Today I'm going
to be discussing a little topic that is extremely important in chemical
engineering and that is separations. Separations is basically exactly what the
name implies but it goes into massive depth in the field of chemical
engineering. I personally am taking a class right now that just serve as an
intro into separations and it's extremely difficult and complicated already. On
the most basic level, separation processes are processes by which things are separated
from other things. Now that seems pretty redundant and almost meaningless so
let’s get a bit more specific. A few weeks ago, I talked about distillation.
Distillation is a specific type of separation and there are even many
subsections of distillation that are further divided into more specific types
of distillation (tray, flash, etc. and there are even subdivisions of these
types honestly).
So we’ve got one example of separations. But what other kinds are
there out there? Let’s go with something simple and then expand it a bit to an
industry example: filters. Filters are everywhere around you. They’re in the
air intake vents in your house or apartment, on your AC unit, in your fridge,
in your Brita, everywhere. Basically, filters are a way of separating things
based on the size of the particles involved. You could separate rocks from
water with a towel, or cheese from whey with cheesecloth and you’d be filtering
in each case. Now let’s look at a specific industry example of filtration. You
may have heard of reverse osmosis from somewhere or another. It’s a filtration
process that relies on specific membranes through which water molecules (and
anything smaller) can pass through unhindered while bigger particles (dirt,
salt, or metals) cannot. Reverse osmosis is extremely useful in regions that
don’t have access to freshwater because it can turn saltwater into drinkable
water. Now there are major downsides to reverse osmosis – it is extremely
energy intense and thus is really only optimal for places without access to
freshwater from other sources. That is not to say that it’s not an incredibly
powerful separation method though. For most of human history, the only way to
separate salt from water was to evaporate it and re-condense the evaporated
water in another container. This is time consuming and very difficult. Reverse
osmosis is continuous and freshwater can be continually generated as the
process goes. See the below picture for a very basic diagram of this process.
It’s just like any other filtration. You’re relying on the fact that smaller
particles will fit through smaller holes while larger ones will not. Now filtration
can be a little more complicated a times – filters can also rely on differences
in chemical or electric properties to remove impurities rather than just pores.
Filtration is just one example of separations though. The field is
quite possibly endless and that’s why I’m going to try to go into just some of
the biggest ones here and give enough depth to show why they’re each important.
Let’s move on to absorption and stripping (not that kind of stripping).
Absorption and stripping are extremely common methods for liquid-liquid,
liquid-gas, or gas-gas separations. The idea is that two streams can be passed
by one another where one stream contains a contaminant that we want to remove
and the other stream will be pure but used to absorb some of the unwanted
material. So say we have some methanol vapor (very bad stuff to consume) contamination
in an air sample and we want to remove it so that we can use the air for
medical purposes. We could use a water stream running counter-current (in the
opposite direction) of the gas stream in a separation unit to pull out the
methanol and purify the air so that it’s safe to use. The methanol gets caught
up in the water much more easily than it does in the air and so as the streams
pass one another, methanol is pulled into the liquid, while the air stream is purified
and passes through the other side. The methanol is now in the liquid phase and
though methanol-water is very hard to separate, it is much less dangerous in its
liquid form than its vapor form. You still shouldn’t drink it though. Methanol
is extremely dangerous and can cause you go to blind in even very small
amounts. It has its uses though. Methanol-water (the mixture of the two) is a
very good solvent and is often used in analytical chemistry.
In industry, separations are easily the most important part of any
chemical production processes. In any process, there are unwanted byproducts
and contaminants that must be removed before they can be taken to market. If
you ever drive past a refinery, the tallest towers you see are the separation
towers and they’re the most expensive component on-site because they’re extremely
important. You can’t sell your gasoline if it has water in it or you’re oxygen
if it’s contaminated with borane gas. You have to separate out the unwanted
components. Now you can never truly get pure separations (100% of one thing).
There will always be some small contaminants. That’s why you shouldn’t freak
out when you hear there’s arsenic in the water. There’s arsenic in all water
but in your drinking water, the amounts are generally of no concern. There can
be cases where the contaminants exceed acceptable levels and then separations
must be done. Look at Flint, Michigan for example. Due to improper handling
with the switch over from Detroit’s water supply to lake water, the acidity of
the lake water caused corrosion in the old lead pipes and now lead has been
seeded into the water supply. To remove this lead, you need a high grade filter
specifically designed for heavy metal filtration. I should also point out that
again, having a tiny amount of lead in the water is not dangerous. All water
around the world contains some small amount of lead but the amounts are
generally so small as to be no concern (I’m talking about parts per trillion,
and not parts per million – PPM is generally a common measure for the amount of
contaminant in something). All that said, being able to separate contaminants
from the target product is extremely important not only to big companies but
also to individuals.
There are easily hundreds of different types of separations and I’ve
only just gone over a few of them today. Many of them are very complicated and
a bit hard to explain in layman’s terms. For example if you really want to go
down the rabbit hole for separations, lookup “tray hydraulic distillation” or “packed
tower” for some really dry and technical (though extremely meaningful) reading.
For most of us, we would never even use the word separation processes to
describe the kinds of things we do that might fit under that heading. We filter
stuff, we boil off stuff, we absorb stuff, but all of those things fall into the
field of separations. Almost ever drink you’ve ever had was probably separated
somewhere in the production line. Fruit juices are separated from pulp via
filtration. Salt can be separated from salt water via boiling to make sea salt
and if you re-condense the air, pure drinking water. I’ve included some interesting
links below that the kids might want to check out if you want to try some basic
separations experiments at home. I hope I’ve done an acceptable job of
introducing the field of separations to you guys. If you have any questions or
want to see a specific topic, post it in the comments and I’ll talk about it
next time.
http://www.rsc.org/learn-chemistry/resource/res00000386/separating-sand-and-salt?cmpid=CMP00005908
Image source
http://puretecwater.com/what-is-reverse-osmosis.html
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