Pharmaceuticals
Powder put to the test Powder testing solves pharmaceutical flow problem, says Ajax Equipment
Strict hygienic processing and containment requirements can make solving pharmaceutical powder handling issues more problematic than for other industries. Ajax Equipment has recently worked with a pharmaceutical company experiencing flow problems and, as a result, contamination issues. Using its predictive tool for powder flow, Ajax has improved powder flow, eliminated the contamination risk and optimised the plant's performance. Two areas of the plant were involved: flow through a sifter when discharging a pyramid hopper containing unmilled powder; and a pyramid hopper fitted with a cone valve, and used as a hopper for milled powder, which discharged to a packing off station.
An initial assessment of both plant areas showed that 'arching' was occurring. Arching is symptomatic of poor gravity flow as powder approaches the hopper discharge valve. Vibratory equipment and air hammers were being used to prevent arching, and in some cases manual intervention was necessary resulting in 'hammer rash' on both hoppers leading to contamination from shredding seals and gaskets. Also of concern was the fact that the cone valve attached to the pyramid hopper needed to be operated at such a high amplitude to ensure reliable flow, that this has also led to contamination from seal shedding. Ajax visited the pharmaceutical manufacturing site to conduct tests on the milled and unmilled powders thus providing flow data with which to assess the existing plant and as the basis for designing new plant equipment.
Powder testing is used to determining flow properties by measuring bulk density, slip characteristics (wall friction angle) and deformation behaviour (shear strength). Bulk density is important, as it provides the driving force for gravity flow and is highly influential for its flow behaviour. Wall friction tests determine the powder's resistance to slip on a contact surface for a given contact load. This measurement is a dominant factor when determining the inclination of a hopper wall for the required mode of flow. Wall friction is also important in the design of chute angles, mixer blades, feeders and conveyors. Shear strength testing is used to measure the powder's flow characteristics or how readily the mass deforms. In addition, the test give a measure of the deformation characteristics of powdered materials and size of outlet required to ensure reliable flow from a converging channel, e.g. a conical hopper. Using the results of the above tests, Ajax applied its spider- diagram flow diagnostic tool for predicting flow behaviour by
taking the measured characteristics of wall friction (φw) shear strength (τs) bulk density (ρb ) and adding three further factors: hopper or reactor wall angle (βc) outlet size (Dcrit) and Hausner Ratio (H.R.) - the ratio of tapped to loose bulk density. The
greater the ratio the more sensitive the powder is to vibration and hence flowability worsens.
26 Solids & Bulk Handling • March 2011
The predictive tool for powder flow showed that the unmilled and milled powders exhibited difficult flow characteristics with bulk densities that are sensitive to handling, high wall friction angles and the potential to develop shear strength.
www.solidsandbulk.co.uk
Using these factors a ‘spider’ diagram was produced comprising a series of three concentric circles divided by intersecting axes for each of the characteristics. An 'easy flow' powder is characterised by a small circle at the axes intersection point. ‘Modest’ and ‘poor flow’ materials are represented by large diameter circles.
Unmilled and Milled Powder Test Results Scale Unmilled Milled
Bulk Density
Hopper Wall Angle Wall Friction Angle Hausner Ratio H.R.
0 - 1000 338
Critical (Outlet) Diameter 0 - 200 189 Shear Strength
0 - 3000 2509 0 - 90 68 0 - 40 20.8 1 – 1.5 1.3
241 86
735 74
25.6 1.2
The following spider diagram summarises the flow test results for the unmilled and milled powder
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