Solvent extraction prepress

Mechanical Pressing. Historically, the first large commercial production of oils from seeds and nuts was carried out using labor-intensive hydraulic presses. These were gradually replaced by more efficient mechanical and screw presses. Solvent extraction was developed for extraction of seeds having low oil content. For seeds and nuts having higher oil content, a combination of a screw press followed by solvent extraction is a common commercial practice (prepress—solvent extraction). 

Bell, J.M., Tyler, R.T. and Rakow, G. (1998) Nutritional composition and digestibility by 80-kg to 100-kg pigs of prepress solvent-extracted meals from low glucosi-nolate Brassica juncea, B. napus and B. rapa seed and of solvent-extracted soybean meal. Canadian Journal of Animal Science 78, 199-203. 

Introduction of the expander has enabled extraction plants to handle additional seed species, with purchase of only minimal cleaning and dehulling equipment where needed. Prepress-solvent extraction facilities are being replaced by expander-direct solvent extraction equipment, leaving two basic extraction processes in modern large volume oilseed extraction plants expander-direct solvent extraction, and hard press for applications... 

Cottonseed 19 Hard pressing or prepressing or direct solvent extraction... 

Peanut (shelled) 47 Hard pressing or prepress solvent extraction... 

Solvent extraction. The press cake emerging from a screw press still retains 3 to 15 percent of residual oil. More complete extraction is done by solvent extraction of the residues obtained from mechanical pressing. The greater efficiency obtained in the solvent extraction process encouraged the industry for direct application to oilseeds. In the United States and Europe, continuous extractor units are used in which fresh seed flakes are added continuously and are subjected to a counterflow of solvent by which intimate contact is achieved between the seeds and solvent. The common solvent for edible oil is commercial hexane or heptane, commonly known as petroleum ethers, boiling in the range of 146 to 156°F (63.3 to 68.9°C). After extraction, maximum solvent recovery is necessary for economical operation. The solvent is recovered by distillation and is reused. The extraction oil is mixed with prepress oil for refining. The extracted meals contain less than 1 percent of residual oil. 

Prepress Solvent Extraction. Solvent extraction is relatively costly and is not well suited for the high-oil content of cottonseed. Mechanical pressing leaves about 5% oil in the press-cake, and it is desirable to recover as much oil as possible. A logical processing step was to combine the two extraction techniques. With prepress solvent extraction, cottonseeds are pressed to remove most of the oil and then the oil remaining in the press-cake is extracted with solvent. This solvent extraction operates on a reduced volume of feed stock (i.e., press-cake, as opposed to full-fat flakes) and, therefore, requires a modest size extractor with modest amounts of desolventizer and solvent. 

For this purification process, the crude miscella source may be from (1) the preevaporator of a direct-solvent extraction plant, (2) a blend of prepressed crude oil and solvent-extracted miscella from the press-cake, or (3) a reconstituted blend of crude oil with solvent. In the process, a mixture of approximately 40% to 58% oil in solvent is heated or cooled to 104°F (40°C) and filtered to remove meal, scale, and other insoluble impurities. Two solvents that have been used commercially for miscella refining are hexane and acetone. 

The low ehd fractioh of meal that resulted from prepress-solvent extraction of safflower seed followed by two fraction tail-end decortication. 

Prepress-solvent extraction of safflower seed without decortication. Typical California, 1992. 

These same expellers, if employed in a prepressing mode wherein 15-17% residual oil remains in the cake that is sent to the solvent extraction unit, can process 45-501 of seed per day. Prepressing of safflower produced under Califomia conditions (or the equivalent) requires no cooking, flaking, or cracking of the seed before extraction and results in oil capable of being heat bleached to 1-3 Gardner color. 

Mechanical expression of rice bran yields less oil, 10-12%, than solvent extraction, 16-18%. Rice bran is treated with steam and dried prior to pressure expression. Prepressing is usually carried out at 70 kg/cm followed by oil expulsion at 105-316 kg/cm (9). As a result of the low yield of oil from mechanical extraction, residual oil in the bran is recovered with hexane. Hexane extraction can be performed by batch or a continuous operation. Continuous operation uses countercurrent flow to improve mass transfer. Solvent extraction at high temperatures results in higher crude oil yield, but the crude oil is of lower quality. A new oil-extraction process, which involves premolding of rice bran at 14% moismre content and <40°C followed by hexane extraction at <15°C, was reported to yield a light-colored crude oil with no wax (9). 

In 1951, the V.D. Anderson Company was again the pioneer in mechanical screw pressing, being first to patent the process of using a mechanical screw press to continuously prepress oleaginous materials ahead of continuous solvent extraction plants. The company was awarded U.S. Patent 2,551,254 (5). 

This is somewhat an oversimplification because this form of leaching assumes all oil is freed from cells, which is not the case and is discussed further later in this chapter. Although many solvents were tried over the years, mixtures of hexanes are mostly used today. Most hexane mixtures used contain about two-thirds -hexane, the remainder being mostly other isomers of hexane. Very little detectable benzene is allowed. Direct solvent extraction is usually preferred for soybeans both prepress solvent extraction and direct solvent extraction are depicted in Fig. 11.3. 

Fig. 11.3. Depiction of prepress solvent extraction and direct solvent extraction (provided by French Oil Mill Machinery Co., Piqua, OH).

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