Pyrolysis kinetic expressions

Analyses of rate measurements for the decomposition of a large number of basic halides of Cd, Cu and Zn did not always identify obedience to a single kinetic expression [623—625], though in many instances a satisfactory fit to the first-order equation was found. Observations for the pyrolysis of lead salts were interpreted as indications of diffusion control. More recent work [625] has been concerned with the double salts jcM(OH)2 yMeCl2 where M is Cd or Cu and Me is Ca, Cd, Co, Cu, Mg, Mn, Ni or Zn. In the M = Cd series, with the single exception of the zinc salt, reaction was dehydroxylation with concomitant metathesis and the first-order equation was obeyed. Copper (=M) salts underwent a similar change but kinetic characteristics were more diverse and examples of obedience to the first order, the phase boundary and the Avrami—Erofe ev equations [eqns. (7) and (6)] were found for salts containing the various cations (=Me). 

A few authors have attempted to extract reaction kinetics from their experimental work. Kinetic expressions reported in the literature include the kinetics of both PTFE pyrolysis and various gas-phase reactions. A summary of these kinetics is presented in Table 5.2. 

These processes can occur in parallel or subsequently, depending strongly on the process configuration. All of them would require dedicated kinetic expressions. In practice, assumptions are taken to simplify the formulation of the kinetic model (e.g., neglecting tars, inert ash, one pyrolysis gross reaction). 

The isothermal pyrolysis in the presence of air proceeds at a much faster rate and higher weight losses are obtained as compared to vacuum pyrolysis at the same temperature. The first order rate constant obtained is linearly related to the expression [%LOR + o-(% crystallinity)]//o with a degree of correlation r = 0.923, where a is the accessible surface fraction of the crystalline regions according to Tyler and Wooding [501], and / is the orientation factor. No correlation could be found with DP due to very rapid depolymerization. The fact that the rate is inversely proportional to the orientation and that it decreases with the increase in the thickness of the fibers indicates that the rate of the diffusion of the oxygen into the fibers controls the kinetics and that oxidation is the predominant process in air pyrolysis. 

The kinetics of coal pyrolysis are complicated because of the numerous components or species which are simultaneously pyrolyzed and decomposed. The chemical expression in coal pyrolysis is represented briefly by an overall reaction ... 

When the pyrolytic process does not occur in gas phase, different problems appear. Although equations of the type (6) with k expressed by rel. (5) or (14) can be used in certain cases, these may lead to incorrect results in many cases. Various empirical models were developed for describing the reaction kinetics during the pyrolysis of solid samples. Most of these models attempt to establish equations that will globally describe the kinetics of the process and fit the pyrolysis data. Several models of this type will be described in Section 3.3. A different approach can be chosen, mainly for uniform repetitive polymers. In such cases, a correct equation can be developed for the description of the reaction kinetics. This is based on the study of the steps occurring during pyrolysis involving a free radical chain mechanism. The subject will be discussed in some detail in Section 3.4. 

The temperature dependence of the rate of reactions is particularly Important for the pyrolytic processes. Relation (5) can be used for the understanding of the common choices for the pyrolysis parameters. As an example, we can take the pyrolysis of cellulose [8]. Assuming a kinetics of the first order (pseudo first order), the activation energy in Arrhenius equation was estimated E = 100.7 kJ / mol. The frequency factor was estimated 9.60 10 s These values will lead to the following expression for the weight variation of a cellulose sample during pyrolysis ... 

Table 3.2.1. Calculated values for WAA/q for pyrolysis of cellulose expressed in %, assuming a first order kinetics and 10 s pyrolysis time (THT).

For pyrolytic reactions, the variation of the molar concentration [A] of a substance during the pyrolysis is not always the most appropriate parameter to be monitored. The calculation of [A] can be a problem for many types of samples, and very frequently during pyrolysis not only one decomposition process takes place. In this case, the overall reaction kinetics must be considered. A more convenient parameter for monitoring pyrolytic reactions is, for example, the sample weight. For a reaction of the first order, by multiplying rel. (2.3.2) with the volume V and the molecular weight M of the substance A, since W = [A] V M, the following expression is obtained ... 

The stepwise dissociation energies Z)(FO—F) and 2)(0— ) were calculated from the data as 40 1 and 50.8 2.0 kcal. These are in excellent agreement with the recent figures obtained by Cl)me and Watson. Houser has expressed scepticism for the kinetic data for this pyrolysis reaction as deduced from the shock-tube experiments of Lin and Bauer. Houser places more trust in the kinetic data obtained from earlier studies using conventional techniques. 

Monnery WD, Hawboldt KA, Pollock A, Svrcek WY (2000) New experimental data and kinetic rate expression for H2S pyrolysis and re-association. <2hem Eng Sci 55 957-966... 

The similar pyrolysis mechanisms of benzoic acid and benzaidehyde are proposed in Figures 8.6 and 8.7 [19] , the optimized structures and their atom numbers of reactants, intermediates, transition states, and products are shown in Figures 8.8 and 8.9 and the energy profiles of the stationary points for benzoic acid and benzaidehyde pyrolysis reactions are shown in Figures 8.10 and 8.11. According to the transition state theory [64], activation enthalpy activation entropy and activation energy can be obtained from Eqs (8.1 )- 8.3), respectively. The rate constant k can be expressed as shown in Eq. (8.4). These kinetic parameters are listed in Table 8.1. 

Product Yield and Intensity Function of the Pyrolysis Condition" kinetic considerations on the Arrhenius expression of the rate constant suggest that, in order to obtain a given conversion of reactant, reaction temperature and residence time are interchangeable to some extent. The intensity function proposed by Linden (3) for pyrolysis of hydrocarbons ( ) is, accordingly, reasonable in principle. 

Pyrolysis of Biomass The kinetic parameters of the pyrolysis of solid materials can be determined by means of non-isothermal thermogravimetric measurements. The rate of release of volatile matter may be described by a simple first-order expression ... 

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