Reference Manual

www.Fisher.com
Chapter 10A
Digesters
Batch Digesters – Kraft Pulping
Kraft batch digesters have been produced in
several different configurations, including rotating,
horizontal, and spherical vessels. By far the most
prevalent configuration is the upright, cylindrical
batch digester.
Typically a batch digester is two to three and a
half stories tall and has 2500 to 7000 cubic feet of
capacity. The quantity of pulp produced per batch
ranges from five to 25 tons.
Wood chips, chemicals, and steam are combined,
coked under pressure to a schedule, and then
dumped to a blow tank on a batch basis. Mills with
batch digesters have been between four and 36
units. Some mills have both batch and continuous
digesters.
There are two methods of heating batch digesters:
D Directly steam batch digesters (figure
10A-1): These units are the least complicated
and are usually of older design. Steam at 50 to
150 psi is injected at the base of the digester into
direct contact with the wood chips and cooking
liquor.
D Indirect steam batch digesters (figure
10A-2): Cooking liquor is extracted from the
digester through a screen to prevent removal of
wood chips or pulp. The liquor is passed through
an indirect heat exchanger and then recirculated
to the top and bottom of the digester. Chip
packing, air evacuation systems, and presteaming
are incorporated with indirect heating to produce a
more modern batch digester design. A relatively
new derivation of the modern, indirectly steamed
batch digester is the “low energy process.” The
low energy process batch digester is covered later
in the chapter.
A drawback to the directly steamed digester is the
dilution effect from condensed steam. Indirectly
steamed digesters; however, require more
maintenance due to the screens, pumps, and
external heat exchangers. Regardless of the
digester steaming method, the process objective
is the same: to elevate the temperature and
pressure of the chip-liquor mass such that the
alkaline component in the cooking liquor can
dissolve the desired amount of lignin and
extractives from the cellulose fiber.
Batch Digester Process Parameters
The Kraft pulping process is also known as the
sulfate or alkaline process. The actual cooking
liquor is a mixture of white liquor from the
chemical recovery boiler and recausticizing
operations, and black liquor (spent white cooking
liquor), which has been separated from previous
batches by brown stock washers. The main
constituents in the white liquor that contribute to
dissolving away the lignin binder material are
NaOH and Na
2
S. The Kraft cooking liquor is
alkaline, or basic, with a starting pH above 13
units. The temperature of the cooking liquor, when
added to the digester, is typically 160 to 190°F.
The temperature of the wood chips is typically 60 -
80°F, but may be much colder in northern
climates. The following paragraphs describe the
various process parameters associated with the
Kraft batch pulping process.
Chemical Concentration of Cooking
Liquors
A key process parameter in the production of Kraft
pulps is the chemical strength of the white cooking
liquor being added to the wood chips in a digester.
To achieve a target pulp yield at a target K (or
Kappa) number, a specific quantity of white and
black cooking liquors must be added per unit of