Page 126 - Introduction to chemical reaction engineering and kinetics

108 Chapter 5: Complex Systems

In their study of the kinetics of the partial oxidation of methane to HCHO, along with CO,
CO,, and H,O (Example 5-l), Spencer and Pereira (1987) observed the following:
.
(l) SHCHO/C& = 0.89 when extrapolated to f& = 0, and decreased as f& increased.
(2) i Aco,cH4 = 0 at f& = 0, and increased as fcH4 increased.
= 0.11 at f& = 0 and remained constant, independent of fcb.
c3) &O&H4
(4) There was no change in the observed selectivity or conversion when the initial molar
ratio of CH, to 0, was varied over a wide range.
(5) In separate experiments on HCHO oxidation over the same catalyst, CO was formed
(but very little CO,).
Construct a reaction network that is consistent with these observations.

SOLUTION

The five points listed above lead to the following corresponding conclusions:
(1) HCHO is a primary unstable product (like B in Figure 5.5); see also (5).
(2) CO is a secondary stable product (similar to C in Figure 5.5, but with no maximum
or drop-off); see also (5).
(3) CO, is a primary stable product (like A in Figure 5.5, but remaining constant).
(4) The rate of any step involving O2 is independent of ccoZ (zero-order).
(5) CO is a primary product of HCHO oxidation.
A reaction network could then consist of two steps in series in which CH, forms HCHO,
which subsequently oxidizes to CO, together with a third step in parallel in which CH,
oxidizes to CO,. Thus,

CH~ + O,-%HCHO + H,O

HCHO + ; O2 3 CO + H,O

CH, + 202 &JO, + 2H20

The corresponding rate laws (tested by means of experimental measurements from a dif-
ferential PFR) are:

(-rCH4) = (kl + k3kCH4
rHCHO = ‘% cCH.j - k2CHCH0
rco, = k3CCH,

(Values of the rate constants, together with those of corresponding activation energies, are
given by the authors.)

5.7 PROBLEMS FOR CHAPTER 5

5-1 Consider a reacting system in which species B and C are formed from reactant A. HOW
could you determine from rudimentary experimental information whether the kinetics scheme
should be represented by