Particle attrition in a mechanically agitated vessel
Creators: name: RAVISANKAR, Vivek
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 400403
subject: Chemical engineering design
classificationCode: 400406
subject: Powder and particle technology
classificationCode: 400499
subject: Chemical engineering not elsewhere classified

description: Solid-liquid mixing in mechanically agitated vessels is a widely used unit operation in chemical and mineral processing industries. Researchers spend considerable time and effort in improving the efficiency of mixing in solid-liquid stirred vessels to maintain the production capacity and product quality. The quality and morphology of particles measured in terms of shape and size distribution after strong mixing influence the efficiency of downstream operations such as washing and filtration, and particle physical properties like bulk density, flow-ability and packing. Good control over the particle size distribution (PSD) of products can be used to achieve enhanced product quality and efficient operating procedures for critical downstream operations. Particle characteristics like size and shape cannot be compromised at the end of the mixing operation. There have been a very few studies in the literature that reports the extent of particle attrition in a mechanically agitated vessel for a range of solids concentration and reactor geometry. This experimental study aims to monitor the particle size changes throughout mixing operation in a solid-liquid mechanically agitated vessel using focused beam reflectance measurement (FBRM) method, which is a number-based laser measurement tool, for solids concentration ranging from 5 to 30 wt%. Three types of impellers, namely Ruston turbine (RT), pitched-blade impeller (PBT) and a hydrofoil impeller (A310) were used under baffled and unbaffled conditions. A majority of experiments were carried out by operating the impellers at speeds (Njs) that ensure just off-bottom suspension of solids for four hours (240 minutes).

FBRM results obtained at ten (T10) and 240 (T240) minutes after the start of mixing showed that the total number of particles at T10 and T240 increases with increasing solids concentration in the case of PBT under baffled condition. The variation ratio (VR), which is the ratio of the total number of particles at T240 and T10, increases from 1.67 to 2.89 as solids concentration increases from 5 to 30 wt% indicating the extent of particle attrition increases as solids concentration increases due to both an increase in the number of particles in the population and increased energy dissipation rate in the slurry with an increase in Njs. 

The VR value obtained under baffled condition generally increases from 1.82 to 3.09, 1.16 to 2.89, and 1.06 to 2.11 for RT, PBT and A310, respectively as solids concentration increases from 5 to 30 wt%. However, under unbaffled condition, the VR value generally decreases from 1.63 to 1.40, 1.68 to 1.09, and 1.33 to 1.02 for RT, PBT and A310, respectively as solids concentration increases in the same range. The results indicate that removing baffles in agitated vessels helps to minimise the extent of particle attrition even for higher solids concentrations due to reductions in Njs and energy dissipation rate. Among the three impellers, A310 operated under unbaffled condition has the least impact on particle attrition at 30 wt% solids concentration leading to a very low VR of 1.02 at the Njs of 654 rpm for this solids concentration. 

Several continuous industrial solid-liquid agitated vessels are normally operated at a constant impeller speed. However, the solids concentration in the feed could vary significantly, thereby subjecting the impeller to operate at solids loading higher than the design value.  To investigate the effect of varying solids concentration on the extent of particle attrition at a constant impeller speed, experiments were carried out in this work by varying the solids concentration from 5 to 30 wt% while operating the impeller at Njs for 25 wt% solids (Njs(25%)). The total particle counts were measured using FBRM at T10 and T240 for the three types of impellers under baffled and unbaffled conditions. Particle attrition was found to occur at all solids concentration irrespective of impeller type and baffling condition used. Under baffled condition, the lowest VR values for PBT and A310 were found to be 1.49 and 1.48, respectively at 25 wt% solids whereas the lowest VR value for RT was found to be 2.54 at 30 wt% solids. Under the unbaffled condition, the lowest VR values for PBT, A310 and RT were found to be 1.44, 1.29, and 1.09, respectively all at 25 wt% solids. Thus, the removal of baffles was found to lead to a reduction in VR value for all the three impellers at all solids concentration used but the highest and lowest VR reductions of 62.6 and 3.6% were found for RT and PBT both at 25 wt% solids. In this case also, the extent of attrition is the lowest for A310 under unbaffled condition for all solids concentration used except at 30 wt% where VR values are nearly the same for all the three impellers. This study thus indicates that solid-liquid agitated vessels can be operated under the unbaffled condition at a constant impeller speed without varying the feed or tank slurry concentration while still minimising particle attrition. Furthermore, employing A310 as the impeller under unbaffled condition could allow process intensification as well as particle attrition minimisation. SEM images obtained before and after particle attrition experiments show that attrition occurs due to a combination of mechanisms like abrasion, fragmentation, and chipping. However, abrasion seems to be the dominant mechanism in particle attrition.