Chemical Engineering Science 74 (2012) 233–243

Contents lists available at SciVerse ScienceDirect

Chemical Engineering Science

0009-25

doi:10.1

n Corr

Univers

E-m

journal homepage: www.elsevier.com/locate/ces

Energy efficient solids suspension in an agitated vessel–water slurry

Steven Wang a,b,n, David V. Boger a,c, Jie Wu d

a Department of Chemical Engineering, Monash University, Clayton Victoria 3800, Australiab CSIRO Earth Science and Resource Engineering, Clayton Victoria 3168, Australiac Department of Chemical and Bimolecular Engineering, The University of Melbourne, Parkville Victoria 3010, Australiad CSIRO Process Science and Engineering, Highett Victoria 3190, Australia

a r t i c l e i n f o

Article history:

Received 8 July 2011

Received in revised form

16 February 2012

Accepted 21 February 2012Available online 3 March 2012

Keywords:

Energy efficiency

Agitator power efficiency

Mixing

Solids suspension

Removal of baffles

Dispersion of solids

09/$ - see front matter Crown Copyright & 2

016/j.ces.2012.02.035

esponding author at: Department of Che

ity, Clayton Victoria 3800, Australia.

ail address: steven.wang@csiro.au (S. Wang).

a b s t r a c t

Power consumption required to suspend water–solid slurries in a mechanically agitated tank has been

studied over a wide range of design and solids conditions with the goal to improve the agitation energy

efficiency. It is demonstrated that the power required for providing off-bottom solids suspension and

solids dispersion could be reduced dramatically with baffles removed, as in comparison with

conventional agitator designs where vertical baffles were used.

Axial-flow impellers in baffled tanks were found more energy efficient to suspend solids off the

bottom, consistent with the literature data. However, with baffles removed, radial-flow impellers were

found more energy efficient than axial flow impellers. The effect of removal of baffles on the solids

dispersion was also included. The impeller geometrical effects include pitch angle of blade, number of

blades on energy efficiency were studied. The impacts of solid concentration and particle size on power

consumption were also investigated.

Crown Copyright & 2012 Published by Elsevier Ltd. All rights reserved.

1. Introduction

In the context of the increasingly urgent need to reduce carbonemissions, it is highly desirable to investigate options to reducepower consumption used in various unit operations in the processindustry. Mechanically agitated vessels are widely used in mostmodern processing plants, where solids/liquid operations invol-ving mixing and suspension are required to achieve a vast rangeof process objectives such as blending, leaching, digestion, crys-tallization, adsorption, ion-exchange, polymerization and othertype of processes (Nienow, 1994). Energy consumption involvedin solids suspension is therefore of particular relevance andimportant to our goal of reducing energy consumption for theprocess industry in general.

Power input required for agitation of solid–liquid suspensionhas been well studied in the mixing research community. It hasbeen generally accepted that radial turbines are less energyefficient in solids suspension than axial- and mixed-flow impel-lers, and that the energy efficiency for off-bottom solids suspen-sion is sensitive to the impeller off-bottom clearance and impellerdiameter, as reported by Nienow (1994), Drewer et al. (1994),

012 Published by Elsevier Ltd. All

mical Engineering, Monash

Wu et al. (2001), Wu et al. (2002) and among others. Refer toZwietering (1958) for basics of solids suspension in mixing tanks.

The pitched-blade impellers perform more efficiently than diskturbines; and the pitched turbine installed down-flow pumpingconsumes less energy than the pitched-turbine up-flow pumpingimpellers (Ibrahim and Nienow, 1996; Frijlink et al., 1990).Raghavo Rao et al. (1998) stated that poor mixing efficiency fora radial-flow impeller is because only partial energy delivered bythe circulation loop is available for solids suspension. It should benoted that most of these studies involved low concentration ofsolids (o0.20 v/v). Information regarding the effect of impellertype on power consumption at high solids concentration is scarcein the literature. Also unavailable in the literature is reference tothe impact of varying impeller geometrical parameters, includingnumber of blades and blade angles, on suspension power con-sumption at high Cv (40.20 v/v).

Relatively less attention has been devoted so far to solidssuspension in un-baffled vessels. The role of baffles in mechanicallyagitated vessels is to prevent the swirling and vortexing of liquid(Lu et al., 1997). With the installation of baffles, an upward wall-jetin the tank is driving the solids suspension (Bittorf and Kresta, 2003)and it is generally agreed that enhanced mass and heat transfers canbe accomplished. However, excessive and insufficient baffling mayresult in reduction of mass flow and localizing circulation in theagitation system (Nishikawa et al., 1979).

Wu et al. (2010 a, b), Wu et al. (2011) and Wang et al. (2012)present results of a study on minimizing the specific power ðejsÞ

rights reserved.

Fig. 1. Mixing rig used in this study (Schematic diagram of experimental set-up).

Table 1Impeller specification, power number measured in water, T¼390 mm.

Impeller

ID

Full name Flow

pattern

No. of

blades

Blade

width W/D

D/T Power

no.

DT6 6-bladed radial

turbine

Radial 6 1/5 0.41 5.598

DT4 4-bladed radial

turbine

Radial 4 1/5 0.41 4.140

DT3 3-bladed radial

turbine

Radial 3 1/5 0.41 3.172

30PBT6 30o pitch

6-bladed turbine

Mixed 6 1/5 0.41 0.720

20PBT4 20o pitch Mixed 4 1/5 0.41 0.267

S. Wang et al. / Chemical Engineering Science 74 (2012) 233–243234

at the just-off-bottom solids suspension condition in mixing tanksat a high solids concentration. It was found that major savings inthe specific power required to suspend solids at high concentra-tions can be achieved by removal of baffles. This is particularlyrelevant to applications where chemical reaction rate is slow andis not affected by the mixing rate, such as often occurs in themineral industry. This is similar to the findings by Brucato et al.(2010) that the mechanical power required to achieve completesuspension in un-baffled tanks was smaller than that in baffledtanks. They suggested that un-baffled tanks are preferred inprocesses where mass transfer is not the limiting factor.

Some industrial storage tank applications are satisfied withunsuspended particles remaining in the bottom as long as all theparticles are kept in motion. As a result, they are often designed atlevels of agitation a bit lower than the just-suspended condition.Nevertheless, it is more common that solids are required to befully dispersed throughout the entire liquid volume. Examples arecatalytic reactors, crystallization reactors and overflow feed tocentrifuges (Hicks et al., 1997; Bujalski et al., 1999). This isbecause the purity, productivity and selectivity of the reactionare highly dependent on relative rates of mixing or homogeniza-tion in such units. If the particles are fully dispersed throughoutthe tank, the rate of mixing can be assumed to be similar to thatfound in single-phase systems. (Bujalski et al., 1999). Consideringthe fact that poor mixing and a low energy dissipation rate in theclear layer also leads to a larger amount of non-reacting fluid andsolid particles (Bittorf and Kresta, 2003), attempts to fully disperseparticles throughout the reactors are also highly desirable.

This paper aims to minimize the agitation power required tosuspend solids off the bottom of a tank, via optimizing a range ofparameters including impeller type, particle size, impeller geo-metry factors such as number and angle of blades and baffleinstallation. In addition, the slurry cloud will be used as aparameter in an attempt to determine the effect of design changeson the power efficiency for achieving homogenous dispersion ofsolids suspension.

4-bladed turbine

30PBT4 30o pitch

4-bladed turbine

Mixed 4 1/5 0.41 0.664

45PBT4 45o pitch

4-bladed turbine

Mixed 4 1/5 0.41 1.220

30PBT3 30o pitch

3-bladed turbine

Mixed 3 1/5 0.41 0.530

A310 Hydrofoil Axial 3 – 0.41 0.320

Blade thickness¼1.5 mm for all impellers.

2. Experimental set-up and study methodology

2.1. Test rig

Experiments were carried out in a mixing research rig (Fig. 1)consisting of a 390 mm diameter cylindrical tank with a flatbottom placed inside a rectangular outer acrylic tank. Thecylindrical tank was fit with four baffles (B/T¼1/12) verticallyand spaced 90o apart. The outer tank was filled with tap water tominimize the optical distortion. Impellers were mounted on acentral shaft equipped with an Ono Sokki torque transducer andspeed detector. Off-bottom impeller clearance was set at T/3.Agitation was provided by an impeller mounted on a shaft drivenby a motor and the speed of the shaft could be varied by means ofoperating a variable frequency drive.

2.2. Impeller type

The speed and power required to satisfy the just-off-bottomsuspension condition were recorded for three types of impellers:disk turbines (DT), pitch-bladed impellers (PBT), and a Lightninimpellers (A310). The impellers were chosen to three types of flowpattern: radial, mixed and axial flow. Their specifications areillustrated in Table 1 and the impellers are schematically shown inFig. 2. All impellers were 0.41 of the tank diameter. The thickness ofthe impeller blades is 1.5 mm. For the dual-impeller system,impeller spacing was fixed equal to impeller diameter, 160 mm.

2.3. Slurry properties

The mixing fluid utilized in the experiments was tap water andthe liquid height in the tank was kept equal to the tank diameter.Spherical glass ballotini with a density of rg¼2500 kg/m3 anddifferent sizes of ballotini particles (Table 2) were used.

2.4. Visual observation

Zwietering (1958) introduced the visual observation methodto determine Njs. He defined Njs as the speed at which no solid isvisually observed to remain at rest on the tank bottom for 1–2 s.This complete off-bottom suspension criterion (CBS) is frequentlyused as measure of the required suspension point of the solids(Dutta and Pangarkar, 1995; Bujalski et al., 1999; Drewer et al.,2000). A drawback of using Zwietering’s CBS, as pointed out byKasat and Pandit (2005), is the excessive energy required to liftsolids from relatively stagnant regions, for example, around theperiphery of the vessel bottom near the baffles or at the center ofthe vessel bottom, where the liquid circulation is not strong

Fig. 2. Schematic drawings of impellers used in this study.

Table 2Solids properties: glass bead, density: 2500 kg/m3.

d10 (mm) d50 (mm) d90 (mm) Terminal velocity

in water (m/s)

Particle A 70 90 100 0.0094

Particle B 110 165 235 0.0142

Particle C 260 320 480 0.0460

1 Note this sedimentation height refers to the sedimentation at the wall; this

measurement approach works for agitator systems where sedimentation starts at

the tank fillet region.

S. Wang et al. / Chemical Engineering Science 74 (2012) 233–243 235

enough compared to the bulk of the liquid in the vessel. They alsopointed out that from a practical point of view the fillets formedin such areas are generally less significant since it was observedthat any extra input of energy of higher impeller speed (typically20–50%) with the intent of raking particles out of the fillets inthese relative stagnant regions tended to homogenize the solidsconcentration in the bulk of tank before complete elimination offillets. Wu et al. (2011) also pointed out that this method isproblematic since a single particle (or a small quantity) stationaryin a corner may not be suspended even at a very high stirredspeed, and that it is meaningless and unreliable to rely on thestatus of a small amount of particles to determine the just-off-bottom conditions. Based on these considerations, a methodbased on Hicks et al. (1997) was used in our experiments:sedimentation bed height (HB) was measured with various agita-tion speeds, and Njs was defined as the speed at which the heightof the settled bed is zero (i.e. HB¼0) and a further reduction inthe impeller speed will give rise to a visible solid bed (i.e. HB40).The bed height, recorded as an averaged sedimentation heightalong the fillet of the tank wall, was typically measured at a point

in the middle between two consecutive baffles.1 The repeatabilityof the Njs measurement using this method was found to be within72%. Details regarding the optical observation as a function ofimpeller speed can be seen in Fig. 3. Fig. 3(a) shows a conditionwhere solids are completely suspended off the tank bottom wherethere is no sedimentation, nor is there a clear liquid layer at thetop liquid surface. Fig. 3(b) shows the formation of the sedimen-tation bed at the bed bottom for a condition in which impellerspeed is slightly lower than Njs. In this condition, a clear liquidlayer can be also seen at the top liquid surface. It must be notedthat a red dye has been used in the liquid phase to make the flowvisualization and the determination of ‘just-off-bottom suspen-sion’ speed easier. Fig. 3(c) shows that the sedimentation bedthickness has increased further at an impeller speed much lowerthan Njs. These figures show clearly that correct bed heights canbe easily visualized at high solids concentration slurry. Fig. 4(a)and (b) show two example plots of the ratio of the settled bedheight (HB) and the total liquid height (H) against the impellerspeed for the suspension of solids at varying concentrations,under baffled and unbaffled conditions respectively.

The sedimentation bed height at the wall is an approximationto the full status of the sedimentation across the bottom; thisapproximation is convenient, and also practical. This approachhas a direct relevance to the minerals slurry tanks, where drainpipes at the wall sides are used extensively, particular for larger

Fig. 3. Visual method to determine Njs in the agitated vessel. (a) N4Njs,

homogenous suspension, HS¼H; (b) NoNjs, HS/Ho1, HB40; (c) N5Njs,

HS/Ho1, HB40.

S. Wang et al. / Chemical Engineering Science 74 (2012) 233–243236

tanks, where tank bottoms are concrete footings. Sedimentationstarting near the wall is thus of more critical concern.

3. Results

3.1. Critical speed for just-off-bottom suspension

The dependence of Njs on baffling condition is shown in Fig. 5for radial, mixed and axial flow impellers, respectively. Notablymost of the data points obtained in the unbaffled tank are belowthe blue points, which implies the most of the Njs values obtainedin the unbaffled tank are smaller than the relevant values in

baffled systems. The only exception is the case of the axial flowimpeller, for which the two values of Njs are almost identical(Fig. 5(c)).

Based on Fig. 5, for radial and axial flow impellers, the depen-dences of Njs on particle concentration in the unbaffled vessel appearto be similar to those in baffled vessels, whereas the dependence onsolid concentration appears to be practically negligible for the mixedflow impeller (30PBT6) used in this study.

3.2. Maximum solid loading: (Cv)max

Fig. 6 shows a typical trend of agitator power (Pjs/V) at the just-off bottom condition (normalized by the tank volume) as afunction of volumetric solids concentration, from 0.05 (v/v) to�0.50(v/v). The impellers used were a down pumping 301 pitch-bladed impeller and a hydrofoil impeller. It is interesting toobserve that Pjs/V increases steadily with the concentration upto �0.30 (v/v) and thereafter it increases rapidly as the concen-tration approaches �0.50 (v/v), which is very high by anyindustrial standard. The exponential increase of the power inputimplies existence of an upper limit of the solids volumetricconcentration, denoted as (Cv)max. Further attempts to increasethe solids concentration beyond (Cv)max in the agitated tank inboth configurations were not successful due to dramatic increasein the power, beyond the practical limit of the motor equipment.Wu et al. 2002 suggested that theoretically, the maximum solidconcentration achievable in a mechanically agitated vessel shouldbe smaller that the solid packing coefficient Cvb, which is usuallydefined as the volume concentration of solids in the settledcondition, without mechanical agitation, e.g. from vibration orextended time aging effect. The slurry was pumped out andanalyzed immediately after the solids were fully settled in thetank bottom and it was found that the value of Cvb is equal to 0.58for the particles used in the present study. Based on Fig. 6, it canbe estimated that, normalized with the solids packing coefficientof Cvb:

ðCvÞmax=Cvb � 0:90, with baffles

ðCvÞmax=Cvb � 0:98, without baffles:

3.3. Minimum specific power concentration: (Cv)min_power

A useful measure for off-bottom solids suspension is the powerrequired at the just-off-bottom condition on per unit mass of

solids suspended. The power per unit mass (or specific power) isdefined as

ejs ¼Pjs

MSð1Þ

where ejs is the specific power (W/kg) at the ‘‘js’’ condition, and MS

is the mass of solid particles (kg) in the tank. Fig. 7 re-plots Fig. 6on power per unit mass of solids, still at the ‘‘js’’ condition. It isinteresting to see the specific power decreases with solids con-centration initially, until a critical value is reached at which thespecific power is the minimum. Beyond this critical value, thespecific power increase with solids concentration. A similarbehavior could be observed with baffles removed as evident inFig. 7, however, the variation of the specific power with solidsconcentration is less significant. The Cv value at which ejs is aminimum could be designated as an ‘optimum solids concentra-tion’; this ‘‘optimum concentration’’ is for a minimization of thepower per solids mass, which might be important for energysaving in certain bulk material storage process.

Fig. 4. HB vs. N at the baffled tank, Njs determination at different solids concentrations. (a) under baffled condition; (b) under unbaffled condition. Impeller: 30PBT6.

d50¼90 mm.

Fig. 5. Njs as a function of solids concentration. (a) radial flow impeller: DT6; (b) mixed flow impeller: 30PBT6; (c) axial flow impeller: A310. d50¼90 mm.

S. Wang et al. / Chemical Engineering Science 74 (2012) 233–243 237

Fig. 8 plots the ejs as a function of solids concentrations, forradial-flow, mixed-flow and axial-flow impellers respectively, withbaffles installed. The radial-flow impeller (DT6) was found to be lessenergy efficient than axial flow and mixed flow impellers (A310,45PBT4) in suspending the solids, regardless of solids concentration.A310 has the lowest ejs values among those three impellers,indicating that it is more energy efficient for suspending solidparticles than other impellers tested here. The results are consistentwith the findings in the literature (Frijlink et al., 1990; Drewer et al.,1994), all with the conventional baffles installed.

Fig. 9(a)–(c) shows the ejs data for the different size of particles(d50¼90 mm, d50¼165 mm and d50¼320 mm respectively) underun-baffled conditions. Reduction in ejs can be observed withbaffles removed (Fig. 9(a)) as in comparison with baffles installedin Fig. 8 and the effect of baffle removal is most significant forDT6, among the impellers.

Data for coarser particles is also included for comparisonpurposes. These results confirm that (Cv)min_power exists for thethree impellers under unbaffled conditions. However, it is veryinteresting to see that the six-bladed turbine DT6 is more energyefficient than the four-bladed turbine and the three-bladedhydrofoil impeller A310 irrespective of solids concentration andparticle size. Based on Fig. 9(b) and (c), it can be also seen that thedependence of impeller type on specific power is more obviousfor coarser particles.

On-going research suggests that the value of (Cv)min_power isdependent on the impeller type, particle size, baffling conditionsand carrier fluid viscosity, and it is estimated that:

ðCvÞmin_power=Cvb � 0:45� 0:60:

The existence of (Cv)osc is most likely linked to a change in flowpattern, which is related to the variation in Reynolds number.

0

500

1000

1500

2000

2500

0.00 0.10 0.20 0.30 0.40 0.50 0.60Cv (v/v)

P js/V

(W/m

3 )

30PBT6, without baffles30PBT6, with bafflesA310, without bafflesA310, with baffles

(Cv)max = 0.52, in the baffled condition

(Cv)max = 0.56, in the unbaffled condition

Fig. 6. Specific impeller consumption (Pjs/V) as a function of solids concentration

Cv. Impellers: 30PBT6 and A310. d50¼90 mm.

0

0.2

0.4

0.6

0.8

1

1.2

0.06 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.47 0.50Cv (v/v)

ε js (W

/kg)

30PBT6, Pjs/Ms, with baffles

30PBT6, Pjs/Ms, without baffles

(Cv)min

(Cv)min

Fig. 7. Specific impeller power input ejs (¼Pjs/Ms) at Njs at different Cv. d50¼90 mm.

Fig. 8. ejs values under baffled conditions: ejs (¼Pjs/Ms). d50¼90 mm.

S. Wang et al. / Chemical Engineering Science 74 (2012) 233–243238

Reynolds number in an agitated tank is defined here as

Re¼rslurryNjsD

2

Zslurry

where rslurry is the slurry density (kg/m3), Njs is the just-off-bottom speed (rps), D is the impeller diameter, Zslurry is the slurryviscosity (Pa s) at the just-off-bottom suspension conditions. The

calculation of Reynolds number in a solid–liquid mixing system iscomplicated, considering that fact that there is axial/radial con-centration gradient throughout the tank, and the homogeneityvaries at different conditions. In this instance, it might be logicalto assume that all the particles are evenly distributed throughoutthe tank when calculating the Reynolds numbers in the solid–liquid systems.

The impact of solids loading on Reynolds number for 30PBT6under baffled condition is shown in Fig. 10. Correlation equations(Zslurry vs. Cv) were based on the models developed by Thomaset al. and Fedor et al., respectively (Honek et al., 2010). This figureshows that Reynolds number decreases with increase in solidsconcentration. It can be seen that at higher solid concentrations(Z(Cv)osc), it is within the transition regime (Reo10,000), indi-cating that there is change in flow pattern at such high solidsconcentrations. As is expected, laminar flow becomes moresignificant, as Cv approaches the ‘maximum achievable solidsconcentration’, (Cv)max, leading to excessively high power require-ment for just-off-bottom suspension. This is because less turbu-lences are produced in the system due to more significant impactsof solid–liquid friction and particle–particle collisions.

3.4. Improved energy efficiency via agitator design change

The results presented above indicate that the average ‘optimum

solids concentration’ for minimum power consumption per unit ofsolids mass is approximately in the range 0.25–0.35 v/v. Thisoptimum is only suitable for process where the energy is thecontrolling factor in design consideration. In many instances, theneed to maximize the product throughput is more important. Thisrequires a consideration to operate at a higher solids concentration,for purpose of process intensification. For this reasoning, solidssuspension experiments aiming to evaluate the performance ofdifferent impellers were carried out with a solids concentration of0.40 v/v, which is higher than the minimum specific energyconcentration mentioned before.

(a) Impact of baffle removalRemoval of baffles has been shown to be a very effective method

to reduce specific power required to suspend particles off bottom(Brucato et al., 2010). The percentage reduction in ejs due to theremoval of baffles for a mixed-flow impeller (30PBT6) as a function ofsolids concentration is shown in Fig. 11. The data indicates thatsignificant power reduction can be accomplished simply by removingthe baffles, regardless of solids concentration. It is also interesting tosee that the effect of removal of baffles is more pronounced athigher concentrations. For example, at a high solids concentration of0.40 v/v, removal of baffles reduces the agitator’s power consumptionby �60%; whilst �20% power reduction could be achieved in arelatively low solids concentration, e.g. 0.15 v/v.

Table 3 provides quantitative information on the impact ofbaffles removal at different operating conditions. It suggests thatejs generally decreases on removal of the baffles in the range of lowto high solids concentrations studied in this work. This table alsoshows that improvement of energy efficiency for solids suspensioncan be achieved for all single and dual-impeller systems by removingbaffles. It can be seen that the effect of baffle removal is moresignificant at low solids concentration in the cases of disk turbines(DT4 and DT3). However, power reductions for disk turbines decreasewith an increase in solids concentration, which is opposite to thefindings observed for pitch-bladed impellers. Of all the impellers, dualDT4 impellers are the most sensitive to baffle removal with a powerreduction E90%. This suggests that at high solids concentration,Cv¼0.40 v/v, power dissipated by the dual impellers (2�DT4) underbaffled conditions is 11 times higher than that under un-baffledconditions.

Fig. 9. ejs values under unbaffled conditions : (a) d50¼90 mm; (b) d50¼165 mm; (c) d50¼320 mm.

S. Wang et al. / Chemical Engineering Science 74 (2012) 233–243 239

(b) Effect of impeller blade angle and number of bladesExperiments were carried out with impellers of both axial flow

and radial flow impellers of different numbers of blades and pitchangles to minimize ejs. These experiments were carried out for bothbaffled and unbaffled conditions at a solids concentration of 0.40 v/v.

Fig. 12(a) shows ejs variation with the pitch angle over a range201–901, for a 4-bladed impeller. As the pitch angle increases, theflow becomes more radial, with the blade angle at 901 similar to a4-bladed disk turbine. The specific power decreases slightly withangle up to 451 and increase substantially thereafter. At a high-pitch angle of 901, which is effectively similar to a 4-bladed diskturbine, ejs is significantly higher than the pitch-blade (down-ward-pumping), consistent with our understanding of baffledtanks. In the case of unbaffled conditions, ejs values decreasecontinuously with an increase in blade angle as shown inFig. 12(b). In all these experiments, the average glass particle sizewas kept constant at d50¼90 mm and baffles were installed.

The effect of the number of blades on ejs for mixed and radial-flow impellers is illustrated in Fig. 13(a) for the baffled condition.The ejs values decrease with an increasing number of bladesregardless of the impeller type (radial or mixed). However, thedecrease in the ejs values with an increase in the number of bladesfor the mixed-flow impellers is rather marginal. Also it is inter-esting to note that the difference between ejs values for mixed andradial-flow impellers decreases as the number of blades increases.Under unbaffled conditions also, the ejs values decrease with anincrease in the number of impeller blades for both mixed andradial-flow impellers (Fig. 13(b)).

(c) Effect of particle sizeThe effect of particle size on ejs for disk turbines and pitch-

bladed turbines under baffled and unbaffled conditions is shown

in Fig. 14(a) and (b) respectively for a solids concentration of 0.40 v/v.For all impellers, an increase in particle size leads to an increase in ejs

even though the mass of solids is the same. It is also very clear fromFig. 14(a) that under baffled conditions, disk turbines requires higherejs values in comparison to pitch-bladed turbines, to suspend solids. Itis useful to note that, ejs required by DT6 is twice that of 30PBT6 insuspending �67 mm particles, and approximately 1.2 times insuspending �159 mm particles. It is also could be observed thatpitch-bladed turbines are more sensitive to change in particle sizethan radial-flow impellers at this high solids concentration.

Under baffled conditions also the specific power ejs requires tosuspend the particles off the tank bottom increases with anincrease in particle size regardless of the impeller type used(Fig. 14(b)), similar to that with baffles installed. The dependenceof specific power ejs on particle size is almost the same for all diskturbines and blade turbines at such high solids concentration. Thedifference in ejs values for disk turbines and pitched-bladeturbines is not that significant under unbaffled conditions. Theejs values for disk turbines are nearly the same as those for pitch-bladed turbines. As particle size increases, it can be seen that diskturbines require more power than pitched-blade turbines forsuspending solids. It can be thus assumed that for any geome-trical setting, there might be a critical size point beyond whichmixed-flow impellers would perform better than the radial-flowimpellers.

3.5. Improving dispersion of solid particles at high Cv

Fig. 15 plots a typical solids concentration vertical profilebased on the data reported by Hicks et al. (1997) in a tank

Fig. 10. (a) Effect of solids concentration on viscosity (viscosity calculations were

based on Honek et al., 2010); (b) Effect of solids concentration on Reynolds

number (Re) at Njs.

Fig. 11. Percentage reduction (%) in impeller power consumption (ejs) at different

Cv owing to removal of baffles. Impeller: 30PBT6. d50¼90 mm.

Table 3Effect of removing baffle for different impellers at

low and high solid concentrations, % reduction in

ejs with baffles removed.

Impellers Cv¼0.06 v/v Cv¼0.40 v/v

DT4 90 80

DT3 88 80

30PBT4 36 56

30PBT3 29 52

2�DT4 N/A 91

2�45PBT4 N/A 41

Fig. 12. Variation of ejs with impeller blade angle at constant particle loading

(Cv¼0.40 v/v) for pitch-bladed impellers: (a) tank configured in baffled condition;

(b) tank configured in the unbaffled condition. d50¼90 mm.

S. Wang et al. / Chemical Engineering Science 74 (2012) 233–243240

operating with a Chemineer HE-3 axial flow impeller and glassparticles (�200 mm) in water. The data suggest that the solidsconcentration is relatively constant throughout the tank belowthe cloud height, denoted as HS. Above this height, the solidsconcentration falls abruptly to zero. Based on this finding and ourlaboratory measurements, the slurry cloud height HS was used asa visual measure of the status of solids dispersion. Although this isa simplification, it offers a convenient parameter for comparison andis considered useful for the purpose of studying solids dispersion.

Our criteria for a complete dispersion can be expressed below:

HS¼H

HB¼0

Fig. 16 shows the normalized slurry cloud height (Hs/H) as afunction of the specific power at 0.40 v/v of solids concentrationfor a radial-flow impeller (DT6). Sedimentation bed height data inthe figure is also included to quantify the solids suspension statusoff the tank bottom. Note a shift toward left of the curves whenbaffles are removed: a clear indication of reduced power con-sumption for both off-bottom solids suspension, and dispersion ofsolids. A significantly lower specific power is required to providefull dispersion of solids with baffles removed. It can be estimatedfrom the figure that, the ‘minimum’ power required to providefull dispersion (i.e. achieving HS/H¼1, while keeping HB¼0)decreased from approximately 0.50–0.15 W/kg, with removal ofbaffles. Similarly, with the 30PBT6 impeller shown in Fig. 17,

Fig. 13. Variation of ejs with number of impeller blades at constant particle

loading (Cv¼0.40 v/v) for pitch-bladed impellers: (a) tank configured in baffled

condition; (b) tank configured in the unbaffled condition. d50¼90 mm.

0.00

0.20

0.40

0.60

0.80

1.00

0 50 100 150 200Particle Size (μm)

ε js (W

/kg)

DT6 DT4

30PBT6 30PBT4

with baffles

Fig. 14. Effect of particle size on ejs at constant particle loading (Cv¼0.40 v/v):

(a) tank configured in baffled condition; (b) tank configured in the unbaffled

condition. d50¼90 mm.

S. Wang et al. / Chemical Engineering Science 74 (2012) 233–243 241

the specific power decreased from approximately 0.30–0.15 W/kg, atHB¼0 and HS/H¼1, with removal of baffles. Fig. 18 shows the resultsoperating with an A310 impeller, showing a similar improvement interms of increased dispersion at a lower power, although a completedispersion was not achieved (HSo1), due the tests over the motorpower range. Overall, it can be stated that a significant power savingcould be achieved for dispersing solids with baffles removed, in therange between 50% and 70% in the examples.

The data presented above indicates that it is possible tomaintain a high level of homogenization with reduced powerinput if baffles are removed, whilst keeping solids suspendedfrom the bottom.

4. Discussion

In agitated slurry tanks used for chemical reactions, thekinetics of a reaction could be either diffusion limited ornon-diffusion limited. For non-diffusion limited slow reactionprocesses, e.g. such as those in minerals hydrometallurgicalrefineries, it is often sufficient to keep solids just suspended offtank bottoms. Given this practical significance, it is meaningful todefine the specific power input into the agitator system at thejust-off-bottom solids suspension, i.e. ejs for the purpose ofstudying energy efficient design. This basic parameter has beenused extensively in this paper to optimize agitator and tankdesigns to reduce the power consumption, resulting in some

dramatic power saving effects from some perhaps rather simplechanges, such as removal of baffles.

Significant benefits in improving the energy efficiency throughremoving the conventional baffles have been demonstrated inthis study. Although a side effect of removal of baffles is anincrease in the mixing time (Wu et al., 2011), this is not usually anissue for many slowly reacting processes, such as those in theminerals processing. For example, a gold-leaching process maytake 20–40 h to complete; the mixing time, even thoughincreased, is still in the order of tens of minutes; therefore themixing is sufficiently fast in comparison with the requiredresidence time scale.

It has been well established in the literature that it is moreenergy efficient to suspend solids off tank bottom using axialimpellers. It is now clear that this is not entirely true. Axial flowimpellers are more efficient, but only in tanks with vertical bafflesas conventionally used and assumed in the literature. With bafflesremoved, radial flow impellers are more energy efficient thanaxial flow impellers.

It is interesting to comment that the pitch angle of an impellerhas an effect on the power efficiency. This effect is morepronounced in a baffled tank, particularly in large pitch angles.This effect is less significant in un-baffled conditions, althoughthere is a moderate trend of reduced specific power ejs as thepitch angle is increased (i.e. with the flow being more radial,Fig. 12(a)), consistent with our finding that radial flow impeller ismore efficient, albeit with baffles removed.

Among somewhat similar features with the effect of number ofblades ejs (Fig. 13(a) and (b)), it is interesting to comment that

Fig. 15. Typical solids concentration profile (Hicks et al., 1997), impeller: HE-3, dp¼200 mm, D/T¼0.35, C/T¼0.25, H/T¼1, solids mass fraction¼0.116.

Fig. 16. Paticle dispersion for radial flow impeller: DT6 at constant particle

loading (Cv¼0.40 v/v): HS vs. specifc power; HB vs. specific power; with and

without baffles. d50¼90 mm.

Fig. 17. Paticle dispersion for mixed flow impeller: 30PBT6 at constant particle

loading (Cv¼0.40 v/v): HS vs. specifc power; HB vs. specific power; with and

without baffles. d50¼90 mm.

Fig. 18. Paticle dispersion for axial flow impeller: A310 at constant particle

loading (Cv¼0.40 v/v): HS vs. specifc power; HB vs. specific power; with and

without baffles. d50¼90 mm.

S. Wang et al. / Chemical Engineering Science 74 (2012) 233–243242

with baffles removed, there is a rather flat trend of the specificpower curve as a function of the number of blades, similar to theeffect of the pitch angle. This illustrates an important mechanism,called ‘‘equal efficiency’’ as described in our study (Wu et al.,2006), where it shows an insensitivity of varying number ofblades and other parameters of axial flow impellers on thecirculation flow efficiency. This is a very useful result which canbe used in practice to solve industrial problems. For example,impeller erosion or particles attrition could be reduced byoperating at a slower rotational speed with increased number ofblades, whilst providing the same off-bottom solids, at the samepower input-due to the same power efficiency.

It is interesting to discuss the measurement results on disper-sion of solids as reported in this paper. The cloud height as alaboratory visual parameter is a simple, convenient measure of animportant underlining aspect of solids suspension: i.e. the homo-geneity of the solids distribution. Whilst for many processes, thismay not be critical for slowly reacting slurry systems in mineralsprocesses as discussed earlier. Dispersion of solids can howeverbecome important in certain operations, for example, productblending in tanks operating in a continuous mode. It is unaccep-table that the discharge slurry shows inconsistency in theproportions of feed slurries, e.g. fluctuations of the percentageof a chemical additive with the bulk slurry. It is thereforeinteresting to see that improved dispersion can be achieved byremoval of baffles, at the same power input.

Finally, it is interesting to comment on the effect of particlesize on the power efficiency. In general, increased agitator specificpower is required to suspend larger particles. This means for thesame amount of solids mass, less power is required if the same

mass of bulk solids are broken down to smaller sizes. This canhave ramifications for many industrial solids suspension opera-tions, e.g. minerals processing, where a large amount of bulk orematerials has to be milled to small sizes for metal extractionprocessing. Smaller ore particle sizes whilst costing more energyin the crushing and milling stage can actually mean a saving ofthe power consumption in the later solids suspension operationsas implied in this paper. This effect may be significant for someprocesses where long residence time for reaction requires therefineries to keep a vast inventory of solids in suspension inhundreds of tanks of large dimensions.

S. Wang et al. / Chemical Engineering Science 74 (2012) 233–243 243

5. Conclusions

Power consumption required to suspend water–solids slurriesin a mechanically agitated tank has been studied over a widerange of design and solids conditions with the goal to improve theagitation energy efficiency using a laboratory mixing tank. It wasdemonstrated that the power required for providing off-bottomsolids suspension and solids dispersion could be reduced drama-tically with baffles removed, as in comparison with conventionalagitator designs where vertical baffles were used. Axial-flowimpellers in baffled tanks were found more energy efficient tosuspend solids off the bottom, consistent with the literature data.However, with baffles removed, radial-flow impellers were foundmore energy efficient than axial flow impellers. The impellergeometrical effects including pitch angle of blades and number ofblades were studied. It was found that with baffles removed, therewas relatively insensitivity in the number of blades on the specificpower at the just-off-bottom suspension condition.

The upper limit of solids concentration for suspension wasfound extended with baffles removed. It was also found that lesspower is required to suspend small particles, for the same amountof solids mass. The dispersion of solids was studied based on theslurry cloud height. It was concluded that improved solidsdispersion could be achieved by removal of baffles at the samepower input.

Nomenclature

B impeller width (m)C impeller clearance (m)Cv solids volume concentration (v/v)Cvb solids packing volume concentration (v/v)(Cv)min_power optimum solids concentration for particle

suspension (v/v)(Cv)max maximum solids concentration for particle

suspension (v/v)d particle size (mm)d50 mean particle size (nm)D impeller diameter (m)H liquid level (m)HB settled bed height (m)HS slurry height (m)N impeller speed (rev/s, rpm)Njs just-off-bottom solids suspension speed (rpm)Ms solids mass (kg)P power (w)Pjs agitator power for just-off-bottom solids suspension (w)Re Reynolds numberT tank diameter (m)V tank volume (m3)W blade width (m)

ejs agitator power per unit solids mass at the just-off-bottomsolids suspension condition (W/kg)

rslurry slurry density (kg/m3)rg particle density (kg/m3)Zslurry slurry viscosity (Pa s)

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