16.1.3 DIRTY PURGE – Contamination Threats In Vacuum Purging

by Mary Babitz, Founder – Cascade Sciences

In my years of working with engineers, the most interesting remains the Aerospace Contamination Engineer.  Working with them on vacuum “bake-out” systems, I learned just how important contamination risks are to NASA and other companies that send things into space. 

There’s the obvious… satellite/rocket components, and electronics that perform in the vacuum of space must be outgassed prior to launch.  All trapped gasses, moisture, molecules must be completely purged here on earth as not to outgas once in orbit and risk damage to sophisticated electronics.  There are no do-overs once launched.

But the NASA Contamination Engineers take it a step further.  They measure rates and types of materials outgassing off the product during vacuum bake-outs.  They want to be able to know that if Mars Rover rock samples show evidence of Yellow #5 food coloring, Earthlings brought it, because it was identified in an outgas analysis during a Rover component bake-out.   

Purging solvents of cannabis extracts is a decontamination process all its own.  The end product will be ingested/consumed into the human body, so what are some of the potential contamination risks associated with this equipment and process?   Here are a few that come to mind:

Oil Rotary Vane Vacuum Pumps = Potential Oil Contamination

Agilent DS-602 Rotary vane vacuum pump

These vacuum pumps are popular due to their lower cost, rugged performance and deep ultimate pressures, but due to how oil molecules behave under vacuum, rotary vane pumps are not appropriate in clean processes like food, pharma and cannabis.  The oil that keeps these pumps humming collects outgassed material, water, etc.. and dirties over time, just like the oil in an automobile. Ick.

The biggest risk is a “back-streaming” event, where a pump in the off position +  a chamber under vacuum will rush to equalize pressure by sucking air up thru the filthy pump exhaust and splatter oil mist inside the oven.  Back pressure valves try to prevent this, but still – it’s a dirty idea to begin with.

Solutions - A better choice would be oil-free pumps such as scroll pumps, diaphragm pumps. If you must use an oil-rotary vane pump, at the very minimum a foreline trap should be installed at the inlet to the oil-filled rotary vane pump.  This filter will potentially catch hydrocarbon molecules that could forcibly backstream into the oven or catches random dirty oil molecules that free-range around the system during molecular flow.

Agilent KF-25 Fore line Trap

Agilent IDP-7 Dry Scroll Vacuum Pump

If you don’t have an exhaust mist filter on your rotary vane pump exhaust, consider that as well.  The oil mist that is filling the ambient lab space is in the same air space the vacuum oven will draw from vent.

Agilent Oil Exhaust Filter

Heated Shelves = Potential outgassing contamination of solders, epoxies, silicone

Because temperature control is tricky under vacuum, some manufacturers will offer heated shelves.  At lower-tier price points, a heated shelf can consist of coated wires, heating elements, solder material, (typically lead and tin), possible epoxies, etc… all used to fasten a heating element to the bottom of a shelf.  Seems fine enough, but once heat and vacuum are introduced, the materials used to construct the heated shelf, can also outgass inside the oven, potentially exposing the product to a variety of contamination risks. 

Properly designed heated shelves are actually hermetically sealed heated platens, a solid chunk of SST or Alum with heating or cooling elements machine grooved out of the center, hi-vac leak tested to allow zero exposure of any contamination to outgass into the product inside the oven.

Door Gaskets = Potential Outgassing of absorbed waters, surfactants, silicones

Tragically, O-Ring seal failure brought the 1989 Space Shuttle Challenger to a horrific end. It’s ironic how such a small part can have critical impact on a process. It’s no comparison, but without a cured, low-outgassing door gasket, your vacuum process can suffer.   Make sure the vacuum door gasket is cured prior to use and ask for material certifications to ensure your Viton or Silicone is pure.

Parchment Shelf Paper – Out gasing Silicone & Terpene Sponge

Parchment paper is paper + silicone. Terpenes dissolve silicone. During vacuum purging, terpenes are breaking down the silicone barrier and are being absorbed into the leftover paper – potentially losing valuable terpenes.

PTFE Solution – PTFE (Polytetrafluoroethylene) material will not absorb purged product.  It is often reusable. PTFE’s smooth, solvent proof, slippery surface prevents bacteria from sticking and embedding in PTFE surfaces. It also prevents unwanted absorption of valuable terpenes and botanical compounds from leeching out of the product and into the tray. PTFE also won’t break down in the presence of alcohols, terpenes, ethers, hydrocarbons, butane and propane.

PTFE Tray

When cleanliness matters, the devil is in the details.  Attention to a few of these small details can make a bid difference to keep your process free from unwanted/unnecessary contamination.

 

Mary Babitz is the founder of Cascade Sciences, a leader in vacuum and other industrial drying processes.  She can be reached at This email address is being protected from spambots. You need JavaScript enabled to view it.

Mary Babitz of Cascade Sciences

 


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