Study Ethane Pyrolysis

P. Dagaut, J.-C. Boettner, and M. Cathonnet. Ethane Pyrolysis and Oxidation A Kinetic Modeling Study. Int. J. Chem. Kinet., 22 641-664,1990. 

Some such studies on the ethane pyrolysis in the presence of nitric oxide have been made by Pratt > who has been particularly concerned with the kinetics of formation of nitrogen, hydrogen, methane and butane. Work has recently been carried out at the University of Ottawa on the pyrolyses of ethane (Esser and Laidler ) and of acetaldehyde (Schuchmann and Laidler ). The technique of gas-liquid chromatography has been used for the most part to analyze the reaction... 

Williams and Williams27 have studied the pyrolysis of both HDPE and LDPE in a fixed bed reactor. In each experiment the temperature was varied between 25 and 700 °C. The products were swept down through the bottom of the reactor by a nitrogen flow and separated into several fractions by condensation at different temperatures. Two main fractions were recovered as products from the HDPE and LDPE pyrolysis gases with a yield of 15-17 wt% and oils with yields in the range 80-84 wt%. The gases were rich in ethylene, propylene and butene, with lower proportions of saturated hydrocarbons (methane, ethane, propane and butane). The oils produced were analysed by FTIR and GPC,... 

The thermal decomposition of pure ethane is a much studied reaction, and the mechanism is well defined. For purposes of comparison with the oxygen reactions, the ethane pyrolysis is redescribed below. 

The pyrolysis of diethyl mercury has been studied using a nitrogen carrier flow system87 both in the presence and absence of toluene. The experimental conditions used were total pressure = 10+1 torr with 0.4 torr partial pressure of toluene, alkyl pressure 1-10 x 10 2 torr, decomposition 10-75 % and contact time 0.1-0.3 sec. The presence of toluene had no effect on the rate coefficient, the observed ethane/ethylene ratio ( 1) or the C4/C2 ratio ( 4). These ratios were essentially independent of temperature. 

Studies of the pyrolysis of these three alkyls may conveniently be discussed in a combined section. The decompositions were carried out in a conventional toluene carrier flow system using contact times of 1-2 sec120,122,123. The conditions used satisfy both plug flow and thermal equilibrium requirements68,69. Toluene to alkyl ratios greater than 50 in the trimethyl gallium system and greater than 200 in the trimethyl indium and thallium studies were required to obtain first-order dependence in terms of the alkyl concentration. Under these conditions methane and ethane are produced by the reactions... 

A flash vacuum thermolysis (FVT) study of 2-bromoethanol108 reported the formation of 80% l-bromo-l-(2-bromoethoxy)ethane and 10% l-bromo-l-(l-bromoethoxy)ethane. However, at 900-950 °C, the pyrolysis product was exclusively the latter compound. The formation of several intermediates was considered responsible for these results. 

If we consider in retrospect the work on the pyrolysis of ethane, we are struck by the fact that, while it has produced much controversy and much travail and has been a great stimulus to further work, very little if any quantitative data of interest have come from it. On the contrary, all of the best available data on the steps in the proposed mechanism have come from quite different studies on the behavior of free radicals. And in fact, even at present the best use one can make of the data on this pyrolysis is to check them qualitatively against a proposed mechanism. It is quite doubtful that they can be used to predict individual rate constants with any reliability. 

The work on ethane indicates quite strikingly that the same saying applies to pyrolysis. Our present understanding of pyrolysis reactions, which is not inconsiderable, has for the most part derived, and will continue to derive, from the studies of the thermodynamic and kinetic properties of individual free radicals observed in very carefully selected model systems. 

The diethyl ether diffusion flame has also been studied [81], and in common with other diffusion flames [27, 28], it was found that pyrolysis was the primary process occurring in the inner regions of the flame. In the central zone where most of the ether disappears and in which the measured temperature is 400—800 °C, the products are acetaldehyde, ethane, ethanol and ethylene, suggesting that pyrolysis of the fuel was taking place according to the two alternative overall reactions... 

The pyrolysis of azomethane which yields mainly nitrogen and ethane, has been extensively studied [138, 139]. At high temperatures the decomposition is explosive [140]. The evidence suggests that the explosion is of thermal origin above 636 °K, while below this temperature there are indications that chain-branching plays a significant part. 

The main products of the methane pyrolysis are ethylene, acetylene and hydrogen, with smaller amounts of ethane this was established in studies made by Gordon " and by Palmer et In addition, carbon is deposited on the walls of the reaction vessel. 

Eisenberg and Bliss and Palmer et al have studied the time-course of the methane pyrolysis there is an initial acceleration followed by retardation. Both studies show that ethane accelerates the reaction. The work of Palmer et indicates that there is a deposition of vitreous carbon on the walls of the vessel, that the reaction is inhibited by carbon, and that the rate is not appreciably affected by the surface volume ratio. They also find that the reaction is strongly accelerated by added naphthalene, which tends to produce carbon nuclei very rapidly. They conclude that the formation of nuclei has a strong effect on the rate of decomposition. The inhibition by hydrogen may then be due to its removal of nuclei. The accelerating effect of added ethane is attributed to its more rapid decomposition, with accompanying formation of nuclei aside from this, ethane is a good source of free radicals. 

Thus, the crucial difference in the proposed pathways is the emergence of C2H4 or C2H6 as the mechanistic indicator. In fact ethane, C2Hg, is the observed product. Based on this and on other data Speckman and Wendt proposed the reaction pathway for the pyrolysis of EtjAs shown in Scheme 1. Their observations are consistent with the data from the low-pressure pyrolysis of Et3As obtained by Jensen and coworkers. Both these groups results are in conflict with observations of Melas and collaborators , who interpreted their thermolysis study of AsEts interms of a / -elimination mechanism. 

Very recently, the gas phase pyrolysis (155-200°) of methyl azide at low conversions (< 1%) was studied . Nitrogen was the major non-condensable gas, in addition to small amounts of hydrogen (6% at lowest initial pressure of azide to less than 1% at the highest pressure) and methane (<2%). Ethane, ethylene, ammonia and hydrazoic acid were not detected, although the ethane, ethylene and hydrazoic acid determinations were subject to some uncertainty. Two solid white products were also obtained which were not characterized. The results showed that the thermolysis of methyl azide is of first-order, homogeneous and free from chains, in agreement with previous work . The Arrhenius activation parameters (see Table 1)... 

In one of the most elegant applications of gas-phase inhibition by nitric oxide, Birss, Danby and Hinshelwood have studied the thermal dissociation of r-butyl peroxide. The low temperatures required for pyrolysis permitted mass spectro-metric determination of t-butyl nitrite, and a fairly complete kinetic analysis of the system was possible. The rate of decomposition of peroxide was related to the consumption of nitric oxide and to the appearance of butyl nitrite during the inhibition period, and curves were obtained which showed the acetone and ethane concentrations as a function of time during and after inhibition. 

Pyrolysis of the formed polysilazane at 750° C generates several alkylsilanes such as dimethylsilane, trimethylsilane, ethyldimethylsilane, tetramethyidisiloxane, pentamethyidisiloxane, methylenebisdimethylsilane, hexamethylcyclotrisiloxane, octamethylcyclotetrasiloxane, etc. The presence of the cyclic compounds similar to those from poly(dimethylsiloxane) was an indication that some polysiloxane sequences may be present in the polymer. Thermal degradation studied between 350° C and 650° C showed the formation of some hydrogen, methane, ethane, and propene. 

Table III shows the effect of shifting furnace operation from propane fresh feed to ethane. Data are from Schutt and Zdonik (54). The reduction of propylene yield from ethane to negligible levels in favor of increased ethylene production cannot be done if a plant has propylene commitments. Because propylene requirements cannot be satisfied with ethane feed, Ericsson (14) has concluded that propane will continue to be the preferred feedstock to make ethylene. Actually, 85% of the U.S. ethylene plants are located in the Gulf Coast area so that they can obtain and operate on economical ethane and propane feeds. The need for propane pyrolysis has resulted in a renewal of experimental interest in this area, and in-depth studies have been made by Crynes and Albright (17) and by Buekens and Froment (7).

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