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Concentration polarization during the enrichment of aroma compounds from a water solution by pervaporation
Journalof Food Engineering19 (1993) 399-407
Research Note Concentration Polarization during the Enrichment of Aroma Compounds from a Water Solution by Pervaporation Eva Bengtsson, Gun Triigkdh* Department
of Food Engineering,
& Bengt Hallstrijm
Lund University, PO Box 124, S-22 100 Lund, Sweden
(Received 19 June 1992; accepted 19 June 1992)
ABSTRACT Pervaporation has proved to be an interesting technique for the recovery and enrichment of aroma compounds from very dilute solutions. In order to study the mass transfer, a 05ppm solution of butyl butyrate, an important aroma compound present in, for example, apple juice, was pervaporated at different circulation velocities along a PDMS (polydimethylsiloxane) membrane. The permeation of butyl butyrate increased with increasing Reynolds number, demonstrating an influence of concentration polarization. In order to minimize the effect of the polarized layer and to improve the mass transfer, optimization of the feed-circulation velocity is very important.
NOTATION bb C D
4
J k
Butyl butyrate Concentration Diffusivity Hydraulic diameter Flux Mass-transfer coefficient
*To whom correspondence
should be addressed.
399 Journal of Food Engineering
0260-8774/93/$06.00
Publishers Ltd, England. Printed in Great Britain
-
0
1993
Elsevier
Science
400
Eva Bengtsson, Gun Trlg&dh, Ben@ Hallstriim
L e Re SC Sh Y=P
Active membrane length Flow rate Reynolds number Schmidt number Sherwood number Experimental yield
f P
Evident factor, C~~~~*~/C~~~~ Polarization layer thickness Density INTRODUCTION
Pervaporation is a membrane operation whereby for instance, minor liquid components of a solution can be removed and enriched. The driving force for this separation is the difference in partial vapour pressures between the liquid feed and the vapour permeate. The low partial vapour pressure on the permeate side can be obtained by using a vacuum pump (vacuum pervaporation) or by employing a carrier gas (sweep-gas pervaporation). The vapour permeate is condensed at a low temperature. One application of pervaporation is the enrichment of aroma compounds, such as esters, aldehydes, and alcohols, from very dilute solutions (Ben~sson et al., 1990). The aroma compounds are preferenti~y permeated through a hydrophobic membrane with an active layer of, for example, polydimethylsiloxane (PDMS). Concentration polarization (depletion layer) of the solute at the membrane-liquid interface, a major problem in membrane-separation technology, has to date been neglected in pervaporation, and very few papers have been pub~shed in this field. Condensation pola~ation may, however, induce some undesirable effects if the faster permeant is present as a trace in the feed mixture, If this is the case, the soluteconcentration gradient in the vicinity of the upstream interface is not very high, and the membrane is not fed rapidly enough with the solute (N&e1 1991). Accordingly, the outcome of the process may depend on flow conditions, i.e. flow velocity and the geometry of the channel. In the case of a very low solute concentration, as in aroma containing condensate from a fruit-juice evaporation plant, the mass transfer of aroma compounds through a pervaporation membrane could thus be limited by concentration pola~za~on. In order to study the mass transfer in more detail, a 0.5ppm solution of butyl butyrate was used as a feed in pervaporation experiments carried out at different feed velocities. The butyl butyrate flux was plotted as a function of the Reynolds number in
Concentration polarization in enrichment of aroma compounds
401
order to evaluate the effect of concentration polarization. Butyl butyrate was chosen as the solute since it was found to have the highest enrichment factor of twelve aroma compounds investigated (Bengtsson et 1990).
af,
MATERIALS
AND METHODS
Feed solution A refrigerated 500-ppm solution of butyl butyrate was diluted prior to each experiment to obtain a 05ppm feed solution. Experimental set-up The experimental set-up was a pilot-plant pervaporator described previously (Bengtsson et al., 1990). A plate module from GFT (Gesellschaft fiir Treaters, FRG) was equipped with composite membranes with an active layer of silicone rubber (PDMS) on a poly~de-polyimide support and a total membrane area of 0.163 m2. The module configuration with the feed inlet situated at the corner of the rectangular plate is shown in Fig. 1. For calculations of the Reynolds number, the hydraulic diameter, I),, was calculated by using the dimensions of the cross-sectional area shown in Fig. 1. Experimental conditions The membranes were rinsed with water for one day prior to the experiments. In the pe~a~ration experiments, the feed tempera~re was
inlet Fig. 1.
Module (channel) configuration
and dimensions
(mm) for calculation of D,.
402
Eva Bengtsson, Gun Triigcirdh, Bengt Halkitriim
maintained at 2-5°C and the permeate pressure at 5-6 mbar. The feed velocity was varied between 0.1 and 3.9 cm/s, corresponding to Reynolds numbers between 1.4 and 51. The experiments were run in a once-through mode. Each experiment was run for between three and five hours. Analysis The feed, the retentate, and the permeate were analysed by gas chromatography, as described previously (Bengtsson et al., 1990), 2-pentanol being used as an internal standard. The enrichment factor of butyl butyrate was calculated as the ratio of the permeate concentration to the feed concentration (0.5 ppm). The experimental yield ( Ye_,)was calculated as:
yexp =Cm,Cpermeatex Qpermeate Qreed bb.feed x
where C,, is the concentration
of butyl butyrate and Q the flow rate.
RESULTS AND DISCUSSION The partial water flux was independent of the Reynolds number and was approximately 94 g/m’ h. A deviation from this figure was noticed at the lowest Reynolds number (1.4) when a higher value, 130 g/m2 h, was obtained. Figure 2 shows the flux of butyl butyrate as a function of the Reynolds number, The permeation of butyl butyrate increased with increasing velocity in the module. If the increase in the butyl butyrate flux with increasing feed velocity was caused by an increase in the effective membrane area, the water flux would at the same time be expected to increase at higher velocities. This was not the case, as stated earlier. After the last experiment at the highest feed velocity (3.9 cm/s), the experiment at the lowest velocity (0.1 cm/s) was repeated with the same flux of butyl butyrate as earlier. For this reason, fouling is not considered to be a problem. The results thus demonstrate the influence of concentration polarization (depletion layer) of the solute at the feed side of the membrane at the low Reynolds numbers considered. The results are in agreement with the results of Psaume et al ( 1988).
Concentration po~rization in enrichment of aroma compound
403
3 2.8 -
0
c1
2.62.4 22 21.8 1.6 -
0
1.4 -j 12 0 I-
0.8 0.6 -
q
0.4 02 0
I
I
2ki
4kJ
I
)
The flux of butyl butyrate j&J as a function of the Reynolds number (Re).
Psaume et al. (1988) derived mass-transfer equations under certain conditions also valid in the experiments described here. Further, they assumed that: * the convective flux is small, i.e. the radial Peclet number compared with unity; and 0 the selectivity is high.
is small
On the assumption that these conditions are also valid in the present experiments (Pe, = 10 -6 % 1), the solute flux may be calculated from: J bb =
kp
ebb, feed
where k = D/S is the mass-transfer coefficient. According to this relationship, k is proportional to J. In Fig. 3, log J is plotted versus log Re. A straight line with the slope 1 f3 is also shown in the diagram. This value of the exponent is from the Leveque correlation for laminar-flow conditions (Psaume etal., 1988): Sh = 162 (Re SC D,/L )1/3 The experimental results agree quite well with the straight line representing the slope of the Sh-Re relationship.
404
Bengtsson, Gun TrigLirdh,Bengt Halistrtim
Fig. 3.
Log Jbh as a function of log Re: D, ex~~mental A Levkque correlation.
results; -,
linear regression;
m 60-
N-
40-
9 30(Ii
20-
10 -
0
0
1
/
lo
I
40
Re
Fig. 4.
The butyl butyrate enrichment
factor as a function of the Reynolds number.
Concentration polarization in enrichment of aroma cornpow&
405
As a result of the increased solute permeation, the enrichment factor increased with increasing Reynolds number, as shown in Fig. 4. The highest enrichment factor obtained in this study, /I = 63 at Re = 5 1, was lower than the previously obtained enrichment factor, p = 125 at Re = 24 (Bengtsson et aZ., 1990), at the same feed temperature. However, these previous experiments differed from the concentration-polarization experiments in a number of ways: a mixture of twelve compounds was used, the initial butyl butyrate concentration was 0.25 ppm, and the feed concentration decreased with tune. When a mixture of compounds is pervaporated, coupling effects between the different components are likely to occur, which may change the separation characteristics of the membrane (Kedem, 1989). Figure 5 shows the yield as a function of the Reynolds numbers. According to Psaume et al. (1988), the yield is only dependent on the Reynolds number for a given system with a defined solvent-solute diffusivity and capillary-membrane geometry. A very low concentration of solute in the inlet stream and high selectivity of the membrane are assumed. As can be seen from Fig. 5, the yield decreased with increasing Reynolds number, which is in agreement with the results of Psaume et al. (1988). When the yield was compared with the part of butyl butyrate retained in the retentate, the mass balance was found to improve with
I-
0.9 OS 0.7 0.6 7
g
050.4 0.3 02 - o
lie
Fig. 5.
The yield of butyl butyrate as a function of the Reynolds number.
406
Eva Bengtsson, Gun Trtigirdh, Bengt Hallkim 110 ml
-
0
90-
0
0
SO70-
0
6050 0 40302010 0
I
I
2b
4b
!
Rc
Fig. 6.
The mass balance of butyl butyrate (bb) as a function of the Reynolds number.
increasing Reynolds numbers, (see Fig. 6). The mass balance of the last experiment at the lowest velocity was similar to that of the first experiment at the same velocity. CONCLUSIONS During the enrichment pervaporation of butyl butyrate, an aroma compound present in, for example, apple juice, the increase in butyl butyrate flux with increasing Reynold number demonstrates an influence of concentration polarization. In order to minimize this effect, it is very important to optimize the feed-circulation velocity during pervaporation of such streams. The module geometry is also of importance. ACKNOWLEDGEMENTS This work was supported by the National Swedish Board for Technical Development. The authors wish to acknowledge Jan Nilsson for valuable advice and discussions, Dan Johansson for help with the experiments, and Gesellschaft fur Trenntechnik mbH, FRG, for providing the membranes.
Concentration polarization in enrichment of aroma compounds
407
REFERENCES E., Tragardh, G. & Hallstrom, B. (1990). Entichment of Aroma Compounds by Pervaporation, in Engineering and Food, Volume 3, Advanced Processes, eds W. E. L. Spiess & H. Schubert. Elsevier Science Publishers Ltd,
Bengtsson,
Barking, Essex, UK, pp. 270-9. Kedem, 0. (1989). The role of coupling in pervaporation.
.I. Membrane Sci., 47,
277-84.
NCel, J. (1991). Introduction to pervaporation. In Pervuporation Membrane Separation Processes, ed. R. Y. M. Huang, Elsevier Science Publishers BV, Amsterdam, The Netherlands, pp. 1- 109. Psaume, R., Aptel, Ph., Aurelle, Y., Mora, J. C. & Bersillon, J. L. (1988). Pervaporation: importance of concentration polarization in the extraction of trace organics from water. J. Membrane Sci., 36,373-84.
Research Note Concentration Polarization during the Enrichment of Aroma Compounds from a Water Solution by Pervaporation Eva Bengtsson, Gun Triigkdh* Department
of Food Engineering,
& Bengt Hallstrijm
Lund University, PO Box 124, S-22 100 Lund, Sweden
(Received 19 June 1992; accepted 19 June 1992)
ABSTRACT Pervaporation has proved to be an interesting technique for the recovery and enrichment of aroma compounds from very dilute solutions. In order to study the mass transfer, a 05ppm solution of butyl butyrate, an important aroma compound present in, for example, apple juice, was pervaporated at different circulation velocities along a PDMS (polydimethylsiloxane) membrane. The permeation of butyl butyrate increased with increasing Reynolds number, demonstrating an influence of concentration polarization. In order to minimize the effect of the polarized layer and to improve the mass transfer, optimization of the feed-circulation velocity is very important.
NOTATION bb C D
4
J k
Butyl butyrate Concentration Diffusivity Hydraulic diameter Flux Mass-transfer coefficient
*To whom correspondence
should be addressed.
399 Journal of Food Engineering
0260-8774/93/$06.00
Publishers Ltd, England. Printed in Great Britain
-
0
1993
Elsevier
Science
400
Eva Bengtsson, Gun Trlg&dh, Ben@ Hallstriim
L e Re SC Sh Y=P
Active membrane length Flow rate Reynolds number Schmidt number Sherwood number Experimental yield
f P
Evident factor, C~~~~*~/C~~~~ Polarization layer thickness Density INTRODUCTION
Pervaporation is a membrane operation whereby for instance, minor liquid components of a solution can be removed and enriched. The driving force for this separation is the difference in partial vapour pressures between the liquid feed and the vapour permeate. The low partial vapour pressure on the permeate side can be obtained by using a vacuum pump (vacuum pervaporation) or by employing a carrier gas (sweep-gas pervaporation). The vapour permeate is condensed at a low temperature. One application of pervaporation is the enrichment of aroma compounds, such as esters, aldehydes, and alcohols, from very dilute solutions (Ben~sson et al., 1990). The aroma compounds are preferenti~y permeated through a hydrophobic membrane with an active layer of, for example, polydimethylsiloxane (PDMS). Concentration polarization (depletion layer) of the solute at the membrane-liquid interface, a major problem in membrane-separation technology, has to date been neglected in pervaporation, and very few papers have been pub~shed in this field. Condensation pola~ation may, however, induce some undesirable effects if the faster permeant is present as a trace in the feed mixture, If this is the case, the soluteconcentration gradient in the vicinity of the upstream interface is not very high, and the membrane is not fed rapidly enough with the solute (N&e1 1991). Accordingly, the outcome of the process may depend on flow conditions, i.e. flow velocity and the geometry of the channel. In the case of a very low solute concentration, as in aroma containing condensate from a fruit-juice evaporation plant, the mass transfer of aroma compounds through a pervaporation membrane could thus be limited by concentration pola~za~on. In order to study the mass transfer in more detail, a 0.5ppm solution of butyl butyrate was used as a feed in pervaporation experiments carried out at different feed velocities. The butyl butyrate flux was plotted as a function of the Reynolds number in
Concentration polarization in enrichment of aroma compounds
401
order to evaluate the effect of concentration polarization. Butyl butyrate was chosen as the solute since it was found to have the highest enrichment factor of twelve aroma compounds investigated (Bengtsson et 1990).
af,
MATERIALS
AND METHODS
Feed solution A refrigerated 500-ppm solution of butyl butyrate was diluted prior to each experiment to obtain a 05ppm feed solution. Experimental set-up The experimental set-up was a pilot-plant pervaporator described previously (Bengtsson et al., 1990). A plate module from GFT (Gesellschaft fiir Treaters, FRG) was equipped with composite membranes with an active layer of silicone rubber (PDMS) on a poly~de-polyimide support and a total membrane area of 0.163 m2. The module configuration with the feed inlet situated at the corner of the rectangular plate is shown in Fig. 1. For calculations of the Reynolds number, the hydraulic diameter, I),, was calculated by using the dimensions of the cross-sectional area shown in Fig. 1. Experimental conditions The membranes were rinsed with water for one day prior to the experiments. In the pe~a~ration experiments, the feed tempera~re was
inlet Fig. 1.
Module (channel) configuration
and dimensions
(mm) for calculation of D,.
402
Eva Bengtsson, Gun Triigcirdh, Bengt Halkitriim
maintained at 2-5°C and the permeate pressure at 5-6 mbar. The feed velocity was varied between 0.1 and 3.9 cm/s, corresponding to Reynolds numbers between 1.4 and 51. The experiments were run in a once-through mode. Each experiment was run for between three and five hours. Analysis The feed, the retentate, and the permeate were analysed by gas chromatography, as described previously (Bengtsson et al., 1990), 2-pentanol being used as an internal standard. The enrichment factor of butyl butyrate was calculated as the ratio of the permeate concentration to the feed concentration (0.5 ppm). The experimental yield ( Ye_,)was calculated as:
yexp =Cm,Cpermeatex Qpermeate Qreed bb.feed x
where C,, is the concentration
of butyl butyrate and Q the flow rate.
RESULTS AND DISCUSSION The partial water flux was independent of the Reynolds number and was approximately 94 g/m’ h. A deviation from this figure was noticed at the lowest Reynolds number (1.4) when a higher value, 130 g/m2 h, was obtained. Figure 2 shows the flux of butyl butyrate as a function of the Reynolds number, The permeation of butyl butyrate increased with increasing velocity in the module. If the increase in the butyl butyrate flux with increasing feed velocity was caused by an increase in the effective membrane area, the water flux would at the same time be expected to increase at higher velocities. This was not the case, as stated earlier. After the last experiment at the highest feed velocity (3.9 cm/s), the experiment at the lowest velocity (0.1 cm/s) was repeated with the same flux of butyl butyrate as earlier. For this reason, fouling is not considered to be a problem. The results thus demonstrate the influence of concentration polarization (depletion layer) of the solute at the feed side of the membrane at the low Reynolds numbers considered. The results are in agreement with the results of Psaume et al ( 1988).
Concentration po~rization in enrichment of aroma compound
403
3 2.8 -
0
c1
2.62.4 22 21.8 1.6 -
0
1.4 -j 12 0 I-
0.8 0.6 -
q
0.4 02 0
I
I
2ki
4kJ
I
)
The flux of butyl butyrate j&J as a function of the Reynolds number (Re).
Psaume et al. (1988) derived mass-transfer equations under certain conditions also valid in the experiments described here. Further, they assumed that: * the convective flux is small, i.e. the radial Peclet number compared with unity; and 0 the selectivity is high.
is small
On the assumption that these conditions are also valid in the present experiments (Pe, = 10 -6 % 1), the solute flux may be calculated from: J bb =
kp
ebb, feed
where k = D/S is the mass-transfer coefficient. According to this relationship, k is proportional to J. In Fig. 3, log J is plotted versus log Re. A straight line with the slope 1 f3 is also shown in the diagram. This value of the exponent is from the Leveque correlation for laminar-flow conditions (Psaume etal., 1988): Sh = 162 (Re SC D,/L )1/3 The experimental results agree quite well with the straight line representing the slope of the Sh-Re relationship.
404
Bengtsson, Gun TrigLirdh,Bengt Halistrtim
Fig. 3.
Log Jbh as a function of log Re: D, ex~~mental A Levkque correlation.
results; -,
linear regression;
m 60-
N-
40-
9 30(Ii
20-
10 -
0
0
1
/
lo
I
40
Re
Fig. 4.
The butyl butyrate enrichment
factor as a function of the Reynolds number.
Concentration polarization in enrichment of aroma cornpow&
405
As a result of the increased solute permeation, the enrichment factor increased with increasing Reynolds number, as shown in Fig. 4. The highest enrichment factor obtained in this study, /I = 63 at Re = 5 1, was lower than the previously obtained enrichment factor, p = 125 at Re = 24 (Bengtsson et aZ., 1990), at the same feed temperature. However, these previous experiments differed from the concentration-polarization experiments in a number of ways: a mixture of twelve compounds was used, the initial butyl butyrate concentration was 0.25 ppm, and the feed concentration decreased with tune. When a mixture of compounds is pervaporated, coupling effects between the different components are likely to occur, which may change the separation characteristics of the membrane (Kedem, 1989). Figure 5 shows the yield as a function of the Reynolds numbers. According to Psaume et al. (1988), the yield is only dependent on the Reynolds number for a given system with a defined solvent-solute diffusivity and capillary-membrane geometry. A very low concentration of solute in the inlet stream and high selectivity of the membrane are assumed. As can be seen from Fig. 5, the yield decreased with increasing Reynolds number, which is in agreement with the results of Psaume et al. (1988). When the yield was compared with the part of butyl butyrate retained in the retentate, the mass balance was found to improve with
I-
0.9 OS 0.7 0.6 7
g
050.4 0.3 02 - o
lie
Fig. 5.
The yield of butyl butyrate as a function of the Reynolds number.
406
Eva Bengtsson, Gun Trtigirdh, Bengt Hallkim 110 ml
-
0
90-
0
0
SO70-
0
6050 0 40302010 0
I
I
2b
4b
!
Rc
Fig. 6.
The mass balance of butyl butyrate (bb) as a function of the Reynolds number.
increasing Reynolds numbers, (see Fig. 6). The mass balance of the last experiment at the lowest velocity was similar to that of the first experiment at the same velocity. CONCLUSIONS During the enrichment pervaporation of butyl butyrate, an aroma compound present in, for example, apple juice, the increase in butyl butyrate flux with increasing Reynold number demonstrates an influence of concentration polarization. In order to minimize this effect, it is very important to optimize the feed-circulation velocity during pervaporation of such streams. The module geometry is also of importance. ACKNOWLEDGEMENTS This work was supported by the National Swedish Board for Technical Development. The authors wish to acknowledge Jan Nilsson for valuable advice and discussions, Dan Johansson for help with the experiments, and Gesellschaft fur Trenntechnik mbH, FRG, for providing the membranes.
Concentration polarization in enrichment of aroma compounds
407
REFERENCES E., Tragardh, G. & Hallstrom, B. (1990). Entichment of Aroma Compounds by Pervaporation, in Engineering and Food, Volume 3, Advanced Processes, eds W. E. L. Spiess & H. Schubert. Elsevier Science Publishers Ltd,
Bengtsson,
Barking, Essex, UK, pp. 270-9. Kedem, 0. (1989). The role of coupling in pervaporation.
.I. Membrane Sci., 47,
277-84.
NCel, J. (1991). Introduction to pervaporation. In Pervuporation Membrane Separation Processes, ed. R. Y. M. Huang, Elsevier Science Publishers BV, Amsterdam, The Netherlands, pp. 1- 109. Psaume, R., Aptel, Ph., Aurelle, Y., Mora, J. C. & Bersillon, J. L. (1988). Pervaporation: importance of concentration polarization in the extraction of trace organics from water. J. Membrane Sci., 36,373-84.