- •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
HOT
DISPLACEMENT
HEATING
AND COOKING
COLD
DISPLACEMENT
PULP
DISCHARGE
Fig. 4.129 Typical displacement batch cooking cycle.
368 4 Chemical Pulping Processes
Chip Filling
The cooking cycle starts with chip filling, as wood chips are fed to the digester
with screw or belt conveyors. Screw conveyors have the advantage that their closed
design avoids the spread of wood dust which often is a nuisance with belt conveyors.
The chips drop from the conveyor into a chute and usually pass a packing device
as they enter the digester. Low-pressure steam is the most-used packing medium.
Introduced at an angle just as the chips enter the digester, the steam sets the chips
in a spiral motion and ensures their distribution across the digester cross-section.
Packing increases the amount of wood that can be charged to a digester by 10–
20%, thus leading to a higher pulp yield per digester volume. Packing also warms
up the chips and improves the homogeneity of the chip column in the digester
which is an important prerequisite for good liquor circulation and displacement
without channeling. Decent chip filling therefore is the starting point for uniform
pulp quality.
As the chips are warmed up, the air is positively displaced from inside the chips
by the increasing partial pressure of wood moisture and by its own increasing volume.
The residual air removal must happen by counter-diffusion of water vapor
against air. During chip filling, gas is evacuated from the digester through the
strainers by means of a blower.
Impregnation
In the next step of the cooking cycle, impregnation liquor is charged to the bottom
of the digester until the digester is hydraulically full. The impregnation liquor fill
can in fact start before the end of the chip fill in order to shorten the cooking cycle
time. At the end of the impregnation liquor fill, the digester is slightly overfilled
to make sure that it is full. Overflowing liquor is returned to the atmospheric
black liquor tank.
During impregnation, the wood is further preheated and residual air is removed
from within the chips as liquor enters their interior. The impregnation step is supported
by pressure generated by a pump to force the impregnation liquor into the
chips. Good impregnation is another key to uniform pulp quality.
Hot Displacement
After the impregnation step, the liquor in the digester is displaced by hot liquor.
Ideally, the hot liquor pushes out the liquor in the digester in a plug-flow manner.
As in all displacement steps, uniformity of penetration of the chip column without
channeling is of critical importance for the pulp quality. There is less risk of
screen pluggage by fibers when the flow rate of displacement and circulation
liquors is gradually ramped up to full flow.
The liquors charged to the digester during hot displacement carry both the
chemicals needed for cooking and the energy to heat the digester content. The
liquors displaced from the digester are routed to their destination in the tank
farm, depending on their temperature levels. At the end of the hot displacement,
the digester is already close to the cooking temperature.
4.2 Kraft Pulping Processes 369
Heating and Cooking
The final temperature increase required to reach the cooking temperature target
can be made by injecting steam into the digester circulation line, by indirect heating
in shell-and-tube heat exchangers, or by continued liquor displacement. Certain
technologies allow the alkali level to be adjusted during cooking.
When the digester content has reached the cooking temperature, the circulation
flow is sometimes reduced in order to protect the strainers from plugging with
fibers. The cooking step continues until the desired H-factor is reached.
Cold Displacement
The significant cooking reactions are terminated by cold displacement using filtrate
from brownstock washing. Again, it is essential to have plug-flow through
the digester, so that the initially displaced hot black liquor can be collected in the
hot black liquor tank at the highest possible temperature for re-use in the next
digester.
At some point during the cold displacement, wash filtrate begins to break
through to the displaced liquor and to bring its temperature down. When the temperature
drops below a set limit, the displaced liquor is switched from the hot
black liquor tank in the tank farm to another tank of lower temperature. The cold
displacement continues until the desired quantity of wash filtrate has been
pumped through the digester. The digester contents should then be cooled down
to a temperature below 100 °C.
In fact, the cold displacement step is the first brownstock washing stage. This
means that, over time, there must be a balance between the wash filtrate collected
from the wash plant and the liquor pumped through the digesters during the cold
displacement steps.
Digester Discharge
In the terminal step of the cooking cycle, the pulp is discharged from the digester
by pumping. The pump discharge is delicate since, even at the end of the cook,
the pulp in the digester still exhibits the physical structure of the wood chips.
Wash filtrate is added for dilution to the lower part of the digester and to the discharge
elbow. The sustainable consistency for pumping pulp from the digester is
typically lower than 5%.
Besides appropriate dilution, the size of the discharge elbow and of the discharge
valve are critical for a successful pump discharge. It is important for the
succeeding cook that the pulp discharge is as complete as possible and that no
pulp is left in the digester when the next cook starts.
Heat Management
Hot black liquor from a previous cook is stored in the tank farm and, as it is
charged to the digester, passes on its heat energy directly to the next cook. The
remainder of hot black liquor not re-used in hot displacement is available for indirect
heat transfer to white liquor and process water in shell-and-tube heat exchangers.
370 4 Chemical Pulping Processes
Typically excess hot black liquor, which is not needed in hot displacement, heats
white liquor coming from the recausticizing plant. The temperature levels in the
other black liquors are normally not high enough to be economically transferred
to white liquor. Nonetheless, they are sufficiently hot for the generation of considerable
amounts of hot water from warm water. The hot water temperature achievable
from cooling of warm 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.
In addition, the temperature of hot white liquor and/or hot black liquor is
adjusted in the tank farm by indirect steam heating. Steam heating in the tank
farm has the advantage of continuous steam consumption as compared to digester
circulation liquor heating, which occurs only during the short period of heating
to cooking temperature.
Fiber Removal from Black Liquor
The holes in the screens which are installed in the batch digesters need to be a
few millimeters in diameter to avoid plugging during displacement and circulation.
Whenever displaced liquor leaves a digester, it carries a certain amount of
fibers to the tank farm. As a consequence, all liquors circulated in the tank farm
contain fibers, as well as the black liquor in the atmospheric black liquor tank
which is bound for chemical recovery. Since fibers are highly unwelcome in the
evaporation plant, the liquor transferred to evaporation from the atmospheric
black liquor tank must be subjected to fiber removal by a liquor filter or a liquor
screen.
Soap Skimming
The soaps which are generated during softwood pulping can cause foaming and
displacement problems if recycled back to the digester in a displacement cooking
plant. Skimming of soap is a tricky undertaking because the separation of soap
from the liquor occurs in narrow ranges of dry solids and pH. Usually, soap
removal functions best from a liquor returning to the atmospheric black liquor
tank. This liquor must then be allowed enough retention time for soap to come
afloat. A dedicated, separate soap separation tank has proven most effective. The
segregated soap is then pumped to the evaporation plant in a separate line or together
with the black liquor.
Gas Management
The gases vented from apparatus and equipment in the digester plant contain malodorous
compounds, and must be collected for reasons of emission control and maintaining
an acceptable workplace environment. Atmospheric tanks are connected to
the high-volume low-concentration (HVLC) gas collection system. Such tanks include
the atmospheric black liquor tank, the wash filtrate tank, and the pulp discharge
tank. The air evacuated during the chip fill goes also to the HVLC gas system.
4.2 Kraft Pulping Processes 371
Besides non-condensable constituents, the gases vented from the pressurized
tanks in the tank farm contain large amounts of moisture due to their elevated
temperature. They are therefore passing condensation before proceeding to the
low-volume high-concentration (LVHC) gas system. When the digester plant is
being used to process softwood, the condensate contains turpentine, which is separated
from the condensate by decanting.
4.2.8.2.4 Rapid Displacement Heating (RDH)
A typical RDH tank farm consists of three pressurized accumulators and two atmospheric
tanks (Fig. 4.130). The liquor accumulators A, B, and C are staged in
temperature. Fresh alkali is provided from the hot white liquor tank. Steam is
used for top-heating of white liquor in the tank farm and for bringing the digester
to cooking temperature by indirect condensation.
HBL
"C"
WBL DIG WF
"B"
BL
"A"
HWL
WHITE LIQUOR
CHIPS
PULP
BLACK
LIQUOR
WASH
FILTRATE
WW
HW
STEAM
STEAM
Fig. 4.130 Simplified Rapid Displacement Heating (RDH) process
flowsheet. BL = black liquor; WBL= warm black liquor;
HBL= hot black liquor; HWL= hot white liquor; DIG = digester;
WF = wash filtrate; WW = warm water; HW = hot water [1].
The RDH cooking cycle is shown schematically in Fig. 4.131. After chip filling,
warm black liquor (WBL) of up to 130 °C from the B accumulator is used for
impregnation. Then, hot white liquor (HWL) and hot black liquor (HBL) from the
C accumulator are charged to the digester during hot displacement. The displaced
liquor is first returned to the A tank and then to the B accumulator. After hot displacement,
the digester is brought to cooking temperature by indirect steam heating.
Circulation is sometimes stopped after the target temperature has been
reached. Subsequent to cooking, wash filtrate (WF) displaces the hot black liquor
first into the hot black liquor accumulator C and then to the warm black liquor
372 4 Chemical Pulping Processes
PULP
WF
STEAM
WF
HBL, WBL
HWL + HBL
BL, WBL
WBL
CHIPS
AIR
CHIP FILL
RDH
COOKING
CYCLE