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Normal Hexane 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; and
(4) recovery of the solvent, for reuse in the extractor. The solvent is almost always Normal 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).

Normal Hexane (n-Hexane) is both an anthropogenic and naturally occurring chemical. n-Hexane is a minor constituent of crude oil and natural gas. Its inclusion in a variety of petroleum products is a consequence of refining operations that separate hydrocarbons within specific ranges of boiling points for such uses as heating oils or automotive fuels. It may also be a metabolic byproduct from certain types of fungi (Ahearn et al. 1996). In commercial products prepared from the distillation of petroleum, n-Hexane has many uses as a special-purpose solvent and oil extractant. In a highly purified form, n-Hexane is used in chemical laboratories as an extractant for a wide range of hydrocarbons and nonpolar organic compounds.

Virtually all n-Hexane is obtained from petroleum mixtures through controlled fractional distillation and other refinery-based processes (Speight 1991). n-Hexane can also be synthesized from sugar cane wastes using special catalysts (SUCRON 1996). This type of synthesis is relatively new and the volume produced is still very limited. The presence of many types of hydrocarbon impurities in many commercial grades of n-Hexane, combined with the intentional denaturing of n-Hexane preparations to discourage substance abuse, make it difficult to establish odor thresholds for many products containing n-Hexane.

n-Hexane is used mainly as an edible-oil extractant for a variety of seed crops such as soybeans, cottonseed, rape seed (canola), flax (linseed), mustard seed, peanuts, safflower seed, and corn germ, which are then processed into foods for humans or livestock (Bhagya and Srinivas 1992; Conkerton et al. 1995; Dominquez et al. 1995; Kim and Yoon 1990; Lawson 1995; Srinivas et al. 1992; Wanasundara and Shahidi 1994). While other petroleum-derived solvents (e.g., pentane) or other organic solvents (e.g., chloroform, methanol, ethanol, or ammonia-alcohol mixtures) are currently being studied or are used for certain processes, n-hexane has been widely used since the early part of this century, especially with soybeans, cottonseed, and linseed (Conkerton et al. 1995). Part of n-hexane’s appeal relates to aesthetic properties such as preserving the colors of the original plant materials. Different extractant mixtures can also have significant effects on the levels of materials that can cause bitter tastes (e.g., tannins) and on  the degree to which certain flatulence-causing sugars are removed. While other solvents could be used in the initial oil extraction phases, several decades of experience in combining the oil-extraction steps with other procedures to preserve desirable colors and eliminate unwanted tastes or other undesirable food properties have worked to maintain a heavy reliance on n-hexane for edible-oil extraction (Lawson 1995). In the 1970s, it was estimated that soybean oil extraction alone accounted for approximately 30% of all uses of n-hexane in the United States (HSDB 1996).

n-Hexane has other major uses as a special-purpose solvent and cleaning agent (degreaser) in such industries as textile manufacture, shoe and leather making, and furniture manufacturing (Jorgensen and Chor 1981). It is used in the printing industry as a cleaner and as a component of some inks (EPA 1996c; Wadden et al. 1995). Facilities that use rotogravure printers (facilities that produce catalogues, magazines, “glossy” newspaper inserts, or telephone directories) or similar rotogravure or flexographic technologies (for labels, gift wrap, metal foils, flexible packaging materials, and some floor coverings) also use n-hexane (EPA 1996c). While not used in most glues or epoxy cements (Rastogi 1993),

n-hexane is the solvent used in “rubber” cement (also known as gum adhesive) widely used in schools and libraries and by artists (McCann 1992). Various glues, adhesives, and leather-dressing preparations, especially those used in assembling shoes, may contain n-hexane (Cardona et al. 1993; Periago et al.

1993; Takeuchi et al. 1993). In bookbinding and leather working, n-hexane, often mixed with other hydrocarbon solvents, is used as a carrier for cedar oil, beeswax, or lanolin dressings (Jorgensen and  Chor 1981; Roberts and Etherington 1996). n-Hexane is used in some typeover correction (“white-out”) fluids (Ong et al. 1993). It has been used in many types of non-mercury thermometers, especially for thermometers used in low temperature ranges (EPA 1994g). It has been used as a denaturing agent in some alcohol preparations (HSDB 1996). New roofing materials using rubber or plastic films and membranes held together by adhesives, sealants, or hardening agents may contain n-hexane (Herbert et  al. 1995). It may be used as a carrier or aerosol (propellant) agent in some perfumes (Bouhamra 1995; Jorgensen and Chor 1981). It is used in the pharmaceutical industry to help shape pills and tablets, which are then dried to vent off the n-hexane before packaging (Jorgensen and Chor 1981).  In the  petrochemical industry, lighter alkane fractions including n-hexane may be used as feedstocks in the manufacture of polyethylene or polypropylene (Jorgensen and Chor 1981). In canning operations, the ends of tin cans are held in place with adhesives that commonly contain n-hexane (Bachmann et al.

1993). The balls used in several sports (e.g., baseball) have cores wrapped with strings or yarns, which are often held in place with adhesives containing n-hexane (Huang et al. 1991). In the manufacture of truck and automobile tires, n-hexane is a solvent in mixtures (called “thinners”) used to adjust the viscosity of the rubber while it is being polymerized and formed into tires (Jorgensen and Chor 1981; Van Ert et al. 1980). n-Hexane is apparently in the adhesives for certain types of tapes, bandages, and dressings used in hospitals (Jorgensen and Chor 1981). Adhesives, cleaners, or lacquers containing

n-hexane are also used to prepare the veneers used in making many types of furniture or ornamental boxes (Graham et al. 1995).

Pure n-hexane is widely used in laboratories as an extractant for nonpolar compounds and in calibrating instruments for analyses of volatile organic compounds (VOC) or total petroleum hydrocarbons (TPH) (Kanatharana et al. 1993). Since such analyses may require very high levels of purity, laboratories sometimes carry out their own fractional distillation or other pretreatment-purification procedures to remove petroleum hydrocarbon impurities found in commercially available grades of n-hexane (Kanatharana et al. 1993).

Finally, n-hexane may be a component of many types of commercial preparations or in mixtures  produced in small batches on-site such as paint thinners, general-purpose solvents, degreasing agents, or cleaners. For instance, until the 1970s, naphtha, a mixture with a high n-hexane content, was widely used as a dry cleaning agent. Since the early 1900s construction workers, metal workers, janitors, furniture workers, motor-vehicle mechanics, and print-shop workers have used these general-purpose mixtures.

Such mixtures have also been used extensively for home repair and hobby projects. These mixtures have wide variations in their compositions but often contain up to 20% n-hexane even when the main components are other petroleum alkane fractions (e.g., kerosene), aromatic hydrocarbons (e.g., toluene), chlorinated hydrocarbon solvents, or other organic liquids (Farmer 1996; Veulemans et al. 1987).

Normal Hexane is a chemical made from crude oil. Pure Normal Hexane is a colorless liquid with a slightly disagreeable odor. It evaporates very easily into the air and dissolves only slightly in water. Normal Hexane is highly flammable, and its vapors can be explosive.

Pure Normal Hexane is used in laboratories. Most of the Normal Hexane used in industry is mixed with similar chemicals in products known as solvents. Common names for some of these solvents are “commercial hexane,” “mixed hexanes,” “petroleum ether,” and “petroleum naphtha.” An older name for these types of solvents is “petroleum benzene.” Several hundred million pounds of Normal Hexane are produced in the United States each year in the form of these solvents. The major use for solvents containing Normal Hexane is to extract vegetable oils from crops such as soybeans. They are also used as cleaning agents in the printing, textile, furniture, and shoemaking industries. Certain kinds of special glues used in the roofing and the shoe and leather industries also contain Normal Hexane. Several consumer products contain Normal Hexane. For example, gasoline contains about 1–3% Normal Hexane. Normal Hexane is also present in rubber cement.

Hexane is a colourless solvent. Food grade Hexane is primarily used in the extraction of Edible Oils. This calls for very high purity levels of Hexane, followed by safe and careful storage, handling and transportation right from production to end use. It also finds usage in a limited manner in preparation of rubber adhesives, can sealing compounds etc.

Specifications of Food Grade Hexane is given below:

Food Grade Hexane(FGH) Food grade hexane is a colorless solvent. They are primarily used in the extraction of edible oils. This calls for very high purity levels of hexane, followed by safe and careful storage. It also finds usage in a limited manner in preparation of rubber adhesives, can sealing compounds etc.

Polymer Grade Hexane(PGH): Polymer Grade Hexane is a fast evaporating hydrocarbon solvent that consists essentially of hexane isomers. A concentration of approximately 40% makes n-hexane the major component inthis mixture Polymer grade hexane is used as polymerization medium and in the manufacture of catalysts.

Isohexane: This is another compound of interest which also can be drawn from the recycle stream. It is a solvent used in industrial, professional, and consumer applications such as manufacturing process solvent, metalworking, and coatings. It is not sold directly to the public for general consumer uses; however, this product may be an ingredient in consumer and commercial product applications such as cleaning agents and coatings.

Special Boiling Point Spirit: Sometimes on the market demand one more product is made from the recycle stream called the Special Boiling Point Spirit 55/115. It is used in the rubber industry particularly during the process of vulcanization in tyre manufacturing or in preparation of certain rubber mixes, cements and adhesives. It is also used as thinner for varnish, paint and printing inks formulation where quick drying is required and as diluent for lacquer, enamels, high grade Leather drops. The table below gives the typical specifications of the products discussed above at a glance.