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We have a strange problem. We are performing CO2 extraction on cannabis and we are seeing considerable decarboxylation of THCA to THC.

We have high quality temperature sensors at the heat exchangers, the extractor vessels, and the separator. We have not observed any temperature spikes that would explain it.

Various parameters have been tried, i.e.

40 - 50C (104-122F) / 115 - 280 bar (1770 - 4050 psi)

We can't find any correlation between the temperatures / pressures and the level of decarboxylation.

The system has been run for 24 hours at 120bar/50C and we are getting approximately equal amounts of THC and THCA out.

The starting materials has been tested to being only slightly decarboxylated. Disturbingly, the spent material is also being decarboxylated.

The same equipment is used to test both the starting and spent material. The results on the starting material have been verified by a 3rd party lab. It seems unlikely to be a testing problem.

We have multiple extractors and we are seeing the same behaviour so it is unlikely to be a sensor issue. The temperature controllers also have their own thermostat that agrees with the extractor sensor. The extractors are all temperature controlled and all temperatures are logged every 30 seconds.

There is a research paper by Rovetto et al. that also observes significant decarboxylation but not to the extent that we are observing it:

https://www.researchgate.net/publication/315417305_Supercritical_carbon_dioxide_extraction_of_cannabinoids_from_Cannabis_sativa_L_plant_material

Is it possible that the normal decarboxylation threshold (e.g. 115C) is lowered when at high pressure?

I understand that THC extracts far more readily than THCA, however looking at the mass balance it seems that selective extraction can not explain it.

If anyone has any insights to share, I would very much appreciate it.

Thanks

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  • $\begingroup$ +1 for a well written first SE question! Have you checked for any leaks in the system? (humor) $\endgroup$
    – uhoh
    Commented Feb 27, 2020 at 0:36
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    $\begingroup$ Thanks for the encouragement! I wish I could find anything that might explain it :( $\endgroup$
    – oll67c
    Commented Feb 27, 2020 at 0:49
  • $\begingroup$ A paper by Iffland et al indicates that decarboxylation occurs at a lower temperature than 110 C. I think in the plant material that THCA predominates, so your own data of an extraction at 120bar/50C indicates conversion at temperatures as low as 50C. So at 50C the decarboxylation seems to be a kinetic problem. A longer time should convert more. // The decarboxylation reaction would seem to be irreversible since you're using CO2. $\endgroup$
    – MaxW
    Commented Feb 27, 2020 at 1:02
  • $\begingroup$ Also see "Decarboxylation Study of Acidic Cannabinoids: A Novel Approach Using Ultra-High-Performance Supercritical Fluid Chromatography/Photodiode Array-Mass Spectrometry" by Wang et al. They give a formula for the reaction rate. I didn't read the paper carefully enough, but I'm assuming that is for pure THCA. There might be something in the plant material that would stabilize the THCA a bit more. $\endgroup$
    – MaxW
    Commented Feb 27, 2020 at 1:24
  • $\begingroup$ Thank you MaxW, I have seen 80C as the start of decarboxylation according to a few sources now, appreciate the links to the papers. I wonder though (and my chemistry knowledge is weak) - could this 80C be lowered significantly if reacting under huge pressures? $\endgroup$
    – oll67c
    Commented Feb 27, 2020 at 20:34

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If this problem has not already been solved by the author, then here: Supercritical extraction, by nature, breaks the carbon bond in the dissolution phase, which is why you are even seeing your biomass decarbed. It's not a heat thing, it's a function of SCF CO2.

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