Turbidity particle size analysis

D < 500 pm, Fraunhofer Diffraction Pattern Analysis (FDPA) can be employed in measuring particle size distributions (4,5). For the particles in the intermediate range, 0.7 pm < D < 10 pm, Mie theory of scattering holds and Turbidity Spectra (TS) can furnish information about particle sizes (6). 

The overall conclusion from the analysis of equation 18 over the complete domain of a is that, in principle, information about the shape of the particle size distribution can be obtained directly from turbidity, or in general, from scattering measurements. However, the discriminating power of the measurements in terms of the particle size distribution depends upon the wavelength range selected for the analysis. The major difficulty in the interpretation of the data arises from the behavior of the extinction itself as function of the size parameter which causes the measurements "see" a different average at every wavelength. 

Turbidity spectra and size exclusion chromatography (SEC) are two techniques which could be used for the on- and off-line monitoring of polymer particle size during the polymerization. An experimental evaluation of the two techniques is reported herein and it is shown that they satisfactorily follow the change of the particle size during the reaction. Information from SEC analysis is exploited to experimentally evaluate the turbidity spectra technique and investigate its potential to be used as an on-line measurement of particle size for latex reactor control. 

Turbidity measurements are simple, fast and reproducible. Specific turbidity can successfully follow the particle size evolution during the course of emulsion polymerization and can be translated into weight average diameters. A combination of an on-line spectrophotometer with an on-line densitometer (to obtain concentration) would provide the potential to estimate Dw on-line, as well. An on-line determination of PSD s, for small particles however, would seem quite difficult due to the high correlation of their parameters. One should bear in mind that the above conclusions have been validated only for poly(vinyl acetate) latexes the analysis is currently being extened to other systems, such as polystyrene and poly(methyl-methacrylate). 

Zollars [52] described an on-line turbidity system for the estimation of particle size distribution, refractive index and solids concentration. In a review and simulation of turbidimetric methods of on-line analysis Brandolin and Garcia-Rubio [53] state that this method is suitable only for... 

The particle size measurement assumes that light scattering by a particle is singular and independent of other particles, and that any multiple scattering wiU complicate the analysis. According to the Mie theory, the turbidity is related to the particle number N and their cross-section (where r is the particle radius) by... 

Hgure 1 Influence of particle size on the angular distribution of scattered light (A) small particles (D < 0.1A.) (B) large particles (D 0.25>.) and (C) larger particles (D > 1A,). (From Vanous RD, Larson PE, and Hach CC (1982) The theory and measurement of turbidity and residue. In Minear RA and Keith LH (eds.) Water Analysis, vol. 1, pp. 163-234. New York Academic Press.)... 

In (Shchyogolev and Khlebtsov, 1992 Shchyogolev et al., 1993) are reported reviews of the modern applications of the turbidity spectrum method to study biological disperse systems. The effects of polydispersity in some specific versions of reverse problems are analyzed, and the optimal types of averaged particle sizes are pointed to. New kinds of reverse problems related to the analysis of particle aggregation are considered. Examples of experimental determination of complexing biopolymeric systems are given. 

Hamielec and coworkers (9) have used turbidity spectra data obtained by a similar arrangement to generate the leading moments of the particle size distribution for vinyl acetate systems. In developing the method of moments analysis, they have eliminated the need to assume a form for the particle size distribution. 

The authors concluded from their results that the size measurement was not very sensitive with respect to particle orientation in the measurement volume, due to the concentric receiving aperture. Also for transparent particles, where refracted light will be also collected, reasonably accurate measurements could be obtained. One should however keep in mind that intensity measurements are very sensitive to variations in laser power, photodetector sensitivity, and contamination of windows. Moreover, variations of the particle concentration within a cross-section of a flow field will result in variations of light absorption for the incident and scattered light, depending on the measurement location (see for example Kliafas et al. (1990) for a detailed analysis of the turbidity). Therefore, the application of this method is again limited to very dilute two-phase flows. 

In this section the sensitivity of the predicted size distributions to fluid turbulence and fluid density is examined above, in, and below an oceanic thermocline. This analysis shows that a coagulating size distribution at steady state has increased suspended particle volume in response to a decrease in fluid turbulence or an increase in fluid density. This may provide an explanation for the observed maximums in particle concentration and turbidity at oceanic thermoclines (Carder et al. (21)). Jer-lov (22) has argued that higher thermocline concentrations result either from the minimum in eddy diffusivity at the thermocline which decreases turbulent transport through the thermocline, or from the increase in fluid density which traps low density particles. 

For water analysis, the formulation of turbid standards is very difficult, so most water laboratories use a synthetic pol5mer suspension as a standard. The formazin polymer suspension is easy to make and more stable and reproducible than adding clay or other particles to water to prepare standards. Alternatively, suspensions of polymer beads of the appropriate size can be used as scattering standards. (See Standard Methods for the Examination of Water and Wastewater for details.)... 

The turbidity of a highly dilute latex sample will provide information about the number and/or size of the polymer particles. If the system is sufficiently dilute to preclude multiple scattering, the turbidity at various wavelengths may be related to the concentration and size of the polymer panicles by Lorenz-Mie theory (see Section 12.3.2). This has been done by Heller and co-workers [31,32]. Since the method involves only sample dilution followed by turbidity analysis by a UV-visible spectrophotometer, it is a natural choice for continuous, online use. 

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