Take a granular approach

Christine Banaszek, application engineer, Charles Ross & Son looks at the different options in planetary mixers for granulation

Throughout the pharmaceutical and supplement industries, granulation is a processing step that can vary quite significantly from one manufacturer to another. The basic objective is the same: to combine raw materials and create bonds between particles so they can be easily compressed into tablets having a uniform composition. However, equipment used for dry or wet granulation and subsequent drying, as well as the techniques for ensuring optimal density, particle size, flow and compression characteristics, ultimately rely on the properties and interaction of each ingredient. In this context, manufacturers stand to benefit from a re-examination of their granulation procedures, especially when developing new formulations, exploring alternative raw materials or upgrading legacy equipment.

Among the various technologies well-proven for granulation requirements is planetary mixing. The classic double planetary mixer offers an efficient way of preparing powders for tablet making. It moves batch materials by rotating two identical blades on their own axes as they orbit on a common axis. The blades continuously advance along the periphery of the mix vessel, removing material from the walls and transporting it towards the interior.

Wet granulation requires the addition of binding fluid to powders that are very fine or fluffy, or difficult to blend with other solids. The resulting wet mass is subsequently dried, and sometimes passed through a mill to produce granules of a certain size. A major advantage to the double planetary mixer is its ability to execute multiple operations in a single vessel: dry powder blending, liquid addition and coating, heating and vacuum drying.

In a double planetary mixer, the orbiting blades impart a very gentle but thorough folding action that carefully mixes minor or trace ingredients even if the product is not free-flowing. After a short mixing period to homogenously blend the starting dry ingredients, liquids may be added through a manifold on the mixer cover. With the use of a metering pump, liquid flowrate is accurately controlled while the batch transforms into a wet granulated state. Double planetary mixers routinely satisfy the requirement of product uniformity and particle size enlargement (granule formation) in the wet massing phase of many granulation recipes.

The final stage of the process is typically vacuum drying to remove excess moisture. As heat and vacuum are applied, the stirrers continue to mix the granulation at lower speeds, just enough to ensure uniform temperature while maintaining the size of the granules. Vacuum drying offers a fast and reliable method for drying heat-sensitive materials without fear of thermal degradation.

Manufacturers that have successfully transferred their process from multiple pieces of equipment to a double planetary mixer report significantly shorter cycle times, some by as much as 60%. In addition to simpler handling, cleaning and maintenance, the risks for product contamination and batch-to-batch inconsistencies are also minimised when the granulation process is streamlined.

For granulations that require higher shear to break up agglomerates, a planetary disperser is recommended. In this style of mixer, a low-speed planetary stirrer works in tandem with one or two disperser blades (saw-tooth disc or sharp-edged chopper) installed on a high-speed shaft that also orbits around the vessel. The planetary disperser can accomplish multiple phases of a wet granulation procedure including powder blending, wet massing, drying and milling (particle size reduction).

In a double planetary mixer or planetary disperser there are no shaft seals, bearings, packing glands or stuffing boxes submerged in the product zone. Agitators are raised and lowered in/out of the mix vessel by a hydraulic lift, enabling easy access for cleaning between batches. A reverse lift design – where the mix vessel is raised to meet the agitators and lowered after mixing – is also available.

Conclusion

Dry powder blending, granulation and drying are unit processes which have traditionally required separate pieces of equipment. The flexibility to complete multiple operations in a single machine provides a competitive advantage to manufacturers seeking to improve product quality, consistency and yield, reduce processing time and simplify maintenance. By eliminating transfer steps, processing is more streamlined and less labor-intensive.