Aroma compound recovery with pervaporation - feed flow effects

163

Journal of Membrane Science, 81 (1993) 163-171 Elsevier Science Publishers B.V., Amsterdam

Aroma compound recovery with pervaporation - feed flow effects* Hans O.E. Karlsson** and Gun Triig&dh Food Engineering, Lund University, P.O. Box 124, S-221 00 Lund (Sweden) (Received November 3,1992; accepted in revised form February 8,1993)

Abstract A multi component aroma model, consisting of five organic model components in a dilute water solution, was pervaporated through polydimethylsiloxane membranes using different crossflow velocities and hydraulic diameters. Strong feed flow effects were observed in the laminar flow regime for model compounds with low membrane resistance and high concentrations in the feed. In the turbulent flow regime only small or no feed flow effects were observed. A model based on the resistance-in-series model and Sherwood correlations could describe this phenomenon. The different flux increments with increasing crossflow velocity for the different model compounds could be used as a tool, to some extent, to control the composition of the permeate. Keywords: pervaporation; aroma compound recovery; concentration polarization; boundary layer; mass transfer

Introduction Aroma compounds are often lost during processing of beverages. This is generally a set back for the final product, e.g. a concentrated beverage, and recovery of the lost aroma compounds would be of great interest. The aroma compounds, i.e. mainly organics, are present in very low concentrations in the beverage. The large potential of pervaporation for aroma compound recovery has been verified in several studies in the literature and was recently reviewed by Karlsson and Trligtidh [ 11. The pervaporation of dilute organic-water *Partly presented at the Sixth International

Conference on Pervaporation Processes in the Chemical Industry, Ottawa, Canada, September 27-30,1992. ‘To whom correspondence should be addressed.

0376-7388/93/$06.00

mixtures, such as pervaporation of aroma compounds, can often be modelled with a resistance-in-series model [ 11. The model considers mass transfer of penetrants from the bulk of the feed to the permeate to occur in four successive steps: (i) a mass transfer from the bulk of the feed to the feed-membrane interface, (ii) absorption to the membrane, (iii) mass transport through the membrane, and (iv) desorption to the permeate. Studies on the mass transfer in the feed during pervaporation have shown that concentration polarization plays an important role in pervaporation of systems where at least one of the components is present in a low concentration [ 2-61. Generally concentration polarization will result in a region in the feed close to the membrane, the boundary layer, where convective mass transport is lim-

0 1993 Elsevier Science Publishers B.V. All rights reserved.

H.O.E. KarLwon and G. TriigiirdhlJ. Membrane Sci. Bl(l993)

164

-a mass transfer in the feed boundary layer, -a membrane mass transfer involving both absorption, membrane transport and desorption, where the flux equation for a model component can be written as,

I

_

____c .__. /”

_ -.-.

‘b

:

Slower Permeant

Fader PeJrmefult

I ,

_

..-....__.____ ‘Y__,_ “...,

I ;

C*

I I

< Fig. 1. Schematic membrane.

(2)

Ji =Iz,v,i G,i

I

and

Cf

6

1 _=‘+L k0v.i k,i

>

concentration

profile

across

the

ited and where diffusive mass transport is rate determining. Consequently the concentration of the preferentially permeated component will drop in the boundary layer (Fig. 1) [ 11. Theory The flux equation for the transport of a single component, i, from the bulk of the feed to the permeate can be written as,

Ji =Iz,v,i(G,i-Cp,i)

163-171

(1)

where the overall mass transfer coefficient, lz,,,i, contains information on boundary layer effects, absorption to the membrane, mass transfer in the membrane, desorption to the permeate, coupling effects, concentration dependence, etc. [ 11. If the model is simplified with the following fundamental assumptions, -the model components are transported independently of each other both in the feed boundary layer and in the membrane, i.e. the process is not affected by flow coupling in any form, -the mass transfer beyond the boundary layer has a linear concentration dependence, -the concentration of the penetrants in the permeate is zero due to their low partial pressures in the permeate, then the model can be considered to consist of only two steps,

(3)

L,i

where h,i and k+ are the mass transfer coeffcients for the mass transfer in the boundary layer and the membrane respectively. The film theory gives an interpretation of the steady-state mass transfer in the boundary layer [ 11. The semi-empirical Sherwood correlations that are based on this interpretation give a mass transfer coefficient for the boundary layer, k,i. Several factors, such as crossflow velocity, flow regime, geometrical profile of the membrane module, viscosity of the fluid and diffusivity of the components affect this mass transfer coefficient. For crossflow pervaporation, these correlations are generally written as ineqn. (4) [7], d

where the Sherwood number, Sh, is the relationship between the hydraulic diameter and the thickness of the boundary layer, Si, for the component i.

dh

Sh=x Introduction of the diffusion coefficient transforms the boundary layer thickness to a mass transfer coefficient, &,i, for the component i across the boundary layer.

The Reynolds number, Re, is the well known

H.O.E. Karkson and G. Triig&dh/J.

Membrane Sci. 81(1993) 163-171

dimensionless description of the hydrodynamic conditions in the feed flow, Re=v

v 4,

(7)

and the Schmidt number, SC, is the relationship between the diffusivity of momentum and the diffusivity of mass in the feed, sc=;

(8)

L

In this study we used two sets of coefficients for the Sherwood correlation, one set for the laminar flow regime and one set for the turbulent flow regime. The values of the coefficients a, b, c and d are given in Table 1. With the assumptions made above the mass transfer in the second step could be described, for each model component, by the constant mass transfer coefficient km,+ Furthermore, if kf,iis much larger than &,i then the mass transfer resistance in the boundary layer will be very small and consequently Cf,iwill be equal to Cb,i. This corresponds to lz,,,i being equal to k,,i, which makes it possible to determine k,,i by increasing the feed flow until no further flux increment is observed. In order to perform mass transfer calculations with this model the diffusion coefficients of the aroma compounds must be known. These were estimated with the method of Wilke and Chang [9] for a dilute water solution with a dynamic viscosity of 0.8937 x 10m3 N-sec/m2, which corresponds to a kinematic viscosity of 0.8964 x 10m6m2/sec at a temperature of 298 K

Experimental Pervaporation experiments were performed with an apparatus schematically shown in Fig. 2. The feed vessel can hold a ten litre feed, which is recirculated through a plate and frame module of our own construction. Recirculation is accomplished by two different gear pumps in order to cover the entire range of feed flows examined in this study. The membrane channels in the module had a length of 230 mm and a width of 20 mm, which correspond to a membrane area of 46 cm2 per channel. By changing the frames, the module can get a variable channel height, 0.78, 1.82 or 2.83 mm, corresponding to hydraulic diameters of 1.50,3.34 and 4.96 mm respectively. The module is placed in a vacuum vessel which is evacuated by a vacuum pump and the permeate is collected in cold traps which are cooled with liquid nitrogen. The temperature of the system is controlled by a temperature control system which pumps tempered water through a secondary temperature control loop. In the module this loop consists of a secondary flow channel in the plates and in the feed vessel it consists of a jacketed wall. The feed is thus never in contact with neither

WI* TABLE 1 Coefficients in the Sherwood correlation Flow regime

a

b

c

d

Ref.

Laminar Turbulent

1.62 0.34

0.33 0.75

0.33 0.33

0.33 0

181

171

165

Fig. 2. Experimental setup.

166

H.O.E.

the electrical cooler nor the electrical heater. During the experiments four membrane channels were used, giving a total membrane area of 184 cm2, and the membranes fitted in the module were polydimethylsiloxane composite membranes (Type 1060, Deutsche Carbone, Geschiiftseinheit GFT, Neunkirchen-Heinitz, Germany). These membranes had a thickness of the active layer of 10 pm [ 111. The multi component aroma model, the composition of which is given in Table 2, consisted of five typical aroma compounds in a water solution [ 121. The aroma compounds were obtained from Janssen Chimica (Janssen Chimica, Geel, Belgium) and used without further treatment. Ten litres of the multi component aroma model were recirculated with different crossflow velocities, ranging from 0.00597 to 5.97 m/set, and with different hydraulic diameters as mentioned above. During the experiments the temperature was maintained at 25’ C and the permeate pressure at 2 mbar. The system was allowed to stabilize for three hours before the measurements were made during the fourth hour. The cold traps were changed every hour and the permeate put back to the feed. Thus, by this procedure the feed concentration was kept constant during the experiments. Each pervaporation experiment was randomly repeated twice

and G.

Membrane Sci.

163-171

giving a total of three subsamples for each flux measurement. The total flux was determined with a balance and the composition of the feed and the permeate was determined with a Varian Vista 6000 gas chromatograph (Varian Associates, Sunnyvale, CA). This gas chromatograph was equipped with a NB-351 column (25 m, 0.32 mm, Nordion Instruments Oy Ltd., Helsinki, Finland). The temperature was programmed from 45 ’ C to 85 ’ C at 10” C/min and then from 85 ’ C to 180 ’ C at 30 ’ C/min with an initial hold of 2 min and a final hold of 2 min. The injector and detector were set to 220 and 230’ C respectively. The carrier gas was nitrogen with a flow rate of 2 ml/min and in the injector the sample was split 1: 100. Results and discussion During the experiments the total flux was relatively constant, with a mean value of 126 g/ m2-hr, which, if the density of the permeate is considered to be equal to the density of water at 25 “C (997 kg/m3), corresponds to a volumetric flux of 3.51 x lo-’ m/set. The partial fluxes of the aroma compounds were, however, strongly affected by the Reynolds number as is shown in Fig. 3, where experimental values are

TABLE 2 Properties of the multi component aroma model P-methyl-propanal Cont. (ppm)

2-methyl-butanal

I-pentene-3-01

trans-2-hexenal linalool

9

7

10

20

50

B.P. (“C)

64.6

92.2

115

146

198

M.W.

72.11

86.14

Di,est hn,i

b/f=)

Solubil. in aq.

9.52x lo-lo

8.48x lo-”

86.13 9.31 x lo-‘0

98.14

154.25

7.96x lo-”

6.52x lo-”

4.87x 1O-6

1.19x 10-s

1.22x10-6

1.03 x 10-s

5.16x lo--’

8.4 (2O”C, [IS])

2.1 (2O”C, [ 161)

9.6 (25”C,

_

0.2 (25”C, [ 161)

(% v/v) ( - ) : Data not found.

1171)

167

H.O.E. Karlsson and G. TrtigCrdh/J. Membrane Sci. 81(1993) 163-171

represented by the symbols for a constant hydraulic diameter of 1.50 mm. Depending on the membrane mass transfer coefficient, Jz~,~,and feed concentration the effects were different for the different compounds. 1-Pentene-3-01 had a low membrane mass transfer coefficient and a low feed concentration and consequently the feed flow effect was low, the partial flux increased by 50% from Re 10 to Re 10,000. 2Methyl-propanal had a rather high membrane mass transfer coefficient but a low feed concentration and its flux increased by 190% for the examined flows. The feed concentration of 2methyl-butanal was the lowest in the multi component aroma model but as it had the highest observed membrane mass transfer coefficient its flux increase was the highest observed, 1020%. Trans-2-hexenal had both the second highest feed concentration and the second highest membrane mass transfer coefficient which resulted in a 440% increase in its partial flux. Finally, linalool had the lowest mass transfer coefficient but the highest feed concentration which resulted in a 170% increase in its flux when the flow was increased from Re 10 to Re 10,000. The determined membrane mass 3.0e-010

x7 0 0 0 l

Z.&Y010

transfer coefficients, Jt,,+ for the aroma model components are given in Table 2. Not only the Reynolds number is important for the mass transfer limitations of the boundary layer, but also geometric factors. When the hydraulic diameter was varied at a constant Reynolds number, Re 1,000, the partial fluxes changed as shown in Fig. 4. An increased hydraulic diameter caused decreased partial fluxes for all the aroma compounds in the multi component aroma model. In this case the reason for the flux decline can be deduced from the Sherwood correlation, eqns. (4)- (8). Simplification of this expression results in an expression for the mass transfer in the boundary layer which contains one part of constants and one part of hydrodynamic variables. If the channel length is considered as a constant whereas the hydraulic diameter is considered as a variable, then this expression is equal to, A$ = (a IIf-”

IF*

L-d) (ubd;+d-l)

(9)

and if the values of a, b, c and d for the laminar flow regime are introduced then this expression is simplified to,

3.0e-010

linalool trans-Z-hexenal 2-lp+lyJ~.t+anal 2-methyl-propanal 1-pentene-3-d

v 0 Q 0 l

2&f-010

Z.Oe-010

2

i.k-010

2 ‘J.- 1.5e-010

linalool tram-2-hexenal 2-methyl;bu@g+ 2-methyl-propanal 1-pentene-3-01

2.0e-010

? .z 6’

l.Oe-010

l.Oe-010

5.0e-011

0.0e+000

+

1 10

100

1000

10000

Re (-)

Fig. 3. Effect of crossflow velocity on partial fluxes. Symbols: experimental data, lines: model calculations.

1

2

3

4

5

6

d, (mm)

Fig. 4. Effect of hydraulic diameter on partial fluxes. Symbols: experimental data, lines: model calculations.

H.O.E. Karkson and G. Tr&&rdh/J. Membrane Sci. 81(1993) 163-171

168

/ h,i

=ei

\ 0.33

(t)

(10)

i.e. the mass transfer coefficient for the boundary layer during laminar flow regime is proportional to a factor expressed as the velocity divided by the hydraulic diameter, raised to a power 0.33. The flux across the membrane, as expressed in eqns. (2) and (3), should therefore increase as this factor increases. This is also the case as can be seen in Fig. 5 where the results from Fig. 3 and Fig. 4 have been plotted versus flow velocity divided by hydraulic diameter, and the importance of considering both these variables in module design is obvious. The flux behaviour presented in Fig. 4 is then easily explained. From eqn. (7) it can be seen that if the hydraulic diameter is increased at a constant Reynolds number then the crossflow velocity must decrease. This will lead to a decrease in the “hydrodynamic factor” in eqn. (10) for two reasons: by a decrease in the crossflow velocity and by an increase in the hy-

z.oe-010

0

twins-2%hexenal

1

Fig. 5. Hydrodynamic effects on partial fluxes. Undotted and dotted values correspond to data from Figs. 3 and 4 respectively. Symbols: experimental data, lines: model calculations.

draulic diameter. This will result in the observed flux decrease. The mass transfer model outlined above is represented in Figs. 3,4 and 5 by the solid lines. The model fits nicely for linalool, trans-2-hexenal and 1-pentene-3-01. For 2-methyl-propanal and 2-methyl-butanal small deviations from the experimental data occur, resulting in higher calculated fluxes than those experimentally observed. This indicates that the model assumption is not totally valid for these compounds. The first assumption, i.e. mass transfer independent of each other, could of course be studied by comparison with single component aroma model experiments. The second assumption, i.e. a linear membrane mass transfer coefficient, has been observed for dilute organic-water mixtures in the literature [2,13151. However, the concentrations of 2-methylpropanal and 2-methyl-butanal in the feed were very low, 9 ppm and 7 ppm respectively, and a non-linearity in the concentration dependence of the membrane mass transfer coefficient, &,i, would have a strong effect on the overall mass transfer coefficient, lz,,,i, in eqn. (2). The third assumption, zero concentration in the permeate, is supported by the low concentration of organic components in the permeate in combination with the low permeate pressure. The boundary layer effects are easy to understand if one considers the thickness of the boundary layer, & i.e. the distance in the feed where the penetrants are transported predominantly by diffusion. This distance can be calculated using the Sherwood correlation, eqns. (4) - (8). However, different molecules have different diffusion coefficients, Dip and consequently the region where diffusive mass transfer is important will be different for different molecules. In this study the molecules are rather small and have high and similar diffusion coefficients in water solutions. The differences in thickness of the boundary layers for the different molecules will therefore be small in com-

169

H.O.E. Karkson and G. TriigCrdhjJ. Membrane Sci. 81(1993) 163-171

parison to how much the thicknesses of the boundary layers is affected by the hydrodynamic conditions in the feed. The mean value of the thicknesses of the boundary layers for the different molecules has been calculated using eqns. (4)-(8) and (11) for a constant hydraulic diameter of 1.50 mm (Fig. 6).

(11) The thickness of the active layer in the membrane was 10 p as mentioned earlier, but from Fig. 6 it can be seen that the mean thickness of the boundary layer is much larger. Thus, in the laminar flow regime the distance in the feed where diffusive mass transport occurs is at least 4 times, and at some conditions as high as 30 times, larger than the diffusive distance in the membrane. Consequently the overall mass transfer from the feed to the permeate will be strongly affected by the feed boundary layer in the laminar flow regime, but when the flow regime switches to turbulence the thickness of the boundary layer becomes small and the diffusive mass transfer in the membrane will be rate determining. 300

,

The discussions above are interesting in optimizing the membrane unit operation, i.e. increased crossflow velocities and decreased hydraulic diameters result in increased organic fluxes or, in other words, better efficiency per membrane area. In an application to aroma compound recovery, as discussed in a previous paper [l], another feature of the feed flow effects becomes interesting if the organic fluxes are normalized to each other.as in eqn. (12). C&l

=-J1

(12)

This is done in Fig. 7 for a constant hydraulic diameter of 1.50 mm and it is obvious that the different feed flow dependencies of the different aroma compounds resulted in different relative concentrations of the aroma compounds in the permeate for different Reynolds numbers. The background to this is that as the crossflow velocity increased the mass transfer coefficients for the different components in the boundary layer also increased, but with different rates. For instance, the increment of the

I

c......-......-. ......... ...... 1-pentene-3-01

l

v z. 0.3 0.2

0

-__

100

1000

10000

Re (-1 Fig. 6. Calculated mean thickness of the boundary layer.

go

l

.

0.

..‘......

10

10

0

100

Re (-1 Fig. 7. Effect of crossflow velocity

composition.

1000

10000

on relativepermeate

H.O.E. Ku&son and G. TrhglirdhjJ. Membrane Sci. 81(1993) 163-171

170

overall mass transfer coefficient for linalool with increasing crossflow velocity was not as large as the overall mass transfer coefficient for all the components taken together and consequently its relative concentration was decreased. The overall mass transfer coefficient of trans-2-hexenal, on the other hand, increased faster than the overall mass transfer coefficient for all the components taken together and its relative concentration therefore increased under the same conditions. For a constant Reynolds number of 1,000, the influence of the hydraulic diameter on the relative composition of the permeate was not as drastic as the effect of the crossflow velocity (Fig. 8). In the investigated range only small effects on the relative composition were observed. The crossflow velocity and the hydraulic diameter of the module can thus be used to control, to some extent, the flavour of the permeate that is to be put back to the processed beverage. Consider a beverage that has a very unique and sensitive aroma compound composition which does not correspond to the permeate composition which is achieved with pervaporation at

o’67 0.5

cj-

v

t--

0.3

0 .... ...

..

v 0 0 0

0.2

l

linalool trans-Z-hexenal .2_methy!-butana! Z-m+ethyl-propanal 1-pentene-3-01

0.1 t 0.0

8

..... ... 8 I

1

2

1,

I

3 4 d, (mm)

,

Conclusions Concentration polarization effects during pervaporation of aroma compounds are important, both as a flux decreasing effect but also as a tool to control the composition of the permeate. In the case of aroma compound recovery with pervaporation the latter aspect could be very important in maintaining a desired aroma compound profile, i.e. a desired concentration ratio between the individual aroma compounds. The flux changes due to changes in crossflow velocities and changes in hydraulic diameters can be described mathematically by a simplified resistance-in-series model, which contains a Sherwood correlation and a membrane mass transfer coefficient. If this model is used to calculate the thickness of the boundary layer it becomes apparent that the diffusive mass transfer in the feed, rather than diffusive mass transfer in the membrane, in some cases is rate determining for the pervaporation process. Acknowledgements

l

l

l I

__I

..~..

high Reynolds numbers, but rather to the permeate composition for low Reynolds numbers, in a given pervaporation module. In such a case it could be acceptable to improve the organoleptic quality of the processed beverage by operating the pervaporation unit at low Reynolds numbers, i.e. at the cost of a reduced total organic flux.

/

I

5

6

Fig. 8. Effect of hydraulic diameter on relative permeate composition.

The authors wish to acknowledge NUTEK, Unilever Research and V&S Vin & Sprit AB for financial support of this study and Deutsche Carbone, Geschtiseinheit GFT for generous supply of the membranes. The authors are also grateful to Mr. Egon Svensson for his skilled craftsmanship.

H.O.E. Karlsson and G. TrtigiirdhjJ. Membrane Sci. 81(1993) 163-171

List of symbols a b zl 4, Yl : D J L Re SC Sh

constant (- ) constant (-) constant (-) constant (- ) hydraulic diameter (m) constant ( m/sec0.66) mass transfer coefficient (m/set ) crossflow velocity (m/set ) volumetric concentration (-) diffusion coefficient ( m2/sec) flux (m/set ) channel length (m) Reynolds number (- ) Schmidt number (- ) Sherwood number (-)

3

4

5

6

7

Greek letters s”

kinematic viscosity ( m2/sec ) boundary layer thickness (m)

8 9

Subscripts b f i

bulk of the feed feed permeating component numerator i m membrane mean mean value n number of components in aroma model overall ov permeate P rel relative

10

11 12

13

14

References H.O.E. Karlsson and G. Triiglrdh, Pervaporation of dilute organic-water mixtures - A literature review on modelling studies and applications to aroma compound recovery, J. Membrane Sci., 76 (1993) 121-146. R. Psaume, P. Aptel, Y. Aurelle, J.C. Mora and J.L. Bersillon, Pervaporation: Importance of concentration polarization in the extraction of trace organics from water, J. Membrane Sci., 36 (1988) 373-384.

15

16 17

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P. Ci% and C. Lipski, Mass transfer limitations in pervaporation for water and waste water treatment, in: R. Bakish (Ed.), Proc. third Int. Conf. on Pervaporation Processes in the Chemical Industry, Bakish Materials Corporation, Englewood, NJ, 1988, pp. 449462. H.H. Nijhuis, Removal of trace organics from water by pervaporation; A technical and economic analysis, Ph.D. thesis, University of Twente, Enschede, The Netherlands, 1990. B. Raghunath and S.-T. Hwang, Effect of boundary layer mass transfer resistance in the pervaporation of dilute organics, J. Membrane Sci., 65 (1992) 147-161. R. Gref, T. Nguyen and J. Nobel, Influence of membrane properties on system performances in pervaporation under concentration polarization regime, Sep. Sci. Technol., 27 (1992) 467-491. V. Gekas and B. HallstrGm, Mass transfer in the membrane concentration polarization layer under turbulent crossflow. I. Critical literature review and adaption of existing Sherwood correlations to membrane operations, J. Membrane Sci., 30 (1987) 153170. E. Matthiasson and B. Sivik, Concentration polarization and fouling, Desalination, 35 (1980) 59-103. R.H. Perry and D. Green, Perry’s Chemical Engineers’ Handbook, 6th edn., McGraw-Hill Book Co, Singapore, 1985, Chap. 3. J.M. Coulson and J.F. Richardson, Chemical Engineering, Vol. 1,3rd edn., Pergamon Press, Oxford, 1985, Table 6. H.E.A. Briischke, Deutsche Carbone, Geschjiftseinheit GFT, Personal communications, 1992. G.B. van den Berg, Unilever Research and T. Lindstriim, V&S Vin & Sprit AB, Personal communications, 1992. T.Q. Nguyen and K. Nobe, Extraction of organic contaminants in aqueous solution by pervaporation, J. Membrane Sci., 30 (1987) 11-22. R. Clement, Z. Bendhjama, Q.T. Nguyen and J. Nobel, Extraction of organics from aqueous solutions by pervaporation. A novel method for membrane characterization and process design in ethyl acetate separation, J. Membrane Sci., 66 (1992) 193-203. K.W. Biiddeker, G. Bengtson and E. Bode, Pervaporation of low volatility aromatics from water, J. Membrane Sci., 53 (1990) 143-158. F. Meskens, Janssen Chimica, Personal communications, 1993. P.M. Ginnings, E. Herring and D. Coltrane, Aqueous solubilities of some unsaturated alcohols, J. Am. Chem. Sot., 61 (1939) 807-808.