Pyrolysis reaction rate constants

This paper presents an outline of the main ideas and results illustrating the fusion like behaviour of biomass pyrolysis. The ablative pyrolysis rate of wood rods applied on a hot spinning disk or on a stationnary heated surface is studied. The observations are quite similar to those made in the same conditions with rods of true melting solids (ice, paraffin,. ..) in agreement with a theoretical approach. The "fusion temperature of wood is found close to 739 K in agreement with a limited number of experiments of wood sawdust fast pyrolysis in a hot cyclone reactor. The results show also that in most of the experimental devices, the direct measurement of wood pyrolysis reaction rate constant is impossible above about 800 K. 

Benzofuroxan 79 can be generated from 2-nitrophenyl azide 80 (Scheme 49). Neighboring-group assistance within the pyrolysis leads to a one-step mechanism with an activation barrier of 24.6 kcal/mol at the CCSD(T)/6-31 lG(2d,p) level [99JPC(A)9086]. This value closely resembles the experimental one of 25.5 kcal/mol. Based on the ab initio results for this reaction, rate constants were computed using variational transition state theory. 

By reducing an elementary reaction model taken fi om the database, a comprehensive gas-phase reaction model of propane pyrolysis was derived objectively. The reaction rate constants that were not accurate under the conditions of interest were found and refined by fitting with the experimental results. The obtained reaction model well represented the effects of the gas residence time and temperature on the product gas composition observed in experiments under pyrocarbon CVD conditions. 

In order to extrapolate the laboratory results to the field and to make semiquantitative predictions, an in-house computer model was used. Chemical reaction rate constants were derived by matching the data from the Controlled Mixing History Furnace to the model predictions. The devolatilization phase was not modeled since volatile matter release and subsequent combustion occurs very rapidly and would not significantly impact the accuracy of the mathematical model predictions. The "overall" solid conversion efficiency at a given residence time was obtained by adding both the simulated char combustion efficiency and the average pyrolysis efficiency (found in the primary stage of the CMHF). 

Table 21.1 Apparent reaction rate constants for the pyrolysis of PE with a first-order reaction model...

This chapter focuses attention on reactors that are operated isotherraally. We begin by studying a liquid-phase batch reactor to determine the specific reaction rate constant needed for the design of a CSTR. After iilustrating the design of a CSTR from batch reaction rate data, we carry out the design of a tubular reactor for a gas-phase pyrolysis reaction. This is followed by a discussion of pressure drop in packed-bed reactors, equilibrium conversion, and finally, the principles of unsteady operation and semibatch reactors. 

Pyrolysis is a first order reaction so the temperature function of the reaction rate constant k and the half life time may be computed easily using the coefficients of the Arrhenius equation activation energy E and frequency factor A which had already been determined (see chapter 3.3.1, equations 3-7 and 3-8). Such data are the basis for the parameters of thermal conversion processes, such as temperature of the plant installations, housing time etc. 

Arrhenius equation gives the best description of the dependence of the reaction rate upon the temperature, for pyrolysis (cracking) and oxidation reactions. It is possible to extrapolate the reaction rate constant and the half life time to higher or lower temperatures. Therefore the coefficients of the Arrhenius equation, (activation energy E and the frequency (pre-exponential) factor A) were determined using the method according to ASTM E... 

The values of the real systems, obtained from experiments at pressures up to 50 bar, may be extrapolated to still higher pressures since E = f(P) and log A = f(F) are continuous functions. The supply of oxygen in the oxidation experiments at 50 bar pressure is sufficient to ensure attainment of the asymptotic limits at least in the first reaction step (LTO). Evaluation of the second reaction step of the oxidation (fuel deposition) is more difficult because an increase of the heating rate provokes the occurrence of additional peaks, which will be flattened as a consequence of a rise of the pressure. For the consecutive and parallel oxidation and pyrolysis reactions in this step, overall values of E and log A have been found, which only give steady functions for the vacuum residue. The data of the last reaction step (fuel combustion) may be evaluated very easily. They also give steady functions for E = f(P) and log A = f(P). All substances tested behave similarly to activated carbon (charcoal). Only the coke residue of -hexylpyrene reacts completely differently and demonstrates different curves in the plots of the reaction rate constant and the half life time versus the pressure. In this reaction step the curves did not reach the asymptote even at pressures of 50 bar, but they may be extrapolated to higher pressures. 

Recently, very fast intramolecular cyclization of singlet nitrene was observed upon photolysis of ort/ o-nitrophenyl azide (76). It is well known,that pyrolysis and photolysis of 76 leads cleanly to benzofuroxan (77). According to the results of recent computational stud) and early experiments, the pyrolysis of 76 produces 77 by a concerted one-step mechanism. However, photolysis of 76 produces 77 through a stepwise mechanism. Formation of singlet nitrene 78 from excited 76 was detected to occur with a time constant 500fs. The lifetime of nitrene 78 is very short - 8.3ps, and corresponds to ring-closure reaction rate constant 1.2 x 10" s. ... 

The most reliable method of preparing benzofuroxans is by decomposition of o-nitrophenyl azides. Decomposition can be achieved by irradiation, or more usually by pyrolysis temperatures between 100° and 1.50° are commonly used. Refluxing in glacial acetic acid is the recommended procedure for 4- or 5-sub-stituted 2-nitrophenyl azides, but with 3- or 6-substituted compounds higher boiling solvents are usually necessary. Quantitative studies on the reaction rate have been made, and a cyclic transition state invoked, an argument which has been used to account for the greater difficulty of decomposition of the 6-substituted 2-nitrophenyl azides. Substituent effects on the reaction rate have also been correlated with Hammett a constants, ... 

Our data give a rate constant of 4 X 10-13 cc. molecule-1 sec.,-1 assuming the production of N02- solely by this reaction. However, one must consider the probability that N02, present as a minor impurity or produced by pyrolysis or N20 of or near the hot filament, would react by Reaction 20. 

Below 565 °C the pyrolysis reaction is essentially first order with a rate constant given by... 

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-... 

The TGA system was a Perkin-Elmer TGS-2 thermobalance with System 4 controller. Sample mass was 2 to 4 mgs with a N2 flow of 30 cc/min. Samples were initially held at 110°C for 10 minutes to remove moisture and residual air, then heated at a rate of 150°C/min to the desired temperature set by the controller. TGA data from the initial four minutes once the target pyrolysis temperature was reached was not used to calculate rate constants in order to avoid temperature lag complications. Reaction temperature remained steady and was within 2°C of the desired temperature. The actual observed pyrolysis temperature was used to calculate activation parameters. The dimensionless "weight/mass" Me was calculated using Equation 1. Instead of calculating Mr by extrapolation of the isothermal plot to infinity, Mr was determined by heating each sample/additive to 550°C under N2. This method was used because cellulose TGA rates have been shown to follow Arrhenius plots (4,8,10-12,15,16,19,23,26,31). Thus, Mr at infinity should be the same regardless of the isothermal pyrolysis temperature. A few duplicate runs were made to insure that the results were reproducible and not affected by sample size and/or mass. The Me values were calculated at 4-minute intervals to give 14 data points per run. These values were then used to... 

Researchers in previous studies generally used lst-order kinetics to describe cellulose pyrolysis, but rarely have they examined 2nd-order kinetics. Thus, discussion of our results for untreated samples will concentrate on lst-order rate constants so that our results can be directly compared with results from prior studies. A true reaction order of cellulose pyrolysis based on TGA data is essentially meaningless, however, since mass loss involves complex competing multiple reactions (2,4,8). In addition, reaction order was calculated on a dimensionless mass value rather than on the correct but uncalculable molar concentration term. 

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... 

Rate constants for elementary reactions involving tin compounds are even rarer. In an important recent investigation, Takahashi et al. performed shock-tube measurements and RRKM analysis from which they obtained rate constants for several important reactions involved in mechanism for SnCU pyrolysis [50] ... 

Some semi-quantitative confirmation of these A factors comes from the consideration that the pyrolysis of C2H8 at 900°K. is a chain reaction in which the data on maximal inhibition indicate a chain length X of the order of 10. Since the only likely homogeneous, initiation process is the fission of C2H8 into 2CH3, the hypothetical first-order rate constant for the pyrolysis can be set equal to this initiation rate constant multiplied by X ... 

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 —... 

Somewhere in the temperature range 450° to 600°C. pyrolysis must compete on nearly equal terms with oxidation of alkyl radicals. The work of Baldwin is therefore particularly important since the rate constants for pyrolysis of alkyl radicals are reasonably well established. There is therefore the strong possibility that we shall soon possess rate constants for oxidation reactions of alkyl radicals at high temperatures. Examination of the oxidation products of the higher alkanes by the Baldwin method should go far toward resolving the problem of the source of fragmentation products at lower temperatures. 

P.R. Westmoreland, J.B. Howard, J.P. Long well, and A.M. Dean. Predictions of Rate Constants for Combustion and Pyrolysis Pressure and Temperature Effects Reactions by Bimolecular QRRK. AIChEJ., 32(12) 1971-1979,1986. 

---