- •Recovered Paper and Recycled Fibers
- •Isbn: 3-527-30999-3
- •Introduction
- •Isbn: 3-527-30999-3
- •Isbn: 3-527-30999-3
- •2006, Isbn 3-527-30997-7
- •Volume 1
- •Isbn: 3-527-30999-3
- •4.1 Introduction 109
- •4.2.5.1 Introduction 185
- •4.3.1 Introduction 392
- •5.1 Introduction 511
- •6.1 Introduction 561
- •6.2.1 Introduction 563
- •6.4.1 Introduction 579
- •Volume 2
- •7.3.1 Introduction 628
- •7.4.1 Introduction 734
- •7.5.1 Introduction 777
- •7.6.1 Introduction 849
- •7.10.1 Introduction 887
- •8.1 Introduction 933
- •1 Introduction 1071
- •5 Processing of Mechanical Pulp and Reject Handling: Screening and
- •1 Introduction 1149
- •Isbn: 3-527-30999-3
- •Isbn: 3-527-30999-3
- •Isbn: 3-527-30999-3
- •Isbn: 3-527-30999-3
- •Introduction
- •Introduction
- •Isbn: 3-527-30999-3
- •1 Introduction
- •1 Introduction
- •1 Introduction
- •1 Introduction
- •1 Introduction
- •1 Introduction
- •150.000 Annual Fiber Flow[kt]
- •1 Introduction
- •1 Introduction
- •Introduction
- •Isbn: 3-527-30999-3
- •Void volume
- •Void volume fraction
- •Xylan and Fiber Morphology
- •Initial bulk residual
- •4.2.5.1 Introduction
- •In (Ai) Model concept Reference
- •Initial value
- •Validation and Application of the Kinetic Model
- •Inititial
- •Viscosity
- •Influence on Bleachability
- •Impregnation
- •Impregnation
- •Impregnation
- •Impregnation
- •Impregnation
- •Impregnation
- •Impregnation
- •Impregnation
- •Impregnation
- •Impregnation
- •Introduction
- •International
- •Impregnation
- •Influence of Substituents on the Rate of Hydrolysis
- •140 116 Total so2
- •Xylonic
- •Viscosity Brightness
- •Xyl Man Glu Ara Furf hoAc XyLa
- •Initial NaOh charge [% of total charge]:
- •Introduction
- •Isbn: 3-527-30999-3
- •Introduction
- •Isbn: 3-527-30999-3
- •Introduction
- •Introduction
- •Isbn: 3-527-30999-3
- •In 1950, about 50% of the global paper production was produced. This proportion
- •4.0% Worldwide; 4.2% for the cepi countries; and 4.8% for Germany.
- •1150 1 Introduction
- •1 Introduction
- •1 Introduction
- •Virgin fibers
- •74.4 % Mixed grades
- •Indonesia
- •Virgin fibers
- •Inhomogeneous sample Homogeneous sample
- •Variance of sampling Variance of measurement
- •1.Quartile
- •3.Quartile
- •Insoluble
- •Insoluble
- •Insoluble
- •Integral
- •In Newtonion liquid
- •Velocity
- •Increasing dp
- •2Α filter
- •0 Reaction time
- •Increasing interaction of probe and cellulose
- •Increasing hydrodynamic size
- •Vessel cell of beech
- •Initial elastic range
- •Internal flow
- •Intact structure
- •Viscosity 457
- •Isbn: 3-527-30999-3
- •1292 Index
- •Visbatch® pulp 354
- •Index 1293
- •1294 Index
- •Impregnation 153
- •Viscosity–extinction 433
- •Index 1295
- •1296 Index
- •Index 1297
- •Inhibitor 789
- •1298 Index
- •Index 1299
- •Impregnation liquor 290–293
- •1300 Index
- •Industries
- •Index 1301
- •1302 Index
- •Index 1303
- •Xylose 463
- •1304 Index
- •Index 1305
- •1306 Index
- •Index 1307
- •1308 Index
- •In conventional kraft cooking 232
- •Visbatch® pulp 358
- •Index 1309
- •In prehydrolysis-kraft process 351
- •Visbatch® cook 349–350
- •1310 Index
- •Index 1311
- •1312 Index
- •Viscosity 456
- •Index 1313
- •Viscosity 459
- •Interactions 327
- •1314 Index
- •Index 1315
- •Viscosity 459
- •1316 Index
- •Index 1317
- •Xylose 461
- •Index 1319
- •Visbatch® pulp 355
- •Impregnation 151–158
- •1320 Index
- •Index 1321
- •1322 Index
- •Xylan water prehydrolysis 333
- •Index 1323
- •1324 Index
- •Viscosity 459
- •Index 1325
- •Xylose 940
- •1326 Index
- •Index 1327
- •In selected kinetics model 228–229
- •4OMeGlcA 940
- •1328 Index
- •Index 1329
- •Intermediate molecule 164–165
- •1330 Index
- •Viscosity 456
- •Index 1331
- •1332 Index
- •Impregnation liquor 290–293
- •Index 1333
- •1334 Index
- •Index 1335
- •1336 Index
- •Impregnation 153
- •Index 1337
- •1338 Index
- •Viscose process 7
- •Index 1339
- •Volumetric reject ratio 590
- •1340 Index
- •Index 1341
- •1342 Index
- •Index 1343
- •1344 Index
- •Index 1345
- •Initiator 788
- •Xylose 463
- •1346 Index
- •Index 1347
- •Vessel 385
- •Index 1349
- •1350 Index
- •Xylan 834
- •1352 Index
Impregnation
During impregnation, the cooking chemicals become distributed inside the chips.
Impregnation starts as soon as the chips are subjected to digester pressure in the
chip feeding system. At that time, impregnation is governed by liquor penetration
into the chip voids under a pressure gradient. Once the chips are in the digester,
diffusion takes control over the mass transfer as the chips and liquor move concurrently
through what is commonly referred to as the impregnation zone.
378 4 Chemical Pulping Processes
Good impregnation is a key to uniform pulp quality and optimal cooking time.
The aspects related to steaming and impregnation and their effects on continuous
cooking have recently been extensively described [5].
Cooking
The impregnation step passes into the cooking stage as the free liquor is displaced
from the chip column by hot cooking liquor. Note that the liquor in the digester
can be divided into a free portion moving between the chips and a bound portion
trapped within the chip volume. Only the free liquor is available to displacement,
whereas the bound liquor is accessible by diffusion and thermal conduction.
The cooking liquor is distributed by the central pipe discharge and flows radially
towards the cooking circulation screens. It is essential that the circulation flow
rate is sufficiently high to ensure uniform chemical and temperature profiles
across the digester cross-section. This is of critical importance also for the other
circulation operations in the digester. Otherwise, different radial temperature
and/or chemical levels prevail that lead to inhomogeneously cooked fibers.
Cooking usually occurs in more than one zone. As the chip column moves
down, the alkali, temperature and solid levels in the digester can be adjusted by
introducing white liquor or wash filtrate into circulation loops, by indirect heating
of the liquor in those loops, or by extracting spent liquor for chemical recovery.
Washing
The transition between cooking and washing in a continuous digester is blurred. At
the high temperature levels applied for washing, a considerable number of reactions
continue in the washing zone. The in-digester washing step – termed Hi-Heat washing
– was originally designed to last for up to 4 h, and this resulted in excellent washing
efficiencies due to the vast time allowed for diffusion. In many mills, however, indigester
washing has been compromised for higher production rates when a part
of the digester’s washing zone volume was converted for cooking.
In-digester washing is the first brownstock washing stage. This means that,
over time, there should be a balance between the wash filtrate collected from the
wash plant and the liquor pumped into the digester. For washing to be efficient,
there must be a net flow of liquor from the wash circulation located near the bottom
of the digester to the liquor extraction above.
Pulp Discharge
The significant cooking reactions are terminated when the cool wash filtrate from
the first stage of brownstock washing brings the temperature of the digester contents
down to 85–95 °C. A rotating scraper reclaims the pulp from the cross-section
at the digester bottom to the outlet device. Besides cooling the pulp, the wash
filtrate provides the necessary dilution before the pulp discharge. The blowline
consistency is typically around 10%. Even at the end of the cook, the pulp in the
digester still exhibits the physical structure of the wood chips. This structure is
finally broken up as the pulp becomes defibrated during the turbulent pressure
reduction at the discharge control valve.
4.2 Kraft Pulping Processes 379
Heat Management
Typically, the heat exchangers use indirect steam to raise the temperature of circulation
liquors. Steam/liquor phase digesters may have direct steam addition to the
digester top. The heat in the extraction liquor is usually transferred to vapor which
is used for steaming of the chips, but it can also be exchanged with cooler process
liquors. Additional live steam may be used for chip steaming if necessary. Some
of the residual heat in the weak black liquor is spent for generation of hot water.
The hot water temperature achievable from the cooling of weak black liquor is
80–90 °C. The cooling of wash filtrate coming from brownstock washing yields
somewhat lower water temperatures, because the filtrate temperature must be
low enough to bring the digester contents safely beneath the boiling point.
Fiber Removal from Black Liquor
The slots in the screens installed in the digester need to be a few millimeters wide
to avoid plugging. As a consequence, the extraction liquor contains fibers which
are highly unwelcome in the evaporation plant. Hence, the liquor must be subjected
to fiber removal before being transferred to evaporation.
Gas Management
The gases vented from the chip bin and steaming vessel contain malodorous compounds,
and must be collected for reasons of emission control and maintaining
an acceptable workplace environment. Besides noncondensable constituents, the
vent gases carry certain amounts of moisture, and must therefore pass condensation
before proceeding to the mill’s gas collection and treatment systems. When a
digester plant processes softwood, the condensate also contains turpentine, which
is separated from the condensate by decanting.
4.2.8.3.3 Chip Steaming and Chip Feeding Systems
A conventional chip steaming and feeding system for a continuous digester is
shown schematically in Fig. 4.138. The chips are fed through the airlock, a rotary
star or screw feeding device, into the chip bin. Flash steam from the second
extraction liquor flash tank enters the chip bin near the bottom and provides atmospheric
steaming, which typically lasts for 15–25 min. The chip meter, which
again is a rotary star or screw feeder, sets the pulp production rate of the digester.
It discharges into the low-pressure feeder, which isolates the pressurized steaming
vessel from the atmospheric chip bin. Pressurized steaming is usually continued
for 1–2 min at a pressure of ca. 1.5 bar(g). A screw in the steaming vessel conveys
the chips to the chip chute.
The high-pressure feeder’s plug-type rotor always keeps a vertical and a horizontal
flow path open for liquor circulation. When one particular pocket of the feeder
is in the vertical position, the chips are sucked from the chip chute into the pocket
by the chip chute circulation pump. The chips are retained in the pocket by a
screen mounted in the casing of the feeder. Liquor passes the screen and returns
to the chip chute via the sand separator. As the pocket turns to the horizontal
380 4 Chemical Pulping Processes
Chips
to digester
Liquor
from digester
CHIP BIN
AIRLOCK
CHIP METER
LOW-PRESSURE FEEDER
STEAMING VESSEL
HIGH-PRESSURE FEEDER
CHIP CHUTE
IN-LINE DRAINER
LEVEL TANK
SAND SEPARATOR
White liquor
Flash steam from
flash tank 2
Flash steam from
flash tank 1
Chips
CHIP CHUTE LEVEL PUMP TOP CIRCULATION PUMP
CHUTE CIRCULATION PUMP
Fig. 4.138 Conventional chip feeding system for continuous digester.
position, the liquor coming from the top circulation pulp pushes the chips out of
the pocket and transfers them to the digester (or impregnation vessel). There is a
certain intentional leakage from the high-pressure side of the feeder (i.e. the top
circulation side) to the low-pressure side (i.e. the chute side). Excess liquor in the
chute liquor loop is extracted via the in-line drainer and pumped back to the top
circulation by the chip chute level pump. White liquor can be added to the suction
side of the chute level pump, which is therefore also referred to as make-up liquor
pump.
More recently developed chip feeding systems attempt to reduce the amount of
equipment installed. For example, Kvaerner Pulping’s Compact Feed system
(Fig. 4.139) does not requires the sand separator, in-line drainer and level tank [6].
The Andritz Lo-Level Feed system skips the steaming vessel and chip chute by
replacing the low-pressure feeder with a helical-screw chip pump, which directly
feeds the high-pressure feeder. Recently, Andritz has developed the TurboFeed
system which also eliminates the high-pressure feeder (Fig. 4.140). Chips are
metered from the chip bin via twin screws into a chip tube. From there, they are
forwarded to the digester by a series of specially designed pumps. The feed circulation
cooler ensures that the liquor returned to the chip chute is below 100 °C.
Besides easing the feed process, the system also controls the digester pressure,
thereby allowing a more stable flow pattern of free liquor in the cooking zones
due to a constant wash filtrate feed rate to the digester [7,8].
4.2 Kraft Pulping Processes 381
Chips
to digester
Liquor
from digester
HIGH-PRESSURE FEEDER
CHIP CHUTE
Chips
CHIP CHUTE LEVEL PUMP TOP CIRCULATION PUMP
CHUTE CIRCULATION PUMP
Fig. 4.139 The Kvaerner Pulping Compact Feed system [6].
Chips
to digester
Liquor
from digester
TWIN SCREW CHIP METER
COOLER
Chips
LIQUOR
SURGE TANK
CHIP TUBE
CHIP PUMPS
Fig. 4.140 The Andritz TurboFeed system [8].
382 4 Chemical Pulping Processes
4.2 Kraft Pulping Processes 383
4.2.8.3.4 Modified Continuous Cooking (MCC)
Modified Continuous Cooking [9,10]was the first in a string of alterations
imposed on the conventional continuous pulping process. A typical configuration
of an MCC single-vessel hydraulic digester is shown in Fig. 4.141. The chips enter
the top of the digester together with the top circulation liquor, and are fed to the
top separator, which is a screw conveyor surrounded by a cylindrical screen. The
vertical screw transports the chips downwards and also keeps the slots of the
screen clean. Circulation liquor is extracted through the screen and returned to
the chip feeding system, where the largest portion of the white liquor is added.
The excess liquor from the top circulation travels downwards concurrently with
the chips and enters the impregnation zone (see also Fig. 4.142).
Impregnation is typically performed at a temperature between 115 and 125 °C
and a pressure above 10 bar(g) for 45–60 min. As the chips approach the first
screen section, liquor is displaced horizontally from the central pipe discharge
through the chip column to the strainers, and is then circulated back to the central
pipe via the concurrent cooking heater. A small portion of white liquor is added to
the cooking circulation loop. The heater is operated with indirect steam and the
hot liquor introduced into the digester brings the temperature of the chip column
up to the cooking temperature of 150–170 °C.
Steam
Steam
Wash filtrate
Circulation transfer
White liquor
WASH
HEATER
COUNTERCURRENT
COOKING
HEATER
CONCURRENT
COOKING
HEATER
Pulp
Extraction
liquor
Fig. 4.141 Typical MCC single-vessel hydraulic digester [9,10].
Hot cooking liquor and chips then continue traveling downwards through the
concurrent cooking zone to the extraction screens. This is where the spent cooking
liquor is taken from the digester. Below the extraction screens starts the countercurrent
cooking zone, where the net flow of liquor is directed upwards. The
temperature in both cooking zones is roughly the same, with the countercurrent
cooking heater being responsible for the temperature in the lower zone. White
liquor is added to the countercurrent circulation liquor to increase the alkalinity
towards the end of the cook. Typically, the total cooking time of 90–150 min is
equally split between the concurrent and the countercurrent zones.
As the chips proceed into the washing zone, the countercurrent flow regime
persists. The temperature in the so-called Hi-Heat washing zone decreases gradually
to about 130 °C, and the dissolved wood components as well as spent cooking
chemicals are removed from the pulp by diffusion washing. The final temperature
in the washing zone is controlled by steam addition to the wash heater, which is
installed in the lowest of the circulation loops. At the digester bottom, the pulp is
cooled and diluted by wash filtrate, before it is eventually discharged from the vessel
through the blow valve. The wash filtrate flow usually controls the pressure in
the digester.
The major force driving behind movement of the chip column in the digester is the
weight of the wood material. Forces acting against the direction of the wood’s weight
are the buoyancy of gas entrapped in the chips, friction between themoving chips and
the digester wall, and – in zones of countercurrent flow – the drag induced by the
upward liquor movement. Efficient air removal and reasonable countercurrent liquor
velocities are therefore important prerequisites for smooth chip columnmovement.
The need to maintain high circulation flow rates brings about a considerable
risk of plugging screens or screen headers because fines and other small material
are carried through the chip column and accumulate at the screen surface, together
with chips, or in the header. This is why techniques must be applied to keep
the screens and headers clear. In a typical set of screens, profile bar screen plates
are arranged at two levels above each other, with independent headers and two
nozzles for each header which are positioned at opposite sides of the digester
shell. This arrangement allows the automated side-to-side switching of headers
and resting of screens – that is, temporary stopping of the extraction through one
level of screens. When a screen rests, the movement of the chip column wipes its
slots clear. When a header is switched to the other side, the flow direction is
inverted, which makes the formation of deposits more difficult. In addition, backflushing
of screens may be necessary at times.
There is always a temperature and concentration gradient from the central pipe
discharge along the radius of the digester to the strainers, even at high circulation
rates. In particular, in large-capacity digesters it can be a major challenge to maintain
gradients that are adequate for uniform cooking. Two-vessel systems provide
the opportunity of heating the bottom circulation liquor returning from the digester
to the impregnation vessel, thus allowing constant temperature and alkali profiles
over the digester cross-section at the beginning of the bulk delignification
phase. A typical impregnation vessel with top separator, outlet device and optional
384 4 Chemical Pulping Processes
4.2 Kraft Pulping Processes 385