Question

3. One of the important stages in Downstream processing is the purification and polishing stage to meet the product criteria / specifications in accordance with applicable regulations. Explain the process and tool for purification and polishing of biodiesel using microalgae, give reasons for choosing the equipment used?

Answer 1

What are microalgae?

Microscopic algae belongs to the kingdom Protista which is restricted to prokaryotes simple unicellular organisms mainly bacteria and cyanobacteria otherwise known as blue-green algae. They are phototrophic so they utilize the sun for energy by performing photosynthesis and are highly adapted to marine or freshwater environments. They have neither tissue specialization nor internal cellular organelles but contain a nucleus for genetic material which may or may not be membrane bound. They also have ribosomes for protein synthesis but those are found existing freely throughout the cytoplasm rather than attached to the endoplasmic reticulum.

Microalgae can exist as individuals or they can colonize into large groups or chains that can become visible to the naked eye.

Microalgae have higher lipid yields than terrestrial plants. They have high growth rates undergoing cell multiplication several times a day and high light to biomass conversion rates on average microalgae produce around 30 times more oil per unit of land and can grow under a variety of conditions.

What is biodiesel?

Biodiesel combines alcohol (usually methanol) with oils (palm oil, coconut oil or canola oil) from plants. It includes vegetable oil, animal oil/fats, tallow and waste cooking oil. The process used to convert these oils to biodiesel is called transesterification.

Steps involved in biodiesel using microalgae

1. Cultivation:

Microalgae requires sunlight, carbon dioxide, water, and inorganic salts to grow. The nutrients required by microalgae are often the same nutrients that are present in waste affluent like sewage and contain significant amounts of phosphorus and nitrogen which are required for micro algal growth.

Photoautotrophic-based microalgal culture systems

Raceway Ponds:  a long shallow pool of water (usually about 1 foot deep) where the water is kept moving with paddle wheels providing circulation and mixing of the algal cells and nutrients, the algae absorbs nutrients from the water, some CO₂ from the atmosphere and some sunlight energy and converts it into fats.

Raceway ponds are great but they have some disadvantages.

The main disadvantage is they often experience a lot of water loss due to evaporation as they are open-air systems and the optimal conditios are difficult to maintain.

Bioreactors: algae are to be grown in specially designed vessels loaded with particular type of nutritive media enable sunlight to be contacted and usually they bubble through either air or enhanced with some extra CO₂.

2. Harvesting:

Harvesting means removing the algae from the ponds and dewatering it. This can be done by a variety of methods all of which depend on several factors like species selection, density size, culture method and other physicochemical conditions.

Some common methods are:

Centrifugation: which uses centrifugal forces to separate biomass from the water by density difference. It is fast and reliable but also limited to small scale operations.

Gravity sedimentation: the algal particles eventually become large enough that they succumb to the forces of gravity and settle to the bottom of the pond this could be applicable to large-scale harvesting but it is limited to strains with high sedimentation rates.

Flocculation: uses chemicals to induce aggregation. The individual cells are forced out of suspension and glom on to each other creating clumps. Flocculation is often used as an additional step to improve other methods like centrifugation sedimentation filtration etc.

3. Extraction:

Extraction involves separating the lipids from the cells.

In involves various techniques depending on the same set of factors species, selection size and so forth.

Mechanical extraction: uses physical means like grinding or crushing also known as bead beating in which the algal slurry is spun at very high speeds with fine beads to impose direct damage on the cell.

Ultrasound uses wave action to disturb the integrity of the cell wall.

Microwave heat the cell and induce intracellular water vapor to disrupt the cell from within.

Electroporation uses electricity to alter the conductivity and permeability of the cell wall.

These mechanical methods generally include high capital, maintenance costs, require skilled personnel and inefficient lipid yields can be inconsistent.

Chemical extraction uses solvents to dissolve the cell wall usually hexane, benzene or chloroform compounds. There are different techniques the flock method, the blind ire method among others basically the algal slurry is washed repeatedly in a solvent mixture until it undergoes phase separation whereupon the lipid layer is then easily extracted.

This method is very effective however at the commercial level using solvents will post significant environmental and health risks.

Biological this primarily consists of enzyme assisted extraction (enzymes like cellulose and trypsin) degrade the polymers on the cell surface which then facilitates other means of extractions mainly mechanical. This method is expensive but at least it is safe and effective.

4. Conversion:

Transesterification is the process where a glyceride reacts with an alcohol (typically methanol or ethanol) in the presence of a catalyst forming fatty acid alkyl esters and an alcohol resulting in biodiesel and glycerine.