Page 35 - Introduction to chemical reaction engineering and kinetics

1.5 Aspects of Chemical Reaction Engineering 17

The balance equation, whether for mass or energy (the two most common uses for
our purpose), is of the form:

Equation 1.5-1 used as a mass balance is normally applied to a chemical species. For
a simple system (Section 1.4.4) only one equation is required, and it is a matter of
convenience which substance is chosen. For a complex system, the maximum number
of independent mass balance equations is equal to R, the number of chemical equations
or noncomponent species. Here also it is largely a matter of convenience which species
are chosen. Whether the system is simple or complex, there is usually only one energy
balance.
The input and output terms of equation 1.5-1 may each have more than one contri-
bution. The input of a species may be by convective (bulk) flow, by diffusion of some
kind across the entry point(s), and by formation by chemical reaction(s) within the con-
trol volume. The output of a species may include consumption by reaction(s) within the
control volume. There are also corresponding terms in the energy balance (e.g., gener-
ation or consumption of enthalpy by reaction), and in addition there is heat transfer
(b), which does not involve material flow. The accumulation term on the right side of
equation 1.5-1 is the net result of the inputs and outputs; for steady-state operation, it
is zero, and for unsteady-state operation, it is nonzero.
The control volume depicted in Figure 1.3 is for one fixed in position (i.e., fixed ob-
servation point) and of fixed size but allowing for variable mass within it; this is often
referred to as the Eulerian point of view. The alternative is the Lagrangian point of
view, which focuses on a specified mass of fluid moving at the average velocity of the
system; the volume of this mass may change.
In further considering the implications and uses of these two points of view, we may
find it useful to distinguish between the control volume as a region of space and the
system of interest within that control volume. In doing this, we consider two ways of
describing a system. The first way is with respect to flow of material:
(Fl) Continuous-flow system: There is at least one input stream and one output stream
of material; the mass inside the control volume may vary.
(F2) Semicontinuous-flow or semibatch system: There is at least one input stream or
one output stream of material; the mass inside the control volume does vary for
the latter.
(F3) Nonflow or static system: There are no input or output streams of material; the
mass inside the control volume does not vary.
A second way of describing a system is with respect to both material and energy
flows:
(Sl) An open system can exchange both material and energy with its surroundings.
(S2) A closed system can exchange energy but not material with its surroundings.
(S3) An isolated system can exchange neither material nor energy with its surroundings.
In addition,

(S4) An adiabatic system is one for which 0 = 0.
These two ways of classification are not mutually exclusive: Sl may be associated with
Fl or F2; S2 with Fl or F3; S3 only with F3; and S4 with Fl or F2 or F3.