Oxidative Processing of Hydrocarbons

Most industrial hydrogen is manufactured by the following hydrocarbon-based oxidative processes steam reforming of light hydrocarbons (e.g., NG and naphtha), POx of heavy oil fractions, and ATR. Each of these technological approaches has numerous modifications depending on the type of feedstock, reactor design, heat input options, by-product treatment, 

Simplified schematics of hydrogen production by SMR. (a) SMR with solvent removal of C02 and a methanation unit, (b) SMR with a PSA unit. HT- and LT-WGS high- and low-temperature WGS reactors, respectively. 

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The intermediates formed in AOPs sometimes are more toxic than the parent compounds and are required to be decomposed completely using either combination of AOPs or combination of AOP and some other treatment methods such as adsorption and biodegradation. Carbonyl compounds, particularly aldehydes, are quite toxic, and some of the secondary compounds formed from aldehydes, especially peroxyacylnitrates are more dangerous than the parent compounds. Organic peroxy radical (ROj) reactions are of significance because they represent an important class of intermediates formed in the oxidation process of hydrocarbons (15). Intermediates such as ethers and alcohols have enhanced reactivity toward hydroxyl radical. The rate constant of oxidation of these compounds is of similar order of magnitude as of the alkanes. 

Molecular oxygen is the major cause of irreversible deterioration of hydrocarbon substrates, leading to the loss of useful properties and to the ultimate failure of the substrate. The oxidation process of hydrocarbons is autocatalytic oxidation starts slowly, sometimes with a short induction period, followed by a gradual increase in the rate, concomitant with the build up of hydroperoxides, which eventually subside, giving rise to a sigmoidal oxidation curve. When initiators such as peroxides are present, the length of the induction period is absent, or very short, but it can be prolonged by antioxidants, as shown in Fig. 1. The basic autoxidation theory of hydrocarbons involves a complex set of elementary reaction steps in a free radical-initiated chain reaction mechanism the basic tenets of this theory apply equally to polymer oxidation. 

Organic compounds involving oxygen are the main stock for synthesis of various plastics, lacquers, resins, and other material. Oxidative processing of hydrocarbons has long ago attracted the attention of chemists as one of the main trends in organic synthesis. 

There are at present three fundamental directions of pursuit in the investigation of oxidative processing of hydrocarbons ... 

It is well recognised that peroxy radicals play a key role in the oxidation processes of hydrocarbons under the conditions of the atmosphere, leading to the formation of tropospheric ozone and other oxidised molecules, in the presence of nitrogen oxides. Their relative reaction rate with nitrogen oxides and witii other radicals is one of the principal factors determining the ozone balance in the troposphere. 

Other nonpolymeric radical-initiated processes include oxidation, autoxidation of hydrocarbons, chlorination, bromination, and other additions to double bonds. The same types of initiators are generally used for initiating polymerization and nonpolymerization reactions. Radical reactions are extensively discussed in the chemical Hterature (3—15). 

Whereas this reaction was used to oxidize ethylene (qv) to acetaldehyde (qv), which in turn was oxidized to acetic acid, the direct carbonylation of methanol (qv) to acetic acid has largely replaced the Wacker process industrially (see Acetic acid and derivatives). A large number of other oxidation reactions of hydrocarbons by oxygen involve coordination compounds as detailed elsewhere (25). 

For the manufacturing of sulfosuccinic acid esters, which belong to a special class of surfactants, maleic acid anhydride is needed. Maleic acid anhydride is an important intermediate chemical of the chemical industry. Its worldwide output amounts to about 800,000 tons (1990) [64]. Maleic acid is produced by catalytic vapor phase oxidation process of benzene or n-C4 hydrocarbons in fixed bed or fluidized bed reactors according the following reaction equations. The heat of reaction of the exothermic oxidation processes is very high. 

In the majority of the homogeneous oxidations of hydrocarbons by oxometal-based catalysts (including metalloporphyrins), there is appreciable decomposition of catalyst ligands by oxidation, and hence appreciable loss in activity after a few turnovers. A similar degradation of organic ligands, often hydrophobic long-chain carboxylates, is also observed in industrial processes of hydrocarbon... 

Trapido, M., Veressinina, Y., and Munter, R., Ozonation and advanced oxidation processes of polycyclic aromatic hydrocarbons in aqueous solutions a kinetic study, Environ. Technol., 16, 729-740, 1995. 

Several compounds were also found to have a seasonal distribution. Kubatova et al. (2002) found that concentrations of lignin and cellulose pyrolysis products from wood burning were higher in aerosol samples collected during low-temperature conditions. On the other hand, concentrations of dicarboxylic acids and related products that are believed to be the oxidation products of hydrocarbons and fatty acids were highest in summer aerosols. PAHs, which are susceptible to atmospheric oxidation, were also more prevalent in winter than in summer. These results suggest that atmospheric oxidation of VOCs into secondary OAs and related oxidative degradation products are key factors in any OA mass closure, source identification, and source apportionment study. However, additional work is much desirable to assess the extent and seasonal variation of these processes. 

Let us discuss in general gas-phase processes of oxidative dehydrogenation of hydrocarbons involving as reagents substances that easily induce free radical transformations of substrates. Many such substances are known that dissociate to free radicals or induce free radical reactions. However, the most widespread in investigations are compounds that are able to shift dehydrogenation and cracking product ratios toward the first process. 

Beltran FJ, Rivas FJ, Alvarez P, Alonso MA, Acedo B. A kinetic model for advanced oxidation processes of aromatic hydrocarbons in water application to phenanthrene and nitrobenzene. Ind Eng Chem Res 1999 38 4189-4-199. 

Non-catalytic partial oxidation (POX) of hydrocarbons from residual fuel oils to methane is commercially proven by two processes, one offered by Texaco and the other by Shell. Davy has experience with both processes. Each process has a large number of plants in operation, with feeds varying from natural gas to high sulfur residual oil. (In fact, so long as the feedstock can be pumped, it is a suitable feestock for a partial oxidation gasifier. 

Many extensive models of the high-temperature oxidation process of methane have been published [20,20a, 20b, 21]. Such models are quite complex and include hundreds of reactions. The availability of sophisticated computers and computer programs such as those described in Appendix H permits the development of these models, which can be used to predict flow reactor results, flame speeds, emissions, etc., and to compare these predictions with appropriate experimental data. Differences between model and experiment are used to modify the mechanisms and rate constants that are not flrmly established. The purpose here is to point out the dominant reaction steps in these complex models of methane oxidation from a chemical point of view, just as modem sensitivity analysis [20, 20a, 20b] can be used to designate similar steps according to the particular application of the mechanism. The next section will deal with other, higher-order hydrocarbons. 

The early applications of fast CV mainly focused on the measurement of the peak-potential separation, AEp, for the reduction or oxidation process of aromatic compounds, to obtain the pertinent standard heterogeneous rate constant k° from the relationship given in Table 2 [22]. The largest k° values of about 4 cm s were found for the reduction of aromatic hydrocarbons such as anthracene at a gold electrode in acetonitrile. The peak-potential separation increased from the 58 mV expected for a reversible process at low v to about 100 mV on going to v values of 10 kV s . This also shows that there is no real need for employing extremely large sweep rates in the determination of k° for the majority of compounds. Rather, it is important to ensure that the measurements at the lower sweep rates are not hampered by the contribution from spherical diffusion if a too small UME is used. 

Substantial improvements in the performance of several processes of hydrocarbon selective oxidation can be achieved solely by developing new reactor configurations. An important step in this direction is exemplified by the circulating fluidized bed reactor, which over the years has been proposed for use in several selective oxidation reactions and has, finally, found application in w-butane selective oxidation to maleic anhydride. Although production at the plant (built in Spain) was later stopped, because it was uneconomic, it remains an interesting example that may find application in other reactions. 

KINETIC MODELS OF C,-C4 ALKANE OXIDATION AS APPLIED TO PROCESSING OF HYDROCARBON GASES PRINCIPLES, APPROACHES AND DEVELOPMENTS... 

The oxidative addition of hydrocarbons to a transition metal center is an important reaction for the development of catalytic processes involving for example the hydrogenation and hydroformylation of olefins [11], The activation... 

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