Note: Descriptions are shown in the official language in which they were submitted.
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SACRIFICIAL RECOVERABLE DISPLACING AGENT FOR
IMPROVED PAPER DEWATERING
BACKGROUND OF THE INVENTION
Field of the Invention
The present invention relates to the papermaking arts. More specifically,
the present invention relates to a reusable and recoverable sacrificial
displacing
agent for use with a press fabric in a press section of a paper machine.
Description of the Prior Art
During the papermaking process, a cellulosic fibrous web is formed by
depositing a fibrous slurry, that is, an aqueous dispersion of the cellulose
fibers,
onto a moving forming fabric in the forming section of a paper machine. A
large amount of water is drained from the slurry through the forming fabric,
leaving the cellulosic fibrous web on the surface of the forming fabric.
The newly formed cellulosic fibrous web proceeds from the forming
section to a press section, which includes a series of press nips. The
cellulosic
fibrous web passes through the press nips supported by a press fabric or belt,
or,
as is often the case, between two such press fabrics. In the press nips, the
cellulosic fibrous web is subjected to compressive forces which squeeze water
therefrom, and which adhere the cellulosic fibers in the web to one another to
turn the cellulosic fibrous web into a paper sheet. The water is accepted by
the
press fabric or fabrics and, ideally, does not return to the paper sheet.
The paper sheet finally proceeds to a dryer section, which includes at
least one series of rotatable dryer drums or cylinders, which are internally
heated by steam. The newly formed paper sheet is directed in a serpentine path
sequentially around each in the series of drums by a dryer fabric, which holds
the paper sheet closely against the surfaces of the drums. The heated drums
reduce the water content of the paper sheet to.a desirable level through
evaporation.
It should be appreciated that the forming, press and dryer fabrics all take
the form of endless loops on the paper machine and function in the manner of
conveyors. It should further be appreciated that paper manufacture is a
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continuous process which proceeds at considerable speeds. That is to say, the
fibrous slurry is continuously deposited onto the forming fabric in the
forming
section, while a newly manufactured paper sheet is continuously wound onto
rolls after it exits from the dryer section.
The present invention relates specifically to the press fabrics used in the
press section. Press fabrics play a critical role during the paper
manufacturing
process. One of their functions, as implied above, is to support and to carry
the
paper product being manufactured through the press nips.
Press fabrics also participate in the finishing of the surface of the paper
sheet. That is, press fabrics are designed to have smooth surfaces and
uniformly
resilient structures, so that, in the course of passing through the press
nips, a
smooth, mark-free surface is imparted to the paper.
Perhaps most importantly, the press fabrics accept the large quantities of
water extracted from the wet paper in the press nip. In order to perform this
function, there literally must be space, commonly referred to as void volume,
within the press fabric for the water to go, and the fabric must have adequate
permeability to water for its entire useful life. Finally, press fabrics must
be able
to prevent the water accepted from the wet paper from returning to and
rewetting the paper upon exit from the press nip.
Contemporary press fabrics are produced in a wide variety of styles
designed to meet the requirements of the paper machines on which they are
installed for the paper grades being manufactured. Generally, they comprise a
woven base fabric into which has been needle-punched a batt of fine,
non-woven fibrous material. The base fabrics may be woven from
monofilament, plied monofilament, multifilament or plied multifilament yarns,
and may be single-layered, multi-layered or laminated. The yarns are typically
extruded from any one of several synthetic polymeric resins, such as polyamide
and polyester resins, used for this purpose by those of ordinary skill in the
paper
machine clothing arts.
The woven base fabrics themselves take many different forms. For
example, they may be woven endless, or flat woven and subsequently rendered
into endless form with a woven seam. Alternatively, they may be produced by a
process commonly known as modified endless weaving, wherein the widthwise
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edges of the base fabric are provided with seaming loops using the
machine-direction (MD) yarns thereof. In this process, the MD yarns weave
continuously back and forth between the widthwise edges of the fabric, at each
edge turning back and forming a seaming loop. A base fabric produced in this
fashion is placed into endless form during installation on a paper machine,
and
for this reason is referred to as an on- machine-seamable fabric. To place
such a
fabric into endless form, the two widthwise edges are brought together, tl3e
seaming loops at the two edges are interdigitated with one another, and a
seaming pin or pintle is directed through the passage formed by the
interdigitated seaming loops.
Further, the press fabric may be formed of several layers. For exarnple,
the fabric may include a woven base and an intermediate layer that are
laminated together.
The present invention relates primarily to an improvement in the
dewatering characteristics of press fabrics through the use of a sacrificial
reusable water displacing agent applied to the surface of the press fabric.
SUMMARY OF THE INVENTION
The present invention is directed to a method of reducing the am unt of
rewet in a press section of paper machine. The method includes steps of
providing a press fabric, depositing a water containing fibrous paper web on
the
press fabric to form a sheet, and depositing a displacing agent on one side of
the
press fabric. After pressing the combination of the press fabric, fibrous
paper
web and displacement agent in a press nip, upon exiting the press nip, the
displacement agent absorbs interfacial water and prevents, or at least
miriimizes
rewet of the paper web.
Another aspect of the present invention is directed to an apparatus for
reducing the amount of rewet of a paper web in a press nip. The apparatus
includes at least one press fabric for carrying a sheet, and a press for
applying
pressure to the press fabric and for forcing water out of the sheet and into
the
press fabric. The apparatus also includes means for applying a reusable
displacement agent to at least one side of press fabric, and recovering the
displacement agent for reuse.
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The present invention will now be described in more complete detail
with frequent reference being made to the drawings identified below.
BRIEF DESCRIPTION OF THE DRAWINGS
The following detailed description, given by way of example and not
intended to limit the present invention solely thereto, will best be
appreciated in
conjunction with the accompanying drawings, wherein like reference numerals
denote like elements and parts, in which:
Figure 1 is a profile view of a press section of a papermaking macliine
according to one einbodiment of the present invention; and
Figure 2 is a profile view of a press section of a papermaking machine
according to a second embodiment of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
A preferred embodiment of the present invention will be described in the
context of papermaking press fabrics. However, it should be noted that the
invention may find other applications with fabrics used in other sections of a
paper machine, as well as to those used in other industrial settings.
Some examples of other fabric types to which the invention is applicable
include papermakers' forming fabrics, papermakers' dryer fabrics, through-air-
drying fabrics and pulp forming fabrics.
One aspect of the present invention is directed to the use of a particulate
dispersion placed at the sheet/press fabric interface that is active in the
press nip
of a papermaking machine to successfully compete with the paper sheet in
absorbing interfacial water that is between the sheet and press fabric.
Typically,
in a pressing process interfacial water migrates into both the press fabric
and
sheet exiting a nip. . However, the migration of water back into the sheet
results
in "rewet", which is a very undesirable occurrence in papermaking.
The mechanism thought to be responsible for this undesirable rewet is
the pore size of the sheet as compared to the press fabric as they both exit
the
nip and expand. The paper sheet typically has a smaller pore size distribution
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than the press fabric. It is believed that the pore size of the paper sheet is
up to
l OX smaller than the pore size in press fabrics containing the finest surface
batt
fibers. Estimates are 0.5-1 micron mean flow pore diameters in the paper sheet
as compared to 10-20 microns in the press fabric. Very fine pores
preferentially
allow more water to re-enter the paper web structure.
Previously, efforts have been made to produce a press fabric surface
agent having pore sizes closer to those of the paper sheet. One of these
efforts
has been through the use of membranes which are integral with the press fabric
and help to expedite water removal. However, these previous attempts have
generally failed because these membranes suffer from poor durability when
compared to a traditional press fabric having a paper contacting surface
formed
of fine batt fibers.
One aspect of the present invention eliminates the durability limitation
of the prior art membranes by eliminating the requirement that the competitive
pore size material be durably affixed to the press fabric. Instead, the
present
invention, which is used to reduce operating costs while decreasing the amount
of paper rewet, is directed to the application, recovery and reformulation of
a
"displacing agent" to assist dewatering in a press nip. A further aspect of
the
present invention is directed to a particulate collection process that
recovers the
water-competitive particulate either from the sheet surface, the press fabric
surface, or the "water spray" exiting the nip.
In Fig. 1, one embodiment of the pressing process using a displacement
agent is shown. The agent 10 can be applied to the surface of press fabric 12
or
bottom side of the paper web or the sheet as a barrier to rewet and may be
either
recovered immediately by the recovery means 14 after the press section 16
action, or recovered after several pressing or drying steps then recovered for
reuse if the particle temporarily attaches itself to the paper web. The
determination of when to recover the agent may depend on the characteristics
of
the agent, as well as the individual press section configurations. The
application
of the agent can be perfonned on any press.
This concept is similar to what a catalyst does in a chemical reaction. In
essence, the agent is not consumed, but likely needs refreshing or
reformulation
in for example a reformulator 18. The agent may be applied by full width
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showers when the agent is dispersed in a fluid. Alternatively, the agent can
be a
solid and be applied from a spool in the form of a film. Also, the agent can
be
applied using conventional coating applications from a roll for example, as
would be used to apply a coating or sizing agent on a paper web.
The two primary requirements for the displacing agent are that it,
compete for boundary layer water as well or better than the paper web to be
dewatered, and that it is not intrinsically attached to the paper web that is
being
dewatered in order for subsequent collection and reformulation to occur.
A further aspect of the present invention is shown in Fig. 2 where
displacing agent is recovered from the surface of the paper sheet by contact
with
another PMC fabric or belt specially designed to attract the displacing agent.
In
this regard a displacing agent may be applied to an underside of a press
fabric as
a particulate and carried to the nip by the press fabric to interface with the
paper
sheet in the nip.
A substantial percentage of the displacing agent may temporarily adhere
to the paper sheet surface exiting the nip; however, this can be easily
removed.
In Fig. 2, a second papermaker's belt or fabric 20 may be used to attract and
remove the agent from the paper sheet 22.
The belt 20 may, for example, have a smoother surface than the paper
sheet, or perhaps a surface that capitalizes on a characteristic of the
displacing
agent to separate it from the paper sheet 22 without adversely effecting the
paper sheet 22.
Modifications to the above would be obvious to those of ordinary skill in
the art, but would not bring the invention so modified beyond the scope of the
appended claims.
EXPERIMENTS
Testing has been undertaken to demonstrate the effectiveness of the
displacing agent described above when compared to either a press fabric alone
or a press fabric in combination with a membrane. In addition, the experiments
were undertaken to show that the displacing agent could effectively be reused
and recycled. Finally, an experiment was undertaken to determine if there were
significant advantages to the use of the displacing agents and a surface
membrane. These experiments were undertaken on a device known as a press
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nip simulator ("SPNS") which can simulated the pressure, residence time, and
pressure pulse that exist in commercially available press nips. The results of
these experiments are shown in Fig. 3. Note, our Fig. 3 reference to "SRDA" is
when the sacrificial recoverable displacing agent is used.
As can be seen with reference to Fig. 3, the press fabric alone had a final
dryness of only 34.6%, whereas when the press fabric was used with the
displacing agent described herein, the final dryness increased to over 45%.
This
is exceeds the dryness achieved by a pressfabric and a surface membrane which
resulted in a dryness of 44.6%. It was found that the drying capabilities of
the
displacing agent were not significantly reduced when the agent was reused and
recycled. Final dryness for the recycled particles was 44.7% which, though
less
than on the first pass remained favorably comparable to the press fabric and
the
surface membrane. Finally, an experiment was undertaken to determine
whether combined use of a displacing agent and a surface membrane resulted in
even greater effect. The result showed that though when used-in combination
there was a somewhat increased dryness achieved, 46.1 % final dryness.
Accordingly, the testing demonstrates that the use of a displacing agent
as described by herein achieves improved drying characteristics which are
comparable or exceed that achieved by the use of surface membranes as
previously known. Further, even when reused these displacing agents retain the
ability to achieve an acceptable final dryness.
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