Reference Manual

8−2
sodium hydroxide (NaOH) and sodium sulfide
(Na
2
S). This solution, known as white liquor,
breaks down the glue-like lignin which binds the
cellulose wood fibers together. Cellulose fibers are
used to form a paper sheet on the paper machine.
The primary piece of equipment in the pulp mill is
the cooking vessel or digester. The digester is a
vessel in which wood chips and white liquor are
steam heated to a predetermined pressure and
temperature. The objective is to remove as much
lignin as possible without decreasing fiber
(cellulose) strength.
Both batch and continuous processes are used to
cook wood chips. The batch process involves
filling a vessel with wood chips and white liquor.
The contents are then heated to a predetermined
cooking temperature and pressure via direct or
indirect steam heating. After a prescribed cooking
time, the contents are blown to a holding tank and
the process repeated.
As its name implies, the continuous digester has a
fairly constant input of chips and outflow of pulp
fibers. The chips are usually preheated in a
steaming vessel before they are conveyed to the
digester. As the chips move down through the
digester (vertical type), they are successively
heated, cooked, and washed prior to cooling and
discharge to the blow tank. Indirect steam heating
of the cooking liquor is used to control the
temperature in each section of the digester. Most
of the newer pulp mills have favored the
continuous digester cooking process over the
batch process.
Following cooking, the pulp from the blow tank
must be washed to remove residual cooking
chemicals. This is sometimes referred to as brown
stock washing. The resulting process stream from
washing the stock is referred to as weak black
liquor.
For many years, the standard method of washing
has been a series of rotary vacuum washers. The
pulp and wash filtrate (black liquor) flow in a
countercurrent sequence with clean water used
only for the final washing stage. This allows an
increase in wash solids as it flows toward the first
stage washer and a decrease in pulp solids as it
moves to the last stage washer.
As mills face growing economic and environmental
pressures, new methods of washing have been
developed. Some of these systems include rotary
pressure filter systems, continuous digester
washing, and pressure diffusion washers. Each of
these systems are designed to achieve clean pulp
and reclaim cooking chemicals with less wash
water.
A final step in the pulping process involves
passing the pulp through knotter and screen
equipment. These steps may occur before, after,
or split around brown stock washing. The knotters
remove uncooked wood chips, knots, and fiber
bundles called shives. Screening is removal of
other tramp rejects such as rocks, steel, plastic or
conveyor parts.
Recovery of Kraft Pulping Liquors
Due to the high cost of pulping chemicals (sodium
and sulfur), effective recovery following cooking
has an important economic impact on mill
operating cost. The primary purpose of the Kraft
recovery cycle (see figure 2) is to reclaim these
chemicals and regenerate them to cooking liquor
form. A secondary objective is efficient heat
recovery and steam generation from the
combustion of wood organics in black liquor fuel.
This complex process will be covered in three
steps: evaporation, burning, and causticizing or
regeneration.
Evaporation
Weak black liquor from the brown stock washers
contains spent cooking chemicals, wood organics
such as lignin, and water. At this stage, the solids
content is typically 12 - 18%. Before burning,
water must be evaporated to raise the solids
content to 65 - 70%. The bulk of this task is
commonly accomplished by multiple-effect
evaporators.
A set of evaporators commonly consists of six
vessels and interconnecting pumps and piping.
Steam is used as a heating medium to evaporate
water from the liquor. Steam typically flows
countercurrent to the liquor for maximum
economy. Since the vessels operate at different
pressures, the vapors from one vessel serve as
the steam supply for the next vessel. Liquor
typically leaves the evaporators at 50% solids with
5 - 6 lbs. of water evaporated per pound of steam
used.