Particle size distribution ultrasonic measurements

On-line particle sizing by ultrasonic (acoustic attenuation) spectroscopy was developed for use during batch crystallization processes.14 Crystallization of the alpha polymorph of (l) -glutamic acid from aqueous solution was monitored by continuously pumping the crystallizing solution through an on-line ultrasonic spectrometer. The method enabled measurement of the crystal size distribution and solid concentration throughout the... 

Although the effect of the concentration of suspension on the results of the measurement of pigments has never been proven, the development of techniques able to cope with concentrations closer to the applications if of interest These would make it possible, for example, to determine the particle size distribution in a dispersion paint or in a reaction vessel where a pigment is produced by the precipitation process. A measurement technique having no problems, in principle, with high concentration dispersions is the scattering of ultrasonic waves. Nevertheless the instruments on the market have up to now failed to realize the great expectations of this technique. 

Malvern (Insitec) ECPS2 is designed to monitor and control particle size distributions from 0.5 to 1,500 pm, at concentrations up to 10,000 ppm, directly in pneumatic powder flow streams. Up to one thousand size distribution measurements per second are carried out at flow velocities from static to ultrasonic. Discrete data point, extracted from the log file, can be viewed. The data can also be viewed in tabular form and as a size distribution curve. Data can also be integrated over any selected range. A Statistical Process Control (SPC) option enables the file data to be viewed in standard control chart format either as an X or R chart. Various interface arrangements have been described, [203] ... 

In-line measurements of particle size distributions are essential in order to maximize production capacity and product quality. Ultrasonic attenuation is emerging as a technique, with capabilities beyond those of light scattering. In addition to the needs of industry for compact, robust instrumentation, this method is capable of operating at high concentrations, thus eliminating the need for an expensive dilution step, which may alter the very properties one wishes to measure [225,226]. 

In later papers they extended the theory to cover multiple scattering effects [241,242]. They also investigated neural network recognition of particle size distribution by ultrasonic spectroscopy [243] for measuring high concentration suspensions. This work formed the basis for an on-line ultrasonic size analyzer (OPUS) which is available from Sympatec (Figure 10.15). 

A personal computer (PC) controls the measuring arrangement. The evaluation algorithm in the PC calculates the ring intensity values of the percentage of certain particle size classes by comparison between measured values and predetermined theoretical values. The intensity values are converted into a size distribution by an iteration process. Ultrasonic measurement has also been used to determine the particle size distribution in emulsions down to 20 nm in size. The attenuation was measured in the frequency range 100 kHz to 185 MHz with computer... 

For liquids, the velocity of ultrasound depends on the compressibility and density of the liquid. For suspensions, the velocity depends also on the drag of particles in the liquid under the influence of the ultrasonic wave. At low frequencies, small particles tend to move in phase with the liquid and the ultrasonic velocity may differ widely from that in the pure liquid. As particle size and ultrasonic frequency increases, the particles tend to lag more and more behind the movement of the liquid and the ultrasonic velocity approaches that of the suspension acting as a uniform fluid. There is a transition frequency range between complete entrainment and no entrainment of the particles that can be used to obtain particle size information. The hydrodynamic model of Marker and Temple [267 ] can be used to calculate ultrasonic velocity. This model takes into account the effects of fluid viscosity, of concentration, density and elastic modulus of both particles and fluid and can predict ultrasonic velocities accurately for volume fractions between 5% and 20%. Ultrasonic velocity measurements in the 50 kHz to 50 MHz can be used to determine particle size distributions in the range of about 0.1 to 30 pm. 

The particle size distribution of the dry stucco was measured using a Sedi-graph 5(XX)D particle size analyzer. Specimens were prepared by dispersing the stucco in Sedisperse A-11 and placing the suspension in an ultrasonic bath for 2 min. Data were acquired in the form of a cumulative mass percent versus equivalent spherical diameter curve. As demonstrated in Fig. 1, this analysis included particles with an equivalent spherical diameter as small as 0.325 xm. For all specimens tested, the particles had equivalent spherical diameters smaller than 90 )im. 

FIGURE 38.15 Particle size distributions in respect to the light scattering (solid lines), particle volume (dashed lines) and particle number (dot-dashed lines) for the aqueous suspensions of fumed silica A-300 at Csio = (d) 0.1 wt.%, (a) 0.25 wt.%, (e) 0.5 wt.% and (b, c, f) 1 wt.% and different pH values suspensions (a-c) strongly sonicated by the disperser for 5 min and (d-f) softly sonicated (pH 5.3-5.6) in the ultrasonic bath for 30 min (d-e) exposure time interval between each measurement was 90 s, the regularization parameter a = 0.01. 

The application of ultrasonics to the monitoring of emulsion polymerisation reactors is considered. The use of acoustic speed measurements to monitor conversion is demonstrated by its apphcation to the control of the emulsion copolymerisation of styrene and butyl acrylate. The potential of acoustic attenuation for the measurement of particle size is discussed and applied to the determination of the particle size distribution of PVC and PTFE latices. 27 refs. 

The determination of particle concentration and particle size distribution in a suspension proceeds from a set of ultrasonic extinction measurements at various frequencies f, i = 1...M. Based on a discretized version of Eq. (1),... 

For particle size analyses, ultrasonic extinction is measured for a number of different frequencies consecutively. The whole procedure of measurement, including frequency selection, extinction measurement and mathematical evaluation is automated. One measurement of particle size distribution and particle concentration takes about 2 to 5 minutes, depending on the resolution and the reproducibility required. Some cumulative area density distributions measured by ultrasonic spectrometry, compared to the results obtained from a microscopic evaluation, are shown in Fig. 3. Broad and narrow distributions are clearly distinguished, owing to the high resolution and reliability of the method. 

Fig. 3 Some particle size distributions measured by ultrasonic spectrometry, compared to results of microscopic evaluation.

Instead of trying to determine ratios of mobilities with and without surfactant as the above authors did, it is possible to obtain the polymer parameters directly from the mobility of the colloid with the surfactant coating. There is a limit to how much information can be obtained from the dynamic mobility spectrum due to its smooth nature, but in principle it should be possible to get up to four parameters with reasonable precision. Thus, if the particle size distribution is known by some other method, such as ultrasonic attenuation, then it should be possible to extract zeta and the three polymer layer parameters from the measured dynamic mobility spectrum. Unfortunately, no one has attempted this to date. 

So far LII has only been applied for aerosol processes without the consideration of particles dispersed in liquids. First, investigations were carried out with re-dispersed carbon blacks. Besides furnace blacks (Printex A, G, 25, 35, and 55), various gas carbon black particles (FW 18, Colour Black SI60 and S170, Printex U and U140) were also considered. The particles were suspended in different liquids and dispersed by ultrasonic excitation. The stability of the suspension was recorded by measuring the aggregate size distribution (diffusion diameter) with DLS. Moreover, this was done before and after the LII measurements in order to control the stability of the particle suspension. To achieve LII... 

Figure 6 Oil droplet size distribution of on olive oil emulsion, stabilized with hydroxy-propyl mcthylceUkiloae, after different emulsification procedures blender (triangles), ultrasonic probe (squares), and ultrasonic homogenize (stars). Theoretical distributions were calculated from Coulter Counter measurements using a software program, assuming spherical particles.

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