A General Hydrocarbon Overview
One primary method of extracting cannabis for its essential oils involves the use of solvents, more commonly known as hydrocarbons or LPG’s (liquefied petroleum gases). Commonly used hydrocarbons for cannabis extraction include butane and propane. In this article, we are going to discuss the use of these hydrocarbons and how they are used to make different BHO (butane hash oil) or PHO (propane honey oil) products. My goal in writing this is to give you a clear understanding of which hydrocarbons you will benefit from using when processing cannabis for extraction. Let’s start by looking at the word hydrocarbon. What is it? A hydrocarbon is an organic compound made up of only carbon and hydrogen atoms. There are two types of hydrocarbons - today we will be focusing on a few of the aliphatic hydrocarbons.
How did we go from using propane to make burgers on our grill, to using it for cannabis extraction as well?! Many years ago, there was a desire to extract the essential plant oils out of cannabis. The process used to extract these cannabis oils is very similar to the process for extracting vegetable and seed oils. When this was first attempted with propane, it was, and still is, much more accessible to the average person than butane. Through years of study with the help of the periodic table, we have come to learn that propane as a molecule is classified as a straight chain alkane, known as C3H8. An alkane is a hydrocarbon in which there are only single covalent bonds, the simplest being methane with the molecular formula CH4. The single carbon atom is the central atom and it makes four single covalent bonds to the four hydrogen atoms. Next comes ethene (C2H6) followed by propane (C3H8)!
Now that we know how we got here and how propane is classified, what does it all mean? Being that propane consists of three carbon atoms to make its straight chain, it is a rather short chain in the world of hydrocarbons. The longest straight chain hydrocarbon has ten carbon atoms! What this means to us is that at room temperature propane is very unstable, as the molecule is quite small or short if you will. It has a very low boiling point of -43.6 degrees Fahrenheit (-42 Celsius). This low boiling point inherently results in propane having very high vapor pressure.
With propane having a naturally high vapor pressure, the use of it is rather straightforward as propane, unless chilled considerably (below -42 C), will constantly be evaporating giving you positive vapor pressure to operate your extraction machine. You need positive vapor pressure during operation to fill your solvent tank or inject your material columns with hydrocarbon liquid, propane in this instance , to extract your essential oils. Constantly having positive pressure not only makes this part of the process simple, but fast as well. Being that there is always an abundance of vapor pressure when using propane, it is imperative to consistently monitor the extraction machine’s pressure gauges to ensure there is no over-pressurization in any area of the machine as propane moves exceptionally fast under its own pressure.
Another positive when using propane as a hydrocarbon when extracting cannabis is the fact that it is the shortest chain hydrocarbon viable for cannabis extraction. This results in higher quality and color of oil from moderate biomass. The longer chain hydrocarbons may extract more oils and compounds from the cannabis than propane would in the same scenario, which doesn’t sound like a win for propane, but in certain scenarios, you need to extract less to achieve the color and quality of oil you are after to make best use of the biomass that you have to work with. Typically, in the modern world of extraction, I see propane used for older biomass as the trichomes have "ambered" and the terpenes have evaporated/degraded. There are multiple methodologies around how to best use propane, this is simply what I have seen working in the field, as well as how I would operate with it myself.
Where the negatives for propane come in is right where the positives left off. What do you do with all that pressure? To relieve or lower the pressure, you need to condense the pressure-causing gas back into a liquid. With propane’s boiling point being -42 degrees Fahrenheit (-42 Celsius), this is not always a simple task. Traditionally, a condensing coil or two would be covered with dry ice to achieve a temperature cold enough to be efficient, but this quickly becomes costly. Today, most people using propane to process and extract cannabis resort to using a recirculating chiller that can condense propane without the use of dry ice. A chiller with enough power to keep up with propane’s evaporation rate at that temperature typically costs north of 100k installed.
All in all, propane is a good choice for use as a hydrocarbon for cannabis extractions, and it has its purposeful use on certain biomass or with specific equipment. However, I do feel that through time and industry innovation we have proven that there are better alternatives 90% of the time. One of these superior alternatives is Butane.
Butane is arguably the most widely used solvent/LPG gas when it comes to cannabis processing. One thing to consider is that there are two different types of butane. Two?! Below, I will discuss in-depth both isomers of butane, the pros and cons, and how they pertain to your extraction!
First up is n-butane, the more common of the two butane isomers. This isomer of butane is a straight chain alkane, just like propane. However, its molecular formula is C4H10. Having an extra carbon atom makes n-butane the longest straight chain hydrocarbon traditionally used in cannabis biomass extraction. The additional carbon atoms make butane drastically more stable at room temperature, although still unstable due to its boiling point.
The boiling point of n-butane is 30.2 degrees Fahrenheit or -1 degree Celsius. With a 73.8 degrees Fahrenheit (41 Celsius) difference in boiling point between the two, they behave quite differently. While this boiling point is quite high compared to propane, it does have its advantages, namely lower vapor pressure. The higher boiling point of n-butane will result in less vapor pressure at room temperature, requiring more energy or heat to evaporate the solvent during processing. However, it will require less energy to condense n-butane back into a liquid. This can be quite helpful as it is rather inexpensive to generate heat to evaporate your solvent for recovery. However, condensing the gas back into a liquid during recovery can also be quite costly to efficiently achieve. To put this simply it requires far less energy to return a vapor back to the 30.4 degrees Fahrenheit evaporation point of n-butane than to -43.6 degrees Fahrenheit of propane. Any temperature below these hydrocarbons’ evaporation point will cause the hydrocarbons to condensate back into a liquid to be reused.
An additional benefit to the higher boiling point is that n-butane evaporates slower than propane! This results in n-butane having lower vapor pressure which then results in slower moving gas. You might ask “Why would I want something to go slower?” The answer is simple; ease of use. Having a less pressurized solvent allows for more time in between processes, as well as easier soaking of your biomass during injection/filling without worry of over-pressurization. The lower vapor pressure also allows for a consistent consistency of your extractions when they come out of your extraction machine, as you need a saturated liquid for several different types of extracts. This is simply not plausible with propane. This is important because if you want your extracted oils to go through the crystallization process to create “diamonds and sauce”, you need a liquid solution. In this case it is easier to start with the correct viscosity of liquid extract right out of the machine, n-butane makes this very easy to control. This is important as the crystallization process involves saturating a solution of extracted oils with a solvent. To achieve crystallization of this solution, proper saturation must occur. The slower evaporation of n-butane makes this window of saturation quite large.
On to the negatives of n-butane; one, ironically, being its boiling point. The higher boiling point requires more energy/heat to evaporate thus taking it longer to evaporate or recover during processing vs propane. You might ask “How much heat do I need to efficiently recover my n-butane?” In reference to the temperature of the water running through the external jacket of a jacketed tri-clamp spool, which is standard to most extraction machines, the typical temperature for either isomer of butane to evaporate at a controlled rate is 104 degrees Fahrenheit (40 Celsius). When extracting cannabis, you should not exceed 122 degrees Fahrenheit (50 Celsius) as any higher of a temperature will result in terpenes evaporating. Terpenes account for the smell and taste of your extractions/concentrates, not to mention a percentage of your overall yield.
As we know from reading above, n-butane has a high boiling point vs propane, which means it will have low vapor pressure in comparison. Low pressure can be tough to navigate if you are not using an inert gas such as argon or nitrogen to assist tank pressure when transferring liquid n-butane from one vessel to another for the injection/filling process. It is highly recommended to use a regulated inert gas set up for “pushing” the n-butane liquid, as it is quite difficult to get it to move naturally vs propane. It will save you quite a bit of time and frustration.
One other potential negative of n-butane is that it is a straight chain compound with the four carbon bonds, meaning it will extract more out of the material/biomass being processed than propane would. This is a good thing right? Generally speaking yes, however some material has a better response (high quality yield) to a shorter chain hydrocarbon such as propane or even isobutane.
This isomer of butane, commonly known as isobutane, is a bit different from the two straight chain alkanes above. To start, isobutane is not a straight chain alkane but rather a constitutional isomer of butane. In the case of isobutane, there is a side chain in the molecule where three carbon atoms form the parent chain and one carbon atom is placed as the side chain at the C-2 of the parent chain. This gives the same C4H10 molecular formula a new structural formula thus changing the connectivity of the atoms. Being that isobutane has a different structural formula than n-butane or propane, it possesses a few unique characteristics.
One of those unique characteristics is isobutane’s boiling point of 10.9 degrees Fahrenheit (-11.7 Celsius). This boiling point finds common ground between propane’s -43.6 F and n-butane’s 30.2F. This boiling point results in a medium amount of positive natural vapor pressure vs propane’s high vapor pressure and n-butane’s low vapor pressure. With this information in mind, it is easy to see which of the three hydrocarbons will evaporate and recover fastest, as well as which one will be the slowest. With isobutane being the median, if you will, of boiling points and vapor pressure between the three hydrocarbons, it provides the extractor an additional, very viable choice of solvent to extract with.
I’m sure you’re wondering how else isobutane will benefit your extracts. With isobutane having a parent body of three carbon atoms it can, in some cases, achieve a better color extract than n-butane, while maintaining a higher yield than propane due to isobutane’s fourth carbon atom in the side chain. In my experience, the use of isobutane has helped in avoiding extracting darker colored and unwanted compounds from particular biomass that was generally older or of less than medium quality. Isobutane is also a good choice for making a variety of products with your extracted oils. Having a median boiling point, isobutane is viable to use for “diamonds and ”. When used for wax or shatter production, isobutane can quicken the production of products typically made with n-butane, as isobutane evaporates quicker, resulting in less time in the vacuum purging oven.
Now on to the few negatives of using isobutane, one of them being availability. In most areas where cannabis extraction has been legal for some time, it should be quite easy to acquire. However, in areas that have just recently legalized cannabis and the extraction of it, isobutane can be tough to find as it is not as commonly spoken or heard of in when searching for what hydrocarbon to use in extraction. Although, with a request to your local gas supplier for isobutane(R600A), they should have no problem getting it sourced for you. With isobutane having a medium vapor pressure, some extractors may find it difficult to achieve a good flow rate through their closed loop extraction system as the pressure from isobutane alone sometimes is not enough. This scenario is very similar to that of n-butane where the use of an inert gas such as argon or nitrogen is needed to up the vapor pressure to achieve better flow rates through the system.
As a conclusion to this article, I would like to point out the fact that the three hydrocarbons in this article are indeed miscible with one another. Some operations prefer a n-butane and propane blend while some prefer a tri-blend with all three, and others may use a single solvent or hydrocarbon for their extractions. Generally, the choice of which hydrocarbons (or mix thereof) is most viable to your situation boils down to your equipment and the biomass you are extracting. Between a number of varying factors and with cannabis extractions being fairly new to our world, it is tough to narrow down one hydrocarbon as being the best for cannabis extraction. In fact, the intent of writing this article is to provide you with enough information so that you may make an informative decision as to which hydrocarbon will best benefit you and your extractions.
Any comments or statements that are of personal belief or from my own personal experience have been noted as such in this article. All other information was cross-referenced with the use of; byjus.com, chem.libretexts.org, purdue.edu, pubchem.ncbi.nlm.nih.gov.