循環式流體化床中氣固流力行為之小波分析

Wavelet Analysis on the Hydrodynamics of Gas-Solid Flow in Circulating Fluidized Beds

楊東昱(Tung-Yu Yang)

指導教授 ： 呂理平

國立臺灣大學/工學院/化學工程學研究所/博士(2008年)

https://doi.org/10.6342/NTU.2008.00075

摘要

由於氣相與固相複雜之交互作用，循環式氣固流體化床(CFB)系統之氣固流經常表現出多尺度及非線性之動態特質。因此，以往學者利由於氣相與固相複雜之交互作用，循環式氣固流體化床(CFB)系統之氣固流經常表現出多尺度及非線性之動態特質。因此，以往學者利用壓力擾動或固體粒子含率擾動訊號之平均值、標準偏差等時均 (time-averaged)參數來探討其系統中流力行為，可能會造成某些訊息或時變(time-variant)特徵之遺漏，而無法獲得正確地分析結果。本論文利用具時頻分析能力之訊號處理工具，小波分析(wavelet analysis)，來探討CFB系統之氣固流力行為。
本論文研究主題包含：以壓力擾動訊號之統計與小波分析探討氣泡流體化至紊流流體化之流態變遷、L-閥中壓力擾動之小波分析、循環式流體化床中FCC (fluid catalytic cracking)觸媒固體粒子絮狀物流力行為之研究。
首先探討FCC觸媒(dp=78 microns, rp=1880 kg/m3)、玻璃珠(dp=159 microns, rp=2593 kg/m3)以及砂粒子(dp=194 microns, rp=2635 kg/m3)，在一內徑0.07 m、高2.2 m (CFB-A)以及一內徑0.108 m、高7 m (CFB-B)之CFB中，由氣泡流體化至紊流流體化之流態轉變機制。以絕對壓力探針為量測工具，擷取床內不同流態下之絕對壓力擾動(APF)訊號。進一步利用統計分析以及小波分析之多層解析度分析(multi-resolution analysis, MRA)界定出流態移轉速度Uc和Uk，並探討靜床高與絕對壓力探針之軸向高度位置對Uc和Uk之影響。統計分析之結果顯示，不同軸向高度位置下，Uc並無明顯之差異；而Uk在床層表面以上或距分散板太近之位置不容易被觀察到。在探針軸向高度與靜床高之比值，介於0.5到1.0之間是偵測Uk之最佳位置。Uk隨靜床高增加而增加，相反地，其隨軸向高度位置增加而減小。另一方面藉由小波分析將APF分解成微觀、中間以及巨觀三個不同尺度大小之子訊號，吾人提出ㄧ均相指數(homogeneous index, HI)來界定流態轉變，並在傳統統計方法無法得到Uc與Uk之量測位置，成功地求得Uc與Uk。同時，HI可明確地表徵出不同種類粒子流力行為之差異性。
其次，在CFB固體粒子回流系統，內徑0.08 m之L-閥中量測其APF訊號，並探討L-閥通氣量、通氣位置、上昇床氣速以及雙成份混合粒子組成比例(194-microns和937-microns砂)，對系統壓力擾動、L-閥中固體粒子回流量以及操作穩定性之影響。根據實驗觀察以及L-閥之APF訊號，吾人歸納出六種流型。其次，根據多尺度子訊號之能量分布顯示，微觀尺度子訊號之能量在APF訊號總能量中所佔比例甚小，幾乎不隨操作變數有顯著的變化，僅在低通氣量下降管中有較大之能量表現。增加L-閥通氣量以及增高通氣位置皆容易造成駐塞現象，以致於在下降管與L-閥中其APF之小波能量分布分別往中間與巨觀尺度偏移。在固定通氣量而逐漸增加上昇床氣速之情況下，APF巨觀尺度之能量有明顯下降之趨勢，顯示上昇床氣速之提高有助於砂粒子在系統中之流動。探討變數雙成份混合粒子比例發現，隨著小粒徑砂粒子比例之增加，L-閥中固體粒子回流量呈現先上升後下降之趨勢，其主要歸因於砂粒子與粒子間以及砂粒子與L-閥管壁間作用力改變所致。同時，L-閥之中間尺度子訊號之能量亦顯示先升後降之趨勢。
最後，探討紊流流體化至快速流體化中固體粒子絮狀物之流力行為。在一內徑0.108 m、高5.75 m之CFB中，以反射型光纖探針量測床內局部之FCC觸媒固體粒子含率(solids hold-up)擾動訊號。吾人藉由MRA，導入多尺度之概念提出ㄧ門檻法則(threshold criterion)來鑑識固體粒子絮狀物。進ㄧ步計算出床內固體粒子絮狀物之流力特性，其中包含絮狀物之顯現頻率、顯現時間分率、平均顯現時間以及絮狀物之平均固體粒子含率，在不同軸、徑向位置與流態之分布情形。與傳統門檻準則比較，小波門檻法則更適用於鑑識CFB床底部濃相區之絮狀物結構，且其同時表徵出固體粒子含率擾動訊號之時變特性，故能更準確地求得絮狀物之流力特性。

關鍵字

循環式流體化床 ； 多尺度 ； 小波分析

並列摘要

The gas-solids flow in the circulating fluidized bed (CFB) systems usually exhibits multi-scale and nonlinear dynamic behaviors conduced by the chaotic interactions between the void and the solid phases. These time-variant features may be neglected by the analytic method employing the time-averaged parameters, e.g. mean and standard deviation of the pressure fluctuation and the solids hold-up fluctuation signals measured in CFBs. A specific tool, wavelet analysis, taking account of the above problems was adopted in three subjects of this study.
The first subject, study of transition velocity from bubbling to turbulent fluidization by statistic and wavelet multi-resolution analysis (MRA) on absolute pressure fluctuations (APF), was carried out in three lab-scale CFBs by using FCC catalyst, glass beads and sand particles as bed materials. According to the standard deviation of APF, the effect of axial positions and static bed heights on transition velocities Uc and Uk was systematically investigated. An appropriate measuring interval of relative axial position was recommended to identify Uk and two correlations were proposed to predict Uc and Uk for APF measurement. By means of MRA of wavelet analysis, a redefined variable, homogeneous index HI, was successfully applied to determine Uc and Uk, even at the measuring positions where no Uk was identified by the standard deviation of APF. HI also demarcated the dynamic behaviors of Geldart group A (spent FCC catalyst) and group B (sand particles and glass beads) particles.
The second subject, MRA of wavelet transform on APF in an L-valve, was carried out in an 80 mm-i.d. L-valve with sand particles as bed material. APF signals, acquired from the center of the standpipe and the horizontal part of L-valve, were used to characterize the flow patterns of L-valve under different operational conditions. In terms of APF and virtual observation, six flow patterns were recognized in the system. Based on the aspect of MRA, the original APF signal were decomposed into three subsignals of different scales, i.e. micro-scale, meso-scale, and macro-scale. The energy contribution of multi-scale subsignals characterized the effects of operational parameters, including the aeration rate, aeration positions, riser gas velocity and compositions of binary particle mixture (194-microns and 937-microns sand particles), on the performance of L-valve in various flow patterns.
The third subject, MRA on identification and dynamics properties of clusters in a CFB, was carried out for gaining further insight about the complicated dynamics of gas-solids flow (clusters) in the turbulent, transition and fast fluidization flow regimes. A new wavelet-threshold criterion was developed to distinguish the cluster phase from the transient solids hold-up fluctuation signals measured at different axial and radial positions. An appropriate level of approximation subsignal was systematically specified as a threshold for cluster identification based on MRA. By the established threshold, the dynamic properties of clusters were determined under the turbulent, transition and fast fluidization flow regimes. The results also described the dynamic properties of clusters and flow patterns in the splash zone and the dense bottom region of CFB. Comparing the traditional threshold criterion, wavelet-threshold criterion was more suitable and accurate for cluster identification, because the time-variant feature and multi-scale behavior of the original solids hold-up fluctuation signals could be characterized by MRA.

並列關鍵字

circulating fluidized beds ； multi-scale ； wavelet analysis

參考文獻

Arena, U., Langeli, C.B., Cammarota, A., (1998). L-valve behaviour with solids of different size and density. Powder Technol. 98, 231-240.

Bai, D., Bi, H.T., Grace, J.R., (1997). Chaotic behavior of fluidized beds based on pressure and voidage fluctuations. AIChE J. 43, 1257-1361.

Bai, D., Shibuya, E., Nakagawa, N., Kato, K., (1996). Characterization of gas fluidization flow regimes using pressure fluctuations. Powder Technol. 87, 105-111.

Bi, H.T., (2007). A critical review of the complex pressure fluctuation phenomenon in gas-solids fluidized beds. Chem. Eng. Sci. 62, 3473-3493.

Bi, H.T., Ellis, N., Abba, I.A., Grace, J.R., (2000). A state-of-the-art review of gas-solid turbulent fluidization. Chem. Eng. Sci. 55, 4789-4825.