Sciencemadness Discussion Board
Not logged in [Login ]
Go To Bottom

Printable Version  
 Pages:  1  
Author: Subject: steam distillation - best method?
BenZeen
Harmless
*




Posts: 37
Registered: 15-1-2010
Member Is Offline

Mood: No Mood

[*] posted on 14-8-2010 at 01:08
steam distillation - best method?


Hi
I have seen two methods for steam distillation: the first one where steam is generated externally and then delivered into the flask via tubing and a long stem inlet adapter. The second method is to drip water into the flask with an addition funnel, thus generating the required steam internally.
I imagine that the first method may result in less water being collected in the receiving vessel, but the second method seems very convenient.
Which method is better/safer?
View user's profile View All Posts By User
peach
Bon Vivant
*****




Posts: 1428
Registered: 14-11-2008
Member Is Offline

Mood: No Mood

[*] posted on 14-8-2010 at 09:34


If you've ever cooked stew or vegetable soup, you may have noticed oils float to the surface when just warming the ingredients for a while.

On that basis, I'd stick your organics in the 'pot' and leave it gently simmering for a while to drive the oils out. Note that good stew often takes a day of slow cooking or more to yield a tasty result.

If the organic will yield the oil under simmering, you can safely leave the flask to cook all day, whilst getting on with something else. When happy that no more is coming to the surface, ramp that bitch up and roast to drive the oil over.

If the oil is particularly prone to leeching under simmering, you may not even have to distill it, you can simply simmer it for days and then scoop the oil off the surface. Cooks (actual kitchen, culinary cooks) do this, 'skimming' or 'defatting' the stew or soup so it doesn't leave a greasy lining on your mouth. It requires low technology and cost equipment, a spoon and a minute or two. That's easier than dealing with pints and pints of steam distilled emulsion, and having to worry about addition funnels and steam generators running out, or the flask going dry and the material burning.

In terms of simplicity and cost, I'd say that's the better option. You won't need external steam generators and you won't need to start poking special steam inlets into the flasks, or buying them.

With regards to the quantities of water involved, I suspect this option may also require less, due to the oils having a chance to 'escape' prior to being pushed over.

Regardless, you'll likely need a large collection vessel on the end. For instance, if you're doing 250ml of oily raw organic material (like twigs, leaves and things), you can expect to see a good 500ml at the other end, with not a huge amount of oil in it.

One option, to deal with the water, would be to add a puddle of a good, immiscible solvent for the material you're distilling across. Give it a stir as the mix comes over. As it approaches the top of the container, stop stirring, wait for it to all settle, and then decant the water. The product should, hopefully, be sat in the immiscible solvent. You're essentially running an odd, Frankenstill by doing so.

This is a little similar to the way product can also be harvested using Dean Stark traps, or their reverse versions, when they have taps on them for solvent splitting and take off.

This all changes if you're distilling a 'pure' oil, as you'll get better turbulence and mixing between it and the water if you blow steam directly into the oil.

[Edited on 15-8-2010 by peach]




View user's profile View All Posts By User
Panache
International Hazard
*****




Posts: 1290
Registered: 18-10-2007
Member Is Offline

Mood: Instead of being my deliverance, she had a resemblance to a Kat named Frankenstein

[*] posted on 15-8-2010 at 03:17


good question, i have been meaning to write a comment on steam distillation for quite sometime as it is often poorly understood by people using the technique. It is also poorly understood by myself, but thats because i'm a phys chem hack, i have good practical points to offer and hopefully in offering these, others with a greater theoretical understanding can explain or correct me.
Unlike an ordinary distillation steam distillation's rely essentially on somethings partial solubilty in water at the temperature and pressure you are running the steam distillation at. this partial solubility allows the systems pressure to be made up of the partial pressures of the two components in the two component system and hence the vapour above the liqiuid to be likewise made up of the two components.

Gee's i could write several pages on this without pausing so for a first post i'll try to be succinct (that's code for i'm running out of time)

In relation to your query the third and most common laboratory method is simply to mix the chemical with water and distil, this has practical problems on a large scale as your ability to recover all the compound you are trying to distil is limited but for lab scale it can be effective/good enough.
As always your choice of setup is very system dependant, ie what are you distilling. Adding salts to the water can greatly affect the speed of the process, slight vacuum likewise, slight superheating of steam likewise.

Bah frustrating to have to stop this communication i have to go however. I'll write more later.




View user's profile View All Posts By User
peach
Bon Vivant
*****




Posts: 1428
Registered: 14-11-2008
Member Is Offline

Mood: No Mood

[*] posted on 15-8-2010 at 04:10


I am also not all that experienced when it comes to steam distillation, as I tend to go straight for the 'pure' oils and vacuum distillation or a high solubility solvent option. I would be willing to bet some of the older chemists here have done a fair bit of it back in the day when solvents were more of an issue to get. But they're staying quiet at the moment it seems. Being 'a youngin' I'm somewhat spoilled with regards to that, with the big 5l bottles of dirt cheap, easily available volatiles. The quantity of water involved in steam distillation is annoying; to be polite.

However, I am interested in how effective the simmering and skimming method is, in terms of yields, as opposed to actual steam distillation.

[edit]I just piled 400ml of cloves into a flask for a simmer. I'll see if any skimming is possible over the next day or three.

As a variation on Panache's salt suggestion, that may also help with the resulting emulsion a steam distillation produces. By heaping salt in until it's saturating, you'll severely ionized the water (increasing the polar difference between it and the oil) and help force it to split; a bit like adding lemon juice to milk, but not quite. Even organic solvents like DCM (which usually splits great around water) will suck up some moisture or cross over the phase boundary to some extent. Salting up the water resulting from a steam distillation (or even in a sep funnel) can help push the DCM away and in with it's friends, taking the oil with it. It's a fairly common 'trick' during separations, particularly when the bastard emulsion child appears; heart attack inducing quantities of salt.

John

[Edited on 15-8-2010 by peach]




View user's profile View All Posts By User
BenZeen
Harmless
*




Posts: 37
Registered: 15-1-2010
Member Is Offline

Mood: No Mood

[*] posted on 15-8-2010 at 05:35


thanks for the advice!
View user's profile View All Posts By User
Panache
International Hazard
*****




Posts: 1290
Registered: 18-10-2007
Member Is Offline

Mood: Instead of being my deliverance, she had a resemblance to a Kat named Frankenstein

[*] posted on 15-8-2010 at 18:32


...continuing on, wrt peach's post.

Peach you're understanding the process in reverse, if you are simply wanting to do a solvent extraction you're far better off doing that directly using a soxhlet.
By boiling or digesting the cloves in water then letting the water separate then 'skimming', regardless of whether you use an extra solvent or not to ensure complete removal of the 'oil', you are performing a solvent extraction and a rather crappy one at that as your solvent is most likely eugenol which is forced out by the conditions and solvates the remaining volatiles not soluble in h20, eugenol is a beautiful emulsifier also if i recall, ensuring the solution is buffered with a little tartaric acid will help no end in this regard.
i'm getting off topic, so why i said you're going in reverse in your rationale is because you want solubility in water for an effective steam distillation, but not too much. Hence the salt advice as it can be used to either decrease the solubility of your volatile fraction, if it is too soluble, you'll get too much water coming over per volatile, salt 'can' adjust the mol fraction of the distillate into your favour, however too much can raise significantly the bp of your solution increasing the solubility so there's always adjusting.
If you want to adjust your system without doing a tedious full optimisation you need to take a few fractions every few minutes, and simply look at the volatile/water ratio collected. If you use a 10mL or a 100mL volumetric flask and by eye estimate this ratio.
Then simply add some salt without adjusting anything else to see if it changes dramatically, if it does its win (most likely), this is a practical thing.
For better science, using a markham still, an optimisation can go fairly quickly (especially if you use a center composite experimental model, i think there free excel overlays around (plug and play for the chemist)), is perfect for doing an optimisation and will save you lots of energy. If you are interested in one i have a couple that i'll get up on a ladder for and ship you for not very much (cf cost of new).
Bah i'm blabbing again.
Also, although its seems hypocritical coming from me, as i am guilty at times for irrelevancies and verbosity, your last post could have been reduced to new information by the following thus.

'The quantity of water involved in steam distillation is annoying.'

i can't really see anything else in the post that is relevant or not repeated in your first post. However i'm not at concerned with the extra irrelevancies, repeats, incorrect information, bad analogies and offensive prejudicial language (bastards are people too), its fine with me, to a certain extent. However i think many people get annoyed with it simply because it means to continue on being readers/contributers to threads they have to wade through it all and its tedious.
So my advice keep to you is zomgnooblrn2filter

Now completely of topic, Ha!, your reference to bmx forums in another thread made me think, 'hey what does bmx stand for.' Well apparently its 'bicycle motorcross'. Which is most inaccurate as a description because WHERES THE FUCKING MOTOR on a bmx, there isn't one. Almost as bad as the expression 'bmx bike'.

As for rating posts, there should be a system where at the end of each post two big buttons are offered, one that says 'great post' and one that says 'shit post', however the buttons do nothing, allowing people frustrated to repeatedly click on something, happy as Larry, content they are contributing.




View user's profile View All Posts By User
Sedit
International Hazard
*****




Posts: 1939
Registered: 23-11-2008
Member Is Offline

Mood: Manic Expressive

[*] posted on 15-8-2010 at 18:41


Your talking the difference between hydrodistillation which is a form of codistillation and steam distillation. Hydrodistillation will destroy many of the more sensitive compounds due to the extended heating period so by all means I would suggest steam distillation for most applications.

I'v been wanting to do a writeup for sometime on steam distilling but since sassafrass is the highest local high oil yeilding plant im abit reluctant to post my writeup for obvious reasons.

Hydrodistillation is simpler then steam distillation but will only work for substances that are not very sensitive.





Knowledge is useless to useless people...

"I see a lot of patterns in our behavior as a nation that parallel a lot of other historical processes. The fall of Rome, the fall of Germany — the fall of the ruling country, the people who think they can do whatever they want without anybody else's consent. I've seen this story before."~Maynard James Keenan
View user's profile View All Posts By User
anotheronebitesthedust
Hazard to Others
***




Posts: 189
Registered: 24-6-2007
Member Is Offline

Mood: No Mood

[*] posted on 15-8-2010 at 21:20


I'm also inexperienced but this video makes it seem pretty simple:

http://www.youtube.com/watch?v=k0w19_vt8QY
http://www.youtube.com/watch?v=8lKYRXjOVYE
http://www.youtube.com/watch?v=lKLHrZecGf8

[Edited on 16-8-2010 by anotheronebitesthedust]
View user's profile View All Posts By User
peach
Bon Vivant
*****




Posts: 1428
Registered: 14-11-2008
Member Is Offline

Mood: No Mood

[*] posted on 23-8-2010 at 11:06


Quote: Originally posted by Panache  

However i'm not at concerned with the extra irrelevancies, repeats, incorrect information, bad analogies and offensive prejudicial language (bastards are people too), its fine with me, to a certain extent.


Honestly, I try paying a nebulous, nameless person a compliment and what do I get in return? A big poo in my mouth... :P

To the point, I realize a better choice of solvent would function more efficiently, the goal was to see if a large amount of simple, tap available boiling water would liberate the oils so they could be easily stripped with a solvent; without having to loose a lot of solvent as it's absorbed by the organic matter.

To that end, I did AND experiment!

I piled about 3 litres (~1.35kg) of cloves into the pressure cooker, cooked for half an hour, ran off through a big honey filtering sieve for the bubblesnatches we keep, mushed the cloves to pulp, recooked for another half hour, filtered. Ended up with around 5l of brown water. Swirled in around half a pint of DCM and whipped it up, returning to swirl some more every now and again, let it settle over night, split, removed solvent, vacuum distilled the resulting oil.

The results? Take a deep breath!

Theoretical minimum oil content 189g

Tube weight 57.64237g
Cap & clip 24.76337g
Filled weight 96.7426g

Raw oil yield 14.3368g

Efficiency 7.58% :D

Yes.

I know.

There are better, far more efficient ways. I was just seeing how it'd respond to a pressure cooker for those without the glass.

With regards to using a steam tube, how's about hooking it up to your wife & or girlfriends steam cleaner and jamming the trigger open? ;)

--------------------------------------------------------------------
Edit:

Whilst I'm on about cloves, I may as well say for anyone interested, that lab guide that's right up as the first result on google for steam A/B extracting the oil from cloves uses quite large excesses of things compared to what I discovered it needs; probably to make it easier for the students to handle the volumes with such small amounts of starting material.

Personally, I wasn't happy with the results of this (in terms of purity), they would need three way vacuum distilling to get a clean result. But I'm sure it'd accelerate and simplify similar extractions.

Running it from raw oil (which is what they yield from the steam distillation), it comes out like this (considerably less solvent);

1 BASE EXTRACTION
Take your oil and add 5% base (KOH / NaOH) aqueous washes until the washes no longer go orange / yellow. You'll need 6ml of base to every 1ml of raw oil, roughly. Make 10:1 and add it in portions until the aqueous additions stop coloring. They will stay opaque for a long time. Ditch the organic, keep the aqueous.

2 CLEAN YOUR BASIC AQUEOUS EXTRACT
Clean the useless muck out of the aqueous phase by rinsing with 1-3ml of DCM per ml of raw oil. Ditch the organic, keep the aqueous.

3 ACIDIFY
Do it carefully. At least with some pH paper and care. It needs a good stirring and time to go, you're looking for around pH1.

4 GET YOUR OIL BACK
Rinse the aqueous phase with 2-3ml of DCM per 1ml of raw oil. Keep the organic, remove the solvent. Bingoz... oil.

These numbers aren't guesses, I specifically worked them out to use the minimal amount of base and solvent.

Recycle your solvent... or I'm calling Greenpeace out.

[Edited on 23-8-2010 by peach]




View user's profile View All Posts By User
Magpie
lab constructor
*****




Posts: 5939
Registered: 1-11-2003
Location: USA
Member Is Offline

Mood: Chemistry: the subtle science.

[*] posted on 23-8-2010 at 15:25


Here's another data point for steam distillation of cloves, which I did as a school lab:

Raw cloves wt: 6.0125g
recovered oil wt: 0.375g

Using the assumption that the raw cloves contain 14wt% oil, as did peach, the yield was 46.0%.

Of course I used glassware, and ground the cloves prior to charging. The direct steam distillation method was used, ie, no sparge tube, just heat a 500mL RBF on a mantle and dribble in water as required for makeup.




The single most important condition for a successful synthesis is good mixing - Nicodem
View user's profile View All Posts By User
Panache
International Hazard
*****




Posts: 1290
Registered: 18-10-2007
Member Is Offline

Mood: Instead of being my deliverance, she had a resemblance to a Kat named Frankenstein

[*] posted on 23-8-2010 at 19:47


Quote: Originally posted by peach  

Honestly, I try paying a nebulous, nameless person a compliment and what do I get in return? A big poo in my mouth... :P



You do know i was parodying the overzealous PC mob (you know those that aren't offended but complain on behalf of those that may be offended, who turn out to not actually exist) with this i hope, i'm fairly certain you do, however just in case people thought a stoush was brewing, sadly no, was just some humor.
That sentence was so over-complicatedly (is that a word) structured it's ridiculous, gotta love the english language.




View user's profile View All Posts By User
densest
National Hazard
****




Posts: 359
Registered: 1-10-2005
Location: in the lehr
Member Is Offline

Mood: slowly warming to strain point

[*] posted on 23-8-2010 at 20:50


Am I confused, or are two different meanings of "steam distillation" being conflated here?

(a) extraction of volatile matter from solids by evaporation, the heat being applied as steam

(b) purification or separation of substances immiscible with water by distillation with water, such that the steam carries the substance through the column
View user's profile View All Posts By User
Magpie
lab constructor
*****




Posts: 5939
Registered: 1-11-2003
Location: USA
Member Is Offline

Mood: Chemistry: the subtle science.

[*] posted on 23-8-2010 at 21:47


Quote: Originally posted by densest  
Am I confused, or are two different meanings of "steam distillation" being conflated here?

(a) extraction of volatile matter from solids by evaporation, the heat being applied as steam

(b) purification or separation of substances immiscible with water by distillation with water, such that the steam carries the substance through the column


"Steam distillation" actually is a very well defined physical process. It occurs when two immiscible liquids are codistilled. The temperature of this distillation will be less than the normal bp of either pure component. Hence you can distill a component of high boiling point (like eugenol) at a temperature less than 100C using water.

The temperature of the codistillation is that in which the sum of the vapor pressures of each pure component adds up to the pressure of the distillation, usually 1 atm, unless distilling under vacuum.

[Edited on 24-8-2010 by Magpie]




The single most important condition for a successful synthesis is good mixing - Nicodem
View user's profile View All Posts By User
peach
Bon Vivant
*****




Posts: 1428
Registered: 14-11-2008
Member Is Offline

Mood: No Mood

[*] posted on 24-8-2010 at 01:01


Quote: Originally posted by Magpie  
Here's another data point for steam distillation of cloves, which I did as a school lab.


Caaaaa, thanks for that magpie. Out of interest, you don't remember a rough weight for the cloves versus oil do you?

If I was going to extract them, I'd probably throw them all into a big sohxlet, but am curious to know.

Quote: Originally posted by Panache  

You do know i was parodying the overzealous PC mob


I did not know.

That's what Data says all the time. "You're meant to be smart Data!" "I did not know" "And you're messing with the warp engines? Who let you on board?" [looks around] "Oh, he's gone..."

Quote: Originally posted by densest  
Am I confused, or are two different meanings of "steam distillation" being conflated here?

(a) extraction of volatile matter from solids by evaporation, the heat being applied as steam

(b) purification or separation of substances immiscible with water by distillation with water, such that the steam carries the substance through the column


In my case, I was purposefully trying to avoid A due to the need for glassware and B due to the volume, time and glassware; by pressure cooking it all.

As the pathetic yield suggests, some quantity of the oils were driven into the aqueous phase by the heating, to some extent. But, unfortunately for those without glass, barely any. I was hopeful, but also doubting, that a larger amount would end up in the water after pressurized cooking, as it does for the fats, waxes and oils I see floating around on the surface of soups I make in a similar manner; which would allow them to be 'skimmed' with not a lot of volatile solvent. Even if they formed an emulsion, the volatile could have dealt with that.

I don't think using a column for steam distillation is such a great idea. Using a big enough vigreux, or otherwise packed column, you'd likely end up with water as the distillate.

Quote: Originally posted by Magpie  

The temperature of the codistillation is that in which the sum of the vapor pressures of each pure component adds up to the pressure of the distillation, usually 1 atm, unless distilling under vacuum.


I've seen that description on wiki as well.

Being the young, questioning mind that I am, I also question whether or not it's actually correct.

If that was all that was happening, standard distillations would have a hard time functioning. If you think of something like crude oil, that'll be rammed full of various components boiling anywhere from tens of degrees to hundreds. Yet they still manage to fraction without a lot of trouble; they don't end up with a load of high boiler floating around in the low boiler. So the vapour pressures can't all be combining to lower the BP of the highers right down near the lowers. There is no explanation in the wiki for why that would be so either, why it doesn't occur in reverse (BP's raising close towards the higher fractions).

Neither is it azeoptroping, as the distillate isn't stable.

My own, possibly incorrect, thinking is that this is more to do with the 'tangible physical' properties involved than the vapour pressures.

By that, I mean, the steam will form a temporary emulsion with the volatile oil due to the heat and mixing action of the steam / boiling. Because the steam is being aggresively, rapidly forced off to the condenser, I expect it's carrying the oil over effectively as a contaminant.

This is evidenced by the lack of fractioning, or even normal, columns in steam distillations. They're driving it way too quickly for a normal distillation, it's using a lot of water and they're making no attempt to 'purify' the vapour; they're trying to get it over from the material as quickly as possible, probably before it splits again on the way.

Even the drug cooks roasting their oils at 10 on the plate manage a decent separation on close boiling points. Steam is waaaaay off clove oils BP.

Also of note is that the petrochemical industries are infamous for the gigantic, very well packed columns. Again, suggesting steam distillation is a 'contaminated' distillation as opposed to a stable property, as per the result splitting it's self on cooling, huge amount of carrier, lack of columns etc.

I seem to remember seeing something like 120 or 130C on my thermometer when steam distilling cloves. But I could be remembering that wrong.

[Edited on 24-8-2010 by peach]




View user's profile View All Posts By User
un0me2
aliced25 sock puppet
***




Posts: 205
Registered: 3-2-2010
Member Is Offline

Mood: No Mood

[*] posted on 24-8-2010 at 05:24


Holy fucking duckshit batman, how fucking hard do you want to make it? Here is a step-by-step procedure for the steam distillation of cloves. Alternatively you could buy clove oil (which works out pretty cheap compared with the amount of cloves needed to make it, look around, there are scam artists everywhere). The isolation of Eugenol with KOH is detailed in this paper (from THIS site).

IIRC there is a paper floating around on the methylation/isomerization of eugenol to 3,4-dimethoxypropenylbenzene (aka isoeugenol methyl ether) in one pot using tetramethylammonium salts.




quam temere in nosmet legem sancimus iniquam
View user's profile View All Posts By User
peach
Bon Vivant
*****




Posts: 1428
Registered: 14-11-2008
Member Is Offline

Mood: No Mood

[*] posted on 24-8-2010 at 05:32


Quote: Originally posted by un0me2  
Holy fucking duckshit batman, how fucking hard do you want to make it?


You should really have a bit more of a read prior to breaking about the big boy words, batty boy.

Quote:

Yes. I know. There are better, far more efficient ways. I was just seeing how it'd respond to a pressure cooker for those without the glass


Quote:
that lab guide [the one you just linked to] that's right up as the first result on google for steam A/B extracting the oil from cloves uses quite large excesses of things compared to what I discovered it needs





View user's profile View All Posts By User
Magpie
lab constructor
*****




Posts: 5939
Registered: 1-11-2003
Location: USA
Member Is Offline

Mood: Chemistry: the subtle science.

[*] posted on 24-8-2010 at 07:28


Quote: Originally posted by peach  

Caaaaa, thanks for that magpie. Out of interest, you don't remember a rough weight for the cloves versus oil do you?


No, I've not seen that information.

In regard to your description of distillation at an oil refinery, that is not steam distillation. The components are miscible. In this case Raoult's Law applies.




The single most important condition for a successful synthesis is good mixing - Nicodem
View user's profile View All Posts By User
peach
Bon Vivant
*****




Posts: 1428
Registered: 14-11-2008
Member Is Offline

Mood: No Mood

[*] posted on 24-8-2010 at 08:14


Just to clarify, which case are you referring to when you say Raoult's law applies, the steam distillation or refinery?

I'm guessing you mean the refinery.

If you mean the steam distillation, those are far from ideal solutions, meaning the law doesn't really apply.

That still doesn't explain why something boiling at less than half the temperature of the component is combining with it's vapour pressure to increase the pair's.

DCM and water are immiscible, I can still easily split them. And the petrochemical guys can split misicibles.

I note a strong lack of references in the Wiki on Steam Distillation.

[Edited on 24-8-2010 by peach]




View user's profile View All Posts By User
ScienceSquirrel
International Hazard
*****




Posts: 1863
Registered: 18-6-2008
Location: Brittany
Member Is Offline

Mood: Dogs are pets but cats are little furry humans with four feet and self determination! :(

[*] posted on 24-8-2010 at 08:33


Miscible liquids eg hydrocarbon oils obey Dalton's law and fractionate.
Immiscible liquids eg eugenol and water obey Raoult's law and codistil.

Really you would be better buying the clove oil unless you can set up a fairly heavy duty steam still. I have seen this done on a light industrial scale and the set up is huge, start thinking of 50litre pots to get hundreds of mls of oil!
View user's profile View All Posts By User
peach
Bon Vivant
*****




Posts: 1428
Registered: 14-11-2008
Member Is Offline

Mood: No Mood

[*] posted on 24-8-2010 at 08:43


Why would immiscible liquids obey Raoults when they're so far from ideal solutions?

I'm not really all that bothered about steam distilling cloves and what not, I have pharmacopedia grade eugenol already. I just gave it a go since I had a bag spare.




View user's profile View All Posts By User
ScienceSquirrel
International Hazard
*****




Posts: 1863
Registered: 18-6-2008
Location: Brittany
Member Is Offline

Mood: Dogs are pets but cats are little furry humans with four feet and self determination! :(

[*] posted on 24-8-2010 at 09:22


Good explanation here

http://www.umsl.edu/~orglab/documents/distillation/dist.htm
View user's profile View All Posts By User
peach
Bon Vivant
*****




Posts: 1428
Registered: 14-11-2008
Member Is Offline

Mood: No Mood

[*] posted on 25-8-2010 at 17:18


That's an excellent explanation of the maths at work! I may have to OpenOffice the maths to a spreadsheet, so numbers can be poked in to get some more out.

Panache, I'd hold off on writing that explanation, this one's great.

Quote:
It must be stable and relatively insoluble in boiling water, and it must have a vapor pressure in boiling water that is of the order of 1 kPa (0.01) atmosphere.


From ChemSpider;

Water, 24.5 mmHg at 25°C
Eugenol, 0.0104 mmHg at 25°C

Quote:
A useful "rule of thumb" is that the vapor pressure of a substance roughly doubles for every increase in 10 °C.


I get that to be around 1.9968 mmHg at 100°C for eugenol.

Quote:
A compound must satisfy three conditions to be successfully separated by steam distillation. It must be stable and relatively insoluble in boiling water, and it must have a vapor pressure in boiling water that is of the order of 1 kPa (0.01) atmosphere.


Check, check and to go from mmHg to kPa we divide by around 7.5

Eugenol, 0.2662 kPa at 100°C

Not quite 1, but within the order.

I think I've found a mistake at the end;

Quote:
Pwater/ Porg = nwater/norg and


That means nwater = (Pwater / Porg) x norg

Quote:
wtwater = (100/1)(18/150) = 120 g water or 120 mL


The vapour pressures part is correct (100/1), as is the multiplication, but why do we now have the molar weights of each element as opposed to the moles of organic present?

That sum also doesn't equal 120, it's 12.

Surely the correct rearrangement is;

wtwater = ((Pwater / Porg) x norg) * 18

For cloves, that's;

Vapour pressure of boiling water = 100 kPa
Vapour pressure of eugenol at 100°C = 0.2662 kPa
Theoretical minimum amount of oil present = 189g
Moles of oil present = 189 / 164.2 (molar weight of eugenol) = 1.2 Daltons eugenol

= ((100/0.26624) x 1.2) x 18
= (375.6 x 1.2) x 18
= 8113 g / ml of water for 189ml eugenol

If I run his number through my rearrangement, PwaterV = 100, PorgV = 1, mass org = 1, g/mol = 150, then I get (100 x (1/150)) x 18.

Which gives 0.67 x 18, 12ml... again.

Either there's a mistake there, or my terrible maths skills are showing through.

[Edited on 26-8-2010 by peach]




View user's profile View All Posts By User
Magpie
lab constructor
*****




Posts: 5939
Registered: 1-11-2003
Location: USA
Member Is Offline

Mood: Chemistry: the subtle science.

[*] posted on 25-8-2010 at 18:23


Quote: Originally posted by peach  

Quote:
Pwater/ Porg = nwater/norg and



That's a correct equation.

Assuming:

Tb = 100C
Pwater = 760mmHg
Porg = 2mmHg
wt org = 189g
MW org = 164.2

norg= 189/164.2

nwater = (760mmHg/2mmHg)/(189/164.2) = 310 moles
wt water = 310(18) = 5580g




The single most important condition for a successful synthesis is good mixing - Nicodem
View user's profile View All Posts By User
chemrox
International Hazard
*****




Posts: 2953
Registered: 18-1-2007
Location: UTM
Member Is Offline

Mood: LaGrangian

[*] posted on 25-8-2010 at 21:47


Single flask steam distillation is not a very good technique. It's much better to run live steam into the flask with the material in it. This way the oils co-distill right away rather than sitting in boiling water where they are more labile to oxidation. I have a pressure cooker converted to release a continuous course of steam and a water collector in line to capture any condensate. Lab instructors often oversimplify in favor of completing the task in the allocated time.



"When you let the dumbasses vote you end up with populism followed by autocracy and getting back is a bitch." Plato (sort of)
View user's profile View All Posts By User
peach
Bon Vivant
*****




Posts: 1428
Registered: 14-11-2008
Member Is Offline

Mood: No Mood

[*] posted on 25-8-2010 at 23:09


[whoops]Found the mistake[/whoops]

I was fine with that, but managed to make a mistake in understanding how he'd rearranged it. Both the arrangements work.

Mandatory excuses: Terrible at maths, been awake all night weighing pennies, making balance beams out of rulers, dividing piles of table salt by eye and then plotting the errors of the three balances, coins and methods to produce some interesting graphs.

Saying that, his example arrangement and values do result in 12ml and not 120ml.

Chemrox, did you design in some method of refilling the pressure cooker or having it switch off when it ran dry?

As I've said, I don't have any pressing need to do stream distillations, but I'd be interested to know how you've worked it out. I'd be concerned about incinerating the contents if it wasn't rigorously under the evil eye of scrutiny.

[Edited on 26-8-2010 by peach]




View user's profile View All Posts By User
 Pages:  1  

  Go To Top