Edible oil is a common ingredient found in households across the globe. But have you ever looked at the humble bottle of oil in your kitchen and wondered how the oil is extracted out of different resources? In a commercial setup, one of the most common ways to procure vegetable oil and fats is solvent extraction.

What is solvent extraction?
Solvent extraction is a chemical method of procuring oil and involves a preferential dissolution of oil where oilseeds come in contact with a liquid solvent.

Understanding Solvent Extraction
According to statistics, the production of vegetable oil has consistently increased around the world, rising from around 90.5 million metric tonnes in 2000/2001 to 207.5 million metric tonnes in 2019/2020 and this trend is only expected to continue in future. Solvent extraction is used to manufacture about 98% of that number.

This technique is still used in rural areas and in large facilities for pre-pressing before solvent extraction to separate the oily phase from the solid phase of the seed material.

There are many naturally-occurring sources of oil, including different vegetables, nuts, and seeds. The edible oil industry mostly uses three main processes to separate the oil or fats from the source and make it available for consumption. The methods are mechanical extraction (pressing), solvent extraction and a combination of both.

Here is How Solvent Extraction Works - A Step-by-Step Guide
Steps in the solvent extraction process depend upon the type and quality of the oil-bearing material in question. A somewhat standard process of solvent extraction, however, has some varitions of the following steps:

PREPARATION: This operation is the most important one because it underpins the entire extraction process economically. Depending on the various types of oil-bearing materials, different preparation techniques are used.

The oil seed is cleaned, cracked, cooked and flaked to the desired size for better extractability. Flake drying is also carried out for seed with less than 20% oil after
flaking to maintain the optimum temperature & moisture required for extraction.

EXTRACTION: The counter-current extraction technique is used for a set amount of time to extract the majority of the oil from the prepared raw material. Spraying the solvent over the material bed causes the oil to entrap with the solvent. Hexane, a petroleum distillate, is commonly used as a solvent extractor for oil and animal fats. Both the oil concentration in the cake and the solvent's (miscella's) concentration fluctuate in a counter-current manner.

DESOLVENTIZATION: 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 conditioned meal is then bagged separately.

DISTILLATION: Most of the solvent from the miscella (oil–solvent mixture) is recovered at the distillation stage. The remaining traces of solvent from the oil exiting the distillation zone are recovered in the oil stripper working under vacuum.

SOLVENT RECOVERY: Vapours are transported to condensers for vapour/solvent condensation and solvent recovery from the DT and distillation separators. Condensate is cycled back to the extraction section after passing through the water separator. To prevent any solvent loss from evaporation, all extraction procedures during this process are performed in closed apparatus under vacuum. Solvent and air are both passed via the recovery system, where solvent traces from vent gases and vapours are recovered through the solvent's absorption in an absorbing liquid.

What Makes Hexane Perfect for Solvent Extraction of Oil?
Hexane has been used in the commercial oil manufacturing industry for a long time. The objective of any successful oil procurement process is to obtain the fat or oil uninjured, free from impurities, and produce byproducts of the highest value possible. Hexane, fits all these criterias. Some of the main reasons why Hexane is the most trusted substance for solvent extraction of oil include the following:

  • It has a low boiling point of about 68º C (154.5ºF), so it evaporates quickly
  • It has low levels of toxicity
  • It can be removed by the process of distillation, collected separately, and used again and again for extraction without compromising the quality of the final product.

Factors That Determine the Success of Solvent Extraction of Oil
There are various factors that are to be considered for the solvent extraction of oil and animal fats. The processes may vary according to the scope of the manufacturing facility, the availability of resources and the quality of the final output.

However, there are a few common factors that can make or break the solvent extraction process of edible oils and fats. These include the following:

Seed Water Content: Oil repels water and vice-versa. Hence the amount of water in the oil seed is of extreme importance. It has to be just right! A seed with a high water content would hinder diffusion, making it difficult for the solvent to penetrate the seed. However, low water content would disturb the seed flake elasticity and make the oil cake crumble, also hindering the penetration of the solvent.

Particle Size and Shape: The shape and size of the particles in the extraction material must encourage the free flow of the solvent with minimum resistance. Additionally, the seed should not be in the form of a meal, as it makes percolation impossible.

Quantity of Extraction Solvent:  The quantity of solvent is proportional to the oil
contain in the flakes/ collets/ cake.

Extraction Temperature: High temperatures improve extract solubility in the solvent and decrease solvent viscosity. Improved extraction results from decreased solvent viscosity and improved solvent performance at high temperatures. Even if there might not be much of a difference, hot extraction agents are nonetheless important. The cost of heating the solvent is outweighed by the increase in oil output.

Extraction Time: The retention time in extractor are depend upon the feed material like flakes/ collets/ cake and majorly it depend upon two factor. First, percolation of solvent
through cake bed. majorly for soya flakes required approximately 40-45 min were as
soya cake/collets required 30-35 min to get desired oil extraction in extractor.

Advantages of the Solvent Extraction Method
Despite several dynamic techniques available to extract oil from naturally-occuring oil seeds, about 98% of the total mass-produced oil is extracted using the solvent extraction method. While there are advantages and disadvantages to all methods of oil extraction, solvent extraction included, it has continued to be the preferred technique for several decades. Some of the advantages that give solvent extraction an edge over other methods of oil extraction include the following:

Reduced Production Cost: Solvent extraction is more mechanically advanced than pressing, and the entire line can be operated automatically. Therefore, fewer operators are required in an edible oil solvent extraction facility than in an edible oil pressing plant.

Lower Energy Consumption: Solvent extraction is a chemical method that does not require hydraulic machinery or an elaborate set-up. This means the whole process can be completed with lower energy consumption.

Standardized Operations and Results: Due to the relative simplicity of the process, solvent extraction plants can be easily automated. This lead to a standardized operation practice that facilitates repeatable outcomes maintaining consistent quality of the finished product.

Higher Yield: It is impossible to totally remove the cells of oil seeds during pressing. When pressing, the temperature and pressure are constrained, so 5-12% of the oil will remain in the oil cakes. The solvent extraction method leaves only about 1% oil in the residual oil cake, thereby reducing wastage, reducing expense, and facilitating high yield.

Superior Quality of Residual Oil Cake: Since the solvent extraction process uses lower temperatures, it protects the protein in meal cakes from degenerating. This makes them a superior choice to be used as animal feed. Manufacturing facilities using the solvent extraction method of oil often use the residual meal cake as a source of parallel revenue.

While solvent extraction offers some evident advantages over other methods of oil extraction, it does have some drawbacks as well. For example, the hexane vapours are flammable in nature and have contributed to many fire mishaps in oil extraction units. Additionally, over dependance on hexane can also give rise to logistical and sourcing issues. However, despite all the possible disruptions, all experts and manufacturers, both large-scale and small-scale, agrees on solvent extraction being the most preferred technique for oil extraction.


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