Wow! I'm not sure where all this misinformation about controlling wash temperature in a still is coming from, but it's going to confuse the bejeezus out of the prospective new distillers. We have to deal with this misinformation on a regular basis, but I've never encountered an epidemic like this before.
The first thing you need to know is that all liquids evaporate at any temperature where they are liquid, and the rate of evaporation of a specific liquid at a specific temperature is determined entirely by its temperature and the pressure of its environment. For the sake of simplicity (and ignoring vacuum stills) I'm going to assume that all examples hereafter are made at "standard" temperature, 1 atmosphere, 760mm of mercury (760mm Hg). So, for all of my examples, rate of evaporation is set by its vapor pressure at some given temperature.
If you set out saucers full of water, methanol, ethanol, ethyl acetate, and acetone, all common
liquids found in a beer-stripping wash, in a room at "room" temperature, say 20C (68F), they will all evaporate until the saucers are dry. Your nose will give you proof that they are evaporating, because when you smell methanol, ethanol, ethyl acetate, or acetone (your nose can't smell the water), what you are actually smelling is molecules of that liquid in the air, and it got in the air by evaporating, even at the lowly temperature of 20C (69F).
Vapor Pressures of liquids at 20C
water 17mm Hg
ethanol 48mm Hg
ethyl acetate 70mm Hg
acetone 189mm Hg
methanol 200mm Hg (These numbers inferred from graphs, so they are not terribly precise)
Because all of these liquids are at the same temperature 20C (68F) in our room, the rate of
evaporation is directly proportional to the vapor pressure of each liquid. For that reason, the methanol, having the highest vapor pressure, evaporates fastest, and the acetone evaporates almost as fast, Next comes ethyl acetate, then ethanol, and finally, water. The experienced distiller will recognize that is the order in which the highest concentration of these liquids come through the still as distillate.
If you were to heat the room, as the vapor pressure of each of the liquids reached the temperature where its vapor pressure equalled the atmospheric pressure in the room, 760mm Hg, that particular liquid will boil, and evaporate quickly, while the lower vapor pressure liquids are still evaporating, but not yet boiling. Because the liquids in the saucers are pure, at least at this point, boiling of each will occur at a specific temperature, the boiling point.
Boiling Points of liquids at 760mm
ethyl acetate 77.1C
As stillers, what we are really interested in is how all these liquids behave when they are mixed together and put in our still boiler, so we can know how to deal with them. First off, the mixture will have a single boiling boint, determined by the vapor pressure of each liquid, the percentage of each liquid in the mixture, and the molecular weight of each liquid, according to something called "Roualt's Law". Actually the last two numbers are combined to get something called the "mole fraction", but we don't need to understand that in this discussion.
As you heat this (initially cool) mixture of liquids in your still boiler, evaporation of ALL
liquids in the
mixture increases with the temperature, although the amount of vapor is so small that you will have essentially NO
distillate coming out of your condenser. ONLY
when the mixture boils, at a temperature determined ONLY
by the concentration of the liquids in the mixture, and according to the law of Francoise-Marie Roualt, do you get enough vapor moving to get distillate coming out of your condenser. In addition, Francoise gets the last word, and determines exactly how much of which of the original liquids is in that
The upshot of this is that you pour your wash (let's say 10% ABV just for kicks) into your boiler, and turn on the heat. I know from my chart that your 10% wash will boil at roughly 93C, so if you used a PID controller to hold the wash at 78C, thinking to extract only ethanol, your wash will be 15 degrees short of the boiling point, and the still will just sit there, producing so
little distillate it may not show at the condenser, and the funny part is this: any distillate that may occur will have the same percentage of ethanol in it as if you had boiled the wash and got some real product. In any event, your first real drops will be about 54% ABV. If oyu do get distillate holding the wash at 78C, bring some good books and a calendar, because you're going to be staring at that still for weeks.
So you HAVE
to boil, and you have no choice
at what temperature it boils at, and you can't get it hotter than its boiling point, no matter how hard you try.
Whew! I hope this clears up this issue, at least until next time.
Zymurgy Bob, a simple potstiller Making Fine Spirits
--- In Distillers@yahoogroups.com, RLB wrote:
> You never want your
wort or wash to boil because it allows water to dilute your Ethanol during a striping run.Â At sea level where water boils at 212 F, you never want to exceed 210 F.Â If you live at a higher altitude like myself,Â where water will boil at a lower temp than 212 F, you need toÂ find out what temp water boils in your area and never go over that temp.
> From: Shane shanemcd76@...
> To: Distillers@yahoogroups.com
> Sent: Tuesday, January 1, 2013 2:48 PM
> Subject: [Distillers] stove top pot still
> once ive reached my tempature of 175 degrees do i need to control the heat or can i just leave the heat on high