Temperature hydrocarbon reactions

The nature of many high-temperature hydrocarbon reactions which potentially can benefit from inorganic membrane reactors (particularly catalytic membrane reactors) is such that the catalysts or the catalytic membranes are subject to poisoning over time. Deactivation and regeneration of many catalysts in the form of pellets are well known, but the same issues related to either catalyst-impregnated membranes or inherently catalytic membranes are new to industrial practitioners. They are addressed in this section. 

Inert combustion gases are injected directly into the reacting stream in flame reactors. Figures 23-22 and 22>-22d show two such devices used for maldng acetylene from light hydrocarbons and naphthas Fig. 23-22 shows a temperature profile, reaction times in ms. 

In chemical laboratories, small flasks and beakers are used for liquid phase reactions. Here, a charge of reactants is added and brought to reaction temperature. The reaction may be held at this condition for a predetermined time before the product is discharged. This batch reactor is characterized by the varying extent of reaction and properties of the reaction mixture with time. In contrast to the flasks are large cylindrical tubes used in the petrochemical industry for the cracking of hydrocarbons. This process is continuous with reactants in the tubes and the products obtained from the exit. The extent of reaction and properties, such as composition and temperature, depend on the position along the tube and does not depend on the time. 

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... 

The direct protonation of isobutane, via a pentacoordinated carbonium ion, is not likely under typical alkylation conditions. This reaction would give either a tertiary butyl cation (trimethylcarbenium ion) and hydrogen, or a secondary propyl cation (dimethylcarbenium ion) and methane (37-39). With zeolites, this reaction starts to be significant only at temperatures higher than 473 K. At lower temperatures, the reaction has to be initiated by an alkene (40). In general, all hydrocarbon transformations at low temperatures start with the adsorption of the much more reactive alkenes, and alkanes enter the reaction cycles exclusively through hydride transfer (see Section II.D). 

At high pressures or in the initial stages of hydrocarbon oxidation, high concentrations of H02 can make reaction (3.45) competitive to reaction (3.44), so reaction (3.45) is rarely as important as reaction (3.44) in most combustion situations [4], Nevertheless, any complete mechanism for wet CO oxidation must contain all the H2—02 reaction steps. Again, a complete mechanism means both the forward and backward reactions of the appropriate reactions in Appendix C. In developing an understanding of hydrocarbon oxidation, it is important to realize that any high-temperature hydrocarbon mechanism involves H2 and CO oxidation kinetics, and that most, if not all, of the C02 that is formed results from reaction (3.44). 

The low-temperature hydrocarbon oxidation mechanism discussed in the previous section is incomplete because the reactions leading to CO were not included. Water formation is primarily by reaction (3.56). The CO forms by the conversion of aldehydes and their acetyl (and formyl) radicals, RCO. The same type of conversion takes place at high temperatures thus, it is appropriate, prior to considering high-temperature hydrocarbon oxidation schemes, to develop an understanding of the aldehyde conversion process. 

The essential point is that the initiation steps provide H atoms that react with the oxygen in the system to begin the chain branching propagating sequence that nourishes the radical reservoir of OH, O, and H that is, the reaction sequences for the complete H2—02 system must be included in any high-temperature hydrocarbon mechanism. Similarly, when CO forms, its reaction mechanism must be included as well. 

The flame speed for a combustible hydrocarbon-air mixture is known to be 30cm/s. The activation energy of such hydrocarbon reactions is generally assumed to be 160kJ/mol. The true adiabatic flame temperature for this mixture is known to be 1600 K. An inert diluent is added to the mixture to lower the flame temperature to 1450 K. Since the reaction is of second-order, the addition of the inert can be considered to have no other effect on any property of the system. Estimate the flame speed after the diluent is added. 

The term prompt NO derives from the fact that the nitrogen in air can form small quantities of CN compounds in the flame zone. In contrast, thermal NO forms in the high-temperature post-flame zone. These CN compounds subsequently react to form NO. The stable compound HCN has been found in the flame zone and is a product in very fuel-rich flames. Chemical models of hydrocarbon reaction processes reveal that, early in the reaction, O atom concentrations can reach superequilibrium proportions and, indeed, if temperatures are high enough, these high concentrations could lead to early formation of NO by the same mechanisms that describe thermal NO formation. 

Nitroalkanes can be formed from the direct nitration of aliphatic and alicyclic hydrocarbons with either nitric acid ° or nitrogen dioxide in the vapour phase at elevated temperature. These reactions have achieved industrial importance but are of no value for the synthesis of nitroalkanes on a laboratory scale, although experiments have been conducted on a small scale in sealed tubes. 

Unlike hydrocarbon-based fuels like methane and gasoline, coal has never been subjected to a comprehensive mechanistic analysis, due to the complexity of its molecular structure. However, coal s complex structure consists of various mono-cyclic units that can be explored aromatic hydrocarbons and heteroaromatic rings are recurring units in coal s structure, even while the overall structure varies geographically. Understanding low- and high-temperature oxidation reactions for these subunits and their reactive radical intermediates will facilitate a better understanding of their chemistry in combustion. 

In this paper selectivity in partial oxidation reactions is related to the manner in which hydrocarbon intermediates (R) are bound to surface metal centers on oxides. When the bonding is through oxygen atoms (M-O-R) selective oxidation products are favored, and when the bonding is directly between metal and hydrocarbon (M-R), total oxidation is preferred. Results are presented for two redox systems ethane oxidation on supported vanadium oxide and propylene oxidation on supported molybdenum oxide. The catalysts and adsorbates are stuped by laser Raman spectroscopy, reaction kinetics, and temperature-programmed reaction. Thermochemical calculations confirm that the M-R intermediates are more stable than the M-O-R intermediates. The longer surface residence time of the M-R complexes, coupled to their lack of ready decomposition pathways, is responsible for their total oxidation. 

Catalytic tests of n-pentane oxidation were carried out in a laboratory glass flow-reactor, operating at atmospheric pressure, and loading 3 g of catalyst diluted with inert material. Feed composition was 1 mol% n-pentane in air residence time was 2 g s/ml. The temperature of reaction was varied from 340 to 420°C. The products were collected and analyzed by means of gas chromatography. A FlP-l column (FID) was used for the separation of C5 hydrocarbons, MA and PA. A Carbosieve Sll column (TCD) was used for the separation of oxygen, carbon monoxide and carbon dioxide. 

TABLE 6.16 Rate Constants at Room Temperature for OH - Aromatic Hydrocarbon Reactions and Branching Ratios for the Abstraction Reaction"... 

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