Vacuum Stills

Using a vacuum allows the use of lower temperatures, and can attain higher alcohol concentrations, eg at 42 mm Hg (cf 760 mm Hg = atmospheric pressure) the column only need be at 35 C (ie just use hot tap water to heat with). The azeotrope (the point where distillation ceases to work because the vapour and liquid purity are the same) moves towards 100 % as the pressure is lowered; below 0.1 atmospheres it disappears, allowing you to distill all the way to 100% alcohol (provided you beef the reflux ratio up to >20). They can also be more energy efficient, and allow for a greater capture of the available alcohol. The lower temperatures also mean that they're use a bit in the distillation of essential oils (which would be otherwise be broken down at higher temperatures).

The graph below shows the reduction in temperature. I'm not quite happy yet with my calculations for this (the Textbook & 1 Atm lines should be the same), so don't go and design from it, but you can see the basic principle at work ...

To create a vacuum, you can either use a mechanical pump, or a a venturi ejector (water jet pump). The venturi ejectors (vecktors) are commonly used by laboratories etc to assist with filtering material. They just use flowing water to create the vacuum, and cost around US$20.

Joel describes his vacuum still
Not only is it all lab glassware with ground glass connections....but the entire system runs under a 25"Hg vaccuum. The vaccuum makes the whole process much more eficient and keeps the boiling temps very "true". The vaccuum is also used to change batches without having to take anything apart.

The tank in the picture is the main coolant resivior. I built a cooling tank that sits outside and uses the cold winter ambient temperatures to chill the coolant. It consists of a 35 gallon garbage can with 125 feet of copper coils inside. The can is filled with automotive anti-freeze and the little pump in the pic pumps winshield washer fluid through the closed loop copper coil system and then through the condenser coils in the picture. On a really cold winter day....I can get a 100 deg. ambient drop between the inlet and outlet ports on the chiller.

After distilling the "product" 6 times and paying close attention to the temperatures (to make sure all the methanol has been separated from the "good stuff") I end up with distilate at 180 proof that WILL NOT give you a hang over.

Ólafur describes his vacuum still ...
My pot is around 60L. It is enclosed in a rather large plastic barrel and I heat the system with hot water. The water is around 80C so I don't need a lot of it and, besides, it doesn't cost a lot here in Iceland. The column is 2" X 60cm SS packed with ceramic rings. The water for the jector I let straight through so as not to create resistance on the out end. The cooling water I let in at the bottom end... The jector suck out of my collecting jar which is in turn connected to the condenser so any vapour rising up the column has to go through the condenser before it gets to the collector before it gets through to the source of the vacuum. The cold water in iceland is somewhere around 4-5 C so cooling is not a problem.

Note that in most cases, the vacuum is applied to the distillate collection container, after the condensor. This way there is little loss of vapour from the process, and there doesnt need to use much vacuum. Many of the comments below are concerning the setup where the vacuum is applied to the vapour line.

There are several problems with Vacuum stills though:
  • you can lose some alcohol vapour out through the vacuum pump,
  • you must have an accurate measure of the vacuum applied (not a big problem, one of the reasons the still is not easy to build),
  • its hard to find leaks (thus requires care during construction),
  • very low vacuums are difficult/expensive to attain (but generally not required - only a small vacuum is needed), and
  • the low temperature can make it difficult to recondense to vapours (need realy cold cooling water & enough condensor surface area)
Often the vacuum can be made by using a "Venturi" or "Vektor" which uses the cooling water, and causing it to go through a small nozzel increases its velocity. This causes a corresponding decrease in pressure, and hence can "suck" vapour from the still. However, this can eject some vapour out of the system.

The heating can be a simple water jacket around the pot, using hot tap water.

What can also be confusing is when the mash begins to boil, the vapours will expand into the vacuum space, and your gauge may go back to zero. This doesn't mean you've lost your vacuum, but if you can have some of your still appliance in glass or plastic so you can see what is going on, this helps so you know you have some action and not a leak.

One of our visitors is in the process of working on an improved vacuum still design. We'll keep you posted once he's happy with the results (including maybe a photo, design, info, history & performance.)
Atm = kPa = mm Hg (mercury) (torr) = mm H2O (water)

David suggest how he would go about setting up a vacuum unit ...
I would go for a batch method where the still, condenser, and receiver were under vacuum; cooling would be circulated car antifreeze which was cooled with an ice/salt mixture. The condenser would be a multi tube high surface area low-ish volume of copper, the salt/brine would attack the copper hense the car antifreeze. This would give a nice big temp change. The pump would only have to maintain the vacuum so it could be a smallish piston perhaps a good diaphram might do. The exhaust routed through a secondary condenser although I wouldn't expect much if the primary condenser was doing it's job, the reciever could sit in a bucket of ice/water to stop any secondary boiling.

Another way of cooling would be to use a freezer, make a dummy door "if you want the freezer after" with a couple of holes in it, inside have a tank of antifreeze and circulate this through the condenser, it could run as low as -18 oC !. Any condenser after the vacuum pump would only have normal cooling as it would be at atmospheric pressure.

Tip if you're going to make a multi tube condenser these hints may save you some time. The endplates can be cut from sheet copper or a piece of tube opened out. Cut a circle out about 0.5 inch dia larger than the tube. Find a socket from a car socket set about 3mm dia less than the bore of the outer tube, trap the annealed copper disc between a thickish "6-7mm thick" metal plate" in a vice, and beat the edge of the copper over the socket. With dividers find the centre of the disc on the inside not out. If you use 32mm bore copper tube seven 10mm pipes will fit in nicely. Draw the 7 circles evenly spaced. Don't try and drill the holes! it's disasterous! Just drill a very small hole and use a metal cutting fret saw better still a jewellers saw if you have one. I use mine frequently it's a great tool. Make two plates. Silver solder or use the copper/phosphorus rods to solder the 7 tubes to the end plates,then slip the unit inside the outer tube, solder in place, make end plates to reduce the size down to a more convenient size "you gone metric yet" 3/4 or 22mm as we use here now.

Laurance adds ..
Don't use any old car antifreeze! Any trance leaks will contain poisonous ethylene glycol. Use propylene glycol (typically RV water line antifreeze or other antifreeze for use in potable water systems)- it will work just about as well and there's that much less chance of poisoning yourself (with antifreeze, at least).     This page last modified Mon, 05 Mar 2012 08:07:08 -0800