Optics: measuring and testing – By particle light scattering
Reexamination Certificate
1998-10-15
2001-04-03
Font, Frank G. (Department: 2877)
Optics: measuring and testing
By particle light scattering
C356S338000, C073S863010
Reexamination Certificate
active
06211956
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method and apparatus for automatic dilution of a starting concentrated fluid suspension of particles for the purpose of optimizing a measurement of the particle size distribution, where the measurement technique chosen is sensitive to individual particles in the suspension over some appropriate range of particle size.
2. Description of the Prior Art
There are many intermediate process materials and final products of technical and/or commercial significance which exist either as a relatively high concentration of solid particles or liquid droplets suspended in a fluid, or as powders, which can be suspended in an appropriate liquid. The physical and/or chemical characteristics of these particle suspensions or dispersions (referred to herein as “sample suspensions”), or of the final products derived from them, often depend critically on the particle size distribution (PSD). Hence, it has become increasingly important to determine the PSD of these particle suspensions with high accuracy, resolution and reproducibility, often in the early stages of production. The solid particles or liquid droplets (in the case of oil-in-water or water-in-oil emulsions)—known as the “dispersed phase”—are suspended in an appropriate fluid, consisting of a simple liquid (e.g. water or an organic solvent) or mixture of liquids, perhaps containing one or more additives of various kinds (e.g. surfactants)—known as the “continuous phase”. Typically, the starting particle suspensions to be analyzed contain a relatively high concentration of the dispersed phase—usually exceeding 10% by weight or volume of the overall fluid suspension, and sometimes reaching 40-50%.
Most methods of particle size analysis (PSA) require that the sample presented to the analyzer be much less concentrated than that which is typically available from an intermediate process stream or final product. This requirement that the starting sample be diluted, sometimes very substantially, prior to determination of its PSD is particularly critical for methods known as single-particle sensing (SPS) techniques. Because these methods demand relatively low concentrations of suspended particles in order to produce PSD results of high accuracy and minimal distortion, they are effective in providing a motivation for the present invention.
The well-known method of single-particle optical sensing (SPOS), described extensively elsewhere, is one particular kind of SPS method. It utilizes the principle of either light extinction or scattering to determine the mean diameters of suspended particles as they pass individually through a very small sensing “zone” or “view volume”. In the recent invention of Wells et al (pending U.S. patent application Ser. No. 08/625,540, filed May 28, 1996, now U.S. Pat. No. 5,835,211), the physical principle of light scattering is combined with that of light extinction in order to increase the sensitivity and dynamic size range of the sensor. The resulting hybrid design thus extends the applicability of the SPOS method to particles smaller than those which would be detectable using only the light extinction technique, while preserving the large size range offered by the latter. This improvement increases the usefulness and overall effectiveness of the SPOS method.
There is another well-known SPS method for particle size analysis which is based on a different physical principle—the “electro-zone”, or “Coulter counter”, method. In this well-known technique, one monitors the conductivity between two volumes of partially conducting liquid, one of which contains the suspended sample particles at relatively low concentration, connected together by a small pore. The conductivity decreases momentarily whenever a particle passes through the connecting pore, caused by a pressure differential applied between the two liquid volumes. The magnitude of the conductivity decrease is proportional to the volume of the particle which momentarily interrupts the current flow through the pore. This represents another viable SPS method which can be used in conjunction with an autodilution system based on the present invention.
Regardless of the specific SPS method which is used to perform a PSA measurement, a concentrated particle suspension typically requires extensive predilution in order to ensure accurate PSD results with minimum distortion and artifacts. Specifically, this step is needed to ensure that the particles pass through the active sensing volume, or zone, one at a time, thereby avoiding all but occasional “coincidences” and consequent distortions of the output signal pulses and resulting PSD. For the purpose of explaining the underlying principles of this invention, it is convenient to focus exclusively on the use of an SPOS-type sensor in conjunction with the automatic dilution method and apparatus to be described. However, no loss of generality is thereby intended or implied. Rather, it is implicitly assumed that other methods of particle size analysis, including but not limited to other SPS methods, such as the electro-zone technique, may be used in place of the SPOS method in conjunction with this invention. Examples include “ensemble-type” methods for PSA, which respond to many particles at the same time, such as dynamic light scattering (DLS) and Fraunhofer diffraction (FD), also known as “laser” diffraction (LD). These techniques also usually require substantial dilution of concentrated particle suspensions, depending on their composition and particle size range (i.e. PSD). However, the extent of dilution typically required for these ensemble techniques is often considerably smaller than that needed for SPS techniques, such as SPOS and electro-zone sensing.
Therefore, PSA measurements, including those performed using the SPOS method, generally require substantial dilution of the original concentrated particle suspension, using an appropriate fluid, or mixture of fluids, as a diluent. This is especially required for automatic “online” particle size analysis of process suspensions in a production environment. In this case a dilution system should be able to accommodate samples with greatly differing PSDs, without requiring knowledge of their concentration, composition or PSD characteristics. The extent of dilution of the starting sample should be arrived at quickly and be optimal for the particle size analysis method being utilized—e.g. SPOS.
There are numerous applications for PSA in which the concentration of the starting particle suspension changes relatively little from one sample to the next. In such cases, it may be sufficient to dilute the starting particle suspension using a fixed, predetermined dilution factor, DF. This fixed DF value can be determined ahead of time, in trial-and-error fashion, for each kind of sample or application. A variety of prior art methods and devices exist for diluting a fluid sample with a predetermined dilution factor. By definition, these prior art methods and devices cannot provide for variable dilution when such would be more useful than the fixed, predetermined dilution which they provide, and therefore they are of limited utility.
Examples of prior art methods and/or devices which can be used to provide a fixed, predetermined dilution of fluid samples are described in: Cruzan, U.S. Pat. No. 4,036,062, Roof et al, U.S. Pat. No. 4,036,063, and Roof, U.S. Pat. No. 4,070,913. All of these patents describe means for diluting a fluid sample with a diluent fluid in which each of the fluids is initially contained in separate conduits. At the start of the dilution process the two conduits are connected together to permit closed-loop circulation and mixing of the two fluids. The extent of dilution—i.e. the value of the dilution factor, DF—is determined ahead of time by preselecting the volumetric relationship (relative volumes) of the two conduits.
This traditional approach to diluting a starting concentrated particle suspension is, by definition, inflexible and therefore quite limited. It is also potentia
Field Milton M.
Font Frank G.
Particle Sizing Systems, Inc.
Punnoose Roy M.
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