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Laser-diffraction particle sizing pays dividends for wet-process monitoring

Sophisticated mathematics extend the range of concentrations over which measurement is feasible

February 01, 2014

By Henrique Kajiyama

The array of emulsions, suspensions and slurries for which particle size is a defining characteristic creates demand for flexible particle-sizing technology, not just for laboratory use, but also for continuous monitoring. Laser diffraction is one such technology.

With a proven track record in numerous wet applications, laser diffraction delivers continuous measurements that bring substantial commercial advantage from instant upset detection and effective troubleshooting to enhanced and automated process control.

From the starting point of an appropriate wet sensor, laser-diffraction systems can be engineered for different process applications. A case study involving emulsification-unit control for production of oil and water emulsions illustrates what can be achieved.

Operations improved by continuous particle-size analysis include emulsification, milling and flocculation. For these processes, particle size defines acceptability in the product or operation. Gathering particle-size information at a sufficiently high frequency to ensure precise control pays dividends. Data supports the highest levels of product quality, but also a platform for minimizing variable costs.

Emulsion droplet size

Expert application of emulsification allows the food industry to deliver products from cream liqueurs to mayonnaise. In food products, the droplet size of the dispersed phase impacts critical factors including:

•      Flavor-release profile

•      Mouth feel

•      Stability (the propensity of the system to separate out or "cream")

•      Color

Further, emulsion droplet size affects its structural characteristics and “whippability,” the ease with which the product holds air, vital in products such as ice cream. Production of finer droplets increases energy consumption. It is better to make as coarse a specification as product-performance targets permit.

More energy efficient than the dry alternative, wet milling is especially suitable if the product can be sold wet or when the feed material is naturally moist, as in minerals processing, a large-scale user of wet technology. Wet milling reduces ores to an optimal particle size for metal extraction or to produce very fine powders.

Calcium carbonate, for example, is milled to down to a Dv50 (median particle size) of around just one micron to provide a product that gives paper a glossy finish.  As with emulsification, reducing particle size further than necessary increases energy use.

Flocculation often is the basis of a separation process — de-watering, filtration or settling, for example — such as in the removal of metal-bearing material from waste ore or contaminants from water. Here, particle- size control ensures clean separation for minimizing flocculating agent use.

In addition, applications arise simply from the need to monitor particles in a suspension, in situ, or on an ongoing basis. Even drilling muds must be measured at the point of use to ensure that they are fit for purpose.

In summary, wet particle-size measurement is diverse in terms of the stream being analyzed and the resulting data application, creating demand for flexible analytical solutions.

Introducing laser diffraction

Laser-diffraction technology is a particle-sizing method of choice because of its speed, versatility, measurement range and automation. Laser diffraction measures all types of wet samples, from slurries through emulsions and sprays.

When laser light passes through a sample it is scattered by any particles or droplets present. Larger particles scatter strongly at narrow angles to the incident beam, whereas smaller particles scatter more weakly at wider angles. By the resulting light pattern, a laser diffraction analyzer or sensor determines sample particle-size distribution, through application of the “Mie” theory of light.

This highlights two key features of laser diffraction that shape its application:

•       Measurement is only feasible if light can penetrate the sample.

•      Scattering of the light by more than one particle prior to detection significantly complicates data analysis

In practical terms, these two constraints limit the sample concentration that can be measured reliably. Malvern Instruments and others have patented multiple scattering algorithms that address the second issue through application of sophisticated mathematics. These significantly extend the concentration ranges over which measurement is feasible. However, sample opacity remains a critical issue, which in the case of wet-sample measurement, may call for sample dilution ahead of measurement.

Applied to wet-process monitoring

A robust particle-sizing sensor is the foundation of any wet-monitoring application. The best laser-diffraction sensors are engineered for process use and the rigorous demands of industry. However, how to construct a solution around the sensor that answers requirements calls for due deliberation. Questions to be addressed include:

•      How often does the process need to be measured for data to meet monitoring goals?

•      Can the sensor be installed in-line or will an on-line system be required, operating on an integrated sampling loop?

•      How should the process interface ensure reliable, representative and continuous sampling, without process disruption?

•      Will the sample need dilution or other form of preparation prior to measurement?

•      What data presentation and automation supports optimal results use and analyzer integration within the existing plant- control platforms?

Consider, for example, a simple system monitoring a small-scale emulsification process, producing a dilute product stream in the order of 10kg/hr through which light can easily penetrate.

Here, sample dilution is unnecessary and flow rate is low enough for in-line monitoring. There is no need to take a cut from the product-process stream. Monitoring only reduces the need for sophisticated automation. Presenting data in a suitable format for operator interpretation is the sole aim. With respect to measurement frequency, system requirements make real-time measurement eminently feasible.

In contrast, another scenario requires a particle-sizing system to support optimized operation and automated control of a mineral-ore grinding circuit. Here flow rates may be several hundred tons per hour and the relatively concentrated slurry must be diluted prior to measurement. An on-line installation, incorporating automated sampling and sample preparation, including sample dilution, is therefore required. Technology for agglomerate break-up ultrasound or a de-magnetizer may also be needed.

Turning to data use, the requirement is for hardware and software that streamlines exploitation of the resulting data-stream within the control architecture. Measurement frequency requirements for process control may, in fact, be relatively low, if the process tends to be stable and changes only slowly, creating an opportunity to share an analyzer over more than one stream. However, the value of rapid upset detection needs to be weighed carefully when considering such a strategy.

Detecting a fault quickly enough to avoid costs associated with an unintended shut down may justify a dedicated analyzer, even absent any further economic gain. Even with a sample preparation required, integrated laser diffraction delivers a complete particle-size distribution every two to three minutes and is effective for upset detection, and a major advance on manual monitoring.

The two examples highlight how laser diffraction can be shaped to meet specific needs, building up from the basic sensor a design that efficiently handles sample preparation, sample dilution and automation. Whether buying an off-the-shelf solution or a bespoke solution, the technology can be fashioned as required.

Stable oil-in-water emulsions

At the Universidade Federal de Itajuba (UNIFEI) of Brazil, engineers research and develop practical solutions to enhance crude oil processing efficiency. One element of the work is evaluation and improvement of oil-in-water emulsion separators, including:

•       Hydrocyclone separators for high oil concentrations

•       Compact electrostatic and microwave coalescers

•       Decanter and disk stack centrifuges for water, crude oil and slop oil treatment; Subsea separators

•       Low-shear valves for crude oil processing

•       Homogenizers for crude oil desalting processing

A research key feature is pilot-scale trials carried out on a unit capable of producing emulsions at a flow rate up to18 m3/h. Trial success relies on producing stable emulsions that represent those encountered in crude-oil processing applications. These are made by manipulating oil concentration and operating pressure to tailor emulsion characteristics appropriately, with droplet size (Dv50) typically in the range 5 to 30 microns.

Off-line analysis is especially difficult in this instance since operational pressures are in the region of 10 bar and the emulsions are potentially unstable. A sharp pressure drop could encourage droplet breakage in the sample. A time delay between sampling and analysis might lead to coalescence, either of which can compromise off-line data integrity. A real-time particle-size analysis is therefore highly beneficial and applied for steady, controlled, test-bed operation.

A trial carried out to monitor the effect of various process changes on measured particle size demonstrates the capabilities of the particle-sizing solution adopted (see Figure 1). In tests one to three, oil concentration in the system was increased, causing a steady increase in obscuration (red line). Obscuration quantifies the amount of light penetrating the sample, with higher values indicating that less source light is reaching the detectors. The observed trend therefore shows the process stream becoming more opaque as oil concentration increases. However, this change has a negligible impact on the volume moment mean of the particles D[4,3] and on the Dv90 (the size below which 90% of the particle population lies).

In tests five through seven, pressure applied to produce the emulsion is steadily increased, driving down particle size: Dv50 (median particle size), Dv10 (the size below which 10% of the particle population lies) and Dv90 all decrease. Oil concentration is increased further in test eight, pushing obscuration up to a very high 97.5% but with no discernible impact on particle size. Finally, in test nine, pressure was raised to 12 bar, to produce a very fine emulsion with droplets with a Dv90 of 26.63 microns.

The results from test eight show that the process stream reaches very high levels of obscuration even at relatively low oil concentrations. At oil concentrations in excess of 1800 ppm, obscuration levels can rise above 95%, compromising measurement and essentially imposing a limit on the experimental conditions applied. To produce a more concentrated oil emulsions, it was decided to install a sample dilution system to work in combination with the particle sizer. Figure 2 shows a schematic of the installed solution which includes a proprietary cascade diluter, a diluent, water-driven unit with no moving parts, that reduces concentration by a defined factor.

To evaluate the impact of the cascade diluter, baseline particle-size measurements were made at an oil concentration of 1800 ppm. Oil concentration was then increased to 7600 ppm, but all other operating conditions kept the same to keep particle size constant. Measurements of the more concentrated system were made by diluting the sample by a factor of four using the two-stage cascade diluter. The results of this trial are shown in Figure 3 and confirm the integrated particle-sizing solution, complete with diluter, measures reliably at the high oil concentrations needed, especially for hydrocyclone and centrifuge trials.

These trials demonstrate how laser-diffraction technology robustly measures droplet size across a range of required operating conditions and at a sufficiently high frequency to support close emulsification-process control. The trial was completed in just one hour. These capabilities deliver the well-defined feed stream needed for separator efficiency analysis.

Final words

Continuous wet-particle size measurement secures reliable product quality in suspensions, emulsions sprays and finished powders, as well as efficient process control, and supports production-economics optimization. Laser diffraction is a proven technology for particle sizing that comfortably spans the majority of wet applications. Building from the starting point of a robust measurement sensor it can be fashioned to meet specific applications through appropriate tailoring of the process interface, sample preparation, and data handling and automation. The resulting solutions underpin truly optimal process operation.

The example presented describes use of real-time measurement for emulsification-process monitoring. Timely process analysis makes it possible to find, set and control operating conditions at the pilot or for commercial-scale use. In research and development this translates directly into greater experimental productivity; in production it results in increased throughput, rapid product changeover, less waste and lower energy use. Gains typically pay back the cost of the analyzer rapidly, often in less than a year.

Henrique Kajiyama is Latin America sales specialist, Malvern Instruments.

Malvern Instruments provides the materials, biophysical characterization technology and expertise that allow engineers to control the properties of dispersed systems. These range from proteins and polymers in solution, particle and nano-particle suspensions and emulsions to sprays and aerosols, industrial bulk powders and high-concentration slurries. Used at all stages of research, development and manufacturing, Malvern’s materials characterization instruments accelerate research and product development, enhance and maintain product quality and optimize process efficiency.

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