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Initiator frequency factor

Figure 10. Effect of the initiator frequency factor on the molecular weight distribution of an addition polymer produced in a tubular reactor constant activation energy and at widely different values of initiator-jacket temperature combination (the conversion is optimized Ea = 32.921... Figure 10. Effect of the initiator frequency factor on the molecular weight distribution of an addition polymer produced in a tubular reactor constant activation energy and at widely different values of initiator-jacket temperature combination (the conversion is optimized Ea = 32.921...
Activation Parameters. Thermal processes are commonly used to break labile initiator bonds in order to form radicals. The amount of thermal energy necessary varies with the environment, but absolute temperature, T, is usually the dominant factor. The energy barrier, the minimum amount of energy that must be suppHed, is called the activation energy, E. A third important factor, known as the frequency factor, is a measure of bond motion freedom (translational, rotational, and vibrational) in the activated complex or transition state. The relationships of yi, E and T to the initiator decomposition rate (kJ) are expressed by the Arrhenius first-order rate equation (eq. 16) where R is the gas constant, and and E are known as the activation parameters. [Pg.221]

Effects of Initiator Parameters. Initiator types can best be characterized by the frequency factor (k ) and the activation energy (E ), and the effect of these parameters on the molecular weight-conversion relationship is shown in Figures 7 and 8. The curves shown are the result of choosing the jacket temperature-inlet initiator concentration combination which maximizes the reactor conversion for each initiator type investigated. [Pg.235]

Figure 7 shows the limiting maximum molecular weight of products from a reactor of fixed size varies directly with the frequency factor of the initiator at a fixed activation energy, while the limiting conversion varies inversely with the frequency factor. In addition, the length of the chain-transfer controlled zone is increased inversely with the frequency factor. [Pg.235]

Figure 7. Tubular plug-flow addition polymer reactor effect of the frequency factor (ka) of the initiator on the molecular weight-conversion relationship at constant activation energy (Ea). Each point along the curves represents an optimum initiator feed concentration-reactor jacket temperature combination and their values are all different, (Ea = 32.921 Kcal/mol In ka = 35,000 In sec ... Figure 7. Tubular plug-flow addition polymer reactor effect of the frequency factor (ka) of the initiator on the molecular weight-conversion relationship at constant activation energy (Ea). Each point along the curves represents an optimum initiator feed concentration-reactor jacket temperature combination and their values are all different, (Ea = 32.921 Kcal/mol In ka = 35,000 In sec ...
Figure 12. Effect of the initiator activation energy on the initiator usage in a tubular-addition polymerization reactor constant frequency factor (the conversion is optimized In k = 26.492 In sec )... Figure 12. Effect of the initiator activation energy on the initiator usage in a tubular-addition polymerization reactor constant frequency factor (the conversion is optimized In k = 26.492 In sec )...
As seen in the figures, the quantity of initiator required to yield a given conversion varies directly with the frequency factor for an initiator with a specified activation energy and inversely with the activation energy for an initiator of specified frequency factor. The relationships do not show a linear proportionality between the reactor conversion and square root of the inlet initiator concentration. [Pg.242]

Table II gives published ( ) half-life data for the two initiators along with values calculated from the optimized values of Yl and Y2. In each case, solvent C data were used to calculate the base activation energies and frequency factors, and the equality of half-life values at Tb illustrates the anchoring of the rate constant for each initiator. Except for initiator 1 at the low temperature, the differences between the optimized and published values are within the range of the differences reported for differing solvents. Table II gives published ( ) half-life data for the two initiators along with values calculated from the optimized values of Yl and Y2. In each case, solvent C data were used to calculate the base activation energies and frequency factors, and the equality of half-life values at Tb illustrates the anchoring of the rate constant for each initiator. Except for initiator 1 at the low temperature, the differences between the optimized and published values are within the range of the differences reported for differing solvents.
Results for styrene - yield Ea 21 kcal. Since Ep — Et/2 was found previously to be 6.5 kcal., we conclude that the activation energy Ei for thermal initiation in styrene is 29 kcal., which would be quite acceptable for the process (21), already rejected on other grounds. For methyl methacrylate, Ea—l kcal. and Ep — Et/2 = b kcal. Hence Ei = 22 kcal. These initiation reactions are very much slower than is normal for other reactions with similar activation energies. The extraordinarily low frequency factors Ai apparently are responsible. For methyl methacrylate, Ai is less than unity. Interpreted as a bimo-lecular process, this would imply initiation at only one collision in about 10 of those occurring with the requisite energy ... [Pg.132]

A change in the reaction temperature affects the rate constant k. As the temperature increases, the value of the rate constant increases and the reaction is faster. The Swedish scientist, Arrhenius, derived a relationship that related the rate constant and temperature. The Arrhenius equation has the form k = Ae-E /RT. In this equation, k is the rate constant and A is a term called the frequency factor that accounts for molecular orientation. The symbol e is the natural logarithm base and R is universal gas constant. Finally, T is the Kelvin temperature and Ea is the activation energy, the minimum amount of energy needed to initiate or start a chemical reaction. [Pg.194]

Availability change to form embryo Initial bubble diameter Frequency factor in nucleation Enthalpy of vaporization Rate of formation of critical-sized embryos per unit volume Jacob numter [Eq. (17)] Boltzmann s constant or thermal conductivity... [Pg.203]

For a purely photochemical polymerization, the initiation step is temperature-independent (Ed = 0) since the energy for initiator decomposition is supplied by light quanta. The overall activation for photochemical polymerization is then only about 20 kJ mol-1. This low value of Er indicates the Rp for photochemical polymerizations will be relatively insensitive to temperature compared to other polymerizations. The effect of temperature on photochemical polymerizations is complicated, however, since most photochemical initiators can also decompose thermally. At higher temperatures the initiators may undergo appreciable thermal decomposition in addition to the photochemical decomposition. In such cases, one must take into account both the thermal and photochemical initiations. The initiation and overall activation energies for a purely thermal self-initiated polymerization are approximately the same as for initiation by the thermal decomposition of an initiator. For the thermal, self-initiated polymerization of styrene the activation energy for initiation is 121 kJ mol-1 and Er is 86 kJ mol-1 [Barr et al., 1978 Hui and Hamielec, 1972]. However, purely thermal polymerizations proceed at very slow rates because of the low probability of the initiation process due to the very low values f 1 (l4 IO6) of the frequency factor. [Pg.273]

Boos and Flauschildt90) obtained for the model copolymerization of phenylglycidyl ether with hexahydrophthalic anhydride activation energies of 96 kJ/mol up to 75% conversion and 27 kJ/mol for higher conversions. Frequency factors are also very different (log A = 13.7 and 5.5, respectively). The frequency factors as well as the temperature coefficients of the solution viscosities depended on the initiator concentration. The activation energy determined by the same authors 90) for the curing of epoxy resins at conversions lower than 75% was 86.4 kJ/mol and the frequency factor log A = 11.8 whereas at higher conversions these values were not obtained. [Pg.130]

Initial values of frequency factors and activation energies of the reactions... [Pg.504]

This model gives a symmetrical peak with its maximum at half conversion. Hence the model is unable to describe non-symmetrical peaks as they are often observed in practice. Moreover, in order to obtain a reaction rate other than zero, some product B must be present in the reaction mass. Therefore, the initial concentration of B (CBo) or the initial conversion (X0) is a required parameter for describing the behavior of the reaction mass. This also means that the behavior of the reacting system depends on its thermal history, that is, on the time of exposure to a given temperature. This simple model requires three parameters the frequency factor, the activation energy, and the initial conversion that must be fitted to the measurement in order to predict the behavior of such a reaction under adiabatic conditions. [Pg.316]

This model comprises eight parameters, that is, two frequency factors, two activation energies, three exponents for the reaction orders, and the initial conversion. It is often used in a simplified form, with all reaction orders equal to one ... [Pg.316]

In order to estimate the kinetic parameters for the addition and condensation reactions, the procedure proposed in [11, 14] has been used, where the rate constant kc of each reaction at a fixed temperature of 80°C is computed by referring it to the rate constant k° at 80°C of a reference reaction, experimentally obtained. The ratio kc/k°, assumed to be temperature independent, can be computed by applying suitable correction coefficients, which take into account the different reactivity of the -ortho and -para positions of the phenol ring, the different reactivity due to the presence or absence of methylol groups and a frequency factor. In detail, the values in [11] for the resin RT84, obtained in the presence of an alkaline catalyst and with an initial molar ratio phenol/formaldehyde of 1 1.8, have been adopted. Once the rate constants at 80°C and the activation energies are known, it is possible to compute the preexponential factors ko of each reaction using the Arrhenius law (2.2). [Pg.25]


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