In soybean oil production, approximately 80% of soybean oil producers in China utilize solvent extraction, while this figure reaches 90% in developed countries in Europe and America. Soybean solvent extraction is the primary method for soybean oil production due to its high oil yield (16%-17%, higher than the 12%-13% yield of pressing) and low cost.
Figure 1 Flaking Mill
The soybean solvent extraction process involves first crushing soybeans into small pieces, then use flaking mill (Figure 1)pressing them into flakes or puffing them. The flakes are then contacted with an organic solvent (typically n-hexane) in a solvent extractor. The oil is extracted by utilizing the mutual solubility of oil and organic solvent. The solvent is then removed through heating and stripping to obtain soybean oil. In the production of soybean oil and its byproducts, solvent extraction is the most crucial step. The quality of the extraction directly affects the overall quality of the process and the success of production. Therefore, controlling the key factors influencing soybean solvent extraction efficiency is essential.
Factors affecting the extraction efficiency of soybean oil include the pretreatment process (moisture content of soybean oilseeds, rolling, puffing) and the extraction process (extraction time, extraction temperature, material-to-liquid ratio, and material height).
Figure 2 Solvent extractor
1 Factors affecting the pretreatment process
1.1 Moisture content of soybean oilseeds
The moisture content of soybean embryos before leaching has a great influence on the leaching process and will ultimately affect the leaching effect. When the moisture content of the oilseeds is low, it will affect the wetting of the embryos by the solvent, slow down the diffusion of oil from the inside of the embryos to the interface layer, cause the embryos to clump, destroy the continuity of the channels between the embryos, and make it difficult for the solvent to penetrate. When the moisture content is too high, the phospholipids, starch, proteins and other substances in the soybean embryos will swell and stick together after absorbing water, wrapping the oil droplets, resulting in most of the oil remaining in the embryos, increasing the burden of the subsequent desolventizing process, leading to a doubling of energy consumption and working time. At the same time, high moisture content of the embryos is prone to "bridging" during unloading. Appropriate moisture can improve the plasticity of the embryos and reduce the powderiness. According to the experimental summary and actual production experience, the moisture content of soybean embryos should be controlled at 7% to 9%.
1.2 Soybean Embryo Thickness
In the pretreatment of soybeans, the rolling process is a crucial process. The rolling process refers to the process of using mechanical action to press oilseed soybeans from granules into flakes. After the rolling process, the soybean embryo becomes an embryo sheet. If the embryo sheet is too thin, there will be too much powder in the embryo, and the solvent penetration will be poor during the leaching process, which is not conducive to the leaching of oil. If the embryo sheet is too thick, the cell structure of soybeans will not be damaged enough, the oil extraction process will be long, which is not conducive to leaching, reducing the oil yield, and also increasing the difficulty of desolventizing and moisture adjustment in the subsequent process, affecting the leaching effect. Before soybean pretreatment, rolling is required. After the thickness of the embryo in the pretreatment process reaches the standard, it can enter the next step of the soybean oil leaching process. derived the effect of embryo thickness on the residual oil rate in soybean embryos in the leaching process based on experiments and actual production experience. The thickness of the embryo should be controlled between 0.25 and 0.35 mm.
1.3 Expansion
The expansion process refers to the process in which soybean embryos undergo intense mixing, heating, extrusion, gelatinization, and gelatinization within a confined space of a machine chamber under high temperature and high pressure steam, resulting in changes in their tissue structure. During this process, the bulk density of the expanded particles increases, the cell structure of the embryos is completely destroyed, resulting in more internal porosity and more free oil on the surface. The particle size and mechanical strength also increase, significantly improving solvent permeability to the material layer during leaching, increasing the leaching rate, and shortening the leaching time. Therefore, the output of the leaching unit can be increased by more than 20%. Thus, the expansion process is crucial. If the expansion process is not carried out properly due to unsuitable expansion conditions, it will affect the state of the embryos entering the leaching unit, thereby affecting the final leaching effect of the soybeans. Conversely, if the expansion process proceeds smoothly, the leaching effect can be greatly improved.
2. Factors Affecting the Leaching Process
2.1 Leaching Time
Leaching time is the time required from the initial leaching of the soybean germ to the final extraction of the meal. Therefore, leaching time is one of the important factors affecting the soybean leaching effect. If the leaching time is too short, the contact time between the soybean germ and the organic solvent (n-hexane) will be too short, and the extraction time will also be too short, resulting in a low oil extraction rate that fails to meet actual production safety requirements. If the leaching time is too long, although it can ensure the contact time between the soybean germ and the extractant and the extraction time, thus improving the oil extraction rate, the excessive time will affect the actual time cost and production efficiency. Moreover, studies have found that the oil content of soybean germ tends to stabilize as the leaching time increases. There is no uniform fixed requirement for leaching time based on product standards and the actual conditions of different production areas. Generally, it is determined according to product standards and actual production conditions. In terms of actual soybean leaching effect, a leaching time of 60–90 minutes is suitable.
2.2 Leaching Temperature
Leaching temperature refers to the temperature at which soybeans are leached. The leaching temperature is an important indicator of the leaching process and one of the important factors affecting the leaching effect of soybeans. If the leaching temperature is low, the energy of the leaching process will be low, the oil molecules will lack activity, thus reducing the oil extraction rate and ultimately leading to poor soybean leaching effect; the higher the leaching temperature, the better it is to reduce the viscosity of the oil, increase the speed of molecular diffusion and convection diffusion, and increase the oil extraction rate [4], thereby improving the soybean leaching effect. However, in actual operation, the closer the leaching process temperature is to the boiling point of the solvent (n-hexane), the stronger the solvent fraction volatilization. For the sake of solvent fraction volatilization and safety considerations, the leaching process temperature should be slightly lower than the initial boiling point of the fresh solvent fraction composition, and controlled at 50-60℃.
2.3 Material-to-liquid ratio
The material-to-liquid ratio is the weight ratio of the leached embryo to the solvent used per unit time, which is another factor affecting the concentration of mixed oil [5]. Insufficient solvent will result in incomplete leaching and degreasing, leading to high residual oil in soybean meal, which will affect the oil extraction rate and cause related product indicators to fail to meet standards. While a larger solvent volume is beneficial for leaching the soybean meal and ensures a good leaching effect, a high solvent ratio in the mixed oil will increase the workload of subsequent processes and increase energy consumption. The larger the amount of solvent supplied per unit time, the more pronounced the concentration difference in the mixed oil. When the solvent increases to a certain limit, the soaking and draining sequence becomes unclear, the material compartments stick together, the concentration gradient between oil compartments cannot be guaranteed, and the final mixed oil concentration gradually decreases. Therefore, the material-to-liquid ratio should not be too low or too high; the ratio must be moderate. Based on relevant literature and actual production experience, the optimal material-to-liquid ratio is controlled at 1:0.8–1.4 (W:W).
2.4 Raw Material Height
Under the condition of equal output, a higher raw material height in the extractor hopper allows for a longer extraction time, which is beneficial for reducing residual oil in soybean meal and increasing oil yield. However, a higher hopper is not always better. If the hopper is too high, the raw material at the bottom will be subjected to excessive force and breakage, increasing the powderiness. Also, if the hopper is too full, the raw material may be flushed out by the solvent or mixed oil from the spray pipes, overflowing from both sides of the hopper into the oil collection hopper. Improper operation can clog the oil pump and spray pipes. The raw material height should be selected based on production needs and the choice of extractor. The specific height requirement should be calculated based on the actual conditions and requirements of the manufacturer.
3 Conclusion
Based on the above analysis, the factors affecting the soybean extraction effect include the moisture content of soybean oilseeds, rolling, puffing, extraction time, extraction temperature, material-to-liquid ratio, and raw material height. Each factor has a significant impact on the leaching effect. When considering these factors, it is necessary to follow the theory while also conforming to the actual situation. In the industrial application of soybean leaching technology, it is essential to combine theory with practice and control every factor that may affect the leaching effect in order to apply the leaching process effectively and ensure the leaching effect is guaranteed.
Our VIC Machinery engineers comprehensively considered the above-mentioned influencing factors and optimized the design when creating the leaching production line and equipment, maximizing the extraction of soybean oil for our customers. Contact us now to purchase!
