Pervaporation of alcoholic beverages—the coupling effects between ethanol and aroma compounds

Pervaporation of alcoholic beverages—the coupling effects between ethanol and aroma compounds

Journal of Membrane Science 264 (2005) 129–136 Pervaporation of alcoholic beverages—the coupling effects between ethanol and aroma compounds Shujuan ...

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Journal of Membrane Science 264 (2005) 129–136

Pervaporation of alcoholic beverages—the coupling effects between ethanol and aroma compounds Shujuan Tan a , Lei Li a , Zeyi Xiao b , Youting Wu a , Zhibing Zhang a,∗ a

Department of Chemical Engineering, School of Chemistry and Chemical Engineering, Nanjing University, Hankou Road 22#, Nanjing, Jiangsu 210093, PR China b Department of Chemical Machinery, Sichuan University, Chengdu 610065, PR China Received 2 December 2004; received in revised form 22 April 2005; accepted 23 April 2005 Available online 1 June 2005

Abstract In pervaporation (PV) of alcoholic beverages, complicated interactions between ethanol and aroma compounds often exist and affect their PV performance. In this paper, a series of model solutions containing ethanol and six typical aroma compounds in alcoholic beverages were experimented. The permeability coefficient was introduced to discuss the coupling effects. The results showed that the solubility thermodynamics, with the feed solution activity coefficient had a dominant effect on the permeability rather than diffusion factor. The presence of aroma compounds decreased the permeability coefficient and separation factor of ethanol from those in ethanol–water binary solutions, while affected little on the fluxes. The effect of ethanol feed concentration on mass transfer of each compound was much related to the solubility properties of the compound in ethanol and water. Little interactions exist in PV process of diluted solution containing these aroma compounds within 1000 ppm. © 2005 Elsevier B.V. All rights reserved. Keywords: Pervaporation; Permeability coefficient; Alcoholic beverage; Multicomponent mass transfer; Coupling effect

1. Introduction During the last years, there has been an increasing market demand for alcoholic beverages such as wine and beer with reduced alcohol content, due to the public’s strong healthcare consciousness and the social initiatives against the abuse of alcohol. However, in many cases, the process of dealcoholization makes the final products subject to the lost of some original aroma, which is so important for their quality to affect and even determine their acceptance by most customers. The aroma of alcoholic beverages consists of a large number of volatile organic compounds (VOCs) at ppm concentrations. To recover these aroma compounds and return them to the final products is therefore of great interest and can be accomplished very effectively with PV [1–7]. ∗ Corresponding author. Tel.: +86 25 83593772/83596665; fax: +86 25 83593772/83317761. E-mail address: [email protected] (Z. Zhang).

0376-7388/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2005.04.028

Pervaporation is a separation process based on selective transport inside a dense membrane associated with vaporization of the permeant. It can be operated at ambient feed pressure and at low temperature, without any additional chemicals for assistance of its process. Therefore, PV is potential to be applied in food industry [8], such as aroma compounds recovery or concentration in fruit juices [2,3], dealcoholization of wine or beer [4], and extraction of edible oil [5]. In general, complicated interactions often take place among the permeates and the membrane during PV process, especially in multicomponent mixtures. The interactions among the permeates called “coupling effects” often change absorption and diffusion rates of solutes through the membrane. The interactions between solute and the membrane, which are called “plasticizing effects”, usually result in swelling of the membrane. Some models used to explain the interactions have been established based on pure component transport [9], Flory–Huggins theory [10], and thermodynamics of irreversible processes [11]. Due to the difficulty in the

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measurement of the interactions quantitatively, most investigations have been focused on the simplest coupled transport of binary or ternary mixtures [12,13]. Only a few PV experiments have been carried out for complicated multicomponent mixtures [14,15]. In this paper, a series of aqueous solutions containing ethanol and six typical aroma compounds in alcoholic beverages were experimented. The objectives of this study were to investigate how the presence of ethanol at different feed concentration affected the mass transfer of aroma compounds, and how the presence of aroma compounds affected the mass transfer of ethanol and water. 1.1. Theory Based on solution–diffusion mechanism, the flux of the component i through membrane (from membrane surface to permeate side) can be expressed as: Ji = kov,i (pf,i − pp,i )

(1)

where kov,i is the overall mass transfer coefficient, Pf,i and Pp,i the upstream and downstream partial pressure, respectively. According to the resistance-in-series theory, the overall mass transfer coefficient can be written as: 1 kov,i

=

1 kbl,i

+

1 km,i

1 + kp,i

(3)

Pi Sm,i Dm,i = lm lm

(4)

where Pi is the permeability coefficient, lm the thickness of the active layer of the membrane, Sm,i and Dm,i are the solubility coefficient and diffusion coefficient of the component in the membrane [6]. am,i γm,i xm,i = af,i γf,i xf,i

(5)

Dm,i = D0,i exp(τam,i )

(6)

Sm,i =

where a is the activity, D0,i the diffusion coefficient at infinite dilution and τ generally termed the plasticizing coefficient that describes the interaction between the permeating species and the membrane.

(7)

where p0i is the equilibrium vapor pressure of the pure compound, γ f,i the activity coefficient and xf,i the mole fraction in the feed solution. Therefore, the flux of component i can be expressed as: Ji =

Pi p0i γf,i xf,i lm

(8)

Thus, if Pi in multicomponent solution differs from the value in binary solution (the component i dissolved in the same solvent), it is known that the difference is due to changed interactions within the membrane or at its surface. In ideal situation that have no thermodynamic interactions and coupling effects existing between the compounds and the membrane, Pi is independent of the concentration and other components, and equal to Pi0 , the permeability coefficient of the pure component i [13]. The separation factor αi/w in multicomponent mixtures is defined as follows:

(2)

The mass transfer coefficient of the membrane includes sorption to, diffusion through and desorption from the membrane. It could be expressed as: km,i =

pf,i = p0i γf,i xf,i

αi/w =

where kbl,i , km,i and kp,i are the mass transfer coefficients for component i of the liquid feed boundary layer, the membrane and the permeate boundary layer, respectively. It has proved in our previous work that as for the same membrane module, kov,i is very close to km,i when the feed flow is over 120 L h−1 [16]. Considering that PV is often operated under very low downstream pressure, Eq. (1) can be simplified as follows when feed flow is over 120 L h−1 : Ji = km,i pf,i

According to Raoult’s law, the upstream partial pressure of the component can be expressed as:

yi /yw Ji /Jw = xi /xw xi /xw

(9)

where x and y is the molar fraction of species in the feed and permeate, respectively. Subscript “i” denotes an organic compound and “w” as water. Combining Eq. (8) with Eq. (9), one can get: αi/w =

Pi p0i γf,i Pw p0w γf,w

(10)

When permeate fluxes and feed concentrations are expressed in terms of weight, αi/w have the same forms as in Eq. (9), while Eq. (8) should be rewritten as: JiM =

¯ Pi p0i γf,i M Cf,i lm

(11)

¯ is the mean molecular weight of the solution. where M

2. Experiments 2.1. Feed solutions A series of solutions consisting of ethanol and six typical volatile aroma compounds in alcoholic beverages, as seen in Table 1, were prepared. The feed concentration ranges of ethanol and aroma compounds were determined according to the usual compositions of various alcoholic beverages. The aroma compounds were obtained as analytical reagents from Shanghai Chemical Reagent Company, China. Distilled and deionized water was used as solvent.

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Table 1 The properties of the aroma compounds used in the model solutions weighta

Formula Boiling point (◦ C)a p0i b (Pa) γ i,∞ c Solubility (%, m/m)a Solubility (%, m/m)a a b c

Ethyl acetate

Methanol

n-Propanol

i-Butanol

n-Butanol

i-Amyl alcohol

88.11 77 16073 153 9.7 Miscible

32.04 64.7 22121 2.38 Miscible Miscible

60.1 97.2 3832 23.25 Miscible Miscible

74.12 108 2035 55.5 10 Miscible

74.12 117.7 1358 55.5 7.4 Miscible

88.15 131 633 153 2 Miscible

The data is obtained from literature [18]. The vapor pressure of the pure components are calculated by the Antoine equation at 30 ◦ C [19]. The activity coefficient at infinite dilution in water is estimated using a UNIFAC method [20].

2.2. Pervaporation membrane The PV membrane used was a PDMS-CA membrane with the effective mass transfer area of 0.024 m2 and skin layer thickness of 8 ␮m. ␣,␻-Dihydroxypolydimethylsiloxane (PDMS) with an average molecular weight of 5000 was supplied by Shanghai Synthetic Resin Company, China. Cellulose acetate (CA) micro filtration membrane, with an average pore size of 0.5 ␮m was supplied by Shanghai Filter Company, China, and used as support layer. The preparation method of the PDMS-CA membrane has been described in our previous work in detail [17]. 2.3. Experimental procedure PV experiments were performed on a continuous apparatus as reported by Li et al. [16]. In the experiments, the feed temperature was maintained at 30 ◦ C and the downstream pressure at 266 Pa. The feed flow were kept 300 L h−1 (Reynolds number about 5000). When operating condition changed, the system was allowed to equilibrate for 3 h before any measurements were made. The experiment was repeated two times for each condition with an interval of 1 h. Only 0.5 ml permeate was used to analyze its composition and the others were added back into the reservoir containing 5 L solution to keep the feed composition unchanged at most. In addition, each time the composition of feed solution and permeate was measured immediately at the same time as experiments. The feed composition in the same condition varied within 8%. The total flux was determined gravimetrically with an experimental error of 1–2%. 2.4. Analytical methods A densimeter (DMA5000, Anton Paar, Austria) was used to measure the liquid densities (the accuracy of 0.000001 g ml−1 ). The concentrations of aroma compounds in both the feed and the permeate were analyzed with gas chromatograph (Agilent 6890N, FID), which was equipped with FFAP capillary column (Dikma, 30 m × 0.53 mm × 1.00 ␮m). The temperature was programmed from 45 to 50 ◦ C at a rate of 1.5 ◦ C/min, and then from 50 to180 ◦ C at a rate of 15 ◦ C/min, with an initial hold of 1 min and a final hold of 3 min. The

injector and detector were both set to 200 ◦ C. The carrier gas was nitrogen at a flow rate of 5.1 ml/min, and the sample was split by 1:5 in the injector. n-Butyl acetate was used as internal standard. Each concentration determination was based on three or four different injections.

3. Results and discussion 3.1. Effects of aroma compounds on PV performance of ethanol and water To explore whether and how the presence of aroma compounds in alcoholic beverages affects the mass transfer of ethanol and water, some experiments were carried on with a series of ethanol–water solutions with and without aroma compounds that were listed in Table 1. The ethanol concentration in this study was determined in the range of 0–40% (m/m) according to its usual concentration in most beers, wines, and distilled spirits. The weight fraction of each aroma compounds present in ethanol–water solution was around 100 ppm. As can be seen from Fig. 1, linear relationships were established between flux and ethanol concentration in the feed, both for ethanol and water. The presence of these aroma compounds in the feed seemed to affect little on the fluxes. Karlsson and Tragardh [14] drew the same conclusions when they studied the effect of some aldehydes at ppm level on the PV performance of ethanol–water mixtures with ethanol concentration in the range of 0–12% (v/v). Fig. 2 showed the dependence of the separation factors of ethanol on its concentration in the feed. The decreased separation factor clearly indicated that the hydrophobic membrane was plasticized by ethanol to some extent [21]. Moreover, with increasing ethanol concentration in the feed, the membrane swelling was more serious relatively. In multicomponent mixtures, the separation factors of ethanol were a little lower to those in ethanol–water binary mixtures. In addition, the difference was greater with decreasing of ethanol concentration in the feed. It indicated that coupling effects existed between ethanol and aroma compounds. Permeability coefficient could well describe the coupling effects on the permeability of ethanol. It was calculated

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Fig. 1. Plot of partial flux vs. feed composition of ethanol: (♦) ethanol without aroma compounds; () water without aroma copounds; ( ) ethanol with aroma compounds; and (䊉) water with aroma compounds.

according to Eq. (11) and showed in Fig. 3. As can be seen that with ethanol feed concentration increasing from 5 to 40% (m/m), Pi of ethanol increased significantly from 14.7 × 10−8 to 46.8 × 10−8 mol m−1 h−1 Pa−1 while that of water changed little around 47.2 × 10−8 mol m−1 h−1 Pa−1 . The activity coefficient of ethanol and water in binary mixture was estimated by UNIFAC method and was showed in Fig. 4. One can see that the coupling effect between ethanol and water made the activity coefficient of ethanol decreased

Fig. 2. Plot of seperation factor vs. feed composition of ethanol: (♦) without aroma compounds; () with aroma compounds.

Fig. 3. Plot of permeability coefficient vs. feed composition of ethanol: (♦) ethanol without aroma compounds; () water without aroma compounds; () ethanol with aroma compounds; and () water with aroma compounds.

from 8.0 to 2.5 and that of water slightly increased from 1.0 to 1.1 with ethanol concentration from 5 to 40% (m/m). According to Eq. (4), the permeability coefficient at ethanol concentration of 5% (m/m) and 40% (m/m) can be written as follows: Pi5 S 5 D5 γ 40 D5 = 40i i40 = i5 40i 40 Pi Si Di γi Di

Fig. 4. The activity coefficient vs. ethanol feed concentration.

(12)

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Therefore, we can get: De5 = 1.01 De40

(13)

5 Dw = 0.92 40 Dw

(14)

The little change of diffusion coefficient strongly indicated that with ethanol concentration from 5 to 40% (m/m), the plasticizing effect was not significant while solubility thermodynamics, with the feed solution activity coefficient apparently having a dominant effect on the permeability. In multicomponent mixtures, the activity coefficient of ethanol and water is difficult to estimate. In this study, it was supposed as the same value as that in binary solutions. In multicomponent solutions, the permeability coefficient of ethanol was a little lower than that in ethanol–water binary mixtures, while that of water changed little. That means coupling effect between ethanol and aroma compounds weakened the permeability of ethanol. As can be seen in Fig. 6, the separation factors of most aroma compounds were much higher than that of ethanol, which indicated that these aroma compounds were more preferable than ethanol to be absorbed by the membrane. As a result, aroma compounds preferentially occupied some of the hydrophobic groups of the membrane and relatively reduced the affinity of ethanol towards the polymer. However, the coupling effects caused by aroma compounds on permeability of ethanol were not significant because of their low concentrations.

Fig. 5. Plot of partial flux vs. feed composition of ethanol: () ethyl acetate; () methanol; () n-propanol; () i-butanol; ( ) n-butanol; and (♦) i-amyl alcohol.

3.2. Effects of ethanol on PV performance of aroma compounds

is the partitioning of the permeate between the membrane and the liquid feed. One can see from Table 1, methanol and n-propanol can be fully miscible with both ethanol and water. The change of ethanol concentration in the feed causes little effect on the partitioning of these compounds. However, for ethyl acetate, i-butanol, n-butanol and i-amyl alcohol, the solubility in water are all no more than 10 in 100 parts solvent, while they all can be fully miscible with ethanol. As a result, the partitioning of these compounds between the

The partial flux and separation factor of aroma compounds at different ethanol feed concentration was showed in Figs. 5 and 6. One can see that the presence of ethanol had different effect on the PV performance of these aroma components. For methanol and n-propanol, both the flux and separation factor are nearly independent of the ethanol feed concentration. However, those of ethyl acetate, i-butanol, nbutanol and i-amyl alcohol decreased with the increasing ethanol concentration. Fig. 7 showed the permeability coefficient of aroma compounds changing with ethanol concentration. Since the feed concentration of these aroma compounds is very low, the activity coefficient of each compound was supposed as the same value at infinite dilution in water, which is estimated using a UNIFAC method. As can be seen in Fig. 7, the coupling effect between ethanol and these aroma compounds affected differently on the permeability coefficient of each component. The permeability coefficient of methanol and npropanol did not changed with ethanol concentration, while that of ethyl acetate, i-butanol, n-butanol and i-amyl alcohol decreased. These phenomena could be explained by different solubilities of aroma compounds in water and ethanol. According to solution–diffusion theory, the absorption step

Fig. 6. plot of seperation factor vs. feed composition of ethanol: () ethyl acetate; () methanol; () n-propanol; () i-butanol; ( ) n-butanol; and (♦) i-amyl alcohol.

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Fig. 7. Plot of permeability coefficient vs. feed composition: () ethyl acetate; () methanol; () n-propanol; () butanol; ( ) n-butanol; and (♦) i-amyl alcohol.

membrane and the liquid feed decreased with the increasing ethanol concentration in the feed. Accordingly, the solubility coefficient and the permeability coefficient decreased, thereby reducing the flux and separation factor. Furthermore, the concentration-independent permeability coefficient of methanol and n-propanol confirmed that swelling effect was not a dominant factor. Based on Eqs. (10) and (11), the effect of permeability coefficient on the flux and separation factor is directly proportional to the product of the equilibrium vapor pressure of the pure compound, the activity coefficient and molecular weight. Therefore, the flux and separation factor of ethyl acetate changed much with ethanol feed concentration while the permeability coefficient changed slightly.

Fig. 8. Plot of partial flux vs. feed composition: () ethyl acetate; () methanol; () n-propanol; () i-butanol; ( ) n-butanol; and (♦) i-amyl alcohol.

a diluted solution with aroma compounds in the range of 10–1000 ppm could be thought as an ideal solution, in which little or no interaction takes place between aroma compounds in the membrane during PV process. Therefore, the permeability coefficient showed in Fig. 10 is equal to that of pure component, which is usually difficult to measure because of serious swelling of the membrane.

3.3. Effects of aroma compounds concentration in the feed on their PV performances The aroma compounds experimented in the present paper are very important volatile organic compounds (VOCs) in most alcoholic beverages with a relatively large content and contribute much to the flavor. The concentration of these aroma compounds in most alcoholic beverages is from several ppm (m/m) to several hundreds ppm (m/m). To investigate the effect of their feed concentrations, a series of diluted solutions containing these aroma compounds in the range of 10–1000 ppm were pervaporated. As can be seen from Fig. 8, the partial fluxes of all aroma compounds displayed a good linear response to their feed concentrations. In Figs. 9 and 10, the separation factors and the permeability coefficients were almost constant and independent of aroma concentrations in the feed. That means

Fig. 9. Plot of seperation factor vs. feed composition: () ethyl acetate; () methanol; () n-propanol; () i-butanol; ( ) n-butanol; and (♦) i-amyl aocohol.

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Nomenclature a C D J k l ¯ M p P S x y

activity weight fraction (m/m) diffusion coefficient (m h−1 ) mole flux (mol m−2 h−1 ) mass transfer coefficient (mol m−2 h−1 Pa−1 ) thickness of the membrane (m) mean molecular weight (g mol−1 ) vapor pressure (Pa) permeability coefficient (mol m−1 h−1 Pa−1 ) solubility coefficient (mol m−2 Pa−1 ) mole fraction in the feed (mol/mol) mole fraction in the permeate (mol/mol)

Greek letters α separation factor γ activity coefficient τ plasticizing coefficient Fig. 10. Plot of permeability coefficient vs. feed: () ethyl acetate; () methanol; () n-propanol; () i-butanol; ( ) n-butanol; and (♦) i-amyl alcohol.

4. Conclusions In the present paper, the permeability coefficient was used to investigate the coupling effect between ethanol and aroma compounds on their permeability. The solubility thermodynamics, with the feed solution activity coefficient had a dominant effect on the permeability rather than diffusion thermodynamics. In the system investigated, the fluxes of aroma compounds, ethanol and water had simple linear responses to their concentrations in the feed. The presence of aroma compounds decreased the permeability coefficient of ethanol to some extent, while it affected little on that of water. The ethanol concentration in the feed affected differently the mass transfer of each compound. It was much related to the solubility properties of the compound in ethanol and water. A diluted solution with aroma compounds in the range of 10–1000 ppm was very similar to an ideal solution. The permeability coefficient of a pure component could be obtained from experiment of its diluted solution. Acknowledgement The present work was supported financially by China National “985” Project (No. 985XK-015).

References [1] J. Olsson, G. Tragardh, Influence of feed flow velocity on pervaporation aroma recovery from a model solution of apple juice aroma compounds, J. Food Eng. 39 (1999) 107. [2] K.W. Boddeker, I.L. Gatfield, J. Jahnig, C. Schorm, Pervaporation at the vapor pressure: vanillin, J. Membr. Sci. 137 (1997) 155.

Subscripts e ethanol bl feed boundary layer f feed or upstream i component m membrane ov overall p permeate or downstream w water Superscripts 0 pure component I ideal situation

[3] N. Rajagopalan, M. Cheryan, Pervaporation of grape juice aroma, J. Membr. Sci. 104 (1995) 243. [4] S.J. Tan, Z.Y. Xiao, L. Li, F.W. Wu, Z.H. Xu, Z.B. Zhang, Experimental research on dealcoholization of wine by pervaporation, Fine Chem. 20 (2003) 69. [5] S.S. Koseoglu, E. Hernandez, V. Shah, D. Tuohey, Opportunities for pervaporation: processing edible oils and fats, in: Proceeding of the Seventh International Conference on Pervaporation Processes in the Chemical Industry, Bakish Materials Corporation, Englewood, NJ, USA, p. 263. [6] H.O.E. Karlsson, G. Tragardh, Pervaporation of dilute organicwater mixtures. A literature review on modeling studies and applications to aroma compound recovery, J. Membr. Sci. 76 (1993) 121. [7] H.O.E. Karlsson, S. Loureiro, G. Tragardh, Aroma compounds recovery with pervaporation–temperature effect during pervaporation of a Muscat wine, J. Food Eng. 26 (1995) 177. [8] H.O.E. Karlsson, G. Tragardh, Applications of pervaporation in food processing, Trends Food Sci. Tech. 7 (1996) 78. [9] R.Y.M. Huang, C.K. Yeom, Pervaporation reparation of aqueous mixtures using crosslinked poly (vinyl alcohol) (PVA). II. Permeation of ethanol–water mixtures, J. Membr. Sci. 51 (1990) 273.

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[10] M.H.V. Mulder, T. Franken, C.A. Molders, Preferential sorption versus preferential permeability in pervaporation, J. Membr. Sci. 22 (1985) 155. [11] O. Kedem, The role of coupling in pervaporation, J. Membr. Sci. 47 (1989) 277. [12] J.P. Brun, C. Larchet, R. Melet, G. Bulbestre, Modeling of the pervaporation of binary mixtures through moderately swelling, non-reacting membranes, J. Membr. Sci. 23 (1985) 257. [13] E. Drioli, S. Zhang, A. Basile, On the coupling effect in pervaporation, J. Membr. Sci. 81 (1993) 43. [14] H.O.E. Karlsson, G. Tragardh, Aroma compound recovery with pervaporation—the effect of high ethanol concentrations, J. Membr. Sci. 91 (1994) 189. [15] I. Habib, Shaban, Separation of binary, ternary and multicomponent organic/water mixtures, Gas Sep. Purif. 9 (1995) 75.

[16] L. Li, Z.Y. Xiao, Z.B. Zhang, S.J. Tan, Mass transfer kinetics of pervaporation by using a composite silicone rubber membrane: (I) the convective transfer on membrane surface, J. Chem. Ind. Eng. 53 (11) (2002) 1169. [17] L. Li, Z.Y. Xiao, S.J. Tan, L. Pu, Z.B. Zhang, Composite PDMS membrane with high flux for the separation of organics from water by pervaporation, J. Membr. Sci. 243 (2004) 177. [18] J.A. Dean, Lange’s Handbook of Chemistry, McGraw-Hill Book Co., 1999. [19] C.L. Yaws, Chemical Properties Handbook, World Book Press Co., Beijing, 1999. [20] R.C. Reid, J.M. Prausnitz, B.E. Poling, The Properties of Gases and Liquids, fourth ed., McGraw-Hill, New York, USA, 1987. [21] D.R. Seok, S.G. Kang, S.T. Hwang, Use of pervaporation for separating azeotropic mixtures using two different hollow fiber membranes, J. Membr. Sci. 33 (1987) 71.