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THE USE OF LIPASE ENZYMES IN TMP FURNISH: A PRACTICAL PERSPECTIVE
Sal Mirza, Guy Harvey, Martin Sénéchal and Stéphane Ouellet
Presented at PAPTAC 2006
The pulp and paper industry is well aware of the operational and quality problems caused by wood extractives. These lipophilic wood extractives in addition to the non-polar triglycerides component cause pitch deposition resulting in interruptions in paper production and product quality issues (examples: machine downtime for wash-ups, holes, or dirt spots).
Over the years, a number of additive methods have been used to minimize these problems. These include dispersion, fixation, encapsulation, stabilization, and absorption. What all of these methods have in common is that none of them really treat the root cause of the problem. The use of enzymes in industrial processes, while not new, has only been prevalent in the pulp and paper industry in the last 10 years. The use of lipase enzymes in TMP furnishes is being explored as one possible option to manage and control these detrimental wood extractive components. This paper presents mill data analyzing the effect of lipase enzymatic treatment of a TMP pulp on triglyceride levels in the pulp, the effect upon paper quality, and on paper machine runnability (deposition). Specifically, data will be presented identifying the effect upon the coefficient of friction of the sheet.
Wood is the main raw material component used in the production of paper and board. However, wood has to be processed before it is suitable for papermaking. The pulping process separates the fibres from the wood and makes them usable for papermaking. The pulping process, specifically thermomechanical pulping (TMP), the subject of this paper, produces detrimental substances that are a major source of reduced paper quality and paper machine runnability. These detrimental substances can be of different organic and inorganic nature and also have different origins. The focus of this paper will be organic substances such as triglycerides and fatty acids and their effect on paper quality and paper machine runnability. The question arises – How do we manage these wood extractive components? Then we must pose a second question: What are the benefits of managing these detrimental wood extractive components?
THE NATURE OF PITCH
Simply put, pitch is the wood resin component of wood extractives. A study of the types of compounds that make up wood pitch would indicate that the major troublesome component is probably the triglycerides. Because these have no charge, fixation methods are ineffective.
The presence of triglycerides appears to account for the differences in pitch deposit formation. Deposit forming pitch contains higher concentrations of triglycerides than pitch that forms no deposits (Allen, 1997). Enzymatic control of triglycerides has been proven through research laboratory analysis (Irie, Matsukura, Usui, & Hata, 1990) as well as full production scale applications. Lipase enzymes have been shown to reduce the triglyceride content in pulps (Fischer & Messner, 1992). Vercoe, Stack, Blackman, and Richardson (2005) confirmed the hypothesis that a three-component pitch colloid consists of a hydrophobic core (TGs) surrounded by a hydrophilic outer shell (resin and fatty acids).
COEFFICIENT OF FRICTION
The coefficient of friction is an important property at various stages of the paper production process. For example, higher friction is needed for controlling paper roll functionality and to obtain good runnability of the paper from a converting or printing press standpoint. Lower friction is preferred, for example, during the corrugating of fluting.
Paper to paper friction is influenced by the surface chemistry of paper (Gurnagul, Ouchi, Dunlo-Jones, Sparkes, & Wearing, 1992). Studies have described how wood extractives and surface contaminates can affect paper friction (Johansson, Fellers, Gundersson, & Haugen, 1998). It has been suggested that thin layers of saturated aliphatic compounds—for example, saturated fatty acids—decrease friction, whereas wood resin acids increase friction (Back, 1991). This effect of contaminates and extractives on paper friction has been attributed to the fact that they modify the surface-free energy of the paper.
LIPASE ENZYMES IN TMP: APPLICATION CONDITIONS AND STRATEGY
As with all chemical programmes, proper application conditions and strategy are important. Even the best technology, if not applied in a suitable manner, can impede performance.
Enzymes have a specific pH and temperature range for maximum effectiveness, which will vary based on the enzyme in question. Therefore, a certain pH and temperature range will allow any enzyme sufficient activity in order for it to be effective. As with most chemical reactions, as temperature increases, the reaction rate increases. This is also true for enzymes: as the temperature increases, the enzyme activity increases. The upper limit for temperature is the point where the enzyme starts to denature or break down. This breakdown is not reversible, and therefore if the enzyme is subjected to a temperature above its upper limit, it will be destroyed.
Having a sufficient amount of contact time is important for any enzyme application. Each enzyme molecule will catalyze a large number of reactions, but it can only work on one at a time. Therefore, the longer the contact times, the more reactions each enzyme molecule can complete. A final factor is the absence of other chemistries that will interfere with the enzyme activity or actually degrade the enzyme. In most cases, one should avoid the use of high levels of oxidizers.
An accurate representation of the problems being caused by the pitch is necessary so that improvements can be tracked and quantified. The first measurement and monitoring tool used was paper machine runnability.
Paper quality measurements were also collected and compared against baseline information; for example, sheet defects (holes, spots), coefficient of friction, and tensile or other strength parameters.
Of course, improved pitch control is the primary advantage of controlling triglyceride levels. This can be seen in a number of ways in the papermaking process: deposition elimination or drastic reduction, and fewer sheet defects due to pitch.
Other benefits of controlling levels of triglycerides have been stated in the literature, such as coefficient of friction of the sheet, and improved strength properties such as tensile and tear.
This paper will explore more of the reasons why controlling triglyceride levels is so important. One explanation is that less pitch buildup on refiner plates allows refiners to operate more efficiently, which helps develop fibre strength. Less pitch in the sheet allows better fibre-fibre bonding, leading to better strength properties.
CASE STUDY: SEASONAL VARIATION IN EXTRACTIVE COMPOSITION AND IMPACT OF LIPASE ENZYME ON TRIGLYCERIDE LEVEL
Stadacona S.E.C. a paper mill located in Québec city, Québec, produces 380 000 tons/year of standard newsprint and 90 000 tons/year of directory paper. The site has four similar paper machines utilizing the Belbond top formation section installed in the mid-eighties.
The furnish mix has changed over the years; currently the mill uses a mix of TMP and deinking pulp. The TMP is using mostly spruce including black spruce as well as balsam fir, aspen, and jack pine. The deinking mill has used old magazine (OMG) and old newsprint (ONP). On certain grades, calcined clay is used to impart desired sheet properties.
Since the introduction of the recycle, Stadacona S.E.C, like many other mills, has been facing the challenge of managing the negative interactions and synergy between wood pitch and stickies, which results in pitch-stickies complex deposition (Allen, 2002).
The continuous improvement project work carried out to date by the mill and its partner supplier has over the last years resulted in significant improvements in managing pitch/stickies deposition. Changes in the process, as well as a balanced chemical programme managing the wet end chemistry, have allowed the mill to keep an excellent record of accomplishment in terms of deposition management.
The chemical programme is based on the use of a copolymer coagulant at strategic stages of the TMP process, and in the approach system of the paper machine. Talc is added to the recycle furnish as well as a polyamine coagulant. As far as the retention program is concerned, a standard cationic polyacrylamide is used post-screen on each paper machine to control white solids. A felt cleaning and conditioning programme eliminated deposition in the press section, especially on the Uhle boxes strip, and has increased felt performance. Changes were made to reduce pH shock between pulp streams as well as with showering.
In terms of process change, strategic purge points have been identified in both the TMP and DIP plants. This systematic "bleeding" allows the management and control of detrimental material in the system to remain below the critical threshold. In addition, a strategy has been put in place to reduce the carryover of metal ions from the DIP to the paper machine. Good screening and stickies concentration follow-up allows the mill to reduce the impact of stickies on the paper machine and in the final sheet. To take full advantage of all these changes, significant work was also carried out on the paper machine to reduce hydrodynamic shear, increase doctoring efficiency, and control calender temperature.
IDENTIFYING PITCH SEASONAL EFFECT
Pitch/stickies control is a journey for mills using TMP/DIP furnishes, requiring teamwork and collaboration and focused efforts to move forward with further changes. Following Blazey, Grimsley, & Chen's (2003) findings regarding the "pitch season," we decided to study the composition of wood extractives during the course of a year. Pulp and paper samples were sent to PAPRICAN, who carried out measurement tests (Zhang, Renaud, & Paice, 2005). Through this longitudinal study, we were able to demonstrate the variation of the resin and fatty acid as well as the triglyceride throughout the year (see Figure 1).
In May 2004, we started to analyze extractives composition. Figure 1 shows the evolution in time of the total extractives, triglyceride (TG), resin and fatty acids (RFA), and the TG/RFA ratio over a twelve-month period.
We see that the total extractives increase slightly with the cold temperature starting in December. One of the reasons may be the higher levels of bark coming into the process. In colder weather, bark is more difficult to remove, so we have higher levels of bark in the process. Since bark contains higher levels of extractives, this may be one reason for the higher levels noted.
What is of interest is the shift in the extractive composition. There is a trend where the level of TGs starts to increase considerably in December to double the values during the winter season. At the same time, the levels of resin and fatty acid decrease, so the TG/RFA ratio goes from 0.5 to 4.0.
At these much lower temperatures, the rate at which the TGs are converted to fatty acid is reduced, thus increasing their concentration compared with what we see in the summer season, where the temperature is much higher.
Because of their high molecular weight and their non-polar characteristic, TGs are well known to be a very troublesome component for TMP wood pitch deposition (Black & Allen, 2000). Because they are non-polar, they cannot be "fixed" to fibre and fines or stabilized using traditional charged- based polymer chemistries. This leads to the explanation of the seasonal pitch outbreak and the variation in TMP turbidity, which are both higher during the winter.
The mill is experiencing a crepe wrinkle and burst problem. Although it has not been statistically proven, there is a belief that crepe wrinkle and burst problems intensify during winter. A team-based systematic problem-solving methodology was used to investigate which factors may influence crepe/wrinkle and burst. Among all the potential causes identified during the problem-solving session, the impact of wood extractives on coefficient of friction and on the crepe wrinkle/burst problem was acknowledged as one to follow up on.
A decision was made to evaluate the effect of a lipase enzyme, based on the data obtained showing the seasonal effect of extractive composition and based upon sporadic industry positive experience of the impact of TGs on coefficient of friction. A plan was developed to evaluate the effect of reducing TG levels with the use of a lipase enzyme on sheet coefficient of friction and on crepe wrinkle/burst frequency.
The trial was started in March 2005; the enzyme selected was Buzyme 2517. Of the enzymes available, this one was chosen because its reactivity was deemed the best for the pH and temperature conditions of the TMP. The enzyme was added at the exit of the disc filter going into the TMP storage tank where we had a residence time of 8-12 hours and a temperature of 650C.
The starting addition rate was set at 0.25 kg/T and was increased to 0.4 kg/T six days later.
Data for transmittance at the TMP and at the headbox showed there was no significant change, indicating that the concentration of colloidal organic had not changed considerably. Thus, no changes to the coagulant polymer, Bufloc 5376, were made to compensate for the increase in fatty acids concentration. In fact, the level of RFA was similar to that which had been observed during the summer season.
TG REDUCTION IN TMP
Figure 2 shows the trend for TGs in the pre- and post-Buzyme 2517 treatment time period. On average, a 45-53% reduction in TG in the TMP main line with the enzyme addition was noted. A point of interest is that the TGs began reduction only 12 hours after the enzyme treatment was started.
In addition to the PAPRICAN test, Buckman Laboratories also used a proprietary test to monitor and compare against the PAPRICAN data. This test had the advantage that it could be carried out onsite with results available within hours. The data was found to correlate very well with those produced by PAPRICAN. The testers found the Buckman methodology to be easy to conduct and replicate.
According to our test, a 53% conversion of the TGs was achieved during the trial. The sample points were used to obtain the TMP sample - before enzyme treatment (disc filter) and after enzyme treatment (storage tank).
TGS REDUCTION IN FINAL SHEET
In preparation for the trial, sheet samples were taken of pretrial conditions and sent to PAPRICAN for TGs testing. The results on the sheet were very impressive; TGs were reduced by 60% compared with samples taken before the trial.
SHEET QUALITY: HOLES AND REJECTED PAPER DUE TO HOLES
To correlate sheet quality, data was collected and analyzed for holes and rejected paper due to holes. This data is presented in Table 1.
Note: 1 = Average number of holes for 50km or paper.
2 = Rejected paper due to holes, dirt, pitch spots, t/day.
3 = The change as compared to the duration of the trial and the average for the same time period for the past six years.
4 = The change as compared to the duration of the trial and the yearly average for the past six years.
As can be noted from the data, a significant reduction in rejected paper was experienced when compared to the same period for the past six years, 4.2 t/day. When compared to the yearly average, the rejected losses were reduced by a significant 2.6 t/day.
IMPACT ON COEFFICIENT OF FRICTION
The effect of coefficient of friction was also one of the goals of this evaluation. The data showed that in this case there was no significant change in the sheet coefficient of friction (see Figure 4).
In conclusion, data supported previous findings of seasonal variation in extractive composition leading to a TG increase during the winter season. TGs are a well known troublesome component of the extractive, which increase deposition on paper machines and cause paper defects. Traditional polymer chemistries are unable to "fix" the TGs to the fibre or fines due to their non-polar nature. The data from this mill trial supports the use of lipase enzymes to manage the levels of TGs in the papermaking process and reduce the deposition tendency. By effectively hydrolyzing TGs to fatty acids, we can achieve better control on fatty acids with a cationic polymer such as Bufloc 5376 to eliminate deposition and paper defects.
The proprietary Buckman Laboratories onsite method testing for TGs is simple to use and is reproducible. The data correlates well with the data obtained from the PAPRICAN analysis.
An indication, at this mill, of a pitch deposition problem is the quantity of small holes and dirt/pitch spots in the final sheet. During the lipase enzyme trial, the numbers of small holes were reduced significantly compared to historical data for the same time period from the past six years. The conclusion made is that with the significant reduction in the reject parameters such as holes, dirt/pitch spots that the troublesome components of pitch were controlled and managed much more effectively.
In this particular case, the treatment of TMP with lipase enzyme did not show any significant changes to the sheet coefficient of friction. This can most likely be explained by the fact that the extra-liberated fatty acids were being fixed in the sheet. Any benefits of reducing TGs in the sheet could have been offset by the increase of fatty acids in the sheet. This would support the suggestions of Back (1991) that thin layers of saturated aliphatic compounds such as saturated fatty acids decrease friction.
An expansion on this suggestion would be to revisit the two possibilities of removing the free fatty acids from the process in order to increase the coefficient of friction:
A process profile of resin and fatty acid levels could identify a better purge point to remove the detrimental substances from the process, rather than "fixation" and removal with the sheet.
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Sal Mirza, Ph.D.: Market Manager - Buckman Laboratories International, Inc.
Guy Harvey, Eng: Process Engineer - Stadacona S.E.C.
Martin Sénéchal, Eng: Mechanical Grade Wet-end Management Team Leader - Buckman Laboratories of Canada Ltd.
Stéphane Ouellet, Eng: Stock Preparation Supervisor - Stadacona S.E.C.