Enzymes for Pulp and Paper Processing - ACS Publications

Chapter 3

Survey of Mill Usage of Xylanase 1

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J. S. Tolan , D. Olson , and R. E. Dines 1

Iogen Corporation, 400 Hunt Club Road, Ottawa, Ontario K1G 3N3, Canada Fletcher Challenge Canada, Mackenzie, British Columbia, Canada R. E. Dines Associates, Vankleek Hall, Ontario, Canada

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Downloaded by UNIV LAVAL on July 11, 2016 | http://pubs.acs.org Publication Date: November 21, 1996 | doi: 10.1021/bk-1996-0655.ch003

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Xylanase enzymes have been used by mills since 1991 to enhance the bleaching of kraft pulp. Canadian mills treated 750,000 tonnes of pulp with xylanase in 1994, representing 8% of Canada's bleached kraft pulp production. This paper describes the results of a survey of the Canadian bleached kraft mills' experiences with xylanase enzymes.

Pulp Production and Bleaching. There are two broad categories of paper produced in roughly equal quantities around the world: high quality paper, which includes paper for writing, books, wrapping, packaging, and other specialized uses, and lower quality paper such as newsprint and that used for telephone directories. The high quality paper is produced from pulp which is primarily made by chemical processes while the low quality paper is primarily based on mechanical action. A general review of pulp and paper manufacturing has been published (/). This chapter is concerned with processing to make high quality paper. Most of the pulp used for high-quality paper is manufactured by the kraft process and is known as kraft pulp. In the kraft process, wood chips are cooked in an alkali cooking liquor to break up thefiberintegrity, solubilize the lignin around the fibers, and produce a brown pulp, known as brownstock. The brown colour is from the lignin remaining in the pulp that cannot be removed in the cooking process without destruction of the pulp fibers. Some brownstock is sold commercially without further chemical processing, such as for brown paper bags used in grocery stores. However, the majority of kraft pulp is bleached to varying degrees to improve its whiteness, strength, and other desired properties that depend on the finished product requirements. For example, writing paper is very white, and the bleaching is carried out to achieve the desired whiteness. Wrapping papers must be strong; in this case, the bleaching is carried out to the point where the strength of the paper is increased sufficiently. Photocopy paper is bleached to the point where it efficiently absorbs ink; this corresponds to a high degree of whiteness. A good review of the manufacture of kraft pulp has been published (2).

0097-6156/96/0655-0025$15.00/0 © 1996 American Chemical Society

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


26

ENZYMES FOR PULP AND PAPER PROCESSING

The bleaching of the pulp is carried out in three to six consecutive chemical treatments, called stages. The layout and conditions of the stages differs between mills, but through the early- 1980's there was a typical pattern of bleaching stages (3). In almost all mills, the first bleaching stage was carried out with chlorine (abbreviated as "C"). Chlorine gas was dissolved in water and then contacted with the pulp as the pulp flowed through a sealed pipe and up a tower, known as the chlorination tower. A typical usage of chlorine was 5% on weight of pulp, and chlorination was carried out a pH of about 2. The stage was designed such that all of the chlorine reacted by the time the pulp reached the top of the tower. The pulp was then washed, and the net result of the chlorination stage was the removal of about 75% of the lignin in the brownstock. Recently, mills have used chlorine dioxide ("D") or a mixture of chlorine and chlorine dioxide ("C D ") in the chlorination stage. In some mills, oxygen ("O") is used before the chlorination stage. The second stage was carried out by adding sodium hydroxide to the chlorinated pulp to adjust its pH to about 10 and heating the pulp to about 70°C for 1 hour, then washing. This is the extraction stage ("E") and it results in the removal of lignin to the point where about 10% of the brownstock lignin remains. This is still enough lignin to maintain a brown colour in the pulp. Recently, mills have included oxygen ("EO") and hydrogen peroxide ("EOP") in the first extraction stage. The third stage is another acidic oxidation stage analogous to the chlorination stage, but this is carried out with chlorine dioxide and known as the chlorine dioxide stage ("D"). Chlorine dioxide is a gas that is unstable and generated on-site at a kraft mill. It was discovered in 1946 to be more effective as a whitening chemical than chlorine. Chlorine dioxide is used at a level of about 1% on pulp, at pH 4 and 80°C. The stage lasts three hours and is carried out in a tower, from which the pulp exits and is washed with water. At the end of this stage, the pulp is off-white and used for some applications, in which it is referred to as semi-bleached pulp. The fourth and fifth stages are extraction (E) and chlorine dioxide (D) stages, as described above. After the fifth stage, the pulp is highly white and sold as market pulp. The interest by the public in environmental implications of processes and the detection of chlorinated compounds in food packaging in the mid-1980's prompted pressure on the industry to decrease or eliminate the use of chlorine in bleaching. Some of the technologies that have been implemented to accomplish this have been the use of modified cooking processes to remove more of the lignin, the use of an oxygen reaction stage before the chlorination stage, the substitution of chlorine dioxide for chlorine in the chlorination stage, and the addition of hydrogen peroxide in the extraction stages. All of these technologies have been implemented by some mills in the past decade. 5O

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Xylanase Enzymes for Bleaching. One new technology has evolved to decrease the use of chlorine for bleaching, and that is treatment of the pulp with xylanase enzymes. Xylanase is used in a mill by adding it to washed brownstock and allowing the enzyme to act on the pulp in the brownstock storage tower prior to the chlorination stage. The enzyme is supplied to the mill as an aqueous solution of protein and


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TOLAN ET AL.

Survey of Mill Usage of Xylanase

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additives such as stabilizers and preservatives. The commercially used xylanases have primarily been of Trichoderma and Bacillus origin. Because of the competitive nature of commercial processes, there have been few published reports of the ongoing industrial use of xylanase enzymes since the initial uses of xylanase in 1992. The use of xylanase enzymes to enhance the bleaching of the pulp was first reported in 1986 (4). Thefirstmill trials of xylanase in North America were carried out at Port Alberni in 1991 and ongoing usage in some mills started in 1992. Mill applications of xylanase have been widely reported (5-JO). Mill Survey. Given the increasing usage of xylanase for enhancing bleaching, a survey of mills regarding their xylanase usage was originated as a project by the Bleaching Committee of the Canadian Pulp and Paper Association. The objectives of the survey were to determine: 1. The major areas of effort of bleaching plants in Canada; 2. The extent to which mills are trialing and using the prominent additives that enhance bleaching: hydrogen peroxide (a common bleaching chemical), anthraquinone (a pulping additive that can decrease the Kappa number and hence bleaching chemical usage), and xylanase; 3. The reasons that motivated mills to trial, use, or discontinue xylanase; 4. The benefits and problems that have been observed with enzyme treatment; 5. The improvements that are desired in xylanase technology for bleaching. The survey was carried out by telephone in September 1994 by co-authors R. Dines and D. Olson. All 40 Canadian bleached kraft mills and two bleached sulfite mills were contacted and responded to the survey. About two-thirds of the respondents were the mill representatives to the Canadian Pulp and Paper Association Bleaching Committee, with the remainder from mill technical and production departments. The results are reported in the following sections. Mill Effort. For the past two years more of the effort in the bleach plants has been focused on decreasing chlorine usage than on other objectives. Of the nine bleaching objectives surveyed, the mills have spent the most effort decreasing A O X (by decreasing chlorine usage), followed closely by meeting customer demands (which in many cases was decreasing chlorine usage) and eliminating chlorine gas (Figure 1). These objectives were followed in effort by decreasing off-grade pulp, decreasing BOD, and cutting costs. The least effort was devoted to increasing throughput (not surprising in a recession), eliminating dioxin (which was largely completed more than two years ago) and converting to TCF, which has not occurred to any great extent yet. Over the next two years, the bleach plant personnel anticipate a reprieve from the many demands and changes that they have been faced with. Effort on all of the objectives is expected to decrease, except cutting costs. Perhaps the mills will be trying to optimize the existing processes. Usage of Additives to Enhance Bleaching. Mills have many bleaching technologies available to meet the objectives described. Some of these technologies involve additives of relatively low capital cost (usually less than $500,000). The survey considered four such additives: 1) xylanase, 2) anthraquinone, a digester additive that can act indirectly to improve the bleaching, 3) hydrogen peroxide at low levels of less than 5 kg/t, and 4) hydrogen peroxide at high levels of greater than 5 kg/t. The results are shown in Table ι.


Jeffries and Viikari; Enzymes for Pulp and Paper Processing ACS Symposium Series; American Chemical Society: Washington, DC, 1996. 2

Past 2 yrs Next 2 yrs

WÊÊ IB

Figure 1. Relative bleach plant effort in several areas, on a scale of zero (no effort) to four (extreme effort).

TCF

Eliminate Dioxin

Increase Throughput

Decrease Cost

Decrease BOD

Decrease Offgrade

Eliminate C12

Satisfy Customers

Lower A O X

1

Relative Effort


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TOLAN ET AL.


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Table I. Mill Trials and Usage of Four Additives


Additive

Number ofMills

Number ofMills Using

Low Peroxide

36

32

High Peroxide

19

11

Anthraquinone

19

7

Xylanase

18

6

Low peroxide is extremely widely used, with regular usage in 80% of Canada's mills. The most common usage is 2 to 3 kg/t of peroxide in the first extraction stage. Perhaps surprisingly, the other three additives have similar trial and usage experience, with around 50% of the mills having trialed high peroxide, anthraquinone, and xylanase, and about one-third of the trials resulting in usage on a regular basis. Clearly the mills do not have a consensus on the additives (beyond low levels of peroxide). This variety of mill practices is no doubt a reflection of the effects of pulp properties, bleaching sequence, and bleaching objectives on the relative benefits of these additives. These results might bode well from the point of view of future xylanase usage, in that xylanase is the most recent of these bleaching technologies and thus might be the most likely to improve with mill and supplier experience. Xylanase Enzyme Usage. Of the 42 bleaching mills surveyed, 18 have run xylanase trials, and six are regular users of the enzyme on at least 20% of their pulp. This accounts for 750,000 tonnes of enzyme treated pulp produced per year in Canada, which is 8% of the total bleached pulp production. All of the enzyme usage is in kraft mills, as xylanase treatment is not generally effective on sulfite pulp. The bleaching sequences of the 10 grades which use enzyme treatment are listed in Table Π. All six of the mills bleach softwood, and one also treats hardwood. Two of the mills have oxygen delignification, which is interesting because only three mills with oxygen delignification have run xylanase trials. All six of the mills have five stage bleach plants. The bleached brightness targets for most of the grades are 88 to 89.5 ISO Brightness, which is 1-2 points lower than typical for Canadian mills. This is somewhat surprising in that the bleaching benefit of xylanase is generally thought to increase with increasing brightness target. Motivation for Xylanase Trials and Usage. For regular users of xylanase, the leading motivations for xylanase use are increasing pulp throughput (such as when the C10 generator is the mill's bottleneck) and obtaining a marketing advantage (Table III). Cost savings, pulp quality, and environmental benefit were cited in decreasing order. The mills who have tried xylanase and decided not to use it regularly have different motivations. For these mills, the leading reasons xylanase trials were run was to meet environmental targets and to obtain a cost savings in bleaching chemicals. 2


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ENZYMES FOR PULP AND PAPER PROCESSING

Table IL Profile of Regular Xylanase Users


Bleaching Sequence

Brightness (ISO)

Grade

Furnish

1

1

SWD

D (EOP)DED

89.0

2

2

SWD

(C D )(EO)DED

91.5

3

SWD

D (EO)DED

91.5

4

HWD

D (EO)DED

89.0

3

5

SWD

OD (EO)D(EP)D

89.5

4

6

SWD

D (EOP)DED

89.5

7

SWD

(C D )(COP)DED

89.5

5

8

SWD

D (EOP)DED

89.5

6

9

SWD

OD (EOP)DED

89.0

10

SWD

O(C D )(EOP)DED

89.0

Mill

100

50

50

100

100

100

100

45

55

100

100

50

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Table ΙΠ. Motivation for Xylanase Usage (Scale: 0=low to 4=high) Factor

Mills Using Xylanase

Mills Tried, Not Using

Increase Throughput

3.6

2.7

Marketing Advantage

3.3

2.9

Cost Savings

3.2

3.4

Pulp Quality

3.1

2.2

Environmental Advantage

1.8

3.5

Enzyme Treatment Operations. The mills have added the enzyme to the pulp in a wide variety of locations between the brownstock decker and the brownstock storage tower, with no one location emerging as a clear preference (Table IV). The variety of treatment locations results from the differences in mill equipment and layout and also, perhaps, to the newness of the technology. Sulfuric acid was used to adjust the pH of the alkali brownstock in all but one of the mills, with the other using chlorination filtrate. Several mills tried sulfurous acid and carbon dioxide for pH adjustment but abandoned these in favour of sulfuric acid. The typical range of pH for xylanase treatment is pH 5.0 to 8.5 . There is significant concern that acidification of the brownstock changes the demand for bleaching chemicals. Of the sixteen mills that addressed this issue, six felt


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TOLAN ET AL.

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Table IV. Enzyme Treatment Locations Location

Number of Mills

Decker Repulper Chute Stock Pump


Hi-Density Storage that adjusting the pH decreased chemical usage, two felt it increased it, and eight detected no effect. The mills that reported a decrease in chemical usage with acidification observed that adding acid to the brownstock decreased the pH to the D100 stage and improved its performance. This is particularly true with low Kappa factor (

Enzymes for Pulp and Paper Processing - ACS Publications

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