- •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
Introduction
The origin of the sulfite process is attributed to the efforts of Benjamin Chew
Tilghman, an American chemist, who was granted U.S. patent 70,485, dated
November 1867, entitled Treating Vegetable Substances for Making Paper Pulp. The
invention was based on the results of the experiments at the mills of W.W. Harding
and Sons at Manayunk, near Philadelphy, in 1866, and covers the pulping of
wood with aqueous solutions of calcium hydrogen sulfite and sulfur dioxide in
pressurized reactors. However, the first sulfite mill started its production in Europe
at Bergvik,Sweden, in 1874 under the direction of C.D. Ekman usingmagnesium
hydrogen sulfite solution, Mg(HSO3)2, as the cooking agent. The mill was equipped
with small rotating digesters heated indirectly by means of a steam jacket. In 1875,
theGerman chemist A. Mitscherlich was developing a sulfite cooking process using a
horizontal, stationary, cylindrical digester lined with brick and indirectly heated by
means of coils of lead or copper pipe. Cooking was carried out under moderate temperature
and pressure conditions. Consequently, the Mitscherlich process was characterized
by a much higher retention time as compared to the directly heated Ritter–
Kellner process, which was developed at the same time in Austria.
The sulfite process was developed around the acid calcium bisulfite process, as
mentioned in the Tilghman patent. It remained the principal process for wood
pulping because of the low costs and high availability until the beginning of the
1950s, when the need to recover the waste liquor and pulping chemicals slowly
emerged, mainly for reasons of environmental protection. Since calcium sulfite is
soluble only below pH 2.3, it can solely be used in acid bisulfite pulping in the
presence of excess SO2. At cooking temperature, the calcium hydrogen sulfite
decomposes to calcium sulfite and hydrated SO2:
Ca HSO3 _ _2__
T CaSO3 SO2 _ H2O _155_
Thus, high charges of free SO2 and low cooking temperatures must be maintained
to prevent the precipitation of calcium sulfite. A further drawback of the use of
calcium as a cation for the acid sulfite process is the formation of calcium sulfate
during the course of the recovery process. The conversion to calcium sulfite is not
practical, since a temperature above 1200 °C is required to achieve its complete
decomposition to calcium oxide and sulfur dioxide. At this high temperature the
crystal structure of calcium oxide changes, thus reducing its reactivity. Furthermore,
calcium sulfite anhydride tends to disproportionate to calcium sulfate and
calcium sulfide. For these reasons calcium has been replaced by more soluble
bases, and is now reserved for a few pulp mills with complete by-product recovery.
Today, the dominating base used in sulfite pulping is magnesium. The corre-
392 4 Chemical Pulping Processes
sponding aqueous magnesium bisulfite solutions are soluble in a pH range up to
5–6, so that acid bisulfite and bisulfite (magnefite) pulping processes in both oneand
two-stage operations can be carried out. The big advantage of the magnesium
bisulfite process compared to the calcium base system lies in its thermochemical
behavior [1]. In contrast to the calcium system, the thermal decomposition of MgSO3
occurs at a rather low temperature, generating only a small amount of sulfide. The
magnesiumsulfate obtained from the combustion of magnesiumsulfite spent liquor
can be decomposed thermally in the presence of carbon from the dissolved organic
substances to give gaseous SO2 and magnesium oxide according to Eq. (156) [2]:
2MgSO4 C __ 2SO2 2MgO CO2 _156_
In order to avoid secondary oxidation of SO2 to SO3 in the absorption unit, the
flue gas must not contain free oxygen in excess of 3%, so that the surplus of air in
the combustion process must not exceed 1.5–2.0% [2].
As alternatives to calcium and magnesium, sodium and ammonium cations are
also used in sulfite pulping. Since both monovalent cations are soluble over the
entire pH range, they can be used in acid, bisulfite (magnefite), neutral and alkaline
sulfite processes. The prevailing sulfite processes are defined according to the
pH range of the resultant cooking liquor, as shown in Tab. 4.51.
Tab. 4.51 Assigament of sulfite pulping processes according to
the different pH ranges.
Nomenclature Initital pH range at 25 °C Base alternatives Acitve reagents
Acid bisulfite 1–2 Ca2+, Mg2+, Na+, NH4
+ H+, HSO3
–
Bisulfite (Magnefite) 3–5 Mg2+, Na+, NH4
+ (H+), HSO3
–
Neutral sulfite 6–9 Na+, NH4
+ HSO3
–, SO3
2–
Alkaline sulfite 10–13.5 Na+ SO3
2–, OH–
The use of monovalent cations, especially ammonium, tends to increase the
rate of delignification at given process conditions [3]. Mill experience indicates
that maximum temperature could be decreased by 5 °C when changing from calcium
to ammonium base while keeping the cooking cycle constant [4]. The reason
for the more rapid delignification in the cooks on soluble cations is not entirely
known. According to the Donnan law, it appears that acidity in the solid phase
decreases as the affinity of the cation to the solid phase increases. It is assumed
that the concentration of the lignosulfonate groups in the solid phase equals about
0.3 N, corresponding to a pH level below 1.0 in the absence of cations other than
protons [5]. The affinity for the solid phase is increasing in the order [6]:
H+ < Na+ < NH4
+ < Mg2+ < Ca2+ < Al3+
4.3 Sulfite Chemical Pulping 393
The acidity of the solid phase should therefore be lower in the presence of aluminum
ions, and highest in the presence of sodium ions. It is likely that the higher
acidity of the solid phase in the case of monovalent bases contributes to a slightly
higher extent of carbohydrate hydrolysis and somewhat greater velocity of delignification.
Although the differences in rate and selectivity of delignification are not
significant, mill application has revealed several advantages, such as higher pulp
yield, viscosity and alphacellulose content at a given kappa number and a lower
amount of rejects [7,8]. These advantages can be attributed to better penetration
with cooking chemicals and a more uniform cook when changing from calcium
to magnesium, ammonium, or sodium base. The brightness of the unbleached
pulps is, however, clearly impaired in the case of ammonium-based pulps. There,
the lower brightness is probably due to a selective reaction between the ammonium
ion and carbonyl groups of lignin. This reaction is also responsible for a
much darker color of the ammonium-based spent liquors. However, no differences
in the bleachability of ammonium-based pulps in comparison to other sulfite
pulps can be observed.
Despite some clear advantages of the monovalent over the bivalent bases with
respect to flexibility (entire pH range available) and pulping operations (more
homogeneous impregnation, higher rate of delignification), their use in sulfite
cooking processes has been limited to a few applications, mainly due to deficiencies
in recovery of the cooking chemicals. For ammonium sulfite waste liquor no
economically feasible solution exists to recover the base. Ammonia recovery processes
based on ion exchange have been developed to the mill level, but have not
gained acceptance in praxis because of high costs. The use of ammonium base in
particular has been shown to be advantageous for the production of highly reactive
dissolving pulp where mill scale operations still exist. Sodium base is predominantly
used in neutral and alkaline sulfite processes. The recovery of sodiumbased
sulfite processes combines the use of a kraft-type furnace and the conversion
of the resulting sodium sulfide to sodium sulfite using carbonation processes
(e.g., liberation of hydrogen sulfide from the smelt by the addition of CO2 from
the flue gas, oxidizing hydrogen sulfide to SO2, reaction of SO2 with sodium carbonate
to give sodium sulfite). The technology employing carbonation of green
liquor was developed in the 1950s and 1960s, but since then no decisive improvements
of this recovery concept have been made. Thus, the recovery of the sodiumbased
sulfite cooking chemicals is significantly less efficient than the sodiumbased
kraft process, and this may be the main reason for the comparatively limited
application of the sodium-based sulfite processes.
During the first 50 years of chemical pulp production, the sulfite process was
the dominating technology, due mainly to the high initial brightness and the easy
bleachability of the sulfite pulps. With the developments of both a reductive recovery
boiler for the regeneration of kraft spent liquor by Tomlinson and chlorine
dioxide as a bleaching agent to ensure selective bleaching to full brightness in the
mid-1940s, the kraft pulping technology became the preferred method because of
better energy economy, better paper strength properties, and lower sensitivity
towards different wood species and wood quality. In the meantime, efficient
394 4 Chemical Pulping Processes
chemical recovery systems have been developed especially to use magnesium as a
base. The high sensitivity to the wood raw material still constitutes a problem in
the case of acid sulfite pulping. Most softwoods except spruce, such as pines,
larches and Douglas fir, are considered less suitable for sulfite pulping. A certain
part of the extractives of phenolic character such as pinosylvin, taxifolin (Douglas
fir) as well as the tannins of bark-damaged spruce and oaks give rise to condensation
reactions with reactive lignin moieties in the presence of acid sulfite cooking
solutions.
Since the 1960s the basic and applied research has been directed almost exclusively
towards alkaline pulping technologies, with kraft pulping as the key technology,
due to the higher overall economic potential. Consequently, the kraft process
has become increasingly important and is now the principal pulping process,
accounting for far more than 90% of world pulp production. For the production of
most paper-grade pulps, the strength properties are of utmost importance. Kraft
pulps show clearly better strength properties, especially with regard to the tear
strength as compared to sulfite pulps. Consequently, new installations for the production
of paper-grade pulps are almost exclusively based on kraft pulping technology.
Unlike paper-grade pulping, the acid sulfite process is the dominant technology
for the production of dissolving pulps and accounts for approximately 70%
of the total world production. Although a clear niche product, the dissolving pulp
production is a firmly established pulp market with a predicted annual growth
rate of about 5% within the next five years. The strong position of sulfite technology
in dissolving pulp production of low-purity grades sufficient for regenerated
fiber manufacture is based on a favorable economy, because of higher pulp yield,
better bleachability and higher reactivity as compared to a corresponding prehydrolysis-
kraft pulp. Therefore, the following presentation of acid sulfite pulping
technology is predominantly oriented toward dissolving pulp production.
4.3.2
Cooking Chemicals and Equilibria
In practice, the terms total, combined and free SO2 are used to characterize sulfite
cooking liquors. The content of stoichiometrically base bound hydrogen sulfite
and sulfite ions is referred to as combined SO2. The difference between the total
SO2 and the combined SO2 is then free SO2. According to this definition, half of
the pure hydrogen sulfite solution is free and the other is bound SO2. The following
definition is rather misleading from a chemistry point of view as no free sulfur
dioxide is present in a pure hydrogen sulfite solution:
2HSO_3 __ SO2_ 3 SO2 H2O _157_
In the central European acid sulfite industry the technical sulfite solutions are
characterized more closely to the chemical state of the constituents. There, the
combined SO2 is equal to the pure hydrogen sulfite, whereas the (excess or true)
free SO2 accounts solely for the sulfur dioxide in its hydrated form (SO2.H2O) and
4.3 Sulfite Chemical Pulping 395
can be calculated as the difference between the total and the combined SO2. The
combined SO2 is calculated from the active base content, usually expressed as oxide
(e.g., CaO, MgO or Na2O). The following equation demonstrates the definition
of combined SO2 using magnesium hydrogen sulfite as an example:
Mg_HSO3_2__ MgO 2SO2 H2O _158_
According to Eq. (158), 1 mol MgO accounts for 2 mol combined SO2. On a weight
basis, one part of MgO corresponds to 3.179 parts of combined SO2. A typical acid
sulfite cooking liquor contains 17.5% total SO2 and 2.20% MgO, equal to 7.0%
combined SO2 on o.d. wood, respectively. The amount of free SO2 calculates to
10.5% (range 7.0–17.5), representing 60% of the total SO2 as free SO2.
For characterization of the composition of the sulfite cooking liquor, the latter
definition will be used.
To better exemplify the differences between the two ways of cooking acid specification
a comparison is provided on the basis of a typical acid sulfite cooking
liquor composition, as depicted in Tab. 4.52.
Tab. 4.52 Specification of a typical acid sulfite cooking liquor
expressed in two different terms (actual definition related the
more actual species concentrations as used in this book vs. the
Palmrose definition according to TAPPI Standard T604 pm-79).
Parameter units Actual definition
used in this booka)