Algae Oil Extraction
On a commercial scale relatively limited volumes of algae oil are currently produced from algal feed stocks. One reason is algae oil extraction suffers from a lack of well-defined and demonstrated industrial-scale methods for extracting and separating of oils and lipids required for enabling biofuel production.
Existing algae oil extraction techniques are another stumbling block on the way to commercialization and are mainly suitable for analytical- and small scale procedures, or for the recovery/removal of high-value products. To produce algal biofuels as competitive bulk commodity, extraction techniques employed must be efficient and effective.
Extraction depends on identifying the particular biological component for extraction, which is dependent on the algal species and growth status. Additionally, different harvest process operations could affect extraction processes, as well as the fuel conversion process.
Expeller pressing (also called oil pressing) is a mechanical method for extracting oil from raw materials. The raw materials are squeezed under high pressure in a single step. When used for the extraction of food oils, typical raw materials are nuts, seeds and algae, which are supplied to the press in a continuous feed. Expeller presses can recover 75% of the oil from algae. As the raw material is pressed, friction causes it to heat up; in the case of harder nuts (which require higher pressures) the material can exceed temperatures of 120 °F.
Mechanical disruption can include
- Cell homogenizers, bead mills (or bead-beating),
- Ultrasounds, and autoclaving.40
Non-mechanical methods include process such as
- application of organic solvents,
- osmotic shock, and
- acid, base, and enzyme reactions.
The use of microwaves to disrupt cells and increase efficiencies of vegetable lipid and oil extraction is a promising development, though applications outside of analytical labs are unclear.
For waste treatment, pretreatment of sewage sludge with “focused pulse” sonication has been shown to improve methane gas production and bio solids reduction in sludge digestion.
Recent work on extraction of lipids from three different types of oleaginous microalgae compared bead beating, sonication, autoclaving, osmotic shock, and microwaves and suggested that microwave disruption prior to solvent extraction is the most efficient method.
It should be said at the outset that using chemicals of this nature can cause cancer. EXTREME care needs to be in place when handling them.
The concept of like dissolves like is the basis behind the earliest and well-known co-solvent extraction procedure. After the extraction reaction is complete, water (which is not miscible with chloroform) is added to the co-solvent mixture until a two-phase system develops in which water and chloroform separate into two immiscible layers. The lipids mainly separate to the chloroform layer and can then be recovered for analysis.
Supercritical Fluid Extraction
Supercritical fluid extraction utilizes the enhanced solvating power of fluids above their critical point. It can be processed using solid and liquid feeds. Supercritical fluid extraction techniques have been used in the commercial extraction of substances from solid substrates, e.g. caffeine from coffee beans, for more than two decades.
Other methods for extraction and fractionation include the production of oils using heterotrophic algae. In this scenario, non-photosynthetic algae are grown using sugars as energy source and using standard industrial fermentation equipment. Some private companies have engineered algae that secrete oil into the fermentation media that can be recovered and later refined into a biofuel; this approach significantly reduces the capital and operating cost for an extraction process.
The potential benefits of this approach are
- The use of standard fermentation systems,
- Higher productivity compared to photosynthetic systems,
- Ease of scale-up,
- Avoidance of expensive extraction scheme(s),
- The ability to maintain the integrity of the fermentation catalyst and
- Use of sugar-based feed stocks.
However, significant downsides to this approach include many of the same feedstock logistics challenges faced by the nascent lignocellulosic industry. Chief amongst is the logistical challenge of securing a sustainable biomass feedstock to supply to feed large-scale heterotrophic “algal-refinery” operations. Algae oil extraction technologies still have a ways to go for mass production or commercialization to take place. However, small scale methods of oil extraction have been proven to be effective.
The Algae Revolution Has Begun