Reference Manual

10B −15
— P = 170 psig 10
— dP = 170 psi
D Typical valve selection:
— These are typically in the NPS 8 size
range for KV-19 A, B, C, D and in the NPS 3
size for KV-19 E and F. 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 very capable of a relatively
long life in this service.
Extraction and Hi-Heat Washing
Chips, having now passed through the cooking
zone, reach the extraction screens. The column is
considerably denser following cooking and has a
better wiping action so that a cycling system is not
required at this zone. The section from the
extraction screens down to the bottom of the
digester is the wash zone. Black liquor is extracted
from two rows of screen plates. The upper screen
extracts primarily the hotter spent liquor from the
downward flow of chips and the lower screen
extracts cooler liquor flowing upward, or
countercurrent, from the washing zone.
The portion of extracted cooking liquor that is not
returned to the digester through the quench
circulation pump goes to the No. 1 flash tank,
which serves as the first stage of the digester heat
recovery system. The liquor leaves the digester at
about 180 psig and discharges to the flash tank at
15-18 psig. The sudden pressure drop causes the
liquor to boil rapidly, or flash, and form steam,
which goes to the steaming vessel. The amount of
steam produced is directly proportional to the hot
liquor flow through the extraction line and reduces
the flow of the fresh make-up steam required at
the steaming vessel. The remaining liquor then
bleeds off to the No. 2 flash tank and is pumped to
the unoxidized weak black liquor storage tank. As
the No. 2 flash tank is under atmospheric
pressure, again flashing occurs and the flash
steam goes to a condenser.
Counter-current hi-heat washing is accomplished
by extracting a greater volume of liquor through
the extraction screens than the liquor volume
coming down with the chips, causing an upflow of
wash filtrate. The wash filtrate is pumped in at the
bottom of the digester where it fills the voids
created by the increased extraction. It also flows
out through the blow line with the pulp acting as
dilution liquor.
Wash filtrate is weak black liquor produced by
extraction and shower displacement of residual
liquor from the pulp. This filtrate is returned to the
digester, mixes with the stronger cooking liquors,
and cools the pulp before it is discharged (cold
blow). The ratio of the number of pounds of
excess filtrate (upflow) added per minute to the
number of pounds of O.D. pulp produced per
minute is called “dilution factor”. For example, a
dilution factor of two would require a net upflow in
the wash zone of two pounds per minute of filtrate
to each pound of A.D. pulp. The amount of filtrate
is determined by the remaining soda content in the
pulp and should be held to a minimum value. More
filtrate produces less soda loss, but at some point,
filtrate addition, which later has to be evaporated
by steam heat, is greater than the cost of soda. It
can, therefore, be seen that a balance must be
made at the most efficient point.
Washing efficiency increases with increased
temperature. Therefore, it is necessary to heat the
wash liquor. Thus, another extraction screen is
located near the bottom of the digester which
draws liquor out. This liquor is circulated via the
wash pump through a steam heater and returned
through yet another chamber located inside the
central distribution chamber. This heated liquor
discharges at the lower wash screen and diffuses
upward through the downflowing chips. It
effectively washes the mass as it replaces
stronger liquor, which has been extracted above.
The reason for injecting the cold liquor into the
bottom and then extracting it to be heated for
washing is explained in the next section. The pulp
column has now reached the final or blowing
stage.
Valve: HV-16 Digester washer
extraction flow
This valve controls the flow of black liquor to the
flash tank.
D Typical process conditions:
— Fluid: Black liquor
— T = 325_F
— P = 130 psig
— dP = 45 psi – 60 psi
— Q = 500 – 2500 gpm
D Typical valve selection:
— This application is typically specified as a
full-bore ball valve. A CV500 in stainless
steel will be a suitable valve for this
application due to its ability to handle scaling
process conditions.