Hydrocarbons, complete oxidation

This process includes two main sections the burner section with a reaction chamber that does not have a catalyst, and a Claus reactor section. In the burner section, part of the feed containing hydrogen sulfide and some hydrocarbons is burned with a limited amount of air. The two main reactions that occur in this section are the complete oxidation of part of the hydrogen sulfide (feed) to sulfur dioxide and water and the partial oxidation of another part of the hydrogen sulfide to sulfur. The two reactions are exothermic ... 

Heat of combustion is the heat liberated or absorbed when one gram mole of the substance is completely oxidized to liquid water and CO2 gas at one atmosphere and 20°C or 25°C. (Cj-C hydrocarbons and cyclohexane at 25°C, others at 20°C). The gross heating value in Btu/ft could be calculated as follows ... 

Supported gold catalysts are, in general, less active than platinum group metal catalysts in the complete oxidation of hydrocarbons however, by choosing... 

Additionally, NO is reduced by H2 and by hydrocarbons. To enable the three reactions to proceed simultaneously - notice that the two first are oxidation reactions while the last is a reduction - the composition of the exhaust gas needs to be properly adjusted to an air-to-fuel ratio of 14.7 (Fig. 10.1). At higher oxygen content, the CO oxidation reaction consumes too much CO and hence NO conversion fails. If however, the oxygen content is too low, all of the NO is converted, but hydrocarbons and CO are not completely oxidized. An oxygen sensor (l-probe) is mounted in front of the catalyst to ensure the proper balance of fuel and air via a microprocessor-controlled injection system. 

However, even if this strategy seems very attractive, the EGR gases densification also shows several drawbacks (see Figure 7.3). Because of a lower temperature range, the combustion no longer allows the complete oxidation of the fuel carbon species that can be released in the exhaust line in the form of unburnt hydrocarbons. With EuroV constraints, the previous NO,/particle compromise is now substituted by a new one the NO,/IIC compromise. 

The gas mixture containing the nitrogen oxides is very important as well. Experiments and modeling carried out for N2/NOx mixtures, or with addition of 02, H20, C02 and hydrocarbons will be discussed. Typical hydrocarbon additives investigated are ethane, propene, propane, 2-propene-l-ol, 2-propanol, etc. As compared to the case without hydrocarbons, NO oxidation occurs much faster when hydrocarbons are present. The reaction paths for NO removal change significantly, in fact the chemical mechanism itself is completely different from that of without hydrocarbon additives. Another additive investigated extensively is ammonia, used especially in corona radical shower systems. 

Heat of combustion the enthalpy change for the complete oxidation of a compound => for a hydrocarbon means converting it to C02 and water. 

Since the complete oxidation of two isomeric hydrocarbons will give the same number of moles of C02 and H20, using the same moles of oxygen, we can find their heats of combustain. For butane and isobutane, the values determined experimentally are ... 

Scheme 9.1 shows a generalized sequence of reactions for the oxidation of an alkane, via alcohol, ketone and carboxylic acid, to the completely oxidized products, water and carbon dioxide. The latter are often referred to as combustion products as they are the same as those formed by burning hydrocarbons. These are not normally desirable chemical products unless it is necessary to destroy a toxic, hazardous or otherwise unwanted waste material. Oxidation itself is not difficult to achieve, and is a highly exothermic or even explosive process. Selective oxidation, however, is a much greater challenge, as it is important to stop the sequence at the desired product without proceeding further down the oxidation pathway. 

It is interesting to review a general pattern for oxidation of hydrocarbons in flames, as suggested very early by Fristrom and Westenberg [29], They suggested two essential thermal zones the primary zone, in which the initial hydrocarbons are attacked and reduced to products (CO, H2, H20) and radicals (H, O, OH), and the secondary zone, in which CO and H2 are completely oxidized. The intermediates are said to form in the primary zone. Initially, then,... 

Steven McIntosh was born in Dundee, Scotland in 1977. He received his Batchelor of Engineering in Chemical Engineering from the University of Edinburgh in 1999. He is currently completing his Ph.D. degree at the University of Pennsylvania. The focus of his thesis is the development and characterization of direct-hydrocarbon solid oxide fuel cells. After a postdoctoral year, he will start as an Assistant Professor in Chemical Engineering at the University of Virginia in 2005. 

Phenols show a two-electron oxidation wave on cyclic voltammetry in acetonitrile at a less positive potential than for the con-esponding methyl ether (Table 6.5) or a related hydrocarbon. Phenol radical-cation is a strong acid with pKg ca. -5 in water [93], so the two-electron oxidation wave for phenols is due to formation of a phenoxonium ion such as 13, where the complete oxidation process is illustrated for 2,4,6-tri-tt rf-butylphenol. Phenoxide ions are oxidised at considerably less positive potentials than the conesponding phenol. They give rise to a one-electron wave on cyclic voltammetry in aqueous acetonitrile or in aqueous ethanol containing potassium hydroxide. 2,4,6-Tri-/ert-butyiphenoxide ion is reversibly oxidised to the radical in a one-electron proces.s with E° = -0.09 V V5. see. The radical undergoes a further irreversible one-electron oxidation with Ep = 1.05 V vs. see on cyclic voltammetry forming the phenoxonium ion which reacts with water [94J. 

C=C bond hydrogenation, olefin + H2-> paraffin C=0 bond hydrogenation, acetone + H2 -> isopropanol Complete oxidation of hydrocarbons, oxidation of CO 3H2 + N2 -> 2NH3... 

Selectox catalyst was developed to oxidize H2S to sulfur and S02 in the presence of hydrogen, paraffin hydrocarbons or ammonia The latter compounds are effectively inert at temperatures which allow complete oxidation of H2S. Figure 3 shows the direct oxidation section of the Lingen plant. About 80 percent recovery of sulfur is achieved, even though the initial concentration of H2S is only 1 to 2 percent. 

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