- Email: [email protected]
Recovery of key components of bilberry aroma using a commercial pervaporation membrane
Desalination 224 (2008) 34–39
Recovery of key components of bilberry aroma using a commercial pervaporation membrane Nazely Diban, Ane Urtiaga, Inmaculada Ortiz* Department of Chemical Engineering, University of Cantabria, Avda. de los Castros, 39005 Santander, Spain Tel. +34 942 201585; Fax: +34 942 201591; email: [email protected] Received 15 January 2007; Accepted 5 April 2007
Abstract Pervaporation, a membrane technology requiring low energetic demands, is considered to substitute the conventional distillation unit employed in the beverage industry in order to recover the key aroma compounds, and that would avoid the damages to the quality of the flavour profile. The analysis of the separation and recovery of some selected aroma compounds belonging to bilberry juice was made by employing a mathematical model previously developed for the pervaporation of VOCs. A PV hollow fiber module provided with a PDMS commercial membrane was considered. For each compound, the characteristic mass transfer parameters are the permeability of the membrane and the diffusion coefficient in the aqueous phase. Enrichment factors over 100 were achieved for the aroma compounds with higher permeabilities. The membrane thickness influence over the enrichment factor and flux of the bilberry impact aroma compound (E-2-Hexen-1-ol) was studied observing higher enrichment factors and lower fluxes with higher membrane thicknesses until an asymptotic value was reached. The predicted permeate composition achieved under the simulated conditions did not keep the same proportion of that in the feed composition. A sensorial analysis should be made to measure the product quality achieved. Keywords: Pervaporation; Bilberry aroma recovery; PDMS membrane; Modelling
1. Introduction Aroma recovery has become a key issue in the beverage industry. The high energetic demands of the distillation unit employed during the manu*Corresponding author.
facture of concentrated juices imply high costs but also damages to the quality of the organoleptical profile of the recovered aroma concentrates. Alternative technologies employing low thermal requirements are considered to substitute the rectification conventional process.
Presented at the 11th Aachen Membrane Colloquium, 28–29 March, 2007, Aachen, Germany. 0011-9164/08/$– See front matter © 2008 Published by Elsevier B.V. doi:10.1016/j.desal.2007.04.076
N. Diban et al. / Desalination 224 (2008) 34–39
Membrane technologies need low energetic requirements and facilitate modular configurations that allow the possibility to easily scale-up the process from pilot to industrial plants. Pervaporation has been widely analysed in previous works [1–3] in order to separate and concentrate valuable aroma compounds from process streams. In the review presented by Pereira et al. [4], it was concluded that the most typical membrane used to perform the pervaporation of aroma compounds was made of polydimethylsiloxane (PDMS) material. The demand for processing bilberries (Vaccinium myrtillus L.) has increased in the last years [5]. Although these fruits can be eaten fresh, are more usually processed to obtain products like jams, fools, juices or pies. Therefore, this wild fruit has become to be cultivated, mainly in North America. Important bilberry producers among the european countries are France, Holland, Germany, Poland and Spain. The aim of this study is to adapt a mathematical model that was previously developed for the pervaporation of VOCs from wastewaters to predict the behaviour of the PV separation of aroma compounds from aqueous solutions. A mixture of 7 volatile compounds, characteristic of the aroma of bilberry juice, has been defined, and the use of PDMS pervaporation membrane with a hollow fiber configuration has been considered. The prediction of the flux of each compound and the concentration factor obtained in permeate are sought. The influence of the membrane thickness on both parameters will be discussed.
Fig. 1. Schematic view of the pervaporation set-up.
A mathematical pervaporation model that was developed and implemented previously [6] has been used. The characteristic mass balance equation for the aqueous feed circulating through the inside of the hollow fiber in dimensionless form appears in Eq. (1):
δCi* δC * 2 δ ⎛ δCi* ⎞ + (1 − R 2 ) i = ⎜R ⎟ δt δξ R δR ⎝ δR ⎠
(1)
This expression is obtained by defining the following dimensionless variables and parameters:
Ci* =
Ci Ci , z =0,t =0
(2)
R=
r rt
(3)
ξ=
zDi 1 = 2 4vrt Pe( z )
(4)
2. Mathematical modelling Pervaporation of a synthetic aromatic feed solution characteristic of bilberry juice was considered. The system used a PDMS membrane with a hollow fiber configuration and operated with recirculation of the aqueous solution. A schematic view of the set-up is plotted in Fig. 1.
35
Shw,i =
2 Pm ,i srt lDi
(5)
where Ci* is the dimensionless concentration of compound i in the fluid circulating inside the pervaporation module, R is the radial coordinate,
36
N. Diban et al. / Desalination 224 (2008) 34–39
at t = 0
Ci*t = 1
(11)
∀t
Ci*,int = Ci*, z = L
(12)
ξ the axial coordinate, and the parameter Shw,i is the Sherwood number of the wall for component i that expresses the ratio of the mass-transfer resistance in the inner fluid to that in the membrane. That means that for Shw 6 4 the membrane resistance is negligible against the resistance exerted by the liquid phase diffusion, while for Shw = 0 is the membrane resistance which determines the mass-transfer phenomena. Di and Pm,i are the diffusivity in the aqueous phase and the membrane permeability of the compound i, respectively. Individual molecular diffusivities of the organic components into a diluted aqueous phase were calculated using the Wilke-Chang correlation [7]. Parameters are described in more detail in the Symbol section. The set of boundary conditions for this equation is the following:
The water flux was calculated by means of Eq. (14) that is considered as a constant value due to the fact that the water molar fraction xw remains practically constant during the whole experiment:
at t = 0 ∀ξ ∀R Ci* = 1
(6)
Jw =
(7)
The enrichment factor βi for the component i, which is time dependent, was defined according to Eq. (15):
∀t at ξ = 0 0 ≤ R ≤1
Ci* = Ci*t
∀t at R = 0 0 ≤ ξ ≤1
1 Pe( L)
(8)
δC =0 δR * i
at R = 1 0 ≤ ξ ≤ 1 Pe( L) −
δCi* Shw,i * = Ci δR 2
The flux of component i was obtained from the mass balance according to Eq. (13) that is a function of time: Ji =
βi =
F ρCi , Z = L ⎛ Ci , z = L ⎞ 2 ⎜⎜ 1 − ⎟⎟ [g/m s] Ap ⎝ Ci , z =0 ⎠
Pm , w l
ρ w xw
Cip Cit,t =0
[g/m 2s]
(13)
(14)
(15)
The mathematical model hereof was implemented in the simulation software Aspen Custom Modeler.
(9)
The mass transfer model is complemented by the mass balance to the feed in the tank:
V
δCi*t = F ( Ci*,int − Ci*t ) [m 3 / s] δt
(10)
With the initial and boundary conditions:
3. Experimental The qualitative analysis of the aroma compounds present in this fruit was performed experimentally by CG-MS [9]. The major chromatographic peaks correspond, in order of appearance, to E-2-Hexen-1-al, 2-butyl-1-octanol, 1-hexanol, E-2-Hexen-1-ol and 1-hexadecanol. Other minor components present in the sample
N. Diban et al. / Desalination 224 (2008) 34–39
37
Table 1 Characteristic data of the aroma compounds Component
Mi, (g/ mol)
hi (cm3/mol)
Di×109 (m2/s)
Pm,i×1010 (m2/s)
Cit,t = 0 [9] (mg/ kg)
E-2-Hexen-1-ol n-Hexanol E-2-Hexen-1-al Linalool Phenyl acetaldehyde Benzyl alcohol Z-3-Hexen-1-ol Ethanol Water
100.16 102.17 98.14 154.25 120.15 108.14 100.16 46.07 18.02
130.7 144.5 115.5 179.4 116.7 103.0 116.5 59.1 17.6
1.00 0.94 1.08 0.83 1.07 1.15 1.07 1.28 —
2.55 [This study] 9.91 [10] 0.81 [11] 0.61 [11] 0.08 [11] 0.07 [11] 0.44 [11] 0.72 [This study] 0.01 [This study]
0.01 0.02 0.06 0.004 0.003 0.08 0.06 7,800 992,200
analysed are Z-3-Hexen-1-ol, linalool, phenyl acetaldehyde and benzyl alcohol. Previous studies agreed that E-2-Hexen-1-ol is one of the characteristic impact components of the flavour [8] of bilberries. For this multi-component mixture of aroma compounds considered upwards, simulation of the PV performance with the mathematical model proposed is studied. Permeabilities of the aroma compounds through the PDMS membrane are needed to apply the model and their values are extracted from literature, except for E-2-Hexen-1ol which value was obtained experimentally: Pm,hex = 2.55× 10!10 m2/s at 30EC. The values of permeability, the aroma diffusivity coefficient values and the initial feed concentration in the tank together with other component features are compiled in Table 1. As no quantitative aroma analysis was made, the feed concentration of the aroma com-pounds was selected according to those found by Hirvy and Honkanen [9]. Four commercial membranes with different thicknesses (1.48, 1.96, 2.53 and 4.33×10!4 m) were available to study, although to complete the profile evolution a wider range of membrane thicknesses between 1.48×10!5 to 1.48×10!3 m was studied. The operating conditions were considered according to the data used in experiences made in Garcia et al. [9] and they are presented in Table 2.
Table 2 Characteristic data of the laboratory membrane module v (m/s) ¯ ξ Module length, L (m) ID (m) Membrane area, Ap (m2) Shape factor, s
1230.4 2.53×10!4 0.375 5.16×10!4 0.0056 1.343
The trend of the enrichment factor, βi, the permeate concentration, Cip , and the partial flux, Ji, of E-2-Hexen-1-ol (subscript “Hex”) with the thickness of the PDMS membrane is plotted in Fig. 2. A clear increase of the enrichment factor and permeate concentration with thicker membranes is observed while the partial fluxes decrease markedly. An asymptotic profile for all the parameters under study is obtained, meaning that a maximum level of permeate concentration and a minimum value of partial flux of the aroma compound can be reached by increasing the membrane thickness. Similar trend appears for the rest of the aroma compounds. The results of the enrichment factor, permeate concentration and partial fluxes for the different aroma compounds at the beginning of the operation using the membrane module employed during the laboratory experiments in Garcia et al. [9] (1.96×10!4 m thickness), are summarised in Table 3.
38
N. Diban et al. / Desalination 224 (2008) 34–39
Fig. 2. Profile of enrichment factor, βHex; permeate concentration, CpHex; and partial flux, JHex, of E-2-Hexen-1-ol with membrane thickness.
Table 3 Simulation results for the aroma compounds Component
βi
Cip (mg/ kg) Ji (g/m2/h)
E-2-Hexen-1-ol n-Hexanol E-2-Hexen-1-al Linalool Phenyl acetaldehyde Benzyl alcohol Z-3-Hexen-1-ol Ethanol Water
120.6 236.9 46.3 49.3 5.5 4.2 27.4 13.7 0.9
1.21 4.74 2.78 0.20 0.02 0.33 1.64 106,934 893,057
3.78×10!5 14.9×10!5 8.70×10!5 0.62×10!5 0.05×10!5 1.05×10!5 5.15×10!5 2.51 20.9
From comparison of the data in Tables 1 and 3, it is concluded that the distribution of the volatile organic compounds in permeate is somehow different from the feed composition. Nevertheless, a sensorial analysis should be made to measure the product quality achieved. Water content in the permeate is not reduced significantly. Other types of pervaporation membranes should be tested to check membranes with lower water fluxes that allow higher aroma enrichment. 4. Conclusions The following obtained:
conclusions
have
been
1. Higher membrane thicknesses give as a result higher enrichment factors for the aroma compounds under study until an asymptotic value is reached, though partial fluxes decrease considerably. 2. The composition achieved in permeate with the PDMS membrane type under study is different than feed composition. Nevertheless: C Sensorial analysis should be made in order to check the organoleptical quality of the aroma concentrate obtained. C Other membrane types must be tried in order to achieve higher aroma selectivity. 5. Symbols Ap C D F J l L M Pe Pm
— — — — — — — — — —
Effective membrane area, m2 Concentration, m2 Molecular diffusivity, m2/s Flow rate, m3/s Partial flux, g/m2 s Membrane thickness, m Module total length, m Molecular weight, g/mol Peclet number Membrane permeability, m2/s
r R rt s
— — — —
Radial coordinate, m Dimensionless radial coordinate Total membrane fibre radio, m Shape factor
N. Diban et al. / Desalination 224 (2008) 34–39
Shw v ¯ V x z
— — — — —
Sherwood number of the wall Linear velocity, m/s Volume of the feed tank, m3 Molar fraction Axial coordinate
— — — —
Enrichment factor Molar volume, cm3/mol Density, g/m3 Dimensionless axial coordinate
Greek β h ρ ξ
Subscripts i in w
— Aroma component — Current entering the feed tank — Water
Superscripts p t *
— Permeate — Tank — Dimensionless parameter
Acknowledgements Financial support from projects PPQ200300934, CTQ2005-02583/PPQ and CTM200600317/TECNO and FPI grant are gratefully acknowledged.
39
References [1] J. Börjesson, H.O.E. Karlsson and G. Trägårdh, Pervaporation of a model apple juice aroma solution: comparison of membrane performance. J. Membr. Sci., 119 (1996) 229–239. [2] A. Figoli, L. Donato, R. Carnevale, R. Tundis, G.A. Statti, F. Menichini and E. Drioli, Bergamot essential oil extraction by pervaporation. Desalination, 193 (2006) 160–165. [3] C.C. Pereira, J.M. Rufino, A.C. Habert, R. Nobrega, L.M.C. Cabral and C.P. Borges, Membrane for processing tropical fruit juice. Desalination, 148 (2002 57–60. [4] C.C. Pereira, C.P. Ribeiro, R. Nobrega and C.P. Borges, Pervaporative recovery of volatile aroma compounds from fruit juices. J. Membr. Sci., 274 (2005) 1–23. [5] D.M. Barret, L.P. Somogyi and H. Ramaswamy, Processing Fruits: Science and Technology, CRC Press, Orlando, FL, 2005. [6] A. Urtiaga, D. Gorri and I. Ortiz, Mass-transfer modelling in the pervaporation of VOCs from diluted solutions. AIChE J., 48(3) (2002) 572–581. [7] R.H. Perry, Chemical Engineer’s Handbook, McGraw Hill, New York, 2001. [8] E. Von Sydow and K. Anjou, The aroma of bilberries (Vaccinium myrtillis L.). I. Identification of volatile compounds. Lebensmittel Wissenschaft Technologie, 2 (1969) 78–81. [9] T. Hirvi and E. Honkanen, The aroma of blueberries. J. Sci. Food Agricul., 34(9) (1983) 992–998. [10] M. Peng and S.X. Liu, Recovery of aroma compounds from dilute model blueberry solution by pervaporation. Food Eng. Physical Prop., 68(9) (2003) 2706–2710. [11] D.M. Kanani, B.P. Mikhade, P. Balakrishnan, G. Singh and V.G. Pangarkar, Recovery of valuable tea aroma components by pervaporation. Ind. Eng. Chem. Res., 42 (2003) 6924–6932.
Recovery of key components of bilberry aroma using a commercial pervaporation membrane Nazely Diban, Ane Urtiaga, Inmaculada Ortiz* Department of Chemical Engineering, University of Cantabria, Avda. de los Castros, 39005 Santander, Spain Tel. +34 942 201585; Fax: +34 942 201591; email: [email protected] Received 15 January 2007; Accepted 5 April 2007
Abstract Pervaporation, a membrane technology requiring low energetic demands, is considered to substitute the conventional distillation unit employed in the beverage industry in order to recover the key aroma compounds, and that would avoid the damages to the quality of the flavour profile. The analysis of the separation and recovery of some selected aroma compounds belonging to bilberry juice was made by employing a mathematical model previously developed for the pervaporation of VOCs. A PV hollow fiber module provided with a PDMS commercial membrane was considered. For each compound, the characteristic mass transfer parameters are the permeability of the membrane and the diffusion coefficient in the aqueous phase. Enrichment factors over 100 were achieved for the aroma compounds with higher permeabilities. The membrane thickness influence over the enrichment factor and flux of the bilberry impact aroma compound (E-2-Hexen-1-ol) was studied observing higher enrichment factors and lower fluxes with higher membrane thicknesses until an asymptotic value was reached. The predicted permeate composition achieved under the simulated conditions did not keep the same proportion of that in the feed composition. A sensorial analysis should be made to measure the product quality achieved. Keywords: Pervaporation; Bilberry aroma recovery; PDMS membrane; Modelling
1. Introduction Aroma recovery has become a key issue in the beverage industry. The high energetic demands of the distillation unit employed during the manu*Corresponding author.
facture of concentrated juices imply high costs but also damages to the quality of the organoleptical profile of the recovered aroma concentrates. Alternative technologies employing low thermal requirements are considered to substitute the rectification conventional process.
Presented at the 11th Aachen Membrane Colloquium, 28–29 March, 2007, Aachen, Germany. 0011-9164/08/$– See front matter © 2008 Published by Elsevier B.V. doi:10.1016/j.desal.2007.04.076
N. Diban et al. / Desalination 224 (2008) 34–39
Membrane technologies need low energetic requirements and facilitate modular configurations that allow the possibility to easily scale-up the process from pilot to industrial plants. Pervaporation has been widely analysed in previous works [1–3] in order to separate and concentrate valuable aroma compounds from process streams. In the review presented by Pereira et al. [4], it was concluded that the most typical membrane used to perform the pervaporation of aroma compounds was made of polydimethylsiloxane (PDMS) material. The demand for processing bilberries (Vaccinium myrtillus L.) has increased in the last years [5]. Although these fruits can be eaten fresh, are more usually processed to obtain products like jams, fools, juices or pies. Therefore, this wild fruit has become to be cultivated, mainly in North America. Important bilberry producers among the european countries are France, Holland, Germany, Poland and Spain. The aim of this study is to adapt a mathematical model that was previously developed for the pervaporation of VOCs from wastewaters to predict the behaviour of the PV separation of aroma compounds from aqueous solutions. A mixture of 7 volatile compounds, characteristic of the aroma of bilberry juice, has been defined, and the use of PDMS pervaporation membrane with a hollow fiber configuration has been considered. The prediction of the flux of each compound and the concentration factor obtained in permeate are sought. The influence of the membrane thickness on both parameters will be discussed.
Fig. 1. Schematic view of the pervaporation set-up.
A mathematical pervaporation model that was developed and implemented previously [6] has been used. The characteristic mass balance equation for the aqueous feed circulating through the inside of the hollow fiber in dimensionless form appears in Eq. (1):
δCi* δC * 2 δ ⎛ δCi* ⎞ + (1 − R 2 ) i = ⎜R ⎟ δt δξ R δR ⎝ δR ⎠
(1)
This expression is obtained by defining the following dimensionless variables and parameters:
Ci* =
Ci Ci , z =0,t =0
(2)
R=
r rt
(3)
ξ=
zDi 1 = 2 4vrt Pe( z )
(4)
2. Mathematical modelling Pervaporation of a synthetic aromatic feed solution characteristic of bilberry juice was considered. The system used a PDMS membrane with a hollow fiber configuration and operated with recirculation of the aqueous solution. A schematic view of the set-up is plotted in Fig. 1.
35
Shw,i =
2 Pm ,i srt lDi
(5)
where Ci* is the dimensionless concentration of compound i in the fluid circulating inside the pervaporation module, R is the radial coordinate,
36
N. Diban et al. / Desalination 224 (2008) 34–39
at t = 0
Ci*t = 1
(11)
∀t
Ci*,int = Ci*, z = L
(12)
ξ the axial coordinate, and the parameter Shw,i is the Sherwood number of the wall for component i that expresses the ratio of the mass-transfer resistance in the inner fluid to that in the membrane. That means that for Shw 6 4 the membrane resistance is negligible against the resistance exerted by the liquid phase diffusion, while for Shw = 0 is the membrane resistance which determines the mass-transfer phenomena. Di and Pm,i are the diffusivity in the aqueous phase and the membrane permeability of the compound i, respectively. Individual molecular diffusivities of the organic components into a diluted aqueous phase were calculated using the Wilke-Chang correlation [7]. Parameters are described in more detail in the Symbol section. The set of boundary conditions for this equation is the following:
The water flux was calculated by means of Eq. (14) that is considered as a constant value due to the fact that the water molar fraction xw remains practically constant during the whole experiment:
at t = 0 ∀ξ ∀R Ci* = 1
(6)
Jw =
(7)
The enrichment factor βi for the component i, which is time dependent, was defined according to Eq. (15):
∀t at ξ = 0 0 ≤ R ≤1
Ci* = Ci*t
∀t at R = 0 0 ≤ ξ ≤1
1 Pe( L)
(8)
δC =0 δR * i
at R = 1 0 ≤ ξ ≤ 1 Pe( L) −
δCi* Shw,i * = Ci δR 2
The flux of component i was obtained from the mass balance according to Eq. (13) that is a function of time: Ji =
βi =
F ρCi , Z = L ⎛ Ci , z = L ⎞ 2 ⎜⎜ 1 − ⎟⎟ [g/m s] Ap ⎝ Ci , z =0 ⎠
Pm , w l
ρ w xw
Cip Cit,t =0
[g/m 2s]
(13)
(14)
(15)
The mathematical model hereof was implemented in the simulation software Aspen Custom Modeler.
(9)
The mass transfer model is complemented by the mass balance to the feed in the tank:
V
δCi*t = F ( Ci*,int − Ci*t ) [m 3 / s] δt
(10)
With the initial and boundary conditions:
3. Experimental The qualitative analysis of the aroma compounds present in this fruit was performed experimentally by CG-MS [9]. The major chromatographic peaks correspond, in order of appearance, to E-2-Hexen-1-al, 2-butyl-1-octanol, 1-hexanol, E-2-Hexen-1-ol and 1-hexadecanol. Other minor components present in the sample
N. Diban et al. / Desalination 224 (2008) 34–39
37
Table 1 Characteristic data of the aroma compounds Component
Mi, (g/ mol)
hi (cm3/mol)
Di×109 (m2/s)
Pm,i×1010 (m2/s)
Cit,t = 0 [9] (mg/ kg)
E-2-Hexen-1-ol n-Hexanol E-2-Hexen-1-al Linalool Phenyl acetaldehyde Benzyl alcohol Z-3-Hexen-1-ol Ethanol Water
100.16 102.17 98.14 154.25 120.15 108.14 100.16 46.07 18.02
130.7 144.5 115.5 179.4 116.7 103.0 116.5 59.1 17.6
1.00 0.94 1.08 0.83 1.07 1.15 1.07 1.28 —
2.55 [This study] 9.91 [10] 0.81 [11] 0.61 [11] 0.08 [11] 0.07 [11] 0.44 [11] 0.72 [This study] 0.01 [This study]
0.01 0.02 0.06 0.004 0.003 0.08 0.06 7,800 992,200
analysed are Z-3-Hexen-1-ol, linalool, phenyl acetaldehyde and benzyl alcohol. Previous studies agreed that E-2-Hexen-1-ol is one of the characteristic impact components of the flavour [8] of bilberries. For this multi-component mixture of aroma compounds considered upwards, simulation of the PV performance with the mathematical model proposed is studied. Permeabilities of the aroma compounds through the PDMS membrane are needed to apply the model and their values are extracted from literature, except for E-2-Hexen-1ol which value was obtained experimentally: Pm,hex = 2.55× 10!10 m2/s at 30EC. The values of permeability, the aroma diffusivity coefficient values and the initial feed concentration in the tank together with other component features are compiled in Table 1. As no quantitative aroma analysis was made, the feed concentration of the aroma com-pounds was selected according to those found by Hirvy and Honkanen [9]. Four commercial membranes with different thicknesses (1.48, 1.96, 2.53 and 4.33×10!4 m) were available to study, although to complete the profile evolution a wider range of membrane thicknesses between 1.48×10!5 to 1.48×10!3 m was studied. The operating conditions were considered according to the data used in experiences made in Garcia et al. [9] and they are presented in Table 2.
Table 2 Characteristic data of the laboratory membrane module v (m/s) ¯ ξ Module length, L (m) ID (m) Membrane area, Ap (m2) Shape factor, s
1230.4 2.53×10!4 0.375 5.16×10!4 0.0056 1.343
The trend of the enrichment factor, βi, the permeate concentration, Cip , and the partial flux, Ji, of E-2-Hexen-1-ol (subscript “Hex”) with the thickness of the PDMS membrane is plotted in Fig. 2. A clear increase of the enrichment factor and permeate concentration with thicker membranes is observed while the partial fluxes decrease markedly. An asymptotic profile for all the parameters under study is obtained, meaning that a maximum level of permeate concentration and a minimum value of partial flux of the aroma compound can be reached by increasing the membrane thickness. Similar trend appears for the rest of the aroma compounds. The results of the enrichment factor, permeate concentration and partial fluxes for the different aroma compounds at the beginning of the operation using the membrane module employed during the laboratory experiments in Garcia et al. [9] (1.96×10!4 m thickness), are summarised in Table 3.
38
N. Diban et al. / Desalination 224 (2008) 34–39
Fig. 2. Profile of enrichment factor, βHex; permeate concentration, CpHex; and partial flux, JHex, of E-2-Hexen-1-ol with membrane thickness.
Table 3 Simulation results for the aroma compounds Component
βi
Cip (mg/ kg) Ji (g/m2/h)
E-2-Hexen-1-ol n-Hexanol E-2-Hexen-1-al Linalool Phenyl acetaldehyde Benzyl alcohol Z-3-Hexen-1-ol Ethanol Water
120.6 236.9 46.3 49.3 5.5 4.2 27.4 13.7 0.9
1.21 4.74 2.78 0.20 0.02 0.33 1.64 106,934 893,057
3.78×10!5 14.9×10!5 8.70×10!5 0.62×10!5 0.05×10!5 1.05×10!5 5.15×10!5 2.51 20.9
From comparison of the data in Tables 1 and 3, it is concluded that the distribution of the volatile organic compounds in permeate is somehow different from the feed composition. Nevertheless, a sensorial analysis should be made to measure the product quality achieved. Water content in the permeate is not reduced significantly. Other types of pervaporation membranes should be tested to check membranes with lower water fluxes that allow higher aroma enrichment. 4. Conclusions The following obtained:
conclusions
have
been
1. Higher membrane thicknesses give as a result higher enrichment factors for the aroma compounds under study until an asymptotic value is reached, though partial fluxes decrease considerably. 2. The composition achieved in permeate with the PDMS membrane type under study is different than feed composition. Nevertheless: C Sensorial analysis should be made in order to check the organoleptical quality of the aroma concentrate obtained. C Other membrane types must be tried in order to achieve higher aroma selectivity. 5. Symbols Ap C D F J l L M Pe Pm
— — — — — — — — — —
Effective membrane area, m2 Concentration, m2 Molecular diffusivity, m2/s Flow rate, m3/s Partial flux, g/m2 s Membrane thickness, m Module total length, m Molecular weight, g/mol Peclet number Membrane permeability, m2/s
r R rt s
— — — —
Radial coordinate, m Dimensionless radial coordinate Total membrane fibre radio, m Shape factor
N. Diban et al. / Desalination 224 (2008) 34–39
Shw v ¯ V x z
— — — — —
Sherwood number of the wall Linear velocity, m/s Volume of the feed tank, m3 Molar fraction Axial coordinate
— — — —
Enrichment factor Molar volume, cm3/mol Density, g/m3 Dimensionless axial coordinate
Greek β h ρ ξ
Subscripts i in w
— Aroma component — Current entering the feed tank — Water
Superscripts p t *
— Permeate — Tank — Dimensionless parameter
Acknowledgements Financial support from projects PPQ200300934, CTQ2005-02583/PPQ and CTM200600317/TECNO and FPI grant are gratefully acknowledged.
39
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