desorption onto commercial granular activated carbon: Equilibrium and kinetics

Journal of Food Engineering 84 (2008) 82–91 www.elsevier.com/locate/jfoodeng

Recovery of the main pear aroma compound by adsorption/desorption onto commercial granular activated carbon: Equilibrium and kinetics Nazely Diban, Gema Ruiz, Ane Urtiaga, Inmaculada Ortiz * Department of Chemical Engineering, University of Cantabria, Avda. de los Castros s/n, 39005 Santander, Spain Received 4 December 2006; received in revised form 12 April 2007; accepted 19 April 2007 Available online 3 May 2007

Abstract Adsorption and posterior desorption of ethyl 2,4-decadienoate from model aqueous solutions on granular activated carbon (GAC) were investigated as efficient technologies in the recovery and concentration of this valuable aroma component, one of the compounds responsible of the typical flavour impact of pears. The equilibrium and kinetics of the separation processes were analyzed at different temperatures. The thermodynamic parameters were obtained from the equilibrium data, being (DH) = 38.35 kJ mol1, indicating the exothermic and physical nature of the adsorption process. A kinetic model that considers the influence of external mass transfer and intraparticle diffusion was developed and permitted the estimation of the effective pore diffusivity, Dp , ranging between 0.47  109 and 6.14  109 m2 s1 when the operation temperatures changed from 283 to 322 K. In the desorption step aroma concentrations could reach values up to 40 times higher than the initial ethyl 2,4-decadienoate solution for the simulations presented in this work thus showing the viability of the concentration process. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Adsorption; Desorption; Aroma compounds; Granular activated carbon; Equilibrium; Kinetic modeling

1. Introduction Fruit juices are usually concentrated during industrial processing, that is, water is removed in order to improve the microbiological stability and to reduce storage and transport costs. During the concentration by means of evaporation loss of the aroma compounds occur that usually lower the quality of the product (Bomben, Briun, Thijssen, & Merson, 1973). The aroma recovery is currently performed by rectification, which is also a thermal treatment with the consequently energy consumption and physical aroma losses. Although the application of adsorption to aroma recovery is not very common, it is considered by certain authors (Edris, Girgis, & Fadel, 2003; Karlsson & Tra¨ga˚rdh, 1997) as an alternative to those thermal treatments. In a previous *

Corresponding author. Tel.: +34 942 20 15 85; fax: +34 942 20 15 91. E-mail address: [email protected] (I. Ortiz).

0260-8774/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2007.04.024

work (Diban, Ruiz, Urtiaga, & Ortiz, 2007) a viability study of an adsorption process for the recovery of the main pear aroma compound was reported. Flavours are key compounds for food industry. After separation of the aroma from the aqueous waste stream, its recovery and concentration into an organic phase becomes necessary in order to obtain a high quality extract. Several techniques of desorption are available in the literature, such as the classical thermal regeneration desorption (Chu, Baharin, Che Man, & Quek, 2004), high pressure and temperature regeneration (Salvador & Sa´nchez Jime´nez, 1999), by means of a stripping phase (Gupta, Mittal, & Gajbe, 2005), and more recent techniques such as ultrasonic desorption (Juang, Lin, & Cheng, 2006). In this work, the separation and concentration of the main pear aroma compound, ethyl 2,4-decadienoate, from aqueous solutions, was analysed by means of adsorption on granular activated carbon (GAC) using a combination of thermal and stripping regeneration as the desorption

N. Diban et al. / Journal of Food Engineering 84 (2008) 82–91

83

Nomenclature ap c C Dm Dp Ds E F DG DH KF KL Km L 1/n OLS q qmax

superficial area of the granular activated carbon particle liquid-phase concentration of the solute inside the particle (mol m3) concentration of the solute in the interstitial fluid (mol m3) liquid-phase molecular diffusivity (m2 s1) effective diffusion coefficient (m2 s1) solid-phase surface diffusivity (m2 s1) axial dispersion coefficient (m2 s1) flow rate (m3 s1) free energy of adsorption (kJ mol1) enthalpy change of adsorption (kJ mol1) Freundlich equilibrium constant (mol kg1)(mol m3)1/n Langmuir equilibrium constant (mol kg1) (mol m3) external mass transfer coefficient (m s1) length of the fixed bed of granular activated carbon (m) Freundlich equilibrium parameter ordinary least squares coefficient solid-phase concentration of the solute (mol kg1) Langmuir maximum capacity of adsorption (mol kg1)

technique. Thermal desorption was selected by its wide use and simplicity using ethanol as the solvent in order to recover the aroma. The equilibrium and kinetics of the system were studied at different temperatures. The thermodynamic parameters were obtained from the equilibrium data. The kinetic model proposed previously for the adsorption operation (Diban et al., 2007), including liquid-film mass transport resistance and intraparticle diffusion, was studied in a wider range of temperatures, from 283 to 322 K, fitting adequately the adsorption and desorption data obtained in a granular activated carbon fixed-bed column. The detailed description of the system allows forecasting the behaviour of the separation process for aroma recovery employing adsorption onto activated carbon and determining the best operation conditions.

2. Materials and methods 2.1. Activated carbon The adsorbent used in this study was the commercial Aquasorb 2000 granular activated carbon (GAC) supplied by Jacobi Carbons. Activation procedure and physical properties determination and characteristic values were reported previously (Diban et al., 2007).

r Rg Rp DS t T u0 V W z

radial coordinate (m) universal gas constant (J mol1 K1) radius of the granular activated carbon particle (m) entropy change of adsorption (J mol1 K1) time (s) absolute temperature (K) interstitial fluid velocity (m s1) volume of ethanol (m3) mass of granular activated carbon (kg) axial coordinate (m)

Greek letters ee bed porosity ep particle porosity qp particle density (kg m3) s average particle tortuosity Subscripts d desorption e equilibrium exp experimental sim simulated t stirred tank 0 feed

2.2. Analytical method The concentration of ethyl 2,4-decadienoate in aqueous/ ethanol samples was measured by means of GC (Thermo Quest, model 8000 TOP) with a flame ionization detector (FID) and equipped with a DB-Wax chromatographic column (30 m  0.25 mm ID). The details of the chromatographic method can be found in a previous work (Diban et al., 2007). 2.3. Adsorption and desorption set-up Model solutions (0.1–1 mol m3) of ethyl 2,4-decadienoate (Sigma–Aldrich) diluted in a mixture of 30/70 v/v% ethanol absolute (Panreac Quı´mica) and Milli-Q water (Millipore Corporation) were used in the adsorption experiments. Desorption experiments were performed using 2.5  104 m3/5.0  104 m3 of ethanol absolute. A fixed bed configuration was used in the adsorption/desorption experiments. Adsorption experiments were performed working in a once-through mode, while a batch system was used during desorption runs. The experimental set-up employed was described in detail in a previous work (Diban et al., 2007). In the present work, the glass column, with an internal diameter of 6  103 m, was provided with a jacket for

84

N. Diban et al. / Journal of Food Engineering 84 (2008) 82–91

Table 1 Experimental conditions

Adsorption (oncethrough mode) ethanol/water % v/v = 30/70

Desorption (batch mode) ethanol/ water % v/ v = 100/0

a

T (K)

F (m3 s1)

C0 (mol m3)

Fixed-bed length (m)

283

6  108

298

5.4  108

310

6  108

0.18 0.24 0.56 0.11 0.20a 0.22a 0.28a 0.39a 0.62 0.11 0.17 0.23 0.42 1.07

0.037 0.037 0.039 0.041 0.030 0.035 0.055 0.060 0.038 0.039 0.036 0.037 0.039 0.039

q0 (mol kg1)

Fixed-bed length (m)

0.137 0.142 0.148 0.159 0.266 0.323 0.333 0.357 0.733 0.856 1.014 1.632 2.744

0.039 0.036 0.038 0.037 0.039 0.036 0.038 0.037 0.039 0.038 0.037 0.038 0.039

322

1.8  107

Experiments published in Diban et al. (2007).

the circulation of the thermostating fluid which permitted the study of the influence of the temperature on the adsorption/desorption process. The column was filled with a fixed bed GAC supported on washed glass wool QP (Panreac Quı´mica). The bed length was kept constant at approximately 3.9 ± 0.2  102 m in all the experiments performed in the present work. The detailed experimental conditions for both adsorption and desorption experiments are shown in Table 1. 3. Results and discussion 3.1. Equilibrium analysis Several adsorption experiments were performed in a once-through mode working in the temperature range of 283–310 K. Feed concentration (C0) was varied from approximately 0.1 to 1 mol m3, keeping constant the rest of parameters (W, amount of GAC; F, feed flow rate and composition of the solvent, ethanol/water ratio). The obtained adsorption kinetic curves of adsorption of ethyl 2,4-decadienoate on GAC are plotted in Figs. 1a–c. Experiments were led to proceed until the solute concentration in the outlet stream was equal to the inlet or feed concentration (C0), at this moment it was considered

that the equilibrium had been reached between the adsorbed solute and the solute in the liquid phase, with a concentration Ce (mol m3) of the same value than the inlet concentration. The values of the equilibrium concentration of the solute adsorbed onto the GAC sorbent, qe (mol kg1), were obtained by integration of the area above the adsorption kinetic curves represented in Figs. 1a–c. This area represents the amount of solute removed from the feed stream. In a previous work (Diban et al., 2007), several experiments were carried out working at room temperature, data given in Fig. 1b. These results have been included in the analysis of the influence of the temperature on the adsorption equilibrium, and new data at 0.11 and 0.62 mol m3 were added to that set of experiments in order to generalise the former isotherm for a wider range of concentrations. Several desorption experiments were performed at 322 K, temperature below the maximum temperature recommended to preserve the quality of the aroma compounds, 338 K (Belitz & Grosch, 1987), in order to recover and concentrate the valuable aroma in an organic liquid phase. A previously loaded GAC bed with ethyl 2,4-decadienoate was employed to perform one set of desorption runs, until almost complete regeneration of the GAC was reached. After each set of desorption runs, the fixed-bed was replenished with fresh GAC and the whole adsorption/desorption procedure was repeated. The loading step was made at 310 K varying the feed solution concentration (C0). As a result, in the desorption runs, the initial amount of adsorbed ethyl 2,4-decadienoate onto the GAC fixed-bed (q0) varied in the range 0.137 < q0 < 2.744 mol kg1 and the rest of the parameters were kept constant. Volumes of 2.5  104 and 5.0  104 m3 of ethanol were employed as solvent to recover the aroma from the granular activated carbon working in recycling mode. Fig. 1d shows the evolution with time of the concentration of ethyl 2,4-decadienoate inside the stirred tank obtained in the desorption runs. The desorption experiments were led to proceed until the solute concentration in the recycling tank, Ct (mol m3), shown in Fig. 1d, remained constant for a period of time longer than 1 h. In that moment equilibrium was considered to be reached and Ct = Cd = Ce. The equilibrium values qe (mol kg1) were determined from mass balance as given in the following equation: qe ¼

VC e W

ð1Þ

where V is the volume of ethanol employed to carry out the desorption run. In this study, the well-known Langmuir and Freundlich models, Eqs. (2) and (3), representative of favourable isotherms of adsorption, were used to correlate the experimental results. This kind of isotherms has been widely employed by other authors (Gimeno, Plucinski, Kolaczkowski, Rivas, & Alvarez, 2003; Gonza´lez-Serrano, Cordero, Rodriguez-Mirasol, Cotoruelo, & Rodriguez, 2004;

N. Diban et al. / Journal of Food Engineering 84 (2008) 82–91

a

283 K

310 K

c

0.6

1.2

0.5

1

0.4

C (mol m-3)

C (mol m-3)

85

0.3 0.2

0.8 0.6 0.4

0.1

0.2 0

0 0

20

40

60

80

100

120

140

0

10

20

time (h) C0=0.18 mol m-3

C0=0.25 mol m-3

C0=0.56 mol m-3

C0=0.11 mol m -3 + C0=1.05 mol m -3

40

50

C0=0.17 mol m -3

C0=0.23 mol m -3

C0=0.42 mol m -3

322 K

298 K

b

30

time (h)

d

0.6

3.5 3

C (mol m-3)

0.4 0.3

2.5 2 1.5

t

C (mol m-3)

0.5

0.2

1

0.5

0.1

0

0 0

20

40

0

60

0.5

1

C0=0.12 mol m C0=0.28 mol m -3 (*)

2

2.5

3

time (h)

time (h) -3

1.5

-3 (*)

C0=0.20 mol m + C0=0.39 mol m -3 (*)

-3 (*)

C0=0.22 mol m C0=0.62 mol m -3

q0=0.137 mol kg-1 q0=0.266 mol kg-1 q0=0.733 mol kg-1 q0=2.744 mol kg-1

q0=0.142 mol kg-1 q0=0.148 mol kg-1 x q0=0.159 mol kg-1 q0=0.323 mol kg-1 + q0=0.333 mol kg-1 - q0=0.357 mol kg-1 q0=0.856 mol kg-1 q0=1.014 mol kg-1 q0=1.632 mol kg-1

Fig. 1. Experimental and simulated data of once-through mode adsorption at (a) 283 K, at (b) 298 K, at (c) 310 K and (d) batch desorption experimental curves at 322 K. Solid lines (––) represent the simulated curves. *Curves published in Diban et al. (2007).

Souchon, Rojas, Voilley, & Grevillot, 1996) in order to describe the equilibrium behaviour of similar systems. qmax K L C e 1 þ K LCe qe ¼ K F C e1=n

qe ¼

ð2Þ ð3Þ

The comparison between the experimental and simulated equilibrium data is shown in Fig. 2. This plot shows an increase in the adsorption capacity when temperature decreases. The calculated values of the parameters for both isotherms, Langmuir and Freundlich are shown in Table 2. In a previous work (Diban et al., 2007) the equilibrium data obtained at 298 K were fitted to the Freundlich and Langmuir isotherms and the characteristic parameters calculated. New experimental data were added to the data already published at 298 K, obtaining new expressions of the isotherms. Freundlich and Langmuir isotherms are based on the theory of a monolayer adsorption, onto a heterogeneous and homogeneous surface bound, respectively (Ruthven, 1984). Although the Freundlich isotherm is widely used,

it is an empirical equation. Langmuir isotherm gives information of the total capacity of the adsorbent (GAC) for the solute ethyl 2,4-decadienoate, by means of the value of the parameter qmax. The equilibrium parameter KL, is related to the thermodynamic parameters DG, (DH) and DS (free energy, enthalpy change and entropy change of adsorption), as given by Eq. (4) and the van’t Hoff equation (Eq. (5)), indicating the affinity for the binding of the adsorbate onto the adsorbent of the system DG ¼ Rg T ln K L DS ðDH Þ 1 þ ln K L ¼ Rg Rg T

ð4Þ ð5Þ

where Rg and T are the universal gas constant (8.314 J mol1 K1) and temperature in Kelvin units, respectively. The values of the thermodynamic parameters (DH) and DS have been obtained from the slope and the intercept of the plot ln KL versus 1/T, Fig. 3. The standard Gibbs free energy was calculated at each of the analysed values of temperature, Table 2.

86

N. Diban et al. / Journal of Food Engineering 84 (2008) 82–91 3.5

2.5 2

2.5

ln KL

qe (mol Kg-1)

3

2 1.5 1

1.5 1 0.5

0.5 0

0 0

1

3

3

2

3.1

3.2

3.3 -3

3.4

3.5

3.6

-1

1/T x 10 (K )

Ce (mol m-3) Fig. 2. Comparison between experimental and simulated (- - - - -) Langmuir and (——) Freundlich isotherms of adsorption of ethyl 2,4decadienoate onto GAC at () 283, (h) 298, (N) 310 and () 322 K (desorption).

In Table 3, a comparison of the thermodynamic parameters obtained in this work with other values reported in the literature corresponding to the adsorption of organic compounds on granular activated carbons is given. The negative values for DG, in the range 1.07 to 5.33 kJ mol1, confirm the feasibility of the process and spontaneous nature of the adsorption of ethyl 2,4-decadienoate. The values of (DH) and DS were 38.35 kJ mol1 and 120 J mol1 K1 respectively, confirming the exothermic nature of the process and the decreasing of randomness at the solid-solution interface during adsorption. Being the calculated value of (DH) < 40 kJ mol1, it can be concluded that the adsorption of ethyl 2,4-decadienoate on granular activated carbon is governed by physical interactions (Lucas, Cocero, Zetzl, & Brunner, 2004; Karlsson & Tra¨ga˚rdh, 1997). The value of qmax, the maximum capacity of adsorption of ethyl 2,4-decadienoate onto GAC Aquasorb shown in Table 2, was 3.42 ± 0.25 mol kg1 for the adsorption experiments performed at 283, 298 and 310 K, showing a constant value of qmax according to Langmuir theory. However, data obtained during desorption at 322 K, led to values of qmax of 0.48 mol kg1 that were considerably lower than the values obtained in the adsorption runs. The observed difference could be attributed to the influence of the liquid solvent on the adsorption capacity as it has been pointed out by other authors (Jarvie, Hand, Bhuvendralingam, Crittenden, & Hokanson, 2005; Komiyama & Smith, 1974; Matsui, Fukuda, Inoue, & Matsushita, 2003). These works agree with the idea that the maximum

Fig. 3. ln KL versus 1/T  103. Linear regression coefficient r2 = 0.97.

capacity of adsorption decreases with the increase of the presence of an organic compound in the solvent, reducing the water content, due to a mechanism of adsorption site competence between the organic adsorbate and the organic solvent, together with a greater attraction of the ethanol to the adsorbate in comparison with water. The adsorption runs were performed using 70%/30% v/v water/ethanol solutions as solvent, while the desorption runs employed 100% ethanol, so that the shift in the value of the maximum adsorption capacity, qmax, can be attributed to the influence of the solvent used in the experiments. 3.2. Kinetic analysis Kinetic analysis becomes a key point in the knowledge of the behaviour of a system. In a previous work (Diban et al., 2007), a mathematical kinetic model was developed to fit the kinetic data of adsorption of ethyl 2,4-decadienoate onto granular activated carbon working in a fixed-bed at room temperature. In this work, the kinetic model has been used to fit the adsorption and desorption results obtained at different temperatures 283–322 K, working in the range of concentrations (0.10–1.00 mol m3) for adsorption and (0.137–2.744 mol kg1) for desorption, obtaining the dependence of the kinetic parameters with temperature. The kinetic model that describes the continuous adsorption of ethyl 2,4-decadienoate in a fixed bed of granular activated carbon is aimed at the prediction of the evolution with time of the concentration of the adsorbate in the fluid leaving the GAC column, as a function of the operation conditions (Rivero, Iban˜ez, & Ortiz, 2002; Rivero, Primo, & Ortiz, 2004), whereas in the desorp-

Table 2 Freundlich, Langmuir and thermodynamic parameters T (K)

283 298 310 322

Freundlich parameters

Langmuir parameters

Thermodynamic parameters

KF (mol kg1) (mol m3)1/n

1/n

r2

qmax (mol kg1)

KL (m3 mol1)

r2

DG (kJ mol1)

DH (kJ mol1)

DS (kJ mol1 K1)

3.2 3.0 2.8 0.3

0.26 0.30 0.65 0.69

0.98 0.95 0.99 0.97

3.20 3.38 3.69 0.48

9.64 5.02 1.98 1.49

0.95 0.94 0.96 0.93

5.33 4.00 1.75 1.07

38.35

0.12

N. Diban et al. / Journal of Food Engineering 84 (2008) 82–91

87

Table 3 Literature comparison of thermodynamic parameters for adsorption of organic compounds onto granular activated carbon Chern and Chien (2002) p-Nitrophenol

Commercial activated carbon

Enthalpy change of adsorption, (DH) (kJ mol1) 14.5

Entropy change of adsorption, (DS) (J mol1 K1) 29.1

Aquacarb 207C Aquacarb 208A Aquacarb 208EA

Free Energy of adsorption, (DG) (kJ mol1) 30.83–34.93 29.58–31.22 27.64–31.41

Gimeno et al. (2003) 4-Chloro-2methylphenoxyacetic acid

Purkait et al. (2005) Eosin dye

Commercial activated carbon

This work Ethyl 2,4-decadienoate

Enthalpy change of adsorption, (DH) (kJ mol1) 19.69

Filtrasorb F400

Aquasorb 2000

Enthalpy change of adsorption, (DH) (kJ mol1) 26.98

Entropy change of adsorption, (DS) (J mol1 K1) 52.62

Free energy of adsorption, (DG) (kJ mol1) 9.98–11.03

Enthalpy change of adsorption, (DH) (kJ mol1) 38.35

Entropy change of adsorption, (DS) (J mol1 K1) 120

Free energy of adsorption, (DG) (kJ mol1) 1.07–5.33

tion experiments, carried out in batch mode, the kinetic model is able to predict the evolution with time of the concentration of the adsorbate in the stirred tank. The model equations are detailed in the Appendix A. The adjustable parameter in this model is the effective diffusion coefficient Dp . The best value of Dp was obtained by comparing the experimental adsorption/desorption data with the simulated results in order to provide the minimum value of the standard deviation, based on the minimization of theP ordinary least squares coefficient (OLS), OLS ¼ ðC exp  C sim Þ2 . The software tool available in the Aspen Custom Modeler process simulator, that uses finite differences for the discretization of differential equations, was used to solve the set of model equations. The estimated values of Dp are reported in Table 4, and the comparison between the experimental and simulated results is shown in Fig. 1. In this work, the values of effective pore diffusivity, Dp , were plotted against the reciprocal of temperature, Fig. 4. It is observed that the higher the temperature, the higher the value of effective pore diffusivity, Dp , in agreement with previous works (Ruthven, 1984). Dp is a lumped parameter that according to a parallel mechanism of diffusion inside the particle pore fits to the dq expression given by Eq. (A.5), Dp ¼ ep Dsm þ qp Ds dC . The first term of the expression corresponds to the diffusion mechanism of the solute in the liquid phase inside the pore that depends on the molecular diffusivity, Dm, parameter

1.00E-07 3

Dp* (m2 s-1)

Otero et al. (2005) Salicilic acid

3.1

3.2

3.3

3.4

3.5

3.6

1.00E-08

1.00E-09

1.00E-10

1/T x 10-3 (K-1) Fig. 4. Dependence of the effective pore diffusivity with temperature.

that follows a dependence with temperature as given by the correlation of Wilke–Chang, Table 4, with particle porosity, ep = 0.62, and with tortuosity, s = 4, as referenced previously (Diban et al., 2007). The second term refers to the surface diffusion mechanism of the adsorbed compound being the characteristic parameter the solidphase surface diffusivity, Ds. Surface diffusivity, Ds, was calculated from the estimated value of the effective pore diffusivity, Dp , the particle density, qp, and the slope of the isotherm, dq/dC, that was assumed to remain constant in the range of concentrations studied. The values esti-

Table 4 Intraparticle diffusion parameters T (K)

Dp ðm2 s1 Þ (estimated)

Dm (m2 s1) (Wilke–Chang)

D m ep s

283 298 310 322

0.47  109 1.28  109 3.89  109 6.14  109

4.40  1010 6.74  1010 8.75  1010 11.3  1010

0.68  1010 1.04  1010 1.36  1010 1.75  1010

ðm2 s1 Þ

Ds (m2 s1)

. dq ðqp dC Ds Þ ðDm ep =sÞ

0.18  1012 0.46  1012 1.28  1012 24.4  1012

5.9 11.3 27.6 34.1

88

N. Diban et al. / Journal of Food Engineering 84 (2008) 82–91

mated for Dp fall into a range that agrees with the results obtained in other works using activated carbon as adsorbent of organic compounds at room temperature in a glass agitated batch adsorber (Calleja, Serna, & Rodriguez, 1993; Fritz, Merk, & Schlu¨nder, 1981; Neretnieks, 1976) as shown in Table 5. The relative contribution of surface diffusivity increases with increasing temperature. This behaviour can be observed in the change of the ratio of the surface diffusion  dq term to the pore diffusion term, ðqp dC Ds Þ ðDm ep =sÞ, as a function of temperature. In Table 4 it is observed that the value of this ratio is higher as the temperature increases indicating that temperature has higher influence on the surface diffusion rate than on the pore diffusion rate. Surface transport is related to the presence of micropores in the particle (Ruthven, 1984). In the granular activated carbon used in this study, approximately 50% of the porous structure corresponds to micropores and the other 50% corresponds to mesopores. The importance of surface diffusion can be associated to the high percentage of micropores present in the particle. Surface diffusivity becomes the limiting resistance to the transport of the adsorbate at low temperatures, but this resistance decreases when temperature rises facilitating the intraparticle transport rate. Although the adsorption capacity of the GAC is favoured at low temperatures, as it was seen in the previous section, the kinetics are limited by the decrease of temperature. Therefore, a mild adsorption temperature such as room temperature becomes a good option, not only because it leads to a compromise between capacity and kinetics but also because it saves energy costs of cooling or heating. 3.3. Desorption experiments Scarce literature exists on the desorption of organic compounds from activated carbon mainly due to the fact that this technology is mainly employed to remove organic contaminants from wastewaters that are of no economical value; once the carbon is saturated it is dumped and the adsorption system is replenished with fresh carbon (Cooney, 1999). When the adsorbed compound is valuable (Otero, Zabkova, Grande, & Rodrigues, 2005) or when

the recovery of the activated carbon is profitable (Salvador & Sa´nchez Jime´nez, 1999) the study of the GAC desorption process becomes necessary. Ethyl 2,4-decadienoate is a valuable compound with applications in food and cosmetic industry. For that reason, after separation of the aroma compound from the aqueous solution, it is necessary to be recovered and concentrated into an organic phase in order to avoid microbiological activity. Di Cesare and Polesello (1987) and Bitteur and Rosset (1988) desorbed aroma compounds from different types of resins by eluting with absolute ethanol. From the previous thermodynamic study, the exothermic nature of the adsorption process was manifested. Therefore, higher temperatures favour the desorption of the adsorbed ethyl 2,4-decadienoate, so in this work, thermal desorption using absolute ethanol as stripping solvent was selected to recover and concentrate the aroma compound. As previously mentioned, aroma compounds are very sensitive to thermal treatments, which cause the loss of its quality and properties. Due to this fact, thermal desorption was carried out at 322 K, a mild temperature that causes no damage to the compound. The temperature restrictions were compensated by the fact that ethyl 2,4decadienoate feels much more affinity for ethanol than for water, because of its lower polarity, and the fact that the adsorption capacity of the GAC using ethanol as solvent becomes strongly reduced. Several desorption experiments were performed to analyse the behaviour of the equilibrium and kinetics. The experimental set-up and runs have been described previously in this work and the experimental conditions are included in Table 1. The obtained results are shown in Figs. 1d and 2 and in Table 3. A concentration factor was defined as the ratio of the equilibrium concentration reached in the ethanol during the desorption process (Cd) to the feed concentration (C0) of the solution used in the adsorption process, (Cd/C0). The values of this concentration factor, (Cd/C0), versus the feed solution concentration, C0, are presented in Fig. 5. The data were obtained after adsorption runs at 310 K and ethanol volumes of 2.5  104 and

Table 5 Comparison of diffusivities at room temperature for organic compounds onto activated carbon in the literature Calleja et al. (1993) p-Nitrophenol Phenol

Effective pore diffusivity, Dp ðm2 s1 Þ 3–50  109 1.3–3.6  109

Fritz et al. (1981) p-Nitrophenol Phenol

Pore diffusivity, 1–10  109 3–11  109

D m ep s

ðm2 s1 Þ

Surface diffusivity, Ds (m2 s1) 1–4  1012 3–15  1012

Neretnieks (1976) 2,4-Dichlorophenoxyacetic acid

Pore diffusivity, 1.4  1010

D m ep s

ðm2 s1 Þ

Surface diffusivity, Ds (m2 s1) 0.14  1012

Diban et al. (2007) Ethyl 2,4-decadienoate

Effective pore diffusivity, Dp ðm2 s1 Þ 2.47  109

Pore diffusivity, 1.04  1010

Dm ep s

ðm2 s1 Þ

Surface diffusivity, Ds (m2 s1) 0.97  1012

N. Diban et al. / Journal of Food Engineering 84 (2008) 82–91 ex p2.5 2.5x10-3 exp x 10-4m3 m3 12

exp x 10 m3 m ex p5.0 5.0x10-3

10

sim

-4

Cd /C0

6 4 2 0 0.1

0.2

0.3

0.4

0.5

-3

C0 (mol m ) Fig. 5. Experimental and simulated curves of the concentration ratio (Cd/ C0) versus feed concentration (C0) using ethanol volumes of 2.5  104 m3 and 5.0  104 m3. GAC beds previously loaded at 310 K.

5.0  104 m3 in the elution process. Solid lines in Fig. 5 correspond to simulated curves that fit quite well to the experimental data. The experimental values of the concentration factor, (Cd/C0), in the range 10–6, shown in Fig. 5 check the viability of the recovery of ethyl 2,4-decadienoate from a synthetic solution of this aroma compound by means of an adsorption/desorption process onto granular activated carbon. The influence of the temperature of adsorption on the concentration reached during the desorption step is analysed in Fig. 6. Simulated curves that correspond to adsorption temperatures of 283, 298 and 310 K using a volume of absolute ethanol of 1.25  104 m3 are represented. The pear brandy concentration of ethyl 2,4-decadienoate usually falls between 0.24 and 0.34 mol m3, (Kralj-Cigic´ & Zupancˇicˇ-Kralj, 1999). In Fig. 6 a wider range was studied, because the concentration varies depending on the type of juice or beverage and the state of ripening of the fruit employed for the juice processing. The lower the temperature of adsorption the higher the value of the concentration factor, (Cd/C0), reaching values up to 40, showing adequate results of the concentration process.

45

Cd / C0

4. Conclusions

3

8

0

89

40

283 K

35

298 K

30

310 K

25 20

In this work, a study of the removal of the main pear aroma compound, ethyl 2,4-decadienoate, from a model aqueous solution and its posterior concentration by means of an adsorption/desorption process onto granular activated carbon was made. The experiments were carried out in a fixed-bed set-up. The influence of the temperature on the equilibrium and kinetic parameters was experimentally studied. From the analysis of the equilibrium results, it is concluded that the best description of the equilibrium data is obtained with the Freundlich isotherm. However, the theoretical basis of the Langmuir isotherm allowed the calculation of the thermodynamic parameters. DG falls in the range 1.07 to 5.33 kJ mol1, being (DH) = 38.35 kJ mol1 and DS =  120 J mol1 K1. These values confirm that the process is spontaneous and exothermic and that the adsorption proceeds by means of physical interactions. The percentage of ethanol present in the solution has a big influence on the maximum capacity of adsorption of the system, qmax, that drops from 3.42 ± 0.25 kg mol1 to 0.48 kg mol1 when the ethanol content rises from 30% to 100% v/v. A mathematical model accounting for external mass transport and internal pore resistances was developed to fit the kinetic data for both adsorption and desorption experimental curves. The estimated values of the effective pore diffusivity, Dp , ranged between 0.47  109 and 6.14  109 m2 s1 when the temperatures changed from 283 to 322 K. The influence with temperature of the two transport mechanisms present inside the GAC particle was studied observing an increase of the diffusion rate with increasing temperature mainly due to the relative importance of the surface diffusion mechanism. The ratio of surface diffusion rate to pore diffusion rate  dq ðqp dC Ds Þ ðDm ep =sÞ, varies from approximately 6 to 34 as the temperature increases. Temperature favours kinetics due to the decrease of the mass transport resistances, but the capacity of adsorption of the GAC is reduced, and so that equilibrium is limited. Therefore, room temperature seems to be a compromising option for both equilibrium and kinetics, together with the consequent saving of energy costs. Recovery of the valuable aroma compound is necessary and thermal desorption using ethanol as stripping phase gives good results observing that in the range of studied variables the concentration of the aroma after desorption can be increased up to 40 times its initial feed value.

15

Acknowledgements

10 5 0 0

0.1

0.2

0.3

0.4

0.5

C0 (mol m -3) Fig. 6. Simulated curves of concentration ratio (Cd/C0) versus feed concentration (C0) at different adsorption temperatures (T).

Financial support of the Spanish Ministry of Education and Science (MEC) under projects PPQ2003-00934 and CTQ2005-02583/PPQ and F.P.I. grant is gratefully acknowledged. The authors are also thankful to Prof. Francisco Salvador (University of Salamanca) for the samples of GAC and recommendations.

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N. Diban et al. / Journal of Food Engineering 84 (2008) 82–91

Appendix A. Modeling equations

t ¼ 0;

0 6 r 6 Rp ;

This model is based on the following main assumptions: (i) fast intrinsic adsorption kinetics, and therefore, instantaneous local equilibrium is considered and (ii) both the external resistance of solute mass transfer from the bulk liquid phase to the particle surface and the internal resistance associated to solute diffusion within the particle pores are considered (Ruthven, 1984). The column mass balance in the interstitial liquid is given in the following equation:

t > 0;

r ¼ Rp ;

oC oC oC 2 ee þ K m ap ðC  cjr¼Rp Þ þ ee u0 ¼ ðE þ Dm Þ 2 ot oz oz

ðA:1Þ

where C is the concentration of the solute in the interstitial fluid, c the concentration of the solute inside the GAC particle, t the time, z the axial coordinate, the external mass transfer coefficient, E the axial dispersion coefficient, the molecular diffusivity of the solute, ap the superficial area of the particle, u0 the interstitial fluid velocity, ee the bed porosity and Rp the radius of the particle, for spherical GAC particles, ap = 3/Rp. The values of Dm, E, and Km, corresponding to the operating conditions of this work have been calculated using standard correlations. The first term of Eq. (A.1) represents the accumulation of solute in the mobile fluid; the second term is the transfer rate of solute across the liquid film around the particle, the third term accounts for the change in concentration along the bed length due to forced convection and the last term accounts for dispersion. The initial and boundary conditions of Eq. (A.1) for the case of adsorption experiments are: t ¼ 0;

0 6 z 6 L;

t > 0;

z ¼ L;

t > 0;

z ¼ 0;

t > 0;

r ¼ 0;

ðA:2Þ

where L is the length of the fixed bed of granular activated carbon and C0 is the concentration of the solute in the feed solution that enters into the GAC bed. Mass transfer of the adsorbate inside the particles occurs by combination of Fickian diffusion in the liquid phase filling the pores and surface diffusion. For spherical particles, the diffusion of the adsorbate within the particle is described in the following equation:   oc oq 2 oc oc2  ep þ qp ð1  ep Þ ¼ Dp þ ðA:3Þ ot ot Rp or o2 r where q is the concentration of adsorbate in the solid phase of GAC, r is the radial coordinate, Dp is the effective pore diffusivity, ep the particle porosity and qp the particle density. Initial and boundary conditions of Eq. (A.3) are, for adsorption:

q¼0

K m ðC  cjr¼Rp Þ ¼ Dp  oc ¼0 or r¼0

  oc  or r¼Rp

ðA:4Þ

The full expression for the effective pore diffusivity is, Dp ¼ ep

Dm dq þ qp D s dC s

ðA:5Þ

where Ds is the solid-phase surface diffusivity, dq/dC is the slope of the adsorption equilibrium isotherm, and s is the tortuosity factor of the pores of the GAC particles. A mass balance to the stirred tank must be added to the previous set of equations to complete the model describing the desorption system. The sampling volume is considered negligible, so the volume of the tank is approximated to be constant (V), and Eq. (A.6) is obtained: V

dC t ¼ F ðCjz¼L  C t Þ dt

ðA:6Þ

This balance is connected to the column mass balance by means of following equation: 8t;

C t ¼ Cjz¼0

t ¼ 0;

Ct ¼ 0

ðA:7Þ

Eqs. (A.1), (A.3) and (A.5) are valid for batch desorption experiments, and in this case the initial and boundary conditions for Eq. (A.1) are: t ¼ 0;

0 6 z 6 L;

t > 0;

z ¼ L;

t > 0;

z ¼ 0;

C ¼ C0

 oC  ¼0 oz z¼L oC ¼ u0 ðC 0  Cjz¼0þ Þ ðE þ Dm Þ oz

c ¼ 0;

C¼0   oC  ¼0 oz z¼L

ðA:8Þ

C ¼ Cjz¼0

and for Eq. (A.3): t ¼ 0;

0 6 r 6 Rp ;

t > 0;

r ¼ Rp ;

t > 0;

r ¼ 0;

c ¼ 0;

q ¼ q0

K m ðC  cjr¼Rp Þ ¼ Dp  oc ¼0 or r¼0

  oc  or r¼Rp

ðA:9Þ

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