Intro
This video is brought to you by Ecoflow, but more on that later.
Why is Desalination so hard? Don’t get me wrong, we desalinate water all around the world today, but it requires huge investment and lots of energy.
Adding salt to water is as simple as dropping it in, and giving it a stir. But to get this salt back out, is a lot more difficult. But ironically, the bonds that hold the salt together are stronger than the bolds that hold the salt in water. So how exactly does it all work, and what new breakthroughs in the world of desalination can help us solve our freshwater shortages? Let’s figure this out together, I’m Ricky and this is two bit da vinci.
Chemistry
You’d think desalination was hard because it’s hard to break the bond between water and salt, right?
I thought so too but no I was wrong. In fact, when I started studying the process, I found that it’s actually the opposite.
To understand what happens when you try to pry water and salt apart, let’s start with what happens when you dissolve salt in water.
Dissolving salts requires disintegrating the little crystals, making room in water for the salt to dissolve, and then actually dissolving it.
From a chemical perspective, this process involves breaking strong ionic bonds within the salt crystal lattice (to break down the crystals), and hydrogen bonds in water (to make room for the ions), which both require an input of energy.
Then, it involves forming new ion-dipole interactions when water molecules surround and separate each ion (something called hydration), which releases energy. These ion-dipole interactions are the forces that stick salt and water together.
Now here’s the thing: ion-ion interactions, like those between sodium and chloride in salt crystals, are some of the strongest types of interactions known in chemistry. The second-strongest are the ion-dipole interactions between salt and water.
So that means that It’s harder to break apart the ions in salt than it is to separate ions from water. So if it was only about the energy in bonds, it should be easier to desalinate water than to dissolve salt in the first place.
This surprised me, and its a little counterintuitive, but luckily it all starts making sense when we think about thermodynamics, because It turns out i was only looking at half the picture.
According to the Second Law of Thermodynamics, we need to factor in Entropy or the measure of how disordered a system is!
The natural tendency is for entropy, or disorder, to increase. Build a house , a highly ordered and organized structure, and abandon it for a 1000 years, and it’ll return to a disorganized pile of rocks and soil. So the next time someone tells you your room or car is dirty, just blame entropy!
We’re trying to take dissolved ions that are running around freely in water, like a bunch of crazy kindergarteners, and stick them together to form a perfectly ordered crystal lattice. So to desalinate water, means we have to fight against nature, and put things into higher order state.
Even though from a chemical standpoint salt would rather be in a crystal than dissolved in water, from a thermodynamic perspective, the desalination process faces an uphill battle against entropy, which is thermodynamically unfavorable.
This means that, even with whatever innovative technology we come up with, desalination will ALWAYS require a significant energy input to overcome this entropic barrier.
Early Desalination technology relied on thermal desalination like this test setup here. We have an induction cooktop as our heat source, and this sealed chamber should condense water on the glass, and gravity should feed it to our collection chamber. Now induction cooktops are really efficient, but if we really wanted to make this setup better, we could use a heat pump for heating the water. By doing so we could produce between 3-5 times as much heat per unit energy.