Applications of Chemical Kinetics in Environmental Systems - Elements of Environmental Engineering: Thermodynamics and Kinetics - page 267

6 Applications of

Chemical Kinetics in
Environmental Systems

In Chapter 2, we discussed the differences between open and closed systems. Natural
environmental systems are open systems. Both material and energy are exchanged
across the system boundaries. However, it is convenient and useful to model them as
closed systems as well if we consider a fixed composition and ignore any changes
in composition. Many waste treatment operations are conducted as closed systems,
where exchange of energy and/or mass occurs among compartments within the
system, but not necessarily with the surroundings.
The rate laws described in Chapter 5 are for closed systems, where the concen-
tration of a pollutant changes with time, but there is no mass exchange with the
surroundings. Rarely this represents chemical kinetics in the natural environment,
where we encounter time-varying inputs and outputs. The discussion that follows
is derived from the chemical engineering literature (Hill, 1977; Levenspiel, 1999;
Fogler, 2006).
Applications of kinetics in environmental engineering F&T modeling and in waste
treatment operations are numerous (Thibodeaux, 1996).The discussion will start with
the conventional reactor theory. Various types of reactors (ideal and nonideal) will
be discussed. This will be followed by several applications in the hydrosphere, atmo-
sphere, and lithosphere. Finally, the chemical kinetics for the biosphere with special
relevance to biodegradation, enzyme kinetics, and bioaccumulation will be discussed.
Several problems are provided, which serve to illustrate the concepts presented within
each section.

6.1 TYPES OF REACTORS
6.1.1 I DEAL R EACTORS
Chemical reactors are classified into batch and flow systems (Figure 6.1). In a batch
reactor , a fixed volume (mass) of material is treated for a specified length of time.
If material flows into and out of the reactor in a continuous manner, it is called a
continuous flow reactor.
To analyze a reactor, we use the fundamental concept of mass balance or conser-
vationofmass . This principle dates back to 1789 when the renowned French scientist,
Antöine Lavoisier, considered to be the father of modern chemistry, first enunciated it

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