Page 32 - Introduction to chemical reaction engineering and kinetics

14 Chapter 1: Introduction
SOLUTION

From equation (A) in Example 1-3,

rC2H6 CA) _ ‘Gh
- 1 1

Similarly from (B) and (C),

cB)rC2H6 rCH4
-1 =2

and

cc)rC2H6 _ rGHz
- 1 1

Since k2H6 = k,H,cA> + k&p) + rC,H,(c)?
1
b-CzH6) = rC2H4 + 2 cH4 + rC~Hz
-’

Similarly,
1
rH2 = ‘C2& - ~kHz, + 2rC2H2

If we measure or know any 3 of the 5 rates, then the other 2 can be obtained from these 2
equations, which come entirely from stoichiometry.

For a system involving N species, R equations, and C components, the results of Ex-
ample 1-5 may be expressed more generally as

i = 1,2,. . . , C; j = 1,2, . . . , R (1.4-11)

corresponding to equation 1.4!8. Equations 1.4-11 tell us that we require a maximum of
R = IV - C (from equation 1.4-9) independent rate laws, from experiment (e.g., one for
each noncomponent). These together with element-balance equations enable complete
determination of the time-course of events for the N species. Note that the rate of
reaction r defined in equation 1.4-8 refers only to an individual reaction in a kinetics
scheme involving, for example, equations (A), (B), and (C) as reactions in Example 1-3
(that is, to r(A), r(B), and rccj), and not to an “overall” reaction.

1.4.5 Kinetics and Thermodynamics/Equilibrium

Kinetics and thermodynamics address different kinds of questions about a reacting sys-
tem. The methods of thermodynamics, together with certain experimental information,
are used to answer questions such as (1) what is the maximum possible conversion of
a reactant, and the resulting equilibrium composition of the reacting system at given
conditions of T and P, and (2) at given T and P, how “far” is a particular reacting