Method for Measuring Particle Size of a Dispersed Phase in a Fluid Stream

1. Field of the Invention

The present invention pertains to a method for measuring the mean particle size of a dispersed phase in a fluid stream such as oil in water, or vice versa, or fine solids dispersed in a liquid, for example.

2. Background

Many fluid treatment processes require or benefit from knowledge of the particle size of a dispersed phase in a carrier liquid. For example, the particle size of oil droplets dispersed in water is important to know to design and operate effective oil-water separation systems. The separation efficiency of many particle removal or separation devices is dependent on the size distribution of the dispersed phase in the continuous or carrier fluid. In this regard, it is desirable to obtain an accurate estimate of the particle size distribution or at least an accurate estimate of the mean particle size of the dispersed phase.

Measurement of particle size of solids particulates dispersed in a fluid may be relatively easily obtained by withdrawing a sample of the fluid from a flowstream and separating the suspended material to measure the particle size. However, for a dispersed liquid, such as oil, in a carrier or continuous phase liquid, such as water, the sample withdrawal and particle separation procedure cannot be carried out since the very act of collecting a sample entails quiescence of the carrier liquid that leads to irreversible coalescence and alteration of the particle size distribution of the dispersed phase. The present invention provides a unique solution to the above-mentioned problem.

The present invention provides a method for measuring the mean particle size of a dispersed phase in a fluid stream, such as a liquid which is immiscible in another liquid and is uniformly dispersed. The invention is also useful for measuring dispersed particulate solids in a fluid stream.

In accordance with an important aspect of the present invention, a method for measuring a characteristic mean particle size of a dispersed phase in a fluid stream is carried out based on the discovery that the mean particle size of a dispersed phase is dependent on a relationship between the turbidity of the fluid stream and the concentration of the dispersed phase in the fluid stream. In accordance with the present invention, it has also been established that a linear relationship exists between the ratio of the turbidity, in terms of the nephelometric turbidity unit (NTU), to the concentration of the dispersed phase and the dispersed phase particle size for a "mono" dispersed phase or the mean particle size for a "poly" dispersed phase. Accordingly, continuous "on-line" measurement of particle size of a dispersed phase in a flowstream, such as oil dispersed in water, may be carried out by measuring the turbidity of the flowstream, the concentration of the dispersed phase in the carrier fluid and by comparing the ratio of the turbidity measurement to the concentration with the particle size of the particular dispersed phase in question in the carrier fluid in question. In particular, calibration curves may be developed using a range of particle sizes of a particulate material which, in a carrier liquid, presents the same refractive index as the fluid being measured.

The system and method of the present invention permits accurate, "on-line" measurement of the mean particle size of a dispersed liquid phase in a carrier liquid or fluid in an in situ manner without requiring collection or preservation of a fluid sample. The knowledge of the in situ size distribution of a dispersed phase can then be used to provide suitable particle separation equipment or methodology and as a method of continuously monitoring the parameters of a process.

Those skilled in the art will recognize the above-described features and advantages of the invention as well as other superior aspects thereof upon reading the detailed description which follows in conjunction with the drawing.

FIG. 1 is a schematic diagram showing the major components of a system for measuring the particle size distribution of a dispersed phase in a fluid flowstream;

FIG. 2 is a diagram showing the nephelometric turbidity unit (NTU) value versus phase concentration for a particular dispersed phase in a fluid flowstream; and

FIG. 3 is a diagram illustrating the relationship between the ratio of turbidity to concentration and the particle size of a particular dispersed phase in a particular fluid.

FIG. 1 illustrates, in schematic form, a system 9 for measuring the particle size of a mono dispersed phase in a fluid flowstream or the mean particle size of a poly dispersed phase in a fluid flowstream. The system 9 illustrated in FIG. 1 includes a suitable conduit 10 for conducting a fluid flowstream therethrough which includes a dispersed phase such as fine particulate solids or a liquid in the fluid flowstream. For example, it is important to know the particle size of a dispersed phase such as crude or refined oil in water to modify certain processes including separation processes for separating the oil from the water. In this regard, a turbidity meter 12 is provided which includes a suitable probe 14 disposed in the conduit 10 in such a way that it measures the turbidity of fluid flowing through the conduit, such as a water flowstream having a dispersed phase of oil droplets or fine particulate solids therein. The turbidity meter 12 may be of a type commercially available and adapted to continuously measure the turbidity of the flowstream flowing in the direction of the arrow 16 through the conduit 10. One example of a commercial turbidity meter which may be used to practice the method of the present invention may be a series MET or MEX manufactured by BTG, Inc., The Woodlands, Texas. This type of turbidity meter is adapted for in-line measurement of medium to low turbidities of liquid mixtures, such as oil in water. The meter 12 is adapted to measure turbidity up to 1000 NTU's (nephelometric turbidity units).

The system 9 also includes an apparatus 18 for measuring the concentration of the dispersed phase in the fluid flowstream, including oil in water. The apparatus 18 may also be of a type commercially available such as a model OMS-40 Concentration Analyzer commercially available from GSA G. Schmied Abwassertechnik, GmbH, Stuhr, Germany. The above-mentioned apparatus 18 works on the principle of solvent extraction of oil from water with subsequent near infrared spectroscopic analysis of the extracted sample. The apparatus 18 is adapted to operate on a substantially continuous basis by obtaining samples of the fluid flowing through the conduit 10 by way of a sample conduit 20 connected to the apparatus. The apparatus 18 is microprocessor controlled to provide output concentrations in units of milligrams per liter which may be converted to parts per million, for example. Output signals from the turbidity meter 12 and the concentration measuring apparatus 18 may be transmitted to a digital computer or central processing unit 24 whereby certain calculations may be made and compared with a predetermined relationship between the particle size and the calculated parameter in accordance with the present invention.

The present invention indicates that a unique relationship exists between the particle size of a mono dispersed phase, or the mean particle size of a poly dispersed phase in a fluid and the amount of light scattered (turbidity) at a particular concentration of the dispersed phase. This relationship is indicated in FIG. 2 wherein the phase concentration of a quantity of fine particulate solids in water is linearly proportional to the turbidity measurement in nephelometric turbidity units (NTU). For example, in conducting tests in accordance with the present invention, selected quantities of polystyrene spheres of different diameters were dispersed in water to develop the curves 30, 32, 34, 36 and 38 of FIG. 2. The particle sizes of the dispersed phase for the curves 30, 32, 34, 36 and 38 were 1 micron, 2 microns, 4 microns, 8 microns and 16 microns, respectively. If a turbidity measurement device such as the meter 12 is used to measure turbidity, it is necessary to use dispersed materials which, in developing the calibration curves of FIG. 2, in the carrier or continuous phase fluid, provide the same refractive index as the dispersed material of interest in its carrier fluid. For example, if it is desired to be able to determine the particle size of a dispersed liquid, such as crude oil in water, a particulate material used to establish the calibration curves of FIG. 2 should, when dispersed in a carrier or continuous phase liquid, such as water, provide the same refractive index as oil in water. The curves of FIG. 2 indicate that, considering the overall scale of the nephelometric turbidity units, the intercept of these curves with the ordinate or "y" axis is virtually zero for zero concentration of the dispersed phase. The actual relationship is indicated for each curve on the diagram of FIG. 2. Accordingly, the ratio of the nephelometric turbidity units to the phase concentration in parts per million (ppm) may be conveniently computed and used to determine particle size, as will be indicated from the discussion hereinbelow.

FIG. 3 illustrates a semilogarithmic plot of the ratio of the NTU (nephelometric turbidity unit) to the phase concentration in ppm (parts per million). As will be noted from FIG. 3, there is a linear relationship, on a semilogarithmic scale, of the ratio of the turbidity measurement unit to the dispersed phase concentration with respect to the particle size of a mono dispersed phase. In order to verify the efficacy of the discovery of the present invention, a test was conducted using a system similar to that of FIG. 1 with selected quantities of polystyrene spheres of four different particle sizes dispersed in water using a normal distribution and a logarithmic-normal distribution. Table I below gives the amounts of mono dispersed materials used to form the poly dispersed mixtures of two different mean particle sizes of 4 um and 8 um, respectively, for the two distributions noted.

The relationships of FIG. 3 with respect to mean particle size, using the distributions of Table I, fall essentially on the curve 44, indicating a linear relationship of mean particle size of a poly dispersed phase corresponding to the particle size of a mono dispersed phase. Accordingly, the particle size of a mono dispersed phase and the mean particle size of a poly dispersed phase in a fluid flowstream may be determined with a high degree of accuracy in accordance with the method of the present invention.

The system and method of the present invention provides for accurate measurement of the mean particle size of a dispersed liquid phase in a carrier liquid in a manner which does not require collection or preservation of a sample. As such, a real time measurement is capable of being provided while circumventing the inherent problems associated with the coalescence and alteration of particle size distribution which occurs with dispersed phases or emulsions such as oil in water. Moreover, knowledge of the particle size distribution can be used as an indicator to assure that particle separation process and equipment are adequately sized and operated to acquire the desired degree of phase separation.

Although a preferred system and method in accordance with the present invention have been described hereinabove, those skilled in the art will recognize that various substitutions and modifications may be made to the preferred embodiment without departing from the scope and spirit of the invention as recited in the appended claims.