Acetylene, combustion

The early theory of methane oxidation assumed that carbon and water ware tile initial products of reaction or that hydrogen burned preferentially to carbon. However, in 1861 Kersten0 declared that carbon monoxide and hydrogen were the primary products, and that although some free carbon may form at times, the carbon is normally oxidized to carbon monoxide before the hydrogeu is reacted upon. This idea, later revived by Misterli,7 involves the preferential combustion of carbon and is thus directly opposed to the hydroxylation theory. Tins theory might possibly apply to the case of acetylene combustion, since this hydrocarbou is sufficiently unstable as to explode alone under certain conditions, but cannot hold for the more saturated hydrocarbons which do not explode alone. 

The HCjO radical, believed to be formed during acetylene combustion, was originally predicted to have the structure shown in the following figure, which predicts rotational constants of 900.0 GHz, 4.42 GHz, and 4.40 GHz. However, subsequent experiments determined rotational constants of 262 GHz, 4.58 GHz, and 4.49 GHz. Draw a different Lewis-type structure for the molecule that could explain the discrepancy. 

A detailed understanding of combustion must start with simple processes such as hydrogen, methane or acetylene combustion in oxygen or air. Normal liquid hydrocarbons are considerably more complex and wood or coal combustion can hardly be attacked on a molecular level. Below we give some "effective" chemical reactions leading to a transformation of fuel and oxidant into carbon dioxide and water. The processes are strongly exothermic, which is, of course, a common feature for combustion processes (Table 10.1). 

Sources of Thermal Energy The most common sources of thermal energy are flames and plasmas. Flame sources use the combustion of a fuel and an oxidant such as acetylene and air, to achieve temperatures of 2000-3400 K. Plasmas, which are hot, ionized gases, provide temperatures of 6000-10,000 K. 

Thermal energy in flame atomization is provided by the combustion of a fuel-oxidant mixture. Common fuels and oxidants and their normal temperature ranges are listed in Table 10.9. Of these, the air-acetylene and nitrous oxide-acetylene flames are used most frequently. Normally, the fuel and oxidant are mixed in an approximately stoichiometric ratio however, a fuel-rich mixture may be desirable for atoms that are easily oxidized. The most common design for the burner is the slot burner shown in Figure 10.38. This burner provides a long path length for monitoring absorbance and a stable flame. 

Several studies of spherical and cylindrical detonation in acetylene—oxygen and acetylene—air mixtures have been reported (82,83). The combustion and oxidation of acetylene are reviewed extensively in Reference 84. A study of the characteristics and destmctive effects of detonations in mixtures of acetylene (and other hydrocarbons) with air and oxygen-enriched air in earthen tuimels and large steel pipe is reported in Reference 81. 

The electric discharge processes can supply the necessary energy very rapidly and convert more of the hydrocarbons to acetylene than in regenerative or partial combustion processes. The electric arc provides energy at a very high flux density so that the reaction time can be kept to a minimum (see... 

Flame or Partial Combustion Processes. In the combustion or flame processes, the necessary energy is imparted to the feedstock by the partial combustion of the hydrocarbon feed (one-stage process), or by the combustion of residual gas, or any other suitable fuel, and subsequent injection of the cracking stock into the hot combustion gases (two-stage process). A detailed discussion of the kinetics for the pyrolysis of methane for the production of acetylene by partial oxidation, and some conclusions as to reaction mechanism have been given (12). 

In summary, the bad features of partial combustion processes are the cost of oxygen and the dilution of the cracked gases with combustion products. Flame stability is always a potential problem. These features are more than offset by the inherent simplicity of the operation, which is the reason that partial combustion is the predominant process for manufacturing acetylene from hydrocarbons. 

The unit Kureha operated at Nakoso to process 120,000 metric tons per year of naphtha produces a mix of acetylene and ethylene at a 1 1 ratio. Kureha s development work was directed toward producing ethylene from cmde oil. Their work showed that at extreme operating conditions, 2000°C and short residence time, appreciable acetylene production was possible. In the process, cmde oil or naphtha is sprayed with superheated steam into the specially designed reactor. The steam is superheated to 2000°C in refractory lined, pebble bed regenerative-type heaters. A pair of the heaters are used with countercurrent flows of combustion gas and steam to alternately heat the refractory and produce the superheated steam. In addition to the acetylene and ethylene products, the process produces a variety of by-products including pitch, tars, and oils rich in naphthalene. One of the important attributes of this type of reactor is its abiUty to produce variable quantities of ethylene as a coproduct by dropping the reaction temperature (20—22). 

Tetrachloroethylene can be prepared direcdy from tetrachloroethane by a high temperature chlorination or, more simply, by passing acetylene and chlorine over a catalyst at 250—400°C or by controlled combustion of the mixture without a catalyst at 600—950°C (32). Oxychl orin a tion of ethylene and ethane has displaced most of this use of acetylene. 

Acetylene black is prepared by the partial combustion of acetylene and has specialty uses in batteries. Only about 3500 t/yr are produced in the United States. 

Production of Acetylene and Ethylene by Submerged Combustion," Khim. Promsl ( Moscow) 49(5), 330 (1973). 

Grade C, Type II is typical of Hquid oxygen used as a rocket propellant oxidizer. Particulate content is limited because of the critical clearances found in mechanical parts of the rocket engine. In addition to water, acetylene and methane are limited because, on long standing, oxygen evaporation could cause concentration of these combustible contaminants to reach hazardous levels. 

A flame-ionization, total hydrocarbon analyzer determines the THC, and the total carbon content is calculated as methane. Other methods include catalytic combustion to carbon dioxide, which may be deterrnined by a sensitive infrared detector of the nondispersive type. Hydrocarbons other than methane and acetylene are present only in minute quantities and generally are inert in most appHcations. 

Heat is transferred by direct contact with solids that have been preheated by combustion gases. The process is a cycle of alternate heating and reactingperiods. The Wulf process for acetylene by pyrolysis of natural gas utilizes a heated brick checker work on a 4-min cycle of heating and reacting. The temperature play is 15°C (59°F), peak temperature is 1,200°C (2,192°F), residence time is 0.1 s of wmich 0.03 s is near the peak (Faith, Keyes, and Clark, Industrial Chemicals, vol. 27, Wiley, 1975). 

Inert combustion gases are injected directly into the reacting stream in flame reactors. Figures 23-22 and 22>-22d show two such devices used for maldng acetylene from light hydrocarbons and naphthas Fig. 23-22 shows a temperature profile, reaction times in ms. 

Burning a portion of a combustible reactant with a small additive of air or oxygen. Such oxidative pyrolysis of light hydrocarbons to acetylene is done in a special burner, at 0.001 to 0.01 s reaction time, peak at 1,400°C (2,552°F), followed by rapid quenching with oil or water. 

Hydrochloric acid may conveniently be prepared by combustion of hydrogen with chlorine. In a typical process dry hydrogen chloride is passed into a vapour blender to be mixed with an equimolar proportion of dry acetylene. The presence of chlorine may cause an explosion and thus a device is used to detect any sudden rise in temperature. In such circumstances the hydrogen chloride is automatically diverted to the atmosphere. The mixture of gases is then led to a multi-tubular reactor, each tube of which is packed with a mercuric chloride catalyst on an activated carbon support. The reaction is initiated by heat but once it has started cooling has to be applied to control the highly exothermic reaction at about 90-100°C. In addition to the main reaction the side reactions shown in Figure 12.6 may occur. 

Copper, chromium, iron, most metals or their salts, any flammable liquid, combustible materials, aniline, nitromethane Fuming nitric acid, oxidizing gases Acetylene, ammonia (anhydrous or aqueous)... 

On the other hand, organic materials of relatively low molecular weight such as acetylene, benzene, ethylene and methane, can produce vapour-grown carbon materials by imperfect combustion or by exposing their vapour to a heated substrate in an electric furnace in the presence of a metal catalyst. The latter process generates VGCFs. Other precursors to VGCF include polyacrylonitrile (PAN), isotropic or mesophase pitch, rayon or nylon [8]. 

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