INSTRUCTOR: In this screen cast,
I'm gonna provide just a brief introduction
to multi-stage batch distillation.
The idea is we have a still, that we start out with liquid
that we want to separate, in this case, a binary mixture.
We're gonna have a column that's going
to improve the separations, the vapor that leaves is
then condensed in a condenser.
And then much of it is recycled as liquid.
And some, is collected as the distillate.
And for the conditions in this particular column,
we're showing that the more volatile component B
is enriched significantly in the liquid
that we start collecting, so this is the distillate
and this is the reflux, on the right we show
the mol fraction of vapor over mol fraction of liquid
for this particular binary mixture.
So this is basically the blue line is equilibrium data,
for example, say from Raoult's Law for ideal solution.
And the red line is the operating line.
So, the operating line goes through this red point
which is the mol fraction of the distillate.
So this is the distillate and you can see
the still mol fraction here and
this is slightly different conditions,
this is starting with a mol fraction of .31.
A reflux ratio of 4.2 and the reflux ratio
is the liquid flow rate over the distillate flow rate.
And then the slope of this line
is related to this reflux ratio.
So, batch stills are used typically
if we have smaller quantities that we want to separate.
They're a lot more flexible, the same system can be used
in sequence for different mixtures.
It also allows each batch to be identified.
You know, what were the exact feed conditions,
the conditions where we carried out how long, et cetera.
And for pharmaceuticals or for food this could be important.
So, the typical way that this would be run is
we start out where we close this valve
and we completely reflux at the start
and let the system get to equilibrium.
This allows us to get the best separation.
Then, we open the valve and start collecting.
Now unlike the diagrams you see,
say for continuous distillation,
the McCabe-Thiele type diagrams,
as we distill the liquid, the mol fractions
in the still change and the mol fractions
of the distillate change which means this red
and the blue points are moving
which means the line is moving, the red line is moving.
The blue line is just equilibrium data
so that doesn't change, but the red line is going
to move as distillation proceeds.
But the slope remains the same, just R plus one.
So, let's look at what this YX diagram
would look like at a later time.
So, here I'm showing the same Y versus X figure,
but now we've evaporated at .23 mols from the still.
So, that means the distillate composition
has changed significantly and the still composition
has changed so the diagram, the operating line moved.
The slope has not changed and so, this continuous change,
we take in account when we do the calculations,
determine what our distillate composition
would be for a given condition.
Well, the other aspect we wanna look at is
what happens to the temperature.
So, temperature is gonna be higher
at the bottom of the still then at the top,
but the temperature and all locations also changes
with time because the composition changed
and the temperature increases because
as the batch distillation proceeds,
we become enriched in the less volatile component,
the component with the higher boiling point.
So, let's look at a TXY diagram.
So, we get a better idea and we'll look
at the at two conditions, so here we're looking
at temperatures versus mol fraction.
Again, the blue line corresponds
to vapor liquid equilibrium and the stages are counted off
by referring to the YX diagram and the operating line there.
The point that's important here is the, this case, 5 stages
and the temperature is 86 degrees
and the still when we start, but it's only 77 degrees
at the top of the column, so 86 degrees you can
see the blue and then 77, it's a little harder
to see this red point here and then,
after we've evaporate at 30%, notice the stages
and the locations have changed.
The temperature's now 93 degrees in the still.
And then, 78 at the top of the column.
Look at one more figure where we've evaporate even more.
So, here is the same starting conditions,
but now the temperature and the still is 103 degrees.
And the temperature at the top is 94 degrees
and this is because we've now evaporated 50% from the still.
Well, if we go back and let's look at this YX diagram again.
The separation, the composition, the distillate
is going to depend on how many stages.
I've shown 5 stages, obviously more stages,
we get better separation, also depends on that reflux ratio
because it's related to the slope at this red line.
If we increase the reflux ratio,
we're going to get better separations.
Now, of course, if we increase the reflux ratio
then it means the process is slower
since we're sending more back into the column
and only collecting a smaller amount.
Now, these columns can be operated where the reflux ratio
is constant, that's essentially what I'm showing here,
the slope of the red line doesn't change.
Or as distillation proceeds, we could increase this
reflux ratio which would result
in increasing our separations giving us
a higher product composition, but this is at the expense
of taking more energy into taking a longer time.
So, in a subsequent screen cast,
I'll discuss how we do the analysis
for a batch reactor, how we use the number of stages
to determine our compositions and how we use them
and how they change as a function of time.