Oxidation higher hydrocarbons

Methane, or rather natural gas (which may contain carbon oxides, higher hydrocarbons, and inert gases), is of great interest as a source of pyrolytically grown fibers because of its relatively low cost. 

The direct methane conversion technology, which has received the most research attention, involves the oxidative coupling of methane to produce higher hydrocarbons (qv) such as ethylene (qv). These olefinic products may be upgraded to Hquid fuels via catalytic oligomerization processes. 

Generally, the most developed processes involve oxidative coupling of methane to higher hydrocarbons. Oxidative coupling converts methane to ethane and ethylene by... 

Ethane. Ethane VPO occurs at lower temperatures than methane oxidation but requires higher temperatures than the higher hydrocarbons (121). This is a transition case with mixed characteristics. Low temperature VPO, cool flames, oscillations, and a NTC region do occur. At low temperatures and pressures, the main products are formaldehyde, acetaldehyde (HCHOiCH CHO ca 5) (121—123), and carbon monoxide. These products arise mainly through ethylperoxy and ethoxy radicals (see eqs. 2 and 12—16 and Fig. 1). 

Activated alumina and phosphoric acid on a suitable support have become the choices for an iadustrial process. Ziac oxide with alumina has also been claimed to be a good catalyst. The actual mechanism of dehydration is not known. In iadustrial production, the ethylene yield is 94 to 99% of the theoretical value depending on the processiag scheme. Traces of aldehyde, acids, higher hydrocarbons, and carbon oxides, as well as water, have to be removed. Fixed-bed processes developed at the beginning of this century have been commercialized in many countries, and small-scale industries are still in operation in Brazil and India. New fluid-bed processes have been developed to reduce the plant investment and operating costs (102,103). Commercially available processes include the Lummus processes (fixed and fluidized-bed processes), Halcon/Scientific Design process, NIKK/JGC process, and the Petrobras process. In all these processes, typical ethylene yield is between 94 and 99%. 

The oxidation of hydrocarbons involves the sequential formation of a number of similar reactions in which various intermediate radicals which are combinations of carbon, hydrogen and oxygen are formed. In the simplest case, the oxidation of medrane, the methyl radical CH3 plays an important part both in direct oxidation to CO(g) and in indirect oxidation duough the formation of higher hydrocarbons such as CaHe before CO is formed. The chain reactions include... 

The OH radical is a primary oxidizer in the atmosphere, oxidizing CO to CO2 and CH4 and higher hydrocarbons to CH2O, CO, and eventually CO2. OH and other radical intermediates can oxidize CH4 and NO in the following sequence of reactions ... 

Oxidation of hydrocarbons. Paraffin wax in the petroleum industry was oxidized in a gamma field at a higher temperature to produce higher alcohols and fatty acids. The yield envisaged was as a few thousand tons per year. 

Oxidative coupling of methane to yield C2 and higher hydrocarbons... 

Oxidative coupling of methane to yield C2 and higher hydrocarbons 358 Direct partial oxidation of methane to produce methanol and other oxygenates 360... 

Initiators (peroxides, azo-compounds, polyphenylbutanes) accelerate the oxidation of hydrocarbons. The rate of initiated oxidation is much higher than the rate of initiator decomposition [9,10,12,13]. 

Compounds of transition metals (Mn, Cu, Fe, Co, Ce) are well known as catalysts for the oxidation of hydrocarbons and aldehydes (see Chapter 10). They accelerate oxidation by destroying hydroperoxides and initiating the formation of free radicals. Salts and complexes containing transition metals in a lower-valence state react rapidly with peroxyl radicals and so when these compounds are added to a hydrocarbon prior to its oxidation an induction period arises [48]. Chain termination occurs stoichiometrically (f 1) and stops when the metal passes to a higher-valence state due to oxidation. On the addition of an initiator or hydroperoxide, the induction period disappears. 

When variable-valence metals are used as catalysts in the oxidation of hydrocarbons, the chain termination via such reactions manifests itself later in the process. This case has specially been studied in relation to the oxidation of paraffins to fatty acids in the presence of the K Mn catalyst [57], which ensures a high oxidation rate and a high selectivity of formation of the target product (carboxylic acids). As the reaction occurs, alcohols are accumulated in the reaction mixture, and their oxidation is accompanied by the formation of hydroxyperoxyl radicals. The more extensively the oxidation occurs, the higher the concentration of alcohols in the oxidized paraffin, and, hence, the higher is the kinetic... 

Metal dialkyl dithiocarbamates inhibit the oxidation of hydrocarbons and polymers [25,28,30,76 79]. Like metal dithiophosphates, they are reactive toward hydroperoxides. At room temperature, the reactions of metal dialkyl dithiocarbamates with hydroperoxides occur with an induction period, during which the reaction products are formed that catalyze the breakdown of hydroperoxide [78]. At higher temperatures, the reaction is bimolecular and occurs with the rate v = k[ROOH][inhibitor]. The reaction of hydroperoxide with dialkyl dithiocarbamate is accompanied by the formation of radicals [30,76,78]. The bulk yield of radicals in the reaction of nickel diethyl dithiocarbamate with cumyl hydroperoxide is 0.2 at... 

Phenols usually terminate two chains in the oxidation of hydrocarbons and solid polymers (see Chapter 15). The study of the/ value dependence on partial dioxygen pressure showed, however, that the stoichiometric coefficient of inhibition has a tendency to increase with decreasing the dioxygen pressure and, in an inert atmosphere, it is markedly higher than in dioxygen [83]. The results of/ value estimation (f—vi/vInH, phenol concentration was measured spectroscopically) are given in Table 19.13. 

A systematic study of differently supported Ru catalysts showed that carbon catalysts provide very high selectivities to higher hydrocarbons (C10-C20) and the CNT-supported catalyst is among the most active systems of all [138]. In parts this is related to the inertness of carbon preventing the formation of hardly reducible mixed metal oxides with the support, such as CoAl204 [139,140], which is, besides coking, the main reason for catalyst deactivation. The carbon surface functionalized with oxygen... 

In general, the oxidation of H2, CO, CH4, and higher hydrocarbons in fuel cells to produce power also produces reject heat. This heat arises from two sources ... 

Carbon monoxide is an important trace gas, which has a mean residence time of about two months and a mean concentration of the order of 0.1 ppm. The principal sources of CO are (1) oxidation of methane and other higher hydrocarbons, (2) biomass burning, (3) traffic, industry and domestic heating, (4) oceans, and (5)... 

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