SYSTEMS AND PROCESSES FOR PRODUCING LOW COST BULK RANDOM CARBON FIBERS USING ASPHALTENE AS A FEEDSTOCK

Description

FIELD OF THE INVENTION

The invention pertains to systems and processes for bulk production of short carbon fibers, and in particular, those that use asphaltene as a feedstock.

BACKGROUND OF THE INVENTION

Carbon fibers are commonly used to reinforce other materials to add strength, stiffness and/or toughness. Various carbon sources can be used as a feedstock for producing carbon fibers. Most carbon fibers that are presently produced use polyacrylonitrile (PAN) as a feedstock, and a large portion of the remaining carbon fibers are produced from using pitch as a feedstock. Other raw materials that have been used include cellulose (e.g., rayon and cotton) and lignin (D. Choi et al, Carbon 2019).

There are typically three main steps in producing carbon fibers. Initially in the spinning step, fibers of the desired geometry are produced from the starting material (these are sometimes referred to as “green fibers”). The second step is to stabilize the spun green fibers. This is required to avoid their melting in the following carbonization step and involves oxidatively treating the material to raise its melting point and so to lock-in the desired fiber geometry, typically by heating in air. Once the melting point of the fibers has been increased above the onset of carbonization, the stabilized fibers can then be safely heated in an inert atmosphere to carbonize the fibers, producing the desired carbon fibers.

Currently, PAN-based carbon fibers dominate the market, however, one of the key drawbacks of PAN-based carbon fibers is the high cost of the starting polymer. PAN-based carbon fibers are the highest strength carbon fibers available and are typically made as long fibers suitable for weaving into cloth or making fiber-wound products. Long carbon fibers are generated by extruding the starting material through a multi-nozzle die while being drawn out and stretched using a take-up wheel to produce a bundle of small diameter fibers (“melt spinning”). This bundle of fibers (referred to as a “tow”) is then pulled through a series of thermal stages using highly specialized equipment and furnaces to carry out the stabilization step (under air) and the carbonization step (under nitrogen). The systems for this “long fiber process” rely on pulling and processing a long strand bundle and so require a high consistency of the feed because any fiber breakage would be highly disruptive to the process. Further, for a given tow size, the scale up of long fiber processing equipment relies on increasing furnace widths and so costs scale relatively linearly, thus economies-of-scale benefits are not as large as for other chemical processes (J. A. Fry, Harper International white paper). This poses a challenge for trying to achieve lower cost fibers through larger-scale production. An additional cost when making carbon fibers from polyacrylonitrile is the release of the nitrogen in the starting polymer during the carbonization process as cyanide gas, requiring a specialized gas treatment unit.

Pitch, which is a residue from petroleum, coal tar or biomass tar processing, might be expected to be a cheaper feedstock, but significant pre-processing of the raw material is often required to make it suitable for conventional carbon fiber manufacturing thus raising its cost. This pre-processing is required to produce a feed with a softening point range compatible with both the spinning and stabilizing steps. The feedstock pitch must have a softening point that allows the material to be melted and spun into green fibers. Stabilization is then used to raise the softening point to allow the green fibers to then be carbonized without melting or softening. Typically, stabilization is done by heating the fibers in air to produce a controlled amount of oxidation causing crosslinking and polar groups that increase the inter-molecular interactions and resulting in an increase in the softening point. To get reasonable rates for this air oxidation, it is preferably carried out starting at around 250° C. As the green fiber's melting point increases, the temperature can slowly be raised to further increase the rate of oxidation until the green fiber's stability is sufficient to withstand the subsequent carbonization process. To avoid fiber softening and loss of structure during initial stabilization, the softening point of the green fibers and so the starting pitch should be at least 50° C. higher than the initial temperature used for stabilization, so preferably >300° C. To avoid coke formation, which can lead to imperfections in the final fiber product, care should be taken to avoid spinning temperatures above ˜400° C. This then results in a preferred range for the softening point of the feedstock material to give a spinning temperature of about 300-400°. This must be achieved through pre-processing of the pitch feedstock raising the cost of this approach.

Most carbon fiber processes produce long carbon fibers suitable for weaving into cloth or making fiber-wound products (e.g. pressure vessels). Short carbon fibers used for random fiber reinforced composites are therefore typically produced by chopping long fibers. A dedicated method to directly produce short carbon fibers could potentially lower the cost for this product.

Asphaltenes are a promising potential feed material due to their low cost and pre-existing aromatic structures. However, asphaltenes are complex mixtures of hydrocarbons of high molecular weight typically containing heteroatoms such as nitrogen, oxygen and sulfur along with trace metals. They may be found in heavy oils and bitumen and are defined as the fraction of material that is soluble in toluene but insoluble in light alkanes (C5 to C7). As such, asphaltenes comprise a wide range of chemical compounds and structures. Asphaltenes are typically a by-product from heavy oil processing with one major potential source being from partial upgrading of bitumen, and in particular, Alberta bitumen. The properties and purity of a particular asphaltene will vary depending on the feed it is removed from (which will vary with its source and also over time), as well as on the process that was used to recover the asphaltenes. In particular, the asphaltene containing feedstock is likely to have a low softening point leading to similar problems as found with petroleum pitches, but also because they are starting with a more undefined chemical composition they will be more complex to purify.

There is thus a need for systems and approaches which would allow low-cost asphaltenes to be used as a feedstock in carbon fiber production without adding excessive costs to modify the asphaltene feed. There is also a need for systems and processes which are sufficiently robust to handle variations in feed quality. There is a further need for lower cost systems and processes which are capable of producing short carbon fibers without first generating long carbon fibers. The overall need is an approach which can deliver a lower cost carbon fiber product. The present invention is thus directed to an improved, low-cost and robust approach for producing short carbon fibers using minimally pre-processed asphaltene as a feedstock.

SUMMARY

A process has been developed that uses a low-temperature stabilization method to avoid problems with low softening point asphaltenes allowing the process to make use of low-cost feedstocks with minimal pre-processing thus allowing the true low cost of the feedstock to be more effectively realized. This is combined with a robust green fiber spinning method such as melt blowing or centrifugal spinning operated using a high degree of fiber stretching in order to break off discontinuous fibers, thus directly producing short green fibers. In turn, this approach allows conventional bulk solids processing methods and equipment to be used rather than highly specialized equipment and furnaces, allowing for a reduced capital cost for the process. Using bulk solids processing methods that can process large volumes of short fibers also provides an approach that provides good economies of scale for larger scale operations. This combination of steps taken together synergistically provides a unique low-cost production method for short carbon fibers starting from minimally processed and therefore low cost asphaltenes feedstocks.

The carbon fiber production process begins with heating an asphaltene-rich feed to produce a feed stream comprising molten asphaltene. The molten asphaltene may then be filtered to remove contaminants such as any solid impurities. The filtered molten asphaltene may be allowed to outgas so as to remove any volatile constituents in the feed. The molten asphaltenes are supplied to a suitable fiber formation unit within which green carbon fibers are produced. A suitable fiber formation unit includes, but is not limited to, a melt blower or a centrifugal spinner. The resulting green carbon fibers may be in the form of clumps of random discontinuous fibers. These clumps of random discontinuous fibers can be transferred within the process by standard bulk material handling equipment such as, but is not limited to, belt conveyors, vibrating conveyors, augers (screw conveyors), chain conveyors, and pneumatic conveyors. In a following step, the green carbon fibers are infused with an oxidizing gas containing nitrogen dioxide (NO₂) in an infusion vessel at close to ambient temperatures. After infusion, the fibers are heated and during which, oxidative stabilizing reactions are completed (referred to herein as the stabilizing step). Because the infusion step has resulted in some fiber oxidation and left oxidative reactive species on and/or in the fibers, the stabilization reactions begin at very low temperatures (essentially immediately on heating above the infusion temperature) and so very low melting point feedstock materials can be processed. This differs from simple heating in air where the oxidative reactions only begin to occur to a significant extent at around 250° C., requiring the starting material to not soften until the starting material is heated to a temperature that is significantly above that temperature. The heating of the infused fibers may take place within a separate stabilization vessel. The stabilized fibers may then be supplied into a carbonization unit maintained with an oxygen-free environment to undergo carbonization to form the resulting carbon fiber products.

Low-cost, and standard bulk material conveying equipment may be used to convey the bulk carbon fibers between the processing units. Such bulk material conveying equipment include for example, belt conveyors, vibrating conveyors, augers (screw conveyors), chain conveyors, and pneumatic conveyors. One or more airlock-type devices may be arranged between processing units and/or downstream of the carbonization unit. Airlock-type devices are used to prevent the transfer of gases between processing units as the solid product is being transferred between the processing units and between the processing units and the surrounding atmosphere.

Further aspects of the invention and features of specific embodiments of the invention are described below.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments are illustrated in referenced figures of the drawings. It is intended that the embodiments and figures disclosed herein are to be considered illustrative rather than restrictive.

FIG. 1 is a process flow diagram illustrating an example embodiment of the invention.

FIG. 2A is a photograph of a potential asphaltene-rich feed obtained from a first source with properties listed in Table 1, column 1.

FIG. 2B is a photograph of a potential asphaltene-rich feed obtained from a second source with properties listed in Table 1, column 2.

FIG. 2C is a photograph of a potential asphaltene-rich feed obtained from a third source with properties listed in Table 1, column 3.

FIG. 2D is a photograph of a potential asphaltene-containing feed obtained from a fourth source with properties listed in Table 1, column 4.

FIG. 3A is a schematic diagram illustrating the key features of a melt spinner which can be used for producing green carbon fibers.

FIG. 3B is a schematic diagram illustrating the key features of a melt blower which can be used for producing green carbon fibers.

FIG. 3C is a schematic diagram illustrating the key features of a centrifugal spinner which can be used for producing green carbon fibers.

FIG. 4 is a plot of concentration as a function of time (minutes) showing pilot plant data from Example 2 obtained from measuring the gases that were detected during the infusion of green carbon fibers with nitrogen dioxide (NO₂) using a Fourier Transform Infrared Gas Analyzer (FTIR-GA). Nitrogen dioxide is shown in percent on the right hand scale while the other gases are shown in ppm on the left hand scale. Note that Fourier Transform Infrared Gas Analyzers do not detect non-polar compounds such as oxygen (O₂) and nitrogen (N₂).

FIG. 5 is a comparison of Attenuated Total Reflectance Fourier Transform Infrared spectroscopy (ATR-FTIR) spectra from the surfaces of NO₂ treated infused fibers versus green fibers. Two measurements are shown for each sample.

FIG. 6 is a plot showing the preferred infusion times (minutes) and effective nitrogen dioxide (NO₂) concentrations (mole %) to produce good quality carbon fibers. The circular markers indicate infusion conditions that led to the production of carbon fibers which have acceptable properties. The triangular markers indicate infusion conditions that led to the production of carbon fibers which have unacceptable properties.

FIG. 7 is a plot of concentration (ppm) as a function of temperature (° C.) showing the pilot plant results from Example 4 of the measured gases released during stabilization and carbonization of NO₂-infused green carbon fibers using a Fourier Transform Infrared Gas Analyzer (FTIR-GA). Fourier Transform Infrared Gas Analyzers (FTIR-GA) do not detect non-polar compounds such as N₂.

FIG. 8 shows Thermal Gravimetric Analysis (TGA) curves for samples from Example 4, showing the amount of weight loss (%) during heating under nitrogen as a function of temperature (° C.) for stabilization and carbonization of green carbon fibers that have been treated with NO₂ infusion and green carbon fibers without NO₂ infusion treatment as a control.

FIG. 9A is a photograph showing an example of random mats of stabilized green carbon fibers produced after the stabilization step and before the carbonization step in some pilot plant experiments.

FIG. 9B is a photograph showing an example of random mats of carbon fiber products produced after the carbonization step in some pilot plant experiments.

FIG. 10A is a Scanning Electron Microscope (SEM) image of the resulting carbon fiber products that were produced in some pilot plant experiments.

FIG. 10B is an optical microscope photo of resulting carbon fiber products that were produced in some pilot plant experiments.

FIG. 10C is a Scanning Electron Microscope (SEM) image of the resulting carbon fiber products that were produced in some pilot plant experiments.

FIG. 11 is a process flow diagram illustrating an example embodiment of infusion gas recycling (e.g., the processes that may be performed in process block 103C shown in FIG. 1).

DETAILED DESCRIPTION

Some aspects of the invention pertain to a combination of process steps and preferred equipment that provides a robust and low-cost method starting from asphaltene-rich feeds for the production of a random material of discontinuous fibers that are suitable for blending into polymers or other matrices to produce random short carbon fiber reinforced products. An example embodiment of the process steps is diagrammed in FIG. 1.

The systems and processes of the present invention can advantageously be used to produce the desired random short carbon fiber reinforcing product from a wide range of low cost asphaltene-rich feedstocks of variable feed quality (e.g., different compositions, properties and purities) with minimal pre-treatment of the asphaltene feed. In some conventional carbon fiber production processes, pre-treatment of feedstocks may be necessary to adjust the properties, such as the softening point range, to prepare the feed for processing. Such pre-treatment step undesirably increases costs and processing time. Potential sources of asphaltene-rich feedstocks include vacuum residue from bitumen upgrading, asphalt produced from bitumen or heavy oil, and precipitated asphaltenes from solvent de-asphalting of bitumen and heavy oil products.

Table 1 lists the compositions of four different example raw asphaltene samples corresponding to the four images in FIGS. 2A-2D. Table 1 illustrates the variability of the compositions of different asphaltene samples. The compositions of asphaltene feeds can be highly variable depending on their source and method of removal from the source. Asphaltenes are an undesirable component of heavy oil or bitumen that can be removed to allow the remaining components to be more easily further processed into typical hydrocarbon products. Such a separation can be carried out by vacuum distillation type processes resulting in asphaltene-rich “vacuum bottoms” or asphalt streams. Another method to remove asphaltenes from a hydrocarbon stream called “solvent de-asphalting” adds a light solvent (e.g. C5 to C7 alkanes), which causes the asphaltene to precipitate as a solid or separate as a dense liquid (depending on the temperature as well as the type and amount of solvent used). After separation, the solvent can then be recovered and recycled, thereby leaving an asphaltene-rich stream. The above approaches can also be combined with techniques such as thermal cracking with or without hydrogenation to fragment some of the asphaltenes and remove impurities (such as sulfur and metals) to increase the yield of upgraded oil. Thus, asphaltene-rich streams are low value by-product streams from heavy oil or bitumen processing. While many properties may vary depending of the source of the asphaltenes, a key property for fiber spinning is the softening point. For many asphaltene sources, the softening points are too low for conventional spinning and stabilizing. FIGS. 2A-C are photographs showing the different physical appearances of asphaltene samples that were obtained from different sources. The asphaltene-containing samples have a range of physical characteristics from dry sandy, to oily sandy, to glassy to a thick viscous liquid. The present invention has features that allow it to treat a wide range of asphaltene containing feedstocks with minimal pre-treatment due to special features of its process and apparatus. However, it is preferred that the feedstock be “asphaltene-rich”, where asphaltene-rich feed contains at least 40 wt % asphaltenes and preferably at least 50 wt % asphaltenes (as measured by heptane precipitation (e.g. by ASTM D3279 or similar method)). Note that sample 4 does not meet this requirement but sample 4 may be blended with other samples to improve the melt flow characteristics for fiber spinning.

TABLE 1
Compositions of raw asphaltene samples

	1	2	3	4

C (wt %)	82.0	78.9	81.5	85.5
H (wt %)	8.2	7.9	8.5	9.9
N (wt %)	1.2	1.2	1.0	0.8
S (wt %)	8.1	8.2	7.25	2.8
O (wt %)	1.1	2.6	1.8	0.6
H/C (mole ratio)	1.19	1.19	1.24	1.38
Asphaltene (heptane insoluble) (wt %)	60.1	60.0	66.8	8.0
Micro carbon residue(wt %)	43.5	44.2	36.0	22.5
FIG.	2B	2A	2C	2D

FIG. 1 illustrates the exemplary embodiments of processes and systems for the low-cost production of a random short carbon fiber reinforcing product using asphaltenes-rich feedstocks. The carbon fiber production system comprises asphaltene preparation units 100A, 100B and 100C, a fiber formation unit 101, infusion/stabilization units 103A, 103B and 103C, carbonization units 104A and 104B, and a vent gas treatment system 106. Each of the units may comprise one or more apparatuses.

The asphaltene preparation systems comprise units 100A, 100B and 100C. The melting unit 100A is configured to heat an asphaltene-rich feed 1 to a temperature sufficient to decrease the viscosity of the feed, resulting in a feed comprising molten asphaltene-rich stream 2 (or molten feed 2). An example of unit 100A may comprise a solids feeding system followed by a heated extruder. Unit 100B is arranged downstream of the melting unit 100A. Unit 100B may comprise one or more apparatuses configured to perform minimal cleanup of the molten feed 2 before spinning. In some example embodiments, unit 100B comprises one or more apparatuses configured for filtration and/or outgassing. The one or more apparatuses configured for filtration may comprise any suitable filtration apparatus for removing solid impurities from the molten feed. Such solid impurities may negatively impact the quality and/or strength of the carbon fiber products and/or plug the downstream fiber spinning apparatuses. The one or more apparatuses configured for outgassing (referred to hereinafter as the “outgassing unit”) may be adapted to remove volatile constituents such as volatile organic compounds (e.g., hydrocarbons) and/or water from the molten feed 2. The presence of volatile components may undesirably give rise to trapped bubbles in the later formed green fibers, thereby resulting in imperfections in the final carbon fiber products. The outgassing unit may be configured to expose the molten feed to a vacuum atmosphere and/or to a sweep gas such as N₂ and/or steam. During operation, the outgassing unit is preferably maintained under substantially oxygen-free conditions to prevent unwanted oxidation of the asphaltenes which could undesirably alter the properties of the compound. Using substantially oxygen-free conditions also avoids producing potential flammable mixtures with the volatile hydrocarbons. In some embodiments, the gases released from the outgassing unit are transported to a condensing unit 100C comprising one or more condensers. Outgassed compounds that are not recovered by the condensing unit 100C may be transported to a vent gas treatment unit 106 before being vented.

The vent gas treatment unit 106 may comprise any suitable apparatuses configured to treat or clean incoming gases to form a resulting exhaust gas 17 with reduced pollutant emission levels that meet environmental standards. In one embodiment, this may be an incinerator configured to combust the outgassed compounds (e.g., outgassed volatile hydrocarbons) in the presence of oxygen and optionally also in the presence of a catalyst. The combustion of the outgassed volatile hydrocarbons produces heat and so one or more heat recovery systems, such as heat exchangers and heat boilers, may be integrated with the incinerator configured to recover the heat generated from the combustion.

In some embodiments, the operating temperature that is maintained within the asphaltene preparation units 100A and 100B is in the range of from about 150° C. to about 330° C., and in some embodiments, from about 180° C. to about 320° C.

The fiber formation unit 101 is arranged downstream of the asphaltene preparation units 100A, 100B, 100C. The fiber formation unit 101 comprises one or more suitable fiber formation apparatuses adapted to produce green carbon fibers 4 from the molten, filtered and/or degassed asphaltene feed 3 by a process which may be broadly referred to as “spinning”. FIGS. 3A, 3B and 3C are schematic diagrams illustrating the key features of commonly used fiber spinning methods (J. G. Lavin, “Ch. 5, Carbon fibres”, CRC Press, 2001). FIG. 3A is a schematic diagram illustrating a melt spinning process, which is commonly used for long fiber production. A molten precursor is fed to a spinning multiple-nozzle die to extrude a bundle of fibers. The fibers are collected on a take-up wheel running at a sufficient speed to stretch and draw the green fibers down to their target diameter. Once the fibers are sufficiently stretched, cooling air 19 is used to harden the fibers allowing them to be collected as a tow on the take-up wheel without fusing together. Note that while this approach allows large bundles of fibers to be spun and is used to feed a long fiber type carbon fiber line, melt spinning requires a highly consistent feed as any fiber breakage can severely disrupt production. Such an approach is therefore not satisfactory for a low-cost feed with potentially varying properties.

FIG. 3B is a schematic diagram illustrating the key features of a melt blowing process. In this process, the molten precursor is extruded through a nozzle. Hot gas injectors are configured to inject highly velocity heated gas 18 towards the molten green fibers emerging from the nozzles, thereby drawing out or stretching the green fibers into small-diameter fibers. Cool gas 19 may be arranged downstream of the hot gas injectors to solidify the extruded molten asphaltene to form green carbon fibers 4 and to lock in the structures produced during the spinning/stretching (the overall fiber geometry and in some cases, internal molecular orientations produced by shear forces during the spinning process).

FIG. 3C is a schematic diagram illustrating the key features of a centrifugal spinning process. In this process, the molten precursor is supplied to a rotating disk with multiple nozzles around its circumference. Feed pressure and/or centrifugal force are used to extrude the molten material through the nozzles to form green fibers. A combination of centrifugal forces, air drag experienced by the fibers being extruded at the spinning disk's rim, and a flow of hot gases 18 act to draw out or stretch the green fibers into small-diameter fibers. Cool gas 19 is then injected to solidify the extruded molten asphaltene to form green carbon fibers 4 and to lock in the structures produced during the spinning/stretching (the overall fiber geometry and in some cases, internal molecular orientations produced by shear forces during the spinning process).

These latter two methods of green fiber spinning (melt blowing and centrifugal spinning) differ from the commonly used melt spinning approaches in that the drag forces of hot gases are used to stretch the fibers, resulting in a more robust process that can continue to operate with fiber breakage and so with poorer quality, low cost feeds. The spinning process of the present invention is preferably operated in a manner to produce discontinuous fibers. For a given molten precursor (the viscosity of which may be influenced by the precursor composition and temperature), the balance between nozzle flow rate and stretching rate will influence the average length of the formed green fibers. These random, discontinuous fibers may be produced in the form of entangled mats as shown in FIGS. 9A and 9B.

The operating temperature (or spinning temperature) that is maintained within the fiber formation unit 101 is chosen to achieve a target viscosity that is low enough to allow good flow through the spinner nozzles but high enough to support fiber stretching. Thus, the preferred spinning temperature depends on the viscosity versus temperature properties of the spinner feed material (e.g., the degassed asphaltene feed 3). Because of the low temperature that can desirably be used in the fiber stabilization step in the method disclosed herein (which such advantage will be discussed in further detail below), the spinning temperature can be chosen without the normal constraints required for the stabilization step, thereby leaving the spinning temperature free to be adjusted over a wide range to match the spinner feed material. A remaining constraint is to avoid spinning temperatures at above about 400° C., which such high temperatures can undesirably cause coke formation, thereby resulting in imperfections in the final fiber product. Also, the cooling air (flow 19 in FIG. 3), is preferably at a temperature sufficient to solidify the green carbon fibers 4 and also cool the fibers 4 to the point where they are not sticky, but are free flowing. Though, this is a constraint on the molten, filtered, degassed asphaltene feed 3 to the fiber formation unit 101 or spinner, that the feed 3 is preferably solid and not sticky below about 40° C.

The random, discontinuous fibers in the form of entangled mats as shown in FIGS. 9A and 9B can be manipulated as a bulk mass, using standard solids handling techniques and equipment. This allows for higher mass throughout, lower cost equipment, and better economies of scale when scaling up such a process. Non-limiting examples of suitable bulk material conveying equipment include belt conveyors, vibrating conveyors, augers (screw conveyors), chain conveyors, and pneumatic conveyors.

The method of producing green fibers may be optimized to drive the production of discontinuous green carbon fibers. While the goal of this process is to produce discontinuous fibers, the fibers are also preferably stretched enough to produce fibers with a small average diameter. A small average fiber diameter will minimize stress build-up in the fibers during the rapid cooling experienced during the spinning process, allow the subsequent stabilization treatment to properly penetrate the fibers to stabilize the fiber structure, and minimize cracking caused by out-gassing during the carbonization process. A smaller fiber diameter will result in a higher length to diameter ratio, leading to improved reinforcing performance when blended into polymers or other matrices. Thus, the spinning process should be operated to produce discontinuous fibers with a sufficient degree of stretching to result in final carbon fibers 7 which comprise a small average diameter, i.e., one with an average diameter of less than about 25 μm and preferably less than about 20 μm.

The viscosity and surface tension of the molten precursor of the green carbon fibers (e.g., the degassed asphaltene feed 3) will influence the balance between fiber stretching versus fiber breaking and thus have an impact on the average fiber length and diameter of the resulting fibers. The viscosity and surface tension of the extruded fiber at any point of the process will be a characteristic of the starting asphaltene-rich feed 1 and the temperature profile the fiber experiences as the molten material is caused to exit the nozzle and to move through the hot and cold gas flows 18, 19 around the nozzle to form green fibers in the spinning process (FIGS. 3A-3C). Thus, the spinning temperature and the hot gas temperature are chosen to achieve a viscosity that is low enough to allow good flow through the spinner nozzles, but a viscosity that is high enough to resist surface tension forces to allow sufficient fiber stretching before breaking. The balance between the flow of molten material through the nozzle and the stretching forces may also influence the produced green carbon fibers 4. In a melt blowing process, the flow of the molten material through the nozzle is controlled by the feed pressure 30 and the stretching by the hot gas flow 18. In a centrifugal spinning process, feed pressure 30, hot gas flow 18, and/or the centrifugal forces generated by the spinner will have an influence on the produced green carbon fibers 4. For both the melt blowing and centrifugal spinning processes, the hot and cool gas streams (18 and 19) may pick up hydrocarbon fumes from the molten feed 2 that is being spun into fibers. For this reason, exiting flows 10 containing such gas streams from the fiber formation unit 101 may be directed to flow into the vent gas treatment unit 106.

The resulting random, discontinuous green fibers 4 which may be in the form of entangled mats may be transferred to the infusion unit 103A using suitable bulk material conveying equipment. Non-limiting examples of suitable bulk material conveying equipment include belt conveyors, vibrating conveyors, augers (screw conveyors), chain conveyors, and pneumatic conveyors. To maintain the infusion gases within the infusion unit 103A, an airlock-type device 102 may be arranged between the fiber formation unit 101 and the infusion unit 103A. This allows passage of the green carbon fibers 4 into the infusion unit 103A without allowing gases to be exchanged. An airlock-type device 102 is any suitable mechanical unit operation which creates an airtight seal between two locations, such as two processing units, which are maintained at different pressures and/or gas atmospheres while allowing solid materials to be transported between the two locations. An example of a suitable airlock could be a rotary valve or a unit comprising an airlock chamber and two valves. In operation, fibers may be arranged to flow through the airlock by gravity and/or pneumatically (for example a powder pump type system). Gases may be removed from the airlock chamber between valve cycles in order to minimize any transfer or release of gases. In some embodiments, the gases may be removed by using a vacuum pump and/or flushing gas into the airlock chamber. Non-limiting examples of suitable valves that may be used together with an airlock chamber include ball valves, flap valves, butterfly valves, slide gate valves and Roto-Disk valves.

The infusion unit 103A comprises an infusion vessel. In some example embodiments the infusion vessel comprises a rotary drum. A “rotary drum” is an industrial vessel that may be rotated about its longitudinal axis. The axis may be positioned on a slight angle. Rotation of the drum allows for mixing of a solid material that is being processed while providing good exposure of the solid material to the gas atmosphere within the drum. The angle and rotation rate may provide a controlled rate of movement of the material along the drum. A rotary drum, may also be heated to provide a controlled temperature. The green carbon fibers 4 are transferred to the infusion unit 103A within which the fibers 4 are exposed to an infusing gas containing nitrogen dioxide (NO₂). Note that when discussing nitrogen dioxide (NO₂) it is to be understood that nitrogen dioxide (NO₂) is in equilibrium with its dimeric form, dinitrogen tetroxide (N₂O₄). For discussing concentrations of nitrogen dioxide and dinitrogen tetroxide herein, the term “effective NO₂” will be used. Effective NO₂ refers to the gas phase concentration of nitrogen dioxide if all dinitrogen tetroxide was considered to be dissociated. Thus:

Effective NO ⁢ 2 = 100 ⁢ % ⁢ C NO ⁢ 2 + 2 ⁢ C N ⁢ 2 ⁢ O ⁢ 4 C NO ⁢ 2 + 2 ⁢ C N ⁢ 2 ⁢ O ⁢ 4 + ( 100 - C NO ⁢ 2 - C N ⁢ 2 ⁢ O ⁢ 4 ) = 1 ⁢ 0 ⁢ 0 ⁢ % ⁢ C NO ⁢ 2 + 2 ⁢ C N ⁢ 2 ⁢ O ⁢ 4 100 + C N ⁢ 2 ⁢ O ⁢ 4 [ 1 ]

For concentrations in mole %.

During the infusion of the green fibers, oxidation reactions occur, thereby producing some carbon dioxide (CO₂) and water (H₂O) as shown in FIG. 4. Other reactions may occur, resulting in the formation of bound nitro (R—NO₂) and possibly nitroso (R N═O) and nitroxy groups (R—O—NO₂) groups. The appearance of these groups on the surface of the infused green fibers can be seen by the appearance of absorption bands appearing around 1556 cm-1 and 1525 cm⁻¹ and at 1326 cm-1 and 1274 cm⁻¹ in spectra measured using Attenuated Total Reflectance Fourier Transform Infrared spectroscopy (ATR-FTIR) of the surfaces of green fibers before and after infusion, as shown in FIG. 5. The infusion reactions may also result in a weight gain by the green fibers. The results also show a decrease in C—H stretches at 2921 cm-1 and 2851 cm⁻¹ indicating a loss of hydrogen. While the reactions that produced the ATR-FTIR identified surface groups are likely complex, the following overall reactions 2-4 may be postulated.

Note that because reaction products such as water (H₂O) and nitrogen monoxide (NO) appear in the off gases shown in FIG. 4, an equilibrium reaction (reaction 5) can occur between nitrogen dioxide and water vapor to produce nitric acid vapor (as is also seen in FIG. 4).

Because both nitric acid and water vapors are present in the atmosphere of the infusion unit 103A, a film of liquid nitric acid solution may be present on the surfaces of the fibers being treated. Such a film may also contribute to the oxidative stabilization of the asphaltenes. Since liquid nitric acid solution is a highly polar water-based solution, it is unlikely to penetrate the more hydrophobic asphaltene fibers as effectively as NO₂ and N₂O₄ gases would. In addition to nitrogen dioxide and its various reaction and equilibrium products, the remainder of the infusion gas may comprise nitrogen (N₂) and/or oxygen (O₂). While nitrogen would be inert, oxygen may react with nitrogen monoxide in an equilibrium reaction to regenerate nitrogen dioxide as illustrated in reaction 6 below.

Thus, while nitrogen dioxide and dinitrogen tetroxide are expected to be the main active gases for the infusion of the green carbon fibers, the gas mixture will contain many other gases and many other reactions and equilibria will occur. Because of the toxic and corrosive nature of NO₂ gas, the apparatuses that are used in the process where such a gas is expected to be present is desirably operated at a pressure that is slightly below atmospheric pressure (approximately −0.35 psig or −10 in. water) to avoid any outward leakage of NO₂ gas. A result of this design is that a slow inward leakage of air will occur. Therefore, the gases in the gas mixture would include by-products from the stabilization reactions (which can include NO, H₂O and CO₂), air that has entered the process (and so N₂, O₂, Ar, H₂O, CO₂, etc.), and nitric acid vapor (HNO₃) from the reaction of the NO₂ gas with water vapor.

The effectiveness of the infusion treatment can be clearly seen in Example 1. When the green fibers spun from an asphaltene-rich feedstock were slowly heated in air at 0.85° C./min to 225° C., the fibers became sticky at around 100-130° C. and began to melt at around 150-190° C. The melting indicates that conventional methods of stabilization were not suitable for these low softening point asphaltene green fibers. However, after infusion, the fibers were successfully heated under nitrogen to achieve carbonization without melting or fusing. Stabilization of the green fibers thus occurred due to some oxidation that had occurred during the infusion treatment and further oxidation that occurred during heating under nitrogen due to reactions of the bound nitro and possibly nitroso and nitroxy groups added during the infusion step.

The range of infusion conditions that can produce successful fibers starting from low softening point asphaltene green fibers according to some example embodiments are shown in FIG. 6 based on Example 1. FIG. 6 maps good performing fibers versus unacceptable fibers in relation to the infusion treatment time and the effective concentration of NO₂ gas used for infusion at ambient temperature. It can be seen that “good” fibers are best produced with an effective concentration of NO₂ gas greater than 10% and less than 50%. Effective NO₂ concentrations of less than 10% can be made to work but require long treatment times (greater than 40 minutes) or possibly elevated temperatures. High effective NO₂ concentrations (greater than 50%) resulted in fibers that melted or in some cases crumbled. It is suspected that this is due to over-oxidation and thus leading to a decrease in molecular weight and a lower melting point. While the infusion results shown in FIG. 6 were measured at ambient temperature, the infusion reactions could be carried out at higher temperatures if compatible with the properties of the green fibers. To illustrate, in one example, Example 1, the green fibers became sticky at around 100-130° C. and so infusion of these fibers would need to be carried out at a temperature significantly lower than 100° C., preferably lower than 60° C. Green carbon fibers which have a higher sticky point (i.e., a higher temperature at which the green carbon fibers become sticky) may be infused at higher temperatures. However, the ability of the infusion to be successful at ambient temperatures underlines its ability to treat a wide range of green carbon fibers, even those with very low sticky and softening temperatures.

In the example process diagram shown in FIG. 1, a stabilization unit 103B is arranged downstream of the infusion unit 103A. In some embodiments, the stabilization unit 103B comprises a heated rotary drum reactor. The NO₂-infused green carbon fibers 5 may be caused to flow out of the infusion unit 103A to be conveyed to the stabilization unit 103B to undergo oxidative stabilization. An airlock-type device 102 may be arranged between the infusion unit 103A and the stabilization vessel 103B. In such embodiments, the NO₂-infused green carbon fibers 5 are arranged to pass through the airlock-type device 102 before entering the stabilization unit 103B. In some embodiments, the infusion unit 103A and the stabilization unit 103B are combined as one unit (e.g., one rotary drum reactor having a chamber which is separated into different heated zones).

The goal of the stabilization step is to produce a controlled amount of oxidation, which promotes crosslinking and adding of polar groups that increases the inter-molecular interactions and thus resulting in an increase in the softening point of the green carbon fibers 4. The softening temperature needs to be increased to a high enough point to allow for the subsequent carbonization reactions to occur without fusing or melting of the green carbon fibers 4. For oxidation of the green carbon fibers 4 in air, the fibers typically need to be heated to around 250° C. to obtain reasonably fast oxidation reaction rates. As the oxidation occurs and the softening point of the green fibers increases, the stabilization temperature can be increased, thereby accelerating the oxidation reactions and further increasing the softening point. Because the carbonization reactions typically begin at around 350-400° C., carrying out stabilization reactions to around 300-350° C. is expected to provide sufficient protection for the green fibers. For the green fibers that have been infused according to the method discussed herein, the bound nitro (R—NO₂) and possibly nitroso (R N═O) and nitroxy groups (R—O—NO₂) groups begin to react at very low temperatures and thus oxidizing the green carbon fibers at very low temperatures. In Example 1, FTIR-ATR showed that the peaks that are associated with bound nitrogen oxide groups disappeared by around 200° C. and the signal around 1700 cm⁻¹ associated with C═O increased, which indicates oxidation of the fiber surfaces. The plot in FIG. 7 shows the off gases that appeared at different temperatures as measured during such a stabilization process which is carried out under nitrogen. The plot shows NO₂ and NO gases appearing as surface groups react, with NO₂ first appearing at around 60° C. and the last of the NO being visible at around 280° C. As well, CO₂ and H₂O products from the oxidation of the green fibers can be seen. Thus, the stabilization reactions that were promoted by the prior infusion step begin at around 60° C. which suggests that green carbon fibers with low softening point can be effectively stabilized.

The two-step treatment to achieve fiber stabilization, i.e., by first infusing the green carbon fibers in the infusion step and then heating the infused fibers in the stabilization step, may allow the stabilization to penetrate more deeply into the fibers. A problem with air driven oxidative stabilization is that the oxygen-driven cross-linking reactions will lower the diffusivity at the fiber surface and slow the rate of further penetration of oxygen deeper into the fiber interior. Embodiments of the present invention may solve this problem by allowing the nitrogen dioxide (NO₂) to penetrate into the fibers and then in a subsequent step, initiate the oxidation reactions. Thus, this two-step treatment may result in a deeper penetration into the fibers.

The combined infusion and stabilization steps have been shown to also improve the final yield of the carbon fibers. This is shown in FIG. 8 where thermal gravimetric analysis (TGA) results for heating green fibers under nitrogen give a final yield of carbon of 34.8% (as a solid piece due to fiber melting). The infused green fibers lose weight more rapidly as the bound nitro and possibly nitroso and nitroxy groups groups react and oxidize the green fibers. Once these reactions are complete (at around 280° C.), the now stabilized fibers show lower losses and give a higher final yield of carbon fibers of 40.7%. Further, in this case the infusion step caused a weight increase of 8.8%, so the overall yield of carbon fibers from the green fibers would be 44.3%.

As was seen in FIG. 7, a number of gases are generated during the stabilization process over the temperature range from ambient up to around 280° C. In some embodiments, a gas supply line may be flowingly connected to the stabilization vessel 103B to supply a purge gas (such as nitrogen and/or oxygen). This purge gas can be used to carry the generated gases 13 to the process unit 103C to recover and recycle the nitrogen oxide gases. To maximize the recovery of the nitrogen oxide gases from the stabilization process while minimizing the mixing with volatile alkyl compounds from the early stages of de-alkylation and carbonization, the maximum temperature maintained in the stabilization unit 103B may be limited to around 260° C. to about 320° C. (see FIG. 7). The resulting stabilized fibers 6 are then transferred to a carbonization step 104A. In some embodiments, the stabilization unit 103B comprises a heated rotary drum with different temperature zones. The maximum temperature maintained in the stabilization unit 103B may therefore correspond to the final heated zone before the fibers exit the rotary drum. Such temperature is referred to herein as the stabilization outlet temperature. Similar to the infusion unit 103A, the stabilization unit 103B is preferably operated at a pressure that is slightly below atmospheric pressure (e.g., approximately −0.35 psig or −10 in. water) to avoid any outward leakage of the nitrogen oxide gases.

In some embodiments, gases that are released from the infusion unit 103A and the stabilization unit 103B (12 and 13 respectively) are processed in process unit 103C to recover the different nitrogen oxide gases (e.g. HNO₃, N₄O₄, NO₂, N₂O₃ and NO). The process unit 103C may be configured to process the gas mixture to produce processed gases that are suitable for re-use in the infusion reactor 103A. The processed gases may for example be returned to the infusion reactor 103A via flow 11. The reactions which occur in the infusion unit 103A and the stabilization unit 103B may result in the consumption of NO₂ and the generation of NO, H₂O, HNO₃ and CO₂. In some embodiments, some of the NO is converted back to NO₂ by adding oxygen (see reaction 6). Such oxygen may be added in a controlled sub-stoichiometric amount to avoid over-oxidation, which can undesirably lead to excess nitric acid formation.

Following this, the gases may be treated within process unit 103C to rebalance the composition of nitrogen oxide gases to be suitable for use in the infusion unit 103A. The process unit 103C may also be configured to remove reaction by-products and ingressed air from the process. Referring to FIG. 11, in some embodiments, in a first step, unit 103C-1 is configured to contact the gases in a counter-current manner with a nitric acid solution (flow 27) to react NO and nitric acid together to produce NO₂ (the reverse of reaction 5). Such a contacting step may be referred to herein as “reactive contacting”. This reactive contacting may be carried out using any standard gas-liquid contacting device, such as a packed column or a plate column. In some example embodiments in which a column-type contacting device is used, the nitric acid solution that is fed to the top of the column of the contacting device may have a temperature of about 20 to 80° C. and a concentration of about 55 to 68 wt %.

Performing the contacting step in unit 103C-1 will result in diluting of the nitric acid solution. In some embodiments, a now slightly more dilute nitric acid solution is caused to exit from the bottom of the column of the contacting device. The discharged slightly more dilute nitric acid solution may be re-concentrated through a distillation process in unit 103C-4. The re-concentrated nitric acid solution may be returned to the top of the column of unit 103C-1 for re-use (flow 27). The re-constituted gas mixture (flow 20) may then be sent to a condenser 103C-2 to remove nitric acid vapor and water vapor, resulting in dried gases which may be enriched in NO₂, and condensed nitric acid and water solution. The condensed nitric acid and water solution may be transported to the distillation process in unit 103C-4. The resulting dried gases, enriched in NO₂ (flow 21) are generally suitable to return to the infusion unit 103A for use in the infusion step. In some embodiments, a portion of these gases is optionally separated to act as a bleed for the process and remove reaction by-products and ingressed air (flow 22).

The separated gas flow which may be used to act as a bleed (referred to herein as the “bleed gas flow”) may be mixed with oxygen (flow 23). The gas mixture may be caused to pass through a scrubber 103C-3. The scrubber 103C-3 may comprise any standard gas-liquid contacting device such as a packed column or a plate column. In some embodiments, water, as the scrubbing liquid, is supplied to the top of the column of the scrubber 103C-3 (flow 24). The scrubbing liquid will adsorb nitrogen oxide gases contained in the bleed gas flow and produce a dilute nitric acid. The dilute nitric acid may be caused to flow out from the bottom of the column. The dilute nitric acid may then be transported to a distillation process in unit 103C-4. The remaining gases may be caused to flow to the vent gas treatment unit 106 before being discharged. As mentioned previously, the distillation process in unit 103C-4 is configured to produce a re-concentrated nitric acid solution for use in the reactive contacting step in unit 103C-1. The distillation process in unit 103C-4 may also produce a relatively pure water stream (flow 26) that acts as an exit for water by-product from the process.

A carbonization unit 104A is arranged downstream of the stabilization vessel 103B. The stabilized green carbon fibers 6 are transported from the stabilization unit 103B to the carbonization unit 104A by standard bulk material handling equipment such as, but not limited to, belt conveyors, vibrating conveyors, augers (screw conveyors), chain conveyors, and pneumatic conveyors. Carbonization involves heating the stabilized fibers 6 in an oxygen-free environment to remove non-carbon elements. To maintain such an oxygen-free environment within the carbonization reaction vessel, air-lock type devices 102 may be arranged upstream of the carbonization unit 104A, between the stabilization unit 103B and the carbonization unit 104A, and downstream of the carbonization unit 104A where the carbonized fibers 7 exit the carbonization unit 104A. In such embodiments, the stabilized fibers 6 are caused to pass through an air-lock type device 102 before entering the carbonization unit 104A. The carbonized fibers 7 are caused to pass through another air-lock type device 102 after exiting the carbonization unit 104A.

In some embodiments, the carbonization unit 104A comprises a high-capacity bulk handling vessel such as a vibrating/fluidizing furnace or an indirectly heated rotary furnace. A high-capacity bulk handling vessel such as an indirectly heated rotary furnace advantageously allows for low cost high capacity processing, good solid-to-gas heat transfer and a controllable temperature profile. The carbonization process in the carbonization unit 104 involves heating the fibers to a carbonization outlet temperature in the range of from about 800 to about 1650° C. to produce carbon fibers which may be referred to as so called “general purpose carbon fibers”. In some embodiments, the carbonization outlet temperature is in the range of from 800° C. to 1250° C. Lower temperatures are desired in order to lower furnace costs by allowing less expensive materials of construction to be used for the furnace.

In some embodiments, the produced carbon fibers 7 may be surface treated in unit 105 to produce a surface treated carbon fiber product 8. Surface treatment of the fibers makes the fibers more compatible with the matrix they are intended to reinforce. Surface treatments may comprise surface oxidation so as to render the surface of the fibers more hydrophilic. Surface oxidation may for example include methods such as thermal oxidations in air possibly with steam, or chemical oxidations such as with ozone, nitric acid or nitrogen dioxide. Surface treatments may also include coating treatments to provide different surface functionalities (sometimes referred to as sizing or compatibilizers).

When carbonizing stabilized fibers that are produced from asphaltene-rich feeds, significant amounts of hydrocarbon vapors are released by de-alkylation reactions of the asphaltenes. Such release of vapors typically occurs at around 350-500° C. This can be seen in the fiber weight loss that is visible in the 350-500° C. temperature range in FIG. 8 and the peaks labelled as decane and methane in FIG. 7. Other gases that are released in significant amounts during the carbonization process include carbon dioxide (CO₂) and carbon monoxide (CO), as also shown in FIG. 7. In some embodiments, off-gases that are discharged from the carbonization unit 104A are recirculated to a condenser 104B (e.g., via flow 14). The condenser 104B is configured to process the off-gases recover hydrocarbons that are easily condensable. Non-condensable gases such as methane, carbon monoxide and carbon dioxide can be partially returned to the carbonization unit via flow 15 and partially be directed to flow to the vent gas treatment unit 106.

The resulting carbon fiber products 7 or 8 made by the carbon fiber production system discussed herein comprise random, discontinuous fibers in the form of entangled mats. Such random carbon fiber material may be suitable for blending into polymers or other matrices to produce short carbon fiber reinforced products. In testing of product from pilot plant experiments, such blended composites have shown a considerable stiffness advantage. Improved stiffness can allow for thinner parts with savings in weight and polymer resin. In one example application, the mats of random short carbon fibers may be melt blended (referred to as compounding) with one or more thermoplastic materials to form a mixture. The mixture may be pelletized, forming carbon fiber loaded plastic pellets. Such carbon fiber loaded plastic pellets may be suitable for use in standard molding equipment to produce carbon fiber reinforced plastic products. In some embodiments, the carbon fiber product 7 may be treated by one or both of sizing treatment for example, by a sizing agent, and surface treatment prior to melt blending with the thermoplastic material. In some embodiments, the concentration of the resulting carbon fiber products in the carbon fiber loaded plastic pellets is in the range of from about 2% by weight to about 30% by weight.

In some other example applications, the resulting random, discontinuous carbon fibers in the form of entangled mats 7 or 8 may be used to produce filtering or insulating materials. Since carbon fibers have electrical conductivity, such resulting random, discontinuous carbon fibers may be used to create conductive composites for use in applications such as bipolar plates in electrochemical cells and electromagnetic shielding.

In further example applications, the random, discontinuous carbon fibers may be added to concrete for use as a reinforcement and/or to create conductive concrete for electromagnetic shielding or for de-icing of surfaces.

EXAMPLES

Systems and processes of the types illustrated in FIG. 1 were used to produce random, discontinuous carbon fibers in the form of entangled mats from asphaltene-rich feedstocks at a small pilot scale.

Example 1

In these experiments, green carbon fibers having an average diameter of about 13 μm diameter were prepared by extruding a starting asphaltene-rich sample through a die at around 190° C. to 220° C. and stretching using a take-up wheel (melt spinning). The composition of the starting asphaltene used as the raw asphaltene feed in this Example is listed in Table 2.

TABLE 2
Composition of starting asphaltene sample

	Element	C	H	N	S	O
	wt %	81.5	8.5	1.0	7.25	1.8

H/C mole ratio 1.24, heptane insoluble asphaltenes 67 wt %, micro carbon residue 36.0 wt %

When the green fibers were slowly heated in air at 0.85° C./min to 225° C. the fibers became sticky at around 100-130° C. and the fibers began to melt at about 150° C. to about 190° C. This suggests that the softening point of this asphaltene sample was too low for conventional methods of stabilization by heating in air at 250-350° C.

The green carbon fibers were infused at ambient temperature using different blends of NO₂ in oxygen over different infusion treatment times. The concentrations of NO₂ and N₂O₄ were measured using ultraviolet and visible light (UV-Vis) absorption spectroscopy. After infusion, the green carbon fibers were transferred to a tube furnace for stabilization and carbonization. The stabilization and carbonization of the green carbon fibers was carried out by heating under nitrogen from ambient up to 1100° C. Depending on the infusion conditions used, carbon fiber product could be successfully produced without any melting or fusing.

The resulting fibers that did not melt or fuse were subjected to tensile testing. In these experiments, a tensile strength of greater than 350 MPa was considered to be acceptable by the inventors. The results of these tests are shown in FIG. 6. FIG. 6 maps acceptable versus unacceptable fibers in relation to infusion treatment time (minutes) and effective concentration of nitrogen dioxide (NO₂) gas used (100%×(C_NO2+2 C_N2O4)/(100+C_N2O4) in mole %). Referring to FIG. 6, the triangle markers indicate operating conditions that resulted in inadequate properties of the resulting fibers and the circular markers show process conditions that resulted in acceptable quality carbon fibers. The results show that effective NO₂ concentrations of less than 10% can produce good quality carbon fibers, but require long treatment times (i.e., of greater than about 40 minutes) or possibly higher treatment temperatures. High effective NO₂ concentrations (i.e., concentrations of greater than about 50%) resulted in fibers that melted, and in some cases crumbled. The inventors suspect that this may be due to over-oxidation of the fibers, which eventually led to a decrease in molecular weight and a lower melting point. At the high effective concentrations of NO₂ used in this work UV-Vis spectroscopy showed significant amounts of dimeric N₂O₄. It is speculated that this enhanced concentration of the dimeric N₂O₄ may be important for the production of nitroxy groups (R—O—NO₂) and nitro groups (R—NO₂) on the green fibers during the infusion step.

After nitrogen dioxide (NO₂) infusion, the green carbon fibers were directly heated under nitrogen for stabilization, followed by carbonization. The stabilization of the green carbon fibers under nitrogen is believed to rely on oxidative cross-linking driven by the nitrogen oxide groups that were introduced during the NO₂ infusion. The inventors found that the nitrogen oxide groups began to react before the pitch began to excessively soften and thus successfully stabilized the green fibers.

During the nitrogen dioxide (NO₂) infusion treatment, the functional groups that were present on the surfaces of the NO₂-infused green carbon fibers were qualitatively measured using Attenuated Total Reflectance Fourier Transform Infrared spectroscopy (ATR-FTIR). ATR-FTIR analysis of the NO₂-infused green carbon fibers indicated that the NO₂ infusion treatment resulted in peaks appearing at around 1556 cm⁻¹, 1525 cm⁻¹, and at around 1326 cm⁻¹ which may be related to N═O stretches of nitro (R—NO₂) and possibly nitroso (R—N═O) groups, and a peak at around 1274 cm⁻¹ which is possibly due to nitroxy groups (R—O—NO₂).

Samples of the fibers were also taken after different stages of stabilization and re-measured by ATR-FTIR. The results showed no significant changes on heating of the NO₂-infusion green carbon fibers to 100° C., but by 200° C., the peaks around 1274 cm-1 and 1557 cm 1 disappeared. At the same time a signal around 1700 cm⁻¹ associated with C═O increased, thus showing oxidation of the fiber surfaces (even though this test was carried out under nitrogen).

Example 2

An in-house built, batch centrifugal spinner that was capable of holding about 20 to 30 g of asphaltene samples was used to spin the asphaltenes to form green carbon fibers. The spinner has an outer diameter of 2.5 in. (˜6.5 cm) and is attached to a DC motor capable of spinning it at up to 4200 rpm. The spinner was designed to hold up to 4 interchangeable nozzles, thus allowing for a range of nozzle diameters to be used. The spinner comprises disk heaters placed above and below the body of the spinner to heat the contained asphaltene to achieve a sufficiently low viscosity for spinning. The properties of the starting asphaltene sample used as the asphaltene feed in this Example are listed below:

TABLE 3
Properties of the starting asphaltene feed used in Example 2

	C	82.5 wt %
	H	8.5 wt %
	N	1.1 wt %
	S	7.3 wt %
	O	1.0 wt %
	H/C mole ratio 1.22
	Asphaltene (pentane insoluble) 77.5 wt %
	Asphaltene (heptane insoluble) 52.3 wt %
	Ash 0.3 wt %
	Micro carbon residue 40.4 wt %
	Toluene Insoluble 1.2 wt %
	Quinoline Insoluble 0.2 wt %

This starting asphaltene sample was heated under nitrogen to about 250° C. and allowed to outgas before being used for spinning. The centrifugal spinner was operated with a 0.016″ diameter nozzle (406 μm diameter) at centrifugal speeds of from 1400 to 4200 rpm with the disc heaters set at temperatures from 305° C. to 360° C. Due to heat transfer limitations, the true spinner temperature is expected to be significantly lower than the heater values. A total of 25.3 g of green carbon fibers were produced in 2 batches. Note that because of the batch nature of the laboratory centrifugal spinner used, rotational speed and temperature were adjusted during the runs to maintain flow through the spinner nozzles as the spinner load changed over the batch.

The green carbon fibers were then infused with about 26% effective NO₂ in oxygen for about 40 minutes at ambient temperature. Gases that were formed in the infusion reactor were monitored using a Fourier Transform Infrared Gas Analyzer (FTIR-GA). FIG. 4 is a plot of concentration (ppm) as a function of time (minutes). FTIR-GA results show that small amounts of nitrogen monoxide (NO), water (H₂O), nitric acid (HNO₃) and carbon dioxide (CO₂) were generated by the infusion reactions. FTIR-GA only detects polar compounds, so nitrogen (N₂) and oxygen (O₂) were not measured. The optical cell FTIR-GA was also kept warm (about 150° C.) to prevent condensation of nitric acid, thus the NO₂—N₂O₄ equilibrium was locally shifted within the optical cell to largely dissociate the N₂O₄. Thus, the measurement reading for NO₂ shown in FIG. 4 does not reflect the true concentration of N₂O₄ in the infusion reactor but is actually closer to the effective concentration of NO₂.

The NO₂-infusion treatment resulted in a weight gain of the fibers of about 20%. The NO₂ infused fibers were then placed in a tube furnace under high purity (99.999%) nitrogen and heated from ambient to 850° C. The resulting stabilization and carbonization resulted in a 46% yield of carbon fibers based on the starting NO₂ infused fibers, or a 55% yield based on the green carbon fibers (i.e. including the weight gain from the infusion step). FIG. 9A is a photograph showing an example of mats of infused carbon fibers. The infused fibers were black in color. FIG. 9B is a photograph showing an example of mats of carbon fiber product produced after the carbonization step. The carbon fiber products were shiny.

FIGS. 10A and 10C are Scanning Electron Microscope (SEM) images of examples of resulting carbon fiber product. FIG. 10B is an optical microscope photo of an example of resulting carbon fiber product.

The carbon fiber mat was mixed with polyamide 6 (Nylon 6 or PA6) polymer pellets using a twin-screw extruder. The mixing was performed under a screw speed of 100 rpm, mixing time of 5 minutes, torque of 4.5 to 5 Nm, and barrel temperature of 235° C. Test samples were produced with 5 wt %, 10 wt % and 20 wt % of carbon fiber loading and were subjected to tensile testing. The tensile strength results are listed in Table 4.

TABLE 4
Tensile strength of carbon fiber composites

		Youngs' modulus	Ultimate tensile
	Sample name	(MPa)	strength (MPa)

	Neat PA6	1723	56
	PA6 - 5 wt % CF	2282	55
	PA6 - 10 wt % CF	2472	62
	PA6 - 20 wt % CF	2867	69

The tensile testing results show a considerable increase in the Young's modulus of the composite using the carbon fiber mat. The resulting composite material thus has increased stiffness and rigidity which can be useful in applications where maintaining the shape and resisting deformation of the material is critical. It should be noted that these results were obtained with the as-produced fibers and so without any fiber surface treatment to improve the fiber-polymer interface adhesion. Such treatments, as are commonly used in the industry, are expected to further improve the results.

Example 3

An in-house built, batch centrifugal spinner that was capable of holding about 10 to about 15 g of asphaltene was used for producing green carbon fibers from the molten asphaltene-rich feed. The spinner has an outer diameter of 2.0 in. (about 5.0 cm). The spinner was attached to a DC motor. The DC motor is capable of driving the spinner at a speed of up to 4200 rpm. The spinner was designed to hold up to 4 interchangeable nozzles, thus allowing a range of nozzle diameters to be used. Disk heaters were placed above and below the spinner body, thereby heating the contained asphaltenes so that a sufficiently low viscosity of the asphaltenes may be achieved to allow for spinning. The starting asphaltene feed in this Example was a blend of asphaltene-rich feeds 1 and 4 in Table 1, with properties listed in Table 5.

TABLE 5
Properties of the starting asphaltene feed used in Example 3

	C	82.0 wt %
	H	8.2 wt %
	N	1.2 wt %
	S	8.1 wt %
	O	1.1 wt %
	H/C mole ratio 1.19
	Asphaltene (pentane insoluble) 89.0 wt %
	Asphaltene (heptane insoluble) 60.1 wt %
	Ash 0.3 wt %
	Micro carbon residue 43.5 wt %
	Toluene Insoluble 1.2 wt %
	Quinoline Insoluble 0.2 wt %

This starting asphaltene was heated under nitrogen gas to 250° C. and was allowed to outgas before being used for spinning to produce green fibers. The centrifugal spinner was operated with a 0.025″ diameter nozzle (635 μm) at speeds from about 1400 to about 4200 rpm with the disc heaters set for temperatures from about 310° C. to about 350° C. Due to heat transfer limitations, it is expected that the true spinner temperature will be significantly lower than the heater values.

The resulting green fibers were then infused with about 32% effective NO₂ for about 23 minutes at ambient temperature. The NO₂-infused fibers were placed in a tube furnace under high purity (99.999%) nitrogen and heated from ambient temperature to 1100° C. to carry out stabilization and carbonization.

The resulting carbon fiber mat material was mixed with polypropylene (PP) polymer pellets using a twin screw extruder, with the following operating conditions: screw speed of 100 rpm, mixing time of 5-10 min, torque of 8.5-9.5 Nm, and barrel temperature of 200° C. Carbon fiber composites were produced with 5 wt %, 10 wt % and 20 wt % carbon fiber loading. The carbon fiber composites were subjected to tensile testing. The tensile testing results are shown in Table 6.

TABLE 6
Tensile strength of carbon fiber composites

		Youngs' modulus	Yield strength
	Sample name	(MPa)	(MPa)

	Neat PP	775	20.8
	PP - 5 wt % CF	870	20.2
	PP - 10 wt % CF	1038	21.6
	PP - 20 wt % CF	1130	18.1

The tensile testing results show a noticeable increase in the Young's modulus of the carbon fiber composites and thus the resulting materials possess increased stiffness and rigidity which can be useful in applications where maintaining the shape and resisting deformation of the material is critical. It should be noted that these results were obtained with the as-produced fibers and so without any fiber surface treatment to improve the fiber-polymer interface adhesion. Such treatments, as are commonly used in the industry, are expected to further improve the results.

Example 4

The same starting asphaltene-rich feed as was used in Example 2 was used to create additional fibers. An in-house built centrifugal spinner that is capable of holding about 10 to about 15 g of asphaltene was used. The spinner has an outer diameter of 2.0 in. (˜5.0 cm) and was attached to a DC motor capable of spinning it at up to 4200 rpm. The spinner was designed to hold up to 4 interchangeable nozzles, thus allowing for a range of nozzle diameters to be used. The spinner body had disk heaters placed above and below to allow heating of the contained asphaltene to achieve a sufficiently low viscosity for the spinning step.

The starting asphaltene feed 1 was heated under nitrogen to about 250° C. and was allowed to outgas before spinning. The centrifugal spinner was operated with a 0.025″ diameter nozzle (635 μm) at speeds from about 2100 to about 4200 rpm with the disc heaters set for temperatures from about 310° C. to about 340° C. Due to heat transfer limitations, the true spinner temperature will be significantly lower than the heater values.

The green fibers were then infused with about 29% effective NO₂ for about 21 minutes at ambient temperature. The NO₂ infusion treatment resulted in a weight gain of 8.8%. The NO₂-infused fibers were then placed in a tube furnace under high purity (99.999%) nitrogen and heated from ambient temperature to 1200° C. to carry out stabilization and carbonization.

FTIR-GA was used to monitor the gases exiting the nitrogen purged tube furnace used for stabilization and carbonization. Referring to FIG. 7, as the NO₂ infused fibers were heated, FTIR-GA results show the release of NO₂, followed by NO and CO₂, with the release of CO₂ being an indication of oxidation of the fibers. Water vapor (H₂O) was also detected showing up from 200-350° C., but the inventors suspect that the water vapor condensed in cold areas of the furnace thereby resulting in its later appearance as the furnace was heated to higher temperatures, thus appearing at such an unexpectedly high apparent temperature range. By around 270° C., stabilization of the fibers is complete with no further nitrogen oxide gases being detected and further heating to carry out carbonization can be continued without melting the fibers. At higher temperatures (e.g., above about 300° C.), significant de-alkylation of the asphaltenes begins giving rise to a range of alkyl compounds, with the resulting complex infrared signal fitted in FIG. 7 as decane. Methane (CH₄) was visible from around 350° C. to 700° C. CO was detected beginning at above 200° C., and continued to be detected to around 900° C. to 1000° C. The apparent continuation of a constant decane signal at above 650° C. is believed to be due to the deposition of alkyl compounds on the windows of the FTIR-GA optical cell. The yield of carbon fibers from stabilization and carbonization was 42.7% based on the weight of the infused fibers. Because the infusion process resulted in a weight gain of 8.8%, this represents an overall yield based on the green fibers of 46.5%.

The stabilization and carbonization were also monitored using Thermal Gravimetric Analysis (TGA). A sample of the infused fiber was placed in a TGA under nitrogen gas flow and heated to 1000° C. Some green carbon fibers without NO₂ infusion treatment were also separately run as a control. FIG. 8 shows Thermal Gravimetric Analysis (TGA) curves for both giving the amount of weight loss (%) as a function of temperature (° C.) during stabilization and carbonization. Referring to FIG. 8, at lower temperatures, the NO₂ treated fibers showed greater weight loss as compared to the untreated fibers. However, the NO₂ treated fibers showed less weight loss in the 350 to 450° C. de-alkylation region. The untreated fibers melted and resulted in final carbon yield of a solid carbon material of 34.8%, while the NO₂ treated fibers gave carbon fibers with a yield of 40.7%. Further, in this case the infusion had caused a weight increase of 8.8% for the infused fibers, so the overall yield of carbon fibers based on the green fibers would be 44.3%. These results are similar to those found with the treatment of the larger batch of infused fibers in the tube furnace.

Single fibers were separated from the produced carbon fiber mat. The separated single fibers were subjected to single fiber tensile testing using a 1″ span. Test results show that tensile strength of as high as 0.51 GPa with a modulus of 31 GPa was achieved.

Example 5

The same asphaltene-rich feed as was used in Example 1 was used in this Example. In this Example, an in-house built, batch centrifugal spinner capable of holding about 20 to about 30 g of asphaltene was used. The spinner has an outer diameter of 2.5 in. (about 6.5 cm) and was attached to a DC motor capable of spinning it at up to 4200 rpm. The spinner was designed to hold up to 4 interchangeable nozzles, thus allowing for a range of nozzle diameters to be used. The spinner body had disk heaters placed above and below to allow heating of the contained asphaltene-rich feed to achieve a viscosity that is sufficiently low for spinning.

The starting asphaltene feed was heated under nitrogen to 250° C. and allowed to outgas before being used for spinning. The centrifugal spinner was operated with a 0.016″ diameter nozzle (406 μm) at speeds from about 3150 to about 4200 rpm with the disc heaters set at temperatures from about 290° C. to about 330° C. Due to heat transfer limitations, the true spinner temperature will be significantly lower than the heater values. A total of 24.7 g of green fibers were produced in 2 batches. Note that because of the batch nature of the laboratory centrifugal spinner used, rotational speed and temperature were adjusted during the runs to maintain flow through the spinner nozzles as the spinner load changed over the batch.

The green carbon fibers were then infused with about 27% effective NO₂ in oxygen for about 40 minutes at ambient temperature. The NO₂ infusion treatment resulted in a weight gain of the green fibers of 27.1%. The NO₂-infused fibers were then placed in a tube furnace under high purity (99.999%) nitrogen and heated from ambient temperature to 1000° C. to undergo stabilization and carbonization. The yield of carbon fibers from stabilization and carbonization was 44.0% based on the weight of the infused fibers. Because the infusion process resulted in a weight gain of about 27.1%, this represents an overall yield based on the green fibers of 56.0%

Single fibers were separated from the carbon fiber mat. The separated single fibers were subjected to single fiber tensile testing using a 1″ span. Tensile testing results indicate that tensile strength of as high as about 0.43 GPa with a modulus of about 40 GPa was achieved. The product carbon fiber mat material was mixed with polyamide 6 (Nylon 6 or PA6) polymer pellets using a twin screw extruder. The twin screw extruder was operated under the following conditions: screw speed of 100 rpm, mixing time of 5 minutes, torque of 4.5 to 5 Nm, and barrel temperature of 235° C. Carbon fiber composites were produced with 5 wt %, 10 wt % and 20 wt % carbon fiber loading. The composites were subjected to tensile strength testing. The tensile strength test results are shown in Table 7.

TABLE 7
Tensile strength of carbon fiber composites

		Youngs' modulus	Ultimate tensile
	Sample name	(MPa)	strength (MPa)

	Neat PA6	1723	56
	PA6 - 5 wt % CF	2046	51
	PA6 - 10 wt % CF	2308	63
	PA6 - 20 wt % CF	2972	66

The tensile testing results show a considerable increase in the Young's modulus of the carbon fiber composites along with a small increase in strength. The resulting materials thus possess increased stiffness and rigidity which can be useful in applications where maintaining the shape and resisting deformation of the material is critical. It should be noted that these results were obtained with the as-produced fibers and so without any fiber surface treatment to improve the fiber-polymer interface adhesion. Such treatments as are commonly used in the industry should further improve the results.

The Examples show that the carbon fiber production processes and systems disclosed herein have at least one or more of the following advantages:

• • ability to utilize an inexpensive, minimally upgraded asphaltene as the feedstock to directly produce short carbon fibers;
  • ability to use high throughput fiber spinning methods that are tolerant of variations in the feed quality to directly produce discontinuous carbon fibers;
  • ability to incorporate a close to ambient temperature infusion method ahead of the stabilization step, thereby avoiding issues with stabilizing low softening point feeds;
  • ability to incorporate the produced mat of random material of discontinuous carbon fibers in a polymer matrix to produce a reinforced product.

All of the following references are hereby incorporated herein by reference as if fully set forth herein for all purposes.

• • D. Choi, H.-S. Kil and S. Lee, “Fabrication of low-cost carbon fibers using economical precursors and advanced processing technologies”, Carbon, 142 (2019) 610-649.
  • J. A. Fry, “Enabling a step change in single-line carbon fiber production capacity through advanced high precision large scale thermal processing equipment”, Harper International white paper.
  • J. G. Lavin, “Ch. 5, Carbon fibres”, in J. W. S. Hearle, “High-performance fibres”, CRC Press, (2001) 156-190.

Throughout the foregoing description and the drawings, in which corresponding and like parts are identified by the same reference characters, specific details have been set forth in order to provide a more thorough understanding to persons skilled in the art. However, well known elements may not have been shown or described in detail or at all to avoid unnecessarily obscuring the disclosure.

As will be apparent to those skilled in the art in the light of the foregoing disclosure, many alterations and modifications are possible in the practice of this invention without departing from the scope thereof. Accordingly, the description and drawings are to be regarded in an illustrative, rather than a restrictive, sense.