Kinetics pyrolysis

Pyrolysis of biomass is a typical gas-solid reaction in the absence of oxygen (anaerobic process) and with very low contact time of vapor. The solid material contained in 

Assuming the model of the unreacted core layer, as deduced in Chapter 10 (10.8), we obtain  

1 Calculate the particle size necessary for the decomposition of cashew nuts by observing the weight loss in function of the time at a constant temperature of 450°C. 

Diffusion coefficient D = 0.1 cm /s Average molecular weight M = 100 g/gmol Initial concentration of [N2]o = 0.4 moUcm  

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Fig. 1. Pyrolysis kinetic results of HDPE mixed with Al-MCM-41...

Thermogravimetric analysis (TGA) has often been used to determine pyrolysis rates and activation energies (Ea). The technique is relatively fast, simple and convenient, and many experimental variables can be quickly examined. However for cellulose, as with most polymers, the kinetics of mass loss can be extremely complex (8 ) and isothermal experiments are often needed to separate and identify temperature effects (9. Also, the rate of mass loss should not be assumed to be related to the pyrolysis kinetic rate ( 6 ) since multiple competing reactions which result in different mass losses occur. Finally, kinetic rate values obtained from TGA can be dependent on the technique used to analyze the data. 

Cellulose pyrolysis kinetics, as measured by isothermal TGA mass loss, were statistically best fit using 1st- or 2nd-order for the untreated (control) samples and 2nd-order for the cellulose samples treated with three additives. Activation parameters obtained from the TGA data of the untreated samples suggest that the reaction mechanism proceeded through an ordered transition state. Sample crystallinity affected the rate constants, activation parameters, and char yields of the untreated cellulose samples. Various additives had different effects on the mass loss. For example, phosphoric acid and aluminum chloride probably increased the rate of dehydration, while boric acid may have inhibited levoglucosan... 

In studying pyrolysis kinetics, Liliedahl et al. (1991) determined the RTD of their apparatus by pulse-injection of a gaseous hydrocarbon tracer. 

Similarly, pyrolysis kinetics was studied mano-metrically by Hauser and Lee (Ref 20). If the components of the reaction are in solution, titrametric methods have been used (Ref 5). 

Pyrolysis Kinetics of Ethyl Nitrate , JPhysChem 71 (11) (1967V 3422-26 21) DX. 

Reactant conversion passed through a minimum as the solvent loading increased while the product spectrum shifted from pyrolysis-like to solvolysis-like. The observed kinetics were consistent with two mechanistic interpretations. The first involves parallel pyrolysis and solvolysis steps, each having pressure-dependent rate constants. The second assumes the formation of a solvent cage, which suppresses the pyrolysis kinetics as the solvent loading increases from zero to above its critical density. 

For combustion reactions Levenspiel (4) gives the constant temperature integration for reaction and gas or ash diffusion controlled cases. The integration of the pyrolysis kinetics will be demonstrated in the following section. 

The polar nature of these reactions finds additional support in the pyrolysis kinetics of vinyl fluoride (Table 1) where the C—F bond polarization, in thesense C5+- -X5", is the limiting factor. Due to the delocalization of the p electrons on the fluorine of vinyl fluoride (la<- lb), the Ca—F bond polarization in the transition state is reduced. Cleavage of the C—F bond becomes more difficult, making the dehydrofluorination of vinyl fluoride slower than that of ethyl fluoride ... 

It is a known fact that the gas-phase pyrolysis kinetics of alkyl bromides have not been extensively investigated due to the experimental difficulties as well as to the complexity of concurrent unimolecular and radical chain mechanisms. However, when these organic bromides are pyrolyzed under maximum inhibition, the reaction in the presence of a free radical suppressor is a molecular elimination. Sometimes, these organic bromides are pyrolyzed under maximum catalysis with HBr gas, and the process may proceed by an autocatalytic molecular mechanism. 

An interesting work on neighboring group participation in the gas phase is the pyrolysis kinetics of l-chloro-4-(methylthio)butane which forms quantitatively tetrahydrothio-phene and methyl chloride179. In addition to the formation of the cyclic product, the rate of decomposition was also rather high as shown in Table 29. 

Solomon, P. R., and Serio, M. A., Evaluation of Coal Pyrolysis Kinetics, in Fundamentals of Physical Chemistry of Pulverized Coal Combustion, J. Lahaye and G. Prado (Eds ), Martinus Nijhoff Publishers, 1987. 

In actual industrial practice, these figures are relatively meaningless, since actual pyrolysis kinetics are normally determined by the rate of heat transfer [10], A number of interesting simulations of the effect of heat transfer upon apparent rates were performed by both Homung et al. and by Vergneaud in the framework of the E.U. Cycleplast project [6]. 

Y.-H. Lin, P. Sharratt, G. Manos, A. Garforth and J. Dwyer Catalytic Pyrolysis Kinetics of High Density Polyethylene by TGA. Proceedings of the 1996 IChemE Research Event IChemE, 576 (1996). 

J. H. Chen K. S. Chen and L. Y. Tong, On the pyrolysis kinetics of scrap automotive tires. Journal of Elazardous Materials, 84, 43-55 (2001). 

Xing Ji, Jianqiu Wang, and Jiaji Qian, Study on the pyrolysis kinetics of polypropylene by seqnential pyrolysis gas chromatography. Petrochemical Technology, 28(2), 82-86 (1999). 

Mechanistic modeling has been useful in studying pyrolysis kinetics at low conversion (4,5,6). Few attempts have been reported at the high conversion levels of commercial cracking (7). This stems from the large number of species and free radicals and of their associated reactions, which increases substantially with conversion and leads to excessive computation time. In addition, when one considers that precise pyrolysis mechanisms, for even a simple feedstock such as propane (8), are still subject to dispute, it is clear that more empirical models will continue to dominate commercial applications. 

Within the context of a study aimed at the determination of the pyrolysis kinetics of polymers using a laminar entrained flow reactor (LEFR), Westerhout et al (1996) developed a comprehensive model that couples a single-particle con-... 

Boudou J. P. and Espitalie J. (1995) Molecular nitrogen from coal pyrolysis kinetic modeling. Chem. Geol. 126, 319-333. 

Pyrolysis kinetics for cylindrical 12 mm diameter straw pellets in a fluidised bed of sand at different temperatures are shown in Fig. 8. The long pyrolysis times around 100 s are a consequence of the large particle dimension. This demonstrates, that pelletisation is not an advantage it is expensive and destroys the high reactivity of untreated straw with thin ca. 0.5 mm thick walls. 

Fig. 8 Pyrolysis kinetics for cylindrical 12 mm straw pellets in a fluidised bed [20]...

As shown in Fig. 3 for different pyrolysis kinetics and a spherical particle which is exposed to a raAation flux from a blackbody dose to Ae isokinetic temperature, Ae time for pyrolysis may vary by a factor of 3. The Afference between Ae different kinetics increases wiA increasing diameter, because of Ae higher temperature difference over Ae particle. 

Antal M.J. and Vdrhegyi G. (1995) Cellulose pyrolysis kinetics The current state of knowledge. Ittd. ng. Chem. Res., 34,703-17. 

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