Acetylene from flames

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. 

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. 

Increase in flame temperature often leads to the formation of free gaseous atoms, and for example aluminium oxide is more readily dissociated in an acetylene-nitrous oxide flame than it is in an acetylene-air flame. A calcium-aluminium interference arising from the formation of calcium aluminate can also be overcome by working at the higher temperature of an acetylene-nitrous oxide flame. 

The determination of magnesium in potable water is very straightforward very few interferences are encountered when using an acetylene-air flame. The determination of calcium is however more complicated many chemical interferences are encountered in the acetylene-air flame and the use of releasing agents such as strontium chloride, lanthanum chloride, or EDTA is necessary. Using the hotter acetylene-nitrous oxide flame the only significant interference arises from the ionisation of calcium, and under these conditions an ionisation buffer such as potassium chloride is added to the test solutions. 

The ketenyl radical (HCCO) is a key intermediate in the oxidation of acetylene in flames. It is mainly formed from the O + C2H2 HCCO + H reaction. In lean flames, the HCCO + O2 reaction is the main pathway for decay of HCCO, and this reaction has recently been shown to be the source of prompt CO2 [44, 45]. 

The degeneracy of the excited state is 2, whereas that of the ground state is 1. The fraction of Na in the excited state in an acetylene-air flame at 2 600 K is, from Equation 21-2,... 

Figure 17.14. Some unusual reactor configurations, (a) Flame reactor for making ethylene and acetylene from liquid hydrocarbons [Patton et al., Pet Refin 37(li) 180, (1958)]. (b) Shallow bed reactor for oxidation of ammonia, using Pt-Rh gauze [Gillespie and Kenson, Chemtech, 625 (Oct. 1971)]. (c) Sdioenherr furnace for fixation of atmospheric nitrogen, (d) Production of acetic acid anhydride from acetic acid and gaseous ketene in a mixing pump, (e) Phillips reactor for low pressure polymerization of ethylene (closed loop tubular reactor), (f) Polymerization of ethylene at high pressure.

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. 

Oxy-acetylene flame is a very useful tool in modem industry as it can cut through steel slabs and weld them together again. In the oxy-acetylene flame, oxygen and acetylene from separate cylinders are fed into a blow torch. The gases are mixed thoroughly and burnt at the tip of the blow torch. The products of this combustion are carbon dioxide, water vapour and heat. 

Another process involves molecular aggregation by means of direct chemical reactions akin to polymerization. The best known example of this is the process of carbon particles in a premixed acetylene-oxygen flame. Evidently particle formation in this case does not involve condensation from a supersaturated vapor, but proceeds directly through the pyrolysis of the acetylene, forming in the process unstable polyacetylenes as intermediates in the flame. 

FIG. 19-13 Noncatalytic gas-phase reactions, (a) Steam cracking of light hydrocarbons in a tubular fired heater, (b) Pebble heater for the fixation of nitrogen from air. (c) Flame reactor for the production of acetylene from hydrocarbon gases or naphthas. [Patton, Grubb, and Stephenson, Pet. Ref. 37(11) 180 (1958).] d Flame reactor for acetylene from light hydrocarbons (BASF), (e) Temperature profiles in a flame reactor for acetylene (Ullmann Encyclopadie der Technischen Chemie, vol. 3, Verlag Chemie, 1973, p. 335). 

The flame is a chemical reaction which takes place in the gas phase. The ideal flame for atomic absorption would generate the correct amount of thermal energy to dissociate the atoms from their chemical bonds. The most commonly used flames are aii -acetylene and nitrous oxide—acetylene. The choice of oxidant depends upon the flame temperature and composition required for the production of free atoms. These temperatures vary the molecular or chemical form of the element. Air and acetylene produce flame temperatures of about 2300°C and permit the analysis by atomic absorption of some thirty or so elements. The nitrous oxide—acetylene flame is some 650°C hotter and extends the atomic absorption technique to around 66 elements. It also permits the successful analysis of most elements by flame atomic emission, in many cases at fractional parts per million levels, providing adequate spectral resolution is available. 

Many of the interelement interferences result from the formation of refractory compounds such as the interference of phosphorous, sulfate, and aluminum with the determination of calcium and the interference of silicon with the determination of aluminum, calcium, and many other elements. Usually these interferences can be overcome by using an acetylene-nitrous oxide flame rather than an acetylene-air flame, although silicon still interferes with the determination of aluminum. Since the use of the nitrous oxide flame usually results in lower sensitivity, releasing agents such as lanthanum and strontium and complexing agents such as EDTA are used frequently to overcome many of the interferences of this type. Details may be found in the manuals and standard reference works on AAS. Since silicon is one of the worst offenders, the use of an HF procedure is preferable when at all possible. 

Radicals.—The measurement of emission intensities from electronically excited small free radicals has become an important means of determining radical concentrations in hostile environments such as flames. When combined with laser excitation, the technique is very powerful, offering temporal, spectral, and spatial resolution. Just has reviewed laser techniques for the measurement of both radical concentrations and local temperatures in flames, and has demonstrated the use of laser-induced saturated fluorescence to measure the concentrations of CH and OH radicals in low-pressure acetylene-oxygen flames. Vanderhoff ei al. used a novel Kr " and Ar laser intracavity technique to... 

Chrysene occurs as a product of combustion of fossil fuels and has been detected in automobile exhaust. Chrysene has also been detected in air samples collected from a variety of regions nationally and internationally. The concentrations were dependent on proximity to nearby sources of pollution such as traffic highways and industries, and was also dependent on seasons (generally higher concentrations were noted in winter months). Chrysene has also been detected in cigarette smoke and in other kinds of soot and smoke samples (carbon black soot, wood smoke, and soot from premixed acetylene oxygen flames). It has been detected as a component in petroleum products including clarified oil, solvents, waxes, tar oil, petrolatum, creosote, coal tar, cracked petroleum residue, extracts of bituminous coal, extracts from shale, petroleum asphalts, and coal tar pitch. 

The LEI signal produced by amplitude-modulated continuous wave (CW) dye laser excitation has been shown to be less concomitant-dependent than signals obtained with pulsed excitation38. CW excitation is almost completely immune to interferences from low ionization potential sample matrices at virtually any position in an acetylene-air flame whereas pulsed excitation produces the maximum signal recovery only near the cathode surface. CW is more tolerant in this regard because convection or diffusion will move the analyte ion into the nonzero field near the cathode surface during the synchronization window for chopping rates less than 500 Hz. 

In the boat technique, the liquid sample is deposited in a narrow, boat-shaped container and carefully dried. The container is made of tantalum or other high-melting-point metal. The boat is placed under the light path of the hollow cathode lamp and the dried sample is vaporized by electrical heating or with a conventional acetylene-oxygen flame. This technique, although simple, is not free from interferences and offers better sensitivity for a few elements only. 

For thallium, determinations at the 276.78 nm line with an acetylene/air flame are used throughout. Matrix problems are very low, but the sensitivity with regard to the low level of occurrence is poor. The sensitivity can be increased by mounting a slotted quartz tube on the burner head STAT = "slotted tube atom trap") (Milner, 1983), which leads to a detection limit of about 20 mg/kg in the solid sample, which is insufficient for the analysis of biological matrices. In MIBK extracts, determination of thallium is much more sensitive in flame AAS than in aqueous solutions (till about 7-fold). This can be used for solvent extraction of thallium from 0.1M HBr (Hubert and Chao, 1985), as xanthate at pH 8 (Aihara and Kiboku, 1980), or as iodide with tri-n-octylphosphinoxide into MIBK, and direct aspiration of the organic phase into the flame. 

The 1,3-butadienyl radical is primarily a by-product of butadiene pyrolysis in this system but results from vinyl addition to acetylene in flames of other aliphatic fuels. In aromatic flames 1,3-butadienyl may be produced by oxidative and pyrolytic decomposition of aromatic species, as suggested in a study of benzene flames (10) ... 

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