Determination of particle size

Paul Boughton

Fluidised bed granulation processes are used in the manufacture of a multitude of products within many process sectors. The particle size distribution of the granules produced plays a crucial role in downstream processing steps and impacts directly on product performanceinfluencing properties such as flow and drying behaviourbulk densitysolubilitytendency to agglomerateand porosity.

In order to ensure the quality of the end productit is therefore important to closely monitor particle size during the preparation process and to optimise control in order to produce the desired material at minimum variable cost.

Furthermorethe online measurement of process relevant variables isof coursea

very important prerequisite for automated process control.

As part of a joint development project between WeimarGermany-based Glatt Ingenieurtechnik and Chemnitz-based Parsumalso in Germanythe use of inline particle size measurement was investigated under various operating conditions. The aim was to assess its potential for fluidised bed process monitoring and control.

Results from this studywhich focuses particularly on fluidised bed granulationare detailed below.

Spray granulation drying in a GLATTAGT150 fluidised bed unit was studied (Fig.1). In this unitwhich can be operated either continuously or batchwisethe key aim is to produce material with a narrow particle size distribution.

Howeverthe size of the spray droplets produced varies as a result of quality variations in the spray solutionspray pressure changes and a series of other influencing factors.

These changes impact directly on the granule formation process. The result is quality deviations in granule size and a relatively wide particle size distribution.

Conventionally this distribution is only measured in the laboratory after granule drying.

The measurement conditions in the fluidised bedare characterised by:

  • Undirected particle movement.

  • Complex flow regimes.

  • The possibility of high moisture content and elevated temperature.

  • A potential for large agglomerate formation.

  • The simultaneous presence of fine powders and large particles at high concentration.

They therefore constitute a significant challenge compared with conventional applications involving pipeline or free-falling material flow.

The fluidising chamber of the AGT150 hasas standardfour flanges at three different heights. These flangesprovided for the installation of nozzle attachments for bottom- and top-spray processesoffer the possibility of installing the measuring probe at three different positions without the need for complex engineering modifications (Fig.2).

The optimal installation position depends upon the density and height of the bed and has been found to be at around half of the height of the fluidised bed.

At this point highly representative measurements can be achieved. It is necessary to ensure that the measurement zone does not lie within the spray cone in order to prevent particle adhesion to the measurement optics.

During the experimentsdifferent instrument configurations were tested in order to assess the suitability of alternative flow cells and dispersion attachments.

With the IPP50the measurement system installeda continuous or pulsed air stream can be used to prevent particle build-up on the measurement zoneeven in relatively moist streams with high fine particle loadings.

The agglomeration of lactose was studied in some detail in a series of experiments. The impact of additional air introductionwater spraying and nozzle spraying of binders or lactose-like solutions was investigated.

Fig.3 shows data collected over a period of 90 minutes using an initial charge of 500g of fine lactose powder (X50 below 50micron) and a lactose solution as a binder. Overallapproximately 2000g of lactose solution were sprayed.

The development of the X50 value of the particles and also mean particle velocity (V50) were recorded.

In this testthe probe was equipped with a dispersion attachment which accelerates the particles within the measurement volume in order to reduce the initially very high particle concentration. The probe was mounted in the lower flange 65mm above the distributor base.

Particle growth

Particle growth can be clearly observed during the spraying process. As particle size increasesthe mean particle velocity decreasessince heavier particles are not accelerated as strongly by the flow conditions in the dispersion attachment of the probe.

About eight minutes before monitoring was stoppedthe product was withdrawn via the continuous discharge of the fluidised bedusing a low air feed rate. After this only dust particles are being fluidised in the granulator and so particle velocity rises again

During the granulationsamples were extracted for sieve analysis.

The results from these analyses track those produced by the inline instrument although the measured particle size is smaller.

This is attributed to the effects of granule dryingdifferent withdrawal and measurement positionsand differences in the principles that underpin the two measurement techniques.

The observed difference is not important for process monitoring applicationsas particle growth is still tracked effectively.

For better comparability and to therefore to improve operational acceptancean adjustment to the reported values can be made easily using a software calibration function.

The IPP50-S is a rod probe (Fig. 4). It has no moving elements and requires no manual adjustment. It can be mountedusing either a flange or standard screw connectionsin pipelinesfalling zones or vessels.

The installation and de-installation is straightforward and can be achieved during operation if necessary as no significant dismantling of the plant is required.

The probe is insensitive to vibrations and slight impactsand has an operational temperature range of -20 to 100°C.

An integral pressurised air feedensures that the optical sapphire windows have a permanent air or protective gas stream.

Velocity and size distribution

Simultaneous measurements of the velocity and size distribution of moving particles are produced using a patented optical method.

The particles pass through a laserintermittently blocking its detection by optics (fibre-optical spatial frequency filter) at the probe tip. From the resulting frequency-coded optical signalthe individual particle velocity and time of flight of a particle are then determined (Fig. 5).

From these two measurementswhen certain coupling conditions are satisfiedparticle size in the direction of motion (chord length measurement) can be determined.

With a measurement rate of up to approximately 4000 individual particles per second (process-dependent)a smooth particle size distribution can be obtained using a ring buffer.

All the necessary adjustments can be made using the measurement software.

The experimental work demonstrated the suitability of the measurement method and the IPP50-S probe for fluidised bed processes.

A special inline dispersion unit was developed to enhance system performance

Inline operation enables process monitoring in real time as opposed to discontinuous laboratory analyses which involve a significant time delay. Trends in the course of production can therefore be easily visually identified.

Direct measurement in the fluidised bedor in the case of continuous operation on the granule dischargeenables for examplethe spray pressure and the amount sprayedor the air throughput or process temperatureto be adjusted automatically in response to a deviation from the intended particle size.

This approach therefore offers opportunities for automated process control based on particle sizeor alternatively allows systematic and effective control procedures for plant operators to be developed more easily.

Stefan Dietrich is Geschäftsführer with PARSUM GmbHChemnitzGermanytel +49 (0)3 71 53 47 3 28fax +49 (0)3 71 53 47 3 27: Oliver Schmitt is Business Line Manager – Process Systems with Malvern Instruments LtdMalvernWorcestershireUnited Kingdomtel +44 (0)1684 892456fax +44 (0)1684 892789email:


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