(42a) Study of the Effect of Flow Intensity and Flow Geometry on the Breakage of Whey Protein Aggregates Using Cfd

Aggregates have a wide number of applications in many fields such as chemical, food and environmental engineering. In many of these applications it is desirable to form and maintain a large particle size, this is why aggregate formation and growth has been widely studied over the past number of decades. On the other hand, there have been a limited number of studies focusing on the breakage of aggregates occurring as a result of post formation processing, for example while transporting aggregates via pipelines of various dimensions in an industrial process. Some researchers studying contractile flow have concluded that flocs breakage occurs just before the pipe diameter contraction but not after it, that is, only in locations where the flow intensity is higher. Our experimental investigation coupled with CFD simulations allow us to conclude however that aggregate breakage occurs all along the pipeline if the flow intensity is high enough and not only at the pipe diameter reduction. In the present work, whey protein aggregates -selected as a case study-, were formed in a standard configuration agitated vessel at average shear rate of 1118 s^-1. This initial 10% w/w solids dispersion was then diluted to less than 0.1% of solids content in order to negate any particle-particle interaction and let us focus the study only in the hydrodynamic forces that can cause breakage in the pipeline. The dispersion was propelled trough a pipe with ID 0.736 mm at different bulk velocities between 5 to 17 ms^-1 in order to study how flow intensity affects particle breakage. Turbulent flow was observed in all cases, and the diameter transition from the wide pipe (ID 2.286 mm) to the narrow one (ID 0.736 mm) was smoother at 45° to avoid significant breakage in that region. A total of 25 recycles were carried out to investigate the effect of the exposure time on the particle size reduction. A sample was taken after each recycle and the particle size was measured using laser diffraction technique. Finally, the extent of particle size reduction through various different flow geometries was investigated: 60 mm straight pipe, 60 mm+45° bend, 100 mm straight pipe, 100 mm+elbow and 1000 mm straight pipe. The flow field for each experimental geometry and each mass flow rate was modelled using the standard κ-ε turbulent model with a commercial CFD code. Particle breakage was observed with exposure time at all mass flow rates considered and the extent of breakage was found to be different for each flow geometry and each flow intensity. Both the experimental and the CFD results showed that the particle diameter reduction (Δd) can be coupled with both the exposure time (Δt) and the turbulent eddy dissipation rate (ε), then a breakage model of the form: Δd=k*Δt*(ε_i-ε_th)/(ε_th)ⁿ was proposed. Here the term (ε_i-ε_th) is the driving force that leads to the breakage and it is the difference between the local dissipation (ε_i) and the maximum energy dissipation (ε_th) that a particle with a given size can withstand before any breakage occurs. The adjustable model parameters were fitted using the 100 mm straight pipe experimental data and the model was then validated using both the CFD results and the experimental results for the other geometries. The simulations demonstrated good agreement between the experimental and the predicted particle sizes. In conclusion, there is significant particle size reduction in turbulent pipeline flow and aggregate breakage is a function of both flow intensity and the exposure time; flow intensity can be associated with the turbulence eddy dissipation rate in the pipe, which in turn has been calculated using CFD.