Note: Descriptions are shown in the official language in which they were submitted.
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MULTIPLE ZONE LIIVViITING ORIFICE DRYING
OF CELLULOSIC FIBROUS STRUCTURES,
APPARATUS THEREFOR,
AND
CELLULOSIC FIBROUS STRUCTURES PRODUCED THEREBY
FIELD OF THE INVENTION
The present invention relates to absorbent embryonic webs which are through
air dried, and particularly to cellulosic fibrous structures which are through
air dried.
BACKGROUND OF THE INVENTION
Absorbent embryonic webs are a staple of everyday life. Absorbent embryonic
webs include cellulosic fibrous structures, absorbent foams, etc. Cellulosic
fibrous
structures have become a staple of everyday life. Cellulosic fibrous
structures are
found in facial tissues, toilet tissues and paper toweling.
In the manufacture of cellulosic fibrous structures, a wet embryonic web of
cellulosic fibers dispersed in a liquid earner is deposited onto a forming
wire. The
wet embryonic web may be dried by any one of or combinations of several known
means, each of which drying means will affect the properties of the resulting
cellulosic fibrous structure. For example, the drying means and process can
influence the softness, caliper, tensile strength, and absorbency of the
resulting
cellulosic fibrous structure. Also the means and process used to dry the
cellulosic
fibrous structure affects the rate at which it can be manufactured, without
being rate
- limited by such drying means and process.
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An example of one drying means is felt belts. Felt drying belts have long been
used to dewater an embryonic cellulosic fibrous structure through capillary
flow of '
the liquid carrier into a permeable felt medium held in contact with the
embryonic
web. However, dewatering a cellulosic fibrous structure into and by using a
felt belt
results in overall uniform compression and compaction of the embryonic
cellulosic
fibrous structure web to be dried.
Felt belt drying may be assisted by a vacuum, or may be assisted by opposed
press rolls. The press rolls maximize the mechanical compression of the felt
against
the cellulosic fibrous structure. Examples of felt belt drying are illustrated
in U.S.
Patent 4,329,201 issued May 11, 1982 to Bolton and U.S. Patent 4,888,096
issued
December 19, 1989 to Cowan et al.
Drying cellulosic fibrous structures through vacuum dewatering, without the
aid of felt belts is known in the art. Vacuum dewatering of the cellulosic
fibrous
structure mechanically removes moisture from the cellulosic fibrous structure
while
the moisture is in the liquid form. Furthermore, the vacuum deflects discrete
regions
of the cellulosic fibrous structure into the deflection conduits of the drying
belts and
strongly contributes to having different amounts of moisture in the various
regions of
the cellulosic fibrous structure. Similarly, drying a cellulosic fibrous
structure
through a vacuum assisted capillary flow, using a porous cylinder having
preferential
pore sizes is known in the art as well. Examples of such vacuum driven drying
techniques are illustrated in commonly assigned U.S. Patent 4,556,450 issued
December 3, 1985 to Chuang et al. and U.S. Patent 4,973,385 issued
November 27, 1990 to Jean et al.
In yet another drying process, considerable success has been achieved drying
the embryonic web of a cellulosic fibrous structure by through-air drying. In
a
typical through-air drying process, a foraminous air permeable belt supports
the
embryonic web to be dried. Hot air flow passes through the cellulosic fibrous
structure, then through the permeable belt or vice versa. The air flow
principally
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dries the embryonic web by evaporation. Regions coincident with and deflected
into
the foramina in the air permeable belt are preferentially dried and the
caliper of the
resulting cellulosic fibrous structure increased. Regions coincident the
knuckles in
the air permeable belt are dried to a lesser extent.
Several improvements to the air permeable belts used in through-air drying
have been accomplished in the art. For example, the air permeable belt may be
made
with a high open area (at least forty percent). Or, the belt may be made to
have
reduced air permeability. Reduced air permeability may be accomplished by
applying
a resinous mixture to obturate the interstices between woven yarns in the
belt. The
drying belt may be impregnated with metallic particles to increase its thermal
conductivity and reduce its emissivity or, alternatively, the drying belt may
be
constructed from a photosensitive resin comprising a continuous network. The
drying belt may be specially adapted for high temperature airflows, of up to
about
815 degrees C. (1500 degrees F). Examples of such through-air drying
technology
are found in U.S. Patent Re. 28,459 reissued July 1, 1975 to Cole et al.; U.S.
Patent
4,172,910 issued October 30, 1979 to Rotar; U.S. Patent 4,251,928 issued
February 24, 1981 to Rotar et al.; commonly assigned U.S. Patent 4,528,239
issued
July 9, 1985 to Trokhan; and U.S. Patent 4,921,750 issued May 1, 1990 to Todd.
Additionally, several attempts have been made in the art to regulate the
drying
profile of the cellulosic fibrous structure while it is still an embryonic web
to be
dried. Such attempts may use either the drying belt, or an infrared dryer in
combination with a Yankee hood. Examples of profiled drying are illustrated in
U.S.
Patent 4,583,302 issued April 22, 1986 to Smith and U.S. Patent 4,942,675
issued
July 24, 1990 to Sundovist.
The foregoing art, even that specifically addressed to through-air drying,
does
not address the problems encountered when drying a mufti-region cellulosic
fibrous
structure. For example, a first region of the cellulosic fibrous structure,
having a
lesser absolute moisture, density or basis weight than a second region, will
typically
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have relatively greater airflow therethrough than the second region. This
relatively
Beater airflow occurs because the first region of lesser absolute moisture,
density or '
basis weight presents a proportionately lesser flow resistance to the air
passing
through such region.
This problem is exacerbated when the mufti-region cellulosic fibrous structure
to be dried is transferred to a Yankee drying drum. On a Yankee drying drum,
isolated discrete regions of the cellulosic fibrous structure are in intimate
contact
with the circumference of a heated cylinder and hot air from a hood is
introduced to
the surface of the cellulosic fibrous structure opposite the heated cylinder.
However,
typically the most intimate contact with the Yankee drying drum occurs at the
high
density or high basis weight regions, which are not as dry as the low density
or low
basis weight regions. Preferential drying of the low density regions occurs by
connective transfer of the heat from the airflow in the Yankee drying drum
hood.
Accordingly, the production rate of the cellulosic fibrous structure must be
slowed,
to compensate for the greater moisture in the high density or high basis
weight
region. To allow complete drying of the high density and high basis weight
regions
of the cellulosic fibrous structure to occur and to prevent scorching or
burning of the
already dried low density or low basis weight regions by the air from the
hood, the
Yankee hood air temperature must be decreased and the residence time of the
cellulosic fibrous structure in the Yankee hood must be increased, slowing the
production rate.
Another drawback to the approaches in the prior art (except those that use
mechanical compression, such as felt belts) is that each relies upon
supporting the
cellulosic fibrous structure to be dried. Airflow is directed towards the
celIulosic
fibrous structure and is transferred through the supporting belt, or,
alternatively,
flows through the drying belt to the cellulosic fibrous structure. Differences
in flow
resistance through the belt or through the cellulosic fibrous structure,
amplify
differences in moisture distribution within the cellulosic fibrous structure,
and/or
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5 creates differences in moisture distribution where none previously existed.
However, no attempt has been made in the art to tailor the airflow to the
differences
in various regions of the celIulosic fibrous structure.
One improvement in the art which addresses this problem is illustrated by
commonly assigned U.S. Patent 5,274,930 issued January 4, 1994 to Ensign et
al.
and disclosing limiting orifice drying of cellulosic fibrous structures in
conjunction
with through-air drying, which patent is incorporated herein by reference.
This
patent teaches an apparatus utilizing a micropore drying medium which has a
greater
flow resistance than the interstices between the fibers of the cellulosic
fibrous
structure. The micropore medium is therefore the limiting orifice in the
through-air
drying process so that an equal, or at best a more uniform, moisture
distribution is
achieved in the drying process.
The limiting orifice through-air-drying apparatus of the Ensign et a1. patent
teaches having one or more zones with either a subatmospheric pressure or a
positive pressure to promote airflow in either direction.
However, this patent (8:17-26) also teaches that as the basis weight of the
embryonic web increased, greater residence time on the micropore medium would
be
necessary, as logic would dictate. Specifically, it taught a common tissue
paper basis
weight (12 pounds per 3,000 square feet) would require a residence time of at
least
about 250 milliseconds on the micropore medium.
Applicants have unexpectedly found that the necessary residence time in the
first zone can be reduced, providing the limiting orifice through-air drying
apparatus
is divided into plural zones. Furthermore, it has unexpectedly been found that
the
overall energy consumption of the apparatus can be reduced utilizing proper
zones.
Specifically, less fan horsepower is required if the zones are properly sized
and
selected. Fan horsepower reductions of up to 10 to 15 percent over the
original
apparatus disclosed in the aforementioned Ensign et al. patent can be by
utilizing the
~
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present invention. At an advertised annual operating cost of $200 to $250 per
horsepower per year the potential savings can be significant.
Accordingly, it is an object of an aspect of this invention to provide a
limiting
orifice through-air-drying apparatus having a micropore medium which can be
used in conjunction with through-air-drying to produce cellulosic fibrous
structures. It is, furthermore, an object of an aspect of this invention to
provide a limiting orifice through-air-drying apparatus which reduces the
necessary residence time and requires less energy than had previously been
through in the prior art.
SUMMARY OF THE INVENTION
The invention comprises a limiting orifice through-air-.drying apparatus
in combination with an absorbent embryonic web having moisture distributed
therein. The embryonic web may comprise a cellulosic fibrous structure. The
embryonic web may have a consistency of at least 18 percent. The apparatus
comprises a limiting orifice for airflow through the embryonic web. The
apparatus further comprises a plurality of distinct zones, in order, at least
a
first zone and a second zone. The zones have mutually different differential
pressures relative to the atmospheric pressure.
In one embodiment, the apparatus has a water removal rate in the
second zone of at least 5 pounds of water per pound of embryonic web per
second. In a second embodiment the apparatus has a water removal rate in
the second zone at least 0.10 times as great as the water removal rate in the
first zone, while the water removal rate in the second zone is at least 5
pounds of water per pound of embryonic web per second. In a third
embodiment, the apparatus has a residence time in the first zone of less than
about 35 milliseconds.
~
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6a
In accordance with one embodiment of the present invention, a limiting
orifice though-air-drying papermaking apparatus in combination with an
absorbent embryonic web having moisture distributed therein, the apparatus
comprising a limiting orifice for airflow through the embryonic web, wherein
the apparatus further comprises a plurality of distinct zones, comprising, in
order, at least a first zone and a second zone, the distinct zones having
mutually different differential pressures relative to the atmospheric
pressure,
whereby the embryonic web has a residence time on the first zone of less
than about 35 milliseconds.
In accordance with another embodiment of the present invention, a
limiting orifice though-air-drying apparatus in combination with an absorbent
embryonic web having moisture distributed therein, and a consistency of at
least 18 percent, the apparatus comprising a limiting orifice for airflow
through
the embryonic web, wherein the apparatus further comprises at least two
distinct zones, a first zone and a second zone, the first and second zones
having mutually different differential pressures relative to the atmospheric
pressure, the apparatus having a water removal rate in the second zone at
least 0.10 times as great as the water removal rate in the first zone, wherein
the water removal rate in the second zone is at least 5 pounds of water per
pound of embryonic web per second.
In accordance with another embodiment of the present invention, a
limiting orifice though-air-drying papermaking apparatus in combination with
an absorbent embryonic web having moisture distributed therein, the
apparatus comprising a limiting orifice for airflow through the embryonic web,
wherein the apparatus further comprises at least two distinct zones
comprising, in order, at least a first zone and a second zone, the zones
having
mutually different differential pressures relative to the atmospheric
pressure,
the apparatus having a water removal rate in the second zone of at least 5
pounds of water per pound of embryonic web per second.
~
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6b
In accordance with another embodiment of the present invention, a
process for limiting orifice through-air-drying a cellulosic fibrous
structure, the
process comprising the steps of:
providing an absorbent embryonic web to be dried and having a moisture
distribution therein;
providing a means for causing airflow through the embryonic web;
providing a support member to support the embryonic web;
providing a limiting orifice through-air-drying apparatus on the side of the
embryonic web opposite the support member, so that the embryonic web
is intermediate the support member and the apparatus, wherein the
apparatus is the limiting orifice for the airflow, the apparatus having a
plurality of distinct zones for airflow therethrough, the zones having
mutually different differential pressures relative to the atmospheric
pressure;
disposing the embryonic web on the support member; and
causing airflow through the embryonic web and the apparatus; and
transporting the embryonic web relative to the apparatus, whereby the
embryonic web has a residence time in the first zone of less than 35
milliseconds.
In accordance with another embodiment of the present invention, a
process for limiting orifice through-air-drying an absorbent embryonic web,
the
process comprising the steps of:
providing an absorbent embryonic web to be dried and having a moisture
distribution therein, the embryonic web having a consistency of at least
18 percent;
providing a means for causing airflow through the embryonic web;
providing a support member to support the embryonic web;
providing a limiting orifice through-air-drying apparatus on the side of the
embryonic web opposite the support member, so that the embryonic web
is intermediate the support member and the apparatus, wherein the
apparatus is the limiting orifice for the airflow, the apparatus
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6c
having a plurality of distinct zones for airflow therethrough, the zones
having mutually different differential pressures relative to the
atmospheric pressure;
disposing the embryonic web on the support member; and
causing airflow through the embryonic web and the apparatus, whereby
moisture is removed from the embryonic web in the second zone at a
rate of at least 5 pounds of water per pound of embryonic web per
second.
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DESCRIPTION OF THE DRAWIrTGS
Figure 1 is a schematic side elevational view of a micropore medium according
to the present invention embodied on a pervious cylinder and having a
subatmospheric internal pressure.
Figure 2 is a graphical representation of relationship between consistency and
residence time on an apparatus according to the present invention.
Figure 3 is a graphical representation of energy consumption and water
removal as a function of time for the present invention (CC), a prior art
micropore
medium drying apparatus (BB) and a prior art apparatus made according to
commonly assigned U.S. Patent 4,556,450 issued December 3, 1985 to Chuang et
al.
(AA).
DETAILED DESCRIPTION OF THE INVENTION
Referring to Figure 1, the present invention comprises a limiting orifice
though-air-drying apparatus 20 in conjunction with a micropore medium 30. The
apparatus 20 and medium 30 may be made according to the aforementioned U.S.
Patent 5,274,930, the disclosure of which is incorporated herein by reference.
The
apparatus 20 comprises a pervious cylinder 32 and the micropore medium 30
circumscribing such a pervious cylinder 32. A support member 28, such as a
through-air-drying belt, wraps the pervious cylinder 32 from an inlet roll 34
to a
takeoff roll 36, subtending an arc defining a circular segment 40. This
circular
segment 40 may be subdivided into multiple zones 41, 42 having mutually
different
differential pressures relative to the atmospheric pressure. Alternatively,
the
apparatus 20 may comprise a partitioned vacuum slot or an endless belt .The
apparatus 20 removes moisture from an embryonic web.
The limiting orifice through-air-drying apparatus 20 according to the present
invention may particularly be divided into a plurality of zones. A preferred
apparatus
20 has two zones, a first zone 41 and a second zone 42. The embryonic web
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encounters, in order, the first zone 41, then the second zone 42, then
subsequent
zone(s), if any. The first zone 41 is maintained at a pressure less than the '
breakthrough pressure of the apparatus 20. The second zone 42 is maintained at
a
pressure greater than the breakthrough pressure of the apparatus 20. The
breakthrough pressure is found according to the Society of Automotive
Engineers'
Aerospace Recommended Practice 901 issued March 1, 1968, and entitled Bubble
Point Test Method, and modified to use a 50 millimeter immersion depth, and
which
Practice is incorporated herein by reference.
Collectively, the first and second zones 41, 42 may subtend an arc from about
180 to 270 degrees, more preferably 210 to 240 degrees. The first zone 41 may
comprise up to 60 degrees of the total arc subtended by the first and second
zones
41, 42 and more preferably 20 to 30 degrees.
The support member 28 transports the absorbent embryonic web relative to
the apparatus 20 and across the zones 41, 42 at a rate providing the embryonic
web
a residence time in the first zone 41 of less than 35 milliseconds, preferably
less than
25 milliseconds, more preferably less than 15 milliseconds. The residence time
in the
second zone 42 should be at least 125 and preferably at least 175
milliseconds.
As used herein, an "absorbent embryonic web" comprises a cellulosic fibrous
structure, or any other web which is deposited wet and must have the water
removed
to be in a dry state to be functional. As used herein, a web is considered
"absorbent"
if it can hold and retain water, or remove water from a surface. As used
herein,
"cellulosic fibrous structures" refer to structures, such as paper, comprising
at least
fifty percent cellulosic fibers, and a balance of synthetic fibers, organic
fillers,
inorganic fillers, foams etc. Suitable cellulosic fibrous structures for use
with the
present invention can be found in commonly assigned U.S. Patent 5,245,025
issued
September 14, 1993 to Trokhan et al., which patent is incorporated herein by
reference.
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By providing two distinct zones 41, 42, the first zone 41 having a pressure
less
than ~ the breakthrough pressure of the limiting drying orifice apparatus 20,
and the
second zone 42 having a pressure greater than the breakthrough pressure at the
aforementioned residence times, it has been found that the fan horsepower
necessary
to provide the differential pressure can be substantially reduced. Applicants
have
unexpectedly found that further drying, and hence increases in consistency, do
not
substantially increase after more than the aforementioned residence times in
the first
zone 41 occur, as illustrated by Figure 2.
By properly selecting the residence time in the first zone 41, then
transferring
the embryonic web to the second zone 42, the e~ciency of the drying process
can be
maximized and the fan horsepower reduced. For the invention described and
claimed herein, the apparatus 20 has a water removal rate in the second zone
42 of
at least 5, and preferably at least 7, pounds of water per pound of embryonic
web per
second.
The proper transition point between the first and second zones 41, 42 is that
point at which the water removal rate of the second zone 42 exceeds the water
removal rate of the first zone 41. The actual transition point is where the
differential
pressure through the apparatus 20, relative to atmospheric, goes from less
than the
breakthrough pressure to greater than the breakthrough pressure. The system is
optimized when the actual and the proper transition points are coincident. It
is
recognized that the exact transition point will depend upon the porosity and
drainage
capabilities of the absorbent embryonic web, the flow characteristics and size
of the
orifices in the micropore medium, and perhaps other factors as well.
The second zone 42 may be partitioned into one or more subzones, each
having a dedicated fan or may be maintained without a partition and have a
single
" 30 large fan as desired. Alternatively, a single zone 41 or 42 may have its
differential
pressure generated by two or more fans. The fans may be arranged in series or
in
parallel. It is generally believed that the horsepower requirements of two
smaller
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5 fans or one larger fan, having the same total horsepower, are very similar
as used in
conjunction with the present invention.
Since the first zone 41 is run at less than breakthrough pressure, it does not
require a fan and may work well with a vacuum pump. Thus, the first zone 41
consumes only minimal energy in the apparatus 20 according to the claimed
10 invention. As used herein, the unit horsepower refers only to the
horsepower
necessary to create the differential pressure in the apparatus 20, and does
not include
horsepower necessary to transport the embryonic web relative to the apparatus
20.
For the invention described and claimed herein, the ratio of the drying rate
of
the second zone 42 to the drying rate of the first zone 41, as measured in
pounds of
water per pound of embryonic web per second per unit horsepower, is at least
0.10
times as great, and preferably at least 0.12 times as great. Of course this
ratio can be
artificially inflated by running an inefficient first zone 41. For purposes of
the
present invention, the first zone has a water removal rate of at least 40
pounds of
water per pound of embryonic web per second. There is minimal horsepower
involved in the water removal rate of the first zone 41, since the first zone
41 relies
upon capillary dewatering which occurs below the breakthrough pressure, and
does
not rely upon a fan to create airflow above the breakthrough pressure.
The aforementioned residence times are useful for an embryonic web having a
pulp filtration resistance (PFR) of 5 to 20, and preferably from 10 to 11.
Pulp
filtration resistance is measured according to the procedure set forth in
commonly
assigned U.S. Patent 5,228,954 issued July 20, 1993 to ~nson et al., which
patent is
incorporated herein by reference.
Referring to Figure 2, it is to be recognized that the drying rate in the
first
zone 41 varies according to PFR. The drying rate in the second zone 42 is the
same
for all three curves A, B and C. Curves A, B and C in Figure 2 show, in order,
increasing PFR.
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Generally, it has been found that the optimum residence time on the apparatus
20 is directly proportional to the pulp filtration resistance. The incoming
embryonic
web has a consistency of at least 18 percent, and possibly at least 19
percent.
The apparatus 20 according to the present invention has a greater water
removal capability for a given PFR than is obtainable with prior art porous
cylinders
which dry the web by capillary attraction and are maintained at less than
breakthrough, as illustrated in commonly assigned U.S. Patent 4,556,450 issued
December 3, 1985 to Chuang et al., the disclosure of which is incorporated
herein by
reference; prior art woven support members 28, and prior art photosensitive
resin
support members 28.
Water removal rate is measured in terms of pounds of water removed per
pound of fiber divided by the time the fibers are subjected to the process
rate = (pounds of water removed/pounds of fiber)/time in seconds
The water removal rate is ascertained by measuring the consistencies of the
embryonic web before and after the zone 41, 42 in question using gravimetric
weighing and connective drying to achieve a bone-dry baseline. The residence
time
can be easily calculated knowing the path length of the zone 41, 42 and the
velocity
of the embryonic web.
Referring to Figure 3, one will note that the water removal rate in zone 2 is
considerably higher in the apparatus according to the present invention than
is the
water removal rate from the cylinder made according to the aforementioned
Chuang
et al. patent
The apparatus 20 according to the present invention has a water removal rate
of at least 5 pounds of water per pound of embryonic web per second, and more
preferably at least 7 pounds of water per pound of embryonic web per second in
the
s 30 second zone 42. The apparatus 20 according to the present invention has a
water
removal rate of at least 40 pounds of water per pound of embryonic web per
second,
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and more preferably at least 50 pounds of water per pound of embryonic web per
second in the first zone 41.
The apparatus 20 according to the present invention has a power consumption
of less than 5, and preferably less than 4 horsepower per square foot of web
area
subjected to the process in the first zone 41. The apparatus 20 according to
the
present invention has a power consumption of less than 20, preferably less
than 18,
and more preferably less than 16 horsepower per square foot of web area
subjected
to the process in the second zone 41.
