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Sizing particles with a laser.

For more than a decade researchers have wanted to be able to analyze particle size distributions in a more efficient manner than was traditionally possible.

Some needed better instruments to analyze aerosols and reactive powders in high-temperature or otherwise extreme environments.

Others needed ways to measure pulverized coal combustion products in the laboratory, at the pilot plant scale, and in large-scale power boilers. These users wanted useful information on char-burnout, flow distributions, ash formation and fouling characteristics, and emissions control capability.

Still other investigators, including a team at Southern Research Institute, Birmingham, AL, had applications in the analysis of powdered limestone for sulfur reduction in "clean" coal applications, as well as hydrofluoric acid mists in studies of accidental emissions releases. Many of these investigations are research-oriented, but in the long-term there would be a need for instrumentation for continuous process control and optimization.

Traditionally, electro-optical analytical instruments have been employed in the laboratory to speed up the particle analysis process. The challenge has been to develop an alternative instrument that could be used in a wide variety of conditions, from laboratory research reactors to industrial process streams. One answer is a laser-optical instrument developed by Insitec capable of performing size distribution analysis in difficult environments.

The primary benefit of using optical techniques is speed. Optical methods can obtain a complete size distribution in less than a msec. This is sufficient for almost all analysis and control conditions.

In-process measurement currently can be achieved using optical techniques in situations where process concentrations are less than 1000 g/[m.sub.3] (0.1 % by volume) and particle size ranges are from 1 to 500 [mu]m. Smaller sizes, down to 0.2 [mu]m, can be measured when concentrations are more dilute.

Insitec's approach uses both single-particle counting methods for dilute flows and ensemble, or multiparticle, diffraction methods to address this wide range of conditions. Both are based on measuring near-forward scattered light.

The working principle is that small particles scatter light over large angles, while large particles scatter light at small angles.

The ensemble diffraction technique can provide measurements of the particle concentration and size distribution. The single particle counting method can provide velocity measurements in addition to information about the particles' size and concentration.

The new optical system, the Model EPCS, requires a standard low-power laser (He-Ne), with beam expansion optics on the transmitter side and a receiver lens with solid-state ring detector on the receiver side.

In all applications, instrument calibration can be periodically checked by inserting a glass reticle with known reference particle size distribution. For hazardous environments, the optical system may be electrically and optically coupled to a signal processor and computer-controlled console over a cable up to 600 ft in length.

The optics are automatically aligned by small stepper motors which correct for possible thermal expansion and distortion of the optical system over time. Display of the information and outputs for external control can be provided at intervals of 1.0 sec.

Operation is either manual or primarily automatic, with no user input required once operating conditions are established. Both hardware and system performance checks are included in the software.

Donald J. Holve is president of Insitec Measurement Systems Inc., San Ramon, CA. He holds a PhD from the Univ. of California, Berkeley, in mechanical engineering and an MS in heat transfer from Imperial College, London.
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Author:Holve, Donald J.
Publication:R & D
Date:Feb 18, 1992
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