“Black Liquor” is a by-product of pulp from mills that make products from trees, such as paper. It is currently used to recover cooking chemicals and produce high-pressure steam used in the pulp and paper-making process. It is composed of different ingredients from these processes such as lignin, hemicellulose, sodium hydroxide (NaOH) and sodium sulfide (Na2S). The lignin compound in black liquor can be used to make biofuel but it can be expensive to produce and so fuel made from it is not very common.
However, and according to the International Energy Agency (IEA), the future of using black liquor as fuel is promising and it is seen as the fifth most important fuel on earth!
Black liquor evaporation
· Black liquor recovered from pulping contains 14-17% dissolved solids
· These solids are composed of about 1/3 inorganic chemicals that were in the white liquor added to the digester
·The remaining 2/3 consist of the organic chemicals extracted from the wood
·Black liquor must be concentrated to above 60% solids so that it will burn without supplemental fuel
Basic process requirements
· Efficient use of energy
·Efficient separation of water vapor from black liquor
·Proper separation of methanol, tall oil soap
·Concentration of black liquor to 75-85% dry solids
Evaporation of weak black liquor to separate water and create a combustible product-strong black liquor
The three main processes that occur in the evaporator are:
Black liquor+heat ⇒ Strong Black liquor + Water + Steam
Condensate + Steam ⇒ Clean + Dirty condensate + NCG
Black liquor ⇒ Black liquor + Soap (only softwood)
How to reach 80-85% dry solids?
Due to the high viscosity
-MP-steam as heating medium
- liquor temperature 175 ℃ in final concentrator
Liquor retention time
-evaporator acts also as a LHT-reactor
Duplex construction material
-high alkali content may cause Stress Corrosion Cracking (SCC)
Evaporation plants are well-proven in installations around the world. There are a variety of configurations and models available – from pre-evaporators to multiple-effect evaporation trains – to accommodate even the most challenging pulp mill or effluent plant evaporation needs
The major advantage to multiple-effect evaporators is the utilization of proven lamella heating surfaces for high efficiency and reliability.
Evaporation plant lamellar
lamella heating surface
Energy-efficiency for reduced costs
Highest dry solids liquors with maximum availability using crystallization technology
Cleanest reusable condensates using the minimum amount of steam
Lowest steam and electricity consumption
Tolerate even non-soluble scaling
Black liquor evaporation is an essential part of the chemical recovery process as it significantly concentrates the dry solid matter in black liquor so that the liquor can be effectively combusted in recovery boiler. The water removed from the liquor can be segregated and processed so that it can be reused in the mill – reducing fresh water requirements, effluents, and saving money.
Evaporators and concentrators, with their lamella heating surfaces, are adept at processing the spent cooking liquors or mill effluents using the minimum amount of energy while maximizing product dry solids, plant availability, and condensate quality. The lamella heating surfaces are inherently non-scaling. This allows the use of various heat sources inside the lamella, allowing the mill to select the most energy-efficient process connection.
Black liquor is a complex solution of water, organic, and inorganic components. This composition varies from mill to mill, so the evaporation system must be flexible in design over a wide range of operating parameters. The physical properties of black liquor change significantly when it is concentrated. Concentrated black liquor has high viscosity, which can cause significant problems with fouling if these characteristics are not accounted for in system design.
The systems employ crystallization technology to produce the highest dry solids product with minimum downtime for washing and cleaning of the equipment internals. As an option, systems for treating the liquor to reduce its viscosity, and systems for de-activating the calcium in the black liquor to prevent scaling, can be integrated directly into the evaporation plant.
The stripper is part of the multiple-effect evaporator system. Typically, the heat source for the stripper is from effect #1 and the stripper vapors are condensed in effect #2 to maximize energy efficiency.
The process inside and evaporator relies on a free flow falling film of liquor over the surface of the lamella. This ensures low vapor velocities and low shear rates inside the evaporator, which eliminates foam generation in difficult applications.
Uniform liquor distribution of the liquor over the lamellas, and the continuous redistribution of the liquor created by the dimpled shape of the lamella surface, ensure a completely wetted heating surface and eliminate local scaling or over-concentration of liquor. The lamella heating surface ensures that water-soluble scaling can be washed away by a simple dilution wash, eliminating the need for time-consuming and costly outages for cleaning.
The design of the evaporator takes full advantage of the lamella heating surface to optimize liquor distribution and internal circulation flows. This results in excellent coverage and distribution of the liquor over the entire heating surface with minimum pumping power – saving capital and energy costs.