Oil Production by Hexane Solvent Extraction
Solvent extraction consists of a sequence of four operations:
(1) physical removal of oil from the seed in the extractor;
(2) desolventizing-toasting of the de-oiled seeds, often combined with drying and cooling of the meal;
(3) distillation to remove the solvent from the extracted oil;
(4) recovery of the solvent, for reuse in the extractor. The solvent is almost always hexane, which satisfies the technical, economical, and operational needs of all oil millers. Several other solvents have been studied but their disadvantages are such that they cannot compete with hexane, which has many compensatory advantages despite being flammable (Dijkstra and Segers 2007).
The industry generally makes a distinction between two types of extractor: percolation type and immersion type. The percolation process, also known as the continuous extraction process, is based upon the principle of uninterrupted passage of the solvent through the bed of oleaginous material; the oil is thus dissolved in the solvent and carried away. In the immersion process, the entire load of seeds is immersed in solvent. The system is static, so it needs to be stirred to balance the differences in the oil–solvent concentration. Stirring inevitably causes abrasion of the extraction material, so the mixture needs subsequently to be filtered out. This method is used when it is not easy to extract the oil from the matrix. Oil extractors can also be classified on the basis of other different criteria, such as basket or belt operation, rotary or straight, or other shapes, full or partial countercurrent operation, etc.; however, it must be underlined that today the systems available in the market are becoming more and more similar to each other (Fils 2000). The oil-saturated solvent obtained from the extraction process is referred as “miscella.” All commercial extractors are today based on the principle of countercurrent extraction. Fresh solvent encounters previously extracted material, whereas new seeds, flakes, or collet encounter solvent already containing some oil. This method is able to remove a high level of oil using a little solvent quantity (Anderson 2011). Temperature is one of the key variables to keep under control and to optimize the extraction process. The boiling point of hexane is about 69°C near ambient pressure. However, it becomes an azeotrope in the presence of water or steam, with a boiling temperature of 61.6°C. It would be desirable to operate close to the temperature point of this azeotrope; it is the hottest temperature reachable before hexane evaporation, thus it would allow to obtain the lowest viscosity of both solvent and oil and consequently to promote a rapid oil solubilization (Anderson 2011). The length of the extraction process is determined by several factors that affect the contact time between the solvent and the oleaginous material, required for a best extraction yield. Among these factors, the oil concentration, the viscosity of solvent and oil, the shape and size of solid particles and their resulting specific internal structure after pretreatment, are essential to calculate the residence time of the solvent in the extractor. Simulations reported that the greatest amount of oil is extracted during the first minutes, being the oil less accessible to the solvent in the last phase due to equilibrium phenomena (Anderson 2011).
After oil extraction, the meal contains 25%–35% of solvent, which must be evaporated and recovered for reuse (Nagaraj 2009). On the other hand, the de-oiled meal is toasted to reduce anti-nutritional factors such as glucosinolates or trypsin inhibitors, which act as antigrowth factors in monogastric animals if the meal is incorporated into animal feed. Moreover, the meal should be dried to minimize the risk of biological contamination and cooled close to room temperature to remain flowable during storage and transport. The process known as desolventizing, toasting, drying and cooling process (DTDC), invented by Schumacher (1985), combine all these operations in a single piece of equipment (Kemper 2011). The most widely used equipment today is the vertical stack consisting of a number of chambers separated by trays. The meal enters at the top and is conveyed downward while being mixed by agitating sweeps anchored to a central rotating shaft. The heat needed for increasing meal temperature and evaporating the solvent is supplied by steam, which is directly and indirectly introduced into the meal via the trays. When indirectly heated using a steam jacket, hexane will evaporate and the temperature will not rise above the boiling point of hexane. Moreover, in this way, live steam will not condense on the flakes, thus allowing a control of the moisture level during the next steps. The reduced moisture, however, provides less protection against overheating, which may lead to a significant decline of the nutritional value during toasting. Subsequently, the material is heated with live steam, which will condense and raise the temperature above the boiling point of hexane that will be completely vaporized. Additionally, the condensed steam humidifies the meal to a point where a good toasting is possible. In the next chamber, the desolventized meal is cooled and dried by air. Heated air is passed over the material to dry it, at the same time, outside air is blown through the material to cool it. Furthermore, the hot air, while drying, also cools the material and the cold air, while cooling, also dries the material (Kemper 2011).
The miscella leaves the extractor with a 25%–30% oil content, which is separated from the solvent by evaporation of the latter. The miscella evaporator, also referred to as economizer, utilizes the latent heat contained in the vapors leaving the desolventizer to evaporate the solvent till an oil concentration of 65%–75%. The concentrated miscella may then undergo to a second step of solvent evaporation, which utilizes the sensible heat of the condensate steam coming from the DTDC. The residual hexane is then removed by vacuum stripping. The evaporated solvent must be cooled in a condenser and cleaned into a mineral absorption system before being reused in the extractor (Dijkstra and Segers 2007).