Transmission Fluctuation Spectrometry in Concentrated Suspensions. Part Three: Measurements

The theory of transmission fluctuation spectrometry (TFS) was developed for particle size analysis in flowing particle suspensions, whereby the statistical transmission fluctuations are used to extract the particle size distribution (PSD) and particle concentration. In the previous parts of this publication high concentration effects on TFS were investigated theoretically and by simulation. This work presents a study on TFS measurements in concentrated suspensions. By introducing an empirical correction to include the high concentration effects from both the monolayer structure and particle overlapping in the inversion algorithm, it is possible to obtain the particle size distribution and particle concentration over broad ranges of particle sizes and concentrations.     

Experimental Study on Particle Size Distribution and Concentration Using Transmission Fluctuation Spectrometry with the Autocorrelation Technique

Based on the statistical characteristics of the transmission fluctuations in the particle suspension, transmission fluctuation spectrometry with autocorrelation (TFS‐AC) is described theoretically, with the assumptions of geometric ray propagation and completely absorbent particles in the suspension. The experiments presented here are realized in a focused Gaussian beam with the TFS‐AC technique. The acquisition of transmission fluctuation signals is achieved by using a high‐resolution digital oscilloscope. The transition function of TFS‐AC is obtained by varying the autocorrelation time. With a modified iterative Chahine inversion algorithm, solving a linear equation retrieves information on the particle size distribution and particle concentration. Some experimental results on spherical and non‐spherical particles are presented and discussed. The experiments cover a particle size range from 1μm to 1000 μm and a particle concentration of up to 12 %.     

基于二阶滤波器的消光起伏谱颗粒测量结果

消光起伏频谱法是一种新的测量两相流系统中颗粒粒径分布和浓度的方法,装置简单,操作方便,适合实时、在线测量。采用二阶低通滤波器对起伏的透射率信号分析,得到消光起伏频谱实验数据,并利用改进的Chahine循环方法计算得到颗粒的粒径分布和浓度信息。重点讨论高浓度情况,包括对特征函数频率响应的修正和对其阶高修正两个方面,得到修正参量并运用到反演算法中从而得到正确的测量结果。测量结果表明,通过高浓度修正,消光起伏频谱法可以在很大的颗粒浓度动态范围得到合理的测量结果,其可测颗粒最大体积分数视颗粒的大小而定。     

Locally Resolving Measurements of PSD,Particle Concentration and Particle Velocity by Means of TFS with Dual Beams

Transmission fluctuation spectrometry with spatial correlation (TFS‐SC) is based on transmitting two beams of radiation through a flowing suspension, whereby the distance of the beam centers is varied. Thus, the spatial correlation of the transmission fluctuations of the two beams is determined as a function of the beam distance. By numerical modeling, the transition functions of the correlation are found as a function of beam distance, beam diameters, particle diameter and beam intersection angle. Experimentally determined spatial correlation spectra can be inverted to obtain the particle concentration and particle size distribution by using the theoretical transition functions for mono‐sized particles. In addition, time correlations can be used to extract information on particle velocity. Some experimental results obtained by TFS‐SC are presented and discussed. This method appears promising for application in the local resolving of measurements of PSD, particle concentration and particle velocity in two‐phase flows, both in the laboratory and in process control.     

消光起伏自相关频谱法颗粒测量技术

消光起伏相关频谱法(TFCS)是一种新的颗粒测量方法。采用一束窄光束照射两相流系统,照射区中颗粒浓度的起伏所导致的透射光起伏信号中包含了颗粒的粒径和浓度信息,对光束的透过率信号作相关处理得到消光起伏相关频谱,可用来同时测量两相流中颗粒的粒径分布和体积浓度。由于在测量原理和结构上非常简单,这种方法可用来实现在线、实时测量。本文介绍消光起伏自相关频谱的测量原理,并给出部分实测结果。     

消光起伏光谱的时间相关处理

消光起伏光谱法(TFS)是一种新的颗粒测量方法,可同时测量两相流中颗粒的粒径分布和体积浓度。由于在测量原理和结构上非常简单,这种方法可用来实现在线、实时测量。然而在实际测量中,消光起伏光谱法对颗粒粒径分布的分辨率还比较低且对高浓度颗粒系的测量须考虑颗粒相互作用效应。本文提出一种新的数据处理方法-消光起伏相关光谱法(TFCS),通过对消光起伏信号在不同相关时间参数下进行相关计算来得到消光起伏光谱以提高消光起伏法对颗粒粒径分布的分辨率。     

Particle Size Analysis by Transmission Fluctuation Spectrometry: Experimental Results Obtained with a Gaussian Beam and Analog Signal Processing

Transmission fluctuation spectrometry (TFS) is a method for the analysis of particle size distributions based on the statistical fluctuations of a transmission signal. Complete information on the PSD and particle concentration can be retrieved by a special transformation of the transmission signal, whereby the expectancy of the transmission square (ETS) is determined after the signal has been subjected to a procedure of spatial and temporal averaging. By varying the averaging parameters over a wide range, a spectrum of ETSs is obtained and introduced into a linear equation system, which yields the PSD. In the experimental realization presented here, variable temporal averaging is realized in the frequency domain with a series of low pass filters at different cutoff frequencies while spatial averaging inevitably occurs as the particles pass through a focused Gaussian beam of finite cross section. Experimental results on spherical particles (glass beads) and non‐spherical particles (SiC) are presented. The PSDs are resolved in 30 intervals within a particle size range from 1–1000 μm, employing a modified Chahine inversion algorithm. So far, the measurements are limited to moderate particle concentrations. Some influences affecting the measurements, especially for higher particle concentrations, are discussed in detail.     

Transmission Fluctuation Spectrometry in Concentrated Suspensions Part One: Effects of the Monolayer Structure

The theory of transmission fluctuation spectrometry (TFS) was recently developed for particle size analysis in dilute flowing particle suspensions, whereby the statistical transmission fluctuations are used to extract the particle size distribution and particle concentration.     

Transmission Fluctuation Spectrometry in Concentrated Suspensions Part Two: Particle Overlapping

Transmission fluctuations measured on a flowing suspension of particles with a high spatial and temporal resolution can be used to measure the particle size distribution and particle concentration. The theory of transmission fluctuation spectrometry (TFS) was recently developed, whereby the statistical behavior of the entire suspension is described on the basis of a single monolayer, in combination with a layer model describing the suspension as a series of independent monolayers. As the monolayers are assumed to be statistically independent from each other, the transmission through the 3‐dimensional suspension is modeled as the product of transmissions through the monolayers.     

基于CCD传感的激光粒度测试技术研究

颗粒测试在工业生产和科学研究中涉及的领域非常广泛,常用的颗粒粒度及其分布的测试方法是激光粒度测试法,其具有测试精度高、测量速度快、重复性好和可测粒径范围宽等突出优点。CCD传感器有灵敏度高、分辨率高、噪声小和较大的动态范围等优点,其作为激光粒度仪的探测器提高光强分辨率的应用已经很普遍了。为提高测量精度,通过对CCD传感技术的研究,应用图像处理的方法来设计光电探测器,搭建了基于米氏散射原理的激光粒度测试系统。实验结果表明,用CCD传感器采集光散射图像,再对图像进行处理,D50与D10误差在6%之内,D90误差在1%之内,降低了测量的重复误差。     

Transmission Fluctuation Spectrometry with Spatial Correlation

Transmission fluctuation spectrometry (TFS) is being developed as a new method of particle size analysis. In the early approaches, the particle suspension was illuminated by one beam with finite beam diameter and the transmission signals underwent a process with variable spatial and/or temporal averaging and a subsequent nonlinear operation. The transmission fluctuations were obtained as a spectrum which included the information on particle size distribution and particle concentration. A new approach presented here employs two narrow parallel beams. While changing the beam separation, the transmission fluctuations of these two beams are expressed in terms of the expectancy of the transmission product (ETP). The analytical expression of the ETP through a monolayer is derived and the ETP of a 3‐dimensional suspension is formulated based on the layer model. The deviation between the transition functions of 3‐dimensional suspensions and monolayers is found to be affected by effects from particle overlapping and monolayer structure.     

光全散射法测量微粒尺寸分布的研究

本文从光全散射法的基本原理出发,提出了测量微粒尺寸分布的独立道模式光全散射测量法和非独立模式光全散射测量法,解决了以往光全散射法只能测量微粒的平均直径,不能给出尺寸分布,并且测量范围小,测量结果有多值性的缺陷,独立模式光全散射测量法还能用于测量多峰分布微粒的尺寸分布,数值计算和实验研究表明用本文方法,测量结果准确,可靠。     

Fractal Character of Dynamic Light Scattering of Particles

Dynamic light scattering signals from particles, exhibit fractal characteristics. This feature can be used to determine the particle size. The use of the fractal dimension, as a quantitative method to analyze the properties of dynamic light scattering signals from submicron particles, is presented. The analysis is performed directly on the time‐resolved scattered intensity, and the Box Dimensions of light scattering signals of particles with diameters 100, 200, 500 and 1000 nm. The experimental results show that the fractal dimensions of light scattering signals correlate well with particle size. In the submicron size range, the smaller the particles, the larger their fractal dimensions. Compared with the PCS technique, only several hundreds of samples are required in the fractal method. Therefore, the data processing is easily accomplished. However, this method only provides the mean particle size, but not the particle size distribution.     

Resolving Concentrated Particle Size Mixtures Using Dynamic Light Scattering

Dynamic light scattering (DLS) is a technique used for measuring the size of molecules and particles undergoing Brownian motion by observing time‐dependent fluctuations in the intensity of scattered light. The measurement of samples using conventional DLS instrumentation is limited to low concentrations due to the onset of a phenomenon called multiple scattering. The problems of multiple scattering have been addressed in a light scattering instrument incorporating non‐ invasive backscatter optics (NIBS). This novel optic arrangement maximizes the detection of scattered light while maintaining signal quality and allows for measurements of turbid samples. This paper discusses the ability of backscatter detection to accurately determine particle sizes at 1 %w/v sample concentrations and demonstrates the correct resolution of different size populations using a series of latex standard mixtures with known volume ratios. The concentration of 1 %w/v is much higher than can be measured on conventional dynamic light scattering instruments.     

光散射法测量颗粒尺寸、浓度的实验研究

为了能够准确快速地求解出微米量级颗粒系的尺寸和浓度,设计了一套基于Fraunhofer衍射,以线阵CCD为接收器件的实验颗粒测量装置.采用Shifrin积分变换方法,分析了给定样品颗粒的粒径分布、峰值、平均值和体积浓度.实验结果表明,与传统的Swithenbank方法采用环形光电管阵列为探测器接收衍射光强来反演颗粒分布方法相比,该方法不需要知道颗粒粒径上下限,各粒径区间间隔等预知信息,而且对粒径、浓度的实验测量值与理论值相差较小,样品峰值粒径为9.849 8 μm,与给定峰值的相对误差为3.432%,具有较高的测试准确度和较好的测试效果.     

Particle Sizes of Fumed Oxides: A New Approach Using PCS Signals

Fumed oxides produced in gas‐phase processes, such as silicas and aluminum oxide, consist of a cluster of aggregated primary particles. The aggregate size of these particles is an important variable in many applications. However, current procedures for measuring particle sizes all assume that the particles have a spherical shape and are thus not truly capable of determining aggregate size. The results of such particle size measurements are consequently called “equivalent spherical diameter” (ESD), but these results vary from method to method. This publication shows that it is feasible to use the number of primary particles per aggregate, rather than the ESD, as a measure for the particle size of clusters of this type. The method is based on dynamic light scattering (photon correlation spectroscopy, PCS), which has proven itself in the analysis of fumed oxides. A numerical simulation based on random, computer‐generated model aggregates is used to modify the well‐known Stokes‐Einstein equation so that the number of primary particles can be determined.     

Estimation of the Effective Measuring Volume of Single-Fibre Reflection Probes for solid volume concentration measurements

The working principle of the single-fibre reflection (SFR) probe is that light emitted by a laser diode is guided into the measuring volume by the same fibre which receives the proportion of light reflected by the particles in the vicinity of the probe tip and transmits it back to a photosensitive element. In contrast to other configurations of fibre optical probes, the SFR probe is characterized by an unambiguous calibration graph over the entire range of solid volume concentration values. SFR probes have been successfully applied to different kinds of multiphase flow systems, e.g. fluidized beds, pneumatic conveying lines, elutriators and thickeners. A particular question for the interpretation of measurements has always been the effective size of the measuring volume, which is mainly determined by the solid volume concentration. In this paper a simplified mathematical model of the signal generation by backscattering of the emitted light at the particle surfaces is given. The theory takes into account the average optical properties of the solids and their particle size distributions. The particle properties are determined on the basis of this model, which finally delivers the shape, size and depth of the effective measuring volume. For particle sizes between 30 and 120 μm the depth of the measuring volume of a 600-μm fibre probe is between 0.2 mm for solid concentrations near the fixed-bed state and approximately 4 mm for solid volume concentrations as low as 0.1 vol.-%.     

Laser‐induced Breakdown Detection (LIBD) for the Highly Sensitive Quantification of Aquatic Colloids. Part II: Experimental Setup of LIBD and Applications

Laser‐induced Breakdown Detection (LIBD) enables the concentration and size of aquatic nano‐particles (colloids), in the range of 10–1000 nm and from about 1 ng/L (ppt) up to some mg/L (ppm), to be determined. Such particles, which are frequently encountered in aquatic systems, cannot be detected using methods based on laser light scattering or obscuration or if detection is possible, then only at a comparably low level of accuracy. The method of Laser‐induced Breakdown Detection uses a high‐energy pulsed laser beam to selectively generate a plasma (dielectric breakdown) on particles. The method is based on non‐linear optics and allows for the determination of both mean particle size and concentration in aqueous samples. Practically, this method is non‐invasive, sample preparation is not required, and measurement can take place online. This second part of the double publication describes the experimental basics as well as the latest developments in the field of LIBD. Furthermore, it shows examples of practical applications and compares the technology to standard methods like dynamic laser light scattering as well as laser light obscuration.     

超声衰减和速度谱测量亚微米乳液的粒径

研究了超声衰减谱法和相速度测量高浓度多分散脂肪两相乳浊液的粒径分布问题.在理论分析基础上,实验测量了多个浓度(2%～20%)脂肪乳试样在2～13 MHz频带下超声衰减和相速度谱,结合反演算法由实验数据计算出乳浊液颗粒的粒径分布,并讨论了相速度法的意义和特点.将原始浓度(20%)下测得粒径分布与消光法在稀释条件下相同试样测量结果作对比,二者比较吻合,表明超声谱法可在无稀释的高浓度条件下作乳浊液粒径分布的表征.     

Investigations on the Capability of the Statistical Extinction Method for the Determination of Mean Particle Sizes in Concentrated Particle Systems

Due to the fast growing number of processes involving particulate systems, simple and robust measurement techniques which enable an inline monitoring of the particle size and their concentration are urgently required, since this ensures control but also process optimization. In this work, an inline measurement technique based on the statistical extinction method is developed that provides a process monitoring for a wide range of particulate processes, such as dispersion processes and spray processes. The method allows the determination of the mean size and concentration for particle systems with the size larger than 1 µm. For this purpose, a light beam illuminates the particle system, whereby the fluctuating light intensity due to the particle movement through the light beam is detected. The statistical fluctuation of the signal can be related to a mean particle size and a particle concentration. Since concentrated particle systems cause effects that additionally influence the signal, such as multiple scattering, approaches are needed to reduce or eliminate these effects. In this work, an approach using a spatial frequency filter is applied. The experimental investigations reveal that the effects can be significantly reduced with the spatial frequency filter.