Kinetic parameters, exothermic reaction

Controlled elimination of mass and heat transport resistances is an important prerequisite for obtaining intrinsic kinetic parameters of the fast exothermic reaction of partial oxidation of methane to synthesis gas. It has been demonstrated that under conditions of strong transport limitations erroneous conclusions concerning the reaction scheme can be derived [7-9]. It was determined in this laboratory that transport limitations are practically absent over a wide range of operating conditions if one portion of the catalyst (< 40 pm) is diluted with -5 portions of an... 

This reaction is very exothermic (A// —180 to —200kJ mol-1) and, therefore, seems to be very probable from the thermochemical point of estimation. The pre-exponential factor is expected to be low due to the concentration of the energy on three bonds at the moment of TS formation (see Chapter 3). To demonstrate that this reaction is responsible for the oxidative destruction of polymers, PP and PE were oxidized in chlorobenzene with an initiator and analyzed for the rates of oxidation, destruction (viscosimetrically), and double bond formation (by the reaction with ozone) [131]. It was found that (i) polymer degradation and formation of double bonds occur concurrently with oxidation (ii) the rates of all three processes are proportional to v 1/2, (iii) independent of p02, and (iv) vs = vdbf in PE and vs = 1.6vdbf in PP (vdbf is the rate of double bond formation). Thus, the rates of destruction and formation of double bonds, as well as the kinetic parameters of these reactions, are close, which corroborates with the proposed mechanism of polymer destruction. Therefore, the rate of peroxyl macromolecules degradation obeys the kinetic equation ... 

Expls and proplnts decomp exothermally at every temp above absolute zero. If the mass of the material is such that the heat produced by the decompn cannot be dissipated as rapidly as it is produced, the mass will heat itself to expln. The lowest constant surface temp above which a thermal expln is initiated is a function of the size, thermal conductance, the heat of reaction, and the reaction kinetic parameters. 

In this equation, CA is the concentration (in mol m 3) of the rate-limiting component A, k is the nth-order rate constant (with units m3(" lf mol1-" s-1), n is the order of the reaction and rA is the rate of reaction (units, mol m 3 s 1). As already mentioned, in the field of reaction calorimetry, qRe lC is generally defined as positive for an exothermic reaction (negative A rH). The aim of the determination is to calculate the kinetic parameters k and (possibly) n. Some methods also determine the thermodynamic parameter ArH on the basis of this reaction model. 

Schneider, M.A. and Stoessel, F. (2005) Determination of the kinetic parameters of fast exothermal reactions using a novel microreactor-based calorimeter. Chemical Engineering Journal, 115, 73-83. 

TABLE 2.3 Reversible Exothermic Reaction Kinetic Parameters... 

Theoretical research into the problem of combustion propagation on the surface of condensed fuels involving heterogeneous oxidation has been reported It has been found that the combustion velocity depends on thermal effects, kinetic parameters of the exothermic oxidation reaction on the surface, and on the rate of... 

The extents of cure were determined utilizing DSC and FT-IR. DSC experiments were conducted on a Perkin-Elmer DSC-2 instrument. The method for obtaining kinetic parameters from scanning DSC has been described elsewhere (8). Isothermal extents of cure were determined by curing in the DSC for a predetermined time under nitrogen and then scanning at 10 K/min to determine the residual exothermic heat of reaction as compared with a scan of uncured resin. 

Section 6 deals with the autocatalytic reactions of inorganic and organic compounds with molecular oxygen in the liquid phase, and the highly exothermic processes in the gas phase, collectively known as combustion, which may involve oxygen, other oxidants or decomposition flames and are so important technologically. Catalysis, retardation and inhibition are covered. The kinetic parameters of the elementary steps involved are given, when available, and the reliability of the data discussed. 

For any interpretation of kinetic parameters from chemical measurements of exothermic elementary reactions it is essential that the reaction be performed under isothermal conditions so that the reactor temperature itself is the control parameter. This is normally the case in experiments which are set up to investigate elementary reactions. However, low-tern-... 

Displacement reactions, such as those conceivably involved in heteroaromatic substitutions, are not well correlated with the overall exothermicity of the process and are found to depend in detail on the specific ionic species, the electronic properties of the substrate, and the nature of the leaving group. In other words, the kinetic parameters and the reaction mechanism of these... 

The sensitivity of this method is directly related to the apparent molar enthalpy of reaction, so that very endo- or exothermic reactions will be most readily followed. Examples of the application of this method to the determination of enzyme kinetic parameters include dihydrofolate reductase, creatine phosphokinase, hexo-kinase, urease, trypsin, HIV-1 protease, heparinase, and pyruvate carboxylase. 

Because the epoxide hydrolysis reaction is exothermic, isothermal conditions were critical to obtain good data in this parameter study. A solvent (1,2-dichloro-benzene) was used to dilute the reaction mixture in order to help maintain a constant reaction temperature. This proved especially useful when exploring higher catalyst concentrations that accelerate the rate of reaction. It will be demonstrated in Section 2.3.3.3 that the use of solvent did not alter the kinetic parameters of the HKR reaction, and thus the estimated kinetic parameters remained valid under solvent-free conditions. Water retained sufficient solubility in the mixed system to maintain homogeneity at the desired rate of addition. A slight excess of water was used to ensure complete conversion of (S)-epichlorohydrin and to compensate for any competitive hydrolysis of the (R)-enantiomer. 

The most common initiation or homolysis reaction is the breaking of a covalent C-C bond with the formation of two radicals. This initiation process is highly sensitive to the stability of the formed radicals. Its activation energy is equal to the bond dissociation enthalpy because the reverse, radical-radical recombination reaction is so exothermic that it does not require activation energy. C-C bonds are usually weaker than the C-H bonds. Thus, the initial formation of H radicals can be ignored. The total radical concentration in the reacting system is controlled both by these radical initiation reactions and by the termination or radical recombination reactions. In accordance with Benson (1960), the rate constant expressions of these unimolecular decompositions are calculated from the reverse reaction, the recombination of two radical species to form the stable parent compound, and microscopic reversibility (Curran et al., 1998). The reference kinetic parameters for the unimolecular decomposition reactions of K-alkanes for each single fission of a C-C bond between secondary... 

For the thermal runaway hazard evaluation, the "chemistry" of the exothermic reaction can be defined in terms of three sets of parameters the thermodynamic, kinetic, and physical parameters. (1) A list of some of the parameters of interest is given in Table I. 

If the kinetics of an exothermic reaction is known, the thermal behavior of the reactor can be evaluated on the basis of three parameters [27] ... 

The Thermal Analyzer was interfaced directly to a Digital Equipment Corporation PDP 1134 minicomputer Q 2)- This computer performed real time data collection during the run and did the subsequent data analysis and plotting. The data analysis included transformations of the data contained in the exothermic reaction peak to provide information about kinetic parameters. 

A final point to consider is whether heat transfer in a tubular reactor is inherently different from heat transfer without reaction. Several workers have claimed that special correlations for the heat transfer parameters are needed for reliable reactor models. In a number of reactor studies [40], the maximum temperature rise was greatly overpredicted by the models used, but others have reported the opposite effect. These differences may have been caused by inaccurate kinetic equations or errors in the heat transfer parameters, but they were probably not due to a fundamental difference between heat transfer with and without reaction. It is true that in steady-state heat transfer tests, the local gas and solid temperatures are equal, whereas with an exothermic reaction, T, is greater than Tg. If the reactor analysis is based on Tg, the heat transfer rate will be slightly underestimated because of increased radiation and conduction. However, for moderate or high Reynolds numbers, convection is the major contributor to radial heat... 

This test allows early detection of initial exothermicity. From the results it is possible to estimate thermo-kinetic parameters and to estimate how the initial temperature for self-sustaining reaction will vary with the quantity of material present. 

The time, temperature and temperature-rate data collected from the exothermic decomposition of materials in adiabatic calorimeters such as adiabatic Dewars or ARC are handled by a single-step A B reaction model. Townsend and Tou 2 first introduced this model as a means of obtaining safety limits and simple kinetic parameters from such adiabatic data. 

Fig. 18. With reference to Fig. lH the interference of the reaction kinetic parameters is not complicated by possible multiplicity of the heat and mass transf profiles, since an activiation energy of around 25 kcal mol" can only give rise to single solutions. The exothermicity of the absorption process results in a rapid increase in the reaction speed along the jet. This is presented in terms of the half-lives of the reacting sulphur trioxide at the jet surface in Fig. 19. For the nominal 10 SO in the gas phase, the half-life has become less than 10 millrseconds at the end of the jet. The much greater surface temperature achieved in the 30% SO case means that the half-life decreases along the jet surface from around 10 milliseconds close to the jet nozzle to less than a microsecond on entry to the receiver.

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