Manufacturers of tissue and towel have always used new
technology and novel processes to create products that are
softer, stronger, and more absorbent. Today, worldwide tissue
and towel markets represent excellent potential for continued growth in
both consumer and commercial products. Developing the right products
to take advantage of this potential is vital for papermakers, as well as the
equipment manufacturers and chemical companies who serve the industry.
The tissue industry is well aware of the operational and quality problems
caused by wood extractives, such as machine downtime for wash-ups,
holes or dirt spots. Over the years, a number of additive methods have
been used to minimize them. These include dispersion, fixation,
encapsulation, stabilization, and absorption. What all of these methods
have in common is that none of them really treats 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 15 years.
The use of lipases is being explored as one possible option to manage and
control these detrimental wood extractive components.
Tissue and towel manufacturers will continue to face new challenges
in the coming years. More strict environmental regulations, global
competition, and new market-driven demands for tissue properties will
induce changes in production technology and processes. Enzymes are
among the new technologies that can help the tissue industry to reach
these new objectives.
The use of enzymes represents another opportunity to improve
operations and quality at tissue and towel mills. In the past few years,
enzymes have proven effective for many applications, such as reducing
pitch problems, controlling slime problems, minimizing problems from
stickies in recycled fiber, cleaning paper machine systems, and enzymes
for modification of cellulose fibers.
ENZYMES
A multitude of different kinds of enzymes exist in nature. Each one
catalyzes a different biochemical reaction, bringing about a different
chemical transformation. Enzymes function as the catalysts of all the
activities of life. They enable living things to exist and grow. As examples,
enzymes digest food and derive from it the energy we use to live and
think.
The use of enzymes in industry continues to grow. There are a surprisin
number of current uses of enzymes in the everyday world. Examples
include the detergents used to clean your clothes, the preparation of various
foods (eg citrus juices), and the manufacture of clothing. Enzymatic fibre
modification has been used in textiles, where enzymes are used to soften
fibres or give the 'stone-washed' appearance to blue jeans.
The use of enzymes in industry is interesting and attractive for several
reasons. Enzymes typically have a very low toxicity, making them quite
safe to use. Enzymes are 'green', produced from natural sources at low
energy requirements from renewable resources, and easily recycled in the
environment. Enzymes are very specific as to the materials they act on,
so their use can be carefully targeted, with few unwanted or unexpected
side effects.
PITCH IN ACACIA WOOD
When papermakers use the word 'pitch', they are talking about tacky
materials that usually contain resins from the wood. Wood chips from
different types of trees are likely to contain from about 1-5% of wood
pitch.
The wood science and chemistry (particularly DCM extractive content)
of acacia wood are very different from those of other hardwood species.
As shown in Table 1, extractive content in acacia is mainly due to longchain
fatty acids, alkanols, sterols, and steryl esters. Fatty acids are
saponified during cooking and most of them are removed from the pulp
during washing, while alkanols, sterols and steryl esters remain unsaponified.
These unsaponified alkanols, sterols, and steryl esters are difficult to with the pulp through O2 delignification, bleaching, and finally to the pulp dryer area.
Due to intense system closure, a buildup of these materials continues
until equilibrium in the white water system is established. That is why,
when acacia pulping is used, extractive content in the pulp is lower for the first couple of days but steadily rises from day three onwards until it stabilizes after a few days, barring process disturbances. High extractive
content lowers the pulp quality leaving the pulp dryer.
USE OF LIPASES
Lipases are formed from a selective living organic matter. Molecular
biology and protein engineering have been used to incorporate desired
characteristics in a protein by many cycles of mutations. This was necessary
to improve the enzyme adaptability and compatibility to mill conditions.
This specific lipase performance is also based on specificity and/or
reactivity.
The lipase treatment is designed on the basis of bio-technological and
physico-chemical compatible methods for extractive content removal from
acacia pulp. The enzyme treatment is targeted after pulp bleaching and is
basically to selectively degrade/modify leftover unsaponifiables,
predominantly alkanols, sterols and steryl esters in the pulp. Lab evaluations
have indicated that it strongly enhances the degradation of extractive
content present in acacia pulp.
The enzyme performs the work of a catalyst by reducing energy barriers
by contacting active sites as shown in the diagram. It means that the
enzymes are regenerated after the reaction and thus can be reused for the
other extractive molecules. Kinetic constants of this enzyme reveal that
it shows a very high affinity and activity on the DCM extractives present
in the acacia pulp (as shown in Table 2), though it would require more
study to predict a clear picture of this enzyme reaction with the extractive
content present in bleached acacia pulp.
As with all chemical programs, 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 sufficient 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.
CONCLUSION
Pitch problems can be very complex due to the many different
hydrophobic materials that can associate with each other in a paper machine
furnish. For this reason, performing a chemical analysis is highly
recommended. This may make it possible to determine the most likely root causes of the problem. By understanding the root problem, it is
possible to engineer the proper enzyme that will allow you to control pitch
on any type of fibre. TW
| Mill Case
A tissue machine in Indonesia was using acacia and eucalyptus fibres to make facial and toilet tissue. Some of the issues the mill was having were:
• Eucalyptus is more expensive ($60 more than acacia)
• MTH is no longer available after 2009
• Plantation supply will dictate more acacia use over the next few years
• Machine runnability is lower with acacia in the stock and usually impossible at greater than 40%
A trial was conducted and converted into an application using Buzyme™ 2538. The product was added to the pulper at a rate of 0.25 kg/ton and dosage was increased to 1 kg/ton over a period of four days. Over that time, the eucalyptus fibre was replaced entirely with acacia. In addition, various conditions of the yankee dryer as well as the sheet quality were observed. With the use of the enzyme, we were able to eliminate sheet floating on the yankee that was always observed when acacia fibre was used. We were able to increase machine speed, while maintaining machine runnability, and achieve better quality, resulting in increased profitability due to cost savings from the pulp. The usage of softener was dramatically reduced due to the acacia delivering a softer sheet and better handfeel. Softness increased from about 34 to 44 on average as measured with the Handle-O-Meter. The ROI of this application (only taking into consideration fiber substitution) is around $1 million/yr.
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