Agglomerate breakage during pneumatic conveying
27 June 2013
Authors: Dr Kevin Hanley, Imperial College London; Dr Kevin Cronin, University College Cork; and Dr Edmond Byrne, University College Cork
Powders sold to consumers are sometimes agglomerated to improve flowability, bulk density or wettability (for instantised powders). The issue of agglomerate breakage during in-plant handling and transport is often significant, since agglomerates tend to be considerably more fragile – and therefore more liable to disintegrate under mechanical loading – than the primary particles from which they are formed. The extent to which agglomerates disintegrate has a major effect on the properties of the powder.
Research conducted in the Department of Process and Chemical Engineering at University College Cork has investigated the breakage of one agglomerated consumer product, infant formula, during dense-phase pneumatic conveying.
This product was selected for two main reasons. Firstly, infant formula manufacture is an important component of the Irish food and drink industry – Ireland is the world’s largest single producer of infant formulae, accounting for approximately 15% of world production and using around 100,000 metric tonnes of Irish dairy ingredients annually.
Secondly, excessive breakage of this product is particularly undesirable because powdered infant formula is dispensed on a volume basis, rather than gravimetrically. Breakage of the agglomerates affects bulk density, hence the quantity of powder which will be dispensed by the consumer. Since regulatory requirements dictate the nutritional content of the reconstituted formula, the amount of breakage needs to be tightly controlled to ensure that these requirements are satisfied.
It is commonly known that the geometry and operating conditions of a pneumatic conveying system have a significant effect on powder breakage. A modular stainless steel pneumatic conveying rig was constructed, as shown in Figure 1, which could be operated in dense or dilute phase.
When one reference product was tested, it was found that the air velocity was the dominant factor affecting the amount of breakage that occurs, which is in agreement with the findings of other researchers.
What was surprising was the comparative unimportance of all other variables, such as bend radii and conveyor length. This indicates that it is critical to use a conveying velocity that is close to the minimum required to ensure reliable flow, if the minimisation of agglomerate breakage is desirable.
It may also be worthwhile investigating the feasibility of installing a pneumatic conveyor with supplementary air injection to enable reliable flow at extremely low velocities.
Making radical alterations to an existing conveying system would require significant capital expenditure. However, it is often the case that a manufacturing facility will produce a range of products and these will often differ in terms of friability. In this study, four different agglomerated infant formulae with different compositions (such as fat and protein content) were tested, which differ in composition.
Those powders with the highest protein contents were least susceptible to breakage, and individual agglomerates of these high-protein powders were strongest when subjected to uniaxial compression.
There is limited scope to use this information to change the composition of infant formulae, as manufacturers are required to remain in compliance with regulatory requirements for energy and micro-nutrient content; there may be greater potential to adjust product compositions in less heavily-regulated sectors. Furthermore, manufacturers who have advance knowledge of the predisposition of certain products to break significantly may be able to take additional precautions when these batches are being produced.
Another advantage of collecting and comparing breakage data for different products and production conditions is that the amount of breakage of a new formulation can be predicted quite accurately before the first batch is run.
Agglomerates experience many transient contacts as they are conveyed. The forces applied to individual agglomerates at any instant are very difficult to quantify experimentally, due to their small magnitudes and short durations, but can be obtained from a suitable numerical model.
In this UCC research study of infant formula breakage, models were developed at two scales: a micro-scale discrete element model (DEM) of an individual agglomerate and a macro-scale probabilistic model of a simple pneumatic conveying system. The parameters of the DEM were calibrated using the established Taguchi experimental design method with data obtained from quasi-static uniaxial compression of single agglomerates.
The developed model was quite complex, as shown in Figure 2, consisting of approximately 700 bonded spheres with a lognormal distribution of diameters. The results of the model were comparable to those which were measured experimentally during quasi-static uniaxial compression tests and dynamic drop tests, such as forces and strains at failure, agglomerate stiffnesses and coefficients of restitution.
The main benefit of developing this model is that it gives an insight into the evolution of the internal geometric structure of the agglomerate during loading, which cannot be obtained by experimental measurement.
A completely different philosophy was used when developing the macro-scale probabilistic model and many simplifying assumptions were made: the geometry of the agglomerates was not considered explicitly and they were treated as monosized spheres; the radial and tangential components of air velocity were disregarded; and the particles were assumed to be uniformly distributed over the cross-sectional area.
The applications for such a model are, of course, quite different to the detailed DEM. It provides no information about the microstructural changes that occur within the agglomerates, instead giving statistics of the distribution in maximum impact forces for collisions of particles and allowing the percentage of the particles which fail due to this loading to be quantified stochastically.
In principle, both models could be combined by replacing the idealised spheres of the probabilistic model with the detailed bonded agglomerates from the DEM; however, the computational requirements for simulating a large population of agglomerates makes this approach infeasible at present.
This study approached the issue of agglomerate breakage during conveying in several different ways: laboratory testing and numerical simulation; micro-scale and macro-scale. The results added greatly to our understanding of breakage during conveying, and some of the lessons learned are applicable to a wide range of powders.http://www.engineersjournal.ie/2013/06/27/agglomerate-breakage-during-pneumatic-conveying/http://www.engineersjournal.ie/wp-content/uploads/2013/06/baby-milk-1024x1024.jpghttp://www.engineersjournal.ie/wp-content/uploads/2013/06/baby-milk-300x300.jpgChemmanufacturing,UCC