RESEARCH ARTICLE

Particle formation of hydroxyapatite precursor containing twocomponents in a spray pyrolysis process

W. Widiyastuti (✉)1, Adhi Setiawan2, Sugeng Winardi1, Tantular Nurtono1, Heru Setyawan1

1 Department of Chemical Engineering, Institut Teknologi Sepuluh Nopember, Kampus ITS Sukolilo, Surabaya 60111, Indonesia2 Politeknik Perkapalan Negeri Surabaya, Kampus ITS Sukolilo, Surabaya 60111, Indonesia

© Higher Education Press and Springer-Verlag Berlin Heidelberg 2014

Abstract The particle formation mechanism ofhydroxyapatite precursor containing two components,Ca(OOCCH3)2 and (NH4)2HPO4 with a ratio of Ca/P =1.67, in a spray pyrolysis process has been studied bycomputational fluid dynamics (CFD) simulation on thetransfer of heat and mass from droplets to the surroundingmedia. The focus included the evaporation of the solvent inthe droplets, a second evaporation due to crust formation,the decomposition reaction of each component of theprecursor, and a solid-state reaction that included thekinetic parameters of the precursor regarding its twocomponents that formed the hydroxyapatite product. Therate of evaporation and the reacted fraction of the precursorboth increased with temperature. The predicted averagesize of the hydroxyapatite particles agreed well with theexperimental results. Therefore, the selected models werealso suitable for predicting the average size of particles thatcontain two components in the precursor solution.

Keywords droplet, hydroxyapatite particle, CFD, tubularfurnace, spray pyrolysis

1 Introduction

Spray pyrolysis is a process that is known for producingfine particles from single and multicomponent materials,with many advantages such as homogenous chemicalcomposition in products, high-purity products that dependonly on the purity of the precursor, a short production time,and continuous processing [1]. In addition, inexpensiveprecursors such as acetates, nitrates, and chlorides can beeasily used as liquid precursors in a spray pyrolysisprocess. The mechanism of particle generation follows the

one-droplet-to-one-particle conversion [2]. The precursoris atomized to produce droplets that are carried by carriergas into a heated tubular furnace. Inside the furnace,solvents in the droplets evaporate and then precipitatewhen the solute concentration reaches critical super-saturation. At an elevated temperature, thermal decom-position occurs to generate preferred particles. The criteriafor correlating particle morphology for single-componentparticles in a spray pyrolysis process have been classifiedinto two categories before chemical reaction takes place:the particle either melts or does not melt [3]. Further, theformation of broken hollow particles has been analyzed asthe partial liquefaction of a hydrated precursor that releasesgas followed by particle inflation [4].In a previous study, the modeling of particle formation in

spray pyrolysis was used to control particle morphology,however, the described mechanism of particle formationwas limited to droplet evaporation [5,6]. Dimensionlessstudy has also been performed to determine the mainlimiting steps of the evaporation stage in the spraypyrolysis process [7]. Thermal decomposition reactionafter the evaporation of the solvent in the droplets alsoplays a role in decreasing the particle size in the spraypyrolysis process. The previous investigation was limitedto only a single precursor in order to produce a zirconiaparticle [8]. Similar to spray pyrolysis, a model for thedrying of droplets containing suspended solids of a singlecomponent was proposed for the spray drying process inorder to predict the morphology of a dried particle [9].In the present study, the simulation of particle formation

in spray pyrolysis with precursors containing twocomponents was considered. In precursors containingtwo components, interactions of the two componentsoccurred through a solid-state reaction mechanism duringthe formation of the particle product, which might not bepresent in a single-component precursor. The synthesis ofmulticomponent precursors is widely used in industry forthe synthesis of composite materials by controlling the

Received July 28, 2013; accepted December 9, 2013

E-mail: widi@chem-eng.its.ac.id

Front. Chem. Sci. Eng. 2014, 8(1): 104–113DOI 10.1007/s11705-014-1406-1

stoichiometry of the solution precursors. In the simulation,droplets were assumed to be monodisperse depending onthe composition of the solute in each droplet. The selectedmodel material with a two-component precursor washydroxyapatite (HAp) particles that are widely found inbiomedical applications such as in bones and teeth or inhuman health-related applications in tissue implantation[10,11]. The spray pyrolysis method has been applied tothe production of HAp particles by adding additive, suchas polyethylene glycol or decomposed salt, into theprecursor to obtain a nano size [12,13]. Spray pyrolysisand flame-assisted spray pyrolysis followed by an anneal-ing process has also been carried out to generate HApparticles [14,15]. In the present study, in order to study theformation mechanism of HAp particles during the spraypyrolysis process, no additive and no further post-processing treatment was used in either the experimentor the modeling. Ca(OOCCH3)2 and (NH4)2HPO4 wereselected as the two components of HAp precursors.Experimental work was also carried out to compare thesimulation results.

2 Experiment

The experimental set-up of the spray pyrolysis apparatus isshown in Fig. 1. A Ca(OOCCH3)2 solution (Merck) and(NH4)2HPO4 (Merck) with a molar ratio Ca/P equal to 1.67was dissolved in distilled water to obtain a totalconcentration of 0.5 M. The solution was atomized togenerate droplets using an ultrasonic nebulizer (1.7 MHz,NE-U17, Omron). The average diameter of the dropletsfrom the ultrasonic nebulizer was 4.3 µm based on Lang’sequation [16], as follows:

D ¼ 0:348πg

�f 2

� �13, (1)

where γ is the surface tension of the solution (N/m), ρ is thedensity of the solution (kg/m3), and f is the frequency of thenebulizer (Hz).Nitrogen was used as the carrier gas with a flow rate of

1 L/min to carry droplets to the heated tubular furnace. Thedroplet number concentration was estimated to beapproximately 1012/m3 based on the volume of theprecursor solution that was atomized by the carrier gaswith a flow rate of 1 L/min. The tubular furnace consistedof a ceramic tube with an inside diameter of 1.8 mm and alength of 300 mm equipped with a temperature controllerin order to maintain the furnace temperature. The furnacewas isothermally heated through the furnace wall at 500,700, 900, and 1000 °C. An electrostatic precipitator wasused to collect particles, and was maintained at 150 °C tohinder water condensation. The morphologies of thegenerated particles were examined using a scanningelectron microscope (SEM, Zeiss) operated at 20 kV. The

average particle size, Dav, and geometric standard devia-tion, σg, were determined from the SEM images of at least400 particles. An X-ray diffractometer (XRD, Philips)using filter CuKα radiation (θ = 1.54 Å) operated at 40 kVand 30 mAwas used to analyze the crystal structure of theparticles.Kinetic data were obtained by thermogravimetric-

differential thermal analysis (TG-DTA). The precursorswere analyzed using the TG-DTA method (DTG-60 H,Shimadzu) with variations in the heating rate of 10, 12.5,16, 17.5, and 20 °C, respectively. The thermal properties ofeach precursor, Ca(OOCCH3)2 and (NH4)2HPO4, wereexamined separately with a heating rate of 10 °C. TG-DTAanalysis was performed under nitrogen atmosphericconditions at a flow rate of 50 mL/min and within atemperature range of 25 to 1000 °C.

3 Simulation methods

The temperature and velocity distributions of fluid flowingin a heating furnace were calculated numerically usingcomputational fluid dynamics (CFD) with ANSYS Fluent13, and were based on a finite-volume technique for athree-dimensional domain. CFD simulation was usedbecause it provides comprehensive modeling capabilitiesfor a wide range of fluid-flow and heat-transfer problems.CFD solved the continuity (Eq. (2)), momentum (Eq. (3)),and energy (Eq. (4)) equations simultaneously, which werebased on the Navier-Stokes equations for laminar flow[17]. The energy equation took into consideration the heattransfer from the furnace wall to the fluid.

r: � v↕ ↓

� �¼ 0, (2)

r: � v↕ ↓

v↕ ↓

� �¼ –rP þr:ð τ↕ ↓Þ þ � g

↕ ↓

, (3)

r: v↕ ↓

� H –P

�þ v2

2

� �þ P

� �� �

¼ r: krT þ τ↕ ↓

v↕ ↓

� �, (4)

where ρ is density, υ is velocity, P is static pressure, τ is thestress tensor, H is sensible enthalpy, k is thermalconductivity and T is temperature. The first two terms onthe right-hand side of Eq. (4) represent energy transfer dueto conduction and viscous dissipation, respectively. Theheat transfer from the wall to a fluid cell was computed byapplying Fourier’s law at the walls. ANSYS Fluent wasused in its discrete form, as in Eq. (5).

q ¼ kf∂T∂n

� �wall

, (5)

W. Widiyastuti et al. Particle formation of hydroxyapatite precursor containing two components 105

where kf is fluid-side local thermal conductivity and n is thelocal coordinate normal to the wall.The calculation domain used the same heating furnace as

in the experiment containing a hexahedral grid with2225000 nodes and 208490 cells. The setting boundaryconditions included the velocity inlet on the furnace inlet,the fixed temperature on the furnace wall, and the outflowon the furnace outlet. These boundary condition valueswere set according to the experimental conditions. Thetemperature and velocity vector in the center of the furnaceobtained from the CFD calculation were used to predict theparticle size according to several models described in thefollowing paragraphs.The mass and heat transfer of water due to the

evaporation process followed that used in a previousstudy by applying uniformity for the initial droplet size [8].The mass transfer and heat transfer coefficient of the pureliquid on the droplet surface to the surrounding atmosphereused the following equation:

Nu ¼ 2þ 0:6Re0:5⋅Pr0:33, (6)

Sh ¼ 2þ 0:6Re0:5⋅Sc0,33: (7)

The rate of evaporation was determined by the change indroplet mass, m, that was due to the mass loss caused bythe evaporation of solvent in the droplets, as follows:

dm

dt¼ 2πDvM

NAðng – nsatÞ, (8)

where Dv is the coefficient diffusion of vapor in the carriergas and NA is the Avogadro constant, M is the molecularweight of water as a solvent, and nsat and ng are theconcentration of vapor at the droplet surface and the gasconcentration around the carrier gas. The vapor concentra-tion at the droplet surface can be calculated using the

equation below:

nsat ¼xwpsatkBTsat

, (9)

where Psat is the vapor pressure concentration at the dropletsurface and xw is the mole fraction of solvent in the dropletsurface by assuming the solution is ideal. The diffusioncoefficient for the water-nitrogen correlation is as follows[18]:

Dv ¼1:13� 10 – 5T2:159

P: (10)

Other physical properties, such as heat capacity andviscosity, were also considered as a function of tempera-ture [19].The second stage of the evaporation rate can be

expressed as the change of droplet mass by using thefollowing equation [20]:

dm

dt¼ πdpDvShðng – nsatÞ

1þ ShDv

2Dc

δ0:5dp – δ

φ, (11)

whereDc is the diffusion coefficient through the crust and δis the thickness of the crust. The second stage of theevaporation rate was applied when the solute concentrationin the droplet surface precipitated.A change in droplet temperature, Ts, was calculated

using the following equation:

dTSdt

¼ 1

mCpΔHv

dm

dt

� �þ πdpNukgðT – TSÞ

� �, (12)

where kg is the thermal conductivity of carrier gas, Cp is thedroplet heat capacity, ΔHv is the latent heat of evaporation,and T is the temperature of carrier gas as a continuous

Fig. 1 Experimental set-up for spray pyrolysis

106 Front. Chem. Sci. Eng. 2014, 8(1): 104–113

phase in the surrounding droplets, which was taken fromthe CFD results.The diffusion rate of the solute inside the droplet was

calculated based on Fick’s Law, which resulted in a soluteconcentration profile, Cr, along the droplet radius as afunction of time according to the following equation:

∂Cr

∂t¼ 1

r2∂∂r

r2Ds∂Cr

∂r

� �, (13)

where Ds is the coefficient diffusion of the solute-solventsystem.The Arrhenius equation was used to obtain the activation

energy as a part of kinetic parameters of the formation ofHAp particles. These were analyzed from the temperatureof the precursor decomposition, which was evaluated forseveral heating rates. The Arrhenius equation is as follows:

ln�

T2R

¼ –EA

RTRþ ln

AR

EA, (14)

where θ is the heating rate (°C/min), TR is the decomposi-tion temperature (K), EA is the activation energy (J/kmol),A is the pre-exponential factor, and R is the gas constant(8,134 J/kmol∙K). By plotting ln (�=T2

R) vs. 1/TR, theactivation energy and the pre-exponential factor can beobtained by using the least square method.

4 Results and discussion

The temperature distribution in the diagonal plane alongthe length of the reactor, as predicted by CFD, is shown inFig. 2 for the wall temperatures of 773 K, 973 K, 1173 K,and 1273 K by using a carrier gas flow rate of 1 L/min. Thetemperature distribution along the center of the reactor andtheir validation, as measured by thermocouple for a

furnace wall temperature of 473 K, are shown inFig. 3(a), which demonstrates that the temperaturedistribution by CFD simulation agreed well with theexperiment results. Therefore, the temperature distributionalong the inside of the furnace was reliable for particleformation simulation. The linear temperature distributionalong the center of the furnace for furnace walltemperatures of 773 K, 973 K, 1173 K, and 1273 K isshown in Fig. 3(b). As shown in the figures, an increase inthe furnace wall temperature led to an increase in thetemperature inside the furnace.Figure 4 shows the percentage change in the precursor

weight during the heating process in TG-DTA with aheating rate of 10 °C/min. The precursor contained amixture of calcium acetate and di-ammonium hydrogenphosphate at a Ca/P ratio of 1.67. The TG-DTA profile ofthe mixture characterized the combination TG-DTA profileof the calcium oxide and ammonium hydrogen phosphatecomponents. At 155 °C, the TG-DTA profile indicated anendothermic peak caused by the melting and decomposi-

Fig. 2 Temperature distribution of the furnace with a carrier gas-flow rate of 1 L/min for furnace temperatures of (a) 773 K,(b) 973 K, (c) 1173 K, and (d) 1273 K

Fig. 3 (a) Temperature distribution, and validation, as predicted by CFD along the axial direction at the center of the furnace with acarrier gas-flow rate of 1 L/min and a furnace temperature of 473 K; (b) temperature distribution by CFD along the axial direction at thecenter of the furnace for different furnace wall temperatures

W. Widiyastuti et al. Particle formation of hydroxyapatite precursor containing two components 107

tion of di-ammonium hydrogen phosphate. At about180 °C, the hygroscopic calcium acetate released H2O.An endothermic peak caused by the melting of mono-ammonium di-hydrogen phosphate appeared at 190 °C. Asthe temperature reached to 400°C, Ca(OOCCH3)2 decom-posed to CaCO3 and CH3COCH3, which was indicated bythe DTA peak. When the temperature was increased to650 °C, CaCO3 further decomposed to CaO and CO2.The mechanism of HAp formation can be explained

based on the results of the TG-DTA analysis.(NH4)2HPO4 firstly melted and then was encapsulated byCa(OOCCH3)2. Both decomposed into CaCO3 andNH4H2PO4. As shown in the TG graph, a significantdecrease in mass occurred at 600 °C to 700 °C, indicating aconversion to HAp. As shown in Fig. 4, HAp formed at659 °C, which can be observed in the sharp peak of theDTA profile. The same phenomenon was also observed via

TG-DTAwhen an air atmosphere was used at a heating rateof 5 °C/min [21]. The heating rate of the decompositiontemperature can be used to estimate the kinetic parametersof the reaction.The decomposition temperatures for HAp formation as

evaluated by TG-DTAwere 650.6, 659.83, 665.2, 674.41,and 678.75 °C at heating rates of 10, 12.5, 16, 17.5, and20 °C/min, respectively. These data were plotted accordingto Eq. (9), as shown in Fig. 5, to obtain the activationenergy, EA = 160.82 kJ/mol, and the pre-exponentialfactor, A = 2.87 � 108 per minute. To determine a kineticequation that would describe a suitable mechanism foreither the decomposition reaction or crystallization, threemodels were examined: normal grain growth (NGG),Johnson-Mehl-Avrami (JMA), and three-dimensional dif-fusion (3D-D), according to equations (15)-(17), respec-tively, as follows:

f ðxRÞ ¼ ð1 – xRÞnþ1, (15)

f ðxRÞ ¼ nð1 – xRÞ½ – lnð1 – xRÞ�nþ1=n, (16)

f ðxRÞ ¼ ½ð1 – xRÞ – 1=3 – 1� – 1, (17)

where n is the order of the reaction obtained by solving thenon-linear equation determined by Kissinger’s equation[22]. The non-linear equations used to solve the reactionorder are as follows:

n ¼ 1:26  S1=2, (18)

S ¼ ð3 – αÞ½β – ð3 – αÞ�ð3þ αÞ½β – ð3þ αÞ�, (19)

α ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi9 – 4 2 –

1

n

� �1 –

2RTREA

� �s, (20)

β ¼ 2 2 –1

n

� �, (21)

where S, α, and β are function parameters of the reactionorder. By solving the non-linear equation, the reactionorder, n, is 1.25. This value does not change significantlyby changing the heating rate.The decomposition reaction equation follows the

general kinetic equation, as follows:

dXR

dt¼ kðTÞf ðXRÞ ¼ Aexp –

EA

RT

� �f ðXRÞ, (22)

where dXR/dt is the rate of the reaction, XR is the reactedfraction, k(T) is the Arrhenius constant, and f(XR) is thefunction of the reaction fraction. Figure 6 is the plotbetween ln[k(T) f(XR)] vs. –ln(1–XR) of the three models

Fig. 4 Thermo-gravimetric and differential analyses (TG-DTA)for a precursor containing both Ca(OOCCH3)2 and (NH4)2HPO4

Fig. 5 Activation energy of decomposition for theCa(OOCCH3)2 and (NH4)2HPO4 mixtures

108 Front. Chem. Sci. Eng. 2014, 8(1): 104–113

and the experiment data taken from TG-DTA. The NGGmodel was suitable to describe the mechanism of thedecomposition reaction of the precursor to hydroxyapatitevia kinetic reaction, as follows:

dXR

dt¼ kðTÞf ðXRÞ

¼ 2:87

� 108exp –160:82� 103

RT

� �ð1 –XRÞ2:25: (23)

The equation above was used to consider the densifica-tion of particles that resulted in a decrease in particle sizedue to the decomposition reaction.

The decrease in droplet size that was caused by theevaporation of solvent is shown in Fig. 7(a). An increase intemperature led to a shorter time needed to obtain the samesize. This was caused by a dynamic whereby a higherfurnace temperature meant a faster evaporation rate in thesimulation results, as shown in Fig. 7(b).The shrinkage of particles after evaporation indicated a

decomposition reaction which occurs when the activationenergy is surpassed. To evaluate the fraction that reacted ata different synthesis temperature, Eq. (18) was solvedusing the fourth-order Runge Kutta method. The reactionfractions of HAp precursor along the furnace at variedtemperatures (973 K, 1173 K, and 1273 K ) are shown inFig. 8.At 773 K, no decomposition reactions of the precursor

occurred. This was because the heat energy supply to thefurnace did not exceed the activation energy for thedecomposition of the precursor.SEM analysis showed that the particles synthesized via

the spray pyrolysis method had a hollow morphology(Fig. 9). From SEM images, the average particle sizes andthe standard deviations could be calculated, and the resultsindicated that the particle size distribution was almostuniform, as shown in Fig. 10. The spray pyrolysis tendedto produce particles with a hollow-shaped morphology dueto a solvent evaporation rate beginning on the surface ofthe droplet that was more rapid than the rate of solutediffusion, which led to the precipitation of the solute on thesurface of the droplet. The degree of the decompositionrate of the particles was influenced by the temperature ofthe reactor. At a higher temperature, particle densificationresulted in smaller particle sizes.XRD patterns for various furnace wall temperatures are

depicted in Fig. 11. At the lowest temperature of 773 K, nodominant peak was observed, indicating that the particles

Fig. 6 The kinetic model for describing the decomposition of aprecursor

Fig. 7 (a) Particle size shrinkage and (b) evaporation rate at various temperatures

W. Widiyastuti et al. Particle formation of hydroxyapatite precursor containing two components 109

were amorphous. Increasing the furnace temperature led toan increase in the HAp crystallinity and a highertemperature was used to promote the crystal growth ofthe formed Hap particles. The XRD pattern agreed with thereference for hydroxyapatite better at the highest tempera-ture of 1273 K than at lower temperatures. The crystallinityseemed to correlate with the fraction of reacted precursor togenerate a hydroxyapatite particle, as shown in Fig. 8. Anincreased crystallinity could be obtained by prolonging thereaction time through the annealing process, and byincreasing the temperature.In Fig. 12, the experimental average particle sizes

calculated from SEM images were compared with thosefrom simulation that takes evaporation and decompositionreactions into consideration. The average particle sizepredicted by simulation was slightly higher than that fromthe experiment with a discrepancy of about 12%. Thediscrepancy might be caused by ignoring the particle

Fig. 8 The fraction reactions of HAp precursor at variousfurnace temperatures

Fig. 9 SEM images of HAp particles synthesized at furnace temperatures of (a) 773 K, (b) 973 K, (c) 1173 K, and (d) 1273 K

110 Front. Chem. Sci. Eng. 2014, 8(1): 104–113

densification which was caused by the sintering process inthe simulation method.

5 Conclusions

The particle formation of hydroxyapatite precursor con-taining two components, Ca(OOCCH3)2 and (NH4)2HPO4

with a ratio of Ca/P = 1.67 in the spray pyrolysis processwas investigated by both experimental and simulationmethods. The temperature profile in the furnace waspredicted using CFD code. The effects of the furnace wall

temperature on the particle size, the reacted fraction ofhydroxyapatite precursor, and the crystallinity werestudied. The simulation on solvent evaporation and thefollowing solid-state reaction in the precursor wasvalidated by the experimental work and agreed well witha positive discrepancy of approximately 12%. The formermodel that used a single-component precursor was alsosuitable for a two-component precursor such as thehydroxyapatite precursor. If particle densification causedby the sintering process is accounted for in the simulationmethod, the discrepancy between the simulation and theexperiment would be reduced, and this is suggested forfuture simulation modeling.

Fig. 10 HAp particle size distribution synthesized at furnace temperatures of (a) 773 K, (b) 973 K, (c) 1173 K, and (d) 1273 K

W. Widiyastuti et al. Particle formation of hydroxyapatite precursor containing two components 111

Acknowledgements Research Grant sponsored by the Directorate Generalfor Higher Education, Ministry of Education and Culture of Indonesia forInstitut Teknologi Sepuluh Nopember (Penelitian Strategis Nasional,Contract No. 10473/I2.7/PM/2009) is gratefully acknowledged.

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