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Removal of melassigenic ions for beet sugar syrups by electrodialysis using a new anion-exchange membrane
DESALINATION ELSEVIER
Desalination
148 (2002) 143-I 48 www.elsevier.com/locate/desal
Removal of melassigenic ions for beet sugar syrups by electrodialysis using a new anion-exchange membrane A. ElmidaouP*, F. Lutinb, L. Chay”, M. Taky”, M. Tahaikt”, My R. Alaoui Hafidi” “Laboratory of Separation Processes, Department of Chemistry, University Ibn TofaiLBP1246 Kenitra, Morocco TellFax+ 37373033;email: [email protected] ‘>EurodiaIndustrie, 14/i6 voie de Montavas, 91320 Wissous, France ‘Group Sunabel, Morocco Received 8 February
2002; accepted 26 March 2002
Abstract Problems hindering the application of electrodialysis in the sugar industry were the short anion-exchange membrane life, the high viscosity and the low operating temperature that had to be maintained below 40°C. To avoid these problems an electrodialysis operation using an improved stack with new anion-exchange membranes was conducted on Moroccan beet sugar syrup to remove melassigenic ions especially Na’, K’ and Ca*+. The demineralisation was operated at a relatively high Brix up to 55Bx and high temperature up to 60°C. The alkali resistance and the behaviour of the AXE 01 membrane in the sugar syrup were controlled. The electrodialysis is a promising process to reduce the melassigenic ions for the sugar industry. Keywords: Melassigenic
ions, Electrodialysis,
Sugar syrup, Demineralisation,
1. Introduction Alkali metal cations were suspected of being highly melassigenic by holding sugar in the molasses and preventing it from being recovered as crystalline sugar. Various systems to eliminate melassigenic ions have been tested such as ion exchange resins, synthetic adsorbents, coagulants and membranes processes [ l--6]. *Corresponding
author.
Presented at the International July 7-12, 2002.
Congress on Membranes
Alkali resistance
Among such membrane processes electrodialysis makes use of the electrical potential to cause the transfer of ions through selective ion exchange membranes. It is a widely used electromembrane process especially for desalination of brackish water and sodium chloride reconcentration from seawater. Recently electrodialysis has been successfully employed to de-ash milk solutions and clarify products as well as deacidify fruit juices. In sugar plant much work has been and Membrane
Processes
OOI I-9164/02/$- See front matter 0 2002 Elsevier Science B.V. All rights reserved HI: SO0 I I-9 164(02)00668-9
(ICOM).
Toulouse, France,
A. Elmidaoui et al. /Desalination
144
carried out on the demineralisation by electrodialysis of molasses, syrups and juices [7-l I]. The Sunabel Company (beet sugar manufacturing company in Morocco) and Eurodia Company (an affiliate of the Tokuyama Corp. Ltd, Japan) have an interest in the application of the electrodialysis process to Moroccan beet sugar. The aim of this work is to study the removal of melassigenic ions especially Na+, K+ and Ca*+ in the beet sugar syrups using an electrodialysis pilot plant equipped with a new anion-exchange membrane. 2. Experimental The stack design characteristics of the pilot plant are given in Table 1. The pilot was supplied by Eurodia Co. The pilot plant was designed for a production rate of 24 m/d. The number of stacks in series was 1 and the number of cell pairs per stack was 50. The total membrane area was 2.5 m*. The tested membranes were a conventional cationexchange membrane (CMX-Sb) and a new anion exchange membrane (AXE 01). All these membranes were manufactured by Tokuyama Co. The plant was operated in batch mode. The principle of the operation was already described [ 121. To prevent scaling and fouling of the membranes the polarity of the direct current was changed at Table 1 Characteristics EUR 5 B P4)
of the pilot plant TS-5-50 P (Model:
Cation exchange membrane Anion exchange membrane Spacers Electrodes Maximal current,A Maximal voltage, V (lV/Cellule) Flow rate of beet sugar syrup, l/h Flow rate of brine stream, l/h Flow rate of electrodes, l/h
CMX-Sb, 52 membranes AXE 01, 50 membranes EUR 6 ESP DSE 20 50 1000 1000 400
148 (2002) 143-148
the end of the test and in addition, hydrochloric acid was added from time to time to the brine stream to maintain the pH value to about 6-7. The beet sugar syrups supplied by Sunabel were first filtered, cooled to a predetermined temperature and introduced into the electrodialysis pilot. Values of the syrups were measured: conductivity, pH, temp-erature, Brix, polarisation, purity, coloration, ash content, Na+, K+ and Ca*+. The amount of Na+, K+ and Ca*+ ions was determined by flame photometer (PFP7, Jenway). The Brix (Bx) was measured with a refractometer (Abbe, type RL3) and expressed in weight percentage. The ash was determined by conductivity (conductivity meter type Orion Research 101) according to the Icumsa method [ 12,131. The coloration was measured with a spectronic (Miltron Roy 21 D) according to the Icumsa method [12,13]. The experimental details for measuring e1ectrica.l resistance, exchange capacity and burst strength have been already described [13-l 51. 3. Results and discussion The technical problems most hindering the application of electrodialysis in the sugar industry were the short membrane life, especially for the anion-exchange membranes, the high viscosity of sugar syrups - especially beet sugar syrups and the low operating temperature that had to be maintained below 40°C. The higher temperature of syrups damages the membrane and spacers. The contamination of membranes by fouling matter such as polysaccharides and proteins makes the electrodialysis operation uneconomical by reducing the electric current efficiency and shortening the life of anionic membranes. These problems can be avoided principally by using a robust resistant membrane to organic poisoning and to the higher temperature and by improving the design of electrodialysis stacks. In this work an improved stack equipped with new spacers, new anion-exchange membrane, and a conventional cation-exchange membrane were
145
A. Elmidaoui et al. /Desalination 148 (200.2)143-148 tested in the electrodialysis of beet sugar syrup to remove melassigenic ions at various temperatures and Brix. 3.1. Alkali resistance of the AXE 01 membrane [16/ Until recently among the Tokuyama membranes, the AFN Neosepta membrane was the most suitable for the sugar industry thanks to a low cross-linking, allowing easy transport of organic anions of molecular weight lower than 300. However, this membrane showed a weak mechanical resistance when cleaned with NaOH at high temperature. The AXE 01 membrane has been developed by Tokuyama Corporation for sugar applications to overcome the above limitations. Tables 2 and 3 give the characteristics of the new AXE 01 membrane, compared to the AFN membrane. Thanks to its higher burst strength, this membrane is easier to handle in industrial ED stacks. To increase membrane life, it is necessary to avoid irreversible organic fouling by opening the polymeric channel with a low DVB content. In addition, it is beneficial to have the possibility of cleaning with a high concentration caustic solution to allow membrane swelling and removal of Table 2 Characteristics of the new AXE 01 and theAFN membranes Neosepta anion membrane
AXE 01
Electrical resistance, Ohm. cm* Exchange capacity, meq$ Burst strength, Mpa. cmThickness, mm
1.4 2.0 0.41 0.17
AFN 0.9 3.1 0.30 0.18
Table 3 Alkali resistance of the AXE 01 membrane compared to the AEN membrane Neosepta anion membrane
AXE01
AFN
Soaking time in 1% NaOH solution at 60°C h Exchange capacity, meq.g-’ Burst strength, MPa.cmm2
0
0
120
3.1 0.3
0.3 0.1
120
2.0 1.8 0.41 0.32
organic molecules trapped inside the membrane. Indeed, when cleaning ion-exchange membranes with caustic, a risk exists of reducing the exchange capacity and the mechanical resistance of the membrane. The AFN and AXE 01 membranes were soaked in 1wt% NaOH + 1wt% NaCl solution during 5 d at 60°C. The exchange capacity and burst strength were measured. The new membrane features a high alkali resistance with only a 10% E.C. and a 22% burst strength decrease, compared to 90% and 67% for the AEN. 3.2. Removal of melassigenic
ions
Sugar solutions at various Brix were obtained by diluting the pure syrup with a weak juice having a Brix of 16. The dilution was obligatory because the high viscosity of the pure syrup. Tables 4 and 5 give the characteristics of the beet sugar syrup after electrodialysis operation. The operation was carried out for syrup at various Brix and under the following conditions: maximum voltage 45 V, temperature 40°C demineralisation rate 75%. The applied current density was 12 mA/ cm* for syrup at 30 Bx, 14 mA/cm* for 40 Bx and 9.8 mA/cm* for syrup at 45Bx. A good reproducibility has been obtained for each test. Figs. 1-3 show the variations of purity, coloration and ash content as a function of the electrodialysis time and for the three Brix. Variation of sodium, potassium and calcium contents are given in Figs. 4-6. The juice pH remains almost stable for the three tested Brix. At the end of operation, the variation of pH did not exceed 4.6%. The discoloration rates increase with time and reach 14.61% for 30 Bx, 7.54% for 40 Bx and 14% for 45 Bx. The juice quality was improved by the electrodialysis operation. The gain in purity reaches 10 for 30 Bx, 8 for 40 Bx and 10.5 for 45 Bx. Nevertheless for industrial plants, the most important limiting factor regarding product concentrations is the difference of syrup viscosity
A. Elmidaoui et al. /Desalination
146
148 (2002) 143-148
Table 4 Conductivity, pH, purity and coloration of the treated beet syrup by electrodialysis Beet sugar syrup
Conductivity, mS/cm
pH
Purity, %
Coloration (Icumsa)
to
tr
%
to
tc
%
to
t/
A
to
!f
%
30 Brix 40 Brix 45 Brix
7.29 7.86 8.34
2.00 2.00 2.15
75 75 75
8.01 8.72 8.60
7.75 8.32 8.32
3.25 4.59 3.26
80.61 83.97 81.00
90.63 92.21 91.47
10.02 8.24 10.47
2560 3010 6149
2186 2783 5288
14.61 7.54 14.00
Table 5 Ash content, sodium, potassium and calcium contents of the treated beet syrup by electrodialysis Ash content, g%Bx
Beet sugar syrup 30 Brix 40 Brix 45 Brix
Na+, mg/l
Ca’+, mgfl
K’, mg/l
to
tf
%
to
tf
%
to
tr
%
to
tr
%
3.774 4.538 5.315
1.177 1.214 1.400
68.81 73.25 73.66
1939 2983 5267
736 1122 1522
62.04 62.39 71.10
3078 4100 6800
700 945 983
77.26 76.95 85.54
361.3 457.1 832.1
77.9 154.8 266.3
78.44 66.13 68.00
g V
E
-.--
tz
---A- 45Bx
24 0
1
I
10
-
I
a0
!
I
Xl
z
,I
I
40
’
50
r
I
4
40Bx
,‘,‘I
fXl iiD
80
!Xl
Time (mn)
Time (mn) Fig. I. Time dependence of purity.
”
Fig. 2. Time dependence of coloration.
A
-a-
30Bx 1
-I-
30Bx
-O-
40Bx
--t
40Bx
OD ?Q
Kt
-A-- 45Bx
0 0
1,. 10
,I a0
I a, 30 40
’
I ’ f ’ I Xl 8 Xl
Time (mn) Fig. 3. Time dependence of ash content.
I - I 83 90
0
10
2D
30
40
4)
Time (mn) Fig. 4 Time dependence of sodium content.
so
A. Elmidaoui et al. /Desalination
-I-
30Bx
-.-
40Bx
-A-
45Bx
147
148 (2002) 143-148
4 0
10
23
33
4
Time
33
clln
8)
SD
0
10 aTI 30
40
50
al
x-l
$0
!xl
Time (mn)
(n-m)
Fig. 5. Time dependence of potassium content.
Fig. 6. Time dependence of calcium content.
between the pure syrup (65 Bx) and the diluted syrups and consequently the pressure drop through the stacks. Moreover operation at low temperature is not desirable because, on the one hand, there is the risk of growth of microorganisms and, on the other hand, the consumption of thermal power to bring the diluted syrup to the initial pure syrup temperature (79°C). Consequently for the industrial plant it’s desirable to operate at the highest possible temperature and concentration of the diluted syrup. For this reason an electrodialysis operation has been conducted on a diluted syrup at 55 Bx and at a temperature of 60°C under a voltage of 1.3 per cell (Table 6). The pressure drop through the stack has not been significantly affected by the light increase in syrup viscosity. This increase has been partially compensated by the increase in temperature.
The juice quality was improved by demineralisation. The gain in purity reaches 9 and the discoloration rate 10%. Variation of pH reaches only 7.2%. At the end of the operation, 75% of Na+, 86% of K+ and 65% of Ca2+ have been removed. The overall demineralisation rate reaches 75%. No contamination and no fouling have been observed during tests (200 h). The characteristics of the AXE 01 and CMX membranes have been controlled after two months of functioning (600 h). Table 7 shows that these characteristics have been not affected by the electrodialysis operation of the sugar syrup at relatively high temperature and Brix. The little increase of the electrical resistance of the AXE 01 membrane can be avoided by a good cleaning.
Table 6 Characteristics of beet syrup (55 Bx) treated at 60°C by electrodialysis
t0
tr %
Conductivity, mS/cm
pH
Purity, %
Coloration (ICUMSA)
Ash content, g%Bx
Na+, mg/l
K’, mg/f
Ca2+ mg/l
10.19 2.55 75
9.18 8.52 7.19
86.87 95.75 -
6294 5665 10
6.192 1.858 70
6440 1620 74.84
7310 995 86.39
916.9 320.9 65
148
A. Elmidaoui et al. /Desalination
Table 7 Characteristics of the used membranes before and after the
electrodialysis campaign Membrane
AXE 01
CMX
148 (2002) 143-148
Acknowledgements This work was supported by Eurodia Co. (France) and Sunabrl Co. (Morocco). The authors express their thanks for this support.
before after before after Electrical resistance, 0hm.cm2 Exchange capacity, meqlg Thickness, mm Burst strength, MPa.cm-’
1.5 2.6 1.4 2.7 2 1.9 1.5 1.5 0.17 0.17 0.18 0.18 0.41 0.39 -
4. Conclusions Using an improved electrodialysis stack with new spacers and new anion-exchange membranes having a high alkali and temperature resistance, a demineralisation operation by electrodialysis was easily conducted at a relatively high Brix up to 55 Bx and temperature up to 6O”C,to remove melassigenic ions in beet sugar syrup. The syrup quality was improved by the electrodialysis operation. Most technical problems hindering the application of electrodialysis in the sugar industry were partially overcome. Electrodialysis can be considered as one of the technologies that can contribute to improvement of the cost effectiveness of sugar plants, since it can help to control organic fouling, reduce waste effluents and pollution load, improve sugar yields, reduce the volume of molasses and to save capital costs.
References HI J.F.T.Oldfield, M. Shore, C.W. Harvey, D. Gyte and GC. Jones, British Sugar Corp. 24th Tech. Conf., 1978. [21 G Quentin, Z. Zuckerind, 14 (1964) 683. [31 J. Huberlant, Sue. Belge, 86 (1967) 505. [41 K. Schoenrock, J.A.S.S.B.T., 16 (1971) 299. [51 V.I. Suragino, VS. Shtherman, A.R.Sapronov and GS. Tereshenko, Sakh. Prom., 8 (1984) 21. 161 H.G Schneider and J. Mikule, Int. Sug. J., 77 (1975) 259. [71 0.1. Cress, 22nd Internat. Societ. Sugar Cane Tech. Congress Cartagana, Colombia, 10-15 Sept. 1995. [81 K. Ciz, Industrie Alim. Agr., 89 (1972) 1037. r91 P. Kadlec, Dissertation, VSCHT Praha, 1967. [lOI T. Yamauchi, T. Kokubu and S. Miyagi, Int. Sug. J., 87 (1985) 1033. [Ill M. Tako and B. Messaoudi, Int. Sug. J., 95 (1993) 1134. [I21 M.O. El Khattabi, My R. Alaoui Hafidi, M. El Amrani and A.Elmidaoui, Sug. J., Nov. 1998. A. Elmidaoui, J. Molenat and C. Gavach, J. Membr. [I31 Sci., 55 (1991) 79. [I41 A. Elmidaoui and C. Gavach, Desalination, 78 (1990) 343. 1151A. Elmidaoui, T. Sarraf, C. Gavach and B. Boutevin, J. Appl. Polym. Sci., 42 (1991) 2551. 1161ELutin, Sugar Conf. SPRI 2000
Desalination
148 (2002) 143-I 48 www.elsevier.com/locate/desal
Removal of melassigenic ions for beet sugar syrups by electrodialysis using a new anion-exchange membrane A. ElmidaouP*, F. Lutinb, L. Chay”, M. Taky”, M. Tahaikt”, My R. Alaoui Hafidi” “Laboratory of Separation Processes, Department of Chemistry, University Ibn TofaiLBP1246 Kenitra, Morocco TellFax+ 37373033;email: [email protected] ‘>EurodiaIndustrie, 14/i6 voie de Montavas, 91320 Wissous, France ‘Group Sunabel, Morocco Received 8 February
2002; accepted 26 March 2002
Abstract Problems hindering the application of electrodialysis in the sugar industry were the short anion-exchange membrane life, the high viscosity and the low operating temperature that had to be maintained below 40°C. To avoid these problems an electrodialysis operation using an improved stack with new anion-exchange membranes was conducted on Moroccan beet sugar syrup to remove melassigenic ions especially Na’, K’ and Ca*+. The demineralisation was operated at a relatively high Brix up to 55Bx and high temperature up to 60°C. The alkali resistance and the behaviour of the AXE 01 membrane in the sugar syrup were controlled. The electrodialysis is a promising process to reduce the melassigenic ions for the sugar industry. Keywords: Melassigenic
ions, Electrodialysis,
Sugar syrup, Demineralisation,
1. Introduction Alkali metal cations were suspected of being highly melassigenic by holding sugar in the molasses and preventing it from being recovered as crystalline sugar. Various systems to eliminate melassigenic ions have been tested such as ion exchange resins, synthetic adsorbents, coagulants and membranes processes [ l--6]. *Corresponding
author.
Presented at the International July 7-12, 2002.
Congress on Membranes
Alkali resistance
Among such membrane processes electrodialysis makes use of the electrical potential to cause the transfer of ions through selective ion exchange membranes. It is a widely used electromembrane process especially for desalination of brackish water and sodium chloride reconcentration from seawater. Recently electrodialysis has been successfully employed to de-ash milk solutions and clarify products as well as deacidify fruit juices. In sugar plant much work has been and Membrane
Processes
OOI I-9164/02/$- See front matter 0 2002 Elsevier Science B.V. All rights reserved HI: SO0 I I-9 164(02)00668-9
(ICOM).
Toulouse, France,
A. Elmidaoui et al. /Desalination
144
carried out on the demineralisation by electrodialysis of molasses, syrups and juices [7-l I]. The Sunabel Company (beet sugar manufacturing company in Morocco) and Eurodia Company (an affiliate of the Tokuyama Corp. Ltd, Japan) have an interest in the application of the electrodialysis process to Moroccan beet sugar. The aim of this work is to study the removal of melassigenic ions especially Na+, K+ and Ca*+ in the beet sugar syrups using an electrodialysis pilot plant equipped with a new anion-exchange membrane. 2. Experimental The stack design characteristics of the pilot plant are given in Table 1. The pilot was supplied by Eurodia Co. The pilot plant was designed for a production rate of 24 m/d. The number of stacks in series was 1 and the number of cell pairs per stack was 50. The total membrane area was 2.5 m*. The tested membranes were a conventional cationexchange membrane (CMX-Sb) and a new anion exchange membrane (AXE 01). All these membranes were manufactured by Tokuyama Co. The plant was operated in batch mode. The principle of the operation was already described [ 121. To prevent scaling and fouling of the membranes the polarity of the direct current was changed at Table 1 Characteristics EUR 5 B P4)
of the pilot plant TS-5-50 P (Model:
Cation exchange membrane Anion exchange membrane Spacers Electrodes Maximal current,A Maximal voltage, V (lV/Cellule) Flow rate of beet sugar syrup, l/h Flow rate of brine stream, l/h Flow rate of electrodes, l/h
CMX-Sb, 52 membranes AXE 01, 50 membranes EUR 6 ESP DSE 20 50 1000 1000 400
148 (2002) 143-148
the end of the test and in addition, hydrochloric acid was added from time to time to the brine stream to maintain the pH value to about 6-7. The beet sugar syrups supplied by Sunabel were first filtered, cooled to a predetermined temperature and introduced into the electrodialysis pilot. Values of the syrups were measured: conductivity, pH, temp-erature, Brix, polarisation, purity, coloration, ash content, Na+, K+ and Ca*+. The amount of Na+, K+ and Ca*+ ions was determined by flame photometer (PFP7, Jenway). The Brix (Bx) was measured with a refractometer (Abbe, type RL3) and expressed in weight percentage. The ash was determined by conductivity (conductivity meter type Orion Research 101) according to the Icumsa method [ 12,131. The coloration was measured with a spectronic (Miltron Roy 21 D) according to the Icumsa method [12,13]. The experimental details for measuring e1ectrica.l resistance, exchange capacity and burst strength have been already described [13-l 51. 3. Results and discussion The technical problems most hindering the application of electrodialysis in the sugar industry were the short membrane life, especially for the anion-exchange membranes, the high viscosity of sugar syrups - especially beet sugar syrups and the low operating temperature that had to be maintained below 40°C. The higher temperature of syrups damages the membrane and spacers. The contamination of membranes by fouling matter such as polysaccharides and proteins makes the electrodialysis operation uneconomical by reducing the electric current efficiency and shortening the life of anionic membranes. These problems can be avoided principally by using a robust resistant membrane to organic poisoning and to the higher temperature and by improving the design of electrodialysis stacks. In this work an improved stack equipped with new spacers, new anion-exchange membrane, and a conventional cation-exchange membrane were
145
A. Elmidaoui et al. /Desalination 148 (200.2)143-148 tested in the electrodialysis of beet sugar syrup to remove melassigenic ions at various temperatures and Brix. 3.1. Alkali resistance of the AXE 01 membrane [16/ Until recently among the Tokuyama membranes, the AFN Neosepta membrane was the most suitable for the sugar industry thanks to a low cross-linking, allowing easy transport of organic anions of molecular weight lower than 300. However, this membrane showed a weak mechanical resistance when cleaned with NaOH at high temperature. The AXE 01 membrane has been developed by Tokuyama Corporation for sugar applications to overcome the above limitations. Tables 2 and 3 give the characteristics of the new AXE 01 membrane, compared to the AFN membrane. Thanks to its higher burst strength, this membrane is easier to handle in industrial ED stacks. To increase membrane life, it is necessary to avoid irreversible organic fouling by opening the polymeric channel with a low DVB content. In addition, it is beneficial to have the possibility of cleaning with a high concentration caustic solution to allow membrane swelling and removal of Table 2 Characteristics of the new AXE 01 and theAFN membranes Neosepta anion membrane
AXE 01
Electrical resistance, Ohm. cm* Exchange capacity, meq$ Burst strength, Mpa. cmThickness, mm
1.4 2.0 0.41 0.17
AFN 0.9 3.1 0.30 0.18
Table 3 Alkali resistance of the AXE 01 membrane compared to the AEN membrane Neosepta anion membrane
AXE01
AFN
Soaking time in 1% NaOH solution at 60°C h Exchange capacity, meq.g-’ Burst strength, MPa.cmm2
0
0
120
3.1 0.3
0.3 0.1
120
2.0 1.8 0.41 0.32
organic molecules trapped inside the membrane. Indeed, when cleaning ion-exchange membranes with caustic, a risk exists of reducing the exchange capacity and the mechanical resistance of the membrane. The AFN and AXE 01 membranes were soaked in 1wt% NaOH + 1wt% NaCl solution during 5 d at 60°C. The exchange capacity and burst strength were measured. The new membrane features a high alkali resistance with only a 10% E.C. and a 22% burst strength decrease, compared to 90% and 67% for the AEN. 3.2. Removal of melassigenic
ions
Sugar solutions at various Brix were obtained by diluting the pure syrup with a weak juice having a Brix of 16. The dilution was obligatory because the high viscosity of the pure syrup. Tables 4 and 5 give the characteristics of the beet sugar syrup after electrodialysis operation. The operation was carried out for syrup at various Brix and under the following conditions: maximum voltage 45 V, temperature 40°C demineralisation rate 75%. The applied current density was 12 mA/ cm* for syrup at 30 Bx, 14 mA/cm* for 40 Bx and 9.8 mA/cm* for syrup at 45Bx. A good reproducibility has been obtained for each test. Figs. 1-3 show the variations of purity, coloration and ash content as a function of the electrodialysis time and for the three Brix. Variation of sodium, potassium and calcium contents are given in Figs. 4-6. The juice pH remains almost stable for the three tested Brix. At the end of operation, the variation of pH did not exceed 4.6%. The discoloration rates increase with time and reach 14.61% for 30 Bx, 7.54% for 40 Bx and 14% for 45 Bx. The juice quality was improved by the electrodialysis operation. The gain in purity reaches 10 for 30 Bx, 8 for 40 Bx and 10.5 for 45 Bx. Nevertheless for industrial plants, the most important limiting factor regarding product concentrations is the difference of syrup viscosity
A. Elmidaoui et al. /Desalination
146
148 (2002) 143-148
Table 4 Conductivity, pH, purity and coloration of the treated beet syrup by electrodialysis Beet sugar syrup
Conductivity, mS/cm
pH
Purity, %
Coloration (Icumsa)
to
tr
%
to
tc
%
to
t/
A
to
!f
%
30 Brix 40 Brix 45 Brix
7.29 7.86 8.34
2.00 2.00 2.15
75 75 75
8.01 8.72 8.60
7.75 8.32 8.32
3.25 4.59 3.26
80.61 83.97 81.00
90.63 92.21 91.47
10.02 8.24 10.47
2560 3010 6149
2186 2783 5288
14.61 7.54 14.00
Table 5 Ash content, sodium, potassium and calcium contents of the treated beet syrup by electrodialysis Ash content, g%Bx
Beet sugar syrup 30 Brix 40 Brix 45 Brix
Na+, mg/l
Ca’+, mgfl
K’, mg/l
to
tf
%
to
tf
%
to
tr
%
to
tr
%
3.774 4.538 5.315
1.177 1.214 1.400
68.81 73.25 73.66
1939 2983 5267
736 1122 1522
62.04 62.39 71.10
3078 4100 6800
700 945 983
77.26 76.95 85.54
361.3 457.1 832.1
77.9 154.8 266.3
78.44 66.13 68.00
g V
E
-.--
tz
---A- 45Bx
24 0
1
I
10
-
I
a0
!
I
Xl
z
,I
I
40
’
50
r
I
4
40Bx
,‘,‘I
fXl iiD
80
!Xl
Time (mn)
Time (mn) Fig. I. Time dependence of purity.
”
Fig. 2. Time dependence of coloration.
A
-a-
30Bx 1
-I-
30Bx
-O-
40Bx
--t
40Bx
OD ?Q
Kt
-A-- 45Bx
0 0
1,. 10
,I a0
I a, 30 40
’
I ’ f ’ I Xl 8 Xl
Time (mn) Fig. 3. Time dependence of ash content.
I - I 83 90
0
10
2D
30
40
4)
Time (mn) Fig. 4 Time dependence of sodium content.
so
A. Elmidaoui et al. /Desalination
-I-
30Bx
-.-
40Bx
-A-
45Bx
147
148 (2002) 143-148
4 0
10
23
33
4
Time
33
clln
8)
SD
0
10 aTI 30
40
50
al
x-l
$0
!xl
Time (mn)
(n-m)
Fig. 5. Time dependence of potassium content.
Fig. 6. Time dependence of calcium content.
between the pure syrup (65 Bx) and the diluted syrups and consequently the pressure drop through the stacks. Moreover operation at low temperature is not desirable because, on the one hand, there is the risk of growth of microorganisms and, on the other hand, the consumption of thermal power to bring the diluted syrup to the initial pure syrup temperature (79°C). Consequently for the industrial plant it’s desirable to operate at the highest possible temperature and concentration of the diluted syrup. For this reason an electrodialysis operation has been conducted on a diluted syrup at 55 Bx and at a temperature of 60°C under a voltage of 1.3 per cell (Table 6). The pressure drop through the stack has not been significantly affected by the light increase in syrup viscosity. This increase has been partially compensated by the increase in temperature.
The juice quality was improved by demineralisation. The gain in purity reaches 9 and the discoloration rate 10%. Variation of pH reaches only 7.2%. At the end of the operation, 75% of Na+, 86% of K+ and 65% of Ca2+ have been removed. The overall demineralisation rate reaches 75%. No contamination and no fouling have been observed during tests (200 h). The characteristics of the AXE 01 and CMX membranes have been controlled after two months of functioning (600 h). Table 7 shows that these characteristics have been not affected by the electrodialysis operation of the sugar syrup at relatively high temperature and Brix. The little increase of the electrical resistance of the AXE 01 membrane can be avoided by a good cleaning.
Table 6 Characteristics of beet syrup (55 Bx) treated at 60°C by electrodialysis
t0
tr %
Conductivity, mS/cm
pH
Purity, %
Coloration (ICUMSA)
Ash content, g%Bx
Na+, mg/l
K’, mg/f
Ca2+ mg/l
10.19 2.55 75
9.18 8.52 7.19
86.87 95.75 -
6294 5665 10
6.192 1.858 70
6440 1620 74.84
7310 995 86.39
916.9 320.9 65
148
A. Elmidaoui et al. /Desalination
Table 7 Characteristics of the used membranes before and after the
electrodialysis campaign Membrane
AXE 01
CMX
148 (2002) 143-148
Acknowledgements This work was supported by Eurodia Co. (France) and Sunabrl Co. (Morocco). The authors express their thanks for this support.
before after before after Electrical resistance, 0hm.cm2 Exchange capacity, meqlg Thickness, mm Burst strength, MPa.cm-’
1.5 2.6 1.4 2.7 2 1.9 1.5 1.5 0.17 0.17 0.18 0.18 0.41 0.39 -
4. Conclusions Using an improved electrodialysis stack with new spacers and new anion-exchange membranes having a high alkali and temperature resistance, a demineralisation operation by electrodialysis was easily conducted at a relatively high Brix up to 55 Bx and temperature up to 6O”C,to remove melassigenic ions in beet sugar syrup. The syrup quality was improved by the electrodialysis operation. Most technical problems hindering the application of electrodialysis in the sugar industry were partially overcome. Electrodialysis can be considered as one of the technologies that can contribute to improvement of the cost effectiveness of sugar plants, since it can help to control organic fouling, reduce waste effluents and pollution load, improve sugar yields, reduce the volume of molasses and to save capital costs.
References HI J.F.T.Oldfield, M. Shore, C.W. Harvey, D. Gyte and GC. Jones, British Sugar Corp. 24th Tech. Conf., 1978. [21 G Quentin, Z. Zuckerind, 14 (1964) 683. [31 J. Huberlant, Sue. Belge, 86 (1967) 505. [41 K. Schoenrock, J.A.S.S.B.T., 16 (1971) 299. [51 V.I. Suragino, VS. Shtherman, A.R.Sapronov and GS. Tereshenko, Sakh. Prom., 8 (1984) 21. 161 H.G Schneider and J. Mikule, Int. Sug. J., 77 (1975) 259. [71 0.1. Cress, 22nd Internat. Societ. Sugar Cane Tech. Congress Cartagana, Colombia, 10-15 Sept. 1995. [81 K. Ciz, Industrie Alim. Agr., 89 (1972) 1037. r91 P. Kadlec, Dissertation, VSCHT Praha, 1967. [lOI T. Yamauchi, T. Kokubu and S. Miyagi, Int. Sug. J., 87 (1985) 1033. [Ill M. Tako and B. Messaoudi, Int. Sug. J., 95 (1993) 1134. [I21 M.O. El Khattabi, My R. Alaoui Hafidi, M. El Amrani and A.Elmidaoui, Sug. J., Nov. 1998. A. Elmidaoui, J. Molenat and C. Gavach, J. Membr. [I31 Sci., 55 (1991) 79. [I41 A. Elmidaoui and C. Gavach, Desalination, 78 (1990) 343. 1151A. Elmidaoui, T. Sarraf, C. Gavach and B. Boutevin, J. Appl. Polym. Sci., 42 (1991) 2551. 1161ELutin, Sugar Conf. SPRI 2000