Page 35 - Principles of Catalyst Development

CATALYTIC FlJNCTIONS 21
commerical catalysts frequently precludes fundamental information and we
are forced to use pilot unit or even plant data similar to this example.
To calculate the pellet rate, proceed as follows. For an "average" naphtha
molecule, C 7 H I6 , vapor density at 25°C, p = 4.46 X 10- g cm- , molecular
3
3
weight, M = 100, and pellet density, d p = 2.5 g cm- 3
(XHc)(GHSV)(p)
ra t e = -'-----=-:..::"--'-----'--'-'-'-
(M)(d p )
3
(0.75)(5000)(4.4 x 10- ) -2
= =7xlO ( 1.31)
(100)(2.5)
Intersection of these two lines in Fig. 1.8 is at a point where the chemical
rate is about 7 x 10- moles g-I hr-\ i.e., 1] = 0.1. Clearly this is well within
1
the internal diffusion regime, even with all the simplifying assumptions.
Doubling the pellet size will decrease conversion to half its value. Even
though this analysis may be tenuous, it certainly justifies further tests with
8-mm particles.

1.5.2. Diagnostic Tests

Although this "back of an envelope" calculation is sufficient for many
purposes, other indications may be used, depending on the amount of
information available. Absence of such data is itself a valuable indication
of where to begin experimental verification.
When faced with the diagnosis of process behavior, it is sometimes
useful to answer the following questions:
1. What is the apparent order of the reaction? Reaction orders tend
towards first order as the process becomes diffusion controlled. (21) First
order should be regarded as potentially suspicious. Reaction orders other
than one probably indicate chemical control.
2. What is the activation energy? Most chemical activation energies
found in catalysis are in the range 25-50 kcal mole-I. Pore diffusion resist-
ance decreases this by a factor of 2. (21) Values from 10 to 20 kcal mole-I
should be regarded as indications of pore-diffusion control. External
diffusion follows a 3/2 order in temperature. This appears to be an activa-
tion energy of about 5 kcal mole-I-definitely suspicious of external dif-
fussion.
3. Does the rate depend on particle size? If the rate of reaction under
constant conditions is inversely proportional to the radius of the particle,
this is a strong indication of pore diffusion resistance. External diffusion
also shows size effects, but not nearly so pronounced.