Reference Manual

10B −17
D Typical process conditions:
— Fluid: Black liquor
— T = 260_F
— P = 185 – 195 psig
— dP = 185 – 195 psi
— Q = 1500 gpm
D Typical valve selection:
— These are typically in the NPS 3 to NPS 8
range. The DSV valve is suitable for this
application. This is a modified 8510 body
with a strengthened shaft and no seal. Used
in conjunction with the 1061 actuator with a
quad seal option, this assembly is capable of
a relatively long life in this service.
Blowing
At high temperatures, particularly over 220_F, the
mechanical action of the chips resulting from the
violent expansion during the blow is harmful to the
fibers. As mentioned before, washing efficiency
increases with increased temperature; however,
damage to fibers will be reduced by blowing at a
lower temperature. In order to overcome any loss
in pulp strength, the pulp mass is cooled to about
190_F by the weak black liquor filtrate. This liquor
is pumped via the cold blow pump into the digester
at two locations: a portion through a screen plate
around the digester shell near the bottom and
another portion through four nozzles located under
the paddles of the outlet device. The liquor
entering through the screens regulates the
digester pressure, whereas the flow under the
outlet device is fixed by the operator and provides
the required dilution before the pulp is actually
blown from the digester.
Summarizing, we find that the chips undergo three
basic temperature changes that take place from
the extraction zone to the blowing of pulp from the
digester:
First, the chips are cooled or quenched 40 to 50_F
at the extraction zone. This drop occurs where the
up-flowing wash liquor meets the down-flowing
residual cooking liquor and both are drawn off.
Most of the residual liquor goes to the No. 1 flash
tank for heat and chemical recovery. The
remainder is re-circulated via the quench
circulation pump before being recovered at the
flash liquor tank. Re-circulation provides a more
uniform temperature control and is important to
avoid over-cooking the pulp.
The chips are then cooled (30_F) gradually
throughout the wash zone until a second sharp
drop occurs in the blowing dilution zone. Here the
chips are cooled rapidly again by about 60_F
before blowing to avoid mechanical damage to the
exploding chip fibers. The blow line temperature
must be maintained at 210_F or less. The gradual
temperature drop over the wash zone is simply a
result of gradual heat exchange between the
up-flowing wash liquor and the down-flowing chips.
The reverse procedure occurs with the cold filtrate
liquor (160_F). After entering the digester, it is
heated to about 190_F by heat transfer from the
chips and is then extracted to be heated by the
wash heater (260 - 280_F) before returning to the
digester wash zone. It gradually increases in
temperature as it picks up heat from the
down-flowing chips and reaches the lower
extraction at about 290_F to provide the quench at
the end of the cook zone. The wash heater control
point is, therefore, set to arrive at the proper
quench temperature and it is affected by the
production rate, cooking temperature, and dilution
factor.
Chips, upon reaching the outlet device, are now
ready to be discharged from the digester. A load
reading ammeter on the outlet device motor
serves to indicate the consistency in the bottom of
the digester. It has a variable speed drive and the
speed at which it runs directly affects the
consistency; the faster it runs, the higher the
consistency and vice-versa. The reason for this is
that the arms of the outlet device are designed to
scrape the chips from the bottom of the column
and carry them into the discharge port. Through
experience, the operator learns the best speeds
for each production rate. It can also be seen from
the above that the outlet device is used in the fine
control of the digester chip level.
The chips then enter a 12-inch blow line and goes
to a small cigar shaped vessel called the blow unit.
The blow unit is equipped with an agitator and two
exit lines to the blow tank. The purpose of the
agitator is to act as a consistency indicator for the
digester operator by a torque or power-sensing
device. As the consistency goes up or down, the
power required to turn the agitator goes up or
down. The variation is recorded on a chart at the
control panel.
The chips and liquor flow out of the blow unit into
one of the blow lines, and the flow is measured
and recorded by a magnetic flowmeter. Cooked
chips are at digester pressure up to the final ball
valve before the blow tank. As they pass through,
they are subjected to a sharp pressure drop which
causes the chips to explode and break up into
individual fiber bundles which are a form of raw