Study On The Whitening Mechanism And The Breakage Characteristics Of Rice Grains In A Horizontal Friction Type Mill

As one of the three major grain crops in China,total paddy yield accounts for one fifth of the total global production.Hence,improvement of paddy yield is benefit to developing grain processing and ensuring national food security,which has an important practical significance.Alternatively,rice processing is an important way to realize the transformation from paddy to commercial rice,but the breakage of rice grains is an importan t reason causing significant reduction in the quality of milled rice and restriction in the development of rice milling industry.Obviously,reduction in breakage of rice grains,which is a very tricky problem,need to be solved by clarifying the whitening mechanisms and revealing breakage characteristics.Unfortunately,there are a few experimental or numerical studies on the whitening mechanisms and breakage characteristics to date,which are not only helpful for providing theoretic reference for designin g and optimizing the rice mill,but also useful for providing an insight into how to solve the problem of breakage of rice during the milling,which has a great significance in science.Taking the horizontal rice mill as an example,this work is the first attempt to reveal the rice milling mechanism and clarity the breakage characteristics of the rice grains.First,the rubbing mechanism of rice mill was analyzed.The whitening pressures and whitening movements were described.The critical impact velocity corresponding the brittle and fatigue fracture for rice grains were deduced.Then,the motion of spherical particle characterizing rice grain in a horizontal rice mill under different static friction coefficients(i.e.,between rice grain and cylinder siev e wall and between rice grains)and structures of convex ribs(i.e.,including amount,height and inclination angle)was simulated using the discrete element method(DEM).The uniformity of motion and milling performance were defined.The effects of static fiction coefficients and structures of convex ribs on rice milling were discussed.Based on this,the milling mechanism was clarified.Next,DEM combining bonded particle model(BPM)was used to constructed rice model.The impact breakage of single rice grain was simulated.Breakage characteristics for single rice grain were revealed.Thereby,a development of a numerical methodology that captures material property dependent particle level dynamics through the combination particle replacement model(PRM)a nd discrete element method(DEM)for describing particle breakage in a horizontal rice mill was established.The critical impact velocity corresponding fatigue fracture was decided.Breakage characteristics of rice grains were analyzed during the rice mill ing.Structures of convex ribs fixed on the roller shaft were optimized.Pilot scale rice mill was tested by compatibility tests.Finally,the rice milling tests were conducted to analyze the influences of operation parameters,including the moisture content of rice grains,the speed of roller shaft and the weight applied on the outlet on the breakage rate,per unit mass milling power,whiteness,respectively.The detailed results and conclusions are given as follows.(1)The theory of rubbing rice milling.The continuum theory and the second Newton’s law can be used to analyze whitening strength and characteristics of rice milling,including axial motion,circular motion,residence time and collision energy.The fracture criterion of normal stress and von Mises,the critical velocities can be applied to estimate brittle or fatigue fracture for rice grains.The results are useful for providing theoretical basis to reveal the rice milling mechanisms and clarify the breakage characteristics of rice grains.(2)The change laws of whitening movement and milling performance.The results indicated that the static friction coefficient between particle and cylinder sieve wall mainly affects the axial motion,while the static friction coefficient between particles primarily i nfluences on the circular motion.The residence time is strongly affected by the uniformity of axial motion while the collision energy is significantly influenced by the uniformity of circular motion.The relationship between friction characteristics and milling performance can be described based on the method of polynomial fitting.The results are useful for providing essential references to analyze the effect of structure of convex ribs on flow behavior and milling performance.(3)Effects of structures of convex rib on rice milling.The results showed that the height,inclination angle and amount significantly affect the uniformity of axial motion and circular motion,residence time as well as collision energy,but the significance order from big to small of these factors is height,inclination angle and amount,respectively.The relationship between height of convex ribs and milling performance conforms to the exponential equation,while the relationship between inclination of convex ribs and milling performa nce or the relationship between amount of convex ribs and milling performance conforms to the linear equation.The results are useful for providing reference for designing and optimizing horizontal rice mills.(4)Breakage characteristics of single rice grain.The results indicated that according to the change in breakage extent of rice grain,breakage pattern can be divided into four modes.For the mode of local disintegration,the damage ratio and size ratio of the largest fragment change linearly with the impact velocity and moisture content,whereas for the mode of fragmentation the damage ratio and size ratio of the largest fragment vary exponentially with the impact velocity and moisture content.The relationship between moisture content and critical impac t velocity conforms to the exponential equation.The findings are useful for providing reference for the study on the impact breakage of other cereal grains and the revelation of breakage mechanism in a rice mill.(5)Breakage characteristics of group of rice grains.The results showed that rice grains always brittle fracture in the early stage of milling,while rice grains always fatigue fracture in the later stage of milling.Rice grains always brittle fracture in the front of the milling chamber,whereas rice grains always fatigue fracture in the back of the milling chamber.The best structure of convex ribs is the combination of height of 5 mm,inclination angle of 3° and amount of 4.The optimum parameter combination does applies to a pilot scale horizontal rice mill due to the great compatibility.The results is helpful for providing reference for designing and optimizing pilot scale horizontal rice mills.(6)Effects of operating parameters for pilot scale rice mill on qualities of milled rice.The results showed that compared to the speed of roller shaft,the moisture content mainly affects the breakage rate and the whiteness,and the weight applied on the outlet primarily affects per unit mass milling power.The best operation parameters for pilot scale hori zontal rice mill is the combination of the moisture content of 15.2 %,the speed of roller shaft of 680 rpm and the weight applied on the outlet of 13 N.Compared to the long-grain rice,pilot scale horizontal rice mill has a great compatibility for the short-grain or middle-grain rice.This work can not only provide the reference for the designing and optimizing industrial scale horizontal rice mills,which aims to improve the milling quality,but also lay the foundation for rice milling theory.