Of the three rate processes, breakage and attrition is the least understood in granulation [15,16]. However, breakage is an important process: breakage reduces the overall growth of granules, can be used for size control, and improves homogeneity [56]. Furthermore, knowing the breakage behaviour of granules can also provide information on the performance of the product during processing after granulation [15].
In the literature, several ways to study the breakage behaviour of granules are reported. Generally, the study of breakage can be divided into two main categories, based on the scale of the experiments [15]. Experiments on the granule scale focus on single or a small number of granules. This is important, since it shows how properties of the granules are linked to their strength. Breakage studies at the process scale instead consider the behaviour of granules during granulation. Both fields are briefly discussed in the following sections.
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2.3.3.1 Breakage behaviour of individual granules
Methods of studying the breakage of individual granules involve measuring the force required to deform or break granules. Such tests result in a strength to express granule resistance to breakage [15]. The strength may vary depending on the technique used and can be measured either by static or dynamic methods.
Static methods include tensile and compressive strength, bending strength and hardness tests. Of these options, tensile and compressive strength tests are most frequently described in the literature [15].
Tensile strength tests involve the application of a tensile force to a particle. Tensile strength can be measured indirectly by applying a compressive force to granules, causing the generation of a tensile stress plane, which eventually leads to breakage. The compressive force required for failure is usually defined as the granule strength. This type of test is straightforward and requires no preparation of the sample [15], although a large number of granules must be sampled to attain reliable results. Antonyuk and et al. [57] investigated the deformation and breakage behaviour of several types of industrial granules with different properties. It was found that granules show different breakage behaviour, depending on their plasticity and microstructure. Elastic granules showed breakage at the major axis of stress, whereas elastic-plastic granules exhibited conical breakage at the area of contact.
For bending tests, granules are usually compressed into shapes that are easier to evaluate, such as bars or cylinders. A typical example of the method is the three-point bending test. The sample is supported at two ends from below, and a force is applied in the middle of the sample with a blunt wedge from the top, causing breakage. In this way, the propagation of a single crack can be evaluated. However, it is difficult to produce a sample that is representative of the original granules [15].
Hardness of a material is usually determined by indentation, which involves applying a load to an indenter at the granule surface [15]. The shape of the tip may vary. An advantage of indentation tests is their ease of use; provided the sample granule is large enough with respect to characteristic length scales like primary particle size and pore size. For smaller samples, nanoindentation can be used. Pepin et al. [58] found that hardness depends on three factors: the liquid binder surface tension and viscosity, and the interparticle friction.
Although static methods are useful and relatively straightforward to perform, their use for granulation is limited. In fact, Iveson et al. [59] state that due to the influence of the strain rate, traditional static strength measurement methods do not even give a qualitative indication of how materials will behave at high strain rates. Therefore, such methods would be invalid to predict any breakage behaviour during granulation. Consequently, different dynamic methods are needed to investigate breakage.
Antonyuk et al. [60] observed the dynamic breakage behaviour of single granules in granule impact tests, using different types of spherical granules. The breakage behaviour resembled that of static tests.
For their investigation of regime maps, Iveson and Litster [14] predicted the dynamic yield strength of granules by dropping granules from different heights and measuring the deformed contact area. In further research on breakage, Iveson et al. [61] used compression at various strain rates to measure the flow stress of pellets. They found that there was a critical strain rate below which strain rate did not matter, but above which flow stress increased with increasing strain rate. A follow-up study [62] using dynamic uniaxial compression tests at
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different speeds revealed a relationship between the dimensionless peak flow stress, Str*, and the capillary number, Ca. These numbers are defined in Equations 2.14 and 2.15, respectively:
𝑆𝑡𝑟∗ = 𝜎𝑝∗ 𝑑3,2
𝛾 ∗ 𝑐𝑜𝑠(𝜃) (2.14)
𝐶𝑎 = 𝜇 ∗ 𝑣𝑝
𝛾 ∗ 𝑐𝑜𝑠(𝜃) (2.15)
where σp, is the peak flow stress, d3,2 is the primary particle Sauter diameter, γ is the surface
tension, θ is the powder-liquid contact angle, μ is the binder viscosity and vp is the relative
granule velocity.
In the obtained relationship, two regimes where found; for low capillary numbers, Str* flow stress was independent Ca, but for high values of Ca, there was a linear correlation between the two numbers. This relationship was further investigated by Smith and Litster [63] using dynamic diametrical compression tests to successfully define the difference between semi-brittle and plastic failure.
2.3.3.2 Breakage behaviour at the process scale
Breakage tests at the process scale are a useful tool to determine breakage behaviour because they directly provide information on what is happening during the granulation process. There are several ways to study breakage behaviour in a granulator. The two methods discussed in this section are the dye tracer and the breakage-only granulator.
Van den Dries et al. [56] used a dye tracer to investigate the influence of primary particle size and viscosity on the breakage behaviour of granules. Tracer granules were produced by mixing powder with the dye tracer, granulating and sieving out appropriate size fractions. Next, a reference batch was granulated under the same conditions, after which the tracers were added to the batch and granulation was continued. From the resulting batch, samples were taken, dissolved in water and analysed using UV-spectroscopy to determine the dye tracer content. It was concluded that increasing viscosity and decreasing particle size increases granule strength and decreases breakage.
Liu et al. [16] used a different approach. They filled a high-shear mixer with a cohesive sand mixture and added pre-made granule pellets. In this type of mixer, the only phenomenon that occurred was the breakage of granules. At set time intervals, the granules were removed from the bed and survivors were placed back in the granulator. Formulations with varying properties like binder saturation, viscosity and surface tension and primary particle size were evaluated. It was found that the extent of breakage decreased with an increase in binder saturation, viscosity and surface tension, and increased with an increase in primary particle size. Furthermore, Liu et al. state that the percentage of broken granules could be related to the Stokes deformation number. A critical Stokes deformation number of 0.2 was proposed, above which breakage occurred.
Smith et al. [17] investigated the effect of impeller speed and shape on breakage behaviour of granules in the breakage-only high-shear mixer. In addition to a bevelled edge impeller, which creates both shear and impact, a modified, flat impeller was used to maximise shear and reduce the effect of impact. The granulator was operated at two different speeds, and
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instead of pellets, 2-5 mm sized granules prepared by single drop nucleation were used. The bevelled edge impeller caused significant breakage of the sample granules, and there was a clear correlation between the Stokes deformation number and breakage behaviour. For the flat impeller, however, only one of the samples showed significant breakage, even at the highest speed. It was concluded that impeller shape and speed have a strong effect on granule breakage. In particular, the impeller shape could be used to significantly control breakage behaviour.