Paraffins kinetics

While the 13 hydrocarbon lumps accurately represent the hydrocarbon conversion kinetics, they must be delumped for the deactivation kinetics. In addition, delumping is necessary to estimate many of the product properties and process conditions important to an effective reformer process model. These include H2 consumption, recycle gas H2 purity, and key reformate properties such as octane number and vapor pressure. The following three lump types had to be delumped the C5- kinetic lump into Cl to C5 light gas components, the paraffin kinetic lumps into isoparaffin and n-paraffin components, and the Cg+ kinetic lumps into Cg, C9, C10, and Cn components by molecular type. 

Bychkov, V. Yu., Sinev, M. Yu., and Vislovskii, V.P Thermochemistry and reactivity of lattice oxygen in V-Sb oxide catalysts for the oxidative dehydrogenation of light paraffins. Kinet. Catal. 2001, 42, 574. 

Only 20—40% of the HNO is converted ia the reactor to nitroparaffins. The remaining HNO produces mainly nitrogen oxides (and mainly NO) and acts primarily as an oxidising agent. Conversions of HNO to nitroparaffins are up to about 20% when methane is nitrated. Conversions are, however, often ia the 36—40% range for nitrations of propane and / -butane. These differences ia HNO conversions are explained by the types of C—H bonds ia the paraffins. Only primary C—H bonds exist ia methane and ethane. In propane and / -butane, both primary and secondary C—H bonds exist. Secondary C—H bonds are considerably weaker than primary C—H bonds. The kinetics of reaction 6 (a desired reaction for production of nitroparaffins) are hence considerably higher for both propane and / -butane as compared to methane and ethane. Experimental results also iadicate for propane nitration that more 2-nitropropane [79-46-9] is produced than 1-nitropropane [108-03-2]. Obviously the hydroxyl radical attacks the secondary bonds preferentially even though there are more primary bonds than secondary bonds. 

Secondly, this mechanism (1,3-carbon-carbon bond activation) applies to both acyclic and cychc paraffins such as hexane and cyclohexane (Scheme 40 and Table 8). Kinetic studies on the hydrogenolysis of these alkanes are note-... 

Schultz and Linden Ind. Eng. Chem. Process Design and Development, 1 (111), 1962] have studied the hydrogenolysis of low molecular weight paraffins in a tubular flow reactor. The kinetics of the propane reaction may be assumed to be first-order in propane in the regime of interest. From the data below determine the reaction rate constants at the indicated temperatures and the activation energy of the reaction. 

The kinetics of hydrocracking reactions has been studied with real feedstocks and apparent kinetic equations have been proposed. First-order kinetics with activation energy close to 50 kcal/gmol was derived for VGO. The reactions declines as metal removal > olefin saturation > sulfur removal > nitrogen removal > saturation of rings > cracking of naphthenes > cracking of paraffins [102],... 

Formation of products in paraffin cracking reactions over acidic zeolites can proceed via both unimolecular and bimolecular pathways [4], Based on the analysis of the kinetic rate equations it was suggested that the intrinsic acidity shows better correlation with the intrinsic rate constant (kinl) of the unimolecular hexane cracking than with the apparent rate constant (kapp= k K, where K is the constant of adsorption equilibrium). In... 

It appears like a miracle how aliphatic chains (mainly olefins and paraffins) are formed from a mixture of CO and H2. But miracle means only high complexity of unknown order (Figure 9.1). Problems in FT synthesis research include the visualization of a multistep reaction scheme where adsorbed intermediates are not easily identified. Kinetic constants of the elemental reactions are not directly accessible. Models and assumptions are needed. The steady state develops slowly. The true catalyst is assembled under reaction conditions. Difficulties with product analysis result from the presence of hundreds of compounds (gases, liquids, solids) and from changes of composition with time. 

The detailed composition, referring to classes of compounds, is shown for C6 in Figure 9.3 with and without precolumn hydrogenation. In addition to paraffins, there are olefins—mainly with terminal double bond—and small amounts of alcohols (and aldehydes). The low detection limit of gas chromatography (GC) analysis allows precise determination even of minor compounds and provides exhaustive composition data also for use in kinetic modeling. Because of the short sampling duration of ca. 0.1 s,8 time-resolved selectivity data are obtained. 

By extending the FT model to the formation of two kinds of products—olefins and paraffins—and including secondary olefin reactions, the kinetic schemes shown in Figure 9.15 are obtained. In parallel primary reactions (from the growth sites), paraffins and alpha-olefins are desorbed—by irreversible associative desorption (the paraffins) and by dissociative desorption (the olefins) (upper scheme in Figure 9.15). 

A very serious problem was to clear up the formation of hydroperoxides as the primary product of the oxidation of a linear aliphatic hydrocarbon. Paraffins can be oxidized by dioxygen at an elevated temperature (more than 400 K). In addition, the formed secondary hydroperoxides are easily decomposed. As a result, the products of hydroperoxide decomposition are formed at low conversion of hydrocarbon. The question of the role of hydroperoxide among the products of hydrocarbon oxidation has been specially studied on the basis of decane oxidation [82]. The kinetics of the formation of hydroperoxide and other products of oxidation in oxidized decane at 413 K was studied. In addition, the kinetics of hydroperoxide decomposition in the oxidized decane was also studied. The comparison of the rates of hydroperoxide decomposition and formation other products (alcohol, ketones, and acids) proved that practically all these products were formed due to hydroperoxide decomposition. Small amounts of alcohols and ketones were found to be formed in parallel with ROOH. Their formation was explained on the basis of the disproportionation of peroxide radicals in parallel with the reaction R02 + RH. 

The initiating action of ozone on hydrocarbon oxidation was demonstrated in the case of oxidation of paraffin wax [110] and isodecane [111]. The results of these experiments were described in a monograph [109]. The detailed kinetic study of cyclohexane and cumene oxidation by a mixture of dioxygen and ozone was performed by Komissarov [112]. Ozone is known to be a very active oxidizing agent [113 116]. Ozone reacts with C—H bonds of hydrocarbons and other organic compounds with free radical formation, which was proved by different experimental methods. 

A comparative analysis of the kinetics of the reactions of atoms and radicals with paraffinic (R1 ), olefinic (R2H), and aromatic alkyl-substituted (R3H) hydrocarbons within the framework of the parabolic model permitted a new important conclusion. It was found that the tt-C—C bond occupying the a-position relative to the attacked C—H bond increases the activation energy for thermally neutral reaction [11]. The corresponding results are presented in Table 6.9. 

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... 

As mentioned earlier, practically all reactions are initiated by bimolecular collisions however, certain bimolecular reactions exhibit first-order kinetics. Whether a reaction is first- or second-order is particularly important in combustion because of the presence of large radicals that decompose into a stable species and a smaller radical (primarily the hydrogen atom). A prominent combustion example is the decay of a paraffinic radical to an olefin and an H atom. The order of such reactions, and hence the appropriate rate constant expression, can change with the pressure. Thus, the rate expression developed from one pressure and temperature range may not be applicable to another range. This question of order was first addressed by Lindemann [4], who proposed that first-order processes occur as a result of a two-step reaction sequence in which the reacting molecule is activated by collisional processes, after which the activated species decomposes to products. Similarly, the activated molecule could be deactivated by another collision before it decomposes. If A is considered the reactant molecule and M its nonreacting collision partner, the Lindemann scheme can be represented as follows ... 

In this particular case, the adsorption process can be used to overcome the distillation limitation. This is demonstrated in Figure 6.2, which represents the relative adsorption of C5 and C(, Hnear, branched and cycHc paraffins from the liquid phase of the 5A adsorbent used in the HOP GasoHne Molex process, licensed by HOP. In this process, only Hnear paraffins can enter the pores of 5A zeolite, while branched and cyclic paraffins are completely excluded due to their large kinetic diameters. Also, the selectivity for Hnear paraffins with respect to other types of paraffins is infinite. Consequently, the separation of Hnear paraffins from branched and cyclic paraffins becomes possible. 

Figure 6.2 illustrates the separation of n-Csis and non-n-Cs/is in CaA molecular sieves or 5A. The separation mechanism is obvious when the kinetic diameter of the molecules and molecular sieve pore size opening are compared. n-Csjc have kinetic diameters of less than 4.4 A which can diffuse freely into the 4.7 A pores of the CaA molecular sieve, while non-n-Cs/ have kinetic diameters of 6.2A. A commercial example of shape-selective adsorption is the UOP Molex process, which uses CaA molecular sieves to separate Cio-C n-paraffins from non- -parafHns (aromatics, branched, naphthenes). 

Haag, W.O., Dessau, R.M., and Lago, R.M. (1991) Kinetics and mechanism of paraffin cracking with zeolite catalysts. Stud. Surf Sd. Catal., vol. 60, Elsevier, Amsterdam, pp. 255-265. 

Exact temperature control is very important in polymerization reactions, since, among other things, the rate and degree of polymerization are strongly dependent on temperature. For accurate work, for example, for kinetic analysis with a dilatometer, a thermostat filled with water or paraffin oil may be used instead of thermostatting in the normal way with the aid of a contact thermometer and an immersion heater. 

Kinetics and Mechanism of Paraffin Cracking with Zeolite Catalysts... 

This kinetic expression has two limiting cases. At high temperature (Kq small) and/or low paraffin pressure and conversion ([O] smail) ... 

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