INVESTIGATION OF THE EFFECT OF LIQUID HEIGHT AND BAFFLE GEOMETRY ON DRAWDOWN OF FLOATING SOLIDS IN STIRRED TANKS

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2024-8-29

Author

Gök, Gökhan

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In solid-liquid mixing operations, stirred tanks are commonly used in industries. The geometry of the stirred tanks significantly affect the flow characteristics of these operations. Baffle configuration and aspect ratio which is ratio of liquid height (H) to tank diameter (T) are some of the main geometric factors. Especially in a fermentor (bioreactor) where it needs adeaquate volume for an efficient aeration, aspect ratio is one the substantial parameters. Moreover, baffles promote mixing so, it increases reaction rates and heat transfer in chemical reactions. Solids drawdown, which is immersion of floating particles from the liquid surface, is one of the main study field in solid-liquid mixing operations in stirred tanks. Solids drawdown is used for avoiding particle clumping on liquid surface and maximizing solid-liquid contact area for mass transfer and reaction. Thus, in this thesis, distribution of particles is taken into account together with drawdown. There are significant studies in solids drawdown and distribution of drawn down particles focusing on baffle geometry but none of these studies has taken the liquid height into account. The solo and synergistic analysis of these factors needs to be done. Hence, objective of this study is investigation of the effect of the liquid height and baffle geometry on drawdown of floating solids in stirred tanks. The key design parameters of the system were just drawdown speed (Njd) which indicates that no floating particles remain on liquid surface more than 1-2 seconds, Cloud Depth (CD), which indicates the distance between liquid surface and the lowest point of the clustered particle region in the liquid. A new parameter was defined as fully distributed speed (Nfd), which is the impeller speed at which all particles are distributed throughout the tank at which cloud depth of the particles equals to liquid height. Furthermore, power consumption at Njd (Pjd) and power consumption at Nfd (Pfd) were measured. A flat-bottomed plexiglass mixing tank with diameter of 24 cm was used. The experiments were conducted with tap water and at ambient pressure. Polypropylene granule particles (PP) whose dimensions are approximately 3 mm x 4 mm x 5 mm were used. In the beginning of the experiments where the liquid height is equal to tank diameter, solid concentration of particles is adjusted to 1% by weight w/w. Then, the mass of solid particles was kept constant while the liquid height varied. Submergence (S) which is the distance from liquid surface to bottom of the impeller blades was fixed to 8 cm (S= T/3). Three types of impellers were used; four bladed 45° up pumping and down pumping pitched blade turbine (PTBU and PBTD) and Rushton turbine (RT). Two different diameters (D) of each of the three impellers were used: 8 cm (D=T/3) and 12 cm (D= T/2). Surface baffles (B=5T/24) and full baffles (B=H) configurations were used. In addition, liquid height was increased by 4 cm from 20 cm to 40 cm (H=0.83T to H=1.67T). In general, increasing the liquid height from H=0.83T to H=1.67T resulted in a slight increase in Njd, Pjd and CD for all baffle and impeller configurations. Morever, for all impellers at varying liquid heights, combining an impeller with the full baffles gave more CD than combination of same impeller with surface baffles. T/2 impellers gave lower Njd and Pjd than T/3 impellers for all baffle configurations and liquid heights. Moreover, for all baffle configurations and impellers, for all liquid heights, RT-T/2-surface baffles combination gave minimum Njd among all configurations whereas PBTD-T/3-full baffles combination had the highest Njd values. In the overall, at H≥1.17T, for all impellers except for PBTD-T/3 at H=1.17T, it is not recommended to use surface baffles because it is impossible to reach Nfd due to swirling movement below the baffles which blocks axial flow at some speed around 100 rpm larger than Njd. Finally, from comparison of Pjd & Njd with Pfd & Nfd, for all combinations, it can be concluded that baffle type was more effective than impeller diameter in terms of full distribution of particles. Keywords: Drawdown of solids, just drawdown speed (Njd), power consumption at Njd (Pjd) Cloud Depth (CD), fully distributed speed (Nfd), power consumption at Nfd (Pfd)

Subject Keywords

Drawdown of solids, just drawdown speed (Njd), power consumption at Njd (Pjd) Cloud Depth (CD), fully distributed speed (Nfd), power consumption at Nfd (Pfd)

URI

https://hdl.handle.net/11511/111563

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Graduate School of Natural and Applied Sciences, Thesis