When the screw oil press is working, the oil is subjected to shearing and squeezing forces in the press chamber, with the rotation of the snail; the oil near the screw thread rotates with the snail. The free volume enclosed by the inner surface of the squeezer cage and the outer surface of the snail follows the forward direction of the snail, and the capacity changes from large to small. In this process, the oil material is squeezed, so that the oil is squeezed out from the oil material and discharged from the crevice of the pressing rod and the oil groove of the pressing ring. Because of the high oil output rate, energy-saving and labor-saving, and maintaining the natural characteristics of the oil, the screw press is a widely used oil extraction method at present.
1.Research status of screw oil press
1.1 Research status of foreign research
Foreign research on screw oil press technology has a history of more than 120 years. After a long time of accumulation of experience, related technologies have become increasingly mature. At present, foreign research on oil presses is more focused on industrial-grade oil presses. In addition to the research on specific models, the effects of pressing conditions such as the speed of the snails, the amount of material input and the screw pitch on the oil yield, the influence of the squeezing method on the physical and chemical indexes of the oil, and the rheological characteristics of the oil during the squeezing process are also studied.
Simo-Rosedowns in the United Kingdom is a well-known screw press manufacturer, this company used the spiral nitriding technology and the Tungsten-chromium-cobalt cemented carbide processing technology to process the inner surface of the machine and developed a screw press that can withstand higher pressures. Most of Simo-Rosedowns' screw oil press products perform water-cooling treatment on the snails to prevent the occurrence of harmful substances in the cake due to high temperature burning in the press chamber. The use of a double screw feeding device is conducive to forced feeding, increasing the feeding amount, and the pressing capacity reaches 40~65 t/d.
Johnston of American Vincent Company designed a counter-rotating parallel twin-screw press based on predecessors' research experience on twin-screw presses, using the principle of two intermeshed interrupted screw ridge screws . The double meshing screw mechanism has significant effects in reducing slippage and strengthening the ability of material to move forward. The toothed scraper fixed on the squeezing cage can be inserted into the spiral spacer to serve as agitation and prevent clogging.
In view of the variability of the oil cake permeability that occurs during the oil crushing process, Bargale explored the law of change between permeability and time. Assuming the one-dimensional flow of oil in the oil layer from top to bottom, through permeability tests, the empirical formula of permeability with time as a variable and the mathematical model of oil volume flow rate were established, and analytical solutions were obtained. Compared with Darcy's law of percolation and Terzaghi's theory of consolidation, Bargale's research on the permeability of the pressing process with variable characteristics is more suitable for actual pressing conditions.
1.2 Domestic research status
Our country's research on oil pressing with screw presses has a history of 70 years. From the initial small capacity models to small capacity, large capacity and intelligent models, the research on oil presses in China has become more systematic. With the rapid development of computer technology, research on the key mechanics of oil presses has been fruitful.
Li Shilong and others proposed a cold pressing structure model with double stage multi-stage pressing to address some key technical problems in the cold pressing process of double-low rapeseed peeled kernel. The 95-type screw press was tested and found that the cake was loose and the oil was difficult to advance in the press chamber during the pressing process. Based on the common structure of screw presses and the study of H.J. Lazerhorn et al , the structure of the press chamber was changed to a double stage multi-stage press, and a conical press ring was added between the segmental press screws to achieve several compression and relaxation in the compression chamber, which increased the press pressure in the main press section and improved the oil yield, and provided a reference for pressing oil seeds with low coarse fiber content.
The SSYZ50 twin-screw oil press with self-cleaning function was developed by Li Wenlin et al. to solve the problem of difficult transport of peeled rapeseed kernels in the press chamber.17 The SSYZ50 twin-screw oil press uses the principle of twin-screw pressing to improve the feeding capacity of the screw and avoid the problem of slippage caused by smooth peeled oil. The SSYZ50 model has a theoretical compression ratio of 23.0, and the actual production data shows that the moisture content of the cold pressed meal is 4% to 6%, the temperature of the cake is around 55°C, and the working capacity is around 45 t/d. This model is a good reference for improving capacity and reducing energy consumption. The development of screw oil presses in China has been very rapid, and many excellent models have been introduced since 2006, such as the LYZX series low-temperature screw oil press from Hubei Tianxing Grain and Oil Machinery Equipment Co. [The LYZX low-temperature screw pre-press has a longer press stroke and more thorough press and oil discharge by appropriately reducing the spindle speed and increasing the length-to-diameter ratio. The ZY338 screw press from COFCO ENGINEERING EQUIPMENT NANPI CO.,LTD. has a free-feeding structure, and the material is relatively loose and not easily blocked.
The SZX12X2 twin-screw oil press was designed by Li Shilong et al. to accommodate the pressing of oil seeds with high oil content, with two screws distributed in parallel up and down. The cage cavity of this machine is in the shape of an "8", which has a strong forward conveying capacity and opens new oil paths continuously during the pressing process, so that the oil stays in the chamber for a long time and is pressed more thoroughly. The results of the test on the prototype show that the machine is not only suitable for pressing pre-treated oil, but also for cold pressing of non-pre-treated oil. The capacity meets the design requirements, the energy consumption per unit is lower than that of the conventional model, and the residual oil rate of the dry cake can reach as low as 3.5%, achieving the effect of energy saving and high capacity.
Improving the efficiency of the oil press by adjusting the structure or parameters has always been the focus of research at home and abroad. In the actual design of the oil press, most of them are optimized based on the previous production experience, and there is a lack of mechanism research on the working process of the oil press. Zheng Xiao et al. integrated the variability and rheology of oil during pressing on the basis of the research of Mrema and Bargale. According to the finite difference method and the finite element method, the seepage field and the rheological field were alternately solved. The numerical solution obtained by the model is basically consistent with the measured value. Liu Rukuan et al. used the seepage model to calculate the oil production rate model, and obtained the relationship between the radius of the barrel and the oil production rate. After several times of changing the inner diameter of the barrel, the calculated oil output rate is compared with the actual test results, and the error is below 2.1%, which provides a theoretical basis for the improvement of the oil press in terms of data. The development of computer simulation software provides a new solution for the theoretical research of the oil press. Zhang Qiang et al.used EDEM software to simulate the working performance of the press chamber, and established a simulation model of peanut particles with the BPM model. The broken ratio and oil yield of the bonding bond are used to discuss the pros and cons of the press cavity.The three-factor and three-level orthogonal experiment was designed based on the three factors of snail speed, feeding amount and squeezing pitch. Analyzing the results of EDEM work simulation, it is found that among the three factors, the snail squeezing speed has the most significant impact on the work performance, and the peanut oil yield rate is 45.20% under the optimal working parameters obtained by the matrix analysis. Compared with the actual situation, the pressing model still has a certain error, and the analysis of the influencing factors is not comprehensive, but the research has a good reference for the future oil press mechanism analysis.
In the use process of the screw press, the screw surface of the screw is subjected to greater impact, so the strength, surface hardness and wear resistance of the screw have high requirements.Zhang Lin et al. took the pressed snails made of 20 steel as the substrate, cleaned the surface of the substrate with acetone and roughened it, and then sprayed the pressed snails with WC12%Co, a cermet coating material, by explosive spraying technology. . The outer surface of the sprayed snails was honed and polished for abrasion resistance test. The coating thickness on the surface of the snails after explosive spraying is 0.3 mm, and the carburization depth of the snails after traditional carburizing and quenching is 0.2 mm. The wear resistance test under the same test conditions shows that the service life of the pressed screw after WC-12%Co explosive spraying under the same conditions is 4.2 times that of the pressed screw after carburizing and quenching.
2. Problems existing in the development of screw oil pressing machinery
In recent years, our China has made certain progress in the research of low-temperature pressing technology of screw oil pressing machinery, squeezing screw and pressing chamber structure optimization, oil penetration model, oil pressing model, etc. But overall, there are still many problems and no systematic research methods have been formed. At this stage, the research of screw press mainly has the following 3 aspects.
2.1 Severe wear on key components
The screw is the most severely worn part of the screw press, as one of the most critical parts of the screw press, its pitch, screw groove tooth depth and other parameters design is more precise. When the screw press is working, the screw chamber is subjected to continuous friction, high pressure and high temperature;After a period of use, the screw parts of the screw press will appear surface wear and screw tooth fracture. The surface of the screw was examined and the wear surface was analyzed by SEM. It was found that the main wear failure forms of the screw were abrasive wear, fatigue wear and erosion wear.Nowadays, the wear of the screw of the screw press seriously restricts the efficiency and yield of the oil press.
2.2 Low pressing efficiency,poor oil quality
The pressing time and temperature of oilseeds in the chamber of screw press are important parameters to determine the pressing efficiency. Too long pressing time will reduce the oil pressing efficiency and yield, while short time will lead to high oil content and low overall oil yield of the cake produced by the oil pressing. The pressing temperature refers to the temperature of the oilseeds when it is squeezed in the press chamber, which affects the plasticity, oil viscosity and fluidity of the oil, and also affects the pressing efficiency. During the pressing process of the screw press, the oil is squeezed and rubbed strongly, causing the temperature in the press chamber to be too high, which will cause problems such as the denaturation of the protein contained in the oil and the destruction of unsaturated fatty acids, resulting in poor oil quality.
2.3 The study of microscopic mechanism is insufficient
At present, the design of oil press mainly relies on experience and experimental methods, and the research on microscopic mechanism is still very scarce. When the oil press is working, the oil in the machine is seepage field coupled with flow field, and the stress-strain is obviously nonlinear. It is difficult to provide a complete and reliable design basis through experiments because of the complexity of the internal flow field. At present, the numerical analysis and simulation of oil press mainly focus on the screw itself, but oil pressing is the process of interaction between the material fluid in the chamber and the screw. Due to the lack of specific theoretical guidance and reference, it is difficult to determine the parameters such as the speed of the screw press, the amount of material input and the screw pitch of the screw, so the phenomenon of incomplete material pressing during the pressing process of the oil press has restricted the development of the oil press machinery.
3. Suggestions for the development of Spiral oil press
3.1 Improve wear resistance of key components
For a long time, the wear and corrosion of the screw of the screw press appear on the surface of parts, but our design optimization focuses on strength, stiffness, fatigue strength and structure, ignoring the research on the surface performance of parts. The finite element analysis method is used to calculate the deformation and stress of the piecewise pressing screw, and the results of the finite element analysis are analyzed to determine the large deformation and stress part. Combined with the domestic actual situation, advanced surface engineering anti-wear technologies such as surfacing and cladding are used to strengthen the wear resistance of key positions and longer their service life.
3.2 Improve work performance
The thermal analysis of the inner and outer surfaces of the press cage and instantaneous simulation calculation can be carried out by ANSYS software. The simulation results of thermal analysis are compared with the actual temperature measured by the infrared thermometer to systematically explore the variation of the temperature of the press cage. Aiming at the problem of oil product decline caused by high temperature in the process of pressing, the technology of increasing water cooling inside the pressing screw can be used to strengthen the cooling capacity in the process of pressing. The length, lead, compression ratio and seam width of the pressing cage of the oil press are studied to improve the residual oil rate and the overall oil yield of the dry cake.
3.3 Strengthen the study of microscopic mechanism
It is necessary to increase the absorption of foreign theoretical research results and establish mathematical and physical models related to the pressing technology with the current advanced computer technology. Comprehensive consideration of fluid and solid on the change and influence of oil pressing process, fluid-solid coupling analysis, more real reflection of the press press process, the test results and computer simulation results are compared, fully understand the complete oil press process, make the study more systematic and reliable.
4. Conclusion
The development of screw oil press is an important support for cereals and oils industry in China. It is very important to fully understand screw oil press and its related technology. This text summarizes the research status of oil pressing machinery at home and abroad, analyzes the existing problems of oil pressing machinery at the present stage and puts forward some development suggestions. With the strong support of the state, the research on oil presses will inevitably rise to a higher level.
