A Novel Approach To Quantify Scale Thickness and Distribution in

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A NOVEL APPROACH TO QUANTIFY SCALE THICKNESS AND DISTRIBUTION IN STIRRED VESSELS Meysam Davoody, Lachlan J.W. Graham, Jie Wu, Inju Youn, Abdul Aziz Abdul Raman, and Rajarathinam Parthasarathy Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b03543 • Publication Date (Web): 15 Nov 2017 Downloaded from http://pubs.acs.org on November 17, 2017

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Industrial & Engineering Chemistry Research

A NOVEL APPROACH TO QUANTIFY SCALE THICKNESS AND DISTRIBUTION IN STIRRED VESSELS Meysam Davoodya,b, Lachlan J.W. Grahamb, Jie Wub, Inju Younb, Abdul Aziz Abdul Ramanc, Rajarathinam Parthasarathy*a a: School of Chemical and Environmental Engineering, RMIT University, Melbourne, VIC 3000, Australia b: CSIRO Mineral Resources, Melbourne, VIC, Australia c: Department of Chemical Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia *Email: [email protected], Tel No.: +61-3- 9925 2941

Abstract A novel approach is proposed for the numerical evaluation of scale thickness and its distribution in a mixing tank. While a majority of the available literature on the scale build-up is focusing on the applications of chemical anti-scalants, few works have been devoted to preventing scale formation through a proper design and operation of stirred vessels. The methodology proposed in the current study consists of two major phases: 1) identifying an accelerated process to grow scales on the walls of a laboratory reactor and 2) analysing the grown scale physically. These objectives were achieved through the fabrication of a reactor that could be disassembled and scanning the reactor’s surface using a coordinate measuring machine (CMM). The proposed approach was used to study the effects of liquid flow velocity and experiment duration on the pattern of the scale formed qualitatively and quantitatively.

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Keywords: Mixing, scale prevention, CMM, fluids mechanics, flow velocity

Introduction A scale can be referred to as an undesirable hard, adherent deposit that precipitates from solution and grows on a surface1. Scaling is a serious issue to industry, in particular, mineral processing. Increased capital expenditure, reduced capacity, and production loss during the de-scaling operations are a few of the ongoing issues caused by scale formation in processing vessels used in the mineral processing industry2. In other industries, the formation of scale can lead to a reduction in flow and heat transfer and additional maintenance or even breakdown of equipment, resulting in increased operational costs 3. These industries include membrane-based desalination process 4, thermal desalination 5

, cooling water systems 6, pulp and paper industry involving alkaline spent pulping liquor

evaporators 7, geothermal systems 8, oil industry 9, power generation industry processes

11

, and tungsten hydrometallurgical processes

unresolved problem in heat transfer

13

12

10

, dairy

. Scaling is regarded as the major

, and it has been estimated that it costs the cooling

industry 0.5 billion US dollars per year in the United Kingdom to tackle the formation of scale

14

. It is also believed that the operating costs associated with removing scales from a

200-million-dollar dairy processing plant can reach up to 33 million US dollars 11. Significant economic impacts of scaling forces engineers and researchers to look for scale inhibition solutions. The most popular technique to overcome scaling issues is to suppress the deposition process using chemical anti-scalants that are capable of inhibiting the growth of a scale layer and weakening its adherence to the flow surface. The extent of scale suppression provided by a specific anti-scalant depends on a number of parameters. The uncertainties in selecting an anti-scalant for a particular application are due to the poor understanding of the fundamentals of the inhibition mechanisms. The available literature on this topic provides 2 ACS Paragon Plus Environment

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limited information on the expected performance of a specific anti-scalant in a given scaling situation. As a result, applications of anti-scalants rely heavily on empiricism 15. It has been observed that feed treatment in the desalination industry usually has negative impacts on the effectiveness of the anti-scalants, and often results in the loss of scale inhibition control 16-18. A number of studies have also found that the anti-scalants designed to control scaling are ineffective for suppressing precipitation 16, 19. There are a number of studies that indicate the flow rate of the solution has significant influence in scale prevention in pipelines

20, 21

, but the role of fluid dynamics in scale

prevention in mixing tanks has received little attention. Identifying an optimum design and operation that leads to mitigation of scale formation can be invaluable for the industry as it provides a reliable anti-scaling solution for stirred vessels regardless of the system. A major obstacle in that regard is lack of quantitative techniques that can be used to analyze the scaling behaviour in different designs. This challenge can be overcome by quantification of scale thickness and distribution on the surface of the reactor’s walls. A popular approach in quantifying scale thickness is installing a coupon on the surface of the studied vessel’s inner wall and measuring its weight before and after scale deposition 22-25. The coupon can become a barrier to the fluid flow and influence the mass of the scale deposited on its surface. As a result, the data obtained using this technique are subject to errors and uncertainties. Therefore, the vessel’s inner wall, where scales grow, needs to be free of any flow barriers. The present work aims at introducing a novel and reliable approach to grow, study, and quantify scale thickness and distribution in a stirred vessel. Quantitative analysis of scale distribution on the inner surface of mixing tanks can be hugely beneficial in developing a suitable design that prevents/mitigates scale growth. Magnesium hydroxide (brucite, Mg(OH)2) is chosen as the scale in the present work. It is a common scale that affects a number of industries including cation exchange membranes, boiler feedwater heaters, cooling 3 ACS Paragon Plus Environment


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systems, and falling film evaporators used in desalination

26-29

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. Moreover, it has a rapid

growth rate and consistent behaviour making it a suitable material for the study of scale formation.

Methodology Experimental setup The stainless steel tank used in this work was 10 cm (T) in diameter. Four equally spaced baffles with the width of 1 cm (0.1T) were attached to the internal wall along the entire depth of the tank to minimise vortex effects. The agitation was provided by a Lightnin A310 impeller attached to a shaft that was placed on the vertical axis of the tank and driven by a motor. The Lightnin A310 impeller is an axial-flow impeller widely used in the mineral processing industry. The A310 is the impeller of choice in several mineral processing operations that are associated with high maintenance costs for scale removal (e.g., precipitation and neutralization tanks). Thus, the A310 impeller was used to provide agitation in the reactor used in this study as a representative of full-scale practice. There are two major challenges in studying scale behaviour in mixing tanks: growing strong scales in a short period and quantifying scale thickness throughout the tank. In the mineral processing, the scale is built-up over an extended period. Therefore, identifying an accelerated process that leads to the production of scales in a short time is compulsory in studying this problem in the laboratory-scale. At the same time, it is crucial to study scale distribution at various depths inside mixing tanks to have a clear understanding of scale behaviour. As mentioned earlier, it is necessary to have unrestricted access to the tank’s inner surface to have a clear picture of scale growth and distribution. Therefore, a stainless steel tank was designed and fabricated in such a way that it could be disassembled into nine


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segments including a base, four walls, and four baffles. The tank has been engineered to be leak-proof as it was essential to ensure the solution inside would not leak into the water bath surrounding the tank during experiments conducted at high temperature using high impeller speeds. An O-ring and a gasket were used to make sure there was no gap between the base and bottom of walls and baffles. Figure 1 shows the schematic drawings of the mixing tank, and Figure 2 shows the fabricated tank at CSIRO’s workshop.

Figure 1 Schematic drawings of mixing tank fabricated for scale studies: W1-4: walls, B1-4: Baffles, and Base: Tank Bottom



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a

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b

Figure 2 The mixing tank fabricated for scale studies: (a) side view, and (b) segments

The tank was placed in a water bath operating at 80°C during scale tests. All parts of the tank were fabricated out of stainless steel to ensure sufficient heat transfer between the bath and solution inside. Growing scales A mixture of calcium hydroxide (Ca(OH)2) and water with 1.6M concentration was used as the starting liquid medium in our experiments. The experiment was commenced by feeding dropwise 500 ml of 2M sodium carbonate (Na2CO3) solution into the tank using a pump. The molar ratio of sodium carbonate added to calcium hydroxide was 1.2, i.e., the amount of sodium carbonate is slightly excessive to that of calcium hydroxide

30

. Then, 10 grams of

magnesium chloride (MgCl2) powder was gradually added to the solution in the tank under agitation. All chemicals used in our experiments were supplied by Sigma-Aldrich in the powder (fine) form. All experiments were run for 60 minutes. The temperature of the reactor


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liquid was kept constant at 80°C by placing the reactor in a water bath. The experimental setup is shown schematically in Figure 3. The details of the experimental set-up are provided in Table 1. The scales formed in our experiments are mainly due to the generation of magnesium hydroxide (Mg(OH)2) from the chemical reaction between Ca(OH)2 and MgCl2 in the form of a precipitate. The formation and growth of scales are enhanced due to the intimate contact between the circulating solution and the hot surface of reactor wall. The hot surface of the reactor wall encourages the precipitation of Mg(OH)2 and thereby accelerates the process of scale growth. The model system employed in our experimental work simulates the scaling phenomena that occur due to chemical reactions in various processes employed in industrial mineral processing operations like the Bayer process 31, 32 and nickel laterite process in stirred autoclaves 33.

Figure 3 Schematic of the experimental setup used in this study



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Table 1 Details of the experimental rig Symbol

Parameter

Value

Symbol

Parameter

Value

HL

Liquid height

120-165

T

Tank diameter

100

mm HB

Water bath liquid

mm

220 mm

W

Baffle width

10 mm

200 mm

C

Impeller clearance from

30 mm

height HT

Tank height

tank bottom D

Impeller diameter

50 mm

Number of baffles

4

At the end of each experimental run, the reactor solution was pumped out of the tank, and the tank was then disassembled to study the scales grown on the inner walls. A small portion of the grown scale was removed and analyzed using XRD for phase identification. The results discussed in this work have all been obtained after three consecutive experimental runs. This is the reason for the different liquid height values shown in Table 1 for each experimental run. At the end of the first run which usually lasted for 1 hour, the impeller was stopped and the reaction mixture was pumped out of the tank, but the scales grown on the walls and the residuals at the bottom of the tank were not removed. To simulate the operation of a continuous process, the second run was commenced by adding fresh reaction mixture to the tank while the scales and residuals from the first run were still inside the tank. Similarly, the third run was conducted by adding fresh reaction mixture to the tank while scales that had been deposited during the first and second runs remained in the tank. As a result, the liquid height in the tank kept increasing in the second and third runs. The residuals at the tank 8 ACS Paragon Plus Environment

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bottom were allowed to remain after each run due to the difficulties involved in removing them considering the reactor was located in a water bath. Also, we did not want to move the reactor frequently in and out of the water bath because it could disturb the fragile scales on the reactor wall leading to its dislodgement. Quantification of scale distribution Once the scales were grown on the inner reactor wall, they were quantified in two steps: 1) binding the scale to the reactor wall through a coating, and 2) determination of the scale thickness using the coordinate measuring machine. Coordinate measuring machine A coordinate measuring machine (CMM) is a device for measuring the physical geometric characteristics of an object. Measurements are made by a probe attached to a moving axis of this machine. Recently, CMM has been used in many erosion studies

34-41

. To the authors’

best knowledge, its application in studying scales in stirred vessels has not been explored. Before the main runs, the surface profile of one of the plain reactor wall segments was documented using a Sheffield Discovery II CMM with a measurement accuracy of ±1 μm and a system accuracy of ±6 μm. Once the reference coordinate values were recorded and stored, the tank was reassembled and placed into the water bath for the scale growth runs. Once scales were formed, the reactor was disassembled, scales were coated with spray paint, and the profile of the coated scales on the wall segment was measured using the CMM. The coordinate values of the scaled wall segment were compared against the original ones, based on which the thickness and the distribution of the scales were determined. Scale measurements were conducted using only one wall segment out of four available. The scale profiles in the other three wall segments are considered to be similar to the one measured due to geometrical symmetry. 9 ACS Paragon Plus Environment


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Coating As explained earlier, CMM has been extensively used in many erosion studies, but its use in studying scale formation and growth is rare. This can be attributed to the fact that the scale grown on reactor walls can be fragile and may not always be firm enough to withstand the physical touch of the machine’s probe. Consequently, physical analysis of the scales formed on the walls of a mixing tank has always been challenging. Therefore, in this work, the scales grown were coated with spray paint to ensure the scale structure was not damaged during the physical scans. In this study, a quick drying acrylic enamel spray paint was used as a binder. The binder used, whose commercial name was ‘Squirts,' was flat white and supplied by White Knight Paints (Australia). The spray paint was used to coat the scales on the reactor wall segment to harden them and bind them to the segment’s surface. Before coating the scales, the sides of the wall segment, which were serving as the reference points for the CMM, were covered with painter’s tape (Fig. 4). This was important as any paint or scale on the sides would provide incorrect references. Once taped, the wall segment containing scales was uniformly coated by applying the spray paint on their surface. The wall segment was then allowed to rest for 24 hours to ensure complete drying of the paint. The painter’s tapes were then removed, and the surface of the wall segment was scanned physically using the CMM. Figure 4 depicts the wall segment that has been coated with the spray paint in the binding process.


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Figure 4. a) Scales formed on the wall segment at the end of the scaling run b) Covering the sides of the segment with blue coloured painter’s tape c) Wall segment coated with spray paint d) Wall segment coated with spray paint after the removal of painter’s tape

During the scanning of scales by the CMM, the coated wall segment was held by a vice on the machine table under the scanning probe as shown in Figure 5. Figure 5a shows the wall segment arrangement on the CMM and Figure 5b provides a closer view of the wall segment during the scan.

a

b

Figure 5 (a) Wall segment arrangement under CMM (A: CMM controller, B: Vice, C: Spray painted wall segment and D: CMM probe). (b) Close-up view of the setup during the scanning operation. To obtain a detailed scan of the scale profile, the CMM was programmed to conduct readings every 1 mm in both the X (70 counts) and Y (180 counts) directions. As a result, 12600 different spots were analyzed during a 13-hour operation to provide a matrix of 70 mm × 180 mm × thickness of coordinate data. The recorded values were then transferred into MS Excel (2013) spreadsheet, where they were compared against the reference values (obtained from measurements of the clean wall segment). The scale thickness values obtained from the



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spreadsheet were then plotted using ParaView 5.3.0 software to obtain two-dimensional images of scale distribution on the wall segment.

Results and discussion Figure 6 shows the scale build-up on the walls of the reactor at the end of the third run. It can be seen that the bottom region of the reactor is almost free of scale, whereas the scale formation at the top part is quite significant.

Figure 6 Scale on the walls of the mixing tank


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XRD phase identification A sample of the grown scale was subjected to XRD analysis for phase identification. The procedure used in XRD analysis is explained in the Supporting Information. The results of the phase identification and Quantities Phase Analysis (QPA) are given in Table 2. The scale sample contains sodium carbonate (Na2CO3), sodium carbonate hydrate (Na2CO3.H2O), calcium carbonate (CaCO3), sodium chloride (NaCl), magnesium hydroxide (Mg(OH)2) and calcium hydroxide (Ca(OH)2). These results show weight percentages of the components and do not include any unidentified or amorphous material which may exist in the sample. Figure 7 shows the output from the Rietveld analysis showing the goodness of fit of the model to the observed data. In this Figure the blue line represents the observed data, and the red is the calculated pattern generated from the crystal structures of the constituent phases. The references to the crystal structure models used for the analysis are also given in Table 2. Table 2 Results of the QPA Sample

Na2CO3

Na2CO3.H2O

CaCO3

NaCl

Mg(OH)2 Ca(OH)2

Scale

14%

25%

1%

4%

55%

1%

Reference

42

43

44

45

46

47

for the peak



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Figure 7 Output of the Rietveld analysis of the scale sample CMM analysis Once the experiments were complete, the tank was disassembled, and scale grown on the wall segments were analyzed after coating using CMM. Figure 8 provides a close-up look at the scale distribution profile on one wall segment at three different impeller speeds. It also includes a picture showing the scale grown on the shaft and impeller from a run carried out at 400 RPM. As can be seen from Figure 8, a reduction in the impeller speed leads to an increase in the extent of scale. It should be noted that the wall segment represents the region in between two consecutive baffles. It can also be seen that, at any given impeller speed, there is no significant scale formation in the lower part of the reactor probably due to the high liquid velocity present near the impeller region. The scale build-up occurs mostly at higher liquid levels, and it is thicker close to the liquid surface.


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Figure 8 Scale profile on a wall segment at different impeller speeds and on the impeller for 400 RPM Quantification of scale thickness and distribution

Figure 9 Scale distribution on the surface of a wall segment, Impeller speed N = 400 RPM Figure 9 shows a picture of the scale distribution profile of a wall segment, which was produced using ParaView 5.3.0 software based on CMM readings. By comparing the map of the scale distribution shown in Figure 9 with that observed in Figure 8 for 400 rpm, it is evident that CMM can read and map the scale thickness throughout the surface. Similar to the patterns observed in Figure 8, the scale-map in Figure 9 shows that scale formation is 15 ACS Paragon Plus Environment


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dominant in the top region of the wall segment near the liquid surface and insignificant at the tank bottom and close to the impeller region. It was of interest to investigate the effect of impeller speed and experiment duration on the scale profiles using the proposed approach. Thus, a number of experiments were conducted at various impeller speeds (N) and time durations (t) to check the level of agreement between the portrayed pictures and the varying scale profiles. Effect of the agitation Impeller speed is one of the major factors that govern the scale growth rate by directly influencing the flow behaviour near the walls. Although there are practical limitations in increasing the impeller speed in the industry (as it leads to increased power consumption and consequently, increased capital and operating costs), it is a suitable parameter to investigate. To study the role of impeller speed on the scale distribution characteristics, experiments were carried out at three different speeds of 400, 430 and 460 RPM for 60 minutes. The lowest speed, i.e., 400 RPM, is above Njs (the minimum impeller speed required to fully suspend solids off the tank bottom) ensuring that crystals are fully suspended during the scale deposition and growth. Figure 10 shows the CMM results for one wall segment obtained for three agitation speeds. For the sake of simplicity, all three pictures are rescaled to the range of 0 - 2.89 mm. In this Figure, Max, Average, and Sum, refer to the highest value of scale thickness on a wall segment, the average scale thickness value based on the total number of readings on a wall segment (12600), and sum of the total scale thickness values recorded by the CMM on a given wall segment, respectively. Figure 10 shows that the scale distribution maps obtained using the CMM results have excellent agreement with the experimental observations. It can be seen that the scale distribution maps produced from the CMM results agree very closely with the build-up of scales observed on the wall segment after the scale 16 ACS Paragon Plus Environment

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growth experiments for all three agitation speeds used. The effect of agitation speed on scale distribution can be thus observed clearly from both CMM and experimental results. At higher impeller speed (N = 460 RPM), a majority of the scales are found in the top region. As the impeller speed decreases, the scale becomes less dense, but it spreads to cover the larger surface area. It is clear that the decrease in the level of turbulence with decreasing impeller speed leads to the deposition of scale on the tank wall to a larger extent. As the impeller speed increases, the liquid flow velocity near the tank wall increases which lowers the rate of scale deposition due to the increased erosion effect. These results are consistent with our previous findings. Wu et al.

48

proposed a qualitative scale-velocity model to explain the

‘flow velocity-scale formation’ relationship. They reported that, at a particular velocity range, the deposition of material on the vessel wall and its removal due to erosion occur simultaneously as parallel processes thereby slowing down the rate of scale growth. Thus, they concluded that stronger erosion caused by higher flow velocity could be exploited as a technique for effective scale suppression. The pictures shown in Figure 10 provide qualitative as well as quantitative results of the effect of the impeller speed on the scale thickness and its distribution on the surface of the mixing tanks. An increase in the impeller speed leads to a reduction in all of Max, Average, and Sum scale thickness values indicating that a lesser amount of scales are deposited on the walls at higher rotation speed. To further analyze the scale profile on the tank’s surface, a term called ‘Average Scale Thickness (AST)’ is defined. AST is determined in this work by calculating the average values of scale thickness within a chosen tank height intervals (every 10 mm in this work). Figure 11 presents the AST values as a function of tank height for an experiment carried out at N = 400 RPM. The AST values shown in Figure 11 indicate that the operating volume of the tank can be divided into two zones called ‘mobile’ and ‘static’ zones. The turbulent flow in the mobile zone mitigates the scale formation, whereas the relatively less turbulent flow at 17 ACS Paragon Plus Environment


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the static zones creates an environment that is more conducive to the deposition and growth of scale. The AST data calculated for all three impeller speeds are presented in Figure 12. The pattern of the AST data at various tank heights is similar for all the three impeller speeds. For the case of N = 400 RPM, a significant accumulation of scale starts at HT = 110 mm, reaches the maximum values of 0.745 at HT = 140 mm and starts decreasing after that. In the case of N = 430 RPM, the AST value reaches a maximum at HT = 160 mm and starts decreasing after that. A similar trend can be observed in the case of N = 460 RPM. These trends imply that the accumulation of the scales occurs more towards the liquid surface as the impeller speed increases. The Average and the Sum scale thickness values presented in Figure 10 indicate that the overall mass of scales decreases with increasing impeller speed. However, the results from Figure 12 reveal that for tank heights greater than HT = 150 mm, the AST values for N = 430 and 460 RPM have exceeded that for N= 400 RPM, implying that the build-up of the scale near the liquid surface is higher at higher impeller speeds.


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Figure 10 Effect of impeller speed on scale distribution profile on a wall segment



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Figure 11 AST values as a function of the tank height, N: 400 RPM


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180 170 160 150 140 130 120 110

Tank Height (mm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


100 90 80 70 60 50 40 30 20 10

460 RPM

0 0

0.1

0.2

0.3

0.4

430 RPM

0.5

0.6

0.7

400 RPM 0.8

0.9

AST (mm)

Figure 12 AST values as a function of impeller speed Effect of experiment duration Three sets of experiments were conducted using three different time frames of 30, 45, and 60 minutes to study the effect of duration on the scaling behaviour. Figure 13 shows the scale profile on the surface of a wall segment as a function of experiment duration. The impeller speed was kept constant at 400 RPM for all these experiments.



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Figure 13 Effect of experiment duration on scale distribution profile According to the results shown in Figure 13, the scale becomes thicker and covers a larger area with an increase in time. The scale level is small and inconsistent at the impeller region for all three cases. The solution in the reactor was found to evaporate rapidly after 60 minutes of experiment thereby reducing the liquid operating volume and affecting the scale growth pattern. Thus, it was not possible to study the scale growth beyond 60-minutes in this work due to the high temperature (80°C) of the operation. However, high temperature is one of the key requirements of our experiments because it is the main driving force in the formation and accelerated growth of scale in our work. Therefore, it was decided to maintain the temperature at 80C and limit the experiment duration to 60 minutes. The AST values obtained using three different experiment times are shown in Figure 14. Figure 14 also confirms that, for any given experiment duration, the top region of the tank has greater amount of scale compared to middle and bottom regions. Also, the AST values increase with an increase in the experiment duration, which is consistent with expectations.



180 170 160 150 140 130 120 110

Tank height (mm)

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100 90 80 70 60 50 40 30 20

30 mins

10

45 mins

60 mins

0 0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

AST (mm)

Figure 14 AST values as a function of experiment duration Coating impact The detailed scale profile depicted in Figure 9 suggests that spray coating is an effective way to harden the grown scale for physical analysis, although the applied paint could artificially enhance the scale thickness on the wall segments. To quantify the effect of sprayed paint on the scale thickness measured, CMM measurement was carried out using a wall segment that has been prepared to contain three distinct sections using painter’s tape and spray paint. They are: 1) zone containing scale and spray paint, 2) zone that has only spray paint (scale-free), and 3) clear zone (scale-free and paint-free). Once the coordinate values for these three


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sections were measured and recorded, it was possible to estimate the impact of the coating due to spray paint on the scale profile using the difference in the CMM measurements for the painted and the clear zones. Figure 15 provides a comparison between the clear (C) and the painted (B) zones on a wall segment that has been scanned by the CMM. This figure shows that CMM has not detected any significant difference between the measured thicknesses in these two zones, which leads to the conclusion that the effect of coating on the machine readings is negligible relative to the scale thickness investigated here.

Figure 15 Effect of coating on surface thickness, A: scaled and painted zone, B: painted zone (scale-free), C: clear zone (scale-free and paint-free)

Conclusion A novel approach has been proposed to study the scale formation in stirred vessels. Fabrication of a tank that could be disassembled provided unrestricted access to the scales grown on the inner wall of the rector and allowed measurement in critical regions such as the impeller zone. Scale growth at various tank heights was studied as a function of agitation speed and experiment duration. Using the proposed approach in this work, it has been possible to conduct a qualitative and quantitative study on the roles of these two parameters on the scale growth patterns. It was noticed that the overall mass of scales had declined with 25 ACS Paragon Plus Environment


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an increase in the impeller speed. At the same time, the results indicate that closer to the liquid surface, the average scale thickness at the higher impeller speed becomes larger, implying that the build-up of the scale at the near-surface zones becomes denser as the speed increases. It was also observed that running the experiments for a longer time leads to the accumulation of the larger amount of scales on wall segments. Overall, this work established that the proposed approach could be successfully applied to quantify and study the scale thickness and distribution in mixing tanks.

Acknowledgment One of the authors (M.D.) gratefully acknowledges the support of the Australian Government Research Training Program (RTP) Scholarship through RMIT University. Dr. Nathan Webster and Dr. Anita D'Angelo are thanked for providing XRD measurements and analysis. Appreciation is also extended to Mr. Dean Harris, Mr. Greg Short, Mr. Bon Nguyen and the CSIRO Clayton workshop.

Supporting Information The procedure for XRD phase identification is explained in the Supporting Information.

Nomenclature AST, Average scale thickness, mm C, Impeller clearance, mm CMM, Coordinate Measuring Machine D, Impeller diameter, mm HB, Water bath liquid height, mm 26 ACS Paragon Plus Environment

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HL, Liquid height, mm HT, Tank height, mm N, Impeller speed, RPM T, Tank diameter, mm t, Experiment duration, min W, Width of baffles, mm

References

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Graphical abstract 380x245mm (96 x 96 DPI)

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A Novel Approach To Quantify Scale Thickness and Distribution in

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