- •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
Viscosity
[mL g–1]
Chain scissions
[0.1·mmol AHG–1]
Ka118 0 1057 47.7 11.7 934 14.5 6.6 43.6 772 1.041
Ka121 107 708 43.7 12.3 1411 8.7 6.4 48.0 1008 1.182
Ka124 1701 708 42.91 9.0 1152 6.9 4.41 51.1 865 1.269
Ka125 210 368 41.4 9.9 1353 5.2 4.1 58.6 957 1.295
Ka127 310 368 40.6 8.8 1304 3.8 3.4 61.4 936 1.288
Ka129 710 368 36.5 9.0 1112 2.2 2.6 71.1 848 1.244
Ka131 1900 368 31.6 11.1 668 1.3 2.0 82.0 578 1.191
Oxygen delignification was performed in a two-stage reaction without interstage
washing, with 15 min retention time in the first and 60 min in the second reactor,
respectively. The reaction temperature was kept constant at 110 °C throughout
both stages. The total alkali charge of 25 kg t–1, was added in the first stage. The
data in Tab. 4.31 indicate that the efficiency of oxygen delignification improves
along with the removal of the xylan content. Parallel with the reduction in the
hemicellulose content, the number of chain scissions increases until a residual
xylan content of approximately 5% is reached. When the residual xylan content is
further reduced to below 2%, the residual cellulose fraction again becomes more
resistant to degradation reactions (Fig. 4.57).
Interestingly, the degree of delignification during the oxygen delignification
stage is linearly correlated with the logarithm of the xylan content of the Eucalyptus
saligna prehydrolysis kraft pulp (Fig. 4.58).
Recently, the correlation between the residual amount of hemicelluloses and
delignification efficiency during oxygen delignification was confirmed for both
softwood and hardwood kraft pulps, with and without pre-hydrolysis [74]. Surprisingly,
the kappa numbers of the pulps after oxygen delignification display a very
similar final lignin content, expressed as Ox-Dem kappa. The kraft pulps without
pre-hydrolysis (paper-grade pulps) contain a considerably higher amount of “nonlignin”
and HexA structures as part of the kappa number as compared to the prehydrolysis
kraft pulps (dissolving pulps). As shown previously, the false lignin
fraction which is predominantly derived from carbohydrate structures is not susceptible
to oxygen delignification. On the contrary, during oxygen delignification
the proportion of “non-lignin” kappa number fractions even increases. The presence
of chemical linkages between cellulose, the residual hemicellulose and
the residual lignin in native wood were reported by Isogai et al. [75], and the
4.2 Kraft Pulping Processes 259
0 2 4 6 8 10 12 14 16
30
40
50
60
70
80
90
Chain Scissions, 104/P
j
-104/P
0
Degree of delignification
Degree of Delignification [%]
Xylan content [%]
1.0
1.1
1.2
1.3
1.4
Chain scissions
Fig. 4.57 Influence of the residual xylan content of a Eucalyptus
saligna prehydrolysis kraft pulp on the delignification efficiency
and number of chain scissions in a subsequent oxygen
delignification stage (OO: 15/60 min, 110 °C, 25 kg NaOH t–1)
(according to [73]).
1
30
40
50
60
70
80
90
5
Delignification efficiency [%]
Xylan content [%]
10
Fig. 4.58 Influence of the residual xylan content of a
Eucalyptus saligna prehydrolysis kraft pulp on the delignification
efficiency in a subsequent oxygen delignification stage
(OO: 15/60 min, 110 °C, 25 kg NaOH t–1) (according to [73]).
260 4 Chemical Pulping Processes
formation of alkali-stable ethers and carbon–carbon linkages during kraft pulping
were reported by Ohara et al. [76]and Gierer and Wannstrom [77]. Iversen and
Wannstrom proposed the alkali-catalyzed formation of ether bonds between carbohydrate
hydroxyl groups and lignin oxiranes derived from the degradation of the
lignin molecule during kraft pulping [78].
The most prominent lignin structures, which are responsible for the reactivity
in subsequent bleaching treatments, are the alkyl-aryl ether linkages (b-O-4-structures),
the methoxyl groups, the aliphatic and aromatic hydroxyl groups and the
hydrophilic substituents, such as carbonyl and carboxylic groups [79]. Moreover,
the macromolecular properties of the residual lignin provide additional information
about the conditions during the delignification reactions. Unfortunately,
there is still no method for the isolation of a representative residual lignin of
unchanged physical and chemical structure. The acidolytic and enzymatic hydrolysis
methods are used for the isolation of residual lignin. Additionally, a combination
of enzymatic and acidic hydrolysis as a two-step procedure was proposed [80].
The latter shows some advantages with respect to the yield and the amount of
impurities in comparison to the one-step procedure. The dioxane acidolysis,
which is still the most common method, produces pure lignin of only about 40%
yield. Unfortunately, the b-aryl-ether and lignin–carbohydrate linkages are cleaved
during the isolation procedure, which is seen as a reduction in the molecular
weight of the lignin and in an increased phenolic hydroxyl group content [81].
According to Gellerstedt et al., the formation of condensed phenolic groups during
acidolysis is not probable [82]. Although residual lignin can be recovered
quantitatively after enzymatic hydrolysis, the isolated lignin contains large
amounts of impurities which aggravate structural lignin characterization to a significant
degree.
There are some indications that modern modified cooking technologies alter
the structure of residual kraft lignin beneficially for subsequent bleaching treatments.
The residual lignin isolated from a hemlock EMCC kraft pulp using a
dioxane acidolysis protocol shows a lower amount of condensed phenolic and
higher amounts of carboxylic acids and uncondensed phenolic units as compared
to the residual lignin structure from a conventional hemlock kraft pulp [83]. Comparative
data from lignin characterizations are listed in Tab. 4.32.
The enrichment of carboxylic groups during kraft cooking is followed by the
elimination of aliphatic hydroxyl groups, which are decreased from 4.27 mmol g–1
in case of the milled wood lignin to 2.14 resp. 2.15 mmol g–1 for the residual lignin
isolated from the hemlock unbleached kraft pulps (Tab. 4.32). This is in agreement
with the growing elimination of the a-hydroxyl groups present in b-O-4
ether units. The relatively high content of primary hydroxyl groups in the wood
lignin can be expected to be diminished during pulping because of the known
reactions in which the c-carbon is eliminated as formaldehyde. The content of the
primary hydroxyl groups is significantly higher in the residual lignin isolated
from the conventional spruce kraft pulp as compared to the residual lignin from
modified kraft pulp (0.24 mol per aromatic unit versus 0.33 mol per aromatic
unit, respectively) [84].
4.2 Kraft Pulping Processes 261
Tab. 4.32 Comparative evaluation of the residual lignin
structures isolated from milled wood lignin, unbleached
conventional and EMCC kraft pulps (according to [83]).
Parameters Units MWLb) Hemlock kraft pulps
Conv. Kraft EMCC Kraft
Kappa number 26.8 26.0
Isolation yielda) % 14.2 49.5 46.7
Elemental composition
C % 61.0 65.2 62.8
H % 5.7 5.7 5.7
O % 32.9 27.9 29.8
S % 1.2 1.7
OCH3 % 15.3 12.1 10.5
Carboxylic groups mmol g–1 0.15 0.32 0.54
Hydroxyl Units
Aliphatic hydroxyls mmol g–1 4.27 2.14 2.15
Phenolic hydroxyls mmol g–1 1.15 2.71 2.50
Type A mmol g–1 0.02 0.02 0.01
Catechol (type B) mmol g–1 0.05 0.21 0.17
Guaiacol (type C) mmol g–1 0.62 0.23 0.56
Type D mmol g–1 0.06 0.38 0.27
Type E mmol g–1 0.35 1.64 1.32
a) Continuous dioxane acidolysis (dioxane-water = 85:15, 0.1 mol/l HCl).
b) Milled wood lignin from spruce.
R
OH
R
OH
OH
R
OH
OCH3
R
OH
OCH3
R
R
OH
R OCH3
(A) (B) (C) (D) (E)
262 4 Chemical Pulping Processes
The content of b-O-4-structures in residual lignin decreases with the extent of
delignification. The residual lignin in the EMCC pulp with kappa number 17.9
contained less b-O-4 structures and a higher content of C5 condensed structures
as compared to the residual lignin of conventional kraft pulp with kappa number
27.4 [85]. This is in accordance with the results obtained from the characterization
of residual lignins isolated from MCC and Super-Batch pulping technologies [86].
George et al., however, made different observations comparing residual lignins
isolated from spruce kraft pulps using also a dioxane acidolysis procedure [84].
The number of alkyl-O-aryl linkages, determined by 13C-NMR, was higher in the
modified residual lignin than in the conventional residual lignins. This observation
is in accordance with the higher amount of free phenolic groups present in
the conventional lignin. In a recent comparative study of conventional and laboratory-
simulated EMCC kraft pulps produced from Pinus elliottii, the residual lignin
of the latter had a higher content of b-O-4-structures and carboxylic groups. At
comparable kappa number, the amount of condensed structures was, however,
similar for both residual lignins [87].
The total phenolic hydroxyl content in the residual lignin continuously
increases during kraft pulping due to progressive cleavage of the b-O-4 bonds.
The guaicol-type of phenolic unit (type C) gradually decreases in parallel with the
progress in delignification. Conditions favoring the formation of unreactive carbon–
carbon bonds prevail, especially during conventional kraft cooking [88]. As
shown in Tab. 4.32, the amount of phenolic units substituted at the C5 position
(type E) continuously rise in both the dissolved and residual lignins. The Ca-C5
and the diphenylmethane units are described as the predominant C5 condensed
structures [89]. The formation of the diphenylmethane moieties has been
described as a considerably more facile reaction under soda pulping conditions as
compared to kraft pulping conditions. This may be one of the reasons why the
bleaching of soda pulps is more difficult compared to a kraft pulp at a given kappa
number [90]. Recently, the accumulation of completely unreactive 5–5′-biphenolic
hydroxyl groups was detected using quantitative 31P-NMR [91]. The final concentration
of the 5–5′structures after softwood kraft pulping was approximately
0.6 mmol g–1, and thus more than three-fold higher than the corresponding value
of 0.2 mmol g–1 detected for the milled wood lignin.
The molecular weight of the residual lignin increases slightly towards the end
of the cook, which may be an indication of progressive condensation reactions
[83]. Lignin from pulps and corresponding spent liquors during kraft pulping of
Pinus sylvestris covering the kappa number range between 116 and 17 were isolated
by acidic dioxane extraction and characterized by GPC, UV and IR spectroscopy
and oxidative degradation methods [72]. The average molar mass of both lignin
precipitated from the spent liquor and lignins isolated from pulps increases
with the progress in cooking. The lignins extracted from pulps showed a higher
molar mass as compared to the spent-liquor lignins.
In accordance with the higher content of phenolic hydroxyl groups, the conventional
kraft residual lignin exhibits a lower molecular mass than the modified residual
lignin at a given kappa number [84]. In extending the cook from kappa
4.2 Kraft Pulping Processes 263
number 30 to kappa number 15, the molecular weight of the modified residual
lignin continues to decrease, whereas that of the conventional residual lignin is
not influenced [84]. Since the number of phenolic hydroxyl groups in the case of
the residual lignins of both pulps remained constant, it may be assumed that rupture
of the ether bonds immediately leads to lignin dissolution. Extending the
conventional cook results in a significant decrease in the number of methoxyl
groups. This trend is less pronounced with modified cooks. The loss in methoxyl
groups may also be accounted for by a slight enrichment in p-hydroxyphenyl units
toward the end of the cook. It is known that the cleavage of alkyl-aryl ether linkages
is favored by the presence of methoxyl groups. Consequently, guaiacyl units
can be assumed to be removed prior to p-hydroxyphenyl units. In contrast to the
modified residual lignin, the content of quaternary carbons is significantly
reduced in case of the conventional residual lignin, which may be attributed to
the enrichment in p-hydroxyphenyl units.