Simple vs Fractional Distillation

distillation

#1

Hey everyone,

Had some good discussions the other day on instagram with one of our posts. I made a couple diagrams up showing an ideal vapor-liquid diagram. The goal was to make a blog post to discuss in some more detail than I was able to do on IG. Here is a link. Check it out, leave nasty anonymous comments I won’t approve, etc etc.

This blog is a collaborative work in progress and we are always going back and updating text and images as well as SEO stuff. Don’t hesitate to point out if you think something is wrong! One of the main points is generating discussion.

What are you waiting for? Get out there and rip us apart!


#2

If you guys have any used condensers in the back of the shop you plan on throwing away hit me up I’d gladly take them off your hands cheap.


#3

What kind of condenser are you looking for? I got a box in the attic, although not everything is up for grabs.


#4

Sent u a message didn’t mean to detail thread lol


#5

This is a great summary. A couple of things that might be good to add are the effect of surface area and temperature gradient.

High surface area is obviously good because it provides more locations for condensation to occur, which is critical for the benefits of a fractionating column. I’ve never seen a spinning band column before but it seems like the auger in the center of the column is accomplishing this quite well.

Establishing a temperature gradient is also critical. It happens naturally as long as the column isn’t too insulated. By having the column hotter at the bottom and cooler at the top, we ensure that the final evaporation event before the condenser is occurring at the minimal required temperature. This makes it less likely that high boiling compounds sneak over with our compound of interest.

It’s often necessary to insulate a fractionating column in order to get distillation to occur at reasonable temperatures. Understanding the importance of the temperature gradient ensures the operator will not use insulation in such a way as to reduce the efficacy of the gradient.

TL;DR Add insulation as low on the column as is effective. Adding too much insulation at the top of the column can have a negative effect on separation.


#6

I see how spinning band could yield a better separation, but I think you are oversimplifying SPD and wiped film. Take for example 1st pass in SPD, you can separate light terpenes to your cold trap, heavy terpenes to your heads flask, followed by distillate. Or in wiped film you have an internal condenser plus 1-2 external condensers, fractionating distillate, light, and heavy terpenes. How is that not fractional distillation? You could theoretically alter your condenser temps or add condensers to increase separation.


#7

@Alister.Frazzle Fractional distillation means refluxing not just taking cuts from three different places. The vapors do not evaporate and condense multiple times. The terpenes in your example evaporate once and then find themselves in whatever container is cold enough to condense them. That is still a single stage where you are not receiving any of the benefits of vapor-liquid interaction. Without this interaction, you cannot enrich your vapor phase.

Yes I admit that I am understating SPD a little. There is absolutely some reflux occurring and the addition of vigs or packing material helps that. However, I doubt you are seeing much more than 2 plates. All depends on how you are running it exactly. One way spinning band circumvents the issue of low reflux is allowing for “equilibration time”. This setting closes the reflux valve temporarily (~5 min) to allow vapors and liquid to interact inside and wet the column. On a more difficult separation you may have an equilibration time of an entire weekend!

Also, if you are blazing through the distillation, your separation suffers. Issues like flooding and entrainment are more likely to happen.

Something unrelated I want to point out is that higher pressures actually give you more theoretical plates. Some think deeper vacuum gives a better separation. And while there are certainly benefits to strong vac, more theoretical plates is not one.


#8

Part of the issue with an explanation like this is the different usage of the term “short path distillation” in the cannabis realm.

The blog post you linked would fit well in a chemistry text book. “Short path distillation” is synonymous with simple distillation. The term “short path” is used because there is no fractionating column. Since there is no fractionating column we assume we have 0-1 theoretical plates. Collecting fractions off a short path in this case makes little sense, because the technique really only separates compounds with bp differences ~80 °C. Short path distillation in the strictest definition of the term is only useful for separating a liquid from a solid, or two compounds with vastly different bp’s.

In cannabis “short path distillation” takes on a very different form. Vendors sell still heads with considerable internal surface area and path length as a “short path distillation apparatus.” With these apparatus we presumably have a large enough number of theoretical plates to separate compounds with bp’s within 20-30°C of each other (this is the max resolution fractional distillation can provide). so collecting multiple fractions makes sense.

this is just a long way of saying that in cannabis, SPD =/= simple distillation, while in a chemistry lab, SPD = simple distillation.