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2.3 Kinetics
The term kinetics derives from the ancient greek (η κινησις, action of moving or moving
oneself, movement, change). Used in the modern chemistry language, it designates the
study of the reaction rate of chemical or enzymatic reactions.
Chemistry and kinetics of the thermal degradation of hydrocarbons is important in
several different domains of process and environmental engineering. These are e.g.
geochemistry, conversion of petroleum, coal, and biomass to liquid fuels, cracking
processes, and recycling of polymers.
The kinetics of polymer decomposition can be studied in order to determine the
appropriate conditions for hindering or limiting the evolution of toxic compounds and
recuperation of raw materials from the thermal treatment of plastic wastes.
The principal objectives common to the majority of kinetic studies are the
determination of the rate equation, i.e. the description of the extent of conversion of
reactant(s) or formation of product(s) with time, and the assessment of the influence of
temperature on the rate of reaction.
The rate of conversion, dα/dτ, is usually assumed to be a linear function of a single
temperature-dependent rate constant, k, and a temperature-independent function of the
conversion, α, i.e., dα/dτ = k.f(α).
The quantitative representation of the rate-temperature dependence of k has been
almost universally expressed by the Arrhenius equation, k = A.exp(-E a /RT), where A is
the “frequency factor” (usually assumed to be independent of temperature), E a is the
activation energy, and R is the ideal gas constant.
Values of the Arrhenius equation, or else the Arrhenius parameters E a , the activation
energy, and A (called also the “pre-exponential factor”) describe quantitatively the
energy barrier to reaction and the frequency of occurrence of the situation that may lead
to product formation, respectively. As such, these parameters facilitate the concise
reporting of kinetic data, and the comparison of different systems from the point of view
of their chemical reactivities. Moreover, they can be used to forecast behaviour at
temperatures outside the intervals of different conditions of the experimental
measurements.
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