Fast quench reactor and method

Details Fast quench reactor and method Fast quench reactor and method

2000-05-11

2002-09-24

Andrews, Melvyn (Department: 1742)

Specialized metallurgical processes, compositions for use therei

Processes

Electrothermic processes

C075S010210, C075S010280, C075S346000, C266S182000, C373S018000, C420S590000, C422S207000, C423S289000, C423S613000, C585S538000

Reissue Patent

active

RE037853

ABSTRACT:

TECHNICAL FIELD
This disclosure pertains to equipment for thermal conversion of reactants to desired end products, which might be either a gas or ultrafine solid particles. It also relates specifically to methods for effectively producing such end products.
BACKGROUND OF THE INVENTION
The present rector 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
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
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
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 of 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
4
and H
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
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
4
gas and H
2
(50% in excess) is led through a 5 mm inside diameter nozzle of a tungsten electrode at a rate of 4×10
−3
m
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
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 (>4000 K) are reached. Prior researcher 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 co

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