Comparison of the hydrodynamic performance of rotor-injector devices in a water physical model of an aluminum degassing ladle

Accepted Manuscript Title: Comparison of the hydrodynamic performance of rotor-injector devices in a water physical model of an aluminum degassing ladle Author: Ernesto Mancilla Wiener Cruz-M´endez Isa´ıas E. Gardu˜no Carlos Gonz´alez-Rivera Marco Aurelio Ram´ırez-Arg´aez Gabriel Ascanio PII: DOI: Reference:

S0263-8762(16)30464-6 http://dx.doi.org/doi:10.1016/j.cherd.2016.11.031 CHERD 2503

To appear in: Received date: Revised date: Accepted date:

22-6-2016 16-11-2016 25-11-2016

Please cite this article as: Mancilla, Ernesto, Cruz-M´endez, Wiener, Gardu˜no, Isa´ıas E., Gonz´alez-Rivera, Carlos, Ram´ırez-Arg´aez, Marco Aurelio, Ascanio, Gabriel, Comparison of the hydrodynamic performance of rotor-injector devices in a water physical model of an aluminum degassing ladle.Chemical Engineering Research and Design http://dx.doi.org/10.1016/j.cherd.2016.11.031 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Comparison of the hydrodynamic performance of rotor-injector devices in a water physical model of an aluminum degassing ladle Ernesto Mancillaa, Wiener Cruz-Méndezb, Isaías E. Garduñoa, Carlos González-Riverab, Marco Aurelio Ramírez-Argáezb, Gabriel Ascanioa,∗

a

Centro de Ciencias Aplicadas y Desarrollo Tecnológico, Universidad Nacional Autónoma de México, Circuito Exterior, Ciudad Universitaria, 04510, Ciudad de México, México b

Departamento de Ingeniería Metalúrgica, Facultad de Química, Universidad Nacional Autónoma de México, Edificio D, Circuito de los Institutos s/n, Ciudad Universitaria, Coyoacán, 04510, Ciudad de México, México

*Corresponding author

Graphical abstract

Highlights:     

Influence of rotor shape on the flow dynamics in degassing ladles was studied. Gas injection decreased liquid velocity magnitudes for the commercial rotors. Pumping capacities are found higher in radial direction than in axial direction. At high gas flow rate, commercial rotors show a notable diminishing in turbulence. The novel impeller presents the best performance at different gas flow rates.

Abstract The hydrodynamic performance of a stirred ladle, for an aluminum degassing system in turbulent regime, is analyzed experimentally. This study explores the dynamic flow features due to the bubble dispersion on the gas-liquid flow. Diverse impellers with geometrical differences are tested with the purpose of comparing the influence in the flow behavior. Three rotor-injector devices are compared, including two conventional designs and one new rotor design. The rotor performance is evaluated at two gas flow rates. The velocity fields are investigated in a water physical model under flow conditions similar to those encountered in the degassing process of molten aluminum. The particle image velocimetry (PIV) technique is employed to obtain the velocity fields during the degassing process. In such a way, instantaneous measurements of the water flow field for gassed and ungassed conditions were obtained. Considering the two gassing conditions, it is found that the flow patterns changed drastically with all the rotors tested. Moreover, the turbulent flow field results showed significant differences under gassing and ungassed conditions. Here, it is demonstrated that the rotor geometries strongly affect the distribution of turbulent intensities. It was found that the new rotor exhibits the better performance under gassed conditions, showing high turbulent intensities, producing a higher gas breakup rate and promoting the formation of small bubbles that can be easily distributed over the entire ladle. This is due to its asymmetrical geometry, as is exposed in the present analysis. Keywords: Aluminum, degassing, gas-liquid, turbulent, rotor-injector

1. Introduction One of the most relevant metallurgical operations for aluminum processing is referred to the process of the dehydrogenation of molten aluminum (Engh, 1992), (Opie and Grant, 1950), (Auyalebechi, 1988), (Sigworth, 1999). The presence of hydrogen into a metal alloy can lead to the onset of defects in the final products (Zhang et al., 2011), (Sigworth, 1987). Basically, the methodology used for degassing purposes depends basically on the mechanical stirring device and the gas injection device. Both elements compose a complex gas-liquid stirred system. Usually, the gas injection methods are based on the use of porous plugs, lances, and nozzles, among others (Camacho-Martínez et al., 2012), (Guttery, 1993), (Nilmani and Williams, 1993), (Díaz et al., 1997). In the last years, the rotor-injector method has been widely accepted in the degassing industry, as a consequence of the promising results in aluminum processes (Mi et al., 2009), (Saternus and Botor, 2009), (Ni et al., 2003), (Johansen et al., 1998). In such systems, the gas is injected through the impeller shaft whilst the rotor

velocity promotes a large gas liquid contact area and larger residence time, in this way improving mixing and increasing gas removal kinetics. The flow signature is very important to evaluate the mixing performance of the system. For such a reason, the impeller geometry plays an important role in terms of quality production. In order to obtain a vigorous agitation, it is necessary to select the adequate impeller geometric configuration. Some authors have explored the influence of diverse impeller shapes on the critical rotor angular speed for diverse gas flow rates configurations, and the two-phase flow was investigated (Nilmani et al., 1992), (Zhang et al., 2002), (Camacho-Martínez et al., 2010). Such attempts include the physical modelling in 1:1 scaled water tanks, in order to observe the hydrodynamic phenomena involved and to find the flow patterns and gas dispersion into the liquid (Zhang et al., 2011), (Warke et al., 2005), (Warke et al., 2005). In these works several important parameters have been recognized, which include the gas injection rate, impeller geometries, and rotor velocity, among others (Nilmani et al., 1992), (Mazumdar and Guthrie, 1995), (Mi et al., 2008). Due to the flow circulation produced by the rotor, the gas is dispersed into the ladle, and small bubbles are formed in order to increase the kinetics of the degassing process (Tovio et al., 2000). One of the most important variables in an aluminum degassing system is the bubble circulation, which is directly related to the operating conditions. In reactors agitated, several gas dispersion states have been defined in the past (Nienow et al., 1997). Such situations describe the behavior of bubbles under different gas flow rates and impeller speeds. These regimes correspond to: a) no gas dispersion, formation of large bubbles rising along the impeller axis; b) dispersion in the upper part of the vessel; c) gas circulation observed in the upper part of the vessel combined with partial circulation at the lower region; d) circulation of bubbles throughout the vessel; e) higher rotational speed of the impeller producing secondary circulation loop of bubbles near to the surface. The foregoing description identifies three hydrodynamic regimes: flooding, loading, and complete dispersion. These regimes are related to different types of interactions, particularly between the bubbles and the flow generated by the impeller (Tatterson, 1991). The flow behavior influenced by the rotor performance controls the hydrodynamic structure within the ladle. The flooding state comes out from the insufficient power supplied by the impeller to disperse the gas correctly, which rises axially as a bubble column. At such a regime, it is reported a decreasing trend in the gas-liquid mass transfer rate and the gas bubbles are not well distributed through the vessel (Grandfield et al., 1990), (Szekely et al., 1988). From a hydrodynamic standpoint, no studies dealing with aluminum systems have been reported. The mixing performance in bubbly stirred tanks has been widely investigated in rotor-stator configurations. In such works, the power consumption have been measured; additionally the flow fields obtained were analyzed employing 2D PIV technique (Paul et al., 2004), (Mortensen et al., 2011). Some of these studies have been addressed to high-shear mixers (Jasińska, et al., 2015). Also, extensive reviews can be found where the influence of the impeller type has been reported. Nienow (Nienow, 1998) highlighted the importance of different fluid properties and gas injection methods. In that seminal work, the lack of knowledge of the link of the hydrodynamic and the quality of the desired products was recognized. In spite of the works mentioned, they are not devoted to analyze the effect of gas injection over the mixing performance, especially in aluminum industrial processes. However, it has been recognized that degassing processes depends on bubble size and gasliquid interactions (Chen and Zhao, 1995), (Johansen et al., 1996). Consequently, a full hydrodynamic description is not available and bubble-liquid interactions during aluminum degassing are not entirely understood. A better knowledge of such couplings will lead to an

adequate design and scale-up of aluminum degassing systems. In this work, a full-scale physical model has been built for emulating an industrial aluminum degassing process. The PIV technique is applied to acquire the flow fields. Two equal gassing conditions are used to evaluate the hydrodynamic performance of different rotor-injector devices. The conventional gas injection method is implemented to supply the gas flow rate. The attention is focused on the change in the flow patterns and in the turbulent features produced by the action of the bubbles. The main objective of this study is to obtain a complete description of the overall flow behavior in such systems, since the point of view of the gas influence on the hydrodynamic conditions, comparing commercial available impeller designs with a new design of a rotor-injection device. It is expected that this research contributes to the actual understanding of aluminum degassing systems and provides a better understanding of the real hydrodynamic phenomena in current industrial systems.

2. Materials and Methods In the literature, some factors influencing the degassing process have received special attention, namely: the impeller rotational speed, the geometrical characteristics of the impeller, and the gas flow rate. In this work, the experimental setup consists of an acrylic container, which is used to model the process of aluminum degassing in a 1:1 scale water tank. The tank is provided with a rotor-injector device and additionally provides the ease to evaluate the performance of three rotors with variable geometry. The tank is filled with tap water up to height 50 cm (H). Although, it is well known that baffles not only minimize vortex formation but also modify the flow patterns in the ladle, in the present study no baffles have been used since commercial batch process do not consider them. The impellers and the container are related through the following geometric relationships: d=D/3, h=H/3, In Fig. 1(a), d is the rotor diameter, D is the tank diameter, h is the height from the bottom of the tank to the rotor and H is the water height. In order to keep the flow characteristics of the actual degassing process, the physical model and the impellers preserve the principles of hydrodynamic similarity. Figure 1(b) shows the experimental setup for visualization experiments, which consists of a tank and the particle image velocimetry system (PIV). In Fig. 1(c) an image of the system is presented. Due to the curvature of the inner vessel, the tank is placed inside a rectangular container filled with tap water to avoid the optical distortion. Both the shaft and the three rotors are built in Nylamid®, which presents high resistance to wear and good mechanical properties. The impellers used in this work are shown in Fig. 2 and described below. First, the called standard rotor (Fig. 2a), which is a widelyused design, consists of a disc with lateral nozzles. The second geometry is a rotor equipped with nozzles and notches (Fig. 2b). As mentioned before, these two impellers are commercial designs widely-used in the industry. The last rotor is a new rotor described elsewhere (Fig. 2c) (Hernández-Hernández et al., 2015), (Hernández-Hernández et al., 2016). The main feature of this rotor is that the bottom part has been designed with jagged edges, which allow dragging fluid from the tank bottom. The importance of determining the right operational

parameters, in such systems, is based on the knowledge of the hydrodynamic behavior of the flow fields. In this way, it is mandatory to analyze the hydrodynamics of the stirred tank due to the presence of bubbles. In this regard, a variable speed motor is used to operate at the required velocity. The experimental rotational speed, monitored through a digital tachometer (DT-1238L, Lutron), is fixed to 600 rpm, which is a reasonable value to operate in the turbulent regime. Concerning the two-phase flow experiments, air was used as the gas phase and was introduced into the tank through the rotor shaft, which is considered the conventional gas injection method widely used in the industry. A flow meter was used to measure the amount of air supplied in a range from 10 to 40 Liters per minute (LPM). The Particle Image Velocimetry (PIV) technique was used for determining the flow fields of the liquid phase. The PIV system consists of a pulsed laser, a high-speed camera, a synchronization system and a computer for the acquisition of the database. Even though the PIV technique is a widely documented technique, brief particularities of the technique are outlined below. First, the working fluid is seeded with tracer particles. In the present study, micro spheres of 50 μm in diameter made of polyamide and coated with rhodamine B (Dantec Dynamics®) were used as tracer particles. For the purpose of discriminating between the tracer particles light and the light scattered from the bubbles within the flow, the camera is provided with a 550 nm optical filter. Measurements were made in an axial plane in the center of the container, parallel to the impeller in the plane formed by the horizontal and vertical axis. Due to the presence of the shaft, only a half of the tank was considered to fully describe the hydrodynamics of the stirred vessel. Such approximation is well justified by the axisymmetry of the impellers investigated. An optical arrangement is coupled to the laser to generate a thin laser sheet (∼ 1 mm). The laser sheet is particularly useful to orient the speed camera, which must pointing out perpendicularly to the laser sheet. Regarding the instantaneous flow fields, they were determined by means of the acquisition of a series of pairs of images, where the time between each pair of images is around 1000 μs. The velocity fields are calculated using the cross correlation method. For this method, the images are divided into interrogation areas of 32 × 32 pixels with an overlap of 50 × 50%, in such a way, 8806 velocity vectors are generated for each image. Both, data acquisition and images processing, were performed using the software Dynamic Studio (Dantec Dynamics®). For each case, 800 images were acquired and a robust statistical convergence of the data flow was achieved. The flow regime is determined by the Reynolds number, which in an agitated system is defined as follows: 𝜌𝑁𝑑 2 𝑅𝑒 = 𝜇 where µ and 𝜌 are the viscosity and density of the working fluid, respectively, N is the impeller rotational speed and d is the impeller diameter. In this study, the Reynolds number is 193,000 for all the rotors.

3. Results and Discussions

The analysis of the mass transfer efficiency in aluminum ladle facilities is important, so then it is fundamental a detailed knowledge of the flow behavior. This becomes particularly important in rotor-injector degassing systems. In the context of gas-liquid stirring systems, the shape of the agitator determines the degree of interaction held between the discharge streams via the impeller and the amount of gas injected into the container. In the cases investigated, the single-phase flow fields are employed to compare the response of the velocity field to the gas injection. The comparison is achieved by the presence of two different gas flow rates. Further, it is observed that the impeller configuration modifies the flow behavior and the gas interactions, as well as promoting distribution of small bubbles along the vessel. In the following section, a description of the hydrodynamic characteristics of three rotor-injector impellers with different geometries is shown. The flow fields, velocity magnitude distributions, pumping capacity profiles and turbulent patterns are analyzed.

3.1. Flow Patterns Figure 3 shows the velocity flow patterns produced by the different impellers geometries at three operating conditions (ungassed and two gassed conditions). The rotor and gas vortex locations are represented by dotted lines. The vertical and horizontal dimensions become dimensionless quantities relative to the radius of the furnace. The experimental measurements, for both gassed and ungassed condition, were acquired at 600 rpm. Figure 3 (a, d, g) shows the velocity patterns for the standard rotor at the mid plane. The flow field for the single-phase case is depicted in Fig. 3 (a). The flow behavior, shown as a fluid stream, is radially pushed away by the rotor to the inner wall surface of the vessel. This particular aspect has been previously addressed by (Gómez et al., 2013). It is shown the formation of two fluid loops due to the jet impingement in the tank lateral walls, one counter clockwise below the impeller , and the other clockwise, located above the impeller. Overall, the fluid driven by the rotor action produces strong circulation in the lower part of the ladle; whilst, in the upper section of the tank a weak flow region is formed and therefore good circulations are not achieved. The effects of gas supplied are exposed through the velocity field of the standard rotor (Fig. 3 (d, g)). Here, both gas injection cases are shown (10 and 40 LPM). The attention is paid for the low gas injection rate case (10 LPM), where the fluid vortex shifts and moves along the axial direction, as a result of the additional momentum provided by the incoming gas flow at the impeller level in that manner the flow in this zone becomes completely radial. This is apparently a combined effect of the fluid pushed away from the impeller, reinforced by the passing bubbles, generating a strong flow, at first in the radial direction and later, upward by the ascending bubbles. In general terms, the fluid vortex mechanism formation is the same than in single-phase condition, such mechanism is mainly due to the collision of fluid with the walls of the vessel. However, it can be noted that in the area near to upper zone of the ladle wall, the fluid moves towards the surface of the container. At this point, it has been probed that the standard rotor-injector is incapable to disperse completely the gas into the whole flow domain ladle, showing the same trends and velocity magnitudes with or without incoming gas flow, which suggest that the fluid in this zone is not interacting with the ascending bubbles, i.e. the ascending current of bubbles interacts with the fluid mainly in the zone near to the symmetry axis of the ladle. Also, in the upper region of the ladle close to the gas vortex, a strong axial flow is observed in the direction to the vessel wall, which is due to the centrifugal forces produced. It can be seen, when comparing the size of the vortex

for the ungassed and the gassed states observed in Figs. 3 (a,d and g), that the ascending bubbles push the vortex upwards, as a result of the momentum transferred from the ascending bubbles, to the liquid phase. This effect is not present for the others rotors under study, which suggest that for this rotor design, an important part of the current of ascending bubbles rise in the zones of the fluid located near to the axis of symmetry of the ladle. At high gas flow injection (40 LPM), similar flow patterns as in the previous case are found, but slight differences are noticed between cases. In this case of high gas flow rate, an increment in the radial flow and a notorious increment in the amount of fluid pushed away by the impeller to the wall of the vessel are detected. In the upper section of the container, the vortex size is reduced and an increase in the radial currents are also observed. A decrease in fluid handling capacity is noted; the gas vortex inertia induced most of the bubble movement in the upper part. As a result of more flow dragged radially by the bubbles, the flow hits the wall and one part goes upwards while the other goes downwards. The part of the flow going downwards feeds and increases the size of the loop located near the wall, below the rotor and the small loop in the upper part of the rotor is completely dissipated. Next, in Figure 3(b, e, h) the results for the notched rotor-injector, corresponding to the ungassed and two gassing configurations are shown, respectively. The results for the ungassed conditions for the notched rotor are illustrated in Fig. 3(b). As for the standard rotor case, the flow is mainly driven in the radial direction at the rotor level. Clearly the fluid is driven by the impeller to the side walls, favoring a strong discharge. Such stream creates a large, counter-clockwise circulation loop, below the rotor level, near the ladle wall. It can be seen that the size of the gas vortex is a little larger relative to the one found for the previous rotor in the ungassed case. It is noted that the inertia generated by the air vortex presence assists in the movement of the fluid to the outer wall. As a consequence, a strong radial flow is generated in the upper region of the vessel. At the middle section of the container, it is observed the interaction between these flows with the stream starting from the rotor. Interestingly, the velocity pattern measured is greatly affected by the gas flow injected. As a result of the gas introduced to the ladle, the flow velocities are considerably altered. In Fig. 3(e, h) shows in the flow patterns for both gassed conditions. The flow pattern obtained for the gassed conditions (10 and 40 LPM) for notched rotor shows similar characteristics, notwithstanding the velocities are slightly smaller than velocities for the ungassed condition. A general decrease in the size of the velocity vectors for the gassed condition is appreciable in the whole flow field, mainly due to the drag produced by the movement of the bubbles. The large circulation loop at the bottom of the ladle is almost dissipated; but the fluid is still pushed away from the rotor. The stream generated by the impeller is significant in the radial direction but not strong enough to reach the vessel wall. The gas is expelled by the rotor and the fluid is dragged by the bubbles passing and is vertically moved towards the free surface. The gas vortex, produced by the impeller rotation, does not show evidence of change in size. From these maps, it is possible to affirm that the bubbles injection do not have a notable influence on the gas vortex generation. Contrary to the single-phase case in the region below the rotor, no large streams are generated. Besides, this at the upper section of the ladle, the air vortex flow helps to displace the liquid in the radial direction. In that way, as the bubbles get closer to the vortex influence, the circulation of bubbles in tangential direction is enhanced. The flow pattern measured with the new impeller is shown in Fig.3(c, f, i) under ungassed and low and high gassed conditions. Fig. 3c shows the velocity field at the central plane for the single-phase case. In the case of the new rotor is noted that due to its complex geometry and its asymmetry at the bottom, the air vortex becomes smaller than in the cases of standard and notched rotor

under the ungassed condition. This gives an indication that most of the fluid is driven by the lower section of this new rotor-injector device. It is perceptible the way the fluid is sucked by the lower part of the impeller, creating a large stream directed in the axial direction. In the current experiment a fluid loop is formed. This loop is larger than for other impellers. The large scale loop is generated by the fluid dragged by the irregular rotor base, which is a different mechanism respect to the standard and notched agitator, where the fluid is pushed toward the lateral walls of the vessel. Such behavior produces recirculation and agitation of the fluid in larger regions of the container. Velocity patterns at gassed conditions for the novel rotor are shown in Figure 3(f, i). It can be seen that as the gas flow is dispersed into the ladle throughout the rotor irregularities, the fluid circulation intensity decreases. Additionally, the circulation loops change its shape and two smaller loops are generated. This change in the fluid loop behavior is due to the gas flow rate injected, which modifies the amount of fluid suctioned by the rotor. Bubbles drag fluid from the bottom of the tank upwards whilst the rotor pushed the fluid towards the side wall. That mechanism is different from the ungassed case and is a similar mechanism present in the single-phase flow for the standard and notched rotors, where the overall flow pattern has both radial and axial components. However, this agitator is still capable of generate a significant circulating flow, with a couple of small loops that contribute to the circulation of the fluid into the vessel. As in all the experimental results previously showed in this research, at the top section of the container the gas vortex produces a strong radial flow and conduct the fluid to the side walls. Comparing the performances of the different rotor geometries, the new impeller design exhibits a better performance to mix liquid and gas at high gas fraction. For all the rotors analyzed, a large gas vortex is formed due to the impeller rotating velocity. Additionally, it is observed that the gas swirl size is affected by the bubbles injection. Using the fluid currents analysis is possible to observe the flow modification, which is useful in the identification of the most important streams capable to handle the amount of gas injected. In general, for all the rotors tested under the ungassed condition, the flow dynamics is explained due to a pressure drop below the impeller, which sucks fluid from the bottom of the ladle and ejects it through the lateral nozzles in radial direction towards the side wall, creating two circulation loops: one strong loop counter-clock wise below the rotor and other weak clock-wise loop above the rotor. The pressure drops comes out to counterbalance the centrifugal forces promoted by the angular motion of the rotor, which creates the vortex to balance the forces in axial direction. For the gassed conditions, the extra axial momentum transferred to the liquid due to the drag of the ascending bubbles, modifies the flow patterns by reducing the pump of fluid, by decreasing the radial stream towards the side wall and by reducing the circulation loop below the impeller.

3.2. Velocity Magnitude The fluid currents analysis allows the observation of the modification in the flow patterns as mentioned in the previous section. It is possible to identify the location of the most relevant streams to handle the amount of gas injected. However, such a description does not provide a quantitative characterization of flow variation. In such a basis, a comparative examination of the velocity magnitude is presented in this section. The velocity magnitude is calculated as follows:

𝑉 = √𝑣𝑟2 + 𝑣𝑧2 in dimensionless form: 𝑉∗ =

𝑉 𝑁𝑑

where N is the impeller rotational speed, d is the impeller diameter, vr and vz are the velocity components in the radial and axial directions, respectively. The velocity magnitude contours are presented in Fig.4, for the gas and ungassed conditions for distinct rotor-injector configurations. Again, the vertical and horizontal dimensions have been normalized with the radius of the vessel. The velocity magnitude becomes in a non-dimensional parameter through the use of the impeller rotational velocity (Vrot=Nd). For the purpose of comparison, the same color scale is used for all cases. Figure 4(a, b, c) shows the velocity magnitude contour maps for the standard, notched and novel rotors for ungassed condition, respectively. The analysis of the ungassed cases shows similar flow fields for all the rotors examined, having equivalent velocities. A region with high liquid velocity magnitudes is located near to the impellers. Notwithstanding, for the notched rotor, a slight increment in the liquid velocity is observed. The increased liquid velocity is around the rotor due to the strong radial flow generated by the rotors as described above in the flow patterns section. In all cases, lowest velocity magnitudes are observed in the major part of the container, mainly in the region far away from the rotor position. In Fig.4 (d, e, f) is observed the fluid motion response at the low gas injection rate. In general, the drag due to the bubbles movement produces an increment in velocity magnitude, which can be appreciated in the flow field. A great amount of liquid is moved upwards by the presence of gas. As a consequence of the gas flow introduced into the ladle, the flow of liquid turns to be radial. Fig. 4(g, h, i) shows the liquid velocity magnitude for the standard, notched and the design proposed cases in the main flow domain as the gas load is increased (40 LPM). It is noted a decreasing trend in the magnitudes of the velocity for the standard and notched rotors. Contrasting with this behavior the new rotor exhibits an increment in velocity at the upper region of the vessel (Fig .4 i). Additionally, the velocity intensity magnitude is constant close to the impeller. In general, as the gas is supplied throughout the impeller shaft, an increment in the liquid velocity magnitude is observed. This increment is mainly due to the gas flow rate injected with appreciable effects in the entire vessel. Also, high liquid velocity magnitudes are detected at the vicinity of the gas vortex. Regarding high gas flow rates scenario, the standard and notched rotors presented similar response due to the presence of gas bubbles. For both impeller geometries, a reduction in the magnitude of the liquid velocity is found. On the other hand, the new rotor at 40 LPM shows a notorious increment in the velocity magnitude contours.

3.3. Turbulent Intensity Fields

In this section, the analysis of the impeller influence on the mixing performance is presented through the turbulence flow characteristics. It is possible to calculate the turbulent intensity (TI) as a function of the velocity fields through the equation expressed as (Hidalgo-Millán et al., 2012):

√𝑣𝑟′2 + 𝑣𝑧′2 𝑇𝐼 = 𝑁𝑑 Here v′z and v′r are the velocity fluctuations in the axial and radial directions, respectively. The turbulence intensity, (TI), can be interpreted as the agitation degree in the stirred tank generated by the action of the rotors. For comparison purposes, the same color scale is used over all maps. Also, the scale range was selected to have a better clarity on the effect of the gas flow rate on the performance of the different rotor geometries used. Now the attention is paid on the turbulent intensity contours for the gassed and ungassed configurations of the different impellers shown in Figure 5. The analysis of turbulent fields in the single-phase case is presented in Fig.5 (a, b, c), for the standard, notched and new rotors, respectively. In these maps it can be seen, as expected, the highest turbulent intensities located in areas near the rotor, showing a maximum at the bottom of the impeller. Such areas corresponds to regions directly affected by the rotor presence, just as it was shown in the analysis of the liquid flow patterns and liquid velocity magnitude contours. The high turbulent intensity areas found around the rotor corresponds to the vicinity of the low pressure zone created below the impellers, which are causing a turbulent flow of the fluid being sucked by the rotor. Additionally, it is remarkable that in all the impellers the magnitude of velocity fluctuations are almost equal, although in the notched rotor a small increment in the area of high turbulent fluctuations is observed (Fig. 5 b). The response of the turbulent intensities for all the three rotors tested at low gassed operations are shown in Fig. 5 (d, e, f). It is seen that the turbulent contours are modified, but there are similar features in the turbulent contours to the singlephase cases. However, the maximum turbulent intensity values were observed again at the sidewards and bottom regions of the vessel, close to the bottom of the rotors. A noticeable decrement in the amount of turbulence is recognized, particularly for the standard rotor (Fig.5 (d)). Moreover, notched and novel impellers, also show a minor decrement (Fig.5 e, f). It is observed that turbulence intensity maps are drastically modified and values decrease as shown in Fig.5 for the highest gas load (g, h, i). It can be noticed for the standard rotor in the velocity fluctuation field, that the large velocity gradients developed in the zone below the impeller are totally dissipated (Fig.5 g). It was found that in the case of notched rotor, the high gas injection also diminishes the turbulent nature of the flow (Fig.5 h). Moreover, the turbulent zone is reduced and its intensity diminishes about 50%. On the other hand, at high gassing phase condition the new rotor exhibits a turbulence intensity range of the same order than in the previous gassed conditions (Fig.5 i, f). Apparently in this case and due to the high momentum transferred to the fluid by the jagged edges located at the bottom of the new rotor and also due to the bigger pressure drop created by this design ,the incoming gas is not able to promote changes in the flow field strong enough to damp considerably the velocity fluctuations in this zone. However, the increase in gas flow rate causes a slight reduction in the high turbulence zone located below the new rotor as a result of the presence of bubbles

which contributes to damp the velocity fluctuations into the container. This promotes the interaction of the different layers of liquid with the gas injected. Furthermore, for the three rotors, at the top part of the ladle are found regions where the influence of the rotor is imperceptible, these zones are dominated by the flow in the azimuthal direction, and no fluid movement is exhibit in the r-z plane. For the new rotor, the largest turbulence intensities are observed at the maximum gas injection rate as presented in Fig.5i. It can be said that although it is not the configuration where the highest speeds are presented, this rotor is capable to generate strong turbulence fluctuations that interacts with the bubbles injection. For this rotor, the gas is trapped below the impeller; due to this confinement of gas and the high fluctuation intensities, the bubbles interact with the liquid in a more efficient and prolonged way. In a given agitated vessel, the independent variables controlling the power dissipated in the fluids and the gas volume fraction, which control the liquid mixing, are the impeller type, the impeller diameter, the impeller speed, and the gas flow rate. In our research, three of these variables are the same and the only remaining variable affecting the power and the gas volume fraction is the impeller type. The effect of impeller type on the power number is the subject of an ongoing research.

3.4. Pumping Capacity Finally, to complement the hydrodynamic analysis, it is important to quantify the amount of fluid pushed by the rotor-injector devices. The mixing will be promoted by the flow pushed by the rotors. An important parameter to characterize these currents is the pumping capacity. This quantity is inferred from the velocity fields and defined as follows (Hidalgo-Millán et al., 2012): 𝑧

𝑄𝑟 = ∫ 𝑉𝑟 (𝑧) 𝑑𝑟𝑑𝑦 0

𝑅

𝑄𝑟 = ∫ 𝑉𝑧 (𝑟) 𝑑𝑧𝑑𝑦 0

The total amount of fluid pumped by the rotors is quantified as: 𝑄𝑇 = ∑ 𝑄𝑖

in non-dimensional form: 𝑁𝑄𝑇 =

𝑄𝑇 𝑁𝑑 3

where Q is the volumetric flow rate, R is the vessel radius, d the rotor diameter and N is the rotor rotational velocity. Figure 6 shows the cross-section in which the pumping number is

determined. The absolute pumping capacities induced by the rotors are depicted in Figure 7. The pumping capacity profiles were calculated at a distance of the rotor radius along the height. The choice of this point is made with the purpose of obtaining the most representative mass transport flux section. The flow pumping profiles describe the streams pumped by the diverse rotor geometries presented in this work. The radial pumping axial profiles (Qr) for all the impellers show a similar behavior, but also exhibit notable differences in the magnitude. In Figure 7, for the three gassing situations two distinct zones of high radial pump are identified, such areas correspond to the rotor position and the zone of the gas vortex location. This is due to the contribution of the gas vortex to push the fluid to the vessel walls. The air swirl generates high centrifugal forces, which produces a strong radial flow. Such a performance is observed for gassed and ungassed for all the geometries used in this study. These high flows stimulate the movement of the bubbles around the entire ladle in the azimuthal direction, thereby increasing mass transfer rates, and enhanced the efficiency of degassing processes. The curves in Fig. 7a shows the capacities for radial pumping for the standard rotor where it is clearly observed that the lower pumping profiles is obtained for the single-phase condition. In these profiles, it is observed that, at both 10 and 40 LPM, the pumping capacity increases in the rotor area about two times, comparing against the ungassed case. Also, it is remarkable that at the upper section of the vessel, as described previously, the gas vortex size is reduced, which implies that the pumping capacity is diminished in the top part of the container. In Fig. 7b, the pumping capacities for the notched rotor are presented. It can be seen that for this rotor case, large pumping rates are found and similar to the standard rotor; highest values are found at gassing conditions. The highest values are encountered at a distance of 0.5 from the bottom of the tank, which coincides with the rotor location. It can be noted that, when the high gas flow is injected into the ladle (40 LPM), a decrease of more than 50% of pumping capacity for the notched rotor is produced. Analyzing the new rotor behavior, in Fig. 7c, it is seen the pumping capacity profile response to the presence of two different gas flow injection ratio (10 and 40 LPM). In this case, two regions of high pumping are identified and located at different positions of the vessel heights. Such heights correspond to the gas vortex region and to the position of the middle section of the rotor. The higher amount of fluid is pushed in the radial direction. On the other hand, the pumping capacity of the novel rotor for ungassed condition is the lower at the same direction, where a minimum value is obtained. Additionally, in Figure 7c, is found that the pumping profiles are quite similar at both low and high gas flow injection, and no significant deviations are observed in the whole curve. Unlike the standard and notched impellers, the new rotor design does not decrease its pumping capacities when gas at any rate is supplied. This can be interpreted, as this rotor is able to manage a large amount of gas injected into the container. In all the previous cases, the main pumping flow in the tank appears near of the rotor region. Also, a noticeable increase in pumping capacities are observed under gassed situations. The mean amount of the fluid displaced by the impellers in both radial and axial directions was calculated. Table 1 summarizes the values of the radial and axial pumping as well as the total pump number for both gassing conditions. These values show the contribution of the radial and axial pumping in the total pump capacity. For all the rotors analyzed in this work, the radial pumping is positively affected by the presence of bubbles. Furthermore, the measurements of the total pumping capacity exhibit a major contribution from the radial pumping to the total pumping. In Fig. 8, a comparison of the pumping number as a function of the gas flow rates is shown. It is important to note that at the single-phase condition,

standard and novel rotors show similar pumping numbers. However, the notched rotor exhibits higher pumping number than the other two designs. In relation with the other rotors is found that the difference in pumping number is about 25%. It is found that at low gas flow rate condition (10 LPM), the pumping number increases for the three rotors. In the case of the standard and notched rotor exhibits increments of around 58%. Even though, the major increment is observed for the new rotor (65%), the difference in pumping number between the impellers remains about 25%. Focusing on the high gas flow injected scenario (40 LPM), it is found that the standard rotor nearly preserves its pumping number with only a small diminishing of about 5%.On the other hand, in the case of the notched rotor at 40LPM, a large decreasing of about 20% in the pumping number is observed. The commercial rotors exhibit close values at such gassing condition. Additionally, for the new impeller with a large addition of gas into the system (40 LPM) a distinct behavior is observed, and the pumping number increases 18%. It is concluded from these findings, that two situations have to be noted. First, under the gassing condition, the notched rotor exhibits a slightly better performance in terms of the pumping number in contrast to the other impellers. Secondly, the notched rotor at high gas amounts, shows a substantially decrease of its pumping capacity, which is different from the new rotor, where the pumping is not affected by the change in gas flow conditions. Therefore, the novel rotor has demonstrated the best performance under a wide range of degassing operations.

4. Conclusions The goal of this study was to investigate the influence of the rotor shape over the flow characteristics in degassing operations. A novel rotor design has been proposed and compared against two commercial designs used in industrial operations. It was observed that rotor geometries directly define the flow behavior and the gas injection directly affects the dynamic features in all the systems. The commercial rotors showed a decrease trend in velocity magnitudes at gassing conditions. The high streams contribute to the gas circulation, enhancing the mass transport, avoiding that the bubbles rise freely to the vessel surface. The pumping capacities are found higher in the radial direction than in the axial direction. At high gas supply conditions, the standard and notched rotors exhibit a notable diminishing in the turbulent profiles. The velocity fluctuations are damped by a stream of bubbles passing. Additionally, a high turbulent degree is generated as a result of the large velocity gradients produced via the bubble breakup and gas dispersion throughout the ladle. The rotors comparison demonstrates that the novel impeller exhibits a better performance in a wide range of gas injection flow rates. Enhanced mass transfer in ladles is promoted at the corresponding turbulent levels developed in degassing systems. Finally, the current analysis shows evidence of the main parameters to lead to an optimal rotor design. However, additional work stills needed to obtain further comprehension in the mass transfer processes involving turbulent degassing operations.

Acknowledgements M.A. Ramírez-Argáez acknowledges to thank UNAM-PAPIIT for the financial support through grant IN-114115.

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Fig 1a

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Fig 7a

Fig 7b

Fig 7c

Fig 8

Table 1: Pumping number and pumping capacities in the radial and axial directions for the all rotors under ungassed and gassed conditions.

𝑄𝑟

𝑄𝑧

𝑚3 [ ] 𝑠

𝑚3 [ ] 𝑠

𝑄𝑇 = ∑ 𝑄𝑖 𝑁𝑄𝑇 𝑚3 [ ] 𝑠

[−] x 103

Standard Rotor ungassed 10 LPM 40 LPM

1.32 0.98 2.39 1.26 1.96 1.57

2.30 3.65 3.53

2.03 3.22 3.11

1.71 1.17 3.05 1.52 2.03 1.55

2.88 4.56 3.58

2.54 4.02 3.16

1.23 0.95 2.22 1.33 2.67 1.58

2.18 3.55 4.25

1.92 3.18 3.75

Notched Rotor ungassed 10 LPM 40 LPM

New Rotor ungassed 10 LPM 40 LPM