Reference Manual

10A −4
dry wood. However, these total liquor-to-wood and
liquor-strength-to-wood ratios are difficult to
enforce because on-line measurement of chip
moisture and weight have proven unreliable, and
the chemical strength of the white liquor solution is
not a stable or directly measurable variable.
Sensor developments in both areas are rapidly
advancing.
Cooking Time
A digester begins to cook slowly as soon as the
white liquor and black liquor solutions are applied
to the wood chips (even at atmospheric pressure,
i.e., before capping). Typically, a digester cooking
cycle is as follows: the digester is capped,
steaming is begun, and the temperature and
pressure ramp up to a predetermined pressure (or
temperature). Further steaming is then regulated
to maintain the desired target pressure (or
temperature). When the target “H” factor has been
reached, the digester’s contents are blown to the
blow tank.
The total length of the cooking cycle per batch
digester will depend on the desired pulp grade and
the mill’s criteria of operations. Consequently, a
batch cooking cycle can range from two hours
“cap-to-cap” for a hard cook (high yield), to five
hours for a soft cook (low yield).
Cooking Temperature
The batch digester temperature is also a
significant factor in achieving cooking uniformity
(delignification) throughout the mass of chips.
Higher temperatures accelerate the rate of
chemical reaction between the wood chips and the
cooking liquors. The quantity of rejects (i.e., knots
or partially cooked chips) in a batch is related to
uneven temperature distribution in the digester.
For example, if poor convection mixing causes
temperature differences between the bottom and
top, it is not uncommon for pulp at the bottom of a
digester to be several Kappa units different from
pulp at the top of a digester.
Typically, a charged batch digester at atmospheric
pressure is at around 165°F to 195°F. Upon
capping and steaming, the maximum desired
cooking temperatures will range from 330°F to
350°F at pressures of 100 to 120 psi. Normally,
there is at least a top and a bottom temperature
sensor on each digester. A middle temperature
probe is encouraged for improved indication of
temperature distribution. The bottom temperature
usually exceeds the top temperature due to
hydrostatic liquor head on directly steamed units,
because the sensor is closer to the entry point of
steam.
Cooking Pressure
Digester pressure rises as the steam flow to the
digester raises the temperature of the chip and
liquor mass. Batch temperature is considered to
be the key variable, but batch pressure is an
easier and faster variable to measure than a
“representative” chip and liquor mass temperature.
The pressure/temperature relationship is based on
saturated steam tables. The implied digester
temperature should include a slight increment for
the boiling point rise of the cooking liquor. The
elevated boiling point over water is due to organic
and inorganic solids in the liquor. Digester
pressure ranges from atmospheric at liquor
charging to a maximum of 100 to 120 psi for the
extended cooking period.
Digester pressure causes the cooking liquor to
more readily impregnate the wood chips so that
the delignification reactions proceed from the
inside of the chip to the outside of the chip, as well
as vice-versa. Pressure in a batch digester ranges
from atmospheric at liquor changing to a
maximum of 100-120 psi for the extended cooking
period.
Pressure Profile
The batch digester cooking cycle is usually
represented in text books by a graph of the
internal digester pressure vs. time, such as that
shown in figure 10A-3. In real life, the cooking
cycle pressure profile is never this rigid. The time
interval at each different phase of the cook can
vary significantly from one grade of pulp to
another and from mill to mill. Figure 10A-4 shows
a more realistic representation of the pressure
profile over the entire cooking cycle. Figure 10A-5
shows the steam demand profile required to
complete this representative cooking cycle. The
vertical axis defining the amount of steam
demanded is not labeled because different pulp
grades or different sized digesters will require
different quantities of steam. However, a typical
3-hour cooking cycle at 100 psig consumes from
4000 to 6500 pounds of steam per ton of pulp
produced. Therefore, a 7-ton digester could
consume about 40,000 pounds of steam per
cooking cycle.