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Mitigation of scale formation in unbaffled stirred tanks-experimental assessment and quantification
Chemical Engineering Research and Design 1 4 6 ( 2 0 1 9 ) 11–21
Contents lists available at ScienceDirect
Chemical Engineering Research and Design journal homepage: www.elsevier.com/locate/cherd
Mitigation of scale formation in unbaffled stirred tanks-experimental assessment and quantification Meysam Davoody a,b , Lachlan J.W. Graham b , Jie Wu b , Peter J. Witt b , Srinivasan Madapusi a , Rajarathinam Parthasarathy a,∗ a b
Chemical and Environmental Engineering, School of Engineering, RMIT University, Melbourne, VIC 3000, Australia CSIRO Mineral Resources, Melbourne, VIC 3168, Australia
a r t i c l e
i n f o
a b s t r a c t
Article history:
A qualitative and quantitative investigation on the formation of scale in mixing tanks was
Received 5 December 2018
conducted. The fabrication of a purpose-built tank that could be disassembled into nine
Received in revised form 22
segments (including a base, four walls, and four baffles or four blanking pieces) allowed
February 2019
the measurement of scale thickness in critical regions such as the impeller zone. The
Accepted 20 March 2019
scale grown on the walls of the tank was physically scanned using a coordinate measur-
Available online 27 March 2019
ing machine (CMM). The CMM readings were then used to plot 3-D graphs of scale thickness and distribution on the walls of the tank. Experiments conducted under unbaffled and baffled conditions showed that the bottom region of the reactor was almost free of scale while
Keywords: Scale formation
the top region near the liquid surface had noticeable scale formation under both configu-
Unbaffled tanks
rations. The scale thickness and distribution were lower in the unbaffled tank compared
Coordinate measuring machine
to the baffled tank, which, in line with CFD simulations, can be attributed to the increased
Mixing
tangential liquid flow velocity near the tank wall under unbaffled condition.
Fluid mechanics
© 2019 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
CFD
Nomenclature (AST) X (AST) Y C CMM D H N T W
1.
Average scale thickness in X direction, mm Average scale thickness in Y direction, mm Impeller clearance, mm Coordinate measuring machine Impeller diameter, mm Tank height, mm Impeller speed, RPM Tank diameter, mm Width of baffles, mm
Introduction
performance, and equipment downtime needed for cleaning scaled components. In addition to its economic impacts, scaling is also a major health and safety hazard. Fig. 1 shows an example of the scale grown on the walls of a neutralisation tank in a mineral processing plant in Australia. The grown scale, estimated to be 200 mm thick, poses a safety risk to the personnel, and damage risk to the other components of the tank during the maintenance operation. Much of the available literature on scale prevention/mitigation focuses on the use of chemical anti-scalants, whereas investigations into scale prevention using an optimum tank design is limited. This is mainly due to the lack of quantitative information on the growth behaviour of scale on the tank wall under various hydrodynamic conditions. A significant number of mineral processing operations involving solid-liquid systems are carried out in mechanically stirred vessels and therefore scaling in these vessels is influenced by the flow generated by the agitator. Proper design and operation of solid-liquid stirred vessels are critical in preventing and minimising the scale formation and
Scale is a significant issue in many mineral processing plants. It leads to significant revenue loss to the industry due to the reduction in process
∗
growth. The removal of baffles in solid-liquid stirred vessels has been shown to minimise the specific power input required to achieve the
Corresponding author. E-mail address: [email protected] (R. Parthasarathy). https://doi.org/10.1016/j.cherd.2019.03.032 0263-8762/© 2019 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
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Chemical Engineering Research and Design 1 4 6 ( 2 0 1 9 ) 11–21
Fig. 1 – Example of scale formation in a mineral processing plant (a) neutralisation tank, (b) scale grown on its walls. complete off-bottom solids suspension although it increases the liquid mixing time (Wu et al., 2010). However, this is not necessarily a shortcoming because the reaction and slurry residence times in many mineral processes are significantly longer than the mixing time. For instance, the residence time in a gold leaching process varies from hours to several days while the required mixing time is only in the
It is commonly accepted that removing baffles in stirred vessels can enhance the near-wall liquid flow velocity, which is one of the key factors in the prevention of scale formation. Yet, to the authors’ best knowledge, there is no quantitative work that studies the patterns of scale growth on the walls of unbaffled stirred tanks. This is mainly due to the absence of reliable techniques for studying the scale formation
scale of minutes (Marsden and House, 2006; Wu et al., 2010). It is well established that scale growth in processing equipment is affected by a number of factors including supersaturation in solution,
of scale growth in stirred tanks.
phase transformation phenomena, and contact times (Nawrath et al., 2006). These parameters, in turn, are governed by fluid flow through its
There are two major challenges in quantifying scale formation in stirred tanks: (1) identifying a reliable approach to grow scale in a short
influence on heat and mass transfer (Wu et al., 2012). Therefore, it can be concluded that fluid velocity plays an important role in influencing the rate and distribution of scale formation in stirred vessels. Further, the presence of baffles in stirred vessels has a direct and significant
period of time, and (2) measuring the thickness of the scale grown on the surface of the vessel. To address these challenges, a novel approach for scale measurement was presented previously for (Davoody et al., 2017, 2018): (1) growing magnesium hydroxide [Mg(OH)2 ] scale at high
influence on the flow velocity near the vessel wall. Velocity of fluid flow is one of the major factors that dictates the rate of scale deposition and growth. Role of flow velocity on scale
temperature within an hour, (2) designing and building a tank that can be disassembled thereby providing unrestricted access to the scale
thickness and deposition rate has been a topic of interest in several ¨ studies (Bansal and Muller-Steinhagen, 1993; Ceylan and Kelbaliyev, 2003; Fahiminia et al., 2007; Hasson et al., 1968; Hoang, 2015; Karabelas, 2002; Lee et al., 1999; Thonon et al., 1999; Walker and Sheikholeslami, 2003; Watkinson and Martinez, 1975). It has been reported that an increase in flow velocity leads to a decrease in scale thickness if the ¨ flow is fully turbulent (Ceylan and Kelbaliyev, 2003). Bansal and MullerSteinhagen (1993) reported a significant reduction in the scaling rate of calcium sulfate on heat exchanger plates as flow velocity was increased from 25 to 85 cm/s. Thonon et al. (1999) also observed that at velocities higher than 1 m/s, the amount of calcium carbonate scale formed on the plates of heat exchangers was insignificant. Lee et al. (1999)
quantitatively. As such, there is a lack of reliable information in the literature that shows how baffles can influence the rate and distribution
formed on the tank wall for the purposes of measuring its thickness, and (3) utilising a coordinate measuring machine (CMM) to physically scan the surface of the wall containing the scale. The results obtained using this novel approach are in agreement with the visual observations thereby providing confidence in the method developed. The present work utilises the same approach to grow, study, and quantify scale thickness and distribution in a stirred vessel. The effect of the tank configuration, especially the influence of baffles, on the scale formation and its growth on the tank wall was investigated experimentally. In addition, a computational fluid dynamics (CFD) model was developed to predict the near wall liquid velocities and wall shear stress in the mixing tank. The results from the CFD simulations are used to explain the scale formation results for baffled and unbaffled tanks.
reported that the increased wall shear rate at the higher cross-flow velocities would create an unsuitable environment for surface crystallization. From the available literature, it is reasonable to envisage that increasing the impeller speed in a mixing vessel can mitigate the deposition and growth of scale on the walls as it enhances the near-wall flow velocity. Unbaffled stirred tanks have been a topic of interest in several studies involving experimental work (Abatan et al., 2006; Brucato et al., 2017, 2010; Busciglio et al., 2017, 2016; Davoody et al., 2016; Galletti and Brunazzi, 2008; Kagoshima and Mann, 2006; Lamberto et al., 1999; Pinelli et al., 2001; Rao and Kumar, 2007; Scargiali et al., 2017; Tamburini et al., 2014, 2012; Tezura et al., 2007; Wang et al., 2012; Yoshida et al., 2008, 2007), and computational fluid dynamics (CFD) studies (Alcamo et al., 2005; Cokljat et al., 2006; Derksen, 2004, 2006; Deshpande et al., 2017; Murthy Shekhar and Jayanti, 2002; Sbrizzai et al., 2006; Shan et al., 2008). A number of the above studies has indicated that unbaffled tanks are more energy efficient compared to baffled tanks for solids suspension (Brucato et al., 2010; Davoody et al., 2016; Tamburini et al., 2013). This can be mostly attributed to the fact that kinetic energy dissipation is lower in unbaffled vessels. However, a major drawback in these vessels is the generation of a vortex at the free liquid surface due to the absence of baffles. This is undesirable as it can lead to gas entrainment especially at high impeller speeds during the experiments. This problem, however, is less significant at full-scale due to the lower operating impeller speeds. Nonetheless, the scale formation and growth behaviour in unbaffled vessels is not yet well understood and the literature on this topic is still scarce.
2.
Methodology
To have an unrestricted access to the scale grown on the tank wall, a stainless-steel reactor with diameter T = 100 mm was designed and fabricated in a way that it could be disassembled into nine separate pieces consisting of a base, four walls, and four gap stainless-steel fillers with a width of 35 mm (Fig. 2a). These fillers served as baffles with a width (W) of 0.1T when the tank is assembled in a baffled configuration. Another set of four stainless-steel fillers with a width of 25 mm replaced the 35 mm wide fillers when the tank was used in an unbaffled configuration. The individual pieces used in assembling the reactor are shown in Fig. 2a, and the baffled and unbaffled tanks assembled using the individual pieces are shown in Fig. 2b and c, respectively. The agitation was provided by a Lightnin A310 impeller with diameter D = 0.6T attached to a shaft that was located at the vertical axis of the tank and driven by a motor. The liquid height H was set at 1.6T in all experiments. The impeller clearance (C) from the tank bottom was set at 0.3T in all experiments. The Lightnin A310 impeller is an axial-flow impeller widely used in the mineral processing industry for achieving off-bottom solids suspension. This impeller was chosen
Chemical Engineering Research and Design 1 4 6 ( 2 0 1 9 ) 11–21
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distribution on the wall. Further details on the scanning and the measurement of scale thickness distribution are provided elsewhere (Davoody et al., 2017, 2018).
3. Computational fluid dynamics (CFD) simulation
Fig. 2 – The stainless-steel tank fabricated for scale growth studies: (a) individual pieces used in the tank assembly, (b) assembled baffled tank, (c) assembled unbaffled tank. Table 1 – Impeller speeds used in this work. Impeller speed (rpm)
Impeller tip speed/ velocity (m/s)
Reynolds number
400 430 460
1.26 1.35 1.44
15,186 16,325 17,464
to provide the agitation in the reactor used in this study to simulate the scale formation processes in full-scale operations. Table 1 lists the impeller speeds used in this work and the corresponding impeller tip speeds and impeller Reynolds numbers. The scale analog used in this study was magnesium hydroxide, formed from the reaction between Ca(OH)2 and MgCl2 , in the form of a precipitate. All experiments were run for 60 min. The temperature of the reactor liquid was kept constant at 80 ◦ C by placing the reactor in a water bath. The impeller speed was varied from 400 to 460 rpm. More details on the experimental procedure, and the arrangement of the wall segment on the CMM can be found elsewhere (Davoody et al., 2017). The scale formed on the tank walls was physically scanned using a CMM after a thin protective spray paint coating was applied to make the scale firm, thereby protecting it from any form of mechanical failure during measurements. Before any scale-growth run, the surface profile of each ‘clean’ reactor wall segment was scanned using a Sheffield Discovery II CMM to provide a set of reference coordinates. Once the reference coordinate values were documented, the tank was assembled and placed into the water bath for a scale growth run. The coordinate values of the scaled wall segment were compared with the reference coordinates to calculate the scale thickness at each coordinate and determine the scale thickness
Geometries for both the baffled and unbaffled stirred vessels were constructed using ANSYS/CFX 19.0. Both models were geometrically identical except for the presence of baffles. The vessel was cylindrical with a diameter of 100 mm and a height of 200 mm. The liquid height used was 157 mm. The liquid used was water with a density of 998 kg m−3 and a dynamic viscosity of 0.00089 Pa s. A single-phase model was used and the top liquid free surface was modelled as a flat free-slip surface. In the unbaffled tank experiments, a vortex near the liquid surface produced a rise in the liquid level near the walls and a depression near the shaft. The magnitude of this surface distortion and its effect on flows in the bulk of the tank was not considered significant enough to warrant increasing the model complexity. The diameter of the A310 impeller was 60 mm and its hub centre was 29 mm above the vessel base. A stirring speed of 430 rpm was used in the simulation. A hybrid mesh was used to represent the flow domain of each vessel with inflation layers at the vessel wall and impeller blade. Each mesh consisted of approximately 6.2 million nodes and 21.4 million elements, with increased mesh resolution near the impeller. For the conditions modelled, the y+ values on the wall and blades ranged from 0.5 to 2.0. Impeller rotation was accounted for by using a transient simulation and a sliding mesh approach where a section of mesh surrounding the impeller was rotated in 10◦ increments at each time step and the mesh for the surrounding vessel and the baffles remained stationary. A General Grid Interface (GGI) was used to connect the rotating and stationary mesh components. ANSYS/CFX 19.0 was used to solve the unsteady RANS equations using the couple solver and the shear stress transport turbulence model with a curvature correction factor of 1.0 (Menter, 1994). Second order backwards integration was used to advance the solution in time and the high-resolution differencing scheme was used for convective terms in the equations. Further details of the modelling approach used in this work can be read in similar studies reported elsewhere (ANSYS, 2017; Lane, 2017). The CFD models were solved in stages. The initial conditions for the transient situation were obtained by solving a steady state model using a frozen rotor approach. This was used to initialise the transient simulation with a time step set to give 10◦ rotation per time step, with 10 iterations per time step. To eliminate start-up effects, several rotations were modelled until consistently repeatable velocities at several monitoring points and the impeller torque were obtained. Thirty rotations were performed for the unbaffled vessel while 50 rotations were performed for the baffled vessel. After these initial rotations, a further 20 rotations were modelled for each vessel and the transient statistics was processed to obtain an average flow field.
4.
Results and discussion
In this section the results from the experimental findings and the CFD simulations are presented and discussed.
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Chemical Engineering Research and Design 1 4 6 ( 2 0 1 9 ) 11–21
Fig. 3 – Scale build-up on the walls of an unbaffled tank.
4.1.
Unbaffled tank configuration
Fig. 3 shows an example of scale build-up on the walls of an unbaffled tank after a scale-growth run. It is evident that the scale formation at the bottom region of the tank is insignificant whereas its presence in the top part is significant. Also, each wall segment has a similar distribution pattern. To measure the thickness of scale, the tank was disassembled after each run and one of the four wall segments was coated with spray paint and analysed. Fig. 4a provides a close-up view of scale growth pattern on the wall of the unbaffled tank after 60 min while Fig. 4b presents a 3D map of the scale thickness distribution on the same wall obtained from the physical scan of it using the CMM. The resolution of the scan is 1 mm, which led to a matrix of 70 (X direction) × 180 (Y direction) readings on the surface of each segment. Each scan therefore generated 12,600 ( = 70 × 180) scale thickness results, which were then analysed and processed for preparing the scale thickness distribution maps. A comparison between the two figures confirms that the CMM scan technique employed in this work provides a quantitative measure of the scale thickness distribution on the walls of the stirred tank with acceptable accuracy. The scale
distribution map in Fig. 4b shows that scale formation is considerable near the liquid surface at the top region of the wall segment and insignificant in the impeller plane and tank base. This difference, which is consistent with the visual observation results (Fig. 4a), can be mostly attributed to the fact that impeller rotation creates a turbulent regime at the tank bottom that mitigates the formation of scale, while the areas close to the liquid surface provide a more conducive environment for the scale to deposit and grow. Fig. 4a shows a patch of scale formation at the bottom of the tank wall which does not appear on the scale-map Fig. 4b. When the impeller was stopped at the end of the experiment, the solid particles suspended inside the vessel started to settle and form a sediment bed at the bottom of the tank. Because the tank was placed inside a water bath, it was not possible to pump the settled solids fully out of the tank. As a result, the sediment formed at the tank bottom left some scale residue on the wall which could be noticed after the tank was disassembled. These residues were removed prior to the CMM scanning because they were not relevant to the mechanisms of scale formation and growth studied in this work. From Fig. 4b, Max, Average, and Sum scale thickness values were determined to be 1.293, 0.123, and 1552.6 mm, respectively. These terms refer to the highest recorded value of scale thickness on the wall segment, average scale thickness value calculated using all the readings on the wall segment ( = 12600 in each scan), and sum of all the scale thickness values, respectively. These values were calculated for each scan and used to compare and evaluate the influence of impeller speed and baffles on the scale thickness. Additionally, a parameter called ‘Average Scale Thickness in Y direction or (AST) Y ’ was calculated by taking the average of the scale thickness values over discretised 10 mm height intervals over the full height of the tank. Fig. 5 shows the (AST) Y values as a function of the tank height for an experiment carried out at 400 rpm under unbaffled condition. The location of the impeller is also shown in the figure for comparison purposes. The (AST) Y values shown in Fig. 5 indicate that the scale thickness distribution is influenced by the magnitude of the liquid flow velocity especially near the tank wall. The liquid
Fig. 4 – The scale formed on a wall segment of the unbaffled tank: (a) experimental observation, (b) scale thickness distribution map generated using results from the CMM scan (N = 400 rpm).
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Fig. 5 – (AST) Y values as a function of the tank height for the unbaffled tank.
Fig. 6 – Effect of the impeller speed on (AST) Y values, unbaffled tank. velocity closer to the impeller region is expected to be higher due to the impeller pumping. However, the liquid velocity is expected to decrease as the liquid flows towards the surface and changes its flow direction there thereby losing its momentum. The higher liquid velocity in the impeller region is expected to slow down the formation and growth of the scale on the tank wall, which explains the smaller (AST) Y values in the bottom half of the tank. In contrast, the lower liquid velocity near the liquid surface is expected to lead to the formation of thicker scale thereby resulting in higher (AST) Y values. This observation indicates that liquid flow velocity near the tank wall is a critical factor in the rate of formation of scale and its distribution on the vessel wall. To better understand the role of liquid flow velocity on scale thickness distribution in
an unbaffled tank, scale growth experiments were conducted also at 430 and 460 rpm, and the results for all three stirrer speeds are shown in Fig. 6. The (AST) Y values for all three impeller speeds increase with increasing height up to the liquid surface (160 mm). The (AST) Y values for 400 rpm are higher compared to those for the other two higher impeller speeds at heights between 30 and 100 mm, whereas those for 460 rpm are greater for heights 10, 110, 130, 140 and 160 mm. At all other heights, the (AST) Y values for 430 rpm are greater than those for other two impeller speeds. The (AST) Y results indicate that, in general, the scale thickness increases at higher axial locations but decreases above the liquid surface (160 mm). There is no clear trend in (AST) Y values as a function of the impeller speed probably
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Fig. 7 – The scale formed on the wall of a baffled tank: (a) experimental observation, (b) quantitative map generated using the results of CMM scan (N = 400 rpm). because the impeller speed interval used in this work (30 rpm) is small. Therefore, the effect of impeller speed with larger intervals on (AST) Y values needs to be studied in future investigations.
4.2.
Baffled tank configuration
To understand how baffles in a stirred tank can influence the patterns of scale growth, experiments were also carried out in a baffled tank under similar operating conditions. Fig. 7a shows the scale formation profile on the wall of a baffled tank while Fig. 7b depicts the quantitative scale formation map plotted using the results of the CMM scan for N = 400 rpm. A comparison between Fig. 7a and b reveals that there is a strong agreement between the observed patterns of scale formation (Fig. 7a) and the quantitative map generated using the CMM scan readings similar to that observed for the unbaffled tank. As explained earlier, the residues observed at the bottom of the wall segment was removed prior to the CMM scanning.
4.3. Comparison of scale formation under baffled and unbaffled conditions The quantitative maps of scale growth on the stirred tank wall for both baffled and unbaffled conditions at 400 rpm are compared in Fig. 8. A common scale thickness range of 0-1.8 mm was selected to represent the scale distribution for both tank configurations. The scale formation pattern on the tank wall was influenced significantly by the presence of baffles during the experiment. The scale grown on the wall was uniform and evenly distributed for the unbaffled tank while it was non-uniform and mostly in the top left corner of the wall for the baffled tank. Considering the clock-wise direction of the impeller rotation, the accumulation of scale on the left of the wall can be attributed to the flow disturbances caused by the presence of baffles. This disturbance slows down the flow velocity, and creates a suitable environment for scale deposition and growth. The Max, Average, Sum scale thickness values for baffled and unbaffled tanks are listed in Table 2. The difference in the three scale thickness values indicate that operating the tank without baffles rather than with baffles can reduce the amount of scale formed on the surface of the walls significantly.
Fig. 8 – Quantitative maps of scale growth on the wall segment of (a) baffled tank (b) unbaffled tank. Table 2 – Quantitative comparison between baffled and unbaffled tanks. Scale thickness (mm)
Max Average Sum
Baffled condition
Unbaffled condition
1.765 0.147 1854.4
1.293 0.123 1552.6
% reduction in scale thickness
27 16 16
The (AST) Y values under baffled and unbaffled conditions are shown in Fig. 9a. The location of the impeller and its rotational direction are also shown in the figure for reference. The (AST) Y values were smaller at the tank bottom and in the impeller region, and increased with an increase in the liquid height. The highest (AST) Y values of 0.62 and 0.39 mm for baffled and unbaffled configurations, respectively were near the liquid surface. This is probably because the lower liquid flow velocity at the liquid surface created a calmer environment for the precipitates to deposit and grow on the wall. The (AST) Y values for the unbaffled configuration were consistently lower than those for the baffled configuration except at a few locations. When compared to the baffled condition, scale was observed at fluid heights greater than 160 mm for
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Fig. 9 – Effect of baffling configuration on average scale thickness values, (a) (AST) Y , (b) (AST) X , (N = 400 rpm). the unbaffled condition. This can be attributed to a slight rise in the liquid level at the tank wall during mixing under baffled condition. The influence of baffles on the scale growth and thickness distribution can also be studied by plotting (AST) X , which is the average of scale thickness values over an interval of 10 mm in the x-direction (circumferential direction). The (AST) X values for the two configurations are shown in Fig. 9b as a function of the fluid height. The (AST) X values ranged from 0.1 to 0.19 mm and 0.11 to 0.13 mm for baffled and unbaffled conditions, respectively. The differences in (AST) X values indicate that the overall amount of scale on the tank wall under baffled condition was higher than that under unbaffled condition. Under baffled condition, the (AST) X values were greater on the left side of the wall, lacking the uniformity found under unbaffled condition.
4.4.
CFD simulation results
Power number values for the baffled and unbaffled tanks were determined from CFD simulation as 0.27 and 0.19, respectively. The velocity and wall shear stress values obtained by solving the CFD simulation model are presented in Figs. 10–15. Velocity vectors are projected onto a vertical plane aligned to one
of the impeller blades, and in the case of the baffled tank they are located midway between the baffles. In Fig. 10, instantaneous vectors coloured by the velocity components tangential to the plane are shown for both baffled and unbaffled tanks. These plots show strong downward and outward flows from the blade tip and several transient vortices through the vessel. For the baffled tank, the flow field shows a degree of asymmetry with the large vortex on the left side of the image, reaching higher up in the vessel than the vortex on the right side of the image. This asymmetry is due to the transient nature of the flow induced by blades passing the baffles. The ‘time averaged’ velocity vectors projected on the same vertical planes shown in Fig. 11 indicate that time averaged flow is symmetrical for both baffled and unbaffled conditions. In the case of the baffled tank, the impeller-generated strong downward flow gave rise to a toroidal vortex in the lower third of the vessel and no significant time averaged flow in the upper part of the vessel. In the unbaffled tank, the downward flow from the blades was deflected towards the wall where it was split into two parts forming lower and upper vortices with the upper vortex extending to the liquid surface. While the radial and axial velocity components are small compared to the tangential velocity, they are important for vertical mixing and suspending particles in multiphase
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Fig. 10 – Predicted projected velocity vectors at an instant in time after 70 rotations for the baffled vessel (left) and 50 rotations for the unbaffled vessel (right).
Fig. 11 – Predicted time averaged projected velocity vectors for the baffled vessel (left) and unbaffled vessel (right).
Fig. 12 – Predicted time averaged tangential velocity on a vertical plane for the baffled vessel (left) and unbaffled vessel (right).
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Fig. 13 – Predicted time averaged velocity magnitude 0.5 mm inward from the vessel wall for the baffled vessel (left) and unbaffled vessel (right).
Fig. 14 – Predicted wall shear stress on the vessel wall at an instant in time after 70 rotations for the baffled vessel (left) and after 50 rotations for the unbaffled vessel (right).
systems. The time averaged tangential or swirl velocity component is plotted in Fig. 12. It shows that the presence of baffles greatly limited the swirl in the vessel with the main rotation present in the lower third of the vessel around the impeller region. Under unbaffled condition, a free-force or Rankine vortex structure was generated by the impeller around the shaft. The time averaged distribution of the liquid velocity magnitude at 0.5 mm from the vessel wall is plotted in Fig. 13. The velocity scale range is set based on the maximum liquid velocity for the unbaffled vessel. The maximum near wall velocity for the baffled condition was 0.38 m s−1 , which was twice that in the unbaffled case. However, the high average liquid velocity in the baffled tank occurred only in the lower 20% of the vessel and average velocities above the mid-liquid height were below 0.02 m s−1 . On the other hand, it is evident that the near wall velocity for the unbaffled tank exceeded 0.1 m s−1 over its entire height. The wall shear stress at the end of each simulation is plotted in Fig. 14 and it shows that the stress dis-
tribution pattern was similar to that for the near wall velocity distribution shown in Fig. 13. Thus, it can be concluded that the higher average velocities above the mid-liquid height in the unbaffled case prevent/mitigate the deposition and growth of scale on the surface of the vessel, which is consistent with the experimental results presented in Fig. 9. Fig. 15 presents a comparison of the time-averaged velocity distribution at a horizontal plane 40 mm below the liquid surface for the baffled and unbaffled modes at 430 rpm. The circulating flow in the unbaffled tank has a uniform yet strong near-wall velocity, which can be associated with the uniform distribution of scale as shown by the (AST) X values depicted in Fig. 9b. In contrast, the fluid flow in the baffled tank has a non-uniform velocity distribution due to the presence of baffles which act as barriers. The low-velocity zones found behind the baffles create a suitable environment for the scale growth, which is confirmed by the larger (AST) X values on the left side of the wall compared to the right side for the baffled tank shown in Fig. 9b.
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Fig. 15 – Velocity distribution on horizontal planes 40 mm below the free surface at 430 rpm for the baffled vessel (left) and un-baffled vessel (right).
5.
Conclusion
A qualitative and quantitative study on the scale formation in stirred tanks was conducted. A purpose-built tank that could be disassembled into nine segments including a base, four walls, and four baffles or four blanking pieces, has been demonstrated to provide quantitative scale growth distribution results when a wall segment was scanned using a CMM. The CMM readings, when presented as 3-D graphs of scale thickness distribution, enable a quantitative comparison of scale growth for baffled and unbaffled tanks. It was observed that the bottom region of the tank was almost free of scale whereas the top part had significant scale formation. It was also noted that the thickness of scale decreased significantly when operating the tank under unbaffled condition which can be attributed, based on the CFD simulations, to the increased liquid flow velocity near the walls when baffles are absent.
Acknowledgements One of the authors (M.D.) gratefully acknowledges the support of the Australian Government Research Training Program (RTP) Scholarship through RMIT University. Appreciation is also extended to CSIRO colleagues Mr. Dean Harris, Mr. Greg Short, and Mr. Bon Nguyen, and the CSIRO Clayton workshop.
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Brucato, A., Busciglio, A., Scargiali, F., 2017. 5 - unbaffled, stirred bioreactors for animal cell cultivation. In: Larroche, C., Sanromán, M.Á., Du, G., Pandey, A. (Eds.), Current Developments in Biotechnology and Bioengineering. Elsevier, pp. 97–142. Brucato, A., Cipollina, A., Micale, G., Scargiali, F., Tamburini, A., 2010. Particle suspension in top-covered unbaffled tanks. Chem. Eng. Sci. 65, 3001–3008. Busciglio, A., Montante, G., Kracík, T., Moucha, T., Paglianti, A., 2017. Rotary sloshing induced by impeller action in unbaffled stirred vessels. Chem. Eng. J. 317, 433–443. Busciglio, A., Scargiali, F., Grisafi, F., Brucato, A., 2016. Oscillation dynamics of free vortex surface in uncovered unbaffled stirred vessels. Chem. Eng. J. 285, 477–486. Ceylan, K., Kelbaliyev, G., 2003. The roughness effects on friction and heat transfer in the fully developed turbulent flow in pipes. Appl. Therm. Eng. 23, 557–570. Cokljat, D., Slack, M., Vasquez, S.A., Bakker, A., Montante, G., 2006. Reynolds-Stress model for Eulerian multiphase. Prog. Comput. Fluid Dyn. Int. J. 6, 168–178. Davoody, M., Abdul Raman, A.A., Parthasarathy, R., 2016. Agitation energy efficiency in gas–solid–liquid stirred vessels operating at ultra-high solids concentrations. Chem. Eng. Res. Des. 111, 34–48. Davoody, M., Graham, L.J.W., Wu, J., Youn, I., Abdul Raman, A.A., Parthasarathy, R., 2017. A novel approach to quantify scale thickness and distribution in stirred vessels. Ind. Eng. Chem. Res. 56, 14582–14591. Davoody, M., Lane, G., Graham, L.J.W., Wu, J., Madapusi, S., Parthasarathy, R., 2018. Scale formation on the wall of a mechanically stirred vessel—experimental assessment and interpretation using computational fluid dynamics. AIChE J. 64, 3912–3922. Derksen, J.J., 2004. Eulerian–Lagrangian modelling of solid–liquid flow in turbulently stirred tanks. Third International Symposium on Two-Phase Flow Modelling and Experimentation. Derksen, J.J., 2006. Long-time solids suspension simulations by means of a large-eddy approach. Chem. Eng. Res. Des. 84, 38–46. Deshpande, S.S., Kar, K.K., Walker, J., Pressler, J., Su, W., 2017. An experimental and computational investigation of vortex formation in an unbaffled stirred tank. Chem. Eng. Sci. 168, 495–506.
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Fahiminia, F., Watkinson, A.P., Epstein, N., 2007. Early events in the precipitation fouling of calcium sulphate dihydrate under sensible heating conditions. Can. J. Chem. Eng. 85, 679–691. Galletti, C., Brunazzi, E., 2008. On the main flow features and instabilities in an unbaffled vessel agitated with an eccentrically located impeller. Chem. Eng. Sci. 63, 4494–4505. Hasson, D., Avriel, M., Resnick, W., Rozenman, T., Windreich, S., 1968. Mechanism of calcium carbonate scale deposition on heat-transfer surfaces. Ind. Eng. Chem. Fundam. 7, 59–65. Hoang, T.A., 2015. Chapter 3 - mechanisms of scale formation and inhibition. In: Amjad, Z., Demadis, K.D. (Eds.), Mineral Scales and Deposits. Elsevier, Amsterdam, pp. 47–83. Kagoshima, M., Mann, R., 2006. Development of a networks-of-zones fluid mixing model for an unbaffled stirred vessel used for precipitation. Chem. Eng. Sci. 61, 2852–2863. Karabelas, A.J., 2002. Scale formation in tubular heat exchangers—research priorities. Int. J. Therm. Sci. 41, 682–692. Lamberto, D.J., Alvarez, M.M., Muzzio, F.J., 1999. Experimental and computational investigation of the laminar flow structure in a stirred tank. Chem. Eng. Sci. 54, 919–942. Lane, G.L., 2017. Improving the accuracy of CFD predictions of turbulence in a tank stirred by a hydrofoil impeller. Chem. Eng. Sci. 169, 188–211. Lee, S., Kim, J., Lee, C.-H., 1999. Analysis of CaSO4 scale formation mechanism in various nanofiltration modules. J. Membr. Sci. 163, 63–74. Marsden, J.O., House, C.I., 2006. The Chemistry of Gold Extraction, 2nd Edition. The Society for Mining Metallurgy and Exploration Inc., USA, pp. 503–651. Menter, F.R., 1994. Two-equation eddy-viscosity turbulence models for engineering applications. AIAA J. 32, 1598–1605. Murthy Shekhar, S., Jayanti, S., 2002. CFD study of power and mixing time for paddle mixing in unbaffled vessels. Chem. Eng. Res. Des. 80, 482–498. Nawrath, S.J., Khan, M.M.K., Welsh, M.C., 2006. An experimental study of scale growth rate and flow velocity of a super-saturated caustic–aluminate solution. Int. J. Miner. Process. 80, 116–125. Pinelli, D., Nocentini, M., Magelli, F., 2001. Solids distribution in stirred slurry reactors: influence of some mixer configurations and limits to the applicability of a simple model for predictions. Chem. Eng. Commun. 188, 91–107. Rao, A.R., Kumar, B., 2007. The use of circular surface aerators in wastewater treatment tanks. J. Chem. Technol. Biotechnol. 82, 101–107. Sbrizzai, F., Lavezzo, V., Verzicco, R., Campolo, M., Soldati, A., 2006. Direct numerical simulation of turbulent particle dispersion in an unbaffled stirred-tank reactor. Chem. Eng. Sci. 61, 2843–2851. Scargiali, F., Tamburini, A., Caputo, G., Micale, G., 2017. On the assessment of power consumption and critical impeller speed
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in vortexing unbaffled stirred tanks. Chem. Eng. Res. Des. 123, 99–110. Shan, X., Yu, G., Yang, C., Mao, Z.-S., Zhang, W., 2008. Numerical simulation of liquid−solid flow in an unbaffled stirred tank with a pitched-blade turbine downflow. Ind. Eng. Chem. Res. 47, 2926–2940. Tamburini, A., Brucato, A., Busciglio, A., Cipollina, A., Grisafi, F., Micale, G., Scargiali, F., Vella, G., 2014. Solid–liquid suspensions in top-covered unbaffled vessels: influence of particle size, liquid viscosity, impeller size, and clearance. Ind. Eng. Chem. Res. 53, 9587–9599. Tamburini, A., Cipollina, A., Micale, G., Brucato, A., 2012. Measurements of Njs and power requirements in unbaffled bioslurry reactors. Chem. Eng. Trans. 27, 343–348. Tamburini, A., Cipollina, A., Micale, G., Brucato, A., 2013. Particle distribution in dilute solid liquid unbaffled tanks via a novel laser sheet and image analysis based technique. Chem. Eng. Sci. 87, 341–358. Tezura, S., Kimura, A., Yoshida, M., Yamagiwa, K., Ohkawa, A., 2007. Agitation requirements for complete solid suspension in an unbaffled agitated vessel with an unsteadily forward–reverse rotating impeller. J. Chem. Technol. Biotechnol. 82, 672–680. Thonon, B., Grandgeorge, S., Jallut, C., 1999. Effect of geometry and flow conditions on particulate fouling in plate heat exchangers AU - B. Thonon, S. Grandgeorge, C. Jallut. Heat Transfer Eng. 20, 12–24. Walker, P., Sheikholeslami, R., 2003. Assessment of the effect of velocity and residence time in CaSO4 precipitating flow reaction. Chem. Eng. Sci. 58, 3807–3816. Wang, S., Boger, D.V., Wu, J., 2012. Energy efficient solids suspension in an agitated vessel–water slurry. Chem. Eng. Sci. 74, 233–243. Watkinson, A.P., Martinez, O., 1975. Scaling of heat exchanger tubes by calcium carbonate. J. Heat Transfer 97, 504–508. Wu, J., Lane, G., Livk, I., Nguyen, B., Graham, L., Stegink, D., Davis, T., 2012. Swirl flow agitation for scale suppression. Int. J. Miner. Process. 112–113, 19–29. Wu, J., Nguyen, B., Graham, L., 2010. Energy efficient high solids loading agitation for the mineral industry. Can. J. Chem. Eng. 88, 287–294. Yoshida, M., Kimura, A., Yamagiwa, K., Ohkawa, A., Tezura, S., 2008. Movement of solid particles on and off bottom of an unbaffled vessel agitated by unsteadily forward-reverse rotating impeller. J. Fluid Sci. Technol. 3, 282–291. Yoshida, M., Shigeyama, M., Hiura, T., Yamagiwa, K., Ohkawa, A., Tezura, S., 2007. Liquid-phase mixing in an unbaffled agitated vessel with an unsteady forward–reverse rotating impeller. Asia-Pac. J. Chem. Eng. 2, 659–664.
Contents lists available at ScienceDirect
Chemical Engineering Research and Design journal homepage: www.elsevier.com/locate/cherd
Mitigation of scale formation in unbaffled stirred tanks-experimental assessment and quantification Meysam Davoody a,b , Lachlan J.W. Graham b , Jie Wu b , Peter J. Witt b , Srinivasan Madapusi a , Rajarathinam Parthasarathy a,∗ a b
Chemical and Environmental Engineering, School of Engineering, RMIT University, Melbourne, VIC 3000, Australia CSIRO Mineral Resources, Melbourne, VIC 3168, Australia
a r t i c l e
i n f o
a b s t r a c t
Article history:
A qualitative and quantitative investigation on the formation of scale in mixing tanks was
Received 5 December 2018
conducted. The fabrication of a purpose-built tank that could be disassembled into nine
Received in revised form 22
segments (including a base, four walls, and four baffles or four blanking pieces) allowed
February 2019
the measurement of scale thickness in critical regions such as the impeller zone. The
Accepted 20 March 2019
scale grown on the walls of the tank was physically scanned using a coordinate measur-
Available online 27 March 2019
ing machine (CMM). The CMM readings were then used to plot 3-D graphs of scale thickness and distribution on the walls of the tank. Experiments conducted under unbaffled and baffled conditions showed that the bottom region of the reactor was almost free of scale while
Keywords: Scale formation
the top region near the liquid surface had noticeable scale formation under both configu-
Unbaffled tanks
rations. The scale thickness and distribution were lower in the unbaffled tank compared
Coordinate measuring machine
to the baffled tank, which, in line with CFD simulations, can be attributed to the increased
Mixing
tangential liquid flow velocity near the tank wall under unbaffled condition.
Fluid mechanics
© 2019 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
CFD
Nomenclature (AST) X (AST) Y C CMM D H N T W
1.
Average scale thickness in X direction, mm Average scale thickness in Y direction, mm Impeller clearance, mm Coordinate measuring machine Impeller diameter, mm Tank height, mm Impeller speed, RPM Tank diameter, mm Width of baffles, mm
Introduction
performance, and equipment downtime needed for cleaning scaled components. In addition to its economic impacts, scaling is also a major health and safety hazard. Fig. 1 shows an example of the scale grown on the walls of a neutralisation tank in a mineral processing plant in Australia. The grown scale, estimated to be 200 mm thick, poses a safety risk to the personnel, and damage risk to the other components of the tank during the maintenance operation. Much of the available literature on scale prevention/mitigation focuses on the use of chemical anti-scalants, whereas investigations into scale prevention using an optimum tank design is limited. This is mainly due to the lack of quantitative information on the growth behaviour of scale on the tank wall under various hydrodynamic conditions. A significant number of mineral processing operations involving solid-liquid systems are carried out in mechanically stirred vessels and therefore scaling in these vessels is influenced by the flow generated by the agitator. Proper design and operation of solid-liquid stirred vessels are critical in preventing and minimising the scale formation and
Scale is a significant issue in many mineral processing plants. It leads to significant revenue loss to the industry due to the reduction in process
∗
growth. The removal of baffles in solid-liquid stirred vessels has been shown to minimise the specific power input required to achieve the
Corresponding author. E-mail address: [email protected] (R. Parthasarathy). https://doi.org/10.1016/j.cherd.2019.03.032 0263-8762/© 2019 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
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Chemical Engineering Research and Design 1 4 6 ( 2 0 1 9 ) 11–21
Fig. 1 – Example of scale formation in a mineral processing plant (a) neutralisation tank, (b) scale grown on its walls. complete off-bottom solids suspension although it increases the liquid mixing time (Wu et al., 2010). However, this is not necessarily a shortcoming because the reaction and slurry residence times in many mineral processes are significantly longer than the mixing time. For instance, the residence time in a gold leaching process varies from hours to several days while the required mixing time is only in the
It is commonly accepted that removing baffles in stirred vessels can enhance the near-wall liquid flow velocity, which is one of the key factors in the prevention of scale formation. Yet, to the authors’ best knowledge, there is no quantitative work that studies the patterns of scale growth on the walls of unbaffled stirred tanks. This is mainly due to the absence of reliable techniques for studying the scale formation
scale of minutes (Marsden and House, 2006; Wu et al., 2010). It is well established that scale growth in processing equipment is affected by a number of factors including supersaturation in solution,
of scale growth in stirred tanks.
phase transformation phenomena, and contact times (Nawrath et al., 2006). These parameters, in turn, are governed by fluid flow through its
There are two major challenges in quantifying scale formation in stirred tanks: (1) identifying a reliable approach to grow scale in a short
influence on heat and mass transfer (Wu et al., 2012). Therefore, it can be concluded that fluid velocity plays an important role in influencing the rate and distribution of scale formation in stirred vessels. Further, the presence of baffles in stirred vessels has a direct and significant
period of time, and (2) measuring the thickness of the scale grown on the surface of the vessel. To address these challenges, a novel approach for scale measurement was presented previously for (Davoody et al., 2017, 2018): (1) growing magnesium hydroxide [Mg(OH)2 ] scale at high
influence on the flow velocity near the vessel wall. Velocity of fluid flow is one of the major factors that dictates the rate of scale deposition and growth. Role of flow velocity on scale
temperature within an hour, (2) designing and building a tank that can be disassembled thereby providing unrestricted access to the scale
thickness and deposition rate has been a topic of interest in several ¨ studies (Bansal and Muller-Steinhagen, 1993; Ceylan and Kelbaliyev, 2003; Fahiminia et al., 2007; Hasson et al., 1968; Hoang, 2015; Karabelas, 2002; Lee et al., 1999; Thonon et al., 1999; Walker and Sheikholeslami, 2003; Watkinson and Martinez, 1975). It has been reported that an increase in flow velocity leads to a decrease in scale thickness if the ¨ flow is fully turbulent (Ceylan and Kelbaliyev, 2003). Bansal and MullerSteinhagen (1993) reported a significant reduction in the scaling rate of calcium sulfate on heat exchanger plates as flow velocity was increased from 25 to 85 cm/s. Thonon et al. (1999) also observed that at velocities higher than 1 m/s, the amount of calcium carbonate scale formed on the plates of heat exchangers was insignificant. Lee et al. (1999)
quantitatively. As such, there is a lack of reliable information in the literature that shows how baffles can influence the rate and distribution
formed on the tank wall for the purposes of measuring its thickness, and (3) utilising a coordinate measuring machine (CMM) to physically scan the surface of the wall containing the scale. The results obtained using this novel approach are in agreement with the visual observations thereby providing confidence in the method developed. The present work utilises the same approach to grow, study, and quantify scale thickness and distribution in a stirred vessel. The effect of the tank configuration, especially the influence of baffles, on the scale formation and its growth on the tank wall was investigated experimentally. In addition, a computational fluid dynamics (CFD) model was developed to predict the near wall liquid velocities and wall shear stress in the mixing tank. The results from the CFD simulations are used to explain the scale formation results for baffled and unbaffled tanks.
reported that the increased wall shear rate at the higher cross-flow velocities would create an unsuitable environment for surface crystallization. From the available literature, it is reasonable to envisage that increasing the impeller speed in a mixing vessel can mitigate the deposition and growth of scale on the walls as it enhances the near-wall flow velocity. Unbaffled stirred tanks have been a topic of interest in several studies involving experimental work (Abatan et al., 2006; Brucato et al., 2017, 2010; Busciglio et al., 2017, 2016; Davoody et al., 2016; Galletti and Brunazzi, 2008; Kagoshima and Mann, 2006; Lamberto et al., 1999; Pinelli et al., 2001; Rao and Kumar, 2007; Scargiali et al., 2017; Tamburini et al., 2014, 2012; Tezura et al., 2007; Wang et al., 2012; Yoshida et al., 2008, 2007), and computational fluid dynamics (CFD) studies (Alcamo et al., 2005; Cokljat et al., 2006; Derksen, 2004, 2006; Deshpande et al., 2017; Murthy Shekhar and Jayanti, 2002; Sbrizzai et al., 2006; Shan et al., 2008). A number of the above studies has indicated that unbaffled tanks are more energy efficient compared to baffled tanks for solids suspension (Brucato et al., 2010; Davoody et al., 2016; Tamburini et al., 2013). This can be mostly attributed to the fact that kinetic energy dissipation is lower in unbaffled vessels. However, a major drawback in these vessels is the generation of a vortex at the free liquid surface due to the absence of baffles. This is undesirable as it can lead to gas entrainment especially at high impeller speeds during the experiments. This problem, however, is less significant at full-scale due to the lower operating impeller speeds. Nonetheless, the scale formation and growth behaviour in unbaffled vessels is not yet well understood and the literature on this topic is still scarce.
2.
Methodology
To have an unrestricted access to the scale grown on the tank wall, a stainless-steel reactor with diameter T = 100 mm was designed and fabricated in a way that it could be disassembled into nine separate pieces consisting of a base, four walls, and four gap stainless-steel fillers with a width of 35 mm (Fig. 2a). These fillers served as baffles with a width (W) of 0.1T when the tank is assembled in a baffled configuration. Another set of four stainless-steel fillers with a width of 25 mm replaced the 35 mm wide fillers when the tank was used in an unbaffled configuration. The individual pieces used in assembling the reactor are shown in Fig. 2a, and the baffled and unbaffled tanks assembled using the individual pieces are shown in Fig. 2b and c, respectively. The agitation was provided by a Lightnin A310 impeller with diameter D = 0.6T attached to a shaft that was located at the vertical axis of the tank and driven by a motor. The liquid height H was set at 1.6T in all experiments. The impeller clearance (C) from the tank bottom was set at 0.3T in all experiments. The Lightnin A310 impeller is an axial-flow impeller widely used in the mineral processing industry for achieving off-bottom solids suspension. This impeller was chosen
Chemical Engineering Research and Design 1 4 6 ( 2 0 1 9 ) 11–21
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distribution on the wall. Further details on the scanning and the measurement of scale thickness distribution are provided elsewhere (Davoody et al., 2017, 2018).
3. Computational fluid dynamics (CFD) simulation
Fig. 2 – The stainless-steel tank fabricated for scale growth studies: (a) individual pieces used in the tank assembly, (b) assembled baffled tank, (c) assembled unbaffled tank. Table 1 – Impeller speeds used in this work. Impeller speed (rpm)
Impeller tip speed/ velocity (m/s)
Reynolds number
400 430 460
1.26 1.35 1.44
15,186 16,325 17,464
to provide the agitation in the reactor used in this study to simulate the scale formation processes in full-scale operations. Table 1 lists the impeller speeds used in this work and the corresponding impeller tip speeds and impeller Reynolds numbers. The scale analog used in this study was magnesium hydroxide, formed from the reaction between Ca(OH)2 and MgCl2 , in the form of a precipitate. All experiments were run for 60 min. The temperature of the reactor liquid was kept constant at 80 ◦ C by placing the reactor in a water bath. The impeller speed was varied from 400 to 460 rpm. More details on the experimental procedure, and the arrangement of the wall segment on the CMM can be found elsewhere (Davoody et al., 2017). The scale formed on the tank walls was physically scanned using a CMM after a thin protective spray paint coating was applied to make the scale firm, thereby protecting it from any form of mechanical failure during measurements. Before any scale-growth run, the surface profile of each ‘clean’ reactor wall segment was scanned using a Sheffield Discovery II CMM to provide a set of reference coordinates. Once the reference coordinate values were documented, the tank was assembled and placed into the water bath for a scale growth run. The coordinate values of the scaled wall segment were compared with the reference coordinates to calculate the scale thickness at each coordinate and determine the scale thickness
Geometries for both the baffled and unbaffled stirred vessels were constructed using ANSYS/CFX 19.0. Both models were geometrically identical except for the presence of baffles. The vessel was cylindrical with a diameter of 100 mm and a height of 200 mm. The liquid height used was 157 mm. The liquid used was water with a density of 998 kg m−3 and a dynamic viscosity of 0.00089 Pa s. A single-phase model was used and the top liquid free surface was modelled as a flat free-slip surface. In the unbaffled tank experiments, a vortex near the liquid surface produced a rise in the liquid level near the walls and a depression near the shaft. The magnitude of this surface distortion and its effect on flows in the bulk of the tank was not considered significant enough to warrant increasing the model complexity. The diameter of the A310 impeller was 60 mm and its hub centre was 29 mm above the vessel base. A stirring speed of 430 rpm was used in the simulation. A hybrid mesh was used to represent the flow domain of each vessel with inflation layers at the vessel wall and impeller blade. Each mesh consisted of approximately 6.2 million nodes and 21.4 million elements, with increased mesh resolution near the impeller. For the conditions modelled, the y+ values on the wall and blades ranged from 0.5 to 2.0. Impeller rotation was accounted for by using a transient simulation and a sliding mesh approach where a section of mesh surrounding the impeller was rotated in 10◦ increments at each time step and the mesh for the surrounding vessel and the baffles remained stationary. A General Grid Interface (GGI) was used to connect the rotating and stationary mesh components. ANSYS/CFX 19.0 was used to solve the unsteady RANS equations using the couple solver and the shear stress transport turbulence model with a curvature correction factor of 1.0 (Menter, 1994). Second order backwards integration was used to advance the solution in time and the high-resolution differencing scheme was used for convective terms in the equations. Further details of the modelling approach used in this work can be read in similar studies reported elsewhere (ANSYS, 2017; Lane, 2017). The CFD models were solved in stages. The initial conditions for the transient situation were obtained by solving a steady state model using a frozen rotor approach. This was used to initialise the transient simulation with a time step set to give 10◦ rotation per time step, with 10 iterations per time step. To eliminate start-up effects, several rotations were modelled until consistently repeatable velocities at several monitoring points and the impeller torque were obtained. Thirty rotations were performed for the unbaffled vessel while 50 rotations were performed for the baffled vessel. After these initial rotations, a further 20 rotations were modelled for each vessel and the transient statistics was processed to obtain an average flow field.
4.
Results and discussion
In this section the results from the experimental findings and the CFD simulations are presented and discussed.
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Chemical Engineering Research and Design 1 4 6 ( 2 0 1 9 ) 11–21
Fig. 3 – Scale build-up on the walls of an unbaffled tank.
4.1.
Unbaffled tank configuration
Fig. 3 shows an example of scale build-up on the walls of an unbaffled tank after a scale-growth run. It is evident that the scale formation at the bottom region of the tank is insignificant whereas its presence in the top part is significant. Also, each wall segment has a similar distribution pattern. To measure the thickness of scale, the tank was disassembled after each run and one of the four wall segments was coated with spray paint and analysed. Fig. 4a provides a close-up view of scale growth pattern on the wall of the unbaffled tank after 60 min while Fig. 4b presents a 3D map of the scale thickness distribution on the same wall obtained from the physical scan of it using the CMM. The resolution of the scan is 1 mm, which led to a matrix of 70 (X direction) × 180 (Y direction) readings on the surface of each segment. Each scan therefore generated 12,600 ( = 70 × 180) scale thickness results, which were then analysed and processed for preparing the scale thickness distribution maps. A comparison between the two figures confirms that the CMM scan technique employed in this work provides a quantitative measure of the scale thickness distribution on the walls of the stirred tank with acceptable accuracy. The scale
distribution map in Fig. 4b shows that scale formation is considerable near the liquid surface at the top region of the wall segment and insignificant in the impeller plane and tank base. This difference, which is consistent with the visual observation results (Fig. 4a), can be mostly attributed to the fact that impeller rotation creates a turbulent regime at the tank bottom that mitigates the formation of scale, while the areas close to the liquid surface provide a more conducive environment for the scale to deposit and grow. Fig. 4a shows a patch of scale formation at the bottom of the tank wall which does not appear on the scale-map Fig. 4b. When the impeller was stopped at the end of the experiment, the solid particles suspended inside the vessel started to settle and form a sediment bed at the bottom of the tank. Because the tank was placed inside a water bath, it was not possible to pump the settled solids fully out of the tank. As a result, the sediment formed at the tank bottom left some scale residue on the wall which could be noticed after the tank was disassembled. These residues were removed prior to the CMM scanning because they were not relevant to the mechanisms of scale formation and growth studied in this work. From Fig. 4b, Max, Average, and Sum scale thickness values were determined to be 1.293, 0.123, and 1552.6 mm, respectively. These terms refer to the highest recorded value of scale thickness on the wall segment, average scale thickness value calculated using all the readings on the wall segment ( = 12600 in each scan), and sum of all the scale thickness values, respectively. These values were calculated for each scan and used to compare and evaluate the influence of impeller speed and baffles on the scale thickness. Additionally, a parameter called ‘Average Scale Thickness in Y direction or (AST) Y ’ was calculated by taking the average of the scale thickness values over discretised 10 mm height intervals over the full height of the tank. Fig. 5 shows the (AST) Y values as a function of the tank height for an experiment carried out at 400 rpm under unbaffled condition. The location of the impeller is also shown in the figure for comparison purposes. The (AST) Y values shown in Fig. 5 indicate that the scale thickness distribution is influenced by the magnitude of the liquid flow velocity especially near the tank wall. The liquid
Fig. 4 – The scale formed on a wall segment of the unbaffled tank: (a) experimental observation, (b) scale thickness distribution map generated using results from the CMM scan (N = 400 rpm).
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Fig. 5 – (AST) Y values as a function of the tank height for the unbaffled tank.
Fig. 6 – Effect of the impeller speed on (AST) Y values, unbaffled tank. velocity closer to the impeller region is expected to be higher due to the impeller pumping. However, the liquid velocity is expected to decrease as the liquid flows towards the surface and changes its flow direction there thereby losing its momentum. The higher liquid velocity in the impeller region is expected to slow down the formation and growth of the scale on the tank wall, which explains the smaller (AST) Y values in the bottom half of the tank. In contrast, the lower liquid velocity near the liquid surface is expected to lead to the formation of thicker scale thereby resulting in higher (AST) Y values. This observation indicates that liquid flow velocity near the tank wall is a critical factor in the rate of formation of scale and its distribution on the vessel wall. To better understand the role of liquid flow velocity on scale thickness distribution in
an unbaffled tank, scale growth experiments were conducted also at 430 and 460 rpm, and the results for all three stirrer speeds are shown in Fig. 6. The (AST) Y values for all three impeller speeds increase with increasing height up to the liquid surface (160 mm). The (AST) Y values for 400 rpm are higher compared to those for the other two higher impeller speeds at heights between 30 and 100 mm, whereas those for 460 rpm are greater for heights 10, 110, 130, 140 and 160 mm. At all other heights, the (AST) Y values for 430 rpm are greater than those for other two impeller speeds. The (AST) Y results indicate that, in general, the scale thickness increases at higher axial locations but decreases above the liquid surface (160 mm). There is no clear trend in (AST) Y values as a function of the impeller speed probably
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Fig. 7 – The scale formed on the wall of a baffled tank: (a) experimental observation, (b) quantitative map generated using the results of CMM scan (N = 400 rpm). because the impeller speed interval used in this work (30 rpm) is small. Therefore, the effect of impeller speed with larger intervals on (AST) Y values needs to be studied in future investigations.
4.2.
Baffled tank configuration
To understand how baffles in a stirred tank can influence the patterns of scale growth, experiments were also carried out in a baffled tank under similar operating conditions. Fig. 7a shows the scale formation profile on the wall of a baffled tank while Fig. 7b depicts the quantitative scale formation map plotted using the results of the CMM scan for N = 400 rpm. A comparison between Fig. 7a and b reveals that there is a strong agreement between the observed patterns of scale formation (Fig. 7a) and the quantitative map generated using the CMM scan readings similar to that observed for the unbaffled tank. As explained earlier, the residues observed at the bottom of the wall segment was removed prior to the CMM scanning.
4.3. Comparison of scale formation under baffled and unbaffled conditions The quantitative maps of scale growth on the stirred tank wall for both baffled and unbaffled conditions at 400 rpm are compared in Fig. 8. A common scale thickness range of 0-1.8 mm was selected to represent the scale distribution for both tank configurations. The scale formation pattern on the tank wall was influenced significantly by the presence of baffles during the experiment. The scale grown on the wall was uniform and evenly distributed for the unbaffled tank while it was non-uniform and mostly in the top left corner of the wall for the baffled tank. Considering the clock-wise direction of the impeller rotation, the accumulation of scale on the left of the wall can be attributed to the flow disturbances caused by the presence of baffles. This disturbance slows down the flow velocity, and creates a suitable environment for scale deposition and growth. The Max, Average, Sum scale thickness values for baffled and unbaffled tanks are listed in Table 2. The difference in the three scale thickness values indicate that operating the tank without baffles rather than with baffles can reduce the amount of scale formed on the surface of the walls significantly.
Fig. 8 – Quantitative maps of scale growth on the wall segment of (a) baffled tank (b) unbaffled tank. Table 2 – Quantitative comparison between baffled and unbaffled tanks. Scale thickness (mm)
Max Average Sum
Baffled condition
Unbaffled condition
1.765 0.147 1854.4
1.293 0.123 1552.6
% reduction in scale thickness
27 16 16
The (AST) Y values under baffled and unbaffled conditions are shown in Fig. 9a. The location of the impeller and its rotational direction are also shown in the figure for reference. The (AST) Y values were smaller at the tank bottom and in the impeller region, and increased with an increase in the liquid height. The highest (AST) Y values of 0.62 and 0.39 mm for baffled and unbaffled configurations, respectively were near the liquid surface. This is probably because the lower liquid flow velocity at the liquid surface created a calmer environment for the precipitates to deposit and grow on the wall. The (AST) Y values for the unbaffled configuration were consistently lower than those for the baffled configuration except at a few locations. When compared to the baffled condition, scale was observed at fluid heights greater than 160 mm for
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Fig. 9 – Effect of baffling configuration on average scale thickness values, (a) (AST) Y , (b) (AST) X , (N = 400 rpm). the unbaffled condition. This can be attributed to a slight rise in the liquid level at the tank wall during mixing under baffled condition. The influence of baffles on the scale growth and thickness distribution can also be studied by plotting (AST) X , which is the average of scale thickness values over an interval of 10 mm in the x-direction (circumferential direction). The (AST) X values for the two configurations are shown in Fig. 9b as a function of the fluid height. The (AST) X values ranged from 0.1 to 0.19 mm and 0.11 to 0.13 mm for baffled and unbaffled conditions, respectively. The differences in (AST) X values indicate that the overall amount of scale on the tank wall under baffled condition was higher than that under unbaffled condition. Under baffled condition, the (AST) X values were greater on the left side of the wall, lacking the uniformity found under unbaffled condition.
4.4.
CFD simulation results
Power number values for the baffled and unbaffled tanks were determined from CFD simulation as 0.27 and 0.19, respectively. The velocity and wall shear stress values obtained by solving the CFD simulation model are presented in Figs. 10–15. Velocity vectors are projected onto a vertical plane aligned to one
of the impeller blades, and in the case of the baffled tank they are located midway between the baffles. In Fig. 10, instantaneous vectors coloured by the velocity components tangential to the plane are shown for both baffled and unbaffled tanks. These plots show strong downward and outward flows from the blade tip and several transient vortices through the vessel. For the baffled tank, the flow field shows a degree of asymmetry with the large vortex on the left side of the image, reaching higher up in the vessel than the vortex on the right side of the image. This asymmetry is due to the transient nature of the flow induced by blades passing the baffles. The ‘time averaged’ velocity vectors projected on the same vertical planes shown in Fig. 11 indicate that time averaged flow is symmetrical for both baffled and unbaffled conditions. In the case of the baffled tank, the impeller-generated strong downward flow gave rise to a toroidal vortex in the lower third of the vessel and no significant time averaged flow in the upper part of the vessel. In the unbaffled tank, the downward flow from the blades was deflected towards the wall where it was split into two parts forming lower and upper vortices with the upper vortex extending to the liquid surface. While the radial and axial velocity components are small compared to the tangential velocity, they are important for vertical mixing and suspending particles in multiphase
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Fig. 10 – Predicted projected velocity vectors at an instant in time after 70 rotations for the baffled vessel (left) and 50 rotations for the unbaffled vessel (right).
Fig. 11 – Predicted time averaged projected velocity vectors for the baffled vessel (left) and unbaffled vessel (right).
Fig. 12 – Predicted time averaged tangential velocity on a vertical plane for the baffled vessel (left) and unbaffled vessel (right).
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Fig. 13 – Predicted time averaged velocity magnitude 0.5 mm inward from the vessel wall for the baffled vessel (left) and unbaffled vessel (right).
Fig. 14 – Predicted wall shear stress on the vessel wall at an instant in time after 70 rotations for the baffled vessel (left) and after 50 rotations for the unbaffled vessel (right).
systems. The time averaged tangential or swirl velocity component is plotted in Fig. 12. It shows that the presence of baffles greatly limited the swirl in the vessel with the main rotation present in the lower third of the vessel around the impeller region. Under unbaffled condition, a free-force or Rankine vortex structure was generated by the impeller around the shaft. The time averaged distribution of the liquid velocity magnitude at 0.5 mm from the vessel wall is plotted in Fig. 13. The velocity scale range is set based on the maximum liquid velocity for the unbaffled vessel. The maximum near wall velocity for the baffled condition was 0.38 m s−1 , which was twice that in the unbaffled case. However, the high average liquid velocity in the baffled tank occurred only in the lower 20% of the vessel and average velocities above the mid-liquid height were below 0.02 m s−1 . On the other hand, it is evident that the near wall velocity for the unbaffled tank exceeded 0.1 m s−1 over its entire height. The wall shear stress at the end of each simulation is plotted in Fig. 14 and it shows that the stress dis-
tribution pattern was similar to that for the near wall velocity distribution shown in Fig. 13. Thus, it can be concluded that the higher average velocities above the mid-liquid height in the unbaffled case prevent/mitigate the deposition and growth of scale on the surface of the vessel, which is consistent with the experimental results presented in Fig. 9. Fig. 15 presents a comparison of the time-averaged velocity distribution at a horizontal plane 40 mm below the liquid surface for the baffled and unbaffled modes at 430 rpm. The circulating flow in the unbaffled tank has a uniform yet strong near-wall velocity, which can be associated with the uniform distribution of scale as shown by the (AST) X values depicted in Fig. 9b. In contrast, the fluid flow in the baffled tank has a non-uniform velocity distribution due to the presence of baffles which act as barriers. The low-velocity zones found behind the baffles create a suitable environment for the scale growth, which is confirmed by the larger (AST) X values on the left side of the wall compared to the right side for the baffled tank shown in Fig. 9b.
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Fig. 15 – Velocity distribution on horizontal planes 40 mm below the free surface at 430 rpm for the baffled vessel (left) and un-baffled vessel (right).
5.
Conclusion
A qualitative and quantitative study on the scale formation in stirred tanks was conducted. A purpose-built tank that could be disassembled into nine segments including a base, four walls, and four baffles or four blanking pieces, has been demonstrated to provide quantitative scale growth distribution results when a wall segment was scanned using a CMM. The CMM readings, when presented as 3-D graphs of scale thickness distribution, enable a quantitative comparison of scale growth for baffled and unbaffled tanks. It was observed that the bottom region of the tank was almost free of scale whereas the top part had significant scale formation. It was also noted that the thickness of scale decreased significantly when operating the tank under unbaffled condition which can be attributed, based on the CFD simulations, to the increased liquid flow velocity near the walls when baffles are absent.
Acknowledgements One of the authors (M.D.) gratefully acknowledges the support of the Australian Government Research Training Program (RTP) Scholarship through RMIT University. Appreciation is also extended to CSIRO colleagues Mr. Dean Harris, Mr. Greg Short, and Mr. Bon Nguyen, and the CSIRO Clayton workshop.
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