Design factors affecting the dynamic performance of soil suspension in an agitated,baffled tank

M.Moayeri Kashani,S.H.Lai*,S.Ibrahim,P.Moradi Bargani

Department of Civil Engineering,Faculty of Engineering,University of Malaya,50603 Kuala Lumpur,Malaysia

1.Introduction

Identifying how sediment is transported is significant to investigating instances of subaqueous and subaerial mud or debris flows.Such occurrences pose a major threat to life and infrastructure besides representing a key natural mechanism of sediment transport[1–3].The density,shape and size of particles are effective in separate suspension at different rates[4].Analysing changes in representative cores' sediment properties must be preceded by identifying which sedimentation conditions considerably influence the spatial diversity of sediment successions[5].Dispersion phase in solid–liquid suspensions is linked to several parameters;therefore,various aspects concerning the behaviour of large and fine particles[6]in close(batch and semi-batched)and open tank using baffled and un-baffled tank have been studied[7–10].Particle size distribution as well as mineralogy and morphology[1,11]all affect flow as well[12].

A mechanical means of studying sediment movement and settling is to employ a mixing instrument.Such instrument can also be used to dewatering fine particulate solids[13],and recovery of valuable metals from pollutant soil[14,15]and sediment mixtures[16]or cement[17]in order to realize the effects of particle size and water content on settling velocity and rheological properties.

Stirred vessels and reactors are conventional mixing units utilized in the process industries that can be used to gain better understanding of particle suspension in liquids.The suspension phenomenon is required in numerous processes,including leaching,catalytic reactions,crystallization[18]and water treatment.The result of mixing soil-water slurries may be an effective method of removing pollutants,such as sediment,nutrients and pesticides from agricultural runoff[19].

Soil dispersion under different conditions in a stirred tank has been modelled using computational fluid dynamics(CFD)to attain superior solid suspension prediction and enhance stirred reactor performance[20–24].Various experiments have emphasized on the behaviour of solid particles in solid–liquid slurries with different scale of impellers,such as solid settling velocity[25],solid dispersion and stirred tank power consumption[26–28].Coulomb mixture theory modelling can represent variable solid and fluid constituent interactions in heterogeneous debris- flow surges with high-friction,coarse-grained heads and low-friction liquefied tails.

In stirred tanks,the Reynolds numberNReand power number Npcontribute to defining the hydrodynamic behaviour of a slurry[29].where ρlis the fluid medium's density,N is the impeller speed(in r·s-1),Diis the impeller diameter andPis the power input to the agitator.According to theNRe,the suspension is divided into laminar flow(NRe＜10)and turbulent flow(NRe＞2×104).Power consumption depend on the impeller,tank geometric design and interactions between the impeller and tank is constant[30].

The principal aim of this study is to investigate the effect of particle size and texture,slurry height as well as impeller type,diameter and rotational speed on the solid dispersion and power consumption in a stirred tank reactor.The power variation for different impellers,slurry heights and soil series is additionally examined.The novelty of this study lies in the empirical gathering of results from six mixed-soil series.The geometric design of each impeller type,location and diameter of the impellers along with impeller clearance from the bottom of the vessel are noted as well.

2.Materials and Method

The focus of the present study is on investigating the suspension in six soil and water slurry series using a mixing setup.Table 1 presents the classification of the six soil samples collected from various regions in Malaysia.

2.1.Experimental setup

Experiments were carried out in a fully baffled,cylindrical, flat based,transparent Perspex tank with internal diameter(T)of 15.5 cm.This tank was placed inside another rectangular tank filled with water to facilitate undistorted observation of the stirred tank content(Fig.1).A mirror was placed at a 30°angle below the tank to view the tank base and ascertain the state of particle suspension.Observation was enhanced with shining a white LED light,a 10×magnifier and anti-glare shades to reduce reflection.The slurry system comprised a mixture of soil(the six soil series' characteristics were studied with LDPSA and SEM)and tap water(ρl=1000 kg·m-3).The cylinder tank was filled at constant solid loading of 20 wt.%and three different slurry heights(5,15 and 20 cm)in order to explore the effect of height on soil settling and particle suspension(Table 2).The impeller speeds were set at 100,250 and 350 r·min-1.The impellers employed were down-pumping 4PBT with 4,45°pitched blades,and HR100 SUPERMIX?impellers from SATAKE Chemical Equipment MFG.LTD.,i.e.axial HR100 with different diameters.The use of impellers with different specifications is to study the effect on the sediment,but it is not the aim of this work to compare the efficiency of the impellers as this work has been published elsewhere[31].

The impeller specifications are provided in Table 3.Both types of impeller were placed 30 mm from the bottom of the tank.Subsequently,the hydrodynamics of particle mixing and power consumption of the six soil series at three different impeller speeds and slurry heights were investigated in turn.Power consumption is calculated from its relationship to torque at the examined impeller speed for the given conditions(Eq.(3)).The power imparted into the fluid on the shaft,not the electrical input.If the electrical input is measured,it would be necessary to minus friction losses from the bearings in order to get actual power imparted into the tank fluid.

Fig.1.Graphical diagram of used baffled stirred tank.

Table 2Slurry properties

Table 3Impeller specifications

Each soil type was added to the tank that was filled with distilled water to the appropriate height.The impeller was then calibrated by rotating for 30 min.Next,the stirred vessel was operated for 10 min to facilitate data collection.

Table 1Real soil sample characteristics(Kashani et al.,2015)

Fig.2.Graph of particle size distribution(laser diffraction method)of 6 soil samples.

2.2.Soil characterization

Six series of soil rich with fine particles were analysed(Fig.2).The soil particle size distribution was measured by Laser Diffraction Particle Size technique(LDPS,Malvern Master size 2000 Particle Size Analyser).The particles were passed through a laser beam and the light they scattered was recorded over a range of angles in forward direction.Subsequently,the diffraction angles were,in the simplest case,inversely related to particle size with a refractive index of 1.4[32,33].The actual soil specifications and laser diffraction results are presented in Fig.3 and Table 1.

2.3.Scanning electron microscopy

It has been reported in former studies that fine sediment composition and texture considerably influence sediment consistency and transport[34].A scanning electron microscope(SEM)[35]was applied with an SEM-FIB-Zeiss Auriga at 1 kV and working distance of 5 mm with no metal or carbon sample coating.The SEM resolution was 1.0 nm at 15 kV and 1.9 nm at 1 kV,and acceleration voltage between 0.1 and 30 kV.

Monochrome photographs were captured with an integrated reflex camera.These provided semi-quantitative information on textural properties,particle shape[36]and surface roughness(Fig.4)[37].

3.Results and Discussion

Ibrahim[38]have shown that mixing in a stirred vessel does not occur randomly but through complex interactions of geometric,rheological and energetic factors.The majority of studies have investigated the lowest speed and power rates required to suspend all particles in a stirred vessel[39–41].In the current study,the goal is to assess the influence of impeller type and size on soil suspension in a tank.The major factor studied during testing is the dimensionless unit of power number(Np).The six soil series with a constant concentration of 20 wt.%(20%soil)mixed with distilled water were used to evaluate the effect of different parameters on stirring the slurry for 10 min(Fig.5).The soil series' textures varied from sand to clay and silt.Thus,particle size ranged from less than 0.06 to nearly 900 μm.Figs.6 and 7 represent the effect of various experimental conditions and impellers on the power number.

3.1.Effect of particle size

Particle size was found to have an effect on stirred vessel operation.The various behaviours were investigated experimentally.Evidently,higher agitator specific power is required to suspend larger particles[31].The power required is less when particles are broken down into smaller sizes[42,43].Sample 1 indicated greater settling velocity at different slurry heights,as it had the highest percentage of coarse particle diameters(＞100 μm)known as sandy soil(Fig.6).At 5 cm slurry height,the finer unsuspended particles in sample 1,with higher viscosity deposited on the bottom of the tank[44],over which a layer of coarser particles also settled.The images taken during the test illustrate this categorization based on particle size and weight.The 4PBT impeller had a smaller diameter(d=60 mm),which consumed higher power per mass(0.3 W·kg-1)to mix sample 1 at higher speed than HR100.This phenomenon was also detected in previous studies related to the effects of particle size in stirred vessels[43,45–47].

Fig.3.Images of soil samples:a/sample 1,b/sample 2,c/sample 3,d/sample 4,e/sample 5,and f/sample 6.

Fig.4.Scanning electron micrographs(SEM)of the six samples:a/sample 1,b/sample 2,c/sample 3,d/sample 4,e/sample 5,and f/sample 6.The comparison between the sample textures illustrates the clay in samples 4 and 5(d and e),and the silt in sample 6(f).The focus of all photos is 10 μm.

At increasing slurry height and at 100 r·min-1particle movement reduced and the tank was divided into three zones:a very thin layer of fine particles at the bottom covered by a thick layer of coarser particles,a region of clear water at the surface and a suspension zone in between[39].Remarkably,this phenomenon was perceived at a higher volume of slurry and greater 4PBT impeller speed than complete suspension(N＞speed at which the particles were deemed completely suspended),which caused a clear liquid layer in the upper part of the vessel.The physical reason for this effect is not clear[21].It was found that sample 2 had the smallest suspended layer and samples 6 and 3 had the greatest soil suspension layer,signifying the low efficiency of loam in suspension(soil specification Table 1).The state of suspension may have a significant impact on flow patterns and impeller power consumption(Bujalskiet al.,1999).Samples 1 and 2 required the highest power per mass consumption of all samples.This result signifies that the increase in power with the increase in particle size was possibly due to corresponds to substantial increase in particle surface area[48],the particle interactions[48],and higher density of particles rather than water and particle resistance in following the flow.The ratios of unsuspended soil to slurry height and suspended soil to slurry height are demonstrated in Fig.8 for 20 cm slurry height.

Sample 2 had a texture akin to sample 1 and behaved similarly but exhibited higher settling velocity.The influence of the different particle sizes in samples 2 and 1 was evident at the unsuspended soil level on the bottom of the vessel during rotation.The height of unsuspended particles in sample 2 was lower than sample 1.The sand was not uniformly mixed in the tank,but the loam was[19].It was observed that the impeller rotating at 100 r·min-1mixed the coarse particles promptly.Therefore,the aggregative force,weight and size of particles caused settling.In sample 3,theNjswas 250 r·min-1at 5 cm slurry height and the power consumed by the impeller decreased with increasing slurry height for this sample.The remaining samples had more clay and silt,so their behaviour was unlike samples 1,2 and 3.While the impeller was rotating,the coarse particles accumulated towards the centre of the tank,resulting in a pileup of solid particles in this area.The pile size reduced as the impeller accelerated.It should be noted that the rheological behaviour of the soil series significantly influenced the mixing process through the pseudoplastic behaviour of fine particles at the bottom of the tank.Thus,the impeller power in the slurry suspension was restricted[41,49,50].

Fig.5.The behaviour of sample 3(above)and sample 4 prepared in different height,varied impeller speed and different impellers.

It was visualized in the laboratory that the rheological behaviour of clay influenced the power required,as the finer particles appeared more plastic and atN=350 they moved upwards(Fig.8).Mardles[51]reported that the rheological character of clay suspension changes depending on the liquid and operating condition specifications.The other perceived effect of stirring at low speed was that the terminal velocity of soil particles is dependent on particle size and experimental duration[52].Different layers with various colours formed on the bottom of the tank with slurry heights of 15 and 20 cm.It was found the impeller was able to mix all clay and silt particles at greater slurry height but not the sand particles.Hence,a homogenous system was revealed in the clay and silt soil series.Although three distinct zones formed during 100 r·min-1speed operation in the soil suspension at 20 cm,the difference between the particle diameters of samples 4 and 5 signifies that at high slurry level the particles in sample 5 settled faster.The slurry of sample 5 was more compact,and it took longer to loosen the solid bed;thus,an increase in power number over sample 4 was evident.According to Ibrahim,Wong,Baker,Zamzam,Sato and Kato[48],εjsincreases rather than decreases for smaller particles,since the increasing frequency of particle-particle interactions arising from more numerous particles results in a greater total surface area.The height of settled particles during the test was lower for sample 6,and the soil particle classification was less different.This condition indicates the formation of a sediment bed at the bottom when the impeller speed is slightly lower than theNjs[53].Mixing clay consumed less power per mass than mixing sand particles.Sample 2 with sand texture affected impeller operation,as it required the highest power per mass.Fig.7 illustrates the effect of slurry height and particle size on the power and Reynolds numbers for different impeller speeds and two impeller types.

3.2.The in fl uence of slurry height

The slurry height for a constant20 wt.%concentration was adjusted to 5,15 and 20 cm.In the current study,an attempt was made to determine the effect of slurry height on the power number and movement of soil particles at a constant concentration.Fig.8 represents two sample plots of the ratio of settled bed height(HB)and total liquid height(H)to impeller speed variation for the suspension of six solid series at 20 cm slurry height in baffled condition[53].Numerous researchers have examined the influence of solid concentration and height of solid particles on slurry suspension[47,50,54,55].Based on the current laboratory observations,higher impeller speed had greater impact on the soil slurry suspension.Increasing the slurry height caused higher power consumptionP(W)—a value with greater variation when the 4PBT impeller was employed.The power utilized for the sample 2 water slurry(5 cm to 20 cm water level)increased from 0.451 watt to 0.744 watt at 350 r·min-1impeller speed.Therefore,it was proven that under the influence of suspended/unsuspended solids(Bujalskiet al.,1999)the flow pattern is very different at such low liquid height.Thus,increasing height raised the power number.Evidently,the movement of particles at 20 cm slurry level and lower impeller speed was affected by soil sample density and viscosity,which divided the slurry into three distinct zones.The reason for this phenomenon was the inadequate impeller speed[39,56].As described by the effect of particle size,at low impeller speed the unsuspended soil particles rested on the bottom of the tank.The lower impeller speed moved smaller particle size upwards,so the unsuspended layer of fine sediments decreased.By increasing the impeller speed to 250 r·min-1,the clear water layer disappeared and most unsuspended soil particles flowed in the slurry;thus,a homogeneous system was observed at N=350 r·min-1.The influence of impeller speed was investigated and similar results were obtained for a down-pumping marine propeller as well as an axial- flow impeller[57,58].The accelerating impeller speed apparently decreased the power number at 20 wt.% fixed concentration for all slurry heights.At 5 cm height,the power number and Reynolds number were the lowest.At a higher water level,the power number increased.With increasing speed,the Reynolds number increased but the power number decreased.The increase in slurry height caused an increase in unsuspended coarse particles on the bottom of the tank(e.g.sample 1).Subsequently,the importance of off-bottom clearance on stirred vessel outcome must be noted[59].This study only focused on a constantC/T,but varying this may improve impeller speed efficiency at higher slurry levels[48,54,57].Smaller off-bottom clearance could assist with the easier dispersion of soil particles and may have a lesser effect of rheology.

Fig.6.The effect of slurry height on power consumption(W)at impeller speeds of 100,250 and 350(r·min-1)for HR100 and 4PBT.

3.3.Performance of impeller type

This study demonstrated that the particle suspension pressure and efficiency are dependent on the impeller shape and diameter[60].For this reason,the influence of impeller diameter and shape was examined.Both impeller types were affected by changes in particle size(Tables 4 and 5).The power per mass for the HR100 and 4PBT impellers increased by going from clay particles in samples 4 and 5 to sand and loam particles in sample 1 and with decreasing slurry height.

Fig.7.The effect of slurry height and particle size on power number and Reynolds number at impeller speeds of 100,250,and 350(r·min-1)for HR100 and 4PBT.

According to these results,the Reynolds number when using 4PBT was half of that determined with HR100,due to the smaller impeller diameter.Since the diameter of HR100 was 80 mm,the impeller was more efficient and consumed less power than 4PBT with 60 mm diameter.Apparently,the slurry rotation when using HR100 increased the power per mass with higher impeller speed,and the power per mass decreased with a rise in slurry height level.The same rise in power per mass value was noted for the 4PBT impeller.Meanwhile,the power per mass value at 15 cm slurry height was the highest compared with 5 cm when using 4PBT.At 5 cm slurry height,90%of soil particles in sample 3 became suspended using 4PBT,therefore this impeller is more efficient than HR100 in making coarser particles flow.Generally,the results indicate that power consumption is proportional to impeller speed.Besides,with increasing particle size,high energy is required to maintain stirred vessel operation[42].Impeller power consumption directly affects the dynamic performance of a continuous flow mixing system[49,60].The data analysis and lab observations demonstrated that the 4PBT impeller is more effective than the HR100 impeller at increasing soil particle mixing for a homogenized slurry and diminishing the high viscosity of soil particles.Moreover,greater impeller speed drew coarser particles into the middle of the tank in a pile.The shape of the sandy silt pile transformed from a star print when using 4PBT to a circle when using HR100.The HR100 propeller exhibited superior performance in dealing with coarser particles,since the bottom of the tank was cleared at high impeller speed and 5 cm slurry height.Consequently,the presence of baffles influenced the energy required,and various soil particles under the impeller force increased solid loading[53,61].

4.Conclusions

Fig.8.HB and H s vs.N in a baffled agitated tank,where N is determined for different soil samples.Impeller:HR100 and 4PBT;slurry height 20 cm.

This study demonstrated the effect of slurry(soil-water)height,impeller type and diameter,and particle size in six soil series as well as various impeller speeds on the power consumption of a stirred vessel.It was concluded that in a mixture of soil and water,impeller speed has a substantial part in suspending all particles.The soil slurry required greater power at higher impeller speed but less power per mass at greater slurry height.It was also found that less power was required to suspend small particles for the same amount of solid mass.The HR100 impeller led to a lower power number and higher Reynolds number than the 4PBT impeller,due to the larger diameter that accelerated the flow speed.The power consumption of both impellers amplified at lower impeller speed,therefore more unsuspended solid accumulated on the bottom of the tank.The proficiency of 4PBT in clearing the bottom was superior for all soil series;nonetheless,the power number isa significant parameter in industry,and HR100 demonstrated superior efficiency.Another achievement relates to the impact of particle size and density on impeller efficiency,where coarser particles and higher density soil series consumed more power per mass.Achieving just suspension in an agitated vessel with a soil slurry involved more soil characteristics,which is required to load all particles and withstand the terminal velocity of particles.Further investigation of the impeller clearance effect on soil slurry suspension is suggested for future studies.

Table 4Comparing power per mass× 10(W·kg-1)using 4PBT impeller under different experimental conditions

Table 5Comparing power per mass×10(W·kg-1)using HR100 impeller under different experimental conditions

Nomenclature

Coff-bottom clearance

cwsolids loading,%

Dimpeller diameter,mm

dpparticle diameter,μm

Hmixing height,cm

HB the ratio of settled bed height

Njsjust-suspension speed

Nppower number

NReReynolds number

Nimpeller speed,r·min-1

Ttank diameter,mm

εjspower per unit mass of slurry,W·kg-1

μ viscosity,Pa·s

ρlliquid density,kg·m-3

ρssolid density,kg·m-3

Acknowledgments

The authors are grateful to the Government of Malaysia for the financial support from University of Malaya through the UMRG grant no.RP008B-13SUS and Ministry of Higher Education(MOHE)through the FRGS grant no.FP028-2012A.The authors also would like to thank Faculty of Engineering,Universiti Malaya for all the facilities provided.

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