- Email: [email protected]
Development of an anion-exchange membrane with increased permeability for organic acids of high molecular weight
Desalination,
249
79 (1990) 249-260
Elsevier Science Publishers B.V., Amsterdam
Development of an Anion-Exchange Membrane with Increased Permeability for Organic Acids of High Molecular Weight W. GUDERNATSCH’,
CH. KRUMBHOLZ2
and H. STRATHMANN2
‘SEMPAS Membmntechnik GmbH, Pappelstr. 19/l, 7240 Herb a.N. 2Fmunhofer-Institut fiir Grenzfliichen- and Bioverfahrenstechnik, Nobelstn$e 7000 Stuttgart 80 (Germany)
12,
(Received August 12,199O; in revised form December 5, 1990)
SUMMARY
Ion-exchange membranes are mainly used today in electrodialysis for desalination of sea and brackish water. The membranes have been optimized for this application in terms of their ion selectivity and their mechanical, chemical and electrical properties. They are, however, unsatisfactory for the separation of organic components such as amino acids because of their poor permeability for larger anions which is required in certain applications in the food and pharmaceutical industry. In this paper the development of an anion-exchange membrane with high permeability for large organic anions and satisfactory mechanical, chemical and electrical properties is described. The membrane is prepared by changing the degree of cross-linking of the basic ion-exchange polymer.
1. INTRODUCTION
Ion-exchange membranes have been developed primarily for the separation of salts containing small inorganic ions such as Na ‘, Kt , Cat ‘, Mg ’ ’ or PO:-, NO;, OH-, SO: and Cl- from water by electrodialysis. They have been optimized with respect to high selectivity, low electrical resistance and good mechanical strength. Low resistance for the permeation of larger ions has not been a major issue in this application. OOll-9164/90/$03.50
0 1990 Elsevier Science Publishers B.V.
250
However, recent trends in biotechnology and food processing indicate a growing interest in the electrodialytic separation of organic acids from aqueous solutions. In contrary to conventional removal of inorganic salts, this application is characterized by the requirement to remove relatively large anions. Since such large anions have not been taken into consideration during the development of the conventional anion-exchange membranes, these membranes are highly cross-linked to improve the mechanical stability and to avoid excessive swelling. This high degree of cross-linking has no significant effect on the permeability of the small salt-ions. However, experiments that have been carried out to explore the applicability of electrodialysis in food and biotechnology yielded very low permeabilities or organic ions in the conventional ion-exchange membranes. Their permeation resistance appeared to increase drastically with increasing molecular weight of the permeating anions. This observation led to the conclusion that a variation in the “mesh” size of a cross-linked anionexchange membrane must have an effect on the permeability of the larger anions. Therefore, it should be possible to improve the permeability of larger anions adjusting the degree of cross-linking and the chain length of the cross-linking agent. It can be assumed that “loosening” the network would yield better permeabilities for organic anions and therefore an improved separation efficiency of electrodialysis in biotechnology and food applications. An ion-exchange membrane, of course, always needs a certain degree of cross-linking density to prevent significant losses of its mechanical stability and permselectivity [ 11.
2.
EXPERIMENTAL PROCEDURES
To study the effect of polymer cross-linking on the permeability of different size anions, membranes were prepared from anion-exchange polymers with various degrees of cross-linking. These membranes have then been characterized in terms of their physico-chemical and chemical properties and their ion permeability.
2.1. Selection of the polymer system The polymer system selected for this study consisted of a basic molecular chain and mono- and bifunctional molecules as cross-linking agents allowing variations in the degree of cross-linkage at constant chain length an charge density. The bifunctional molecules generate cross-linkages as well as quaternized amines, i.e. positive charges, whereas the monofunctional
2.51
components only generate positive charges. The polymer system used in this study is based on poly(4-vinyl-pyridine). Therefore, halogenized alkanes could be used as mono- and bifunctional components. The monofunctional molecule is used to quaternize the nitrogen atom of the pyridine and thus generate one positive charge. The bifunctional is both quatemizing two nitrogen atoms and cross-linking two polymer chains between two pyridine rings. Fig. 1 shows a two-dimensional section of the polymer network between two poly(4-vinyl-pyridine) backbones. Di-bromo-hexane has been used as a bifunctional cross-linking agent. Therefore, the length of crosslinking molecule is 6 C-atoms. The monofunctional molecule is iodomethane. This system allows the synthesis of the wanted anion-exchange polymer with constant charge density and variable cross-linking density.
CL2 \
“2
\
“2
cd
c 4 2 t?
HZ
Fig. 1. Size comparison between the polymer used in this study and the permeants.
On the left side of Fig. 1 a mesh of 50% cross-linked polymer is shown in comparison to the 100% cross-linked part on the right side. Furthermore, the ideal molecular diameters of a Cl- and lactobionate-ion are plotted in order to demonstrate the size relation between polymer network and permeants. It becomes evident that the bigger lactobionate-ion will have problems to penetrate a 100% cross-linked network, whereas the 50% crosslinked network will ahow permeation more easily. The small Cl-ion can penetrate both structures.
252
2.2. Membrane preparation The membranes prepared for the following experiments have been varied in cross-linking density at a constant length of the cross-linking molecule which contained 6 C-atoms. The degree of cross-linking is defined as:
where v is the degree of cross-linking in %, nBi the number of bifunctional molecules and n the number of quaternizable nitrogen atoms. The number of the monofunctional components nM,, required to quaternize the remaining nitrogen atoms is given by: nMo = n-2nai Anion-exchange been prepared.
(2) membranes
cross-linked at 5, 20, 50, 70 and 100% have
2.3. Membrane characterization The membranes have been characterized with respect to their swelling behavior, their electrical resistance, their permselectivity and their anion permeability. The commercially available Asahi Glass AMV membrane was thereby used as reference. 2.3.1. Membrane swelling To determine the degree of swelling, the membrane samples have been equilibrated for several days in deionized water of 25°C. Then they are removed from the water, wiped clean at the surfaces and weighed. After being vacuum dried in an exsiccator over calcium chloride for 24 h, they are weighed again. Fig. 2 depicts the results. The degree of swelling is defined as the ratio of the mass of-water absorbed by the membrane to the mass of the swollen membrane. The diagram of Fig. 2 indicates that the membranes studied in this paper showed only a very moderate dependence on the degree of cross-linking. According to Fig. [l] the degree of swelling increases slightly with decreasing degree of cross-linkage up to 30%. The membranes with lower cross-linkage show an opposite behaviour, however. Their irregular behaviour is explicable by the formation of partially crystalline areas in the polymer matrix.
253
80 T -&
60
.-L? =
40
20
40
60
8b
lo.0
Degree of cross-linking (%) Fig. 2. Membrane swelling as a function of the degree of cross-linking.
2.3.2. Electrical resistanceof the membrane The electrical resistance of the anion-exchange membranes was determined by measuring the conductivity of the membrane in a 0.5 molar aqueous solution of potassium chloride. Fig. 3 shows the test results. The electrical resistance of the membrane decreases slightly with increasing degree of cross-linking between 40-100% cross-linking. Below 40% crosslinking, however, the electrical resistance increases drastically. Again this irregularity can be explained as a consequence of the formation of crystalline areas in the membrane polymer, since crystalline areas are no longer ionconductive. 2.3.3. Membrane permselectivity According to the initial hypothesis, cross-linking can be varied in two manners: either the length of the cross-linking molecular chain or the degree of cross-linking can be changed. Variation of the chain length would increase the mean spatial distance between the positive charges in the pyridine rings. This must lead to a decrease of permselectivity since the repulsive forces on a cation trying to permeate the anion-exchange membrane are weakened. If the degree of cross-linking is varied, only a slight decrease of permselectivity is expected due to the slightly increased mobility of the chain segments between the points of cross-linkage. The work described in this paper was focused on the variation of the degree of cross-linking only;
254
therefore, no big changes in permselectivity with changing degree of crosslinking were expected.
1 0
20
40
60
80
100
Degree of cross-linking (%) Fig. 3. Electrical resistance of the membrane as a function of the degree of cross-linking.
The permselectivity of an ion-exchange membrane is calculated from the ratio of the experimentally determined diffusion potential of a given membrane between ion solutions of different concentrations on each side of the membrane to the theoretically calculated potential difference for a 100% permselective membrane [2]. The apparent permselectivity P”+ of the membrane is given by
is the measured potential difference between the two electrowhere AE is positive for a cation- and negative for lytes. The zsolute value of AE an anion-exchange membrane. ? or a system consisting of, as an example, standardized aqueous solutions of 0.1 N and 0.5 N KC1 at 25”C, this theoretical potential difference amounts to 36.94 mV. The potential difference between 0.1 and a 0.5 KCl-solution separated by the anion-exchange membrane was measured using two calomel electrodes. Fig. 4 shows the test results.
255
85
80 20
40
60
80
100
Degree of cross-linking (Oh) Fig. 4. PermseAectivity as a function of the degree of cross-linking.
These results indicate that the permselectivities of the anion-exchange membranes prepared in this study are in the same range (91.0%-92.3%) as the permselectivity of the reference membrane (91.2%). The initial concern that a variation of the degree of cross-linkage might not severely affect the repulsion of cations has been confirmed by these results. 2.3.4. Permeabilityof monovdent anions To test the permeability of the prepared membranes with respect to the anion size and independently of the anion charge, four different monovalent ions have been selected. Their molecular weight ranges from 35.5 g/mole to 357.3 g/mole. All of them have been tested as monovalent acids. In Fig. 5 these anions, their structure, their molecular weight (MW) and their molecular diameter (d) are summarized [3]. The chemical nature of these substances had no influence on the selection of the ions since the electrical charge is the main interactive force between membrane material and permeant. The idealized molecular diameters increase to roughly the double and the corresponding molecular weights about tenfold. According to the hypothesis illustrated in Fig. 2, the smallest anion should remain unaffected by the degree of cross-linkage of the membrane, whereas the lactobionate and probably the intermediates should strongly depend on the degree of cross-linking.
2%
structure
anion
cr
OH
MW (g/mol)
d (nm)
chloride
35.5
propionate
73.1
galacturate
211.2
0.60
lactobionate
357.3
0.71
0.33
Fig. 5. Structures, molecular weights and diameters of the monovalent test anions used in this Study.
3. ELECIRODIALYSIS
TEST
The membranes used in this study were characterized in terms of their permeability in an electrodialysis test. The stack used in these experiments consisted of three cell pairs. The effective membrane area was 100 cm2 and the cell thickness 0.5 mm. The flow velocity within the compartments was adjusted to 6.5 cm/s, the temperature was kept at 20°C and the voltage drop across the stack was 10 V. The acid concentration in the feed solution was 0.1 molar. The system was run in a batch operating mode with volume in both the diluate and concentrate being identical. The electrodes were rinsed with 0.5 molar Na2S0, solution.
3.1. Determination of the anion permeability The electrodialysis experiments are evaluated by plotting the concentrations of the concentrate and the diluate solution vs. time. Fig. 6 shows the concentration change of a galacturonic acid solution in the diluate and the concentrate as a function of time using anion-exchange membranes with various degrees of cross-linking and a conventional Asahi Glass AMV as a reference membrane.
257
4 149
I
a AMV concentrate q
AMV diluate
6 100% cross-linked cont. 0 100% cross-linked diluate 50% cross-linked cont. d ? G
80
t
60
50% cross-linked diluate 20% cross-linked cont. 20% cross-linked diluate 5% cross-linked cont. 5% cross-linked diluate
40 0
15
30 time (min)
45
60
Fig. 6. Electrodialysis experiment using 0.1 molar galacturonic acid as test solution and anionexchange membranes with various degrees of cross-linking.
In order to get characteristic relation has been used:
d,
ni=-=-=4
4 4
numbers for the anion flux, the following
Co- cl5 15 min
with n being the molar flow through the anion-exchange membranes in the described stack and c the concentration of the acids in the diluate. The subscripts 0 and 15 refer to the concentration at the beginning and after 15 min of the test. The linear part of the diluate concentration curve has been taken as the relevant concentration difference for the flux determination. This number quantifies the initial flux in the batch experiment. It is characteristic for the permeability because the driving force and the dimensions of the apparatus have been kept constant. Fig. 7 shows the relative permeabilities of the four test anions illustrated in Fig. 5 as a function of the degree of cross-linking of the membrane. The relative permeability is defined as the quotient of the anion flux across the synthesized membrane and the anion flux across the reference membrane.
258
q 4 8 4
0
’ I
20 Degree
0
Fig. 7. Relative permeabilities lil&lg.
I I
P HCI rel P Prop rel
P Gal rel P Lac rel
1 I
40 60 of cross-linking
I
80 (Oh)
100
of the different anions as a function of the degree of
CTOSS-
5 r:
“E
4
3 z g
3
5 = -0
2
id .g .= ”
1
0
0
20
40
SO
SO
lb0
degree of cross-linking (%) Fig. 8. Flux of citric acid in an electrodialysis test in an Asahi reference membrane and in the membrane developed in this study as a function of the degree of cross-linking.
259
3.2. ElectrodiaZysis in the recoveryof citricacid Citric acid is a biotechnical mass product. In the conventional production it is recovered from the bioreactor by precipitation or solvent extraction [4]. Electrodialysis appears to be an attractive alternative to these downstream processes. As an organic acid citric acid is expected to suffer sterical hindrance while permeating conventional anion-exchange membranes. In order to evaluate the potential improvement of permeability, citric acid has been permeability tested in the same way as the monovalent test anions described above with the membranes developed in this study. Fig. 8 shows the test results. The permeability can be improved about 100%. This means that electrodialysis plants in the citric acid recovery can be installed with about half of the conventional membrane area, which significantly lowers the investment costs.
4.
DISCUSSION OF THE RESULTS The
basic assumption that a loosening of the polymeric network will yield improved permeabilities for organic anions of higher molecular weight has been confirmed experimentally. It was shown that the polymer network can be loosened by two different measures: The length of the cross-linking molecules can be increased or their density can be decreased. The latter has been realized experimentally. The permeabilities of these membranes towards large anions turned out to increase with decreasing degree of crosslinking in the range from about 25100% cross-linking. Below 25% crosslinking irregularities were observed. Water swelling of the membrane as well as the permeabilities of the smaller anions, i.e. chloride and propionate, decreased again. An explanation for this behavior is the increased electrical resistance of membranes with a low degree of cross-linking. The large lactobionate ion, however, shows the expected behavior. Its permeability increases with a decreasing degree of cross-linking over the whole range. This fact clearly shows that the lactobionate ion actually suffers sterical hindrance as suggested by the size comparison plotted in Fig. 2. Its permeability can be improved threefold just by decreasing the degree of crosslinking. The galacturate ion shows only a slight dependence on the degree of cross-linking. The potential described of membrane improvement for the permeability of large organic anions has not yet been proven for the long term in technical installations. However, the possible cost reduction due to the reduction in the required membrane area should lead to an increased use of electrodialysis in the food and drug industry.
260 ACKNOWMEDGEMENT
The authors would like to thank Bernd Bauer for his chemistry advice.
REFERENCES 1 F. Helfferich, Ion-Exchange, McGraw-Hill, New York, 1962. 2 E. Komgold, Electrodialysis-Membranes and Mass Transport, in Synthetic Membrane Processes, G. Belfort, ed., Academic Press, New York, pp. 192-219,1984. 3 Handbook of Chemistry and Physics, 64th Edition, CRC Press, Boca Raton, Florida, 1984. 4 J.D. Enzminger and JA. Asenjo, Use of cell recycle in the aerobic fermentative production of citric acid by yeast, Biotechnology Letters, 8(l) (1982) 7-12.
249
79 (1990) 249-260
Elsevier Science Publishers B.V., Amsterdam
Development of an Anion-Exchange Membrane with Increased Permeability for Organic Acids of High Molecular Weight W. GUDERNATSCH’,
CH. KRUMBHOLZ2
and H. STRATHMANN2
‘SEMPAS Membmntechnik GmbH, Pappelstr. 19/l, 7240 Herb a.N. 2Fmunhofer-Institut fiir Grenzfliichen- and Bioverfahrenstechnik, Nobelstn$e 7000 Stuttgart 80 (Germany)
12,
(Received August 12,199O; in revised form December 5, 1990)
SUMMARY
Ion-exchange membranes are mainly used today in electrodialysis for desalination of sea and brackish water. The membranes have been optimized for this application in terms of their ion selectivity and their mechanical, chemical and electrical properties. They are, however, unsatisfactory for the separation of organic components such as amino acids because of their poor permeability for larger anions which is required in certain applications in the food and pharmaceutical industry. In this paper the development of an anion-exchange membrane with high permeability for large organic anions and satisfactory mechanical, chemical and electrical properties is described. The membrane is prepared by changing the degree of cross-linking of the basic ion-exchange polymer.
1. INTRODUCTION
Ion-exchange membranes have been developed primarily for the separation of salts containing small inorganic ions such as Na ‘, Kt , Cat ‘, Mg ’ ’ or PO:-, NO;, OH-, SO: and Cl- from water by electrodialysis. They have been optimized with respect to high selectivity, low electrical resistance and good mechanical strength. Low resistance for the permeation of larger ions has not been a major issue in this application. OOll-9164/90/$03.50
0 1990 Elsevier Science Publishers B.V.
250
However, recent trends in biotechnology and food processing indicate a growing interest in the electrodialytic separation of organic acids from aqueous solutions. In contrary to conventional removal of inorganic salts, this application is characterized by the requirement to remove relatively large anions. Since such large anions have not been taken into consideration during the development of the conventional anion-exchange membranes, these membranes are highly cross-linked to improve the mechanical stability and to avoid excessive swelling. This high degree of cross-linking has no significant effect on the permeability of the small salt-ions. However, experiments that have been carried out to explore the applicability of electrodialysis in food and biotechnology yielded very low permeabilities or organic ions in the conventional ion-exchange membranes. Their permeation resistance appeared to increase drastically with increasing molecular weight of the permeating anions. This observation led to the conclusion that a variation in the “mesh” size of a cross-linked anionexchange membrane must have an effect on the permeability of the larger anions. Therefore, it should be possible to improve the permeability of larger anions adjusting the degree of cross-linking and the chain length of the cross-linking agent. It can be assumed that “loosening” the network would yield better permeabilities for organic anions and therefore an improved separation efficiency of electrodialysis in biotechnology and food applications. An ion-exchange membrane, of course, always needs a certain degree of cross-linking density to prevent significant losses of its mechanical stability and permselectivity [ 11.
2.
EXPERIMENTAL PROCEDURES
To study the effect of polymer cross-linking on the permeability of different size anions, membranes were prepared from anion-exchange polymers with various degrees of cross-linking. These membranes have then been characterized in terms of their physico-chemical and chemical properties and their ion permeability.
2.1. Selection of the polymer system The polymer system selected for this study consisted of a basic molecular chain and mono- and bifunctional molecules as cross-linking agents allowing variations in the degree of cross-linkage at constant chain length an charge density. The bifunctional molecules generate cross-linkages as well as quaternized amines, i.e. positive charges, whereas the monofunctional
2.51
components only generate positive charges. The polymer system used in this study is based on poly(4-vinyl-pyridine). Therefore, halogenized alkanes could be used as mono- and bifunctional components. The monofunctional molecule is used to quaternize the nitrogen atom of the pyridine and thus generate one positive charge. The bifunctional is both quatemizing two nitrogen atoms and cross-linking two polymer chains between two pyridine rings. Fig. 1 shows a two-dimensional section of the polymer network between two poly(4-vinyl-pyridine) backbones. Di-bromo-hexane has been used as a bifunctional cross-linking agent. Therefore, the length of crosslinking molecule is 6 C-atoms. The monofunctional molecule is iodomethane. This system allows the synthesis of the wanted anion-exchange polymer with constant charge density and variable cross-linking density.
CL2 \
“2
\
“2
cd
c 4 2 t?
HZ
Fig. 1. Size comparison between the polymer used in this study and the permeants.
On the left side of Fig. 1 a mesh of 50% cross-linked polymer is shown in comparison to the 100% cross-linked part on the right side. Furthermore, the ideal molecular diameters of a Cl- and lactobionate-ion are plotted in order to demonstrate the size relation between polymer network and permeants. It becomes evident that the bigger lactobionate-ion will have problems to penetrate a 100% cross-linked network, whereas the 50% crosslinked network will ahow permeation more easily. The small Cl-ion can penetrate both structures.
252
2.2. Membrane preparation The membranes prepared for the following experiments have been varied in cross-linking density at a constant length of the cross-linking molecule which contained 6 C-atoms. The degree of cross-linking is defined as:
where v is the degree of cross-linking in %, nBi the number of bifunctional molecules and n the number of quaternizable nitrogen atoms. The number of the monofunctional components nM,, required to quaternize the remaining nitrogen atoms is given by: nMo = n-2nai Anion-exchange been prepared.
(2) membranes
cross-linked at 5, 20, 50, 70 and 100% have
2.3. Membrane characterization The membranes have been characterized with respect to their swelling behavior, their electrical resistance, their permselectivity and their anion permeability. The commercially available Asahi Glass AMV membrane was thereby used as reference. 2.3.1. Membrane swelling To determine the degree of swelling, the membrane samples have been equilibrated for several days in deionized water of 25°C. Then they are removed from the water, wiped clean at the surfaces and weighed. After being vacuum dried in an exsiccator over calcium chloride for 24 h, they are weighed again. Fig. 2 depicts the results. The degree of swelling is defined as the ratio of the mass of-water absorbed by the membrane to the mass of the swollen membrane. The diagram of Fig. 2 indicates that the membranes studied in this paper showed only a very moderate dependence on the degree of cross-linking. According to Fig. [l] the degree of swelling increases slightly with decreasing degree of cross-linkage up to 30%. The membranes with lower cross-linkage show an opposite behaviour, however. Their irregular behaviour is explicable by the formation of partially crystalline areas in the polymer matrix.
253
80 T -&
60
.-L? =
40
20
40
60
8b
lo.0
Degree of cross-linking (%) Fig. 2. Membrane swelling as a function of the degree of cross-linking.
2.3.2. Electrical resistanceof the membrane The electrical resistance of the anion-exchange membranes was determined by measuring the conductivity of the membrane in a 0.5 molar aqueous solution of potassium chloride. Fig. 3 shows the test results. The electrical resistance of the membrane decreases slightly with increasing degree of cross-linking between 40-100% cross-linking. Below 40% crosslinking, however, the electrical resistance increases drastically. Again this irregularity can be explained as a consequence of the formation of crystalline areas in the membrane polymer, since crystalline areas are no longer ionconductive. 2.3.3. Membrane permselectivity According to the initial hypothesis, cross-linking can be varied in two manners: either the length of the cross-linking molecular chain or the degree of cross-linking can be changed. Variation of the chain length would increase the mean spatial distance between the positive charges in the pyridine rings. This must lead to a decrease of permselectivity since the repulsive forces on a cation trying to permeate the anion-exchange membrane are weakened. If the degree of cross-linking is varied, only a slight decrease of permselectivity is expected due to the slightly increased mobility of the chain segments between the points of cross-linkage. The work described in this paper was focused on the variation of the degree of cross-linking only;
254
therefore, no big changes in permselectivity with changing degree of crosslinking were expected.
1 0
20
40
60
80
100
Degree of cross-linking (%) Fig. 3. Electrical resistance of the membrane as a function of the degree of cross-linking.
The permselectivity of an ion-exchange membrane is calculated from the ratio of the experimentally determined diffusion potential of a given membrane between ion solutions of different concentrations on each side of the membrane to the theoretically calculated potential difference for a 100% permselective membrane [2]. The apparent permselectivity P”+ of the membrane is given by
is the measured potential difference between the two electrowhere AE is positive for a cation- and negative for lytes. The zsolute value of AE an anion-exchange membrane. ? or a system consisting of, as an example, standardized aqueous solutions of 0.1 N and 0.5 N KC1 at 25”C, this theoretical potential difference amounts to 36.94 mV. The potential difference between 0.1 and a 0.5 KCl-solution separated by the anion-exchange membrane was measured using two calomel electrodes. Fig. 4 shows the test results.
255
85
80 20
40
60
80
100
Degree of cross-linking (Oh) Fig. 4. PermseAectivity as a function of the degree of cross-linking.
These results indicate that the permselectivities of the anion-exchange membranes prepared in this study are in the same range (91.0%-92.3%) as the permselectivity of the reference membrane (91.2%). The initial concern that a variation of the degree of cross-linkage might not severely affect the repulsion of cations has been confirmed by these results. 2.3.4. Permeabilityof monovdent anions To test the permeability of the prepared membranes with respect to the anion size and independently of the anion charge, four different monovalent ions have been selected. Their molecular weight ranges from 35.5 g/mole to 357.3 g/mole. All of them have been tested as monovalent acids. In Fig. 5 these anions, their structure, their molecular weight (MW) and their molecular diameter (d) are summarized [3]. The chemical nature of these substances had no influence on the selection of the ions since the electrical charge is the main interactive force between membrane material and permeant. The idealized molecular diameters increase to roughly the double and the corresponding molecular weights about tenfold. According to the hypothesis illustrated in Fig. 2, the smallest anion should remain unaffected by the degree of cross-linkage of the membrane, whereas the lactobionate and probably the intermediates should strongly depend on the degree of cross-linking.
2%
structure
anion
cr
OH
MW (g/mol)
d (nm)
chloride
35.5
propionate
73.1
galacturate
211.2
0.60
lactobionate
357.3
0.71
0.33
Fig. 5. Structures, molecular weights and diameters of the monovalent test anions used in this Study.
3. ELECIRODIALYSIS
TEST
The membranes used in this study were characterized in terms of their permeability in an electrodialysis test. The stack used in these experiments consisted of three cell pairs. The effective membrane area was 100 cm2 and the cell thickness 0.5 mm. The flow velocity within the compartments was adjusted to 6.5 cm/s, the temperature was kept at 20°C and the voltage drop across the stack was 10 V. The acid concentration in the feed solution was 0.1 molar. The system was run in a batch operating mode with volume in both the diluate and concentrate being identical. The electrodes were rinsed with 0.5 molar Na2S0, solution.
3.1. Determination of the anion permeability The electrodialysis experiments are evaluated by plotting the concentrations of the concentrate and the diluate solution vs. time. Fig. 6 shows the concentration change of a galacturonic acid solution in the diluate and the concentrate as a function of time using anion-exchange membranes with various degrees of cross-linking and a conventional Asahi Glass AMV as a reference membrane.
257
4 149
I
a AMV concentrate q
AMV diluate
6 100% cross-linked cont. 0 100% cross-linked diluate 50% cross-linked cont. d ? G
80
t
60
50% cross-linked diluate 20% cross-linked cont. 20% cross-linked diluate 5% cross-linked cont. 5% cross-linked diluate
40 0
15
30 time (min)
45
60
Fig. 6. Electrodialysis experiment using 0.1 molar galacturonic acid as test solution and anionexchange membranes with various degrees of cross-linking.
In order to get characteristic relation has been used:
d,
ni=-=-=4
4 4
numbers for the anion flux, the following
Co- cl5 15 min
with n being the molar flow through the anion-exchange membranes in the described stack and c the concentration of the acids in the diluate. The subscripts 0 and 15 refer to the concentration at the beginning and after 15 min of the test. The linear part of the diluate concentration curve has been taken as the relevant concentration difference for the flux determination. This number quantifies the initial flux in the batch experiment. It is characteristic for the permeability because the driving force and the dimensions of the apparatus have been kept constant. Fig. 7 shows the relative permeabilities of the four test anions illustrated in Fig. 5 as a function of the degree of cross-linking of the membrane. The relative permeability is defined as the quotient of the anion flux across the synthesized membrane and the anion flux across the reference membrane.
258
q 4 8 4
0
’ I
20 Degree
0
Fig. 7. Relative permeabilities lil&lg.
I I
P HCI rel P Prop rel
P Gal rel P Lac rel
1 I
40 60 of cross-linking
I
80 (Oh)
100
of the different anions as a function of the degree of
CTOSS-
5 r:
“E
4
3 z g
3
5 = -0
2
id .g .= ”
1
0
0
20
40
SO
SO
lb0
degree of cross-linking (%) Fig. 8. Flux of citric acid in an electrodialysis test in an Asahi reference membrane and in the membrane developed in this study as a function of the degree of cross-linking.
259
3.2. ElectrodiaZysis in the recoveryof citricacid Citric acid is a biotechnical mass product. In the conventional production it is recovered from the bioreactor by precipitation or solvent extraction [4]. Electrodialysis appears to be an attractive alternative to these downstream processes. As an organic acid citric acid is expected to suffer sterical hindrance while permeating conventional anion-exchange membranes. In order to evaluate the potential improvement of permeability, citric acid has been permeability tested in the same way as the monovalent test anions described above with the membranes developed in this study. Fig. 8 shows the test results. The permeability can be improved about 100%. This means that electrodialysis plants in the citric acid recovery can be installed with about half of the conventional membrane area, which significantly lowers the investment costs.
4.
DISCUSSION OF THE RESULTS The
basic assumption that a loosening of the polymeric network will yield improved permeabilities for organic anions of higher molecular weight has been confirmed experimentally. It was shown that the polymer network can be loosened by two different measures: The length of the cross-linking molecules can be increased or their density can be decreased. The latter has been realized experimentally. The permeabilities of these membranes towards large anions turned out to increase with decreasing degree of crosslinking in the range from about 25100% cross-linking. Below 25% crosslinking irregularities were observed. Water swelling of the membrane as well as the permeabilities of the smaller anions, i.e. chloride and propionate, decreased again. An explanation for this behavior is the increased electrical resistance of membranes with a low degree of cross-linking. The large lactobionate ion, however, shows the expected behavior. Its permeability increases with a decreasing degree of cross-linking over the whole range. This fact clearly shows that the lactobionate ion actually suffers sterical hindrance as suggested by the size comparison plotted in Fig. 2. Its permeability can be improved threefold just by decreasing the degree of crosslinking. The galacturate ion shows only a slight dependence on the degree of cross-linking. The potential described of membrane improvement for the permeability of large organic anions has not yet been proven for the long term in technical installations. However, the possible cost reduction due to the reduction in the required membrane area should lead to an increased use of electrodialysis in the food and drug industry.
260 ACKNOWMEDGEMENT
The authors would like to thank Bernd Bauer for his chemistry advice.
REFERENCES 1 F. Helfferich, Ion-Exchange, McGraw-Hill, New York, 1962. 2 E. Komgold, Electrodialysis-Membranes and Mass Transport, in Synthetic Membrane Processes, G. Belfort, ed., Academic Press, New York, pp. 192-219,1984. 3 Handbook of Chemistry and Physics, 64th Edition, CRC Press, Boca Raton, Florida, 1984. 4 J.D. Enzminger and JA. Asenjo, Use of cell recycle in the aerobic fermentative production of citric acid by yeast, Biotechnology Letters, 8(l) (1982) 7-12.