- •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
International
(Palmrose) definition
Total SO2 % od wood 17.5 17.5
Free SO2 % od wood 10.5 14.0
of total SO2 60 80
Combined SO2 % od wood 7.0 3.5
MgO % od wood 2.2
Liquor-to-wood ratio 3.5
Actual SO2 concentration
Total SO2 mol L–1 0.78
Free SO2 mol L–1 0.47
Bound SO2 (HSO3
–) mol L–1 0.31
a) Denoted also as true free and true combined SO2; this definition
may be used to convert both definitions into each other: % True
free SO2 = (% Free SO2 – % Comb. SO2)
% True Comb. SO2 = 2* % Comb. SO2.
396 4 Chemical Pulping Processes
The concentrations of the sulfur(IV) species in the aqueous cooking liquor are
defined through the following equilibria:
SO2 H2O _ H2SO3 SO2 _ H2O _ __H HSO_3 _159_
It has been shown that the major part of the sulfur dioxide in an aqueous solution
is not hydrated to sulfurous acid [6]. The hydrated and non-hydrated form of
the free SO2 are combined to express the first equilibrium constant Ka,1:
Ka_1 _
H _ _ HSO_3 __SO2 __H2SO3 _ _160_
The dissociation constant Ka,1 of combined SO2 decreases clearly with increasing
temperature, as seen in Tab. 4.53.
Tab. 4.53 Temperature-dependence of the first equilibrium
constant of free SO2 (according to [6]).
Temperature
[ °C]
pKa,1
25 1.8
70 2.3
100 2.6
110 2.8
120 3.0
130 3.1
140 3.3
150 3.5
Hydrogen sulfite ions are also in equilibrium with monosulfite ions and protons
according to the following expression:
HSO_3 _ H SO2_ 3 _161_
Hydrogen sulfite is a weak acid, and its equilibrium constant derived from
Eq. (159), and denoted as second equilibrium constant, Ka,2, is expressed as:
Ka_2 _
_H _ SO2_ 3 HSO_3 _ _ _162_
4.3 Sulfite Chemical Pulping 397
The pKa,2 can be approached by a value of about 7.0 at 25 °C. The change in ionization
of the hydrogen sulfite ion with temperature is unknown, and is assumed to
be insignificant. Consequently, pKa,2 is kept constant in the temperature range
prevailing in acid sulfite cooking.
The concentrations of the active cooking chemicals in a pure aqueous acid sulfite
cooking liquor, [H+], [HSO3
– ]and [SO3
2–], can be calculated by the following
simple equations:
The total SO2 concentration at any time and any pH is calculated as:
SO2_tot _ Ctot _ SO2 _ H2O _ HSO_3 SO2_ 3 _163_
The concentrations of [SO2.H2O], [HSO3
– ]and [SO3
2–]can be calculated accordingly:
SO2 _ H2O _ _Ctot _ HSO_3 SO2_ _ 3_ _164_
HSO_3 _ Ctot __SO2 _ H2O SO2_ _ 3_ _165_
SO2_ 3 _ Ctot _ SO2 _ H2O _ HSO_3 _ _ _166_
The pH-dependent concentrations of sulfur(IV) species can be calculated by using
the equilibrium equations:
Ka_1 _ Ctot _ HSO_3 SO2_ 3 _ _ _ __ H _ _ HSO_3 _167_
The hydrogen sulfite ion concentration can be calculated by rearranging Eq. (167):
HSO_3 _
Ka_1 _ Ctot _ SO2_ _ 3_ _Ka_1 _H _ _168_
A similar procedure can be applied to calculate the monosulfite ion concentration:
Ka_2 _ Ctot __SO2 _ H2O SO2_ _ _ 3____H _ SO2_ 3 _169_
SO2_ 3_
Ka_2 __Ctot __SO2 _ H2O _
_Ka_2 _H _ _170_
The course of pH as a function of the concentrations of the sulfur(IV) species in a
pure sulfite cooking liquor can be calculated by considering the equilibrium conditions
for the titration of a weak two-basic acid with strong alkali according to the
following expression:
398 4 Chemical Pulping Processes
__A_ _OH_ __H _AH _171_
Assuming the total concentration of the sum of the conjugated bases [A– ]and the
acid [AH]to be Ctot (in mol L–1), the acid–base equilibria can be calculated as:
_H _
Ka_1 _ Ctot
_Ka_1 _H _
Ka_2 _ Ctot
_Ka_2 _H _
10_14
_H _ C* _172_
where C* is the molar amount of the titrator base NaOH.
As an example, the course of pH of a pure aqueous sulfite solution with a total
SO2 concentration of 50 g L–1 (0.78 mol L–1) is calculated as a function of the free
SO2 concentration (Fig. 4.151). In the first case, the titration curve is calculated
according to Eq. (172), using sodium hydroxide as a titrator base. In the second
approach, the titration curve is calculated by means of ASPEN-PLUS simulation
software, using magnesium hydroxide as a titrator base. ASPEN-PLUS uses a
high-performance electrolyte module based on the NRTL model (nonrandom,
two-liquid) to calculate the thermodynamic properties of aqueous electrolyte systems
[9]. The model provides an accurate description of the nonideality of concentrated
aqueous solutions.
The titration curve estimated by means of Eq. (172) agrees well with that calculated
by ASPEN-PLUS in the pH range 1 to 4.5, until any of the free SO2 is quantitatively
converted to hydrogen sulfite ions. The course of pH beyond this point
1.0 0.5 0.0 -0.5 -1.0
0
2
4
6
8
10
Titrator base, NaOH: 25 .C 140 .C
Titrator base, Mg(OH)
2
25 .C
pH-value
Free SO
2
, mol/l
Fig. 4.151 Course of pH as a function of the
free SO2 concentration assuming an initial
total SO2 concentration of 0.78 mol L–1 at 25 °C
and 140 °C. Two calculation modes: (a) titration
curve calculated according to Eq. (170), using
NaOH as titrator base; (b) titration curve simulated
by means of ASPEN-PLUS using
Mg(OH)2 as titrator base.
4.3 Sulfite Chemical Pulping 399
develops differently for the two bases. The addition of Mg(OH)2 causes a rather
even slope of pH until the equilibrium is shifted to monosulfite ions, while the
addition of NaOH raises the pH more steeply.
The concentrations of ionic species of a sulfite cooking liquor are given as a
function of the liquor composition (e.g., the molar content of free SO2 and active
base) in Tab. 4.54.
Tab. 4.54 Concentrations of ionic species of sulfite cooking
liquor with increasing amount of active base concentration;
initial free S02 concentration 0.78 mol L–1; [H+]calculation
according to Eq. (172), [HSO3
– ]according to Eq. (168), [SO3
2– ]
according to Eq. (170) and [SO2-H2O]according to Eq. (164).
Free SO2
[moI L–1]
Base
[moI L–1]
[H+]
[moI L–1]
pH-Value [SO2.H2O]
[moI L–1]
[HSO3
– ]
[moI L–1]
[SO3
2– ]
[moI L–1]
[H+]*[HSO3
– ]
0.78 0.00 1.04·10–1 0.98 6.77·10–1 1.04·10–1 1.00·10–7 1.07·10–2
0.58 0.20 3.65·10–2 1.44 5.44·10–1 2.37·10–1 6.48·10–7 8.63·10–3
0.39 0.39 1.47·10–2 1.83 3.76·10–1 4.05·10–1 2.76·10–6 5.96·10–3
0.28 0.50 8.49·10–3 2.07 2.72·10–1 5.09·10–1 5.99·10–6 4.32·10–3
0.08 0.70 1.79·10–3 2.75 7.92·10–2 7.02·10–1 3.92·10–5 1.26·10–3
0.00 0.78 3.94·10–5 4.40 1.93·10–3 7.77·10–1 1.97·10–3 3.06·10–5
–0.02 0.80 3.97·10–6 5.40 1.91·10–4 7.62·10–1 1.92·10–2 3.02·10–6
–0.22 1.00 2.57·10–7 6.59 9.10·10–6 5.62·10–1 2.19·10–1 1.44·10–7
–0.42 1.20 8.64·10–8 7.06 1.97·10–6 3.62·10–1 4.19·10–1 3.13·10–8
–0.62 1.40 2.62·10–8 7.58 2.67·10–7 1.62·10–1 6.19·10–1 4.24·10–9
–0.74 1.52 5.68·10–9 8.25 1.51·10–8 4.20·10–2 7.39·10–1 2.39·10–10
–0.78 1.56 3.58·10–11 10.45 3.04·10–13 2.79·10–4 7.81·10–1 1.00·10–14
The relative concentrations of sulfur dioxide, hydrogen sulfite, and sulfite are
determined by the pH of the aqueous solution. Figure 4.152 shows that sulfur
dioxide is present predominantly as SO2.H2O and hydrogen sulfite ions at pH 1–
2, typical for acid sulfite cooking. With increasing pH, the proportion of hydrogen
sulfite ion increases significantly, and in the pH range characteristic for magnefite
cooking (3–5), sulfur dioxide is present almost exclusively in the form of hydrogen
sulfite ions. Above this pH level, the sulfite ions start to become the dominating
ionic species in the sulfite cooking liquor.
400 4 Chemical Pulping Processes
0 2 4 6 8 10
0
20
40
60
80
100
SO
2
.H
2
O HSO
3
- SO
3
2-
mol percentage
pH-value
Fig. 4.152 Relative molar percentage of SO2·H2O, hydrogen
sulfite and sulfite ions as a function of the pH at 25 °C. Data
based on information in Tab. 4.54.
Due to the decrease in the acid dissociation constant of hydrated sulfur dioxide,
Ka,1, with increasing temperature, the pH level of the acid sulfite cooking liquor is
shifted to higher values at cooking temperature (Fig. 4.151, Tab. 4.51). This must
be considered in acid calcium sulfite pulping by increasing the proportion of the
free sulfur dioxide concentration to avoid the formation of insoluble calcium sulfite.
The ionic product, [H+].[HSO3
– ], is said to be proportional to the rate of delignification
in the course of sulfite pulping [6]. According to Tab. 4.54, this ionic product
increases exponentially with decreasing pH, which is equal to an increase in
the free SO2 concentration. This result corresponds well with industrial experience.
Increasing the proportion of free SO2 in the cooking acid continuously
reduces the cooking time at given process conditions.
Moreover, the presence of free SO2 largely determines the vapor pressure of the
cooking acid at the prevailing temperature. Figure 4.153 illustrates the development
of the partial pressure of SO2 of a cooking acid with a total SO2 concentration
of 0.78 mol L–1 containing two different amounts of free SO2, 0.39 mol L–1
(50% of total) and 0.23 mol L–1 (30% of total), respectively, at varying temperature
levels.
The inter-relation of partial SO2 pressure, and free and total SO2 is exemplified
in Fig. 4.154 for two temperatures, 100 °C and 140 °C, the latter being typical for
the cooking phase.
4.3 Sulfite Chemical Pulping 401
0 50 100 150
0
2
4
6
0.78 mol/l ΣSO
2
/l; 0.39 mol/l free SO
2
0.78 mol/l ΣSO
2
/l; 0.23 mol/l free SO
2
partial pressure of SO
2
[bar]
Temperature [. C]
Fig. 4.153 Development of partial pressure of
SO2 of a cooking acid comprising two different
proportions of free SO2, 50% and 30% of total
SO2 concentration (0.78 mol L–1), as a function
of temperature. The equilibrium conditions
were simulated by means of ASPEN-PLUS [10]
based on the pioneering studies of Hagfeldt et
al. [11].
0,1 0,3 0,5 0,7
1
4
7
10
pH value
pSO2: 100 °C 140 °C
partial pressure of SO2 [bar]
Free SO2 [mol/l]
1
2
3
4
Total SO2: 0.78 mol/l
pH: 100 °C 140 °C
Fig. 4.154 Development of partial pressure of
SO2 and pH of the three-component system
magnesium oxide-sulfur dioxide-water as a
function of free SO2 concentration for two different
temperatures, 100 °C and 140 °C,
respectively, while keeping the total SO2 concentration
constant at 0.78 mol L–1. The equilibrium
conditions were simulated by means of
ASPEN-PLUS [10]based on the pioneering studies
of Hagfeldt et al. [11].
402 4 Chemical Pulping Processes
The total pressure of sulfite cooking acids containing large quantities of free
SO2 is largely determined by the partial pressures of SO2, water and, in the case of
hardwood pulping, also by considerable amounts of volatile carbonic acids (e.g.,
acetic acid, furfural, etc.) and carbon dioxide. Digester pressures are usually limited
to 8–10 bar, which means that gas must be released through the relief pressure
valve during the entire cooking phase. New cooking digesters are designed to
operate at higher pressures (>12 bar), which is an effective measure to further
reduce cooking time.
4.3.3