Optimizing supercritical CO2 solid-liquid extraction with Coriolis mass flow

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?

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 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

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

CODA Coriolis mass flow instruments can be used to measure and control supercritical CO₂ flow in many processes, 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.