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Reading about water vapor distillation, I found two different approaches:

  1. Direct
  2. Indirect

The direct approach has an arrangement like this:

enter image description here

Here the chamber contains the water and the plant. It is cooled and as the water vapor flows away it carries the low volatile compound vapor of the plant/vegetable. Both condense and the oils can be obtained.

The plant matter is in the same pot as the water.

The indirect approach is setup like this:

enter image description here

Here the water vapor is generated in the first container (the one furthest to the left) and when flowing through the second flask, it carries the oil vapor. I should remark: the plant matter is in a different pot than the water.

Question

Do you see any fundamental differences in these processes or something related?

It might have to do with the aggregation state of the material we need to extract, but I'm not sure.

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    $\begingroup$ Larger diagrams where we can actually read the text and/or clear labels would be really helpful here... $\endgroup$
    – Zhe
    Commented Jun 4, 2018 at 21:34
  • $\begingroup$ The images are fine, but I have no idea what's going on because I don't know what the substances are. $\endgroup$
    – Zhe
    Commented Jun 4, 2018 at 23:38
  • $\begingroup$ The upper picture is very small and grisly. Upon a second look, I get it, and the problem is that extracted stuff drops back in the water vessel below. $\endgroup$
    – Karl
    Commented Jun 5, 2018 at 5:58
  • $\begingroup$ Of course not. The idea with the second setup is that you don't have your vegetables or whatever in the high-powered steam can, which walls might be considerably hotter than 100°C. Steam distillation is used to distill non-volatile, high-boiling (>100°C) compounds without heating them above 100°C. Picture 1 looks cute but is a technical nonsense imho. More AD1500 alchemy-style. $\endgroup$
    – Karl
    Commented Jun 5, 2018 at 6:24
  • $\begingroup$ Partial pressure of what? I said the wall of the steam can can be hotter than 100°C. Unless you run it at low power, in which case you will never finish distilling your product, which has a low vapour pressure at 100°C. You need to go through a lot of water for a successful steam distillation, unless your compound actually boils below 100°C. In which case I wouldn't usually do steam distillation. $\endgroup$
    – Karl
    Commented Jun 5, 2018 at 6:37

1 Answer 1

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There is no fundamental difference between the two methods. In both cases steam is formed and it is passed through the plant material. The intention is to extract the non volatile water insoluble essential oils from the plant. The method is called steam distillation.

On the other hand, normally it is not done in this way. Since generally only the essential oil is of interest, the integrity of the plant does not matter, it is placed in water and the whole is heated and the water is distilled. The students do this in the lab with cinnamon and orange peels, for instance. It is much easier because you only have to heat one vessel. In these two step setups, you must isolate the part containing the plant very well so that the steam does not condense, once the whole system is hot enough. Preferably, you should also heat the part containing the plant, especially in your second example.

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  • $\begingroup$ Thanks! Possibly the direct method is what you described, I choose that image because I suppose them almost the same... $\endgroup$
    – user43021
    Commented Jun 5, 2018 at 8:48
  • $\begingroup$ Well, not exactly. In your first image the plant is above the water. What we normally do is to put it inside the water. $\endgroup$
    – Raoul Kessels
    Commented Jun 5, 2018 at 8:49
  • $\begingroup$ But we can call it direct method $\endgroup$
    – Raoul Kessels
    Commented Jun 5, 2018 at 8:50
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    $\begingroup$ It complicates the setup but it might have sense if you need the plant for a second different extraction, maybe with solvent. When you boil the plant you get a mass that can not be handled very well. If you need to extract it after the distillation and then filter it, it might be very complicated. Then it is possibly better to use a two stage steam distillation to maintain the plant as whole as possible. $\endgroup$
    – Raoul Kessels
    Commented Jun 5, 2018 at 9:04
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    $\begingroup$ I like how people here think they know who downvoted them. :-/ And I still say method one and even more so the all in one pot variant are inefficent both speed-wise and in being dangerous to your precious product, which comes in contact with the hot (>100°C) wall of the vessel. It's a lousy technique, which might work for some separation problems, but is also slow. The one advantage is that it doesn't need the separate steam can. Poor people's steam distillation, who become even poorer when the energy bill comes in later. $\endgroup$
    – Karl
    Commented Jun 6, 2018 at 19:13

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