Use of Xylanase in the Totally Chlorine-Free Bleaching of Eucalyptus

Chapter 17

Use of Xylanase in the Totally ChlorineFree Bleaching of Eucalyptus Kraft Pulp

Downloaded by STANFORD UNIV GREEN LIBR on October 8, 2012 | http://pubs.acs.org Publication Date: November 21, 1996 | doi: 10.1021/bk-1996-0655.ch017

M. B. Roncero, T. Vidal, A. L. Torres, and J. F. Colom Especialidad Papelera y Gráfica, Escuela Técnica Superior de Ingenieros Industriales de Terrassa, Universidad Politécnica de Cataluña, Colόn, 11 E-08222 Terrassa, Spain

Environmental pressures are forcing the pulp and paper industry to develop new technologies that reduce or eliminate the presence of various contaminants in bleaching plant effluents. Oxygen delignification techniques, replacement of elemental chlorine with chlorine dioxide, ozone, hydrogen peroxide and new bleaching agents as well as the use of xylanase enzymes for biobleaching, reduce or eliminate the production of chlorinated organic substances. This paper compares the sequence XOZP with OZP in the bleaching of Eucalyptus globulus kraft pulps. It has been studied the influence of enzymatic treatment on the consumption of bleaching agents: ozone and hydrogen peroxide. Chemical, physical, optical and refining properties of pulps, as well as COD and colour of effluent are studied as well. The xylanase treatment is positive and it is possible to manufacture fully bleached pulps at high brightness and viscosity without using chlorine compounds at a low ozone and hydrogen peroxide consumption.

Reducing the environmental effect of mill effluents has been a major concern for the pulp and paper industry in recent years. Chlorine-free bleaching represents an alternative for eliminating undesirable chlorinated compounds. Enzymatic prebleaching of pulp opens new possibilities for non-polluting rearrangement of bleaching processes. Especially the use of xylanases for improving the bleachability of kraft pulp has become important (1-5). Xylanase enzymes are liquid solutions of protein which hydrolize a portion of the xylan in the pulp. There has been recent evidence of direct brightening but in general it is considered that enzymes decrease the subsequent amount of oxidative chemicals required to bleach the pulp (3, 6 -7). The incorporation of xylanase enzymes into a pulp mill bleaching sequence is simple and economically feasible (8). The effectiveness of xylanases for totally chlorine-free bleaching is currently being studied using peroxide and ozone. This paper

0097-6156/96/0655-0219$15.00/0 © 1996 American Chemical Society

In Enzymes for Pulp and Paper Processing; Jeffries, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.

ENZYMES FOR PULP AND PAPER PROCESSING

220

describes laboratory results of a enzyme treatment for preventing dioxin formation during the bleaching of pulp. X O Z P and OZP sequences in the bleaching of Eucalyptus globulus kraft pulps are compared. Chemical, physical, optical and refining properties of pulps, as well as COD and colour of effluent are also studied. The results are compared with the ECF (XODPD and ODPD) sequences (9 - 18).


Materials and Methods Pulp. A n industrial Eucalyptus globulus kraft pulp of E N C E mill (Pontevedra, Spain) with a kappa number of 13.6, brightness of 31.7 % ISO and viscosity of 1,090 cm /g was used. The bleaching sequences have been performed in the laboratories of the E.T.S.I.I. of Terrassa (Spain) and they were: OAZ0.4RP, OAZ0.S5RP, XOAZ0.4RP (TCF sequences) and OD1PD2, XOD1PD2 (ECF sequences). In order to evaluate the physical properties, the fully bleached pulps were refined with a PFI mill and evaluated according to standard methods. 3

Terminology of Bleaching Stages. The terminology of bleaching stages used in this paper is as follows: X : Xylanase pretreatment (Pulzyme HC); Ο : Oxygen delignification; A : Treatment with oxalic acid without final wash;-Z : Ozone bleaching stage; R : Post-treatment with sodium borohydride; Ρ : Hydrogen peroxide bleaching stage; D : Chlorine dioxide bleaching stage. Conditions in Bleaching Stages. Three TCF sequences (OAZo 4RP, OAZ0.55RP, XOAZ0.4RP) have been carried out. The OAZ0.4RP and XOAZ0.4RP sequences were to study the effect of xylanase pretreatment, when a same charge is used in the ozone bleaching stage. The OAZ0.55RP has been performed to study the influence of enzymatic treatment on the consumption of bleaching agents (ozone) at the same final brightness. Conditions in bleaching stages are shown in Table I. Treatment with oxalic acid without final wash before Z-stage is carried out because this additive prevents viscosity loss during ozone bleaching, and increases brightness values, in comparison with blank tests (19). During ozone bleaching some carbonyl groups are created on the cellulose chains. These groups are responsible for the rapid depolymerization of the cellulose chains in a subsequent peroxide stage (alkaline stage). It is necessary to reduce these groups and to make the cellulose quite stable during a Ρ stage. Sodium borohydride is used (19-22).

Results and Discussion Effect of Xylanase Pretreatment on Pulp Properties. Results of brightness, viscosity and kappa number after oxygen delignification stage for enzymatic treated and untreated pulps are presented in Table Π. After O-stage, XO-pulp has higher brightness and viscosity, and lower kappa number than O-pulp. Addition of a xylanase pretreatment (Table III) to an OAZ0.4RP bleaching sequence reduced the final kappa


RONCERO ET AL.

Use of Xylanase in TCF Bleaching

Table I. Conditions in bleaching stages


STAGE

Χ 10 60 120 7-8 500 -

Consistency, % o.d.p. Temperature, °C Reaction time, min. Initial pH Enzyme, EXU/kg NaOH, % o.d.p. MgS0 -7H 0, %o.d.p. 4

-

2

Ο

A

R

Ζ

10 10 110 amb. 10 60 1.5 0.5

Ρ

10 10 amb. 85 60 180 10.5 1.8 0.2

40 amb. -

-

6

-

-

-

-

-

2 -

0.4 or 0.55 35

Flow Ο3/Ο2,1/h

-

-

-

150

-

-

NaBHt, % o.d.p. N a C 0 , % o.d.p.

-

-

-

-

0.5

-

-

1 -

2

no

no

no

no

yes

0 pressure, kg/cm

2

2

Oxalic acid, % o.d.p. Ozone, % o.d.p. In. Conc. 0 , mg/1 3

2

3

H 0 , % o.d.p. Acid wash with C 0 2

no

2

2

Table II. Effect of xylanase pretreatment XO

Ο

Kappa number

7.1

8.4

Brightness, % ISO

53.9

52.1

956

912

3

Viscosity, cm /g

Table III. Final results of pulp properties of studied T C F sequences Sequence Kappa number Brightness, % ISO 3

Viscosity, cm /g Residual H 0 , % 2

2

OAZ0.4RP*

OAZ . RP**

XOAZ0.4RP*

1.3

0.6

0.7

87.3

90.3

90

813

793

807

0.6

0.6

0.99

0

55

*: Z-Stage with 0.4% ozone; **: Z-Stage with 0.55% ozone


222


Table IV. Comparison physical properties of ECF and TCF sequences for 3,000 and 4,500 PFI revolutions 3,000 PFI revolutions


Sequence

XOAZ RP ODPD XODPD

OAZoOlP

OAZ055RP

25

27

25

25

23

155

155

148

153

148

38

38

38

42

40

1.56

1.56

°SR WRV (%) Power consump., W*h

04

Bulk, cm /g Permeability B., ml/min Tensile Index, N-m/g

1.65

1.63

1.58

1,685

1,428

1,845

2,391

2,779

60.3

62

55.9

72.7

61.7

Folding Endurance

3.14

3.28

3.01

3.09

2.95

10.7

11.1

12.5

11.8

13.1

4.14

4.3

4.05

4.43

3,83

Sequence

OAZ04RP

OAZo RP

XOAZ0.4RP

°SR WRV (%)

33

36

32

33

30

166

160

160

158

154

56

56

57

62

61

1.6

1.55

1.55

1.50

1.51

565

595

960

1,232

1,682

65.4

73.2

67.4

82.3

68.9

3.56

3.52

3.39

3.18

3.15

-

11.6

12.2

11.0

12.6

4.99

4.47

4.69

4.39

3

e

2

Tear Index, mNm /g Burst Index, kN/g 4,500 PFI revolutions

e

Power consump., W h 3

Bulk, cm /g Permeability B., ml/min Tensile Index, N-m/g Folding Endurance 2

Tear Index, mN*m/g Burst Index, kN/g

4.46

55

ODPD XODPD

Table V. Comparison of pulp properties (ECF) XODPD

ODPD

Kappa number

0.84

0.67

Brightness (% ISO)

88.3

87.7

923

868

Sequence

3

Viscosity (cm/g)


17. RONCERO ET AL.

223


by almost half (from 1.3 to 0.7) and icreased brightness by almost three points (87.3 to 90). In a side-by-side comparison, it was necessary to increase the Z-stage bleaching from 0.4% to 0.55% in order to obtain an equivalent kappa and brightness. Even so, the xylanase-treated pulp showed higher viscosity. Diminutions of kappa number and viscosity, and increase of brightness follow the same pattern in these three TCF pulps. OAZ0.4RP and XOAZ0.4RP have the same viscosity (about 800 cm /g), but brightness is three points higher with enzymatic treatment (X) and the kappa number is lower in XOAZ0.4RP. At the same reagents consumption, xylanase treated pulp has higher brightness than when xylanase is not applied. OAZ0.55RP (untreated pulp) and XOAZ0.4RP (enzymatic treated pulp) have the same viscosity and brightness, but a higher ozone charge is required in untreated pulp (0.55 % 0 ) than in treated pulp (0.4 % 0 ). XOAZ0.4RP has a lower hydrogen peroxide consumption, so it is posible to reduce H2O2 charge in this sequence, with no impact on the final results. So, at the same final brightness, the enzymatic treated pulp can save around 27 % in ozone charge. Alternatively savings are possible in hydrogen peroxide charge. We can conclude that the introduction of xylanase has positive effects and makes it possible to produce high bleached pulp and to reduce chemicals consumption without using molecular chlorine.


3

3

3

Effect of Xylanase Pretreatment in Physical and Refining Properties.

Power

consumption is the same in the three pulps (Table IV), but °SR and W R V are higher for untreated pulps (OAZ0.4RP, OAZ0.55RP) than for the treated pulp (XOAZ0.4RP) due to the presence of more hemicelluloses (Figures l a and lb). Thus, refining is easier with untreated pulp. Results of bulk property do not show any possible effect of enzymatic pretreatment about this characteristic. For treated pulp the permeability and tear properties (Figures l c and Id) are higher than in the case of untreated pulp, because the papers of treated pulp have a more open structure, due to the lesser content of hemicelluloses, and as a consequence less fiber bonding. At the same reagents consumption, tensile and burst properties make no difference when xylanase is applied. But at same brightness, these properties are higher than when enzymatic treatment is not applied. In burst index there is a slight difference, no very important. Results of folding endurance are similar in three pulps, but results are slightly lower in xylanase treated pulp. Perhaps, the main differences in refining and physical properties are related to the quantity of hemicelluloses in pulp fibers. Comparison of Results with ECF Sequences. Results of TCF (OAZ0.4RP, OAZ0.55RP, XOAZ0.4RP) sequences are compared with E C F (XODPD, ODPD) sequences. Tables ΙΠ and V present the results of TCF and ECF bleached pulps. The same conclusions are valid in TCF and E C F sequences. At the same reagent consumption levels for E C F sequences, higher brightness and viscosity are achieved with enzymatic treatment. The use of xylanase in Eucalyptus globulus kraft pulp bleaching improves bleachability in subsequent stages. Physical and refining properties of TCF and E C F pulps for 3,000 and 4,500 PFI revolutions, are shown in Table V . Because the power consumption is higher in ECF pulps, refining is easier with TCF, for about the same °SR and WRV. The same conclusions about physical properties for



224


Figure la. °SR vs. PFI revolutions of TCF sequences.

Figure lb. W R V vs. PFI revolutions of TCF sequences.



17. RONCERO ET AL.


Figure le. Permeability vs. PFI revolutions of TCF sequences.

Figure Id. Tear Index vs. PFI revolutions of TCF sequences.


225

226



TCF sequences hold true for ECF sequences. Permeability and tear properties are higher for treated pulp (XODPD) than untreated pulp (ODPD), while for tensile and burst properties the effect is opposite. We conclude that the physical properties are similar and very high for the TCF and ECF sequences studied, so we can obtain high bleached TCF pulps comparable to ECF pulps without resorting to chlorine compounds. Effect of Enzyme on Effluents. The effluents from ECF and TCF bleaching were characterized by determining color and COD. In the case of COD, the effluent from enzymatic treated pulp had approximately twice the total COD value that untreated pulp. The enzymatic treatment accounted for 50% of total COD in the TCF sequence and 51% in the ECF sequence. The higher COD occurs because xylanase hydrolyzes a fraction of the hemicellulose xylan in the pulps. This releases carbohydrates and some lignin. In the ECF sequences, the X and Ο stages contributed to the highest COD values: XO-stage represents a 85% of total COD in treated pulp and O-stage represents a 70% of total COD in untreated pulp. The D (chlorine dioxide) and Ρ (hydrogen peroxide) contibuted to the lowest total COD value. In the TCF sequences, the Χ, Ο and R stages contributed to the highest COD values, whereas the Ρ stage presents a small percentage of the COD value. In the case of color, the effluent from enzymatical treatment stage contributed to about 28% in ECF and 32% in TCF sequences. These values of color can be explained by a probable bad washing of the brown stock. The X , O, R and Ζ stages contributed to the highest color values in effluents. Otherwise, P-stage presents the lowest color value, because of the presence of hydrogen peroxide residual in the effluent, which works as a bleaching agent.

Conclusions Xylanase pretreatment (X) of a TCF bleached pulp following a sequence X O A Z R P is effective and makes it possible to produce high bleached pulp without using chlorine compounds. Xylanase improves brightness and viscosity and lowers the final kappa number when the same chemical charges are applied. When using xylanase it is possible to reach a predetermined brightness with a significant saving of chemicals. In the case studied the evaluated savings are 27% in ozone charge and 28% in hydrogen peroxide charge for a final brightness of 90% ISO. In TCF pulps, physical properties are slightly different for the treated and untreated pulps. At the same brightness it is possible to reduce chemicals consumption, retaining the strength properties. Refining is easier with untreated pulp due to the presence of more hemicelluloses. The effects of xylanases on physical and optical properties are similar for TCF and ECF sequences. The effluent from enzymatic treated pulp had approximately twice the total COD value that untreated pulp, because xylanase treatment is used to hydrolyze a certain fraction of the hemicellulose xylan in the pulps. This results in the release of carbohydrates and some lignin linked to these carbohydrates, which contributes to higher COD in effluents.


17. RONCERO ET AL.


227

The effluentfromenzymatic treated pulp contributed to about 28% in ECF and 32% in TCF sequences, of the total color value.

References


1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22.

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Use of Xylanase in the Totally Chlorine-Free Bleaching of Eucalyptus

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