Optimizing your RotoVap

Hash and Stuff Solvent Recovery
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One of the most common methods of recovering solvent from hash in the lab is with a rotary evaporator.

This distillation apparatus operates under vacuum to reduce the boiling point of the solvent, allowing us to evaporate off our solvent without damaging the cannabinoids, at much lower temperatures. The boiling flask rotates to create a thin film, which speeds up the evaporation.
After the solvent is evaporated, and is traveling towards the vacuum pump in the vapor stream, it passes over the condenser which forces the solvent back into its liquid phase, allowing gravity to pull the liquid solvent into the receiving flask.
This device, on paper, can be summarized as a simple equation.

Heat (energy) Into the boiling flask = Heat (energy) Out of the condenser

And since deeper vacuum lowers the amount of heat (Energy) needed on the boiling flask, the equation dictates that less heat (energy) is needed on the condenser side. It’s important to note that Heat (energy) is being removed with the condenser via cold fluid, fluid which required (energy) to create.

So how do we apply this knowledge to improve the performance of our roto?

  1. In order to maximize vacuum performance, it is necessary to condense as many volatile molecules in the vapor stream as possible. This means that a cold trap is in order. A dry ice dewar style, similar to those standard on a SPD setup, will suffice.
  2. In order to maintain the equilibrium of our still (heat in = heat out) it makes sense to preheat our incoming feed solution. This can be easily achieved by incorporating a heat exchanger into the feed line. Simply, a coil submerged in a bucket of souz vide heated water, will bring the incoming feed up to temp cheaply and easily without significantly increasing hot residence time.
  3. As we improve the efficiency of our Heat In side, we must balance the equation on our Heat Out side. While buying a larger chiller is the most efficient step, sometimes ingenuity is all we have at our disposal. Another heat exchanger, the coil for example, can be submerged in a cooler full of dry ice, which can be inserted between the output of the chiller, and the input of the roto condenser. It is important to place the exchanger here vs between the chiller and condenser on the fluid input to the chiller so as not to confuse the chillers internal thermostat.

If you find that after these steps you still are at a solvent recovery bottle neck, its time to consider a Falling Film.

36 Likes
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can you elaborate on the heat exchanger, trying to go straight from cold ethanol to particle filtration to roto with pre heated solution

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A simple stainless coil in a bucket of hot water is a good start

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Ping-pong balls in the water bath helps with evaporation. Switching over to oil instead of water is another option, especially if you want to crank up the heat and maybe decarb in the roto.

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Ping Pong balls in the bath is a crucial tip I need to add!

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how does the coil link to heating the incoming feed line? sorry im just not seeing the picture, also what do ping pong balls added to the bath do? i have a hard time keeping my A.I. chiller cool when getting agressive with the water bath (<45C) and have alot of iso coming through my welch pump even with a cold trap.

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You need a more powerful chiller.

Bucket of crude solution>coil in bucket of hot water>roto.

Ping pong balls keep the water from evaporating

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I like the ping pong ball idea. Or making a little plastic dome over the bath to re-collect the water vapor.

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Most of the roto manufactures supply those domes

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I like chilling my recovery flask to run deeper vac rather than jumping the heat on the water. Toss some dry ice in a sack and tie it around the flask and she doesn’t boil the lower you go. On my 20L bvv, not my choice :(, I’ve had her recovering around 4l an hour. Wish I had vaccum take off like the AI and nicer rotos so I could continuous feed.

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That’s a good point. You’ll lose vac depth as the alcohol in your receiving flask warms up and builds up. I’ve never tried cooling it down though.

At the moment I’m recovering 5 gallons in 85 minutes on a 20l roto.

I have a preheating coil, 2 cold traps and I run the water bath at 80c.

You’ll end up with terpy alcohol but I think it’s worth it.

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Can anyone override the low temp shut down on polyscience chillers? I would like to put a coil in dry ice and really only use the pump because these suuuck. I tried it before but it kept shutting down at -20, I’m using there -10 unit for a 20L

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Hey, don’t know if this helps. I use a chill king chiller for hydroponics, and a little Giant pump to move the glycol. I have it on a 50 gal resivour. It can push as low as -20 f and keeps up with a line of rotos. It was cheap to do, and really effective. If I’m stripping from crude I’ll push the bath as high as 65c and load 3/4 of the condenser with condensate. I don’t get too much in the traps. How is your condenser hooked up? It is important to have the top of the condenser be your coldest zone.

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Put that coil between the output of the polysci and the inlet of the roto. The (relatively) warm returning fluid will tell the chiller to keep operating

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Still didn’t work, she was too cold.

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Then why do need the chiller?

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For the pump and storage.

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If you want the compressor to do more work, use less dry ice. If the cooling fluid comes out cold, you need more condenser SA

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There is a Low Temp Limit setting on most chillers. If you check into your setting you may be able to lower the shut down temp. when you’re running i’m assuming there an alarm noise when it kicks off. it should also be displaying a fault code. Mine threw up an FT 02 you can check into the manual to change that factory setting. Hopefully that will take care of your problem.

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Do you know off hand the diameter of the coil stack you’re running or the total over all length?