.
By Rosa M. Covarrubias
In the coming years, stricter environmental regulations, global competition and new-market driven demands for the tissue properties will induce changes in production technology and processes in tissue production. Enzymes are among the new technologies that can help the tissue industry to reach these new objectives.
Enzymes have been used in the paper industry for many years. Typical applications include starch modification, microbiological deposit control, stickies control, and others. While these applications have been successful, there are a number of other cases where enzymes could potentially be very useful. One interesting application of enzymes is to modify cellulose fibers. Certain enzymes can be effective to change fibre characteristics, and this application is just now proving useful in the paper industry.
FIBRE MODIFICATION
Wood fibres are mainly composed of cellulose and hemicellulose microfibrils encrusted in lignin-carbohydrate matrices. They are multilayered structures that can have internal delamination and external fibrillation after chemical and/or mechanical processing.
Wood pulp can be treated with enzymes, and some of the cellulose in the fibre is hydrolysed. This biochemical treatment reduces the amount of mechanical treatment needed to reach the desired fibre properties. Less mechanical action and less energy are required. Since refining requires significant energy input, as well as capital investment for equipment, facilitating the refining process could provide numerous benefits, including stronger paper, more use of recycled paper, elimination of other chemical additives, reduced energy usage, and improvement in various tissue properties.
Beating and refining are mechanical processes that can enhance fibrillation and internal fibre bonding. Properly applied enzymes can enhance fibre strength, reduce refining time, and increase inter-fibre bonding though fibrillation. The main challenge in using enzymes to enhance fibre bonding is to increase fibrillation without reducing pulp viscosity. Viscosity decreases when cellulases cleave cellulose chains lowering the degree of cellulose polymerisation and destroying the fibre integrity.
The exact mechanism by which enzymatic pulp fibrillation occurs is still not understood, so basic research is still underway in this area.
By treating the fibre with the enzyme, the refiner curve will move towards the lower energy side of the untreated refiner curve. As a result, tissue makers will choose whether they want to increase the bonding strength of the fibre, by maintaining the applied energy, or if they want to maintain the bonding, but reduce the applied refiner energy. Figure 2 shows a graphical representation of the decision-making process.
CURRENT APPLICATIONS
Mill applications prove the benefit of this technology, with a variety of advantages seen: increased paper sheet strength, which in turn allows manufacture of paper with less fibre, giving a saving in raw materials (and natural resources) used. Also, with this increased strength more recycled fibre can be used. Additionally, less petroleum-based chemicals are required to give strength to the sheet. In some cases enzymes can be used in place of purchasing additional equipment for refining. Where the product directly contacts food, enzymes can be preferred because of their nontoxic nature. Enzymes are a very attractive green chemistry. They are produced from renewable resources and are completely recyclable.
Below are some examples of the applications we currently have using fibre modification enzymes in tissue and towel.
Example 1
Application in recycled paper for napkins. This North American mill uses primarily recycled fibre. Historically at this mill there has been difficulty attaining the sheet strength objectives due to problems getting sufficient refining. The problem is related to the type of refiners used and excessive wear of the refiner plates. One solution is to purchase virgin pulp to reach the strength specifications. The cost of purchased long fibre can be a problem, and even with the long fibre strength specs are not always met.
Trials with conventional polymer-based dry strength chemicals gave only marginal results. Prior to mill evaluation of the enzyme, laboratory testing was done with various enzymes to determine most effective product to use. A certain enzyme may be very efficient on one fibre type but not on others. The best performer was selected and mill evaluations started. The enzyme was applied at the pulper at dosages ranging from 0.5–1.0 kg/ton, which gave about three hours of contact time with the fibre prior to refining.
During the first trial, an application rate of 0.5 kg/ton was used. After starting up the trial, tensile strength got progressively better, finally reaching a maximum which was approximately 12% above the pre-trial level. While 6% may not seem significant, at the same time, the mill removed 50% of the total softwood kraft being used. The 6% strength increase was maintained at this level of softwood kraft usage.
In subsequent trials to optimise the application of the product, slightly higher dosages of enzyme were used, up to 1.0 kg/ton. With this dosage, the strength target was still achieved, and two additional benefits were realised. First, the mill completely eliminated the usage of softwood kraft for this grade. Additionally, the mill was able to cut its refiner energy applied by as much as 15%.
Table 1 gives some details on the beneficial results. A number of benefits are documented including:
- Elimination of virgin pulp: it is possible to use recycled fibre only. This is a large savings for the mill, amounting to $1 million/yr reduction in raw material cost.
- Increased water removal from the sheet in manufacturing: this reduces the amount of steam required to dry the sheet, giving a significant saving in energy use.
- Improved sheet strength: a 12% increase in sheet tensile strength was achieved; this was the goal of the project. This enables the mill to manufacture an acceptable product.
- Reduction in refining energy: a 15% decrease is seen with the enzyme.
- Reduced refining: this also results in less wear on the refiners, an additional savings in maintenance and equipment replacement.
This mill has now used the enzymatic fibre modification for several years.
This example illustrates several of the potential benefits of these enzymes:
- In some cases refiner energy can be reduced; often this is not a large savings, but it is substantial in other mills. Also, it may allow the mill to avoid capital expenditures for more refining equipment.
- In other cases the major advantage is to allow the mill more flexibility in fibre sources. In this case, a more costly pulp was eliminated from this grade of paper. In other cases it may be possible to use lower quality recycled fibre and still maintain quality.
- Enzymes can increase sheet strength. Again, this provides more flexibility for furnish changes. This customer was able to reach a strength target that previously it had been unable to achieve economically. Enzymatic fibre modification enabled the mill to produce a grade that it otherwise could not provide to its customers.
Example 2
Application in tissue grades: A Canadian mill produces various tissue grades on two machines. The objectives for fibre-modifying enzymes were as follows: reduce softwood use on PM1 and reduce refining on PM2. The enzyme product used here is a specific blend of cellulases that was lab tested and found to work well on the specific species of hardwood and softwood used at this mill. The enzyme is added to the softwood pulper at a maximum addition rate of 200 g/ton. The long residence time, pulp temperature and ample agitation make this an ideal addition point.
As shown in Graph 1, there is a clear correlation between the addition of enzyme and an apparent decrease in softwood usage. The time period shown is a 48-hour trial. During this trial the refining was held constant to determine the extent that softwood use could be lowered. The tensile strength was used to determine the amount of softwood used in the furnish. About three hours after the enzyme was introduced, both the CD and MD tensile began to climb. Softwood flow to the blend chests was reduced to maintain strengths within tolerance. When the enzyme addition rate was increased to 120 g/ton, it was necessary to decrease in softwood percentage again to keep tensile down.
During the evaluation on PM2 at this mill, the tensile reacted in the same manner as on PM1, but the goal on this machine was to decrease refining. As shown in Graph 2, it was possible to reduce the refining by more than 50%. The refining energy was decreased to a point at which machine operators determined that the refiner could have been bypassed as the plates opened during the night due to low load. This application has been running now for several months with much success.
Example 3
Application in tissue grades: This Australian mill producing tissue grades uses 70% CTMP and 30% softwood fibres. The main goals of the evaluation were to reduce refining while maintaining MD tensile, improve bulk to weight ratio and to reduce fines generation.
A short trial was run to qualify the enzyme and a extended evaluation is currently running with very good success. One machine has already been converted and another is being converted. Reduced fines generation was difficult to confirm; however, the improvement in drying capacity implied that the sheet was freer draining. The online backwater consistency instrument was not working, so we were not able to track fines generation. Benefits seen during the evaluation were:
- Reduced refiner energy usage
- Increased drying capability (reduced gas or steam usage)
- Less dust
- Increased production because the machine speeded up by 50 m/min
- Bulk to weight ratio was improved
- The crepe ratio was reduced from 26 to 21.4 and bulk was maintained
- There was a noticeable reduction in hood temperatures during the trial
Figures 3 to 6 show some of the results mentioned above.
CONCLUSIONS
Significant progress has been made over the past two years in the area of enzymatic fibre modification, especially in the tissue and towel industry. We have seen the benefits of using enzymes for fibre modification in mill applications. The benefits of improved tissue quality, increased use of recycled fibers, reduction of starch, improved softness and reduction in energy use are beneficial economically and environmentally.
There are a number of steps involved in running an effective enzymatic fibre modification program. First, determination of the desired end result is critical. Another step in determining the appropriate lab work is understanding the papermaking process.
After product determination, the next step is trial planning. Keeping the end result in mind, the trial plan should include the variables that will be used to take advantage of the fibre modification.
While time needs to be invested in the development of all of this information and learning how to run these programs, the end result for tissue maker can be substantial. Energy costs continue to escalate, and small changes in quality can affect the ability to be in a particular market. Having the flexibility to rationalise fibre sources, depending upon current fibre pricing, will have significant impact on the bottom line. TW
Rosa M Covarrubias is Group Manager Product Development with Buckman Laboratories International in the USA.
