An experimental study of fluidized-bed coating: influence

of operating conditions on growth rate and mechanism

K. SALEH, R. CHERIF and M. HEMATI *

Laboratoire de Gnie Chimique (UMR-CNRS, 5503), ENSIGC, INPT, 31078 Toulouse,France

Received 12 October 1998;accepted 17April 1999

AbstractThis study investigates the influence of fluidizing gas velocity, atomizing air, and liquidflow rates, liquid concentration, initial bed mass, and particle size on the mechanism of growth ofsand particles in a batch fluidized-bed coater. An aqueous solution of NaCl was used as the coatingliquid and sprayed in the bed by means of a pneumatic atomizer. The results showed that for a givenparticle size, the fluidizing air velocity was the most important factor affecting the coating kinetics and stability. The dominant mechanism was the onion-ring layering, especially at excess gas velocitieshigher than 0.27 m/s. For a fixed value of the mass ratio of solute introduced in the bed to the initial particle mass, binder concentration, liquid flow rate, and the initial bed weight had no effect on the growth mechanism. The deposition quality was found to be affected by the droplet size. A decrease of droplet size resulting from increasing the atomizing air flow rate permitted homogenous coating ofthe solid surface.

Keywords: Coating; fluidizedbed; atomization; growth mechanisms; silica sand; NaCl.

NOMENCLATURE

a gap distance between colliding particles cm)C NaCl concentration (kg/m3)DB Bubble diameter (m)

dp Sauter mean diameter (ttm)

di mean diameter of size interval I (pm)

dp initial Sauter mean diameter (item)e particle coefficient of restitution

f adhesion probability

Fcap capillary contribution to bridge force (N)

*To whom correspondence should be addressed.

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Fyis viscous contribution to bridge force (N)

h liquid layer thickness covering colliding granules (pm)

characteristic length scales of surface asperities (ttm)

m weight of the sample removed at time t before washing (kg)

mo weight of the sample removed at time t after washing (kg)

M` total mass of the particles in the bed (kg)

Mo initial mass of the particles in the bed (kg)

s bubble spacing (m)

Stv viscous Stokes' number

t time (s)

u superficial fluidizing gas velocity (m/s)

Vo relative particle collisional velocity (m/s)

?7g bubble velocity (m/s)

Bumf minimum fluidizing velocity (m/s)

wA atomizing air flow rate (kg/s)

w?, atomizing liquid flow rate (kg/s)

ws solute (NaCI) flow rate (kg/s)

X growth rate (%)

Xi particle mass fraction of size interval i

Greek

y liquide surface tension (dyn/cm)

8 dimensionless bubble spacing 2s/DB

g impingement efficiency

? coating efficiency (%)

0 center angle

11. liquid velocity (P)

liquid density (kg/m3)

pp particle density (kg/m3)

solute (NaCI) density (kg/m3)

Ts solute content (%)

Other

* critical value of associated parameter

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1. INTRODUCTION

Fluidized bed coating of particles is a multiphase size enlargement process wheresolid, liquid, and gas come into contact with each other. Generally the purposeof coating particles is to increase the particle size, improving appearance, taste, orodors of particles, or protecting them from oxygen, humidity, light, or incompatibleactive agents. In a majority of cases a liquid (solution, suspension or melt)containing a coating agent, 'binder', is injected or sprayed into or onto a hot fluidized bed of solid particles. The particle coating proceeds by successivewetting of the fluidized solids with sprayed liquid followed by their solidification through drying. Particle growth in the bed is mainly governed by two mechanisms:'agglomeration' and 'layering'. Agglomeration results from the adhering of small particles by forming liquid or solid bridges between them, leading to formation of larger particles, called 'agglomerates', at least twice as big as the initial particles.'Coating' or 'onion-ring' layering corresponds to the deposition of an ingredient onthe entire surface of particles. There could be two objectives: size enlargement by deposition of a thick layer of coating agent on the particles surface and film coating.This type of operation is widely used in fertilizer and detergent manufacturing, foodprocessing, nuclear technology, and especially in the pharmaceutical industry.

In addition to the desirable characteristics of conventional fluidized bed such as isothermicity, high heat and mass transfer rates, and good particle mixing, fluidized-bed coating permits several operations such as wetting, mixing and drying to be carried out in the same apparatus. Therefore, contrary to the classical rotatingdrum (or pan) granulator or coater, there is no need for subsidiary drying unitsto evaporate the added solvent. However, these advantages, responsible for the successful use of fluidized beds in industrial operations, may be upset by somedisadvantages when operating in the presence of spraying liquids, by defluidizationphenomenon occurring due to formation of large agglomerates. This is a potentiallyserious problem that must be kept in mind for coating and agglomeration processesbecause when it occurs the behavior of a fluidized bed can change drastically. Thisadds a degree of risk to the production process that could result in the whole batch being rejected. Another problem when operating fluidized beds is the attritionphenomenon which results in losses in coating agent deposition and then operationefficiency. The latter is an important parameter in the case of costly binders and indicates whether or not the operation is economically acceptable.

On the other hand, the influence of operating conditions on the kinetics and governing mechanisms of growth as well as on the stability of the operation is notwell understood.

Consequently, the successful use of fluidized-bed coating depends on understand- ing the mechanisms which govern particle growth, operation efficiency, and stabil- ity, and on overcoming its disadvantages.

This study, relative to the fluidized-bed coating of silica sand particles by anaqueous solution of NaCl, deals with clarifying the parameters which influence growth mechanisms and operation efficiency, and with determining rules for the

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Figure 1. Phenomena occuring during particle coating.

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avoidance of bed defluidization. The effects of excess gas velocity, atomizerlocation, liquid flow rate and concentration, atomizing air flow rate, initial particlemean size, and initial bed mass are investigated.

2. LITERATURE REVIEW

2.1. Analysis of phenomena occurring during particle coating in a fluidized bed

Generally, the coating process in a fluidized bed is affected by the consecutive and competitive elementary steps: particle mixing, liquid spreading, and partialevaporation of solvent from the particles surface, agglomeration, abrasion, andfragmentation. Several authors [ 1-4] have reported a description of the different phenomena occurring during particle growth in fluidized bed. These phenomenaare summarized in Fig. 1 and described below.

Coating liquid containing a binder is sprayed into the bed and particles are wettedby liquid droplets. If excessive liquid is present or if it is unevenly distributed insuch a way that liquid droplets are too large with respect to the particles, then wet agglomerates can develop by formation of liquid bridges. If wet agglomerates aretoo strong to be fragmented and too large to be fluidized then large regions of the bed may defluidize and stick together as large wet clumps. This phenomenon istermed 'wet quenching'. Note that if the break-up forces exerted by the fluidizedenvironment exceed liquid bridge strength, the wet clumps will be transformed in to smaller wet agglomerates which are still fluidizable.

Alternatively, if the droplet size is less than the particle size, two situations are distinguished:

(1) Fast drying before a collision between wet particles. Consequently, the growthoccurs by layering.

(2) Collision of two or more wet particles leading to the formation of moving(mobile) liquid bridges and wet agglomerates.

In the second case, if the cohesion strength is weak in comparison with the break- up forces induced by the fluidized bed, the break-up of the bridges could lead to the formation of individual wet particles that can be dried and grow by the layeringmechanism. On the contrary, the solidification of liquid bridges occurs due to evaporation of the solvent and then agglomerates become stabilized. Whether or not the particles remain together depends on the relative magnitude of the bindingforces and the break-up forces arising from the movement of particles throughoutthe bed.

If the cohesive forces are in excess of the break-up forces, particle growth occursby agglomeration. If excessive particle growth occurs, the minimum fluidization velocity of particles will exceed the operating velocity and 'dry quenching' of the bed will follow.

On the contrary, if the break-up forces completely predominate, the agglomeratemay break down to smaller agglomerates or individual particles with a small amount

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of coating material attached to the surface of each. Hence, the interactions between

build-up and break-up forces, and consequently the strength of solid and liquidbridges between particles, play a crucial role in determining the growth mechanism.

Note that a parasite phenomenon takes place during the spraying of the liquidcorresponding to the droplets drying before the particles surface is attained (spraydrying). This step favors the formation of fine solid particles that can be carried out

by the gas or introduced in the bed and, in consequence, grow or adhere to other

particles.

2.2. Forces exerting on particles and granulation regimes

According to models described by Rumpf [5] and by Newitt and Conway-Jones [6],presented in Fig. 2, the main cohesive forces that operate during the binder

agglomeration process result from liquid bridges formed between the solid particles.According to the so-called 'Laplace-Young' theory the strength of the cohesiveforce (F) depends on the surface tension of liquid, y, the wettability of the solidsurface with respect to liquid, the gap distance between the particles, a, and the

particle diameter, dp.Furthermore, experimental results from Adams et al. [7] and Mazzone et al. [8],

and more recently theoretical and experimental studies from Ennis et al. [9],demonstrated that the cohesive strength of the dynamic liquid bridges may exceedthat of the static by at least an order of magnitude due to the additional energydissipation resulting from binder viscosity. According to Ennis et al. [9], both the

Figure 2. Adhesion forces brought about by liquid bridge formation.

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capillary and viscous contributions were found to significantly affect the bondingmechanism of colliding particles.

In order to establish the regimes of granulation, Ennis et al. [10] defined the viscous Stokes number, St", as the ratio of the relative kinetic energy between

colliding particles to the viscous dissipation brought about by the pendular bond:

where Uo is the relative velocity of particles, pp is the particle density and p isthe viscosity of the binding liquid. It is to be noted that the calculation of Stvpresumes a knowledge of the interparticle velocity, Uo, which reflects the effect of break-up forces imposed by the granulation system. Ennis estimated Uo to be

equal to 12UBdp/DB as a maximum and to 12UBdpl DBo2 on average for fluidized bed granulators, where 8 is the dimensionless bubble spacing, and UB and DB arebubble velocity and bubble size, respectively. Hence, (1) becomes:

A critical viscous number St must be surpassed for rebound of colliding particles to occur:

where e is the particle coefficient of restitution, h the thickness of the binder layerand a measure of the particle's surface aspirities (Fig. 2).

Three granulation regimes were defined in terms of the magnitude of St, in

comparison to St*:Stv v non-inertial regime (all collisions successful),St" N St v inertial regime (some collisions successful),Stv v coating regime (no collisions successful).Despite the limitation of the theoretical analysis of Ennis due to a number of

simplifications, this theory can be used, at least qualitatively, with experimentalresults for fluidized-bed granulation.

3. EXPERIMENTAL

The experimental set-up is represented in Fig. 3. It consists of a stainless steel

cylindrical column, 0.1 m in diameter and 0.35 m in height, topped by a conical freeboard section 0.2 m high with a 45 included angle. The air distributor is a

perforated plate made from stainless steel with 1.6% porosity. Before entering thebed, the fluidizing gas flow rate is measured by a rotameter and preheated in anelectrical heater (1.8 kW). The elutriated fine particles are collected at the column outlet by a cyclone 50 mm in diameter.

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Figure 3. Schematic view of the experimental set-up.

The coating liquid, a 200 kg/m3 solution of NaCl, is fed in by a peristaltic pumpfrom the reservoir, maintained at 25C and I atm to an external mixing two-fluidspray nozzle (Spray System Co.; J67147, J1650). The atomizing air and liquid flowrates are controlled by niddle valves and measured by rotameters. The atomizer is a downward facing nozzle and is located in the bed.

The bed temperature is controlled by means of a PID regulator, and monitoring oftemperature and pressure droplet is achieved during coating.

The experiments were conducted at 130C with batches of fresh solid particles.To begin a coating experiment, in order to reduce the thermal perturbation caused bythe liquid atomization, distilled water at the same flow rate of the coating liquid wasinitially sprayed. When the bed temperature returned to the set value, the distilledwater was switched with the coating solution.

During experiments samples of solid were regularly removed from the bed bymeans of a particular sampling system located 50 mm above the distributor. The sieve analysis and washing of the samples by distilled water at 40C allowed us to determine the particle size distribution and the quantity of solute (NaCl) present inthe samples. This information allowed us to estimate the Sauter mean diameter,growth rate, solute content and efficiency (coating criteria) as follows:

Sauter mean diameter

Growth rate

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where d, and dpo are, respectively, the mean diameter at time t and the initial diameter.

Solute content

where m and mo are the mass of the sample removed from the bed at time t before and after washing.

Coating efficiency

This criterion was defined as the ratio of the quantity of solute deposited on the solid

particles during the time t to that introduced in the bed for the same duration:

Mo and ws are the initial weight of solid particles in the bed and the solute (NaCl)mass flow rate, respectively.

In agreement with the results obtained by Smith et al. [3], the preliminaryexperiments showed that the most appropriate position of the atomizing nozzle was when the tip of nozzle coincided with the surface of the packed bed and thus became

submerged when the bed was fluidized. This permitted us to avoid cake formation

'caking' reported by Smith et al. [3] and Jonke et al. [11], and to maintain a stable

operation with maximum efficiency.

4. RESULTS AND DISCUSSION

Table 1 summarizes the operating conditions maintained for different experimentsconducted in order to investigate the influence of process variables upon coatingcriteria.

The initial particle size maintained for all experiments (except for those relative to the effect of initial particle size) was 229 pm. The initial size distribution of

particles used is given in Table 2.

4.1. Influence of fluidizing air velocity

The fluidizing gas velocity is a parameter which influences both the operationstability and coating parameters. The hydrodynamic behavior of the fluidized bed coater is strongly dependent on of the fluidizing gas velocity and a properchoice of this parameter is essential to avoid unplanned agglomeration and to keepmaintain stable operation for long periods. It is well known that defluidization ischaracterized by a rapid decrease in pressure droplets, because most of the gas goes through the slumped bed. Consequently the defluidization point can be determined

by measuring the pressure droplet through the bed [12]. Figure 4 illustrates the

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Table 1. Operating conditions maintained during the coating experiments (T = 130 C)

Table 2. Initial size distribution of the utilized sand

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Figure 4. Time evolution of total pressure droplets through the bed as a function of the fluidizing gas velocity.

effect of fluidizing gas velocity on the evolution of total pressure droplets with time. Gas velocities below 0.32 m/s are shown to cause defluidization of the bed. The lower the gas velocity, the faster the defluidization of the bed takes place.

This behavior can be qualitatively explained using Ennis' model of cohesive forces of mobile liquid bridges. According to Ennis in the case of a fluidized bed the transition to the inertial regime will occur when the granules have reached a sufficient size such that the largest collision velocity will cause particle reboundand prevent agglomeration. Note that this corresponds to the maximum size of

agglomerates which can be formed at a given velocity in the bed. The critical sizeat which the transition takes place depends on the humidity content and relativevelocity of particles, and is given by [10]:

In this equation the humidity content and relative velocity are represented through h and UB/DB. Using Davidson and Harison's relationship to estimate the bubblevelocity [13] and Mori and Wen's correlation [14] for bubble diameter, it can be shown that the term UB/DB increases linearly with fluidizing gas velocity. Therefore, by reduction of the fluidizing gas velocity, a decrease in relative velocityand increase of humidity content lead to higher values of dp*.Under these conditions

dp* could exceed dpo and all collisions between particles are successful and the growth is then governed by agglomeration. If the size of agglomerates formed is too large to be fluidized by fluidizing gas (U < Umf), segregation followed bybed quenching takes place. On the contrary, the growth of agglomerates havinga size greater than dp* occurs by a layering mechanism. Note that if the initial particle size is greater than or equal to the critical value dp , no agglomerates can be

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formed. Under our experimental conditions, as the fluidizing gas velocity becomes

higher than a critical value, U* (0.32 m/s), which is high enough to fluidize the

largest agglomerates formed in the bed, the bonding forces are just balanced byforces resulting from the motion of the fluidizing gas in the bed. It means that the

growth occurs in an inertial regime. However, a further increase in the gas velocityresults in a transition from the inertial to coating regime. Sieve analysis of the final product obtained for a gas velocity of 0.44 m/s showed that the agglomeratemass fraction does not exceed 3% and that growth by layering was the dominant mechanism.

The influence of fluidizing gas velocity on growth rate, operation efficiency and

agglomerates fraction, (dp > 2dpo), was studied between 0.32 and 0.55 m/s. The results are illustrated in Fig. 5. It can be concluded that increasing the fluidizinggas velocity leads to a diminution of the all these criteria. The efficiency droplets from 92 to 77% when increasing the air velocity from 0.32 to 0.55 m/s. This maybe due to the increase of the attrition rate with increasing fluidizing gas velocity. Several authors [15-17] have reported a direct relationship between the attrition rate in fluidized beds and the excess gas velocity. In addition, a higher fluidizinggas velocity results in a higher spray drying rate.

The influence of other parameters was studied for a fixed value of the excess

fluidizing gas velocity of 0.39 m/s that permits us to minimize the agglomerationphenomenon and to maintain a coating operation for a long period with a reasonable

efficiency (80%).

Figure 5. Effect of the fluidizing gas velocity on the efficiency, growth rate, and percentage of formed agglomerates (t = 110 min).

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4.2. Influence of atomizing conditions

The atomization air and liquid flow rates can influence the mechanism and qualityof deposition by modifying the size of liquid droplets. Generally it is acceptedthat the mean droplet size, dg, decreases with increasing atomizing air flow rate or decreasing liquid flow rate. The mean droplet size and size distribution weremeasured by the laser diffraction method using a Malvern 2600 particle sizer. The effects of atomizing air flow rate and liquid flow rate on the mean droplet size and size distribution were studied. Figure 6 illustrates the effects of atomizing gas and

liquid flow rates on the Sauter mean diameter of particles for a 200 kg/m3 NaClsolution. As can be seen, under operating conditions only the fluidizing air flow rate has an effect on droplet size. A sharp decrease in the droplet size (from 60 to 20 p,m) can be observed when the atomizing air flow rate increases from 5 x 10-5to 20 x 10-5 kg/s. Beyond this value the droplet size decreases much more slightlywith increasing atomizing air flow rate. A comparison of the experimental resultswith some relationships proposed in the literature for the pneumatic nozzles showed that the well-known Nukiyama's correlation [18] can properly predict the Sauter mean diameter of liquid droplets with an accuracy of

4.2. l. Influence of atomizing air flow rate at constant liquid flow rate. It should be noted that the atomizing air flow rate influences both the droplet size and the

droplet velocity (droplet momentum). The influence of the atomizing air flow ratein the coating criteria was studied between 6.4 and 30.6 x 10-5 kg/s for a liquidflow rate maintained at 13.6 x 10-5 kg/s.

Figure 6. Effect of the atomizing gas and liquid flow rates on Sauter mean diameter of liquid droplets.

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Figure 7 shows the particle growth rate and operation efficiency after 110 min as a function of atomizing air flow rate. Starting from low atomizing gas flow rates, the

operation efficiency first increases when the atomizing gas flow rate increases up to a maximum of about 87% for 23.9 x 10-5 kg/s. Beyond this point any further increaseof atomizing gas flow rate causes the operation efficiency to decrease slightly toa value of about 83%. It is interesting to note that the same trend was reportedby Link and Schliinder [19] concerning the coating of a single freely suspendedaluminum sphere with a 10 wt% hydrated lime [Ca(OH)2] suspension. This can be explained by considering that the binder deposition on the particles surface is controlled by two competitive phenomena: collision between liquid droplets andsolid particles, and attrition. By increasing the atomizing gas flow rate at a constant

liquid flow rate, the ability of a droplet to come into contact with the particle,called the impingement efficiency, s [20], increases because due to both the highervelocity and higher number of droplets, more of them reach the particle surfacebefore spray drying occurs. However, increasing the atomizing air flow rate leadsto an increase in the attrition rate which is a power law of the jet velocity with an

exponent 3 [21]. As can be seen from Fig. 6, the impingement efficiency, whichfor a constant liquid flow rate is inversely proportional to the droplet size, increaseswith increasing atomizing air flow rate up to 20 x 10-5 kg/s. Beyond this value,this parameter becomes independent of the atomizing air flow rate, whereas the attrition rate increases continuously. Hence, the trend of the efficiency curve can be

explained by the competition between these two phenomena.In order to elucidate the effects of the atomizing gas flow rate upon the growth rate

of particles and the quality of deposited layer, it was found to be useful to comparethe experimental results with a layering model. In the case of mono-size spherical

Figure 7. Effect of the atomizing gas flow rate on efficiency, growth rate, and percentage of formed agglomerates (t = 1 10 min).

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particles with a uniform distribution of solute over particles, this model leads to the following relation for estimating of the evolution of mean particle size as a functionof time:

Substitution of (7) into (9) gives:

The experimental values obtained for two series of experiments are compared withthe predictions of the model in Fig. 8. It can be noted that the model underestimatesthe experimental results. Furthermore an increase of the atomizing gas flow ratetends to reduce the deviation from the model predictions. As can be seen from Fig. 7, the efficiency of operation and the percentage of agglomerates are almost the same for these two experiments, and consequently the improvement in agreementbetween experimental results and model should not be attributed to a change inthe growth mechanism. The low percentage of agglomerates indicates that for both cases the growth is mainly governed by layering. Therefore, the reduction of deviation between experimental and calculated data can be attributed to the mechanism of deposition of NaCl crystals at the particle surfaces. Scanning electronmicroscope (SEM) images ( x 2000) of the surface of coated sand particles (Fig. 9a)revealed that coating occurs by superposition of isolated NaCI monocrystals at the solid surface rather than a smooth and homogeneous layer. This can be due to the

Figure 8. Effect of the atomizing gas flow rate on the growth rate of particles [comparison withlayering model (10)].

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Figure 9. SEM images of coated particles obtained at two different atomizing gas flow rates(t = 240 min).

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Figure 10. Evolution of the particle growth rate as a function of total binder quantity (influence ofliquid flowrate).

inadequate distribution of liquid in the solid surface. As seen from Fig. 9b, theNaCl distribution over the particle surface is improved considerably by increasingthe atomizing air flow rate which results in a decrease of droplet size and in anincrease of droplet momentum and number of droplets. In this case the depositedlayer is more compact and homogeneous.

4.2.2. Influence of liquid flow rate at constant atomizing air flow rate. The effect of the liquid flow rate was investigated between 7.6 and 16.6 x 10-5 kg/s at a constant atomizing air flow rate of 17 x 10-5 kg/s. Figure 10 shows that for a given value of the ratio of the NaCl quantity introduced to the initial particlesmass, Ms/Mo, the increase of the liquid flow rate (decrease of the duration of the operation) has no influence on the growth rate. In addition, the low fraction of agglomerates indicates that layering is the predominant mechanism. The variationof liquid flow rate does not influence the growth mechanism nor the operatingefficiency (Fig. 11). This is because the droplet mean size does not vary significantlywith the liquid flow rate in the studied range of the liquid flow rate.

4.3. Influence of solution concentration

The solution concentration is a parameter that affects the duration of the operationas well as the mechanism of the growth. The influence of this parameter wasinvestigated for four values between 100 and 250 kg/m3. The liquid flow rate was maintained at 13.6 x 10-5 kg/s.

Figure 12 shows the effect of the NaCl concentration on the coating criteriaafter 110 min of operation, indicating that solute content, growth rate, efficiencyand agglomerates fraction increase as the solute concentration increases. The weight fraction of agglomerates remains low but increases from 1.05 to 3.52% when increasing the concentration from 100 to 250 kg/m3. This effect may be

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Figure 11. Effect of the liquid flow rate on the efficiency, growth rate, and percentage of formed agglomerates (t = 110 min).

Figure 12. Effect of the binder concentration on the efficiency, growth rate, and percentage of formed agglomerates (t = I 10 min).

attributed to the variation of solution viscosity with concentration. According toKaufmann [22], increasing the NaCl concentration from 100 to 250 results in an increase of solution viscosity from 1.136 to 1.815 cP (60%). This leads to strongerliquid bridges which leads in turn to a higher agglomeration rate. The increase of

operating efficiency may be attributed to a higher adherence probability, f, dueto the higher viscosity of more concentrated solutions which increases the liquidadhesion to the particle surface.

Figure 13 represents the evolution of the growth rate as a function of the ratio of mass of NaCl introduced during a given time to the initial mass of solids MS/Mo. Itcan be concluded that for a given value of MS/Mo, the solute concentration has no

significant influence on the growth rate.

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Figure 13. Evolution of the particle growth rate as a function of total binder quantity (influence ofbinder concentration).

Figure 14. Evolution of the particle growth rate as a function of total binder quantity (influence ofinitial mass of particles).

4.4. Influence of initial particle weight

The influence of this parameter was studied between 1.25 and 2.53 kg, correspond-ing to a ratio of the initial bed height to the bed diameter of between 1.3 and 2.63.A 200 kg/m3 NaCl solution with a flow rate of 13.6 x 10-5 kg/s was used and the flow rate of the atomizing air was maintained at 17 x 10-5 kg/s.

Figure 14 indicates that for a given ratio of the introduced mass of solute to initial bed mass, the initial particle mass does not influence the growth rate. Therefore, itcan be concluded that the growth mechanism is independent on this parameter. Notethat these results are in agreement with those reported by Dencs and Ormos [23]and Ormos et al. [24, 25]. On the other hand, Fig. 15 shows that the efficiencyand mass fraction of agglomerates formed appeared to be independent of the initial bed mass. These results together with those obtained concerning the variation of

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Figure 15. Effect of the initial mass of particles on the efficiency, growth rate, and percentage offormed agglomerates (t = 110 min). ).

the liquid flow rate may furthermore be taken as an indication that particle wetting in a fluidized-bed spray coater occurs only in a limited volume of bed called the 'atomization zone' which is independent of the total mass of particles. The size of this zone is determined by the penetration depth of the spray. This is a functionof the gas velocity, the nozzle position, physical properties of the atomizing and

fluidizing gas, and particle momentum [26]. The existence of such a zone in the coater was reported by Smith et al. [27] by measuring the temperature gradientsnear the nozzle. Since the total bed weight has no effect on the penetration depth ofthe spray there is no effect of this parameter in the coating criteria.

4.5. Influence of initial particle mean size

The effects of particle size on the growth behavior of particles were investigatedusing three batches of sand with different mean sizes (Table 3). As alreadymentioned, particle growth in a fluidized-bed coater is governed by the followingthree mechanisms: layering, agglomeration and attrition. In order to investigatethe influence of the particle mean size on the first two mechanisms, the operatingconditions must be fixed in such a way that the attrition rate could be considered

independent of the studied parameter. Previous works [ 1 5- 1 7, 28, 29] have shown that attrition in fluidized beds occurs only after the minimum fluidization velocityis exceeded. According to these investigations, the main parameter which affects the attrition rate in fluidized beds is the fluidizing excess gas velocity above theminimum for fluidization, U - Umf. In addition, other investigations [29, 30] indicate that the attrition rate is affected only slightly by the particle mean size. In fact, the particle size affects the attrition rate only through its effect on Umf.Hence, in this study, the fluidizing excess gas velocity, U - U",f, was held constant at 0.39 m/s. The experiments were conducted with 1.53 kg sand particles.

Table 3 summarizes the growth rate, solute content, efficiency and agglomeratemass fraction after 1 10 min of coating for different particle sizes. It is clear from

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Table 3 that the smaller the mean particle size, the greater the solute content andthe efficiency of operation. This can be explained by the fact that smaller particlescapture more binder than larger particles. This is essentially due to their greaterspecific area and more frequent contact with the spray in the atomizing zone, whichtend to reduce the losses by spray drying and increase the operation efficiency.

In addition it can be concluded that the growth rate has, on the whole, a tendencyto increase with decreasing particle size. The negligible amount of agglomerateformed when coating particles larger than 221 pm indicates that the growth is

governed by surface layering. Furthermore, it can be seen that there is a significantincrease in the amount of agglomerates for finer particles (dp = 165 p,m). This

shifting of the granulation regime from coating toward the inertial and perhaps non-inertial regime takes place because Stv decreases with decreasing particle size (2).Consequently there is a disproportionate increase in the adhesive forces comparedwith the break-up forces exerted by the fluidized bed, which leads to an increase in agglomerate formation. It is clear from this result that the particle mean size

significantly affects both the efficiency and the mechanism of growth as well as the

stability of operation.

5. CONCLUSION

The fluidized-bed coating of silica sand particles with an aqueous solution of NaCl was investigated. The effect of fluidizing gas velocity, atomizing air and liquid flow

rates, liquid concentration, initial bed mass, and particle size on the mechanism of

growth, stability and operation efficiency was studied. It was found that for a givenparticle size, the fluidizing air velocity was the most important factor affecting the

coating kinetics and stability. Bed quenching can be avoided by increasing the ex- cess gas velocity. This can be attributed to the higher circulation rate of particlesallowing a more uniform distribution of liquid and to higher break-up forces induced

by the fluidized bed which causes the agglomerates to split off. For a fixed value of the mass ratio of solute introduced in the bed to the initial particle mass, binderconcentration, liquid flow rate and the initial bed weight had no effect on the growthmechanism. The deposition quality was found to be improved by increasing the at-

omizing air flow rate. In this case, the finer droplet size and higher droplet momen-tum permits us to ensure a homogenous and compact coating of the solid surface.

Decreasing the initial particle size results in a higher rate of agglomerate forma-tion and a shift from the coating regime toward the inertial regime. This is due to the stronger interparticle adhesive forces for finer particles.

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