Solvent Extraction of Oil: A Modern Approach

When it comes to extracting oil from seeds at an industrial scale, solvent extraction stands out as the most efficient method available today. More than 98% of global oil production is carried out using this technique, and for good reason – it achieves oil recovery rates that mechanical pressing simply cannot match. Whether the raw material is whole oilseeds, partially pressed flakes, or leftover oil cakes, solvent extraction ensures almost no oil goes to waste.

What is solvent extraction?
Why n-hexane is the preferred solvent
The solvent extraction process: step by step
Step 1: Seed preparation
Step 2: Solvent extraction
Step 3: Distillation to separate oil from solvent
Step 4: Desolventizing the meal
Solvent recovery and the closed-loop system
Comparing solvent extraction with mechanical pressing
Safety and environmental considerations
Where solvent extraction is most effectively used

What is solvent extraction?

Solvent extraction is a chemical process in which oil is dissolved out of oilseeds or oil cakes using an organic solvent – most commonly n-hexane. The principle is straightforward: oil and hexane are both non-polar substances, so they dissolve readily in each other. When n-hexane contacts oil-bearing material, it draws the oil out of the seed cells, forming a liquid mixture of oil and solvent known as miscella. The solvent is then removed from this mixture through distillation, leaving behind pure crude oil.

Among conventional extraction methods – including hydraulic pressing, expeller pressing, and solvent extraction – solvent extraction has been the most widely adopted for both economic and practical reasons. It works equally well on whole seeds, pre-pressed flakes, and oil cakes (the solid residue left after mechanical pressing), making it extremely versatile across different processing scenarios.

Why n-hexane is the preferred solvent

Not every solvent is suitable for large-scale oil extraction. The ideal solvent needs to dissolve oil efficiently, separate cleanly, and be economical to use and recover. N-hexane has maintained its dominant position among extraction solvents because it balances all of these requirements better than any alternative tried so far.

Specifically, n-hexane offers several technical advantages that make it the industry standard:

High oil recovery: N-hexane is highly effective at dissolving triglycerides, allowing processors to extract more than 99% of available oil in the seed – far beyond what mechanical pressing achieves.
Low boiling point: With a boiling point of approximately 69°C, n-hexane is volatile enough to be easily evaporated from the oil and meal, enabling near-complete solvent recovery and reuse.
Low energy demand: Its low latent heat of vaporization means solvent can be removed from both the oil and the meal with relatively little energy input.
Oil selectivity: Hexane is preferred due to its ability to effectively dissolve non-polar substances like oils and fats, while leaving behind unwanted polar compounds like water and certain pigments.

Hexane is the primary solvent used to extract edible and industrial vegetable oils from the world’s top five commodity oilseeds: soybean, canola, sunflower seed, cottonseed, and peanut. In commercial settings, processors typically use not pure n-hexane but a mixture of hexane isomers – sometimes called extraction hexane or commercial hexane – which performs similarly and may even have slightly greater extractive power.

The solvent extraction process: step by step

Industrial solvent extraction is a continuous, tightly controlled operation. It involves four distinct stages, each critical to achieving maximum oil yield and product quality.

Step 1: Seed preparation

After oilseeds are cleaned, cracked, cooked, and dried, the flaked seed material is bathed in hexane at a solvent plant. Proper pretreatment helps rupture the oil-bearing cells and increase the surface area of the seed, allowing the solvent to penetrate more effectively. For seeds with high oil content (above about 23%), this step is preceded by pre-pressing in a mechanical expeller, which removes a large portion of the oil first. The pressing operation removes the higher percentages of oil which are by far the easiest to squeeze out of the solids, and the solvent extraction process is best at removing oil from about 20% down to near ½%. The resulting press cake then enters the solvent extractor for final oil recovery.

Step 2: Solvent extraction

The prepared material – whether flakes or press cake – is loaded into an extractor. In immersion-type extractors, flaked oilseeds are completely submerged in solvent, while percolation-type extractors allow solvent to trickle down through a bed of material – the latter being far more common at commercial scale. As hexane flows through or around the oil-bearing material, it dissolves the oil.

During solvent extraction, the miscella diffuses through the cell walls to the oil bodies inside the cells. As the miscella continues to penetrate and dissolve the oil, the pressure in the cell increases and the concentrated miscella diffuses back out of the cell. Six parameters affect how well this step performs: contact time, particle thickness, number of extraction stages, miscella flow rate, extractor temperature, and solvent retention in the meal.

Step 3: Distillation to separate oil from solvent

Once extraction is complete, the miscella – typically containing 25-35% oil – is piped to a distillation unit. The bulk of the solvent from the miscella is recovered at the distillation stage. The final traces of solvent from the oil exiting the distillation zone are recovered in an oil stripper working under vacuum. The result is crude oil with virtually no solvent residue. This crude oil then undergoes further refining to remove color, odor, and other impurities before it is fit for consumption or industrial use.

Step 4: Desolventizing the meal

The solid residue from the extractor – called wet meal – still contains a significant amount of hexane absorbed into its structure. Wet meal discharged from the oil extractor usually contains 25-35% solvent. This meal is transferred to a Desolventizer-Toaster-Dryer-Cooler (DTDC), where steam is applied to drive off the remaining hexane. Extracted meal from the extractor is transferred to the Desolventizer Toaster to recover the entrapped solvent and to condition the desolventized material by controlling and adjusting the temperature and moisture. The finished meal is protein-rich and is widely used as a high-quality animal feed ingredient.

Solvent recovery and the closed-loop system

One of the key economic and environmental features of modern solvent extraction is the almost complete recovery and recycling of hexane. Vapours from the desolventizer toaster and distillation separators are carried to condensers for vapour/solvent condensation and solvent recovery. Condensate is passed through a water separator and re-circulated to the extraction section. This closed-loop design means that the same solvent is used over and over, dramatically reducing operating costs and minimizing environmental releases.

Modern extraction facilities use closed-loop systems to capture and recycle n-hexane, ensuring safety and environmental compliance. Regulatory bodies like the U.S. Environmental Protection Agency (EPA) restrict plant-wide hexane emissions from each affected facility rather than controlling individual emission points, pushing industry toward tighter containment systems.

Comparing solvent extraction with mechanical pressing

Mechanical pressing has its place – particularly for high-value, cold-pressed specialty oils and in small-scale operations – but it cannot match the efficiency of solvent extraction at industrial scale. Solvent extraction is generally considered to be a more efficient method than mechanical pressing for larger capacity processes above 500 tons per 24-hour day.

The residual oil left in the solid after processing also tells the story clearly. Solvent extraction can typically reduce the oil content in the solids to less than 2.0% and on some materials down to 1.0% or less, whereas the residual oil in press cake from mechanical pressing typically ranges from 4-8%. For oilseeds with lower oil content – such as soybeans – solvent extraction with hexane is the standard practice in modern processing plants.

Safety and environmental considerations

N-hexane is highly flammable and its vapors can be explosive, which means industrial extraction facilities require strict safety engineering. Modern plants use explosion-proof equipment, continuous gas monitoring, and fully enclosed systems to contain the solvent at all times. Beyond fire risk, health concerns exist: n-hexane has been shown to be neurotoxic to humans and has been listed as a cause of occupational diseases in several European countries since the 1970s. Trace residues can also remain in finished oil and meal, necessitating careful process control and post-extraction refining.

These concerns have prompted ongoing research into alternative solvents. Green solvents and technologies – including aqueous enzymatic extraction and terpene-based solvents – are considered viable alternatives to n-hexane because of their non-toxic, eco-friendly profiles. However, as the American Oil Chemists’ Society (AOCS) notes, the existing hexane-based technology has sufficient virtues to maintain credibility, and any proposed replacement must exceed hexane’s core advantages while requiring significant development investment to perform comparably at scale.

Where solvent extraction is most effectively used

Solvent extraction is particularly valuable in two situations: processing oilseeds with relatively low oil content (such as soybeans at 18-20% oil), and recovering residual oil from press cakes after mechanical pressing. In both cases, mechanical methods alone would leave too much oil behind to be commercially viable. Both mechanical and hexane extraction methods have their place in the oilseed market, serving distinct industry needs and demands ranging from organic food to biofuel to hundreds of different edible and industrial applications.

Beyond commodity oilseeds, the technique is also applied in rice bran oil production, where the bran’s tendency to go rancid quickly makes mechanical pressing under heat undesirable. It is equally useful for extracting oil from materials with very low oil content – such as oil cake residues from earlier pressing – where mechanical methods would simply not be economically feasible.

What do you think? Given that over 99% oil recovery is achievable with solvent extraction, should this method be the default choice for all industrial oilseed processing – or are there situations where mechanical pressing still makes more sense? And as regulatory scrutiny of n-hexane increases globally, how quickly do you think the industry can realistically transition to greener solvent alternatives without compromising efficiency or cost?

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References

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Table of Contents

What is solvent extraction? 🔗

Why n-hexane is the preferred solvent 🔗

The solvent extraction process: step by step 🔗

Step 1: Seed preparation 🔗

Step 2: Solvent extraction 🔗

Step 3: Distillation to separate oil from solvent 🔗

Step 4: Desolventizing the meal 🔗

Solvent recovery and the closed-loop system 🔗

Comparing solvent extraction with mechanical pressing 🔗

Safety and environmental considerations 🔗

Where solvent extraction is most effectively used 🔗