salt concentration. Department of Chemical Engineering, University of Burgos, Burgos. Spain

Transcription

1 Actvty coeffcents at nfnte dluton of aroma compounds n water: effect of temperature and salt concentraton. Rodrgo Martínez, María Teresa Sanz, Sagraro Beltrán and Elena Corcuera Department of Chemcal Engneerng, Unversty of Burgos, 0001 Burgos. Span ABSTRACT Actvty coeffcents at nfnte dluton n water have been determned for some aroma compounds detected n brown crab lqud effluent produced durng bolng (1-octen-3-ol, 1- penten-3-ol, 3-methylbutanal, hexanal, benzaldehyde, 2,3-pentadone and ethyl acetate) by usng the headspace gas chromatography technque (HSGC). Expermental data have been obtaned over the temperature range of 40ºC to 50ºC. In ths work, actvty coeffcents at nfnte dluton for dfferent type of systems have been consdered: one component solute + water and multcomponent solute + water. No sgnfcant dfferences were observed between actvty coeffcents obtaned n these two types of systems. Addtonally the effect of salt concentraton at 40ºC has been studed by varyng salt concentraton from 0 to 1.71 Correspondng author. Tel.: Fax: E-mal address tersanz@ubu.es 2

2 mol/kg. Expermental data were ftted as a functon of salt concentraton by usng the Setschenow equaton obtanng the saltng out coeffcent. KEYWORDS: aroma compounds, actvty coeffcents at nfnte dluton, salt effect, temperature effect. 1. INTRODUCTION Knowledge of the thermodynamc behavor of dlute aqueous solutons s necessary for a correct desgn of processes to separate dlute compounds such as aroma recovery from food streams. In prevous work (1), t has been reported that the pervaporaton technque s a promsng alternatve to recover the volatle fracton from brown crab processng effluent. Pervaporaton s a membrane process that has been consdered n the last years as an alternatve to conventonal methods to recover aroma compounds, snce the addton of chemcal solvents s avoded (2). Based on the soluton/dffuson model the flux of component through the membrane s proportonal to the dfference n partal vapor s pressure at both sdes of the membrane ( x γ p y p ) p. In pervaporaton of dlute aqueous solutons, actvty coeffcents at nfnte dluton n water ( γ ) are used as feed-sde actvty coeffcents due to the very low concentratons of aroma compounds n the feed. Actvty coeffcents at nfnte dluton provde an nsght nto the chemcal and physcal nteractons between water (solvent) and aroma compounds (solute molecules) n the absence of solute-solute nteractons. The affnty of a solvent towards a volatle compound can be expressed usng Henry s law: H p = lm [1] x 0 x 3

3 where p s the partal pressure of the volatle component and x ts mole fracton at nfnte dluton. The actvty coeffcent at nfnte dluton of the volatle compound n the solvent s determned by takng nto account the vapor pressure of the volatle compound at the same temperature, s p (3): γ H = p s [2] Both, Henry constant and the solvent (water) (4). γ, allow the evaluaton of affnty of the aroma compound and Dfferent technques have been developed to determne γ n water. Sherman et al. (5) emphasze that each method s most sutable n a certan range of relatve volatlty at nfnte dluton, α, w, defned as: α,w = p p s s w γ [3] In ths study the headspace gas chromatography has been used to determne γ. The headspace gas chromatography s useful not only for analytcal purposes (6), but also provdes a valuable tool to obtan thermodynamcally relable data (7). Statc headspace methods are based on measurements of gas-lqud thermodynamc equlbrum. Ths technque has a range of applcablty from low relatve volatlty systems (around 0.5) to systems of relatve volatltes up to 1000 (5). In ths work, the actvty coeffcents at nfnte dluton of seven volatle compounds found n the brown crab processng effluent (1-octen-3-ol, 1-penten-3-ol, 3-methylbutanal, hexanal, benzaldehyde, 2,3-pentadone and ethyl acetate) have been determned by usng 4

4 the headspace gas chromatography technque (HS-GC). These coeffcents were determned at three dfferent temperatures over the range 40ºC to 50ºC. In food systems the volatlty of aroma compounds s dependent on the presence of nonvolatle components such as sugars, salts, lpds and other macromolecular compounds (8). In the partcular case of aroma recovery from brown crab effluent, the effect of the presence of salts must be consdered. Therefore, the effect of sodum chlorde on actvty coeffcents at nfnte dluton has been analyzed by varyng the salt concentraton n the range 0 to 1.7 mol/kg at 40ºC, snce t s well known that the presence of salts and other electrolytes may ncrease or decrease the value of γ (). 2. EXPERIMENTAL SECTION 2.1. Materals The volatle compounds, whose actvty coeffcents at nfnte dluton have been determned n thes work, belong to dfferent chemcal classes: alcohols such as 1-octen-3- ol (Sgma Aldrch, 8 %), 1-penten-3-ol (Sgma Aldrch, %); aldehydes such as 3- methylbutanal (Sgma Aldrch, 7 %), hexanal (Sgma Aldrch, 8 %), benzaldehyde (Sgma Aldrch, >= %); ketones such as 2,3-pentadone (Sgma Aldrch, 8 %) and esters as ethyl acetate (Sgma Aldrch, HPLC grade). The volatle compounds were used as receved. Mll-Q-Mllpore ultra pure water was used n ths work. Water was degassed by ultrasound (Selecta, Ultrasons-H). Sodum chlorde was suppled by Sgma (.5 % purty) Systems 5

5 Actvty coeffcents at nfnte dluton, γ, have been determned for systems of varous types. Frst, γ has been obtaned for bnary systems consstng of a volatle compound and water at three dfferent temperatures, 40ºC, 45ºC and 50ºC. The headspace oven could not be regulated below 40ºC and ths value was the lowest temperature studed n ths work. Subsequently, γ has been obtaned for a system formed by all the aroma compounds prevously consdered and water at 40ºC. Fnally the effect of salt on γ has been determned for a multcomponent solute and water system at 40 ºC n the range 0 to 1.7 mol/kg of salt concentraton Apparatus and procedure Actvty coeffcents at nfnte dluton were determned by headspace gas chromatography (HS-GC). The HS-GC conssts of a gas chromatograph (Hewlett Packard GC 680) and a headspace sampler (Hewlett Packard 764E). To determne γ, glass vals (ca. 20 cm 3 ) were flled gravmetrcally wth dfferent mxtures of the correspondng system varyng the mole fracton of the volatle compound n the lqud phase. To obtan one actvty coeffcent at nfnte dluton, seven measurements were performed to determne the vapor solute partal pressure as a functon of the solute concentraton. The nterval of nfnte dluton regon cannot be evaluated a pror. For hghly assocated compounds, ths regon s smaller than 10-4 mole fracton and can be as small as 10-6 or 10-7 (10). In our case, mole fracton of the aroma compounds was kept below n all cases. In ths concentraton range results are obtaned under Henry s law condtons (see secton 3.1). Solutons were prepared from a concentrated soluton (approxmately 1000 ppm) n water or n a salty water mxture. Ths mxture was stored at 6

6 4ºC to avod losses of volatle compounds and dluted for the dfferent solutons concentratons. The vals were flled about half way (ca. 10 cm 3 ) and mmedately sealed properly wth a pressure-tght rubber septum and a specal alumnum ld to ensure that the headspace gases do not escape. Equlbrum between gas and lqud phases s reached n the headspace oven. After reachng equlbrum, an alquot of the vapor phase s wthdrawn and transported and analyzed n the GC. The GC column was a 007 FFAP 25 m 0.25 mm bonded phase fused slca capllary column. The njector and flame onzaton detector were at 200 ºC and 250ºC respectvely. The oven was operated at programmed temperature, from 40ºC to 220ºC. At least three replcates of each experment were made. Equlbrum tme was determned for each of the nvestgated systems. For that, dfferent glass vals were prepared wth the same concentraton and kept n the headspace oven for dfferent ncreasng tme ntervals. When the peak areas obtaned n the GC were constant, phase equlbrum was assumed to be reached. Calbraton was performed accordng to Whtehead and Sandler (11) by usng pure components at dfferent temperatures to determne the relatonshp between solute vapor pressure and peak area. Ths way, for mxtures, the solute partal pressure n equlbrum wth the dlute soluton can be obtaned from the saturaton pressure calbraton curves. For all the components and range of condtons consdered n ths work, the pure component peak area was lnearly proportonal to vapor pressure wth a lnear correlaton coeffcent above 0.. Vapor pressure correlatons of the pure compounds were obtaned or predcted consderng expermental data found n the lterature by usng Aspen Plus (2008) (12) except for 2,3-7

7 pentadone, whch Antone constants were obtaned from the lterature (13). The equaton for the extended Antone vapor pressure model s: C s 2, C ln ( p ) ( ) ( ) ( ) 7, kpa = C1, + + C 4, T / K + C5, ln T K + C6, T / K [4] + C ( T / K) 3, Coeffcents for the extended Antone equaton are lsted n Table 1. The uncertanty n the pure solute vapor pressure has not been consdered n γ calculaton snce the way the vapor pressure data have been obtaned s unknown. Smlar procedure has been followed n the lterature (14, 15) 3. RESULTS AND DISCUSSION When determnng γ by headspace, t must be taken nto account that the lqud phase composton n equlbrum wth the vapor phase does not correspond wth the lqud composton calculated from the amounts weghed, snce a certan amount has been vaporzed durng equlbraton. Ths correcton has been calculated as ndcated by Brendel and Sandler (). Due to the large dfference n the molar volume of a lqud and a gas, the correcton n the lqud phase was not very mportant. For the systems studed n ths work the relatve devaton between the ntal and the real lqud phase composton was always lower than 2 %. Ths correcton, though small, was ncluded n all the results. Addtonally, as t was ponted out by Whtehead and Sandler (11) the greatest source of expermental error n the actvty coeffcent at nfnte dluton calculaton comes from the solute peak area determnaton. 3.1 Bnary systems: solute + solvent 8

8 The values of γ for the aroma compounds selected n ths work have been determned at three dfferent temperatures, 40ºC, 45ºC and 50ºC. The partal pressures of the volatle compounds n the vapor phase n the vals were calculated from the calbraton wth pure components and the detector response. Ths solute partal pressure has been found to be a lnear functon of the aroma mole fracton n the lqud phase. As an example, Fgure 1 shows ths behavor for benzaldehyde at the three dfferent temperatures studed n ths work. In ths graph, the uncertantes for the mole fracton and the partal pressure have been also represented. Henry s law constant can be calculated from the slope of the varaton of partal pressure wth mole fracton accordng to Equaton 1 (3). Ths slope s ndependent of mole fracton, whch ndcates that results were obtaned under Henry s law condtons n the nterval of nfnte dluton. Actvty coeffcents were drectly deduced from H values by usng Equaton 2. The H and γ values for each volatle compound at the three temperatures are lsted n Table 2 together wth the uncertantes for the actvty coeffcents calculated. The uncertantes for γ are expressed through the relatve standard devatons calculated from the uncertantes of the expermental varables (24). Relatve standard devatons range from 4% to a maxmum of 17%. For most expermental ponts relatve standard devaton s less than 10 %, wth a mean value of %. The maxmum value of 17% corresponds to 1-octen-3-ol. Brendel and Sandler () ponted out that the error of the HS-GC technque can be as hgh as 25%, especally for compounds wth low solublty and hgh values of actvty coeffcent at nfnte dluton, as s the case of 1- octen-3-ol. The temperature dependence of γ can be expressed by an Arrhenus type relatonshp (26): ln γ = a + b ( T / K) [5]

9 In general γ slghtly ncrease wth ncreasng temperature. Fgure 2 shows the logarthms of γ as a functon of the recprocal temperature and the correspondng Arrhenus ft. Table 2 also reports some actvty coeffcents found n the lterature for the compounds studed n ths work. For some of the compounds, dfferences can be apprecated among the dfferent values reported n the lterature. In ths regard Barrera Zapata et al. (25) emphasze that accurate data for γ are not abundant and even for common systems lke ethanol n water at room temperature, the expermental values reported for γ can vary by a factor of two. Fgure 3 shows the logarthms of the actvty coeffcents at nfnte dluton obtaned n ths work for ethyl acetate as a functon of the recprocal temperature, together wth the data found n the lterature. From lnear regresson analyss of all the data, an actvty coeffcent of 67.8 can be estmated for ethyl acetate at K. For the rest of the compounds lmted publshed data do not allow precse comparson wth the results of the present work. For hexanal, benzaldehyde and 1-octen-3ol a value of γ of 64, 501 and 1753 respectvely at K can be extrapolated from the expermental data reported n ths work. 3.2 Multcomponent solute + solvent system Actvty coeffcents at nfnte dluton for each volatle compound prevously consdered n ths work were also determned n a mxture formed by all the volatle compounds (multcomponent solute) and the solvent (water). Table 3 shows the γ values obtaned for each compound n ths multcomponent mxture at 40ºC and the values obtaned at the same temperature n a sngle component solute mxture. Ths Table also presents the uncertanty of γ through the percentage of the relatve standard devaton. As an example, Fgure 4 10

10 shows the solute partal pressure n the vapor phase as a functon of the mole fracton of the lqud phase n the sngle component solute mxture and n the multcomponent solute mxture for ethylacetate. The values obtaned for multcomponent solute-water mxtures at 40ºC are smlar to those obtaned for sngle solute component-water mxtures concludng that no nteractons take place among the volatle compounds n the range studed n ths work. So, n ths case, a mxture wth (n-1) components at a composton close to zero and the solvent at a composton close to 1 s smlar to the stuaton of havng (n-1) bnary mxtures formed by the solvent and the (n-1) components, always nfntely dluted (10). Smlar results were obtaned for Bao and Han (27) n the study of the nfnte dluton actvty coeffcents for varous types of systems. 3.2 Effect of salt on actvty coeffcents at nfnte dluton The effect of salt concentraton on γ has been evaluated by varyng the sodum chlorde concentraton from 0 to 1.71 mol/kg (0% to 10 wt%) at 40ºC. Henry s constants have been calculated through the slope of solute partal pressure n the vapor phase as a functon of mole fracton of the solute n the lqud phase. Actvty coeffcents were then deduced from equaton (2). The expermental values are lsted n Table 4. As a general trend, the values of γ ncrease wth ncreasng salt concentraton; ths effect s referred as saltng out snce an ncrease of the actvty coeffcent value nvolves lower solublty values. Brendel and Sandler () proposed the followng equaton to correlate γ n salty solutons based on the Setschenow emprcal equaton to correlate solubltes of substances n salty solutons: ln γ = k c γ,o s [8] 11

11 where γ s the actvty coeffcent at nfnte dluton n salty solutons, γ, o the actvty coeffcent at nfnte dluton n pure water, c s the salt concentraton and the proportonalty factor, k, s the saltng-out coeffcent. Brendel and Sandler () report a dependence of k on temperature but ths effect has not been studed n ths work. The values of the saltng out coeffcents at 40ºC are provded n Table 4. The correlaton factor was hgher than 0.8 for the all the compounds consdered n ths work. Accordng to equaton 3, the relatve volatlty would also ncrease wth salt concentraton due to the saltng out effect. 4. Conclusons The Henry s law constant and the actvty coeffcent at nfnte dluton of seven volatle compounds found n brown crab bolng effluent have been determned by usng the headspace gas chromatograpy technque. Expermental data have been obtaned at three dfferent temperatures 40ºC, 45ºC and 50ºC. The temperature dependence of actvty coeffcents at nfnte dluton can be expressed by an Arrhenus type expresson. Comparng the γ obtaned n a sngle component solute aqueous soluton wth those obtaned n a multcomponent solute aqueous soluton, t can be concluded that no, or lttle, nteractons take place among the volatle compounds n the concentraton range studed n ths work. However, one should keep n mnd that the number of volatle compounds dentfed n the brown crab bolng effluent was more than 150 compounds, ncludng aldehydes, ketones, alcohols, esters, aromatc compounds and sulphur and ntrogencontanng compounds (1). Fnally, the effect of salt concentraton has been studed by varyng the sodum chlorde concentraton from 0 to 1.71 mol/kg. As a general rule, actvty coeffcents at nfnte 12

12 dluton for all the volatle compounds consdered I nths work ncrease as the salt concentraton ncreases, showng a saltng out effect. Nomenclature c s = salt concentraton H = Henry constant k = saltng out coefffcent p = pressure R = gas constant T = temperature γ = actvty coeffcent α = relatve volatlty Upperscrpts: s: saturaton : Infnte dluton Subscrpts: = component w = water 13

13 ACKNOWLEDGMENTS Fnancal support from the MICINN through CTQ PPQ s gratefully acknowledged. R. Martnez acknowledges the PIRTU program of the JCyL Educaton Mnstry for a PFI PhD grant. References (1) Martínez, R.; Sanz, M.T.; Beltrán, S. J. Food Eng. 2011, 105, (2) She, M.; Hwang, S.-T. J. Membr. Sc. 2006, 27, (3) Hadjoudj, R.; Monner, H.; Rozard, Ch.; Lapcque, F. Ind. Eng. Chem. Res. 2004, 43, (4) Bay, K.; Wanko, H.; Ulrch, J. Chem. Eng. Res. Desgn 2006, 84 (A1), (5) Sherman, S.R.; Trampe, D.B.; Bush, D.M.; Schller, M.; Eckert, Ch. A.; Dallas, A.J.; L, J.; Carr, P.W. Ind. Eng. Chem. Res. 16, 35, (6) Kolb, B. J. Chromatography 176, 122, (7) Kolb, B. J Chromatography 175, 112, (8) Sadafan, A.; Crouzed, J. J. Flavour and Fragrance 187, 2, () Brendel, M.L.; Sandler, S.I. Flud Phase Equlbr. 1, 165, (10) Aless, P.; Fermegla, M.; Kkc, I. Flud Phase Equlbr. 11, 70, (11) Whtehead, P.G.; Sandler, S.I. Flud Phase Equlbr. 1, 157, (12) Aspen Plus V7.1 (2008) Aspen Technology, Inc., (13) Son, M.; Ramjugernath, D.; Raal, J.D. J. Chem. Eng. Data 2008, 53, (14) Hertel, M.O.; Scheuren, H.; Sommer, K.; Glas, K. J. Chem. Eng. Data 2007, 52, (15) Hertel, M.O.; Sommer, K. J. Chem. Eng. Data 2006, 51,

14 (16) Sancho, M.F.; Rao, M.A.; Downng, D.L. J. Food Eng. 17, 34, (17) Druaux, C.; Le Thanh, M.; Seuvre, A.M.; Volley, A. J. Am. Ol Chem. Soc. 18, 7(2), (18) Kojma, K.; Zhang, S.; Hak, T. Flud Phase Equlbr. 17, 131, (1) Ras, A.; Aroujalan, T.; Kaghazch, T. J. Membr. Sc. 2008, 332, (20) Carell, A.A.; Crapste, G.H.; Lozano, J.E. J. Agrc. Food Chem. 11, 3, (21) She, M.; Hwang, S.-T. J. Membr. Sc. 2004, 236, (22) Le Thanh, M.; Thbeaudeau, P.; Thbaut, M.A.; Volley, A. Food Chem. 12, 43, (23) Lammer, T.; Rohart, M.S.; Volley, A.; Baussartb, H. J. Membr. Sc. 14, 0, (24) Chrco, R.D.; Frenkel, M.; Dky, V.V.; Marsh, K.N.; Wlhot, R. J. Chem. Eng. Data 2003, 48, (25) Barrera Zapata, R.; Vlla, A.L.; Montes de Correa, C. Flud Phase Equlbr. 200, 275, (26) Ge, M.-L.; Ma, J.-L.; Wu, C.-G. J. Chem. Eng. Data 2010, 55, (27) Bao, J.-B.; Han, S.-J. Flud Phase Equlbr. 15, 112(2),

15 Table 1. Coeffcents for the extended Antone Equaton, Eq. [4]. Compound C 1 C 2 C 3 C 4 C 5 C 6 C 7 Reference Water Aspen Plus Ethylacetate Aspen Plus 3-Methylbutanal Aspen Plus 2,3-Pentadone [13] 1-Penten-3-ol Aspen Plus* Hexanal Aspen Plus Benzaldehyde Aspen Plus 1-Octen-3-ol Aspen Plus* (*) predcted 16

16 Table 2. Expermental values of Henry s law constant (H ), γ obtaned wth Eq. 2 andlterature values of γ. Compound T (K) H, kpa γ RSD ( γ %) γ (lterature) Ethylacetate ± ± ºC,[16] ºC,[18] ºC,[17] ºC,[18] ± ºC,[18] ºC,[18] Methylbutanal ± ± ± _,[1] ºC,[14] 2,3-Pentadone ± ± ± Penten-3-ol ± ± ± Hexanal ± ± ± ºC,[5] ºC,[16] ºC,[14] 20 25ºC,[20] Benzaldehyde ± ± ± ºC,[21] 55 25ºC,[16] 25ºC,[22] ºC,[15] Octen-3-ol ± ± ºC,[17] ºC,[23] ±

17 Table 3. Actvty coeffcents at nfnte dluton obtaned n a sngle component solute aqueous soluton and n a multcomponent solute aqueous soluton at 40ºC. Compound γ Sngle component solute mxture RSD (%) γ Multcomponent solute mxture RSD (%) Ethylacetate Methylbutanal ,3-Pentadone Penten-3-ol Hexanal Benzaldehyde Octen-3-ol

18 Table 4. Actvty coeffcents at nfnte dluton at 40ºC at dfferent salt concentratons And saltng out coeffcents, k, at 40ºC. Compound c s (mol/kg) γ RSD (%) r 2 k Ethylacetate ± Methylbutanal ± ,3-Pentadone ± Penten-3-ol ± Hexanal ± Benzaldehyde ± Octen-3-ol ±

19 Lst of Fgure Capton Fgure 1. Partal pressure of benzaldehyde as a functon of mole fracton n the lqud phase at three dfferent temperatures ( 40ºC, 45ºC, 50ºC). Standard devaton for each data has been drawn. Fgure 2. Arrehnus plot of the actvty coeffcents at nfnte dluton for the volatle compounds studed n ths work. Standard devaton for each data has been drawn. Fgure 3. Values of the actvty coeffcents at nfnte dluton for ethyl acetate as a functon of temperature ( data obtaned n ths work, lterature values from Table 2). Fgure 4. Partal pressure of ethylacetate as a functon of mole fracton n the lqud phase at 40ºC ( Multcomponent solute mxture, sngle component solute mxture). Standard devaton for each data has been drawn. 20

20 Fgure 1. Partal pressure of benzaldehyde as a functon of mole fracton n the lqud phase at three dfferent temperatures ( 40ºC, 45ºC, 50ºC). Standard devaton for each data has been drawn. 21

21 Fgure 2. Arrhenus plot of the actvty coeffcents at nfnte dluton for the volatle compounds studed n ths work. Standard devaton for each data has been drawn. 22

22 Fgure 3. Values of the actvty coeffcents at nfnte dluton for ethyl acetate as a functon of temperature ( data obtaned n ths work, lterature values from Table 2). 23

23 4E-01 3E-01 p, kpa 2E-01 1E-01 0E E E E E E E-04 Mole fracton, x Fgure 4. Partal pressure of ethylacetate as a functon of mole fracton n the lqud phase at 40ºC ( Multcomponent solute aqueous soluton, sngle component solute aqueous soluton). Standard devaton for each data has been drawn. 24