Reference Manual

10B −2
mass throughout the chip column which explains a
growing use of the two vessel digester system.
Process
Regardless of the type, a Kamyr continuous
digester combines the digesting and washing
process into one vessel so that only one washing
stage is required following the blow tank. Chips
from the wood yard are fed to a surge bin (chip
bin) in the digester building. They are then
metered continuously through a chip meter to a
low pressure feeder. The chips then fall into the
steaming vessel, and are conveyed to the chip
chute and the high pressure feeder. Chips are
then sluiced to the top of the digester and the top
separator at a predetermined temperature and
pressure. Cooking liquor is also fed continuously
into the top of the digester in the desired ratio to
the wood chips. The chips move slowly by gravity
to the bottom where they are discharged as pulp.
Along the way, they are heated to simulate the
heating of a batch digester and its contents. The
temperature is varied in the mid-section, or
cooking zone, to suit different production rates.
The bottom digester section is used as a washer
and, at this point, any similarity between batch and
continuous cooking ends.
With a vacuum drum washer, liquor is drained
from the pulp and replaced by water. In the Kamyr
digester wash system, the liquor is displaced by
introducing very weak black liquor from the
vacuum washer, diffuser or even warm water at
the bottom of the digester, making it flow
counter-currently to the pulp mass which is
moving to the bottom. This is known as “hi-heat”
diffusion washing. The up-flowing weak black
liquor wash displaces the stronger residual black
liquor, which is drawn off through a screen in the
wall of the digester located about half-way up from
the bottom, but below the cooking zone. This
allows the pulp to be effectively washed before it
is blown to the blow tank.
Although all components of the Kamyr digester
are closely interrelated, the following is a logical
step by step explanation of controlled loops, which
will provide an understanding of the various stages
of the Kamyr system.
Pressurizing
Weak black liquor (filtrate) or warm water,
approximately 170_F, is pumped to the digester
through the cold blow pump. There, it is used to
maintain digester pressure. The digester pressure
control system has three major components:
D An automatic pressure control valve on an
input liquor line.
D An automatic pressure relief control valve on
the digester relief line.
D A pressure pump kick-out switch on the
digester.
The digester pressure can be controlled by moving
the set point on the input control valve to 165 psig
(normal digester pressure). A rise in the set point
would then cause the valve to open to admit more
liquor to the digester and, therefore, increase the
pressure. The reverse occurs when the pressure
set point is lowered. This is a rapid response
controller.
The secondary pressure control element consists
of the automatic relief valve, which bleeds liquor
from the lower cooking zone header to the No. 2
flash tank. The relief valve is set slightly higher (15
psig) than the input valve and bleeds off liquor
when the pressure exceeds this set point. The
relief valve should always be in a closed position
under normal conditions. It is only used as a
pressure relief valve.
The third control device is primarily an emergency
safety device, which is activated when the first two
control devices fail. A pressure switch is mounted
on the digester shell and stops the cold blow pump
when the pressure rises too high. Digester
pressure is normally set at 165 psig, the pressure
relief valve is set on 180 psig, and the pressure
switch at 225 psig. As the pressure continues to
rise, the make-up liquor pump will kick-out.
Chip Feeding
Processed chips from the wood room are
transmitted to the chip bin, which also serves as a
short term chip storage bin. This storage can
facilitate continued digester operation during small
upsets between the wood room and the digester
building. Chips flow by gravity from the bin through
a tapered hopper into the chip meter. The chip
meter is a rotating star feeder with seven pockets
yielding a certain volume of chips per revolution.
Digester production is regulated by a variable