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Advantages of twin-screw extruder in aquafeed application
According to relevant statistics, China's current annual demand for aquatic feed is about 30 million tons, but only one-third use industrial blended aquatic feed, and most of them are hard-particle aquaculture feeds produced by traditional granulation processes. With the development of aquaculture industry in the direction of scale, intensification and specialization, the requirements for aquafeeds are getting higher and higher. The combination of aquatic products produced by traditional granulators has poor stability in water, rapid sedimentation speed and easy to cause. Disadvantages such as loss of feed and water pollution have become less and less suitable for the development of modern aquaculture. Aquatic feed processed by extrusion technology can solve these problems better, and can produce ideal feed forms suitable for the development of modern aquaculture industry, such as floating materials, slow materials and sinking materials, and has great development space. It has a good market prospect in China.
At present, due to the limitations of the original production process conditions and equipment, China's aquatic feed producers only account for a small portion of extruded aquatic feeds, and mainly rely on single-screw extruders, mainly due to imported twin-screw extrusion. The investment cost of the extruder is too high, and there are still gaps in the technical performance and quality of the domestic twin-screw extruder, and there is also a lack of large-volume models for the characteristics of the feed industry, despite the extrusion from single screw and twin screw. The principle and structure analysis show that the twin-screw extruder has obvious advantages and better cost performance, but it is still much less effective than the food processing industry.
In recent years, with the introduction, digestion and absorption of foreign advanced technology by domestic enterprises, the production technology level of domestic twin-screw extruder has been continuously improved, especially for the production of aquatic feed, a special model of twin-screw extrusion is used, and single-screw extrusion is used. The investment in the extruder can also be selected as a twin-screw model, which has changed the concept that people used to avoid the use of twin-screw models due to high investment costs. In order to meet the needs of China's feed industry development and improve product quality, it is necessary to pay attention to and promote the application of twin-screw extrusion technology in feed processing, and to play a wider adaptability and product adaptability of twin-screw extruder. The advantages of wider, intrinsic product and appearance quality, higher output under the same power, better maturing and homogenization effect, simpler process operation, lighter wearing parts and lower production cost, so that feed processing enterprises When building or expanding special feed production lines such as aquatic feed and pet food, there are more good reasons to choose a twin-screw extruder.
1 extrusion principle
Extrusion processing is defined as the process of forcing the feed/food material to flow through a die under one or several process conditions (eg, agitation, heating, shearing) to shape and/or evaporate the material. (Rossenand Miller, 1973). In other words, the extrusion process combines several operations, including agitation, maturation, kneading, shearing, forming, and granulation. In today's feed and food industries, extrusion extruders are machines that have a spiral feature (i.e., a rotating screw with blades that are very tightly mounted in a cylinder to deliver fluid) to continuously process a product. The extrusion extruder can be designed to include various operations such as pulverization, agitation, homogenization, maturation, cooling, vacuuming, forming, and cutting. With the wide application and development of extrusion technology, the types of extrusion equipment are increasing, and the classification methods are also diverse. It can be divided into high shear, medium shear and low shear extrusion extruder according to the degree of shear strength. It can be divided into self-heating and external heating according to heating mode or dry and wet extrusion extruder. The most common and typical classification is to divide the wet extrusion extruder into a single screw extrusion extruder and a twin screw extrusion extruder according to the number of screws.
2 single screw extrusion extruder
The single-screw extruder has a simple structure, usually adopts a belt transmission mode, and the spindle rotation speed is constant and not adjustable, and the transmission efficiency is low and the process control is difficult. The screw of the single-screw extruder is composed of a shaft connecting the screw units of various structures. The entire screw consists of three sections: a feed section, a kneading section or a melting section and a forming section. After the material enters the barrel from the feeding port, it undergoes a solid conveying and melting process in the screw to change the material from a loose state to a continuous plastic dough shape.
The single-screw extruder is mainly a device that performs the transportation by dragging the flow. Since the structural characteristics of the single screw determine the friction between the barrel and the material when the material advances in the barrel, the greater the friction, the higher the efficiency of forward advancement, but the friction between the material and the screw. The larger the block, the blockage of the above-mentioned flow will cause the material to stick to the screw. If the material adheres to the screw, it cannot be transported forward to cause blockage. When the moisture and oil content of the material is higher, this kind of material The more obvious the trend, the more limited the processing of low-viscosity materials, especially high-fat raw materials, in single-screw extruders.
The single-screw extruder has a poor conveying capacity under high pressure conditions, and its processing output is also low compared to the twin-screw extruder of the same power. This is mainly because the conveying method is sensitive to high pressure, the pressure will generate reflux, and the conveying efficiency is lowered. Since the delivery amount is equal to the tow flow minus the pressure flow, the high pressure often causes a decrease in the total production. The majority of the energy of a single-screw extruder is derived from the external heating conducted by the barrel and the mechanical energy conversion heat generated by the shear force when the screw rotates. The mechanical energy is determined by the screw speed and the screw structure. Since the shear rate is proportional to the screw speed, for a given extrusion extruder, the screw structure is generally pre-configured and the possibility of modification is small. For large single-screw extruders, heat exchange and increased throughput and pressure increase will become more difficult, as the larger the size of the extruder, the smaller the surface area to volume ratio of the material. Therefore, for larger single-screw extruders, the larger the size of the screw and barrel, the smaller the heat transfer area per unit volume of the barrel, resulting in the formation of material transport, heat transfer and pressure. difficult.
The poor homogenization effect of the material of the single-screw extruder is the biggest defect. The most difficult part is the improvement of the mixing performance. In order to improve the mixing and homogenization effect, the spiral of the screw must be The structure of the barrel is made up of some special designs. Good mixing requires the material to flip frequently during the extrusion process. In addition to leakage current, the structure of the single-screw extruder has greatly limited the mixing of materials. Some extrusion extruders are inserted into the screw structure by special throttling elements to improve their mixing characteristics, but the effect is extremely limited. . Since these mixing elements also cause a large pressure drop, the poor pumping characteristics of the single screw extruder also limit the length of these mixing elements.
3 twin screw extrusion extruder
The twin-screw extruder is developed on the basis of a single-screw extruder. In the barrel of the twin-screw extruder, two screws are placed side by side, according to the relative position of the screw and the rotation according to the screw. Engagement in the same direction of rotation, non-engagement relative rotation, meshing in the same direction of rotation, and meshing relative rotation. The fully meshed co-rotating twin-screw extruder has greatly improved material handling, pumping and mixing characteristics. Although the twin-screw extruder is also a towed flow machine, it has an additional function of a positive pressure pump because its two screws rotate in the same direction and are fully engaged. This allows the twin-screw extruder to be used to transport highly viscous materials as well as low-viscosity materials for high-viscosity materials, low-viscosity materials or high-moisture, high-fat formulations. At the same time, it has a self-cleaning function, so after the feed processing and production, the machine will automatically push the internal residual materials out of the machine, without having to remove them one by one to clean up, saving time.
In the co-rotating twin-screw extruder, the nature of the co-rotating twin-screw pressure zone is different. The material in the sleeve cavity is rotated by the screw to produce a high-pressure zone and a low-pressure zone. Obviously, the material will follow two directions. It flows from the high pressure zone to the low pressure zone. One is to form two left and right C-shaped material flows along the inner wall of the sleeve in the direction of rotation of the screw, which is the mainstream of the material, and the other is to form a counterflow through the gap of the meshing portion of the screw. The reason for the countercurrent is that the left screw pulls the material into the meshing gap, and the right screw pulls the material out of the gap. As a result, the material advances in a "∞" shape, and the spiral edge produces both forward conveying and material change direction. This not only contributes to the mixing and homogenization of the materials, but also provides better mixing and heat exchange, and also causes grinding (ie shearing) and rolling action between the slots of the screw, and the calendering effect occurs. The reverse screw calendering effect is much smaller. Of course, the calendering effect is small, and the wear of the material on the screw is also reduced. The material is conveyed, sheared, mixed, and heated by the casing of the barrel, and matured under the action of high temperature and high pressure, and finally extruded outside the cylinder.
The source of heat required for the material, except for the same portion as the single screw, is mostly from the meshing gap. The heat is generated by the shearing, squeezing, and mixing of the engaged threads to homogenize the heat. The size of the gap has a great influence on the quality of the extruded puff, the gap is small, the shearing force is large, but the amount of material passing through is reduced. The gap is large, the amount of material passed increases, but the shear force decreases. The twin-screw forced delivery and self-cleaning properties make the material residence time in the barrel short and uniform. The good mixing performance of the twin-screw makes the heat obtained by the material uniform in time, accelerates the degree of ripening of the material, reduces the fluctuation of the material temperature, and improves the output and quality of the extruded product.
The combined screw structure and the segmented barrel can meet various special processing requirements and can be combined into twin-screw extruders of various structural forms. Each of the components that make up the screw can be designed to have specialized functions of conveying, kneading, mixing, shearing, and increasing pressure. The reasonable combination of these components along the length of the screw allows precise control of the processing conditions, providing the system with a variety of advantages. The shear ring (shear lock) is the most important mixing element that makes up the screw member and is used to input the added mechanical energy into the product. Different mixing elements will affect the diffusion and mixing distribution of the material. The type of shear ring (shear lock) will depend on the requirements of the final product. A narrow shear ring is used to maximize the flow separation per unit length, while a wide disc shear ring is used for high shear and diffusion mixing.
The twin screws are kneaded and rubbed against each other to make the energy consumption lower than that of the single screw. In this way, the material can be kneaded and matured more fully, the overall degree of ripening can reach more than 95%, the stability of the product in water can be maintained above 24h, and the surface smoothness and particle uniformity of the particles are also better. After the extruded puffed feed is fully matured, the animal not only tastes good, but also has high digestibility and easy nutrient absorption. Therefore, when purchasing an extrusion extruder, it must be considered whether the maturity of the material can reach more than 90%.
The biggest feature of a twin-screw extruder is its ability to mix various materials without regard to their composition. Therefore, such diverse components as grains, soy flour, animal meal, fresh meat, fat, nutrients, protein, vitamins, starch, and wet fish residue can be easily processed. Therefore, the twin-screw extruder has a large variety of processing feeds, and can be adapted to the processing of highly viscous materials, low-viscosity materials or high-moisture, high-fat formula products. In the aspect of aquatic feed, such as fish, shrimp feed, river otter, high-grade ornamental fish feed, etc., it is necessary to use twin-screw extruder, especially particulate aquatic feed (0.5-1.5 mm in diameter), high-fat aquatic feed and production. Feeds that are small in size but often change in formulation must be produced using a twin-screw extruder. In pet food processing, the main components of pet food are generally composed of cereal flour (such as wheat flour) and higher protein content of animal powder, fresh meat and fat. In the processing of dry pet food, when the fresh meat content is as high as 20% to 30%, it is necessary to strongly mix and knead when extruding, which can only be completed by a twin-screw extruder. Because the twin-screws are mutually twisted and squeezed, they will not stick together and affect the operation of the machine even in the case of high-oil grease. Compared with the single-screw extruder, it can be added in the twin-screw extruder. Almost twice the amount of fat into the pet food formula, so the formula contains more than 20% oil. This not only improves the quality of the formulation, but also slows down the wear and resistance of the parts and reduces production costs. This is not possible with a single-screw extruder. In a single-screw extruder, the fat content is limited because if the fat content is too high, the extruder will fluctuate drastically and thus lead to product quality. Unstable. Once the oil content in the material is too high, the material will stick and the machine will not operate. In addition, the twin-screw extruder can complete the change of raw materials in 30s, which can be used for color identification and allows products of multi-color mixture to be processed directly in the extrusion extruder.
The twin-screw extrusion extruder adopts the direct-coupled variable-frequency motor transmission mode. The spindle rotation speed can be adjusted according to the requirements of the processed products. The product variety can be replaced or the floating water and submerged water feed can be produced without changing the screw parameter setting without stopping the machine. The operation is particularly convenient and results in a completely satisfactory result.
4 advantages of twin-screw extruder
(1) The material is more adaptable and can be adapted to high viscosity, low viscosity, high oil content, high moisture or viscous, oily, very wet and other raw materials that can slip in single screw (SSE).
(2) The particle size limitation of the raw material is less, and it can be adapted to the processing of raw materials having a wide particle size or a raw material from the fine powder to the coarse powder particles and a particle size outside the specific range of the single screw processing.
(3) The material flow is more uniform in the barrel, which can be more accurately scaled and achieve the actual demand effect regardless of the addition of steam, water and other auxiliary processing.
(4) The product has better internal and appearance quality, can achieve very good homogenization state and make the material molecular structure uniform, the surface is smooth during the extrusion process, the product particles are uniform and the uniformity is good.
(5) The ripening and homogenization effect is better. Generally, the starch ripening degree can reach more than 95%, so that the processed aquatic feed can be kept stable in water and the nutrients of the product are not lost and easily digested and absorbed.
(6) The output is higher under the same power, and the good mixing performance makes the heat obtained by the material uniform in time, accelerates the maturity of the material, reduces the fluctuation of the material temperature, and improves the output of the extruded product.
(7) Product diversity and adaptability, can process micro-aquatic feed, high oil formula, high moisture, high adhesion products and multi-color, sandwich, special shape products.
(8) The process operation is simpler, and the spindle speed can be adjusted according to the needs of the processed products. Due to the self-cleaning characteristics, the cleaning is very convenient, and there is no need to disassemble the equipment after each processing.
(9) The wearing parts are lighter in wear, and usually a misconception is that the wear of the single screw is small. In fact, due to the stable material conveying and material flow characteristics during the twin-screw extrusion process, the material is determined to be the screw and the barrel. The wear of the inner sleeve is smaller than that of the single screw. Although it is one more than the screw, the cost of the accessory is still lower than that of the single screw.
(10) The production cost is lower. Due to the good operational stability of the twin-screw model, the feed processing cost is less, the water and gas waste is less, the working time is less, the heat transfer efficiency is high, the yield is high, and the power is high. The high production index and the low cost of accessories also result in a much lower final production cost than a single screw.
5 twin-screw extrusion extruder is more cost-effective
So far, 70% of the aquatic feed in the United States and 100% of the aquatic feed in South Korea have been extruded and processed. In China, more and more feed processing companies are gradually replacing the traditional granulator with aquatic granulators to produce aquatic feed. Other special feeds. In the current Chinese aquatic feed market, since most aquatic feed production enterprises are limited by investment funds, domestically produced single-screw extruders with lower prices are usually used, but because of their simple structure and technical level, the manufacturing level is not high enough. It is difficult to produce submerged water and other high-grade aquatic feed products in the production of lower grade freshwater fish floating water in aquafeed processing. Some imported single-screw models (such as WENGER products) can meet the production of high-grade submerged feed products, but their prices are too high, and there is no good cost performance. We choose extrusion equipment not only to see the price of twin-screw extruders higher than single-screw extruders, but also to consider the overall cost performance and cost per ton of production. The technical performance and manufacturing level of domestic high-quality twin-screw extruder have reached a certain advanced level. Although the price is higher than the domestic single-screw extruder, the price is much lower than that of the imported single-screw model. It has surpassed the imported single-screw model and can completely replace the imported twin-screw model. It can not only meet the demand of high-grade aquatic feed in the breeding industry, but also achieve high cost performance. It should be the best choice for the current aquatic feed processing industry. With the advent of large-size, large-volume and special-purpose twin-screw models, the use of twin-screw extruders can achieve greater efficiency, and can fully utilize the wide range of raw materials used in extrusion and expansion processing technology. The degree of gelatinization is high, the stability in water is much higher than that of ordinary pellet feed, the density of the product can be adjusted to process floating water, slow, fast sinking and high moisture, high fat feed. The loose structure of the pellet is beneficial to the post-spraying technology. Avoid the loss of heat-sensitive raw materials, prevent pollution of the environment and prevent disease and disease, improve the nutritional value of feed, reduce production costs and other unique advantages.