Part 29: Other Things - Distillation by Feinne

Alright, so let's talk distillation for a moment.

Distillation is a process that separates mixtures based on differences in phase behavior, specifically their boiling point. The idea is that we want to bring the mixture to a temperature at which part of it will boil and part of it won't. We'll then end up with two phases, a gas phase enriched in the low-boiler(s) and a liquid phase enriched in the high-boiler(s). The actual composition of the two phases at equilibrium will depend on the mixture you're dealing with.

So, the most simple distillation process just boils the mixture and takes the gas and liquid. For things with very different boiling points, this might actually be enough to get you a pretty good separation. For example, the idea of a Bromine/Chlorine mixture. But if we want to separate more problematic mixtures and improve our purity, we need to start adding a bit of complexity to our system. So, let's introduce the concept of stages now to our process and move to a three stage distillation. Instead of just one vessel, we have three of them, each at a different temperature. So, for example, if our first pressure vessel is at T, we'll put our second at T+x and the third at T+2x. Things get a bit tricky, now. See, we still only actually have two process outputs, one vapor and one liquid. It'll be helpful now to envision that these three vessels are on top of each other, with the coolest on the top and the hottest on the bottom. Imagine now that the liquid phase of each is able to run into the vessel below it and the vapor phase into the one above at some rate. The actual vapor and liquid we take as output comes from the top and bottom of the system.

Now that we've discussed that thought experiment, let's talk about how it's relevant to a real process. I suggested you think of the vessels as being on top of each other because the real workhorse of distillation is the distillation tower. A distillation tower is a large single vessel filled with a series of trays, able to interchange vapor and liquid with each other. Each tray works like its own single distillation process, and by stripping things to their bare minimum we're able to condense many of these processes into as small an area as possible. Small is, of course, a relative term and for very difficult separations and high throughputs your towers can end up quite large in practice.

So, distillation is an incredibly powerful process, but there are a few things to remember about it. The first, obviously, is that it can only separate two things at a time. If you've got a mixture of more than two things, a distillation process can only result in one pure output (after all, everything you put in has to come out somewhere). Another, though, brings us to the concept of an azeotrope.

So, an azeotrope is a mixture composition that we can't distill past. Basically, at that composition the liquid and vapor phases will be equally enriched in all of the components. These tend to form from mixtures of strongly interacting compounds with similar boiling points, for example ethanol and isopropyl alcohol. They represent a real problem for us because they provide us with a hard limit on what we can do with a simple distillation. That said, there are ways to go around them (literally, in fact). So, one thing we can do to get around an azeotrope is to actually add an extra component that we know will be easy to separate back out again. For example, we might add ethylene glycol to ethanol and IPA. While there's a binary EtOH/IPA azeotrope, there isn't a ternary EtOH/IPA/EG azeotrope. It's therefore possible to separate the pure ethanol from the ternary system and then separate the IPA from the EG. Generally, then, we'd just recycle the EG back to the start of the process. Another possible solution is to add one of our components in a pure or more pure form to the stage of the column where the azeotrope develops. The goal is to change the concentration of that stage such that it's past the azeotrope, at which point we can continue driving to where we want.

Those are the basics. It's really more complicated than most of that, but enough for you to understand what's what.