Pyrolysis rate constants

Assuming that pyrolysis selectivity Is not affected by SC water, rate constants of pyrolysis at various SC water densities were calculated with naphthalenes as pilot compounds and the "dry experiment as the reference pyrolysis rate. The rate constant of hydrolysis ki, 2 subsequently followed as the difference of total ether conversion rate constant and pyrolysis rate constants. 

Figure 12. Correlation of pyrolysis rate constant with ionic strength and solvent density Symbol legend Figure 10...

The pyrolysis of CR NH (<1 mbar) was perfomied at 1.3 atm in Ar, spectroscopically monitoring the concentration of NH2 radicals behind the reflected shock wave as a fiinction of time. The interesting aspect of this experiment was the combination of a shock-tube experiment with the particularly sensitive detection of the NH2 radicals by frequency-modulated, laser-absorption spectroscopy [ ]. Compared with conventional narrow-bandwidth laser-absorption detection the signal-to-noise ratio could be increased by a factor of 20, with correspondingly more accurate values for the rate constant k T). 

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. 

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. 

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. 

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-... 

Thermogravimetric analysis (TGA) measures cellulose pyrolytic mass loss rates and activation parameters. The technique is relatively simple, straightforward and fast, but it does have disadvantages. One disadvantage is that determination of the kinetic rate constants from TGA data is dependent on the interpretation/analysis technique used. Another disadvantage of TGA is that the rate of mass loss is probably not equivalent to the cellulose pyrolysis rate. 

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. 

The substitution of bromide for ammonia in [Au(NH3)4]3+ occurs in a stepwise fashion to give [AuBr(NH3)3]2+, fra/w-[AuBr2(NH3)2]+, [AuBr3(NH3)] and [AuBr4], and the corresponding second-order rate constants follow the sequence ki < k2 k3 complex trans-[AuBr2(NH3)2]Br can be isolated from partially reacted mixtures.570 The complex [AuC13(NH3)] has been isolated in a different way by pyrolysis of NH4[AuCU], and its structure has been determined.119... 

The vacuum pyrolysis of thin samples of polytetrafluoroethylene follows first-order kinetics with monomer as the major decomposition product in the temperature range from 360—510° C (Siegle, Muus, and Lin). The rate constant does not depend on either the molecular weight or the type of polymer and is characterized by an activation enthalpy of 83.0 kcal/mole and a frequency factor of 3 x 1019 sec-1. The melt viscosity decreases during pyrolysis. 

Thne-of-flight (TOF) mass spectrometric analysis of the pyrolysis fragments of di-t-butyl peroxide suggests t-BuCO as the primary product, followed by decomposition of this radical into CHj.253 Elsewhere, the kinetics of the pyrolysis of dimethyl, diethyl, and di-t-butyl peroxides in a modified adiabatic bomb calorimeter have been investigated.254 The lifetime of acyloxy radicals, generated by the photolysis or thermolysis of acetyl propionyl peroxide, have been studied. Chemical nuclear polarization has been used to determine the rate constant for the decarboxylation of these radical intermediates.255... 

The products of the thermolysis of 3-phenyl-5-(arylamino)-l,2,4-oxadiazoles and thiazoles have been accounted for by a radical mechanism.266 Flash vacuum pyrolysis of 1,3-dithiolane-1-oxides has led to thiocarbonyl compounds, but the transformation is not general.267 hi an ongoing study of silacyclobutane pyrolysis, CASSF(4,4), MR-CI and CASSCF(4,4)+MP2 calculations using the 3-21G and 6-31G basis sets have modelled the reaction between silenes and ethylene, suggesting a cyclic transition state from which silacyclobutane or a trcins-biradical are formed.268 An AMI study of the thermolysis of 1,3,3-trinitroazacyclobutane and its derivatives has identified gem-dinitro C—N bond homolysis as the initial reaction.269 Similar AMI analysis has determined the activation energy of die formation of NCh from methyl nitrate.270 Thermal decomposition of nitromethane in a shock tube (1050-1400 K, 0.2-40 atm) was studied spectrophotometrically, allowing determination of rate constants.271... 

Reactivities and activation parameters for pyrolytic unimolecular first-order elimination reactions of A -acetylurea, A -acetylthiourca, /V,7V -diacetylthiourea and N-acetylthiobenzamide have been interpreted with reference to those for other amide derivatives.55 The first-order rate constants for pyrolysis of RCONHCSNHCe R (R = Me, R = H R = Ph, R = H, 4-N02, 3-C1, 4-C1, 4-Me) have also been measured at 423-500 K and correlated with Hammett [Pg.378]

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