Kinetic Pyrolysis Reaction

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-... 

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. 

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. 

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 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-... 

There are several hundreds of papers studying the kinetics of reactions involved in the pyrolysis step (27). Unfortunately for the gasifier modelling there is neither a unified approach nor an overall kinetic equation(s) describing the pyrolysis step for all possible biomass under all possible circumstances. Besides, most of the kinetic studies have been made in thermobalances or related apparatuses with low heating rates (around 25 "C/min), far from the high heating rates in fluidised beds of up to a claimed 1000 C/s or even 10000 C/s. Kinetic equations obtained in thermobalances would provide results very different from the ones in fluidised bed, which is the present case. 

Graham R.G., Bergougnou M.A., Free B.A. (1994) The Kinetics of Vapour-Phase Cellulose Fast Pyrolysis Reactions. Biomass and Bioenergy, 1, 33-47. 

Four independent, parallel reactions with kinetic data, K1-K4, empirically derived from measurements of the mass loss of small samples of birch wood [9] have previously been shown to describe the pyrolysis reaction [I ]. 

During the pyrolysis process, the final conversion mainly depends on three phenomena the heat transfer from the reactor to the feedstock, the feedstock movement in the reactor and the kinetics of pyrolysis reactions. The heat transfer rate determines the rate of temperature increase of the feedstock. The feedstock movement behaviour determines the residence time of the feedstock particles in the reactor. In turn the heating rate and the residence time control the quantity of energy transferred and thus the ten Jerature distribution throughout the feedstock in the reactor. Once the tenqserature distribution is known, the kinetic behaviour of the feedstock determines the final conversion at the reactor outlet. 

The term on the left side of the equation represents the flow of internal energy in and out of the system, where m is determined by the kinetic Equations (1-9), Ac is determined by the total number of nodes generated, N, and the feedstock residence time in the reactor, which can be calculated by equations 4-7. The first term on the right side represents the heat transfer from the bottom heating plates TVs is the temperature of the heating plate and Ohai is the heat transfer coefficient which is determined by the heat transfer equations (Eq. 1-3), The second term is the radiation heat transfer contribution from the reactor wall. The last term represents the kinetic energy released during the pyrolysis reaction, which is assumed to be proportional to die rate of pyrolysis reaction (Eq.8-9). 

It is assumed that pyrolysis kinetics have a first-order ether dependence and that pyrolysis reactions are complementary to hydrolysis reactions. The first-order rate constant k follows now... 

Reference Kinetic Parameters of Pyrolysis Reactions (Units are kmol, m, s and kcal)... 

There is a great abundance of groundwork in the scientific literature on both the fundamental and applied chemical kinetics of pyrolysis reactions relating to... 

The kinetic aspect common to all the topics discussed in this chapter is the pyrolysis reactions. The same kinetic approach and similar lumping techniques are conveniently applied moving from the simpler system of ethane dehydrogenation to produce ethylene, up to the coke formation in delayed coking processes or to soot formation in combustion environments. The principles of reliable kinetic models are then presented to simulate pyrolysis of hydrocarbon mixtures in gas and condensed phase. The thermal degradation of plastics is a further example of these kinetic schemes. Furthermore, mechanistic models are also available for the formation and progressive evolution of both carbon deposits in pyrolysis units and soot particles in diffusion flames. 

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