipitation process is a crystallization problem, and achieving
higher concentrations requires that evaporation equipment be
designed as crystallizers to allow these salts to form in the bulk
of the liquor,and not as scale on the heat transfer surfaces.
2. High liquor viscosity
As its concentration increases, black liquor rheological behavior
changes from a Newtonian fluid to a pseudo- plastic fluid extremely
viscous. Such high viscosities translate into poor heat transfer in
concentrators (low Reynolds number hence low turbulence) but also
represent an impediment to crystal growth within the bulk of the
liquor. In addition,storage of the concentrated liquor, especially
if well above 75%TS, may have to be in a pressurized tank in order
to maintain the ability to pump the liquor to the boiler as well as
proper spraying patterns. To address these viscosity issues, black
liquor concentrators are typically operated at substantially
elevated temperatures and proper control of the liquor temperature
under varying operating conditions becomes a critical parameter of
the design as a mere 20 °F increase in liquor temperature can
translate into a viscosity reduction of 50% in some cases.
Operation at elevated temperatures enhances the breakdown of
calcium-organic complexes present in the liquor and, as a result,
the risk of precipitation of calcium carbonate on the heat transfer
surfaces is substantially increased. Precipitation of other water
insoluble compounds, such as silica and oxalate salts if present in
the liquor, can also occur at these higher temperatures, increasing
the risk of scaling of the concentrator units.
Heat treatment of the liquor prior to the concentrator can
permanently reduce the liquor viscosity by thermal- cracking of the
long lignin and other organic compounds responsible for the liquor
viscosity. Such treatment typically takes place in a continuous
reactor operated at high pressure and temperature (above 350°F).
Over 30 min of residence time in the reactor must be provided to
achieve maximum viscosity reduction.
Two types of black liquor concentrators are in use today and those can be broadly classified as Falling Film (FF) and Forced Circulation (FC)designs.
Mechanical Vapor Recompression MVR Evaporator consumption compared with traditional evaporation equipment calculated by evaporating 1T water
Name | Steam | Electric Power | Total Cost (RMB) | ||
Evaporation Capacity (kg/h) | Consumption (T) | Cost (RMB) | Consumption (kw) | Cost (RMB) | |
Single Effect Evaporator | 1.1 | 220 | 3 | 2.1 | 222.1 |
Double Effect Evaporator | 0.55 | 110 | 3 | 2.1 | 112.1 |
Three Effect Evaporator | 0.4 | 88 | 3 | 2.1 | 90.1 |
MVR Evaporator | 0.02 | 4.4 | 30 | 21 | 25.4 |
Falling Film Concentrator
Falling Film Concentrators are really an adaptation for high solids service of the FF evaporator design discussed above. By nature, FF concentrators, where evaporation takes place from a liquor film within the heating element result in high supersaturation levels being developed within the liquor. This can result in uncontrolled scale formation due to excessive crystal nucleation rather than gentle crystal growth.
Forced Circulation concentrator
Forced Circulation concentrators have been around since the 1950's and are well proven in black liquor service. The design consists ofa heat exchanger and a vapor/liquor flash chamber connected to each other through a recirculation loop.