Simple Distillation: How It Works

Simple distillation separates a liquid mixture in one vaporisation-condensation step: you boil the mixture, the vapour travels straight to a condenser, and the condensate is collected. It works when one component is far more volatile than the rest. Textbooks put the cut-off at boiling points about 25 °C apart — but vacuum moves that line.

We build the rotary evaporators that run this separation under vacuum, so we spend a lot of time arguing with that 25 °C number. It is a rule of thumb written for a flask boiling at atmospheric pressure on a hot plate. Change the pressure and the rule shifts.

What is simple distillation, and how does it work?

Heat a mixture of two miscible liquids and both of them contribute to the vapour above the surface. The more volatile component contributes more than its share — its molecules leave the liquid more readily at a given temperature. So the vapour that forms is richer in the light component than the liquid it came from. Condense that vapour and you have separated something.

That is the whole mechanism, and the important word is once. Simple distillation gives you one equilibrium stage. The vapour enriches, you condense it immediately, and whatever enrichment happened in that single step is what you get. Nothing in the apparatus re-boils the condensate to enrich it again.

So four things have to be present: a heat source, a boiling flask, a condenser cold enough to take the latent heat back out, and a receiver. Everything else — vacuum, rotation, a lift, a temperature controller — exists to control where on the temperature scale that single step happens.

Why does the textbook demand 25 °C between boiling points?

Because one stage only enriches so much. If the two components boil far apart, the light one dominates the vapour and a single condensation gives you a clean cut. If they boil close together, both are well represented in the vapour, and one pass hands you a mixture that is only slightly better than what you started with. You would have to run the distillation over and over to get anywhere — which is exactly what a fractionating column does, in one piece of glass.

So the 25 °C rule is not a law of physics. It is a statement about how much enrichment one stage buys you before the effort stops being worth it. Two facts hide inside it that matter for equipment selection:

• It assumes atmospheric pressure. The boiling points being compared are the ones in the reference book, measured at roughly one atmosphere.
• It says nothing about whether your compound survives. A pair well outside the 25 °C gap passes the rule easily and still ruins your product if the heavy one starts decomposing before the light one boils off.

That second point is where most of the real failures happen, and it is the reason our customers buy vacuum equipment for separations the textbook says need no help at all.

How does vacuum change that rule?

Drop the pressure over the liquid and it boils at a lower temperature — the liquid only has to generate enough vapour pressure to match what is pushing down on it. Our rotary evaporators pull to -0.095 MPa, which leaves a small fraction of atmospheric pressure in the flask. Every boiling point in the system comes down.

Two consequences, and people usually only think about the first:

1. You can boil below the decomposition temperature. This is the obvious win, and the reason vacuum distillation exists at all. A separation that is thermally impossible in an open flask becomes routine once the boiling happens far enough below the point where the product starts breaking down. The bath on an RE-501 runs from room temperature to 200 °C with ±1 °C accuracy — but under vacuum, most of the useful work happens at the bottom of that range, not the top.

2. The gap between the two boiling points does not stay put. Two compounds have two different vapour pressure curves, and those curves are not parallel. Lower the pressure and the temperature difference between them can widen or narrow. This is the part the textbook rule cannot express, because the rule quotes a single pair of atmospheric numbers as if they were fixed properties.

The honest instruction here is: do not assume, and do not let us hand you a number either. Whether vacuum improves the separability of your pair depends on the two vapour pressure curves, which you look up or measure for the specific compounds. What we can tell you is that the pressure knob is a real variable in the separation, not just a thermal safety measure — and a rig that only boils at atmospheric pressure has thrown that variable away.

There is a third effect that is pure hardware. Rotating the flask spreads the liquid into a thin film across the glass wall, so evaporation happens over a large heated surface instead of at one hot spot in a static pool. You get the vapour rate without driving the bulk liquid into a violent, bumping boil. On our units the drive motor is 40 W and the speed is adjustable across 0–120 rpm, and that speed is a process setting — a foamy extract and a thin solvent do not want the same film.

What is in a vacuum simple distillation rig?

Strip a rotary evaporator down and it is a simple distillation apparatus with the four elements rearranged for control. The flask still boils, the condenser still condenses — the difference is that each element has a knob on it.

That last row sounds trivial and is not. Being able to pull the flask out of the bath ends the run in seconds, which matters when the thing you are protecting is a heat-sensitive residue. On a static rig you turn off the heat and the mantle keeps radiating.

Here are the actual figures we build to on the 5 L machine — if you want the mechanism turned into a purchase decision, this is the table to read:

Note what is not in that table: the vacuum pump and the cooling source. The -0.095 MPa figure is what the glassware and seals are built to hold — the pump you attach has to actually reach it against your solvent’s vapour load, and the condenser needs a cold source that can absorb the latent heat you are putting in. Those are separate purchases, and mismatching them is the single most common way a good machine underperforms. We walk through the pairing in our guide to matching a vacuum pump and chiller to the evaporator you have chosen. If the rotary evaporator itself is new to you, the working principle and typical lab uses are worth ten minutes before you look at models.

Simple distillation vs fractional distillation: which do you need?

Fractional distillation puts a packed or plated column between the flask and the condenser. Vapour rises, partially condenses on the packing, re-boils, condenses again — dozens of small vaporisation-condensation steps stacked in series. Each one enriches a little, and the total is a separation a single stage cannot touch.

The decision is not “which is better”. It is a sequence of questions:

• Are you separating two products, or removing a solvent? Removing a solvent from a residue you want to keep is a one-stage job by definition — the volatility difference is enormous. That is the bulk of what rotary evaporators do, and a column would be dead weight.
• How close do the components boil under the pressure you will actually run? Not at atmospheric. At your pressure.
• How pure does the cut have to be? A single stage that gets you to “mostly” is a failure if the spec says “mostly” is not enough, and a waste of column if it already clears the spec.
• Does the mixture survive the column? A column adds residence time at temperature. For a thermally fragile compound, the very thing that improves the separation degrades the product.

Our honest position as the people selling the equipment: most of the separations described to us as “we need a column” are actually solvent removals where the customer had been told the 25 °C rule and never checked which side of it they were on. Answer the four questions before you buy glass.

Which flask size does your batch actually need?

Once the mechanism is settled, scale is the next decision, and it is mostly about the evaporating flask. Here is the range we build across the standard line:

The 1 L and 3 L machines (RE-101, RE-201D, RE-301) share a 260×150 mm bath and a 1000 W heater; the 5 L RE-501 steps up to a 280×160 mm bath and 1500 W. The drive motor is 40 W across all of them, and every model holds the same -0.095 MPa and the same ±1 °C bath control. What changes with size is how much liquid you can put through per run and how much heat the bath can push into it — not the quality of the control.

Two direct pieces of advice. Skip the 5 L if your batches never outgrow what the 3 L RE-301 handles; you will pay for a bath you are not filling and handle glassware heavier than the job needs. And do not size on the flask nameplate — you never fill an evaporating flask to its rated volume, because the whole point is a rotating film, not a full bottle sloshing. Size on the batch you actually run, then go up one step.

The full parameter sets are on the product pages: the 5 L RE-501 with its 1500 W bath and 280×160 mm heating well for pilot-scale batches, and the 1 L–3 L range covering RE-101, RE-201D and the 50# flange-port RE-301 for bench work. The rest of the line, including the configurations not listed here, sits in the rotary evaporator category. If you would rather work through the choice as a sequence of questions than compare tables, start with our equipment selection guide.

Where does simple distillation still lose?

Vacuum widens the range of what one stage can do. It does not make one stage into ten. Simple distillation — with or without a pump attached — is the wrong tool when:

• The components boil close together at every accessible pressure. No amount of pump gets you a clean cut from one equilibrium stage. Use a column.
• The mixture forms an azeotrope. At the azeotropic composition the vapour and the liquid have the same composition, so the enrichment mechanism has nothing to work with. Neither simple nor ordinary fractional distillation gets past it.
• The compound decomposes even at the lowest pressure your rig reaches. This is where the conversation moves to short-path and molecular distillation, where the vapour travels a few centimetres to a cold surface and the residence time at temperature collapses. If you have hit that wall, our molecular distillation equipment selection guide covers the next tier up.
• The volatile is the impurity and it is present in traces. One stage removes bulk. Removing the last fraction of a percent is a different problem.

Fragen und Antworten

Is a rotary evaporator doing simple distillation?

Yes — mechanically it is a single-stage distillation with three modifications: the pressure over the liquid is reduced, the evaporating surface is a rotating thin film instead of a static pool, and the heat comes from a controlled bath rather than a plate. There is no column, so there is no fractionation. One vaporisation, one condensation, one receiver.

Why does vacuum let simple distillation handle separations a textbook says need a column?

Two reasons. Lowering the pressure lowers the boiling temperature, so separations that would decompose the product at atmospheric pressure become possible at all. And because the two components’ vapour pressure curves are not parallel, the temperature gap between them changes with pressure — it can open up. The atmospheric boiling points quoted in reference tables are one point on a curve, not a fixed property.

What vacuum level do these rotary evaporators reach?

Our units are rated to -0.095 MPa. That is what the GG3.3 borosilicate glassware and the seals are built for; the pump you attach determines whether you get there in practice against your solvent’s vapour load.

How do I pick between the 1 L, 3 L and 5 L models?

Size on your working batch volume, not on the flask rating — the flask is never filled. The 1 L and 3 L units (RE-101, RE-201D, RE-301) run a 1000 W bath at 260×150 mm; the 5 L RE-501 uses a 1500 W bath at 280×160 mm. Vacuum rating, 40 W drive, 0–120 rotation and ±1 °C bath control are identical across the line, so the choice is throughput, not capability.

Does rotating the flask improve the separation itself?

Not the thermodynamics — the enrichment per stage is set by the components and the pressure. Rotation improves the rate and the control: a thin film over a large heated area evaporates fast without superheating the bulk liquid, which is how you avoid bumping and how you keep the residue’s exposure to heat short. Speed is adjustable from 0 to 120 for exactly that reason.

Choosing the rig, not just the method

Work in this order. Decide whether you are separating two products or stripping a solvent — that alone settles simple versus fractional for most jobs. Check your components’ boiling behaviour at the pressure you intend to run, not at atmospheric, and check the decomposition temperature of whatever you are keeping. If the answer is one stage under vacuum, then size the flask on your real batch volume and go up one step; the bath power and size follow from that, and the control specs do not change across the line.

Then treat the pump and the cold source as part of the machine, not accessories. A -0.095 MPa rating is a promise the glassware makes, not one the pump makes for you. If you want the flask volume, condenser type, vacuum pump, cooling source, bath temperature, lift mode and solvent compatibility confirmed against your actual process before you commit, send us the mixture and the batch size and we will work through it — that is the conversation we would rather have than a parts list. Common questions we get at that stage are collected in our equipment FAQ.