James G. Speight - Encyclopedia of Renewable Energy

In the first step of physical extraction, the oil must be separated from the rest of the algae. The simplest method is mechanical crushing. When algae are dried it retains its oil content, which can then be recovered using an oil press. Many commercial manufacturers of vegetable oil use a combination of mechanical pressing and chemical solvents in extracting oil. Since different strains of algae vary widely in their physical attributes, various press configurations (such as the screw, expeller, and piston configurations) work better for specific algae types. Often, mechanical crushing is used in conjunction with chemical solvents.

Ultrasonic extraction is a type of physical extraction that 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, the resulting shock waves and liquid jets cause those cells walls to break and release their contents into a solvent. Ultrasonication can enhance basic enzymatic extraction. The combination sono-enzymatic treatment accelerates extraction and increases yields.

In chemical extraction, chemical solvents are often used in the extraction of the oils. A common choice of chemical solvent is hexane, although other hydrocarbon solvents can also be used. Another method of chemical solvent extraction is Soxhlet extraction in which oils from the algae are extracted through repeated washing, or percolation, with an organic solvent, under reflux in specialized equipment. The value of this technique is that the solvent is reused for each cycle.

Supercritical carbon dioxide can also be used as a solvent. In this method, carbon dioxide is liquefied under pressure and heated to the point that it becomes supercritical (having properties of both a liquid and a gas), allowing it to act as a solvent.

Enzymatic extraction uses enzymes to degrade the cell walls with water acting as the solvent. This makes fractionation of the oil much easier. The enzymatic extraction can be supported by ultrasonication.

See Algae, Algae Fuels, Aquatic Plants, Biomass.

Algal blooms refer to the rampant growth of certain microalgae, which in turn leads to the production of toxins, disruption of the natural aquatic ecosystems, and increases the costs of water treatments. The blooms take on the colors of the algae contained within them. Graham states that the main toxin producers in oceans are certain dinoflagellates and diatoms. In freshwaters, cyanobacteria are the main toxin producers, though some eukaryotic algae also cause problems. Under natural conditions, Graham notes that algae use the toxins to protect themselves from being eaten by small animals and only need a small amount to protect themselves.

The main cause of algal blooms is nutrient pollution in which there is an excess of nitrogen and phosphorus, which can push algae toward unrestrained growth. The phenomenon is caused by a variety of human activities in which the fertilizers used in agriculture and animal manures are rich in nitrogen, while improperly treated wastewater is high in both nitrogen and phosphorus.

Alkali washing (often referred to as caustic scrubbing) for hydrogen sulfide removal with caustic scrubbing is only economical in small amounts if hydrogen sulfide is present and suitable means of disposing the spent solution are available. The chemistry is simples and to some extent, depending upon the concentration of hydrogen sulfide in the gas stream, efficient. Thus:

Carbonate washing is a mild alkali process for emission control by the removal of acid gases (such as carbon dioxide and hydrogen sulfide) from gas streams and uses the principle that the rate of absorption of carbon dioxide by potassium carbonate increases with temperature. It has been demonstrated that the process works best near the temperature of reversibility of the reactions:

In the Benfield process, acid gases are scrubbed from the feed in an absorber column using potassium carbonate solution with Benfield additives to improve performance and avoid corrosion.

Water washing, in terms of the outcome, is analogous to washing with potassium carbonate (Kohl and Riesenfeld, 1985), and it is also possible to carry out the desorption step by pressure reduction. The absorption is purely physical, and there is also a relatively high absorption of hydrocarbon derivatives, which are liberated at the same time as the acid gases.

See also: Gas Cleaning, Gas Processing, Gas Treating, Hot Potassium Carbonate Process, Scrubbing.

Alicyclic hydrocarbons contain cyclic structures in all or part of the chemical skeleton. The saturated alicyclic hydrocarbons have the general formula C nH 2n. When the molecular formula of a saturated hydrocarbon corresponds to the general formula C nH 2n-2, then the compound contains two rings; if the formula corresponds to C nH 2n-4, it contains three rings, etc. Their boiling points and densities are higher than alkanes having the same number of carbon atoms. In crude oil, the most frequently found rings are those having five or six carbon atoms. In these rings, each hydrogen atom can substituted by a paraffinic alkyl chain that is either a straight chain or branched. Monocyclic naphthenes with one ring are major constituents of the light fraction. Monocyclic naphthene of carbon numbers C 20, to C 30, with long side chains can be isolated from paraffin waxes.

Cycloalkanes (naphthenes) catalytically crack by both ring and chain rapture and yield olefins and paraffin. In the case of hydrocracking process, polycyclic hydrocarbons aromatic are partially hydrogenated. Naphthene rings in polycyclic compounds are readily removed by ring–opening followed by cracking. Single–ring naphthenes and paraffin are more resistant to cracking. It was also found that single-ring naphthenes appear to react more slowly at high conversion levels. In the catalytic reforming process, the fundamental reaction occurring is (i) dehydrogenation of six members rings naphthenes by platinum catalyst, and (ii) isomerization of alkyl cyclopentane derivatives to cyclohexane derivatives.

The stability of the cycloalkane derivatives increases up to the six member ring, then decreases from seven to eleven member ring, and from the twelve member onwards attains the stability of two six member rings ( Table A-12).

Table A-12Properties of selected alicyclic hydrocarbon derivatives.