Biomedical Engineering Reference
In-Depth Information
time ranging from about 30 to 45 minutes depending upon the degree of delignification
desired. The resulting pulp exiting the digesters is diluted and washed with water to
remove the black liquor (spent pulping chemicals and dissolved wood solids). Large
quantities of water are used, resulting in dilute black liquor - typically between 12 and
18% solids. The diluted black liquor must be concentrated before it can be efficiently
burned in a recovery boiler to produce energy and recover the chemicals for reuse in
the pulping process. The recovery of chemicals to form fresh pulping liquors is a vital
part of the pulping operation. The energy generated by burning the black liquor in the
recovery boiler is used in pulping and papermaking operations and significantly reduces
purchased energy requirements. The black liquor is concentrated to 70-78% solids
in steam-heated, multiple-effect evaporators. The evaporators are typically the second
largest energy users in pulp mills and the largest source of steam consumption, at around
4.4 GJ (4.2 million BTU) of steam per metric ton of pulp.
Causticizing is a multistep process in chemical recovery process chain aimed at regen-
erating the original pulping liquor ('white' liquor) from the molten smelt of inorganic
chemicals (sodium sulfide and carbonate) exiting the chemical recovery boiler. The pro-
cess starts by dissolving the molten salts in water to form an aqueous solution called
'green liquor.' Lime (calcium oxide) is then mixed with the green liquor in a slaker to
form sodium hydroxide and calcium carbonate. The chemical reactions that form the
white liquor are completed in a series of reactors called 'causticizers'. The spent car-
bonate sludge ('lime mud') from the slaker and causticizers is removed in a clarification
step, thickened, washed, and calcined in a lime kiln to recover calcium oxide for reuse
in the slaking process. Causticizing is an old technology that has not benefited from
innovation in many years. It is extremely capital intensive and suffers from very high
operating and maintenance costs. The lime kiln in particular is a high-energy user and
prone to maintenance problems. Lime kilns are typically fueled by oil or natural gas
and represent one of the largest consumers of purchased energy in the pulp mill. Lime
kilns and causticizers can also be a production bottleneck, limiting the mill's production
capacity.
Nanotechnology opportunities/needs in pulping are to: (1) use nanotechnology to
reduce steam use for black liquor evaporation to achieve energy savings; (2) develop
low corrosion nanocoatings and nanomaterials to prolong the life of capital equipment
especially in bleach plants; (3) develop cost-effective alternative nanocatalyzed, simpler
means of regenerating white liquor, that regenerates sodium hydroxide in the recovery
boiler and smelt-dissolving tank; (4) develop new nanocatalysis techniques to rapidly
delignify wood (e.g. in 10 minutes or less) at lower temperatures (i.e. below the boiling
point of water so as to not require pressurized vessels) that would also allow for easier
separation of spent pulping liquor components, easier solids concentration, and easier;
and (5) develop new nanocatalysis techniques for separating wood cell wall constituents
without altering native structures of wood constitutive components (i.e. hemicellulose,
cellulose, and lignin).
The kraft pulp industry has traditionally been a source of odorous emissions (primar-
ily methylmercaptans) that, although not a health risk, are regarded as a nuisance in
nearby communities. In recent years, the industry has made great progress by installing
state-of-the art systems to reduce in-mill sources of the odors. However, odors from
wastewater treatment operations continue to be a cause for community concern in many
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