Desolventizer toaster

Fig. 34.16. Schumacher type desolventizer-toaster-dryer-cooler (DTDC). (Courtesy of Crown Iron Works Company, Minneapolis, MN.)...

Mechanically Extracted Meals. Solvent extracted oilseed meals typically contain less than 1.5% residual fat unless the gums (hydrated phosphatides) or soapstock have been added back to the meal before the desolventizer-toaster or meal dryer. Mechanically extracted (expeller or screw-pressed) meals can contain 4—9% oil, which can be a significant calorie source in animal feeds. Fat contents of extracted meals are not part of the definition, although typical analyses are shown below. 

Wet extmders are also used ahead of a number of soybean solvent extraction plants. In plants with undersized extractors, the energy used by the extruder can be compensated by additional oil yield from the extractor. Steam consumption is reduced in the desolventizer toaster. Additionally, the wet extruder in a soybean plant increases temperature sufficiently to stop the phospholipase enzyme from converting additional hydratable phosphatides into nonhydratable phosphatides. This reduces acid, caustic, and silica consumption in the downstream refinery. However, in soybean plants with adequately sized extractors where potential oil yield... 

After the prepared material has had its oil extracted in the solvent extractor, it is conveyed to the meal desolventizer toaster, commonly referred to as the DT. The material entering the DT is typically at the extractor temperature of 60°C, and it contains 25-35 %(w/w) of solvent. The primary purpose of the DT is to remove the solvent from the meal fraction so that the solvent can be recovered. 

Figure 10. (a) Desolventizer toaster. Courtesy of Crown Iron Works, (b) Phase equilibrium. 

This countercurrent desolventizer/toaster was originally developed in Germany by Heinz Schumacher and put into service in about 1982. Crown Iron Works Co. (Minneapolis, Minnesota), Extraction de Smet (Zaventem, Belgium), and Krupp Maschinentechnik (Hamburg, Germany) obtained licenses to build and sell this desolventizer in various countries of the world. 

High nitrogen solubihty index (NSI), soy protein for manufacture of meat analogs is prepared in flash desolventizers. Some soybean extraction plants send part of their marc through a flash desolventizer to serve the high NSI soy flour market and the balance of their marc through desolventizer/toasters to serve the animal feed soy meal market. The first flash desolventizer was commissioned by EMI Corporation (Em. Corp., Des Plaines, Illinois) in 1960 (214). 

Recovery of used solvents by distillation, on site or off site at a central facility, is much easier if each solvent is stored separately. A single solvent for the whole plant would be desirable if a solvent must be used. Sandoz Pharmaceutical (now Novartis) has done this for a process, with the result that they now produce only 1.5 lb of waste per pound of product instead of the former 17.5 lb.16 For an investment of 2.1 million dollars, they save 775,000 dollars/year. Processes that use solvent mixtures should be restudied to see if a single solvent can be used instead. Solvent recovery almost never reaches 100%. A plant extracting oil from soybeans recovered 99.92% by use of desolventizer toasters with fully countercurrent stripping steam.17 It still lost 0.3 gal solvent per ton of... 

Solvent (hexane) extraction of soybeans is a diffusion process achieved by immersing solid in solvent or by percolating solvent through a bed of solids. Rotary (deep-bed), horizontal belt, and continuous loop extractors are used for soybeans (Woerfel 1995). Solvent is recovered from the mixture of solvent and extracted oil (miscella) by double-effect evaporator and steam stripping and from flake by a desolventizer-toaster, and is recycled. 

The meal goes through a desolventizer/toaster and is cooled and stored. High heat in the de-solventizer/toaster is an important step for the complete removal of solvent and for the deactivation of tripsin inhibitor which is critical for feeding ruminants. 

Condensed solvent and water from the meal desolventizer/toaster and the oil stripper must be separated in gravity water separators before recycling solvent to the extractor. The water phase is then heated to remove residual hexane. Hexane losses must be minimized, and much engineering has gone into reducing hexane loss, which is now typically - I L/metric ton of soybeans processed. 

The residual solvent in the meal as it leaves the extractor is usually about 25%-35% by weight. The desolventizer-toaster heats the meal and solvent and vaporizes and steam strips the solvent from the meal at atmospheric pressure. 

Pressing followed by hexane extraction leads to the highest oil yield. The meal contains only about 1% residual oil. After solvent extraction, the used hexane has to be removed from the meal which is done in desolventizer toaster (DT) systems. During this process step, the proteins within the meal are partially damaged leading to losses of functionality (Becker, 1983 Natsch, 2006 Krause, 2007). 

The meats are heated, with the addition of steam to increase the moisture content if needed. Heaters (cookers), desolventizer-toasters (DTs) and coolers used in the oilseeds processing industry often are of the shallow circular pan, sweep arm, multi-stack design shown in a later figure. In earlier techniques. 

Fig. 8.9. Cutaway drawing of Crown/Schumacher desolventizer-toaster-dryer-cooler (DTDC). Note the stacked design of tray (ring) heaters-coolers with sweep arms, and steam-jacketed and perforated bottoms. Stack cookers generally have solid steam-jacketed bottoms for heating oilseeds. (By permission of Crown iron Works Company, Minneapolis, MN.)...

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