循環式流體化床中粒子速度之探討

Abstract

本研究利用內徑為0.108 m、高5.75 m，頂端接有內徑0.25 m、高1.5 m之擴大管的循環式流體化床體，使用粒子為平均粒徑78 microns、密度1880 kg/m3之FCC catalyst粒子，以光纖探針為量測工具。將光纖探針裝置在不同之軸向及徑向位置，以得到在不同氣體表觀速度及固體迴流量下，局部平均粒子速度以及局部粒子濃度之徑向分佈圖。在量測床中粒子速度時，先利用光纖探針測得上游以及下游之粒子光訊號，將兩組訊號經由cross-correlation function之運算以得到延遲時間。經由統計分析後，則可得到床中局部平均粒子速度。 由結果得知，平均粒子速度幾乎都是在管中心為最大值，隨著往管壁區域附近接近而逐漸減小，至管壁處則為最小值且可能為負值。代表粒子之平均通量，在管中央區域為向上運動，在管壁則可能為向下運動。 當操作在氣泡流體化床及紊流流體化床時，平均粒子速度會隨著量測高度增加而減小。當操作在紊流流體化床與快速流體化床間之過渡區、快速流體化床及核與環稀相氣體輸送時，平均粒子速度會隨著量測高度增加而增快。 當量測高度固定在35 cm與55 cm處而逐漸增大氣體表觀速度，而流態由氣泡流體化床轉變成紊流流體化床時，平均粒子速度會先增大；至流態為紊流流體化床與快速流體化床間之過渡區時則減小；之後再隨著氣體表觀速度增加而變大。當量測高度固定在75 cm處而逐漸增加氣體表觀速度，而流態由氣泡流體化床逐漸轉變成快速流體化床時，平均粒子速度隨之增大，至流態為核與環稀相氣體輸送時，平均粒子速度則減小。當量測高度固定在115 cm處，而流態由快速流體化床轉變為核與環稀相氣體輸送，平均粒子速度隨著氣體表觀速度增加而增大。

The local time-mean particle velocity profile in the different flow regimes and solid circulation rate was acquired by optical fiber probes which detected at the different axial and radial positions in a 0.108 m i.d., 5.75 m high with 0.108 m i.d., 5.75 m high expanded-top circulating fluidized bed for FCC catalyst (dp=78 microns, particle density =1880 kg/m3). Upstream and downstream light signals which reflect from particles were received by optical fiber probes and were used to compute delay time by cross-correlation function. Then the local time- mean particle velocity can be acquired by statistics. The results show that the local time-mean particle velocity was always the maximum value in the central region of the riser, then decreased toward the wall, even became negative value at the wall. It means particles move upward in the central region of the riser and a downward particle movement exists near the wall. When operated in the bubbling fluidized bed and the turbulent fluidized bed, the time-mean particle velocity decreased with measuring height. When operated in the fast fluidized bed and core-annulus dilute phase flow and the flow regime which between the turbulent fluidized bed and the fast fluidized bed, the time-mean particle velocity increased with measuring height. When measuring height was fixed at 35 cm and 55 cm and increased the superficial gas velocity, the time-mean particle velocity first increased with the superficial gas velocity, then decreased in the flow regime which between the turbulent fluidized bed and the fast fluidized bed, and finally the time-mean particle velocity increased with the superficial gas velocity again. When measuring height fixed at 75 cm and increased the superficial gas velocity, the time-mean particle velocity first increased with the superficial gas velocity, then decreased in the core-annulus dilute phase flow. And when measuring height was fixed at 115 cm and increased the superficial gas velocity, the time-mean particle velocity was increased with the superficial gas velocity.