Reference Manual

10A −6
False Pressure
A batch digester is basically a large pressure
cooker. As steam is applied to the mass of chips
and liquor, a quantity of resinous vapors are
distilled off. These vapors, along with air initially
entrained with the chips and a small quantity of
non-condensed steam, migrate to the top of the
digester. These vapors and gases are
systematically drawn off through the digester relief
(gas off) piping. The vapors are a major source of
the distinctive Kraft mill odor.
If the non-condensable portions of these gases
are not relieved from the digester they would
eventually accumulate sufficiently in the top of the
digester to indicate a falsely high pressure relative
to the steam saturation temperature. Under such
conditions, the correct control action would be to
reduce the steam flow to the digester. The cook
would then take place at a steam saturation
temperature corresponding to say, 102 psig
instead of the 100 psig target. The resulting batch
would be very undercooked and possibly ruined.
Therefore, the non-condensable portion of the
relieved gases must be removed from the digester
in order to maintain the correct
temperature-pressure relationship.
Overpressure
Overpressure of a digester means that the actual
digester pressure is above the desired target.
Overpressure may result from trying to maintain
the proper temperature while false pressure exists
(overshooting), or via exothermic reactions once
the target pressure has been reached. For
insurance and safety purposes, each digester will
have an upper pressure limit rating. Overpressure
exposes production personnel to a hazardous
environment and is a contributor to off-quality
pulp.
Relief or Gas Off
The previous discussions of digester overpressure
and digester false pressure outlined the necessity
for relieving excess gases from a digester. Figure
10A-1 shows a typical relief piping arrangement.
When the gas off valve is open, the blow back
valve must be closed. This interlock must exist for
both safety and economic reasons.
In general, a relief line is connected to the neck of
the digester through a relief screen. The screen
prevents large quantities of cellulose fibers or
chips from entering the relief line piping system.
Liquor, being a fluid, can readily pass through the
screen, but it is undesirable to allow any chemical
loss to occur. All relief gases and liquids pass from
their respective digesters into a common header.
This header directs all such materials to a central
separating device which separates
non-condensable gases from condensable gases
and liquor, pulp, etc. The non-condensable gases
are quite odorous. These gases are usually
scrubbed and/or burned in a lime kiln. The
condensable gases, however, can contain, in
addition to steam, a significant quantify of a
valuable byproduct, i.e., crude turpentine. Three to
four gallons of crude turpentine can be distilled off
and recovered per ton of resinous southern pine
pulp produced.
Blow Back
Blow back is basically a short reversal of steam
flow through the gas off line to the top of the
digester for the purpose of cleaning the relief
screen and/or collapsing the steam bubble, which
may form within the chip mass in the lower area of
the digester. The sequence is typically: (1) shut off
the high pressure steam valve to the base of the
digester, (2) shut off the gas-off valve to the
common header, and (3) open the blow back valve
(see figures 10A-1 and 10A-2). This sequence
allows high pressure steam to be briefly injected
through the relief lines and into the top of the
digester. The surge of high pressure steam blows
the screen clean while the increased pressure
from the top forces the chip mass down,
collapsing the bubble. A blow back typically lasts
only 10-30 seconds, and then the valves revert to
the original status.
Blow Tank
When a digester cooking cycle is completed, the
blow valve is opened to connect the digester with
a common blow tank. The blow tank is a low
pressure receiving vessel, which is usually
capable of holding several blows. Several batch
digesters producing the same grade of pulp will
discharge into the same blow tank. The high
pressure in the digester will blow the entire mass
of chips and black liquor into the blow tank.
Typically, a blow tank is equipped with both
vacuum and pressure relief valving systems
because 100 to 120 psi is released when the blow
valve is opened. A significant quantity of vapor will
be flashed off the pulp and liquor as it enters the
blow tank. A blow tank is not designed to