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Ind. Eng. Chem. Res. 2009, 48, 976–982
GENERAL RESEARCH Annatto Powder Production in a Spouted Bed: An Experimental and CFD Study Fabiano G. Cunha,† Kassia G. Santos,† Carlos H. Ataı´de,† Norman Epstein,‡ and Marcos A. S. Barrozo*,† School of Chemical Engineering, Federal UniVersity of Uberlaˆndia, AVenida Joa˜o NaVes de A´Vila 2121, Bloco 1K, Campus Santa Moˆnica, Uberlaˆndia, MG, Brazil 38400-902, and Department of Chemical and Biological Engineering, UniVersity of British Columbia, 2360 East Mall, VancouVer, BC, Canada V6T 1Z3
The mechanical extraction of the bixin from Bixa orellana seeds using a spouted bed was investigated in this work. The experimental program was divided into two main steps. In the first step, a two-level factorial experimental design was used to analyze the influence of the main process variables on the mechanical extraction responses. The second step of the experiment was carried out to evaluate the effect of the distance between the draft tube and the conical base (ht). Computational fluid dynamic technique was used to understand seed flow and the effect of ht on the mechanical extraction process. The results obtained showed that the presence of the draft tube was the variable that most strongly affected the powder extraction. The best condition for the bixin extraction from B. orellana seeds was the one when the draft tube was positioned at 4 cm from the air inlet. 1. Introduction There have been recent concerns about the use of artificial dyes in foodstuffs and a trend toward the use of natural dyes. It is considered that annatto extract ranks as the second most economically important natural color in the world.1 Annatto dye is a natural reddish-yellow extract obtained from seeds of Bixa orellana L., a tropical shrub native to South America. The name comes from the Spanish conquistador, Francisco de Orellana, who is credited with discovering the Amazon River in 1541.1 Its seeds are composed of an “inner seed” with a shelled kernel containing oils, waxy substances, mineral ash and alkaloid compounds, a peel composed of cellulose and tannins, and an outer cover containing pigments, moisture, and a small amount of oils. About 90% of the total pigments in this outer cover are the red oil-soluble carotenoid bixin.2 The use of annatto is an ancient art, since its pigments were manipulated by Brazilian native tribes for skin and artifacts painting.3 This natural dye is noteworthy because of its lack of toxicity, its intense coloring capacity, and its range of color, comprising red, orange, and yellow hues. Rare characteristics, such as the possibility to obtain hydrosoluble and liposoluble extracts from the same source and its stability due to its property of bonding to certain proteins, make the annatto extract one of the main natural pigments utilized in food worldwide. Annatto is widely used for coloring cheese, butter, chocolate, ice cream, sausages, cereals, snack food, and mayonnaise and also for baking purposes. Other applications include formulation of medicines and cosmetics.4 Organic solvents such as chloroform, dichloroethane, or acetone are used commercially to extract pigment from the B. orellana seed. This process is quite expensive and requires several extraction steps to achieve the desired pigment content. In addition, other disadvantages are low recovery of solvents, occurrence of thermal and oxidative degradation, and limited * To whom correspondence should be addressed. Fax: 55-3432394188. E-mail:
[email protected]. † Federal University of Uberlaˆndia. ‡ University of British Columbia.
use of extracted residues. Heat is generally applied to the extract to reduce solvent residue within tolerable limits. High temperatures (>50 °C) are known to degrade the pigment.5 An alternative method is the mechanical attrition of the seeds for wearing of the pigment layer. This involves the use of simple equipment, such as a ball mill or a spouted bed.6-8 The advantage of a spouted bed is better process control.5 Spouted beds6 are gas-particle contactors in which the gas is introduced through a single nozzle at the center of a conical or flat base. This technique has been applied in many industrial processes, such as drying of granular materials, blending of polymer chips, coating of tablets, and granulation of fertilizers and other materials.9 It is an alternative technique to fluidization for particulate solids that are too coarse for good fluidization. In addition to their ability to handle coarse particles, spouted beds have structural and cyclic flow patterns with an effective fluid-solid contact that offers an excellent alternative for the mechanical extraction of the bixin from the surface of the B. orellana seeds. The fluid dynamics characteristics of the spouted bed allow the combination, in the same unit, of impact and seed drying, ensuring the good quality of the final product.5 The gas-solids flow in a conventional spouted bed can be divided into three regions: a spout zone in the center of the bed, where the gas and particles rise at high velocity and the particle concentration is low (lean phase); an annulus zone between the spout and the column wall where particles move slowly downward and inward (dense phase) with counter-current percolation of the fluid; and a fountain zone, where particles rise to their highest positions in the bed and then rain back to the surface of the annulus. Thus, a cyclic pattern of solids movement is established. The gas enters the column through an orifice and flows up the spout and up through the void space in the annulus zone. The introduction of a cylindrical tube (draft tube) above the gas nozzle influences the characteristics of the spouted bed markedly. The spouted bed with draft tube also exhibits a stable dense-phase solids recirculation in the annulus zone and a leanphase conveying up the center with disengagement at the top.
10.1021/ie801382d CCC: $40.75 2009 American Chemical Society Published on Web 12/11/2008
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But, unlike the conventional spouted bed, the configuration with draft tube prevents cross-flow of gas and solids between the two counter-flowing streams.10 Knowledge about the fluid dynamics of the seed flow in the spouted bed is very important for a better understanding of the mechanical extraction results. It is difficult to measure gas and particle dynamics without disturbing the flow field. The literature contains studies of spouted beds, with particle velocities measured in half-cylindrical beds with a flat front wall. It was found, however, that the flat wall could influence the motion of the particles.7 Measuring particle and gas flows by inserting probes throughout the spouted bed zones may also lead to error due to disturbances caused by these probes.10 Studies of numerical simulation techniques and computational fluid dynamics (CFD) have become popular in the field of gas-solid two-phase flow. Some examples of systems examined by these studies are fluidized beds and spouted beds.9-11 The main advantage of numerical simulation is that a wide range of flow properties can be measured simultaneously without disturbing them. In light of the aspects mentioned above, the goal of this work was to study the mechanical extraction of the bixin from B. orellana seeds in a spouted bed. The main variables of the process analyzed were the influence of the draft tube, the air flow, and the seed charges. The responses analyzed were the extracted mass of powder, the productivity, and the final product quality (bixin content). CFD techniques were used to understand both the particle flow and the effect of the distance between the draft tube and the conical base (ht) on the mechanical extraction process. The software used was Fluent 6.3.26, and the mathematical model adopted was the Granular Eulerian multiphase model. 2. CFD Model and Numerical Methodology 2.1. Granular Eulerian Multiphase Model. The Eulerian multiphase model9-13 allows multiple separate yet interacting phases to be modeled. The phases considered here were gas (air) and solid (B. orellana seeds). The Eulerian approach was used for each phase, taking into account every possible intraand interphase interaction. The conservation equations were derived from the ensemble average of the local instantaneous balances for each phase. Mass conservation for gas (g) and solid (s) phases is represented by the following equations. The continuity equation for phase q, q ) (g, s) is given by: f ∂ (R F ) + ∇ · (RqFq ν q) ) 0 (1) ∂t q q Equations 2 and 3 define the momentum balance for the continuous and granular phases, respectively.
∂ (R F b V ) + ∇ · (RgFgb Vgb Vg) ) -Rg ∇ p + ∇ · cτg + RgFgb g+ ∂t g g g bg + b b Flift,g + b Fvm,g) (2) Rgs + (F ∂ (R F b V ) + ∇ · (RsFsb Vsb Vs) ) -Rs ∇ p - ∇ ps + ∇ · cτs + ∂t s s s bs + b Flift,s + b Fvm,s) (3) g+b Rsg + (F RsFsb in which b Rgs ) Rsg ) b
n
∑ [K
bs - b V g) + m ˙ sgb Vsg - m ˙ gsb Vgs] sg(V
p)1
The Reynolds stress for phase q (q ) g, s) is:
(4)
2 cτq ) Rqµq(∇V bq + ∇ b VqT) + Rq λq - µq ∇ · b VqIc 3
(
)
(5)
Fq where µq is the shear viscosity and λq is the bulk viscosity. b is the external body force, b Fvm,g and b Fvm,s are the virtual mass Flift,s are the lift forces forces for gas and solid phases, b Flift,g and b for gas and solid phases, b Rsg is the interaction force between the two phases, p is the pressure of both phases, b g is the gravity Vg are the velocity vectors for solid and gas phases, force, b Vs and b and Fs is the solids density. In this work, the lift and virtual mass forces were disregarded because the particle density is several orders of magnitude higher than the fluid density. Therefore, only the drag force and gravitational force were considered. The momentum exchange coefficient was calculated following the drag model of Gidaspow et al.,14 which is a combination of the Wen and Yu model15 for the dilute phase and the Ergun16 equation for the dense phase: For Rg > 0.8 (Wen and Yu15): bs - b Vg| -2.65 3 RsRgFg|V Rg Ksg ) CD 4 ds
(6)
24 [1 + 0.15(RgRes)0.687] RgRes
(7)
CD )
For Rg e 0.8 (Ergun16): Rs2µg
FgRs|ν s - ν g| + 1.75 Ksg ) 150 2 ds Rgds f
f
(8)
The constitutive equations for shear and bulk viscosities estimation are required for closure of the solid-phase momentum equation and are defined by the granular kinetic theory derived by Lun et al.17 The solids shear viscosity is calculated by adding the collisional and the kinetic contributions. Collisional term (Lun et al.17):
()
θs 4 µs,col ) RsFsdsg0,ss(1 + ess) 5 π 18 Kinetic term (Syamlal and O’Brien ): µs,cin )
1/2
(9)
RsFsds√θsπ 2 1 + (1 + ess)(3ess - 1)Rsg0,ss 6(3 - ess) 5
[
]
(10)
The solids bulk viscosity is given by Lun et al.:17
()
θs 1/2 4 λs ) RsFsdsg0,ss(1 + ess) (11) 3 π The granular temperature θ is assumed to be analogous to the thermodynamic temperature of gases and is proportional to the kinetic energy of the fluctuating particle motion, defined by: θ ) 〈Vp2 〉/3
(12)
The transport equation related to granular temperature takes the form: 3 ∂ (F R θ ) + ∇ · (FsRsb V sθs) ) (-psIc + cτs) : ∇ b Vs + 2 ∂t s s s ∇ · (kθs ∇ θs) - γθs + φls (13)
[
]
where kθs is the diffusion coefficient of granular temperature (Gidaspow et al.14), -γθs is a term for collisional dissipation of energy (Lun et al.17), and φgs represents the energy exchange between phases. The solids pressure is given by (Lun et al.17):
978 Ind. Eng. Chem. Res., Vol. 48, No. 2, 2009
Figure 1. Computational grid.
Figure 3. Variation of the extracted mass of powder (MP) with the processing time for MS ) 2.0 kg.
Figure 2. Experimental apparatus. Table 1. Experimental Results of the First Step presence of the MS experiment draft tube Q/Qms (kg) 1 2 3 4 5 6 7 8
yes yes no no yes yes no no
1.2 1.2 1.2 1.2 1.1 1.1 1.1 1.1
2.5 2.0 2.5 2.0 2.5 2.0 2.5 2.0
MP (g)
PR (g kg-1 h-1)
CB (%)
67.58 69.52 24.28 21.67 69.56 44.32 22.78 13.97
6.76 8.69 2.43 2.71 6.96 5.54 2.78 1.75
33.1 44.3 23.4 24.5 35.5 48.0 23.5 25.6
Table 2. Significant Statistical Effects (First Step Experiments) statistical effects
mean (1) seeds charge: Ms ) 2.0 f 2.5 kg (2) draft tube absent f present (3) air flow Q/Qms ) 1.1 f 1.2 (1) e (2) (1) e (3) (2) e (3) R2
MP (g)
PR (g kg-1 h-1)
CB (%)
+41.71
+4.63
+32.26
+8.68
-6.71
+42.07
+4.70
+15.94
+8.11
+1.02
-1.83 -5.12
-8.35
-1.04
0.976
0.974
ps ) RsFsθs + 2Fs(1 + ess)Rs2g0,ssθs
-1.24 0.999
(14)
where g0,ss is the radial distribution function. 2.2. Boundary Conditions. At the entrance, the solids velocity is zero. At the outlet, the axial velocity gradients for both phases are zero and the pressure is set as atmospheric (∂ψ/ ∂x ) 0, ψ ) u, V, θs, Rs, Rg). At the spout axis, the radial velocity gradients for both phases are zero (∂ψ/∂y ) 0, ∂ψ/∂z ) 0; ψ ) u, V, θs, Rs, Rg), while on the wall a no-slip boundary condition is assumed, that is, the fluid and particle velocities are zero (u ) V ) θs ) Rs ) Rg ) 0). 2.3. Numerical Methodology. A typical aspect of the computational grid adopted in this study can be seen in Figure 1. In the conical part and at the beginning of the cylindrical part to a total height of 19 cm, an unstructured grid formed by
triangular cells was adopted, and in the cylindrical part that remained a structured grid formed by rectangular cells was used. The grid used in this study had a total of 5600 cells on average. The set of balance and constitutive equations was solved using the finite volume technique, employed by the CFD software Fluent 6.3.26. The algorithm SIMPLE was adopted to establish the coupling velocity-pressure. The maximum time step was 10-4 s, and the criterion for convergence established was 10-3. 3. Experimental Methodology B. orellana seeds used in this work were obtained from plantations in the Jequitinhonha Valley region located in the southeast of Brazil. The seeds had the following physical properties: particle size (sphere diameter of the same volume as the particle, dp) of 3.57 mm, density (Fs) of 1.27 g/cm3, and sphericity (φ) of 0.68. The experimental apparatus for mechanical extraction of bixin used in this study is shown in Figure 2. The spouted bed apparatus consists of a stainless steel cylindrical section with a conical base. The internal diameter of the cylindrical section is 0.21 m. The conical base has an internal angle of 60°, and the gas inlet orifice is 0.035 m in diameter. During tests, air was provided by an air compressor of 7.5 hp (2), the flow rate of which was controlled by a pressure regulator and measured by an orifice plate connected to a pressure transducer (3). The pressure drop across the bed of particles was measured at the wall using a pressure transducer connected to a pressure tap located on the conical base 10 mm above the air entry slot. The signal was transmitted to a microcomputer by an A/D data acquisition card and processed by LabVIEW 7.1 software. Data were sampled for 10.24 s at a sampling frequency of 100 Hz, so that 1024 data points were recorded for each condition. An eight-mesh sieve was fixed between the column and the top cone to avoid seed loss from the bed. The concentrated bixin powder extracted was collected in a receptacle (7) connected to the cyclone underflow, type Stairmand (8), 10 cm in diameter. A spectrophotometric technique1 was used to determine the bixin content (CB) with acetone as the extractor solvent. The
Ind. Eng. Chem. Res., Vol. 48, No. 2, 2009 979 Table 4. Minimum Spouting Velocities (m/s): CFD Results and Experimental Data MS ) 2.0 kg MS ) 2.5 kg
Figure 4. Variation of productivity (PR) with the processing time for MS ) 2.0 kg.
experimental data
simulated data
error (%)
21.30 24.00
21.97 26.00
+3.1 +8.3
factorial experimental design was used to analyze the influence of the independent variables. The variables studied were the seed charge in the bed (MS), the influence of the draft tube, and the ratio of the air flow rate to the minimum spouting air flow rate (Q/Qms). A second step of experiments was carried out to evaluate the effect of the distance between the draft tube and the conical base (ht). In this second step, several ht values were used, keeping constant the seed charge in the spouted bed (MS ) 2.5 kg), the air flow (Q ) 99.70 m3/h), and the processing time (4 h). The draft tube used in this work was 0.035 m in diameter. In the experiments of the first step, the value used as a reference for Qms was the one with the spouted bed configuration without the draft tube. Therefore, for the charge of 2.0 kg, Qms ) 73.75 m3/h, and for the charge of 2.5 kg, Qms ) 83.08 m3/h. 4. Results and Discussion
Figure 5. Variation of the extracted mass of powder (MP) with the processing time for several values of ht.
Figure 6. Total mass of extracted powder (MT) as a function of ht. Table 3. Bixin Content and Productivity Obtained in the Present Work and Others
CB (%) PR (g kg-1 h-1)
Passos et al.5
Massarani et al.7
Barrozo et al.19
this work
4 cm). The other operational conditions for these CFD simulations were MS ) 2.5 kg, He ) 19 cm, and Q ) 99.74 m3/h. It can be seen in
Figure 7. Solids volumetric fraction distribution for MS ) 2.5 kg. (a) Ums ) 26.00 m/s, without draft tube. (b) U ) 28.78 m/s, without draft tube. (c) U ) 28.78 m/s, with draft tube.
Figure 8. Solids velocity vector plots (MS ) 2.5 kg and U ) 28.78 m/s). (a) Configuration without draft tube. (b) Configuration with draft tube.
these CFD results that in the central area (spouted region) the air velocity was higher for the configuration with ht ) 4 cm and that the air velocity in this region decreased with increase in ht. The CFD results for the B. orellana seed velocity along the spouted bed are shown in Figure 10, for the same operating conditions as in the previous simulations. It can be clearly seen in these CFD results that the spouted bed configuration with draft tube at ht ) 4 cm presented the highest levels of particle velocity. The CFD results of Figures 9 and 10 explain the trend of the experimental data (section 4.2) (i.e., they show why the distance between the draft and the conical base that promote conditions to maximize the mechanical extraction of the bixin in the spouted bed is ht ) 4 cm). This value of ht is the one that favors the higher solid circulation rates and minimum deviation of entrance air to the annular region. This value can be recommended for experimental conditions (inlet and draft
Ind. Eng. Chem. Res., Vol. 48, No. 2, 2009 981
The CFD simulations showed that the solids volumetric fraction distribution and seed velocities were affected by the presence of the draft tube. Also observed was good agreement between the CFD simulation and experimental data for the minimum spouting velocity. Acknowledgment We are thankful for the financial aid from FAPEMIG and CNPq. Nomenclature
Figure 9. Air velocity along the spouted bed with draft tube for different distances between the draft tube and the conical base (ht ) 4, 5, 6, and 7 cm).
Figure 10. B. orellana seed velocity along the spouted bed with draft tube for different distances between the draft tube and the conical base (ht ) 4, 5, 6, and 7 cm).
tube diameters, bed diameter) similar to those investigated in this work. For other conditions, the methodology of CFD used in this work can be used to find the best distance between the draft and the conical base. 5. Conclusions In the present work, it was possible to quantify the effects of the main independent variables (seed charge in the bed, the presence and clearance of the draft tube, and the air flow rate) on the response variables for the mechanical extraction of bixin in a spouted bed. The presence of the draft tube was shown to be the variable that most strongly affects the powder extraction. The experimental data and CFD results show that the best condition for the bixin extraction from B. orellana seeds was the one when the draft tube was positioned vertically at 4 cm from the air inlet (ht ) 4 cm). This value of ht promotes the most appropriate conditions for circulation of the particles (seeds) and for the air and seed velocities.
CD ) drag coefficient CB ) bixin content [%] ds ) particle diameter [mm] DC ) column diameter [cm] Di ) orifice diameter [cm] eSS ) elastic restitution coefficient b F ) external body force [N] b Flift ) external body force b Flift ) lift force [N] b Fvm ) virtual mass force [N] b g ) gravity acceleration [m s-2] g0,ss ) radial distribution function ht ) distance between the draft tube and the conical base [cm] He ) packing bed height [cm] Ksg ) momentum exchange coefficient [kg m-3 s-1] kθs ) diffusion energy coefficient [J kg-1] MP ) extracted mass of powder [g] MS ) seeds charge in the bed [kg] MT ) total mass of extracted powder [g] p ) total pressure of all phases [Pa] PR ) productivity [g kg-1 h-1] pS ) solids pressure [Pa] Qms ) air flow at minimum spouting condition [m3 h-1] Rsg ) interaction force between phases [N] Re ) Reynolds number Res ) particle Reynolds number t ) processing time [h] U ) gas velocity [m s-1] Ums ) gas velocity at minimum spouting [m s-1] Vp ) particle velocity fluctuation [m s-1] Vs ) particle velocity [m s-1] Greek Letters Rg ) gas volume fraction Rs ) solids volume fraction Rs,max ) maximum solids volume fraction ε ) voidage γθs ) energy dissipation due collisions [J] λs ) solid bulk viscosity [Pa s] µ ) gas viscosity [Pa s] µs ) solid shear viscosity [Pa s] Φc ) angle of conical part θs ) granular temperature [m2 s-2] Fg ) gas density [kg m-3] Fs ) particle density [kg m-3] cτg ) stress tensor [N m-2] cτs ) solid stress tensor [N m-2] φgs ) energy exchange between phases [J]
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982 Ind. Eng. Chem. Res., Vol. 48, No. 2, 2009 (2) Ribeiro, J. A.; Oliveira, D. T.; Passos, M. L. A.; Barrozo, M. A. S. The Use of Nonlinearity Measures to Discriminate the Equilibrium Moisture Equations for Bixa orellana Seeds. J. Food Eng. 2005, 66, 63. (3) Shuhama, I. K.; Aguiar, M. L.; Oliveira, W. P.; Freitas, L. A. P. Experimental Production of Annatto Powders in Spouted Bed Dryer. J. Food Eng. 2003, 59, 93. (4) Alves, R. W.; Souza, A. A. U.; Souza, S. M. A. G. U.; Jauregi, P. Recovery of Norbixin from a Raw Extraction Solution of Annatto Pigments Using Colloidal Gas Aphrons (CGAs). Sep. Purif. Technol. 2006, 48, 208. (5) Passos, M. L. A.; Oliveira, L. S.; Franc¸a, A. S.; Massarani, G. Bixin Powder Production in Conical Spouted Bed Unit. Drying Technol. 1998, 16, 1855. (6) Mathur, K. B.; Epstein, N. Spouted Beds; Academic Press: New York, 1974. (7) Massarani, G.; Passos, M. L.; Barreto, D. W. Production of Annatto Concentrates in Spouted Beds. Can. J. Chem. Eng. 1992, 70, 954. (8) Barreto, D. W.; Jaeger, L. M.; Massarani, G.; Sartori, D. J.; Silveira, A. M. Production of Bixin Concentrates. Proc. XVII Meeting Flow Through Porous Media 1989, 175. (9) Duarte, C. R.; Murata, V. V.; Barrozo, M. A. S. A Study of the Fluid Dynamics of the Spouted Bed Using CFD. Braz. J. Chem. Eng. 2005, 22, 263. (10) Vieira Neto, J. L.; Duarte, C. R.; Murata, V. V.; Barrozo, M. A. S. Effect of a Draft Tube on the Fluid Dynamics of a Spouted Bed: Experimental and CFD Studies. Drying Technol. 2008, 26, 299. (11) Du, W.; Bao, X.; Xu, J.; Wei, W. Computational Fluid Dynamics (CFD) Modeling of Spouted Bed: Assessment of Drag Coefficient Correlations. Chem. Eng. Sci. 2006, 61, 4558.
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ReceiVed for reView September 12, 2008 ReVised manuscript receiVed November 4, 2008 Accepted November 5, 2008 IE801382D
