Effect of oxygen delignification process on the lignin content and wastewater quality from kraft-pulped Eucalyptus pellita

Abstract

The aim of this research is to observe the decrease in lignin content and wastewater quality from oxygen delignification process of kraft-pulped Eucalyptus pellita. In pulp production, conventional kraft cooking is carried out with five active alkali charge (16%, 17%, 18%, 19%, and 20%), with 25% sulfidity in addition to H factor of 800. Oxygen delignification occurs in medium consistency (10%) and 2 bar oxygen pressure, in addition to delignification with an alkaline charge of 20 g/L and a temperature of 85°C. In the kraft pulping process, the increase of alkali active causes a decrease in lignin levels. In the next process, oxygen delignification shows that longer reaction time causes a decrease in the levels of lignin, COD, and BOD5. The information from this study will be helpful to the future implementation of kraft pulping integrated with oxygen delignification process in actual pulp mill applications of Eucalytus pellita.

Full text

AIP Conference Proceedings 2197, 110002 (2020); https://doi.org/10.1063/1.5140955 2197, 110002
© 2020 Author(s).
Effect of oxygen delignification process on
the lignin content and wastewater quality
from kraft-pulped Eucalyptus pellita
Cite as: AIP Conference Proceedings 2197, 110002 (2020); https://doi.org/10.1063/1.5140955
Published Online: 02 January 2020
Aria Darmawan, Yanuar Ananda Putri Ramadhan, Nita Rahayu Dewi, Hikmatun Ni’mah, Achmad
Roesyadi, and Firman Kurniawansyah
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Effect of Oxygen Delignification Process on the Lignin
Content and Wastewater Quality from Kraft-pulped
Eucalyptus pellita
Aria Darmawan1, Yanuar Ananda Putri Ramadhan1, Nita Rahayu Dewi1, Hikmatun
Ni’mah1, a), Achmad Roesyadi1 and Firman Kurniawansyah1
1 Department of Chemical Engineering, Faculty of Industrial Technology, Institut Teknologi Sepuluh Nopember
(ITS), Surabaya, East Java, Indonesia, 60111.
a)Corresponding author: hikmatunn@gmail.com
Abstract. The aim of this research is to observe the decrease in lignin content and wastewater quality from
oxygen delignification process of kraft-pulped Eucalyptus pellita. In pulp production, conventional kraft
cooking is carried out with five active alkali charge (16%, 17%, 18%, 19%, and 20%), with 25% sulfidity in
addition to H factor of 800. Oxygen delignification occurs in medium consistency (10%) and 2 bar oxygen
pressure, in addition to delignification with an alkaline charge of 20 g/L and a temperature of 85ºC. In the kraft
pulping process, the increase of alkali active causes a decrease in lignin levels. In the next process, oxygen
delignification shows that longer reaction time causes a decrease in the levels of lignin, COD, and BOD5. The
information from this study will be helpful to the future implementation of kraft pulping integrated with oxygen
delignification process in actual pulp mill applications of Eucalytus pellita.
Keyword: eucalyptus, lignin content, oxygen delignification, wastewater quality
INTRODUCTION
In general, the principle of the pulping process is separating the cellulose from impurities contained in wood. The
kraft cooking process is used to degrade the lignin polymer which acts as a kind of adhesive between the fibers. This
kraft cooking process is done by the active hydroxide and hydrogen sulfide ions presented in the cooking liquor in
combination with energy in the form of elevated temperatures. The lignin is not only degraded during the cooking
process, but cellulose and hemicellulose could also degraded and dissolved[1]. The purpose of the cooking process is
to separate the fibers in the wood and dissolve most of the lignin contained in the fiber wall or to cook the chip
according to the kappa number target. Kappa number is one of the most important parameters measured in chemical
pulping. It indicates the pulp delignification degree, which is defined as the volume (ml) of 0,1 N potassium
permanganate solution consumed by 1 g of moisture-free pulp in an acidic medium[2]. The quality of pulp is not only
related to kappa number but also related to viscosity. The viscosity of pulp was used extensively as a parameter to
control the quality of pulp at different phases of the pulping process. The viscosity parameters are related to the average
degree of polymerization and molecular weight of the polymer. These measurements are used indirectly to estimate
the degree of polymerization and to know the extent to which carbohydrates were broken down during the process of
obtaining cellulose. An increase of delignification and reduction of kappa promotes higher pulp degradation which
results in the production of pulp with higher fines fibers fragmentation and lower viscosity[3].
The resulting pulp is brown because some lignin isn’t degraded, so it is separated in oxygen delignification (OD)
and chemical bleaching to achieve the desired brightness. Oxygen delignification is defined as usage of oxygen and
alkali to degrade most of the residual lignin from unbleached pulp. Low kappa number after oxygen delignification
Proceedings of 2nd International Conference on Chemical Process and Product Engineering (ICCPPE) 2019
AIP Conf. Proc. 2197, 110002-1–110002-6; https://doi.org/10.1063/1.5140955
Published by AIP Publishing. 978-0-7354-1948-3/$30.00
110002-1

results in lower active chemical charge needed for pulp bleaching. This reduces the costs of chlorine dioxide and other
chemical requirements to achieve the desired brightness[4].
Extended oxygen delignification could lower the lignin content in pulp, increase the efficiency of delignification
and pulp viscosity, and lower the carbonyl content. But, the oxygen consumption that isn’t validated by dissolved
lignin could be improved by increasing the cooking temperature and alkali charge[5]. Comparison between the alkali
charge and the temperature factor shows that the alkali charge is more influential in the oxygen delignification process.
Furthermore, with the increase of cooking temperature, it is possible to lower the kappa number by decreasing more
selectivity. Temperature is an active agent in chemical reaction, given that for each alkaline charge applied, the
residual alkali concentration decreases with increasing temperature[6]. Oxygen gas is then added to the pulp ahead of
an oxygen mixer, which reduces the size of the oxygen gas bubbles and produces intimate contact between the pulp
and oxygen. The result of this process is a decrease in KaNo, so the use of bleaching chemicals will decrease. The
degree of delignification at the same sodium hydroxide charge was dependent on initial kappa number. The target of
cooking softwood kraft for unbleached pulp is above the kappa range 25−30 and further delignification is using an
alkaline oxygen process to the kappa level 15-20[7]. The Process scheme of oxygen delignification process can be
seen in Figure.1
FIGURE 1. Scheme of Medium consistency oxygen single-stage system [5]
Pulp washing process plays an important role in achieving effective and selective oxygen delignification. Washing
at the oxygen stage is the amount of dissolved material from the digester and the material currently present from the
oxygen stage with the filtrate from washing[8]. It has also been clearly shown that the efficiency of delignification in
the oxygen stage depends not only on the levels of COD and BOD5 but also on the character of dissolved organic
matter in the filtrate being carried[4].
TABLE 1. Kappa number currently achieved with different delignification technologies and comparison of the
calculated
effluent COD without considering the washing losses [11]
Delignification technologies
Kappa for
hardwood
Kappa for
softwood
Calculated COD load (kg/t)
Hardwood
Softwood
Conventional cooking
14-22
30-35
28-44
60-70
Conventional cooking + oxygen
delignification
13-15
18-20
26-30
36-40
Extented cooking
14-16
18-22
28-32
36-44
Extended cooking + oxygen
delignification
8-10
8-12
16-20
16-24
One of the goals of washing pulp is to remove the dissolved components from the pulp suspension by using as
little washing powder as possible. Washing yields, washing losses, are usually evaluated by measuring the amount of
COD and BOD5[9]. In optimizing the efficiency of removing lignin by oxygen delignification in the case of pulp
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washing, it is important to reduce the amount of dissolved solids. An important criterion for designing a moderate
consistency oxygen delignification system is the number of pre-oxygen and post-oxygen washing steps[10].
The present work aims to investigate lignin content and wastewater in the oxygen delignification process of kraft-
pulped Eucalyptus pellita.
MATERIALS AND METHOD
Materials
The materials used is Eucalyptus pellita wood from Jambi Province (Table 2). The chips and fiber are filtered
(SCAN-CM 40:94), dried and stored at 25% water content. The concentration standard of NaOH and Na2S for pulping
was set to 97.0 g / l and 32.4 g / l respectively.
TABLE 2. Chemical Properties of Chip Eucalyptus pellita [3], [12], [13]
Wood and Fiber Characteristics
Chip Eucalyptus pellita
Density (g/cm3)
0.469
Cellulose
68.85%
Hemi cellulose
18%
Lignin
28.09%
Moisture content
31.6%
Ash content
0.4%
Kraft Pulping
For conducting oxygen delignification trials, pulps obtained by the conventional kraft process were obtained. The
raw material for the cooking were industrial wood chips of Eucalyptus pellita with thickness from 3 to 5 mm. Six
replication of cookings were performed in rotating digester Haato 6 chambers, with a capacity of one liter and 300 g
of wood chips in each. The purpose was to obtain pulp with kappa number 18±2. The cooking conditions are described
in Table 3.
Cooking liquor including sodium hydroxide and sodium sulfide were prepared from Merck products.
Concentrations of cooking liquor were analyzed according to SCAN–N 2:88 standard. Then, on the basis of
concentration of cooking liquor and the ratio of liquor to chips weight (L/W). The cooking was carried out in a rotating
digester Haato 6 chambers with active alkali charge of pulping liquors. The cooking conditions are shown in Table 2.
Each chamber of digester was filled with 300 gram of oven-dried chips and the wood ratio between chips and liquid
was 1: 3,5. The chips, white liquor, and solvent were added into chambers and placed in digester. Impregnation time
consisted of 3 stages, the first stage was from 27 to 100˚C for 30 minutes, the second stage was from 100 to 132˚C for
30 minutes and the third stage was from 132 to 165˚C for 30 minutes. The cooking process was at a maximum
temperature of 165˚C for 90 minutes. The resulted pulp was sieved through a screen somerville. Pulping yield was
obtained gravimetrically by weighing the oven-dry pulp and reported as percentage of the original wood. Viscosity
and kappa number were determined by TAPPI T 236 cm-85 and TAPPI T-230 om-89 respectively.
TABLE 3. Cooking conditions
Parameters
Conditions
Active Alkali (%)
17
Sulfidity (%)
25
Heating time (min)
60
Time at maximum temperature (min)
90
Maximum Temperature (oC)
165
H Factor
800
Liquid/wood ratio
3.5/1
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Oxygen Delignification
Oxygen delignification is carried out in a rotating reactor with an electric heater and a volume of 20 liters,
consisting of 6 individual tubes of 100 mL each. Pulp samples with 4 grams each were placed into the reactor in a
medium consistency in Table 4 and heated to a predetermined temperature. The pulp is washed with distilled water.
Pulp that has been washed, then filtered, and dried were then tested for its quality. The experimental design used is
completely random in factorial settings. The factors involved are various time (20, 40, 60, 80, and 100 minutes). For
all combinations with reaction temperature, oxygen pressure and constant consistency, which are 85-90oC, 2 bars and
10% consistency, respectively. The yield, kappa number, viscosity, COD and BOD5 of pulps were determined
according to the same standard test methods as used for the cooked pulps shown in Table 5.
TABLE 4. Conditions of Oxygen Delignification
Parameters
Conditions
Pulp consistency (%)
10
Reaction time (min)
20, 40, 60, 80, 100
Maximum temperature (oC)
85-90
Alkaline charge (Kg/ADT)
15
Wood chips dry mass (g)
4
Oxygen load (Kg/ADT)
20
Delignified Pulps Analysis
Kappa number of pulp and pulp viscosity were determined in accordance with the procedures of TAPPI T 236 cm-
85 and TAPPI T-230 om-89 respectively. COD and BOD5 of wastewater pulp were determined in accordance with
the procedures of ISO 5815-1:2003 and ISO 6060 respectively. Lignin content pulping [14] and Degree of
Polymerization (DP) were calculated following the formula of:
ܮ݅݃݊݅݊ܥ݋݊ݐ݁݊ݐሺܭ݈ܽݏ݋݊ሻൌ ͲǤͳͷʹ ൈ ܭܽ݌݌ܽܰݑܾ݉݁ݎ
ܦܲ଴Ǥଽ଴ହ ൌͲǤ͹ͷሾߟሿ
ሾߟሿൌ ݅݊ݏݐݎ݅݊ݏ݅ܿݒ݅ݏܿ݋ݏ݅ݐݕ
RESULT AND DISCUSSION
Kraft Pulping
The kraft process is carried out to determine the pulp properties of abaca. The kraft method is 25% sulfidity, 17%
active alkali (AA) at a cooking temperature of 165oC and a cooking time of 90 minutes at maximum temperature, as
is commonly done in the pulp and paper industry. The main purpose of this cooking process is to reduce the lignin
content measured from the kappa number (KaNo) according to the minimum standard as a reject pulp.
The main purpose of this cooking process was to reduce the lignin content that measured from kappa number
(KaNo) in accordance with the minimum standard as the rejected pulp. In the kraft cooking process, the total yield
was 56.21% with screen yield 52%. The brown pulp kappa number obtained was 16.23 with a viscosity value of 907
mL/g. The yield of lignin obtained was 2.46%. The results obtained during the pulping stage may be considered typical
if we refer to eucalyptus chips processed by conventional batch cooking.
The high kappa number shows the high lignin content in the pulp produced after the cooking process. Pieces of
raw wood and fiber contain cellulose, hemicellulose and other impurities. Active alkali charge was observed to get
the target kappa number. Within the normal range, the kappa number is expected to be <20 according to the alkaline
preparation standard where the range of the kappa standard for Eucalyptus pellita is 12-18 [3].
(1)
(2)
(3)
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Oxygen Delignification
Effect of oxygen delignification on lignin content of pulp
Pulp with a good degree of delignification is indicated by a low kappa number value which indicates a complete
delignification process. The high kappa number indicates that there are still levels of lignin contained in the pulp
produced after the chip cooking process[15]. The lignin reactions were quick during the first 60 min, with a very
intense stage taking place during the first 20 min. After the first 60 min, the reactions decelerated, while after 100 min,
the reactions did not advance significantly.
The oxygen delignification operation of kraft pulp eucalyptus reduced the lignin content by 43.06%, 46.88%,
48.42%, 49.78%, and 51.44%, respectively (Table 5). The values reported are typical of those achieved by softwood
pulp mills using medium consistency oxygen delignification reactors[6].
Viscosity and kappa number are used to determine the lignin content in the pulping process. It can be seen that
there is a significant decrease between kappa number and viscosity during the oxygen delignification process,
respectively (Table 5). Lignin has been extensively reducing during the delignification process. Lignin has been
reducing by breaking the bond between the α- and β-aryl ether bonds between the propane units and subsequently
breaking the liquor[3]. The lower the kappa number after cooking, the lower the kappa number after oxygen reduction.
The results showed that the lignin content in the oxygen-delignification stage pulp one level could be reduced to a low
level either by reducing lignin residues during cooking or by increasing the alkali content at the oxygen delignification
stage. However, pulp with lower lignin content (kappa number 7-9) and great viscosity can only be obtained if the
amount of lignin residue is quite low after cooking. Great viscosity of pulp after oxygen removal has a degree of
polymerization for pulp of ≥ 1200 [16] (according to ISO 5351: 2012 standards) except for pulps cooked with reaction
time 40-100 min shown in Table 5. This result is very interesting for modern pulp mills, which aim, for example, to
use ECF or TCF bleaching techniques. However, more work needs to be done to clarify the bleaching capability, and
especially the feasibility of the pulp strength, of this pulp with low residual lignin content.
TABLE 5. Characteristics of the eucalyptus pulps
Characteristics
After
cooking
After O
2
Delignification
Reaction time (min)
20
40
60
80
100
Total Yield (%)
56.21
49.71
48.94
49.39
48.94
49.59
Kappa Number
16.23
9.24
8.62
8.37
8.15
7.88
Viscosity (mL/g)
907
838
802
790
778
756
Degree of
polymerization
1349.02
1236.08
1177.54
1158.09
1138.67
1103.14
Lignin content (%)
2.46
1.40
1.31
1.27
1.24
1.20
Lignin reduction (%)
43.06
46.88
48.42
49.78
51.44
COD (mg/L)
50
48
45
40
36
BOD5 (mg/L)
25
23
24
23
19
Effect of oxygen delignification on wastewater quality of pulp washing
Proper post-washing oxygen delignification pulp well is also very important. It minimizes the carryover into the
bleach plant, which increases the consumption of bleaching chemicals. In the absence of good washing, the resulting
increase in bleaching chemical use will increase pulp bleaching costs, and also increase discharges of BOD5 and COD
from the bleach plant[17].
Oxygen delignification operations of eucalyptus pulp kraft produce a decrease in the value of COD and BOD5 over
time (Table 5). The total COD and BOD5 levels are dependent on the kappa numbers. It can be seen that there is a
decrease in wastewater quality similar with the value of kappa produced. The COD compositions of eucalyptus and
softwood effluents are significantly different, where the effluents from the eucalyptus pulps are more biodegradable.
The compounds forming the kappa number in softwood and hardwood (especially eucalyptus) differ as well in
softwood[11].
The viscosity represents the strength of the pulp and it depends on cellulose chain, long cellulose chain gives high
viscosity. Table 5 shows the relationship between the viscosity and wastewater quality carryover at the same time,
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temperature, pressure, and alkali concentration. The viscosity for clean pulp was 756 ml/g by producing COD of 36
mg/L and BOD of 19 mg/L.
CONCLUSIONS
In this study, Eucalyptus pellita was delignified by kraft pulping and the subsequent oxygen delignification to pulp
with a low kappa number. Viscosity and kappa number are used to determine the lignin content in the pulping process.
It can be seen that there is a significant decrease between kappa number and viscosity during the oxygen delignification
process. The oxygen delignification operation reduced the lignin content by 43.06-51.44%. The values reported are
typical of those achieved by softwood pulp mills using medium consistency oxygen delignification reactors.
Oxygen delignification operations of eucalyptus pulp kraft produce a decrease in the value of wastewater quality.
The total COD and BOD5 levels are dependent on the kappa numbers. It can be seen that there is a decrease in
wastewater quality similar with the value of kappa produced.
ACKNOWLEDGEMENT
This work has been financially supported by a research grant of “Penelitian Dasar Unggulan Perguruan Tinggi
(PDUPT)” for year of 2019 from DRPM, Indonesia, to which the authors express their gratitude
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