Pre-pressing solvent leaching and extrusion solvent leaching are two different processes in the oil production process, and they differ in many aspects.
1. Process principle
• Pre-pressing solvent leaching
• Pre-pressing leaching is to first press the oil and use mechanical pressure to squeeze out part of the oil in the oil. This process is mainly carried out through equipment such as screw presses. The oil is subjected to strong pressure in the pressing chamber, and the oil is squeezed out from the oil cells. For example, in the pre-pressing process of rapeseed, the rapeseed enters the pressing chamber of the screw press. As the pressing screw rotates, the pressure in the pressing chamber gradually increases, and the oil is squeezed out through the gap between the pressing bars to obtain pre-pressed crude oil, and the remaining cake with a higher oil content enters the leaching stage. Leaching is to extract the oil in the pre-pressed cake using organic solvents (such as hexane). Organic solvents can dissolve oil, and dissolve the oil from the cake by soaking or spraying, and then separate the solvent from the oil through evaporation, stripping and other processes to finally obtain leached crude oil.
• Extrusion solvent leaching
• The first step of the extrusion leaching process is to puff the oil. In the extruder, the oil changes its structure under the action of high temperature, high pressure and high shear force. For example, in the extruder, due to the increase in temperature and the extrusion of the screw, the oil cells are completely destroyed, the internal oil is released, and the volume of the oil increases, forming a loose and porous expanded material. The structure of this expanded material is conducive to the penetration of the solvent. In the subsequent leaching process, the solvent can contact the oil more quickly and evenly and dissolve it. The principle of leaching is the same as the leaching link in the pre-pressing leaching. It also uses organic solvents to extract oil from the expanded material, and then recovers the solvent through evaporation, stripping and other steps to obtain crude oil.
2. Process and equipment
• Pre-pressing solvent leaching(Main equipment:pre-pressing oil press machine)
• Pre-pressing link: The main equipment is the pre-pressing screw press with stem cooker. The oil enters the screw press after pre-treatment such as cleaning and crushing. The pressing chamber of the press consists of a screw and a bar. The screw rotates to push the oil forward, the pressure gradually increases, and the oil is squeezed out. The cake after pre-pressing is compact and has a relatively high oil content, generally around 10% - 20%.
• Extraction stage: The extraction equipment is usually a horizontal extractor or a ring extractor. After the pre-pressed cake enters the extractor, it is sprayed or soaked with a solvent. The mixture of solvent and oil (commonly known as mixed oil) is separated from the cake by filtering and other methods, and then passes through an evaporation system (including the first evaporator, the second evaporator, etc.), and the solvent in the mixed oil is evaporated by steam heating, and then the residual solvent is further removed through a stripping tower to obtain leached crude oil. At the same time, the cake after desolventization is discharged after the residual solvent is removed by equipment such as a degassing machine.
• Extrusion solvent extraction(Main equipment:extruder)
• Extrusion stage: The core equipment is the extruder. The oil enters the extruder after cleaning and conditioning. There are screws and barrels inside the extruder. Under the rotation of the screw and the heating of the barrel, the oil is squeezed and heated, and the pressure can reach 3-5MPa and the temperature can reach 100-130℃. After the oil is squeezed out of the outlet of the extruder, the volume expands instantly to form an extruded material. The extruded material has a loose structure, high porosity, and the oil content is equivalent to that of the pre-pressed cake, but the oil distribution is more uniform.
• Extraction stage: The equipment is similar to the extraction equipment for pre-pressing extraction, but due to the characteristics of the extruded material, the extraction process may be more efficient. After the extruded material enters the extractor, the solvent is easier to penetrate, and the quality of the mixed oil may be better. The subsequent solvent recovery and crude oil extraction process are similar to pre-pressing extraction.
3. Oil yield and oil quality
• Oil yield
• Pre-pressing extraction: In the pre-pressing stage, a part of the oil can be extracted first. Generally, the pre-pressing oil yield is about 70%-80%, and then the remaining oil is extracted by extraction. The overall oil yield is high, but if the oil content of the pre-pressed cake is not properly controlled, it may affect the oil yield efficiency of the leaching process. For example, if the oil content of the pre-pressed cake is too high, uneven solvent penetration may occur during leaching; if the oil content is too low, too much oil may be lost during the pre-pressing process.
• Extrusion leaching: Since the extrusion process can fully break the oil cells and release the oil more thoroughly, the solvent can extract the oil more effectively during the leaching process. Generally speaking, the oil yield of extrusion leaching can reach about 95% - 98%, which has certain advantages over pre-pressing leaching, especially for some oils with high oil content and difficult to destroy cell structure, such as soybeans, the improvement of oil yield is more obvious.
• Oil quality
• Pre-pressing leaching: The quality of pre-pressed crude oil is relatively poor and contains more impurities, such as phospholipids, free fatty acids, etc. This is because the pre-pressing process is mainly mechanical pressing, which cannot effectively remove these impurities. After leaching, the impurity content in the crude oil is still high, and a more complex refining process is required to obtain high-quality oil. However, some natural antioxidants in the pre-pressed crude oil may be relatively well preserved, which is beneficial to the oxidative stability of the oil.
• Extrusion extraction: The crude oil obtained by extrusion extraction is of relatively good quality. Because the extrusion process can denature impurities such as phospholipids to a certain extent, it is easier to separate from the oil during extraction. Moreover, due to the uniform structure of the extruded material, the distribution of impurities in the extracted crude oil is also relatively uniform, which is easier to handle during the refining process. The refined oil obtained has a light color, good flavor, and relatively low content of impurities such as phospholipids and free fatty acids.
4. Cake quality
• Pre-pressing solvent extraction
• The texture of the pre-pressed cake is relatively compact due to mechanical pressing. The degree of protein denaturation is relatively low, and when used as feed, the digestibility of protein may be affected to a certain extent. However, the shape of the pre-pressed cake is relatively complete, which is convenient during storage and transportation. For example, when producing soybean meal, the particles of pre-pressed soybean meal are larger, and they can be crushed and processed as needed in the feed formula.
• Extrusion solvent leaching
• The cake after extrusion leaching is the product of extrusion, with loose structure and high porosity. This structure makes the protein in the cake highly denatured, which is more conducive to animal digestion and absorption. In the feed industry, extruded cake has higher nutritional value, and due to its loose structure, it can be better mixed with other raw materials during feed processing. However, the density of extruded cake is relatively low, and it takes up more space during storage and transportation.
5. Energy consumption and production cost
• Energy consumption
• Pre-pressing leaching: The pre-pressing link needs to consume a certain amount of mechanical energy to drive the screw press for pressing, and the energy consumption is mainly used to provide pressure. The leaching link needs to heat the solvent and mixed oil, and the evaporation and stripping process also consumes a lot of steam energy. In general, the energy consumption of pre-pressing leaching is relatively high, especially in the connection process between the pre-pressing and leaching links. If it is not handled properly, it may lead to energy waste.
• Extrusion leaching: The extrusion link needs to consume higher electrical energy and thermal energy because the oil needs to reach a high temperature and high pressure state in the extruder. However, since puffed materials are more conducive to solvent penetration during the leaching process, the leaching time may be shortened, and the energy consumption of the leaching process may be reduced compared to pre-pressing leaching. From the perspective of the entire process, the total energy consumption of puffing leaching is equivalent to or slightly lower than that of pre-pressing leaching.
• Production cost
• Pre-pressing leaching: In terms of equipment cost, the investment in screw presses and leaching equipment is relatively large. Operating costs include labor costs, equipment maintenance costs, energy costs, etc. Since the process of pre-pressing leaching is relatively complex and there are many equipment, labor costs and equipment maintenance costs may be high.
• Puffing leaching: The price of the puffing machine is relatively high, which increases the equipment investment cost. However, from the perspective of long-term operation, due to the economic benefits brought by the possible reduction in energy consumption and higher oil yield, as well as the advantages of high-quality cakes in the feed market, its production cost can be balanced to a certain extent, and it may have more cost advantages in some large-scale production.
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