Page 90 - The engineering of chemical reactions

74 Reaction Rates, the Batch Reactor, and the Real World

vary with conditions. Catalytic processes also exhibit deactivation, and all properties will
typically vary with the time the processes have been in operation, and a major engineering
question is how long will be required before the process must be shut down to reactivate or
replace the catalyst.
This is the fun (and frustration) of chemical reaction engineering. While thermody-
namics, mass and heat transfer, and separations can be said to be “finished” subjects for
many engineering applications, we have to reexamine every new reaction system from
first principles. You can find data and construct process flowsheets for separation units
using sophisticated computer programs such as ASPEN, but for the chemical reactors in
a process these programs are not much help unless you give the program the kinetics or
assume equilibrium yields.
These complications show why we emphasize simple and qualitative problems in
this course. In reactor engineering the third decimal place is almost always meaningless,
and even the second decimal place is frequently suspect. Our answers may be in error by
several orders of magnitude through no fault of our own, as in our example of the temperature
dependence of reaction rates. We must be suspicious of our calculations and make estimates
with several approximations to place bounds on what may happen. Whenever a chemical
process goes badly wrong, we are blamed. This is why chemical reaction engineers must be
clever people. The chemical reactor is the least understood and the most complex “unit” of
any chemical process, and its operation usually dominates the overall operation and controls
the economics of most chemical processes.

REACTION-RATE DATA

We consider next the acquisition of data on the kinetics of a chemical reaction. We want
rate expressions r(Cj, T) which we can insert into the relevant mass balance to predict
reactor performance. The methods of acquiring these data, in order of increasing difficulty
and expense, are

1. Literature values. If the process is simple and well known, there may be rate expressions
in the literature that can be used.
2. Estimations. If one can find a process similar to the one of interest, then rates can be
estimated from these data. For example, if one finds a reactor for which a specified
conversion is obtained with a specified reactant composition and temperature, then one
may guess the orders of the reaction with respect to each species (guess first order) and
proceed to formulate a reasonable rate expression.
3. Theoretical rate calculations. Statistical mechanics permits one in principle to compute
reaction-rate expressions from first principles if one knows the “potential energy surface”
over which the reaction occurs, and quantum mechanics permits one to calculate this
potential energy surface. In Chapter 4 we consider briefly the theory of reaction rates
from which reaction rates would be calculated. In practice, these are seldom simple
calculations to perform, and one needs to find a colleague who is an accomplished
statistical mechanic or quantum mechanic to do these calculations, and even then
considerable computer time and costs are usually involved.