# CO2 extraction



## BlazinBob (Sep 30, 2008)

Do these directions make sense

Supercritical Fluid Extraction (SFE) is the process of separating one component (the extractant) from another (the matrix) using supercritical fluids as the extracting solvent. Extraction is usually from a solid matrix, but can also be from liquids. SFE can be used as a sample preparation step for analytical purposes, or on a larger scale to either strip unwanted material from a product (e.g. decaffeination) or collect a desired product (e.g. essential oils). Carbon dioxide (CO2) is the most used supercritical fluid, sometimes modified by co-solvents such as ethanol or methanol. Extraction conditions for supercritical CO2 are above the critical temperature of 31°C and critical pressure of 74 bar. Addition of modifiers may slightly alter this. The discussion below will mainly refer to extraction with CO2, except where specified.SFE is an alternative to liquid extraction using solvents such as hexane or dichloromethane. There will always be some residual solvent left in the extract and matrix, and there is always some level of environmental contamination from their use. In contrast, carbon dioxide is easy to remove simply by reducing the pressure, leaving almost no trace, and it is also environmentally benign. The use of SFE with CO2 is approved by the Soil Association for organic products. [1] The CO2 used is largely a by product of industrial processes or brewing, and its use in SFE does not cause any extra emissions. [2]


[edit] Selectivity
The properties of a supercritical fluid can be altered by varying the pressure and temperature, allowing selective extraction. For example, volatile oils can be extracted from a plant with low pressures (100 bar), whereas liquid extraction would also remove lipids. Lipids can be removed using pure CO2 at higher pressures, and then phospholipids can be removed by adding ethanol to the solvent.[3]


[edit] Speed
Extraction is a diffusion based process, with the solvent required to diffuse into the matrix, and the extracted material to diffuse out of the matrix into the solvent. Diffusivities are much faster in supercritical fluids than in liquids, and therefore extraction can occur faster. Also, there is no surface tension and viscosities are much lower than in liquids, so the solvent can penetrate into small pores within the matrix inaccessible to liquids.


[edit] Limitations
The requirement for high pressures increases the cost compared to conventional liquid extraction, so SFE will only be used where there are significant advantages. Carbon dioxide itself is non-polar, and has somewhat limited dissolving power, so cannot always be used as a solvent on its own, particularly for polar solutes. The use of modifiers increases the range of materials which can be extracted. Food grade modifiers such as ethanol can often be used, and can also help in the collection of the extracted material, but reduces some of the benefits of using a solvent which is gaseous at room temperature.


[edit] Procedure
The system must contain a pump for the CO2, a pressure cell to contain the sample, a means of maintaining pressure in the system and a collecting vessel. The liquid is pumped to a heating zone, where it is heated to supercritical conditions. It then passes into the extraction vessel, where it rapidly diffuses into the solid matrix and dissolves the material to be extracted. The dissolved material is swept from the extraction cell into a separator at lower pressure, and the extracted material settles out. The CO2 can then be cooled, re-compressed and recycled, or discharged to atmosphere.


Figure 1. Schematic diagram of SFE apparatus
[edit] Pumps
Carbon dioxide is usually pumped as a liquid, usually below 5°C and a pressure of about 50 bar. The solvent is pumped as a liquid as it is then almost incompressible. As a supercritical fluid, much of the pump stroke will be "used up" in compressing the fluid, rather than pumping it. For small scale extractions (up to a few grams / minute), reciprocating CO2 pumps or syringe pumps are often used. For larger scale extractions, diaphragm pumps are most common. The pump heads will usually require cooling, and the CO2 will also be cooled before entering the pump.


[edit] Pressure vessels
These can range from simple tubing to more sophisticated purpose built vessels with quick release fittings. The pressure requirement is at least 74 bar, and most extractions are conducted at under 350 bar. However, sometimes higher pressures will be needed, such as extraction of vegetable oils, where pressures of 800 bar are sometimes required for complete miscibility of the two phases.[4]

The vessel must be equipped with a means of heating. It can be placed inside an oven for small vessels, or an oil or electrically heated jacket for larger vessels. Care must be taken if rubber seals are used on the vessel, as the CO2 may dissolve in the rubber, causing swelling, and the rubber will rupture on depressurization.


[edit] Pressure Maintenance
The pressure in the system must be maintained from the pump right through the pressure vessel. In smaller systems (up to about 10 mL / min) a simple restrictor can be used. This can be either a capillary tube cut to length, or a needle valve which can be adjusted to maintain pressure at different flow rates. In larger systems a back pressure regulator will be used, which maintains pressure upstream of the regulator by means of a spring, compressed air, or electronically driven valve. Whichever is used, heating must be supplied, as the adiabatic expansion of the CO2 results in significant cooling. This is problematic if water or other extracted material is present in the sample, as this may freeze in the restrictor or valve and cause blockages.


[edit] Collection
The supercritical solvent is passed into a vessel at lower pressure than the extraction vessel. The density, and hence dissolving power, of supercritical fluids varies sharply with pressure, and hence the solubility in the lower density CO2 is much lower, and the material precipitates for collection. It is possible to fractionate the dissolved material using a series of vessels at reducing pressure. The CO2 can be recycled or depressurized to atmospheric pressure and vented. For analytical SFE, the pressure is usually dropped to atmospheric, and the now gaseous carbon dioxide bubbled through a solvent to trap the precipitated components.


[edit] Heating and Cooling
This is an important aspect. The fluid is cooled before pumping to maintain liquid conditions, then heated after pressurization. As the fluid is expanded into the separator, heat must be provided to prevent excessive cooling. For small scale extractions, such as for analytical purposes, it is usually sufficient to pre-heat the fluid in a length of tubing inside the oven containing the extraction cell. The restrictor can be electrically heated, or even heated with a hairdryer. For larger systems, the energy required during each stage of the process can be calculated using the thermodynamic properties of the supercritical fluid. [5]


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## King Bud (Sep 30, 2008)

Yea.. I _guess _it makes sense.. but it's not for marijuana. The concept is in there, between all the fancy words, and stuff we don't need.. but you know, there's easier ways to make hash. Check out the bubble hash, the shake method, and maybe even the iso method (though this one is a little dangerous).

Did you snag that out of a textbook or something? Seems like it's instructions for some lab equipment.

It also refers to some very serious controls, nothing you'd want to mess around with (without some REAL knowledge), if it is a machine or whatever.

There's safe and easier ways to make hash.. personally I prefer the real deal. Pure bud. The higher the quality the better.


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## thc is good for me (Oct 17, 2008)

If you couldmake hash oil with C02 people would be doing it.  I think the butane way works fine.


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