POWDERS ON-LINE particle size does matter

Particle size analysis has always been an important facet of processes involving the production, handling and transportation of bulk and finely divided solids. A knowledge of particle size distribution can be a boon to the efficient control of many process unit operations, from crystallisation, through spray drying to pneumatic conveying. Unfortunately, acquiring that knowledge has traditionally been a relatively slow, off-line operation usually carried out in the plant's QC lab.

Today's flexible processing plants, however, now demand in-process measurements of parameters that can have a crucial bearing on process performance. This demand is being fuelled by industries, such as pharmaceuticals, food and fine chemicals, that need to understand and optimise their often high value, low volume processes.

To meet this demand Malvern Instruments introduced its EPCS (Ensemble Particle Concentration and Size) in-line particle size analyser. Based on the light scattering technique of laser diffraction, the EPCS can be used in any dry powder stream or spray to measure particle sizes from 0.5 to 1000microns. The software of the system - which can run in a PC up to 1500m away from the actual in-line instrument - employs multiple scattering algorithms that allow measurements at high concentrations.

By using a by-pass system to continuously channel samples from the powder stream through a laser beam, the EPCS produces real-time results of particle size and concentration data that can be fed back into process control systems.

While the EPCS is available as a standalone system, Malvern has also recently entered into a strategic alliance with Hosokawa Micron for the use of the EPCS on air jet milling systems such as Hosokawa's Alpine AFG mill.

In many processes milling efficiency can be a critical issue, with continuous monitoring of the mill's output extremely helpful in ensuring consistent product quality at maximum process efficiency. The 100 AFG air jet mill has both a grinding and a classifying section. In the mill itself, material is ground to a fine powder in a pulverising chamber by a combination of the highly turbulent fluid energy of the transport gas (usually air) and the attrition effect of the particles themselves.

A mechanical classifier attached to this chamber ensures a very narrow particle size distribution and prevents overgrinding. Fines exit the mill through the rotating classifier, with coarse particles being recycled to the grinding section. Cut size is controlled by adjusting the classifier's speed.

Malvern's EPCS is installed as a by-pass system on either the exit stream from the mill (for control of the mill) or after the final cyclone on the final product stream (for quality control). Particle size distribution is thus monitored continuously throughout the grinding process, giving exact control of the fineness specification. On applications such as cement processing, where energy is a high cost factor, these combined mill/sizer units have been shown to have very short payback periods.

As well as collaborating with other equipment manufacturers such as Hosokawa, Malvern Instruments is also involved in an EPSRC research project at Heriot-Watt University in Edinburgh. This is looking at other ways of implementing in-process particle size analyses, including acoustic-type sensors which, says Malvern's marketing director Duncan Roberts, `are proving to be the way forward in process measurement at the moment.'

While we wait to hear more of these techniques, another interesting application of laser technology can be found in Erwin Sick's new Bulkscan 210 instrument. Not so much a particle measuring device as more of a bulk solids conveyor monitor, the Bulkscan operates on the principle of laser beam transit time. The system is zeroed on an empty conveyor belt and then, in operation, the transmitted laser pulse is deflected across the conveyor by a rotating mirror. Material on the belt then reflects the pulse back to a sensor, producing a profile or contour measurement of the material. Integrating these contours or `slices' in the system's software over time gives a volume flow rate, or a mass flow rate if the density is known.