October 22, 2019

Short-Path Distillation For Pure Cannabis Distillate

Producing high-quality and pure cannabis distillate is a complex process with multiple stages that require different equipment. That being said, the process of distillation is a crucial part of the cannabis manufacturing process. That being said, it is hardly exclusive to cannabis extraction; for example, it is also commonly used in alcohol production. But short-path distillation (also known as molecular distillation) is a specific distillation process that is especially useful and effective for cannabis production, albeit for a smaller-scale operation.

What Exactly Is Distillation?

Let’s take a moment to explore what distillation is. In order to fully understand this chemical process, let’s discuss what “distillation” means outside of this context. When not referring to chemical extraction, it refers to the act of extracting the fundamental meaning or most important aspects of something. For example, the phrase “to distill something down to its essence” means breaking something down to its essential parts.

Well, in the context of chemical extraction, distillation is essentially the same thing. If you have a liquid solution, then its distillation is the act of purifying it via heating (vaporizing) and cooling (condensation). If we break this down into fundamental steps, they look something like this:

Liquid solution → Vaporization → Condensation → Collection of distillate

That is a basic breakdown of how distillation works. Notice that the final product is known as distillate; this means that a solution with multiple compounds gets “purified” (or “distilled”) into a final product known as a “distillate”.

Now that we have a general notion of what happens during distillation, let’s take a moment to discuss some basic chemistry. Remember that matter can be divided into states. Technically, there are five, but there are only four that can be observed in everyday life:  

  1. Solid
  2. Liquid 
  3. Gas
  4. Plasma

For the purposes of this discussion, let’s only consider the first three: solid, liquid, and gas. As you probably remember, adding heat to a solid turns into a liquid and adding heat to a liquid turns into a gas. This process of vaporization can be reversed via condensation, in which cooling a gas turns it into a liquid and cooling a liquid turns it into a solid. For the purposes of this discussion, it’s also crucial to remember that vapor means gas.

Keeping this in mind, let’s expand those steps of distillation and look at them in a little more detail:

  • Add heat to a liquid solution or mixture that has at least two different constituent compounds. 
  • As this solution heats up, the compounds that have lower boiling points will begin to vaporize first and rise up through the distillation column. Another way to say this is that substances with high volatility will vaporize first. 
  • The higher these vapors climb, the purer the distillate becomes. This is because the heavier molecules will “fall off” and return back to the liquid stage. That means that at any given point in the column, there will be vapors rising and liquid falling back down. It should be noted that the height of the column will affect the percentage separation; imagine that a higher column will naturally result in more stages of separation. 
  • Once these vapors reach the top of the column, a condenser cools them back down to a liquid form. This distillate is then piped to some kind of collection tank or storage. 
  • Any substances that remain in the column continue the process of distillation. Some systems use a continuous process (more base solution is added on a continual basis) while others use a batch system (one batch is completed and another is then added). 

Distillation can be performed multiple times to increase the purity level; it simply depends on what the manufacturer is looking for in terms of the final product.

Furthermore, short-path distillation (or molecular distillation) is a kind of distillation done in a vacuum (also known as vacuum distillation), although the separation is not executed via solvent affinity but by differences in boiling points of the constituent compounds. We’ll examine this process more closely below, but first let’s go over some relevant concepts.

Which Comes First: Extraction or Distillation?

There is some confusion about where distillation falls in the overall order of production for cannabis distillates. Is distillation the same thing as extraction? No, it’s not. Does extraction come before distillation? Yes, it does. In fact, let’s just quickly break down the basic steps as follows:  

Extraction → distillation (short-path) → chromatography (reverse chromatography) → analytics → distillation (multiple passes that are optional) 

As you can see, extraction comes first. As far as cannabis extraction, it could be alcohol (usually ethanol), carbon dioxide (CO2), or hydrocarbon (like butane or propane). Each of these extraction methods have their respective pros and cons, but it’s important to note that short-path distillation is a distinct step and can be implemented with different extraction methods.

What Exactly Are Volatility, Vapor Pressure, and Boiling Points?

Before we dive into discussing short-path distillation specifically, let’s just take a quick moment to discuss the concepts of volatility, vapor pressure, and boiling points. These are crucial when discussing short-path distillation, as these specific properties are what allow this process to be so effective and efficient for the production of cannabis distillate.

First, let’s start with a few formal definitions:

Volatility is the likelihood that a specific substance will exist as a vapor at a given temperature or pressure. A substance with high volatility is more likely to be a gas while a substance with low volatility is more likely to be a liquid or solid. You can also break it down like so: the higher the volatility, the faster it will vaporize out of a solution that is being heated. 

The vapor pressure (or equilibrium vapor pressure) is the pressure at which the gas state of a compound is in equilibrium with its more condensed phases (liquid or solid). This occurs in a closed system (like a piece of distillation equipment under a vacuum) and it is ultimately an indication of the rate of evaporation of a liquid solution. 

The boiling point is the temperature at which the vapor pressure is equal to the surrounding pressure, thereby enabling bubbles to form and release vapors. The boiling point is highly dependent on the atmospheric pressure (or the surrounding pressure). 

But what does all this mean for the operating principles of short-path distillation? Well, let’s break this down into a few key concepts, laid out simply and concisely:

  • A compound with higher vapor pressure will be more volatile. 
  • Higher vapor pressure/volatility equals a lower boiling point
  • Increasing temperature will increase the vapor pressure.

Remember that a successful distillation process requires the vaporization of the solution in order to actually distill it down its constituent compounds. Therefore, a distillation system operating in a vacuum with higher vapor pressure will have lower boiling points for its constituent compounds. A solution of cannabis material, post-extraction, is made up of a series of constituent compounds that are sensitive to heat. As a result, any system that decreases boiling points would be preferable for this type of distillation.

What Is Short-Path Distillation?

So now that we’ve covered the overall process of distillation, let’s focus in on short-path distillation, also known as molecular distillation. This is one of the most efficient distillation processes to use for cannabinoids, resulting in final cannabis distillates that can reach 99% purity. Short-path distillation gets its name from the fact that the cannabis distillate only has to travel a very short distance between vaporization to condensation. This is usually a distance of only a few centimeters; in other words, the vapors travel a very “short path”.

The use of this short path effectively means that the heated evaporation surface is in close proximity to the condensing surface. This also means that the short-path apparatus is generally quite compact, making it especially useful for smaller-scale distillation operation. Furthermore, since all of this is happening in a vacuum, the entire process can be conducted at:

  • Lower temperatures – The boiling points of various compounds are reduced by 300 K when compared to their boiling points in standard atmospheric conditions. 
  • Lower operating pressures – These can be as low as 1 to 0.001 millibars (mbar). Remember that standard atmospheric pressure at sea level is 1013.25 mbar, so these operating pressures are vastly lower!

The short path between the evaporation surface and the condensing surface also means that pressure drop is minimized. This makes it easier to maintain a vacuum and keep those operating pressures so low. Additionally, the overall temperature (also known as the operating temperature or operation temperature) is kept lower as well, thereby preserving the structural integrity of the volatile compounds in cannabis.

What Are the Parts of a Short-Path Apparatus?

These will vary, depending on the specific equipment manufacturer and the fact that many cannabis producers customize their equipment, but the most basic parts of a short-path apparatus include:  

  • Heating/evaporating component (also known as a heating mantle) – This is usually a magnetically stirred hot plate used to heat up the initial solution.
  • Distillation head – Vapors rise through this passage before condensation.
  • Condenser – This can be just one or multiple condensers used to condense the vapors boiling off of the heated solution.
  • Chillers – These are usually recirculating chillers and are used to cool the condensers.
  • Multi-position receiving/collection vessels (also known as evaporating flasks or cow receivers)– These can be adjusted to collect the different cannabinoids, terpenes, flavonoids, or contaminants that are being condensed.
  • Vacuum pump – A pressure vacuum is maintained by sucking out and trapping the air present in the system, thereby keeping it a closed system.

Basically, the heating mantle vaporizes the solution, those vapors travel upwards through the distillation head until they reach the condensers and then drip down into the collection vessels. It’s really quite simple! But the simplicity of the system is what makes it so efficient, easy, and cost-effective. For example, the most basic short-path apparatus is known as a kugelrohr and consists of an electric heater, glass bulbs joined via ground glass joints, and a bulb condenser cooled by ice. Simple!  

Furthermore, at various intervals the vapors can be collected post-condensation with one or more of the collection vessels. For example, THC has a lower boiling point than CBD. So, if you are looking to create a final product that is rich in CBD but not THC, then you can easily vaporize the THC before the CBD reaches its boiling point. If you want to save the isolated THC for a different product, then you can easily collect the condensed vapors by clicking one of the multi-position receivers into place. 

As a result, the operator will have to monitor the temperature and pressure gauges to keep track of what is vaporizing so they can collect it at various intervals. Remember, this is a closed system and it must maintain a vacuum in order for the boiling points to stay low. However, short-path distillation is best for producers working with lower volumes; a distillation system that would work better with high volumes would be wiped film distillation.

How short-path Distillation Work On A Molecular Level?

This method is also known as a liquid-liquid separation technology, because both the initial extract solution and the final distillate are liquids. It relies on the different boiling points of the constituent compounds, but it also relies on the different molecular mean free paths to perform full separation at the molecular level (hence the term molecular distillation).

In physics, mean free path refers to the average distance that is travelled by a moving molecule in between two collisions or impacts, with each of these successive impacts modifying the energy and direction of these molecules.  

Furthermore, let’s consider that the constituent compounds will have different molecular weights, so when they are vaporized they will not travel the same distance because they have different mean free path values. This is because there is a simple correlation between these two values: the heavier the molecule, the slower it will move as a vapor.

That’s how molecular separation is achieved! These different compounds will reach the condenser at different times, thereby allowing for complete separation between constituent compounds. This allows cannabis producers to have tremendous control in which constituent compounds they choose to be collected in the final product. Furthermore, the low operating temperatures and pressures have tremendous benefits for the overall production process.

Why Should Cannabis Distillation Be Done At Lower Temperatures?

Now that we’ve established that short-path distillation allows for a closed system to operate at lower temperatures, let’s examine why that’s so important in cannabis production. Many of the constituent compounds in cannabis have high boiling points (see section below), making distillation at atmospheric pressure highly undesirable.  

Consequently, cannabis distillation is best performed at lower temperatures because:

  • Cannabis is made up of constituent compounds that are volatile/unstable at higher temperatures.
  • Exposure to oxygen at higher temperatures (like the boiling points at standard atmospheric pressure) can result in oxidation.  
  • Prolonged exposure to heat can result in thermal decomposition.

Let’s take a moment to discuss oxidation. This is the chemical process by which the addition of oxygen results in a compound losing electrons. We can see this process all the time in our daily lives; metal will rust or fat will turn rancid when exposed to oxygen for prolonged periods. 

Oxidation can also alter the chemical makeup of the cannabis solution. For example, the cannabinoid tetrahydrocannabinol (THC) is arguably the most desirable constituent compound in cannabis distillate. It is the primary psychoactive component and, aside from cannabidiol (CBD), it has the most medicinal benefits. 

However, THC that has been oxidized will lose its four hydrogen atoms. Once this happens, THC becomes cannabinol (CBN); “tetra” means four in Greek, so “tetrahydro-” means “four hydrogen. CBN is not psychoactive, so a distillate made up of CBN will not have the same effects. Although CBN has various benefits itself, it is emphatically not the same as THC.

Additionally, thermal decomposition, also known as thermolysis, is a form of chemical decomposition created by heat. Basically, this means that the chemical bonds which make up these various compounds are broken by the heat energy. Because a certain threshold of temperature energy is needed to cause this decomposition, it does not occur right away. Rather, prolonged exposure to heat will cause thermolysis, especially in substances that are more volatile, like cannabis.

Constituent Compounds: Cannabinoids, Terpenes, Flavonoids, and Contaminants

Following extraction, you will have a solution of raw cannabis extract that needs to be distilled and purified. Short-path distillation can individually isolate and concentrate the constituent compounds that are present in this cannabis extract. Because certain cannabinoids can be targeted, many manufacturers will create final products like CBD distillate or THC distillate.

But let’s take a step back: what exactly are these constituent compounds? What are the various parts to a solution of cannabis extract? Let’s break them down into three categories:

  • Cannabinoids (phytocannabinoids) – Controlling temperature and pressure allows individual extraction of the various cannabinoids. Essentially, you can choose which cannabinoids you want present in the final product after you condense the vapors back into a final solution.
  • Contaminants – Distilling the cannabis extract can also remove unwanted byproducts like pesticides that have been used in the cultivation of the original cannabis plant. Also, you can remove any amount of solvent, both in large volumes or residual amounts.  

Each of these various compounds will vaporize individually and can then be condensed and collected. Furthermore, here are the boiling points of the major phytocannabinoids: 

  • THC: 157 C
  • CBD: 160 C to 180 C
  • CBN: 185 C
  • CBC: 220 C
  • CBG: 220 C
  • Delta-8-THC: 175 C to 178 C
  • THCV: less than 220 C

While here are the boiling points of some of the major terpenes in cannabis: 

  • Limonene: 177 C
  • Caryophyllene: 119 C
  • Linalool: 198 C
  • Myrcene: 166 C to 168 C
  • Pinene: 156 C
  • Eucalyptol: 176 C

As you can see, many of these constituent compounds have similar boiling points. That means that the distillation will have to occur at precisely controlled temperature and pressure intervals in order to be highly selective in the vaporization/condensation process. Short-path distillation is particularly useful for cannabis as the short distance between vaporization and condensation allows for quick and easy collection under finely tuned temperature and pressure levels.

Furthermore, mean free path also factors into molecular separation. That means if two constituent compounds have the same boiling point, then their separation will occur due to their differing molecular weights. Remember, they will hit the condenser at different times, thereby allowing the short-path operator to still effectively select which compounds are to be collected and separated.

The Great Terpene and Flavonoid Debate

There has been an ongoing debate rumbling about whether or not to include terpenes and flavonoids in the final distillate. There is a growing body of research indicating that these constituent compounds have a variety of health benefits, especially terpenes. Furthermore, when all of the constituent compounds in cannabis are consumed together, then their beneficial effects are magnified (or potentiated) in a biochemical process known as the entourage effect. Additionally, a distillate that has had its terpenes preserved has what is known as a terpene profile.

It’s important to remember that short-path distillation allows for the cannabis producer to individually collect these compounds. These can then be: 

  • Distilled further for additional purity.
  • Consumed/sold separately, like THC/CBD distillates. These are known as isolates.
  • Recombined into a final product that is known as full-spectrum.

Isolates that have had the terpenes and flavonoids removed do not have the telltale color or smell of cannabis. Some consumers do not like the bitter taste of cannabis and consider a distillate free of terpenes and flavonoids to be “purer”. Ultimately, this is still a source of debate and is essentially a matter of personal taste and depends on what the consumer is looking for.

What Are The Advantages to Short-Path/Molecular Distillation?

Short-path/molecular distillation has several distinct advantages, including:

  • Pressure is kept low, resulting in lower boiling points. 
    Operating/operation temperature is kept low. This is lower than the boiling point at standard atmospheric pressure. 
    Heating time is kept short (usually a few seconds) due to the short-path between vaporization and condensation. 
    The efficiency and degree of separation is high, especially when compared to traditional methods of distillation.  
  • It is especially easy to maintain high quality from the raw extract because the process of separation is physical (occurring on a molecular level).
  • Contaminants and pollutants can be easily removed via vaporization and then collected via condensation.
  • Specific constituent compounds can be collected individually (like THC or CBD) and then recombined or sold separately. 
  • The standard equipment/apparatus is small and compact, making it easy to add to a pre-existing extraction system. 

Consequently, this distillation method is especially useful for isolating the constituent compounds of heat-sensitive liquids or for solutions that have high boiling points. Since it is a liquid-liquid separation method, cannabis producers can also use various extraction methods as long as the raw extract is a solution (in liquid form). Remember that the short-path/molecular distillation process is best when working with lower volumes. If a producer is looking to distill higher volumes or scale up their production, then wiped film distillation would be the better option.

Ultimately, innovative extraction/distillation methods are giving cannabis products a bright and promising future. By using a precise distillation method that preserves the purity and structure of the original extract while selectively removing various constituent compounds, cannabis producers can then recombine these various parts into exciting new products.

For example, the use of short-path/molecular distillation coupled with reverse chromatography has resulted in a full-spectrum cannabis oil that is CBD-rich but does not have THC. If a consumer is looking for a powerful cannabis oil with all the health benefits but none of the intoxication, this kind of product will meet that specific demand in the marketplace. As the cannabis market continues to grow and expand, an increasing number of consumers will be looking for products that are tailor-made for their specific needs.

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