Parallel Reactions and Selectivity - Chemical Kinetics Video Lecture | Physical Chemistry

Here I want to discuss multiple reactions
and selectivity. So in most situations, more
then one reaction takes place in a chemical
reactor. So selectivity can be more important
than rate of reaction, namely how much of the
desired reaction takes place compared to the
undesired reaction. So I am looking at 2 reactions
here, 2 reactions in parallel, where this
is our desired reaction, but we also have
a second undesired reaction, and we are talking
about selectivity. A typical way of defining
it is, though this is not unique, is the rate
of formation of our desired product, the rate
this first reaction occurs, divided by the
rate of formation of all products, namely
the rate of the first reaction plus the second
reaction occurs. This is often multiplied by
100 to make selectivity a percentage. The
idea is that selectivity is important in a
chemical reactor, and can dominate how we
carry out the reaction. So what we want to
look at is how it is affected by things like
concentration and temperature and type of
reactor. So the typical way then, we might
write selectivity in symbols for this system
of parallel reactions, is the flow rate at
the exit of our desired product over the flow
rate of the desired plus the flow rate of
the undesired, so these are moles per time
for say a plug flow reactor. We could also
define selectivity say for a batch reactor.
The number of moles of D in the reactor at
any given time over the number of moles D
plus the number of moles of U for these parallel
reactions. Lets discuss how concentration
affects selectivity. Higher concentrations
favors higher order reaction. Again looking
at these 2 parallel reactions, if our desired
reaction is second order, so this is the rate,
the rate constant, concentration squared,
and the undesired reaction is first order, then
if we wanted a higher selectivity we
want to carry out the reaction at high concentrations
of our reactant. There are varies ways we can
do this, such as increasing the pressure in
the gas phase reaction, changing the type
of reactor we use, but higher concentration
favors this higher order reaction. Next variable
I want to look at is temperature, because
temperature also has a large effect on selectivity.
Higher temperature favors the reaction with
the higher activation energy. So again two
parallel reactions, the activation energy, I'm just looking now at the rate constant, the
pre-exponential factor, and an exponential term
for the activation energy. So the activation
energy is going to be proportional to the
slope in this rate vs 1 over temperature diagram.
So in this diagram as we raise the temperature
as we go to the left, raise the temperature
then the rate increases. It increases exponentially.
So I am plotting the log of the rate vs the
inverse temperature, a straight line. The
desired reaction, slope in absolute values
since the slope is minus the activation energy
divided by r. The desired reaction activation
energy is higher, therefore its absolute values
of slope is higher. So our desired reaction
means that if I raise the temperature, so
this is higher temperature at this end, then
the desired reaction rate is higher than the
undesired reaction at this higher temperature.
At lower temperature the rates would be equal
or even less. The rate constants, and so if
we want to have a high selectivity for our
desired reaction, we want to run the reaction
at higher temperatures. Now other things may
occur as we do this, so we have to be aware
of them. Additional reactions might take place,
energy demand of higher temperatures, and
multiple things that we would have to worry
about. The type of reactor also influences
selectivity, and it influences it because of the concentration.
As I indicated earlier, higher concentrations then
favor the reactions that have the higher order
of reaction. So batch, tubular, or plug
flow reactors have a high concentration of
reactant at the inlet, and that concentration
decreases with time. CSTR we immediately drop
to our final concentration. So if want to
favor this reaction with a high order, we
should run it in a batch or a plug flow reactor
to get that reaction preferentially taking
place at the higher concentration. If we wanted
to favor instead this reaction then we are
better off running in a CSTR to get higher
selectivity for the second reaction, and then
the semi-batch reactor gives us additional
control. So you can imagine in that case we
want to look at for example A plus B reacting
in a parallel reaction to desired and undesired
products. Lets say R also. So here let's look
at order of reaction. So the rate of this
first reaction or rate of desired product,
so rate constant, and lets say it's second
order B, whereas the undesired let's say it
is first order in A and B. So I want the concentration
of B to be high. I might run the reaction
by starting with B, and then adding A to keep
the concentration of B high, and by carrying
out the reaction in semi-batch reactor, this
way I am going to favor the desired reaction
because the concentration of B is going to
stay as high as possible for this type of
system.