Application Of AQ In Oxygen Delignification Biology Essay

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Oxygen delignification of wheat straw soda pulp with AQ addition was studied. Through a L9(34) orthogonal test, the effects of alkali charge, oxygen pressure, maximum cooking temperature and holding time at the maximum temperature were evaluated. The optimal conditions were as follows: alkali charge on oven dry pulp (as NaOH) was 3%, oxygen pressure of 0.5MPa, maximum temperature of 120°C and the holding time at the maximum temperature was 110min. Compared with the conventional oxygen delignification of wheat straw soda pulp, the oxygen delignification with AQ addition produced pulp with close kappa number but with higher yield, viscosity and brightness.

Keywords: wheat Straw, soda pulp, oxygen delignification, AQ

INTRODUCTION

Oxygen delignification technology began in the 1960's. Because of less environment pollution, higher yield, better quality and lower cost, oxygen delignification has become a main process of ECF and TCF bleaching till now [1]. As it development, many attempts have been made to improve the efficiency and selectivity of oxygen delignification, such as installing capital-intensive two stage or multi-stage oxygen delignification system, using additives and pretreatment before bleaching [2-4]. Commonly used additives include surfactants [5,6], hydrogen peroxide [7], sodium ethane-thiolate [8], polyoxo-metalate [9] and so on. So far, Anthraquinone (AQ) is an agent often used in pulping, especially in alkaline pulping. During the cooking process, AQ works as a catalyst, repeating oxidation-reduction cycles to accelerate delignification and protect carbohydrates [10,11]. In this investigation, AQ was used in oxygen delignification of wheat straw soda pulp to study the role of AQ in oxygen delignification.

EXPERIMENTAL

Materials

The raw pulp with kappa number 22.4, viscosity 1043ml/g, and brightness 42.3% ISO was provided by a pulp mill in Shandong Province, P. R. China.

Oxygen Delignification

Oxygen delignification was carried out in a 1.5L stainless steel autoclave. Each autoclave was charged with 20g oven dried pulp. The pulp and alkali liquor were mixed in polyethylene plastic bags and transferred to the stainless steel autoclave completely. The autoclaves were then connected to an oxygen cylinder with a controlled pressure and then the autoclaves were placed in a 15L rotary electric digester to heat to the target temperature by water-bath and held the appointed time at the maximum temperature. The oxygen delignified pulp was washed with tap water, dewatered in a centrifuge and homogenized,then placed in sealed polyethylene plastic bags to balance water for further analysis, such as kappa number, viscosity, brightness.

Analytical Test

The kappa number and brightness of pulp were measured by Tappi standard methods. Viscosity was determined by the method described in Analysis and Detection Techniques for Pulping and Papermaking [12].

RESULTS AND DISCUSSION

Oxygen Delignfication Orthogonal Test

Oxygen delignification is a well defined industrial process. Variables affecting its performance include alkali charge, oxygen pressure, maximum cooking temperature, holding time at maximum temperature and so on. In this investigation, A four factor three level orthogonal design L9(34) was selected to evaluated the effects of alkali charge, oxygen pressure, maximum cooking temperature and holding time at maximum temperature. Levels of each factor were listed as follows:

alkali charge on oven dry pulp (%, as NaOH): 2, 3, 4;

oxygen pressure (MPa): 0.3, 0.5, 0.7;

maximum temperature (°C): 90, 105, 120;

holding time at the maximum temperature (min): 60, 85, 110.

Other conditions were: pulp concentration of 20%, MgSO4 amount of 0.5%, AQ amount of 0.1%.

The effects of alkali charge, oxygen pressure, maximum cooking temperature and holding time at the maximum cooking temperature on oxygen delignification with AQ addition of wheat straw soda pulp were present in Table 1. After oxygen delignification, final pH value was in the range of 9.40 to 12.69. Kappa number was in the range of 6.89 to 17.59. Yield was among 81.35% to 88.00%. Viscosity was among 757.24ml/g to 947.76ml/g and brightness was in the scope of 37.49 -54.43% ISO. In order to facilitate the analysis, the correlations and range of four factors which include alkali charge, oxygen pressure, maximum cooking temperature and holding time at the maximum cooking temperature were listed in Tables 2-5 respectively.

Table 1. Design and results of L9(34) orthogonal test of oxygen delignification with AQ addition

No.

1

2

3

4

5

6

7

8

9

Conditions

Alkali Charge (%)

2

2

2

3

3

3

4

4

4

Oxygen Pressure (MPa)

0.3

0.5

0.7

0.3

0.5

0.7

0.3

0.5

0.7

Temp. (℃)

90

105

120

105

120

90

120

90

105

Holding Time (min)

60

85

110

110

60

85

85

110

60

Results

pH Value

11.92

10.52

9.40

11.21

10.70

12.42

10.62

12.69

12.23

Yield (%)

83.22

85.74

87.69

86.06

88.00

86.13

81.35

87.05

83.92

Kappa Number

17.59

12.91

10.36

10.51

9.22

12.85

6.89

11.96

9.01

Viscosity(ml/g)

757.24

947.76

878.50

905.51

902.82

947.52

836.29

903.54

919.95

Brightness (% ISO)

37.49

44.44

49.27

46.54

49.34

43.33

54.43

44.13

48.80

Table 2. Range and variance analysis of L9(34) orthogonal test for yield

Factors

Alkali

Charge

(%)

Oxygen

Pressure

(MPa)

Temp.

(°C)

Holding

Time

(min)

K1

85.55

83.54

85.47

85.05

K2

86.73

86.93

85.24

84.41

K3

84.11

85.91

85.68

86.93

Range

2.62

3.39

0.44

2.53

From Table 1 and Table 2, it can be found that the range of alkali charge, oxygen pressure, maximum cooking temperature and holding time at the maximum cooking temperature to the wheat straw soda pulp yield after oxygen delignification with AQ addition are 2.62, 3.39, 0.44 and 2.53, respectively. Oxygen pressure is the most important factor than the other three and its range arrives up to 3.39. The effects of alkali charge and holding time to yield are very close. All these show that oxygen pressure has the greatest, while maximum cooking temperature has the minimal impact on pulp yield.

Table 3. Range and variance analysis of L9(34)orthogonal test for Kappa number

Factors

Alkali

Charge

(%)

Oxygen

Pressure

(MPa)

Temp.

(°C)

Holding

Time

(min)

K1

10.61

11.25

12.34

11.62

K2

11.44

11.30

11.32

11.19

K3

11.85

11.35

10.24

11.10

Range

1.23

0.10

2.10

0.52

From Table 1 and Table 3,it can be obtained that the range of alkali charge, oxygen pressure, maximum cooking temperature and holding time at the maximum cooking temperature to the wheat straw soda pulp kappa number after oxygen delignification with AQ addition are 1.23, 0.10, 2.10 and 0.52, respectively. Maximum cooking temperature is the most effective factor than the other three and its range arrives to 2.10. But the range of oxygen pressure is only 0.10, which is the least effective one to kappa number. Thus, maximum cooking temperature is the most effective factor for kappa number of soda pulp, and the least one is oxygen pressure. As the temperature increases, kappa number decreases dramatically. It means that the temperature of cooling is the most important factor that influences the process of delignification.

Table 4. Range and variance analysis of L9(34) orthogonal test for brightness

Factors

Alkali

Charge

(%)

Oxygen

Pressure

(MPa)

Temp.

(°C)

Holding

Time

(min)

K1

43.73

46.15

41.65

45.21

K2

46.40

45.97

46.59

47.40

K3

49.12

47.13

51.01

46.65

Range

5.39

1.16

9.36

2.19

From Table 1 and Table 4, it can be found that the range of alkali charge, oxygen pressure, maximum cooking temperature and holding time at the maximum cooking temperature to the wheat straw soda pulp brightness after oxygen delignification with AQ addition are 5.39, 1.16, 9.36 and 2.19, respectively. Maximum cooking temperature is the most effective factor and its range arrives to 9.36, while the least effective factor is oxygen pressure, with a range of 1.16.

Table 5. Range and variance analysis of L9(34) orthogonal test for viscosity

Factors

Alkali

Charge

(%)

Oxygen

Pressure

(MPa)

Temp.

(°C)

Holding

Time

(min)

K1

861.17

833.01

869.43

860.00

K2

918.62

918.04

924.41

910.52

K3

886.59

915.32

872.54

895.85

Range

57.45

85.03

54.97

50.52

From Table 1 and Table 5, after the L9(34) orthogonal test on oxygen delignification with AQ addition of wheat straw soda pulp it can be seen that the range of four factors to pulp viscosity are 57.45, 85.03, 54.97 and 50.52 respectively. Compared among these four factors, oxygen pressure is slightly more significant than other three to the pulp viscosity and its range is up to 85.03. Alkali charge, maximum temperature and holding time at the maximum temperature have the similar impact on the pulp viscosity, with a range from 50.52 to 57.45.

According to the range and variance analysis of the L9(34) orthogonal test on oxygen delignification with AQ addition of wheat straw soda pulp which were listed in Tables 2-5, the optimal oxygen delignification with AQ conditions are as follows: alkali charge on oven dry pulp (as NaOH) of 3%, oxygen pressure of 0.5MPa, maximum temperature of 120°C, holding time at the maximum temperature for 110 min, pulp concentration 20%, MgSO4 amount on oven dry pulp 0.5% and AQ amount on oven dry pulp 0.1%.

The Effect of AQ in Oxygen Delignification

The oxygen delignification was conducted with alkali charge on oven dry pulp (as NaOH) 3%, oxygen pressure 0.5MPa, maximum temperature 120°C and holding time at the maximum temperature for 110min, pulp concentration 20%, and MgSO4 amount on oven dry pulp 0.5%. Meanwhile, the oxygen delignification with AQ addition was performed in the same condition with AQ amount on oven dry pulp 0.1%. The results were shown in Table 6. (1: conventional oxygen delignification; 2: oxygen delignification with AQ addition)

Table 6. The results of oxygen delignification

No.

Yield

(%)

pH Value

Kappa Number

Viscosity

(ml/g)

Brightness

(%ISO)

1

83.86

9.46

7.88

755.66

61.14

2

86.04

9.38

7.85

745.00

60.04

From Table 6, it can be found that under the optimal conditions of oxygen delignification with AQ addition, the delignified pulp obtained was with the yield of 86.04%, final pH of 9.38, kappa number of 7.85, viscosity 745.00ml/g and brightness 60.04% ISO. Compared with the conventional oxygen delignification, the yield of oxygen delignification with AQ addition was increased by 2.18%. The pH value and kappa number after oxygen delignification with AQ addition were all close to those of conventional oxygen delignification.

Therefore, the pulp obtained after oxygen delignification with AQ addition was with the same kappa number to the conventional oxygen delignification, but a higher yield. Viscosity and brightness of pulp were also remained in high level.

CONCLUSIONS

Through orthogonal experiment and variance analysis, it can be concluded that the four factors, including alkali charge, oxygen pressure, maximum cooking temperature and holding time at the maximum temperature have great effects on the properties of oxygen delignified wheat straw soda pulp. Oxygen pressure has the greatest impact on yield and viscosity but little impact on kappa number and brightness. While maximum cooking temperature has the marked effect on kappa number and brightness but least effect on yield. In addition, holding time at the maximum temperature has little impact on viscosity.

Based on the orthogonal experiment test and variance analysis, optimized oxygen delignification with AQ addition conditions were determined as follows: alkali charge on oven dry pulp (as NaOH) was 3%, oxygen pressure was 0.5MPa, maximum temperature was 120°C, and the holding time at the maximum temperature was 110min. Other parameters such as pulp concentration, MgSO4 amount on oven dry pulp and AQ amount on oven dry pulp were kept at constant at 20%, 0.5%, 0.1%, respectively.

Under the optimal condition of oxygen delignification with AQ addition, the resulting delignified pulp with the yield of 86.04%, kappa number of 7.85, viscosity of 745ml/g, brightness of 60.04(% ISO) and final pH of 9.38 can be achieved. If compared with the conventional oxygen delignification, the yield of oxygen delignification with AQ addition increases by 2.18%. The pH value and kappa number after oxygen delignification with AQ addition are all close to the conventional oxygen delignification. Meanwhile, the viscosity and brightness are also remained in high levels.

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