Note: Descriptions are shown in the official language in which they were submitted.
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PROCESS AND APPARATUS FOR THE ;~
HIGH SPEED OXIDATION OF ORGANIC FIBER
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1. Field of the Invention
This invention relates to a process and apparatus for
the continuous oxidation of organic fibers. -
2. Description of the Prior Art
It is well known that carbon fibPrs may be produced from
organic fibers, particularly polyacrylonitrile fibers. The
fibers are first oxidized in an oxidizing gas and then
carbonized in an inert atmosphere. ~he focus of the present
invention is on the oxidation step.
U.S. Patent No. 4,609,540 discloses a process for ~-
producing a carbon fiber from a polyacrylonitrile-type
polymer fiber by subjecting the fiber to an oxidizing
atmosphere at 200 to 400C in a treatment furnace. Driving -
rolls external to the furnace are employed and controlled so
as to achieve the desired multistep elongation. This
reference is not concerned with heat balance or close
temperature control of the fiber.
U.S. Patent Nos. 4,671,950; 4,069,297; 4,517,169;
4,536,448; 4,545,762 and 4,559,010 also disclose methods or
oxidizing an acrylic fiber or strand. These references show
multiple rollers external to the oxidizing furnaces. The
references are conc~rned with various aspects of the
oxidation process such as control over fiber shrinkage,
recycle of spent oxidizing gas, coating of the acrylic fiber
with an ammonium salt prior to oxidation, controlling
temperature variances in the furnace and spraying water into
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various parts of the oxidizing apparatus to control
temperature and quench fires.
Japanese Application No. 57-40923 shows oxidation of a
strand of polyacrylic yarn via a series of passes of the yarn
through a hot active atmosphere with cooling of the yarn
between passes by rollers which also serve to reverse
direction of each pass.
U.S. Patent No. 4,461,159 teaches dissipating the
exothermic heat of reaction from the oxidation of "tows" of
acrylic fiber bundles in an oxidation chamber by having walls
in the oxidation chamber of high thermal conductance and
emissivity for absorption of heat from the fibers and, where
there are multiple fiber layers, avoidance of exchange of
radiant heat between layers.
It is also known in the art to oxidize a web of
polyacrylonitrile fibers by passing the web in parallel
layers of alternating direction through an oxidizing chamber
while using external rollers to reverse direction of the web.
Although some heat dissipates from the web by convection with
and radiation into the air when the web passes around the
rollers, no aktempt is made to remove significant amounts of
heat via the rollers or maintain control of heat removal at
the rollers. These prior art means rely on very large
amounts of air blown through the oxidation chamber to keep
the web temperature from becoming excessive and the highest
linear velocity at which the web can be passed through the
chamber is about 4-7 meters/minute.
None oP the above prior art techniques are concerned
with conservation of heat in the oxidation furnace as well as
control of the fiber temperature so as to enable maximization
of the rate of production of oxidized fiber layers.
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SUMMARY OF THE INVENTION
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It is thus a primary objective of the present invention
to continuously oxidize a continuous web of organic fibers
and make highly efficient use of the heat supplied to the
oxidation chamber while controlling the fiber web temperature
and maximizing the rate at which the web is passed through
the chamber.
Accordingly, in one embodiment, the present invention is
a process for continuously oxidizing organic fiber~. The -
process comprises introducing a continuous web of organic
fibers as a first layer into an oxidizing chamber through an
entrance opening into the chamber. An oxidizing atmosphare ~ -
and oxidizing conditions are maintained throughout the
intPrior of the chamber. The interior walls of the chamber
comprise reflective surfaces. The first layer of the web is
passed through the chamber and out of an exit opening. The
web is then passed over the circumference of a first cooling
roll external to the chamber which reverses the direction of
travel of the web. The web of reversed direction is the~
passed as a second layer into the chamber through a second
opening, the first layer and second layer being substantially
parallel and in close proximity. The second layer is passed
throuyh the chamber and out of a second exit opening. The
above sequence is repeated with the continuous web with
additional cooling rolls, entrance openings, parallel layers
of close proximity and exit openings. Also controlled are
the temperature and circulation of the oxidizing atmosphere
in the chamber and the amount of heat removal of the cooling
rolls to the extent necessary to achieve the desired degree ;~
of oxidation of the organic fibers, while avoiding excessive
temperature on any part of the web and while maximizing the
linear velocity of the web through the chamber.
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In a second embodiment the present invention is an
apparatus suitable for the continuous oxidation of organic
fibers comprising:
an oxidizing chamber having internal walls with highly
reflective surfaces;
two opposite facing walls of the oxidizing chamber, each
wall having a series of slots therethrough able and
aligned in a manner to accommodate tight passage of a
continuous web of fibers in alternating parallel layers
passing back and forth through the chamber;
means for feeding the continuous web through a ~irst of
the slots;
cooling cylinders external to the chamber and aligned
with the chamber in a manner to enable layers of the web
exiting the chamber to pass around the circumference of
a cooling cylinder and be in contact with the surface
thereof and reenter the chamber through another slot so
as to comprise the next alternating layer;
means for passing a cooling medium through each cooling
cylinder to enable absorption of heat transferred from
the web;
drawing and collecting means to pull the web through the
la~t slot of the chamber through which the web exits at
the desired linear velocity, and to retain the web;
means for providing heated oxidizing gas to the
oxidizing chamber and for circulating and withdrawing
the gas from the chamber; and
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control means to control the rate of flow of cooling
medium through the cooling cylinders, to control the
circulation and temperature of oxidizing gas to and
through the oxidizing chamber and to maximize the rate
of drawing the web through the chamber while achieving
the desired degree of oxidation of the fibers in the web
while preventing their melting or other degradation
because of excessive temperature on any part of the web.
BRIEF DESCRIPTION OF THE DRAWING
Figure 1 is a plane view of the apparatus of the present
invention, including oxidizing chamber 1, web 2, entrance
slots 3, 3' and 3", exit slots 4, 4' and 4", cooling
cylinders 5, 5' and 5" and various temperature control
systems associated with the oxidizing chamber and cooling
cylinders. ;~
DETAILED DESCRIPTION OF THE INVENTION ~ :
The present invention encompasses a unique design for a
highly efficient oxidation system for use in carbon fiber
manufacture. In view of the prior art discussed above, the
chemistry of such manufacture is well known. Typically
organic fibers such as polyacrylonitrile fibers are contacted
with an oxidizing atmosphere, such as air, at a temperature
of from about 200C to about 300C to raise~the oxygen `~
content of the fibers to the desired level and the resultant
preoxidized fibers are then carbonized at temperatures as
high as 3,000C in a non-oxidizing atmosphere. Carbon fibers
have numerous uses, most notably in the construction of high
strength composites.
Although the prior art references teach the importance
of maintaining control over the maximum temperature of the ~
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fibers in the course of oxidation so as to avoid degradation
of the fibers through melting or burning, those references
indicate that almost no thought was given to the optimization
of the process so as to minimize energy requirements and
maintain close control over fiber temperature while
maximizing the throughput of the fiber through the oxidation
chamber or furnace. As will be later discussed, such
optimization cannot be achieved where the process is dealing
with a single fiber or fiber strand that is being passed
through an oxidizing chamber, even with multiple passes and
the use of cooling cylinders external to the chamber. In the
one reference found (U.S. Patent No. 4,461,159) where the
fiber is pulled through the chamber in the form of a layer,
the design of the chamber is such that transfer of heat from
one layer to another is precluded and the internal walls of
the chamber are specifically designed to absorb the
exothermic heat of oxidation of the fibers.
In marked contradistinction to the teaching of the prior -~
art the present invention retains heat to the extent possible
within the oxidizing chamber and maintains close control over -
the maximum temperature likely to be reached by the fiber web
by the precisely controlled heat removal from the web via the
cooling cylinders or rollers external to the chamber.
Retaining heat within the oxidizing chamber is accomplished
by having the internal walls comprise reflective (mirrored)
surfaces and by having adjacent alternate layers of web which
are passed back and forth through the chamber being in as
close proximity as possible so as to maximize the transfer of
heat by radiation from the hot part of one layer (the part
leaving the chamber) to a relatively cool part of an adjacent
layer (the part entering the chamber). ~t least one way of
precisely controlling the maximum temperature of the fiber -
web with the cooling cylinders is to measure the difference
in temperature of a layer of the web leaving the oxidation
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chamber and the temperature of the web after it contacts the ~ - -
cooling cylinder in question and using that measurement as
input to an automatic controller that compares the measured
temperature di~ference with a "setpoint" and regulates a
valve in accordance with the difference between the measured
difference and setpoint which in turn regulates the quantity
of flow of cooling medium, such as water, through the
interior of the cooling cylinder.
Of great importance to the present invention is the
10 nature of the fiber web. Ideally, the web would comprise a `
solid sheet of fiber with no gaps between fibers or fiber
bundles so that cool portions of a layer could completely
absorb heat radiated from hot portions of an adjacent layer.
It is this distribution and conservation of heat between
15 layers that cannot be achieved by prior art processes that
treat only single strands of fiber.
The interior highly reflective surfaces of the oxidation
chamber reflect heat back to the web and minimize
uncontrolled heat loss from the chamber. This is an
20 exceptionally important consideration for the interior
surfaces at the ends or opposite walls of the chamber where
the slots for the ingress and egress of the web are cut. The
slots are positioned and aligned with respect to the
oppositely facing walls to accommodate passage of the
25 continuous web in alternating parallel layers passing back
and forth through the chamber, a layer becoming the adjacent
alternating layer upon reversal of direction when passed
around a cooling cylinder. Such passage through the slots
should be tight, meaning that the dimensions of the slots are -~
30 just large enough to allow passage of the web without undue
friction due to contact between the web and edges of the ;
slot~.
The primary functions of the cooling cylinders are to
absorb heat from the fiber web with which it is in contact
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and to reverse direction of the web and serve as a guide to
the web in the course of its passage out of and into the
oxidizing chamber. To facilitate the former function the
cylinders are constructed of materials and have internal and
external accommodation for the cooling medium in accordance
with principles well known to those in the art dealing with
heat exchangers. The latter function may be best served by
having one or more of the cylinders rotate about its
longitudinal axis, perhaps even by being motor driven.
The carrying out of the process and operation of the
apparatus of the present invention may be illustrated by
reference to Figure 1. Oxidizing chamber 1 is shown in a
preferred embodiment as essentially a large box. There are
slots 3, 4' and 3" shown through one wall of chamber 1 and
slots 4, 3' and 4" through an opposite facing wall, but there
can be any number of slots, depending on the number of passes
of web desired as will be later discussed. The interior
surfaces of chamber 1 are highly reflective, even mirrored,
so as to minimize the loss of heat through its walls.
Fiber web 2 is a flat sheet comprising "tows" of
continuous multifilament bundles of organic fibers,
particularly polyacrylic fibers, each bundle containing about
1,000 to about 160,000 individual fibers. The sheet of
fibers, which can be as wide as the apparatus is able to
accommodate, is supplied via a feeding means not shown,
probably from a large roll mounted in close proximity to a
first entrance slot 3. The web is pulled through slot 3 ~ ~
which like all of the slots has dimensions just large enough ~ ~;
to accommodate the web. The pulling means for the web is not
shown, but it could be a motor driven roller that collects
the web after it passes around cooling cylinder 5".
An oxidizing atmosphere maintained inside and circulated
through chamber l is preferably air introduced into chamber 1
by conventional means not shown. It is important to maintain
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an elevated temperature in the oxidizing atmosphere, high
enough to enable oxidation of the organic fiber, but not so
high as to cause degradation of the fiber through burning or
melting. ~he oxidizing gas introduced into chamber 1 will be
heated, but a large part of the heat in chamber 1 will be
generated by the exothermic heat of reaction as the organic
fiber is oxidized.
Web 2 passes through chamber 1, preferably horiæontally,
and exits through first exit slot 4 after which it passes
around while in contact with the surface of cooling cylinder
5. Cooling cylinder 5 as well as the other cooling
cylinders, including 5' and 5", functions as a heat exchanger
by absorbing heat from web 2 through the surface of the
cylinder and into a cooling medium such as water which
circulates through the internals of each cooliny cylinder.
The cooling cylinders also serve to change or reverse the ~ ~;
direction of travel of web 2 and, to facilitate their
interaction with web 2, may rotate freely about their i~;~
longitudinal axis or have such rotation powered by motors.
Coolant flow controllers 9, 9' and 9" receive input from
infrared temperature sensors 7, 7' and 7" which measure the
temperatures of web 2 after each chamber exit, but before
contact with the next cooling cylinder, and infrared
temperature sensors 8, 8' and 8" which measure the
temperatures of web 2 after passing around eaah cooling
cylinder. A coolant flow controller calculates the
temperature differential of web 2 before and after contact
with the cooling cylinder with which it is associated and ;
controls the volume of flow of coolant through that cylinder
in response to the calculated temperature differential with
the objective being to maintain a preset or "setpoint"
temperature differential in the web before and after each
cooling cylinder.
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Thermocouple 6 shown in the oxidation chamber may be
located anywhere in the chamber, but probably about where the
oxidizing gas is introduced into the chamber. The objective
is to maintain the temperature of the gas at this location at
a desired predetermined level. Although not shown in Figure
1 thermocouple 6 would be associated with the controller,
heat source, fans, etc. necessary to deliver the oxidizing
gas in the amount and at the temperature required.
Also not shown in Figure 1 are means for circulating the
oxidizing gas within chamber 1, such as fans and internal or
external conduits so as to obtain as homogeneous an oxidizing
atmosphere within chamber 1 as possible.
Although Figure 1 shows only three passes of the web
through the oxidizing chamber and associated cooling ~ -~
cylinders with controllers, it should be understood that the
number of passes may and probably will be much higher. Such
number depends on the degree of oxidation required and the
desired quantity of throughput of web through the apparatus. -
Also not shown is the disposition of oxidiæed web 2 after
contact with cooling cylinder 5", but it is understood that
the web will be further processed, probably carbonized at
high temperature in a non-oxidizing atmosphere. - ;~
It may also be desirable to have a second or more
oxidation chambers in series with the first having internal
and external construction and control means, similar to that
of the first, particularly if further oxidation is required
perhaps, but not necessarily at different conditions, either
less or more severe.
Assuming fiber filaments of from 0.5 to about 10 denier,
a sheet having fiber spaced at about 20,000 filaments per cm
would be about 90% closed (free of gaps). However, a
countervailing consideration is that the thinner the ~heet
the more effectively heat can be radiated from the sheet and
conducted from the interior of the sheet to its surface, so
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there is an optimum filament count between 20,000 per cm and ~-
10,000 per cm the latter of which would be the ideal count if
only the efficiency of conduction of heat from the interior
of the web was being considered. The count is, therefore,
preferably between 10,000 per cm and 20,000 per cm. The
width of the sheet may be as large as the dimensions of the
oxidation chamber and associated apparatus will permit.
The alternating layers of web are substantially parallel
and in close proximity so as to facilitate transfer of heat
by radiation and convection from the hot portions of one ~ ;
layer to the relatively cool portions of the adjacent layers.
How close the adjacent layers might be to each other is
dictated by practical considerations in the manufacture of
the apparatus, particularly with regard to the drawing or
feeding of the continuous web through the oxidation chamber
and the minimum diameter of the cooling cylinders possible in
view of the countervailing requirements of minimum surface
area of the external surface of the cylinder in contact with
the web to achieve the desired heat transfer and internal
accommodation in the cooling cylinders for passage of the
cooling medium. An acceptable distance from a surface of one
layer to the closest surface of the next adjacent layer is
from about 2 cm to about 20 cm.
The essential purpose achieved by the process and
apparatus of the present invention is to oxidize organic
fiber as efficiently as possible without degradation of the
fiber, including a rate of production not heretofore realized ;~
by prior art devices. The combination of conserving the heat
in the oxidation chamber, distributing it evenly between
layers of fiber web within khe chamber and the precise
control of web temperature enables the desired high rate of
production. The expected rate of production in terms of the
linear velocity of the fiber web through the apparatus of ths
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invention is from about 10 meters/minute to about 50
meters/minute.
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