Effect of agglomerate properties on agglomerate stability in fluidized beds

Chemical Engineering Science 63 (2008) 4245 -- 4256

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Chemical Engineering Science

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Effect of agglomerate properties on agglomerate stability in fluidized beds

Sarah Webera, Cedric Briensa,∗, Franco Berrutia, Edward Chanb, Murray Grayc

aDepartment of Chemical and Biochemical Engineering, The University of Western Ontario, London, Ontario, Canada N6A 5B9bSyncrude Canada Ltd., Research Centre, Edmonton, Alberta, Canada T6H 1H4cDepartment of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta, Canada, T6G 2G6

A R T I C L E I N F O A B S T R A C T

Article history:Received 1 February 2008Received in revised form 21 May 2008Accepted 23 May 2008Available online 11 July 2008

Keywords:FluidizationAgglomerationAttritionParticulate processesWet agglomerateAgglomerate growth

Fluidized bed agglomeration is used to stabilize particulate mixtures and reduce dust emissions. Thistechnology is applied to a variety of production processes for the pharmaceutical, chemical, fertilizer andfood industries. In most of these applications, agglomerate stability is an essential criterion. Agglomeratesand granules that do not conform to size and shape specifications may create problems in downstreamprocesses, such as tableting, thus compromising process efficiency and product quality. When an agglom-erate is formed in a fluidized bed, it can grow by incorporating other bed particles, split into smallerfragments, or be eroded by fluidized bed solids. The objective of the present study is to determinethe critical agglomerate liquid content at which the rates of agglomerate growth and shrinkage arebalanced when artificial agglomerates made from glass beads and water are introduced into a fluidizedbed. This study examined the effects of agglomerate size, agglomerate density, liquid viscosity, binderconcentration, and fluidizing gas velocity on the critical initial liquid content. This study found that smallagglomerates and low density agglomerates displayed higher critical initial moisture contents. Whenthe viscosity was increased by using sugar solutions, agglomerates were very stable and had very lowcritical initial moisture contents. The study also found that as the superficial gas velocity increased, theagglomerates started to fragment, rather than erode.

© 2008 Elsevier Ltd. All rights reserved.

1. Introduction

Particulate operations play a very large role in many industriesand their performance is essential to the success of many processes.In industries such as the pharmaceutical industry, agglomerates areintentionally produced (Simons, 1996). In other processes, such asfluid coking, however, agglomerates are not desired (House et al.,2004). Agglomerate properties can potentially be manipulated toensure survivability, if they are desired, or to enhance destruction ifthey are undesired.

The objective of this study is to investigate how factors such asagglomerate size, agglomerate density, agglomerating liquid proper-ties, and superficial gas velocity affect the stability of agglomeratesand, specifically, their critical initial moisture content. The criticalagglomerate liquid content is the liquid content at which the ratesof agglomerate growth and shrinkage are balanced when artificialagglomerates made from glass beads and water are introduced intoa fluidized bed. Agglomerates containing a moisture level above thiscritical value will survive the fluidized bed conditions, whereas those

∗ Corresponding author. Tel.: +15196612131; fax: +15196613498.E-mail address: [email protected] (C. Briens).

0009-2509/$ - see front matter © 2008 Elsevier Ltd. All rights reserved.doi:10.1016/j.ces.2008.05.035

with moisture contents below this critical value will begin to bedestroyed in the fluidized bed. Water and glass beads, a systemwhich does not tend to form agglomerates because the contactangle between the liquid and solids is low, were used in thisstudy (McDougall et al., 2005). Previous studies have looked atthe effect of different parameters on agglomerate growth (Hematiet al., 2003) and on agglomerate growth and destruction (Weberet al., 2006). The present study differs from that of Hemati et al.(2003) because it controls initial agglomerate properties to examinestability, and it expands on our previous work (Weber et al., 2006)by investigating the effect of different parameters on agglomer-ate behavior. Agglomerates can be destroyed according to variousmechanisms. These include external mechanical stresses caused byfluidized bed conditions, internal mechanical stresses caused bychemical reaction and production of vapor, and migration of liquidfrom the agglomerate to drier bed particles. This study investigatedhow agglomerate characteristics affected the mechanisms of ag-glomerate destruction in the absence of reaction or vapor evolution.

1.1. Agglomeration forces

Agglomerates are formed by the aggregation of particulate solidsthat are held together by short-range physical or chemical forces

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4246 S. Weber et al. / Chemical Engineering Science 63 (2008) 4245 -- 4256

Fig. 1. A liquid bridge between two identical particles with diameter d. The liquidbridge here is attracting the particles together (adapted from Simons et al., 1994).

acting among particles, by chemical or physical modification of theparticles triggered by specific process conditions, or by substancesthat act as binders by adhering physically or chemically to formmaterial bridges among particles (Pietsch, 2003). One importantbinding mechanism for agglomeration requires liquid to be present.In agglomerates and granules, there are three situations where liquidcan produce cohesive forces: mobile liquid bridges, adsorbed liquidlayers on particles, and adhesive or viscous binders (Sherrington andOliver, 1981). In the liquid–solid agglomerates studied in this paper,the cohesive forces were generated by liquid bridges.

The force exerted by a liquid bridge is caused by the pressuredifference across the liquid–gas interface. The force generated byliquid bridges may be repulsive or attractive (Rondeau et al., 2003).The Young–Laplace equation, defining the pressure difference acrossa liquid bridge, is a function of the surface tension and the curvatureof the interface of the liquid bridge (Rondeau et al., 2003):

�P = �[1/R1 + 1/R2] (1)

R1 and R2 are the principal radii of curvature of the liquid bridge asdefined in Fig. 1. The contact angle of the liquid on the solid deter-mines the radii of curvature. When the contact angle is 0◦, the pres-sure difference is negative and there is an attractive force betweenthe two particles (Rondeau et al., 2003). When the contact angle isgreater than 90◦, the force between the two particles becomes re-pulsive. For the intermediate case of partially wetting substances,the liquid bridge may begin in an equilibrium state, and forces maybecome attractive as the liquid evaporates from the agglomeratestructure (Rondeau et al., 2003). Although researchers have devel-oped criteria to indicate when a liquid bridge will rupture (Dai andLu, 1998), these were not tested because the focus of this study waslooking at macro-scale agglomerate failure.

The amount of liquid in an agglomerate determines the state ofliquid saturation in that agglomerate. In the pendular state of liq-uid saturation, liquid forms bridges at point contacts within the ag-glomerate structure. As the pore space of the agglomerate is filledwith liquid, the agglomerate saturation state goes to the funicularand finally to the capillary states. In the pendular state, the agglom-erate strength is caused by liquid bridges. In the capillary state, theagglomerate strength is caused by capillary pressure. The funicularstate is a combination of the two forces (Sherrington and Oliver,1981).

As discussed above, an important aspect of the forces inliquid–solid agglomerates is the interaction of liquids with the solidparticles, which can be characterized by the contact angle. Contactangle measurements can be used to assess the wettability of solids.Wetting behavior in agglomerates can lead to two different typesof agglomerates. One occurs when the liquid covers the powder,producing a dense and less porous granule, and the other occurswhen the powder covers the liquid, producing a more porous gran-ule (Buckton, 1993). Spreading coefficients can be calculated andused to determine properties of granules such as density and sizedistribution (Buckton, 1993). While smaller contact angles providebetter wetting, higher degrees of saturation are then required forsuccessful agglomerate growth (McDougall et al., 2005).

1.2. Agglomerate growth

Many studies have been done to examine the effect of differentparameters on the growth of agglomerates in granulating pro-cesses. Several of these studies have attempted to model the growthof agglomerates by defining the different growth mechanisms(Hemati et al., 2003; Iveson and Litster, 1998). Traditionally, growthmechanisms were divided into categories such as layering, nucle-ation, coalescence, crushing and layering, and abrasion transfer.These classifications are somewhat arbitrary, therefore a morerecent division of agglomerate growth mechanisms includes wet-ting and nucleation, consolidation and growth, and breakage andattrition (Iveson and Litster, 1998).

Some work has been done to investigate growth regime mapsfor liquid-bound granules. Iveson and Litster (1998) constructed aregime map to qualitatively explain the variations observed in gran-ulation behavior and were able to confirm the regime map experi-mentally. There are some limitations to these types of regime mapsas the boundaries need to be better defined to be very effective(Iveson and Litster, 1998). Researchers have modeled the agglom-eration process. Peglow et al. (2006) developed a model based onmonodisperse nuclei formation, which allows the extraction of rateconstants for agglomeration and growth processes. Saleh et al. (2003)examined fluidized bed coating and agglomeration using the conceptof population balance to predict the time evolution of particle sizedistributions. Granule properties can be linked to granulation ratesusing multi-dimensional population balance equations that can bereduced to one-dimensional population balance equations by usingsimplifying assumptions. Biggs et al. (2003) were able to describethe results of high-shear granulation using this approach. Their workdefined two model parameters: the aggregation rate constant andthe critical binder volume fraction (Biggs et al., 2003). Attempts atmodeling the agglomeration process are now considering simulta-neous aggregation and breakage behavior. Tardos et al. (1997) intro-duced a general theoretical framework to look at granule formation,growth, and breakup. More success has been found modeling aggre-gation behavior rather than breakage kinetics (Tan et al., 2005). Morework is required to be able to fully understand the fundamentalsof the agglomeration process and manipulate it to increase productconsistency.

1.3. Agglomerate destruction

Controlling particle and agglomerate size is important for manyindustrial applications. To modify the size distribution, impact com-minution is widely used. Difficulties can arise when unintentionalattrition by impact occurs, causing problems in the process due todegradation of particles and granules. By understanding breakage inparticles and granules, manufacturing efficiency and product qualitycan be improved (Salman et al., 2004).

In a study by Salman et al. (2004), wet granules were studied us-ing impact tests. They found that granule failure could be classifiedby the impact velocity. For low impact velocities, plastic deforma-tion was observed in wet granules. At high impact velocities, wetgranules were greatly reduced in size due to fragmentation. Thisstudy also found that small wet granules showed significant plas-tic deformation before failure. This differed from large wet granules,which exhibited localized debris formation and chipping (Salmanet al., 2004). Salman et al. (2003) also found that increasing granulesize caused agglomerate breakage to occur at lower impact veloc-ities in impact studies of fertilizer granules. Breakage patterns andfailure modes are dictated by the agglomerate structure and the ve-locity of the agglomerate impact (Salman et al., 2003, 2004; Suberoand Ghadiri, 2001). The breakage pattern affects the size distributionof the impact product (Subero and Ghadiri, 2001). Researchers have

S. Weber et al. / Chemical Engineering Science 63 (2008) 4245 -- 4256 4247

found that it is very difficult to model agglomerate failure becauseof the complex processes that are occurring, as well as the difficultyaccounting for the structure of the agglomerate material (Subero andGhadiri, 2001). The impact tests conducted by Subero and Ghadiri(2001) indicate that there are twomain types of breakage. The first islocalized damage and the second is distributed damage. Tardos et al.(1997) looked at the use of a critical Stokes number to define whengranules will break when exposed to shearing in different types ofgranulators. Moreno-Atanasio and Ghadiri (2006) used amechanisticmodel that relates the number of broken contacts in an agglomeratedue to impact velocity, properties of the particles that form the gran-ule, and the interparticle adhesion energy. This study found that theamount of damage was related to a dimensionless group, �, definedby the following equation (Moreno-Atanasio and Ghadiri, 2006):

� = (D5/3E2/3V2�)/�5/3 (2)

Other studies have examined the effect of hydrodynamic forces onthe dispersion of agglomerates. It has been found that there aretwo mechanisms of agglomerate dispersion; adhesive and cohe-sive failure. Adhesive failure occurs in agglomerates that are par-tially infiltrated with fluid. Cohesive failure occurs in agglomeratesthat have no infiltrating liquid or in fully infiltrated agglomerates.Erosion and fragmentation of agglomerates is caused by cohesivefailure. The cohesivity of an agglomerate depends on the physicalnature of the powder and its packing structure. Increasing the hydro-dynamic forces causes agglomerate dispersion to shift from erosionto fragmentation (Boyle et al., 2005). Shamlou et al. (1990) studiedthe hydrodynamic influences on particle breakage in fluidized bedsby studying a fluidized bed of granules made from soda glass beadsand polymer. This study found that the size of the bed particles dropscontinuously until a plateau value is reached. This study also foundthat the mode of bed material breakage occurs predominantly by at-trition rather than fragmentation and that this attrition is most likelycaused by low energy impacts in the bed core (Shamlou et al., 1990).

The complexity of both the agglomerate material and the forcesexerted on the agglomerate by the fluidized bed environment re-quire more study to fully understand agglomerate destruction mech-anisms.

1.4. Segregation in fluidized beds

Agglomerates can tend to segregate in fluidized bed operations.In the polymer industry, the size distribution of particles taken fromthe bottom of a reactor can be very different from the size distri-bution of particles taken from the top of the reactor (Kim and Choi,2001). The most influential parameter in fluidized bed segregationis particle density differences. Severe axial particle size distributionscan also be observed in a fluidized bed of powder of a continuoussize distribution at velocities that are much higher than the mini-mum fluidization velocity according to the study by Hoffmann andRomp (1991) (Kim and Choi, 2001). To model the segregation of par-ticles in a gas phase fluidized bed, Kim and Choi (2001) postulatedthat the segregation of a particle mixture of equal density will oc-cur because of the size-dependent transfer of particles between thewake and bulk phases as gas bubbles rise in the fluidized bed. Influidized beds of group C powders, agglomerates are segregated bysize (Xu and Zhu, 2005). Xu and Zhu (2005) found large differencesin particle sizes depending on where in the fluidized bed the parti-cles were removed. Large agglomerates were found at the bottom ofthe fluidized bed and small agglomerates were found at the top ofthe fluidized bed.

Segregation can affect the behavior of agglomerates within thefluidized bed. Agglomerates that tend to settle at the bottom of the

bed will experience different forces and stresses within the fluidizedbed than agglomerates that remain fluidized.

2. Materials and methods

2.1. Agglomerate preparation

A syringe–piston system was used for agglomerate fabrication.For predetermined moisture contents, known masses of solidsand liquid were combined in a container and thoroughly mixed.A constant mass of wet solids was placed in the syringe–pistonsystem and compressed to a predefined length. To test the ef-fect of agglomerate size on agglomerate stability, three cylindricalmolds were used. The largest mold had a diameter of 0.0169mand a height of 0.0169m, the smallest mold had a diameter of0.0067m and a height of 0.0070m, and the intermediate mold hada diameter of 0.0116m and a height of 0.0120m. One of thesemolds was placed inside the syringe and the piston was fully com-pressed. The mold was pushed out of the end of the syringe andremoved, leaving an agglomerate sitting on the bench top. Fig. 2

Fig. 2. Examples of agglomerates used in this study after fabrication. (A) A largeagglomerate (diameter=0. 0169m). (B) A small agglomerate (diameter=0. 0067m).

Table 1Agglomerate properties before fluidization

Agglomeratediameter (m)

Agglomerateheight (m)

Averagemass beforefluidization (g)

Averageagglomeratedensity(kg/m3)

Porosity(dry basis)

0.0067 0.0070 0. 37 ± 0. 01 1484 ± 37. 8 0.400.0067 0.0070 0. 34 ± 0. 01 1375 ± 43. 9 0.450.0116 0.120 1. 93 ± 0. 02 1524 ± 13. 3 0.390.0169 0.0169 5. 86 ± 0. 04 1547 ± 9. 7 0.38

The intervals are the standard deviation of the measured value. The properties of theprimary particles used to make the agglomerates are:dpsm=179�m, �p=2500kg/m3


Fig. 3. Schematic diagram of experimental fluidized bed equipment.

shows an example of the largest and smallest agglomerates used inthese experiments after fabrication. The reproducibility of the fabri-cated agglomerates is given in Table 1 based on agglomerate massbefore fluidization. For this study, the agglomerates were made withglass beads and water or aqueous sugar solutions. The particle den-sity of the spherical glass beadswas 2500kg/m3 and the Sauter-meandiameter was 179�m. On average, agglomerates were made with adensity of 1520kg/m3. To observe the effect of agglomerate density,some agglomerates were also made with a density of 1375kg/m3.To observe the effect of the concentration of binder solutions on ag-glomerate stability, different aqueous solutions of white table sugarwere used to form the agglomerates. The concentration of the sugarsolutions ranged from 0 to 40wt% sugar.

2.2. Critical desktop experiments

Agglomerates were formed with different moisture contents todetermine the minimum and maximum moisture contents that arerequired for an agglomerate to maintain its cylindrical shape whenformed and placed on a flat horizontal surface. The effect of agglom-erate size on the minimum and maximum values was investigatedusing the three agglomerate molds described above.

2.3. Fluidized bed experiments

A bed formed with the same glass beads as the agglomerates wasfluidized in a clear column with an internal diameter of 0.10m. Thestatic bed height was approximately 0.15m. The bed was fluidizedwith compressed air through a distributor consisting of a polyethy-lene disk with 70�m pores. A large expansion section above the bedhelped return entrained particles back to the bed. A schematic dia-gram of the fluidized bed equipment is shown in Fig. 3.

Agglomerates were formed with a specified moisture content andweighed to determine their initial mass (m0). All agglomeratemasseswere determined using an accurate scale. They were then insertedinto the bed at approximately minimum fluidization conditions. Thefluidization air was then turned off and the expansion section of thefluidized bed column was attached. A second fluidization line wasthen opened to fluidize the bed at the desired superficial velocityfor a predetermined time interval. The fluidization air was turnedoff, the expansion section of the column was removed, and the bedcontents were emptied. The agglomerates, if they had survived flu-idization, were recovered and weighed to determine their mass after

fluidization (m). If the agglomerates had fragmented during fluidiza-tion, the mass of all of the agglomerate fragments were weighed todetermine their cumulativemass,mfragments. The number of agglom-erate fragments was also estimated. When aqueous sugar solutionswere used to form agglomerates, some fluidized bed experimentswere done using humidified air with a 52–55% relative humidity.

2.4. Segregation experiments

Agglomerates were made using both the large and smallagglomerate molds. These agglomerates had an average density ofapproximately 1500kg/m3. Three large agglomerates or six smallagglomerates were added at a time to the fluidized bed. The bedwas fluidized at 0.17m/s because this was the velocity used to in-vestigate the effects of agglomerate size and liquid viscosity in thisstudy. The fluidizing air was turned off 30 s after the introductionof agglomerates, causing the bed to quickly defluidize. The positionof the agglomerates in the bed was determined by removing bedparticles slowly using a vacuum cleaner. As each agglomerate wasuncovered, its position was measured.

3. Results and discussion

3.1. Critical desktop experiments

The minimum and maximum amount of moisture for an agglom-erate to maintain its shape on a horizontal flat desktop was inves-tigated. This was used to investigate the effect of the cohesive forceon the agglomerate structure versus the stress due to gravity. Theeffect of the agglomerate size on the minimum and maximum mois-ture contents was studied. The results are shown in Fig. 4.

The error bars in Fig. 4 represent the moisture contents thatbracket the minimum and maximum moisture contents for each ag-glomerate size. The true minimum and maximum moisture contentslie between these two values. The minimum moisture content re-quired to form stable agglomerates is very low (<1wt%), and Fig. 4shows that agglomerates with a small agglomerate diameter requiremore moisture to form a stable agglomerate on a flat horizontal sur-face than agglomerates with larger diameters. This was a surprisingresult. An explanation for this behavior is the effect of the agglomer-ate size on the internal structure of that agglomerate. Agglomerateswere made to have the same density, independent of the size of theagglomerate. It can be assumed that the packing and pore structure


Fig. 4. Relationship between the minimum and maximummoisture contents requiredto form a stable agglomerate on a flat horizontal surface and the agglomeratediameter. The error bars represent the moisture contents that bracket the trueminimum and maximum values.

of the agglomerate is the same, independent of size. If the pores arethe same size in all of the agglomerates, then a pore in the small ag-glomerate is a larger percentage of the structure than the same porein the large agglomerate. This may cause the smaller agglomerate tobe weaker than the larger agglomerate. If the small agglomerate isweaker, therefore, less stable, then more moisture is required to cre-ate a stable agglomerate for this agglomerate size than for the largeragglomerates. Without the influence of the fluidized bed, these ex-periments show that when agglomerates are formed with very lowmoisture contents, smaller agglomerates require more moisture toform a stable agglomerate.

The maximum moisture content that can form a stable agglom-erate on a flat horizontal surface is also shown in relation to theagglomerate diameter in Fig. 4. The maximum moisture content val-ues are between 20 and 23wt%. The agglomerates with a large di-ameter were able to maintain stable agglomerate structures withslightly higher moisture contents than the agglomerates with thesmallest agglomerate diameter. Small agglomerates are unable tosustain extreme conditions in their structure, whether it is very littlemoisture or high moisture contents. Small agglomerates are moreunstable.

These values represent the extreme conditions that agglomeratescan withstand without the interference of external forces other thangravity acting on their structures. These experiments help identifythe extreme conditions that agglomerates can survive, especially forthe fluidized bed as fluidization time goes to zero. As the time de-creases, the amount of force that the agglomerate is exposed fromthe fluidized bed decreases.

3.2. Fluidized bed experiments

Experiments were conducted to determine a critical initial mois-ture content where the mechanisms of agglomerate destructionand agglomerate growth are balanced and no net change in theagglomerate mass is observed. This critical concentration was de-termined by graphing the ratio m/m0 as a function of the initialmoisture, as shown in Fig. 5. The superficial gas velocity of 0.17m/s

(U/Umf =6. 5) was chosen because it was used in a previous study toobserve the effects of different agglomerate properties on stability(Weber et al., 2006). The maximum fluidization time was selectedto be 90 s because, in some reactors, it has been found that agglom-erates may be stabilized by reaction products and survive for verylong periods of time.

The points bracketing the value m/m0 = 1 were used with lin-ear interpolation to determine the critical initial moisture contentas well as the 95% confidence interval for this value. During thesefluidized bed experiments, some evaporation of the water used toform the agglomerates will occur, causing the moisture content ofthe agglomerate to change with time which might affect the surviv-ability of the agglomerate in the fluidized bed. Some experimentswere conducted using humidified air to greatly reduce evaporationand showed only small differences with the drier air case. This willbe discussed further in Section 3.2.4.

3.2.1. Effect of agglomerate size—erosion regimeThe effect of agglomerate size on the critical initial moisture con-

tent was studied using three mold sizes. The agglomerates were flu-idized at U = 0. 17m/s for different time intervals. At this superficialgas velocity, the agglomerates were observed to erode rather thanfragment. The results are shown in Fig. 6.

Fig. 6 shows that for short residence times in the fluidized bed,the critical moisture content required for the agglomerate to main-tain its initial mass decreases with increasing agglomerate size. Theconfidence intervals for these values are quite narrow. As the ag-glomerate residence time in the fluidized bed increases, the criticalinitial moisture content increases. When the residence time is 60 s,the increasing trend levels off and the critical initial moisture con-tent becomes the same for both large and medium agglomerates. Forfluidization times greater than 60 s, the medium and large agglom-erates reach a dynamic equilibrium between the addition of solidsand the erosion of solids, although they eventually slowly lose mois-ture. Therefore, the agglomerate size will then slowly decrease withtime as erosion acts on the agglomerate structure. The critical initialmoisture content for the smallest agglomerate size is different fromthe other two sizes and shows an increasing trend over the timesstudied.

At the superficial gas velocity of 0.17m/s (U/Umf =6. 5), the mainmechanism of agglomerate destruction was erosion. When the flu-idized bed contents were screened after fluidization, a single largeagglomerate core was observed with no other fragments. The dataof Fig. 6 suggest that the smallest agglomerates, under these condi-tions, required more moisture in the agglomerate structure to over-come the erosion forces. This finding disagrees with the findings ofTardos et al. (1997), who found that a granule that is larger thana critical value will become unstable and fragment. However, ero-sion, not fragmentation, was the observed behavior under our ex-perimental conditions, and the erosion mechanism may be affectedby agglomerate size differently than the fragmentation process ofagglomerate destruction. The ratio of surface area to volume of thelarge agglomerates is the smallest of the three agglomerate sizes.In the large agglomerates, it is likely that there is enough liquid inthe agglomerate to provide sufficient moisture on the agglomeratesurface to incorporate bed particles into the agglomerate structureand compensate for erosion forces acting on the agglomerate over alonger period of time than the smallest agglomerates.

To investigate whether agglomerates fluidized differentlydepending on agglomerate size, segregation experiments were per-formed. The results are shown in Fig. 7. The segregation of agglom-erates was investigated by fluidizing the largest and the smallestagglomerates for 30 s at a superficial gas velocity of 0.17m/s to ob-serve the two extremes. The experiments using large agglomerateswere repeated six times and the experiments using small


Fig. 5. Example of graphs used to determine the critical initial moisture content. This graph shows the behavior of an agglomerate made with the large mold, U = 0. 17m/s.Fluidization time = 30 s (A) and 60 s (B).

Fig. 6. Effect of agglomerate size on critical initial moisture content at U=0. 17m/s.The error bars are 95% confidence intervals for each value.

agglomerates were repeated three times. The results were pooledtogether to form one large population for each agglomerate size. Thepercentages presented in Fig. 7 were calculated from these largerpopulations. Segregation of agglomerates in fluidized beds can in-fluence the behavior of agglomerates. Agglomerates may experiencedifferent forces and may be destroyed by different mechanismsdepending on their position in the fluidized bed.

Fig. 7. Agglomerate position within the fluidized bed. Agglomerates were fluidizedfor 30 s at U = 0. 17m/s.

The distribution of agglomerates in the fluidized bed was foundto depend on the size of the agglomerate. The highest percentage ofsmall agglomerates was found in the middle portion of the fluidizedbed. The highest percentage of large agglomerates was found in thearea above the distributor region. There were no large agglomeratesin the middle portion of the fluidized bed, although 17% of largeagglomerates made it to the top portion of the bed. After 30 s of


Fig. 8. Effect of agglomerate density on the critical initial moisture content. Agglom-erates were made using the small mold (diameter = 0. 0067m) and U = 0. 17m/s.The error bars are 95% confidence intervals for each value.

fluidization with a superficial gas velocity of 0.17m/s, only smallportions of both sizes of agglomerates were found to be settled onthe distributor. There are two possible explanations of why no largeagglomerates were observed in the middle portion of the bed. Largeagglomerates may not travel to the middle and top portions of thefluidized bed as frequently as smaller agglomerates or they maytravel as frequently to these areas but settle more quickly throughthe emulsion phase than smaller agglomerates. Large agglomeratesappear to have more of a tendency to segregate to the lower regionsof the fluidized bed, however, not many were found to be directlyon the distributor.

The results shown in Fig. 7 indicate that the increased stabilityof large agglomerates observed in fluidized beds is not due to seg-regation alone. The behavior of agglomerates in the fluidized bed isa combination of many effects dictated by the size of agglomerates.

3.2.2. Effect of agglomerate density—erosion regimeThe effect of agglomerate density was investigated by making ag-

glomerates using the small mold. Two agglomerate densities weretested and the results are shown in Fig. 8. The superficial gas velocitywas 0.17m/s and the dominant mechanism of agglomerate destruc-tion at this velocity was erosion.

Fig. 8 shows that the density of an agglomerate does play a rolein the survival of agglomerates in a fluidized bed. Different agglom-erate densities were achieved by changing the agglomerate poros-ity. Agglomerate density was increased by adding more agglomeratematerial to the mold and compressing it to the same height. Whenagglomerates were formed with higher densities, a lower critical ini-tial moisture content was observed. This indicates that denser ag-glomerates are better able to withstand erosion. As the time thatthe agglomerate was exposed to the fluidized bed environment in-creased to 60 s, more variability was seen in the data, as shown bywider confidence intervals. Lower agglomerate densities are associ-ated with more pore space in the agglomerate structure. This makesthe agglomerates weaker and more susceptible to damage in thefluidized bed. This agrees with the findings of Subero and Ghadiri(2001) who found that agglomerate structure plays a very large rolein the destruction of agglomerates. The lower density may also cause

the agglomerate to travel throughout the fluidized bed more thanthe more dense agglomerate, or to fall at a lower velocity throughthe emulsion. Increased motion in the fluidized bed may expose theagglomerates to more shear and erosion forces, requiring a higherliquid saturation to recruit and maintain particles in the agglomeratestructure. Fig. 8 shows that higher moisture contents are requiredto balance the agglomerate growth and reduction processes in thefluidized bed for lower density agglomerates.

3.2.3. Effect of superficial gas velocityTo determine the effect of the superficial gas velocity on agglom-

erate stability, agglomerates were made using all three agglomeratemolds. Three agglomerate moisture contents were tested: 9.1, 4.8,and 1wt%. Agglomerates were then fluidized in a bed of small glassbeads at different superficial gas velocities. These superficial gas ve-locities were 0.17m/s (U/Umf =6. 5), 0.26m/s (U/Umf =10. 0), 0.31m/s(U/Umf =11. 9), 0.34m/s (U/Umf =13. 0), and 0.40m/s (U/Umf =15. 4).

During the experiments, two different behaviors were observed.When the superficial gas velocity was low, the agglomerates wereseen to erode if the moisture content was low, or gain mass if themoisture content was high. Agglomerates were recovered from thebed in one piece. When the velocity was high, however, agglomer-ates were seen to fracture into several smaller pieces. When this oc-curred, all of the agglomerate fragments were weighed to get theircumulative mass (mfragments). The number of fragments was alsoestimated. When erosion was the dominant mechanism of agglom-erate destruction, the number of fragments was equal to 1. The re-lationship between mfragments/m0 and superficial gas velocity forlarge agglomerates is shown in Fig. 9(A). The relationship betweenthe number of agglomerate fragments and superficial gas velocity isshown in Fig. 9(B).

Fig. 9(A) shows that at the lowest superficial gas velocity, ag-glomerates with 1wt% moisture content lose mass. Agglomerateswith higher initial moisture contents gain mass from the fluidizedbed at this superficial gas velocity. Fig. 9(B) shows that at the low-est superficial gas velocity, all agglomerates remained in one piece.As the superficial gas velocity increases to approximately 0.26m/s,Fig. 9(B) shows that the number of agglomerate fragments increasesfor agglomerates made with moisture contents of 1 and 4.8wt%,while agglomerates with the highest moisture content showed noincrease in the number of fragments. At this superficial gas velocity,agglomerates with a moisture content of 4.8wt% have enough mois-ture at the surface of the fragments to continue to recruit more bedparticles than are lost to erosion. This is shown by mfragments/m0greater than 1 in Fig. 9(A). Agglomerates with a moisture contentof 1wt% do not have enough moisture at the surface of the frag-ments to recruit more bed particles than are lost to erosion and thisis shown by mfragments/m0 less than 1. As the velocity increases fur-ther to approximately 0.31m/s, the agglomerates with the highestmoisture content begin to fragment as shown in Fig. 9(B). The frag-mentation of these agglomerates causes the mfragments/m0 ratio toincrease as shown in Fig. 9(A). This indicates that these agglomer-ates have enough moisture at the surface of the fragments to recruitmore bed particles than agglomerates with lower moisture contents.Agglomerates made with the lowest moisture content continue tolose mass as erosion is dominant over particle recruitment. As thevelocity continues to increase, agglomerates with 4.8 and 9.1wt%moisture contents have increasing numbers of fragments. The num-ber of fragments is very similar for these two groups of agglomerates.Agglomerates with the lowest moisture content continue to exhibitfragmentation behavior, however, the number of fragments is lowercompared with the other agglomerates. The number of fragmentsalso eventually starts to decrease. This behavior may be caused bythe complete destruction of the smallest fragments of these agglom-erates, decreasing the number of fragments observed.


Fig. 9. (A) Relationship between mfragments/m0 and the superficial gas velocity. Agglomerates were made using the largest mold (diameter = 0. 0169m). Measurementswere taken after 30 s of fluidization. (B) Effect of superficial gas velocity on the number of agglomerate fragments. Agglomerates were made using the largest mold(diameter = 0. 0169m). Measurements were taken after 30 s of fluidization. Legend values are agglomerate initial moisture contents.

Fig. 10. (A) Relationship between mfragments/m0 and the superficial gas velocity. Agglomerates were made using the medium mold (diameter = 0. 0116m). Measurementswere taken after 30 s of fluidization. (B) Effect of superficial gas velocity on the number of agglomerate fragments. Agglomerates were made using the medium mold(diameter = 0. 0116m). Measurements were taken after 30 s of fluidization.

The effect of the superficial gas velocity was also observed inagglomerates made with the medium mold (diameter = 0. 0116m).The results are shown in Figs. 10(A) and (B).

As was observed in Fig. 9(A), medium sized agglomerates withlow moisture contents had mfragments/m0 ratios less than 1 andthe agglomerates with higher moisture contents had mfragments/m0


Fig. 11. (A) Relationship between mfragments/m0 and the superficial gas velocity. Agglomerates were made using the smallest mold (diameter = 0. 0067m). Measurementswere taken after 30 s of fluidization. (B) Effect of superficial gas velocity on the number of agglomerate fragments. Agglomerates were made using the smallest mold(diameter = 0. 0067m). Measurements were taken after 30 s of fluidization.

ratios greater than 1. One difference that was observed was that themfragments/m0 ratio reached 0 at a lower superficial gas velocity formedium sized agglomerates with a moisture content of 1wt% com-pared with the large agglomerates. Fig. 10(B) shows the number offragments for medium sized agglomerates. As was seen in Fig. 9(B),the number of fragments increased as the superficial gas velocityincreased for agglomerates with high moisture content. For drier ag-glomerates, there was an initial increase in the number of fragmentswith increasing superficial gas velocity. The number of fragmentsstarted to decrease as the superficial gas increased further, show-ing that some of the fragments were eroded away. One differencebetween Figs. 9(B) and 10(B) is the number of fragments that wereobserved. The medium sized agglomerates fragmented into fewerpieces than large agglomerates.

Figs. 11(A) and (B) show the effects of superficial gas velocity onagglomerates made with the smallest mold (diameter = 0. 0067m).As shown in Figs. 9(A) and 10(A), the effect of moisture contentis generally very similar for all sizes of agglomerates. The effect ofagglomerate size is seen in the magnitude of mfragments/m0. Smallagglomerates with a moisture content of 1wt% were unable to in-corporate enough bed materials into their structures to balance theerosion and fragmentation process as was seen for larger agglomer-ates with the same moisture content. The difference in the behaviorobserved in Fig. 11(A) is that the mfragments/m0 ratio is lower for thesmallest agglomerates at this moisture content. Smaller agglomer-ates with higher moisture contents were able to recruit enough bedparticles to maintain the mfragments/m0 ratio greater than 1. Thisratio was not as high for the smallest agglomerates with the high-est moisture content as larger agglomerates with the same mois-ture content. Fig. 11(B) shows that the smallest agglomerates hadthe lowest number of fragments of the three agglomerate sizes. Thedriest agglomerates had very few fragments survive at the highestsuperficial gas velocity.

Fig. 12 shows a large and a small agglomerate with a moisturecontent of 9.1wt% after recovery from the fluidized bed after 30 s offluidization. Fig. 12 shows the difference in the number of fragmentsas agglomerate size decreases when agglomerate behavior is in thefragmentation regime.

Two destruction processes are occurring simultaneously inthe fluidized bed. As fragmentation is occurring, the fragmentsare also undergoing erosion. This contributes to decreasing themfragments/m0 ratio for the agglomerates with moisture contents of1 and 4.8wt% for all agglomerate sizes. Erosion has a large impacton the decrease of the mfragments/m0 ratio for the driest agglomer-ates at higher velocities, accounting for the low values of this ratioand the decreasing number of fragments at the highest superficialgas velocity. As the size of the agglomerates decreases, the numberof fragments also decreases. Agglomerates fragment until the piecesreach a size that can survive in the fluidized bed. Smaller agglom-erates do not have to fragment as much to reach the critical size tosurvive at a certain fluidized bed condition. That is why a decreasein the number of fragments was observed with decreasing size.

The reduction in mfragments/m0 ratio and number of fragmentsobserved as further erosion of the fragments occurs is in agreementwith the findings by Shamlou et al. (1990). Several studies have usedimpact testing of agglomerates to examine breakage phenomena.Salman et al. (2004) examined the impact failure modes of sphericalparticles, including wet agglomerates. Although impact destructionin their study is different from the mechanism of destruction in thefluidized bed, the fragmentation of the agglomerates in the fluidizedbed looked very similar to the high velocity failure of wet agglom-erates in this study (Salman et al., 2004). Lower moisture contentsin the fluidized bed study caused the agglomerates to be more sus-ceptible to fragmentation because granule strength depends on localagglomerate structure (Subero and Ghadiri, 2001). With less mois-ture available in the agglomerates, fewer liquid contact points can be


Fig. 12. Examples of large (0.0169m) and small (0.0067m) agglomerate behavior in fluidized beds. Moisture content was 9.1wt%. Large agglomerate (A) before fluidization,(B) after fluidization at 0.17m/s (growth), and (C) after fluidization at 0.40m/s (fragmentation). Small agglomerate (D) before fluidization, (E) after fluidization at 0.17m/s(growth), and (F) after fluidization at 0.40m/s (fragmentation).

Table 2Properties of sucrose solutions (adapted from Reiser et al., 1995)

Concentration ofsolution (wt%)

Viscosity at20 ◦C (cP)

Surface tension(mN/m)

0 1.005 72.6830 3.187 75.8940 6.167 77.08

maintained within their structure, causing them to be weaker thanthat of the agglomerates made with more liquid.

Fragmentation can cause many problems in processes where ag-glomerates are not desired because it can potentially increase theamount of agglomerate material as more fragments are created andcan recruit surrounding particles into the structure. If agglomeratedmaterial is not desired, then reducing the moisture content of theagglomerates can promote fragmentation and erosion, reducing theamount of agglomerate material left in the fluidized bed. If agglom-erates are desired, the opposite is true. Decreasing the size of ag-glomerates will cause an agglomerate to lose mass more quickly, butwill decrease the number of fragments.

3.2.4. Effect of binder solution concentrationAqueous sugar solutions were used to investigate the effect of

binder concentration on the critical initial moisture content. Theywere also used to investigate the effect of liquid viscosity on thecritical initial moisture content. These experiments were done usingthe largest agglomerate mold. The viscosity of the sugar solutionswas measured and found to be very similar to literature values foraqueous sucrose solutionswhen adjusted for temperature. Propertiesof pure sucrose solutions at the same concentrations as those usedfor the experiments are found in Table 2.

The sugar solutions have very different viscosities when com-pared with pure water, although the surface tension remains verysimilar. The critical initial moisture content was found for each sugarsolution and they are shown in Fig. 13 in comparison with the dis-tilled water case. The superficial gas velocity was 0.17m/s and thedominantmechanism of agglomerate destruction at this velocity waserosion.

Fig. 13. Effect of sugar concentration on the critical initial moisture content. Agglom-erates were made using the largest mold (diameter = 0. 0169m) and U = 0. 17m/s.The error bars are 95% confidence intervals for each value.

Fig. 13 shows that there is a dramatic difference in the stability ofagglomerates made from sugar solutions when compared with ag-glomerates made with distilled water. Much less liquid is requiredto balance the agglomerate growth and reduction mechanisms whensugar solutions are used. Another difference between the two situa-tions is that the critical initial moisture content is not greatly affectedby the fluidization time and this is shown in an almost horizontalline for the agglomerates made with sugar solutions. Less solution


Fig. 14. Effect of humidification of fluidization air. The liquid used to make theagglomerates was 30wt% sugar solution. Agglomerates were made using the largestmold (diameter = 0. 0169m), U = 0. 17m/s, and fluidization time was 90 s.

is required to create a stable agglomerate structure when comparedwith the pure water case. The increase in the sugar concentrationfrom 30 to 40wt% causes the viscosity of the solution to approxi-mately double. It was expected that this would have a large impacton the critical initial moisture content observed, but this was not thecase. The critical initial moisture contents for both sugar solutionsover time were very similar. In most cases, overlap was observed.The results from Fig. 13 show that the initial increase in liquid vis-cosity from 1 to 3.2 cP had a large impact on the critical initial mois-ture content. Beyond a liquid viscosity of 3.2 cP, in this agglomeratesystem, increasing the viscosity did not greatly impact the criticalinitial moisture content. This indicates that beyond a certain liquidviscosity, increasing the liquid viscosity further does not necessarilyhave a great impact on the stability of the agglomerate. This agreeswith the findings of Hemati et al. (2003) and Pont et al. (2001) whofound that the dominant forces in granulation are capillary forcesand that the viscosity of the solution does not influence agglomerategrowth as much as interfacial parameters. In this study, if the liquidviscosity was sufficiently low, the findings of Hemati et al. (2003)and Pont et al. (2001) did not apply. To determine whether the be-havior in Fig. 13 occurred because of water evaporating from thesolution during fluidization, causing the sugar concentration and theviscosity to increase, experiments were conducted using humidifiedair. Humidified air had a relative humidity that was between 52%and 55%. Normally, the air used to fluidize the bed had a low relativehumidity, less than 10%. The superficial gas velocity was once again0.17m/s and the dominant mechanism of agglomerate destructionat this velocity was erosion. The results are shown in Fig. 14.

It was observed that when the fluidization air had a relative hu-midity of 60%, the bed could not be fluidized because the particleshad caked together. This can happen at a relative humidity less than100% because of fine dust that may be stuck on the surface of theparticles (Williams and Nosker, 1988). For this reason, the relativehumidity of the fluidization air was kept between 52% and 55%. Thebed appeared to be well fluidized at this level of humidification.Fig. 14 shows that humidifying the air increased the amount ofparticles incorporated into the agglomerate structure, although the

increase is not very large. Fig. 14 also shows that when theagglomerate moisture content was higher, the humidification madethe agglomerate less stable. It was expected that humidifyingthe fluidization air would prevent evaporation of water from hesugar solution used to form the agglomerate. This would preventan increase in the sugar concentration, and its resulting increase inliquid viscosity. For this reason, the decreased stability observed inagglomerates with the highest moisture content using humidifiedair was an expected result. In general, Fig. 14 shows that humidifi-cation did not have a large effect on the agglomerate stability. Thechange in viscosity of the sugar solution due to the evaporation ofwater during fluidization was not occurring to a large extent duringthe previous experiments.

4. Conclusions

Agglomerate behavior in fluidized beds is affected by manyvariables. The variables studied in this paper include fluidizationvelocity, agglomerate size, agglomerate density, and binder solutionconcentration. Critical desktop experiments were also done to ob-serve the effect of agglomerate diameter on agglomerate stabilitywithout the influence of forces exerted by a fluidized bed.

This study found that:

• Agglomerate diameter does affect the minimum and maximummoisture contents required to maintain its shape on a horizontalsurface. Larger agglomerates can tolerate more extreme maximumand minimum moisture contents.

• When erosion is the dominant mechanism of destruction, largeragglomerates are more stable in fluidized beds. This increased sta-bility was not caused by complete segregation of the agglomeratesto the bottom of the fluidized bed.

• When erosion is dominant, agglomerates with higher densitieswere more stable and required less moisture in their structure tomaintain their initial mass.

• Increasing the superficial gas velocity caused agglomerates to frag-ment and secondary fragmentation and erosion destroyed frag-ments if the initial moisture content was low. Agglomerates withhigh initial moisture contents fragmented and recruited more bedparticles than drier agglomerates. Decreasing agglomerate sizecaused the number of agglomerate fragments to decrease.

• Increasing the liquid viscosity to a certain level, 3 cP in this study,caused the agglomerates to bemuchmore stable. Further increasesin the liquid viscosity did not make the agglomerates more stable,agreeing with literature findings.

The results from this study show that more workmust be done to un-derstand agglomerate destruction in fluidized beds but provides di-rectional information about agglomerate destruction. More researchneeds to be done to see how these principles apply to industrial ag-glomerating systems.

Notation

d particle diameter, mD primary agglomerate diameter, mE elastic modulus, Pam mass of agglomerate after fluidization, gmfragments mass of fragments recovered from fluidized bed after

fluidization, gm0 mass of agglomerate after agglomerate formation, g�P pressure difference across a liquid bridge, PaR1 principal radius of curvature (as defined in Fig. 1)R2 principal radius of curvature (as defined in Fig. 1)t time


U superficial gas velocity, m/sUmf minimum fluidization velocity, m/sV particle velocity, m/s

Greek letters

� liquid surface tension, mN/m� interface energy, J/m2

� dimensionless group for description of breakage� agglomerate density, kg/m3

�p particle density, kg/m3

Acknowledgements

The authors acknowledge the support of the Natural Sciencesand Engineering Research Council of Canada and Syncrude CanadaLtd. for their funding and support for this study. The authors wouldalso like to thank Ahmad Abu Romeh for his assistance with theexperimental program.

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Effect of agglomerate properties on agglomerate stability in fluidized beds