Ethane oxygenation

Cg portion and then switching to the TCD for carbon dioxide, ethane, oxygen (if present), nitrogen, and methane, a complete analysis is shown in Figure 6.30. The signal was automatically switched to the thermal conductivity detector at 8 min. 

Methane reacts only slowly with oxygen below 400° C. Ethane oxidation was observed by Bone and Hill (S) at 290° to 323° C. Formaldehyde, a reaction product, was found to increase, reach a maximum, and then decrease. Addition in amounts of 1% to a 3 to 1 ethane-oxygen mixture at 316° C. and 720 mm. eliminated the induction period, but other additives such as nitregen dioxide, acetaldehyde, ethyl alcohol, or water, were also more or less effective. 

Formaldehyde, in sufficient quantities, can suppress cool-flame formation. Jost (27) presents evidence indicating that cool flames are a form of branched-chain explosions. It has been suggested that the cool-flame reaction is quenched by its own reaction product, formaldehyde, and arrested short of complete release of chemical enthalpy. This seems unlikely, however, because in systems exhibiting multiple cool flames the concentration of formaldehyde after the first cool flame does not drop in some cases it increases, and yet does not suppress subsequent cool flames. Bardwell (5), and Bard well and Hinshelwood (4) explain cool flame phenomena by a modified theory of Salnikov. This thermal theory is further supported by the results of Knox and Norrish (30) in the ethane-oxygen system. The key intermediate is presumed to be a peroxide by Bardwell and Hinshelwood (4). Formaldehyde is considered an inert, stable product with little effect on the reaction. 

It would seem worth while, therefore to restudy the explosion limits of methane-oxygen and ethane-oxygen and also to study the effects of these hydrocarbons on the carbon monoxide-oxygen limits, with a view toward establishing whether these systems are connected in any way. In any case, valuable clues to the mechanisms of combustion of hydrocarbons can probably be obtained. 

Combustion occurs with a large number of intermediate steps and even simple processes, such as the ones listed in Table 10.1, occur through dozens of coupled elementary reactions. With computer simulations it is possible to describe the interaction between the reactions, and concentration profiles can be calculated. In order to perform the computer calculations it is necessary to know the rate constants for the individual elementary reactions. Comparisons between theory and experiments are best made for a flat, premixed flame, which in its central part can be considered to have only onedimensional (vertical) variation, allowing computer calculations to be performed comparatively easily. The most important reactions are included in the computer description. In Fig. 10.1 experimental and theoretically calculated concentration curves are given for the case of low-pressure ethane/ oxygen combustion. As examples of important elementary processes we give the reactions... 

Fig. 10.1. Experimental and theoretical concentration profiles for an ethane/oxygen flame [10.3, first Ref.] (Copyright 1982 by Scientific American, Inc.. All rights reserved)...

Further improvements of the selective production of ethylene were obtained by Bodke et by co-feeding H2 to ethane-oxygen mixtures over a Pt-Sn coated monolith ethylene was produced at 80-85% selectivity with over 70% ethane conversion. Apparently the Pt-Sn alloy could favour the selective oxidation of hydrogen, which thermally drove the selective dehydrogenation of ethane to ethylene. 

Vanadium-cobalt substituted aluminophosphate molecular sieve of AEI structure (VCoAPO-18) was found to be active and selective in the ODH of ethane. Its catalytic behavior can be related to the presence of redox (probably related to and Co " ") and acid sites (related to Co + cations) in addition to its unique structural properties. The conversion and ethene selectivity decreases in the order VCoAPO-18 >VO c/CoAPO-18 > CoAPO-18 [38]. At 873 K, the VCoAPO-18 catalyst showed a 50% ethene selectivity at 60% ethane conversion for an ethane/oxygen molar ratio of 4 8. Acid SAPO-34-based microporous catalysts with chabasite structure have been tested for the ODH of ethane in the temperature range of 823 to 973 K. Pure acid and La/Na containing SAPO-34 were catalytically active and a 75 ethene selectivity for 5% ethane conversion and a 60% ethane selectivity for 30% ethane conversion was observed [39]. 

Phase equilibria data have been obtained for the methane-oxygen, ethane-oxygen, ethylene-oxygen, propane-oxygen, and propylene-oxygen systems by the visual observation method. 

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