EFFECT OF BAFFLES ON SOLID-LIQUID MASS TRANSFER COEFFICIENT IN HIGH SOLID CONCENTRATION MIXING

EFFECT OF BAFFLES ON SOLID-LIQUID MASS TRANSFER COEFFICIENT IN HIGH SOLID CONCENTRATION MIXING Eng Ying Bong 1, Rajarathinam Parthasarathy 1 *, Jie Wu 2 and Nicky Eshtiaghi 1 1 School of Civil, Environmental and Chemical Engineering, RMIT University, Melbourne, 3001, Australia 2 CSIRO, Division of Process Science and Engineering, CSIRO, Highett, 3190, Australia *Corresponding author. E-mail: rchrp@rmit.edu.au ABSTRACT Suspension of solids plays an important role to promote mass transfer between solids and liquid in industrial mechanically agitated vessels employed in mineral and chemical industries. This becomes especially important when high solids concentrations are used for the purpose of process intensification. In mineral industry, there is a strong demand for intensifying existing mixing operations so that more materials can be processed without significant increase in energy consumption and major changes in the geometry of the existing infrastructure. This study aims to investigate the effect of baffles on solid-liquid mass transfer coefficient in slurries with high solids concentration up to 30 % (v/v) under just off-bottom solid suspension condition. Experiments were carried out in a 0.2 m diameter mixing vessel equipped with a six-bladed Rushton turbine. Four vertical baffles spaced at equal distances inside the tank were used to study the effect of baffles on mass transfer. Aqueous NaOH solution and cationic ionexchange resin particles were used as the liquid and solid phases, respectively. Experimental results show that solid-liquid mass transfer coefficient (k SL ) increases rapidly with an increase in C v from 0.08 to 0.20 (v/v). This study also indicates that similar k SL values could be obtained under both baffled and unbaffled conditions at N js. It has been also found that operating the vessel under unbaffled condition is more energy efficient compared to baffled condition. These results imply that it is possible to intensify the process and still attain significant achievement in mass transfer coefficient simply by removing the baffles and increasing the solids concentration in the system. INTRODUCTION Mass transfer coefficient between solid and liquid in agitated vessels has attracted considerable attention from researchers and engineers, as it is one of the important design parameters for mixing vessels used in mineral processing industry for operations such as adsorption, ionexchange, dissolution, leaching and other chemical processes. There have been many papers in the literature on solid-liquid mass transfer systems, but not much attention has been paid on the effect of solids concentration on mass transfer coefficient. Many of the past studies on solid-liquid mass transfer involved dissolution operations with fairly high inert particle concentration but low active solids concentration. Similarly, most of the previous studies on mass transfer in agitated adsorption systems were often restricted to systems with solids concentration less than 1% (v/v) where the interaction between the particles is negligibly small [Kato et. al., 1998; Kasat et. al., 2005; Pangkara et. al., 2002]. Harriott (1961), however, reported that solids concentration between 0.12 and 5.53% (v/v) has no effect on mass transfer coefficient. Few other investigators also made similar observation up to 10% (v/v) solids concentration (Lal et. al., 1988). On the other hand, Cline (1978) used solids

concentration of 0.5 40% (v/v) and showed that the mass transfer coefficient decreased with increasing solids concentration. It is therefore clear that the knowledge on the effect of solids concentration on solid-liquid mass transfer coefficient in agitated vessels is not complete and hence the applicability of literature results in the process design of industrial solid-liquid mass transfer vessels is limited. Many industrial solid-liquid agitated systems are required to process slurry with high solids concentration for the purpose of process intensification. Process intensification in agitated vessels requires that the production rate per unit volume to be increased without major changes in the geometry of the existing infrastructure. Since it is often impractical to reduce the size or volume of existing vessels, process intensification can be achieved by increasing the throughput or through improved physical process such as efficient mixing (Wu et. al., 2010). In such instances, process intensification can be achieved also by retaining the vessel size or volume but changing impeller or baffle geometry. Recently, Wang et. al. (2012) have suggested that operating a solid-liquid mixing vessel at a relatively higher solids concentration of 20% (v/v) is preferable due to higher energy efficiency at this condition and the possibility of suspending more solids for every unit of power input. Despite many publications on solidliquid mass transfer in agitated vessels, knowledge on the effect of baffles on solid-liquid mass transfer coefficient is lacking especially for systems with high solids concentration. The main reason for operating the impeller at a critical speed is to ensure that all the solid surface area is available for mass transfer. The critical impeller speed (N js ) was defined by Zwietering (1958) as the just off-bottom solids suspension condition at which all particles are in motion and no particles remain on the vessel bottom for more than 1 or 2 seconds. At this condition, entire surface area of solid particles will be in contact with the liquid ensuring maximum availability of the interfacial area for effective mass transfer (Levins and Glastonbury, 1972). The mass transfer rate at just off-bottom solids suspension condition is the most important parameter controlled by the bulk flow pattern associated with a particular impeller (Nienow and Miles, 1978; Upadhyay et. al, 1994). Baffles have been used widely in agitated vessels to study the solid-liquid mass transfer as it is generally agreed that the presence of baffles enhances mixing and, consequently, increases mass and heat transfer. The main reason for using baffles is to prevent swirling and vortexing of liquid in the tank. Barker and Treybal (1960) reported that, in a baffled agitated vessel, higher mass transfer coefficient is observed when particles are completely suspended. Yet this observation was based only on low solids concentration. Unbaffled agitated tanks have been used recently to examine the influence of baffles on impeller power required to suspend high concentration slurry (Wu et al., 2010, Wang et al., 2012, Tawaga et al., 2011). These studies have reported that impeller power consumption decreases significantly in the absence of baffles. It was believed that baffles, which create turbulent liquid flow and convert the rotational liquid motion into axial motion, lead to a significant increase in impeller power consumption (Tawaga et al., 2011). Although the influence of baffle removal on impeller power at N js is recognised now, its effect on solid-liquid mass transfer is not yet clear especially when high solids concentrations are used. Therefore, it will be valuable to investigate the influence of baffles on solid-liquid mass transfer coefficient. This study aims to investigate the effect of baffles on solid-liquid mass transfer coefficient at N js using high solids concentration (up to 0.30 (v/v)). The changes in mass transfer coefficient due to the variation in solid concentration are analysed on the basis of the changes in impeller power consumption at N js. It is also aimed to determine the maximum achievable or optimum 2

solids concentration that will provide higher mass transfer coefficient value at highest possible impeller energy efficiency. EXPERIMENTAL All experiments were carried out in a 0.2 m diameter cylindrical, flat-bottom perspex tank placed inside a square outer perspex tank. The cylindrical tank was equipped with four equally spaced baffles with width (B) to tank diameter ratio (B/T) of 1/12. The space between the inner and outer tanks was filled with tap water to minimize the optical distortion due to the curvature of cylindrical tank during flow visualisation. Off-bottom impeller clearance was set at T/4 in all experiments. A six-bladed Rushton turbine impeller of diameter D = T/3 was used as the agitator. The impeller was mounted on a centrally driven shaft attached to a torque sensor and speed detector. The impeller speed was read from the motor display. Aqueous NaOH solution was used as the liquid phase. An initial NaOH concentration C eo = 0.25 M was used in all experiments. The liquid height in the tank (H) was maintained equal to the tank diameter. The solid phase used was cationic ion-exchange resin (Amberlite IRN-77 supplied by Sigma-Aldrich, Australia) with an average diameters d p of 0.6 0.7 mm and density ρ s of 1220 kg/m 3. Solid concentration (C v ) was varied from 0.08 to 0.30 (v/v) to study its effect on mass transfer coefficient. Mass transfer rate was determined by measuring the changes in the conductivity of NaOH solution at various intervals due to the transfer of cations from the liquid to solid phase using an electrical conductivity meter (HACH, sension TM 40d). The conductivity probe was placed close to the centre of the tank at mid-liquid height position. The volumetric solid-liquid mass transfer coefficient, k SL a p was obtained from the change in NaOH concentration using the following equation (Levins and Glastonbury, 1972; Kato et. al., 1998, Tezura et. al., 2008): [ C Na ] ln = ksla pt C (1) [ Na ] 0 where k SL a p is the product of the solid-liquid mass transfer coefficient k SL and the solid-liquid interfacial area per unit volume of solid a p, [C Na ] 0 and [C Na ] are the sodium concentrations at time = 0 and time = t, respectively. Using equation (1), k SL a p was obtained from the slope of the linear plot of ln([c Na ] /[C Na ] 0 ) versus t. Then, k SL was determined by dividing k SL a p by the interfacial area a p (m -1 ) which was determined using the following equation: 6C a = v p d (2) 32 where C v is the volume fraction of solids in the slurry and d 32 is the Sauter-mean (or surfacevolume mean) solid diameter. The value of d 32 was determined to be 0.67 mm from the size analysis of resin particles obtained using Malvern particle size analyser. Mass transfer experiments were conducted at the critical impeller speed N js required to just suspend the solid off the tank bottom. N js was determined in this work using a technique involving the measurement of settled solid-bed height. This method has been demonstrated to be quite reliable for suspensions with high solids concentration by Wang et. al. (2012) and Wu et. al., (2010). To determine N js according to this method, the impeller speed was initially increased to a sufficiently high value so that all the particles were fully suspended and no particles remained stationary at the tank bottom. The impeller speed was then varied up and down a few times until a thin solid bed disappears at the tank bottom. The impeller speed at 3

which the solids bed disappeared was designated as N js. Fluorescein dye was added in the liquid and UV light was used to illuminate the tank bottom to observe the settling of solids. A torque sensor (Burster 8645) was used to determine the torque experienced by the impeller shaft. Electrical signal from the torque sensor was converted into digital signal by an A/D converter. A personal computer was used to process the digital data and determine the torque values. The impeller power draw P (W) was determined using the following equation: P = 2πNτ (3) where τ is the torque (N.m) experienced by the impeller shaft and N is the impeller rotational speed in revolutions per second (rps). Specific impeller power consumption ε (W/kg) was calculated using equation (4). P 2πNτ ε = = (4) M s M s where M s is the mass of solids suspended in the vessel. This equation for ε was based on the consideration that the rate of mass transfer in a solid-liquid agitated system is independent of agitation and vessel volume once the suspension of solids is achieved in majority of solidliquid operations except for those which requires homogeneous suspension such as crystallisation (Drewer et. al., 2000). Drewer et. al suggested that, for such processes, the mass transfer rate is controlled by the solid surface area and therefore it is reasonable to evaluate the effect of solids concentration on specific impeller power based on the mass of solids suspended. RESULTS AND DISCUSSION Effect of solids concentration and baffles condition on specific impeller power (ɛ js ) The impeller power input at various solids concentration has been considered on the basis of mass of solids suspended and used in the evaluation of impeller power efficiency. Figure 1 shows the experimental data of ɛ js = P js /M s at N js as a function of solids concentration C v under both baffled and unbaffled conditions. When ɛ js value is plotted in this form, it decreases with an increase in C v, until a critical value is reached and begins to increase thereafter. Higher ɛ js values represent lower impeller energy efficiency. This trend was observed also by Wang et al. (2011). They designated the C v value at which ɛ js is minimum as the optimum solids concentration (C v ) osc, because it represents a condition at which the energy input through impeller rotation is used most efficiently. According to this definition, the optimum solids concentration for baffled conditions in the present case is found to be 0.20 (v/v) (Figure 1). It appears that the minimum value of ɛ js is at C v = 0.15 (v/v) in this case. However, when ɛ js values are compared, ɛ js for C v = 0.20 (v/v) is only 2% higher than that for C v = 0.15 (v/v). Therefore, 0.20 (v/v) was selected as (C v ) osc in this work because it is much beneficial to operate the system at a higher C v and a similar ɛ js. In other words, more solids are suspended per unit of impeller power input (kg solids/w) at C v = 0.20 (v/v) compared to C v < 0.20 (v/v). To illustrate this point, mass of solids suspended per unit of power input are also shown in Figure 1. For example, at C v = 0.20 (v/v), the impeller can suspend about 0.38 kg solids per Watt, whereas at lower C v of 0.08 (v/v), it can suspend only 0.30 kg solids per Watt. These results indicate that the energy efficiency of solid-liquid mixing vessels can be increased by 4

operating them at higher solids concentration up to a critical concentration where ɛ js value is at minimum. Similar trend can be observed for unbaffled condition (Figure 1). The (C v ) osc for unbaffled condition is also found to be at 0.20 (v/v). These results reinforce the point that, regardless of the baffles condition used, operating the tank with higher C v (up to (C v ) osc ) could enhance the overall impeller energy efficiency. Under unbaffled condition, ɛ js values are much lower than those under baffled condition for all solids concentrations used. For example, at C v = 0.20 (v/v), 1.26 kg solids can be suspended per Watt under unbaffled condition while only 0.38 kg solids can be suspended per Watt under baffled condition. These results indicate that operating the system under baffled condition is less energy efficient especially for high C v. This is most likely due to the strong swirling flow present at the tank bottom under unbaffled condition which promotes the suspension of solids. Recently Wu et al., (2010) and Wang et al., (2011) reported similar findings for ultrahigh solid concentrations up to C v = 50%. Based on these findings, it be concluded that the removal of baffles is highly beneficial in decreasing the specific impeller power input, especially when high solids concentration are used in agitated vessels. Consequently, it is of interest to examine the effect of baffle removal on solid-liquid mass transfer coefficient. 4.5 0.22 kg/w 0.30 kg/w 3.0 0.38 kg/w Baffle Unbaffle ɛ js (W/kg) 1.5 1.01 kg/w 1.26 kg/w 1.00 kg/w 0.0 0 0.1 0.2 0.3 C v (v/v) Figure 1: Specific impeller power input at N js at different C v under baffled and unbaffled condition. Effect of solids concentration and baffles on solid-liquid mass transfer coefficient Mass transfer experiments were carried out in this study by varying C v to examine its effect on solid-liquid mass transfer coefficient (k SL ) under baffled and unbaffled conditions. Mass transfer coefficient values (k SL a p and k SL ) obtained at N js under baffled condition are shown in Figure 2 for a C v range of 0.08-0.30 (v/v). Many investigators have proven that it is essential to operate the system at N js so as entire solid surface area is utilized in mass transfer 5

effectively. Therefore, mass transfer experiments in this study were carried out only at N js. From Figure 2, it can be seen that volumetric mass transfer coefficient, k SL a p increases rapidly with an increase in C v from 0.08 up to 0.25 (v/v). This increase is due to an increase in solidliquid interfacial area a p with increase in C v (equation 2) and possibly due to an increase in k SL too. However, k SL a p starts decreasing with increase in C v beyond C v = 0.25 (v/v). When compared to k SL a p value at C v = 0.25 (v/v), those at C v = 0.30 (v/v) is 40% lower. This decrease is certainly due to a decrease in k SL because a p values at these C v are higher than that at C v = 0.25 (v/v). In order to understand the influence of C v on k SL a p clearly, therefore, it is essential to study the trend in k SL as a function of C v. Experimental k SL value increases with increase in C v up to 0.15 (v/v). Between C v = 0.15 and 0.20 (v/v), it remains more or less constant and thereafter starts decreasing. This trend is an interesting observation and has not been reported in the literature. Cline (1978) reported that solid-liquid mass transfer coefficient decreases with increase solids concentration but his experiments were carried out at constant impeller speed (not at N js ) by increasing the solids concentration. The increase of k SL observed in Figure 2 with an increase of C v is mainly due to the increase of impeller speed. The speed to achieve off-bottom solid suspension would increase accordingly with an increase in solids concentration that may lead to an upsurge of turbulence around the solid particle. The intensity of turbulence hence enhances the convective velocity distribution in the vessel causing an increase in k SL (Paul et al., 2004). Nienow (1969) has reported that mass transfer coefficient increases with impeller speed and proposed a relationship of the form k SL α N. Therefore the results obtained in this work imply that operating the impeller at N Njs is important to achieve enhanced values of mass transfer coefficient. The reasons for the decrease in k SL a p and k SL with increase in C v beyond C v = 0.20 (v/v) could be many. One possible reason could be the increased apparent viscosity of the solid-liquid mixture at these conditions due to increased particle-particle interaction thereby decreasing the interfacial area available for mass transfer (Tagawa et al., 2011, Conway et al., 2002). The reduction in k SL a p and k SL could be also due to the dampening of liquid turbulence (Conway et al., 2002) or change in flow pattern at high C v. The influence of C v on turbulence can be recognised by studying impeller Reynolds number (Re) for solid-liquid agitated vessels. It can be determined using equation (5). ρ Re = slurry η N js slurry D 2 where ρ slurry is the slurry density (kg/m 3 ), D is the impeller diameter (m) and η slurry is the apparent slurry viscosity (Pa.s). A number of empirical correlations are available in the literature to estimate η slurry as a function of C v. In this work, η slurry was calculated using a correlation (equation 6) proposed by Thomas (Honek et al., 2005). 2 η =η (1 + 2.5C + 10.05C + 0.0273exp(16.6C )) (6) slurry r v v η slurry and Re values calculated using equations 6 and 5 are shown in Figure 3a and 3b, respectively for a range of C v. It is evident from Figure 3a that apparent slurry viscosity increases with increasing solids concentration. Consequently, Re at N js decreases with increasing C v (Figure 3b). Re values for C v 0.2 (v/v) indicate that the slurry flow is in turbulent region (Re 10000) (Paul et al., 2004). On the other hand, Re values for C v 0.2 (v/v) indicate that the slurry flow is approaching the transition region ( Re < 10,000). These observation suggest that operating the system at C v 0.25 (v/v) will lead the flow into v (5) 6

transition or laminar region which will result in the dampening of liquid turbulence hence causing less effective solid-liquid mass transfer. The trends observed in k SL values as a function of C v under baffled condition have also been observed under unbaffled condition. The k SL values for baffled and unbaffled conditions are compared in Figure 4. It can be noted that k SL values under baffled condition are almost the same as those under unbaffled condition for all C v. The k SL value under unbaffled condition also increases with C v up to 0.20 (v/v), then starts to decrease. When these results are considered along with the trends in specific impeller power input data shown in Figure 1, it is clear that the removal of baffles helps in not only enhancing the impeller energy efficiency, but also in leading to mass transfer coefficient values that are similar to those for baffled condition. The optimum solids concentration (C v ) osc obtained on the basis of impeller energy efficiency is 0.20 (v/v). The maximum achievable k SL value is also found at C v = 0.20 (v/v) for both baffled and unbaffled conditions (Figure 4). Based on these results, it can be concluded that operating the system at (C v ) osc = 0.20 (v/v) under unbaffled condition is advantageous in terms of both mass transfer and energy efficiency. 0.25 1.4 0.20 1.2 1.0 k SL a p (1/s) 0.15 0.10 0.8 0.6 0.4 k SL x 10-4 (m/s) 0.05 kslap k SL a p k SL ksl 0.2 0.00 0.0 0.1 0.2 0.3 C v (v/v) 0.0 Figure 2: k SL a p versus k SL at N js for solid concentration range 0.08 0.30 (v/v) under baffled condition. 7

(a) Figure 3: Effect of solids concentration on (a) viscosity and (b) Reynolds number. (b) Figure 4: Solid-liquid mass transfer coefficient k SL at N js at different C v for baffled and unbaffled conditions. CONCLUSION Solid-liquid mass transfer at high solids concentrations was investigated experimentally using NaOH solution cation exchange resin system under baffled and unbaffled conditions in a mechanically agitated vessel. Results show that solid-liquid mass transfer coefficient increases with increasing solid concentration from 0.08 to 0.20 (v/v). Mass transfer coefficients of similar values can be achieved under both baffled and unbaffled condition. Specific power input ε js expressed as impeller power input per unit mass of solids suspended decreases with increasing solid concentration up to 0.2 (v/v) and increases thereafter. Similar results were obtained from experiments carried out under unbaffled condition. However, specific power input ε js values for unbaffled condition are lower than those for baffled conditions for all solids concentrations used. These results indicate it is beneficial to carry out solid-liquid mass transfer operation using unbaffled vessel because it helps to intensify the process and still attain significant achievement in mass transfer. 8

NOTATIONS a p solid-liquid contact area per unit volume of liquid, m -1 B baffle width, m C Na NaOH concentration, mol m -1 C v solid concentration, % (v/v) D impeller diameter, m d 32 Sauter mean diameter, m d p average diameter of ion-exchange resin, mm H liquid height, m k SL mass transfer coefficient, m s -1 k SL a p volumetric mass transfer coefficient, s -1 M s mass of solids, kg N impeller speed, rps N js just off bottom solids suspension speed, rps P impeller power consumption, W Re Reynolds number T tank diameter, m t time, s ɛ js specific power consumption at the just suspension condition, W kg -1 ρ s density of solids, kg m -3 ρ slurry density of slurries, kg m -3 τ torque, N m Ƞ slurry viscosity of slurries, Pa s viscosity of liquid, Pa s Ƞ r REFERENCES BARKER, J. J. & TREYBAL, R. E. 1960. Mass transfer coefficients for solids suspended in agitated liquids. AIChE Journal, 6, 289-295. CLINE, H. B. 1978. A correlation for predicting the mass transfer coefficient in agitated vessels that accounts for the effect of solids concentration on the particle slip velocity, University of Maryland. CONWAY, K., KYLE, A. & RIELLY, C. D. 2002. Gas Liquid Solid Operation of a Vortex- Ingesting Stirred Tank Reactor. Chemical Engineering Research and Design, 80, 839-845. DREW, G. R., AHMAD, N. & JAMESON, G. J. 2001. An optimum concentration for the suspension of solids in stirred vessels. Mixing and Crystallization, Kluwer Academic Publishers, Netherlands. HARRIOTT, P. 1962. Mass transfer to particles: Part I. Suspended in agitated tanks. AIChE Journal, 8, 93-101. HONEK, T., HAUSNEROVA, B. & SAHA, P. 2005. Relative viscosity models and their application to capillary flow data of highly filled hard-metal carbide powder compounds. Polymer Composites, 26, 29-36. 9

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