Methane from carbides

Methane from Carbides.—Another method of preparation is of interest and importance because of its connection with theories as to the formation of methane and other hydrocarbons in petroleum. With some metals carbon forms compounds which are very stable at high temperatures, and which have been artificially produced in the electric furnace (about 35O0°C.) by Moissan. These metallic carbon compounds, known as carbides, are, most of them, easily decomposed by water at ordinary temperatures, and when so decomposed they yield various members of the hydrocarbon group of compounds. A familiar example of this class of reactions is the one by which acetylene gas is made by the action of water on calcium carbide. The carbide of aluminium decomposes with water and yields methane according to the foUowing reaction ... 

The second principle was the great defensive strength of an established, capital intensive procedure. In the overall process for making acrylonitrile via acetylene, very big plants were needed for making the acetylene either by partial oxidation of methane or from carbide furnaces. Manufacture of HCN from methane involved further expense ... 

Production of acetylene from natural gas and other petroleum hydrocarbons has grown sharply and it is predicted to exceed that from carbide within about 10 years (26). One of two important processes, the Sachsse process (3, 6, 8, 10, 36, 63) involves the formation of acetylene in flames in the partial combustion or oxidation of methane. 

Methane-14 was prepared (Ciranni and Guarino, 1966) from aluminium carbide and pure T2O. The crude CT4, specific activity of 116,500 Curies per mole, was immediately diluted with a large excess of CH4 and subjected to a rigorous purification, including a preparative gas-chromatographic separation over a special capillary column that allows (Bruner and Cartoni, 1965) the complete resolution of the four tritiated methanes, from CH3T to CT4. The final sample, whose isotopic purity is illustrated in Fig. 5, was further diluted with CH4 to a specific activity of 0-2 Curies per mole to carry out the decay experiments. Nefedov et al. (1968) used CT4 prepared by essentially the same procedure in their study of the reactions of methyl ions in hydroxylic compounds. 

The methods for the synthesis of alkynes have been extensively reviewed in the past 20 years two books, which deal particularly with the preparative aspects of alkyne chemistry, have been published. Except for the syntheses of acetylene and propyne, which are prepared in technical processes from carbides, from methane by oxidation, or by electric arc processes, all carbon-carbon triple bonds must be generated by an elimination reaction. Again, as in the synthesis of alkenes, the most important is the de-hydrohalogenation. 

Following earlier work in which the intermediate in the formation of methane from carbon monoxide and hydrogen was found to be carbon, McCarty and Wise carried out a thorough study of the system. Four types of carbon were found to be formed from carbon monoxide on nickel at 550 50K. Chemisorbed carbon atoms reacted readily with hydrogen as did the initial layers of nickel carbide. Further deposits of the carbide, amorphous carbon, and crystalline elemental carbon were much less reactive and the kinetics of the reaction should be described by the established rate laws. Conversion of the more active to the less active forms of carbon occurred above approximately 600 K. 

Calcium carbide (original route). Methane from coal by partial combustion and by arc process (1935-45)... 

Acetylene manufactured from carbide made in the United States and Canada normally contains less than 0.4 percent impurities other than water vapor. Apart from water, the chief impurity is air, in concentrations of approximately 0.2 percent and 0.4 percent. The remainder is mostly phosphine, ammonia, hydrogen sulfide, and in some instances, small amounts of carbon dioxide, hydrogen, methane, carbon monoxide, organic sulfur compounds, silicon hydrides, and arsine. Purified acetylene is substantially free from phosphine, ammonia, hydrogen sulfide, organic sulfur compounds, and arsine. The other impurities are nearly the same as in the original gas. 

In order to produce methanol the catalyst should only dissociate the hydrogen but leave the carbon monoxide intact. Metals such as copper (in practice promoted with ZnO) and palladium as well as several alloys based on noble group VIII metals fulfill these requirements. Iron, cobalt, nickel, and ruthenium, on the other hand, are active for the production of hydrocarbons, because in contrast to copper, these metals easily dissociate CO. Nickel is a selective catalyst for methane formation. Carbidic carbon formed on the surface of the catalyst is hydrogenated to methane. The oxygen atoms from dissociated CO react with CO to CO2 or with H-atoms to water. The conversion of CO and H2 to higher hydrocarbons (on Fe, Co, and Ru) is called the Fischer-Tropsch reaction. The Fischer-Tropsch process provides a way to produce liquid fuels from coal or natural gas. 

There is also the promise of finding large amounts of deep methane formed not from biomass but by some abiological processes from carbonates or even carbides formed from carbon-containing asteroids that hit the earth over the ages under the harsh prebiological conditions of our planet. 

Total carbon in beryUium is determined by combustion of the sample, along with an accelerator mixture of tin, iron, and copper, in a stream of oxygen (15,16). The evolved carbon dioxide is usuaUy measured by infrared absorption spectrometry. BeryUium carbide can be determined without interference from graphitic carbon by dissolution of the sample in a strong base. BeryUium carbide is converted to methane, which can be determined directly by gas chromatography. Alternatively, the evolved methane can be oxidized to carbon dioxide, which is determined gravimetricaUy (16). 

The methanation reaction is carried out over a catalyst at operating conditions of 503—723 K, 0.1—10 MPa (1—100 atm), and space velocities of 500—25,000 h . Although many catalysts are suitable for effecting the conversion of synthesis gas to methane, nickel-based catalysts are are used almost exclusively for industrial appHcations. Methanation is extremely exothermic (AT/ qq = —214.6 kJ or —51.3 kcal), and heat must be removed efficiently to minimise loss of catalyst activity from metal sintering or reactor plugging by nickel carbide formation. 

Four pilot plant experiments were conducted at 300 psig and up to 475°C maximum temperature in a 3.07-in. i.d. adiabatic hot gas recycle methanation reactor. Two catalysts were used parallel plates coated with Raney nickel and precipitated nickel pellets. Pressure drop across the parallel plates was about 1/15 that across the bed of pellets. Fresh feed gas containing 75% H2 and 24% CO was fed at up to 3000/hr space velocity. CO concentrations in the product gas ranged from less than 0.1% to 4%. Best performance was achieved with the Raney-nickel-coated plates which yielded 32 mscf CHh/lb Raney nickel during 2307 hrs of operation. Carbon and iron deposition and nickel carbide formation were suspected causes of catalyst deactivation. 

The saline carbides are formed most commonly from the metals of Groups 1 and 2, aluminum, and a few other metals. The s-block metals form saline carbides when their oxides are heated with carbon. The anions present in saline carbides are either C>2 or C4. All the C4 carbides, which are called methides, produce methane and the corresponding hydroxide in water ... 

Other compounds have been deposited by fluidized-bed CVD including zirconium carbide (from ZrCl4 and a hydrocarbon), hafnium carbide (from HfC and methane or propylene), and titanium carbide (from TiCl3 and propylene). 

The deposition of pyrolytic graphite in a fluidized bed is used in the production of biomedical components such as heart valves, ] and in the coating of uranium- and thorium-carbides nuclear-fuel particles for high temperature gas-cooled reactors, for the purpose of containing the products of nuclear fission.fl" The carbon is obtained from the decomposition of propane (CgHg) or propylene (CgHg) at 1350°C, or of methane (CH4) at 1800°C. Its structure is usually isotropic (see Ch. 4). 

Zirconium carbide has also been deposited from the tetrachloride with methane or cyclopropane as the carbon source,... 

Vanadium carbide, VC, deposited from the chloride and methane. 

Example describes the synthesis of acetylene (C2 H2) from calcium carbide (CaC2). Modem industrial production of acetylene is based on a reaction of methane (CH4) under carefully controlled conditions. At temperatures greater than 1600 K, two methane molecules rearrange to give three molecules of hydrogen and... 

Van Ho and Harriott (54) have done similar transient experiments on H2/CO over 10% Ni/Si02, and a result is shown in Fig. 23. Even though the bulk of the nickel is not carbided, the methane production rate rises from zero, as shown. 

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