Method of and an apparatus for ultrasonic measuring of the solid

Measuring and testing – Particle size

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Details

73 28, 73 61R, G01N 1502, G01N 1506, G01N 2902

Patent

active

047065095

DESCRIPTION:

BRIEF SUMMARY
The invention relates to a method of simultaneously measuring the concentration of solids and the particle size distribution in a suspension, wherein the suspension is excited by ultrasonic waves of more than one frequency and the absorption thereof by the solid particles is measured and evaluated. The invention also relates to an apparatus for carrying out such a method.
Suspensions are encountered frequently in the recovery and processing of raw materials, in the chemical industry, and in the preparation of foodstuffs. A suspension in the instant case is to be understood not only as a fine distribution of very small particles of a solid substance in a liquid but, quite generally, also as the distribution of minute droplets of an insoluble liquid or of gas bubbles in a fluid. The monitoring and control of such processes of industrial processing engineering often requires measurements to be taken of the concentration and dimensional distribution of the particles. As the measurement and control usually are being made with a flowing system, the following requirements must be met:
(a) The measurement is to be made instantaneously, in other words, the result of the measurement is to be available after a period of time during which the flowing suspension has not covered any substantial distance;
(b) the flowing system is not to be influenced by the measurement; rement;
(c) sampling is to be avoided, especially if the substance concerned is poisonous, caustic, hot, or highly pressurized.
As a result of these conditions the particle concentration and particle size distribution cannot be determined by screen or sedimentation analyses (sedimentation scale or photosedimentation meter). Other methods, like the measurement of the scattering of a laser beam, although meeting the above requirements, are restricted to transparent media. Also the method of the so-called Coulter counter is limited to certain electrically conductive fluids and, moreover, very susceptible to disturbances as the narrow capillary needed for measurement is plugged up easily.
Methods employing ultrasonic waves, on the other hand, are applicable with any fluid which can be excited by the same. As is well known, the reduction in intensity of radiation in an absorbing medium is defined by Beer's law into which enters the coefficient of absorption. A method of measuring the solids concentration in a suspension by means of ultrasound is known from DE-OS No. 22 57 802. In that case the suspension is excited by ultrasonic waves of two frequencies, and the dampening of the radiation intensity by absorption is measured. The absorption data among others are used for calculating the percentage volume share of solid particles and the mean geometric particle diameter. If the laws of statistics according to which the particles are formed were known, these data might be used for an exact determination of the particle size distribution. However, as a rule, this is not the case, in other words, the particle size distribution does not follow a known function. For this reason the method known from DE-OS No. 22 57 802, although permitting indications of individual parameters of the particle size distribution, does not permit a definition of the particle size distribution itself. The two frequencies of the ultrasonic waves in that case are selected such that the absorption is sensitively dependent on the characteristic parameters of the suspension. As a result, the frequencies are selected rather closely together.
It is the object of the invention to provide a method of and an apparatus for measuring the concentration of solids and the particle size distribution in a suspension which will meet the above mentioned requirements (a) to (c) and permit an accurate measurement of the particle size distribution. bution.
A method solving this problem and a corresponding apparatus are characterized in the claims.
With any given suspension, normally, the limits within which the diameters of the particles lie are well known. For this reason it is possible to fix two frequencie

REFERENCES:
patent: 3779070 (1973-12-01), Cushman et al.
patent: 3802271 (1974-04-01), Bartelson
R. J. Urich, "The Absorption of Sound in Suspensions of Irregular Particles," The Journal of the Acoustical Society of America, vol. 20, No. 3, (May 1948), pp. 283-289.

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