Reaction kinetics is a subject that studies the chemical reaction rate and the influence of various factors on it. Traditionally, it belongs to the category of physical chemistry, but in order to meet the needs of engineering practice, chemical reaction engineering has also carried out a lot of research work in this field during its development.

Most chemical reactions are not completed in one step according to the stoichiometric formula, but are composed of several elementary reactions with certain procedures (one or several reaction components are directly converted into other reaction components in one step, or simple reactions). The actual process of reaction is called reaction mechanism.

Generally speaking, chemists focus on the reaction mechanism and try to predict the kinetic law of the whole reaction according to the theoretical calculation of the elementary reaction rate. Chemical reaction engineers mainly determine the relationship between the concentration and temperature of each component in the reactant system and the reaction rate through experimental determination to meet the needs of reaction process development and reactor design.

Reaction rate:

Reaction rate ri is the reaction amount of component I per unit time in reactant system and unit reaction zone, which can be expressed as follows: the volume of reaction zone can be the volume of reactant system, the mass of catalyst or the area of phase interface as required. In the same reactant system, there is a certain proportional relationship between the reaction rates of different components, which obeys the chemometrics law.

For example, for reaction: for reactants, the reaction rate ri is preceded by a negative sign; For the reaction products, the plus sign is used before ri.

Rate equation:

The reaction rate equation represents the quantitative relationship between the reaction temperature and the concentration of each component in the reactant system and the reaction rate, namely:

Where c is the concentration vector of the reactant; T is the reaction temperature (absolute temperature). A large number of experiments show that temperature and concentration usually affect the reaction rate independently, so the formula (3) can be rewritten as:

In formula (4), fT(T) is the reaction rate constant k, which indicates the influence of temperature on the reaction rate. For most reactions, k follows the Arrhenius relation (that is, the reaction kinetics equation established by Swede S. Arrhenius in 1889):

Where a is the frequency factor, or the pre-index factor; E is the activation energy of reaction; R is the molar gas constant. Frequency factor is a parameter related to the number of collisions of reactant molecules in unit time and unit volume; The activation energy of the reaction indicates the peak energy that must be overcome when the reaction occurs. High activation energy makes the reaction difficult, while low activation energy makes the reaction easy.

Both frequency factor and activation energy determine the reaction rate at a certain temperature and concentration. FC(C) in Formula (4) indicates the influence of concentration on the reaction rate, which can usually be expressed in the form of power function or hyperbola. The reaction rate equation of (1) power function type can be written as:

Where n 1 and n2 are the reaction orders of reaction components A and B, respectively; N 1+n2 is the total reaction order, or simply the reaction order.

Hyperbolic equation is often used to study the kinetics of gas-solid catalytic reaction.

For example, reaction A-r consists of molecular adsorption of component A, surface reaction and molecular desorption of component R. When the surface reaction is the control step, the rate equation can be written as follows: where pA and pR are the partial pressures of components A and R respectively; K is the rate constant including the adsorption equilibrium constant; KA and kR are the adsorption equilibrium constants of components a and r, respectively; K is the chemical equilibrium constant.

The quantitative relationship between reaction rate and reaction conditions in the operating range of industrial reactors was re-studied by kinetics. Therefore, a series of dynamic experimental research methods have been developed.