Investigation on Soda and Soda-Anthraquinone (AQ) Pulping of Saccharum spontaneum

M. Sarwar Jahan1, M. Khalidul Islam1, AJM Moynul Hasan2 and D.A. Nasima Chowdhury

1. Pulp and Paper Research Division, BCSIR Laboratories, Dhaka, Dr. Qudrat-E-Khuda Road, Dhaka-1205, Bangladesh

2. Fiber and Polymer Research Division, BCSIR Laboratories, Dhaka, Dr. Qudrat-E-Khuda Road, Dhaka-1205, Bangladesh

ABSTRACT

Soda and soda-AQ pulping of Saccharum spontaneum were studied with different variables, such as active alkali, cooking temperature, time and liquor to fiber ratio. Desired delignification of S. spontaneum was achieved with only 14% of NaOH on oven dried (o.d) raw material in 1 hour of cooking. S. spontaneum pulp showed pulp yield about 53-61% with Kappa number about 16-29. The strength properties of the pulp were almost similar to bamboo pulp, which is now being used in the Karnaphuli Paper Mills of Bangladesh. The addition of 0.05% anthraquinone to the alkaline liquor reduced the alkali concentration of 4% on o.d. raw material or reduced the cooking time and improved strength properties except tear strength. About 16 % tear index was lost with the addition of 0.05% AQ. The kinetics of the soda and soda-AQ delignification of S. spontaneum was characterized by an apparent high reaction order.

The basic raw materials of paper and paper products come from forests. Therefore, random deforestation results in severe ecological problems. One of the best possible remedies is to use annual plants, agricultural and other lignocellulosic wastes as raw material in the pulp and paper industry.

In Bangladesh the conventional raw materials for pulp and paper industries are bamboo, mixed hardwood, bagasse, Gewa wood etc. Bagasse is a by-product of the sugar industry. The sugar industry uses it as a fuel for steam generation. Therefore, bagasse based paper industry in Bangladesh could not run in full swing. On the other hand, Gewa wood is grown in Sundarban. Recently United Nation declared Sundarban as a world heritage. So, Sundarban authority can not supply Gewa wood to the pulp and paper industries to meet their demand. Therefore, it is utmost need to find out alternative source of raw materials to survive our paper industries. One of such possible alternative raw materials is Saccharum spontaneum. In Bangladesh, it is called "Kash" and in India  "Kans grass".

S. spontaneum is a perinial grass with slender culms, growing in stools or forming continuous cane-brakes with most often aggressive rhizomatous tillering, distributed widely in the  sub-tropical and tropical parts in Asia, Africa and ascending  upto an altitude of 1,800 m (1). The forms of S. spontaneum show a wide range of variation in habitat and in the morphological character of stem, leaf and root. S. spontaneum is a coarse grass normally not relished by the cattle, and is generally used as fodder only during scarcity. A few investigators (2-5) showed the possibility to use it as a pulping raw material but the intensive study could not proceed.

Soda-AQ
Nowadays interest has been given on sulfur free pulping. Therefore, studies on different pulping processes have been done (6). Among these processes used to get improved quality pulp and higher yield, soda-anthraquinone (AQ) process has given encouraging result. It is believed that AQ is still the most cost-effective sulfur free accelerator for alkaline pulping. The soda-AQ process also offers a direct advantage of eliminating the air pollution associated with the kraft process.

Delignification kinetics
Kinetics of pulping is also necessary to effectively control the cook. Alkaline delignification kinetics is generally divided into three stages (7): initial, bulk and residual stages. The initial stage is characterized with slow delignification, high carbohydrate dissolution and heavy consumption of alkali. Most of the lignin is removed in the bulk stage. Carbohydrate dissolution and alkali consumption is also low in this stage. Residual delignification is slow and selectivity is poor in this stage. The yield loss and carbohydrate degradation make pulping in the residual stage detrimental to the process.

The objective of the present study was to investigate the pulping behavior of S. spontaneum during soda and soda-AQ pulping under different pulping conditions. Also kinetics behavior of soda and soda-AQ delignification were investigated. 

EXPERIMENTAL

Material
S. spontaneum was collected from the Jamuna Char and its leaves, roots were removed. It was cut into about 2-3 cm in length.

Proximate analysis
Before carrying out the pulping experiments an approximate chemical analysis of S. spontaneum was performed in accordance with Tappi Test Methods.

Pulping
Pulping was carried out in a 20-l capacity batch cylindrical reactor heated by means of electrical resistance and was rotated by a motor. The normal charge was 1 kg of moisture free S. spontaneum. The following parameters were maintained:

  • The alkali concentration was varied from 10 to 16% as NaOH on o.d. S. spontaneum for soda process and 8-14% for Soda-AQ process.
  • The cooking temperature was varied from 150-1800C
  • The cooking time was varied from 0 to 120 min at the maximum temperature.
  • The liquor/ material ratios were 5, 6 and 7.
  • 0.05% AQ was in soda-AQ process.

After digestion the pulp was washed until free from bleach liquor, disintegrated in a standard disintegrator and screening through a screen of 1-mm mesh size. Hard cooked materials were defibrated in a laboratory model Sprout Waldron disk refiner. The pulp yield was determined as percentage of oven-dry raw materials. The kappa number of the pulps was determined according to Tappi Test Methods (T 236).

For kinetic study, pulping was carried out in a 5-l capacity digester. The alkali concentration was 14% for soda and 10% for soda-AQ process. The lignin content of the pulp was determined by multiplying the kappa number with the factor 0.153. The lignin yield on o.d. S. spontaneum was calculated by the following formula:

   % of lignin on o.d. S. spontaneum = % of lignin on pulp X % of total pulp yield / 100

Evaluation of pulps
S. spontaneum was beaten in a Valley beater. The samples were collected at different freeness and hand sheets of about 60g/m2 were made in a Rapid Kothen Sheet Making Machine according to German Standard Methods DIN 106. The sheets were tested for tensile (T 404os 61), burst (T 403m 53) and tear strength (T 414m-49), double fold (T 423m-50) according to TAPPI Standard Methods.

RESULTS AND DISCUSSION

Characteristics of S. spontaneum
The proximate chemical analysis and fiber characteristics of S. spontaneum compared with bagasse and bamboo, presently used in Bangladesh is shown in Table 1.

Table 1. Characteristics of Saccharu

Chemical and morphological characteristics

Saccharum spontaneum

Bagasse

Bamboo

Holocellulose, %

75.39

49.4

75.5

Pentosan, %

24.01

27.6

18.4

Lignin, %

16.82

17.8

28.0

Ash, %

3.89

1.8

2.46

1% alkali solubility

26.9

41.0

27.3

Alcohol-benzene solubility, %

3.72

8.0

6.21

Hot water solubility, %

7.6

12.9

9.38

Avg. fibre length, mm

1.52

1.70

2.57

Avg. fibre diameter, Fm

16.0

20.0

23.7

m spontaneum
with bagasse (8) and bamboo (9)

 

 

 

 

 

 

 

From the Table it can be seen that lignin, ash, 1% alkali and alcohol-benzene solubility are lower for bagasse (8) and bamboo (9) than for S. spontaneum. Holocellulose content in S. spontaneum is almost similar to bamboo but higher than bagasse. Average fiber length of S. spontaneum is lower than bamboo. The chemical and fiber characteristics of S. spontaneum is comparable to hardwood species (10). Therefore, it may be a suitable raw material for pulp production.

Pulp yield and Kappa number
Pulp yield and kappa number are given in Table 2.

Table 2. Pulping conditions and pulp properties obtained from experiments with Saccharum spontaneum

Alkali, % as NaOH

Cooking temp. oC

Time, min

Liquor ratio

Pulp yield, %

Reject, %

Kappa number

Breaking length, m

Burst Index. kPa.m2/g

Tear Index, mN.m2/g

Double fold number

Bright-
ness, Tapp%

Soda

 

 

 

 

 

 

 

 

 

 

 

10

170

60

6

60.2

0

29.6

4606

2.8

7.6

16

39.3

12

170

60

6

57.2

0

23.8

4819

2.9

8.1

20

41.6

14

170

60

6

54.7

0

17.6

4970

2.9

8.0

23

43.0

16

170

60

6

52.8

0

16.6

4989

3.1

7.9

29

44.2

14

170

0

6

57.7

0

25.0

4618

2.7

7.8

13

38.9

14

170

30

6

55.4

0

18.3

4727

2.8

8.0

25

40.5

14

170

60

6

54.7

0

17.6

4970

3.0

8.0

23

43.0

14

170

90

6

54.0

0

17.4

5011

3.2

8.1

25

43.4

14

170

120

6

53.2

0

17.8

5029

3.2

8.1

28

43.2

14

150

60

6

56.4

0

26.7

4519

2.6

7.7

12

39.9

14

160

60

6

55.3

0

19.6

4788

2.8

7.9

23

43.1

14

170

60

6

54.7

0

17.6

4970

3.0

8.1

23

43.0

14

180

60

6

52.3

0

22.2

4629

2.8

7.9

21

37.2

14

170

60

5

54.9

0

21.4

4934

3.0

8.3

22

43.2

14

170

60

6

54.7

0

17.6

4970

3.0

8.0

23

43.0

14

170

60

7

53.4

0

17.7

4977

3.1

8.1

26

43.2

Soda-AQ

 

 

 

 

 

 

 

 

 

 

 

8

170

60

6

61.3

0

30.7

4992

2.8

6.9

21

40.2

10

170

60

6

57.9

0

20.0

5536

3.2

6.5

32

42.5

12

170

60

6

55.9

0

16.9

5586

3.3

6.6

34

43.6

14

170

60

6

54.1

0

16.0

5612

3.3

6.6

37

43.8

10

170

0

6

64.8

0

26.1

5422

3.0

6.7

20

39.8

10

170

30

6

61.8

0

22.5

5508

3.1

6.8

23

40.5

10

170

60

6

57.9

0

20.0

5536

3.2

6.5

29

42.5

10

170

90

6

55.3

0

19.6

5631

3.3

6.7

33

43.5

10

170

120

6

53.1

0

19.1

5639

3.3

6.6

31

42.1

10

150

60

6

62.3

0

24.0

5077

2.8

6.5

18

39.3

10

160

60

6

59.1

0

22.6

5329

2.9

6.8

23

41.5

10

170

60

6

57.9

0

20.0

5536

3.2

6.5

23

42.5

10

180

60

6

55.3

0

22.1

5098

2.8

6.8

17

39.1

14

170

0

6

57.1

0

21.2

5322

2.8

6.9

25

42.1

14

170

30

6

55.1

0

16.5

5065

2.9

6.6

22

42.8

14

170

60

6

21.2

0

16.1

5008

2.7

6.3

20

43.1

14

170

90

6

21.5

0

16.0

4981

2.7

6.1

21

41.7

14

170

120

6

20.2

0

16.1

4912

2.6

6.1

18

41.1

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

In the soda process, pulp yield and kappa number are decreased with increasing alkali concentration, which corresponds to other alkaline pulping studies (11). For a 6% alkali concentration, the pulp yield and kappa number drop to 7.4 and 13 points, respectively. Increasing the alkali concentration from 10 to 14%, the kappa number reduces by 12 points.

Further 2% alkali concentration increase, the kappa number reduces only 1 point. Therefore, pulping is reached to the residual stage and should not exceed 14% alkali. At 14% alkali, pulp yield and kappa number are decreased with increasing cooking time. After 60 min of cooking delignification is not prominent. Minimum reduction of kappa number is obtained at 1800C. This is possibly due to condensation and redeposition of lignin on carbohydrates at higher temperature (12). Pulp yield and kappa number is not more sensitive to liquor/ solid ratio.

In soda-AQ process when the alkali concentration is increased from 8 to 14%, there is a reduction in the pulp yield from 61.3 to 54.2% but decrease in kappa number from 30.7 to 16. At 10% alkali, with an increase of cooking time from 0 to 120 min pulp yield and kappa number decrease from 62.8 to 53.1 % and 26.1 to 19.1, respectively.

Fig. 1 & 2 show the effect of AQ on the delignification of S. spontaneum.

Figure 1. The effect of time on Kappa number in soda and soda-AQ pulping of S. spontaneum

Figure 1

Figure 2. The effect of alkali on Kappa number in soda and soda-AQ pulping of S. spontaneum

Figure 2

As expected, soda-AQ moves to a lower kappa number at the same time or alkali charge. In Figure 1 at a cooking time of 60 min, soda-AQ process requires only 10% alkali to reach the targeted kappa number 19, whereas soda process needs 14% alkali. Fig. 2 shows AQ's advantages in terms of reduction of cooking time. An addition of 0.05% AQ, cooking time reduces to 20 min. These results of S. spontaneum are in good agreement with those reported for soda-AQ pulping of wood and non-wood (6,13,14).

These results of AQ may be used in two ways; either keep the alkali concentration constant, and use the AQ to achieve a higher delignification rate by faster cooking time, or keep the cooking time constant, and reduce the alkali ratio by 4% on o.d. S. spontaneum.

The relationship between the kappa number and pulp yield obtained from 10 and 14% alkali charge for soda-AQ process and 14% alkali charge for soda process are presented in Fig. 3.

Figure 3. Total pulp yield as a function of Kappa number in soda and soda-AQ pulping of S. spontaneum

Figure 3

The pulp yield at any kappa number is higher in soda-AQ process than the soda process. At kappa number 20, pulp yield of soda-AQ process from 10 and 14% alkali charge is almost same, which is higher than the soda process (Figure 3). This increase in yield arises from the stabilization of carbohydrates against peeling reaction (15). This reaction is confirmed as well as a mechanism proposed by other researcher (16) to explain the AQ enhancement of the delignification rate. But at 14% alkali, pulp yield from soda-AQ process is almost similar to soda process although delignification is continued to much lower level. This indicates that AQ does not contribute to the retention of hemicellulose at this higher alkali charge.

Physical properties
It is seen from Table 2 that the breaking length, burst index, double fold number and brightness of S. spontaneum are increased with the increase of cooking time or alkali concentration in both soda and soda-AQ processes. The tear index does not follow any general relationship with the process variables. At 1800C temperature, all physical properties and brightness are deteriorated. Tear index of soda-AQ pulps is always lower than that of soda pulp. This result fully agrees with the result of soda-AQ jute, bamboo and wood pulps (6,13,14,17). Figure 4 shows the tensile -tear relationship of soda and soda-AQ S. spontaneum pulps. The tear index of soda-AQ pulp are lower than the soda pulp at any breaking length but as far as breaking length is concerned the soda-AQ pulp is superior. Figure 4 shows that an average soda-AQ pulp is about 16 % lower tear index at any breaking length.

Figure 4. Breaking length-Tear relationship of soda and soda-AQ pulping of S.spontaneum of almost similar Kappa number

Figure 4

Soda and soda-AQ processes produce brighter pulps from S. spontaneum (Table 2). Therefore, for newsprint, these pulps do not require bleaching.

Table 3 compares soda-AQ S. spontaneum pulp with kraft pulps from bamboo and mixed hardwood.

Table 3. Comparison of soda-AQ S.spontaneum pulp with kraft pulps from bamboo (17) and mixed hardwood (18)

Species

Kappa number

Pulp yield, %

Breaking length, m

Burst index kPA.m2/g

Teat index mN.m2/g

S. spontaneum

20.0

57.9

5536

3.2

6.5

Bamboo

24.6

45.9

5511

4.9

18.1

Mixed hardwood

45.0

45.5

8600

6.3

10.5

 

 

 

 

In respect to pulp yield and kappa number soda-AQ S. spontaneum pulp shows superiority over bamboo and hardwood pulp (Table 3) (17,18). The breaking length and burst index are almost similar and tear index is lower than the bamboo pulp. This pulp has potential to replace some kraft pulp from bamboo that is used in Bangladesh. Tear index of S. spontaneum pulp could be improved with the blending of jute pulp (19).

Delignification kinetics
Figure 5 shows the lignin yield on o.d. raw material during soda and soda-AQ pulping of S. spontaneum at different pulping temperature. In view of the need for quantitative kinetics description of the alkaline pulping of S. spontaneum, this work relating to delignification reaction with respect to reaction rate and order of reaction.

Figure 5. The delignification of S.spontaneum with respect to time during soda and soda-AQ pulping

Figure 5

The kinetic data could be correlated using the general kinetic model given by equation 1.

    dL/dt = k Ln ------------------------------(1)

In its logarithmic form, this becomes:

    ln (-dL/dt) = lnk + nlnL----------------(2)

where, L is the lignin yield on o.d. S. spontaneum.

Accordingly, by plotting ln(dL/dt) Vs lnL, straight lines were obtained (Figure 6). Where slope represents the order of reaction 'n' and intercept represents lnk.

Figure 6. Reaction rates (k) and order of delignification (n) soda and soda-AQ pulping of S.spontaneum

Figure 6

The delignification reaction is higher order with respect to lignin yield and ranged from 4.8- 5.6 for soda and 4.2-5.3 for soda-AQ at different pulping temperature (Table 4).

Table 4. Observed specific reaction rates (k) and order of reaction (n) of soda and soda-AQ pulping of S.spontaneum

Pulping process

Pulping temperature oC

Order of reaction, n

Reaction rate (k), min-1x10-4

Soda

150

4.8

2.9

160

5.6

2.7

170

5.4

4.9

180

5.4

1.7

150

4.5

2.7

Soda-AQ

160

4.2

4.7

170

5.3

2.4

180

4.2

5.5

 

 

 

 

 

 

The high rate order results are due to slow second phase of delignification. The first phase of delignification was completed before reaching the maximum cooking temperature. Axegard et al. (20) suggested that the slow second phase of delignification reaction can be explained in terms of some short hindrance to the delignification process. This could be lignin condensation reaction, diffusion resistance. The high reaction order could be also explained that the large number of parallel reactions of lower order (20).

The observed reaction rate constants of delignification are varied with pulping temperature (Table 4). The values are 1.7 X 10-4 to 4.9 X 10-4 for soda and 2.4 10-4 to 5.5 X 10-4 for soda-AQ. Table 4 indicates that the rate of reaction does not follow systematic relationship with cooking temperature. This is due to maximum amounts of lignin was dissolved before reaching the cooking temperature. This was also observed during alkaline pulping of wheat straw (21), where 60% lignin was removed before reaching 1000C. In the applied conditions the activation energy of S. spontaneum pulping is not possible to determine due to unsystematic results of reaction rates on temperature variation. In the next study, the initial stage of delignification will be performed therefore; energy of activation will be possible to determine.

CONCLUSION

  • Pulp yield and kappa number of S. spontaneum pulp are decreased with increasing concentration of alkali or cooking time. Kappa number reduction is maximum up to 14% alkali after that the rate of reduction of kappa number is reduced.
  • An addition of AQ to the cooking liquor reduces the cooking time or alkali to reach the desired degree of delignification. Pulp yield and delignification are increased with an addition of AQ to the cooking liquor.
  • All physical properties except tear strength are improved on addition of AQ to the soda liquor. An average of 16% lower tear strength was obtained in soda-AQ pulp than that of soda pulp at any breaking length.
  • The delignification reaction orders of S. spontaneum are 4.8-5.6 for soda and 4.2-5.3 for soda-AQ process. The delignification rate constants are varied unsystematically with the temperature.

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