Balance and equilibrium.
They’re ideas that seem peaceful and harmonious
– surely that’s what we want in life?
Well, as an engineer, sometimes you need to
upset the balance in the name of solving problems.
Last time, we discussed mass transfer
and how two substances can form a mixture
through processes like diffusion,
where the individual molecules are driven
by their concentration differences towards
a state of chemical equilibrium.
That means having the same concentration on either
side of a boundary, or uniformly throughout a single
container, like food dye mixed into a beaker of water.
But what if one of the components of the mixture
is more useful than the other?
Worse still, what if one of the components
is actively harmful and you need to isolate it?
It’s time to separate the wheat from the
chaff.
[Theme Music]
Much like we’ve seen for momentum and heat,
different types of molecules, especially in liquids,
are driven towards equilibrium by differences in
their concentrations.
For some liquids, this happens naturally
through the process of diffusion,
when the particles of a substance spread
from a region of higher concentration to a region
of lower concentration.
But, let’s consider something a little different.
Imagine thoroughly stirring some caramel sauce
into butterscotch pudding.
No complaints here.
But accidentally mixing a dollop of mustard
into your pudding?
Well, that’s not going to taste so great.
You’re probably going to want to get it out of
there, unless you have some very unusual tastes.
Not that we’re judging.
OK, so separating mustard from pudding
might not sound too vital,
but separating different types of chemicals
from one another is something engineers need
to do pretty often!
More specifically, we often want to be able
to purify chemicals or extract one particular
product from an evenly balanced mixture.
That can be a crucial step in tackling an
engineering problem.
To achieve this, engineers make clever use of
temperature, pressure, and flow, to create the right
conditions for separating two or more substances.
Concentration differences might cause substances
to mix, but with the right apparatus, you can use
mass transfer to isolate substances instead.
Let’s take a closer look at three particularly
useful separation techniques: distillation,
liquid-liquid extraction, and reverse osmosis.
Now, if you’ve ever been to a bar, you’ll
know that the most popular drinks on the menu
have one thing in common: ethanol.
That’s the type of alcohol people drink.
And along with causing tipsy Saturday nights
and terrible headaches on Sunday mornings,
ethanol has lots of important industrial uses
as a disinfectant, a preservative, and even a
source of renewable fuel!
But in the form we produce it, it’s often
mixed in with too much water for those purposes.
Whatever you’re using it for, you’ll need
to find a way to separate it into a purer state.
And that’s where a process like distillation
can help.
You might have heard of distillation before,
and for good reason!
It’s really important for engineering.
It’s based on the fact that often the
components in a mixture have different
boiling points and volatilities;
that is, their tendency to vaporize at a
given temperature and pressure.
A distillation column uses temperature to
separate the chemicals in a mixture based
on their different boiling points.
You start by introducing the mixture into the column,
which can contain several round platforms, called plates,
that divide the column at different heights up its length.
Each of the plates has little holes in it,
like a sieve.
And those columns, by the way, can be as tall
as two stories high!
One of the important features of a distillation
column is that it’s heated from the bottom,
so that the top is much cooler, setting up
a gradient of warmer to cooler temperatures
from bottom to top.
That’s going to be important for separating
the substances!
There are two things going on throughout the
column.
Liquid is falling down the column through the holes
in the plates or into the spillover, the gap between
the plates and the walls of the column.
Meanwhile, gas is rising up throughout the
column, passing through the liquid and those
same holes in the plates.
Depending on the temperature at a given plate,
the parts of the mixture that are still firmly in the
liquid phase will tend to be drawn downwards,
while those chemicals that are at the right
temperature to become a gas will be drawn
upwards by the rising vapor.
So the more volatile substances transfer into
the gas phase and join the rising vapor,
while the less volatile substances, which are below
their boiling points, channel downwards as a liquid.
There’s also some energy transfer going on.
If some of the gas has risen high enough to
cool off and become a liquid again, it releases
some energy as it condenses.
When that energy is absorbed by the more volatile
substances in the liquid, it causes them to undergo a
phase change, turning into a gas and traveling upwards.
So you want the liquid and gas to interact as
much as they can at every point in the column.
The plates are positioned at certain heights
up the column so they’re at the specific temperatures
that work for the chemicals you want to separate.
Over time, they’ll collect concentrations
of different chemicals in the mixture.
And there you have it!
Chemical separation.
Like all processes in engineering distillation
isn’t 100% efficient at yielding products.
The concentration of the product might be higher
than it was when it started, but it might not be high
enough after just one pass through the column.
Well, no problem!
You can simply do what’s known as reflux, where
you pass the liquid back down to the bottom.
There, it enters a reboiler and rises as a
gas to be further separated in the column.
You can keep doing this until you’ve
separated the mixture into all its components
at the purities you want.
Besides separating ethanol and water, this sort of
process is also used to separate crude oil into different
useful components, like motor fuel and lubricants.
For other types of mixtures, you might need
a different strategy.
Consider alcohol again, but this time mixed
with oil.
Introducing alcohol to vegetable oil, the
kind you cook with, is part of the process of
extracting lecithin, a mixture of fats.
You can use it for all kinds of things, from
making chocolate flow better to intensifying
the colors in paint.
As an added bonus, extracting lecithin also
helps refine vegetable oil.
But you generally don’t want that alcohol
still in the oil when you come to actually use it.
That’s where liquid-liquid extraction comes
in.
Liquid-liquid extraction, is exactly what
it sounds like: extracting something from
one liquid into another.
It’s nice when engineers give these things
sensible names.
This time, instead of separating by temperature,
you’re separating liquids by density.
Distillation works for chemicals that are
willing to mix, like ethanol vapor and liquid water
so they can exchange volatile particles.
For liquids that don’t mix so easily, liquid-liquid
extraction provides an alternative.
In an extraction unit, the idea is to decontaminate the
mixture by transferring the contaminant, in this case
alcohol, to another liquid with a different density.
The liquid you’re transferring the contaminant
into is called the solvent.
With the alcohol-oil example, the solvent
would be water.
The important thing is that the solvent can mix with
the contaminant, but it won’t mix with the product –
the purified liquid that you want at the end.
That way, they can pass through one another
while only exchanging the contaminant in the
mixture.
So to do the extraction, you put the denser of the
two inputs – so either the mixture or the solvent – at
the top, and the lighter of the two inputs at the bottom.
That way, when they enter the extraction unit
they’ll swap places as the denser liquid sinks
and the other floats up,
forcing the solvent and the mixture
through one another.
In this case, water is denser than the oil-alcohol
mixture, so you’d feed the solvent in at the top and
the mixture at the bottom,
but different scenarios might require that the
solvent and the mixture to be the other way around!
Either way, you want to pick a solvent that the contaminant is more chemically attracted to than
in the original mixture –
in other words, that the contaminant is
more soluble in.
That way, the contaminant will transfer to
the solvent and get carried away with it, leaving
behind a purer version of the product.
Since alcohol mixes more easily with water than
oil, it gets drawn into the water as the water passes
through the mixture, leaving a purer form of oil.
So even when temperature differences can’t
help you, solubility can!
Other times, the best way to separate things
is through reverse osmosis.
That might sound like a science-fiction
surgical procedure,
but it’s really just a fancy term for filtering
molecules or small particles from the medium
they’re mixed into.
For example, of all the water on Earth, only
2.5% of it is fresh water, the kind we drink,
and most of that is locked up in glaciers!
The rest of it is undrinkable salt water – the
kind you find in the ocean.
And the amount of fresh water available per
person on Earth is rapidly shrinking.
But if you can find a way to separate out
the salt, you can make more fresh water from
salt water.
In other words, you need a way to tip the
see-saw of chemical balance to isolate water
from the stuff it’s mixed with.
To understand reverse osmosis, let’s take
a look at plain old osmosis first.
Osmosis is based on the ideas of mass transfer
we looked at last episode.
You have two liquids on either side of a semipermeable
barrier, which allows the solvent to pass through,
but not the other molecules in the mixture.
In regular osmosis, the solvent moves from the side of
the barrier with a higher concentration to the side with a
lower concentration until both sides are in equilibrium.
If we can reverse the process by forcing only the
solvent through the barrier while leaving the other types
of molecules behind, we can separate the two out.
To make things less abstract, let’s go back
to our salt water conundrum.
We wanted to filter out the salt from the salt
water, and we can do that with reverse osmosis.
All you have to do is pressurize the water
and force it through a semipermeable barrier.
With the right kind of filtering material,
the water is forced through,
while ions like salt and bigger things like
other molecules, tiny bits of dirt, and bacteria,
are held back and removed.
The stuff that gets filtered out is known
as a reject stream.
At its best, reverse osmosis can remove over 99%
of the salt and other unwanted stuff from water!
Some countries that don’t have much fresh water, like
Saudi Arabia, have giant plants that use reverse osmosis
to generate over a million liters of fresh water every day.
For the record, there are lots of other methods for
separating the different mass species in a mixture,
like absorption columns, stripping columns,
driers, humidifiers, evaporators, and more.
Diffusion, liquid-liquid extraction, and reverse
osmosis are three methods that come up a lot,
but lots of the products we use every day
are made of materials derived from some
of the other processes.
Unfortunately, rescuing your butterscotch
pudding is more difficult than it seems.
You’d have to use a combination of different
techniques to separate components of the mustard
and pudding, like distillation and filtration.
And even then, in the process you’d probably
also separate the pudding itself into streams of
isolated fats, sugars, dairy, starch, and salt.
On second thought, maybe you should just have
an apple instead.
Today we’ve covered the need for separating
chemicals from one another, and three different
processes engineers use to achieve that separation:
distillation, which separates substances 
based on their different boiling points;
liquid-liquid extraction, which uses
differences in solubility to transfer a
contaminant into a solvent;
and reverse osmosis, which filters molecules
from a solvent by pressurizing it through a
semipermeable barrier.
Next time, we’ll be taking a broader look at
materials, what kinds there are, the properties
they have,
and how we use them to build everything
from screwdrivers to skyscrapers.
Crash Course Engineering is produced in association
with PBS Digital Studios.
For more inventive approaches to daily life,
check out our sister channel ReInventors,
and meet scientists, inventors, and tinkerers
who are working to create a more sustainable future.
Crash Course is a Complexly production and this
episode was filmed in the Doctor Cheryl C. Kinney Studio
with the help of these wonderful people.
And our amazing graphics team is Thought Cafe.
