Heating a concentrate to its melting point in a vacuum oven or chamber, so as to reduce surface tension enough for bubbles to escape, and then reducing the atmospheric pressure on it that are preventing the bubbles from escaping, is the fastest way we've found to remove residual solvents from a concentrate.
The manner that we do that has changed some over the last decade sooo please allow me to share a quick trip down memory lane? Here is our first vacuum chamber, a borosilicate bell with a small hot plate inside, in which we were able to process samples using 6" petri dishes.
Early vacuum purging experiments
Vacuum purged raw BHO extract
Next came our DIY vacuum chamber, whomped up out of an available pressure cooker to which we added plumbing and a 1 1/2" thick polycarbonate lid. Initially we heated using a reptile mat , but ultimately switched to silicone heat pads controlled by a Variac variable transformer.
DIY Vacuum chamber from pressure cooker
Vacuum purged winterized BHO
Our next endeavor was to rebuild and re-engineer a used NAPCO vacuum drying oven.
Modified NAPCO drying oven
After sharing an R&D project to refine a vacuum oven for the cannabis market with Cascade TEK, our eyes were opened wide enough that we just had to have a couple for our very own selves and have never looked back, except for just that one little time, when Specialized Formulations purchased the AI Elite oven she had been testing.
Here is Joe’s Cascade TEK oven with Inficon residual gas analyzer attached:
TVO-2 Cascade TEK
Inficon Residual Gas Analyzer
Gas Analyzer readout
Here is some shatter Pharmer Joe made in a Cascade TEK TV02 vacuum oven.
Slab of vacuum purged BHO
Here is a picture of Carla’s AI Elite oven. See her evaluation under Equipment Testing:
AI Elite 2.3 oven
If any of the above methods fit into your needs and budget, look in the DIY section for details:
Meanwhile back to the present, we are only using the vacuum ovens, one use of which is purging residual solvents from a concentrate. There are different thoughts on exactly how to do that while keeping the material in carboxylic acid state while retaining maximum monoterpene flavor and aromatics.
The process also differs some with changes in solvent, like for instance butane and ethanol, because of the way they bind themselves to the essential oil mixture.
For purging BHO, here is how I do it:
Place a thin film of the concentrate on parchment paper and place it in a vacuum oven preheated to 43C/110F and bring up to temperature. Raise the temperature as required until the concentrate is "just fully molten", usually between 43C/110 and 46C/115F.
If you are in a hurry, just start at 46C/115F and skip the nuance.
Once the material is fully melted, start pulling the vacuum and when the material reaches a frenetic bubbling state, hold the vacuum at that level until it calms down, before continuing to reduce the oven's partial pressure.
Once you reach 10,000 microns/-29.5" Hg and the bubbling is calm, back fill with Nitrogen, remove the paddy from the oven and flip. Repeat step one until the paddy no longer produces solvent bubbles at 10,000 microns/-29.5" Hg, back fill with N2 again, and remove to cool.
For purging alcohol we do it basically the same way except alcohol and cannabis oleo resin love each other so much that at 46C/115F it produces huge bubbles whose surface tension is so high that they resist bursting, sooo I increment up the temperature until they do more readily burst, usually closer to 52C/125F.
For difficult bubbles at 125F, we sometimes back fill enough with N2 to collapse them, then pull down again and resume purging.
Boiling is defined as when the internal vapor pressure of a solvent exceeds the atmospheric pressure suppressing it from boiling, so when we remove that atmospheric pressure, the solvent boils at much lower temperatures. A key issue when we are trying to limit decarboxylation or evaporating off some of the higher boiling point constituents, which also have high vapor pressures.
Here is a chart showing the boiling point of ethanol at different vacuum levels by Skyhighler:
Ethanol boiling points under vacuum
Here's one for Isopropyl:
Isopropyl boiling points under vacuum
An issue regardless of which solvent you are purging, is what altitude you are operating at, because while there is 760,000 microns/29.92" atmospheric pressure at sea level, as we gain altitude that pressure becomes less. That atmospheric pressure drops off at approximately 1" Hg per 1000 feet of altitude gained, but below is a more exact chart:
Atmospheric pressure at altitude
Lastly, you may ask why we limit the vacuum levels to 10,000 microns/-29.5" Hg, and the answer is that not only does it lower the boiling point of the solvent, it lowers the boiling point of the target elements that we are trying to hold on to, and some have such high internal vapor pressures that they evaporate away rapidly at temperatures considerably below their boiling points.