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Topic: Does temperature of the distillate and high pressure chemically affect the oil?  (Read 5010 times)

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Offline Maydi

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Hi there

I've just joined the forum. I am a newbie to distillation and I have 2 burning questions:

1. Does the temperature of the distillate affect the oils chemically?

2. How does high pressure (10 lbs per square inch) affect the oils chemically?

As background information, I am steam distilling frankincense (more specifically Boswellia carterii and B. frereana). At lower pressures (3-4 lbs psi) we are distilling for 8 hours; but at higher pressure (8-10 lbs psi) we run out of distillate in under 5 hours.

Looking forward to your expert feedback

Many thanks

Maydi

Offline discodermolide

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Well talking about "burning questions" in a forums like this, I'm not sure.
The compounds may decompose at their boiling point, or isomerise or react.
Pressure, I assume you are talking about steam pressure. A fast rate of steam will accelerate the distillation process,which is what you observe.
If the compounds have appropriately situated double bonds they may undergo a Diels-Adler or other thermal process.
So I would suggest to lower the steam pressure and do a relatively slow distillation.
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Offline Maydi

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Hi Discodermolide,

Thank you very much for your interesting insight. You are right to assume that in "pressure" I am talking about steam pressure. I do not have a strong organic chemistry background and am not sure if frankincense has double bonded compounds that may undergo any thermal processes. Are these double bonds you've mentioned isomers by the way?

A client of ours has asked that we distill the gums at higher pressures (10 lbs psi). We want to submit oil samples to him very soon but are kind of not sure what is happening here with the hot distillate.

In any case, thanks for the feedback again.

Maydi

Well talking about "burning questions" in a forums like this, I'm not sure.
The compounds may decompose at their boiling point, or isomerise or react.
Pressure, I assume you are talking about steam pressure. A fast rate of steam will accelerate the distillation process,which is what you observe.
If the compounds have appropriately situated double bonds they may undergo a Diels-Adler or other thermal process.
So I would suggest to lower the steam pressure and do a relatively slow distillation.

Offline discodermolide

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A quote from Wiki: "The oil's chemical components are 75% monoterpenes, sesquiterpenes, monoterpenoles, sesquiterpenols, and ketones.".
So yes there are lots of double bonds and alcohols.
So if the wrong conditions are chosen for distillation I would say that some of these may be unstable and react or isomerise.
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Offline fledarmus

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Natural products are very complicated mixtures. Occasionally you will find a single compound in high enough concentration to isolate, or that will have a very specific property that will allow a separation technique that will isolate it. More frequently, you will have the situation the Discodermolide reports - a method of isolation will give a complex mixture. In that case, any change in isolation conditions will result in changes in composition of the products, even if there are no chemical reactions. When you add the possibility of thermal reactions, the situation just gets more complicated.

Ideally, you adjust your separation technique to give you a suitable product, then try to make that technique as consistent as possible. Changes to the technique will change the product.

Offline curiouscat

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Have you considered applying a vacuum? Is vacuum steam distillation an option?

Offline discodermolide

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Have you considered applying a vacuum? Is vacuum steam distillation an option?

Applying a vacuum will just suck the steam out, so your compound(s) will not steam distill.
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Offline curiouscat

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Applying a vacuum will just suck the steam out, so your compound(s) will not steam distill.

Wikipedia says:

If the substances to be distilled are very sensitive to heat, steam distillation can also be combined with vacuum distillation.

Or this blog:

http://www.pelletlab.com/blog

Or this article: Nectarine volatiles: vacuum steam distillation versus headspace sampling

http://pubs.acs.org/doi/abs/10.1021/jf00081a037

Or this patent: High temperature vacuum steam distillation process...

http://www.google.com/patents/US4789554

Are you sure steam distillation cannot be done under a vacuum?

« Last Edit: October 23, 2012, 02:17:35 AM by curiouscat »

Offline discodermolide

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There are lots of things I don't know. This seems like a contradiction in terms to me.  ???
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Offline curiouscat

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There are lots of things I don't know. This seems like a contradiction in terms to me.  ???

Can you elaborate on what seems the contradiction? I'd be curious to hear your reasoning.

PS. I've myself never tried this in person. So I cannot be 100% sure it works either. :)

Offline discodermolide

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Well the purpose of applying a vacuum is to reduce the boiling point of the stuff you are distilling. By passing a gas (steam) through the mixture you will destroy the vacuum. I guess it is like pushing and pulling at the same time. ::)
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Offline curiouscat

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Well the purpose of applying a vacuum is to reduce the boiling point of the stuff you are distilling. By passing a gas (steam) through the mixture you will destroy the vacuum. I guess it is like pushing and pulling at the same time. ::)

The way I saw it was that the goal of reducing the Boiling point can be met by (a) A Vacuum (b) By adding an immiscible relatively high Vap. Pr. fluid i.e. steam

We agree that both are approaches that work by themselves so why not combine them?

Steam is not necessarily a "gas" if you have a condenser before your vacuum pump. If you condense out a large fraction of the steam before the pump it shouldn't be grossly overloaded.

The question remains about the relative merits of steam vs a vacuum i.e. How much steam do you need to get the same BP depression as say each 100 mm increment of applied vacuum. Which is more economical and is there a synergy at some stage in the calculation?

Pushing and pulling at the same time may not be a bad idea so long as the directions are right.  ;D


Offline discodermolide

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A steam distillation purifies only those compounds that are steam volatile i.e. at circa 100°C. This allows easier separation from the other non-volatile components.
A vacuum distillation for such a mixture may well require a long fractionating column with many plates to improve the separation or a spinning band distillation to achieve the same desired separation.
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