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: Harvest the algae from its growth medium (using an appropriate separation process), and extract the oil out of it. Extraction can be broadly categorized into two methods:

1. Mechanical methods

The mechanical methods are further classified into:

• Expression/Expeller press
• Ultrasonic-assisted extraction

1. Chemical methods

The chemical methods are further classified into:

• Hexane Solvent Method
• Soxhlet extraction
• Supercritical fluid Extraction 

Each of these methods has drawbacks: 

1. The mechanical press generally requires drying the algae, which is energy intensive
2. The use of chemical solvents present safety and health issues
3. Supercritical extraction requires high pressure equipment that is both expensive and energy intensive. 

Many manufacturers of algae oil use a combination of mechanical pressing and chemical solvents in extracting oil.

Apart from these, there are some other methods which are not well-known. This includes the following:

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.

CHALLENGES IN OIL EXTRACTION FROM ALGAE:

• Microscopic algae suspended in water are virtually indestructible
  • Cell wall has a high elasticity modulus
  • Even when free water has been removed, wet biomass retains sufficient interstitial water to act as lubricant
• Rupture of cell wall through mechanical friction and steam explosion is only possible when dry

BREAKTHROUGHS IN OIL EXTRACTION FROM ALGAE:

1. a.Single-Step Extraction:

The OriginOil’s algae Single-Step oil extraction process is simpler and more efficient than current systems, without requiring chemicals or significant capital expenditure for heavy machinery.

The Single Step Process harvests, concentrates and extracts oil from algae, and separates oil, water and biomass in one step. The process does not use chemicals or heavy machinery and no initial dewatering is required, and separates the oil, water and biomass in less than an hour. The company’s Quantum Fracturing technology combines with electromagnetic pulses and pH modification to break down cell walls and release oil from the algae cells.

1. b. Continuous algal oil extraction system:

Cavitation Technologies Inc. (CTI) has developed a technology that is able to extract oil from algae on a continuous basis utilizing cavitation based extraction. CTI’s Nano reactor is 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. The company plans to license the technology to algal fuel developers.

1. c. Extraction using nanotechnology:

Catilin and Iowa State University - Center for Catalysis (ISU-CCAT), members of the National Alliance for Advanced Biofuels and Bioproducts (NAABB), will build on their pioneering algal oil extraction technology using mesoporous nanoparticles to selectively extract and sequester targeted fuel-relevant and high value compounds within the algal lipid mixture. The balance of the algal oil, which contains free fatty acids (FFA) and triglycerides, will be converted to Biodiesel using Catilin's commercially available T300 catalyst. This technology is efficient and solid catalyst provides a cost effective conversion route.