Extraction based on supercritical CO2
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The use of classic organic dissolvents in the food industry cause environmental and health risks. That’s the reason why for years now they’ve been studying which dissolvents can be used at extraction processes to replace these organic dissolvents. CO2 is one of them.
A substance is called supercritical when the transition phase of fluid to gas is no longer detectable. This phenomena occurs after surpassing a specific pressure and temperature and depends on the substance. At supercritical extraction – Supercritical Fluid Extraction (SFE) – specific components are extracted from the basic material by use of a supercritical dissolvent. Different dissolvents can be used for this but in general one uses CO2. Also water is well dissolving in supercritical CO2. This gives the technology the potention to be used as drying technique = specific extraction of water as alternative to freeze-drying.
The critical temperature of CO2 is 31.3 ˚C at a critical pressure of 72.9 bar, see Figure 1. (To compare: for water the critical temperature is 374.4˚C at a critical pressure of 226.8 bar.)
Physical-chemical properties can change drastically the moment a substance becomes supercritical. In this condition the physical-chemical properties of CO2 in some cases resemble those of fluid CO2 and in other cases those of gaseous CO2. Supercritical CO2 (scCO2 for short) has the physical properties of gaseous CO2 (high diffusivity, low viscosity, no surface tension) and the chemical properties of fluid CO2 (high density and large dissolvability). These physical-chemical properties can be rather drastically altered by adjusting the pressure and/or the temperature. In other words, it is possible to adapt the properties of scCO2 to a specific application, which makes it usable in a number of processes.
Principle of operation and Applicability
The general principle of operation of a scCO2-extractor can be reproduced as follows. Gaseous CO2 is cooled, compressed to above the critical pressure and warmed to above the critical temperature at which the supercritical properties are obtained. Next the scCO2 flows through an extraction barrel which contains the raw material from which certain components need to be extracted. The scCO2 containing the extracted components are decompressed, after which the gaseous CO2 is being separated from the extract in one or more separators. The vapor CO2 at the exit of the last separator can be reused by making it fluid again (condensate), while the extract can be drawn from the separators. Supercritical extraction with CO2 is already being used for a long time for the extraction of caffeine from coffee beans. In addition the use for the extraction of hop is an important application. Over the past few years new application areas are found in extracting several components from herbs, spices and algae, like colorants and flavorings, essential oils and antioxidants. Because scCO2 can be used for the extraction of water from solid matters, it is also a drying technique that can be executed at low temperatures, as a substitute for freeze-drying, while maintaining a high product quality.
By using the inert CO2 as dissolvent and executing the process at a low temperature, the finished product will be of high quality.
Pros and Cons
- No (toxic) residues in extract
- Chemical inert
- Particularly suitable for temperature sensitive substances through low operating temperature
- Particularly suitable for oxidation sensitive substances through absence of oxygen
- Components that normally are difficult to separate can show a very different level of solvability in CO2, providing a good separation.
- CO2 is reusable
- Used as a drying technique considerably cheaper than freeze-drying
- An important con of this technology is that it isn’t a continuous process; materials need to be treated batch after batch. The past few years Wageningen UR have developed a continuous variant that should end this problem. Industrialization of the process is in progress.
Costs of supercritical extraction strongly depends on the size of the installation (the volume) and of the components being extracted. It is suggested that the costs for investments are higher and the variable costs lower than for the conventional methods. Therefor the total cost are comparable or lower to those for using organic dissolvents. Also for drying applications, model calculations show the technique to be cheaper than freeze-drying.
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