METHODS AND SYSTEMS FOR HCN REMOVAL

Abstract

Hydrogen cyanide removal during transformation of a carbon fiber precursor into carbon fiber is provided. The method may comprise heating a carbon fiber body precursor at a substantially atmospheric pressure to above about 1200° C. in a furnace in a first stage, expelling a stream of effluent gas outside the furnace, wherein the stream of effluent gas comprises a cyanide, thermally oxidizing the cyanide during the first stage, and heating the carbon fiber body precursor to a temperature of between about 1600° C. and about 2200° C. in a second stage.

Description

FIELD OF INVENTION

The present invention generally relates to methods and systems for HCN removal, and more particularly, to methods and systems for HCN removal during transformation of a carbon fiber precursor into carbon fiber.

BACKGROUND OF THE INVENTION

Industrial applications of ceramics have become increasingly important over the last fifty years. Monolithic ceramics and cermets, however, exhibit low impact resistance and low fracture toughness. Ceramic Matrix Composites (CMCs) exhibit some useful thermal and mechanical properties and hold the promise of being very good materials for use in high temperature environments and/or in heat sink applications. CMCs generally comprise one or more ceramic materials disposed on or within another material, such as, for example, a ceramic material disposed within a structure comprised of a fibrous material. Fibrous materials, such as carbon fiber, may be formed into fibrous bodies suitable for this purpose.

Carbon fiber bodies are typically formed from carbon fiber body precursors. For example, preoxidized polyacrylonitrile (PAN) is commonly used as a carbon fiber body precursor. Carbon fiber body precursors may be manipulated and fabricated in a manner similar to a textile (e.g., weaving, knitting, etc) to form desired structures. To transform the carbon fiber body precursor into a carbon fiber body, various methods and techniques may be used. For example, during transformation of PAN materials, the PAN fiber may be carbonized and then processed to eliminate metals or other impurities that may be found in the PAN fiber.

Transformation of carbon fiber body precursors, such as PAN fibers, often occurs in a two stage process. The first stage may be a carbonization stage. A carbonization stage is typically performed at temperatures of less than 1100° C., and most typically between about 800° C. and 950° C. The second stage may be a high temperature stage, typically using temperatures over 1400° C.

However, the carbonization causes the PAN fibers to release various cyanides (such as HCN) in a gaseous state, and the subsequent management of the residual impurities is often problematic. For example, cyanides like HCN are toxic and pose an environmental hazard. Conventional means of managing cyanides such as HCN are not satisfactory because cyanides are often released along with other hazardous materials and/or problematic gaseous materials. Moreover, it may be difficult to separate HCN from other materials for appropriate processing. Further still, where HCN is emitted in a second stage, HCN may be drawn into downstream equipment, such as a steam vacuum. The presence of HCN in a steam vacuum may damage the steam vacuum and, in some case, endanger those in the vicinity. Conventional means make it difficult for cyanides to be released during a carbonization stage.

Accordingly, there is a need for managing cyanide release in a controlled manner such that the cyanides can be managed appropriately. For example, there is a need to release HCN in a carbonization stage so that HCN may exit a furnace system prior to the use of a vacuum generating device, such as a steam vacuum.

SUMMARY OF THE INVENTION

In various embodiments, methods and systems for HCN removal are provided. For example, a method may include transforming a carbon fiber body precursor into a carbon fiber body, comprising heating carbon fiber body precursor at a substantially atmospheric pressure to above about 1200° C. in a furnace in a first stage, expelling a stream of effluent gas outside the furnace, for example, by using the furnace's internal pressure, wherein the stream of effluent gas comprises a cyanide, thermally oxidizing the cyanide during the first stage, and heating the carbon fiber body precursor to a temperature of between about 1600° C. and about 2200° C. in a second stage.

Various embodiments also include a system for transforming a carbon fiber body precursor into a carbon fiber body, comprising a furnace having a gas inlet, a thermal oxidizer, wherein the furnace is configured to heat a carbon fiber body precursor at a substantially atmospheric pressure to above about 1200° C. in a first stage, wherein the furnace is configured to expel a stream of effluent gas outside the furnace, wherein the stream of effluent gas comprises a cyanide, wherein the thermal oxidizer is configured to thermally oxidize the cyanide during the first stage to yield a less toxic material.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of an exemplary embodiment including a furnace; and

FIG. 2 is a flow chart illustrating an exemplary embodiment of a method.

DETAILED DESCRIPTION

The detailed description of exemplary embodiments herein makes reference to the accompanying drawings, which show exemplary embodiments by way of illustration and its best mode. While these exemplary embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, it should be understood that other embodiments may be realized and that logical, chemical and mechanical changes may be made without departing from the spirit and scope of the invention. Thus, the detailed description herein is presented for purposes of illustration only and not of limitation. For example, the steps recited in any of the method or process descriptions may be executed in any order and are not necessarily limited to the order presented. Moreover, many of the functions or steps may be outsourced to or performed by one or more third parties. Furthermore, any reference to singular includes plural embodiments, and any reference to more than one component or step may include a singular embodiment or step. Also, any reference to attached, fixed, connected or the like may include permanent, removable, temporary, partial, full and/or any other possible attachment option. Additionally, any reference to without contact (or similar phrases) may also include reduced contact or minimal contact.

As noted above, in various embodiments, methods and systems are provided herein that manage cyanide (e.g., HCN) in a carbon fiber production process. In particular, systems and methods are disclosed herein that improve the separation of HCN from other materials in a gaseous stream so that HCN management may be improved. In this regard, it has been found that, contrary to conventional methods, use of a higher carbonization stage temperature assists in the separation and elimination of HCN from a gaseous stream of a carbon fiber production process.

As used herein, a carbon fiber production process may be any process that results in forming a carbon fiber body. Additionally, as used herein, a carbon fiber body may comprise any material containing carbon fiber. As noted above, carbon fiber bodies are typically formed from carbon fiber body precursors in a carbon fiber transformation process. As used herein, a carbon fiber body precursor may comprise any material containing a carbon fiber precursor. Carbon fiber precursors include preoxidized polyacrylonitrile fiber, rayon fibers, and pitch fibers.

As noted above, to transform the carbon fiber body precursor into a carbon fiber body, various methods and techniques may be used. For example, during transformation of PAN materials, the PAN material may be carbonized and then processed to eliminate impurities (often comprising metals and metallic compounds) that may be found in the PAN material. HCN and/or other cyanides may be released from PAN fibers during carbon fiber precursor processing.

Transformation of carbon fiber body precursors, such as PAN fibers, often occurs in a two stage process in a process vessel, such as a furnace. The first stage may be referred to as a carbonization stage or carbonization. A carbonization stage is conventionally performed at temperatures of less than about 1100° C., and most typically between about 800° C. and about 950° C. Carbonization is typically performed from about atmospheric pressures (e.g., about 1 atm) to elevated pressures (e.g., about 1.5 atm).

The second stage may be a high temperature stage, typically using temperatures over about 1400° C., up to and about 2400° C. The second stage may be performed in a vacuum or partial vacuum. For example, a pump or steam vacuum may be used to pull a vacuum in a furnace suitable for the second stage. The second stage is referred to herein as a second stage or a second stage of a carbon fiber transformation process.

During carbonization in stages such as those described above, HCN and other cyanides may enter a gas form and exit the furnace in a gaseous stream, usually by way of a pipe. During the second stage, HCN and other cyanides also may enter a gas form, often in higher concentration than during carbonization, and exit the furnace in a gaseous stream, usually by way of a pipe. As HCN and other cyanides are toxic and pose environmental hazards, carbonization processes benefit from systems and methods to manage and/or eliminate HCN.

Conventional carbonization is performed below temperatures of about 1100° C. because it is generally believed that temperatures above about 1100° C. may cause an unwanted elimination of metallic impurities (e.g., sodium) during carbonization, when it is desired that such impurities are not removed until the second stage. Conventionally, most, if not all, cyanides are released in the second stage. However, because the second stage occurs in a vacuum created by a vacuum device (e.g., pump or steam vacuum), cyanides may become trapped in the vacuum device. This may damage the vacuum device and may pose a hazard for those near the device.

It has been found that, contrary to conventional teachings, performing carbonization at temperatures above about 1100° C. allows HCN and other cyanides to be expelled from a furnace. The HCN and other cyanides may be conducted to a thermal oxidizer and thermally oxidized (i.e., burned) to yield other, less or nontoxic materials, such as carbon dioxide, water, and nitrous oxides.

Further, it has been found that, in various embodiments, performing carbonization at temperatures above about 1100° C. allows HCN to be separated from other materials that are released by the carbon fiber precursor bodies.

Still further, it has been found that, in various embodiments, performing carbonization at temperatures above about 1100° C. allows HCN to be removed in the carbonization stage. As the carbonization is performed under substantially atmospheric conditions, HCN need not come into contact with a vacuum device and, instead, may be forwarded to a thermal oxidizer, for example, by virtue of the internal pressure of the furnace itself.

Accordingly, with reference now to FIG. 1, an embodiment of HCN elimination system 100 comprising a furnace 108 which is used to contain and carbonize carbon fiber body precursors is illustrated. In various embodiments, furnace 108 is any furnace capable of achieving the temperatures used in the improved carbon fiber production process. Furnace 108 has inlet 102 and outlet 104. Inlet 102 and outlet 104 are configured to conduct gas in and out of furnace 108. Furnace 108 may also have seal flanges 110 that may be assembled (e.g., bolted) together to prevent unintentional gaseous emissions.

During conventional carbonization, non-cyanide impurities such as hydrocarbons may be released into gaseous form at temperatures below about 1100° C. At such temperatures, various emissions management systems are used to address these emissions. However, significantly, at temperatures above about 1100° C., such as at about 1200° C., hydrocarbon emissions decrease. Thus, in various embodiments, when carbonization is performed at about 1200° C., the gaseous emissions are comprised substantially of HCN. For example, gaseous stream 121 resulting from carbonization may contain HCN.

Thus, to address the HCN that is created, in various embodiments, carbonization is performed in furnace 108 at temperatures above about 1100° C. For example, in various embodiments, carbonization may be performed at temperatures from above about 1100° C. to about 1400° C., from about 1200° C. and about 1300° C., and above about 1200° C. Carbonization may be performed in furnace 108 at pressures of from about 1 atm to about 1.5 atm. For example, in various embodiments, carbonization may be performed at about 1 atm and at about 1.5 atm. Carbonization performed within these temperature and pressure ranges may improve the release of HCN and other cyanides from a furnace. For example, HCN and other cyanides may be expelled (i.e., automatically eliminated) from the furnace, for example, by virtue of the internal pressure of the furnace, requiring little to no pump or steam vacuum assistance. This is beneficial because pumps and/or steam vacuums may be damaged by prolonged exposure to HCN, in addition to creating a hazardous environment for those in proximity to such equipment. In this regard, such a process enhances the useful life of vacuum devices.

In this regard, HCN may be caused to flow out of a furnace via the internal pressure of the furnace. In various embodiments, to assist HCN flow, a furnace may have an inlet for the acceptance of a secondary gas which may be pumped into a furnace inlet to push HCN and/or other cyanides out of the furnace. For example, with reference to FIG. 1, a furnace may have a secondary gas injected into it via inlet 102 to assist in the expulsion of gaseous materials from inside the furnace. A secondary gas may comprise any substantially inert gas such as nitrogen gas (N₂) and any noble gas such as helium, argon, neon, and krypton.

In various embodiments, carbonization may be performed at atmospheric or substantially atmospheric pressures. However, the pressure in furnace 108 may be raised by, for example, a secondary gas pumped into inlet 102.

In an embodiment, with continued reference to FIG. 1, upon expulsion from a furnace, HCN in gaseous stream 121 is directed to a thermal oxidizer 116. In various embodiments, HCN is directed to a thermal oxidizer via a pipe. For example, pipe system 106 may be used to conduct gaseous stream 121 to thermal oxidizer 116. Pipe system 106 may be made from any material suitable for this purpose. For example, pipe system 106 may be made from metal and may be wholly or partially lined (e.g., refractory lined).

In various embodiments, various valves and valve like devices may be used to conduct HCN in gaseous stream 121 to thermal oxidizer 116. For example, valve 124 and valve 120 may be used to direct the flow of gaseous stream 121. Valve 124 and valve 120 may be made from any material suitable for this purpose. For example, valve 124 and valve 120 may be made from metal and may be wholly or partially lined (e.g., refractory lined). Valve 120 may be configured to be at least partially open to allow gaseous stream 121 to proceed to thermal oxidizer 116. Valve 124 may be configured to be closed to prevent gaseous stream 121 from proceeding to steam vacuum system 112. At times when gaseous stream 121 is not present, and thus no HCN is present, valve 124 may be configured to be at least partially open.

As noted above, in various embodiments, a thermal oxidizer may be provided. As used herein, a thermal oxidizer may be any device capable of housing a combustion reaction. For example, a thermal oxidizer may mix air and/or added oxygen to the gaseous HCN and provide an ignition source. The combustion of HCN may yield compounds that are less toxic than HCN, including carbon dioxide, water, nitrous oxides, and other like compounds. As used herein, less toxic compounds refer to compounds that result from the combustion of HCN. For example, thermal oxidizer 116 is in fluid communication with pipe system 106. Thermal oxidizer is configured to incinerate or otherwise burn HCN and other materials for emission. Thermal oxidizer 116 may comprise any known or hereinafter developed form of thermal oxidizer. For example, thermal oxidizer 116 may be of a direct fired (i.e., afterburner) type, a recuperative type, a regenerative type, a catalytic type and/or a flameless type. Suitable thermal oxidizers are available from John Zink Company, LLC, 11920 East Apache, Tulsa, Okla. USA 74116. In addition, suitable thermal oxidizers are available from Process Combustion Corporation, 5460 Homing Road, Pittsburgh, Pa. USA 15236, MRW Technologies, 1910 West C Street, Jenks, Okla. USA 74037 Inc., EPCON Industrial Systems, 17777 Interstate 45 South Conroe, Tex. USA 77385.

In various embodiments, the resulting compounds from combustion in the thermal oxidizer may be forwarded to an environmental mitigation device (e.g., a “scrubber”) or a chimney for emission into the air. For example, various environmental mitigation devices may be used to reduce the concentrations of various materials, such as nitrous oxides. In various embodiments, the resulting compounds from combustion in the thermal oxidizer are emitted via a chimney. For example, the resulting compounds from combustion in the thermal oxidizer 116 are released in chimney 114.

With continued reference now to FIG. 1, furnace 108 has inlet 102 and outlet 104. Furnace 108 also has seal flanges 110 that may be bolted together to prevent gaseous emissions. Carbonization may take place as furnace 108 is heated to above about 1100° C., and in various embodiments, furnace 108 may be heated to above about 1200° C. Gaseous stream 121 may be formed at temperatures above about 1100° C. as HCN and other cyanides are emitted from the carbon fiber precursors within furnace 108. Accordingly, gaseous stream 121 comprises HCN.

Gaseous stream 121 is emitted from furnace 108 at point 109. In various embodiments, nitrogen gas may be pumped into furnace 108 to assist expulsion of gaseous stream 121. Pipe system 106 conducts gaseous stream 121 away from furnace 108. In various embodiments, pipe system 106 comprises multiple pipes, though, in alternate embodiments, pipe system 106 comprises a single, continuous pipe. Pipe system 106 may also be at least partially lined or insulated, and/or at least partially unlined or uninsulated.

In various embodiments and as noted above, valves 124 and 120 are coupled with pipe system 106. Valve 124 separates pipe system 106 to steam vacuum system 112. Valve 120 separates pipe system 106 from thermal oxidizer 116. Valves 124 and 120 may be open or closed, as appropriate, during carbonization and during the second stage. For example, valve 120 may be open during carbonization so that gaseous stream 121 will continue to thermal oxidizer 116. Valve 124 may be closed during carbonization so that the steam vacuum system 112 is protected from gaseous stream 121 and the HCN contained therein. However, during the second stage, valve 124 may be open so that steam vacuum system may pull a vacuum in furnace 108 and valve 120 may be closed so that effluent gases during the second stage are not drawn into the thermal oxidizer. This may be beneficial, as second stage effluent gases may contain residual impurities (such as sodium) that often require separate material management strategies.

Vacuum system 112 may comprise any suitable system configured to pull a vacuum in a furnace 108. For example, vacuum system 112 may comprise a steam vacuum or a mechanical vacuum.

In an embodiment, thermal oxidizer 116 is configured to oxidize gaseous stream 121 and/or the HCN contained therein. Thermal oxidizer 116 may accomplish oxidization by combustion of gaseous stream 121, either in combination with air or supplied oxygen.

Now, with reference to FIG. 2, in an embodiment, a method of HCN removal 200 comprises heating a furnace to above about 1200° C. in step 201, although step 201 may comprise heating to a temperature in the range of from above about 1100° C. to below about 1600° C. HCN and/or other cyanides may be expelled from the furnace in step 203. Optionally, an inert gas may be pumped into the furnace to assist in the expulsion of HCN (not shown in FIG. 2). HCN may be oxidized in step 205 in a thermal oxidizer. The second stage may be performed in step 207. Optionally, a vacuum may be drawn using a vacuum device (e.g., pump or steam vacuum) prior to step 207.

Benefits, other advantages, and solutions to problems have been described herein with regard to specific embodiments. However, the benefits, advantages, solutions to problems, and any elements that may cause any benefit, advantage, or solution to occur or become more pronounced are not to be construed as critical, required, or essential features or elements of the invention. The scope of the invention is accordingly to be limited by nothing other than the appended claims, in which reference to an element in the singular is not intended to mean “one and only one” unless explicitly so stated, but rather “one or more.” Moreover, where a phrase similar to “at least one of A, B, or C” is used in the claims, it is intended that the phrase be interpreted to mean that A alone may be present in an embodiment, B alone may be present in an embodiment, C alone may be present in an embodiment, or that any combination of the elements A, B and C may be present in a single embodiment; for example, A and B, A and C, B and C, or A and B and C. Furthermore, no element, component, or method step in the present disclosure is intended to be dedicated to the public regardless of whether the element, component, or method step is explicitly recited in the claims. No claim element herein is to be construed under the provisions of 35 U.S.C. 112, sixth paragraph, unless the element is expressly recited using the phrase “means for.” As used herein, the terms “comprises”, “comprising”, or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus.

Claims

1. A method of transforming a carbon fiber body precursor into a carbon fiber body, comprising:

heating a carbon fiber body precursor to a temperature of above about 1200° C. and a pressure of about 1 atm to about 1.5 atm in a furnace in a first stage;

expelling a stream of effluent gas outside the furnace, wherein the stream of effluent gas comprises a cyanide;

thermally oxidizing the cyanide during the first stage; and

heating the carbon fiber body precursor to a temperature of between about 1600° C. and about 2400° C. in a second stage.

2. The method of claim 1, further comprising drawing a vacuum using a steam vacuum prior to entering the second stage.

3. The method of claim 1, wherein the first stage temperature does not exceed about 1600° C.

4. The method of claim 1, wherein the thermal oxidation occurs in an incinerator.

5. The method of claim 1, wherein a secondary gas is pumped into the furnace during the expelling of the stream of effluent gas.

6. The method of claim 1, wherein the secondary gas is at least one of nitrogen and argon.

7. The method of claim 1, wherein the first stage temperature is between about 1200° C. to about 1300° C.

8. The method of claim 1, wherein the cyanide is hydrogen cyanide.

9. A system comprising for transforming a carbon fiber body precursor into a carbon fiber body, comprising:

a furnace having a gas inlet;

a thermal oxidizer in fluid communication with the furnace;

wherein the furnace is capable of heating a carbon fiber body precursor to above about 1200° C. in a first stage at a pressure of about 1 atm to about 1.5 atm;

wherein a stream of effluent gas is expelled outside the furnace in response to the furnace reaching a temperature of above about 1200° C.;

wherein the stream of effluent gas comprises a cyanide; and

wherein the thermal oxidizer oxidizes the cyanide during the first stage.

10. The system of claim 9, further comprising a steam vacuum.

11. The system of claim 9, wherein the first stage temperature does not exceed about 1600° C.

12. The system of claim 9, wherein the thermal oxidizer comprises an incinerator.

13. The system of claim 9, wherein secondary a gas is pumped into the inlet.

14. The system of claim 13, wherein the secondary gas is at least one of nitrogen and argon.

15. The system of claim 9, wherein the first stage temperature is between about 1200° C. to about 1300° C.

16. The system of claim 1, wherein the cyanide is hydrogen cyanide.

17. A method of transforming a carbon fiber body precursor into a carbon fiber body, comprising:

heating the carbon fiber body precursor at a substantially atmospheric pressure to above about 1200° C. in a furnace in a first stage;

introducing a secondary gas into the furnace at a positive pressure;

expelling a stream of effluent gas outside the furnace, wherein the stream of effluent gas comprises a cyanide;

thermally oxidizing the cyanide during the first stage using a thermal oxidizer;

closing a first valve so that the thermal oxidizer is no longer in fluid communication with the furnace;

opening a second valve in fluid communication with a steam vacuum so that the steam vacuum becomes in fluid communication with the furnace; and

heating the carbon fiber body precursor to a temperature of between about 1600° C. and about 2200° C. in a second stage.

18. The method of claim 17, wherein the thermal oxidizer is an incinerator.

19. The method of claim 17, wherein the secondary gas is nitrogen gas.

20. The method of claim 17, further comprising using the steam vacuum to form a vacuum in the furnace.