Enzymes are more widely used in Canada and Europe, but North American mill interest is on the rise

Enzyme Use Can Lower Bleaching Costs, Aid ECF Conversions

HISTORICAL PERSPECTIVE. One of the first applications of enzymes to modify pulp properties was reported by Paice and Jurasek in 1984.¹ In this instance, a crude mixture of degradative enzymes, including xylanases, was applied to dissolving pulp to remove xylan. The resulting increase in alpha cellulose content was expected to allow the pulp to be derivatized to higher value products. Although no significant reductions in hemicelluloses were observed, a subsequent study by Finnish workers demonstrated that a similar enzyme treatment surprisingly reduced the requirement of chemicals needed to bleach the pulp.² Other work showed that the xylanase in the crude enzyme mixture was responsible for the pre-bleaching effect.³

The Enso Gutzeit mill in Imatra, Finland conducted the first mill trial which was reported in 1989.⁴ A reduction in chlorine used for bleaching allowed the AOX level to be lowered by over 50%. More notable was that the level of chlorinated dioxins formed was reduced below the detection threshold. The positive results were tempered, however, by a loss in pulp viscosity and yield. This problem was due to the presence of cellulases which contaminated the early enzyme preparations.

As a result of this work, much of the development efforts at the industrial scale have focused on the removal of cellulase contamination. Cellulase-free preparations are now available. Xylanases are produced using microbes that are either naturally free of cellulase-producing ability or have been mutated or genetically engineered to eliminate the cellulases. The most significant advancement is development of xylanases such as Ecopulp TX-200C that function in alkaline pHs and at high temperatures. Advances in production technology have drastically reduced production cost, making enzymes a sensible economic choice in bleaching. Enzymes are now available to work in all possible bleach plant scenarios.⁵

MECHANISM OF ACTION. Xylanase enzymes hydrolyze the xylan polymer that exists within pulp fibers. Xylans are intimately linked to cellulose and lignin, thus it follows that disruption of the xylan backbone affects their separation during bleaching. Xylanase was also shown to increase fiber wall swelling and in turn increase the speed of diffusion through the walls.⁶ Since it is believed that extraction of depolymerized lignin from pulp is a diffusion-limited process, xylanase treatment ultimately improves the extraction of lignin from pulp.⁷ Thus, for enzyme-treated pulp, subsequent bleaching stages are more efficient, and higher brightness can be expected. Other work suggests that if lignin covalently bound to xylan was made smaller by enyzme use, it would be more easily extracted.⁸

Another hypothesis that came from research was that xylanase enzymes catalyze the hydrolysis of xylan that has reprecipitated on the fibers during alkaline pulping.⁹ Removal of this xylan was thought to remove a physical barrier preventing the extraction of residual lignin. However, recent work has shown that pulp prepared under conditions that prevent xylan reprecipitation also responds well to the xylanase bleach boosting effect.¹⁰

BENEFITS OF ENZYME USE. Following the first application of xylanases in pulp bleaching, other studies have demonstrated the benefits of enzymes in the bleaching of softwood¹¹, ¹² and hardwood pulps.¹³ These benefits can be exploited in a bleach plant in several different ways, which have been described in a recent survey of xylanase use in Canadian mills.¹⁴

Brightness Gain. Xylanases improve bleaching chemical efficiency, leading to higher pulp brightness. This benefit is particularly attractive for mills with shortened bleaching sequences needing a high final brightness.

Chemical savings. If the xylanases increase brightness in the bleach plant, the mill can cut chemical use and still attain the original target. Saving money with lower chemical use is the primary driving force for mills to adopt enzymes. At the time of this writing, typical net savings range between US$1 to $3 per ton of pulp.

AOX and dioxin reduction. Using enzymes to reduce chlorine dioxide charge can also reduce AOX in the mill's effluent. This has been documented in laboratory¹⁵ and mill situations.^{16, 17} Mills still using chlorine can often reduce chlorine charge below the threshold level where chlorinated dioxin formation begins.^{4, 18}

C1O₂ limitation. The ability to generate an adequate supply of chlorine dioxide may be a bottleneck. Enzymes can effectively make more C1O₂ available since less is used per ton, leading to increased production.¹⁴

Other Benefits. Two Scandinavian mills are using enzymes to eliminate the D0 stage completely, while maintaining brightness targets. This has allowed the mills to send bleach filtrates prior to D1 stages back to recovery and resulted in partial closure of their water loop.

Xylanase can reduce the use of caustic soda, hydrogen peroxide, ozone and other chemicals such as activated oxygen.^{19, 20} Therefore, enzymes can be used for total chlorine free (TCF) bleaching and, occasionally a bleach sequence can be modified or stages eliminated.

MILL OPERATION. The most conventional method is to add xylanase to the brownstock pulp prior to the high-density (HD) tower (Figure 1).

Figure 1: Typical xylanase and acidification sites.

The enzyme reaction takes place in the tower and the treated pulp then passes into the bleach plant. Various ways to add enzymes have been used, including: spraying on the decker pulp mat; adding to either the decker repulper or discharge chute; adding into the stock of medium consistency pump leading to the HD tower; and adding directly into the HD tower. Xylanase has also been added later in the bleaching sequence, rather than to brownstock pulp.

The latest generations of alkali-tolerant enzymes require little, if any, addition of acid to adjust the pH. Earlier generations of enzymes had pH optima ranging from 5 to 6.5 and required acid addition to brownstock pulp. Instances of corrosion problems were seen when acid was incorrectly applied. New xylanases have higher pH optima and function optimally without pH adjustment.

The acid of preference by far has been sulfuric acid. However, with the development of alkaline xylanases, non-corrosive carbon dioxide is an excellent choice and also improves washer performance.²¹ The addition of acid prior to the D0 stage in ECF bleaching has also been shown to improve the performance of the D0 stage.¹⁴ This is because the higher acidity in the stage prevents decomposition of chlorine dioxide to chlorate,²² and because the chemistry of delignification with chlorine dioxide favors an acidic environment.²³ During trials the benefit of acid alone should be established.

Typical sites of acidification are also indicated in Figure 1. Acid added to the low consistency pulp prior to the washer vat provides the benefit of reducing pitch deposits; however, acid charges here tend to be much higher due to the large volume that must be treated. Acid can also be added on the washer shower bars washer or in the repulper discharge section. Experience has shown that prevention of corrosion must be a priority.

Retention in high-density towers has generally needed to be greater than one hour. However, new thermo-tolerant enzymes now permit higher brownstock thus shortening retention requirements.

MILL EXPERIENCES. Xylanase use in bleach plants is more common in Canada than the U.S. because of more stringent AOX levels, and thus a greater percentage of Canadian mills also produce ECF pulp. Reducing ECF cost is the main benefit of xylanases. A similar drive to reduce bleaching costs and AOX is occurring now that the guidelines for U.S. Cluster Rules have been implemented. Mill-based applications and benefits are discussed below.

Increased Brightness. A Western Canadian market pulp mill, producing 770 tpd to 880 tpd of softwood pulp using coastal woods, wanted higher brightness. The mill employed a DEoWD sequence with the water soak stage taking place in an unused hypo tower. Typical brightness produced was 86% ISO.

Spraying a dilute solution of SO₂, dissolved in water, onto the pulp mat of the decker to achieve a 6.2 pH during the enzyme trial acidified the pulp. The enzyme was diluted and hosed into the repulper section of the decker and the pulp was then pumped to the HD tower at temperatures between 55°C to 58°C.

Figure 2: Xylanase application for increased final brightness.

For example, an increase in pulp brightness was obtained using xylanese in Figure 2. On average, an additional 1.5% ISO brightness points were achieved using enzymes for the duration of the two-week trial. Use of enzymes in this simple application provides help to mills experiencing problems with brightness and who are looking to penetrate higher-brightness markets.

Chlorine Dioxide Reduction. Xylanase enzyme was applied at an Eastern Canadian mill to reduce chlorine dioxide use and its inherent costs. This mill produced 992 tpd of softwood market pulp of >90% ISO brightness using MCC pulping and a DEoDEpD sequence. The pulp was acidified to pH 6 to 7 with dilute sulfuric acid sprayed over the repulper of the decker. The enzyme was diluted and hosed into the discharge chute of the decker. Pulp was pumped using a medium consistency (MC) pump to the HD tower with a retention time of one to two hours at 59°C to 61°C. A control test period without enzyme was conducted for two weeks prior to enzyme addition. When enzyme was applied, an increase in the DEo brightness signaled that the enzyme was working. At this point, chlorine dioxide application in the Do stage was cut. The mill operated under these conditions for one week and was able to maintain the >90% ISO target brightness as well as strength properties. The reduction in chlorine dioxide, expressed as a reduction in kappa factor, can be seen in Figure 3.

Figure 3: Reduction in ClO2 use by application of xylanase.

During the enzyme application period, the kappa factor was reduced from an average of 0.165 to 0.12-0.13. The mill was able to reduce actual chlorine dioxide use by 11 lb per ton (5.5 kg per metric ton). This estimation was made using pre-trial control data gathered over a three-month period.

The reduction in chlorine dioxide use was also expected to reduce the AOX released in the final mill effluent. In a later trial, AOX discharges into the local river were monitored. An average of >0.35 kg/mt of AOX was measured during a one-month control period prior to enzyme addition. AOX in effluents released during the xylanase application period ranged between 0.08 kg/mt and 0.23 kg/mt (Figure 4) and returned to pre-trial levels after the enzyme was turned off.

Figure 4: AOX levels in final effluents during xylanase application.

In a similar application, a Western Canadian mill that has used enzymes since 1992 has recently reported a savings of 7.8% in bleaching chemical costs.24 ClO2 use was reduced 8.2% and caustic soda use 21%. Enzyme use lowers the kappa factor, which reduces the acidity of the pulp carried into extraction stages, leading to lower caustic soda use in extraction.

In another recent example, a Finnish mill produced up to 600 tpd of softwood pulp bleached using an ECF sequence. This mill has been running full-time on xylanase for the last two years. Enzyme is applied at 55°C for two hours on brownstock batch pulp. As can be seen in Figure 5, the mill trial resulted in a 23% reduction in chlorine dioxide (expressed as total available chlorine) application and experienced better overall brightness. This mill currently reports a 15% chlorine dioxide savings. Several of the half-dozen mills running full-time in Finland also have bulk storage facilities for enzymes. The majority of these also use oxygen delignification.

Figure 5: Effect of xylanase use on final pulp brightness and applied chlorine dioxide use.

Xylanase Applications at High pH. Recently, a high pH tolerant xylanase has been developed. This enzyme allows enzyme use with minimal or no acidification of the brownstock pulp. The following mill data shows the application of this enzyme (Ecopulp TX-200C).

No acidification of the northern softwood brownstock pulp was done prior to the bleach plant. Thus, the pH for the enzyme reaction in the high-density tank was 10.0 to 10.3 with a retention of about one hour at 65°C. A reduction in the bleaching kappa factor of approximately 15% was achieved (Figure 6).

Figure 6: Mill trial with xylanase applied at high pH.

Current Use. As is the case for most new technologies, the rate of adoption of enzymes has been relatively slow. Various enzymes have been available for almost ten years. Many trials have been conducted over the years but few have translated into full-time application on the mill level. In the last year, new high-temperature and high pH enzymes have been developed at considerably lower cost. Thus, interest has been rekindled as is shown by a significant increase in the number of trials and planned trials that are taking place. In the case of Ecopulp TX-200C, which has been available in Scandinavia for a year and half, there are nine mills using the enzyme (Table 1).

Presently there are three Canadian users and there is a large list of mills in Canada and the United States preparing for trials.

David J. Senior and Janice Hamilton are with PCI Chemicals Canada Inc.; Pasi Taiplus and Jaana Torvinen are with Rohm Enzyme Finland Oy.