Soybean is not only an important oil crop, providing edible vegetable oil, but also an important source of vegetable protein. In the soybean press oil plant, the solvent extraction process is usually used to extract soybean oil. The wet meal after extraction usually contains 55~70% dry material, 25~35% residual solvent, 5~10% moisture, and less than 1% residual oil. Generally, the temperature of wet meals is 55~60 ℃. In most cases, the de-fatted wet meal contains anti-nutritional factors, which may inhibit digestion. Soybean wet meal cannot be directly used commercially, and transportation is unsafe, requiring further processing.
There are two main methods to treat wet meal after solvent extraction:
- (1) Most of the soybean wet meal goes through the process of desolvation, toast, drying, and cooling (DTDC) to produce protein-rich soybean meal as an animal feed ingredient.
- (2) Another soybean wet meal is produced by low-temperature desolventizing, we call the final product Soybean White Flake, which is used for the production of edible soybean powder, soybean protein concentrate, soybean protein isolate, etc., and for the application of human and animal special feeds.
There are two kinds of technology in low-temperature desolventizing fields.
First is FDS, Flash Desolventizing System, The so-called “Flash desolventizing” refers to the instantaneous desolventizing of wet meal at high temperature, removing most of the solvents, and then directly stripping the white cake under vacuum. The desolventizing material is then cooled and unloaded to a lower temperature area, so as to minimize the denaturation of protein.
The second is Chemsta Tank A-B System, which is a two-step desolvation process, the first step is desolvation under normal temperature, and the second step is desolvation under vacuum. These two processes are applied around the world, and the Chemsta Tank A-B system has many advantages over than FDS system.
1. FDS System
The wet meal is fed into the circulating tube of superheated solvent vapor with a high flow rate through the conveying equipment, and the temperature of the superheated solvent vapor is about 150℃. The solvent vapor transports the wet meal in the annular tube at a speed of about 20m/s. In the superheated solvent vapor flow, the solvent vapor is in full contact with the wet meal and exchanges heat rapidly. The heat of the superheated vapor is released to provide latent heat for the evaporation of liquid solvent and water in the wet meal.
After the wet meal enters the circulating tube for 2 seconds, the outlet temperature reaches about 100~105℃, containing 1~2% liquid solvent and 6~8% water. After the white flake is discharged from the annular tube, the temperature of the overheated solvent flow drops to about 110℃, then the speed is restored by a blower, and the solvent vapor is reheated to 150℃ by the solvent heater. Finally, the overheated solvent vapor reaches the wet meal inlet of the annular tube. The excess solvent and steam generated at the outlet of the annular tube are sent to the condenser through the automatic pressure control valve, and the solvent gas enters the condensation system for condensation, recovery, and recycling.
After hot steam flashing, the white embryo flakes still have a solvent concentration of 1~2%, which cannot be safely treated. Therefore, it is further conveyed to a vacuum desolventizing and contacted with superheated steam to remove the remaining solvent until it is below 2000 ppm.
In modern process equipment, the stripping equipment is maintained at a vacuum of about 0.5bar to help reduce the residual solvent to 500ppm. The stripping equipment can be vertical cascade type, horizontal conveying type, or horizontal paddle type hybrid type. In order to maintain the PDI of the meal as much as possible, any superheated steam cannot condense. Therefore, the key is that these pieces of equipment need hot water to achieve very good insulation. For the production of protein isolate and special powder, the ideal PDI is as high as possible(PDI85～90). In these cases, the heating surface of the stripper is maintained at 90~100℃. For the production of protein concentrate products, the ideal PDI is usually about 70. In this case, steam is introduced into the cascade stripper and indirectly heated to maintain a high operating temperature.
The discharge temperature of the cascade stripper is usually 90~100℃. Through air convection cooling, the temperature is reduced to 10~20℃ of the ambient temperature. In small-scale factories, this kind of cooling is completed in the dilute phase pneumatic conveying system, while in large-scale factories, this kind of cooling is completed in DC (like the hot meal drying cooler). This product is usually called white flake. In special soybean powder products, a biofilter should be used to remove all bacteria. For white flakes entering the protein concentrate or protein isolate process, this is not important, because any bacteria are eliminated in the subsequent process.
2. Tank A-B System
After extraction by the solvent extractor, the soybean wet meal enters Tank A by the Airlock (Input Air Lock Feeder A). In Tank A, the wet meal is heated by the high-temperature solvent steam (generated by Solvent Steam Heater and Hot Wind Circulating Blower) flowing in the reverse direction, and the liquid solvent in the wet meal is gasified and goes out from the feed end of Tank A with the high-temperature solvent steam. The soybean meal and solvent gas in the gas are separated by the cyclone, and the gas is blown into the Solvent Steam Heater again for recycling, while a small amount of solvent gas goes to the Wetting Type Catcher with hot water and then to the condenser for condensation, and the non-condensing exhaust gas goes to the paraffin recovery system. The soybean meal from Tank A goes to Tank B through Input Air Lock Feeder B. The Tank B barrel is under vacuum to remove the residual solvent in the meal and adjust the moisture, and the finished meal is brought to the meal cooler, and then measured and bagged. The gas is extracted from Tank B, trapped by Wetting Type Catcher hot water, then goes to the condenser for condensation, and the non-condensable exhaust gas goes to the paraffin recovery system.
3. FDS V.S Tank A-B System
In FDS, the transportation of materials is completely dependent on hot air wind, and the materials will collide with the wall of the pipe many times leading to low-temperature meal crushing, and the same thing will still happen when unloading, which will lead to the progress of broken material. So the powder degree is high.
In the Tank A-B system, the well-designed rotor will lift the material from the bottom of the equipment, and the material will fall through its own gravity, the whole process will not occur with violent friction, and the material will not be broken.
Wet meal desolvation takes a certain amount of time, in FDS, the speed of hot air is very fast, so the length of the duct can only be increased, which leads to very large floor space.
In the Tank A-B system, the compact structure of equipment, usually Tank A is located on the second floor of the workshop and Tank B is located on the first floor, the process flow is smooth and the floor space is small.
In FDS, the material is transported entirely by hot air, so the power of the fan is very high.
In Tank A-B System, the hot air is only used for desolvation, and the material is transported by the rotor of the equipment to push the material forward.
In the past few years, Chemsta has built several soybean white flake production lines with capacities from 500TPD to 800TPD. In the two years of 2020-2022 alone, Chemsta has built one 500TPD soybean white flake production line and two 600TPD soybean white flake production lines. Chemsta’s Tank A-B System low-temperature desolvation technology has now reached the world’s leading level, and we hope to cooperate with more customers in the future to share the fruits of technology development.