The present reactor and method are intended for high temperature reactions that require rapid cooling to freeze the reaction products to prevent back reactions or decompositions to undesirable products. They use adiabatic and isentropic expansion of gases in a converging-diverging nozzle for rapid quenching. This expansion can result in cooling rates exceeding 10.sup.10 K/s, thus preserving reaction products that are in equilibrium only at high temperatures.
The concepts of this reactor were originally developed in a study of hydrogen reduction of titanium tetrachloride. When the concept was found to provide the high quench rates required to produce titanium, the concept was then applied to other processes requiring rapid quenching, including conversion of methane to acetylene.
Titanium's properties of high corrosion resistance and strength, combined with its relatively low density, result in titanium alloys being ideally suited to many high technology applications, particularly in aerospace systems. Applications of titanium in chemical and power plants are also attractive.
Unfortunately, the widespread use of titanium has been severely limited by its high cost. The magnitude of this cost is a direct consequence of the batch nature of the conventional Kroll and Hunter processes for metal production, as well as the high energy consumption rates required by their usage.
The large scale production processes used in the titanium industry have been relatively unchanged for many years. They involve the following essential steps: (1) Chlorination of impure oxide ore, (2) purification of TiCl.sub.4 (3) reduction by sodium or magnesium to produce titanium sponge, (4) removal of sponge, and (5) leaching, distillation and vacuum remelting to remove Cl, Na, and Mg impurities. The combined effects of the inherent costs of such processes, the difficulty associated with forging and machining titanium and, in recent years, a shortfall in sponge availability, have contributed to relatively low titanium utilization.
One of the most promising techniques currently undergoing development to circumvent the high cost of titanium alloy parts is powder metallurgy for near net shape fabrication. For instance, it has been estimated that for every kilogram of titanium presently utilized in an aircraft, 8 kilograms of scrap are created. Powder metallurgy can substantially improve this ratio. Although this technology essentially involves the simple steps of powder production followed by compaction into a solid article, considerable development is currently underway to optimize the process such that the final product possesses at least equal properties and lower cost than wrought or cast material.
One potential powder metallurgy route to titanium alloy parts involves direct blending of elemental metal powders before compaction. Presently, titanium sponge fines from the Kroll process are used, but a major drawback is their high residual impurity content (principally chlorides), which results in porosity in the final material. The other powder metallurgy alternative involves direct use of titanium alloy powder subjected to hot isostatic pressing.
Several programs are currently involved in the optimization of such titanium alloy powders. Results are highly promising, but all involve Kroll titanium as a starting material. Use of such existing powders involves a number of expensive purification and alloying steps.
The present disclosure is the result of research to develop a new plasma process for direct and continuous production of high purity titanium powder and/or ingot. The previously-described steps (1) and (2) of the Kroll or Hunter processes are retained in this process, but steps (3), (4), and (5) are replaced by a single, high temperature process. This new process can directly produce high purity titanium from TiCl.sub.4 and eliminates the need for subsequent purification steps.
Depending upon collection conditions encountered in the present process, the resulting titanium product can be either a powder suitable for the elemental blend approach to powder metallurgy or in an ingot or sponge-substitute. Titanium alloy powders and other materials can also be produced in a single step process by such direct plasma production systems.
The formation of titanium under plasma conditions has received intermittent attention in the literature over the last 30 years. Reports have generally been concerned with the hydrogen reduction of titanium tetrachloride or dioxide with some isolated references to sodium or magnesium reduction.
The use of hydrogen for reducing titanium tetrachloride has been studied in an arc furnace. Only partial reduction took place at 2100 K. The same reaction system has been more extensively studied in a plasma flame and patented for the production of titanium subchloride (German Patent 1,142,159, Jan. 10, 1963) and titanium metal (Japanese Patents 6854, May 23, 1963; 7408, Oct. 15, 1955; U.S. Pat. No. 3,123,464, Mar. 3, 1964).
Although early thermodynamic calculations indicated that the reduction of titanium tetrachloride to metallic titanium by hydrogen could start at 2500 K, the system is not a simple one. Calculations show that the formation of titanium subchloride would be thermodynamically more favorable in that temperature region.
U.S. Pat. No. 3,123,464, Mar. 3, 1964, claims that reduction of titanium tetrachloride to liquid titanium can be successfully carried out by heating the reactants (TiCl.sub.4 and H.sub.2) at least to, and preferably in excess of, the boiling point of titanium (3535 K). At such a high temperature, it was claimed that while titanium tetrachloride vapor is effectively reduced by atomic hydrogen, the tendency of H.sub.2 to dissolve in or react with Ti is insignificant, the HCl formed is only about 10% dissociated, and the formation of titanium subchlorides could be much less favorable. The titanium vapor product is then either condensed to liquid in a water-cooled steel condenser at about 3000 K, from which it overflows into a mold, or is flash-cooled by hydrogen to powder, which is collected in a bin. Since the liquid titanium was condensed from gas with only gaseous by-products or impurities, its purity, except for hydrogen, was expected to be high.
Japanese Patent 7408, Oct. 15, 1955, described reaction conditions as follows: a mixture of TiCl.sub.4 gas and H.sub.2 (50% in excess) is led through a 5 mm inside diameter nozzle of a tungsten electrode at a rate of 4.times.10.sup.-3 m.sup.3 /min and an electric discharge (3720 V and 533 mA) made to another electrode at a distance of 15 mm. The resulting powdery crystals are heated in vacuo to produce 99.4% pure titanium.
In neither of the above patents is the energy consumption clearly mentioned. Attempts to develop the hydrogen reduction process on an industrial scale were made using a skull-melting furnace, but the effort was discontinued. More recently, a claim was made that a small quantity of titanium had been produced in a hydrogen plasma, but this was later retracted when the product was truly identified as titanium carbide.
In summary, the history of attempts to treat TiCl.sub.4 in hydrogen plasmas appears to indicate that only partial reduction, i.e., to a mixture of titanium and its subchlorides, is possible unless very high temperatures (&gt;4000 K) are reached. Prior researchers have concluded that extremely rapid, preferential condensation of vapor phase titanium would be required in order to overcome the unfavorable thermodynamics of the system.
A second exemplary application of the present equipment and method pertains to production of acetylene from methane.
Natural gas (where methane is the main hydrocarbon) is a low value and underutilized energy resource in the U.S. Huge reserves of natural gas are known to exist in remote areas of the continental U.S., but this energy resource cannot be transported economically and safely from those regions. Conversion of natural gas to higher value hydrocarbons has been researched for decades with limited success in today's economy.
Recently, there have been efforts to evaluate technologies for the conversion of natural gas (which is being flared) to acetylene as a feed stock for commodity chemicals. The ready availability of large natural gas reserves associated with oil fields and cheap labor might make the natural gas to acetylene route for producing commodity chemicals particularly attractive in this part of the world.
Acetylene can be used as a feed stock for plastic manufacture or for conversion by demonstrated catalyzed reactions to liquid hydrocarbon fuels. The versatility of C.sub.2 H.sub.2 as a starting raw material is well known and recognized. Current feed stocks for plastics are derived from petrochemical based raw materials. Supplies from domestic and foreign oil reserves to produce these petrochemical based raw materials are declining, which puts pressure on the search for alternatives to the petrochemical based feed stock. Therefore, the interest in acetylene based feed stock has currently been rejuvenated.
Thermal conversion of methane to liquid hydrocarbons involves indirect or direct processes. The conventional methanol-to-gasoline (MTG) and the Fischer-Tropsch (FT) processes are two prime examples of such indirect conversion processes which involve reforming methane to synthesis gas before converting to the final products. These costly endothermic processes are operated at high temperatures and high pressures.
The search for direct catalytic conversion of methane to light olefins (e.g. C.sub.2 H.sub.4) and then to liquid hydrocarbons has become a recent focal point of natural gas conversion technology. Oxidative coupling, oxyhydrochlorination, and partial oxidation are examples of direct conversion methods. These technologies require operation under elevated pressures, moderate temperatures, and the use of catalysts. Development of special catalysts for direct natural gas conversion process is the biggest challenge for the advancement of these technologies. The conversion yields of such processes are low, implementing them is costly in comparison to indirect processes, and the technologies have not been proven.
Light olefins can be formed by very high temperature (&gt;1800.degree. C.) abstraction of hydrogen from methane, followed by coupling of hydrocarbon radicals. High temperature conversion of methane to acetylene by the reaction 2CH.sub.4 .fwdarw.C.sub.2 H.sub.2 +3H.sub.2 is an example. Such processes have existed for a long time.
Methane to acetylene conversion processes currently use cold liquid hydrocarbon quenchants to prevent back reactions. Perhaps the best known of these is the Huels process which as been in commercial use in Germany for many years. The electric arc reactor of Huels transfers electrical energy by `direct` contact between the high-temperature arc (15000-20000 K) and the methane feed stock. The product gas is quenched with water and liquefied propane to prevent back reactions. Single pass yields of acetylene are less than 40% for the Huels process. Overall C.sub.2 H.sub.2 yields are increased to 58% by recycling all of the hydrocarbons except acetylene and ethylene.
Although in commercial use, the Huels process is only marginally economical because of the relatively low single pass efficiencies and the need to separate product gases from quench gases. Subsidies by the German Government have helped to keep this process in production.
Westinghouse has employed a hydrogen plasma reactor for the cracking of natural gas to produce acetylene. In the plasma reactor, hydrogen is fed into the arc zone and heated to a plasma state. The exiting stream of hot H.sub.2 plasma at temperatures above 5000 K is mixed rapidly with the natural gas below the arc zone, and the electrical energy is indirectly transferred to the feed stock. The hot product gas is quenched with liquefied propane and water, as in the Huel process, to prevent back reactions. However, as with the Huels process, separation of the product gas from quench gas is needed. Recycling all of the hydrocarbons except acetylene and ethylene has reportedly increased the overall yield to 67%. The H.sub.2 plasma process for natural gas conversion has been extensively tested on a bench scale, but further development and demonstration on a pilot scale is required.
The Scientific and Industrial Research Foundation (SINTEF) of Norway has developed a reactor consisting of concentric, resistance-heated graphite tubes. Reaction cracking of the methane occurs in the narrow annular space between the tubes where the temperature is 1900 to 2100 K. In operation, carbon formation in the annulus led to significant operational problems. Again, liquefied quenchant is used to quench the reaction products and prevent back reactions. As with the previous two acetylene production processes described above, separation of the product gas from quench gas is needed. The overall multiplepass acetylene yield from the resistance-heated reactor is about 80% and the process has been tested to pilot plant levels.
Like the Huels reactor, the present fast quench reactor can use an electric arc plasma process to crack the methane, but it requires no quenchant to prevent back reactions. In this manner it eliminates any need for extensive separation.