To separate these into smaller fractions at atmospheric pressure would need extremely high temperatures (and a taller tower). So we use the fact that the boiling point of a liquid is less at lower pressure. This is something that mountaineers know about.
They have always found that it is difficult to make a decent cup of tea up Mount Everest. This is because, up there, water boils at 72°C rather than 100°C. And this isn't hot enough to make tea (that tastes nice!). So why is the boiling point lower?
The boiling point is lower because the air pressure is less at the top of the mountain. And boiling point depends on air pressure. In fact, the lower the pressure, the lower the boiling point. It's even possible to boil water at room temperature by reducing the pressure to nearly a vacuum.