Page 190 - Modeling of Chemical Kinetics and Reactor Design

160 Modeling of Chemical Kinetics and Reactor Design

p = C RT (3-206)
A
A
where

p = Pascal Pa (atm)
,
A
C = mol  mol 
A
m 3  l 

3
mPa l atm
•
R =8 314 or 0 082 •
.
.
•
•
mol K mol K
Caution must be exercised when the rate equations with gas phase
reactions are expressed in terms of the partial pressure as compared
to concentration. This is because each rate gives different activation
energy for the same data and for the same reaction. Levenspiel [4]
suggests that the difference can be ignored for reactions with reasonably
high activation energies as the amount is only a few kJ.
Pressure measurement devices such as a manometer are used with-
out disturbing the system being monitored. Another type of reacting
system that can be monitored involves one of the products being
quantitatively removed by a solid or liquid reagent that does not affect
the reaction. An example is the removal of an acid formed by reac-
tions in the gas phase using hydroxide solutions. From the reaction
stoichiometry and measurements of the total pressure as a function of
time, it is possible to determine the extent of the reaction and the
partial pressure or concentrations of the reactant and product species
at the time of measurement.
Consider the following gaseous reaction aA + bB → cC + dD.
Pressure and concentration are related, and for a constant volume
reactor with changing number of moles during reaction, the total
pressure (π) changes with time, t. For an ideal gas, with any reactant
A or B, the partial pressure is expressed as:

p = C RT = p + a (π − ) π
∆ n
A A AO O (3-207)
where

∆n = (c + d) – (a + b) (3-208)