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Title: Multi-stage granulation in a high shear mixer
Author: Al Hassn, Ali Z.
ISNI:       0000 0004 7966 9582
Awarding Body: University of Sheffield
Current Institution: University of Sheffield
Date of Award: 2018
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The wet granulation process is generally conducted by adding the formulation to the granulator and running the granulator under fixed operating conditions from the starting point until the end point. This type of granulation process is referred to as normal granulation (NG). In the current work, a novel granulation method was used termed as Multi-Stage Granulation (MSG), as the granulation process was performed over three stages of process variables rather than one stage as in the NG. The MSG process, besides to the NG process, were carried out in a lab scale Eirich mixer using calcium carbonate (as a powder) and Polyetheylene Glycol (as a binder). The effect of varying the granulation time and the impeller speed on the granule properties was studied. In the NG process, the granule size was noticed to increase progressively with time, but with different tendencies depending on the impeller speed and liquid to solid ratio. The rate of increase was higher at 2000 RPM compared to 7000 RPM and was higher for a L/S=0.15 compared to L/S=0.14. While, the porosity, and the specific surface area of the granules decreased as the granulation time was increased. The size distribution of the granules also narrowed with time. This was inferred by examining the span of the size distribution with granulation time. For instance, at impeller speed 2000 RPM and L/S=0.14 the span was 1.47 at 400 sec while it was 1.26 at 600 sec. The amount of the binder was noticed to affect the span as well, where it was slightly higher at L/S=0.14 compared to the L/S=0.15. In the MSG process, the speed of the impeller was adjusted from a moderate speed to a high speed then to moderate speed again, creating three stages of the process parameters. The granule median size changed clearly during the process due to occurrence of different granulation mechanisms (growth and breakage) during the different stages. However, the granule porosity did not change enormously during the process. Although the granule porosity did not increase satisfactorily, the granule size distribution interestingly became narrower at the beginning of the third stage of MSG compared with corresponding points in NG process. For example, the span decreased from 1.47 (NG) to 0.99 (MSG) at L/S=0.14 and 400 sec, while at 600 sec it decreased from 1.26 (NG) to 0.88 (MSG). This means the span was reduced to about 30% when using MSG process compared to the NG process. The surface area of the granules was also affected by the stages of the MSG process. The surface area of granules decreased with time in the first stage then increased in the second stage, and it was high in the third stage, as well. This, consequently, affected the dissolution time of the granules. The dissolution time for the granules at the third stage was reduced, by approximately 21%, compared with granules from the first stage, which could be attributed to the increase in the surface area. Since the flow of the granular bed has an impact on the granule properties and there is lack of studies available in the literature about it, the velocity distribution of granular bed surface was studied using PIV . The flow in the Eirish mixer is different to the typical vertical high shear mixer. The area inside Eirich mixer was divided into different zones depending on the velocity of the granular bed monitored. The motion in the zone after the impact of the impeller to the granular bed (zone 2) was the most intensive one, which is believed to have caused granule breakage. While in the zone before the contact with the impeller (zone 1) was the lowest intensive motion condition, which could motivate the growth process. The highest and lowest intensive motion was happening during the second stage and the first stage of MSG, respectively. Power consumption during the different stages was also monitored and correlated with the granulation process of growth/breakage. The flow of the particle bed was also examined by simulating the motion of the particle using Discrete Element Method (DEM),. The simulation work was validated by experimental work using PIV results. The velocity distribution obtained by DEM was similar to the PIV results. The particles had high velocity after been impacted by the impeller and low velocity before the impact. The mixing of the particles was also characterized using Lacey's Mixing Index (LMI). The results showed that the LMI was higher in the second stage of MSG compared to the first/third stage. This gives an indication that the mixture after the second stage was increasing in homogeneity, i.e. the binder distribution was better during the second stage of MSG process. Finally, the outcomes from this research provide valuable insight into how granular properties from the high shear mixer can be adjusted by utilising a multi-stage approach. This is useful as it allows methods, which can be adopted to optimise granular properties for the end purpose and offers industries a novel route for tailoring products for consumers and for assuring product quality in the global market.
Supervisor: Salman, Agba D. ; Hounslow, Michael J. Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available