Effect of temperature and time on acid sulfite cooking for dissolving pulp

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Karlstad University 65188 Karlstad Tfn 054-7001 00 00 Fax 054-700 14 60

www.kau.se information@kau.se

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

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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

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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.

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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

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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.

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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.

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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.

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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].

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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

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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].

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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 NH⁴⁺. 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.

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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

Sulfitolysis Hydrolysis

Figure 3: lignin degradation reactions

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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.

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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

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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 Acid 24 % 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) Cent Screening rifug

ation

Dissolving Pulp Analysis

pH of spent liquor

Total Residue SO2

Reject Total

Yield

Kappa no.

Viscosity

R18

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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

1 hour 2 hours 3 hours

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.

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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 25oC ^{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

R_18,17 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

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0 2 4 6 8 10 12 14 16 18

0,5 5,5 10,5 15,5 20,5 25,5

Time (hrs) Total Residual SO2 (g SO2/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)

Total Yield (%) ^{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

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0 2 4 6 8 10 12 14 16 18

0 5 10 15 20 25 30 35 40 45

Time (hrs)

Kappa 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

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20

0 2 4 6 8 10 12 14 16 18

50 250 450 650 850 1050 1250

Time (hrs)

Viscosity (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) R18 %

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

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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)

Reject (%)

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

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22

8 3

2

0 2 4 6 8 10 12 14 16 18 20

135 140 145 150 155 160 165

Time (hrs) Temper at ur e (

o

C)

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

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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.

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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

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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

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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

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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)

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)

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

Effect of temperature and time on acid sulfite cooking for dissolving pulp

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

Contents

Abstract

Executive Summary

Abbreviations:

Objective

1. Introduction

2. Material and Method

3. The Process flow diagram of experiment

4. Results & Discussion

Time (hrs) Temper at ur e (

C)

5. Conclusion

6. Recommendation for future work

7. Acknowledgement

8. References:

9. Appendix: