Soybeans are the most processed seed of about 56% of oil seeds processed by solvent extraction plants.Soybeans are about 7% hull by weight.
Soybeans are the most processed seed of about 56% of oil seeds processed by solvent extraction plants.Soybeans are about 7% hull by weight. The hull is a paper-thin shell surrounding each seed that is high in fiber and low in oil; it is very low density unless ground quite fine, and therefore takes up a lot of space in costly process equipment. Being very low in oil, it is not particularly profitable to extract. It is also very low in protein and, when it is removed early in the process, the extracted meal has a significantly higher protein percentage and market value.
For all these reasons, one of the first steps in the total process for soybeans is often a dehulling of the seed. In summary, removal of perhaps 90% of that 7% of hull reduces the volume going through extraction, reduces the energy for desolventizing, increases the meal protein percentage, and gives a by-product that has at least some value – the hulls.
This dehulling function is usually integrated with other preparation systems to crack, heat and flake the material to make it ready for the solvent extractor. The dehulling process and the other steps in preparation are as given below.
Soybean Preparation: This should provide material with the following characteristics:
* Cracked into 4 to 8 pieces per soybean using sharp serrated rolls. Cracks should be large enough to provide good distortion during flaking, but not so large as to require excessive flaker power or reduced capacity.
* Dehulled: This is done mostly to improve the protein level of the meal. However, it also reduces the tonnage going to extraction and increases the density in the extractor - leading to more retention time.
* Conditioned: Material to the flakers should generally be about 158°F (70°C) and final flakes at the extractor should be about 147-154°F (64-68°C) and from 9% to 10% moisture.
* Flaked to cause breakage of cell structure to release the oil for ease of extraction. For shallow bed extractors, perhaps flakes of 0.012" to 0.015" (0.30 mm - 0.38 mm). For deeper beds, perhaps flakes of 0.013" to 0.017" (0.33 mm - 0.43 mm).
* Avoid unflaked particles! Less than 0.3% is reasonable to avoid high average residual oil and very uneven test results.
* Low fines content so that it will not block drainage of miscella or leave excessive fines in the miscella.
* Good draining material, appropriate to the extractor.
With properly prepared soybean material and sufficient extraction time, it is often possible to achieve about 0.35 to 0.60% residual oil in spent flakes.
Horizontal Continuous Extractors are employed around the globe and works on the principle of gravity percolation. The prepared material from the preparatory section is transported to the continuous extractor unit using conveyor system. Extractor unit comprises of a number of solvent sprayers which sprays the solvent over the entire bed of raw material transporting the material from feed to the discharge end. Extractor bed with efficient filtration system for holding the material besides providing clean miscella moves over the rails inside the extractor unit. Miscella is combination of oil and the solvent (mostly hexane) percolated through the material bed. Miscella is then subsequently taken to heaters for separating hexane from oil in three to four stages. Light and sight viewing glasses are provided to view the distribution of spray inside the extractor unit.
The wet meal from the extractor unit is de-oiled material and absorbed solvent is transported by means of bulk flow conveyor to the Desolventising section to recover the hexane.
Desolventiser-Toaster (DT) consists of a series of heating jackets mounted vertically one over the other. Each jacket has a specially designed double bottom in which high pressure steam is introduced. Provision for open steam is also made such that the entire surface of the material is subjected to its action.
De-oiled material from the continuous extractor unit contains 20 to 35% of solvent. This material enters from the top of desolventiser toaster and passes from stage to stage of steam jacket while being rotated with an agitator shaft fitted with blades. The basic principal involved in Desolventisation is direct and indirect heating of the de-oiled material with steam to a temperature well above boiling point of the solvent and to ensure that no solvent is left over in the de-oiled material. The material flows from one steam jacket to the next through discharge chutes. Solvent vapours are sent to the Condensers via a wet scrubber where the solvent is washed for any fines.
The de-oiled and desolventised meal called as De-Oiled bran is an excellent ingredient for cattle and poultry feed manufacturing is conditioned and cooled to desired moisture level before being transported to the bagging Section with the help of a conveyor.
Miscella from the extractor unit contains nearly 12% to 18% of solvent in case of oil cakes or rice bran and up to 25% to 35% in the case of sunflower seed or rapeseed and hence the two liquids of miscella have to be separated. Separation of two liquids is done by the evaporation of solvent due to its lower boiling point of 64 to 67 degree Celsius and thus leaving clean oil behind.
Distillation process is usually carried out in three stages under vacuum condition (zero oxygen for better efficiency of evaporation at elevated temperatures) to retain the better oil characteristics. First stage of distillation is carried out in Economizer followed by a Flasher to evaporate the solvent and thus leaving practically only oil behind. Thus obtained oil is further treated with open steam to ensure that no solvent stays behind.
The solvent vapours thus produced passes through an Oil Vapour separator to separate out any oil particles trapped with the solvent vapours before passing on to a Condenser.
The oil and solvent mixture is distilled later to recover the solvent. The hot oil from the evaporator is passed through a cooler to cool to room temperature, and finally transported to the storage tank.
Condensation of hexane
Hexane vapours from Desolventisation Section and also from Distillation Section need to be condensed. Condensers usually are of floating head type and with tube bundles to carry the cooling water. Cooled water at room temperature is circulated inside the tube bundles and with vapours outside of the tubes. Thus the solvent vapours are cooled and gets condensed into liquid. The condensate liquid so obtained is passed to solvent water separator where the difference in densities of water and solvent and their immiscibility accomplishes complete separation. The so obtained solvent from this tank is again fed to the extractor continuously for the final washing of the meal bed. The uncondensed gases from the condensor are led to contact cooler where they are washed with cold water spray.
Final solvent recovery
The air that is being ejected out of the system contains traces of solvent. In order to recover these traces, a special final vent air stripping column has been provided. It comprises of a main Absorber to give a large contact surface. It is partly filled with mineral oil to absorb the solvent vapors from the air.
The residual vapours from these condensers are evacuated by ejector to a vent condenser cooled by water in close circuit. The residual air is aspirated through an absorption tower where mineral oil absorbs residual hexane in air before being condensed using vent condenser.