Apparatus for forming carbon fibres

Abstract: In a preferred embodiment, an apparatus (10) for forming carbon fibres in a continuous gas-phase reaction is disclosed wherein a reactor (12) defines a vertical reaction chamber (14) with a closed upper end (16) and a lower outlet (18), with a vertical reactant feed tube (20) which extends generally axially therewithin. Gaseous reactants are injected into the reactor (12) through an opening (24) of the feed tube (20). The feed tube (20) and chamber (14) are constructed and arranged to define a gas flow path (26) which extends generally axially upwards through the feed tube (20) into the chamber (14) and thereafter generally axially downwards through the chamber (14). Preferably, the feed tube (20) and reactor chamber (14) are radially spaced and concentric to one another. The feed tube (20) may enclose a capillary tube (28) which supplies a vaporizable liquid reactant. The reactor (12) and feed tube (20) are each elongated and of a material which is relatively resistant to heat and corrosion, such as mullite. The feed tube (20) extends more than one-half of the axial length of the reactor (12). Desirably, the feed tube (20) extends at least three-fourths of the axial length of the reactor (12), and preferably extends at least five-sixths of the length thereof and is concentric with the reactor chamber (14) defined by the tubular reactor (12).

Description: This invention relates to forming carbon fibres. More particularly, this invention relates to an apparatus as specified in the preamble of claim 1, for example as disclosed in US-A-1,823,503. It concerns such an apparatus for making carbon fibres in a continuous gas-phase reaction, which provides desired conditions for each step of the reaction including decomposition of a compound containing nuclei, formation of nuclei, decomposition of a carbonaceous compound and reaction of the decomposed carbonaceous compound with the nuclei, thereby forming carbon fibres. Carbon fibres are used to make composite materials. Such carbon composite material provide the advantages of relatively high strength and low weight as compared to other materials commonly used to form articles such as automotive, boat, airplane and other parts. High strength and low weight are key advantages needed to reduce vehicle fuel consumption and increase fuel economy. The strength of the composite material is directly related to the quality of the carbon fibres used therein. Preferred fibres have basal planes arranged in concentric circles and are of microscopic size for improved strength to weight ratio. Such microscopic fibres effectively achieve the advantages of improved strength, low weight and, therefore, improved fuel economy in vehicles formed from composite materials containing such fibres. Prior methods of forming carbon fibres basically consist of conducting a gas-phase batch reaction in a vessel. Specifically, such methods include dispensing iron particles, in the form of a powder or in a solution, into a vessel whilst simultaneously dispensing therein a liquid or gas hydrocarbon compound, then decomposing the hydrocarbon compound to form carbon-containing compounds which react with the iron particles. A diluent gas, such as hydrogen, is injected into the vessel to control the rate at which decomposition and growth proceed. The vessel is typically arranged so that convective currents are generated. These currents often cause a loss of heat which reduces the temperature of the reactants to below that desired for optimum fibre formation. The currents also cause losses of feedstock reactant material, including expensive catalyst material, due to thermophoretic deposition on the walls. Typically, the batch reaction has proceeded in the vessel for a period of time, fibres are collected, and then the reaction is started again. Typically, large fibres, that is fibres having dimensions greater than the desired microscopic fibres, are produced along with undesirable soot. Often a mixture of soot and large fibres results when prior processes and vessels are used. This may occur because it is difficult to control the conditions at which the various steps of the reaction occur in conventional batch processes, particularly when convective currents arise in conventional vessels. Prior methods and vessels typically include a pre-conditioning period. The vessel is pre-heated and/or the surface thereof is prepared using iron particles before the reaction is initiated. The pre-preparation effort is typically extensive in relation to the reaction period during which the product is actually produced. Such methods are not cost-effective for use in commercial-scale plants. An apparatus for forming carbon fibres by a continuous gas-phase process is characterised by the features specified in the characterising portion of claim 1. It is an object of this invention to provide an apparatus for forming carbon fibres in a continuous gas-phase reaction; to improve the process of forming carbon fibres by providing an apparatus constructed and arranged to achieve optimum conditions for forming microscopic carbon fibres in a gas phase reaction; to maintain essentially steady-state, controlled reaction conditions in a continuous flow process in which microscopic carbon fibres are formed; to minimize convective currents; and to minimize thermophoretic losses of feedstock reactant material. It is also an object to provide an apparatus which produces an increased yield of carbon fibres, provides a larger proportion of smaller fibres, substantially eliminates soot, and decreases the cost of producing carbon fibres. In accordance with the invention, these and other objects are accomplished by providing an apparatus for forming carbon fibres in a continuous gas-phase process by reacting a carbonaceous compound in the presence of nuclei entrained in a gas stream. Each of the nuclei forms a centre about which a carbon fibre grows. The apparatus comprises a reactor and reactant feed tube constructed to provide a continuous gas flow path with an upward flow portion and a downward flow portion arranged in heat transfer relationship to one another. The reactor defines a vertical reaction chamber with a closed upper end and a lower outlet. The reactant feed tube is disposed in the reactor and extends generally axially upwards therein. Gaseous reactants are injected into the reactor through an opening of the feed tube. The feed tube and chamber are constructed and arranged to define a gas flow path which extends generally axially upwards through the feed tube into the chamber and thereafter generally axially downwards through the chamber. Preferably, the tube and reactor chamber are radially spaced and concentric to one another. In one embodiment of the invention, the feed tube may encompass a capillary tube which supplies a vapourizable liquid reactant. The reactor and feed tube are each elongated and of a material which is relatively resistent to heat and corrosion and are preferably formed of a mullite material. The feed tube extends more than one-half of the axial length of the reactor. Desirably, the feed tube extends at least three-fourths of the axial length of the reactor, and preferably extends at least five-sixths of the length thereof. In order to supply the necessary heat for the reaction, the apparatus has means for heating the reactor to establish a hot zone for thermal decomposition, i.e., a pyrolysis reaction within the chamber adjacent its closed end. A relatively cool zone is adjacent the reactor outlet. The hot and cool zones are constructed and arranged to establish a thermal gradient within the reactor so as to minimize convective currents within the reactor. As a result, in the reactor, hot gases are disposed over cold gases. Preferably, the heating means is a furnace which encompasses at least the closed upper end of the reaction chamber and a portion of the chamber adjacent the closed end. The apparatus also has means for continuously supplying a gaseous reactant stream into the feed tube at a rate sufficient to entrain the nuclei within the gas stream, so as to minimize thermophoretic deposition, and so as to disperse the nuclei into the reactor hot zone. The gaseous reactant stream may include a vapourizable constituent. To facilitate fibre growth, the reactor chamber has an average cross-sectional area substantially greater than the average cross-sectional area of the feed tube. In this arrangement, gas moves upwardly through the feed tube quickly, minimizing reactant deposition and other losses, and into the hot zone at a velocity sufficient to entrain nuclei and disperse and scatter the nuclei in the reaction chamber. However, the flow must not be so rapid as to cool the reaction zone. Since the reactor chamber has a much greater average cross-sectional area, it accommodates the gas stream at a reduced downward flow rate suitable for sustained fibre growth. Preferably, the reactor chamber is tubular and substantially concentric with the feed tube along the axial extent thereof. Desirably, the reaction chamber has a diameter which is at least three times greater than the diameter of the feed tube, and preferably five times greater. It will be appreciated by those skilled in the art that the average mass flow rate of the upwardly moving gas stream and the downwardly moving gas stream are essentially the same as there is no noticeable accumulation of reactants within the reaction chamber. Due to the expansion of the gas stream, the downwardly flowing gas stream will have a reduced flow rate, that is, a reduced average velocity in accordance with the greater cross-sectional area available in the reactor chamber as compared to the feed tube. The apparatus includes means for collecting carbon fibres from the exhaust gas which exits the reactor outlet. In use, preferably a pressurized gas stream comprising a gaseous carbonaceous compound is supplied to the inlet of the feed tube. A compound which forms nuclei is injected into the gas stream. The pressurized gas stream and injected compound flow upward in the feed tube, are heated from an ambient inlet temperature to an elevated temperature, and are discharged at the opening into the hot zone of the reactor chamber. The stream flows at a reduced flow rate down through the relatively large reaction chamber wherein the nuclei each form a lengthening filament from carbonaceous compounds produced by the decomposition of the carbonaceous compound reactants in the feedstock. The gaseous stream in which the carbon fibres are entrained then exits the reactor at the outlet thereof. Thereafter, the carbon fibres are separated from the gaseous stream, preferably by cyclone separators. Preferably, in use, the hot zone is at a temperature sufficient to initiate at least the formation of catalyst particles i.e., nuclei, in the 5 nm in diameter range, whilst the gaseous mixture is in the feed tube. At the same time, the temperature of the hot zone is controlled so that complete pyrolysis of reactants in the feed tube does not occur, as this may lead to sooting, with undesired deposition of material, rather than the formation of microscopic carbon fibres. Carbon fibres formed in the apparatus of the invention have basal planes arranged in concentric circles and are microscopic in size. The carbon fibres are in the range of 20 to 100 nanometres in diameter (microscopic) and have a length of at least several micrometres. The carbon fibres pack so loosely in a fibrous matrix that their apparent bulk density is in the range of 0.001 to slightly over 0.02 grams per cubic centimetre and preferably have a density below 0.01 grams per cubic centimetre. Thus, the invention provides a continuous apparatus for forming carbon fibres in a gas-phase reaction, improves the process of forming microscopic carbon fibres, permits the continuous formation of carbon fibres in a continuous reaction whilst maintaining an essentially steady state, controls desired reaction conditions, minimizes undesirable convective currents, increases the yield of carbon fibres, minimizes thermophoretic wastage of feedstock, substantially eliminates soot, increases the proportion of smaller, microscopic carbon fibres, and decreases the cost of producing carbon fibres. The invention and how it may be performed are hereinafter particularly described with reference to the accompanying drawings, in which: Figure 1 is a diagrammatic cross-sectional view of an apparatus embodying this invention with a reactor defining a vertical reaction chamber and a reactant feed tube extending axially upwards therein. Figure 2 is a fragmentary exploded view of the encircled portion of Figure 1. Figure 1 illustrates a preferred apparatus 10 for forming carbon fibres by a continuous gas-phase process comprising reaction of a gaseous carbonaceous compound in the presence of nuclei entrained in a gas stream. Apparatus 10 includes a reactor 12 defining a chamber 14 with a vertical axis, a closed upper end 16 and a lower outlet 18. Gaseous reactants are injected into the reactor 12 by a reactant feed tube 20 which is disposed in the chamber 14 and extends generally axially upwards. The feed tube 20 has an inlet 22 at one end and a discharge opening 24 at the other end spaced from the closed end 16 of the reactor 12. The opening 24 of the feed tube 20 is adjacent the closed end 16, and the feed tube 20 extends more than one-half of the axial length of the reactor. Desirably, the feed tube extends at least three-fourths of the axial length of the reactor, and preferably five-sixths of the length thereof. A gas flow path 26 is defined by the gas feed tube 20 and reactor 12. The gas flow path 26 extends generally axially upwards through the feed tube 20 into the chamber 14 and thereafter generally axially downwards through the chamber 14. Desirably, the reactor 12 defines a tubular, thermal reaction chamber 14 which is substantially concentric with the opening 24. Preferably, the chamber 14 is radially spaced from the feed tube 20 and concentric therewith throughout its entire axial extent. Desirably, the reactor 12 and feed tube 20 are each elongated, that is, each has a length greater than its diameter. If it is desired to supply a vapourizable liquid reactant, preferably, a capillary tube 28 supplies the vapourizable liquid reactant through a discharge end 30 thereof. Capillary tube 28 is disposed within the feed tube 20 and is concentric therewith. In order to provide surface area from which any droplets of vapourizable liquid reactants will vapourize, a screen web 32, preferably of metal such as stainless steel, is disposed around the capillary tube 28 adjacent its discharge end 30 within the feed tube 20 (see Figure 2). Desirably, the liquid includes the nuclei and is supplied to capillary tube 28 from pump 33. In order to supply the necessary heat for the reaction, the apparatus 10 has heating means such as a furnace heater 34 to heat the reactor 12 to establish a hot zone 36 within the chamber 14 adjacent the closed end 16. The temperature of the hot zone 36 is sufficient to sustain carbon fibre growth. There is a relatively cool zone adjacent the reactor outlet 18. The hot zone 36 adjacent the closed end 16 of the chamber 14 and the cool zone adjacent the reactor outlet 18 establish a thermal gradient within the reactor 12 to minimize convective currents within the reactor. As a result, in the reactor, hot gases are disposed over cold gases and convective currents are thus minimized. Preferably, the temperature of the hot zone 36 is sufficient to decompose any of a variety of gaseous carbonaceous compounds which may include, for example, one or more vapourizable hydrocarbons. Thus, the reactor 12 defines a thermal decomposition, pyrolysis chamber 14. Desirably, a first temperature is maintained adjacent the inlet 22, a second temperature greater than the first temperature is maintained within the hot zone 36, and a third temperature less than the second temperature is maintained adjacent the outlet 18. Desirably, the inlet 22 is at an ambient temperature, the hot zone 36 is at a temperature greater than 1000°C and the outlet 18 is at a temperature between ambient and 1000°C. Preferably, a temperature over 1050°C is maintained near zone 36. In order to establish a desired temperature condition, preferably, the furnace 34 is an electric furnace which encompasses at least the closed upper end 16 of the chamber 14 and a portion of the chamber 14 adjacent the closed end 16. The furnace 34 is arranged to transfer heat to the chamber 14 which in turn transfers heat to the feed tube 20. The reactor 12 and feed tube 20 are each constructed of a material which is relatively resistant to heat and corrosion. Suitable materials include stainless steel, quartz, ceramic, and refractory materials. Desirably, a refractory such as alumina or mullite is used. Preferably, mullite is used, which is a silicate of aluminium. Mullite is more resistant to corrosion than stainless steel and it has better heat transfer properties than quartz. Mullite is better able to withstand the temperature shock that occurs when liquid is injected. The apparatus 10 comprises means for continuously supplying a gaseous reactant stream into the feed tube 20 at a rate sufficient to entrain the nuclei within the stream and disperse the nuclei into the reactor hot zone 36. Preferably, pressurized, compressed gas is used, supplied from compression and flow control system 39, and then through line 40. Specifically, the gaseous reactant stream must move at a velocity sufficient to entrain the nuclei within the gas stream for dispersion into the reactor hot zone 36. If it is desired to supply nuclei entrained in a gas stream at inlet 22 of feed tube 20 rather than nuclei in a liquid, a pressurized gas is supplied from compression and flow control system 41. The gas is then bubbled through a reservoir 42 which holds nuclei bound in a liquid. Line 43 connects system 41 to reservoir 42. This bubbling of the gas into the liquid held in reservoir 42 forms a gas/nuclei mixture which is then supplied from reservoir 42 through line 44 by gas pressure rather than a pump and then through line 46 to tee-junction 48 where it mixes with gas in line 40 before entering feed tube 20 at inlet 22. Valve 49 is closed when nuclei are supplied as a gaseous mixture through line 44, and valve 50 is closed when nuclei are supplied in a liquid through capillary tube 28. When gaseous reactants are provided through lines 44 and 40 to feed tube 20, apparatus 10 may be operated with valve 49 closed; alternatively, apparatus 10 may be operated with capillary tube 28, web 32 and pump 33. In order to provide desired conditions for microscopic carbon fibre growth, the reaction chamber 14 has an average cross-sectional area substantially greater than the average cross-sectional area of the feed tube 20. Desirably, the reactor chamber 14 has an inside diameter at least three times greater than the diameter of the opening 24, and preferably five times greater. By this arrangement, gas moves quickly through the inlet 22, is warmed in feed tube 20, and then moves into the hot zone 36, where it is dispersed and scattered remote from the surfaces within the hot zone 36. The gas then moves slowly and migrates down through the chamber 14 for an extended period of time, at a reduced flow rate suitable for sustained carbon fibre growth. Preferably, the velocity of the reactant gas stream in the feed tube 20 is at least 10 cm/s and this velocity is at least nine times greater than the velocity of the downwardly-migrating gas stream in the chamber 14. In this arrangement, gases around the feed tube 20 can be thought of as essentially controlling the heat flow from the hot zone 36 to the feed tube 20 to warm the incoming gases without encouraging premature carbon fibre growth. The reactor 12 includes means for collecting carbon fibres from exhaust gas exiting the outlet 18, which desirably comprises at least one cyclone-type separator 51 and filters 52 for trapping the microscopic carbon fibres formed in the reaction chamber 14. Preferably, a portion of the carbon fibres are first collected in a trap 54 adjacent the outlet 18 at the bottom of an angled exhaust line 56, and then the rest are collected by two cyclones 51 separately in series with one another. The apparatus 10 of the invention is used to form microscopic carbon fibres in accordance with the following examples which are meant to be illustrative and not limiting. Example 1 The apparatus 10 of the invention was used to form carbon fibres by a continuous gas phase reaction in the presence of nuclei introduced in a gas stream. A pressurized gas stream comprising a gaseous carbonaceous compound such as methane, ethane, hexane, carbon dioxide, carbon monoxide, ethylene and/or acetylene was supplied to the inlet 22 of the reactant feed tube 20. A compound which forms nuclei was injected into the gas stream. The pressurized gas stream and injected compound flowed upwards in the feed tube 20, were heated from an ambient inlet temperature to an elevated temperature, and were discharged at the opening 24 into the hot zone 36 of the reactor chamber 14. The pressurized gas stream flowed at a reduced flow rate down through the relatively large reaction chamber 14 wherein the nuclei present in the gas stream grew carbon fibres, formed from gas-phase carbonaceous compounds. The gaseous stream in which the carbon fibres were entrained then exited the reactor 12 at the outlet 18. The carbon fibres were then separated from the stream by the trap 54 and cyclone separators 51. Example 2 The apparatus 10 of the invention was used in a process to form carbon fibres in accordance with Example 1 wherein the reactants included a hydrocarbon compound such as methane, ethane, hexane and ethylene and a metal compound which formed metal nuclei. The hydrocarbon compound included a vapourizable solvent in which the metal compound was dissolved. The solution comprising the solvent and the dissolved compound were injected at the inlet 22 and vapourized within the feed tube 20. The metal compound formed submicron metal nuclei and carbon fibres were produced as the hydrocarbon compound decomposed. That is, the hydrocarbon compound was dehydrogenated and carbon filaments grew on the metal nuclei, thereby forming carbon fibres. Thermal decomposition was achieved in the reactor 12 by maintaining a temperature in the hot zone 36 in excess of 1000°C. Example 3 The apparatus 10 of the invention was used to form carbon fibres in accordance with Example 2. The apparatus 10 further included a reactor 12 of a mullite material defining a 13 centimetre internal diameter (ID) reaction chamber 14 and a feed tube 20 of mullite with a 19 mm internal diameter (ID). The closed end 16 of the reactor 12 extended about 102 cm into the furnace 34. The feed tube 20 extended into the reactor chamber 14, so that the opening 24 of the feed tube 20 was spaced about 7 centimetres from the closed end 16 of the reactor. In this position, the feed tube 20 extended about 94 cm into the chamber 14. The feed tube 20 enclosed a 0.5 mm diameter capillary tube 28 which had a discharge end 30 adjacent the inlet 22 of the feed tube 20, and concentric therewith. The capillary tube 28 extended about 3 cm into the furnace 34 and was spaced about 99 cm from the closed end 16 of the reactor. Metal nuclei of iron were formed from an iron compound, ferrocene, which was dissolved in hexane to form a solution. To make the hexane/ferrocene solution, 2.5 g of ferrocene were dissolved in hexane, producing a total volume of 100 cc; this solution was pumped through the capillary tube 28 and into the feed tube 20. In order to provide a warm surface area from which any droplets of liquid solution would vapourize, a stainless steel web 32 was placed around the capillary tube 28 adjacent its discharge end 30 and disposed between the capillary tube 28 and the feed tube 20. Any droplets of liquid which wetted the web 32 were vapourized. The gaseous stream was injected into the feed tube 20 at a pressure of about 101.325 kPa (760 Torr). The flow rates of the reactants were measured at 22°C and 101.325 kPa (760 Torr) pressure. The gaseous flow rates for the various constituents were approximately as follows: hydrogen at 5000 cc/min, air at 1365 cc/min, methane at 1500 cc/min, hydrogen sulphide at 110 cc/min. The hexane/ferrocene liquid was injected at a rate of about 4 cc/min. The process provided microscopic carbon fibres having a density of about 0.005 grams per cubic centimetre. A yield of approximately 12 percent was achieved. Example 4 The apparatus 10 of the invention in accordance with Example 3 was used in a process as described therein except that the initial flow rates were approximately as follows: hexane/ferrocene at 4.5 cc/min; air at 2000 cc/min; methane at 2500 cc/min; hydrogen sulphide at 9 cc/min. The total flow rate was varied while maintaining the proportionality of the reactants described immediately above. Microscopic carbon fibres were formed having a diameter in the range of 50 to 100 nanometres and a length of several micrometres with a density in the range of 0.002 to 0.014 grams per cubic centimetre. At a total flow rate of 10,000 cc/min, a yield of 36 percent was obtained, and the carbon fibres had a density of about 0.009 grams per cubic centimeter, at a total flow rate of 1,800 cc/min, a yield of 19 percent was obtained, and the carbon fibres had a density of about 0.002 grams per cubic centimetre. Example 5 The method of Example 4 was used except that the internal diameter of the feed tube 20 was 8 mm and it was constructed of a (quartz) silica material. As in Example 4, the total flow rate was varied whilst maintaining the proportionality of the reactants as described in Example 4. Microscopic carbon fibres were formed having a diameter in the range of 50 to 100 nanometres and a length of several micrometres with a density in the range of 0.002 to slightly over 0.02 grams per cubic centimetre. At a total flow rate of 5,000 cc/min, a yield of 36 percent was obtained, and the carbon fibres had a density of 0.004 grams per cubic centimetre. Example 6 The method of Example 4 was used except that the internal diameter of the feed tube 20 was 22 mm and it was constructed of 304 stainless steel. As in Example 4, the total flow rate was varied whilst maintaining the proportionality of the reactants described in Example 4. Microscopic carbon fibres were formed having a diameter in the range of 50 to 100 nanometres and a length of several micrometres with a density in the range of 0.002 to slightly over 0.02 grams per cubic centimetre. At a total flow rate of 7,000 cc/min, a yield of 28 percent was obtained, and the carbon fibres had a density of 0.002 grams per cubic centimetre. Example 7 The apparatus 10 of the invention was used in a process to form carbon fibres in accordance with Example 1 wherein the reactants included a gaseous hydrocarbon compound such as methane and ethane and a metal compound which formed metal nuclei. The gaseous hydrocarbon compound stream was supplied from controller 39 through line 40. Helium gas at a flow rate of 72 cc/min was bubbled through reservoir 42, which was a standard laboratory bubbler containing 250 cc of liquid iron carbonyl. Because of its vapour pressure (3199.7 Pa (24 Torr) at 20°C), appreciable iron carbonyl vapour was entrained and mixed with the flowing helium. The helium/carbonyl gas stream flowed into lines 44 and 46. The helium/iron carbonyl gas mixture in line 46 and gaseous reactants supplied through line 40 were mixed at tee-junction 48 before entering reactant feed tube 20. The gaseous reactants in line 40 were methane at 2500 cc/min, air at 2500 cc/min, and hydrogen sulphide at 5 cc/min. Helium gas was used to avoid problems with oxidation and degradation of the iron carbonyl; however, other gases such as air and methane could be substituted under suitable conditions. Microscopic carbon fibres were formed having a diameter in the range of 50 to 100 nanometres and a length of several micrometres with a density of about 0.003 g/cc. A yield of about 26 percent was obtained and there was relatively little soot admixture. In this example, iron pentacarbonyl was used as a source of iron to form nuclei. The gaseous iron pentacarbonyl simplified the process of forming carbon fibres because the step of volatilizing liquid feedstock reactants, such as ferrocene, is not required. Comparative Example For comparative purposes, the apparatus 10 of Example 3 was used in a process as described therein except that the feed tube 20 was only extended about 13 cm into the reactor chamber 14. In this position, the feed tube 20 was spaced about 88 cm from the reactor closed end 16. A colloidal, black soot-like product was obtained with a density of 0.05 grams per cubic centimetre. This product was 10 times as dense as that of Example 3 and only contained a few fibres. As shown in Table I, three different feed tubes 20 were used: a 22 mm ID 304 stainless steel tube, a 19 mm ID mullite tube, and an 8 mm ID (quartz) silica tube. The 304 stainless steel tube had sufficient heat capacity that evaporation of the liquid solution was accomplished effectively. However, it was not a completely satisfactory material as it corroded too rapidly. The 8 mm silica tube has a low conductivity and a specific heat; therefore, any hexane flow above saturation rapidly cooled this tube and resulted in the formation of soot. Mullite was preferred because it is more resistant to corrosion than stainless steel and it has better heat transfer properties than quartz. Mullite is also resistant to thermal shock which occurs when the liquid is injected. Although not wishing to be limited to any particular theory, the inventors believe that the following observation is the theoretical explanation of carbon fibre formation in apparatus 10. Microscopic carbon fibres are formed from metal nuclei, preferably of iron having a diameter of about 5 nm. This is believed to be the optimum nucleus size for growing carbon fibres. Ferrocene decomposes and nucleates into Fe particles in the feed tube 20. In the carbon-rich environment within the reactor 12, it is likely that carbon filaments (fibre precursors) lengthen for a relatively short time before each nucleus is surrounded by excess carbon products. Filament lengthening is not appreciable in the feed tube 20 because the tube may become blocked. The lengthening period is always, of course, less than the period of time each nucleus spends in the reactor 12. Once the filaments reach the larger diameter chamber 14, their linear flow (velocity) slows down and the filaments may lengthen and thicken for a relatively longer period of time, forming fibres in chamber 14 before they reach the outlet 18. As the filaments thicken, they move downward in the larger diameter chamber 14, so their motion will be assisted by gravity. As the carbon fibres and accompanying gases leave the furnace 34, they cool as they flow downward, avoiding convective currents which would occur if the carbon fibres exited at the top of the reactor 12. Therefore, the apparatus 10 is constructed and arranged so that convection is minimized by suspending hot gases over cold gases. This is an important advantage which is only achieved by the apparatus 10 of this invention. Accordingly, it is believed that a key factor in improving carbon fibre growth is the construction and arrangement of the feed tube 20 and reactor chamber 14, which provides conditions conducive for fibre growth, particularly when liquid reactants, such as the hexane/ferrocene solution, are used. Hexane is suitable for this process because it has a relatively low heat of vapourization, which means cooling adjacent the inlet 22 of feed tube 20 is minimized. The optimum flow of the hexane/ferrocene liquid solution is believed to be about 4.5 cubic centimetres per minute in the apparatus 10 as described in Example 4, with a ferrocene concentration in the solution greater than 0.6 grams for every 100 cubic centimetres of hexane. At that flow rate, the hexane was effectively dispersed in the mixture of gases which includes 2000 cubic centimetres per minute of air, 2500 cubic centimetres per minute of methane, and 9 cubic centimetres per minute of hydrogen sulphide. Under these conditions, the hexane had a partial pressure of 14.932 kPa (112 T), which is very close to its vapour pressure at 21°C of 14.799 kPa (111 T). Under these conditions, the heat transferred from the feed tube 20 to the flowing gases was small enough so that the gas temperature within the feed tube 20 remained near room temperature. Also, under these conditions, the mixture of reactants was lean enough in carbon so that formation of soot was avoided. Below 1000°C, dense sooty material was produced. Over 1050°C, the desired low-density microscopic fibre materials were produced. The apparatus 10 could not be operated at a hot enough temperature to define a maximum temperature of carbon fibre growth. Generally, as the temperature increased, there was an increase in the efficiency of conversion of the reactants to carbon products; that is, the yield of carbon fibres increased with increasing temperature. Other conditions which contributed to the formation of acceptable carbon fibres included providing a quantity of sulphur, by weight which is approximately equal to the quantity of iron nuclei, and providing oxygen in the feed stream. Oxygen is known to preferentially attack the aromatic organic compounds formed during pyrolysis, which are the precursors of soot. Thus, the oxygen acts to diminish soot formation. It should be noted that insufficient oxygen for complete combustion was provided. However, even partial oxidation of the incoming hydrocarbons diminished the soot produced compared to what would have existed in the absence of oxygen. Acceptable carbon fibre products are not formed when a stoichiometric amount of oxygen is introduced, and preferably no more than a fraction of the stoichiometric amount of oxygen should be used. The quality of carbon fibre product may be roughly assessed by measuring its apparent density. This parameter is very useful for monitoring and optimizing carbon fibre production in apparatus of the invention. For example, the apparent density of soot produced with no nuclei material present was well over 0.04 grams per cubic centimetre. Fibrous material, however, is much less dense, that is, near 0.002 grams per cubic centimetre. Thus, density measurements are useful for discriminating between microscopic carbon fibres and soot. Low densities always correlated with high quality carbon fibres. This was verified by beginning the process as described in Example 4 above with no ferrocene, and progressively adding ferrocene until the density dropped to about 0.002 grams per cubic centimetre. Adding more than 0.5 grams of ferrocene per 100 cubic centimeters of hexane did not further improve the density. Densities of carbon fibres produced under the optimized conditions presented in Example 4 are included in Table I. The apparatus 10 of the invention provided for continuous growth of carbon fibres whilst minimizing thermophoretic and convective losses. That is, the apparatus 10 of the invention minimizes the loss of heat and particularly the loss of the ability of the reactants to undergo reaction due to the loss of heat. The apparatus 10 maximized production of microscopic carbon fibres, and minimized the production of soot and carbon fibres of undesirable dimensions.

Claims: An apparatus (10) for forming carbon fibres by a continuous gas-phase process comprising reaction of a gaseous carbonaceous compound in the presence of nuclei entrained in a gas stream, said apparatus comprising: a reactor (12) defining a reaction chamber (14) having a vertical axis, a closed upper end (16) and a lower outlet (18), a reactant feed tube (20) having an opening (24) spaced from the closed end (16) of the reactor (12), said feed tube (20) extending generally axially upwards within the reactor (12), whereupon said feed tube (20) and said reactor (12) co-operate with one another to define a gas flow path (26) that extends generally axially upwards through the feed tube (20) into the chamber (14) and thereafter generally axially downwards through the chamber (14), means (34) for heating the reactor (12) to establish a hot zone (36) within the reaction chamber (14) adjacent the closed end (16) thereof, and flow control means (39,40,41,42,43,46,48,50; 28,33,40,46,48,49) for continuously supplying a gaseous reactant stream into the feed tube (20), characterised in that the means (34) for heating the reactor (12) establishes said hot zone (36) within the reaction chamber (14) adjacent the closed end (16) thereof at a temperature sufficient to sustain carbon fibre growth and a relatively cool zone adjacent the reactor outlet (18), thereby establishing a thermal gradient within the reactor (12) that minimizes convective currents therewithin; said flow control means (39,40,41,42,43,44,46,48,50; 28,33,39,40,46,48,49) continuously supplies a gaseous reactant stream containing sources of nuclei into the feed tube (20) at a rate sufficient to entrain the nuclei within the gas stream for dispersion into the hot zone (36) of the reactor (12); said reactor chamber (14) has an average cross-sectional area greater than the cross-sectional area of said feed tube (20) so as to accommodate said gas stream (26) at a reduced downward flow rate suitable for sustained carbon fibre growth, and there are means (51,54) for collecting carbon fibres from exhaust gas leaving the reactor outlet (18). An apparatus (10) according to claim 1, in which said reactor chamber (14) is tubular in shape and is arranged substantially concentric with said opening (24) in the feed tube (20), and said reactor chamber (14) has a diameter which is at least three times greater than the diameter of said opening (24). An apparatus (10) according to claim 1, in which said feed tube (20) extends upwards for at least three-fourths of the axial length of said reactor (12). An apparatus (10) according to any one of claims 1 to 3, for forming carbon fibres by a continuous gas-phase process comprising thermal decomposition of a gaseous hydrocarbon compound in the presence of metal nuclei entrained in a gas stream, in which said flow control means (39,40,41,42,43,44,46,48,50; 28,33,39,40,46,48,49) for continuously supplying a gaseous reactant stream supplies a gaseous reactant stream comprising a metal compound thermally decomposable to produce submicron metal nuclei into the feed tube (20) at a rate sufficient to entrain the nascent metal nuclei within the gas stream for dispersion into the hot zone (36) of the reactor (12). An apparatus (10) according to claim 4, in which said reactor chamber (14) is tubular and is generally concentric with said feed tube (20) along the axial extent thereof and has a diameter which is at least three times greater than the diameter of said feed tube (20). An apparatus (10) according to claim 4, in which said apparatus includes a capillary tube (28) which has a discharge end (30) within the feed tube (20) and is concentric therewith. An apparatus (10) according to claim 4, in which said reactor (12) and said feed tube (20) are formed from mullite. An apparatus (10) according to claim 1, for forming carbon fibres by a continuous gas-phase process comprising pyrolytic decomposition of a gaseous hydrocarbon compound in the presence of iron nuclei entrained in a gas stream, in which said reactor is an axially-elongated tubular thermal reactor (12) defining a hydrocarbon pyrolysis chamber (14) having a closed upper end (16) and a lower outlet (18); said feed tube is an axially-elongated reactant feed tube (20) having an inlet (22) and an opening (24) adjacent the closed end (16) of the reactor (12), said feed tube (20) extending axially upwards within the reactor (12) for at least three-fourths of the axial length of the reactor (12) and being radially spaced therefrom; said means (34) for heating the reactor (12) to establish said hot zone (36) within the chamber (14) adjacent the closed end (16) thereof at a temperature sufficient to sustain carbon fibre growth and a relatively cool zone adjacent the reactor outlet (18) thereby provides a first temperature adjacent said inlet (22) of the feed tube (20), a second temperature greater than said first temperature within said hot zone (36) and a third temperature less than said second temperature adjacent said outlet (18) to minimize said convective currents within the reactor chamber (14) and the feed tube (20); said flow control means (39,40,41,42,43,44,46,48,50; 28,33,39,40,46,48,49) continuously supplies a gaseous reactant stream comprising an iron compound thermally decomposable to produce submicron iron nuclei into the feed tube (20) at a rate sufficient to entrain the nascent iron nuclei within the gas stream for dispersion into the hot zone (36) of the reactor (12); and said reactor chamber (14) is generally concentric with said feed tube (20) along the axial extent thereof and has a diameter that is at least three times greater than the diameter of said feed tube (20) to accommodate said gas stream at said reduced downward flow rate suitable for sustained fibre growth.

Inventors: GORKIEWICZ DANIEL WILLIAM, HAMMOND JR DEAN CRAIN, TIBBETTS GARY GEORGE, GORKIEWICZ, DANIEL WILLIAM, HAMMOND JR., DEAN CRAIN, TIBBETTS, GARY GEORGE
Applicants: GEN MOTORS CORP, GENERAL MOTORS CORPORATION
Application year: 1991
Main classification: D01F9
IPC codes: D01F9, B01J19, B01J12