1. The latest development of puffing technology
1.1 Density Control System
Density control is the most challenging aspect of expanded feed production, especially for aquafeed production, where sinking feed should sink in the desired manner. If the sinking feed floats on the surface of the water, it not only reduces the feed conversion rate, but also pollutes the environment as a waste of nutrients. The difficulty is even greater when using a "low-energy formula" sinking feed that produces a relatively low oil content. At present, it is commonly used in China to puff the raw materials and then granulate them.
Generally, formula adjustment and operating parameter adjustment can be adopted to control the product density, such as reducing the spindle speed, adding more steam and adding water, increasing the formula oil content, reducing the feed amount and enhancing the expansion chamber cooling. Some more powerful measures can also be taken. , such as:
- venting or decompression zone on the puffing chamber, which is commonly used by manufacturers of extruders (Wenger);
- increase the template opening rate or change the thickness of the template, reduce the pressure difference at the template;
- changing the structure of the spiral and puffing chamber;
- Adjust the formula, especially to reduce the amount of carbohydrates.
Although these measures have a role in controlling the degree of expansion, they are not sufficient to produce a sinking material in a controlled manner. As a result, Sprout-Matador has developed a new density control system for the production of aquatic materials, which can be called the most significant advancement in puffing technology in recent years.
In the extruder, the material is subjected to mechanical shearing and high temperature and high pressure. Due to the high pressure, the temperature does not reach the boiling point of the water, but when the material is extruded from the template, it enters the normal pressure, the boiling point appears, and the water forms a "flash". The material is expanded into a porous structure containing a large number of pores, causing a change in product density. The higher the carbohydrate content, the more pores are formed. High porosity means low density and the material can float in the water. For high oil products, the porous structure facilitates the absorption of sprayed grease (especially when vacuum sprayed) and forms a higher density product. However, it is more difficult to control the production of heavy oil and low fat.
The density system developed by Sprout-Matador uses pressurized cutting or post-die pressurization to maintain a certain positive pressure in the cutting chamber, as the boiling point of the water increases with pressure as the material enters from the expansion chamber. After the cutting chamber, the flashing can be reduced to control the expansion of the material. Since the starch molecules are solidified instantaneously in the cutting chamber, the material does not expand again after entering the normal pressure from the cutting chamber. The positive pressure of the cutting chamber is generally maintained at 0.3-2.0 bar. In this range, the pressurization has a significant effect on the product expansion, but the pressurization above 2.0 bar has little effect on the product expansion. For example, for sea bream, squid and squid feeds with medium fat content, the expansion degree is 50% (water boiling point 100 °C) under normal pressure cutting, and pressure cutting is used, and the positive pressure of 1 bar can reduce the expansion degree by 50%. That is to say, the product density is increased from about 440 g/l to 550 g/l. This technology is mainly used to produce difficult feeds, such as low-fat products, and it can be used without positive pressure on the oil products. Compared with other density control methods such as venting and decompression zones, this pressure cutting is unique in producing some challenging products:
- Accurate control of product density (±5g/l) is any other means;
- Compared with the model of the open exhaust port, the material can be increased by 25 to 50% in the production of low fat or high starch-based feed because the material is less disturbed during the entire expansion chamber;
- There is no need to control other expansion factors, such as spiral and puffing chamber structure, feed volume, etc., thereby reducing the need for manual operations;
- Since the degree of expansion is controlled outside the extruder, the operator only needs to monitor the visual quality of the product, and it is easier to control the applied positive pressure than to control the pressure of the expansion chamber.
Domestic extruders basically have no density control measures. They only make simple considerations in the configuration of spirals and puffing chambers. Some manufacturers imitate Wenger's exhaust devices, but they are less maneuverable due to some factors in technology and processing. Pressurized cutting is relatively easy to implement, the air pump can be used to maintain the required pressure, the discharge can be used to shut off the fan, and because the required pressure is not high, the sealing of the moving parts is not too big a problem. This technology is easy to realize from the aspects of equipment production, operation and use, and density control of puffed products. It is a new direction for the development of puffing technology.
1.2 quenching and technology
The quenching and tempering technology is an indispensable part before puffing. At present, the research on the tempering device is still deep in China, and there are products for both single-axis and double-axis. The quenching and tempering involved in this paper is actually pre-conditioning with waste steam. In principle, it is very simple, that is, the steam released from the puffed "flash" and the hot air at the front end of the cooler are sent back to the conditioner, thereby reducing fuel consumption and reducing CO2 and SO2 emissions, and at the same time, in the exhaust gas. The odorous substance is absorbed. Dust emissions are also reduced due to partial circulation of the cooling air. As a world-renowned extruder manufacturer, Kahl has developed an Eco-processor and has conducted detailed research on the production of expanded feed.
Since the environmentally-friendly conditioner uses the extruder/expander outlet steam and the hot air at the front of the cooler for pre-conditioning, the material temperature can be raised from 20 °C to 40 °C without steam addition, which usually takes 2%. Steam (20Kg / t product). In other words, it is equivalent to saving 1.2Kg of fuel per ton of product, and accordingly CO2 and SO2 emissions are also reduced (see Table 16). At the same time, the odorous substances in the exhaust gas are absorbed by the materials, and it is particularly important to reduce the odor emission in the vicinity of the residential area. Compared with ordinary granulation, expansion production can save 5 degrees per ton of product.
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