Page 244 - Modeling of Chemical Kinetics and Reactor Design

214 Modeling of Chemical Kinetics and Reactor Design

2. Steps 1, 3, and 4 are in equilibrium and step 2 is the rate-
determining step.
3. The conditions at which the reaction N + 3H → 2NH are
2
3
2
performed are such that the reverse rate is negligible.
C = total concentration of sites and defined as
T
 total moles of sites 
 
 unit mass of catalyst
C = concentration of vacant sites
S
C = concentration of adsorbed hydrogen
HS
 moles of hydrogen adsorbed
 
 unit mass of catalyst 

C = concentration of the most abundant surface intermediate
NS
C NH S = concentration of adsorbed ammonia
3
 moles of ammonia adsorbed
 
 unit mass of catalyst 

p = partial pressure of nitrogen
N 2
p = partial pressure of ammonia
NH 3
(−r ) = rate of reaction of nitrogen, moles of nitrogen disappearing
N 2
per unit time per unit mass of catalyst
Since step 2 is the rate-determining step, then the rate expression is:

− ( r ) = k p C 2 (3-340)
N 2 2 N 2 S

The net rate of the disappearance of H is given by
2
) 2 2
− ( r = k p C − k C (3-341)
H 2 1 H 2 S −1 HS
net
On the basis of the pseudo-steady state assumption, the net rate of
disappearance is zero, therefore

2
kp H 2 C = k C 2 HS
−
1
S
1