Hey friends I am back again. I know you must have missed my article so yes I have yet another blog written just for you. Primarily the blog is all about supercritical CO₂ extraction that is so prevalent in these days. But the sad part is not everyone has idea what it is all about.

I hope I wrap this blog by giving you that much needed idea.

Supercritical CO₂ extraction is a type of supercritical fluid extraction process. Now we should first know what is supercritical fluid extraction, right? So come along and read with me. Supercritical fluid extraction is a process of separating one component from other with the use of supercritical fluids suc as solvent. Extraction generally takes place from solid matrix but can also take place from liquids.

The supercritical fluids include carbon dioxide, propane, butane and ethylene. Supercritical CO₂ extraction is originated from this process, as the supercritical fluid mostly preferred in this is carbon dioxide (CO₂). Supercritical fluids above their critical point exhibit liquid like and gas like properties.

Supercritical fluid extraction allows processing at low temperatures and so it prevents thermal degradation. It also avoids use of toxic solvents. This is very basic introduction; we will now head towards supercritical fluids. To grasp command over this extraction method, it is important to understand this.

Supercritical fluids

Two parameters that play crucial role in complete chemistry are temperature and pressure. Any substance is distinguished by a critical point which is obtained at specific temperature and pressure. This critical point acts as a barrier and when any compound is subjected to temperature and pressure above this critical point the barrier breaks and compound reaches into supercritical stage and such compounds are termed as supercritical fluids.

In the supercritical stage no distinct liquid and gas phases exist. Another definition suggests that the fluid is said to be ‘supercritical’ when it is heated above its critical temperature and compressed above its critical pressure.

I would like to simplify it little more.

Think as if there is high temperature greater than critical temperature so now kinetic energy of molecules is high enough to beat any inter molecular forces that would condense the sample into liquid phase. Also if there is high pressure above critical point that would not allow sample to exist in pure gaseous state. 

And to create stability substance exists in a state between gas and solid. The most widely used such fluids are CO₂ and water.

Properties of supercritical fluids: It isquite obvious that these fluids have some special properties so we are emphasizing so much on this.Properties are as follows:

• Supercritical fluids show an intermediate behavior between liquid and gas.

• They possess liquid like densities and gas like viscosities.

• Diffusities are intermediate to that of liquid and gas. 

• Reduction in surface tension.

• Ability to act as solvent.

• These fluids flow through solids like a gas and dissolve materials like a liquid.

• All supercritical fluids are completely miscible with each other.

Now we are heading towards supercritical CO₂.

Supercritical CO₂:

We now well know that carbon dioxide is most widely used supercritical fluid. At most pressure and temperature s carbon dioxide exists in a form of gas. As pressure increases it becomes a liquid and eventually a solid. When pressure along with heat is added it becomes supercritical fluid.

The reason why this is a favorite super-critical fluid, it is cheap, chemically inert, non-toxic, non-flammable and easily available at high purity. It should also be noted thatcritical point of CO₂ is easily reached.

Critical temperature for CO₂ is 31°C and critical pressure is 74 bars. This is helpful as it can be used at mild conditions of temperature and thus will not emit harmful organic residues. Because of all these properties supercritical CO₂ is also known as “green” solvent.  It is evident that carbon dioxide contains all those properties of supercritical (SC) fluid.

History

Solvent properties of supercritical fluid were first reported over 100 years ago in 1879 by Hannay and Hogarth. He measured solubility of inorganic salts in supercritical ethanol. This was the start of supercritical fluid emergence. Since 1980s and 1990s SCF has been used in several industrial processes.

We should know quickly jump into the extraction process along with applications.

Supercritical CO₂ extraction process

Supercritical CO₂ extraction is the process of separating one component from another using carbon dioxide as the extracting solvent. This extraction process creates phase change in CO₂ utilizing temperature and pressure.

These phase change actually works in creating an environment to drop out differing weights of components in the plant material. It is okay if you don’t have any plan for buying CO2 extractor, knowing the complete process is part of curiosity. Let’s jot the basic steps of the process:

• A chamber in the CO₂ extractor is filled with plant material of interest. The plant material should be grounded. The type and quality of plant material is crucial along with extraction process for high quality end product or extract.

• CO₂ pump pushes gas under pressure and at right temperature into the chamber containing plant material.

• Supercritical CO₂ now comes into action and does it work on plant material. The soluble particles get dissolved while rest is left behind. These particles are carried by SCF through a pressure release valve to the separator.

• In the separator, pressure drops so the carbon dioxide can separate from the particles. CO₂ becomes normal gas again and travels to the top of the separator.
• CO₂ gas can be rerouted and fed into the gas tank so that it can be used again. This makes this process environment friendly.

• Oils from the particles fall to the bottom of the separator and gets collected. It can then be analyzed and processed as product for sale.

This is the whole process in general. I am further going to discuss about the components that are must in a supercritical CO₂ extraction unit.

Components of supercritical CO₂ extraction process

Now when you know a general picture of how the process takes place, it will be easier to relate and grasp components of the unit.

In the complete process, the most important and unavoidable is carbon dioxide that acts as solvent. This means system must contain a gas pump that pumps this CO₂. There should be a pressure cell that contains sample. Maintaining pressure in the complete unit is foremost so there should be a means to support that. Lastly there should be collecting vessel and separator in which the particle of interest collects and separates respectively.

Let us know more about components.

Pumps: Pumps form an important component of super-critical CO₂ process. Carbon dioxide is usually pumped as a liquid where temperature is set below 5°C and pressure about 50 bars. The solvent which is pumped is liquid and so it is almost in-compressible.

To act as super-critical fluid much of the pump stroke will be used up in compressing the fluid rather than pumping it. There are different types of pumps according to scale of extraction. For small scale extractions reciprocating CO₂ pumps or syringe pumps are preferred. For large-scale extractions diaphragm pumps are common. The pump heads usually entail cooling and CO₂ will also be cooled before pumping. 

Pressure cell: Pressure vessel or pressure cell is used for containing sample. It should be capable to withstand high pressure. The design of pressure vessels varies from just simple tubes to sophisticated ones.

The pressure requirement in this vessel is at least 74 bars. Higher pressure of 800 bars is preferred for extraction of vegetable oils or else most extractions takes place at pressure below 350 bars. The vessel must contain some means of heating. For smaller vessels, oven can be opted and for larger vessels electrically heated jackets are preferred.

Maintenance of Pressure: It is really important to maintain pressure in the system right from the pump and to the pressure cell. According to the vessel size simple restrictor can be used. It can be a needle valve or capillary tube that maintains pressure at different flow rates. No matter what method is opted, heating must be supplied to prevent adiabatic expansion of CO₂.

Collection: The solvent used for extraction is passed into vessel at lower pressure than the extraction vessel. Density and the dissolving power of SCF are connected with pressure and thus vary sharply. Hence the solubility in the lower density CO₂ is much lower and the material precipitates for collection. Dissolved material can be fractionated using different vessels at reducing pressure. The CO₂ gas can be discharged to atmosphere or rerouted and reused which is the specialty of the process.

Cost of Extraction

The whole process sounds not so complex and results are even desirable but the process is performed by experienced individual. Yet another hurdle in using this technique is cost associated with it. Equipment needed for the process is very expensive. The highest-end machine is quite outstanding in working as it can even separate specific individual compounds.

But hold on, the fancier the machine the more it costs. It is simple the more advanced features an equipment possess the more will be its cost. Though the machine is expensive, it becomes more cost-effective when it is compared with other solvent based extraction methods.

Application of Extraction process:

Research in super-critical extraction technology initiated about two decades ago. In the meantime, several hundred such plants have been designed to operate at extremely high pressure worldwide. It has been an effective method for separation of phenolic compounds. Ethanol, water and super-critical CO₂ are all classified as generally recognized as safe (GRAS). The technology has wide variety of industrial applications which are as follows:

• Food industry has a huge application of this technique. Supercritical CO₂ as extraction solvent for natural products is the oldest and most practiced process. Decaffeination of coffee is the first as well as best example of this industrialization.

• Fruits and vegetables are rich in antioxidants like polyphenols, carotenoids and tocopherols. These antioxidants can be selectively extracted using this technique.
• The process is highly preferred for extraction of natural plant extracts i.e. essential oils. Among the spectrum of these extracts terpenes can be extracted which can prove fruitful in pharmaceutical company.

• The technique also has role in polymer and plastic industries as CO₂-SC is a good plasticizing agent.

• It also plays role in catalysis and biocatalysis of organic compounds.

Advantages of CO₂-SCF

Though we now know many benefits of this process, it will be easy if I quickly note it down:

• The process offers selective extraction that means properties of the supercritical fluid can be altered by varying temperature and pressure.

• Extraction is a diffusion based process and diffusivity is much faster in supercritical fluids than in liquids. This clearly depicts that extraction is faster in such process.

• In this process CO₂ is used as solvent which is easily available, non-toxic and non-explosive.

• Critical temperature and pressure of carbon dioxide is easier to achieve.

• It offers easy separation of solvent and extract.

• The process is environment benign as carbon dioxide is easy to remove just by reducing pressure and thus leave no traces of solvent.

Disadvantages of CO₂-SCF:

• Carbon dioxide itself is non-polar and has limited dissolving power. So it cannot always be used as solvent on its own particularly for polar solutes.

• The technique operates at high pressure which results in higher capital and operating costs.

I’m Sure you must have got an idea of Super-critical Co2 Extraction. For more you can write us in comments and we will like to answer as many questions of yours as possible . Also, share if something important you have to say on Supercritical Extraction or Essential Oils.