In¯uence of feed ¯ow velocity on pervaporative aroma recovery froma model solution of apple juice aroma compounds

Jenny Olsson *, Gun Tr�ag�ardh

Department of Food Engineering, Lund University, P.O. Box 124, SE-221 00 Lund, Sweden

Received 5 March 1998; received in revised form 2 November 1998; accepted 2 November 1998

Abstract

Pervaporation is a membrane separation technique being developed for aroma recovery from various aqueous solutions. This

study concerns pervaporative aroma recovery from apple juice. The objective was to study the concentration polarisation phe-

nomenon in this application and to ®nd a possible relationship between the relative composition of the permeate, i.e. the aroma

concentrate, and the feed ¯ow conditions (Re. 50±1000). The study was based on a model solution of apple juice aroma compounds.

The relative content of esters and aldehyde, trans-2-hexenal, increased as the hydrodynamic conditions were improved at the expense

of the alcohols. For the alcohols, the membrane resistance dominated over the feed boundary layer resistance to mass transfer. For

most of the esters, the opposite relation was found. For the aldehyde and ethyl acetate, the resistances were of the same order. The

Sherwood correlation was found to overestimate the mass transfer coe�cient of the feed boundary layer in cases of severe con-

centration polarisation. Ó 1999 Elsevier Science Ltd. All rights reserved.

Keywords: Pervaporation; Hydrophobic membranes; Apple juice; Aroma recovery; Concentration polarisation

1. Introduction

The aroma complex of apple juice is a highly volatilefraction of principally esters, aldehydes and alcohols,but also includes ethers, fatty acids, lactones, terpenesand ketones. The total concentration of aroma com-pounds is approximately 200 ppm. Each group of aromacompounds gives a typical character to the apple juice¯avour. The esters give the juice a sweet, fruity ¯avoura�ording a sensation of ripeness. The aldehydes give afresh, grassy ¯avour that gives the juice a ¯avour ofimmature apple. The alcohols contribute to both the

Journal of Food Engineering 39 (1999) 107±115

Nomenclature

CPM concentration polarisation modulus ())

D di�usivity coe�cient (m2/s)

J mole ¯ux (mole/m2s)

L length of the ¯ow channel (m)

P pressure (mbar)

P0 equilibrium vapour pressure (mbar)

Pe Pecl�et number ())

R resistance to mass transfer (m2 s mbar/mole)

Re Reynolds number ())

Sc Schmidt number ())

Sh Sherwood number ())

Vp velocity of the ¯uid perpendicular to the

membrane surface (m/s)

X mole fraction ())

dh hydraulic diameter (m)

k mass transfer coe�cient (mole/m2 s mbar)

kv mass transfer coe�cient (m/s)

v mean velocity of the ¯uid (m/s)

y length co-ordinate (m)

z relation between the volume fraction and the

mole fraction ())

Greek letters

b enrichment factor ())

c activity coe�cient ())

d thickness of boundary layer (m)

m kinematic viscosity ())

q mole density (mole/m3)

Subscripts

app apparent

bl feed boundary layer

f feed or feed side

i component

int intrinsic

m membrane

ov overall

p permeate or permeate side

tot total

Superscripts

b bulk

exp experimentally obtained

m membrane

theor theoretically obtained

* Corresponding author. E-mail: Jenny.Olsson@livstek.lth.se.

0260-8774/99/$ ± see front matter Ó 1999 Elsevier Science Ltd. All rights reserved.

PII: S 0 2 6 0 - 8 7 7 4 ( 9 8 ) 0 0 1 5 4 - X

fruity and the immature ¯avour. Esters and aldehydesare present at considerably lower concentrations thanthe alcohols, but as they have very low aroma thresholdvalues, typically at ppb levels, they are responsible forthe main part of the total aroma intensity of the juice.The aroma threshold value is often de®ned as the lowestconcentration in a water solution at which the aromacompound is perceptible. The alcohols have consider-ably higher aroma threshold values, typically at ppmlevels, but as they are the quantitatively largest group ofaroma components in apple juice, they still make animportant contribution to the fruity ¯avour of the juice(Morton & Macleod, 1990; Dimick & Hoskin, 1981;Simpson, 1979).

The industrial production of concentrated apple juiceinvolves an evaporation step. Di�erent techniques canbe applied to recover the aroma compounds from theloss stream, e.g. distillation and partial condensation.Pervaporation is another technique under developmentfor this application. The objective here was to study theconcentration polarisation phenomenon and to ®nd apossible relationship between the relative aroma com-position of the aroma concentrate and the feed ¯owconditions. The results could be compared with theSherwood correlation for estimation of the concentra-tion polarisation e�ect in this application. The study wasbased on a model solution of apple juice aroma com-pounds.

The aroma complex of apple juice is sensitive to heattreatment, which means that pasteurisation and evapo-ration may cause chemical and physical losses due toevaporation, leading to a major loss of aroma com-pounds and changes in the ¯avour. Instead of recoveringthe aroma compounds from the loss stream, per-vaporative aroma recovery can be performed on thejuice prior to the evaporation step and heat treatment ofthe sensitive aroma compounds can thus be avoided.

2. Pervaporation

In this application, vacuum pervaporation is per-formed with a hydrophobic polymer membrane in orderto selectively transport the volatile aroma compounds,present at ppm levels, across the membrane. Thus, thearoma concentrate is obtained as the permeate, while thedearomatised solution is obtained as the retentate. Adriving force for the process is established by main-taining a di�erence in partial pressure between the feedand the permeate. The ¯ux varies linearly with the gra-dient in partial pressure, according to

Ji � kov;i P bf;i ÿ P b

p;i

� ��1�

In vacuum pervaporation this is accomplished by low-ering the total pressure on the permeate side of the

membrane to a few mbars. According to Raoult's law,which is a model assumption, the partial pressure ofcomponent i in the bulk of the feed is

P bf ;i � ci X b

f;i P0;i �2�where ci is the activity coe�cient, X b

f;i is the mole fractionin the bulk of the feed and P0;i is the equilibrium vapourpressure of component i. The partial pressure of com-ponent i in the permeate is assumed to be the permeatepressure, Pp, times the mole fraction of component i ofthe permeate, X b

p;i, i.e.

P bp;i � X b

p;i Pp �3�The ¯ux of component i, Ji, is proportional to thedriving force and kov;i, the overall mass transfer coe�-cient, corresponds to the overall resistance to masstransfer of component i in the process. As the concen-trations of the aroma compounds in this study were atppm levels, it could be assumed that they were trans-ported through the membrane independently of eachother. According to the resistance-in-series theory, theresistance from the bulk of the feed to the bulk of thepermeate can be divided into several consecutive steps:di�usion through the liquid feed boundary layer fromthe bulk of the feed to the feed side of the membrane,adsorption into the membrane, di�usion through themembrane, desorption into the vapour phase at thepermeate side of the membrane and di�usion to the bulkof the permeate. The fundamental assumption that the¯ux through all the mass transfer resistances is equalyields

Ji � kov;i P bf ;i ÿ P b

p;i

� �� kbl;i P b

f;i ÿ P mf;i

� �� km;i P m

f;i ÿ P mp;i

� �� kp;i P m

p;i ÿ P bp;i

� ��4�

where kbl;i, km;i and kp;i are the mass transfer coe�cientsfor component i of the liquid feed boundary layer, themembrane and the vapour permeate boundary layer,respectively (Karlsson, 1996). At the very low permeatepressures considered in vacuum pervaporation, the masstransfer resistance of the vapour permeate is commonlyassumed to be negligible in comparison with the othermass transfer resistances. Using this assumption, therelationship of the mass transfer coe�cients can bewritten as follows

1

kov;i� 1

kbl;i� 1

km;i�5�

The mass transfer coe�cient of the membrane, km;i, in-cludes adsorption to the membrane, di�usion throughthe membrane and desorption from the membrane forpermeant i. As the resistance to mass transfer is de®nedas the inverse of the mass transfer coe�cient, Eq. (5) canbe written

Rov;i � Rbl;i � Rm;i �6�

108 J. Olsson, G. Tr�ag�ardh / Journal of Food Engineering 39 (1999) 107±115

During experimental steady state conditions, km;i wasconsidered to be constant, as the only parameter variedwas the feed ¯ow velocity. This a�ected pm

f ;i and thus X mf ;i .

It has been found in several studies on pervaporativearoma recovery that km;i remains una�ected due to thevery low concentrations of organics encountered.

The mass transfer coe�cient for the boundary layer,kbl;i, can be estimated using semi-empirical Sherwoodcorrelations. For laminar ¯ow and cross¯ow across themembrane, according to Porter (1972), and Karlssonand Tr�ag�ardh (1993), the Sherwood correlation is

Sh � 1:62 Re Scdh

L

� �0:33

�7�

where the Sherwood number, Sh, is the ratio betweenthe hydraulic diameter and the thickness of the boun-dary layer, di, for the component i.

Sh � dh

di�8�

The Reynolds number, Re, and the Schmidt number, Sc,are de®ned according to:

Re � v dh

m�9�

Sc � mDi

�10�

where v is the mean velocity of the ¯uid, dh is the hydraulicdiameter, m is the kinematic viscosity of the ¯uid and Di isthe di�usion coe�cient of component i. By introducingthe di�usion coe�cient, the mass transfer coe�cient ofthe boundary layer, kbl;

mi , can be obtained from:

kmbl;i �

Di

di�11�

in which the thickness of the boundary layer is obtainedfrom the Sherwood number.

The mass transfer coe�cients for the boundary layermust be corrected according to

kbl;i �km

bl;i qi zi

ci P0;i�12�

where qi is the mole density and zi is the relation be-tween the volume fraction and the mole fraction(Karlsson, 1996). When using the Sherwood correlation,it is assumed that the contribution of convection to themass transfer across the boundary layer is negligible incomparison with the contribution from di�usion. Thisassumption is valid as here the Pecl�et number in all theexperiments was less than 0.03 for all the permeants.The Pecl�et number is de®ned according to

Pe � Vp

kmbl;i

�13�

At steady state, the sum of the convective and the dif-fusive transport through the liquid feed boundary layer

is equal to the amount permeated through the mem-brane (Wijmans, Athayde, Daniels, Ly, Kamaruddin &Pinnau, 1996)

VpXf ;i ÿ DidXf;i

dy� VpXp;i �14�

where Vp is the velocity of the ¯uid perpendicular to themembrane surface.

Vp � Jtot

q�15�

After integration with the following boundary condi-tions:

at y � 0 Xf ;i � X bf;i

at y � d Xf ;i � X mf;i

the concentration polarisation modulus, CPM, which isde®ned as the relationship between the concentration ofthe component near the membrane surface on the feedside of the membrane and the concentration of thecomponent in the bulk of the feed solution, is obtainedaccording to (Wijmans et al., 1996)

X mf;i

X bf ;i

�X b

f ;i ÿ Xp;i

� �e Vp=km

bl;i� � � Xp;i

X bf ;i

�16�

However, if the mass transfer coe�cient of the mem-brane is known from experiments, CPM can be directlyobtained by combining Eqs. (2), (3) and (4)

X mf;i

X bf ;i

�Xp;i Pp � kov;i

km;iX b

f ;iciP0;i ÿ Xp;iPp

� �X b

f ;iciP0;i�17�

The performance of the pervaporation process is notonly dependent on the mass transfer properties, butalso on the selectivity of the membrane. The apparentenrichment factor of permeant i, bapp ;i, is a measure ofthe selectivity of the membrane and it is de®ned asthe quotient of the concentration of the permeant inthe condensed permeate and the concentration of thepermeant in the bulk of the feed

bapp;i �X b

p;i

X bf ;i

�18�

However, due to concentration polarisation in the feedboundary layer, the concentration of the permeant inthe bulk of the feed deviates considerably from the feedconcentration just outside the membrane. A bettermeasure of the real selectivity of the membrane isobtained with the intrinsic enrichment factor, bint;i, de-®ned as the quotient of the concentration of the per-meant just outside the membrane on the permeate sideof the membrane and the concentration of the permeanton the feed side just outside the membrane.

J. Olsson, G. Tr�ag�ardh / Journal of Food Engineering 39 (1999) 107±115 109

bint;i �X m

p;i

X mf ;i

�19�

The assumption that no concentration polarisation oc-curs on the permeate side of the membrane yields

bint;i �X m

p;i

X mf ;i

� X bp;i

X mf ;i

�20�

In an earlier study (B�orjesson, Karlsson & Tr�ag�ardh,1996), the enrichment factors were determined for themembrane used in this study. The e�ect of concentrationpolarisation on the feed side was very limited due toboth the hydrodynamic conditions, corresponding to Re22,000, and the geometrical design of the pervaporationmodule, and the enrichment factors in this study willthus be close to the intrinsic enrichment factors.

3. Materials and methods

3.1. Feed solution

As feed solution, a previously developed model so-lution of apple juice aroma compounds was used(B�orjesson et al., 1996). Aroma compounds in applejuice made from Spanish Granny SmithÕs apples wereidenti®ed by gas chromatography (GC) and by gaschromatography together with mass spectroscopy (GC-MS). Ten of the identi®ed compounds were then selectedfor a model solution of apple juice aroma compounds.The procedure is thoroughly described in a previousstudy (B�orjesson et al., 1996). In the model solution,each aroma compound was present at a concentration of10 ppm (w/w), see Table 1. Demineralised and distilledMilli-Q water, with a resistance greater than 18 MX cm,was used as solvent. The aroma compounds used toprepare the solution had a purity of about 99%.

3.2. Pervaporation membranes

The pervaporation membrane used was a POMS-PEImembrane, a poly-octyl-methyl-siloxane membranesupplied by GKSS Forschungzentrum, Geesthacht,Germany.

3.3. Pervaporation apparatus

The experimental set-up is shown in Fig. 1. A plate-and-frame module with a membrane area of 184 cm2

was used. The dimensions were 0.75 mm ´ 20 mm andthe hydraulic diameter was 1.45 mm. An accurate gearpump was used to recirculate the retentate to the tem-perature-controlled feed vessel (101). The permeate wascollected in cold traps by condensation, either with liq-uid nitrogen at )196°C or with dry ice at )56°C. Vac-uum was achieved by a vacuum pump, which alsoseparates inert gases from the permeate.

3.4. Experimental procedures

The membrane was conditioned for 12±16 h withMilli-Q water before each experiment. The feed temper-ature was 20.0°C and the permeate pressure was less than1 mbar. The Reynolds number was varied in the range50±1000. During conditioning, dry ice was used forcondensation, whereas during the experiments liquid ni-trogen was used. Three hours after starting the pervap-oration process using the model solution, the feedtemperature was stable and the mass transfer equilibriumwas deemed to have been established. Every hour, thecold trap was exchanged and the permeate was thawedand poured back into the feed tank to limit the loss ofaroma compounds from the feed. During the fourthhour, permeate was collected and samples from thefeed tank were taken for analysis. The total ¯ux was

Table 1

Data for the aroma compounds used in the model apple juice aroma fraction

Aroma compound P0; i

(mbar)

ci

())

Pf ; i á103

(mbar)

bint; i

())

kov; i ;av á105

(mole/m2 s mbar)

Ethyl-2-methyl butanoate 25.88 5600 133 3400 0.27

Ethyl butanoate 17.46 1860 40 2000 1.2

Ethyl acetate 98.44 153 28 370 1.3

Isoamyl acetate 5.95 3800 24 2900 1.8

Hexyl acetate 1.34 11000 11 3400 2.8

Isobutanol 11.84 55.6 1.6 38 5.0

trans-2-Hexenal 8.12 400 5.1 620 8.3

Butanol 6.11 55.5 0.8 40 10

Isoamyl alcohol 2.99 153 0.9 73 12

Hexanol 0.89 426 0.7 150 24

Pf ; i is the partial pressure of the component in the feed solution. P0; i is the equilibrium vapour pressure for the pure component. ci is the activity

coe�cient of the component at in®nite dilution. bint; i is the intrinsic enrichment factor of the component obtained through Eq. (16). kov; i ;av is an

averaged overall mass transfer coe�cient of the component determined from experiments performed with the model solution of apple juice aroma

compounds using the POMS-PEI membrane. (Olsson & Tr�ag�ardh)

110 J. Olsson, G. Tr�ag�ardh / Journal of Food Engineering 39 (1999) 107±115

determined gravimetically with an experimental error of1±2%.

3.5. Analysis

The aroma compound concentrations in both the feedand the permeate were analysed with a Varian Vista 6000gas chromatograph (Varian Associates, Sunnyvale, CA,USA). The gas chromatograph was equipped with afused silica capillary GC column (NB-351, 25 m, 0.32mm, Nordion Instruments Oy Ltd, Helsinki, Finland).The temperature was programmed from 55°C to 93°C ata rate of 4°C/min, and then from 93°C to 110°C at a rateof 30°C/min, with an initial hold of 2 min and a ®nal holdof 3 min. The injector and detector were set to 180°C and190°C, respectively. The carrier gas was nitrogen at a¯ow rate of 0.85 ml/min, and the sample was split 1:125in the injector. As internal standard 1-pentanol was used.Each concentration determination was based on three tofour di�erent injections. The values of aroma concen-trations varied by 5±10% (B�orjesson et al., 1996).

3.6. Estimation of di�usivity coe�cients, equilibriumvapour pressures and activity coe�cients of the aromacompounds

The mutual di�usivity coe�cients of the aromacomponents at in®nite dilution were estimated accord-ing to the Wilke-Chang method (Reid, Prausnitz &Poling, 1987). The standard deviation of the di�usivitycoe�cients with this method is about 10%.

In order to express the mass transfer coe�cients inmole ¯ux per unit partial pressure, some properties ofthe aroma compounds must be known. The equilibriumvapour pressure of the pure aroma compounds at 20°Cand 1 atm were obtained through Antoine's constantsfrom the literature, for all of the compounds except fortrans-2-hexenal (One, 1976; Lide, 1994; Gmehling &Onken, 1995). The equilibrium vapour pressure of trans-

2-hexenal was estimated through a method based on theClausius-Clayperon equation (Reid et al., 1987). Theactivity coe�cients of the aroma compounds at in®nitedilution at 20°C and 1 atm were estimated using aUNIFAC method (Reid et al., 1987). The equilibriumvapour pressures and the activity coe�cients are pre-sented in Table 1.

4. Results and discussion

The mass transfer coe�cients of the membrane, km;i,were determined experimentally in a previous study byB�orjesson et al. (1996). In this study, the experimentswere performed at hydrodynamic conditions which al-lowed the assumption to be made that the mass transfercoe�cients of the liquid feed boundary layer were neg-ligible in comparison with the mass transfer coe�cientsof the membrane itself. These values of the mass transfercoe�cients of the membrane together with the masstransfer coe�cients of the feed boundary layer, esti-mated as described above through the Sherwood cor-relation for laminar ¯ow, were used to obtain theoreticaloverall mass transfer coe�cients for the aroma com-pounds, ktheor

ov;i , according to Eq. (5). The theoreticallyobtained overall mass transfer coe�cients were thencompared with the experimentally obtained ones, k exp

ov;i .See Fig. 2. The overall mass transfer coe�cients areplotted as a function of the Reynolds number. From the®gures it can be seen that the theoretically and the ex-perimentally obtained overall mass transfer coe�cientsagreed well. However, the theoretically obtained masstransfer coe�cients for the esters, the aldehyde and forone of the alcohols, i.e. hexanol, seemed to be somewhatoverestimated, by 10±30%. The reason for this could bethat the membrane mass transfer coe�cients for thesecomponents were overestimated due to some concen-tration polarisation in the previous study by B�orjessonet al. (1996). Another explanation could be that the

Fig. 1. Experimental set-up of the pervaporation apparatus.

J. Olsson, G. Tr�ag�ardh / Journal of Food Engineering 39 (1999) 107±115 111

Sherwood correlation overestimates the mass transfercoe�cient of the liquid feed boundary layer. Three ofthe alcohols, i.e. isobutanol, isoamyl alcohol and buta-

nol, showed constant behaviour and were not a�ectedby an increase in Reynolds number. The overall masstransfer coe�cient for hexanol increased slightly as theReynolds number was increased. The overall masstransfer coe�cients of both the aldehyde and the estersincreased rapidly with increasing Reynolds number.When comparing the di�erent aroma compounds stud-ied, it can be seen that the esters had the lowest overallmass transfer coe�cients, whereas the alcohols had anorder of magnitude higher than the overall mass transfercoe�cients.

As the values of the membrane mass transfer coe�-cients were determined in a previous study by B�orjessonet al. (1996), the CPM could be obtained through ex-perimental results according to Eq. (17). In Fig. 3, theexperimentally obtained values of the CPM as well asthe values obtained via the Sherwood correlation incombination with Eq. (16) for ethyl-2-methyl butanoate,ethyl butanoate, trans-2-hexenal and butanol are plottedas a function of the Pecl�et number. When comparingthese values with those in the literature (Wijmans et al.,1996), the experimentally obtained CPM values variedwith the Pecl�et number in an expected way, in the Pecl�etnumber region studied. Thus, the CPM decreased withincreasing Pecl�et number and the CPM exhibited a limitapproaching one, as the Pecl�et number approaches zero.In the log±log plot it can be seen that for larger values ofthe Pecl�et number CPM decreased linearly with in-creasing Pecl�et number. Eventually, at Pecl�et numbersgreater than one, the CPM will level o� at a constant

Fig. 2. The overall mass transfer coe�cients of the aroma compounds

are plotted as a function of the Reynolds number. The ®lled symbols

correspond to theoretically calculated coe�cients, according to Eq. (5),

while the open symbols correspond to experimentally measured

coe�cients. The aroma compounds are abbreviated as follows:

I Bu ± isobutanol; Bu ± butanol; I Am ± isoamyl alcohol; He ± hexanol;

EtBu ± ethyl butanoate; Et-2-MeBu ± ethyl-2-methyl butanoate;

I AmAc ± isoamyl acetate; HeAc ± hexyl acetate; EtAc ± ethyl acetate;

t-2-He ± trans-2-hexenal.Fig. 3. The concentration polarisation modulus, CPM, of four dif-

ferent aroma compounds are plotted as a function of the Pecl�et

number, Pe. The open symbols represent experimentally obtained

CPM through Eq. (16), while the ®lled symbols represent CPM ob-

tained via the Sherwood correlation through Eq. (15). For abbreviat-

ions of aroma compounds, see Fig. 2.

112 J. Olsson, G. Tr�ag�ardh / Journal of Food Engineering 39 (1999) 107±115

value. This is not seen in the plot as the levelling o�takes place outside the studied region.

However, the values of the CPM obtained via theSherwood correlation deviated considerably from theexpected values, especially in the case of extensive con-centration polarisation, see the plots for ethyl-2-methylbutanoate and ethyl butanoate in Fig. 3. The reason forthis behaviour is illustrated in Fig. 4, in which themembrane resistances to mass transfer, Rm;i, obtainedvia the Sherwood correlation and Eqs. (5) and (6), areplotted versus the Reynolds number. As can be seen,Rm;i appears to vary with the Reynolds number for someof the aroma compounds. As discussed previously, themass transfer properties through the membrane are in-dependent of the feed ¯ow velocity, i.e. Rm;i is constant.The explanation for obtaining a variation of Rm;i withthe Reynolds number lies in the way the Sherwoodcorrelation estimates the variation of the mass transferresistance of the feed boundary layer, Rbl;i, with theReynolds number. Rm;i obtained from Eq. (6) can onlybe constant if the variation of the overall resistance tomass transfer, Rov;i, coincides with the variation of Rbl;i

with the Reynolds number. Evidently, this is not thecase. The in¯uence of Rbl;i on Rov;i is signi®cant in caseof extensive concentration polarisation, which explainsthe large deviations from expected values for ethyl bu-tanoate and ethyl-2-methyl butanoate in Fig. 3. Whenconcentration polarisation is small, the in¯uence of Rbl;i

on Rov;i is limited or even negligible. This is the reasonwhy the two plots of CPM for trans-2-hexenal and bu-tanol in Fig. 3 are brought closer to each other. Fromthese results it can be concluded that the variation of themass transfer coe�cients of the feed boundary layerwith the Reynolds number was not satisfactory esti-mated by the Sherwood correlation.

In Table 1, the aroma compounds are listed in orderof increasing average overall mass transfer coe�cients.The data implied that the partial pressure of the com-ponent in the feed is of major importance for the masstransfer properties of the permeant. The general trendappears to be: the higher the partial pressure of thepermeant in the feed, the lower the overall mass transfercoe�cient, and vice versa. The pervaporability of low-volatility organic compounds from water has been in-vestigated in a series of studies (B�oddeker, Bengtson &Bode, 1990a; B�oddeker, Bengtson & Pingel, 1990b;Bengtson & B�oddeker, 1988). The investigators statedthat if a component forms a non-ideal solution withwater, it has a low, but non-vanishing equilibrium va-pour pressure, and if it is preferentially permeated by themembrane, then it can be pervaporated. As can be seenin Table 1, none of the compounds studied formed anideal solution with water, as all the activity coe�cientsare far from one. The components with relatively lowequilibrium vapour pressures exhibited the highestoverall mass transfer coe�cients. The most hydrophobiccompounds studied, which also have the greatest activitycoe�cients, exhibited the greatest selectivities, as themembrane itself was hydrophobic. These results are inagreement with the studies referred to above (B�oddekeret al., 1990a,b; Bengtson & B�oddeker, 1988).

However, not only the mass transfer properties, butalso the selectivity of the membrane is of importance forthe recovery of the aroma compounds. Using Eqs. (17)and (19), the real enrichment factors of the membranecould be obtained from the experimental results, seeTable 1. When comparing these enrichment factors withthe real enrichment factors obtained in the previousstudy by B�orjesson et al. (1996), it can be seen that theyagree within an error of 10%. The esters and the alde-hyde exhibited the highest selectivities, which compen-sates for their relatively poor mass transfer properties.The total recovery of the esters and the aldehyde will behigher than for the alcohols.

In Fig. 5, the overall resistance to mass transfer andthe membrane resistance to mass transfer are plotted asa function of the Reynolds number. As can be seen inFig. 5(a), the overall resistance to mass transfer for thealcohols was dominated by the resistance of the mem-brane, i.e. the recovery of these alcohols was not sig-ni®cantly a�ected by improved ¯ow conditions. Themass transfer resistance of ethyl acetate and trans-2-hexenal, see Fig. 5(b), was in¯uenced by both the re-sistance of the feed boundary layer and the resistance ofthe membrane. However, regarding the resistances ofthe remaining esters studied, i.e. ethyl butanoate, iso-amyl acetate, hexyl acetate and especially ethyl-2-methylbutanoate see Figs. 5(c) and (d), it is evident that theoverall resistance to mass transfer was dominated by theresistance of the feed boundary layer. In the case ofethyl-2-methyl butanoate, the resistance of the mem-

Fig. 4. The membrane resistances to mass transfer, Rm; i, obtained via

the Sherwood correlation and Eqs. (5) and (6) of four di�erent aroma

compounds, are plotted versus the Reynolds number. For abbreviat-

ions, see Fig. 2.

J. Olsson, G. Tr�ag�ardh / Journal of Food Engineering 39 (1999) 107±115 113

brane was negligible in comparison with the resistanceof the feed boundary layer.

The water ¯ux was not signi®cantly a�ected byimproved hydrodynamic conditions in the Reynolds

number range studied, which was expected as theconcentration of organics in the feed was very low.It remained constant at a level of 8.80 á10ÿ4 mole/m2 sin all the experiments, with a standard deviation of2.5%.

5. Conclusions

For the esters studied, except for ethyl acetate, thedominating resistance to mass transfer in the pervap-oration process was the resistance in the liquid feedboundary layer, whereas for the alcohols studied, theresistance to mass transfer in the membrane itselfdominated. For the aldehyde and ethyl acetate, thesetwo resistances were of the same order. In other words,increasing the feed ¯ow velocity did not signi®cantlyincrease the recovery of the limiting alcohols, whereas itincreased the recovery of the other components studied.Improved hydrodynamic conditions will thus improvethe recovery of the esters and the aldehyde, while therecovery of the alcohols will remain almost una�ected.Due to the very high aroma threshold values of the estersand the aldehydes in comparison with the alcohols, thesecomponents contribute the main part of the ¯avour tothe apple juice. The altered composition due to improvedfeed ¯ow conditions would, for this reason, probably notgive a permeate with a signi®cantly altered ¯avour.

For the components causing extensive concentrationpolarisation, it was concluded that the Sherwoodcorrelation overestimated the mass transfer coe�cient ofthe boundary layer. This study has shown that theconcentration polarisation phenomenon is betterstudied experimentally without the use of Sherwoodcorrelations.

Acknowledgements

This work was supported by the European Com-mission through the AAIR project PL 94 1931. Theauthors wish to acknowledge GKSS Forschung-zentrumfor generously supplying the membranes.

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