Here I want to briefly discuss what aspects of chemical reactor design we should be taking
into account when we choose the reactor temperature.
The reactor temperature actually has a big influence on a number of aspects, and of course
there's not just one temperature, the temperature can change for example in a batch or semibatch
reactor with time, and in a plug flow reactor it can change with distance down the reactor,
and for, for example, CSTRs in series, each one can be at a different temperature.
So a number of things are affected by the temperature, and certainly one of the most
critical ones is the reaction rate.
For most reactions, the rate of formation, rate of reaction, is related to some rate
constant, a pre-exponential factor, and an exponential dependence on temperature.
Now certainly the concentration dependence could be more complicated and have other temperature
dependence, but as a first approximation, this exponential dependence on temperature
is quite important.
Small temperature increase increases the rate alot, which affects the reactor size, and
so this certainly affects the reactor economics, and therefore, reaction rate is one of the
critical things that we need to take into account.
Second one is quite important, is equilibrium limitations.
So if we were to look at equilibrium conversion versus temperature, as we raise the temperature
for an exothermic reaction, the equilibrium conversion goes down, so this is exothermic,
and of course most large-scale reactions are exothermic.
So as we raise the temperature, equilibrium conversion goes down even though the rate
goes up, so this is going to affect how we choose the temperature in the reactor.
The third thing that is affected by temperature is the presence of side reactions.
Most chemical reactions have multiple reactions taking place, and so selectivity is really
critical.
Sometimes it really governs the overall process.
So I've plotted log of rate versus inverse temperature.
This exponential dependence, we expect straight lines.
So if we have two reactions, and let's say reaction 1 is our desired reaction, this is
desired, and at low temperature it's favored, at high temperature it's certainly not favored,
and so selectivity changes dramatically with temperature, and selectivity matters because
now much of our reactant could be going to a product that we don't want at the higher
temperature, and we could be creating a waste product at the higher temperature that we
actually have to spend money to get rid of, and so again the economics get affected quite
a bit by the side reactions and the selectivity.
And fairly closely related to this, but not quite the same thing, is the product distribution,
and what I mean by product distribution is for example in polymers, where we have a molecular
weight distribution, the length of the chains, the distributions of those lengths, can be
affected by the reactions.
We're not in this case having selectivity to products, so we don't want so much as changing
the product, and again changing the economics.
So again, temperature is going to affect this because it affects the rate constants of the
different reactions, say initiation, propagation and termination, differently.
So certainly something that's very important in thinking about the temperatures is, what
is our potential for thermal runaway?
We have an exponential dependence on temperature, so as the rate increases exponentially, this
means the rate of heat generation increases exponentially.
We certainly have major safety issues that we have to keep in mind, reactor stability
issues, and temperature is pretty critical in these.
The next thing in our list is the materials in the reactor.
You think about carrying out a chemical reaction, we can be at high temperature, we can be at
high pressure, and we have reactive molecules, and therefore what we make the reactor out
of and how fast the reactor material could actually react certainly is a function of
temperature, and this again very much affects economics.
The next aspect is the physical properties of the reactants and products, things like
vapor pressure, boiling point, does the liquid stay in the liquid phase at the temperature
we pick for the reaction.
Solubility, when we look at gas-liquid reactions solubility is a function of temperature, and
therefore that's going to affect the rate.
Miscibility, other physical properties, viscosity, are going to change by temperature.
And the last thing I'll add to the list here is the heat transfer area.
So exothermic reaction typically means we have to remove heat.
We have a jacket around the batch reactor.
Do we have a sufficient heat transfer area?
What's the maximum temperature we're going to get in the reactor?
Can we control the temperature?
Again, it relates to safety issues, economics, as we're paying for the heat transfer area,
paying for the heat transfer system, and this is very much going to be affected by the rate
of reaction, which is affected by the temperature.