The Science Behind Castor Oil Refining: A Closer Look

What is castor oil?

Castor oil is a commercially important oil produced from castor seeds which come from the Ricinus communis plant, native to Africa and Asia. In recent years, Brazil, China and India have been the world’s leading castor oil producers. While India is a major exporter of the oil, the US and the EU are major importers. Due to castor oil’s commercial significance, there has been increasing scientific investigation aimed towards optimizing castor oil production and castor oil refining.

Why is a castor oil refining plant a good investment?

Castor oil is believed to have a number of health benefits and has been used in traditional medicine systems. It is used as a laxative, but is also purported to be beneficial for the face and skin, though research supporting such claims is insufficient. Nonetheless, this vegetable oil has applications in a vast number of industries – castor oil can be used to synthesise a variety of renewable or biodegradable polymer materials; it can be used to make soaps, greases, waxes, lubricants and brake fluids; the byproducts of castor oil production, husks and meal, make excellent organic fertilizers as they, especially the meal, contain high levels of nitrogen and phosphorus; dehydrated castor oil has also been shown to be useful in making paints and coatings; and we’ve already discussed its medicinal uses, though those are not as significant as its use in the chemical or biofuel industries.

Speaking of the biofuel industry, it is important to point out that castor oil is an important renewable energy source. Research published in the journal Lipid Insights outlines some of the scientific studies which have investigated castor oil, its properties and applications. One of these studies found that methyl esters of castor oil can be a biodiesel alternative feedstock when it is blended with diesel fuel. Another study established a 15% blend of castor oil-biodiesel as an optimised ration which may improve engine performance; a different investigation also reveal that lower blends increased break thermal efficiency and reduced fuel consumption. Such work has shown that castor oil is a viable diesel alternative in a compression ignition engine. There are still barriers to its wide use as a diesel replacement, but we cannot deny its useful properties.

The science behind it all

Castor oil properties

Castor oil is the only commercial source of hydroxylated fatty acids. So although it forms only about 0.15% of the vegetable oils produced around the world, it is extremely important and demand for it is only growing with the years, especially from the specialty chemical industry.

The castor plant grows well in wet tropical regions, with the warm climate imparting its seeds a high oil content. The oil produced from the plant is a pale yellow, viscous vegetable oil which is non-volatile and non-drying, with a flash point of 145°C and a melting point of -2 to -5°C. Compared to standard lubricants, its viscosity, density, pour point, and thermal conductivity are fairly high.

Chemically, castor oil is composed of mainly ricinoleic acid (90%), which is what makes it particularly useful in the chemical industry. Additionally, castor oil comprises linoleic, oleic, stearic and linoleic fatty acids in smaller amounts. Castor beans also contain some allergenic proteins as well as the toxic ricin, which makes castor oil refining all the more important. Without utilising a castor oil refining plant to take out these and other potential hazardous impurities, it is impossible to safely use castor oil for pharmaceutical purposes or any other application which would expose humans to their ill effects.

Castor oil production

The seeds of the Ricinus communis L. plant contain about 30 to 50% oil, so mechanical pressing (or pre-pressing) with an oil expeller usually needs to be followed up with solvent extraction to draw as much oil from the oil-bearing material as possible.

Castor seeds need to undergo a number of preparatory processes to achieve a high yield from the castor oil production process. Harvested castor seeds are dried to allow the seed hull to split open; once the hull is removed, the seeds are cleaned to remove undesirable foreign particles. The castor seeds are then conditioned with heat to draw as much moisture out of them as possible. Then comes mechanical extraction – the first step in which we begin to expel oil from the seeds. While the expelled oil proceeds to the decantation process, the remaining cake moves to the solvent extraction process, in which hexane is used to procure even more oil from the seeds. All of the extracted oil makes its way to a castor oil refining plant after visible physical impurities are eliminated using a filtration system, usually a filter press.

Castor oil refining

Now comes the most crucial part of producing clean and clarified castor oil which can then be sent to the various industries which rely on it: castor oil refining. In a castor oil refining plant, processes like degumming, bleaching and/or deodorization are employed to remove impurities. These impurities include colloidal matter, phospholipids, excess free fatty acids, and compounds which impart castor oil its distinct colour. Refining not only makes the oil more fit for use by other industries, but it also extends the shelf life of the final oil produced.

During degumming, as the name suggests, gums are removed from the oil. Castor oil has hydratable gums, so water is added at high temperatures and the mixture is given time to sit, before an aqueous layer separates and can be removed. Some castor oil refining plants will include a neutralisation step after degumming, using compounds like sodium hydroxide to remove more impurities. Then, bleaching eliminates colour, phospholipids, and oxidation products. If necessary, odoriferous compounds are also removed from the castor oil by employing deodorizers.

At present, the world relies on only a few countries to supply its demand for castor oil. All of the processes mentioned above collectively result in refined castor oil which can be stored safely for about a year. But the demand for castor oil is growing, as is the need for more advanced processes for castor oil refining and production. Researchers have used artificial neural networks and predictive modelling to enhance oil yield from solvent extraction. Other research efforts have also attempted to understand how genetic modification of castor seeds might improve their oil content. Currently, research shows that a leaching time of two hours at 50°C with a solute-solvent ration of 2g:40ml are the optimum conditions for castor oil production. More such modelling as well as advances in genomics and transcriptomics can target different parts of the process to improve the quality and quantity of castor plant cultivation as well as castor oil processing to ultimately comfortably service the demands of this oil which could transform our approach to renewable energy.