INSTRUCTOR: Here I'm going to
briefly overview chemicals reactors
and describe four types of ideal reactors,
and indicate a few important aspects.
And so what we're looking at is
where we have reactants A and B entering the reactor,
and this is in this case a flow reactor.
And we have A and B that have not reacted,
leaving and then two products, D and U.
And the situation we're looking at
is if we were to have two reactions.
A plus B going to D,
A plus 2B going to U.
And the important thing here is
this is the desired product that we want to make.
And an example might be
we're looking at partial hydrogenation.
We had one hydrogen, or we had two hydrogens.
So typically,
selectivity is one
of the most important aspects of a chemical reactor.
And it's a flow reactor, one way we can define it
is the flow rate of the desired product,
or the flow rate of these undesired product
where this is moles say per second
leavin' the reactor.
So F representing molar flow rate.
The other way, if it where a closed reactor,
or a batch reactor, we'll talk about,
will be number of moles of D
over number of moles of U.
So this has units now of just total moles of product D.
And why selectivity is so important,
is because it affects the subsequent separation
of these four species.
And separations is one of the most expensive aspects
of reactors and plants.
Also if we react a lot of A and B to make a product U
that we can't use, then we really essentially throwing away
some of our reactant that we have to buy.
So the real purpose the reactor is to take something
that's less expensive, A and B,
make a product D that's more expensive.
And what governs behavior and what we're interested in
for chemical reactors, or mass balances,
and energy balance for the reactor,
and chemical equilibrium.
So this of course affects
how much product we can make,
and the important thing that we use
in these mass balances rate of reaction.
So we're gonna be careful in how we define that.
So rate of reaction, gonna use it as
moles per volume per time.
And this can have positive or negative sign.
If it's, in this case, a reactant,
it's gonna have a negative sign,
and it'll be a rate constant
which depends on temperature,
and for example, let's say
for the first reaction the rate of reaction is
first order A and first order B.
Now the important thing to keep in mind
this is not the same as the change in concentration
of A respect of time.
In some special circumstances that's true,
but in general it's not the case
if we're looking at this flow reactor
that's a steady state, the change in concentration of time
is equal to zero.
So we have to be careful,
when we talk about rate of reaction
that we use in the mass balances.
So let's look at types of ideal reactors
that we're gonna most commonly use,
and sometimes these are good representations
or real reactors, sometimes real reactors
are a combination of ideal reactors.
So I'm showing here four types of ideal reactors.
A Batch reactor, which you could think of
essentially as a laboratory beaker that's scaled up
for mixing it, and in general,
of course the concentration would change to time,
and temperature would change with time, right.
So both of these in general will change with time.
Continuous Stirred-Tank Reactor, looks like a batch reactor
where we continuously add, continuously remove material
at steady state.
So that means the concentration
and the temperature in the reactor are both constant,
they're not changing with time.
Derivatives respect of time are equal to zero,
because it's steady state.
A variation on the Batch reactor is a Semibatch reactor,
and this can take different forms.
In this case I've drawn it where for example,
we start with A in the reactor
and then we add B.
And this means it's not at steady state,
and therefore like Batch, both concentration
and temperature can change with time.
And then the final one we look at, Plug flow reactor
which we envision as a tube
that we're flowing material through.
It's again at steady state.
So that means derivatives respect to time
are equal to zero.
So the important thing, the concentration of A
and the other species of course,
and the temperature they change with location.
In other words, if we consider this volume
or a variable as we move down the reactor,
here the volume is zero, here is our total volume.
As we move down CA reacts,
CB reacts to form products.
So these three reactors, we model them,
we assume that they're uniform spatially,
or the other term used is they're gradientless.
Now if we look at the mass balances for these,
for the Batch reactor, change the number of moles.
So A is the number of moles of A respect to time,
is the rate of reaction times the volume
of the reactor contents.
And in that case where volume is constant
the number of moles, of course,
to the volume times the concentration.
So you can see it's independent of volume,
if the density doesn't change.
CSTR, steady state, the mass balance
is the molar flow rate entering.
So FA zero,
here this is the molar flow rate leaving,
minus FA plus the rate reaction times volume.
Semibatch reactor can take different forms
depending on exactly how we're running it.
The general form was we have a flow rate in,
of A or B, I'm just writing the balance for A now.
Flow rate out, which may or may not exist,
and rate of reaction times volume.
And then the plug flow reactor
is the change in the molar flow rate
as a function of cumulative volume
as we move down the reactor,
and that's equal to the rate of reaction.
I won't talk about the energy balance here, right,
I'll just mention one other term that's commonly used,
and that's conversion, where if we look
at a plug flow reactor conversion
would be flow rate in of A,
flow rate out divided by a flow rate in.
So this is a fractional conversion.
This also the case for CSTR,
that's both of these are steady state.
For a Batch reactor it would be number of moles of A.
We start with number of moles,
later time over the number of moles of A
that we start with.
And for Semibatch we have to be pretty careful,
that conversion may not be a good variable.
So this just a brief overview of ideal reactors
and their mass balances.