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To extract essential oils from plant matter (amongst other purposes), steam distillation is often used. I've seen a lot of different glassware setups for this. I've sometimes seen them broken into two categories:

  • Internal steam distillation
  • External steam distillation

Here's my (admittedly shaky) understanding of each:

Internal

With internal steam distillation, you have one boiling flask containing plant matter and liquid water. You simply boil the mixture and send the gas to to the condenser.

External

With external steam distillation, you have two flasks. One contains water only. You heat this one and route the steam into the second flask. The second flask contains a mixture of water and plant matter. The steam from the first flask combines with the liquid mixture in the second flask. Optionally, the second flask also has a heating mantle. The output of the second flask then goes to the condenser.

Differences?

What is the effect of using one or the other setup? External is substantially more complex, so I can't imagine it would be used if it didn't offer some advantages in some circumstances. When would you use external, and why?

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The main purpose is to distill the component (essential oil) with quite high boiling point. You have to think of distillation of immiscible liquids, http://www.chemguide.co.uk/physical/phaseeqia/immiscible.html where you can take advantage of the equillibria in liquid and gas phase separately.

In short - two very beneficial things happen:

  1. boiling point of the mixture is lower than boiling point of water (and substantially lower than BP of the oil)
  2. the composition of vapor is independent of composition of liquid, i.e. you can distill out even the smallest fraction of oil (c.f. Raoult's law for miscible liquids)

On the other hand, the partial pressure of the oil is small, let's say 5% that of the water, therefore you need to evaporate and condense 20x more of water than of oil. To achieve this, you can either put all the needed water in the flask and do the "internal" distillation, or add just a small amount of hot water and use steam from external source in "external" setup.

In general, it is simpler to just boil pure water (or even have a direct source of steam) and use it for distillation, than to take care of the whole process in one flask.

As such, I would see the "external" setup simpler and more robust, even though it needs little bit more glassware.

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  • $\begingroup$ I've never understood this too well myself. I assume that internal steam distillation is worse because you rely on dissolving the compounds you want in the boiling water (often quite low solubilities) before they're allowed to exert their vapour pressure (most of the material being under a sheet of water)? I imagine the vapour pressure of a substance in a dilute solution is smaller than that of the pure substance at a given temperature. $\endgroup$ – Nicolau Saker Neto Nov 19 '13 at 14:23
  • $\begingroup$ Meanwhile external steam distillation theoretically has the undissolved substances directly exposed to the atmosphere in the flask, so they have maximum evaporation while the steam flow ensures matter transport outwards of the flask containing the desired substances without using a pump. Is that it? $\endgroup$ – Nicolau Saker Neto Nov 19 '13 at 14:23
  • $\begingroup$ "The main purpose is to distill the component (essential oil) with quite high boiling point."--This is the description of steam distillation in general. But my question is about two different sub-types of steam distillation: internal and external. Both are steam distillation, and both are said to allow distillation of substances with high boiling points. But what are the advantages and disadvantages of each? $\endgroup$ – rlkw1024 Nov 19 '13 at 15:36
  • $\begingroup$ Summarized illustrated here: erowid.org/archive/rhodium/chemistry/equipment/… The partial pressure of water is much higher, than of the compound distilled, therefore you need LOT of water to go through. You have to supply it, somehow. To supply directly steam looks as simple solution... Once you are happy with explanation, I will reiterate it into the answer itself. $\endgroup$ – ssavec Nov 19 '13 at 21:07
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Suppose you have 100 tonnes of harvested spearmint from a farm. It is degrading in air and biologically as you look at it. Batch internal steam distillation will be a procedural nightmare. You will burn the biological where it touches the heated inside of the flask. With external steam distillation, you continuously feed in biological and live steam, capture the mixed distillate (carvone), then discard the exhausted outflow. You now have excellent process control for the temperature and volume of steam admitted, and efficient continuous throughput.

If it were peppermint, the menthol product is a solid at room temp. That is a whole different ballgame re clogging. A production line requires trained skilled people. If it requires intelligent people, you only have a pilot plant.

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