BACKGROUND OF THE INVENTION
1. Field of the Invention
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This invention relates to a process and apparatus for the continuous oxidation of organic fibers.
2. Description of the Prior Art
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It is well known that carbon fibers 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. The focus of the present invention is on the oxidation step.
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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 400°C 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.
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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 for oxidizing an acrylic fiber or strand. These references show multiple rollers external to the oxidizing furnaces. The references are concerned 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 various parts of the oxidizing apparatus to control temperature and quench fires.
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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.
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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.
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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 attempt 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.
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None of 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.
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.
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Accordingly, in one embodiment, the present invention is a process for continuously oxidizing organic fibers. 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 atmosphere and oxidizing conditions are maintained throughout the interior 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 then 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 through 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 first 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 last 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
- · 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
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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
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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 200°C to about 300°C 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 3,000°C in a non-oxidizing atmosphere. Carbon fibers have numerous uses, most notably in the construction of high strength composites.
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Although the prior art references teach the importance of maintaining control over the maximum temperature of the 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.
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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). At 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 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 difference 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.
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Of great importance to the present invention is the 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 layers that cannot be achieved by prior art processes that treat only single strands of fiber.
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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 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 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 just large enough to allow passage of the web without undue friction due to contact between the web and edges of the slots.
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The primary functions of the cooling cylinders are to absorb heat from the fiber web with which it is in contact 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.
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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.
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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''.
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An oxidizing atmosphere maintained inside and circulated through chamber 1 is preferably air introduced into chamber 1 by conventional means not shown. It is important to maintain 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. The 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.
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Web 2 passes through chamber 1, preferably horizontally, 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 cooling 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 longitudinal axis or have such rotation powered by motors.
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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 each 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.
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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.
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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 oxidized 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.
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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.
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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 sheet the more effectively heat can be radiated from the sheet and conducted from the interior of the sheet to its surface, so 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.
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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.
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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 the 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 the invention is from about 10 meters/minute to about 50 meters/minute.
