以分散板與銳孔流量計壓降探討B族粒子之最小流體化床速度

Abstract

流體化床擁有廣大的非勻相接觸面積，提供優異的質傳以及熱傳效率，因此被廣泛利用於工業製造。最小流體化床速度的決定對於流體化床是否良好運作佔有重要的地位。雖然前人已提出許多估計最小流體化床速度的經驗式，但多數僅考慮床質物性、流體黏度與密度。實際操作時，最小流體化床速度亦受到分散板設計、床高床徑比等的影響。本研究使用內徑11 cm的圓柱流化床，以四種分散板、五種床重，探討分散板以及床重對最小流體化床速度之影響。 使用不同分散板分析時，實驗結果所得的最小流體化床速度與經驗式預測結果不同。對於粒徑為595.7 μm的玻璃珠，經驗式高估最小流體化速度；對於粒徑為1059 μm的玻璃珠，經驗式低估最小流體化速度。分散板開孔率越大，經驗式與實驗所得最小流體化床速度差距越大。使用不同床重分析時，實驗所得最小流體化床速度並非隨著床重的增加而單調遞增或遞減。 除了可藉由流化床整體壓降隨流速變化曲線判斷最小流體化床速度外，吾人提出並以實驗驗證可藉由氣體通過分散板壓降隨流速變化曲線，或是由空氣通過系統上游的銳孔壓降隨流速變化曲線判斷最小流體化床速度。在不同實驗系統中，若以氣體通過上游銳孔壓降出現劇烈變化判斷最小流體化床速度，更具有便利性。

A fluidized bed has a large contact area between different phases, and high mass and heat transfer rates, leading to its wide applications in industry. Whether a fluidized bed operated efficiently depends on the accurate determination of the minimum fluidization velocity. Although there are several empirical equations for the minimum fluidization velocity prediction, most of them are only functions of the particle and fluid physical properties. In reality, the minimum fluidization velocity also depends on the design of the system, such as the distributor design and the bed aspect ratio. In the current study, experiments are performed in a cylindrical fluidized bed with a 11 cm inner diameter. Four different gas distributor designs and five different particle weights are employed to study their effects on the minimum fluidization velocity. The results show that there is difference between the empirical equation predicted minimum fluidization velocity and that from the experimental observation. The empirical equation presents a higher minimum fluidization velocity for glass beads having a diameter of 595.7 μm, and a lower minimum fluidization velocity for glass beads having a diameter of 1059 μm. For the design of the distributor, the higher the open area ratio is, the larger the difference between the empirical equation prediction and the experimental observed velocity. Except from the bed pressure drop versus the superficial gas velocity (Us) diagram, we propose and prove two new methods to determine the minimum fluidization velocity. The minimum fluidization velocity can be determined from the distributor pressure drop versus Us diagram, and from the upstream orifice pressure drop versus Us diagram. Among them, the upstream orifice pressure drop monitoring is a more convenient method to determine the minimum fluidization velocity.