Miscellas recovery

Hexane in miscella and cake is recovered for reuse in succeeding operations. Escaping hexane vapors are trapped by cold mineral oil spray to preclude hazards and maximize solvent recovery. 

Solvent Fractionation. This process is the most expensive because of solvent loss, solvent recovery equipment, much lower temperature requirement, and stringent safety features. The process involves the use of solvents such as hexane or acetone. The oil is first dissolved in the solvent followed by cooling to the desired temperatures to obtain the desired crystals. Cooling is effected by brine if very low temperature is required. The miscella containing the partially crystallized oil and solvent is then filtered under vacuum suction in an enclosed drum filter. The olein miscella and stearin miscella are then separately distilled to remove the solvent and recover the fractions. Yield of olein is about 80%. The solvent process nowadays is only viable in the production of high value products such as cocoa butter equivalent or other specialty fats. 

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. 

As the temperature of the miscella exiting the first stage evaporator is low, it is a good heat sink for heat recovery. In various facilities, heat from hot finished oil, heat from steam ejector exhausts, or recovered flash steam is used to preheat the miscella to approximately 75°C in temperature. The preheated, concentrated miscella is then typically heated to 110°C in a steam-heated exchanger prior to entering the second rising film evaporator. 

The solvent extraction process consists of the unit operations of solvent extraction, meal desolventizing, meal drying and cooling, miscella distillation, and solvent recovery. These unit operations are highly interrelated, primarily because of various heat recovery methods that link the operations together. A process upset in any one of the unit operations will typically cause abnormal operation in the others. The key to efficient solvent extraction operation is process consistency. Consistency of seed preparation, consistency of rate, and consistency of solvent extraction operating parameters are all important factors in operating a safe, environmentally friendly, and cost-effective solvent extraction process for oil extraction. 

For oleaginous materials having a low oil content (18-20%), such as soybean and rice bran, solvent extraction is often applied for oil recovery. Hexane is widely accepted as the most effective solvent used today. Most of the extractors currently used are designed as countercurrent flow devices. The solid material flows in an opposite direction of solvent-oil miscella with an increasing oil concentration. The miscella containing around 25-30% oil after extraction is subjected to solvent distillation to recover the oil. The extracted solid material, commonly known as white flakes, is also conveyed to the desolventizing process. 

Pagliero et al. [53] evalnated the recovery of solvent in miscella degummed snnflower oil/hexane with concentrations of oil between 25% and 45% (w/w). Manbranes were synthesized from PVDF, prepared by the process of phase inversion, and evaluated for their flow and selectivity toward the oil. Tests were performed in a 400 mL filtration unit, with an effective area of membrane equal to 31.66.10 m and agitation of 750 rpm. The best separation was achieved at pressures between 4 and 6 bar, and a tanperature of 50 C and 25% oil in miscella (w/w), corresponding to a flow of 30 L m h. 

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%. 

The extraction operator can, within limits, control bed depth, solvent input and to some degree its (solvent/miscella) distribution within the extractor as well as extractor temperatures. However, the operator must accept the quality and quantity of press cake produced in the preparation plant. The quality of extracted product is highly dependent on a good press cake. Poor quality cake commits the extraction plant to poor oil recovery with high solvent loss and even high levels of solvent in the meal, both of which are beyond the extraction operator s power to effectively control. 

Another accident to be avoided is an overflow of miscella into the extracted meal. The results are overloading of the desolventizer and higher than normal residual solvent in the meal. In addition the solvent recovery system is subjected to an overload with attendant higher than normal losses. 

The recovery of solvent from both the miscella and the leached seeds or beans is an essential part of the vegetable-oil leaching process. In a typical arrangement, the filtered miscella is passed to an evaporator for removal of solvent, sometimes followed by final stripping in a tray column, to produce the solvent-free oil. The wet seeds are steamed to remove residual solvent and air-cooled. Vented gas from condensers may be sent to an absorber to be scrubbed with petroleum white oil, and the resulting solvent-white-oil solution stripped to recover any solvent. 

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