粉塵餅過濾的壓力降和揮發損失

Abstract

Pressure drop and evaporation loss of dust cake during filtration have been st udies in this thesis. Three kinds of dust materials, fly ash, limestone and S AE fine dust, have been tested in a dust chamber and filtered through a Teflon filter at filtration velocity ranging from 1 to 9 cm/sec. Dust cake porosity and specific dust cake resistance coefficient have been investigated. It has been found that higher filtration velocity will result in larger amount of ir reversible dust cake compression and lesser amount of elastic dust cake compre ssion. This leads to higher packing density or lower porosity of the dust cak e for higher filtration velocity. The experimental specific dust cake resista nce coefficient, K2, is also found to increase with an increasing filtration v elocity. For the three dusts tested, K2 is found to relate to filtration velo city Vf as K2=fV^n, where n is 0.52, 0.38 and 0.43 for fly ash, limestone and SAE fi ne dust respectively.The theoretical K2 values predicted by the Rudnick-Happel equation are found to agree better with the experimental K2 values than those by the Kozeny-Carman equation. When the dust cake porosity is larger than 0. 6, the difference between the K2 values predicted by the Rudnick-Happel equati on and the experimental values is about 16 %, However, when the dust cake poro sity is small than 0.6, the difference becomes larger.In another study, the ev aporation loss of submicron, monodisperse ammonium nitrate particles during fi lter sampling when the upstream saturation ratio is zero has also been examine d. A theoretical model has been proposed which considers the upstream particl e concentration, the porosity of the particle bed, particle diameter, upstream temperature, pressure drop. According to the experimental results, upstream particle concentration heavily influences the evaporation loss of ammonium nit rate particles.Theoretical results agree reasonably well with the experimental data obtained in the laboratory under well controlled conditions. Results pr esented herein confirm that the simplified theory by Zhang and McMurry (1992) provides a reasonable yet somewhat lower collection efficiency of volatile spe cies during filter sampling owing to the assumptions in the downstream saturat ion ratio and pressure drop. 本論文探討在粉塵過濾過程中，流體流經粉塵餅的壓力降及揮發損失。本研究係利用飛灰 、碳酸鈣和SAE細粉塵於不同過濾速度下進行實驗，以觀察各粉塵餅的孔隙度、比粉塵餅 阻抗係數及粉塵餅的壓縮特性。根據實驗結果，當粉塵餅受到一個較高的過濾速度時，此 粉塵餅會有較大量的不可逆壓縮及少許的彈性壓縮發生。當粉塵餅被壓縮後，粉塵餅的孔 隙度會變小，而導致壓力上升、K2值變大。且比粉塵餅阻抗係數會隨著過濾速度的增加而 變大，兩者的關係可表示為K2=fV^n。飛灰、碳酸鈣和SAE細粉塵的指數n分別為0.52、0.3 8和0.43 。使用Rudnick-Happel模式預測K2理論值所得結果較Kozeny-Carman模式所預測 值接近實驗值，當粉塵餅的孔隙度大於時0.6，使用Rudnick-Happel模式預測K2理論值和 實驗值僅有16%的相差。但是當粉塵餅的孔隙度小於時0.6，模式預測K2值和實驗值會有較 大的差異。本研究亦探討在濾紙上游硝酸蒸氣飽和度為零的條件下，濾紙採樣過程中次微 米、單一粒徑硝酸銨微粒的揮發損失。理論模式中探討影響揮發損失的因素包含濾紙上游 的微粒濃度、微粒床的孔隙度、微粒粒徑、溫度及通過微粒床的壓降。研究結果顯示濾紙 上游的微粒濃度對硝酸銨微粒的揮發損失影響很大。理論模式的計算結果和從實驗室所獲 得的數據相吻合，結果可確認Zhang and McMurry(1992)所提出的理論在假設濾紙下游硝 酸蒸氣會達到飽和度及壓力降維持不變的簡化下會低估揮發性微粒的收集效率。