Biermann Ch. Handbook of Pulping and Papermaking
Подождите немного. Документ загружается.
244 9, PAPER MANUFACTURE
The simplest design used on older machines
is
the
basic steam and condensate control system, which
supplies steam
at
controlled pressure and removes
condensate
for
reuse
in
the steam plant.
The cascade steam system
is
slightly more
complex and
is
useful
for a
system using high and
low pressure dryers. The bleed-off from the high
pressure dryers
is
reused through the low pressure
dryers.
The
thermo-compressor takes
low
pres-
sure bleed-off steam
and
puts
it
into
the
high
pressure inlet header
to
recover
its
heat value.
The action
of
the thermo-compressor
is
based
on
the venturi effect where the pressure
of a
flow
of
gases
is
decreased
by
moving
it at
high speed
(i.e.,
through
a
constriction).
Dryer hood
The dryer hood
is an
enclosure around
the
dryer section
and is
used
to
improve drying effi-
ciency by removing the moist
air
near the surface
of the web.
If
this
air is not
removed,
it
quickly
becomes saturated with water, preventing further
water removal from the web.
Yankee dryer
The Yankee dryer is
a
large dryer drum (3.5-
4.5
m or
12-15
ft in
diameter)
for
drying tissue
papers that
are not
strong enough
to
endure
nu-
merous felt transfers (Fig. 9-53).
The
Yankee
dryer
is
normally
the
only dryer used
to dry
tissue.
A
Yankee dryer with
a
smooth chrome
surface
is
used
in
some printing papers (prior
to
other dryers)
to
impart
a
machine glaze
on the
surface
of
these papers.
An air
cap
is a
hood
mounted close
to the
surface
of
the Yankee dryer
that forces heated
air on the
web, improving
the
drying efficiency.
The
creping blade (doctor
blade)
is a
thin metal blade that scrapes
the dry
tissue
off
the Yankee dryer.
It
also compresses or
crepes
the
tissue
for
increased flexibility
in the
machine direction. Fig. 9-54 shows an example of
creped paper. Because
of its
large size,
the
yankee dryer must be heated with care when cold.
Impulse drying
The use
of a
long press nip with one roll
at
175-400°C (350-750°F) to remove water from the
web is known as impulse drying. The objective
is
to vaporize some
of
the water quickly
so
that
the
CREPED SHEET
DOCTOR BLADE
WET SHEET
Fig. 9-53.
A
Yankee dryer with
air cap and
doctor blade. Courtesy
of
W. Bublitz.
generated steam front forces much
of
the remain-
ing water
out as a
liquid. This eliminates
the
energy required to vaporize much of
the
water and
reduces the dryer section size. Impulse drying
is
still an experimental technique.
It
was developed
at the Institute
of
Paper Science and Technology.
The method leads
to
sheet delamination, although
a recent publication claims
to
have solved this
problem.
Size
press
The size press
is
located between dryer
sections
and
consists
of a
pair
of
squeeze rolls
mounted horizontally, vertically,
or at a
45° angle
(Fig. 9-55).
It is
used
to
apply surface size
(usually
a
starch solution)
to
papers.
In the
horizontal size press,
the
bottom roll rotates
in a
pan filled with size solution, and the same solution
is delivered
to the top
surface
of the web by a
shower pipe. The excess solution
is
squeezed
off
the sheet. The base sheet
is
then redried, using,
of course, much energy. Fig. 9-56
is a
size press
using
a
flooded
nip. Fig. 9-57 is a
gate roll
(transfer roll) size press.
Defoamers are typically used in the size press
at 10-100 ppm. Defoamers that contain glycols,
modified glycols,
or
polyalkyloxylated phenols
must
be
avoided
in the
size press because small
amounts can greatly decrease sizing. One should
THE DRYER SECTION 245
-rr'Mv.
Fig. 9-54. Creped giblet paper. The insert is magnified 4X.
also consider the possibility of defoamer carry-
over through pulp from the deinking plant.
Surface sizing
Surface size uses polymeric materials, com-
monly starch, but sometimes starch reacted with
ethylene oxide (hydroxyethyl starch with a rela-
tively low degree of substitution), carboxymethyl
cellulose, polyvinyl alcohol, methyl cellulose,
alginates, or wax emulsions using alum that are
applied as a solution to the surface of the web at
the size press, followed by drying.
Surface sizing improves the water resistance
of paper and bonds surface fibers to it to improve
printing properties. Except for wax, surface sizes
are generally not hydrophobic like internal sizes;
instead, they act by sealing the sheet, that is,
filling the capillaries and pores with sizing formu-
lation. This reduces the absorption of water by
capillary action. (Fig. 17-5 demonstrates capillary
action.)
However, in the last several years, nonpolar
synthetic polymers have been used. The three
major types include polystyrene maleic anhydride
copolymers (SMA, which may be partially hydro-
lyzed to convert about 10% of the maleic anhy-
dride groups to maleic acid) that are used with
tackifiers, polystyrene acrylic acid copolymers,
and polyurethanes. (Tackifiers are usually coordi-
nating cations, such as alum, that bind the carbox-
ylate groups so the polymers are not "sticky".)
These polymers are effective in providing ink
holdout. Ammonium zirconium carbonate is
another sizing agent that has been used for many
years in certain applications.
Typically about 30-50% wet pick-up is used
or 300-500 kg/t (600-1000 lb/ton). Papers without
internal size may pick up higher levels. Sizing
efficacy may by measured by the Gurley porosity
or the sizing tests.
9.9 POST DRYING OPERATIONS
On-line
paper testing
There are a number of on-line (Fig. 9-58)
paper tests that can be accomplished and the
246
9.
PAPER MANUFACTURE
SHOWER PIPE
DRY
SHEET ^O^
\
TO DRYERS
/^STARCH
SOLUTION
Vertical Size Press
DRY SHEET
SHOWER PIPES O
SOUEEZE V^^y
ROLLS
^-^1^-^
TO
DRYERS
V
Horizontal Size Press
Inclined Size Press Gate Roll Size Press
Fig. 9-55. Size press conHgorations.
Fig. 9-56. Flooded nip size press.
POST DRYING OPERATIONS 247
Fig. 9-57. Gate roll size press. The top insert shows the roll coated with sizing solution. The
bottom insert shows the size press nip.
Fig. 9-58. Basis weight and moisture content measurement with a scanning detector.
248 9, PAPER MANUFACTURE
Loading Cylinder
Fig. 9-59. Paper machine calender stack.
Zellerbach Corp., with permission.
Reprinted from Making Pulp and
Paper^
©1967 Crown
number is increasing. The most common ones
used are basis weight measured by a
jS
(beta) ray
detector (it measures the absorption by paper of
electrons emitted by a radioisotope) and moisture
content by infrared (IR absorption
is
taken at about
4 different wavelengths to determine moisture
content with a correction factor for IR absorbed by
the paper fibers). These devices are combined
into a unit which scans the paper across the paper
machine just before the calender stack. Other
devices measure
brightness,
strength properties (by
ultrasonic properties of the paper), thickness,
color, opacity, gloss, ash content (by low energy
X-ray analysis), etc.
Calender, calender stack
The calender stack is a series of solid rolls,
usually steel or cast iron, mounted horizontally
and stacked vertically (Fig. 9-59). During ma-
chine calendering, the dry paper passes between
the rolls under pressure, thereby improving the
surface smoothness (for example, from imperfec-
tions caused by felt marks, cockle, lumps, fibrils,
etc.) and gloss and making the caliper more
uniform, if
not
decreasing the caliper and porosity.
These improvements make the paper better suited
for printing and decrease problems during print-
ing. (If uniform caliper is the only goal, one nip
is often sufficient to achieve this.) Pressures of
850-85,000 N/cm (100-1000 PLI) are used. There
is some loss of opacity, brightness, and stiffaess
on calender finished papers and paperboards.
The bottom roll is called the king roll. It is
usually larger in diameter than the other rolls and
crowned (it has a larger diameter in the center) to
permit even pressure when loaded on the ends. A
POST DRYING OPERATIONS 249
Fig. 9-60. A single-nip calender for caliper
control of linerboard.
variable crown can be achieved using a pressur-
ized, oil-filled roll where the oil pressure controls
the degree of crown. The roll above the king roll
is the queen roll. The trend is towards the use of
calender stacks with fewer rolls. Fig. 9-60 shows
a calender with a single nip used for caliper
control of linerboard.
Cooling air is supplied through a series of
controllable jets across one of the rolls for caliper
control in the cross machine direction. This
arrangement is known as the actuator. The air
cools the roll and causes a local decrease in roll
diameter and an increase in sheet thickness.
Recent work on a newsprint machine (Journeaux
et al., 1992) shows that the actuators might be
most effective on the queen roll or one of the rolls
immediately above the queen roll, especially if
they are unheated, thin walled rolls. Using a roll
with a single nip, such as the king roll or top roll,
was not very effective. Since the caliper control
is in the early nips, the actuator is more effective
on the roll above the queen roll compared with
rolls farther away from the queen roll.
Because supercalendering (described below)
has high operating costs, many paper machines are
using hot calendering with soft rolls. These rolls
differ from those of supercalenders and are not
Fig. 9-61. Supercalender. Reprinted from
Making Pulp and Paper^ ©1967 Crown
Zellerbach Corp., with permission.
filled
rolls.
Generally, two nips are used to
calender each side of the paper or, if necessary,
four nips are used, which may be through two
calenders with three rolls to make two nips. Soft
calendering of papers made from mechanical pulps
has been discussed by Tuomisto (1992). This
method can be done on-line.
Supercalender
A supercalender is similar to the calender but
uses alternate hard and soft, heated rolls (Fig. 9-
61).
While considered here for the purpose of
comparing it to the calender section, it is usually
used off-machine such as after paper coating. For
example (Fig. 9-62), it is used to impart high gloss
250 9. PAPER MANUFACTURE
Fig. 9-62. Uncoated groundwood paper (top), coated groundwood paper (middle), and
supercalendered, coated groundwood paper (bottom).
POST DRYING OPERATIONS 251
to clay coated papers and to produce glassine
paper from grease-proof paper. Pressures of
17,000-34,000 N/cm (2000-4000 PLI) and temper-
atures of 40-70°C (105-160^F) are used.
Supercalendering may actually increase fiber
bonding with an increase in strength, but the
compact sheet has decreased opacity. It is usually
done off-machine because of the relatively low
speeds required.
Reel
The reel is the last unit on the paper machine
that collects the paper (Fig. 9-63). The paper is
wound on a spool that rotates against a drum. A
new spool is started on a secondary arm before the
llil^iliii
filled reel is discharged, for changing reels "on the
fly". Plate 37 shows reel handling.
9.10 PAPER MACHBVE BROKE SYSTEM
During startup, threading, and the (hopefully
rare) event of paper machine breaks, broke (un-
usable paper) will be produced at various parts of
the paper machine. Fig. 9-64 shows a representa-
tive broke handling system. Broke at the wet end
is easily repulped and metered back into the
system, such as by use of the Agi-Pulper'^^.
Broke in the dryer sections requires more severe
pulping. Coated papers, of course, may have to
be handled separately.
Fig. 9-63. Autoflyte reel. The gooseneck and cutter knife flower right) are used for positive
automatic sheet transfer. Courtesy of Black Clawson-Kennedy.
252 9. PAPER MANUFACTURE
AghPuiper Agi-PuipBr WSyiimf^per
Fig. 9-64. A paper machine broke system in two segments. Courtesy of Black Clawson-Kennedy.
9.11 CONVERSION
Converting
Conversion is a general term for operations
on paper following its manufacture. Converting
operations include trimming, rewinding, sheeting,
coating, printing, saturation, and box making.
Winder
The winder is a device that receives the large
reels off the paper machine and slits the paper into
smaller widths while rewinding it onto smaller
rolls for shipping to customers (Fig. 9-65) or for
further conversion within the paper mill.
CONVERSION 253
Fig. 9-65. Banding a roll for shipping after
rewinding from the reel.
Slitter knives
The slitters cut the paper into narrower rolls
during rewinding (Fig. 9-66). Sharp circular
knives for trimming reels into rolls are called
slitter knives.
Sheeter
A sheeter is a machine holding multiple rolls
of paper that cuts webs into large sheets and stacks
the sheets on a pallet.
Guillotine
The guillotine is a device for cutting stacks of
large sheets from the sheeter into commercial size
sheets (for example, SVi in. x 11 in. sheets).
Saturation
Saturation involves treatment of paper
whereby the saturating material is distributed
completely through the sheet, in contrast to a
coating, which lays on the surface. The process is
used to increase the strength and durability of the
paper. Thermosetting resins (such as phenol-
formaldehyde or melamine-formaldehyde) are used
to make hard, stiff papers for use in hardboard
overlay, countertops, and laminates with high
Fig. 9-66. Slitting durmg rewmdmg. The msert is a close-up of a knife slittmg linerboard.