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- •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
Xylan and Fiber Morphology
During pulping, the xylan is dissolved and also modified to a large extent. Towards
the end of the cook [96,97], part of the xylan – which is still of oligomeric nature –
180 4 Chemical Pulping Processes
is re-precipitated at the cellulose fibers [98–102], thereby increasing pulp yield
[103], changing the mechanical characteristics (increasing strength) of kraft pulps
[104,105], and affecting the fiber quality. Hereby, the structure of the xylan affects
the adsorption characteristics to a large extent, and increasing the removal of carboxyl
groups thereby favors the retake of xylan [106].
The interaction of cellulose and hemicellulose in kraft pulps can be addressed
by a number of modern analytical approaches. Differences in hemicellulose concentrations
are observed between the surface and the inner layer of a kraft pulp
fiber. In the pulps investigated (softwood and hardwood), the amount of hemicelluloses
is generally larger at the surface as compared to the inner layer. The MWD
and sugar composition of the hemicellulose deposits can be studied with MALDI
and CE [107,108]. Analysis of the solid material involves CPMAS-13C-NMR studies
[109–112], association and localization of hemicellulose on pulps are studied by
GPC [113–116], and the use of enzymes in combination with other analytical techniques
provides insights into bonding types [117–119], as well as LCC and hemicellulose
structures [120,121].
Carbohydrate-derived Chromophores
Low molecular-weight carbohydrates can undergo further reactions under the
alkaline conditions of a kraft cook. Besides a series of isomerizations and further
fragmentations, condensation to cyclic compounds and phenols can also occur
(cf. Scheme 4.21). A number of catechols, phenols and acetophenones could be
detected in model studies with glucose and xylose [122]. Some of these can also
form stable radicals [123], which can also be detected in alkaline solutions from
hot caustic extractions of cellulose [124]. Chromophores are also produced by oxidation
reactions under alkaline conditions [125].
OH
H
H OH
H OH
H O
H
HO
CH2OH
OH
H
H O
H
HO
H
O
H
H
OH
HO
H
H
OH
H
H O
H
OH
H
H O
HO
H
H
2
O
H
H O
H
H
O
CH3
O
H
O
H C 3
O
H
O
O
OH
R1 OH
R2
O
R1 O
R2
+
_
OH
OH
+
Scheme 4.21 Formation of chromophores from carbohydrate monomers.
4.2.4.3 Reactions of Extractives
Extractives are complex mixture of terpenes, fats, waxes, resin acids, fatty acids,
phenols and tannins. Most extractives are soluble in alkaline solutions, and a
good solubility permits the processing of wood species that are rich in extractives
(including tropical woods). In kraft pulping, however, high extractive contents of
wood may result in a considerable reduction in pulp yield. This in turn leads to an
4.2 Kraft Pulping Processes 181
increase in the consumption of chemicals, since extractives react rapidly with
alkali and thus the amount of available hydroxyl ions is reduced [126]. The dissolution
of extractives during pulping is of primary importance. Extractives are
responsible for pitch problems in papers, they may also prevent delignification by
covering parts of lignin with resinous material or simply reduce the penetrability
of cooking chemicals into the wood [127], and they add to the toxicity of kraft mill
effluents. The total amount of extractives which can be recovered from pulp mills
varies greatly with the wood species and the storage conditions of the wood
(Scheme 4.22). The highly volatile fraction is called turpentine, sulfate turpentine
or tall oil (from the Swedish “tall” = pine), and is recovered from the digester relief
condensate [128]. The sulfur-containing fractions (mercaptanes) need to be
removed from the distillates.
COOR
COO Na
+
( )
n
pulping
( )
n
saponification
isomerization
acidification
crude tall oil
extractives
fatty acids resin acids
pitch residue
soap skimmings
destillation
turpentine
-
neutral
compounds
(e.g. sterols)
light oil
extraction
Scheme 4.22 Fractions of extractives obtained after kraft cooking [128].
Fatty acids and resin acid esters are saponified in alkaline pulping and recovered
as tall oil soap [1]. Acidification of the crude tall oil yields the corresponding
free acids. This deacidification process consumes a large amount of sulfuric acid,
which can be reduced by a carbon dioxide pretreatment.
Wood terpenes undergo mainly condensation reactions during pulping, and are
collected as sulfate turpentine. The major reactions of extractive components are
as follows:
_ Fatty acids [129]: these undergo isomerization reactions (the shift
of double bonds in the fatty acid chain from cis to trans, or vice
versa) under alkaline pulping conditions, and are mainly dis-
182 4 Chemical Pulping Processes
solved. Nonconjugated double bonds are transformed to mainly
conjugated isomers. The degree of conjugation is highly influenced
by the prevailing conditions during the cook. For linoleic
acid, almost no isomerization was observed at 150 °C, whereas at
180 °C almost 98% were isomerized [130]. The incorporation of
fatty acids into residual lignin has recently been demonstrated [12].
_ Resin acids [129]: these are also mainly dissolved. Part of the levopimaric
acid (65) is converted to abietic acid (66), though the
extent of this reaction during pulping is variable (Scheme 4.23).
The acidification and heating of sulfate soap finally converts most
of the levopimaric acid [131,132].
COOH COOH
Levopimaric acid Abietic acid
65 66
Scheme 4.23 Conversion of levopimaric acid to abietic acid during the kraft process.
_ Waxes: sterol esters and waxes are saponified much more slowly
as compared to the glycerol esters. Waxes and triglycerides are
hydrolyzed during alkaline pulping; hence, no esters are detected
in sulfate soaps [129]. The sterol esters, waxes and free sterols do
not form soluble soaps as do free acids, and therefore have a tendency
to deposit and as such cause pitch problems.
A number of extractives survive the cook more or less unchanged, and this portion
is referred to as the “non-saponifiable” fraction.
4.2.4.4 An Overview of Reactions During Kraft Pulping
The course of dissolution of lignin and carbohydrates reveals three distinct phases
of a kraft cook: initial, bulk, and residual delignification which affect the single
wood components as summarized in Scheme 4.24.
_ Initial phase: the initial stage is characterized by losses in the carbohydrate
fraction, which is more pronounced for hardwoods as
compared to softwoods [133]. The hemicelluloses undergo deacetylation
and physical dissolution, and peeling reactions also start.
Cellulose degradation by peeling is negligible in terms of yield
loss. Reactive phenolic lignin units, such as a-O-4-ethers, are
cleaved as early as the initial phase.
4.2 Kraft Pulping Processes 183
_ Bulk phase: The core delignification occurs in the bulk phase and,
importantly, both phenolic and nonphenoplic b-O–4-ether bonds
are cleaved. About 70% of the lignin is removed. The reactions of
the carbohydrates are characterized by secondary peeling (i.e.,
alkaline cleavage of the glycosidic bonds), but also by stopping
reactions, which are favored at elevated temperature. Methanol is
liberated from 4-O-methylglucuronic acid side chains, and hexenuronic
units are formed.
_ Residual phase: the residual phase begins at a delignification rate
of about 90%. Delignification has slowed down considerably due
to depletion of reactive lignin units. It is believed that the chemical
nature of the residual lignin hampers further degradation
reactions. A slow delignification is accompanied by rapid carbohydrate
degradation, causing disproportionate carbohydrate losses.
0 40 80 120 160 200 240