Paper
Manufacturing
Background and Applications
Paper
Mill Boilers
Pulp
Manufacturing
The
Kraft Liquor Cycle
Paper
Machines
Tissue
Machines
Yankee
Dryer
Paper
Drying
Converting
Paper
Coating
Background
and Applications
Paper manufacturing is
a very energy intensive process. About one-half of the total
energy used is in the pulping process, which is the cooking of the
wood chips to open the fibers that create the paper materials.
Therefore, a mill that is 'integrated' (it includes pulping and paper
manufacturing), will use at least twice as much energy as the same
size mill that uses 'market pulp' (pulp produced at another location
and sold as a raw material). The largest single energy user
at a paper mill is boiler fuel. The graphic below shows the
major thermal energy flow through the conventional integrated mill.
Paper
Mill Boilers
Packaged Boilers are typically
natural gas or oil. Power House boilers are typically coal,
wood waste, black liquor, oil, natural gas, or multi-fueled, depending
on the size of the mill and if it includes a pulping operation.
It is common for several different kinds of boilers to be used at
the same mill.
Most mills also include
a steam turbine(s) to generate electricity. This may be because
the mill is older than the public electric system in the area, but
continues to make operational sense. The boilers produce high
pressure steam (in excess of 500 psi) that first passes through
the steam turbine. The steam turbine takes energy out of the
steam by reducing its pressure to about 100 psi, which then runs the
rest of the mill. Mills may also have direct steam drives in
areas that would otherwise have electric motors. Steam drives
resemble mini-steam turbines, and are much smaller than an electric
motor of the same horse power.
For more information about
Steam and Boilers, please visit the Energy Solutions Center's Boiler
Burner Consortium web site at www.CleanBoiler.org
Pulp
Manufacturing
There are three basic methods
to produce pulp:
Mechanical
Pulp (yield 90%)
Mechanical pulping uses
mechanical abrasion to separate cellulose fibers which are held together
by a material called lignin. In the process called "Groundwood"
wet wood is ground by large stones. In Thermo Mechanical
Pulping (TMP), metallic plates rub steam heated chips at high speeds,
separating fibers
Chemical
Pulp (yield 50%)
Uses chemicals to dissolve lignin. Kraft pulp is most common
pulp.
Semichemical
Pulp
Uses chemicals to soften
lignin, and mechanical abrasion in refiners - Chemi Thermo Mechanical
Pulping
The kind of paper that
can be made is determined by the wood fiber and other raw or recycled
materials used and the pulping method.
If the pulp will not be
used on-site it becomes "Market
Pulp", and shipped to another location. Due to
many factors but mostly environmental and local wood supply, many
mills no longer produce their own pulp.
Before the pulp is shipped,
it is dried on a machine called a Flakt Dryer. The Flakt Dryer has multi-level
decks or webs with steam-heated air jetted through the pulp and rollers
that squeeze it to remove the water.
The
Kraft Liquor Cycle
The pulping process produces
a liquid byproduct called " -color- liquor"; it's -color-
is dependant on the process and where in the process the liquor is
at. Liquor has many propertied, including flammability, and
can therefore be used as a source fuel. For more information
Paper Kraft Liquor Cycle
Paper
Machines
The same paper machine
line can be used to produce a hundred different kinds of paper, depending
on the kind of pulp, thickness and line speed. Paper can vary
from very thin, high quality "Bible Paper" to very heavy,
low quality box or kraft paper used to make 'cardboard boxes'.
Typically mills are broadly separated according to "High Quality
Papers" or "Kraft Mills". Kraft mills make the
same paper all the time, with small variations for basis weight or
coatings. Quality Paper mills tend to make a variety of papers
that mostly serve the printing industry. Newsprint falls someplace
in the middle.
The Fourdrinier is the
most common paper making machine. The above picture shows all
of the major sections, but is simplified when compared to most machines
in use today. Today's machines are much larger (longer) to allow for
faster line speeds. The Dryer Section may include over 100
steam cans (drums). Line speed is almost always above 500 feet
per minute, and may exceed 1,000 feet per minute. Machines
have also become wider. The average width exceeds 100 inches.
Head
Box - receives the liquid pulp, with all ingredients ready
to make the paper
Flow
Spreader - controls the pulp distribution on the Table
Fourdrinier
Table - a perforated conveyor belt that supports the pulp solids
and allows the water to drain through
Press
Section - rollers that squeeze water from the pulp
Dryer
Section - typically steam drums (cans) that progressively dry
the pulp as it turns to paper; the drying section may also include
electric or gas infrared heaters and convection heating hoods.
Calendar
Stack - metal rollers that compress the paper to form uniform
thickness; may be smooth or include some sort of pattern or texture.
Reel
- takes up the paper as it is finished
Converting
Paper is manufactured in
very large rolls. In a separate process called "converting"
the large rolls are sliced and cut into sizes used by the end-user.
The converting process may also include additional calendaring
to change the basis weight of the paper and give it various textures.
Linen papers are typically calendared with a screen-like finish.
Tissue
Machines
Tissue Machines are very
similar to paper machines in over-all process, but they are much smaller
in size and have a different kind of dryer section. Tissue
is what is used to make toilet paper, paper towels, napkins, etc.
Like paper, tissue is made from a pulp material, but is more
often recycled and much lower quality materials. It is made
in large rolls and later in a separate process "converted"
to its final product.
Yankee
Dryer
The Yankee Dryer uses a
very large steam cylinder (18 - 25 ft diameter) surrounded by an air
cap (hood). The hood supplies hot, high velocity air that impinges
on the sheet. This way, drying is accomplished by a combination
of conduction (the steam drum) and convection (moving air).
The drying work is about evenly divided between the drum and hood.
Paper
Coating
Many papers require a "coating"
to be complete. The coating process may be connected to the
paper machine where the machine is dedicated to making only one kind
of paper, or may be a complete separate process in a totally different
location. Common coatings include various clays to improve
whiteness and absorption (such as premium ink jet papers) and silicone
for water resistance and release for things like labels.
The coating will make the
paper wet or require curing. Dryers used on coaters are typically
non-contact convection hoods and/or electric or gas infrared.
Paper
Drying
After the pulping process,
paper drying is the second most energy intensive process in the paper
mill. Historically, paper was dried almost 100% with steam
cans. Later, hoods were added. The most recent technology
advancement has been infrared (IR). Initially the IR heaters
were all electric, because of the concern with flammability and controllability
of the gas units. New gas units have been specifically designed
or adapted to the paper machine application, and now much of this
market has gone to gas IR dryers.
The IR units do not replace
the steam cans (although in some cases, a couple steam cans may need
to be removed to make room for the IR units) but are used to supplement
steam drying. Sophisticated high speed controls read the moisture
content across the paper sheet and adjust the IR burner temperature
accordingly in a process called "profiling". This
has allowed production line speed to be increased.
It is important that paper
be dried to a specific moisture content. If it is too wet,
it will cause later production and quality problems; if it is too
dry, it may curl, become a fire hazard, break or rip, and cost more
to produce.
If a paper machine is "dryer
limited" it means that the line speed cannot be increased beyond
what the dryer section can dry. It is extremely expensive to
add length to a paper machine to accommodate more steam cans.
IR burners and other direct fired gas technologies can remove more
water in a smaller space and therefore are often used to speed up
production lines and/or increase paper drying quality.
For More
Information
Paper
Drying
IR
Paper Dryers
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Source:
Aqeel Zaidi, PE, UnionGas Co, CIGC- IGT Presentation, 7/2000
