Controlling immersion heating element

Yulaw:
I am, as I think some of you have guessed correctly, building a system to destill a mash/wash after fermentation. It’s goal ultimately is to separate ethanol from the water and from all other nasties (a bit oversimplified but hey you get the point). So it is true my ‘liquid’ does not have a single boiling point as its composition changes over time.

The goal of the fractionating column is to make the separation of the ethanol and all other compounds easier. This is basically done by letting the liquid evaporate out of the boiler, up through the column, condensing it with the cooler at the top and then re-evaporating it while it falls back down. Without going too much into the theory behind it, what happens after a while is that the compound with the lowest boiling point is concentrated at the top and the one with the lowest is at the bottom. By slowly removing the top one, making sure not to disturb the equilibrium too much, this still is capable of ultimately collecting nearly all ethanol at azeotropic level of 95%. By measuring the temperature of this top compound, it becomes possible to determine where the process is at.

So that means I do not want to control this temperature but, on the contrary, I monitor it (sort of as a variable) as it tells me what compound is at the top (the ethanol I’m looking for, some nasty stuff or just mostly water vapor).

I think the process would be as follows:

  • Turn on, heat the solution to a boil, as fast as possible, max power (waiting is boring, right?)
  • Reduce power somewhat to be able to remove nasty stuff with boiling points lower than ethanol.
  • Set power so that the evaporation rate is matched to the condensing rate of the cooler and create the equilibrium with ethanol at the top.
  • Increase power slightly (when I’ve got nearly all ethanol) to collect some stuff with boiling points higher than ethanol but lower than water.
  • turn off, job done!

I think that these power settings can indeed just be a few fixed values and I’m thinking of just doing some trial-and-error to find the right ones (and using the SSR from Option 3). I might have been over-complifying things in my OP.

Just one question: if I for example want 25% power, does it matter if the element turns on-off for 250ms-750-ms or 1s-3s or 20s-60s or whatever? Is one better than the other?

the primary mixture has constituent parts that have different boiling points. the water, 212, anything above that and you will have steam .

to that end, if you can reach and hold 211, then everything with a boiling point under 212 will boil off.
I was using the silliness about global warming as an example. if you take the time to look at how much energy is required to change phase of s substance. you will come the realization that the amount of energy is so vast that it is impossible to come to any other conclusion.

it takes 250 calories to raise 50 grams of ice, 10 degrees.
it takes 4000 calories to have it melt ice and not raise the temperature.
it takes 5000 calories to raise water from 32 to 212
and a whopping 27,000 calories to change phase from water to steam
http://www.school-for-champions.com/science/heat_ice_steam.htm#.Vg-4TW5HBh8

assuming the heat source, yours or the planets, does not change, the same energy needed to melt ice would then heat the resulting water to over 170.

what goes for ‘science’ is akin to have fallen off a cliff and as you are near the rocks, you realize you are wearing silk. you do not ask how you fell off the cliff, how it happened, only that in the final moments, or how your actions have anything to do with how you got there. just what you are doing now, in the most intimate details.

your problem is similar. you can put a bunch of heat into the water without actually boiling the water.
however, your other fluids, with a lower boiling point, will hold the temperature at boiling as the energy is transferred from liquid to gas. this is an important point. with a constant heat, the entire body of liquid will raise to a temperature, then hold, then boil off the spirits (or melt glaciers), then, once they are no longer present, the water temperature will again rise.

you should be able to dump energy into the fluid to bring the temperature up to the boiling point of the spirits, and then hold it there, constant power. watch the temperature level off as the spirits evaporate, and then rise once they have evaporated.

for that, the Arduino offers a lot of functionality. you can watch for that pause in temperature rise.

since your real goal is to deal with the gasses, you do not want to run the temperature past the boiling point of water.
if you leave the heat on, the natural result is that the water will begin to boil and will boil off.
physics, (not politics) dictates the equilibrium.

feedback control should be built in.

if your vessel is insulated, then your heat gain will be higher than the heat loss.

[rant] please do not add insulation after it rose to temperature and pretend that the insulaiton had anything to do with the energy needed to get to that point [/rant]

Boardburner2: Complicifcations,- more information the better.

yes. Complicifcations ... not sure what they are... more information is better.