Sciencemadness Discussion Board

How to increase the yield of steam distillation?

Tacho - 5-6-2006 at 08:24

It can, very often, replace vaccum distillation. It is nice and clean, but yields are sometimes terribly low.

Adding salts to the water raises its boiling point (0,512ºC per mol of ions/liter, or, 1,024ºC per mol of, say, NaCl), and increases yield, but if you add too much salt it crystalizes at the bottom of the flask causing nasty bumping.

What about epson salts? They are so soluble they dissolve in their own water of crystalization when heated. They are quite inert and one could add lots of it to the water.

I plan to make some experiments on the subject, but I would like to discuss it here first.

Any other ideas? Any experience? Links?

[Edited on 5-6-2006 by Tacho]

Nicodem - 5-6-2006 at 13:13

Using ethylene glycol or glycerol to reduce the partial pressure of water might be better. Most organic compounds that don't dissolve in water don't dissolve in glycol/glycerol either. Therefore such addition should not break the required biphasic system. This way you could reduce the partial pressure of water and consequently increase the rate of distillation of the product considerably more efficiently than with an inorganic salt.
But obviously there is nothing to gain since every compound that can be steam distilled can also be vacuum distilled with a vacuum of an aspirator way faster and efficiently. Steam distillation is only a better alternative when one can selectively distill the product directly from the reaction mixture.

leu - 5-6-2006 at 15:58

The best way to speed up a steam distillation is by using an aspirator to reduce the pressure, one must use a suitable condenser and cool the receivers as well :cool:

BromicAcid - 5-6-2006 at 18:38

Are you talking about boiling an aqueous mixture containing the organic molecule and letting the boiling water carry over the organic phase alone? Or are you talking about doing the afformentioned process along with bubbling steam through the reaction vessel?

I had done the first of those two in an orgo lab to distill lemon oils but the second method I used to distill aminobenzaldehyde. In the case of the second it is much faster and the steam works much better.

Tacho - 6-6-2006 at 03:46

Thanks, good tip. I'll try ethylene glycol and/or glycerol.

1-When you are extracting volatile oils from vegetal matter, steam distilling is a must.
2 - when you have your organic product in a large amount of aqueous reaction mixture, steam distilling is outstanding. Avoids painfull extractions and distills at the same time. Pristine purity.
3 - vacuum distilling very small amounts of product requires special glassware and implies in great losses. The mere wetting of the walls might take away half of your product.

Edit: sorry leu, I see now that I misread your post. I thought you meant vacuum distillation INSTEAD of steam distillation.

I am talking about extracting organics from an aqueous mixture with steam generated in situ. I never tried distilling with external steam. Isn't it necessary to heat the mixture close to boiling before I apply the steam? This sounds like a lot of trouble if you don't have a steam supply like a boiler. Two heaters, three flasks, lots of tubes.

[Edited on 7-6-2006 by Tacho]

Rosco Bodine - 6-6-2006 at 08:32

There is a special receiver which looks much like a
pressure equalized addition funnel having an oversized
bore equalization tube on the side as does a soxhlet extractor , and the stopcock valve is a three way oblique
which can drain straight downward from a short tip
observable through the clear part above the male
ground joint which functions as a " drip observation window " , or the stopcock valve can be turned 90 degrees to drain through a 45 degreee angled external side drain tip , or turned further to shutoff both drain paths . The domed upper part of the cylindrical receiver
is equipped with a small female ground joint like a thermometer port , which can be used as such or as
a topside reservoir access point for introducing or
withdrawing material . Kontes or Ace are one of
manufacturers who will be familiar with this type
of distillation receiver .

In use the receiver is placed vertically just above the flask from which the distilling vapors will rise through
the side tube on the receiver and into the top portion
of the receiver continuing upwards into a bulb type Allihn reflux condenser . A few ml of solvent ( benzene is ideal ) is added to either the contents of the distilling
flask or the receiver . The top of the reflux condenser
is fitted with a vent line , and the system is otherwise
closed off from atmosphere . Distillation proceeds
with two layers forming in the accumulated liquid in
the receiver , sufficient benzene being added to keep
the organic layer less dense than the separated water
below which accumulates faster in the receiver . At
a point when a workable quantity of water has accumulated , the stopcock valve is adjusted to a
slow drip so that about as much water as is accumulating
is returned to the distilling flask , so that the predominating accumulation in the receiver is the organic layer of material which is being steam distilled from
the flask below . The benzene accomplishes several things , a clean separation of what will otherwise often
be a milky emulsion product from steam distillation ,
and holding the temperature of the receiver contents
to the much lower steam distillation temperature of
a benzene water mixture , while the warm receiver
actually becomes the residing place for the simulataneously extracted and separated organic layer
benzene can be observed actually boiling away from
the receiver and mingling with the steam vapors rising ,
most of the benzene co-condensing preferentially with
the organic material so that separation occurs at the
vapor to liquid phase change in the reflux condenser .
And beyond these things , the vapor pressure and
slight loss of benzene as uncondensed vapor through
the vent line excludes atmosphere from the apparatus
by virtue of the slight continuous venting flow of vapor .

The low boiling point of benzene makes its subsequent
removal from the organic distillate very simple under
vacuum distillation to further purify the steam distilled
product .

I have done this many times and it works very well for
products which have good volatility with steam at
the normal boiling point , where no separate steam
source is required beyond just the largely aqueous reaction mixture which is desired to be steam distilled ,
simply being brought to a productive boil and the
vapors managed as described . The process is slow
because of the small percentage of organic material
vapors mingled with the steam , for example it may
require all day to steam distill a couple of hundred ml
of phenylacetone from three liters of mixture in a
five liter distillation flask , but the process works
reliably ....slow but sure produce an exceptionally
pure product from one pass , and subsequent vacuum distillation of the product results in purity exceeding
99.9% or pure to the limitation of any usual physical
methods of identification .

Magpie - 6-6-2006 at 09:54

Tacho this is just speculation but it is an idea for your consideration:

With the live steam method you have the capability of adding superheated steam. This would raise the temperature of the steam distillation thereby raising the vapor pressure of the organic. This might give you a better yield.

I agree that generating live steam is more trouble than using the direct steam method. I have done this for a steam bath using an old pressure cooker as a steam generator.

leu - 6-6-2006 at 18:48

Using an aspirator to reduce the pressure speeds up the process of steam distillation greatly. Wallpaper removing steamers work far better than pressure cookers as a source of live steam. Plumbing fittings are available which will attach to the outlet thread. Some rather informative illustrated posts on this topic from a website not on the internet anymore:

For Rapid Steam Distillation
Laboratory steam distillation is often a slow process if the compounds concerned have vapor pressures lower than that of aniline. Increase in speed can be attained by increasing the temperature through superheating, increasing the rate of steam flow, or by securing a more nearly perfect equilibrium between the steam and the organic vapor. The open tube often used as a steam inlet in laboratory work has the effect of bypassing a considerable portion of the steam. Introduction of steam through a number of smaller orifices is an obvious improvement. A steam inlet made of 1/4" copper tubing is often more convenient and more satisfactory than one made of glass. Eight or ten slots are cut near the lower end with a hacksaw, and a 1/16" hole is drilled at a point that will be just above the liquid surface in the distilling flask. The slots admit steam in thin jets, the hole permits equalization of pressure when the steam is cut off so fluid is not sucked back into the steam line or generator. SAE tube fittings are much superior to rubber tubing for connecting such a steam inlet the supply line.

The common superheater, made from a coil of copper tubing, may be made considerably more efficient by winding it in the form indicated below. The several turns are held in position by interlacing stays of iron wire. A cone shaped sheet of iron in the center spreads the flame, and the iron cover forces all of hot gases to flow out between the coils. The outer case is open at the top.

The rate of steam flow is usually limited in laboratory practice by the capacity of the condensing system. The condenser shown below will handle six to eight liters of distillate per hour. A short length of 24 mm pyrex tubing is sealed to the bottom of a three liter round-bottom flask, and is is fitted to a water jacket made from a ten quart pail by means of a rubber stopper and some gasket cement. A water outlet is provided near the top the pail by soldering an old cork borer into a 1/2" hole. A two-hole cork the mouth of the flask carries the inlet tube from the still and a long slender Hopkins condenser. Such a large condenser is best mounted on a shelf at level of the table top, and the receiver is placed directly below it on a lower shelf or on the floor.

Apparatus for the Steam Distillation of Essential OilsIn the production of essential oils, speed of operation and fidelity of the odor and flavor characteristics of the final oil are imperative. This apparatus works by condensing the vapor directly on a cold finger within the distillation trap, as shown in the left illustration (dimensions in mm). Off odors are thus avoided. Continuous contact of the material to be extracted with the boiling water is avoided by a slightly more complicated apparatus shown in the right illustration. It consists of a modification of the cold finger distillation trap, two flasks (one for steam generation and one to hold the sample to be extracted), a glass tube which protrudes into the sample flask and is small enough in diameter to allow steam to emerge around it, and connecting taper joints. The hydraulic seal existing below the sample flask avoids immediate passage of the vapors to the cold finger and also promotes the drainage of the condensed steam from the trap to the generation flask. For use with larger or smaller flasks, the dimensions of either design may be changed appropriately.


Rosco Bodine - 6-6-2006 at 22:29

Originally posted by leu
The best way to speed up a steam distillation is by using an aspirator to reduce the pressure, one must use a suitable condenser and cool the receivers as well :cool:

Have you ever performed a steam distillation ?

The boiling point of the aqueous mixture is determined
by the sum of the vapor pressure of water added to
the vapor pressure of the organic material reaching normal
pressure at given temperature which is already less than
the normal boiling point of water . The percentage of
the distillate which is organic material carried over with the steam is exactly the same on a molar basis compared with
water , as is the ratio of the vapor pressures at the boiling point of the mixture . The volatility of the organic material
which will be steam distilled is usually low , even at the normal boiling point of water , and a bit lower still at the reduced boiling point of the mixture . Using a vacuum aspirator to lower the boiling point of the mixture further
will cause a disproportionate reduction of the vapor pressure
of the higher boiling component of the mixture compared
with the vapor pressure of the more volatile water ......
which would be counterproductive in terms of the percentage
of the organic component which would be present in the distillate coming over at lower temperature and reduced pressure .

If anything would help , it would seem that a slightly elevated pressure and the superheating of the steam
would speed the process , but whether any of the added
speed of distillation would be worth the complication and
equipment demands is dubious for lab processes , and
enters into the industrial realm of pressure safe reaction
vessels which are specially designed for the purpose .

leu - 7-6-2006 at 03:50

Have you ever performed a steam distillation ?

More times than can counted, reduced pressure steam distillation has been used for over a century by those who are practiced in the art :P

Tacho - 7-6-2006 at 04:06

Sorry leu, I misread your first post. See my edited post above. Thanks for the very informative pictures and text.

Reducing the pressure of vacuum distillation, however, is counterproductive for the reasons exposed by Rosco.

I tested a mixture of 75ml ethylene glycol, 25ml of water and 5g of sodium carbonate (to test the ethylene glycol for a hot alkaline medium) and the boiling happened only at a sharp 130ºC. --- That is very good! --- I would like to know more about ethylene glycol reactivity though.

Is live steam so much better than in situ?

Does it worth the trouble of generating and superheating it?

Does the extracted mixture have to be heated close to boiling to avoid condensation or the superhot steam just passes by, removing the volatiles?

Can the superheated steam decompose the organics or the water in the extracted mixture prevents superheating?

An electric steam generator/superheater may be a nice project for the future.

[Edited on 7-6-2006 by Tacho]

leu - 7-6-2006 at 13:07

What Roscoe posted is contrary to real world experience, but clearly one's mileage may vary depending on equipment, facilities and skills :P See:


[Edited on 7-6-2006 by leu]

Rosco Bodine - 7-6-2006 at 15:49

The equipment I was using is a bit more advanced
than anything shown in those links , the vertical
15 bulb allihn condenser had a 1 meter length
water jacket and was manufactured by Wittig
Precision Glass in west germany . The five liter
reaction flask was equipped with a gearbox driven
rapid reversal stirrer which kept the contents
emulsified , and the efficiency of distillation was
generally about 98% at normal pressure .
The flask was heated by a thermostat controlled
oil bath which was supported on a motor driven
lifting platform , everything mounted in a structural
steel framework . The entire apparatus was about
seven feet tall and weighed about four hundred pounds
and could accomodate reaction flasks up to 50 liters .

I know , it is a modest scale ....but it is as much as
I could afford for a home laboratory .

So when it comes to skill in the art .....
forgive me for speaking , I should leave it to
those who are better equipped to know .

Anyway , the method and apparatus I described
always worked quite well for me , contrary to
others real world experience or not .

Nicodem - 8-6-2006 at 00:31

Regarding steam distillation under reduced pressure

It has already been evaluated theoreticaly and practically on another forum. The theory, by using the Claussius-Clapeyron's equation, shows that:

...the results are (very!) strongly dependent on evaporation deltaH which is a specific property of each compound. Small differences in deltaH cause huge variations in the steam distillation. More specifically:
- if the molar deltaH is higher than that of the water, vacuum steam distillation causes a lowering of the non-polar fraction content in the distillate;
- if the molar deltaH is lower than that of the water, vacuum steam distillation causes a lowering of the water fraction content in the distillate;
- if deltaH are equal or very similar, no considerable effect should bee noted.

while practice shows that:
But as the distillation itself proceeds ~50% (??) faster with vac...
...IMHO I believe vac steam distillation is quicker.

Taken from Vacuum steam distillation of P2P (the table was fucked up, but the rest of the relevan posts are there).

However, this has to do with the speed of distillation and not really about efficiency. It can, for certain compounds even lower efficiency. While the original question was about efficiency only.

Rosco Bodine - 8-6-2006 at 06:20

If it was my choice , stability of the distillation for
a valuable material is more important than throughput
anyway . Vacuum introduces the shake rattle and roll
of bumping and burping to what is a smooth boil at
atmosphere even under slight variations of temperature and stirring , and volume changes as the liquid is reduced
or foams in varying degee . If I was trying to increase throughput , the way I would go is a rotating flash
evaporator blowing superheated steam through it
from the 15psi vent on a pressure cooker .....but
still leaving the glass system at normal pressure just
to simplify things , and not have to babysit the process
minute by minute for the hours it would still take to
produce useful amounts of product .

I still think the second stage vapor phase extraction
using benzene vapors is something which will be confirmed to provide a superior separation , unless
a milk like emulsion of phenylacetone in water is
desired to be the product managed in a separate step
of extraction and separation .....which will involve losses
that my method does not introduce .

Anyway I know how I would do this , having done it
a few times using equipment dedicated for the task ,
and vacuum is something I use for final distillation
only , to isolate the concentrated product which is
already quite pure and very easily gotten by vacuum
distillation which proceeds quickly for a small quantity of
material , using vacuum as the finish work to " polish "
the purity of the end result .

P2P , benzyl methyl ketone , phenylacetone .....we are
talking about the same thing .

leu - 8-6-2006 at 17:03

The largest apparatus sold by heart magic is 85 gallons which sells for over $11,000 :P The experiment using vacuum steam distillation that prompted that post in Stimulants was legal to possess all over Planet Earth, so obviously it wasn't BMK. The inspiration for this experiment was found in this article from Thorpe's Dictionary for Applied Chemistry:

Steam distillation is the distillation of a substance in a current of steam. The term is usually applied to the co-distillation of water with a substance which is practically insoluble in it, but also, less commonly, to distillation of it homogeneous aqueous solution (particularly glycerol-water mixtures). Water may be present in liquid form in the still (which is then heated externally) or it may be introduced as steam, which may be either saturated or superheated.
The chief advantage offered by steam distillation is that a substance of fairly low volatility can be separated from non-volatile impurities at a temperature much below its normal boilingpoint. This is particularly useful, if the substance is liable to decompose at its normal boiling-point (e.g., fatty acids), or if it is present in small quantity (e.g., essential oils in plant material). Steam distillation has therefore much the same effect as low-pressure distillation (which, however, tends to be preferred for some substances in modern practice), but the plant required is simpler, and the entrainment effect helps to carry over the required component if it is present in small proportion.
On the other hand, steam distillation is thermally less efficient than low-pressure (" vacuum ") distillation, since large quantities of water are distilled simultaneously. Further, there is the liability of loss of a proportion of the distillate if it is appreciably soluble in or finely dispersed in droplet form in the aqueous layer. In some cases this layer is returned to the still to obviate loss.
Steam distillation necessarily requires a much larger condenser surface and more cooling water than vacuum distillation. Conversely, it has the advantage of being less liable to damage the substance being distilled through superheating (e.g., by charring or decomposition of natural oils, cracking or tar formation of hydrocarbons, etc.) and the jet of steam is said to keep the still bottom clean when heavy oils are being distilled, thus prolonging the life of the still. Further, the agitation of the jet of steam increases the efficiency. of removal of the volatile constituent. . In addition, close control of the .boiling tempera, ture is possible when direct steam is used in the still (as compared with external heating of vacuum stills).
Steam distillation under reduced pressure offers a compromise between simple steam distillation at atmospheric pressure and the technically more difficult distillation under very high vacuum. With miscible liquids such as glycerol, the better the vacuum the smaller the proportion of water which comes over in the distillate at a given still temperature. Alternatively, with immiscible liquids, the better the vacuum the lower the temperature of distillation-a particular advantage with unstable substances.
Superheated steam is frequently used to increase the temperature and, hence, rate of distillation. It is by a combination of superheated steam with a very good vacuum that glycerol is obtained almost water-free. Since the temperature of superheated steam is not subject to any automatic maximum (as is that of saturated steam) there is more danger of damaging labile substances such as essential oils.
In principle, any immiscible liquid or unreactive gas could be used in place of steam for this so-called " entrainment distillation." (See also " Partial Pressure Processes," by Othmer, Ind. Eng. Chem. 1941, 33, 1106.) Gases are impracticable since a very large cooling surface would be required (see, however, the use of gas under SUBLIMATION, this Vol., p. 172d). Water is obviously most convenient for many organic compounds. However, entrainment distillation with anhydrous media has been suggested (Fester and Collados, Anal. Assoc. Quim. Argentina, 1942, 30, 36): it has been found possible to distil gallic acid and pyrogallol by entrainment with kerosene.
Similarly, the well-known method of determining water in biological materials by distillation with, e.g., toluene, is based on exactly the same principle as steam distillation.

Neither Nicodem or I are saying that steam distillation at atmospheric pressure doesn't work very well, we're merely saying that in some circumstances when the physical constants are in the proper range; that reduced pressure can be beneficial. In the previous experiment, it cut the time used by at least a factor of four times, and doubled the yield per liter of water as well :) You're not the only one who possesses a two meter tall lattice frame, though one suspects that yours cost much more since it's probably a standard laboratory supply company product as opposed to one made from components that aren't nearly so expensive :D The entire article is attached, it goes into the theory of steam distillation quite thoroughly, clearly the author of the textbook you're quoting from should have read this article:


[Edited on 9-6-2006 by leu]

[Edited on 9-6-2006 by leu]

[Edited on 9-6-2006 by leu]

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Rosco Bodine - 8-6-2006 at 21:47

The condition for distillation which I was using was
one where efficiency was maxed out , the contents
being boiled were an emulsion being continuously
stirred in a rapid reversal agitation like a washing machine only making about six complete revolutions
one way to pull a vortex and then tearing backwards
through that vortex , like a churn cycling every three
seconds , while the mixture was at a rolling boil ,
and the cooling vapors were being counter current
extracted with benzene vapor . When the process
is already maxed out at normal pressure , there is
no getting a four hundred per cent increase by
applying vacuum to a process already running at 98%
efficiency .

I'm sorry to doubt your figures , but the only way of getting that sort of gain is if you aren't running things
in the groove in the first place . There's only so much vapor throughput you can distill through a fixed bore condenser , and the vapors were rising as a visible fog about three quarters of the distance up the column even at normal pressure you would need a ten foot column at the rates you are describing . Now you want to talk about real world experience like it is some mystery
to me .....go ahead with this but I am not amused .

bio2 - 9-6-2006 at 02:52

..... unless a milk like emulsion of phenylacetone in water is desired......

A very simple way that eliminates this
problem is to place about 1/4 or so volume of benzene or PhMe to expected product
into the reciever along with enough powdered NaCl to nearly saturate the water when filled.

There is so little organic in the water phase
that it's hardly worth the effort to extract but Joe does it once anyway.

Your vapor phase 2nd stage extraction is very clever and simple enough.

Easiest way to increase take-off when steam distilling is to circulate ice water
in the condenser and immerse the reciever
in ice/water. Turn up the heat and stirring
until the condensate is no more than warm.

Doing only this will at least double the
take-off using a standard glass lab condenser. A metal condenser speeds it up even more.

Still takes a long time though compared to vacuum fractionating although old Joe
still does it after steaming, with virtually no residue and a tight collecting range of about
+/- 3-4deg.

leu - 23-8-2006 at 16:36

Now you want to talk about real world experience like it is some mystery to me .....go ahead with this but I am not amused .

It's clear that you are sharing what you know about the subject, but the subject is far more complicated than you seem to realize :P As Nicodem was trying to explain, the partial pressures of the components at the boiling point determine what actually happens, there is no uniform result due to this fact; since the properties differ for each component :) The attached article is by Donald Othmer, of the Polytechnic Institute; the same person who's name is on the Kirk-Othmer Encyclopedia of Chemical Technology :cool: The primary purpose of this forum is to share knowledge, for entertainment there's Whimsy :D

[Edited on 24-8-2006 by leu]

[Edited on 24-8-2006 by leu]

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