Cyclone dust collectors – an underestimated technology for the pharmaceutical industry?

Cyclone technology is currently in vogue. Colin Hayward, product manager, MikroPul, looks at the opportunities that could make it a useful technique in the pharma industry

In consumer markets the phenomenal success of the bagless, dual cyclone-based Dyson vacuum cleaner and its latest rival, the 'triple vortex' machine, has brought all the merits of this apparently simple air/dust separation technique to householders throughout the world. The benefits are very similar to industrial applications – cost savings with minimum maintenance and no need to invest in spare bags or change dirty filters.

The theory of cyclonic separation is relatively simple and is based on the principle that the induced spiralling action of the gas/particle stream subjects particles to substantial centrifugal forces, effectively adding G-force weight to the airborne solids. This alters the relationship between the particles and the minimum gas velocity required to keep them airborne (known as saltation velocity).

As the effective mass of the particles increases, they will drop out of suspension and be discharged from the cyclone hopper, leaving the clean air to exit through the central outlet tube.

However, in industrial applications cyclones have long been regarded as inefficient dust collectors with limited uses. It is usually the lack of knowledge regarding the correct applications of cyclones and the inability to accurately predict performances of such machines that has somewhat restricted their wider acceptance and use.

To exploit the potential of cyclones fully, industrial filtration experts are able to create computer models of the saltation phenomenon, resulting in very accurate performance predictions for their range of high efficiency cyclones.

The following case study highlights the advantages for a pharmaceutical manufacturer. Operations at the site surrounded the manufacture of a range of pharmaceutical products, including one containing the active ingredient selenium sulfide, which is harmful to the eyes and lungs. This ingredient is drawn under suction from an existing mill through a modified knock-out chamber into a mixer/blender. The carry-over from the knockout chamber passes through a second unit before ending up in the bag filter.

In the past, emptying and maintenance of the second collection unit was highly labour intensive as well as inefficient. The operators had to empty the drum beneath the filter at frequent intervals using breathing apparatus. Acute space limitations added to the problem of equipment selection.

The solution was to replace the second collection unit with a new, specially computer-modelled cyclone on top of the mixer/blender vessel so that a higher proportion of the material could be put straight back into the process without manual handling.

Advanced computer modelling was used to custom design a high efficiency compact cyclone. A compact model 18 (300mm barrel diameter) with a rotary airlock fitted to the discharge was selected. Where as previously 100kg would have been carried over and manually handled, the quantity of dust has been reduced to approximately 3kg, a dramatic 97% reduction in carry-over dust going into the bag filter. This means the operators now only have to go into the area for a fraction of the time that was necessary with the old system.

The size and range of cyclones available is vast, and a wide variety of materials of construction can be used: refractory lined cyclones with hastalloy outlet tubes have been supplied. Construction codes to BS5500 and ASME VIII can be undertaken and there are cyclones operating at 32 bar in the petrochemical industry. Also available are high temperature cyclones for applications in excess of 1,000°C. Removable wear liners are incorporated in most cyclones and explosion relief or 10 bar pressure shock resistant units can also be manufactured. Installing a cyclone in front of a dust filter, and designing it with 'quick release-easy strip' cyclone body parts and polished internals can cut downtime significantly and increase product collection where previously good product was disposed of in the filter.

The cyclone has been regarded as a relatively inefficient collector, particularly where dust is fine. However, versatility and improved technology has made industry look closely at the cost effective use of cyclones. Successful applications, to suit specific jobs, explain why cyclones may well be on the brink of renaissance.