Soybean is not only an important oil crop, providing edible oil, but also an important source of vegetable protein. In soybean pressing oil plants, solvent extraction is usually used to extract soybean oil. The extracted wet meal usually contains 55 to 70% dry materials, 25%-35% residual solvents, 5%-10% moisture, and less than 1% residual oil. The wet meal temperature is usually 55-60°C. In most cases, skim meal (white cake) contains anti-nutritional factors that may inhibit digestion. White cake cannot be used directly for commercial purposes, nor is it safe to transport, and requires further processing.
There are two possible ways to treat wet meal after solvent leaching. More than 95% of wet meal is passed through the desolvent, toasting, drying and cooling (DTDC) process path to produce protein-rich soybean meal as an animal feed ingredient. Less than 5% of the wet meal is processed by flash evaporation to desolvent and cooling. It is used to produce edible soybean meal, protein concentrate, protein isolate, etc., used for human and animal special feed applications.
The so-called "Flash desolventizing" refers to the instant desolvent of wet meal at high temperature, removing most of the solvent, and then direct steam stripping of the white cake under vacuum. Cooling the desolvent material and discharge to a lower temperature area to minimize protein denaturation.
Flashing evaporation: The wet meal is fed into a high-flow superheated solvent vapor circulation tube through a conveying devic,the temperature of the superheated solvent vapor is about 150°C. The solvent vapor transports the wet meal in a circular pipe at a speed of about 20 meters/second. In the superheated solvent vapor stream, the solvent vapor is in full contact with the wet meal and quickly exchanges heat. The heat of the superheated steam is released, providing latent heat for the evaporation of the liquid solvent and moisture in the wet meal.
After the wet meal enters the circulation tube for 2 seconds, the outlet temperature reaches about 100 to 105°C, contains 1%-2% of liquid solvent and 6 to 8% of moisture. After the white flakes discharged from the annular tube, the temperature of the superheated solvent steam drops to about 110°C then the speed is restored by a blower and the solvent vapor is reheated to 150°C by a solvent heater. Finally, the solvent superheated vapor reaches the circular tube Entrance of wet meal. Excess solvent and water vapor generated at the outlet of the loop pipe are sent to the condenser through the automatic pressure control valve, and the solvent gas enters the condensation system for condensation and recycling.
Vacuum desolvent: The white flakes after hot steam flashing still has a solvent concentration of 1-2% and cannot be safely processed. As a result, it was further transported to a vacuum desolventizer and contacted with superheated steam to remove the remaining solvent until below 2000 ppm.
In modern process equipment, the stripping equipment is maintained at a vacuum of about 0.5 bar to help reduce residual solvents to 500 ppm. The stripping equipment can be a vertical stacking type, a horizontal conveying type or a horizontal paddle type mixing type. In order to maintain the PDI of the meal as much as possible, any superheated steam cannot be condensed. Therefore, the key is that these devices use hot water to accompany the heat to achieve very good insulation. For the production of protein isolates and special powders, the ideal PDI is as high as possible (PDI85 ~ 90). In these cases, the heating surface of the stripping tower is maintained at 90-100°C. For the production of concentrated protein products, the ideal PDI is usually around 70. In this case, water vapor is passed into the cascade stripper while being heated indirectly to maintain a high operating temperature.
Cooling: The temperature of discharged material from the stacked stripper is usually at 90-100°C. By air convection cooling, the temperature is reduced to 10-20°C of the ambient temperature. In small-scale plants, this cooling is done in a dilute phase pneumatic conveying system, while in large-scale plants, this cooling is done in a DC (like a hot meal dryer cooler). This cool product is often called white flakes. In special soy flour products, a biological filter is used to remove all bacteria. For the white flakes which entering the protein concentrate or protein isolate process, this is not important because any bacteria are eliminated in subsequent processes.
For the application of protein concentrates, the integrity of the flake shape is important and the powder material needs to be screened out. For specialty soy flour and protein isolate products, maintaining the shape of the flash meal is not important, as reducing size is a normal step in subsequent processes.
The advantages of flash evaporation desolvent process: simple process system, protein dispersion index is adjustable, convenient operation, small loss of active protein during desolvent process, low power consumption. The disadvantage is that the meal powder is large.
The foreign low temperature desolvent technology started earlier and a study in 1954 showed that the thermal denaturation of soybean meal after flash desolvent was low. Professor Liu Fuguang of Jiangnan University was the first to conduct low temperature desolvent research in China. The result of the "intermediate test of low temperature desolvent of soybean leaching meal" won the second prize of the Ministry of Commerce in 1987. Low-temperature desolvent technology is mainly used in the production of soybean edible protein products. In addition to flash evaporation desolvent, another low-temperature meal production process is a two-step process. The first step is desolvent at normal temperature and the second step is desolvent under vacuum, it’s typically a two-stage horizontal low temperature desolvent process, that is A and B cylinder processes. These two processes in China have applications. In the later article the AB tube low temperature desolvent process will be specifically introduced.
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