Dr Eddie McGee, technical director, Ajax Equipment, looks at the causes of powder segregation during handling along with design implementations that help to prevent it
All the good work put into achieving a proper mix or blend of pharmaceutical compounds can be undone whenever the batch is moved and segregation opportunities arise. Segregation is the process of involuntary separation of fractions of a bulk solid that were previously homo-geneously dispersed throughout the mass of material. As such, it is the direct opposite of blending and mixing.
Segregation is pervasive and pernicious. Opportunities to segregate occur in every stage of handling and processing of pharmaceutical powders, and are especially common during transfer to an intermediate vessel. As the powder is transferred into a hopper, its flow and discharge pattern can cause ‘repose segregation’ where small powder particles are concentrated in the middle channel of the hopper and larger particles closer to the vessel wall. If the flow out of the intermediate hopper is non-mass flow, then the first powder out is too fine and the last out too coarse, leading to quality issues at the beginning and end of the batch.
All too often the place where segregation effects become apparent is not necessarily where the segregation process has taken place. The detectable results may be the consequences of accumulative behaviour during many handling stages as the powder moves along the process path.
Tell-tale signs of segregation are product quality inconsistencies at the beginning and end of the batch operations, particularly if the product is stored at an intermediate stage and refill takes place before total hopper discharge. Concentration of fines can also lead to flow stoppages and hold-ups.
counter measures
Ways of reducing the effect of segregation in pharmaceutical processing include modifying the powder particle size, changing powder transfer processes and altering the design of the storage hopper.
Modifying the powder’s particle size by milling, for example, to achieve a consistent particle size can reduce the tendency for the material to segregate. Particle size – or more correctly a range of particle sizes – is a major causal factor for segregation.
Alternatively it may be possible to consider ‘mechanical’ design means for countering segregation by ‘conditioning’ the material to minimise or oppose the differential nature of the separating forces between particles. For example, the addition of a tiny amount of moisture can severely inhibit segregation as cohesive products tend to de-mix or segregate with difficulty. But care is needed in addressing one difficulty, not to introduce different problems of a more objectionable nature.
When transferring pharmaceutical powders long repose slopes should be avoided when filling hoppers. Multi point fill is preferable as it avoids concentrated separation of the fine and coarse fractions. And batches of bulk, such as mixer discharge, should be transferred as quickly as practicable, with a confined dense flow preferable to ‘hold’ the mass together. This restricts the opportunities and time over which segregation processes can act.
Also situations where there is biased transfer of powder to process lines, conveyors or storage hoppers should be avoided. For example, the flow stream of a belt conveyor delivering material to a hopper will eject the coarse powder particles further than the fine powder particles, allowing the separated size portions to land and slide down on the opposite sides of repose slopes of a forming pile. To prevent this either focus the flow stream, or intersperse a flow stream divider to divert the flow stream across the vessel’s cross-section to different filling regions.
A mass flow discharge pattern, in which the entire hopper contents move during discharge, is a good way of re-mixing products segregated during filling a storage container. However, it is not a ‘cure-all’. When the level of powder in the storage hopper falls to the point at which the walls start to converge, the velocity of flow in the centre of the cross section is higher than that at the walls. Consequently, the last portion of the contents of a batch to discharge comprises material originally local to the wall, mainly coarse particles.
A central insert, (not necessarily circular), can help by distending the collecting flow channel to an annulus, to mitigate the velocity profile effects and increase the area of draw-down, or dilute local aggregations by drawing product from multiple radial locations.
Alternatively, a non-mass flow hopper may be completely suitable for storing and homogeneously discharging a material that tends to segregate. It all depends upon how the container is filled and the extraction pattern that is achieved during its discharge. With well dispersed in-feed, segregation is minimised and the pattern of discharge is then less significant. Simple plate inserts close to the container wall can be used to collect free-flowing material from multiple points of the storage vessel, even in non-mass flow bins.
Segregation can be significantly reduced by means of ‘tributary’ extraction mechanisms to blend various zones of the stored contents into a single discharge stream. ‘Laminated’ plates offer even more feed tributaries, and can be applied to conical, pyramid and V-shaped hoppers.
modifying flow
Inserts work by modifying the flow regime in the hopper to a form that is more favourable to flow. They may be installed as integral to an original design or be used to rectify problems in an existing hopper. There are various technical approaches to insert design, the simplest being wall liners that allow the stored material to slip more easily at the hopper walls. These are generally aimed at reducing wall friction to allow the material to slip more easily.
However, in some cases high friction can be used to resist slip in selected regions of the hopper walls, thus modifying flow behaviour. Internal inserts can either convert axi-symmetric flow to plane flow type, or to complex but even more favourable flow forms. These geometrical techniques address mechanisms that allow the bulk strength of the solid to exceed the failure criteria in the critical arching dimensions of the flow channel.
Segregation prevention inserts take many forms. Table 1 on p35 provides an overview of the common forms of inserts categorised by location in hopper.
wall surface
Wall friction is dependent on the bulk solid, the wall material and its surface finish. Table 2 provides a comparison of wall friction angles.1
Significantly, the wall friction angle for coal dust is lowest with ultra high molecular weight polyethylene (UHMWPe), while a polyethylene powder against various stainless steel finishes shows that the wall friction reduces with coarse polishing, but has higher friction with finer polishing.
Wall-mounted inserts that alter the flow channel geometry can have a profound effect on overcoming arching, ratholes (see figure 1), mitigating segregation, expanding the flow channel and converting the contents to mass flow. This technique has been used successfully in a wide range of hoppers at scales ranging from pharmaceutical 100kg batch containers to 3000 tonne coal bunkers.
Externally, the hopper maintains its poor flow aspect but the wall profile inserts fitted internally radically alter the flow path towards the outlet.
The simplest method of creating a pseudo planar flow channel within a conical hopper is to place an inverted cone centrally above the hopper outlet (see figure 2). This provides an obstacle that diverts flow through an annular gap, and acts as a shield over the outlet. The flow channel is changed from a converging, radial flow pattern to a curved form of V-hopper simulating plane flow.
The inclination of the insert can be less steep than the hopper wall because flow on the surface of the conical insert is diverging from its axis; for example, limestone stored in a hopper required a wall angle of almost 80º to generate true mass flow. A static inverted insert enabled slip along a much more shallow central cone, (52º to the horizontal).
cone-in-cone
The cone-in-cone concept (see figure 3) uses a small, steep-walled, concentric conical hopper supported from the walls of the main hopper. This creates two separate flow systems, each of mass flow form. The internal cone widens flow in the central region of the hopper because the material will slip on the insert cone wall more easily than on a shearing layer. The peripheral material is no longer subject to the restraining ‘core’ pressure and mass flow takes place in the annulus region in a pseudo-wedge-shape channel that has a negative rake inner wall.
The effect of having two mass flow regions within a hopper allows the outer wall angle to be considerably shallower than that required for mass flow in a simple cone. This form of insert can be used to increase holding capacity within a plant of restricted headroom.
Inserts are a powerful and relatively unexploited method of combating segregation and enhancing hopper performance. However, care is needed in their application. In particular, it is not recommended for those unfamiliar with the magnitude of stresses that an apparently innocuous flow obstruction can impose. Their proper application and use can carry enormous benefits, in some cases being the only alternative to massive process modifications.
The application of innovative design techniques places this field of advanced hopper design in the domain of the specialist, but initial advice is usually free and the value of using inserts can be quickly established.
