1 The purpose and requirements of low-temperature desolventizing of soybean meal
1.1 Reduce the thermal denaturation of protein in the process of soybean meal desolventizing, reduce the loss of NSI or PDI, and improve the value of subsequent processing.
1.2 In the process of desolventizing, the formation of crushed white flakes and powders should be minimized to improve the yield of products and facilitate the subsequent extraction of proteins of various qualities (soybean protein concentrate SPC and soybean protein isolate SPI).
1.3 Reduce the degree of blockage and wear of the desolventizing equipment, extend the service life of the equipment, reduce the cost of system maintenance and overhaul, and thus reduce the production cost.
1.4 Keep the color of soybean white flakes after low-temperature desolventizing to be uniform light white and light yellow.
1.5 Thoroughly remove the residue solvent from soya meal, save solvent and steam consumption, and reduce production costs.
2 Process introduction
Low-temperature desolventizing technology is mainly used for edible soybean meal (soybean white flake). At present, there are two mainstream desolventizing methods in the world. One is flash desolventizing, which is referred to as “FDS”. The other is a two-stage horizontal low-temperature desolventizing process, which is referred to as the “Tank AB System“.
There are two forms of flash desolventizing process, one is composed of flash tube and vertical vacuum desolventizer (see Fig 1, referred to as FDS 1#), and the other is composed of flash tube and horizontal plate desolventizer (referred to as FDS 2#).
The FDS is composed of evaporation tube, cyclone discharger, airlock, feeder (sometimes horizontal three-way feeder can also be selected), circulating fan, solvent gas heater, and cyclone for discharging solvent gas. The extracted wet meal falls into the evaporation pipe through the airlock and feeder, where the wet meal is mixed and held up by the superheated solvent gas with a temperature of 150~160 ℃ for pneumatic conveying, and the solvent gas is supplied by the circulating fan. In the evaporation pipe, the wet meal and the superheated solvent gas conduct heat exchange, so that the solvent of the wet meal can be quickly evaporated and escaped. When the meal and solvent gas enter the cyclone discharger, the solvent gas and the meal are centrifugally separated, and then the meal is discharged for further treatment. A part of the separated solvent gas is sent to the solvent gas heater by the circulating fan, and the heated solvent gas is re-supplied to the FDS for solvent evaporation of the new wet meal; the other part of the solvent gas enters the cyclone through the automatic control valve for powder separation. The purified solvent gas enters the first evaporator for heat exchange to participate in the evaporation of the miscella, and then enters the condensing system for condensation. The solvent obtained is reused. There are two subsequent technologies for the material obtained after passing through the flash tube. (1) One is to enter the vertical vacuum desolventizer, use indirect steam and direct steam to continue to remove the residual solvent in the meal under negative pressure, and finally get the finished soybean white flakes. (2) The other is to enter the horizontal plate desolventizer, and the material will continue to be heated on several layers of plate heated by indirect steam and desolventizing under negative pressure. The escaping solvent gas enters the catcher under negative pressure and condenses after solvent gas purification.
2.2 Tank AB System
Tank AB System (see Fig 2) is mainly composed of airlock, tank A, tank B, Cyclone 1#, circulating fan, solvent gas heater, Cyclone 2#, wet fines catcher for Tank A, dry fines catcher, wet fines catcher for tank B, etc. The extracted wet meal enters tank A through the 1# air lock at the inlet of tank A. There is a partition at the inlet of tank A. One side of the partition is the channel for the wet meal to enter tank A, and the other side is the channel for the solvent gas after heat exchange to exhaust upwards from tank A. The solvent gas and part of the meal foam are extracted by the negative pressure of the circulating fan, and then enter the cyclone 1# catcher, after catching fines, and then enter the circulating fan. The solvent gas discharged from the circulation fan is divided into two parts. One part enters the solvent steam heater, and then the solvent gas is heated and sent to the air inlet of tank A, where the solvent in the wet meal in tank A is rapidly gasified and removed, thus achieving the effect of desolventizing; another part of solvent gas enters cyclone 2#, passes through the wet fines catcher for tank A, enters the first evaporator shell side for miscella evaporation, and the heat is recycled, and finally enters the condenser for condensation. The meal foam discharged from cyclone 1# and cyclone 2# both enter tank B. The materials that have been desolventized through tank A enter tank B of the horizontal desolventizer through 2# air lock, and is heated or insulated by indirect steam in the jacket of tank B, the residual solvent in the meal is slowly released and escaped under vacuum condition, and the moisture is adjusted properly and the discharge temperature is reduced. Finally, the soybean white flakes are discharged through 3# air lock. The solvent gas extracted from tank B by the vacuum pump first enters the dry fines catcher, and the caught powder is returned to tank B again. The solvent gas after preliminary purification enters the wet fines catcher for tank B for solvent gas purification again and finally enters the condenser for condensation.
3 Analysis and characteristics introduction of two kinds of low-temperature desolvation technology
3.1 Lost of NSI：
After the wet meal enters the system, it is heated by superheated solvent gas with a temperature of 150~160 ℃, and strong heat exchange is carried out in the process of pneumatic conveying. The time of wet meal in the flash pipeline is only a few seconds. With the continuous evaporation of the solvent, the mass of wet meal decreases and the temperature increases. The temperature of solvent gas decreases to the lowest when discharging, and the temperature of wet meal rises to the highest. It belongs to the opposite heat transfer process. The temperature difference between solvent gas and material is small. The temperature of solvent gas after passing through the solvent heater is required to be high, so the risk level is high. Because the contact time between the material and the solvent gas at high temperature is relatively short, the desolventizing rate is only 90%. After flashing, the material enters the vertical or horizontal desolventizer. Under the negative pressure condition, the surface temperature of the bottom interlayer heated by steam is high, and a part of materials are in direct contact with the heating surface, which will inevitably lead to inconsistent heating of the material, increase the rate of loss of water-soluble protein (NSI) of the material, and greatly affect the water-soluble protein content of the material.
3.1.2 Tank AB System
The wet meal material enters the tank A through the air lock, and the solvent gas at 130~140 ℃ enters the tank A tangentially and rotates close to the tank wall with the same direction as the rotor and moves towards the solvent extraction outlet. The wet meal is continuously turned up along the rotation direction of the solvent gas under the action of the integral flip board, and moves towards the discharge outlet under the push of the rotor spiral blade. Due to the gap of several centimeters between the flip board and the tank wall, the solvent gas rotates in the outer circle, and the wet meal is drawn into the inner circle by the solvent gas to rotate slowly. The wet meal and the solvent gas conduct strong heat exchange, and the moving direction of the wet meal is opposite to that of the solvent gas, which is a reverse heat exchange process. The highest temperature of wet meal at the outlet is 85 ℃, and the highest temperature of solvent gas at the outlet of wet meal (i.e. the inlet of solvent gas) is 130~140 ℃. At the inlet of wet meal, the temperature of the material and solvent gas are the lowest, but the negative pressure value of tank A here is the highest (- 1500Pa), which improves the heat exchange effect. Although the wall of tank A is also heated by indirect steam, the wet meal in tank A is not in close contact with the wall, and there is no uneven heating, so the loss of water-soluble protein of the material is less. The time of wet meal in tank A is several minutes, and the desolventizing rate reaches 99%, and the loss of water-soluble protein is less.
The wet meal discharged from the discharge outlet of tank A with a temperature not higher than 85 ℃ enters tank B. The structure of tank B is basically similar to that of tank A, and indirect steam heating is also provided. On the one hand, the material releases slowly and continues to escape solvent under negative pressure condition, and at the same time, the temperature at the time of discharging from tank B is not higher than 72 ℃, so the temperature of the white flakes in tank B is slowly reduced, and the indirect steam heating in tank B only plays the role of preheating the equipment at the initial stage or heat preservation when the ambient temperature is too low. Therefore, the loss of water-soluble protein in tank B is less.
Through the above analysis and practice, it is proved that the loss of water-soluble protein in soybean white flakes caused by FDS technology is within the range of 3.0~5.0%.
In the Tank AB system process, the temperature of solvent gas used for desolventizing is about 20 ℃ lower than that in the FDS system, and the material is not in close contact with the tank wall in tank A, so although the time for the wet meal to turn up and move in tank A reaches several minutes, the loss of water-soluble protein is less. After the wet meal enters tank B, the solvent will be slowly released and escape and cooled properly under negative pressure condition. Therefore, the loss of water-soluble protein of materials in tank B is less.
The loss of water-soluble protein in soybean white flakes caused by Tank AB System technology is not more than 2%.
3.2 Analysis of crushing phenomenon of low-temperature soybean meal in the process of desolventizing:
The process of strong heat exchange and mass exchange of wet meal materials in the flash pipeline is actually a process of pneumatic conveying. In order to achieve the effect of desolventizing, the air flow speed is relatively high, generally 16~25m/s, and the conveying speed is also increasing as the wet meal solvent continues to evaporate and escape; due to the continuous collision and friction between material particles and between particles and pipe wall, the two-phase flow movement will cause material crushed, which is also the disadvantage of pneumatic conveying. Especially when the material enters the Cyclone, under the action of centrifugal force, the material closely contacts with the tank wall of the discharger, which causes great wear, and the material crush rate increases more. At the same time, there are crushed materials mainly formed by the shearing of the feed and discharge air lock.
After the flash evaporated soybean white flakes enter the vertical or horizontal desolventizer, the material collides and rub with the bottom plate and tank wall under the stirring action of the stirring blades, and the discharge outlet is equipped with a air lock, which will also lead to a certain degree of crushing.
3.2.2 Tank AB System
Due to the distance between the flip board and spiral blade of tank A and the tank wall, the rotor rotates in the direction of rotation of solvent gas. Under the action of wet meal friction, the wet meal close to the inner circle of the rotor drives the outer circle of wet meal to continuously turn up and move, and the wet meal is not in close contact with the tank wall in tank A; although it takes several minutes for the material to turn up and move in tank A, and there is powder generated by friction, but the degree of formation of crushed material is light and the quantity is small. After entering tank B, because the structure of tank B is basically the same as that of tank A, and the rotating speed is slow, the degree and quantity of crushed material formation are also small.
From the above analysis, it can be seen that the Tank AB double-drum desolventizing process has a small degree of crushing on the material.
3.3 Equipment maintenance cycle
No matter what kind of process, it is necessary to ensure that the solvent gas circulating pipe is unobstructed, the circulating fan runs smoothly, the solvent gas heater tube is not blocked, the first evaporator tube and the condenser tube are not dust bonded, so as to ensure the stable production of customers.
The process of solvent evaporation in the flash tube is actually the process of strong heat exchange and mass exchange of wet meal in the solvent gas pneumatic conveying. If the conveying speed is too high, it will cause material crushing, equipment wear and power consumption waste; if the conveying speed is too low and the material solvent evaporation time is long, it is easy to cause pipeline blockage, material omission, etc., affecting continuous production. Therefore, the material flow and solvent gas flow should be stable and uniform. After passing through the flash tube, the wet meal and solvent gas enter into Cyclone for centrifugal separation of materials and gas. After passing through the circulating fan, part of the solvent gas enters the solvent gas heater for heating, and then enters the system for desolventizing. Although the cyclone discharger is a centrifugal separation discharge, which has a certain dust separation function, its main function is to discharge after air conveying. In addition, the material speed is also very fast, so the equipment is not good for dust separation, so there will be a lot of dust entering the solvent gas heater, which will lead to the blockage of the tubes of the solvent gas heater, which needs to be shut down regularly for cleaning; another part of solvent gas enters another cyclone for centrifugal separation and purification of solvent gas, and then enters the first evaporator for heat exchange, and then condenses in the condenser; the solvent gas separated by cyclone has only been separated once, and no wet fines catcher is set to strengthen the purification of solvent gas. Fine dust will enter the first evaporator and condenser, which not only requires regular cleaning, but also increases the difficulty of cleaning. At the same time, due to the limited dust removal effect of cyclone, the separated solvent gas passing through the circulating fan will also lead to dust binding on the fan blades, affecting the smooth operation of the circulating fan.
3.3.2 Tank AB System
In the process of Tank AB double-drum desolventizing, the solvent gas after heat exchange from tank A first enters 1# cyclone dust collector for gas and dust centrifugal separation, and the solvent gas after dust removal enters the circulating fan, and the fan blades will not bind dust, which improves the operating conditions of the circulating fan. Part of the solvent gas from the circulating fan enters the solvent vapor heater, and then participates in the desolventizing through tank A. The cyclone can remove dust particles larger than 20 microns, so the solvent heater will not be blocked and the cleaning cycle will be extended; another part of solvent gas enters 2# cyclone dust collector for centrifugal separation of fine dust. Due to the limited dust removal effect of the cyclone dust collector on fine dust particles, a wet fines catcher for tank A is set up after it. The fine dust in the solvent gas is collected by hot water and then enters the first evaporator and condenser. At the same time, the dust from 1# cyclone and 2# cyclone does not enter tank A, but enter tank B, which effectively reduces the dust content in solvent gas from tank A and reduces the subsequent dust removal load.
To sum up, there is no material dropping and pipeline blockage in the tank AB double-drum desolventizing process, the circulating fan operates stably, the solvent gas heater is not blocked, and the first evaporator shell and condenser need not be cleaned.
3.4 Color of soybean white flake
3.4.1 FDS System
In the gas flow flash desolventizing process, after the flash tube, whether the vertical desolventizer or the horizontal desolventizer is used, there is always a part of the wet meal in direct contact with the heating surface, which will inevitably lead to inconsistent heating of materials and obvious heat accumulation, and darken the color of some materials.
3.4.2 Tank AB System
In the process of tank AB double-drum desolventizing process, the material is in non-tight contact with the wall of tank A. Although the material is in relatively tight contact with the tank wall when it first enters tank A, at this time, the material temperature is low, the moisture content is high, the negative pressure value is large, and the phenomenon of thermochromism is not obvious. After the material enters tank B, the solvent will be released slowly and cooled properly under the condition of negative pressure, so the material will no longer have the phenomenon of thermochromism tank B.
3.5 Solvent consumption and steam consumption
3.5.1 Solvent consumption
Based on the above analysis, it can be seen that both processes can better remove the solvent in the wet meal, but the solvent consumption still exists in the operation of shutdown maintenance, system cleaning and troubleshooting, so Tank AB System has certain advantages in this respect.
3.5.2 Steam consumption
When the wet meal in the flash tube and tank A is desolventized, the heat is recycled and the excess solvent gas is used for the evaporation of the miscella, so the steam consumption in this section has little difference.
However, when FDS subsequently uses a vertical desolventizer or horizontal plate desolventizer, it needs to use steam to evaporate the last remaining 10% solvent from the wet meal (about 90% solvent is removed from the flash tube). In Tank AB System, since 99% of the solvent is removed from tank A, the material is slowly released in tank B, and the remaining 1% solvent gas escapes, and the temperature when the material is discharged from tank B is not more than 72 ℃, so tank B does not need steam during normal production.
To sum up, the steam consumption of Tank AB system is lower.
The above contents, based on the principle of objective analysis, have compared the FDS System and Tank AB System in NSI loss, the degree of crush of soybean white flakes, the blockage of equipment, the color of finished soybean white flakes and the consumption of solvent and steam. In a comprehensive view, Tank AB system technology has certain advantages. This is also in line with the actual situation. After decades of technological development, China has produced most of the white flakes in the world. In China, the FDS system has been basically eliminated, and replaced by the tank AB system low temperature desolventizing technology.
Chemsta has absolute advantages in the field of low-temperature desolventizing technology of Tank AB system. We have contracted to build most of the soybean white flake factories in China and applied this technology to factories in other countries. If you are interested in this technology, you are welcome to visit China at any time.