Does the weather affect your powders?

Of the many variables known to impact powder behaviour, moisture is perhaps one of the most easily recognised and some plants do exhibit significant changes in performance as the seasons, and atmospheric humidity, change.

Powder handling problems caused by moisture can be avoided by controlling storage and/or processing conditions but such control typically incurs extra cost, not least because of the high energy costs associated with water removal.

Those looking to optimise humidity control strategies, therefore, need to precisely understand the impact that moisture will have on powder properties that directly influence process performance.

For a powder to flow, the particles within it must move relative to one another. The common perception is that introducing water makes powders flow less freely, and there is mechanistic support for this view. Water in a powder may form liquid bridges between particles that inhibit their movement and compromise flowability.

However, there are also ways in which moisture can improve flow behaviour. Particles with a rough surface, for example, may be lubricated by water, while in a system with fine electrostatically charged particles, water may transform flow properties by improving electrical conductivity and dissipating the strong interparticulate electrostatic forces of attraction.

Because moisture can induce these, and many other, very different effects it is crucial to measure its influence in ways that support effective process design and operation. Recent studies suggest that dynamic powder testing can be extremely useful in this regard.

 Impact of humidity

Dynamic powder testing involves measuring the rotational and axial forces acting on a blade as it rotates through a powder sample, to determine parameters, such as ‘basic flowability energy’ (BFE) that directly quantify how easily a powder flows under specific conditions.

The graph above how the BFE of microcrystalline cellulose [PH200, FMC] and lactose [Flowlac 100, Meggle] change as a function of moisture content. The lactose exhibits a steady reduction in BFE suggesting here the moisture has a lubricating effect.

With the MCC, however, BFE passes through a minimum as moisture content is increased from the initially desiccated condition. During testing, the desiccated MCC sample tended to coat the test vessel, suggesting that it may be electrostatically charged.

For this material, it is likely then that low levels of moisture reduce inter-particulate forces of attraction by dissipating electrostatic charge while higher moisture levels induce liquid bridging between particles, increasing adhesion in the system and causing an increase in flow energy.  

Multi-faceted powder characterisation

By directly measuring powder flowability, dynamic testing quantifies the impact of humidity in a process relevant way.

The presented data provide just a snapshot of the information that dynamic testing can generate, as the technique can be applied to consolidated, conditioned, aerated and even fluidised powders, and equally importantly, of the insight that can be gained by applying the technique in combination with others such as bulk and shear measurement.

The impact of humidity can be complex and is most comprehensively and productively assessed through multi-faceted powder characterisation that maximises the understanding of all relevant aspects of powder behaviour.

With this approach engineers can secure the firmest of foundations for the specification of humidity control strategies, and effectively protect the process from the adverse effects of moisture, even from the weather.

 Article by Jamie Clayton is operations manager at Freeman Technology, based on a white paper ‘Quantifying the impact of humidity on powder properties’ which can be downloaded from the Freeman Technology website: www.freemantech.co.uk