气固流化床中声发生机理及在工业装置中的应用

Frequency analysis of acoustic emission and applicati on in gas-solid fluidized bed

利用声测量技术,结合频谱分析,建立了颗粒碰撞的声波频率模型,可定量描述声波主频随颗粒粒径、弹性模量和密度的变化规律.通过改变流化颗粒的粒径、弹性模量参数和密度,发现声波主频与频率模型计算值之间的最大偏差为8.3%,说明声波主频可以代表颗粒在壁面的碰撞频率.讨论了热态和冷态条件下声波主频之间变化规律,通过对弹性模量参数的校正,声测量技术可以用于预测工业装置中物料的平均粒径变化,并将该模型应用于线性低密度聚乙烯、高密度聚乙烯和双峰聚乙烯工业生产装置中的平均粒径测量,发现与传统的取样筛分方法所得测量结果十分接近.同时,发现当系统产生聚合物颗粒结块时,声波主频将急剧降低,声波频谱的能量分布将明显集中增大,这可作为判断流化床稳定运行的一个判据.

A model of signal main-frequency measurement was presented for the gas-solid fluidized bed to represent the quantitative relation of main-frequency and particle size, temperature, Young＇s modulus and density. The acoustic emission （AE） measurement was completely non-intrusive and not affected by the background noises, and caused to human harm. The experiment based on AE measurement was performed by using a fast Fourier transform （FFT） analyzer in this study. It was found the signals main-frequency decreased with the increasing particle size, temperature, Young＇s modulus and density. The experiment in the fluidized bed with i. d. of 150 mm showed that satisfactory agreement was obtained between calculated and experimental data with respect of the main-frequency （LLDPE, HDPE and bimodal polyethylene）, and the average errors were all less than 8. 3 %. The scale-up feasibility of the technical process was discussed by adjusting the elastic partameter. When the AE signals main-frequency model was applied to the industrial fluidized bed of LLDPE, HDPE, bimodal polyethylene, the mean errors between calculated and experimental data were 3.9%, 8. 2% and 8. 8% respectively. By the criterion of the sharp decrease of main-frequency and the sharp increase of amplitude, a novel approach could be developed to judge whether the fluidization runs stably, and it could replace the γ-ray measurement in the future.