温度和粉碎粒度对不同能量饲料原料热物理特性的影响

Effect of temperature and particle size on thermophysical properties of different energy feedstuffs material

为探究常见能量饲料原料在不同温度、不同粉碎粒度下的热物理特性差异,该文以4种谷物(玉米、小麦、大麦和高粱)和4种加工副产品(小麦麸、木薯渣、甜菜渣和米糠)原料为研究对象,分别粉碎过孔径1.5、2.0和2.5 mm筛片,得到3种不同粉碎粒度的粉料,利用差示扫描量热法(differential scanning calorimetry,DSC)和线热源法分别测定了不同粉料在25～100℃范围内的比热和导热率,通过计算得到相应的导温系数,分析了温度、粉碎粒度对原料热特性的影响以及不同原料之间的热特性差异,并建立了热特性参数关于温度的回归预测模型。结果显示:8种原料粉碎过1.5 mm筛孔的粉料比热、导热率和导温系数随温度的升高分别在1.580～2.671 kJ/(kg·K),0.054～0.362 W/(m·K)和6.694×10^(-8)～23.254×10^(-8) m^2/s范围内变化。整体上,原料的热特性均随温度的升高而呈线性或非线性上升趋势。不同粉碎粒度的同一原料,在相同温度下的比热差异均不显著(P>0.05);随着粒度的增大,粉料的导热率和导温系数均有逐渐下降的趋势。4种谷物的比热与温度之间呈线性关系;小麦麸、木薯渣和米糠的比热可用温度的三次多项式表示;而甜菜渣的比热与温度呈二次关系。4种谷物和甜菜渣在3种粒度下的导热率均可用温度的三次多项式表示,而小麦麸、木薯渣和米糠的导热率则可用温度的二次多项式表示。小麦麸和米糠的导温系数与温度呈二次关系,其余6种原料的导温系数则可用温度的三次多项式表示。研究结果可为配合颗粒饲料配方变换所需的调质、制粒等热加工过程的工艺参数的调整、优化提供理论依据。

Energy feedstuffs,the most important and indispensable components in the pelleted complete feeds,provide energy in the form of soluble carbohydrate,oil and fat.Energy feedstuffs mainly includes cereals,cereal brans,molasses,oil,tuber and tuberous root,among which corn,wheat,barley,sorghum,wheat/rice bran,pelleted cassava residue and beet pulp are widely utilized in diets for livestock and poultry.Knowledge of the thermal properties of different feedstuffs is required for designing and parameter optimization of thermal process such as conditioning and pelleting processing,especially when energy costs,pellet quality and uniformity are the main consideration.The specific heat,thermal conductivity and thermal diffusivity are the most important thermal properties.In this paper,eight energy feed ingredients(4 cereal grains:corn,wheat,barley,sorghum;4 processed by-products:wheat bran,pelleted cassava residue,pelleted beet pulp,rice bran)were obtained from a feed mill in the suburb of Beijing,and ground in a mini-mill equipped with screens having a aperture size of 1.5,2.0 or 2.5 mm to obtain feed meals with 3 particle sizes(except rice bran,which could pass through the 1.5 mm screen completely without being ground).The moisture content of all feed meal samples was conditioned to 12%wet basis in order to eliminate the effect of moisture content difference on thermal properties.The specific heat,thermal conductivity of feed meals at the temperature range of 25-100℃were determined by DSC(differential scanning calorimetry)and KD2 Pro(thermal properties analyzer),respectively.Thermal diffusivity of feed meals was calculated using measured thermal conductivity,specific heat,and bulk density.Based on the experimental data,prediction models of three thermal properties as a function of temperature were established.The effects of particle size on thermal properties of feedstuffs were analyzed.The particle size of 8 feedstuffs grinding through screens with aperture size of 1.5,2.0 and 2.5 mm ranged from^257.61 to 511.79μm,325.65 to 594.83μm and 335.28 to 671.05μm,respectively,which increased significantly with the increase in mesh size.There were significant differences between the values of particle size of 8 feedstuffs through the same mesh size.The values of specific heat of 4 grain meals(passing a screen with aperture size of 1.5 mm)increased with temperature(25-100℃)and ranged from 1.678 to 2.421,1.644 to 2.402,1.642 to 2.385,and 1.649 to 2.441 kJ/(kg K),respectively.For wheat bran,cassava residue and beet pulp,the values were found to range from 1.621 to 2.540,1.580 to 2.195,1.746 to 2.351 kJ/(kg K).The specific heat of rice bran raised from 1.966 kJ/(kg K)at 25℃to 1.985 kJ/(kg K)at peak temperature(28.0℃),then declined to 1.868 kJ/(kg K)at end-set temperature(33.5℃),and finally increased to 2.671 kJ/(kg K)at 100℃.There was no significant effect of particle size on the specific heat of all feedstuffs(P>0.05).The specific heat followed linear relationships with temperature at the range of 25-100℃for 4 grains,and cubic polynomial relationships for wheat bran,cassava residue and rice bran,and a second order polynomial relationship for beet pulp.The values of thermal conductivity of 4 grain meals(passing a 1.5 mm screen)increased nonlinearly with temperature(25-100℃)and varied from 0.089 to 0.299,0.078 to 0.336,0.088 to 0.276 and 0.082 to 0.288 W/(m K),respectively.For 4 processed by-products,the thermal conductivity was found to raise from 0.054 to 0.190,0.092 to 0.362,0.085 to 0.263,and 0.064 to 0.202 W/(m K),respectively.Significant differences were observed between the thermal conductivity of 8 feed mills(passing a 1.5 mm screen)at a higher temperature(85 or 100℃),and the thermal conductivity of cassava residue was the highest,followed by wheat,wheat bran with the lowest value.For the same feedstuffs,the thermal conductivity of feed mills,at the same temperature,decreased generally with the increase in particle size,and the higher temperature,the stronger decrease.Cubic polynomial relationships of thermal conductivity with temperature(25-100℃)were established for 4 grains and beet pulp with different particle sizes,and quadratic polynomial relationships were obtained for wheat bran,cassava residue and rice bran.The values of thermal diffusivity of 4 grain meals(passing a 1.5 mm screen)varied from 7.459×10-8to 17.320×10-8,6.694×10-8 to 19.195×10-8,7.878×10-8 to 17.045×10-8,and 6.857×10-8 to 16.109×10-8 m^2/s at temperature range of 25-100℃,respectively.The values was observed to ascend from 9.543×10-8 to 20.892×10-8,8.217×10-8 to 23.254×10-8,7.258×10-8 to 16.646×10-8 and 8.092×10-8 to 18.718×10-8 m^2/s for 4 processed by-products,respectively.In general,the thermal diffusivity of the 8 energy feedstuffs had a slow increase with temperature at the range of 25-55℃,and a sharply raise with temperature at the range of 55-100℃.The thermal diffusivity of all 8 feedstuffs tended to decline with increasing particle size.The thermal diffusivity displayed second order polynomial relationship with temperature at the range of 25-100℃for wheat bran and rice bran,and cubic polynomial relationships for the other 6 feedstuffs.