Why supercritical CO2 is super cool

Why supercritical CO2 is super cool

CO2 is used in all kinds of industrial applications; for example, liquid CO2 is commonly used as a cooling agent and solid CO2 (dry ice) as a dairy preservative. Here, we focus on the industrial applications of supercritical CO2 (sCO2).

What is sCO2?

Phase state diagram showing conditions for supercritical CO2

CO2 phase diagram

A supercritical fluid is one that exhibits both gas and liquid properties simultaneously. Supercritical fluids have gas-like surface tensions and viscosities and liquid-like densities. A fluid transitions into the supercritical state when the fluid is heated and compressed beyond its critical point – for CO2 this is when temperature ≥ 31°C and fluid pressure ≥ 1071 PSIA.

sCO2 is an excellent solvent

Supercritical CO2 is the most commonly utilized supercritical fluid, often used as a solvent in commercial applications such as decaffeination, THC extraction, and essential oil production; it can even be used to make the vanilla extract found at your local grocery store.

Supercritical CO2 is a good solvent for small, nonpolar compounds – comparable to hexane and pentane. So what makes sCO2 advantageous over organic solvent alternatives? There are several things: sCO2 is non-toxic, non-flammable, doesn’t leave harmful organic residue, and it is more environmentally friendly than many conventional organic compounds (which are oftentimes under restriction by the EPA)!

There are a few additional characteristics that make sCO2 stand out as an ideal solvent for extraction processes. Like many other supercritical fluids, the combination of its liquid-like density and gas-like surface tension and viscosity allow sCO2 to effectively penetrate into porous solids. And its relatively low critical pressure and temperature make sCO2 preferable over many other supercritical fluids. CO2 is also cheap, so it is an economically beneficial choice.

Using mass flow to optimize the sCO2 solid-liquid extraction processes

Whether extracting essential oils, THC, or caffeine, the basic sCO2 extraction process is as follows:

  1. Gaseous CO2 is condensed into its liquid form.
  2. Liquid CO2 is then compressed and heated until it reaches the supercritical state.
  3. The sCO2 is circulated through the raw material in a solid matrix – for example, in a caffeine extraction, the coffee beans are mixed with the sCO2.
  4. The mixture is depressurized, and the CO2 phase shifts back into gas so it can be reused. The extract remains in the separator where it can be collected.
sCO2 solid-liquid extraction process

sCO2 solid-liquid extraction process

There is generally a tradeoff between extraction rate and sCO2 use – the faster the extraction rate, the more excess sCO2 used. This extraction rate is directly dependent on the flow rate, so an increased flow rate means increased extraction rate.

To optimize this process, it is best to consider specific application needs. Take for example the case of essential oils, where an incomplete extraction may yield sufficient product while limiting sCO2 usage. Whatever your process may be, precise flow control is necessary to optimize the extraction rate and meet production needs for time and sCO2 usage, and then maintain this rate throughout the process.

Alicat Scientific’s CODA-Series of Coriolis mass flow instruments is a great way to measure and control sCO2 flow in many processes. CODA instruments are able to meet the low-flow requirements for sCO2 applications with high accuracy and precision. They can also measure and control all phases of CO2 and can therefore be used throughout the various steps of the extraction process.

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