Before we discuss extraction techniques, lets discuss safety and social responsibility. Butane and Propane are both high flammable and typically under pressure in their containers, due to their low boiling points.
NFPA-58 outlines the rules for handling safely. They can be reviewed at: http://www.nfpa.org/assets/files/aboutthecodes/58/58-98-pdf.pdf
Their intent is clearly to control the three factors leading to a conflagration or explosion. Fuel, oxygen, and ignition source, as well as where extraction labs are located.
For ignition to occur, the LPG mixture must be between its Lower and Upper Explosive Limits mixed with atmosphere containing 21% oxygen, and have an ignition source.
Propane vapors are about 1.5 times as heavy as air and Butane about 2 times, so both pool at floor level as they build up to Lower Explosive Limits, which is about 2.1 for Propane and 1.86% for Butane. Upper explosive limits for Propane is 10.1% and Butane is 8.41%.
Propane and Butane vapors not only burn at over 1900C/3500F, if ignited in a confined space will explode, because a fuel mixture rising from 18C/65F/291K ambient, to 1970C/3578F/2243K, expands about 7.2 times, moving everything out of it way in doing so.
You may have noticed that there are more and more incidences of brothers and sisters blowing up folks, critters, houses, motel rooms, and cars of late, which brings us to the number one rule of LPG extraction, which like real estate, is location, location, location.
For starters, that location is never ever indoors, even with an ASME certified extraction system, unless it is in a properly engineered NEMA 7, Class I, Div I extraction booth, which fire safety and zoning laws exclude from residential zones or zones containing high occupancy buildings.
We certified our WolfWurx Mk IVC/VC's through PSI to meet IBC, IEC, NFPA-58, in addition to ASME in OR, WA, CO, NV, and MD, as well as Kirkland Dynamics for a customer in the idyllic land of WA, but final certification signoff required that the extraction room also meet those codes, and so were in industrial or agricultural zoned buildings.
Most of the indoor explosions have been from open blasting, but some have occurred even with closed loop systems, due to leaks, operational errors, and procedures like early pour off.
Before they made it a Class B felony to do so, we successfully extracted for close to a decade outdoors, using fans to rapidly disperse the vapors and minimizing ignition sources. Though most of it involved closed loop systems, prior to that time, we danced with the devil open blasting using either a tube or a thermos.
In open blasting LPG/essential oil solution is typically discharged into an open container sitting in hot water and allowed to boil off to atmosphere. The only thing missing is an ignition source.
Even with the closed loop systems we sometimes experienced leaks with no ignition, but consider that explosions occur because when you not only have fuel, oxygen, and ignition source, but things are also in the way of the rapidly expanding gas.
Without confinement there is only a 1970C/3578F/2243K degree flash, which is hot enough to melt steel, so even as a brief flash is contraindicated in the presence of living tissue, however nothing constraining its expansion is blown apart.
In our case that didn't prove to be an issue, because even outdoors we used non sparking bladed fans to blow the vapors away so they didn't accumulate, and minimized ignition sources, including synthetic fiber clothing prone to static charges.
Some luck was involved as well, because there were times we were between LEL and UEL, just didn't have ignition sources, and times that we had ignition sources, but no leaks within explosive limits.
More than once even open blasting I grabbed someone's hand poised to light up while standing at ground zero. If you smoke, leave your lighter somewhere else, lest you go on auto pilot.
Even if you have the latest and greatest certified extraction booth, leave your lighter somewhere else, because your room is supposed to be free of ignition sources.
Wear cotton or other not static building clothing even in a certified extraction booth, and don't forget about your socks. If you need further convincing, pull a polyester or other plastic fiber garment off over your head in a dark room and watch the fireworks. Unless you have big feet and a small head, you probably can't do that with your socks, but believe you will accept it with a leap of faith after trying the sweater and consider that your socks are down where the vapors try to pool.
Which brings us to leaks. Except for brainless acts, all of our leaks have been valve stem seals, loose fittings, or worn gaskets. All minor, but without ventilation would have pooled and indoors without ventilation could have built up to be explosive.
We cover start up steps under in The Generic Terpenator Operator's Manual, which outlines how to test for leaks, but in summary, the way we minimize those leaks is that every day when we fire the unit up, we both pressure and vacuum check it, including the pump seals. We also pressure and vacuum check it every column we change, and the whole system every time we change the collection pot.
Every time we open and close the collection pot, we clean the gasket surface and use a clean gasket, as well as use a torque wrench to tighten all the critical seals in the system.
With any flammable gas, safety is in using properly engineered systems, and operating them properly. Somehow we've learned to operate LPG forklifts and other LPG powered vehicles indoors, as well as somehow surviving outdoor barbeques, and in Europe they've expanded on LPG in indoor refrigeration systems, so prima facie evidence would suggest that in an of itself LPG can be safely used in professionally designed and operated systems, even of some of the operators are certifiable as mechanically inept or even total morons.
The number of home explosions suggests that there can be issues with non professionally designed and operated systems, especially when operated by morons or in some cases sociopaths, whom by definition have no regard for pain and suffering of others.
The infamous last words of the most recent perpetrator of an explosion that leveled his house, killed a contractor instantly and the perpetrator shortly there after, were "I'm sorry." Don't be! Be smart and play safely by the rules instead