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Karlstad University 65188 Karlstad
Tfn 054-7001 00 00 Fax 054-700 14 60
www.kau.se [email protected]
Faculty of Technology and Science
Department of Chemical Engineering
Muhammad Adeel Shahzad
Effect of temperature and time on acid sulfite
cooking for dissolving pulp
Degree Project of 30 credit Points
Master of Science in Engineering
Degree Program in Chemical Engineering
Date: 2012-11-19
Supervisor: Ulf Germgärd
Assistant supervisor: Niklas Kvarnlöf
Examiner: Lars Järnström
2
Contents Abstract ...................................................................................................................................... 3
Executive Summary ................................................................................................................... 4
Abbreviations: ............................................................................................................................ 5
Objective .................................................................................................................................... 6
1. Introduction ............................................................................................................................ 6
1.1 Wood ................................................................................................................................ 6
1.2 Pulp ................................................................................................................................... 7
1.3 Softwood and Hardwood .................................................................................................. 7
1.4 Cellulose ........................................................................................................................... 8
1.5 Hemicellulose ................................................................................................................... 8
1.6 Lignin ............................................................................................................................... 8
1.7 Extractives ........................................................................................................................ 9
1.8 Dissolving Pulp ................................................................................................................ 9
1.9 Sulfite Process ................................................................................................................ 11
2 Material and Method ............................................................................................................. 14
2.1 Screening of Chips: ........................................................................................................ 14
2.2 Preparation and analysis of cooking liquor: ................................................................... 14
2.3 Acid Sulfite cooking: ..................................................................................................... 14
3. The Process flow diagram of experiment ............................................................................. 15
4. Results & Discussion ........................................................................................................... 16
4.1 Effect of cooking time and temperature on spent liquor pH .......................................... 17
4.2 Effect of cooking time and temperature on total residual SO2 ....................................... 18
4.3 Effect of cooking time and temperature on total yield ................................................... 18
4.4 Effect of cooking time and temperature on kappa number ............................................ 19
4.5 Effect of cooking time and temperature on limiting viscosity ....................................... 20
4.6 Effect of cooking time and temperature on R18 .............................................................. 20
4.7 Effect of cooking time and temperature on reject .......................................................... 21
4.8 Black cooks at different cooking temperatures and times .............................................. 22
5. Conclusion ............................................................................................................................ 23
6. Recommendation for future work ........................................................................................ 23
7. Acknowledgements .............................................................................................................. 23
8. References: ........................................................................................................................... 24
9. Appendix: ............................................................................................................................. 26
9.1 Equations: ....................................................................................................................... 26
9.2 Summary of Results ....................................................................................................... 27
3
Abstract
The aim of this study was to investigate the effect of cooking temperature and cooking time
on spruce sulfite dissolving pulps. Dissolving pulp is used for the manufacturing of cellulose
derivative products like viscose fiber, rayon and synthetic material. The sulfite (magnesium
base) pulping process was used liquor to wood ratio was 4:1 and the total SO2 charge was
24%. Three different temperatures 140 ⁰C, 150 ⁰C and 160 ⁰C were used and various pulp
properties were measured at different time intervals until the black cook was obtained. After
sulfite, pH and total residual SO2 of spent liquor were measured. The yield, reject, kappa
number, limiting viscosity and R18 of the cooked pulp were analyzed according to ISO
standards. When the cooking time was increased at a specific temperature pH, total residual
SO2, reject, total yield, kappa number and limiting viscosity decreased and R18 value
increased. It was noted that when pulp was over cooked, kappa number and reject increased
due to lignin condensation which forms black cook.
Black cook was obtained after 17 hours, 5 hours and 3 hours at 140 ⁰C, 150⁰C and 160 ⁰C
respectively.
Keywords: Cellulose, Hemicellulose, Lignin, Sulfite pulp, Dissolving pulp, Cellulose derivatives
4
Executive Summary
The demands for dissolving pulps have substantially increased in the last decade because of
its high cellulose purity. Currently, dissolving pulps are produced by the acid sulfite and the
vapor-phase prehydrolysis kraft processes. Acid sulfite pulping renders a good basis for the
realization of biorefinery concept in which cellulose, hemicellulose and lignin may be
recovered to optimize the process. Viscose fiber market has boosted up the demand for the
production of dissolving pulps. Therefore, the industries and the academia are researching on
the processes available for the production of dissolving pulp to optimize the process. In view
of this aspect, an effort was made towards the production of dissolving pulp.
Purpose of this work was to investigate the effect of cooking temperature and time on acid
sulfite for dissolving pulp production. Acid sulfite cooking was done at 140 ⁰C, 150 ⁰C and
160 ⁰C with different time intervals, varying from 1.5 hours to 17 hours for different cooking
temperatures. After sulfite cooking, pH and total residual SO2 were analyzed from spent
liquor. The total yield, reject, kappa number, limiting viscosity and the R18 values of the
produced dissolving pulp were analyzed according to international standards.
Experimental work for this study was performed at Karlstad University, Sweden. Spruce
wood chips were collected from pulp mill StoraEnso Skoghall, Sweden dried and screened in
the lab of the company named Metso Paper Karlstad, Sweden. Overlarge and over thick chips
were separated. Only chips good in quality that passed through 8mm screening slots and
retained at 7mm round whole tray were used for the pulping process. 2.5 l stainless steel
autoclaves were used during cooking. The cooking liquor was prepared by mixing 5g of MgO
in deionized water to make the solution of 800g. The solution was stirred until 50g of SO2
was added to the solution. Total SO2 and combined SO2 were measured by titration against
KIO3 and NaOH respectively.
200 g of 93 % dried chips were added to the autoclave and steamed for 30 minutes at 2.5 bar
pressure and amount of water (steam) added to the wood chips was calculated. Calculated
amount of already prepared cooking liquor was added to the autoclave. Wood to liquor ratio
was kept 1:4 for the cooking process. Autoclaves were placed in PEG bath at 90 ⁰C as its
initial temperature and the PEG bath temperature was ramped up at the rate of 2.5 ⁰C /min
upto 115 ⁰C for 30 minutes for sulfonation reaction. PEG bath temperature was ramped up at
the rate of 0.41, 0.58 and 0.75 ⁰C /min upto 140 ⁰C, 150⁰C and 160 ⁰C respectively. Cooking
times for 140 ⁰C were 2,3,5,8 and 17 hours, cooking times for 150 ⁰C were 1.5, 2,3,4,5 and 6
hours and cooking times for 160 ⁰C were 2,3, and 4 hours. After cooking, spent liquors were
collected from autoclaves for measuring pH and total residual SO2.
Pulp was washed with deionized water, disintegrated and screened. The reject was collected
and dried overnight in the oven at 105 ⁰C. The screened samples were centrifuged for 10
minutes and dried at room temperature. After wards, total yield, kappa number, limiting
viscosity and R18 of the produced pulp were measured.
In this study, focus was to determine the effect of cooking time and cooking temperature with
a constant total SO2 charge i.e 24%. As the cooking time was increased, pH, total residual
SO2, reject, total yield, kappa number and limiting viscosity decreased and R18 value
increased with the increasing time at a specific temperature. When the pulp was overcooked,
kappa number and reject increased drastically due to lignin condensation which forms a black
cook. Black cook was obtained after 17 hours, 5 hours and 3 hours at 140 ⁰C, 150 ⁰C and 160
⁰C respectively.
5
Abbreviations:
SO2 sulfur dioxide
MgO Magnesium oxide
KI Potassium iodine
KIO3 Potassium iodate
Na2S2O3 Sodium thiosulfate
NaOH Sodium hydroxide
HSO3-
hydrogen sulfite ions
PEG polyethylene glycol
CED Copper-ethylene-di-amine
DP Degree of polymerization
ºC Degree centrigrade
6
Objective
This thesis focuses on the acid sulfite pulping process of spruce softwood for the production
of dissolving pulp, mainly because dissolving pulp production growth rate has significantly
increased due to various cellulose derivatives products production. Purpose of this work is to
investigate how the acid sulfite pulping process influences the pulp properties because under
acidic conditions lignin solubilizes through the addition of hydrophilic sulfonate groups and
its effects were studied. In the manufacturing of dissolving pulp it is most important to
remove the hemicelluloses, lignin and extractives to have cellulose as pure as possible.
1. Introduction
1.1 Wood
Wood is a natural product and a sustainable resource when used responsibly that need not
result in damage to the environment. It is one of the most important raw materials for human
beings. It is significant not only for the use in hundreds of products, but is also a renewable
natural resource because it is a cellular material of biological origin [1]
. Wood is produced by
the seed bearing plants and has hierarchic structure that is responsible for mechanical and
physical properties of all its products including pulp [2]
. Wood is a complex material with
different properties. It is a hygroscopic (ability to attract moisture from air) and anisotropic
(wood structure and properties vary in different directions) material of biological origin.
Biological origin of wood indicates the diversity and variation among different species of
trees. Wood properties and behavior depend fundamentally on the structure of wood from
molecular to cellular or anatomical level.
Figure 1: Elemental composition of dry wood
Wood is an organic material which consists of mainly carbon, hydrogen and oxygen.
Elementary composition of dry wood substance is about 50% carbon, 6% hydrogen and 44%
oxygen, with small variations in species like softwoods and hardwoods. These elements form
macromolecules at higher level called polymers which represent the main cell wall
compounds of cellulose, hemicelluloses and lignin and are the main constituents of wood. [1]
Wood is a raw material in the forest industry for the production of pulp and paper. But
nowadays, new technologies are emerging and the wood is also being used as the production
of other valuable products like cellulose derivative products. In Sweden, forests consist of
different types of trees but the main types are Spruce, Pine and Birch. However, Spruce is
available abundantly followed by Pine. Forest industry of Sweden used for different pulping
process is shown in figure 2.
7
Figure 2: Distribution of wood tree species in Sweden
Wood is composed of highly ordered axial and radial cell systems .These cells vary in both
size and shape. Wood is mainly classified into two major groups: softwoods and hardwoods.
[2]
1.2 Pulp
The term pulping indicates the process by which wood or other lignocellulosic material is
reduced to a fibrous mass known as pulp. Pulps are produced from different methods and
have different properties that make them suited to particular products. Normally pulps are
produced by chemical pulping process globally [1]
. The wood consists of cellulose,
hemicellulose and lignin. Fibers are separated by the dissolution of the lignin and for this
purpose; chemical and mechanical processes are employed [3]
. In chemical pulping process,
wood chips are cooked with chemicals which dissolve the lignin and separate the fibers [4]
.
Main chemical wood cooking techniques are kraft or sulfate process and sulfite process. For
dissolving pulp production, sulfite process is a dominating technology because lignin and
hemicelluloses are removed in the same step. [5]
1.3 Softwood and Hardwood
Softwoods and hardwoods are distinguished by the structure of their wood and cell elements.
Softwoods show a simpler structure than hardwoods. Softwoods are basically composed of
tracheids which are mainly oriented in the longitudinal direction while a few amounts of
tracheids are radially oriented within the rays. In hardwoods, vascular and vasicentric
tracheids are associated with vessels and fluid conducts through these vessels. Hardwoods
have much variation in rays width and height as compared to softwoods. Softwoods have
longer fiber length than the hardwoods and hardwood fibers have thicker cell walls, smaller
cell lumina. Softwoods and hardwoods have different composition of wood cell wall layers.[2]
Most softwoods like pines and larches are less suitable for sulfite pulping due to the presence
of certain part of extractives of phenolic character, which gives rise to the condensation
reactions with reactive lignin in the presence of acid sulfite cooking solutions.[1]
Hemicellulose in softwoods have a higher proportion of mannose and galactose than
hardwoods[6]
. As it is known that wood is no uniform substance but consists of many
chemical components that are different in quantity with different species. These components
are classified according to the available analytical methods, where an originally analytical
term has been applied to a chemical compound. These important terms are cellulose,
hemicelluloses and lignin.
8
1.4 Cellulose
Cellulose is the main component in cell wall of the plant [2]
. It is the most abundant compound
in the plants and wood [7].
It is a long homo saccharide of sufficient chain length which is
insoluble in water [3]
. Cellulose is a biopolymer and renewable raw material and this property
makes it important and fascinating. Many cellulosic materials consist of crystalline (ordered)
and amorphous regions (disordered) in different proportions [8]
and these crystalline and
amorphous regions affect the accessibility and chemical reactivity of the fibers [9]
. The degree
of polymerization of cellulose is very high; value of 15000 residues in single chain unit makes
cellulose to the longest of all known polysaccharides. Cellulose chains are not completely
straight and it may have extended helix [2]
.
1.5 Hemicellulose
Generally, hemicellulose occurs as heteropolysaccharides and it is the second most abundant
polysaccharide group in plants [10]
. Wood hemicelluloses are short in chain length. Therefore,
it has a lower degree of polymerization: up to 200 [2]
and lower molecular weight than the
cellulose. They are often branched rather than the linear polymer like cellulose. Hemicellulose
DP and composition depends on wood species. Hemicelluloses are embedded within the cell
wall and associated with cellulose and other components. Hemicelluloses are non-crystalline
because of the heterogeneity which makes it easier to hydrolyze [11]
. Chemical and thermal
stability of hemicelluloses are lower than the cellulose. Hemicelluloses are also found in the
matrix between cellulose fibrils in the cell wall and it may serve as an interface between the
cellulose and lignin [2]
. Hemicelluloses are assigned to those carbohydrates which degrade by
acid hydrolysis more rapidly than cellulose. Hemicelluloses are amorphous in structure.
Therefore, most chemical agents reach the hemicelluloses much more easily than the cellulose [12]
.
1.6 Lignin
Lignin is the third main component of wood. Wood is actually defined by the presence of
lignin in the cell wall structure. Infact the stiff woody appearance is due to the lignin [12]
.
Lignin is the hydrophobic polymer which is in between the cellulose microfibrils and
hemicelluloses, fixating them towards each other, resulting woody properties. Lignin has
complex structure and it has amorphous region. It is neither a polysaccharide nor a nucleotide.
Lignin is not a linear polymer as cellulose or a branched polymer as the hemicelluloses, but it
has three dimensional web structures [2]
. Lignin is a highly complex polymer consisting of
phenolic compounds. Lignin distribution within the cell wall and lignin contents of different
parts of a tree are different [1]
.
9
1.7 Extractives
Major structural components of wood include cellulose, hemicelluloses and lignin. Other
wood components contain exceedingly various types of high and low molecular weight
(organic) compounds which are called extractives [1]
. Extractives are extractable from wood
with hot water or organic solvents [12]
. Extractives amount varies in certain parts of the wood,
it is not constant throughout the wood [1]
.
1.8 Dissolving Pulp
For the production of dissolving pulp, chemistry of cellulose and hemicelluloses have the
most significance to optimize the production of high purity dissolving pulp, since the
hemicellulose and the cellulose reactivity with chemicals differ. Hundreds of cellulose
molecules aggregate to form microfibrils because of strong tendency of hydrogen bonds to
form intra and inter molecular hydrogen bonds. The microfibrils layering give the crystallinity
and non crystallinity regions (amorphous) in the cellulose chain [13]
.
Normally, softwoods are used for the production of dissolving pulp but sometimes hardwoods
are also used. Dissolving pulp is also being produced from cotton linters (soda pulping) and
wood via prehydrolysis kraft or acid sulfite processes. However, paper grade pulp is also
converting into dissolving pulp with selective removal of hemicellulose while ensuring the
high pulp reactivity [14]
. During dissolving pulp, manufacturing controlled variable is the
degree of polymerization (DP) of the cellulose because it gives an indication of the average
length of the polymer chains.
High grade cellulose pulp is known as dissolving pulp because it contains low amount of
hemicelluloses, lignin and resin. Dissolving pulp is a chemically refined bleached pulp,
consisting more than 90% alpha cellulose content. Quality of the dissolving pulp depends on
both the properties of raw material wood and pulping process [15].
Dissolving pulp production
is mainly done by acid sulfite and prehydrolysis kraft process. Sulfite pulp produces relatively
pure and uniform molecular weight distribution. Among these processes, acid sulfite process
is the most common and beneficial technique, including high recovery rates of the inorganic
cooking chemicals and the totally chlorine free bleaching. There is only one disadvantage that
acid sulfite process results in pulps with a broad molecular weight distribution of cellulose [7]
.
Different cellulose sources are being used for the production of dissolving pulp. About 85%
of the total volume of the wood (hardwoods and softwoods) uses wood derived celluloses.
The morphology of these two types of wood is different. The appearance of the fiber from its
natural state in wood and also how the pulping process has affected the fiber is of utmost
importance for the end user of dissolving pulp. Fiber properties and fiber morphology like
fiber length, fiber length distribution, roughness and fiber shape are vital to the physical
properties of many cellulose derivative products.
Dissolving pulp has different chemical and physical properties from kraft pulp. Different type
of hemicelluloses and their quantity present in pulp has most significant chemical variation
and thus has different impact on the processability to the quality of the end products. Different
dissolving pulps possess different chemical and physical properties because dissolving pulp
cannot be categorized by their origin of wood type or process type but also for their purpose
of final use because the demand on quality can vary [6]
.
Most common use of pulp is in paper making but dissolving pulp is the cleanest pulp.
Therefore, dissolving pulp is well suited as a raw material for different types of cellulose
derivative products including viscose textile fiber, rayon, carboxy methyl cellulose, cellulose
ester, cellulose acetate, and staple fibers [16]
. Sulfite pulp production is quite small as
10
compared to kraft pulp, the sulfite pulp production towards pulps for integrated production of
certain papers like wood fee printing and writing papers, tissue papers and grease proof paper
[2]. The dissolving pulp cellulose is distinguished by high reactivity, means cellulose ability to
form filterable solution. Cellulose from sulfite delignification for viscose textile fiber gives
high homogeneity [17]
.
11
1.9 Sulfite Process
Sulfite pulping name was derived from the use of a bisulfate solution as the delignifying
medium. Sulfite pulping process has many advantages like high initial brightness and easy
bleachability of sulfite pulps. Acid sulfite pulping process is the dominant technology for the
production of dissolving pulps and responsible for approximately 70% of the total world
production. The sulfite pulp technology in dissolving pulp production of less purity pulp
grades sufficiency for the regenerated fiber. Manufacturing is based on a favorable economy
because of higher pulp yield, higher reactivity of pulp as compared to a corresponding
prehydrolysis kraft pulp and better bleachability. These are the reasons for the domination of
sulfite pulping process for the production of dissolving pulp [1]
.
Sulfite pulping process is sensitive to the wood species. In acid sulfite pulping, wood species
with low extractive contents are used safely because acid sulfite cook has poor ability to
dissolve extractives [18]
.
During sulfite cooking, phenolic compounds in lignin cause problems in the process because
lignin condensation at low pH or low concentration of SO2 can easily occur. In sulfite process,
acid conditions promote the cleavage of glycosidic bond in the cellulose and hemicelluloses
because of the lower degree of polymerization and amorphous state. Weaker glycosidic bonds
and hemicelluloses are depolimerized easier than the cellulose and dissolved in the cooking
liquor as monosugars. Cellulose chains are also affected by acid hydrolysis during the
cooking process and major depolymerization of cellulose does not happen until the end of the
delignification [6]
.
Normally during the process, cation used is calcium, magnesium, sodium or ammonium.
Different sulfite solutions have different solubilites. These solubilites set the selection of
cation like calcium requires pH 2 to stay in solution, magnesium requires pH 4, sodium and
ammonium sulfite solutions may be strongly alkaline without precipitation. Acid process has
pH 2-3, bisulfate processes operated at pH range 3-5, neutral sulfite processes have pH range
6-9 and alkaline sulfite processes have pH range above 11. [18]
Usually sulphite cooking liquors are analyzed for total SO2, free SO2 and combined SO2.
These are determined by iodometric titration, followed by another titration with NaOH. Total
SO2 sums up the SO2- content in the cooking liquor of SO2, HSO3- and SO32-
. The combined
SO2 value is the measured amount of cation in the system and is defined as the amount of
SO2that is needed to produce XSO3, where X is the cation i.e. Ca2+
, Mg2+
, Na+, or NH
4+.
Calcium can be used for acid sulfite cooking. Calcium in the spent liquor cannot be recovered
for continued used and has therefore been substituted primarily by sodium or magnesium
ions. Magnesium base can be used at higher pH but then in slurry which is gradually
dissolved after chelation by means of dissolved wood material. Due to the formation of this
slurry, it is not possible to use Mg base at large scale. However, Na can be used at all pH
levels. NH4+
can also be used to some extent and at all pH levels but it cannot be recovered
and the pulp is darker than normal. [2]
During sulphite cook, the composition of sulphite liquor varies very much. SO2 is consumed
for the sulphonation of lignin, lignosulphonic acids are dissolved which increase the acidity of
the liquor, etc. The rate at which lignin is sulphonated and hydrolyzed depends on the
composition of the liquors. These rates vary during the cooking process which is very hard to
predict. At 130 ºC maximum sulfonation reaction takes place. Therefore, the cook is heated at
130 C, little longer than the usual heating for the sake of maximum sulfonation [18].
Lignin degrading reactions in the acid sulfite process are characterized in the figure 3 below.
12
Sulfonation is the main reaction under acidic conditions. Sulfonation reaction is the strong
dependence on pH and is fast reaction at low pH. Hydrolysis reaction is somewhat slow as
compared to the sulfonation reaction and decreases the molecular weight of lignin because
hydrolysis linkages between lignin and carbohydrates. During these reactions, condensation
reaction also takes place but these lignin condensation reactions are undesired processes and
counteracting the delignification. [1]
Temperature is one of the most important parameter because temperature determines the rate
of the delignification in sulfite process. Increase in temperature increases the pH, decreases
SO2 solubility, increases lignin dissolution and helps in decomposition of carbohydrates. In
acid sulfite cooking, hydrolytic carbohydrate reactions take place. In this reaction, cellulose
and hemicelluloses both take part. But attach on cellulose is minor in this reaction due to the
low accessibility of cellulose. However, dissolution of hemicelluloses through hydrolysis is
substantially low. [18]
The dominating sulfite pulping process today is magnesium because the corresponding
aqueous magnesium bisulfate solutions are soluble in a pH range upto 5-6. Magnesium base
sulfite process also has an advantage that the thermal decomposition of MgSO3 occurs at a
rather low temperature, producing only a small amount of sulfide. The obtained magnesium
sulfate from the combustion of magnesium sulfite spent liquor can be decomposed thermally
in the presence of carbon from the dissolved organic substances to render gaseous SO2 and
MgO according to equation 1.
2MgSO4 + C → 2SO2 + 2MgO + CO2 (1)
The chemical reactions during sulfite cooking are complicated system of bonding between
inorganic compounds, lignin, cellulose, hemicellulose, wood extractives and between the
organic compounds exclusively [12]
.
During acid sulfite pulping process, three species are active which are H+, SO2, and HSO
3-.
These species take part in the delignification which is desirable and are not desirable in
carbohydrate reactions. Most important lignin reactions are splitting of α alkyl-aryl ethers and
formation of a carbanion, sulphonation, formation of a benzyl alcohol and condensation of
lignin units.
For the dissolution of lignin, degree of sulphonation is important and the probability for
condensation of lignin increases with a low content of total SO2 or a low amount of combined
SO2 in the cooking liquor. If total SO2 and combined SO2 amounts are very low then the
disturbances appear in the cook and the lignin gets dark and delignification totally stops. This
Lignin
Sulfonation
Dissolution degradation
condensation
SulfitolysisHydrolysis
Figure 3: lignin degradation reactions
13
is called burnt cook or black cook. Whereas SO2 has a strong influence on the delignification
rate, high initial SO2 content in the acid sulphite cooking liquor and maximum pressure gives
a good cooking rate [1]
.
During sulphite cooking, pH of the cook must be well controlled. This could be done by the
addition of correct amount of SO2 for a given case. For instance, in an acid sulphite process
which is used for papermaking, the amount of SO2 is high and pH will be relatively constant
due to the high buffering capacity. However, the combined SO2 is lower in sulfite cook
intended for viscose and the pH of the cook will drop in the end, leading to higher α-cellulose
content. After the main part of the lignin dissolution, lower pH in the final part of the cook
decreases the hemicellulose content. As the kappa number decreases during sulfite pulping
process, yield also decreases to give high amount of cellulose because more amount of
hemicellulose is removed [2]
. When acid sulfite pulping process is used for the dissolving
pulp production, cooking temperature is 140-150 ºC, leading to a high hemicellulose
dissolution. This is beneficial because hemicellulose content should be low in dissolving pulp. [1]
14
2. Material and Method
2.1 Screening of Chips:
Spruce wood chips were collected from StoraEnso Karlstad Sweden, dried and screened in
Metso, Sweden by using SCAN CM 40:88 (46 E). Over large and over thick chips were
separated, whereas the chips that were good in quality and passed through 8mm screening slot
and retained at 7mm round whole tray were collected for experiment. Bark was removed
manually and dryness of chips was checked.
2.2 Preparation and analysis of cooking liquor:
The cooking liquor was prepared by mixing 5 g of MgO and distilled water was added to a
total weight of 800 g. Mixer was started with the speed adjusted to 250 rpm. Then 50 g of SO2
was added to the solution and the small valve was closed in order to stop the SO2-gas flow.
The total and free amount of SO2 in the solution was then determined by titration analysis. For
this purpose, approximately 50 ml of distilled water was added to the E-flask and 1 ml of the
cooking liquor was added with the help of pipette. 5 ml of starch (10 g/l) , 5 ml of Kl (1 M)
and 7-10 drops of methyl red (0.1%) as an indicator were added in the flask. Solution was
titrated with KlO3 (1/60 M). The amount of KlO3 was registered when the solution turned
blue. Then one or two drops of Na2S2O3 (0.2 M) were added and titrated with NaOH (0.1 M).
The amount of NaOH was registered when the solution turned yellow. The total SO2, free
SO2, bound SO2 and total free SO2 were calculated by using equations A, B, C, D (Appendix)
respectively.
2.3 Acid Sulfite cooking:
200 g dry chips of 93% dryness were added to the autoclave. The bottom valve was opened at
half rotation. The difference of autoclave was weighed before and after 30 minutes steaming.
The charge of 24 % of total SO2 and the total amount of liquid (cooking acid+ water) of 800 g
was poured to each autoclave as the liquor to wood ratio was 4:1.The amount of cooking acid
and water was calculated by using equations (E) and (F) shown in Appendix.
Then the autoclave was closed with a lid ensuring that there was no leakage from the upper
and lower valves of the autoclave. Autoclaves were placed in PEG bath with 90 C as its initial
temperature. The temperature of PEG- bath was ramped up at the rate of 2.5 ºC /min upto 115
ºC for 30 minutes in order to get better sulfonation reaction. The temperature of PEG-bath
was ramped up at the rate of 0.41, 0.58 and 0.75 C/min up to 140 ºC, 150 ºC and 160 ºC
respectively. Pressure of autoclaves at 140 C, 150 C and 160 C was 6 bar, 7 bar and 8 bar
respectively. The cooking times for 140 C were 2, 3, 5, 8, 17 hours. Cooking times for 150 ºC
were 1 ½, 2, 3, 4, 5 and 6 hours, whereas cooking times for 160 C were 2, 3 and 4 hours.
The autoclaves were taken out from the PEG-bath after cooking and cooled down. The spent
liquor was collected from each autoclave for pH and total residual SO2.
For analysis of total residual SO2, 50 ml distilled water was poured in E flask and pipette 10ml
of residual liquor into flask. 5 ml starch and 5 ml KI were added and titrated with KIO3 until
the solution turned dark blue. The amount of KIO3 was then registered and total residual SO2
was calculated by using equation (G) shown in Appendix.
The washed pulp samples were soaked overnight in distilled water and screened after
disintegrating according to ISO 5263-1:2004. The screened pulp samples were then
centrifuged for 10 minutes. The reject was kept overnight in the oven at 105 ºC. The yield was
15
calculated by the equation (I) shown in Appendix 1. Kappa number, limiting viscosity and R18
of screened pulp were measured according to ISO 302:2004(E), ISO/FDIS 5351:2009(E) and
ISO 699-1982(E) respectively.
3. The Process flow diagram of experiment
Figure 4: process flow diagram of experiment
Cooking
Acid24 % of total
SO2
Cent
rifug
ation
Swedish
Spruce
Chips
Cellulose 40 %
hemicellulose 28.5 %
Lignin 27.7 %
extractives 3.5 %
Screening(SCAN CM
40:94) over 7 mm Round
holes
Cooking
at [140 ,150, 160]
oC
Pressure [6, 7, 8] bar
for Sulfonation (115
oC , 30 mints)
Steami
ng
(2.5
bar, 30
mints)
Washing
Dissolvi
ng Pulp
Defibrillation
(ISO 5263-1:2004) Screening Cent
rifug
ation
Dissolving Pulp
Analysis
pH of
spent
liquor
Total
Residue SO2 Reject Total
Yield
Kappa no.
Viscosity
R18
16
4. Results & Discussion
In this study, spruce wood was processed by acid sulfite process. Different dissolving pulps
were produced by varying the cooking temperature and cooking time and different properties
of spent liquor and produced pulps were investigated like pH, residual total SO2, reject, yield,
kappa number, limiting viscosity and R18. The dissolving pulp was produced at 140 ºC, 150
ºC and 160 ºC at different cooking time intervals with the same cooking liquor charge until
black cook was produced at each temperature. Physical appearance of the cooked pulp at
different time and different temperatures are shown in figures 5, 6 and 7.
When the pulp was cooked at 140 ºC for two hours, the pulp gave a bit stiff feeling and the
pulp was getting softer when it was prolong cooked. After sometime pulp started detoriating
and black cook was obtained at 17 hours.
When the pulp was cooked at 150 ºC for one and half hour, the pulp was soft in feeling. As
the cooking time increased, pulp was getting harder and black cook was obtained at five
hours. When the pulp was cooked at 160 ºC for one hour, the pulp was soft in feeling and
when cooking time was increased and at three hour, black cook was obtained.
2 hours 3 hours 5 hours 8 hours 17 hours
Figure 5: different cooked pulp samples at 140 ⁰C and different time intervals.
1, 5 hours 2 hours 3 hours 4 hours 5 hours 6 hours
Figure 6: different cooked pulp samples at 150 ⁰C and different time intervals.
1 hour 2 hours 3 hours
Figure 7: different cooked pulp samples at 160 ⁰C and different time intervals.
Different reject and R18 samples were obtained at black cook are shown in figures 8 and 9.
Due to lignin deposition on fibers it became difficult to pass through screen slots and it also
increased the weight of reject. Lignin deposition on fibers also made difficulty during R18
test.
17
0 2 4 6 8 10 12 14 16 18
0,5
0,7
0,9
1,1
1,3
1,5
1,7
Time (hrs)
pH
at
25
oC
140 °C
150 °C
160 °C
17 hrs, 140 ⁰C 5 hrs, 150 ⁰C 6 hrs, 150 ⁰C 3 hrs, 160 ⁰C
Figure 8: different reject samples at different critical times and temperatures
R18, 17 hrs, 140 ⁰C R18, 6 hrs, 150 ⁰C R18, 3 hrs, 160 ⁰C
Figure 9: different R18 samples at different critical times and temperatures
4.1 Effect of cooking time and temperature on spent liquor pH
During acid sulfite cooking, lignin and hemicelluloses were dissolved simultaneously. As the
cooking time increased at a specific temperature, spent liquor pH had decreased. Longer the
cooking time, lower will be the spent liquor pH because during cooking, acetic acid is formed
from acetylated polysaccharide of the hemicelluloses. Acetic acid formation enables the
hydrolysis for the dissolution of the hemicelluloses and cleavage of lignin carbohydrate
bonds. [19, 20, 21, 22] as shown in figure 10.
Figure 10: effect of cooking time and temperature on pH of spent liquor
18
0 2 4 6 8 10 12 14 16 18
0,5
5,5
10,5
15,5
20,5
25,5
Time (hrs)
Tota
l Res
idu
al S
O2 (
g SO
2/l
)
140 °C
150 °C
160 °C
0 2 4 6 8 10 12 14 16 18
0,5
10,5
20,5
30,5
40,5
50,5
60,5
70,5
Time (hrs)
Tota
l Yie
ld (
%)
140 °C
150 °C
160 °C
4.2 Effect of cooking time and temperature on total residual SO2
Figure 11 shows the effect of cooking time on total residual SO2. As the cooking time had
increased, the total residual SO2 decreased. As the cooking begins, SO2 consumption starts
which is too quick to dissolve the lignin and hemicelluloses. But after certain time when
lignin and hemicelluloses amount decreases in the wood, SO2 is not too effective to get rid of
these hemicelluloses and lignin because of the acetic acid formation in the spent liquor and
some amount of SO2 remains unreacted. The amount of total residual SO2 was too high at 140
ºC and cooking time two hours gives high amount of total residual SO2 probably because of
the excessive temperatures and time is required to react the pulp with the chemicals. At 150
ºC total amount of SO2 was not varied too much because this high temperature allowed better
chemical reaction of the pulp. When black cook condition was reached at 150 ºC and five
hour cooking time, no further change was observed in total residual SO2 because of lignin
condensation. At high cooking temperature 160 ºC and high pressure 8 bar facilitates, the
pulping reaction occured for a shorter time. However when the cooking time was increased
from one hour to three hour lignin condensation started and resulted black cook.
4.3 Effect of cooking time and temperature on total yield
Figure 11: effect of cooking time and temperature on total residual SO2 in Spent liquor
Figure 12: effect of cooking time and temperature on total yield
19
0 2 4 6 8 10 12 14 16 18
0
5
10
15
20
25
30
35
40
45
Time (hrs)
Kap
pa
no
. 140 °C
150 °C
160 °C
Figure 12 shows the relation between cooking time and total yield at different cooking
temperatures. When cooking temperature and cooking time increased total yield had
decreased because high temperature and more cooking time helps the cooking liquor to
dissolve more lignin and hemicellulose. Therefore low total yield was obtained because better
penetration and higher rate of delignification was obtained.
Pulp yield also depends on the degradation of carbohydrates and these carbohydrates are
degraded by peeling, chain cleavage and the dissolution of short chain carbohydrates. During
cooking hemicelluloses which mainly consist of glucomannan and xylan are degraded. Both
are degraded at specific conditions and reduce the pulp yield.
As shown in the figure 12 lower temperature gives high yield as compared to high
temperature. This is because rate of delignification is much higher at higher temperature as
compared to lower temperature and at higher temperature we get lower yield because higher
temperature also affects and damages the fiber which contributes in decreasing the yield.
Higher temperature also affects the lignin condensation because at higher temperature,
chemical penetration takes place rapidly which also reduces the yield. When the cooking
temperature increases by 10 ºC, the rate of delignification doubles which also reduced the
total yield. At higher temperature, degradation of both hemicelluloses and celluloses occur
which results in reducing the total yield.
4.4 Effect of cooking time and temperature on kappa number
Figure 13 shows the relationship between kappa number and time. As the cooking
temperature had increased, kappa number decreased because lignin decreased in the pulp,
resulting lower kappa number. When pulp is cooked for longer time, condensation of lignin
on fiber takes place which results in black cook. When the condensed lignin is precipitated
onto the fiber it resists the delignification from wood. That is why black cook is obtained after
a certain time. As cooking time increases dissolution of lignin takes place which results in
reducing the combined SO2 or total SO2. This leads towards the lignin condensation to give a
black cook. Cooking temperature at 140 ºC and pressure 6 bar prolongs the lignin
condensation i.e. seventeen hours because the consumption of cooking liquor is slow as
compared to the high temperature i.e.160 ºC and 8 bar. At higher temperature, lignin
condensation started after a short time period as in the case of 160 ºC three hours shown in
figure 13.
Figure 13: effect of cooking time and temperature on kappa number
20
0 2 4 6 8 10 12 14 16 18
50
250
450
650
850
1050
1250
Time (hrs)
Vis
cosi
ty (
ml/
g)
140 °C
150 °C
160 °C
0 2 4 6 8 10 12 14 16 18
60
65
70
75
80
85
90
95
Time (hrs)
R1
8 %
140 °C
150 °C
160 °C
4.5 Effect of cooking time and temperature on limiting viscosity
Limiting viscosity gives the idea about the degradation of the cellulose. During acid sulfite
cooking hemicelluloses and short chain carbohydrates are degraded easily and dissolved in
the cooking liquor. During cooking some of the cellulose is also degraded. Limiting viscosity
gives the average length of the cellulose chains. The greater the cellulose chain length is, the
higher the limiting viscosity and the degree of polymerization will be. As we increased the
cooking time and temperature, long chain of carbohydrates had also shortened to reduce the
pulp viscosity. It is clear from the figure 14 that lower temperature has higher viscosity as
compared to higher temperature because higher temperature degrades the cellulose. At higher
cooking temperature i.e. 160 ºC for a longer cooking time i.e. three hours, limiting viscosity
was found to be very low due to heavy chain cleavage.
4.6 Effect of cooking time and temperature on R18
Figure 14: effect of cooking time and temperature on limiting viscosity
Figure 15: effect of cooking time and temperature on R18
21
0 2 4 6 8 10 12 14 16 18
0,0
0,5
1,0
1,5
2,0
2,5
3,0
3,5
4,0
4,5
5,0
Time (hrs)
Rej
ect
(%) 140 °C
150 °C
160 °C
at 140 ⁰C ,2 hr = 9,07
To measure the pulp purity, R18 method is now widely used. For R18 analysis, pulp samples
were treated with 18% sodium hydroxide solution to dissolve the alkali soluble impurities.
18% sodium hydroxide dissolves pulp contents other than the crystalline cellulose and the
value of R18 gave the amount of alphacellulose. R18 test at room temperature gives the
information about the pulp quality like the amount of hemicellulose present in the pulp and
the extent of cellulose degradation. As the cooking time increases more and more
hemicelluloses and lignin are removed from the pulp which gives the high amount of
alphacellulose. But when cooking time is increased to more than some extent, cellulose is
degraded which also dissolves in the 18% sodium hydroxide solution. At 160C and three hour
time, pulp has high amount of lignin condensation on it which has very low R18 value because
fibers were damaged and large amount of lignin were deposited on the black cook. While at
140C and eight hour cooking time, the value of R18 was maximum but when pulp was more
cooked at 140 ºC and seventeen hours, the value of R18 decreased shown in figure 15 because
lignin condensation and fibers damaged on the pulp.
4.7 Effect of cooking time and temperature on reject
Figure 16 shows the relationship between reject and cooking time at different cooking
temperatures. As the cooking temperature and time increased, the reject percentage had
increased due to the dissolution of lignin and hemicelluloses. In case of 140 ºC and two hour,
the reject amount was so high because rate of delignification was too slow at that condition,
therefore less amount of lignin and hemicelluloses dissolved. However, when the cooking
time was prolonged for 140 ºC, reject percentage decreased because of favorable conditions of
cooking. But after that particular point reject amount increased because longer cooking time
favors the deposition of lignin on the fiber surface which was difficult to pass through the
screening slots same as with temperature 150 ºC. In case of 160 ºC reject percentage was very
low for the first hour because lignin and hemicelluloses dissolved into the cooking liquor. But
at three hours, amount of reject was negligible because all the cooked pulp was like slurry
which easily passed through the screening slots.
Figure 16: effect of cooking time and temperature on reject
22
8
3
2
0 2 4 6 8 10 12 14 16 18 20
135
140
145
150
155
160
165
Time (hrs)
Tem
pe
ratu
re (
oC
)
140 °C
150 °C
160 °C
Area of Acceptable cooks
4.8 Black cooks at different cooking temperatures and times
Figure 17 shows the end points of the pulp cooking at specific liquor charge. At 140 ºC the
black cook was obtained at seventeen hour pulp cooking. At 150C the black cook was
obtained at five hour pulp cooking. At 160 ⁰C the black cook was obtained at three hour pulp
cooking. Lower temperature gives longer time for lignin condensation during pulp cooking.
140 ⁰C is the safest temperature to obtain different pulp properties while temperature at 160
⁰C is highly risky because after one hour cooking lignin condensation chances are more to
produce the black cook.
In sulfite cooking SO2 has strong influence on delignification rate. Therefore in the beginning
of the cooking, rate of delignification is much higher as compared to the later cooking. This is
because initially higher amount of combined SO2 is available for the chemical reaction and as
the combined SO2 consumes in the reaction rate of delignification decreases. Dissolution of
hemicellulose is high at 140-160 ºC.
Figure 17: Black cooks at different temperatures & times
23
5. Conclusion
Higher cooking temperature and cooking time gave faster both lower pH and total
residual SO2.
Higher cooking temperature and cooking time gave faster lower pulp yield.
Kappa number decreased with increasing cooking time and temperature but after a
certain point kappa number increased due to lignin condensation.
Limiting viscosity of the pulp decreased with increasing cooking temperature and
time.
Reject percentage decreased with increasing cooking temperature and time and reject
percentage had increased in black cook area.
R18 value gave higher alphacellulose content and had increased with increasing
cooking temperature and time until the cellulose was not affected by cooking
conditions.
6. Recommendation for future work
Sulfite cooking could be done with different combined SO2 charges to minimize the lignin
condensation at high cooking temperature.
7. Acknowledgement
I take this opportunity to express my profound gratitude and deep regards to my supervisor
Ulf Germgärd and assistant supervisor Niklas Kvarnlöf for their constant supervision and
sincere guidance. I also thank Stora-Enso Skoghall for providing spruce wood chips and
special thanks to Metso Paper Karlstad for providing chip screening facility.
24
8. References:
1. Sixta, H. (2006): Hand book of pulp, vol 1, Weinheim: Wiley-VCH verlag GmbH &
Co. Germany.
2. Ek, M., Gellerstedet, G., Henriksson, G. (2012): Ljungberg text book pulp and paper
chemistry & technology, Stockholm: Fiber and Polymer Technology, KTH, Sweden
3. Sjödahl, R.G. (2006): Some aspects on the effects of dissolved wood components in
kraft pulping. Doctor Dissertations, KTH, Sweden
4. Kassberg, M. (1999): The Swedish forest industry, In pulp manufacture-a review,
published by: Skogsindustrins Utbildning i Markaryd, Sweden pp 7-11
5. Schild, G., Sixta, H., Estova, L. (2010): Multifunctional alkaline pulping,
delignification and hemicellulose extraction. Cellulose chemistry and technology Vol
4 (1) pp 35-45
6. Strunk P. (2012): Characterization of cellulose pulps and the influence of their
properties on the process and production of viscose and cellulose ethers, Doctor
Dissertation, Umeå University, Sweden
7. Christoffersson, K. E. (2005): Dissolving pulp-Multivariate characterization and
analysis of reactivity and spectroscopic properties. 28-01-2005. Doctor Dissertations,
Umeå University, Sweden
8. Ciolacu, D., Ciolacu, F., Popa, V.I. (2011): Amorphous cellulose structure and
characterization. Cellulose chemistry and technology 45 (1-2) pp 13-21
9. Ciolacu, D. (2007): On the supramolecular structure of cellulose allomorphs after
enzymatic degradation. Journal of optoelectronics and advanced materials Vol. 9 (4)
pp 1033-1037
10. Schädel, C., Blöchl, A., Richter, A., Hoch, G. (2012): Quantification and
monosaccharide composition of hemicelluloses from different plant functional types.
Plant physiology and biochemistry. Vol. 48 (1) pp 1-8
11. Lisa X., Lia, (2010): Bio products from sulfite pulping. Master´s Thesis, University of
Washington, USA
12. Rydholm, S. A. (1985): Pulping processes, New York, USA: Robert E. Krieger
Publishing Company
13. Almlöf H. (2010): Extended mercerization prior to carboxymethyl cellulose
preparation, Licentiate thesis, Karlstad University, Sweden
14. Gehmayr, V., Sixta, H. (2011): Dissolving pulp from enzyme treated kraft pulps for
viscose application. Lenzinger berichte Vol. 89 (1), pp 152-160
25
15. Sarwar, J., M., Sabina, R., Nasima, C., D., A., Al-Maruf, A., (2008): Alternative
pulping process for producing dissolving pulp from jute. Bio Resources Vol. 3 (4), pp
1359-1370, Austria.
16. Sixta, H., Schild, G., (2009): A new generation kraft process. Lenzinger Berichte Vol.
87(1) pp 26-37
17. Mazur, N., A., Zubakhina, N., L., Khizhnyak L., G. (2006) Evaluation of the quality of
cellulose for chemical processing. Fiber chemistry Vol. 38 (1) pp 23-25
18. Sten H., Bengt O. L., Ulla S. (1953): The rate dominating reaction of the
delignification of wood powder with sulfite solutions. Svensk Papperstidning Vol. 56
(17) pp 645-690
19. Paredes, H., J., J. (2009): The influence of hot water extraction on physical and
mechanical properties of OSB, 01-01-2009. Doctor Dissertation, University of Maine,
Orono
20. Tunc, M., Heiningen, V., A., R., P. (2008): Hemicellulose extraction of mixed
southern hardwood with water at 150 °C. Effect of time. Industrial engineering and
chemistry research, 47 (18), pp 7031-7037
21. Liu, Z., Ni. Y., Fatehi, P., Saeed, A. (2011): Isolation and cationization of
hemicelluloses from pre hydrolysis liquor of kraft based dissolving pulp production
process. Biomass bioenergy 35(5), pp 1789-1796
22. Hage, R., E., Chrusciel, L., Desharnais, L., Brosse, N. (2010): Effect of autohydrolysis
of Miscanthus x giganteus on lignin structure and organosolve delignification.
Bioresources technology 101(23), pp 9321-9329
23. Borrega, M., Nisminen, K,. Sixta, H. (2011): Effects of hot water extraction in a batch
reactor on the delignification of birch wood. Bio Resources Vol. 6 (2) pp 1890-1903
26
9. Appendix:
9.1 Equations:
Equations for calculating the total, bound and free concentration of SO2 in the solution:
Total SO2: g SO2/l (A)
Free SO2: g SO2/l (B)
Combined SO2: g SO2/l (C)
Total free SO2: g SO2/l (D)
Where a = amount of KlO3 used during titration
b = amount of NaOH used during titration
The amount of cooking liquor
(E)
Where
e = Charge 24 % of total SO2
f = amount of dry wood chips
g = concentration of the total SO2
The amount of water to add (F)
Where
h = amount of water present in chips (93% dryness)
i = amount of water due to steaming
Total Residual SO2: g SO2/l (G)
Where
j = amount of KlO3 used during titration
Rejects (%)
(H)
Yield %:
(I)
R18
(J)
Where
m1 = oven dry mass of alkali insoluble fraction in grams
m2 = mass of the test portion calculated on an oven dry basis in grams
27
9.2 Summary of Results
Table 2: Results of different parameter of acid sulfite pulping at 140 ⁰C and different time intervals
with standard deviation. Standard deviation is based on four samples.
Time
(hrs)
pH of
spent
liquor at
25 ⁰C
Total
Residual
SO2 (g
SO2/l)
Reject
(%)
Total
Yield
(%)
Kappa
no.
Viscosity
(ml/g)
R18 (%)
2 1,55
±0,84
20,03
±0,27
18,15
±0,84
58
±0,28
33
±0,84
1079
±10
84,28
±0,12
3 1,45
±0,35
12,10
±0,30
4,44
±0,34
50
±0,38
23
±0,34
993
±2
85,94
±0,45
5 1,33
±0,13
11,14
±0,14
0,38
±0,62
43
±0,32
9
±0,23
766
±6
87,37
±0,08
8 1,17
±0,32
10,88
±0,32
0,12
±0,31
34
±0,14
5
±0,48
430
±4
90,71
±0,18
17 0,70
±0,24
7,68
±0,01
0,70
±0,02
18
±0,21
11
±0,23
264
±7
80,48
±0,34
Table 2: Results of different parameter of Acid sulfite pulp at 150 ⁰C and different time intervals
Time
(hrs)
pH of
spent
liquor at
25 ⁰C
Total
Residual
SO2 (g
SO2/l)
Reject
(%)
Total
Yield
(%)
Kappa
no.
Viscosity
(ml/g)
R18 (%)
1,5 1,50
±0,15
9,96
±0,29
2,58
±0,15
46
±0,10
16
±0,15
776
± 5
82,45
±0,10
2 1,40
±0,52
9,60
±0,13
1,85
±0,52
43
±0,28
14
±0,52
651
±12
86,24
±0,21
3 1,34
±0,35
9,16
±0,50
0,95
±0,35
36
±0,19
11
±0,35
415
± 7
90,13
±0,47
4 1,29
±0,42
8,96
±0,32
0,88
±0,12
34
±0,32
6
±0,43
292
±8
90,58
±0,09
5 1,23
±0,21
7,96
±0,48
0,93
±0,34
27
±0,12
8
±0,30
185
±10
87,56
±0,51
6 1,20
±0,43
7,96
±0,51
1,32
±0,24
24
±0,50
9
±0,12
170
±4
79,81
±0,42
Table 3: Results of different parameter of acid sulfite pulping at 160 ⁰C and different time intervals
Time
(hrs)
pH of
spent
liquor at
25 ⁰C
Total
Residual
SO2 (g
SO2/l)
Reject
(%)
Total
Yield
(%)
Kappa
no.
Viscosity
(ml/g)
R18 (%)
1 1,35
±0,39
13,54
±0,02
0,54
±0,33
48
±0,33
13
±0,39
754
±4
87,35
±0,31
2 1,12
±0,31
8,87
±0,36
0,60
±0,31
30
±0,49
7
±0,31
166
±6
73,82
±0,26
3 0,90
±0,97
4,48
±0,22
0,42
±0,69
26
±0,25
15
±0,97
115
±13
72,10
±0,35