(113d) Development of an Abrasion Tester for Testing Coated Granules

Many products in the chemical, pharmaceutical and the food industry are produced in the form of granules. These granules consists of agglomerated powder and have beneficial properties over the original powder. To enhance granule properties granules are often coated with a polymer layer. Benefits include: · Less dust formation · Design of controlled release particles · Tuning product appearance for marketing purposes · Better flowability · Lower bulk density

And many more?.

During handling and transportation of coated granules, they are exposed to forces of different directions and magnitudes. The type of forces is summarized in the matrix below:

Figure 1: The influence of magnitude and direction of a force on the breakage mechanism.

In industrial processes particle breakage often occurs, in order to quantify the damage during each unit operation a tester is required. In literature there are lots of tests available simulating process conditions that cause problems during the process. Another way of looking at particle damage is not by simulating the process but look at the mechanism of damage and try to isolate it. To account for differences in e.g. particle size and shape the test should preferably be a multiple particle tester. In Figure 1 different type of forces are shown, the goal is to find for each regime a different tester isolating the breakage mechanism.

Beekman et al. [1] and later Pitchumani et al. [2] developed a multiple particle impact tester, the Repeated Impact tester (RIT). In the RIT particles are subjected to forces in the normal direction only. By this, satisfying the requirement of isolating one component only. In real processes almost always both normal and tangential components are present.

Recently Reynolds et al.[3] reviewed current methods for testing particle breakage. The majority of particle tests reviewed in this paper, test forces consisting of both normal and tangential components. Salman et al. [4] impacted single granules at an oblique target and found a lower attrition rate to impacts at a target under an angle lower than 50°. This result shows the importance of the tangential component in particle breakage. Unfortunately this test does not specify the difference between normal and tangential forces in a different test, both components are always present.

Frye and Peukert [5] use two types of degradation testers, an attrition tester and a sliding friction tester. In the sliding friction tester a particle is uniaxially pressed on a rotating disk. In the attrition tester the particles are centrifugally accelerated. The particles obtain a spin and are impacted perpendicular on an impact ring target. In this setup the particles obtain a tangential velocity component. But again there the normal and tangential components are not separated.

Abou Chakra et al. 6] used a multiple particle impact tester similar to the tester Frye and Peukert used in their work. The difference is that in this setup the target plate hence the impact angle can be selected.

In order to isolate the tangential component during particle breakage a new tester is needed. The goal of this work is to develop a true abrasion tester.

In order to test the new abrasion tester, particles need to be tested. In earlier experiments Micro Crystalline Cellulose (MCC) granules appeared to show no attrition in the RIT. The MCC granules are sold as cellets 700® produced by IPC process centre GmbH. The size range used in these experiments are 850 mm ? 1000 mm. These granules are coated in a lab scale fluidized bed coater from Niro Aeromatic Ltd, model STREA.

The coated MCC granules will be tested in the Repeated Impact Tester and attrition behaviour will be compared with results from the new abrasion tester.

Bibliography

1. M. Gebert, R. Pitchumani, W.J. Beekman, G.M.H.Meesters, B. Scarlett. ?Repeated impact apparatus and method for characterisation granule strength.? WO. O2/04923 A2 US 20000611631. Genencor International Inc., Palo Alto, USA 2. R. Pitchumani, S. Arce Strien, G.M.H. Meesters, S. H. Schaafma, B. Scarlett. (2004)? Breakage of sodium benzoate granules under repeated impact conditions?, Powder Technology 140, 240-247 3. G.K. Reynolds, J.S. Fu, Y.S. Cheong, M.J. Hounslow, A.D. Salman (2005), ?Breakage in Granulation : A review?, Chemical Engineering Science 60, 3969-3992 4. A.D. Salman, D.A. Gorham, A Verba (1995), ?A study of solid particle failure under normal and oblique impact?, Wear 186-187, 92-98 5. L. Frye, W. Peukert (2005), ?Identification of material specific attrition mechanisms for polymers in dilute phase pneumatic conveying?. Chemical engineering and processing 44, 175-185 6. H. Abou-Chakra, U. Tüzün, I. Bridle, M. Leaper, M.S.A. Bradley, A.R. Reed (2003), ?An investigation of particle degradation by impact within a centrifugal accelerator type degradation tester?, Proceedings of the Institution of Mechanical Engineers part E-Journal of process mechanical engineering 217 (E3): 257-266