Powder Technology 209 (2011) 53–64

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Stability of agglomerates made from fluid coke at ambient temperature

Sarah Weber a, Cedric Briens a,⁎, Franco Berruti a, Edward Chan b, Murray Gray c

a Department of Chemical and Biochemical Engineering, The University of Western Ontario, London, Ontario, Canada N6A 5B9b Syncrude Canada Ltd., Research Centre, Edmonton, Alberta, Canada T6H 1H4c Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta, Canada T6G 2G6

⁎ Corresponding author.E-mail address: cbriens@uwo.ca (C. Briens).

0032-5910/$ – see front matter © 2011 Elsevier B.V. Aldoi:10.1016/j.powtec.2011.02.004

a b s t r a c t

a r t i c l e i n f o

Article history:Received 1 December 2009Received in revised form 6 November 2010Accepted 11 February 2011Available online 18 February 2011

Keywords:Agglomerate stabilityFluid cokingAgglomerate destructionAgglomerate growth

Fluidized bed agglomeration is used to reduce dust problems and stabilize mixtures of various particulatecomponents. Agglomeration in fluid coking, however, can create heat and mass transfer limitations whichincrease reactor fouling and decrease process efficiency. The objective of this study was to investigateagglomerate survival in fluidized beds in the absence of reaction using materials that have similar properties tothose used in the industrial fluid coking process. Agglomerates consisting of fluid coke and oils were madeartificially outside of the fluidized bed, fluidized, and then recovered to determine whether agglomeratebreakage, erosion, or growth occurred. This study investigated the effects of agglomerate liquid content, liquidbinder viscosity, and fluidizing gas velocity on agglomerate stability. It was found that the onset of completefragmentation occurred at 0.25 m/s (U/Umf=26) when the liquid binder viscosity was 6.44 cP and 0.34 m/s (U/Umf=35) when it was 64.7 cP. During fragmentation, more agglomerate material was recovered from thefluidized bed when the initial agglomerate diameter was 0.007 m compared with larger agglomerates (initialdiameters of 0.012 m and 0.0167 m). This was due to the effects of secondary erosion on the fragmentationproduct.

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1. Introduction

Particulate operations play a very large role in many industries andtheir performance is essential to the success of many processes. Inindustries such as the pharmaceutical industry, agglomerates areintentionally produced [1]. In other processes such as fluid coking,however, agglomerates are not desired. Agglomerate properties canpotentially bemanipulated to ensure survivability, if they are desired, orto enhance destruction if they are undesired.

The objective of this study was to investigate how factors such asliquid content, superficial gas velocity, liquid viscosity, and initialagglomerate size affect the stability of agglomerates made usingmaterials similar to those found in the industrial fluid coking process.Agglomerates were made using fluid coke particles, which is the samematerial used in the commercial fluid cokers. Oil and biodiesel mixtureswere used as representative liquid because their properties at roomtemperature are similar to the properties of bitumen at reactiontemperature. Previous studies have investigated the effect of differentparameters on agglomerate growth [2] and on agglomerate growth anddestruction [3]. The present study differs from that of Hemati et al. [2]because the initial agglomerate properties have been controlled toexamine stability, and the research expands onour previouswork [3] by

investigating the effect of different parameters on agglomerate behaviorin a solid–liquid system that is similar to the industrial fluid coking case.

Agglomerates can be destroyed according to various mechanisms.These include external mechanical stresses caused by fluidized bedconditions, internal mechanical stresses caused by chemical reactionand vapor production, and migration of liquid from the agglomerate todrier bed particles, which weakens the agglomerates. This studyinvestigated how agglomerate characteristics affect the mechanismsof agglomerate destruction in the absence of reaction or vapor evolutionand compared the results with the reacting liquid binder case.

1.1. Fluid coking process

Coking technology is the most popular method for upgradingbitumen and petroleum residue to more valuable petroleum products.In this process, it is essential to maximize the yield of condensablecracked products while minimizing coke formation [4]. In fluid coking,heavy oil is sprayed into a bed of inert hot fluidized fluid coke particles.The hot particles provide the heat required for heating the liquid feed toreaction temperature and for the endothermic cracking reaction. Theproduct vapors risewhile coke deposits onto the surface of the bed cokeparticles [5]. The vapors produced by the reaction rise to the top of thereactor and are removed and quenched while coke flows down to astripper where interstitial vapors are displaced with steam. The coke isthen conveyed to a burner where a portion is burned to supply heat tothe reactor [6].

54 S. Weber et al. / Powder Technology 209 (2011) 53–64

One problem encountered in fluid coking technology is fouling,when particles wet with heavy oil stick to reactor internals, such asstripper sheds [7]. Heavy oil may be present because the cracking of theliquid is slowed down by mass and heat transfer limitations inagglomerates formed when liquid bitumen is injected in the fluidizedbed of coke particles [8]. Most of the heavy oil feed injected into afluidized bed initially forms liquid-solid agglomerates [8]. Poor initialliquid distribution increases the fraction of liquid that forms agglom-erates and results in wetter agglomerates. This causes the coke yield toincrease [9]. Because of mass and heat transfer limitations inagglomerated material, the yield of valuable vapor products decreasesas overcracking occurs [9]. Poor heat transfer within agglomeratesmeans that the fluidized bed must be operated at higher temperaturesto achieve the required feed conversion [9] and reduce the fouling ofinternals [10]. Higher bed temperatures increase the vapor-phasecracking of desirable products to undesirable products such as lightgases and coke [6]. It is, therefore, important tominimize the formationof stable agglomerates. Agglomerates produced during the spraying ofheavy feed into the reactor may further compound operationalproblems by causing defluidization and fouling.

Manipulatingoperational conditions toeliminateagglomerates, or atleast to reduce their stability, is vital to improve coker operation andperformance. It is essential to determine the agglomerate characteristicsthat will facilitate their subsequent destruction in the fluidized bed.

1.2. Agglomerate growth and destruction

Agglomerates formwhen particles are held together by short-rangephysical or chemical forces, by chemical or physical modification of theparticles triggered by specific process conditions such as sintering, or bysubstances that act as binders by adhering physically or chemically toform material bridges among particles [11]. Seville et al. [12] reviewedsome interparticle forces that occur during fluidization that can causeagglomeration and defluidization. The interparticle forces they inves-tigated were liquid bridges, van der Waals forces, and sintering [12].David et al. [13] looked at modeling agglomerate growth andcrystallization. During crystallization, agglomerate formation occurs intwo steps. Thefirst is the initial collision of the primary particles and thesecond is the sticking growth of crystalline bridges between particles[13]. In the current study, wet agglomeration and the effect of liquidbridges were investigated because this corresponds to the case whenbitumen is sprayed into the fluid coker and initially forms agglomeratesbefore the binder starts to react and form solid bridges in theagglomerate. In agglomerates and granules, there are three situationswhere liquid can produce cohesive forces: mobile liquid bridges,adsorbed liquid layers onparticles, and adhesive or viscous binders [14].

The amount of liquid in an agglomerate determines the liquidsaturation state in that agglomerate. In the pendular state of saturation,liquid forms bridges at point contacts within the agglomerate structure.As the pore space of the agglomerate is filled with liquid, theagglomerate saturation state goes to the funicular and finally to thecapillary states. In thependular state, theagglomerate strength is causedby liquid bridges. In the capillary state, the agglomerate strength iscaused by capillary pressure. The funicular state corresponds to acombination of the two forces [14].

Many studies have examined the effect of different parameters onthe growth of agglomerates in granulating processes. Several of thesestudies have attempted to model the growth of agglomerates [2,15].Agglomerate mechanisms are divided into the processes of wetting andnucleation, consolidation and growth, and breakage and attrition [15].Someworkhas beendone to investigate growth regimemaps for liquid-bound granules during granulation, however, there are limitations forthese types of regime maps. They do not consider nucleation andwetting effects. They also do not predict the rate and extent ofagglomerate growth [15]. McDougall et al. [16] investigated regimemaps that indicate when agglomerates form in relation to contact angle

and liquid viscosity. It was found that beyond critical values for theseproperties, the liquid will start to form agglomerates with the fluidizedbed particles, even if the liquid is originally well distributed overindividual particles [16]. Researchers have also modeled the agglomer-ation process using different approaches including population balances[17–19]. Attempts at modeling the agglomeration process are nowconsidering simultaneous aggregation and breakage, but modelingaggregation has been more successful than modeling breakage kinetics[20].

Some studies have focused on examining the destruction ofagglomerates and granules. Salman et al. [21] studied wet granuledestruction using impact tests outside of thefluidized bed environment.They found that granule failure could be classified according to theimpact velocity. For low impact velocities, plastic deformation wasobserved in wet granules while at high impact velocities, wet granuleswere greatly reduced in size due to fragmentation. This study also foundthat small wet granules (b0.5 mm) showed significant plastic defor-mation before failure. This differed from larger wet granules, whichexhibited localized debris formation and chipping [21]. Salman et al.[22] also found that increasing granule size caused agglomeratebreakage to occur at lower impact velocities in impact studies offertilizer granules. Breakage patterns and failure modes are dictated bythe agglomerate structure and the velocity of the agglomerate impact[21–23]. The breakage pattern affects the size distribution of the impactproduct [23]. Researchers have found that it is very difficult to modelagglomerate failure because of the complex processes that areoccurring, as well as the difficulty accounting for the structure of theagglomerate material [23]. Shamlou et al. [24] studied how hydrody-namics affects particle breakage by studying a fluidized bed of granulesmade from soda glass beads and polymer. Their study showed that thesize of the granules dropped continuously until a plateau was reached,that themodeof breakage occurs predominantly byattrition rather thanfragmentation, and that attrition is most likely caused by low energyimpacts in the bed core [24].

The complexity of both the agglomerate material and the forcesexerted on the agglomerate by the fluidized bed environment meansthat more study is required to fully understand agglomeratedestruction mechanisms.

1.3. Contact angle and the Washburn technique

Contact angle measurements can be used to assess the wettabilityof solids with certain liquids. Wetting behavior in agglomerates canlead to two different types of agglomerates. One type occurs when thebinder covers the powder, producing a dense and less porous granule,and the other occurs when the powder covers the binder, producing amore porous granule [25]. Spreading coefficients can be calculatedand used to determine properties of granules such as density and sizedistribution [25]. While smaller contact angles provide better wetting,higher degrees of saturation are then required for successfulagglomerate growth [16]. Although there are many different methodsfor measuring contact angle, most methods cannot produce accuratemeasurements of contact angles for powders. Some methods includethe measurement of a sessile drop on a compressed powder cake or avacuum balance technique [26].

This study used the Washburn technique to measure the rate atwhich liquid was absorbed into a packed bed of solids. The Washburntechnique was specifically developed for contact angle measurementswith porous materials. It is based on the following equation [27]:

M2 = KWBρ2Lγ cosθ2μ

t ð1Þ

Eq. (1) was used to assess the interaction of liquids with powders toexplain observed fluidized bed results.

Fig. 1. Particle size distribution of fluid coke used in this study.

55S. Weber et al. / Powder Technology 209 (2011) 53–64

2. Materials and methods

2.1. Agglomerate preparation

A syringe-piston system was used for the fabrication of agglom-erates. For predetermined liquid contents, knownmasses of solids andliquid were combined in a container and thoroughly mixed. Aconstant mass of wet solids was placed in the syringe-piston systemand compressed to a predefined length. To test the effect ofagglomerate size on agglomerate stability, three cylindrical moldswere used. The largest mold had a diameter of 0.0169 m and a heightof 0.0169 m, the smallest mold had a diameter of 0.0067 m and aheight of 0.0070 m, and the intermediate mold had a diameter of0.0116 m and a height of 0.0120 m. One of these molds was placedinside the syringe and the piston was fully compressed. The mold waspushed out of the end of the syringe and removed, leaving anagglomerate sitting on the bench top. The reproducibility of thefabricated agglomerates is shown in Table 1 based on agglomeratemass before fluidization (mo). For this study, the agglomerates weremade with fluid coke and motor oil and biodiesel mixtures. Theparticle density of the fluid coke was 1 500 kg/m3 and the Sautermean diameter was 140 μm. The particle size distribution is shown inFig. 1.

On average, agglomerates were made with a density of 948 kg/m3.To observe the effect of liquid viscosity on agglomerate stability,different grades of Mobil 1 motor oil were used as binder, as well asmixtures of motor oil and biodiesel. The fluid properties will bepresented in the results section.

The reproducibility of the agglomerates was good within each sizegroup. The density of the agglomerates was very similar between thelargest and medium sized agglomerates. The smallest agglomerateshad a higher average density than the other two agglomerate sizesand there was more variation within this group. This occurredbecause very little mass was required to form the smallestagglomerates. Small deviations in the mass placed in the mold causedmore noticeable differences in the agglomerate density whencompared with the larger agglomerate sizes.

2.2. Changing solid–liquid interactions

One aspect of this study was to investigate the effect of changingthe solid–liquid interactions in this system. This had been done for theglass beads–water system using a silanization treatment for the glassbeads [3]. In the current coke-biodiesel system, the solid–liquidinteraction was changed using a surfactant. The surfactant wasintroduced to the system by mixing it first with a small amount ofethanol and then mixing it with the biodiesel. The surfactant tested inthis study was sodium lauryl polyoxyethylene ether sulfate (AKScientific Inc., USA).

2.3. Fluidized bed experiments

A bed formed with the same fluid coke as was used to make theagglomerates was fluidized in a clear column with an internaldiameter of 0.10 m. The static bed height was approximately

Table 1Agglomerate properties before fluidization. The values in brackets are the standarddeviations of the measured values.

Agglomeratediameter (m)

Agglomerateheight (m)

Average mass beforefluidization (g)

Average density(kg/m3)

0.0067 0.0070 0.26 (0.01) 1060 (51)0.0116 0.0120 1.14 (0.02) 900 (13)0.0169 0.0169 3.36 (0.07) 890 (19)

0.15 m. The bed was fluidized with compressed air through adistributor consisting of a polyethylene disk with 70 μm pores. Alarge expansion section above the bed helped disengage entrainedparticles, which were returned back to the bed. A schematic diagramof the fluidized bed equipment is shown in Fig. 2.

Agglomerates were formed with a specified liquid content andweighed to determine their initial mass (mo). They were thenintroduced into the bed at approximately minimum fluidizationconditions. The fluidization air was then turned off and the expansionsection of the fluidized bed column was attached. A secondfluidization line was then opened to fluidize the bed at the desiredsuperficial velocity for a predetermined time interval. The fluidizationair was turned off, the expansion section of the column was removed,and the bed contents were emptied. The bed contents were passedthrough a 710 μm sieve which allowed non-agglomerated bedparticles to easily pass through the screen and retained the intactagglomerates, if they had survived fluidization, or agglomeratefragments if they had not survived fluidization. This limited additionalbreakage outside of the fluidized bed. Intact agglomerates wereweighed to determine their mass after fluidization (m). If theagglomerates had fragmented during fluidization, the mass of all ofthe agglomerate fragments were weighed to determine theircumulative mass (mfragments). This allowed a ratio of the mass ofagglomerate material after fluidization to the initial agglomerate massto be calculated. The number of agglomerate fragments, Nfragments,was also estimated by counting.

2.4. Washburn technique

The Washburn technique was used to compare the spreadingcharacteristics of different liquids with fluid coke and silica sand. Anumber 3 Whatman filter was fitted to the end of a cylindrical holder.Particles were packed in the holder so that the sample voidage wasapproximately 0.41. Approximately 200 g of the test liquid was placedin a small container on the scale. The end of the cylindrical holder wasthen submerged by slowly tipping it into the liquid. Care was taken sothat the solids holder was not touching the scale so that the change inmass was caused only by the migration of liquid into the packed bed.The amount of liquid that migrated into the solids bed was used tocalculate the contact angle.

Fig. 2. Schematic diagram of the fluidized bed equipment used in this study.

Table 2Measured viscosities at room temperature of liquids used to form agglomerates in thisstudy.

Liquid Viscosity (cP)

Biodiesel 6.4475 wt.% biodiesel, 25 wt.% mobil 1 5W20 9.7950 wt.% biodiesel, 50 wt.% mobil 1 5W20 16.10Mobil 1 5W20 64.70Mobil 1 5W30 84.75

56 S. Weber et al. / Powder Technology 209 (2011) 53–64

3. Results

3.1. Liquid properties

Because the experimental work in this study was completed atroom temperature, liquids were selected to have similar fluidproperties as those of Athabasca vacuum residue at industrial processtemperatures. The fluid properties of Athabasca vacuum residue werepreviously determined for reacting conditions by Aminu et al. [28]using the measurement of the forces on a liquid bridge between twocrossed rods at different temperatures and reaction times. The authorsfound that the surface tension of Athabasca vacuum residue decreasedwith reacting temperature but did not change with reaction time. At400 °C, the surface tension of Athabasca vacuum residue was ap-proximately 15 mN/m and at 503 °C, the surface tension wasapproximately 6 mN/m [28]. When no significant reaction wasoccurring, the surface tension of the Athabasca vacuum residue waslinearly related to temperature. However, as soon as reaction becamesignificant, there was a large decrease in the surface tension and itbecame insensitive to temperature [29]. Biodiesel and motor oilmixtures were used in the current study. Allen et al. [30] investigatedthe surface tension of biodiesel and found that the average surfacetension was 27 mN/m. Dasch et al. [31] measured the surface tensionof synthetic grinding oil and found it to be 28.5 mN/m. Aminu [29]reported the surface tension of standard oils ranged from 30.7–32.7 mN/m. The surface tensions of biodiesel and motor oil are higherthan the surface tension of Athabasca vacuum residue at 400 °C. Thismust be kept in mind when comparing the room temperatureagglomerate results to the reacting case where Athabasca vacuumresidue was used as the binder. The contact angle of bitumen withcoke after injection is 0° [32]. The motor oils used in theseexperiments wetted the coke well. The contact angle of the biodieselwith the coke was measured and found to be 0° using the Washburntechnique. This will be discussed in more detail in Section 3.2.3.

The viscosity of Athabasca vacuum residue has also been studied.When no significant reaction was occurring, the viscosity of thebitumen decreased exponentially with temperature [29]. When thistrend was extrapolated to temperatures of 300 and 350 °C, assumingno reaction, the viscosity of the Athabasca vacuum residue would beapproximately 13 and 4 cP respectively. Previous researchers foundthat when the temperature of the Athabasca vacuum residue wasincreased to 400 °C and reaction began occurring, the viscosityincreased dramatically with increasing reaction time and increasing

temperature. At 400 °C and after 22 s of reaction time, the residueviscosity was approximately 4900 cP [29]. Mixtures of motor oils andbiodiesel were used in this study to investigate a range of liquidviscosities. The liquid properties of different grades of Mobil 1 motoroils and mixtures of oil and biodiesel were measured at roomtemperature using a Brookfield viscometer. The results are presentedin Table 2.

The viscosity of biodiesel is within the range of viscosities forAthabasca vacuum residue when no significant reaction is occurring.The liquids tested encompass a wide range of viscosities and wereused to investigate how liquid viscosity affects agglomerate behavior.

3.2. Fluidized bed results

This experimental study focused on the effects of liquid viscosity,superficial gas velocity, and initial agglomerate size on howagglomerates survived in fluidized beds of fluid coke particles. Thelargest agglomerates made with oil (μ=84.75 cP) with a liquid tosolid mass ratio (L/S) of 0.10 or 0.05 were tested to determine theirsensitivity to changing superficial gas velocity. These agglomerateswere tested at two time intervals, i.e. after a fluidization time of either30 s (Fig. 3) or 60 s (Fig. 4).

Figs. 3 and 4 show that agglomerates with these liquid contentsexhibited growth behavior. Figs. 3(B) and 4(B) show that the onset offragmentation occurred for agglomerates with a liquid to solid massratio (L/S) of 0.05 at 30 m/s when fluidized for 30 s and at 0.17 m/swhen fluidized for 60 s. Generally, there was little to no fragmentationobserved for agglomerates when L/S 0.10. When no fragmentationwas occurring, all agglomerates were recovered in one piece. Therewere no error bars for these results. Once fragmentation started tooccur, larger error bars were observed because of the large variation innumbers of agglomerate fragments observed. To increase the

Fig. 3. (A) Effect of liquid content on relative agglomeratemass recovered. (B) Effect of liquidcontent on the number of agglomerate fragments recovered. Dagglomerate=0.0169m, t=30 s,μ=84.75 cP. The error bars are the standard deviation of themeasurements (3–5 replicates).

Fig. 4. (A) Effect of liquid content on relative agglomeratemass recovered. (B) Effect of liquidcontent on thenumberof agglomerate fragments recovered.Dagglomerate=0.0169 m, t=60 s,μ=84.75 cP. The error bars are the standard deviation of themeasurements (3–5 replicates).

57S. Weber et al. / Powder Technology 209 (2011) 53–64

sensitivity of the agglomerates to changing conditions and to observewhat happens during erosion, agglomerates were made such that L/S=0.01 for the rest of the experiments in this study.

3.2.1. Effect of superficial gas velocity and liquid viscosityThe effect of superficial gas velocity, liquid viscosity and time were

investigated. The results are shown in Tables 3 and 4. A representativegraph of some results is shown in Fig. 5.

The results in Fig. 5 show that when agglomerates were fluidizedat very low velocities, they remained intact. As the superficial gasvelocity increased, agglomerates began fragmenting, however, thisfragmentation was incomplete. Some agglomerates remained intact.As the superficial gas velocity further increased, the fraction ofagglomerates recovered fragmented increased until complete frag-mentation was observed. This was representative of what wasobserved when the viscosity of the liquid binder and the fluidizationtime were investigated.

Tables 3 and 4 show that there was no agglomerate growth, evenat low superficial gas velocities for all liquid viscosities tested. After60 s of fluidization (Table 3), the effect of increasing the viscosity ofthe liquid binder can be seen on the fraction of agglomeratesrecovered fragmented from the fluidized bed and the number ofagglomerate fragments. Increasing the liquid binder viscosity de-creased the amount of fragmentation. Increasing the liquid viscositycaused the superficial gas velocity when complete fragmentation wasobserved to increase. At 6.44 cP, complete fragmentation wasobserved at 0.25 m/s (U/Umf=26.3) and at 64.7 cP, completefragmentation was observed at 0.34 m/s (U/Umf=35.1). At a liquidviscosity of 6.44 cP, it was observed that the number of fragmentspeaked when the superficial gas velocity was 0.25 m/s. The number offragments decreased when the superficial gas velocity was increasedto 0.34 m/s. This was not observed for the other liquid viscosities. Ingeneral, increasing the superficial gas velocity caused a decrease inmfragments/mo for agglomerates made with all liquid viscosities,indicating that secondary erosion processes were occurring. It appears

Table 3Agglomerate properties after 60 s of fluidization, initial L/S=0.01. The values in brackets are the standard deviations of the measured values (3–5 replicates).

6.44 cP 9.79 cP 16.10 cP 64.7 cP

U(m/s)(U/Umf)

mfragments/mo

(−)Nfragments Percent

fragmented(%)

mfragments/mo

(−)Nfragments Percent

fragmented(%)

mfragments/mo

(−)Nfragments Percent

fragmented(%)

mfragments/mo

(−)Nfragments Percent

fragmented(%)

0.10 (10.4) 0.996(0.009)

1.0 (0.0) 0 0.996(0.004)

1.0 (0.0) 0 0.994(0.005)

1.0 (0.0) 0 0.996(0.006)

1.0 (0.0) 0

0.17 (17.6) 0.849(0.111)

29.2(17.3)

80 0.937(0.027)

12.6(12.4)

60 0.942(0.024)

9.0(11.3)

40 0.946(0.037)

4.2 (7.2) 20

0.25 (26.3) 0.574(0.099)

33.5(5.2)

100 0.486(0.154)

34.4(5.6)

100 0.761(0.082)

27.0(10.0)

100 0.707(0.271)

24.8(16.0)

80

0.34 (35.1) 0.180(0.057)

23.0(8.7)

100 0.442(0.070)

47.0(2.5)

100

58 S. Weber et al. / Powder Technology 209 (2011) 53–64

that the effect of liquid viscosity on mfragments/mo was minimal at lowgas velocities, while more differentiation was observed at higher gasvelocities. When the fluidization time was increased to 120 s(Table 4), a transition from no fragmentation to complete fragmen-tationwas again observed. Increasing the liquid viscosity did not seemto have quite the same effect in reducing the fraction of agglomeratesfragmented and the number of fragments as observed in Table 3. Thenumber of fragments increased with increasing viscosity. As super-ficial gas velocity increased at this fluidization time, the number offragments peaked for agglomerates made with liquids of 6.44 cP and9.79 cP. This was only observed for 6.44 cP when the fluidization timewas 60 s.

3.2.2. Effect of initial agglomerate sizeThe effect of initial agglomerate size was also investigated using

the three agglomerate sizes described previously. The experimentswere conducted by forming agglomerates using 6.44 cP liquid. Theresults are shown in Fig. 6.

Fig. 6 (A) shows that the effect of initial agglomerate size is verysmall when the superficial gas velocity was 0.10 m/s. The erosion andgrowth processes were relatively balanced, so the ratio mfragments/mo

was close to 1. When the superficial gas velocity was increased to0.17 m/s, the initial agglomerate size affected the amount ofagglomerate material recovered from the fluidized bed, especiallywhen comparing the two largest agglomerate sizes. As the initialagglomerate size increased, the ratio mfragments/mo decreased.Agglomerate size also affected the number of fragments recoveredfrom the fluidized bed (Fig. 6 (B)). Fragmentation did not occur whenthe superficial gas velocity was 0.10 m/s. When the superficial gasvelocity was increased to 0.25 m/s, fragmentation occurred and theeffect of increasing the initial agglomerate diameter from 0.0067 m to0.0116 m was very pronounced on the number of fragments.Increasing the diameter further showed some effect, but it was notas significant. The relationship between the percentage of agglomer-ates fragmented and the agglomerate size is shown in Fig. 6 (C). Theobserved trends were similar to those in Fig. 6 (B).

3.2.3. Effect of wettabilityThe effect of changing the wettability of the liquids and solids was

investigated in the fluidized bed environment by using surfactant-

Table 4Agglomerate properties after 120 s of fluidization, initial L/S=0.01. The values in brackets

6.44 cP 9.79 cP

U(m/s)(U/Umf)

mfragments/mo

(−)Nfragments Percent fragmented

(%)mfragments/mo

(−)Nfra

0.10 (10.4) 0.993 (0.002) 1.0 (0.0) 0 0.974 (0.013) 10.17 (17.6) 0.751 (0.177) 26.6 (24.3) 80 0.662 (0.059) 36.20.25 (26.3) 0.136 (0.061) 20.4 (9.2) 100 0.244 (0.135) 33

biodiesel mixtures to make the agglomerates. While pure biodieselwetted the coke particles well with a contact angle of 0o, adding0.5 wt.% surfactant to the biodiesel changed the liquid-solid interac-tion, giving a contact angle of 42.5o. The results of fluidized bedexperiments are shown in Fig. 7.

Fig. 7 shows that there was a large amount of overlap between thetwo experimental conditions tested and the data are not statisticallydifferent. This is especially true at the lowest superficial gas velocitywhere the results were almost identical. The surfactant results do,however, become very variable at the highest superficial gas velocitytested.

3.3. Comparison of coke agglomerate results with other conditions

The results from fluidized bed experiments in the current fluidcoke–oil agglomerate system were compared with another liquid–solid system. This was done to see if the observations were universalwhen several system parameters were matched. The results from thecurrent study were also compared with reacting condition results.

3.3.1. Comparison with the silica sand–water agglomerate systemThe results in this study were compared with results from

agglomerates made with silica sand and water. In order to compareboth agglomerate systems fairly, several parameters were matched.These parameters included particle size distribution, agglomeratesize, liquid viscosity, liquid content, contact angle, and the relation-ship between superficial gas velocity and minimum fluidizationvelocity. Another reason why silica sand was selected was becauseof its non-spherical shape. Fluid coke is also non-spherical, however,the sphericity of the two particles is not identical. Fig. 8 shows themicroscopic images of both fluid coke and silica sand. The differencesin the shape can be seen.

The silica sand was sieved to have a similar particle sizedistribution as the fluid coke. The viscosity of water was increasedusing sugar solutions to match the viscosity of biodiesel. Because theparticle densities of the fluid coke and the silica sand were different(1500 kg/m3 and 2650 kg/m3 respectively), some considerationswere required to ensure that the liquid content was comparablebetween the two systems. Sherrington and Oliver [14] definedsaturation as the volumetric fraction of the agglomerate pore space

are the standard deviations of the measured values (3–5 replicates).

16.10 cP

gments Percent fragmented(%)

mfragments/mo

(−)Nfragments Percent fragmented

(%)

.0 (0.0) 0 0.975 (0.010) 4.4 (7.6) 205 (6.1) 100 0.611 (0.266) 30.2 (18.0) 80.4 (9.8) 100 0.247 (0.078) 36.0 (9.9) 100

Fig. 5. Effect of superficial gas velocity on the relative mass of agglomerates andfragments recovered from the fluidized bed and the percentage of agglomerates thatwere fragmented after fluidization. L/S=0.01, Dagglomerate=0.0169 m, μ=6.44 cP, andt=60 s. The error bars are the standard deviation of the measurements (3–5replicates).

59S. Weber et al. / Powder Technology 209 (2011) 53–64

taken up by the liquid. The agglomerate saturation was calculatedusing Eq. (2) [14].

S =εL

εg + εL

!ð2Þ

The results of the comparison of the two systems are shown inFig. 9. Fig. 9 (A) shows the results plotted versus U/Umf and Fig. 9 (B)shows the results plotted versus U-Umf.

Because sugar solutions were used, the saturation of theagglomerate was calculated using two bases. The first used the vol-ume and density of the sugar solution in the calculation (Δ). Thesecond used the volume and density of the water in the solution in thecalculation (□). Because the density of the sugar solution was higherthan the density of purewater, themass of liquid required to reach thesame saturation is less than the mass of pure water required. Whenusing the solution as the basis for calculating the saturation, theamount of water in the agglomerate is less than the pure water casebecause some of the solution is made up of sugar. This second basiswas used to determine if the results were related more to the amountof water in the agglomerate or to the amount of solution in theagglomerate. It can be seen from Fig. 9 (A) that there is more overlapin the data for the coke-biodiesel and silica sand-sugar solutionsystems when the saturation was calculated using the volume ofwater in the solution. The amount of agglomerate material recoveredfrom the fluidized bed when silica sand and sugar solutions were usedwas smaller than that found using coke and biodiesel. This may havebeen caused by the difference in the sphericity of the particles.Initially, both types of agglomerates can maintain their initial mass.When the superficial gas velocity was increased further, agglomeratemass began to be lost to the surrounding fluidized bed. Fig. 9 (B)shows that, when plotted versus U–Umf, the results for the differentsolid–liquid systems do notmatch aswell. The silica sand agglomerate

Fig. 6. The effect of agglomerate size after 60 s of fluidization. L/S=0.01 and μ=6.44 cP.(A) Relative mass recovered from the fluidized bed. (B) Number of fragments recoveredfrom the fluidized bed. (C) Percentage of agglomerates fragmented. The error bars arethe standard deviation of the measurements (3–5 replicates).

Fig. 7. Effect of surfactant addition to biodiesel on agglomerate stability. L/S=0.01,μ=6.44 cP, fluidization time=60 s. The error bars are the standard deviation of themeasurements (3–5 replicates).

60 S. Weber et al. / Powder Technology 209 (2011) 53–64

curve is shifted further to the right than the fluid coke agglomeratecurve.

The percentage of agglomerates fragmented during fluidizationwas plotted versus both U/Umf and U–Umf. The results are shown inFig. 10. Saturation values in Fig. 10 were calculated using Eq. (2).

As seen in Fig. 9, the results of both systems more closely matchedwhen they were plotted versus U/Umf. When U–Umf was used, theagglomerates made with sand and sugar solutions exhibited completefragmentation at higher values of U–Umf (0.50 m/s) compared withcoke-biodiesel agglomerates (U–Umf=0.20 m/s).

Fig. 8. Microscopic images of fluid coke (A) and silica sand (B). Images are 50×magnification.

3.3.2. Comparison with reacting conditionsThe results obtained from the coke-biodiesel experiments were

also compared with results obtained from agglomerates made fromcoke and bitumen at reacting conditions. These experiments wereconducted using an apparatus with two fluidized beds. One fluidizedbed was capable of reaching high temperatures (N500 °C) and theother fluidized bed was used as a quench and maintained at roomtemperatures. More details about the reacting experiments andexperimental equipment can be found in [33]. Agglomerate liquidcontent (L/S), fluidization time, and U/Umf were matched. Because theliquid viscosity in the reacting experiments changes with time, the

Fig. 9. Comparison of fluid coke-biodiesel and silica sand-sugar solution (40 wt.%)agglomerates. Comparison using (A) U/Umf and (B) U–Umf. The error bars are thestandard deviation of the measurements (3–5 replicates).

Fig. 10. Comparison of the percentage of agglomerates fragmented by the fluidized bedfor coke-biodiesel and silica sand–sugar solution (40 wt.%) agglomerates. (A) U/Umf

and (B) U–Umf.

Fig. 11. Comparison of room temperature results using coke-biodiesel agglomerateswith results using coke-bitumen agglomerates at 525 °C. L/S=0.10 and fluidizationtime=60 s for room temperature experiments and 70 s for high temperatureexperiments. The error bars for the room temperature results are the standarddeviation of the measurements (3–5 replicates).

61S. Weber et al. / Powder Technology 209 (2011) 53–64

results for the reacting case were compared with the lowest viscositytested in the room temperature experiments. If the room temperatureresults were found to be much lower than the reacting temperatureresults, then a higher viscosity liquid would be used in the roomtemperature results for comparison. It was acceptable to matchagglomerate liquid content using a mass ratio because the densities ofbiodiesel and bitumen are very similar and fluid coke was used as thesolids in both cases. The results are shown in Fig. 11.

Extensive fragmentation was observed at all conditions in Fig. 11for both the high temperature and room temperature experiments.For the high temperature experiments, several agglomerates were

injected simultaneously into the reactor and a very large number offragments were recovered. Fig. 11 shows that the mfragments/minitial

ratio decreased with increasing U/Umf for both experimental studies.For the room temperature experiments, there is originally growthfollowed by rapid reduction in the amount of agglomerate materialrecovered from the fluidized bed with increasing superficial gasvelocity. For the high temperature experiments, there is only areduction in the amount of agglomerate material recovered and thedecrease is not as dramatic as in the room temperature experimentalcase.

4. Discussion

The results of this study illustrate the complexity of agglomeratebehavior in a fluidized bed environment. How agglomerates survivewithin a fluidized bed is dependent on the interaction of manydifferent parameters. Changing one parameter may improve onefactor for stability, but it may also decrease a different one.

4.1. Effect of superficial gas velocity and liquid viscosity

Superficial gas velocity has been found to determine themethod ofagglomerate destruction [3,34]. Increasing the superficial gas velocitychanged the method of agglomerate destruction from erosion to acombination of fragmentation and erosion. The results in Tables 3 and4 show that increasing the superficial gas velocity decreased theamount of agglomerate material recovered from the fluidized bed andincreased the percentage of agglomerates fragmented in the fluidizedbed. The superficial gas velocity determines the erosion rateexperienced by the agglomerates within the fluidized bed. Thisaccounts for the decrease in the amount of agglomerate materialrecovered from the fluidized bed. The velocity also increased thenumber of agglomerate fragments produced for most liquid viscos-ities. These results agree with the findings of Salman et al. [21],Salman et al. [22] and Subero and Ghadiri [23]. These authors foundthat agglomerate structure and the velocity of impact dictatesagglomerate breakage patterns including size and number offragments. In the case investigated in the current study, the shearingforce of the surrounding fluidized bed is dictated by the superficial gas

62 S. Weber et al. / Powder Technology 209 (2011) 53–64

velocity and acts on the agglomerates in a similar manner as theimpact velocity in the work of Salman et al. [21], Salman et al. [22] andSubero and Ghadiri [23]. There was a peak in the number of fragmentswhen the liquid viscosity was 6.44 cP in Table 3 and when the liquidviscosity was 6.44 cP and 9.79 cP in Table 4. This illustrates the tworeduction processes acting on the agglomerate fragments. Initialfragmentation occurs and then the erosion process acts on thefragments produced. The decrease in the number of fragments occursbecause secondary erosion and fragmentation make some of thefragments so small that they pass through the sieve used to recoverthe agglomerate material from the fluidized bed. This peak was notobserved at higher liquid viscosities because the granule fragmentswere stronger and more resistant to erosion and to furtherfragmentation. Agglomerate breakage occurs when the granulestrength is less than the shearing force of the surrounding fluidizedbed [35]. Stronger agglomerates and fragments can exist in largersizes at certain fluidized bed conditions than weaker agglomeratesand fragments [35]. As the superficial gas velocity increased, morefragmentation resulted in smaller fragments and increased secondaryerosion of these fragments. This caused a decrease in the amount ofagglomerate material recovered which agrees with the findings ofWeber et al. [3,34].

Increasing liquid viscosity was expected to improve the ability ofagglomerates to resist fluidized bed erosion and fragmentation. Thesuperficial gas velocity corresponding to complete fragmentationincreased with increasing liquid viscosity (Tables 3 and 4). Liquidviscosity did not have a pronounced affect on the amount ofagglomerate material recovered from the fluidized bed except whencomparing the extreme viscosities tested. This indicates that theerosion process is not as influenced by liquid viscosity as it is by otheragglomerate parameters. Increasing the liquid viscosity increasesoverall granule strength [36]. It was observed byWeber et al. [34] thatwhen the liquid viscosity increased within a certain range, less liquidwas required to balance the agglomerate growth and erosionprocesses in a fluidized bed environment. This agrees with thefindings of Salman et al. [22] who found that the probability ofbreakage is dependent on thematerial strength of the tested granules.

4.2. Effect of initial agglomerate size

The size of the agglomerate also plays a role in determining theamount of agglomerate material that survives fluidization and theonset of fragmentation. The onset of fragmentation occurred at lowersuperficial gas velocities for larger agglomerates when compared withsmaller agglomerates. In the erosion regime, previous work hasshown that increasing the agglomerate size increases the stability ofagglomerates [34]. Fig. 6 shows that when no breakage is occurring(U=0.10 m/s, U/Umf=10.4), the amount of agglomerate materialrecovered from the fluidized bed is not greatly affected byagglomerate size. Because the viscosity of the liquid is higher andthe superficial gas velocity is lower in the current study than theconditions in [34], the erosion rate is very low. When the superficialgas velocity increased, agglomerates fragmented during fluidization.When fragmentation occurred, larger agglomerates fragmented intomore pieces than smaller agglomerates. This agrees with theobservations from [34] and the findings of Salman et al. [22] whofound that lower impact velocities were required to fragment largeragglomerates. In Fig. 6 at a superficial gas velocity of 0.17 m/s (U/Umf=17.6), the smallest agglomerates fragmented into an average of1.2 pieces. By comparison, the largest agglomerates fragmented intoan average of 29.2 pieces. The average size of a fragment from thesmallest agglomerate was larger than fragments from the largestagglomerate. When agglomerates and granules are exposed to forcesin a fluidized bed, there is a stable maximum size that will survivethose conditions [35]. When the smallest agglomerates were exposedto conditions where they would fragment, they did not fragment as

extensively as larger agglomerates. The smallest agglomerates couldlose small pieces from their structures and quickly get to a stable size.The large agglomerates, on the other hand, fragment much moreextensively more quickly. This may cause the agglomerate fragmentsto go past their maximum stable size and instead come to a size that issmaller than the maximum stable size. When secondary erosion wasacting on these fragments, smaller fragments from initially largeragglomerates were more susceptible to the fluidized bed erosionforces. This explains why initially smaller agglomerates had highermfragments/mo ratios in the fragmentation regime. When the velocitywas increased to 0.25 m/s (U/Umf=26.3), the smallest agglomeratesfragmented into more pieces. This caused the average size of thefragments to decrease and made them more susceptible to fluidizedbed erosion. Weber et al. [34] found that smaller agglomerates aremore susceptible to erosion than larger agglomerates. More workneeds to be done to investigate how much of the fragmentationproduct is a result of initial breakage and fragmentation, which isnearly instantaneous, and how much is a result of time dependent,secondary processes.

4.3. Effect of wettability

In previous studies, it has been shown that the interaction of theliquid and the solid plays an important role in agglomerate stability atlow liquid contents. Increasing the contact angle of the liquid with thesolids in agglomerates with low liquid saturations can decrease theamount of agglomerate material recovered from the fluidized bed [3].If the contact angle of oil with fluid coke can be changed, then theability of the agglomerates with low liquid contents to survivefluidization was expected to decrease. In the Washburn technique,the slope of the square of the mass of liquid absorbed versus timegraph illustrates the affinity of the liquid for a particular solid.McDougall et al. [16] measured the contact angle of fluid coke andwater to be 75°. This value was used to confirm that the contact angleof biodiesel and fluid coke is very close to 0°. The contact angle waschanged to 42.5° when surfactant was added to the system in thebiodiesel. The effect of this change in the fluidized bed experimentswas not very great, especially at low superficial gas velocities. Therewas very little difference in the mean values of the measurements.Although the contact angle is much higher, it is not quite high enoughto greatly affect the strength of the liquid bridges that are holding theagglomerate structure together. In [3], the contact anglewas increasedto 65°. This was found to affect the amount of agglomerate massrecovered from thefluidized bed. It is expected that if the contact anglein the fluid coke-biodiesel system was greater than or equal to 65°,then differences in the agglomerate behavior in the fluidized bedwould be observed. At high superficial gas velocities, the standarddeviation of the measurements in the surfactant results are very large,illustrating that there was a lot of variability in these measurements.This may have been caused by variability in the liquid distribution inthe agglomerate structure.When the contact angle increases, it ismoredifficult to homogenously mix the liquid and particles together. Thismay cause some areas in the agglomerate structure to have differentliquid contents although the overall structure has a uniform liquidcontent. During fragmentation, the agglomerates are breaking intosmaller pieces. Any areas where the liquid content is less thanexpected will be weaker than other areas that have higher local liquidcontents. At low velocities, the agglomerates fragment into fewerpieces so the discrepancies in the liquid content of the fragments arenot as influential as in smaller fragments. The opposite would beoccurring at the highest superficial gas velocities.

4.4. Comparison of coke agglomerate results with other conditions

The findings from the fluid coke agglomerates were comparedwith one of the solid systems studied in [3] to determine whether the

63S. Weber et al. / Powder Technology 209 (2011) 53–64

results were system specific. The results were also compared with thereaction case. The results in Figs. 9 and 10 show that using U/Umf asthe basis of comparison caused the results from different systems tobe very similar when all other parameters were matched. Otherresearchers have suggested using U–Umf because the number andrelative velocity of the collisions relies on the number of bubbleswithin the fluidized bed. This can be quantified using U–Umf for groupB powders [37]. In the current study, the results of the two systemswere more closely matched using U/Umf and this indicates thatagglomerate destruction is not just related to the bubble velocitywithin the fluidized bed. Bubble velocity dictates the shear forces,however, agglomerate destruction also occurs by erosion. This is moredependent on the relative movement of the fluidized particles.

The results in Figs. 9 and 10 also show that the definition of liquidsaturation is an important parameter to consider. In previous studies,agglomerate liquid content was defined based on the mass ratio ofliquid to solids [3,9,34]. In those studies, only one type of solid andliquid were used or the solids had very similar densities. Whenmaterials of different densities are to be compared, using the samemass ratios causes the volume of components to be different which inturn affects agglomerate saturation and how liquid exists in theagglomerate structure [14]. Using agglomerate saturation as definedby Sherrington and Oliver [14] resulted in similar results for bothsolid–liquid systems.

The current study results were compared to results obtained whenagglomerates made from fluid coke and bitumen were fluidized at hightemperatures. At temperatures greater than 500 °C, the bitumen liquidbinder is thermally cracked to more valuable distillate products, whichare recovered for further processing, and solid coke deposits on bedparticles and in agglomerates [5].When comparing the current study tothe reaction case, extensive fragmentation was observed at bothtemperatures. Fig. 11 shows that although the actual values ofmfragments/minitial are not similar between the two experimentaltemperatures, decreasing trends were observed in both systems.Based on the amount of agglomerate material recovered from thereactor, the agglomerates at reacting conditions behavedmore similarlyto drier agglomerates at room temperature. This observation was notsurprising because the liquid bitumen binder in the agglomerates testedat high temperatures was reacting during fluidization. This causes theamount of liquid to decrease with reaction time. This explanation wasconfirmed by results obtained by fluidizing coke-bitumen agglomeratesat lower reactor temperatures. At lower temperatures, more liquid isavailable for longer periods of time because the reaction rate is slower.The amount of agglomerate material recovered from the fluidized bedincreased with decreasing reactor temperature [33]. A further discrep-ancy between room and high temperature experiments was theinsensitivity of the high temperature results to increasing superficialgas velocities. This can also be explained by the effect of reaction.Although reaction reduces the amount of liquid binder available, it alsocauses solid bridges to form between the particles as the coking processoccurs. This strengthens the bonds holding the particles in theagglomerate structure and makes them more resistant to secondaryerosion and fragmentation. It is predicted that testing roomtemperatureagglomerates made from biodiesel at higher U/Umf values would showthat the room temperature results will actually have lower mfragments/minitial values. This would occur because of the effect of secondaryerosion at high superficial gas velocities without the strengtheningeffect of the solid bridges. Although results fromroomtemperature testsand high temperature tests cannot be compared directly, roomtemperature results do directionally identify how different parametersaffect the behavior of agglomerates in fluidized bed environments atboth non-reacting and reacting conditions. Adjustments are required toaccount for the changes in liquid properties caused by reaction.

When comparing results between two liquid–solid agglomeratesystems, the ratio of the superficial gas velocity to the minimumfluidization velocity (U/Umf) was a better predictor of agglomerate

behavior than the excess velocity (U–Umf) when the water saturationwas matched. When room temperature experiments were comparedwith high temperature experiments, the results were not directlycomparable, although decreasing trends were observed in both cases.Room temperature agglomerates behaved as high temperatureagglomerates with higher initial liquid contents.

5. Conclusions

This study investigated how different agglomerate and fluidizationproperties affect agglomerate behavior in a liquid-solid agglomeratesystem that is more consistent with the industrial case. The wayagglomerates grow, erode, and fragment in a fluidized bed environmentis affected by a very complicated interaction of several parameters.

• Increasing the superficial gas velocity shifted the reductionmechanism from a no fragmentation regime to a completefragmentation regime with a transition region between the two.The onset of complete fragmentation occurred at 0.25 m/s (U/Umf=26.3) when the binder viscosity was 6.44 cP and at 0.34 m/s(U/Umf=35.1) when the viscosity was 64.7 cP.

• Increasing initial agglomerate size greatly influenced how agglom-erate material survived fluidization in the fragmentation regime buthad little effect in the no-fragmentation regime. Larger agglomer-ates fragmented, on average, into more pieces which resulted in asmaller average fragment size (1.30×10−7 m3) than the averagefragment size of smaller agglomerates (2.06×10−7 m3). Thisincreased the amount of secondary erosion experienced byfragments of the larger agglomerates, causing them to lose moremass than smaller agglomerates.

• Adding surfactant did change liquid–solid interactions, but theeffect was negligible until extensive fragmentation was occurring.

• When comparing different liquid solid systems, U/Umf was found tobe a better predictor of agglomerate behavior than U–Umf.

Agglomerate destruction behavior is a complicated processbecause of the nature of the agglomerate material and the complexforces acting within the fluidized bed. Further research should morefully quantify the fragmentation products generated by fluidization.This includes parameters such as the size distribution and the liquidcontent of the fragments. Further research should also look into whenand where fragmentation occurs in the fluidized bed. This additionalinformation could help make more general predictions regardingagglomerate stability.

Nomenclature

Dagglomerate Agglomerate diameter (m)KWB Material constant for use in Washburn equationL/S Liquid to solid mass ratio (−)M Mass of agglomerate after fluidization (g)M Weight of the sample in the Washburn techniquemfragments Cumulative mass of agglomerate fragments recovered

after fluidization (g)minitial Mass of agglomerates after formation (g)mo Mass of agglomerate after agglomerate formation (g)Nfragments Number of fragments recovered after fluidization (−)S Agglomerate saturation (−)t Time (s)U Superficial gas velocity (m/s)Umf Minimum fluidization velocity (m/s)γ Liquid surface tension (N/m)εg Gas volumetric fraction in an agglomerate (−)εL Liquid volumetric fraction in an agglomerate (−)θ Contact angle (degrees)μ Liquid viscosity (Pa s)ρL Liquid density (kg/m3)

64 S. Weber et al. / Powder Technology 209 (2011) 53–64

Acknowledgements

The authors acknowledge the support of the Natural Sciences andEngineering Research Council of Canada and Syncrude Canada Ltd. fortheir funding and support for this study.

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