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New insights into retention aids dosage and mixing

Abstract In this work, critical design and operational parameters for retention aids dosage are studied through a combination of computational fluid dynamics (CFD), experimentation and pilot-scale production trials. In the first part of this work, three different retention aids dosage strategies are investigated in conjunction with pilot scale production trials. In all dosage strategies, a maximum in the percentage filler retention was observed at a speed ratio of 1.1, while considerably lower retention levels were observed when the speed ratio was greater than 2.2. However, the different dosage strategies led to markedly different retention of filler material. In the second part of this work, two-phase computational fluid dynamics (CFD) was used to model the three different dosage strategies implemented in the pilot production trials. The location and magnitude of maximum strain in each nozzle was determined and for each dosage case this was found to occur just outside the dosage nozzle at the point of impingement between the dosage and outer flows. In the third part of this work, conditions leading to the onset of retention polymer degradation were determined using an experimental flow loop. The effect of dosage speed and elongational strain created inside the dosage nozzle were studied systematically. These experiments showed that the effect of relative dosage velocity on polymer degradation was minimal. However, large levels of polymer degradation were observed when the elongational strain in the dosage nozzle was increased, i.e. when the exit nozzle diameter was decreased. Together, the three sets of experiments suggest that elongational strain during dosage degrades retention aids polymers and therefore hinders filler retention during production.