Refining miscella

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. 

In the process, miscella leaves the extractor at about 30-35 percent oil and is concentrated to approximately 65 percent oil by evaporation. The FFA in the concentrate then is reacted with alkali (sodium hydroxide solution) to produce soaps that are removed with other water-soluble compounds by centrifugation. Next, the solvent is removed from the miscella-refined oil by further evaporation, and the soapstock is spread on the meal in the DT to recover its solvent. Hexane vapors from the miscella and the DT are condensed, and the solvent is recycled to the extractor for reuse. The noncondensable gases are passed through a mineral oil stripper to recover the last traces of hexane. 

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. 

Hydrolysis of phosphatides and pigments in the crude oil miscella requires an acid pretreatment, which usually varies between 100 ppm and 500 ppm by weight of the oil, depending on the quality of the crude oil. An acid such as phosphoric or glacial acetic has been found effective in improving oil quality and reducing refining losses. Phosphoric acid is used more commonly because of its less corrosive properties and its availability. The acid is mixed with the miscella in a static mixer to provide an intimately dispersed acid phase that immediately reacts with the crude miscella. 

The process developed by Gupta (19-22) processed crude oils with a combination of UF membrane separation and silica gel column percolation. The soybean and rapeseed-hexane miscellas were passed through various UF membranes. The solvent was removed from the permeate, and the degummed oil was then treated with bleaching clay, silica, and/or acid and finally was steam deodorized to produce refined, bleached, and deodorized oil. The color readings of the oils obtained... 

Bhattacharyya, A.C., Majumdar, S. and Bhattacharyya, D.K. (1986) Edible quality rice bran oil from high FFA rice bran oil by miscella refining. J. Am. Oil Chem. Soc., 63, 1189-1191. 

Steam reforming Steam cracicing Refining Qj-NaOH plants Miscella-Total neons (... 

Obvious disadvantages for the miscella refining process that may have discouraged many processors from adopting this processing system include (Norris, 1982) ... 

Hendrix, W.B. Current practices in continuous cottonseed miscella refining./. Am. Oil Chem. Soc. 1984, 61, 1369-1372. 

Raman et al. [30] examined NF membranes (provided by Kiryat Weizmann, Israel), resistant to hexane in the recovery of the solvent of miscella (constituted 20% of refined soybean oil dissolved in hexane). In the first stage, at an average flow of 9 L m h, a pressure of 2.76 MPa, and a temperature of 24°C, a retentate with 45% oil was obtained. This was again concentrated through nine successive filtrations on similar membranes, with an average flow of 20 L h, in the same pressure and temperature. The separation of oil in the combined systan was approximately 99%. 

Miscella refining (Cavanagh, 1976) is a special case of continuous caustic soda refining. The most favourable conditions for the process are those of the solvent extraction plant where it is also desired to refine the oil and perhaps further process it by winterization. For this reason cottonseed is the oil most usually processed in this manner. The miscella from the extraction plant is evaporated to the desired oil hexane proportion, approximately 65 35 in the case of cottonseed oil. The corrected miscella is then pretreated with phosphoric acid, neutralized and the soapstock centrifugally separated as for the normal 100% oil process. Soapstock separation is very efficient and therefore the miscella does not need to be washed. The solvent is evaporated from the refined oil and the oil is bleached and deodorized. The process is capable of giving higher yields of refined oil and improved colour of the bleached oil by comparison with non-miscella refined oils. 

Austrian patent 3 562 28 is concerned with refining in miscella. Degumming is achieved by membrane ultrafiltration of the miscella which is then refined by passing it through a silica or aluminium oxide column. If necessary the oil can be bleached in miscella after which the solvent is distilled off and the oil deodorized. UK Patent application 2 118 568A also uses membrane ultrafiltration of a miscella. In this case a phosphatide solute and ammonia or other specified neutralizing chemical are added to the miscella before ultrafiltration. 

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