Karlstads Universitet 65188 Karlstad Tfn 054-7001 00 00 Fax 054-700 14 60 Faculty of Technology and Science Department of Chemical Engineering Muhammad Asim Effects of prehydrolysis prior to kraft cooking on Swedish Spruce Wood Degree Project of 30 credit Points Master of Science in Engineering Degree Program in Chemical Engineering Date/ Term: 2012-06-04 Supervisor: Ulf Germgärd Assistant supervisor: Niklas Kvarnlöf Examiner: Lars Järnström
The aim of this study was to investigate the effect of prehydrolysis on wood chips prior to kraft cooking to produce dissolving pulp. Dissolving pulp is used for the manufacturing of textile fibers, viscose etc. Wood chips were processed through two stages, first the prehydrolysis of wood chips and then the kraft cooking. In the prehydrolysis stage both the time and the temperature were changed alternatively, but in the kraft cooking stage both time (2 hours) and temperature (160 oC) were kept constant for all samples. For this purpose spruce wood chips were collected from a local pulp mill. After drying and screening of the wood chips, they were prehydrolyzed with deionized water at 4:1 liquor to wood ratio. The prehydrolysis stage was carried out at 150 oC, 160 oC, and 170 oC and for each temperature wood chips were prehydrolyzed for different times. After the prehydrolysis stage the pH of spent liquor was noted. Then all the prehydrolyzed samples were processed according to kraft cooking process. For the kraft process parameters like time, temperature, and wood charge conditions were kept constant. After defibrillation and screening of the pulp, the yield, kappa number, intrinsic viscosity and R18 of the pulp were determined according to ISO standards. Residual alkali in black liquor was also measured for each sample. It was found that the pH of the spent liquor from the prehydrolysis stage decreased with increasing prehydrolysis time and temperature. After kraft cooking it was found that the yield and the intrinsic viscosity of the pulp decreased with increasing prehydrolysis time and temperature. The R18 of the pulp and residual alkali in the black liquor increased with increasing prehydrolysis time and temperature. Kraft cooking was performed with two effective alkali charges (20 % and 25 %). Kraft cooking with effective alkali charge of (25 %) was done also for 160 oC prehydrolyzed wood chips and same trends of results were observed as for the 20 % effective alkali case. Keywords: Dissolving pulp, Prehydrolysis, Kraft cooking, Hemicellulose, Cellulose, Lignin
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Executive summary
Nowadays many of the researchers have focus on such type of products that are sustainable and renewable. Day by day new technologies are being developed and due to population growth the consumption of sources like fossil fuel and viscose or textile fiber are increasing. From last few years there has been an increase in the interest of dissolving pulp, regenerated cellulose, and derivatives of cellulose in the field of fiber pulping.
The purpose of this work was to investigate the effect of the prehydrolysis time and temperature prior to kraft cooking for the production of dissolving pulp. The prehydrolysis was done for different time at 150 oC and similarly for 160 oC, and 170 oC respectively. Kraft cooking was done at constant time (2 hours) and temperature (160 oC). The final product was analyzed for yield, kappa number, intrinsic viscosity, and percentage of cellulose present in the dissolving pulp. Kappa number, intrinsic viscosity, and R18 were measured according to the ISO standards.
Experimental work was performed at Karlstad University Sweden. Spruce wood chips were collected from the local pulp mill StoraEnso and were dried and screened in the lab of the local company Metso Fiber. The cooking study was performed in 2.5 l stainless steel digesters. Six autoclaves were used for prehydrolysis and kraft cooking. Liquid to wood ratio of 4:1 was maintained for prehydrolysis as well as for kraft cooking. Prehydrolysis and kraft cooking were performed in two stages respectively. The prehydrolysis parameters were changed while kraft cooking parameters (time, temperature, and alkali charge) were kept constant.
The prehydrolysis was done to extract the wood chips of hemicellulose, and a little amount of lignin. Hemicellulose and lignin both can be extracted from the prehydrolysis spent liquor and can further be used for the production of energy and for many value added products. The prehydrolyzed wood chips were then processed through kraft cooking to dissolve residual hemicellulose, lignin, and extractives. When the prehydrolysis was done for longer time and at higher temperature some sticky materials were produced. These sticky materials are sometime problematic for kraft cooking. It could be considered that there may be two reaction phenomena in the prehydrolysis stage dissolution and condensation. During dissolution hemicellulose is dissolved into the water and the pores of the wood are opened, while in the case of condensation lignin is condensed onto the fiber and resist the further delignification from wood/ fiber. Dissolution is carried out in the prehydrolysis stage at shorter time and at lower temperature. But condensation is carried out in the prehydrolysis stage at longer time and at higher temperature. Longer time and higher temperature of prehydrolysis gave high amount of residual alkali in the black liquor. Longer time and higher temperature of the prehydrolysis are good for extraction of hemicellulose. It was also observed that high temperature and long prehydrolysis time shortened the cellulose chains and reduced intrinsic viscosity of the dissolving pulp. These low molecular weight cellulose fragments are dissolved in 18 % NaOH during the measurement of R18. Alkali charge is also responsible for depolymerization of cellulose, thus a high charge will result in cellulose of low molecular weight.
In this study the focus was on the prehydrolysis stage rather than the cooking, due to the changing parameters (time and temperature) of the prehydrolysis stage rather than the kraft cooking. One of the most important finding in this study was the condensation of the lignin in the prehydrolysis stage. Due to the condensation high lignin contents were observed which could not be dissolved even during kraft cooking stage except when using longer kraft cooking time, higher temperature, and higher alkali charges.
5
Abbreviations
PH Prehydrolysis KC Kraft cooking e.a Effective alkali K Kappa number temp. Temperature α Alpha β Beta DP Degree of polymerization t Time pH Potential Hydrogen
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1. Introduction
1.1 Background New technologies are continuously being developed, and as the world population is increasing.
The amount of natural sources is not sufficient enough to fulfill the growing population’s demands. Due to the growing population there is a shortage of raw materials [1], [2]. Natural resources such as fossil fuels are known as non-renewable resources. Natural resources are formed in millions of year. The fuel reserves are being depleted faster than they are being produced. Most of the world economy depends on the fossil fuels. Due to the depleting fuels reserves, and the impact of carbon dioxide emission on the environment, the search for sustainable and reusable raw material is important [3].
The concept of converting biomass into production of valuable biofuels/biochemical is referred as biorefinery. Biorefinery is strategically significant because it produces renewable and sustainable value added products [4]. In biorefinery biomass of corn, wheat, sugar cane, barley, oils and agricultural residue are used. The consumption of these products for biomass can however reduce food and animal feed, so there has been a need to exploit new biomass resources. Wood is a good resource for production of biofuel that will not influence the world’s food supply. Wood is also a good raw material for the production of dissolving pulp [3]. Wood biorefinery includes separation of hemicellulose, cellulose, and lignin for production of high value added products [5]. A substantial interest has been developed in dissolving pulp from over the last few years; regenerated cellulose and cellulose derivatives are becoming part of fiber pulping named as biorefinery products.
Cotton is mostly used for the manufacturing of textile fiber, and the current cotton growth rate is not enough to fulfill the consumer demand. The cotton production demands a large amount of farming land, water, and pesticides. Much of the farming land and water will soon be used for the production of food. The use of pesticides by farmers is a great environmental problem. So it is necessary to switch textile raw material from cotton to regenerated and sustainable raw material [6],
[7]. Energy crises and shortage of textile fiber or viscose raw materials are increasing the demand of dissolving pulp.
The shortage of raw material forces development of new sources and raw materials that are reusable and sustainable. Many technologies are being investigated to produce value added products from wood and dissolving pulp is one of them. Wood is being used as raw material in the forest industry for the production of pulp and paper, fiber board and biorefinery. Products manufactured from wood are sustainable and reusable. The Swedish forest consists mainly of spruce, pine, and birch. Spruce is the most abundantly available wood followed by pine. The forest industry of Sweden contains 47 % of spruce, 37 % of pine, and 16 % of birch and other wood species.
Figure 1 Distribution of the wood tree species in Sweden [8]
1.2 Pulp Fiber is obtained from wood, and the properties of the fiber depend on the type of wood and
its origin. Softwood fibers are longer than and hardwood fibers. Wood mainly consists of cellulose, hemicellulose, and lignin. Cellulose is a long homopolysaccharide chain, hemicellulose is a branched
Spruce
47%Pine
37%
Birch &
others
16%
7
heterogeneous polysaccharide, and lignin is a glue like material that binds the wood fibers together. For the separation of fiber dissolution of the lignin is necessary. To accomplish this, chemical and mechanical processes are employed [8].
Chemical pulping starts by chipping wood into chips. The wood chips are then cooked together with chemicals, which dissolve the lignin causing the fiber separation of the wood [9].
The main chemical wood cooking techniques are sulfite, kraft or sulfate, and soda cooking. The kraft process is nowadays dominating. But in the case of dissolving pulp, the sulfite process is the dominating technology, because hemicellulose and lignin are removed in the same step while in case of the kraft process hemicellulose and lignin are removed in two separate steps. Dissolving pulps should have a high level of purity, uniform molecular weight distribution, high reactivity, and good accessibility of the cellulose to the chemicals. The chemicals process percentages on a global scale are shown in the given figure 2 [10].
Figure 2 The relative fraction of different chemical pulping processes used globally
1.3 Theory of prehydrolysis To produce a dissolving pulp, prehydrolysis was done before kraft cooking. Wood chips were
pretreated with water and this process is known as prehydrolysis of wood chips [10]. Prehydrolysis prior to the kraft process gave a high quality pulp and higher cooking capacity. In prehydrolysis kraft process hemicellulose was usually extracted in two stages i.e. before and after pulping that makes easy extraction of some wood components [10]. The prehydrolysis kraft based dissolving pulp production process is a demonstrable example of the integrated forest biorefinery. Because the spent liquor obtained from the prehydrolysis is very useful for different value added products. Remaining wood can further be used for production of pulp. Hemicellulose and hemicellulosic sugars were depolymerized and solubilized in the prehydrolysis process. The prehydrolysis liquor also contains other bio products e.g. lignin, acetic acid, and furfural [2], [4], [11], [12]. The main objective of the prehydrolysis was to remove hemicellulose from the wood by heating it at specific temperature, without damaging the cellulose [3]. Some lignin was also solubilized in the prehydrolysis process [3],
[11]. The pH of the extracted spent liquid from the prehydrolysis stage was between 2.5-4.0. The spent liquor was acidic due to the formation of acetic acid, which was formed from acetylated polysaccharide of the hemicellulose [3], [11], [13]. The formation of acetic acid enabled the hydrolysis for the dissolution of a great part of the hemicelluloses and cleavage of lignin carbohydrate bonds [14]. Hemicellulose can be extracted from prehydrolysis liquor and can further be treated to produce many value added products like bioethanol, furfural, and hydroxymethylfurfural. Hemicellulose can also be used as functional paper making chemicals [4], [15], [16], [17]. Extracted lignin can be used as fuel energy and starting material in the production of polyurethane and carbon fibers [18]. Acetic acid and furfural can be used for industrial applications [4].
Hemicelluloses are strongly linked to cellulose by hydrogen bonds and lignin is linked with hemicelluloses by covalent bonds [3]. Hydrogen ions are formed during the hydrolysis process by the production of acetic, uronic, and phenolic acids. The formation of hydrogen ions catalyzes the depolymerization of hemicellulose [5], [19]. In such acidic environment depolymerization of the lignin
sulfate
89%
sulfite
5%
others
6%
8
also occur due to the cleavage of α-o-4 and β-o-4 bonds. These bonds are abundantly available in softwood [5]. The depolymerization reaction is followed by condensation of the lignin fragments already dissolved in the liquor. The condensed lignin precipitates onto the fiber, and thus increases the amount of residual lignin in the wood. Condensation of the lignin and degradation of the hemicellulose reactions are favored by elevated time and temperature [5]. Some of the lignin is solubilized in the extracted liquor, and lignin of higher molecular weight may condense during cooling, and form dark sticky precipitates. The condensation of lignin during the prehydrolysis treatment increased the delignification resistance of the pulping process [5].
Figure 3 is showing the change in color of wood chips after prehydrolysis. It was observed that the color did change from brown to blackish brown as the prehydrolysis time was being increased. The colour change might be due to the condensation of lignin onto the fiber during the prehydrolysis.
Chips after 30 minutes PH at 160 oC Chips after 60 minutes PH at 160 oC
Chips after 120 minutes PH at 160
oC Chips after 180 minutes at 160
oC
Figure 3 Change in color of the wood chips after prehydrolysis (PH)
It was found in another article [21] that quantifiable extractions of hemicellulose begins at 120 oC and degradation of cellulose starts at 140 oC, and Research work concludes that 140 oC is implementable for industrial purpose [1], [19], [22]. At higher temperature (170 oC) some of the lignin and most of the hemicellulose are dissolved or extracted [1], [21]. At elevated temperature lignin is dissolved in aqueous acidic medium and forms sticky precipitates in prehydrolyzation liquor during the production of dissolving pulp. The formation of sticky precipitates can be reduced by lowering the prehydrolysis reaction time and temperature [10]. Prehydrolysis step require high investment and energy and disposal of wastage of prehydrolyze liquor makes the process expensive [23]. It was physically observed that dissolving pulp obtained from high temperature and long time prehydrolysis treatment was little harder and separated fibers of wood after treatment were strongly stuck together. It may be due to the formation of sticky precipitates.
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1.4 Kraft cooking process The kraft process is used for the conversion of wood into wood pulp. Kraft process is the
dominating process as compared to the sulfite process. The kraft process is applicable to all types of wood (hardwood and softwood). It was introduced in 1879 to cover the losses in the recovery cycle of the cooking chemicals. It also improved the quality of the pulp [24].
During kraft cooking lignin is degraded and dissolved into the cooking liquor, and the wood fibers are separated. The active cooking species of cooking liquor are hydroxide and hydrogen sulfide ions. The depolymerization of the lignin in alkaline process is particularly due to the cleavage of β- aryl ether linkages. The depolymerization of lignin depends on the hydroxide ions concentration and the hydrogen sulfide ions concentration, cooking time, cooking temperature. Dissolved lignin can also be precipitated onto the fibers. The yield of the pulp is also dependent on the degradation of carbohydrates and lignin. Carbohydrates are degraded by different phases peeling, chain cleavage, and dissolution of short chain carbohydrates. Hemicellulose contains mainly glucomannan and xylan; both are degraded during kraft cooking at specific conditions [8] and reduce the yield of the pulp.
Figure 4 Compositions of the pulp after prehydrolysis and kraft cooking, the initial wood is shown as a reference
Figure 4 shows the degradation of lignin, hemicellulose, and cellulose after prehydrolysis and kraft cooking. The spruce wood initially contains 40 % cellulose, 28.5 % hemicellulose, 27.7 % lignin, and 3.5 % extractives [27].
Some advantages of the kraft process are good pulp strength, the process is feasible for all kind of wood types and maximum recovery of the chemical is possible. Disadvantages are low brightness of the pulp and a high consumption of bleaching chemicals [8].
Cellulose is a major part of the wood. Cellulose is divided in two parts, crystalline and amorphous. If cellulose is arranged more or less parallel to each other, it is called crystalline. If cellulose chains are ordered evenly with respect to each other, it is called crystalline cellulose. If cellulose chains are not evenly ordered it is called amorphous. Amorphous cellulose is hygroscopic while crystalline are not [25]. During the prehydrolysis hemicellulose and a little amount of lignin are dissolved in water and in kraft cooking lignin is dissolved in white liquor. For a longer cooking time or for higher alkali charge, it may happen that some of the cellulose is dissolved in white liquor or may be shorten the chain of cellulose chain.
The aim of this study was to extract the maximum contents of hemicellulose and lignin by prehydrolysis kraft cooking process. Wood chips were then first pretreated with water for different times and temperatures and were then cooked by the kraft process. The final product was analyzed on the basis of the yield of the pulp, kappa number of the pulp, intrinsic viscosity of the pulp, and R18 of the pulp.
Original After After wood prehydrolysis prehydrolysis
wood kraft cooking Lignin
Hemicellulose
Cellulose
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2. Materials and Methods
2.1 Prehydrolysis Spruce wood chips were taken from the mill and were dried and screened (thickness of 3mm
to 7mm). Bark and over size wood chips were removed manually before adding 200 g of chips into a 2.5 l steel autoclave. After closing the lower valve, 800 g of deionized water was poured into the autoclave to gain a liquor to wood ratio of 4:1. Then the autoclave was closed with a lid and made sure that there was no leakage from the upper and the lower valve of the autoclave. The __PEG- (polyethylene glycol) -bath temperature was initially maintained at 90 oC. The temperature of PEG—bath was ramped up at the rate of 2, 2.34, and 2.67 oC/minute up to 150 oC, 160 oC, and 170 oC in 30 minutes, respectively. The prehydrolysis time for 150 oC was 60, 120, 180, 240, and 300 minutes. The prehydrolysis time for 160 oC was 30, 60, 120, and 180 minutes. The prehydrolysis time for 170 oC was 30, 60, and120 minutes. After the prehydrolysis cook the autoclaves were taken out from the PEG__bath and were cooled down. The liquor from the autoclaves was drained off and the pH of the spent liquor was then measured.
2.2 Kraft cooking
Cooking liquors of 20 % effective alkali and 40 % sulphidity were prepared for each batch of prehydrolyzed wood chips separately. Cooking liquor for 6 autoclaves was prepared according to the liquor to wood ratio of 4:1, and was then poured into the autoclaves. The bottom valves of the autoclaves were closed and the upper valves remained open for the impregnation of the wood chips. Nitrogen gas was added to each autoclave until 10 bars pressure was reached. This facilitated the impregnation of the wood chips and to check that there was no leakage of the autoclaves. Then autoclaves were fixed into the PEG__bath and its temperature was maintained at 90 oC. The __PEG__ bath temperature was kept at 90 oC for 30 minutes for the impregnation of the wood chips and,__ after 30 minutes the gas pressure was released and the temperature was ramped up to 160 oC in one hour at the rate of 1,17 oC/minute. The kraft cooking time for the pretreated wood chips was two hour at 160 oC. All prehydrolyzed wood chips were cooked at the same kraft cooking time and temperature conditions. The same experiment was repeated by changing the kraft cooking white liquor charge to 25 % effective alkali and 40 % sulphidity. The residual alkali of black liquor was measured according to ISO 699-1982(E). After the cooking the pulps were disintegrated according to ISO 5263-1:2004 and screened. Yield, kappa number, intrinsic viscosity and R18 of the pulps were analyzed according to the standards. Kappa number was measured according to ISO 302:2004(E), limiting viscosity was measured according to ISO 5351:2004(E), and R18 was measured according to ISO 699-1982 (E). The process is easily understandable from the following process flow sheet, figure 7.-
Figure 5 Flow sheet of prehydrolysis kraft cooking batch process.
Water White Liquor Wood Dissolving Pulp Chips Spent Black Liquor
PH Cook
Kraft Cook
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3. Results and Discussions
In this study wood was processed in two stages (prehydrolysis and kraft cooking). Most of the dissolving pulp results (kappa number, intrinsic viscosity, and R18) obtained after kraft cooking are affected by the prehydrolysis stage but not with kraft cooking stage, due to the changing parameters (time and temperature) in the prehydrolysis stage. The reason behind this was that in the kraft cooking stage all the parameters (time, temperature, and alkali charge) were constant. The prehydrolysis was done for different time intervals at specified temperatures. In the prehydrolysis stage cleavage of hydrolysable bonds of carbohydrates and lignin takes place, but also condensation of lignin can take place in the prehydrolysis stage. The condensation of lignin at severe condition (long times and high temperatures) may be so extensive that the delignification ability of the kraft cook is impaired. Delignification of wood is delayed/ stopped in kraft cooking due to the severe conditions of the prehydrolysis stage. When delignification reaction is resisted in the kraft cook, increased alkali charge, time, and temperature will be helpful for the degradation of lignin [24].
3.1 Yield after prehydrolysis and after kraft cooking The yield of the wood chips after prehydrolysis was decreased due to the extraction of
hemicellulose and lignin. The kraft cook also showed a little decrease in the yield. The decrease in yield after the kraft cooking was mainly due to the changed parameters (time and temperature) in the prehydrolysis stages the-kraft cook parameters were kept constant for all samples.
Figure 6 The total yield after prehydrolysis and kraft cooking versus prehydrolysis time
Figure 7 The total yield after prehydrolysis and kraft cooking versus prehydrolysis time
20
40
60
80
100
0 100 200 300
Yie
ld (
%)
Time (minutes)
PH: 150 oC, KC: 20 %e.a
After Prehydrolysis
After Kraft Cooking
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30
40
50
60
70
80
90
100
0 50 100 150 200
Yie
ld (
%)
Time (minutes)
PH: 160 oC, KC: 20 %e.a
After Prehydrolysis
After Kraft Cooking
12
Figure 8 The total yield after prehydrolysis and kraft cooking versus prehydrolysis time
The decrease of the yield after prehydrolysis was due to the cleavage of hydrolysable bonds of carbohydrates and lignin. Long prehydrolysis time degraded a high amount of hemicellulose but low amount of lignin. Due to the extraction of hemicellulose, and lignin the total yield obtained was low for long prehydrolysis time. Similarly a high temperature of the prehydrolysis stage degraded a high amount of hemicellulose, and low amount of lignin. Figure 6, 7, and 8 are showing the yield of the wood after prehydrolysis as well as the dissolving pulp yield after kraft cooking.
Figure 9 The total yield after prehydrolysis and kraft cooking versus prehydrolysis time
Figure 9 is showing the prehydrolysis yield as well as the dissolving pulp yield at 25 % e.a and 160 oC in the kraft cook. The only difference found was a lower yield at 25 % e.a as compared to the 20 % e.a. The reason for the low yield at the same temperature was the higher alkali charge. At high alkali charge hemicellulose, and lignin are dissolved in excess as compared to a low alkali charge. A low total yield was thus obtained.
The yield of the pulp is also dependent on the degradation of carbohydrates. The carbohydrates are degraded by different phases: peeling, chain cleavage-, and dissolution of short chain carbohydrates. Hemicellulose contains mainly glucomannan and xylan; both are degraded during kraft cooking at specific conditions [8] and reduce the yield of the pulp.
Screening reject percentage obtained was less than 1 %, so screen yield % will be the same as the total yield. As shown in appendix table 5.
20
40
60
80
100
0 50 100 150
Yie
ld (
%)
Time (minutes)
170 C, 20 %e.a
After Prehydrolysis
After Kraft Cooking
20
40
60
80
100
0 50 100 150 200
Yie
ld (
%)
Time (minutes)
PH: 160 oC, KC: 25 %e.a
After Prehydrolysis
After Kraft Cooking
13
3.2 Effect of the prehydrolysis on pH of the spent During the prehydrolysis acetic acid is formed via separation from the polysaccharide. Due to
the formation of acetic acid the prehydrolyzed liquor became acidic. High amount of acetic acid in the liquor will decrease the pH of the liquor. Acetic acid is formed from the acetylated polysaccharides hemicellulose. It was found from the results that a long prehydrolysis time and high temperature resulted in low pH as compare to a short prehydrolysis time and low temperature. A high amount of acidic acid may be produced at high prehydrolysis times and high prehydrolysis temperatures.
Figure 10 The pH of the liquor extracted from the prehydrolysis stage
3.3 Effect of prehydrolysis on Kappa Number As discussed earlier in this study, most of the dissolving pulp properties were dependent on
the prehydrolysis stage. Figure 11 shows the kappa number of the dissolving pulp produced by the prehydrolysis kraft cooking process. In the prehydrolysis stage not only the hydrolysable bonds of the carbohydrates and lignin were cleaved, but also condensation of the lignin occurred. In the prehydrolysis stage two cooking phenomena can take place: dissolution and condensation of the lignin. Condensation is a critical phenomenon. It may be considered that the condensation of the lignin occurred at a long prehydrolysis time and high temperature.
Figure 11 The kappa number of the dissolving pulp after kraft cooking at prehydrolysis time
The results show that a slightly longer prehydrolysis time and lower temperature were more advantageous as result found at 150 oC. At higher prehydrolysis time and higher temperature the
3,15
3,2
3,25
3,3
3,35
3,4
3,45
3,5
0 50 100 150 200 250 300 350
pH
Time(minute)
150 C
160 C
170 C
10
15
20
25
30
35
0 100 200 300
Ka
pp
a N
um
be
r
Time (minute)
PH:150 C, KC: e.a 20%
PH:160 C, KC: e.a 20%
PH:170 C, KC: e.a 20%
14
condensation of lignin occurred. The condensed lignin precipitated onto the fiber and resists the delignification from wood. So it was found that the wood prehydrolyzed for higher time and higher temperature contained high amount of lignin, as was observed for 160 oC and 170 oC. Figure 11 shows that high prehydrolysis time and high temperature gave high kappa number.
Formation of acetic acid during prehydrolysis stage forms the liquor acidic. Under acetic conditions degradation of lignin generated by the cleavage of α and β-aryl ether bond. During prehydrolysis carbonium ion is formed in the intermediate. Carbonium ion may be responsible for the condensation of the lignin. The condensed lignin precipitated onto the surface of the fiber and increases the amount of residual lignin (by resisting delignification) in final product. At high temperature lignin condensation reaction started after a short time period, as in the case of 170 oC [5].
Figure 12 The kappa number of the dissolving pulp after kraft cooking at prehydrolysis time
Delignification was delayed/ stopped in the kraft cooking due to the severe conditions in prehydrolysis stage. When delignification is delayed, then the high alkali charge of the kraft cook accelerated the delignification reaction. Prehydrolysis at 160 oC gave a high kappa number even after a short time with 20 % e.a in the kraft cooking. However for the charge of 25 % e.a in the kraft cooking the kappa number was lower after a short time prehydrolysis as compare to 20 % e.a.__Figure 12 shows that a high alkali charge dissolves more lignin and resulted in a lower kappa number as compare to 20 % e.a.
10
12
14
16
18
20
22
24
26
28
30
0 50 100 150 200
Ka
pp
a N
um
be
r
Time (minute)
PH:160 C, KC: 20 % e.a
PH:160 C, KC 25 % e.a
15
3.4 Effect of prehydrolysis on intrinsic viscosity The viscosity measurement is used for measure the degradation of the cellulose. The
prehydrolysis also causes cleavage of carbohydrate bonds. During prehydrolysis a considerable part of the hemicelluloses and a little part of the cellulose are hydrolyzed into short chains. In the kraft cook the remaining hemicelluloses are degraded and easily dissolved in the liquor. In the kraft cook, delignification reactions took place as well as some degradation of cellulose. The viscosity of the dissolving pulp depends on the chain length of the cellulose. The pulp containing longer cellulose chains will have a higher viscosity as compare to a shorter one.
Figure 13 The viscosity of the dissolving pulp after kraft cooking for prehydrolysis time
Long prehydrolysis time and high temperature hydrolyzed more cellulose into shorter chains. From figure 13 it is clear that the pulp prehydrolyzed for longer time gave lower viscosity at the same temperature as compare to shorter prehydrolysis time. Similarly a high temperature gave lower viscosity [24]. As shown in the figure 13.
Figure 14 The viscosity of the dissolving pulp after kraft cooking at prehydrolysis time
A high alkali charge on the wood chips during the kraft cook also increased the depolymerization of the cellulose as compared to the low alkali charge at the same temperature. From figure 14 it was found that the viscosity was decreased for 25 % e.a_ compared to 20 % e.a.
400
500
600
700
800
900
1000
1100
1200
0 50 100 150 200 250 300
Vis
cosi
ty (
ml/
g)
Time (minute)
150 C, e.a 20%
160 C, e.a 20%
170 C, e.a 20%
400
500
600
700
800
900
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1100
1200
0 50 100 150 200
Vis
cosi
ty (
ml/
g)
Time (minute)
PH: 160 C, KC 20 % e.a
PH: 160 C, KC: 25 % e.a
16
3.5 Relationship of prehydrolysis time and residual alkali In the prehydrolysis stage lignin condensed onto the fiber inhibited the delignification
reaction in the kraft cooking stage. In kraft cooking hydroxides ions and hydrogen sulfide ions are used for delignification. Good delignification will consume a much of alkali. Due to the precipitation of lignin onto the fiber the consumption of alkali was reduced in the kraft cooking. The precipitated lignin due to the condensation in the prehydrolysis stage may block the pores of the fiber.
Figure 15 The residual alkali in the black liquor at kappa number
Figure 16 The residual alkali in black liquor for prehydrolysis time
There may be two possibilities 1: The condensed lignin becomes inert for the cooking liquor. 2: The condensed lignin is precipitated onto the fiber and does not allow the alkali to attack the lignin inside the fiber. In both cases residual alkali will be high in the black liquor. Figure 15 and 16 are showing that a long prehydrolysis time and a high temperature gave a high amount of residual alkali in black liquor. Also high kappa number was observed for high residual alkali. By increasing the alkali charge in the kraft cooking the alkali will be in excess. Excessive alkali (25 % e.a) will dissolve higher amount of lignin as compared to 20 % e.a. Due to excess alkali in white liquor residual alkali will be high in black liquor.
10
15
20
25
30
35
10 15 20 25 30
Ka
pp
a N
um
be
r
Residual Alkali (g/l)
150 C, e.a 20%
160 C, e.a 20%
170 C, e.a 20%
160 C, e.a 25 %
0
5
10
15
20
25
30
0 50 100 150 200 250 300
Re
sid
ua
l A
lka
li (
g/l
)
Time (minute)
150 C, e.a 20 %
160 C, e.a 20 %
170 C, e.a 20 %
160 C, e.a 25%
17
3.6 Effect of prehydrolysis time and temperature on R18 For R18 analysis a pulp samples were treated with 18 % NaOH to dissolve pulp contents other
than the crystalline cellulose. The value of R18 gave the amount of alphacellulose. In the prehydrolysis stage the extraction of cellulose was tried to be avoided. At higher prehydrolysis temperature and longer time hemicellulose and a small amount of lignin were dissolved. So the pulp obtained from longer prehydrolysis time and higher temperature contained a higher percentage of cellulose as compared to shorter time and lower prehydrolysis temperature. As shown in figure 17, longer prehydrolysis time at the same temperature gave higher value of R18. Higher prehydrolysis temperature also gave higher value of R18.
Figure 17 The R18 of the dissolving pulp for prehydrolysis time t
When pulp is dissolved in 18 % NaOH, hemicellulose will be dissolved in NaOH. But it is not true all the time, some of the degraded cellulose (amorphous cellulose) are also dissolved in 18 % NaOH, as in the case of 25 % e.a. when wood chips is cooked with a high alkali charge, it will shorten the chain of the cellulose and result in a low degree of polymerization. These short chain celluloses will also be dissolved in 18 % NaOH [26] and be drained out during washing.
Figure 18 The R-18 of the dissolving pulp for kraft cooking yield
Longer prehydrolysis time and higher temperature dissolved more hemicellulose which resulted in a low yield of the pulp. A low yield pulp may however contain a high percentage of
88
90
92
94
96
98
100
0 50 100 150 200 250 300
R-1
8
Time(minute)
150 C, e.a 20%
160 C, e.a 20%
170 C, e.a 20%
160 C, e.a 25%
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94
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30 32 34 36 38 40 42
R-1
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Yield %
150 C, e.a 20%
160 C, e.a 20%
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alphacellulose. As shown in figure 18, a low yield pulp resulted in a high R18 value. High cooking alkali charge may be responsible for the degradation of some of the cellulose. Degraded cellulose may dissolve in 18 % NaOH and give a low value of R18
[26].
4. Conclusions
• The prehydrolysis treatment of wood chips decreased the yield of wood by the extraction of hemicellulose and some lignin.
• The kraft cooking further extracted the hemicellulose, lignin and extractives_, which decreased the yield further.-.
• The pH of the spent liquor from the prehydrolysis stage was decreased with longer prehydrolysis time and high temperature, due to the formation of acetic acid.
• Dissolution and condensation reactions of lignin occurred during the prehydrolysis treatment of wood. For the dissolution, the lignin content was slightly reduced, and during the condensation lignin was condensed and precipitated onto the fibers and inhibited the delignification of pulp during the cooking.
• The intrinsic viscosity was decreased with increasing prehydrolysis time and temperature. Longer prehydrolysis time and high temperature shorten the chain of the cellulose and reduced the viscosity of the pulp.
• The precipitation of lignin onto the fiber in the prehydrolysis stage reduced the consumption of alkali in the kraft cooking and for higher Kappa number; higher amount of residual alkali was observed in the black liquor.
• Long prehydrolysis time and high temperature gave a high amount of alphacellulose. It was also observed that some of the cellulose was also dissolved in the 18 % NaOH due to the high degree of polymerization of cellulose.
• Standard deviations has been calculated, deviations in the results were very small. So the error bars were small.
5. Recommendation of future work
Shorter prehydrolysis time and low temperature (greater than 120 oC) may be better for the extraction of hemicellulose and to avoid lignin precipitation on the fibers. Prehydrolysis temperature of 120 oC, 130 oC, and 140 oC would thus be of interest. It would be probable that these temperatures will give a low kappa number in the following kraft cook.
6. Acknowledgement
I want to say special thanks to my supervisor Ulf Germgåärd and assistant supervisor Niklas Kvarnlöf for supervision and guidance. I also want to say thanks to Mikael Andersén for providing chemicals in time.
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