Reference Manual

10A −8
Batch Digester – Low Energy
Process
Much has been written about the relatively new
low energy cooking process. Studies indicate
improved pulping properties and operating
efficiencies over conventional batch digesters.
Some of the reported benefits of the low energy
process are:
D Significant steam savings
D Reduced evaporator load
D Lower black liquor viscosities
D Lower alkali consumption
D Fewer brown stock washing stages
D Stronger pulp
D Lower environmental impact
The Process
The main difference between the low energy
cooking process and a conventional batch digester
is the tank farm associated with the heat recovery
system. Accumulator tanks are added with each
“stage” of process. A typical three-stage design is
shown in the process schematic (see figure
10A-6). Actual mill installations have duplicate sets
of pumps, valves and piping to handle odd and
even digesters. Also there are typically sets of “A”,
“B”, and “C” accumulator tanks. This arrangement
allows for operations flexibility.
The function of the tank farm and management of
the transfer of liquors is key to understanding the
low energy process and cooking cycle. First,
empty digesters are filled with wood chips. If
desired, packing of the chip bed can be
accomplished with steam or liquor. Steam
provides higher compaction. Compaction of the
bend increases capacity of the cook.
Cool black liquor from the atmospheric “A” tank is
added to provide a liquor pad in the bottom of the
digester. Warm black liquor from the pressurized
“B” tank is then pumped into the digester
displacing entrained air. The discharge valves are
then closed. The warm liquor pump brings the
digester up to pressure by pre-impregnating the
chips and hydraulically filling the vessel.
The chips are further heated by pumping both hot
black and hot white liquor into the digester. The
white liquor is preheated through an indirect heat
exchanger between the “C” and “B” tanks using
hot black liquor as the heat source. The hot white
liquor is then stored in a pressurized accumulator
tank for delivery to the digesters. Warm black
liquor is displaced to the “A” tank where soap is
skimmed and excess liquor is sent to the weak
liquor filters.
After completion of the hot liquor fill operation, the
pulp mass is generally close to the required cook
temperature and pressure. If necessary, further
heating is done by an external liquor heater.
As the pulp is cooked, resinous vapors are given
off. These vapors, along with any remaining
entrained air, migrate to the top of the digester.
These gases are systematically drawn off through
the digester relief valve. If these gases were not
drawn off a false pressure relative to the steam
saturated temperature would be indicated. Under
such conditions the control action might be to
reduce steam to the digester and thus undercook
the pulp. From time to time between relief cycles,
steam is blown back through the relief line to clean
debris off the relief screens.
Once the proper degree of cooking (H-factor) has
been reached, cooking is stopped by pumping
washer filtrate into the bottom of the digester.
Most of the cooking liquor remains hot and is
displaced to the “C” tank, ready to use for the next
cook. The cooler liquor goes to “B” and “A” tanks.
As a result, the pulp in the digester is washed and
cooled below flash point at atmospheric
conditions. This in-digester washing reduces load
on both the brown stock washers and evaporators.
The pulp is then transferred to the blow tank by
cold-blowing the digester with compressed air. A
high pressure air receiver is used for this air
supply. Since the pulp is blown cool a number of
benefits arise including improved pulp quality and
lower emissions of total reduced sulfur.