Ch17. Heterogeneous Reactions:
Introduction
Classification of Reactions
Phase: state of matter, i.e., solid, liquid, and gas phases
Homogeneous system : in which, reaction takes place in one phase
(i.e., single gas phase system, single liquid phase system) Heterogeneous system : in which, reaction takes place in multiple phases
(i.e., solid-gas phases system, solid-liquid phases system,
gas-liquid phase system, liquid-liquid phases system)
Factors to be considered in heterogeneous reactions 1) Complication of the rate equation
2) Complication of the contacting patterns for two-phase systems
Since more than one phase is present, the movement of material from phase to phase must be considered in the rate equation.
Thus the rate expression will incorporate mass transfer terms in addition to the usual chemical kinetics term.
These mass transfer terms are different in type and numbers in the different kinds of heterogeneous systems.
In heterogeneous systems
r = f (composition, temperature,
mass transfer, heat transfer, and etc.) In homegeneous systems
r = f (composition, temperature, and etc.)
Example 17.1 The Burning of a Carbon Particle in Air
Example 17.1 The Burning of a Carbon Particle in Air
2 steps in series are involved
1) mass transfer of oxygen to the surface
2) burning reaction at the surface of the particle
Example 17.2 Aerobic Fermentation
7 possible steps in series are involved
Definition of Reaction Rate
- Based on unit volume of reacting fluid
- Based on unit mass of solid in fluid-soild systems
- Based on unit interfacial
surface in two-fluid systems or based on unit surface of solid in gas-solid systems
- Based on unit volume of solid in gas-solid systems - Based on unit volume of reactor
Based on unit volume of reactor fluid
Based on unit mass of solid
Based on unit surface
Example 17.3 Overall Rate for a Linear Process
Example 17.3 Overall Rate for a Linear Process
Example 17.4 Overall Rate For a Nonlinear Process
Example 17.4 Overall Rate For a Nonlinear Process
Fortunately, most real reactors for heterogeneous systems can be satisfactorily approximated by one of the five ideal flow patterns
There are many ways that two phases can be contacted, and for each the design equation will be unique.
