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Concentrating xylose solution by reverse osmosis with cellulose acetate dry membrane
Desalination, 62 (1987) 193-201 Elsevier Science Publishers B.V., Amsterdam -
193 Printed in The Netherlands
Concentrating xylose solution by reverse osmosis with cellulose acetate dry membrane* LIU YURONG, CA1 BANGXIAO, CHEN YIMING and LANG KANGMIN Second Institute of Oceanography, State Oceanographic Administration, P.O. Box 507, Hangzhou (China)
SUMMARY
This paper presents the results of research into concentrating xylose solution by using cellulose acetate reverse osmosis ( CA RO ) dry membranes which contain different additives. According to the feasibility test, CA-CTA,,, type mixed membranes and CA,,, membranes were found effective to concentrate aqueous xylose solutions. The membranes were fitted into plate type desalters, and the desalters replaced the original Three Boiling System in the xylitol production process. After 3f years, the rejection of the membranes was still good although the flux had slightly decreased; the used membranes could be used again after mechanical cleaning. This paper also discusses the factors which affect the service life of the membranes and suggests the methods to eliminate the effects.
INTRODUCTION
The reverse osmosis technique was developed more than 20 years ago and has found applications in food and pharmaceuticals concentration during the last ten years. However, in China, despite the success of studies on the application of this technique in recent years, no success on an industrial scale was reported until last year. In xylitol production in China the Three Boiling System is a commonly used process for concentrating the intermediate, xylose. The technique is ripe and practical, but it is highly energy and labour intensive, and would cause xylose to be burned, and pollute the environment. The concentration process with the reverse osmosis technique takes place at ambient temperature, so no burned sugar or new pigment can be produced. The reverse osmosis process can also *Presented at the International Symposium on Synthetic Membrane Science and Technology, Dalian, China, April 13-18,1986.
Ooll-9164/87/$03.50
0 1987 Elsevier Science Publishers B.V.
194
remove part of the acid and a small amount of dust from the solution. Therefore the new process can reduce the load of ion-exchange resin needed in the process after reverse osmosis and shorten the production period, and it also reduces the cost of the production and improves the qualtity of the xylitol. The application of the reverse osmosis technique will make the process of xylose purification preferable. A laboratory test for concentrating xylose solutions began in August 1981 in China; after reliable data from feasibility tests had been obtained, the technique of reverse osmosis was chosen for the process of manufacturing xylitol in the First Chemical Plant of Jilin. EXPERIMENTAL
Membrane
materials and solvents
Cellulose acetate (CA) - acetyl content, 38.46; viscosity, 228 cP; transparency, 13.2 cm. Cellulose triacetate (CTA) - acetyl content, 43.38; degree of polymerization (DP), 321.8; transparency, 21 cm. Acetone ( chemically pure ) . Dioxane (chemically pure). Preparation
of membrane
The CA or CA-CTA mixtures, and additives were added into solvents in proper proportion with stirring. After the polymers were dissolved, the cast solution was filtered and stilled until bubble free. The membranes were cast under controlled temperature and air humidity conditions. The cast solution was decanted into the slot of the casting knife mounted on the casting machine, and then the machine cast the solution on a polyester web. The film being produced was passed through a gelation bath, a heat treatment bath and a drying bath in turn; finally, it dried at ambient temperature until a dry membrane had formed. The conditions of membrane preparation were as follows: environment temperature lo-3O”C, environment humidity above 80%, evaporation time lo-60 s, gelation tap water temperature lo-3O”C, annealing water temperature 75-85°C (annealing process may be cancelled), drying bath aqueous glycerol or surfactants solution. Feasible concentration
test conditions and apparatus
WNJ-II high pressure water pump, stainless steel high pressure reverse osmosis cells; operation pressure 40 kg/cm2 at the start, later increased. Feed xylose solution refractive index from 3% at the beginning to 15% at the end.
195
Feed solution cell 40 cm’.
pH 3, feed flow rate 230 mg/min,
effective
membrane
area of a
Measurement instruments and calculation equation WZS-1 Arby-refractometer; Rejection =
refractive
WYT Saccharometer.
index of feed-refractive index of permeate refractive index of feed
x 100%
RESULTS AND DISCUSSION
Choice of membrane As the molecular weight of xylose is higher (MW = 152) than that of a number of salts, the required rejection of the membranes for xylose concentration can be comparatively low, but the membranes must satisfy the following requirements. (1) Rejection to xylose is over 95%, ( 2) the flux should be high enough and the slope of flux decrease should be low, (3) they should be suitable for acid feed ( pH = 2-6 ) , ( 4 ) the membranes should not be degraded by microbial degradation, (5) they must have high intensity and long service life, (6) they should be convenient to bond, store and transport, (7) their mechanical production should be easy. According to the above principles, several cellulose acetates or their derivatives were chosen as polymers, and phosphoric acid, methanol, n-propanol, water, triethanolamine, N,N-dimethylformamide, magnesium perchlorate, dioxane etc. were chosen as additives, to make reverse osmosis dry membranes which were reinfored with a polyester web. After some concentration tests using different membranes, considering both the rejection and the flux of the membranes, the CAzOi type membrane with magnesium perchlorate and dioxane as additives, and the CA-CTA,,, type membrane with methanol as additive were selected for xylose solution treatment [ 21. Effect of the conditions of preparation on membrane performance It is well known that membranes with different performances can be produced from the same cast solution simply by changing the conditions under which the membranes are made. We carried out the following tests to prepare membranes suitable for concentrating xylose solutions. Effect of solvent evaporation time on membrane performance. The period it takes
196
I 5
10
15
Gelation
20
water
I 25
I
1
30
35
temperature
40
PC
45
1
Fig. 1. Effect of the temperature of the gelation water on the membrane performance. (-_)
CA,
(---) CA-CTA; (X ) flux, (0) rejection.
for the film to go from the casting knife to the gelation bath is called solvent evaporation time here. In this period the solvent evaporates from the surface of the membrane, the chains of the polymers in the casting solution become close to each other, and a thin, dense layer is formed in the upper layer of the film. The structure of the thin layer will directly affect the membrane performance. Some casting solutions were cast under controlled atmosphere, temperature and humidity conditions. The results are that for CA,,, membranes the flux decreases and the rejection increases as the evaporation time increases, and for CA-CTA,,, membranes both the flux and rejection change only slightly within an evaporation period from 5-60 s, but when evaporation time exceeds 60 s, the flux increases quickly and the rejection decreases correspondingly [ 3, 41. Effect of gekztion temperature on membrane performance. After solvent evaporation, the film entered a water bath and gelation began. In the bath, the solvent and additive in the film diffuse outwards, and the diffusion rate is closely related to the temperature of the gelation bath. From Fig. 1 the relationship between the membrane performance and the temperature of the gelation bath can be seen. The fluxes, no matter whether the membrane is CA,,, or CA-CTA,,, type, as shown in Fig. 1, decrease with increase of gelation temperature. The rejection of the CA membrane has a maximum at 30°C; that of the CA-CTA membrane changes little when the gelation temperature is over 35”C, but the surface of the membranes would become wrinkled if the temperature would be over 35 ‘C. From a production viewpoint, it is preferable to set the gelation temperature at about 30’ C. Effect of drying temperature on membrane performance. At ambient tempera-
197 1 h
9-
-100
5 a-
o-
-_~_--
_~~:ZX==S----
-60
“E ‘-
2
‘;; x-__
--‘%-.___
6-
-60 c 20 'Z
--1x---X__
\
36 IL
---IL
XL"_ 4-
+
x-x-20
J10
20
Drying Fig. 2. Effect of
m
40
tempemture
50
60
PC 1
drying temperature on membrane performance. (-1
flux, (0 ) rejection.
CA, (___) CA-CT& (X )
ture membranes which have just passed through the drying treatment bath, take a long time to dry. To find the shortest practical drying time which would allow constant and reliable production of the membranes, drying tests were carried out on the membrane at different temperatures. The results of the tests are shown in Fig. 2. The rejection increases and the flux decreases both when the drying temperature is increased and when the drying time is increased. Tests of the membrane performance Effect of concentration of xylose solution on membrane performance. In order to allow reverse osmosis techniques to be applied process of concentrating xylose solution, the effect of the concentration of the xylose solution on the performance of the membrane must be known. Therefore circular concentrating tests were carried out in which different membranes were used. The test results were that the fluxes always decrease and the rejections are not markedly influenced as the concentration of xylose increases, for all kinds of membranes tested. As an example the date for the CA,,, membrane are listed in Table I. In view of operation pressure, feed solution pH and membrane performance, it is preferable to use the CA,,, membrane and CA-CTA,,, mixed membranes in the industrial concentration process. There are two methods to assemble desalters in the concentration process. The first is a two-stage method because of the considerable difference in concentration between feed and final concentrated solution. In the first section membranes with high rejection are used to treat the solution with low concentration and in the second section membranes with high flux and lower rejection are used to treat the solution with high concentration; the two sections are connected and product permeated through the membranes of the second section will be sent back to the feed, because it
198 TABLE I EFFECT OF CONCENTRATION OF XYLOSE SOLUTION ON THE MEMBRANE PERFORMANCE Feed solution temp. 29-29.5” C, effective membrane area 40 cm’, feed flow rate 230 ml/min. Concentration of xylose solution (Wl
3.23
6.80
8.50
9.50
12.25
14.50
15.00
Flux (ml/cm’
4.50
3.15
3.00
2.40
2.55
2.55
2.55
h)
Operation pressure ( kg/cm’ 1
40
Feed solution pH
40
3.0
50
4.0
60
5.0
5.4
70
70
5.6
70
5.6
contains some xylose. A second method applies the same membranes rejection throughout the whole process. The choice of the method based on economic efficiency.
5.6
with high should be
performance in xylose solution concentration. It is obvious that different rejections are found when the same membrane is used to treat tap water and xylose solution, respectively. From Table II it can be seen that a membrane with 85% rejection to tap water has a corresponding rejection of more than 95% to xylose. Therefore membranes with 85% rejection to tap water were chosen to be fitted into the desalters to concentrate the xylose solution, and the results were satisfactory. This is in agreement with Nielsen’s conclusion that cellulose acetate membranes with 8590% rejection to low salinity brackish water have a 100% rejection to sugar [ 51. Membrane
TABLE II THE RELATIONSHIP OF MEMBRANE PERFORMANCE BETWEEN THE REJECTION TO TAP WATER AND THE REJECTION TO XYLOSE SOLUTION Operation pressure 40 kg/cm*, feed solution temperature 26” C, effective membrane area 40 cm’, feed flow rate 230 ml/min. Membrane performance
Feed solution Tap water (Hangzhou)
Flux (ml/cm* h) Rejection ( % )
3.9 90.0
5.4 87.6
7.8 82.5
Xylose solution 6.5 85.0
6.0 85.0
2.3 100
3.0 100
(4.6% ) 5.0 89.2
4.5 95.0
4.0 98.5
199 TABLE III THE RESULTS OF FOUR DESALTERS USED TO TREAT WATER AND XYLOSE SOLUTION Operation pressure (tap water) 28 kg/cm’ (inlet), 23 kg/cm’ (outlet); operation pressure (first section, xylose) 35 kg/cm* ( inlet) ,30 kg/cm* ( oulet ) ; operation pressure (second section, xylose ) 45 kg/cm’ (inlet), 40 kg/cm* (outlet). Temperature of feed solution 26”C, effective membrane area of each desalter 18 m*, date of measurement: October 16th, 1985. Number of desalter
1 2 3 4
Section
Feed solution and membrane Rejection to tap water (%)
1 2
96.55 96.98 92.83 92.68
performance
xylose from 4% to 8.5%
xylose from 8.5% to 20%
Rejection (%)
Flux (l/h)
Rejection (W)
Flux (l/h)
100
480 480 85.05 85.05
432 432
100
The desalter on the production line In the production process, we selected the two-stage method to treat the xylose solution. In the first section feed with concentration 3.0-4.5% and pH 3-5 was concentrated to 7-lo%, and then the concentrated product was passed through the second section, where it was further concentrated to 1517% (see Table III). The CAzol and the CA-CTA,,, type membranes proved to have longer service life for the concentration of xylose solution in industrial production.
Fig. 3. Photographs
of membranes
after different
work periods; left 0.5 y, right 1 y.
Fig. 4. Photograph
of a membrane
after 3 y work period.
From May 1982 till October 1985, in all six desalters were used in production. Two desalters began service in May 1982. In January 1984 four new desalters replaced the two old ones, but some membranes which had been used in the two old desalters were reassembled into the four new desalters and used again after mechanical clearing. Until October 1985, the rejection of the membranes to xylose was still satisfactory, as was rejection to tap water. The results are listed in Table III. It is a pity that the flux decreased slightly. Perhaps one reason is that the membranes became more dense under pressure. Another reason is that the membrane surface was polluted and blocked by dust and colloids in the solution. However, the flux would rise after clearing the surface of the membrane. As can be seen from the photographs in Figs. 3 and 4 showing membranes at different time in their service life, pretreatment of the feed solution is very important. It has a direct effect on the service life and concentrating efficiency of the membrane. To reduce the cost of changing membranes, more attention should be point to feed solution pretreatment. CONCLUSIONS
Reverse osmosis membranes made from cellulose acetates or their derivative can take the place of the Three Boiling System to concentrate xylose solutions on an industrial scale. CA,,, and CA-CTA,,, type membranes can concentrate xylose solutions from 3% to more than 15% and both the fluxes and the rejections of the two membrane types are satisfactory. Compared with the Three Boiling System, the reverse osmosis process has some advantages. It can avoid xylose burning, save energy while keeping the same standard of purification. At the same time, it allows acid in the feed to
201
permeate the membranes and so the acidity of the solution, decreases, increasing the service life of the exchange resin which is assembled behind the desalters. As the concentration of the xylose solution increases during the processing, the concentration efficiency must be raised. Therefore the process is divided into two stages. In the first section high-rejection membranes are used, and in the second section high-flux membranes are used. High efficiency can be achieved with CA-CTA,,, type membranes in the first section and CASO1type membranes in the second section. To increase the service life of the membranes, feed pretreatment can be applied. The pH value of the feed should be between 3 and 6 and the operation pressure below 42 kg/cm’. Membranes reinforced with polyester webs and treated into dry membranes possess high intensity, and are convenient to store, transport, bond and seal.
REFERENCES 1
2 3 4
5
Liu Yurong, Lang Kongmon, Chen Yiming and Cai Bangxiao, Effect of heat-treating and dry conditions on the performance of cellulose acetate reverse osmosis membrane, Desalination, 54 (1984) 185-195. Liu Yurong and Cai Bangxiao, Technology of Water Treatment, (1985) 9. Liu Yurong and Lang Kangmin, Dry cellulose acetate (CA) reverse osmosis membrane reinforced with polyester fabric and cast by machine, Technology of Water Treatment, (1985) 6. K. Sir Kar et al., The effect of short air exposure periods on the performance of cellulose acetate membranes from casting solutions with high cellulose acetate content, J. Appl. Polym. Sci., 22, No. 7 (1978). A.F. Turbak. ACS. Symp. Series, 154 (1981) 207-219.
193 Printed in The Netherlands
Concentrating xylose solution by reverse osmosis with cellulose acetate dry membrane* LIU YURONG, CA1 BANGXIAO, CHEN YIMING and LANG KANGMIN Second Institute of Oceanography, State Oceanographic Administration, P.O. Box 507, Hangzhou (China)
SUMMARY
This paper presents the results of research into concentrating xylose solution by using cellulose acetate reverse osmosis ( CA RO ) dry membranes which contain different additives. According to the feasibility test, CA-CTA,,, type mixed membranes and CA,,, membranes were found effective to concentrate aqueous xylose solutions. The membranes were fitted into plate type desalters, and the desalters replaced the original Three Boiling System in the xylitol production process. After 3f years, the rejection of the membranes was still good although the flux had slightly decreased; the used membranes could be used again after mechanical cleaning. This paper also discusses the factors which affect the service life of the membranes and suggests the methods to eliminate the effects.
INTRODUCTION
The reverse osmosis technique was developed more than 20 years ago and has found applications in food and pharmaceuticals concentration during the last ten years. However, in China, despite the success of studies on the application of this technique in recent years, no success on an industrial scale was reported until last year. In xylitol production in China the Three Boiling System is a commonly used process for concentrating the intermediate, xylose. The technique is ripe and practical, but it is highly energy and labour intensive, and would cause xylose to be burned, and pollute the environment. The concentration process with the reverse osmosis technique takes place at ambient temperature, so no burned sugar or new pigment can be produced. The reverse osmosis process can also *Presented at the International Symposium on Synthetic Membrane Science and Technology, Dalian, China, April 13-18,1986.
Ooll-9164/87/$03.50
0 1987 Elsevier Science Publishers B.V.
194
remove part of the acid and a small amount of dust from the solution. Therefore the new process can reduce the load of ion-exchange resin needed in the process after reverse osmosis and shorten the production period, and it also reduces the cost of the production and improves the qualtity of the xylitol. The application of the reverse osmosis technique will make the process of xylose purification preferable. A laboratory test for concentrating xylose solutions began in August 1981 in China; after reliable data from feasibility tests had been obtained, the technique of reverse osmosis was chosen for the process of manufacturing xylitol in the First Chemical Plant of Jilin. EXPERIMENTAL
Membrane
materials and solvents
Cellulose acetate (CA) - acetyl content, 38.46; viscosity, 228 cP; transparency, 13.2 cm. Cellulose triacetate (CTA) - acetyl content, 43.38; degree of polymerization (DP), 321.8; transparency, 21 cm. Acetone ( chemically pure ) . Dioxane (chemically pure). Preparation
of membrane
The CA or CA-CTA mixtures, and additives were added into solvents in proper proportion with stirring. After the polymers were dissolved, the cast solution was filtered and stilled until bubble free. The membranes were cast under controlled temperature and air humidity conditions. The cast solution was decanted into the slot of the casting knife mounted on the casting machine, and then the machine cast the solution on a polyester web. The film being produced was passed through a gelation bath, a heat treatment bath and a drying bath in turn; finally, it dried at ambient temperature until a dry membrane had formed. The conditions of membrane preparation were as follows: environment temperature lo-3O”C, environment humidity above 80%, evaporation time lo-60 s, gelation tap water temperature lo-3O”C, annealing water temperature 75-85°C (annealing process may be cancelled), drying bath aqueous glycerol or surfactants solution. Feasible concentration
test conditions and apparatus
WNJ-II high pressure water pump, stainless steel high pressure reverse osmosis cells; operation pressure 40 kg/cm2 at the start, later increased. Feed xylose solution refractive index from 3% at the beginning to 15% at the end.
195
Feed solution cell 40 cm’.
pH 3, feed flow rate 230 mg/min,
effective
membrane
area of a
Measurement instruments and calculation equation WZS-1 Arby-refractometer; Rejection =
refractive
WYT Saccharometer.
index of feed-refractive index of permeate refractive index of feed
x 100%
RESULTS AND DISCUSSION
Choice of membrane As the molecular weight of xylose is higher (MW = 152) than that of a number of salts, the required rejection of the membranes for xylose concentration can be comparatively low, but the membranes must satisfy the following requirements. (1) Rejection to xylose is over 95%, ( 2) the flux should be high enough and the slope of flux decrease should be low, (3) they should be suitable for acid feed ( pH = 2-6 ) , ( 4 ) the membranes should not be degraded by microbial degradation, (5) they must have high intensity and long service life, (6) they should be convenient to bond, store and transport, (7) their mechanical production should be easy. According to the above principles, several cellulose acetates or their derivatives were chosen as polymers, and phosphoric acid, methanol, n-propanol, water, triethanolamine, N,N-dimethylformamide, magnesium perchlorate, dioxane etc. were chosen as additives, to make reverse osmosis dry membranes which were reinfored with a polyester web. After some concentration tests using different membranes, considering both the rejection and the flux of the membranes, the CAzOi type membrane with magnesium perchlorate and dioxane as additives, and the CA-CTA,,, type membrane with methanol as additive were selected for xylose solution treatment [ 21. Effect of the conditions of preparation on membrane performance It is well known that membranes with different performances can be produced from the same cast solution simply by changing the conditions under which the membranes are made. We carried out the following tests to prepare membranes suitable for concentrating xylose solutions. Effect of solvent evaporation time on membrane performance. The period it takes
196
I 5
10
15
Gelation
20
water
I 25
I
1
30
35
temperature
40
PC
45
1
Fig. 1. Effect of the temperature of the gelation water on the membrane performance. (-_)
CA,
(---) CA-CTA; (X ) flux, (0) rejection.
for the film to go from the casting knife to the gelation bath is called solvent evaporation time here. In this period the solvent evaporates from the surface of the membrane, the chains of the polymers in the casting solution become close to each other, and a thin, dense layer is formed in the upper layer of the film. The structure of the thin layer will directly affect the membrane performance. Some casting solutions were cast under controlled atmosphere, temperature and humidity conditions. The results are that for CA,,, membranes the flux decreases and the rejection increases as the evaporation time increases, and for CA-CTA,,, membranes both the flux and rejection change only slightly within an evaporation period from 5-60 s, but when evaporation time exceeds 60 s, the flux increases quickly and the rejection decreases correspondingly [ 3, 41. Effect of gekztion temperature on membrane performance. After solvent evaporation, the film entered a water bath and gelation began. In the bath, the solvent and additive in the film diffuse outwards, and the diffusion rate is closely related to the temperature of the gelation bath. From Fig. 1 the relationship between the membrane performance and the temperature of the gelation bath can be seen. The fluxes, no matter whether the membrane is CA,,, or CA-CTA,,, type, as shown in Fig. 1, decrease with increase of gelation temperature. The rejection of the CA membrane has a maximum at 30°C; that of the CA-CTA membrane changes little when the gelation temperature is over 35”C, but the surface of the membranes would become wrinkled if the temperature would be over 35 ‘C. From a production viewpoint, it is preferable to set the gelation temperature at about 30’ C. Effect of drying temperature on membrane performance. At ambient tempera-
197 1 h
9-
-100
5 a-
o-
-_~_--
_~~:ZX==S----
-60
“E ‘-
2
‘;; x-__
--‘%-.___
6-
-60 c 20 'Z
--1x---X__
\
36 IL
---IL
XL"_ 4-
+
x-x-20
J10
20
Drying Fig. 2. Effect of
m
40
tempemture
50
60
PC 1
drying temperature on membrane performance. (-1
flux, (0 ) rejection.
CA, (___) CA-CT& (X )
ture membranes which have just passed through the drying treatment bath, take a long time to dry. To find the shortest practical drying time which would allow constant and reliable production of the membranes, drying tests were carried out on the membrane at different temperatures. The results of the tests are shown in Fig. 2. The rejection increases and the flux decreases both when the drying temperature is increased and when the drying time is increased. Tests of the membrane performance Effect of concentration of xylose solution on membrane performance. In order to allow reverse osmosis techniques to be applied process of concentrating xylose solution, the effect of the concentration of the xylose solution on the performance of the membrane must be known. Therefore circular concentrating tests were carried out in which different membranes were used. The test results were that the fluxes always decrease and the rejections are not markedly influenced as the concentration of xylose increases, for all kinds of membranes tested. As an example the date for the CA,,, membrane are listed in Table I. In view of operation pressure, feed solution pH and membrane performance, it is preferable to use the CA,,, membrane and CA-CTA,,, mixed membranes in the industrial concentration process. There are two methods to assemble desalters in the concentration process. The first is a two-stage method because of the considerable difference in concentration between feed and final concentrated solution. In the first section membranes with high rejection are used to treat the solution with low concentration and in the second section membranes with high flux and lower rejection are used to treat the solution with high concentration; the two sections are connected and product permeated through the membranes of the second section will be sent back to the feed, because it
198 TABLE I EFFECT OF CONCENTRATION OF XYLOSE SOLUTION ON THE MEMBRANE PERFORMANCE Feed solution temp. 29-29.5” C, effective membrane area 40 cm’, feed flow rate 230 ml/min. Concentration of xylose solution (Wl
3.23
6.80
8.50
9.50
12.25
14.50
15.00
Flux (ml/cm’
4.50
3.15
3.00
2.40
2.55
2.55
2.55
h)
Operation pressure ( kg/cm’ 1
40
Feed solution pH
40
3.0
50
4.0
60
5.0
5.4
70
70
5.6
70
5.6
contains some xylose. A second method applies the same membranes rejection throughout the whole process. The choice of the method based on economic efficiency.
5.6
with high should be
performance in xylose solution concentration. It is obvious that different rejections are found when the same membrane is used to treat tap water and xylose solution, respectively. From Table II it can be seen that a membrane with 85% rejection to tap water has a corresponding rejection of more than 95% to xylose. Therefore membranes with 85% rejection to tap water were chosen to be fitted into the desalters to concentrate the xylose solution, and the results were satisfactory. This is in agreement with Nielsen’s conclusion that cellulose acetate membranes with 8590% rejection to low salinity brackish water have a 100% rejection to sugar [ 51. Membrane
TABLE II THE RELATIONSHIP OF MEMBRANE PERFORMANCE BETWEEN THE REJECTION TO TAP WATER AND THE REJECTION TO XYLOSE SOLUTION Operation pressure 40 kg/cm*, feed solution temperature 26” C, effective membrane area 40 cm’, feed flow rate 230 ml/min. Membrane performance
Feed solution Tap water (Hangzhou)
Flux (ml/cm* h) Rejection ( % )
3.9 90.0
5.4 87.6
7.8 82.5
Xylose solution 6.5 85.0
6.0 85.0
2.3 100
3.0 100
(4.6% ) 5.0 89.2
4.5 95.0
4.0 98.5
199 TABLE III THE RESULTS OF FOUR DESALTERS USED TO TREAT WATER AND XYLOSE SOLUTION Operation pressure (tap water) 28 kg/cm’ (inlet), 23 kg/cm’ (outlet); operation pressure (first section, xylose) 35 kg/cm* ( inlet) ,30 kg/cm* ( oulet ) ; operation pressure (second section, xylose ) 45 kg/cm’ (inlet), 40 kg/cm* (outlet). Temperature of feed solution 26”C, effective membrane area of each desalter 18 m*, date of measurement: October 16th, 1985. Number of desalter
1 2 3 4
Section
Feed solution and membrane Rejection to tap water (%)
1 2
96.55 96.98 92.83 92.68
performance
xylose from 4% to 8.5%
xylose from 8.5% to 20%
Rejection (%)
Flux (l/h)
Rejection (W)
Flux (l/h)
100
480 480 85.05 85.05
432 432
100
The desalter on the production line In the production process, we selected the two-stage method to treat the xylose solution. In the first section feed with concentration 3.0-4.5% and pH 3-5 was concentrated to 7-lo%, and then the concentrated product was passed through the second section, where it was further concentrated to 1517% (see Table III). The CAzol and the CA-CTA,,, type membranes proved to have longer service life for the concentration of xylose solution in industrial production.
Fig. 3. Photographs
of membranes
after different
work periods; left 0.5 y, right 1 y.
Fig. 4. Photograph
of a membrane
after 3 y work period.
From May 1982 till October 1985, in all six desalters were used in production. Two desalters began service in May 1982. In January 1984 four new desalters replaced the two old ones, but some membranes which had been used in the two old desalters were reassembled into the four new desalters and used again after mechanical clearing. Until October 1985, the rejection of the membranes to xylose was still satisfactory, as was rejection to tap water. The results are listed in Table III. It is a pity that the flux decreased slightly. Perhaps one reason is that the membranes became more dense under pressure. Another reason is that the membrane surface was polluted and blocked by dust and colloids in the solution. However, the flux would rise after clearing the surface of the membrane. As can be seen from the photographs in Figs. 3 and 4 showing membranes at different time in their service life, pretreatment of the feed solution is very important. It has a direct effect on the service life and concentrating efficiency of the membrane. To reduce the cost of changing membranes, more attention should be point to feed solution pretreatment. CONCLUSIONS
Reverse osmosis membranes made from cellulose acetates or their derivative can take the place of the Three Boiling System to concentrate xylose solutions on an industrial scale. CA,,, and CA-CTA,,, type membranes can concentrate xylose solutions from 3% to more than 15% and both the fluxes and the rejections of the two membrane types are satisfactory. Compared with the Three Boiling System, the reverse osmosis process has some advantages. It can avoid xylose burning, save energy while keeping the same standard of purification. At the same time, it allows acid in the feed to
201
permeate the membranes and so the acidity of the solution, decreases, increasing the service life of the exchange resin which is assembled behind the desalters. As the concentration of the xylose solution increases during the processing, the concentration efficiency must be raised. Therefore the process is divided into two stages. In the first section high-rejection membranes are used, and in the second section high-flux membranes are used. High efficiency can be achieved with CA-CTA,,, type membranes in the first section and CASO1type membranes in the second section. To increase the service life of the membranes, feed pretreatment can be applied. The pH value of the feed should be between 3 and 6 and the operation pressure below 42 kg/cm’. Membranes reinforced with polyester webs and treated into dry membranes possess high intensity, and are convenient to store, transport, bond and seal.
REFERENCES 1
2 3 4
5
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