Miscella extraction

Wash time can be further subdivided into contact time and dormant time. Contact time is the time a particle of oleaginous material spends in the washing zone of the extractor where the particle is in contact with miscella. Extraction only takes place during contact time. Dormant time is the time a particle of oleaginous material spends in the washing zone of the extractor where it is not... 

Screw-pressed oil is aUowed to stand to settle out suspended soUds, filtered through plate filter presses, and then pumped to storage. The oil-rich solvent (miscella) from the solvent-extraction process is filtered or clarified, and most of the solvent is removed in a long tube evaporator. FinaUy, the concentrated oil passes through a stripping column where sparging steam is injected to remove the residual solvent. A metric ton of cottonseed yields ca 91... 

Hardly any batch-type oilseed solvent extractors remain. Three of the more popular types currently manufactured include (1) shallow bed-type extractors, where a 0.5-1.5 m thick layer of collets or flakes is pulled across a linear screen and extracted by drenching with a countercurrent flowing miscella consisting of solvent and solubilized oil (Fig. 34.13) (2) diffusion belt type, where deeper beds of collets or flakes are conveyed on a woven mesh or folding-pan belt while drenched in countercur-... 

In processing most oilseeds, hexane is stripped from the miscella by distillation to produce a crude oil that subsequently is alkali or physically refined. However, gossy-pol and other pigments become extremely difficult to bleach if left in warm cottonseed oil for more than a few days. It is normal practice for cottonseed oil mills to send their crude oil immediately to an alkali refinery or to operate an on-site miscella refinery, where phosphatides, FFA, and color pigments are removed by alkali treatment of the oil-extraction solvent mixture. Cooling the crude oil as produced, until refining, also slows fixing of color. 

Copra undergoes an accelerated preliminary extraction with a controlled residual oil content of 14-18% in the expeller meal expeller throughput rate is almost doubled. Hexane (bp 68.7°C) is widely used as the solvent for extraction. In an extraction unit that operates on a countercurrent system, the cake is met by oil-rich miscella (hexane + oil) and leaves the extractor as it is rinsed with pure hexane. The solvent-extracted cake has a residual oil content of approximately 3.5%. 

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 X-M process combines solvent extraction and milling of the rice (41). Brown rice is pretreated with warm rice oil (0.5%) for 2-3 hours to soften the bran. The rice is then milled in the presence of a rice oil miscella. The solvent slurry is then removed from the rice and the rice oil is recovered. Advantages are that stabilization is not required and the resultant oil had a minimum FFA level. This process is no longer used. 

Attempts have been made to recover the wax using cold and hot extraction (2). Wax yields of 1.29-1.82% of the crade oU are obtained. Continuous dewaxing of rice bran oil by chilling the oil or miscella to less than 20°C followed by filtration through plate and frame filters is practiced. Kinsey and Hummell (44) reported on the use of sodium silicate as an aid for dewaxing. The characteristics and physical properties of a purified rice bran wax are similar to carnauba wax (45). 

Recently, polymeric ultrafiltration membranes were used for degumming crude soybean oil and removing phospholipids from the crude oil/hexane miscella (168). Crude soybean oil also can be de-acidified by methanol extraction of the free fatty acids and the extract separated into fatty acids and solvent by a membrane filter (169). A surfactant-aided membrane degumming also has been applied to crude soybean oil, and the degummed oil contained 20-58 ppm of phosphorus (170). Supercritical carbon dioxide extraction was shown to be an effective means of degumming (171). In this process, soybean oil countercurrently contacted supercritical carbon dioxide at 55 MPa and 75°C. The phosphorus content of the oil was reduced from 620 ppm to less than 5 ppm. Ultrasonic degumming was also successfully used to reduce the gum content of soybean oil (172). 

The oil-solvent mixture and the meal is stripped of the solvent to recover solvent-free oil and meal. The solvent-enriched meal is conveyed to vertical desolventizer where heat and vacuum facilitate removal of solvent vapors. Desolventizer contains trays with sweeping arms to agitate the meal for improved efficiency. Some plants purge the cake with steam to remove the solvent, whereas others use hot air, although application of vacuum is most common. The solvent oil miscella are stripped of solvent in a three-stage evaporator. The hexane is reused for the extraction of oil. 

Hexane extraction removes about 50% of phospholipids from the meal (114). The presence of fines in the miscella is undesirable for making lecithin and should... 

Although the dry lecithin can be filtered, many processors prefer to filter the crude oil before degumming (111). Usually, the crude oil is filtered through large plate-and-frame filter presses. If the degumming operation is conducted at a solvent extraction oil mill, then either the miscella, i.e., oil-solvent mixture, or the oil can be filtered. If all of the fines and filter aid are not removed, the dry lecithin will appear cloudy. 

Because of special safety considerations, the solvent extraction process is constructed in a separate facility from the seed preparation process. The solvent extraction process consists of five closely interrelated unit processes solvent extraction, meal desolventizing, meal drying and cooling, miscella distillation, and solvent recovery. 

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