Page 34 - Elements of Chemical Reaction Engineering Ebook
P. 34
Sec. 1 .l Definition of the Rate of Reaction, -r, 5
NoOH- CHSCOOC~H,
Figure El-1.1 Well mixed reaction vessel.
where A E NaOH . Substitution of Equation (El-1.1) into Equation (1-1) leads to
r, = 0 (E 1- 1.2)
which is incorrect because C,H,OH and CH,COONa are being formed from NaOH
and CH3COOC2H5 at a finite rate. Consequently, the rate of reaction as defined by
Equation (1-1) cannot apply to a flow system and is incorrect if it is dejined in this
manner.
By now YOU should be convinced that Equation (1-1) is not the definition
Definition of rJ of the chemical reaction rate. We shall simply say that r, is the rate offorma-
tion of species j per unit volume. It is the number of moles of species j gener-
ated per unit volume per unit time. The rate equation for rl is solely a function
of the properties of the reacting materials [e.g., species concentration (i.e.
activities), temperature, pressure, or type of catalyst, if any] at a point in the
system and is independent of the type of system (Le., batch or continuous
flow) in1 which the reaction is carried out. However, since the properties of the
reacting: materials can vary with position in a chemical reactor, rJ can in turn
be a function of position and can vary from point to point in the system.
What is -rA The chemical reaction rate is an intensive quantity and depends on tem-
a function Of? perature and concentration. The reaction rate equation (i.e., the rate law) is
essentially an algebraic equation involving concentration, not a differential
equation. For example, the algebraic form of the rate law -r, for the reaction
,4 ___) products
may be a linear function of concentration,
or it may be some other algebraic function of concentration, such as
For further elaboration on this point, see Chem. Eng. Sci., 25, 337 (1970); B. L.
Crynes and H. S. Fogler, eds., AIChE Modular Instruction Series E: Kinetics, Vol. 1
(New York AIChE, 1981$, p. 1; and R. L. Kabel, “Rates,” Chem. Eng. Commun., 9,
15 (1981).
