APPARATUS

Abstract: An apparatus is described for effecting high temperature reactions between reactants comprising a reactor means (1), an open ended insert means (2) disposed within the reactor means and spaced from the walls thereof, gas inlet means (3) at one end of the reactor means for discharging into one end of the inlet means a jet of a gaseous mixture comprising at least one reactant and a plasma forming gas, outlet means (4) for the reaction product or products from the reactor means, and plasma discharge means (5, 6) for heating the gaseous medium by plasma discharge to an elevated temperature, the insert means being so disposed in relation to the gas inlet means that an extended path for gas is formed between the gas inlet means and the gas outlet means. This apparatus can be used, for example, to produce silicon carbide by reaction of silica or silicon monoxide with particulate carbons, produced for example by cracking a hydrocarbon, at a temperature in the range of from about 2000 DEG K up to about 2350 DEG K.

Description: APPARATUS This invention relates to an apparatus for effecting high temperature reactions between reactants, such as between a gaseous reactant and a solid reactant. In particular it relates to an apparatus for such purposes which uses a plasma gas discharge to produce the requisite ' high temperatures. The use of thermal plasma technology for heating gas and particulate materials to temperatures which may be in excess of 3000°K has been proposed on several occasions, Publications on this topic include, for example, papers entitled "The Plasma Bed : Performance and Capabilities" by . . Goldberger, Chem. Eng. Progr. Symp. Ser. No. 62, 62, 42-46 (1966) and "A multiple arc discharge reactor for materials processing" by J.E. Harry and R. Knight, Symposium Proceedings Seventh International Symposium on Plasma Chemistry, Eindhoven, July 1985, Paper No. B-8-5, pages 1192 to 1195. The present invention seeks to provide an improved form of apparatus for effecting high temperature reactions utilising a plasma arc to generate the necessary high temperatures. According to the present invention there is provided an apparatus for effecting high temperature reactions between reactants comprising a reactor means, an open ended insert means disposed within the reactor means and spaced from the walls thereof, gas inlet means at one end of the reactor means for discharging into one end of the inlet means a jet of a gaseous mixture comprising at least one reactant and a plasma forming gas, outlet means for the reaction product or products from the reactor means, and plasma discharge means for heating the gaseous medium by plasma discharge to an elevated temperature, the insert means being so disposed in relation to the gas inlet means that an extended path for gas is formed between the gas inlet means and the gas outlet means. In one form of such apparatus the insert means is so disposed that recycle of gas from the other end of the insert means to the said one end of the inlet means is induced in the space between the insert means and the walls of the reactor by the incoming jet o.f said gaseous medium. The reactants may be introduced into the reactor in gaseous, liquid or solid form, or in any combination of two or more thereof. For example, the reactants may include a gas and a solid. Hence the gaseous mixture introduced into the reactor may comprise a mixture containing, besides a plasma-forming gas, at least one gaseous, liquid or solid reactant or any combination of two or more thereof, such as a mixture of two solids dispersed in the plasma forming gas. Upon heating by the plasma arc at least one of the reactants may be wholly or partially vaporised. The apparatus of the invention is of particular applicability in the manufacture of ceramic powders. Such powders are finding increasing usage in the manufacture of engineering components as a replacement for metal. For example, it has been reported that an internal combustion engine has been produced in which such items as the cylinder block and pistons have been produced from ceramic materials, thereby obviating the need for lubrication. To produce an intricately shaped component it is necessary, according to conventional techniques, to compact a ceramic powder of defined morphology and purity. An appreciable proportion of components produced in this way have to be rejected because of the difficulties inherent in their manufacture. In some cases failure of a component to attain the desired quality control standard can be ascribed to use of powders which do not meet the required morphology specification. A typical specification for a silicon carbide powder for this purpose requires that it should be in the form of β-silicon carbide, that it should have a low level, e.g. less than about 2% by weight, of by-product impurities, such as silicon dioxide, carbon and silicon, and that it should have a small range of size distribution. Usually it is specified that 90% or more of the particles should lie in the range 0.1 to lOum, whilst it is preferred that over 95% of particles should lie in the range 0.1 to 5um. Particles of regular shape, e.g. spherical particles, are preferred. Silicon carbide can be produced in lump form by reactions of quartz and coke in an electric furnace. The resulting lumps of silicon carbide can then be milled and the appropriate fraction of the required particle size can be selected for use as the powder for production of ceramic components. However, the resulting powder is relatively impure and its particles are of a very irregular shape. Moreover, as the milling operation gives rise to a wide particle size range, an appreciable proportion of the powder has to be rejected as being of unsuitable size for compaction. In addition milling is a process requiring high cost equipment and consumes much energy. An alternative method of producing silicon carbide powder involves production of metallurgical grade silicon as a first step. This is then reacted, in a second step, with chlorine to produce silicon tetrachloride, which is then purified in a third step. Next the purified silicon tetrachloride is reacted with a gaseous source of carbon, e.g. methane, to produce silicon carbide. In a fifth and final step the chlorine released is recovered and recycled for production of further silicon tetrachloride. Because of the difficulties inherent in production of silicon carbide powder of suitable particle size and morphology, it is a relatively expensive raw material. This factor, plus the relatively high rejection rate of compacted components, currently makes the cost of producing ceramic engineering components very high. The apparatus of the invention can be used for the production of ceramic powders and enables powders of defined morphology to be obtained by a continuous process in one stage or in at most a few stages, e.g. 2 or 3 stages, with acceptable energy requirements and with low production of by product waste. It can be used, for example, for the production of silicon carbide powders suitable for compaction, as well as for the production of silica or alumina powders of defined morphology. The key to producing silicon carbide of the required size and morphology is to react silica or silicon monoxide with particulate carbon produced in a specific particle size range under controlled conditions of temperature and residence time. It is therefore very important to provide a reactor design to produce both particulate carbon and silicon carbide. The simplest form of process for producing silicon carbide uses particulate carbon with a controlled particle size produced in a separate reactor. Finely divided silica and the particulate carbon are intimately mixed and reacted at high temperature under controlled conditions of temperature and residence time. An example of a process for production of silicon carbide involves reaction under the high temperature conditions generated by the plasma arc of gaseous silicon monoxide with a dispersion in a carrier gas of finely divided carbon particles. The gaseous silicon monoxide can be produced by reacting silicon dioxide of suitable purity with a carbonaceous reducing agent to produce a gaseous stream comprising silicon monoxide in admixture with carbon monoxide. Preferably an approximately stoichiometric mixture of silicon dioxide and the carbonaceous reducing agent is used. To produce silicon carbide of acceptable purity for ceramic powder compaction the silicon dioxide used as feed material should contain at least 99% Siθ2 and minimum amounts of Fe and/or Al. The carbonaceous reducing agent may be a particulate carbon source well mixed with the silicon dioxide or it may comprise a gaseous or vaporous hydrocarbon such as an alkane of the general formula ^*n H 2n+2' an lkene of the general formula C n H2 n , or an alkyne of the general formula C n H n , where n is an integer from 1 to about 20, or a mixture of two or more thereof. Examples of suitable hydrocarbons include methane, ethane, propane, n- and iso-butane, pentane, hexane, octane, nonane, decane, ethylene, propylene, butene-1, butene-2, acetylene, and the like, as well as mixtures thereof. The use of cyclic hydrocarbons, such as benzene, cyclohexane, cyclopentane, cyclohexene, naphthalene, tetralin and decalin, as well as mixtures thereof is not excluded. In one arrangement an apparatus in accordance with the invention is used to heat a mixture of particulate carbon or a hydrocarbon and silica to a temperature of at least about 1700°K to produce a gaseous stream containing gaseous silicon monoxide and carbon monoxide which is then reacted in a second reactor with a dispersion of finely divided carbon particles in the gaseous medium, preferably at a temperature below about 2350°K. When a hydrocarbon is used the dispersion of finely divided carbon particles in the gaseous medium can be formed in situ by thermal cracking of a hydrocarbon supplied to the second reactor. Such a second reactor may also use a plasma arc to generate the high temperature needed to crack the hydrocarbon. In a preferred arrangement it comprises an apparatus according to the invention. In a variant of this process a dispersion of finely divided carbon particles in a gaseous medium is formed by thermally cracking a gaseous or vaporous hydrocarbon in a third reactor so as to form a gaseous dispersion of carbon particles in a gaseous medium comprising hydrogen which is introduced into the second reactor. Typically the third reactor is operated at a temperature of at least about 1000°K. Again plasma arc heating can be used; thus the third reactor may also be an apparatus according to the invention. Alternatively the finely divided carbon may be produced by thermal cracking using the heat generated by burning part of the hydrocarbon. When a second reactor is used this is preferably operated at a temperature below that used in the first reactor, for example at a temperature of in the range of from about 1300°K to about 2350°K. If the second reactor is operated at temperature below about 2350°K, and preferably below about 2000°K, then the possibility of producing alpha-silicon carbide is reduced. It is further possible to carry out the reactions in a single reactor so that both silicon monoxide and the dispersion of carbon particles are produced in the same vessel. Again an apparatus according to the invention can be used for this purpose. In this case it is preferred to use an appropriate selected hydrocarbon:silica ratio to maximise conversion to silicon carbide and minimise the impurity content in the silicon carbide powder. Normally this will involve use of a stoichiometric or near stoichiometric hydrocarbon:silica ratio such that there will be a slight excess of carbon supplied to the reactor. If necessary the silicon carbide particles produced in the process of the invention can be sieved or classified to produce a fraction or fractions of suitable morphology for use in production of engineering components. The apparatus of the invention may further be used in the production of silica or alumina powder for compaction. In this case the plasma arc is used to produce the suboxide of either silica or alumina from impure or coarse feeds which is then passed to a second reactor, which may also be an apparatus according to the invention, for reaction with air or oxygen to produce oxide powder with controlled particle size under controlled conditions. The important features of the reactor are the provision of a high intensity source of heat to supply the energy required in a small volume and the predictability and control of both residence time and the temperature of the reactor. The high intensity source of heat is conveniently a plasma arc, whilst the residence time is controlled by control of the length of path which the reactants must travel and the degree of mixing in the reactor. The temperature in the reactor is controlled by the degree of mixing in the reactor and by the temperature and position of the heat source and by the rate of throughput of the reactants. In a preferred form of apparatus according to the invention, the reactor means is preferably circular in section, e.g. cylindrical. The insert means is also preferably of circular section, e.g. cylindrical. Preferably the insert is coaxially positioned within the reactor. The gas inlet means is conveniently coaxially arranged with respect to the reactor means and/or to the insert means. In order that the invention may be clearly understood and readily carried into effect, some preferred embodiments thereof will now be described, by way of example only, with reference to Figures 1 to 6 of the accompanying diagrammatic drawings, each of which is a vertical section through an apparatus constructed in accordance with the invention. Referring to Figure 1 of the drawing, an apparatus for effecting high temperature reactions between reactants, such as methane or particulate carbon generated externally and finely divided silica, comprises a generally cylindrical reactor 1 with an open ended cylindrical insert 2 positioned coaxially therein. Appropriate supports (not shown) are provided to hold insert 2 in spaced relationship to the walls of reactor 1. A gas inlet duct 3 is connected to reactor 1 coaxially with insert 2. Reactor 1 also has a gas outlet duct 4 radially connected thereto. Upstream from reactor 1 gas inlet duct 3 has a pair of electrodes 5 and 6, between which a plasma arc 7 may be struck upon application of a suitable high current dc or ac voltage between electrodes 5 and 6. A plasma-forming gas, such as argon, is fed, as indicated by arrows 8, at a pressure of just above atmospheric pressure through the narrow gap 9 between electrodes 5 and 6 to assist in sustaining the plasma arc 7. A reactant mixture comprising methane or particulate carbon and finely divided silica in approximately stoichiometric proportions, possibly in admixture with a non-oxidising gas, such as hydrogen, carbon monoxide, argon or helium, is supplied along gas inlet duct 3 as indicated by arrow 13. A gas, such as hydrogen, carbon monoxide, argon or helium, is supplied as indicated by arrow 10 and passes through the plasma arc 7. As a result the gas of arrow 10 is heated rapidly to a high temperature, typically at least about 3000°K. The jet of extremely hot gas is projected at high velocity into the reactor 1 as indicated by arrow 11. This hot jet induces rapid recirculation of gases containing the reactants, as indicated by arrows 12, within reactor 1 around the open-ended cylindrical insert 2. As a result reactor 1 operates essentially as an isothermal and adiabatiσ reactor and the reactions to form silicon carbide occur, typically at at least 2000°K up to about 2350°K, according to the following equations : CH 4 > C + 2H 2 (A) 2C + Si0 2 > SiO + CO (B) SiO + 2C > SiC + CO (C) Reaction (A) can be carried out at about 1000°K or above, while reaction (B) begins at about 1675°K. Reaction (C) becomes significant at about 2100°K. The gas introduced into reactor 1, as indicated by arrow 13, also induces circulation and serves to control the reaction conditions. When the feed mixture introduced at 13 is in the ratio and under the required reaction conditions to produce silicon monoxide and CO by reactions (A) and (B) the gaseous mixture can be introduced to a second reactor of similar design, which is also fed with further particulate carbon, so as to form silicon carbide according to equation (C) above. Such a second reactor can be supplied with the gaseous mixture along the path indicated by arrow 10 or along the path indicated by arrow 13. Reaction temperature control within reactor 1 can be achieved by controlling the energy input to the plasma arc 7 and the feed rate of the reactants. Isothermal conditions are maintained by the very rapid circulation of gases 12 around insert 2, induced by the jet 11 of plasma gas. Mean residence time in reactor 1 is governed by the ratio of the volume of reactor 1 to the total input gas flow rate plus product gas production rate. For a specified feed rate the required residence time may therefore be selected by selecting the volume of reactor 1. The distribution of residence times may be predicted by controlling the ratio of recycle gas flow to input gas flow so as to ensure that the percentage of reactants with the residence time below the minimum to achieve the desired product does not exceed the feed impurity specification of the desired product. It will be appreciated by those skilled in the art that reactor 1 and insert 2 should be constructed of materials which will withstand the temperature and chemistry of the reaction system. It may be convenient to construct both from the same ceramic material as the required final product, e.g. silicon carbide. Graphite is another material that can be used as it is relatively cheap and is easy to fabricate and install and as it can, in some cases, aid the process to be carried out in the reactor by helping to create a reducing atmosphere. The size of the silicon carbide particles is determined largely by the size of the carbon particles. Hence by appropriate control or the conditions it is possible to influence the size of the silicon carbide particles by providing hydrocarbon cracking conditions to produce particulate carbon of the appropriate size or providing particulate carbon of the required size from an external source. The product gas stream from reactor 1, or from the final reactor, if more than one such reactor is used, contains silicon carbide particles dispersed in a gaseous medium comprising a mixture of carbon monoxide, hydrogen, and the plasma forming gas. This gas stream can be passed to a quenching zone in which recovery and separation of the silicon carbide particles is effected. Quenching can be achieved by addition of a solid or liquid with a high latent heat of vaporisation which will remain in the vapour phase at the quenching temperature. Water and liquid sodium are examples of suitable quenching media. Alternatively quenching can be achieved by admixture with a large volume of gas, such as methane, hydrogen, cooled process recycle gas, or nitrogen. A third possibility involves rapid separation of the solid from the gas at reaction temperature, for example by use of cold wall cyclones, electrostatic precipitation, diffusion of the gas through a porous medium, or by high temperature diffusion. Another use for the apparatus of Figure 1 is in the production of silica or alumina powders for compaction. To produce silica powders the feed mixture of arrow 10 comprises a mixture of a gas, such as nitrogen, a hydrocarbon and a coarse or impure sand, whilst air or oxygen is injected as indicated by arrows 13. The gaseous silicon monoxide produced by reaction (B) above is oxidised by the air or oxygen to silica, according to the following equation : 2SiO + 0 2 > 2Si0 2 (D) To produce alumina powder for compaction, a coarse or impure alumina is used in place of silica in the feed mixture of arrow 10. Again air or oxygen is supplied as indicated by arrows 13. These reactions are : Al 2 0 3 + C > 2A1 2 0 + 2C0 (E) and Al 2 0 + 0 2 > A1 2 0 3 (F) The apparatus of each of Figures 2 to 4 is generally similar to that of Figure 1, except for the method of producing a plasma arc. Like reference numerals have been used throughout to indicate like features of the various forms of apparatus illustrated. In the apparatus of Figure 2 electrodes 5 and 6 are replaced by cathode 15 and anode 16 between which is struck a long radiating plasma arc 17 that provides the heat energy required by the reactions occurring in the reactor 1. Additional arc stability may be induced by injecting feeds or other gases tangentially through the top of the walls of reactor 1, as indicated by arrows 18 or 19, to produce a vortex effect in the space within insert 2. Figure 3 shows an alternative embodiment in which a hollow electrode 20 and the insert 2 itself form the electrodes between which a plasma arc 21 is struck. In this case an electrical contact with insert 2 will be required; this contact may be part of the support structure for the insert 2. Arc 21 may be operated either as an ac arc or as a dc arc. Normally it will be preferred to operate using dc power, with the insert 2 acting as anode, in order to maximise heat efficiency. A similar arrangement is shown in Figure 4 in which an axial electrode 22 is used in conjunction with insert 2 to provide an ac or dc plasma arc 23. Again d.c operation is preferred, in which case it is preferable to use the insert 2 as an anode. A further embodiment of the reactor concept is shown in Figure 5 which is particularly suitable for reacting particulate carbon and finely divided silica. The apparatus comprises a cylindrical reactor 1 with a cylindrical insert 24 positioned coaxially and connected to the inlet means 3 and the furnace roof 25 to form an extended path between the inlet means 3 and the exit means 4. The reactant mixture comprising an approximately stoichiometric mixture of methane or particulate carbon and finely divided silica, possibly in admixture with a non oxidising gas such as hydrogen, carbon monoxide, argon or helium is supplied along a duct 3 as indicated by arrow 13. A source of heat such as a plasma arc or a hot gas is also supplied to the reactor as indicated by arrow 26 using any of the means illustrated in Figures 1 to 4. The apparatus is operated in the manner illustrated above so that reactions (A), (B) and (C) take place to produce particulate carbon, silicon monoxide and silicon carbide in the manner described above. In this case the extended path for reactants and the control of residence time and temperature is provided by the cylindrical insert means connected to the furnace roof and is assisted by the feed injected tangentially into the inlet means 3 so that it forms a spiral path down the inner wall of the insert means. Reaction heat is provided by the plasma heat source. On reaching the bottom of the insert means the reactants and reaction products are picked up in the gas stream and pass to the reactor exit 4. A modification of this reactor concept is illustrated in Figure 6 in which the reactor consists simply of the cylinder 24.

Claims: CLAIMS 1. Apparatus for effecting high temperature reactions between reactants comprising a reactor means, an open ended insert means disposed within the reactor means and spaced from the walls thereof, gas inlet means at one end of the reactor means for discharging into one end of the insert means a jet of a gaseous mixture comprising at least one reactant and a plasma forming gas, outlet means for the reaction product or products from the reactor means, and plasma discharge means for heating the gaseous medium by plasma discharge to an elevated temperature, the insert means being so disposed in relation to the gas inlet means that an extended path for gas is formed between the gas inlet means and the gas outlet means. 2. Apparatus according to claim 1, in which the insert means is so disposed that recycle of gas from the other end of the insert means to the said one end of the inlet means is induced in the space between the insert means and the walls of the reactor by the incoming jet of said gaseous medium. 3. Apparatus according to claim 1 or claim 2, in which the reactor means is circular in section. 4. Apparatus according to claim 3, in which the reactor means is cylindrical. 5. Apparatus according to any one of claims 1 to 4, in which the insert means is circular in section. 6. Apparatus according to claim 5, in which the insert means is cylindrical. 7. Apparatus according to any one of claims 3 to 5, in which the insert means is coaxially positioned within the reactor. 8. Apparatus according to any one of claims 1 to 7, in which the gas inlet means is coaxially arranged with respect to the reactor means. 9. Apparatus according to any one of claims 1 to 8, in which the gas inlet means is coaxially arranged with respect to the insert means. 10. A process for producing silicon carbide which comprises reacting in an apparatus according to any one of claims 1 to 9 silica or silicon monoxide with particulate carbon at a temperature in the range of from about 2000°K up to about 2350°K. 11. A process according to claim 10, in which an apparatus according to any one of claims 1 to 9 is used to heat a mixture of particulate carbon or a hydrocarbon and silica to a temperature of at least about 1700°K to produce a gaseous stream containing gaseous silicon monoxide and carbon monoxide which is then reacted in a second reactor with a dispersion of finely divided carbon particles in the gaseous medium. 12. A process according to claim 10, in which a dispersion of finely divided carbon particles in a gaseous medium is formed by thermally cracking a gaseous or vaporous hydrocarbon in a third reactor so as to form a gaseous dispersion of carbon particles in a gaseous medium comprising hydrogen which is introduced into the second reactor. 13. A process according to claim 12, in which the temperature in the third reactor is at least about 1000°K. 14. A process according to claim 12 or claim 13, in which the third reactor is an apparatus according to any one of claims 1 to 9. 15. A process according to any one of claims 11 to 14, in which the second reactor is operated at a temperature below that used in the first reactor. 16. A process according to claim 15, in which the second reactor is operated at a temperature in the range of from about 1300°K to about 2350°K. 17. A process according to claim 16, in which the second reactor is operated at a temperature below about 2000°K. 18. A process for producing silicon carbide which comprises reacting in an apparatus according to any one of claims 1 to 9 a mixture of silica and a hydrocarbon at a temperature in the range of from about 2000°K up to about 2350°K. 19. A process according to claim 18, in which a stoichiometric or near stoichiometric hydrocarbon:silica ratio is used. 20. A process for the production of a silica powder for compaction in which a mixture of a gas, a hydrocarbon and a coarse or impure sand is heated in an apparatus according to any one of claims 1 to 9 to a temperature of at least about 1700?K to produce gaseous silicon monoxide which is then oxidised by admixture with air to produce a silica powder. 21. A process for the production of an alumina powder for compaction in which a mixture of a gas, a hydrocarbon and a coarse or impure alumina is heated in an apparatus according to any one of claims 1 to 9 to a temperature of at least about 1700°K to produce A1 2 0 vapour which is then oxidised by admixture with air to produce an alumina powder.

Inventors: BAGHA SHAHIN, NADEN DAVID, BAGHA, SHAHIN, NADEN, DAVID
Applicants: DAVY RES & DEV LTD
Application year: 1990
Main classification: C01B31
IPC codes: B01J8, B01J19, C01B31, C01B33, C01F7