Wednesday, October 8, 2008

Chemical Reaction Rate

From an engineering viewpoint, reaction kinetics has these principal functions:
Establishing the chemical mechanism of a reaction obtaining experimental rate data
Correlating rate data by equations or other means; Designing suitable reactors, Specifying operating conditions, control methods, and auxiliary equipment to meet the technological and economic needs of the reaction process.

Reactions can be classified in several ways. On the basis of mechanism they may be:
1. Irreversible
2. Reversible
3. Simultaneous
4. Consecutive

A further classification from the point of view of mechanism is with respect to the number of molecules participating in the reaction, the molecularity:
1. Unimolecular
2. Bimolecular and higher

Related to the preceding is the classification with respect to order. In the power law rate equation r = k(Ca)p. (Cb)q, the exponent to which any particular reactant concentration is raised is called the order p or q with respect to that substance, and the sum of the exponents p + q is the order of the reaction. At times the order is identical with the molecular, but there are many reactions with experimental orders of zero or fractions or negative numbers. Complex reactions may not conform to any power law. Thus, there are reactions of:

1. Integral order
2. Nonintegral order
3. Non–power law; for instance, hyperbolic

With respect to thermal conditions, the principal types are:
1. Isothermal at constant volume
2. Isothermal at constant pressure
3. Adiabatic
4. Temperature regulated by heat transfer

According to the phases involved, reactions are:
1. Homogeneous, gaseous, liquid or solid
2. Heterogeneous:

  • Controlled by diffusive mass transfer
  • Controlled by chemical factors

A major distinction is between reactions that are:
1. Uncatalyzed
2. Catalyzed with homogeneous or solid catalysts

Equipment is also a basis for differentiation, namely:
1. Stirred tanks, single or in series
2. Tubular reactors, single or in parallel
3. Reactors filled with solid particles, inert or catalytic:

  • Fixed bed
  • Moving bed
  • Fluidized bed, stable or entrained

Finally, there are the operating modes:
1. Batch
2. Continuous flow
3. Semibatch or semiflow

Clearly, these groupings are not mutually exclusive. The chief distinctions are between homogeneous and heterogeneous reactions and between batch and flow reactions. These distinctions most influence the choice of equipment, operating conditions, and methods of design.

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