【摘要】：菜籽油是我國主要的食用植物油之一,是世界上第三大食用植物油,僅次于大豆油和棕櫚油,并在全球的經(jīng)濟發(fā)展上具有非常重要的作用和地位,從而受到全世界各國的普遍重視。然而油菜籽在高溫高濕環(huán)境下特別容易生長霉菌,導(dǎo)致高水分油菜籽霉變,需盡可能在短時間內(nèi)將其干燥至安全儲藏水分的范圍內(nèi)。流化床干燥(熱空氣)可滿足油菜籽安全儲藏的要求,也能提高干燥效率和干燥品質(zhì),是油菜籽較理想的一種干燥技術(shù)。因此,本文結(jié)合大量的實驗和理論分析,深入系統(tǒng)地研究流化床干燥特性和氣-固兩相傳熱傳質(zhì)規(guī)律,確定適合于油菜籽流化床干燥的干燥動力學模型,優(yōu)化干燥工藝條件。并在此基礎(chǔ)上,通過理論分析、試驗研究和數(shù)值模擬,優(yōu)化設(shè)計布風板的開孔率、布孔方式和結(jié)構(gòu)尺寸,以確定最佳布風板的具體結(jié)構(gòu),進而揭示最佳布風板的氣-固傳熱傳質(zhì)規(guī)律,為新干燥設(shè)備的開發(fā),以及現(xiàn)有設(shè)備性能的改進提供理論依據(jù)。主要的研究內(nèi)容如下：(1)通過實驗深入研究了油菜籽初始含水率、熱空氣溫度和熱空氣流速3個影響因素對油菜籽流化床干燥過程的影響,研究結(jié)果表明：油菜籽初始含水率越大、熱空氣溫度越高和熱空氣流速越大,油菜籽流化床干燥速率、干燥熱效率、對流傳熱傳質(zhì)系數(shù)以及水分有效擴散系數(shù)越大：3個影響因素對干燥速率和平均對流傳熱傳質(zhì)系數(shù)的敏感性順序依次為：油菜籽初始水率熱空氣溫度熱空氣流速；對水分有效擴散系數(shù)的敏感性順序依次為：熱空氣溫度熱空氣流速油菜籽初始含水率。(2)采用目標函數(shù)法對油菜籽流化床干燥速率進行優(yōu)化,利用MATLAB7.0軟件的非線性規(guī)劃求解進行優(yōu)化計算,優(yōu)化結(jié)果發(fā)現(xiàn)3個影響因素的最優(yōu)水平組合為：最佳優(yōu)化工藝條件是油菜籽初始含水率為23.44%、熱空氣流速為2.25m/s和熱空氣溫度為65℃。(3)通過4種常見薄層干燥動力學模型對試驗數(shù)據(jù)進行非線性擬合,比較相關(guān)系數(shù)R2、卡方值X2和均方根誤差RMSE 3個評價指標,發(fā)現(xiàn)Page模型是描述油菜籽流化床干燥失水規(guī)律的最優(yōu)模型(R2=0.9993)。在此基礎(chǔ)上,通過試驗數(shù)據(jù)的擬合和響應(yīng)曲面分析,分別建立了模型參數(shù)n和k的二次多項回歸方程,從而進一步建立了油菜籽流化床干燥動力學模型,該模型能夠較好地預(yù)測油菜籽流化床干燥過程的失水規(guī)律,其試驗值與模型預(yù)測值的符合程度高,最大相對誤差僅為3.2%。(4)通過Design-Expert 8.0.6軟件對試驗數(shù)據(jù)進行響應(yīng)面分析,分別建立了平均對流傳熱傳質(zhì)系數(shù)和水分有效擴散系數(shù)的標準回歸模型,通過試驗驗證,采用直接誤差分析和成對雙樣本均值分析的t-檢驗分析試驗值與模型預(yù)測值的差異性,結(jié)果表明：平均對流傳熱傳質(zhì)系數(shù)以及水分有效擴散系數(shù)的試驗值與模型預(yù)測值的最大直接相對誤差僅為4.4%、3.3%和2.8%,且無差異概率分別為0.9238、0.8633和0.9126,都大于給定顯著水平0.05,表明這3個模型的預(yù)測值與試驗值沒有顯著性差異,即平均對流傳熱傳質(zhì)系數(shù)和水分有效擴散系數(shù)的標準回歸模型擬合較好,可靠性較高。(5)通過響應(yīng)曲面分析方法,對任何2個因素的交互效應(yīng)對平均對流傳熱傳質(zhì)系數(shù)和水分有效擴散系數(shù)的敏感性進行分析與評價,結(jié)果表明：初始含水率與熱空氣流速的交互作用對平均對流傳熱傳質(zhì)系數(shù)的影響最大,對水分有效擴散系數(shù)的影響最小;油菜籽初始含水率與熱空氣溫度的交互作用的影響次之;熱空氣流速與熱空氣溫度的交互作用對平均對流傳熱傳質(zhì)系數(shù)的影響最小,對水分有效擴散系數(shù)的影響最大：其中與平均對流傳質(zhì)系數(shù)的影響相比,各影響因素的交互作用對平均對流傳熱系數(shù)的影響稍小。(6)根據(jù)菲克第二定律,對試驗數(shù)據(jù)進行線性回歸擬合計算水分有效擴散系數(shù)和平均活化能,結(jié)果表明：水分有效擴散系數(shù)數(shù)量級為10-10～10-9 m2/s,分布在脫水食品的正常范圍內(nèi)(10-12～10-8m2/s)。油菜籽初始含水率在14.41%-29.72%范圍內(nèi)所對應(yīng)的水分有效擴散系數(shù)為6.485x10-10-10.133×10-10m2/s：熱空氣流速在1.75-2.25m/s范圍內(nèi)所對應(yīng)的水分有效擴散系數(shù)為7.296×10-10～9.525×10-10m2/s；熱空氣溫度在45～65℃范圍內(nèi)所對應(yīng)的水分有效擴散系數(shù)為5.269x 10-10～8.917x10-10 m2/s。油菜籽流化床干燥的平均活化能為22.84kJ/mol。(7)針對開孔率和布孔方式2個方面,優(yōu)化設(shè)計了5種布風板(12.27%開孔率正三角形布孔、14.10%開孔率正三角形布孔、15.84%開孔率正三角形布孔和15.84%開孔率正三角形不均勻布孔),通過試驗結(jié)果和數(shù)值模擬的結(jié)果進行對比分析,結(jié)果表明：與其他4種布風板相比,15.84%開孔率圓形不均勻布孔的布風板是油菜籽流化床干燥最佳的布風板,其不僅增強了熱空氣的流通性,減少了局部區(qū)域的熱空氣聚集,實現(xiàn)了氣固兩相流動的正常流態(tài)化,達到了流化床內(nèi)油菜籽顆粒均勻分布,無溝流和死區(qū)的目的。同時也提高了流化床層內(nèi)熱空氣溫度沿軸向和徑向分布的均勻性,擴大了徑向高溫區(qū)域的范圍,增加了熱空氣與油菜籽顆粒以及油菜籽顆粒間的傳熱傳質(zhì)效率,增大了恒速干燥對流傳熱傳質(zhì)系數(shù),從而提高了油菜籽流化床的干燥速率和干燥熱效率,縮短了油菜籽流化床的干燥時間,減小了所需的單位能耗。(8)針對最佳布風板(15.84%開孔率正三角形不均勻布孔),根據(jù)歐拉(Euler)方法的雙流體模型和標準k-ε湍流模型,采用計算流體力學(CFD)分析軟件Ansys Fluent 15.0進行數(shù)值模擬分析了流化床層氣-固傳熱傳質(zhì)規(guī)律,結(jié)果表明：在整個熱空氣流化床干燥油菜籽顆粒的過程中,熱空氣溫度發(fā)生了明顯的變化,其先緩慢地下降,而后迅速地降低,最后在整個流化床上部的熱空氣溫度趨于穩(wěn)定。在布風板區(qū)域附近,熱空氣溫度、熱空氣速度和油菜籽溫度顯著地增大并達到最大,遠離布風板后都急速下降,最后趨于穩(wěn)定,而表壓強則沿著流化床高度的增加而急劇下降最后趨于零。在整個流化床層內(nèi)氣-固熱量傳遞的主要推動力是氣-固流速差、溫度差和油菜籽顆粒體積分數(shù),其中在布風板附近起主要控制作用的影響因素是氣-固溫度差,在遠離近布風板位置,油菜籽顆粒體積分數(shù)和氣.固流速差是主要的影響因素；氣,固質(zhì)量傳遞的主要推動力是油菜籽顆粒體積分數(shù)、氣一固流速差和壓強差,其中在布風板區(qū)域內(nèi)起主要控制作用的影響因素是氣一固速度差,在遠離近布風板位置時,油菜籽顆粒體積分數(shù)和壓強差是質(zhì)量傳遞的主要控制因素。(9)結(jié)合數(shù)值模擬的結(jié)果和傳熱傳質(zhì)系數(shù)的經(jīng)驗公式,對最佳布風板傳熱傳質(zhì)系數(shù)的變化規(guī)律進行了分析,結(jié)果表明：沿著流化床高度(軸向),氣,固傳熱傳質(zhì)系數(shù)都呈現(xiàn)出先升高后逐漸降低再略有上升的趨勢：沿著流化床徑向,氣,固傳熱傳質(zhì)系數(shù)均呈現(xiàn)出中心處氣,固傳熱傳質(zhì)系數(shù)較高,且變化平坦,基本保持不變,而近流化床壁面的兩側(cè),都逐漸遞減。
[Abstract]:Rapeseed oil is one of the main edible vegetable oils in China, which is the third largest edible vegetable oil in the world, second only to the soybean oil and palm oil, and has a very important role and status in the global economic development, thus being widely valued by all the countries around the world. However, in that high-temperature and high-humidity environment, the rapeseed is particularly susceptible to the growth of mold, resulting in the mildew of high-moisture rapeseed, and it is necessary to dry it to a safe storage moisture in a short time as much as possible. The fluidized bed drying (hot air) can meet the requirements of the safe storage of the rapeseed, and also can improve the drying efficiency and the drying quality, and is an ideal drying technology for the rapeseed. In this paper, a large number of experiments and theoretical analysis are combined to study the drying characteristics of the fluidized bed and the heat and mass transfer law of gas-solid two-generation. The drying kinetic model suitable for the drying of the fluidized bed of rapeseed is determined, and the conditions of the drying process are optimized. On the basis of this, through the theoretical analysis, the test research and the numerical simulation, the opening ratio, the hole distribution method and the structure size of the air distribution plate are optimized, so as to determine the specific structure of the optimal air distribution plate, and then the air-solid heat transfer and mass transfer law of the optimal air distribution plate is disclosed, and the development of the new drying equipment is provided. And provides a theoretical basis for improving the performance of the existing equipment. The main contents of the research are as follows: (1) The effect of the initial water content, hot air temperature and hot air flow rate of the rapeseed on the drying process of the rapeseed fluidized bed is studied in detail. The results show that the higher the initial water content of rapeseed, The higher the hot air temperature and the higher the hot air flow rate, the higher the drying rate and the drying efficiency of the rapeseed, the greater the heat and mass transfer coefficient and the effective diffusion coefficient of the moisture. The more sensitive order of the three influencing factors to the drying rate and the average convective heat transfer coefficient is as follows: The hot air temperature and the hot air flow rate of the initial water of the rapeseed; and the sensitivity sequence for the effective diffusion coefficient of the moisture is the initial water content of the hot air temperature hot air flow rate rapeseed. and (2) optimizing the drying rate of the rapeseed fluidized bed by using a target function method, carrying out optimization calculation by using a non-linear programming solution of the MATLAB7.0 software, and finding that the optimal level combination of three influencing factors is as follows: the optimal optimization process condition is that the initial water content of the rapeseed is 23.44 percent, The hot air flow rate was 2.25 m/ s and the hot air temperature was 65 鈩，

本文編號：2499607