Series Reactions and Selectivity - Chemical Kinetics Video Lecture - Chemistry

Here we are going to discuss series reactions
in chemical reactors and then how we would maximize
selectivity. So we have more then one reaction
taking place in almost all chemical reactors, and
so often more important than rate would be
selectivity, and a common way of defining
selectivity is the rate of formation of a
desired product over the rate of formation
of all the products. So in a series reaction,
so series A reacting to B, B subsequently
reacting to C, and this is a generic reaction. For
example if we were to look at hydrogenation
or oxidation reactions, certainly a common
type where I have a reactant, say reacting
with oxygen to make some product, and this
can react with additional oxygen to make some
other product. This might be CO2. This might
be a partially oxidized organic, and we certainly
want to make the partially oxidized organic
instead of CO2. So it turns out then for series
reactions that the reaction time is our most
important variable. Certainly temperature
is going to change the relative rates of formation
of B and C, and higher temperature will favor
the reaction with a higher activation energy, as discussed for parallel reactions in
another screencast, but if we just
concentrate on the effects of reaction time,
because that is what we can control. So as
the concentration of A decreases, then the
concentration of B, our desired product increases,
but as the B concentration increases then
the rate that B goes to C increases, because
typically the rate is proportional to the
concentration of B. So that means picking the
right type reactor or the right type of reaction
time would give us high selectivity. If we
run for this amount of reaction time we do
not convert all of our reactant, but we would
have high selectivity. If we run for much
longer of time we would have much lower selectivity.
So I am looking at concentrations here, and
of course concentrations in a plug flow reactor
are going to be proportional to flow rates
at a given location, and so the selectivity
at the intermediate times, selectivity here
would be something like I have product B.
It looks like it is about 2/3 of the total
B plus C. So selectivity here is around say
0.7 at the longer time. So this is the short
time. The selectivity out here at the longer
time, a very small amount of B maybe its
0.05. So a rather dramatic difference with
time. If we want to maximize selectivity for
series reactions, then we have to control the
reaction time.