Monday, May 14, 2007

Oil Extraction From Algae

Oil extraction from algae is a hotly debated topic currently because this process is one of the more costly processes which can determine the sustainability of algae-based biodiesel.

In terms of the concept, the idea is quite simple: Extract the algea from its growth medium (using an appropriate separation process), and use the wet algae to extract the oil. (Note: The algae need not be dried before oil extraction)

There are three well-known methods to extract the oil from oilseeds, and these methods should apply equally well for algae too:

1. Expeller/Press

2. Hexane solvent oil extraction
3. Supercritical Fluid extraction


Expression/Expeller press-When algae is dried it retains its oil content, which then can be "pressed" out with an oil press. Many commercial manufacturers of vegetable oil use a combination of mechanical pressing and chemical solvents in extracting oil.

While more efficient processes are emerging, a simple process is to use a press to extract a large percentage (70-75%) of the oils out of algae.

Hexane Solvent Method

Algal oil can be extracted using chemicals. Benzene and ether have been used, but a popular chemical for solvent extraction is hexane, which is relatively inexpensive. The downside to using solvents for oil extraction are the inherent dangers involved in working with the chemicals. Benzene is classified as a carcinogen. Chemical solvents also present the problem of being an explosion hazard.

Hexane solvent extraction can be used in isolation or it can be used along with the oil press/expeller method. After the oil has been extracted using an expeller, the remaining pulp can be mixed with cyclo-hexane to extract the remaining oil content. The oil dissolves in the cyclohexane, and the pulp is filtered out from the solution. The oil and cyclohexane are separated by means of distillation. These two stages (cold press & hexane solvent) together will be able to derived more than 95% of the total oil present in the algae.

Supercritical Fluid Extraction

This can extract almost 100% of the oils all by itself. This method however needs special equipment for containment and pressure

In the supercritical fluid/CO2 extraction, CO2 is liquefied under pressure and heated to the point that it has the properties of both a liquid and gas. This liquefied fluid then acts as the solvent in extracting the oil.

Other Less Well-known Extraction Methods

Enzymatic extraction - Enzymatic extraction uses enzymes to degrade the cell walls with water acting as the solvent, this makes fractionation of the oil much easier. The costs of this extraction process are estimated to be much greater than hexane extraction.

Osmotic shock - Osmotic shock is a sudden reduction in osmotic pressure, this can cause cells in a solution to rupture. Osmotic shock is sometimes used to release cellular components, such as oil.

Ultrasonic-assisted extraction
- Ultrasonic extraction can greatly accelerate extraction processes. Using an ultrasonic reactor, ultrasonic waves are used to create cavitation bubbles in a solvent material, when these bubbles collapse near the cell walls, it creates shock waves and liquid jets that cause those cells walls to break and release their contents into the solvent.
References for Ultrasonic-assisted extraction – see
here, Ultrasound – A New Technique to Harvest Microalgae? (PDF), Sonochemistry – from Food Technology Centre, Ultrasonic Irradiation for Synthesis of Biodiesel Fuels

See also:



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