Pyrolysis Reaction, Kinetics

Numerous kinetic mechanisms have been proposed for oil shale pyrolysis reactions (11—14). It has been generally accepted that the kinetics of the oil shale pyrolysis could be represented by a simple first-order reaction (kerogen — bitumen — oil), or... 

Dente and Ranzi (in Albright et al., eds.. Pyrolysis Theory and Industrial Practice, Academic Press, 1983, pp. 133-175) Mathematical modehng of hydrocarbon pyrolysis reactions Shah and Sharma (in Carberry and Varma, eds.. Chemical Reaction and Reaction Engineering Handbook, Dekker, 1987, pp. 713-721) Hydroxylamine phosphate manufacture in a slurry reactor Some aspects of a kinetic model of methanol synthesis are described in the first example, which is followed by a second example that describes coping with the multiphcity of reactants and reactions of some petroleum conversion processes. Then two somewhat simph-fied industrial examples are worked out in detail mild thermal cracking and production of styrene. Even these calculations are impractical without a computer. The basic data and mathematics and some of the results are presented. 

A great many reactions follow first-order kinetics or pseudo first-order kinetics over certain ranges of experimental conditions. Among these are many pyrolysis reactions, the cracking of butane, the decomposition of nitrogen pen-toxide (N205), and the radioactive disintegration of unstable nuclei. 

Kinetic Data. The pyrolysis reaction obeys first-order kinetics with a rate constant equal to 3.98 x 1012e 59,loo/jR r sec-1, where T is ex-... 

In most of the other investigations of boron hydride pyrolysis no kinetic data are reported usually only the reaction products and in some cases also active intermediates are detailed. For these studies the reader is referred to existing literature reviews81,85. 

Dibenzothiophene has been isolated in low yield from the pyrolysis of benzene and sulfur dioxide in glass ampuls at between 400° and 540° and the reaction kinetics determined. 

Graphical Interface for the Study of Gas-Phase-Reaction Kinetics Cyclopentane Vapor Pyrolysis 230... 

D.S. Ross et al, Study of the Basic Kinetics of Decomposition. . , AFRPL-70-29, SRI, Menlo Park, Contract F04611-69-C-0096 (1970) [From their work the authors conclude that there is no way to distinguish between the very low pressure pyrolysis reactions UDMH - NH3+CH2 N-CH2 (1) and UDMH ->(CH3)2N. +.NH2 (2). The reported pyrolysis fall-off rate constants kx are listed as log k(1 = 13.0 —... 

The unit of the velocity constant k is sec-1. Many reactions follow first order kinetics or pseudo-first order kinetics over certain ranges of experimental conditions. Examples are the cracking of butane, many pyrolysis reactions, the decomposition of nitrogen pentoxide (N205), and the radioactive disintegration of unstable nuclei. Instead of the velocity constant, a quantity referred to as the half-life t1/2 is often used. The half-life is the time required for the concentration of the reactant to drop to one-half of its initial value. Substitution of the appropriate numerical values into Equation 3-33 gives... 

The basic assumption inherent to heat transfer limited pyrolysis models is that heat transfer rates, rather than decomposition kinetics, control the pyrolysis rate. Consequently, thermal decomposition kinetics do not come into play, other than indirectly through specification of Tp. This approximation is often justified on the basis of high activation energies typical of condensed-phase pyrolysis reactions, i.e., the reaction rate is very small below Tj, but then increases rapidly with temperature in the vicinity of Tp owing to the Arrhenius nature, and the high activation energy, of the pyrolysis reaction. 

In kinetically limited models, the pyrolysis rate is no longer calculated solely from a heat balance at the pyrolysis front. Instead, the rate at which the condensed-phase is volatilized depends on its temperature. This gives a local volumetric reaction rate (kg/m3-s) by assuming that all volatiles escape instantaneously to the exterior gas-phase with no internal resistance, the fuel mass flux is obtained by integrating this volumetric reaction rate in depth. One consequence is that the pyrolysis reaction is distributed spatially rather than confined to a thin front as with heat transfer limited models and the thickness of the pyrolysis front is controlled by decomposition kinetics and heat transfer rates. For a pyrolysis reaction with high activation energy or for very high heat transfer rates, the pyrolysis zone becomes thin, and kinetically limited models tend toward heat transfer limited models. 

Polysaccharide pyrolysis at 375-520°C is accompanied by a higher rate of weight loss and evolution of a complex mixture of vapor-phase compounds preponderantly of HsO, CO, C02, levoglucosan, furans, lactones, and phenols (Shafizadeh, 1968). The volatile and involatile phase compositions are conditional on the rate of removal of the vapor phase from the heated chamber (Irwin, 1979), inasmuch as the primary decomposition products are themselves secondary reactants. The reaction kinetics is described as pseudo zero order (Tang and Neill, 1964) and zero order initially, followed by pseudo first order and first order (Lipska and Parker, 1966), suggesting an... 

In the following sections some background information on stiff ordinary differential equations will be given and the general finite difference approximations for particle temperatures will be derived. Later, the technique will be applied to coal pyrolysis in a transport reactor where the difference equations for reaction kinetics will be discussed and the calculation results will be compared with those obtained by the previously established techniques. 

A comparison with industrial data shows that the kinetic data from Table 7.5 gives somewhat conservative results. The temperature should be raised to more than 550 °C to achieve conversions of about 50%. It is known that modern processes operate at much lower temperatures and make use of initiators. To obtain more realistic results the pre-exponential factor for the pyrolysis reaction was modified to 1.14 x 1014, while the pre-exponential factor of the acetylene production increased to 5 x 10+14. The reactions (23) to (25) were neglected, while the reactions (26) to (31) were accounted for by a stoichiometric approach. 

Keywords carbon nanofibres, pyrolysis, x-ray diffraction, reaction kinetics... 

In conclusion, pseudo-kinetic models cannot be extrapolated beyond the range of the experimental data they are derived from, cannot incorporate the progress achieved in the whole field of fundamental chemical kinetics, both experimental and theoretical, and cannot be used for designing new reactors. In all these domains, mechanistic simulation is obviously superior, at least theoretically, and this seems also to be true in practice. Indeed, Goossens et al. [77—79] have carried out a comparison of the value for prediction of their mechanistic model and of the molecular reaction schemes proposed by Ross and Shu [55] and Sundaram and Froment [60]. Goossens et al. concluded that there is an actual superiority of the mechanistic model. Froment himself now seems to agree with this conclusion since, after having developed the molecular reaction schemes with co-workers [57—61], he and Sundaram [186] have lately proposed free radical schemes for pyrolysis reactions. 

The description of reaction kinetics for circumstances where an exceptionally large number of species are involved or where well-defined molecular entities cannot be specified poses a particularly difficult challenge. Such situations can arise, for example, in the cracking of petroleum, the pyrolysis of biomass, and the liquefaction of coal. The best way to treat such systems is to either lump classes of reactions or follow the dynamics of functional groups rather than specific molecular species. 

The kinetics of these pyrolysis reactions were followed by several complementary methods under conditions as close to the product studies as possible. The most frequently-used ampule technique14 17) with gc analysis of 5 and the scavenger technique, with chloranil or Koelsch radical as scavenger 18), for very labile compounds 5 were complemented by the DSC method, in which the heat flow under conditions of linear temperature increase is analysed. It proved to be a particularly convenient and reliable technique 18- 21). Rates were followed over a temperature span of at least 40 °C with temperature control of 0.1-0.2 °C. All rate data and activation parameters were subjected to a thorough statistical analysis including statistical weights of errors. The maximum statistical errors in k were 3%, in AH 1 kcal mol-1 in AS 513 e.u. and in AG (at the temperature of measurement) g0.5 kcal mol-1. 

This mathematical model describing the inhomogeneous pyrolysis reactions by a set of apparent kinetic data (which are changing with the progress of the pyrolysis) should be understood as a first attempt to set up a mechanism to predict pyrolysis. The target of the application of this model would be to evaluate the influence of temperature programs on baking behavior. 

Because pyrolysis reactions do not occur at sharply defined temperatures, the heating rate has a marked effect on the nature and distribution of pyrolysis products, as summarized in Table 19.14. Solomon and coworkers conducted extensive work on the kinetics of coal devolatilization, and many reviews are available.36... 

The first general mechanism to account for the presumed first-order kinetics of these and other organic pyrolysis reactions was proposed by... 

The work on ethane indicates quite strikingly that the same saying applies to pyrolysis. Our present understanding of pyrolysis reactions, which is not inconsiderable, has for the most part derived, and will continue to derive, from the studies of the thermodynamic and kinetic properties of individual free radicals observed in very carefully selected model systems. 

The kinetics of these pyrolysis reactions were followed by several complementary methods under conditions as close to the product studies as possible. The most frequently-used ampule technique with gc analysis of 5 and the scavenger technique,... 

Thus, before the rate of coke formation can build up to a steady level, the coke precursors must reach some suitable concentration. At this point, the most logical candidates for coke precursors are the / -resins. Plotting the amount of coke formed as a simple function of the quantity of / -resins produces the curve shown in Figure 5. The / -resins/coke data obtained at 800°, 825°, and 850°F (430°, 440°, and 450°C, respectively) lie approximately on the same curve, while the data obtained at 980°F (530°C) follow a separate curve. At relatively low levels of / -resin formation the coke concentration increases only slowly, but as the /3-resin concentration approaches 14-16%, the amount of coke formed rises rapidly and the two curves converge. In the pyrolysis runs where the reactions were terminated before the / -resin concentration had risen much above 10-12%, a substantial portion of the / -resin producing reaction follows first-order kinetics. However, if the pyrolysis reaction is allowed to continue, the concentration of / -resins levels off at about 16-17%, regardless of any further reaction, and the first-order relationship no longer... 

The fact that flame retardants and salts alter the kinetics, as well as the products, of the pyrolysis reactions is confirmed by the investigations of Tang and Neil involving thermogravimetric and differential thermal analysis methods (see Section 11,6 p. 446). These investi-... 

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