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Polysulfone amide (PSA) asymmetric RO membrane
Desalination, 83 (1991) 271-278 Elsevier Science Publishers B .V ., Amsterdam
POLYSULFONE AMIDE (PSA) ASYMMETRIC RO MEMBRANE
GAO CONGJIE, LU XUBREN and BAO ZHIGUO Second Institute of Oceangraphy, SOA Hangzhou, China
Abstract Polysulfone amide is a very good material for making RO membrane . Comparing with other polyamides, its outstanding feature is resistance to free chlorine and oxidation, but the flux of corresponding membrane is low . In this work, the casting solution formula and membrane making condition have been modified to suit the requirement for industrial production, and also much afford have been done on improving the flux of the membrane by chemical modification of PSA . Introduction Since the research on CA RO membrane got a breakthrough in 1960, the RO technique has developed very quick . It is sooner found that CA has a narrow allowed pH range, and a poor resistance to biodegradation . This has led scientists to search for new polymers with better quality, one kind of the polymers is aromatic polyamide . Some of Soviet and Chinese membrane scientists widely worked on PSA membrane from the early of 1970'x . It was found from the research that PSA has superior properties to resist oxidation and degradation by oxygen, chlorine and Cr 6+ et al . and also a lot of laboratory research has been done in the formula of cast solution and technological conditions in making PSA membranes, The disadvatage for PSA as a membrane material is that PSA membrane has a low flux . This can be anticipated from PSA molecular structure .
271
272 The purpose of this research is to improve the flux of PSA membrane by modification of PSA chemically and to optimize membrane making conditions . Polysulfone amide PSA is obtained by solution condensation polymerization of 3,3' or 4, 4' bisaminophenylsulfone with terephthaloyl chloride at low temperature . The membrane properties from most of aromatic polyamide are very easily deteriorated by the attack of free chlorine, but those from PSA are not . This is related to its molecular structure showed bellow .
n Whole macromolecule forms a large bond by conjugation and the density of electron cloud tends to share equally . This make PSA more stable to oxidation and hydration, as well as the substitution of chlorine on aromatic ring is also became very difficult, Preparation of the membrane The membrane making steps are briefly given bellow . Adequate amount of additives is added to PSA solution, stir the solution until the additives are fully mixed and dissolved, then filter the solution and still for bobble free, pour the PSA solution on a piece of polyester web, and cast the solution into a very thin layer, evaporate the solvent at high temperature for a few minutes, finally put the formed membrane into water at about 25°C . Results and discussion The experiments proceeded in this study include evaporation temperature, evspor*ition period, gelation temper=. .ure, cast solution concentration, additives and modifying agents and dry membrane, as well as parts of operating parameters and the test of resistance free chlorine . 1 . Evaporation temperature It can be seen from Table 1 that the evaporation temperature at 100-110 °C is better than others . The evaporation at higher temperature
273 causes thick skin membrane to form, therefore, the flux o_` the membrane is lower than that at low evaporation temperature . On the other hand, if the evaporation temperature is too low, the evaporation of the solvents with high boiling point is not enough for forming an integrated skin layer, so the rejection of such membrane is very low .
Table 1 . Batch
Effect of evaporation temperature on membrane performance Evaporation temperature ( 00)
Rejection
Flux (ml/cm2 .h)
(%)
1 .23 1 .20
Test condition :
92 .7
10-3 10-4
110
93 .9
1 .24
conductivity of 100-200
100 90 BC
91 .0 51 .0
1 .43
10-5
16 .5
" .47
us/cm Operating pressure : 3 .OMPa Temperature : 25 ° C
10-1 10-2
10-6
140 130
94 .5
feed :
5 .10
tapwater with
2 . Evaporation Period : A lot of work was done here at different evaporation temperature for various period . Obviously, the higher evaporation temperature, the shorter the evaporating period . A two minute evaporation period at 120 0 C was enough for forming a dense skin, while it took 5 minutes at 105 ° C . The blowing in evaporation sharply shortened the evaporation period, and did not change tie membrane performance very much . Table 2 3 and 4 indicate these results . Table 2 .
Batch
Effect of evaporation period on membrane performance (at 125 00)
Evaporation period (min)
Rejection (%)
9-6
6
95 .3
9-5 9-4
5 4
91 .5 90 .4
9-3 9-2 9-1
3 2
90 .4 91 .5 31 .9
1
Flux (MI/cm 0 .8 1 .1 1,1 1 .1 1 .2 1 .1
2.
h)
274 Table 3 . Effect of evaporation period on membrane performance (at 105-110° C) Batch
Evaporation period
Rejection
(min)
(%)
Flux (ml/c,:2 . h)
11-1
10
95 .1
0 .75
11_2
8
93 .4
0 .68
11-3
6
11-4
5
91 .1 95 .6
0 .75 1 .00
11-5
4 2
57 .6
3 .70 16 .60
11-6
7 .5
Table 4 . Effect of evaporation period on membrane performance(blowing) Batch 100-105° C 13-4 13-3 13-2 13-1
Evaporation period 90-95° C (min)
13-8 13-7 13-6 13-5
1 2 3 4 5
100-105 °C
90-95 °C
Rejection Flux Rejection (%) (ml/cm h) (%)
Flux (ml/cm2 .h)
8 .7 0 .98 0 .75 0 .90
0 .83 0 .75 0 .53 0 .75
39 .4 94 .5 93 .8 94 .5 -
92 .4 93 .6 95 .4 96 .0
3 . Additives Two types of additives were used in the experiment : inorganic and organic . The role of inorganics is only for pore-forming agents, but the organics are not only as pore-forming agents, but also as modifying agents . Various inorganic salts have different effect on membrane performance . In general, LiNO 3 , LiCl and Ca(N03 ) 2 are very suitable as membrane additives . It is enough for making the membrane with good performance that the concentration of the additives in cast solution equals one half of that of polymer in cast solution . This is shown in Table 5 and 6 . Prom Table 7, it is found that organic additives are mainly the compounds containing carboxyl, hydroxyl groups, and that containing nitrogen . As an additive, some of them are as good as inorganics . It seems that there is still a big potential for further improvement .
275 Table 5 . Effect of inorganic additives on membrane performance Batch
seat
Rejection (%)
18
CaC12
30
Ca(N03 ) 2 4H 20 86 .5
31
LiCl LiNO 3
32
Table 6 .
Flux (ml/cm2 .h) 1 .27 1 .05
63 .0
1 .15
85 .5 92 .7
0 .90
Effect of concentration of Ca(N03 ) 2 4H20 on membrane
performance Batch
Rejection (%)
Concentration (%)
58
150
57
100
89 .6 89 .8
56
50
91 .0
1 .10 0 .95 0 .90
Table 7 .
Effect of organic additives
Batch
Additive
60-1
tannic acid
41 .6
59-1 52-1
DAPS
58 .5
H5I0I
70-2
Inositol
92 .5 87 .3
13-9 65-1
PEG DC DA
72 .0 92 .9
Rejection (%)
Flux 2 (ml/cm h)
Flux (ml/cm 2 .h) 3 .90 1 .80 1 .65 1 .30 1 .50 0 .85
4 . Resistance to chlorine Resistance of membrane to chlorine is very important because water disinfection by chlorine is widely used in most water treatment technology . If the membrane is poor for resistance to chlorine, the membrane will be deteriorated by chlorine and the service life of the
Operating pressure 3 .0 MPa
* R:
1 .30
90 .0
PSA-52
95 .0 0 .60
0 .60
92 .4
Rejection (%)
F2
86 .6 1 .10 93 .1 0 .80 92 .3 0 .60
B2
0 .80 0 .60
1 .10
F1
97 .0 1 .15
F : Flux (ml/cmN)
0 .60
95 .1 0 .55 97 .1 0 .60
94 .9 0 .55
F4
93 .3 95 .1
R4
92 .0 1 .10 96 .8 0 .75
F3
40
5 540
1 .10 0 .75
B3
28
4 418
88 .6
8
2
0
83 .5 91 .3 90 .6
PSA-27
R• 1
3 245
2 180
1 138
PSA-30 PSA-31 PSA_32
Membrane Performance Batch of membrane
Period (day) Feed conductivity(us/cm) Chlorine concentration in feed(mg/L)
Table 8 . Test of resistance to chlorine for PSA membrane
95 .8 97 .6
97 .0
R5
52
770
6
0 .55 0 .60
0 .75
F5
110
7 1270
26
96 .6 1 .00
R6
135
8 1500
F7
94 .2 1 .00
R7
277 membrane is very short, or the water to be treated by the membrane must dechlorinate first . As indicated in Table 8, the PSA membrane is very good for resistance to chlorine . This may be related to the conjugation of the molecular structure . From this point of view, PSA membrane should be further explored in detail .
5 . Effects of feed pressure and concentration on membrane performance As it can be expected, the flux and rejection of the membrane increased with the increase of pressure . The gradient of flux rising is very slow, this is because the membrane is very dense . The rejection has got a greater rising as increasing pressure from 2 .0 to 3 .0 MP a . This may be the result that more water permeates through the membrane than the salt under higher pressure (Fig .1)
c e J 72'd
N i
30-D BB .*
2.0
86 .0
// O
84.0
Lu 20
3.0
4.0
pressu re ( MQa) Fig . 1 Effect of pressure on membrane performance
It can be seen from Fig .2 that water flux remarns nearly anchange and rejection is improved a lot when the feed concentration is lower than 100 mg/L . On the other hand, water flux decreased nearly 30%, and rejection only increased a little . It can be concluded that the membrane is suitable for desalination of the feed aith high salinity at higher pressure .
6 . SEM photomicrograph of the membrane SEX photomicrograph of cross-section of PSA membrane in Fig .3 reveals the dense structure of both skin layer and substrate of the
278
98 .0 0ac
,a, 9¢0
9ia
1 tonccntraf
o,it"'ft)
Fig .2 Effect of feed concentration on membrane performance
Fig,3 SEM photomicrograph of PSA membrane
membrane . This may be one of the reason for low flux of the membrane . Therefore, to overcome the problem, i,e, to make a looser substrate, is a key factor for making the membrane with high flux .
Conclusion Comparing with other polyamides, PSA can be considered as a good material for making membrane, oweing to its resistance to chlorine and oxidation . This property of PSA is related to the molecular stability of conjugation . The membrane reinforced with polyester web was produced under proper control of cast solution formula and membrane making conditions, The membrane has high rejection and lower flux . There is still a potential for further flux improvement by chemical modification .
POLYSULFONE AMIDE (PSA) ASYMMETRIC RO MEMBRANE
GAO CONGJIE, LU XUBREN and BAO ZHIGUO Second Institute of Oceangraphy, SOA Hangzhou, China
Abstract Polysulfone amide is a very good material for making RO membrane . Comparing with other polyamides, its outstanding feature is resistance to free chlorine and oxidation, but the flux of corresponding membrane is low . In this work, the casting solution formula and membrane making condition have been modified to suit the requirement for industrial production, and also much afford have been done on improving the flux of the membrane by chemical modification of PSA . Introduction Since the research on CA RO membrane got a breakthrough in 1960, the RO technique has developed very quick . It is sooner found that CA has a narrow allowed pH range, and a poor resistance to biodegradation . This has led scientists to search for new polymers with better quality, one kind of the polymers is aromatic polyamide . Some of Soviet and Chinese membrane scientists widely worked on PSA membrane from the early of 1970'x . It was found from the research that PSA has superior properties to resist oxidation and degradation by oxygen, chlorine and Cr 6+ et al . and also a lot of laboratory research has been done in the formula of cast solution and technological conditions in making PSA membranes, The disadvatage for PSA as a membrane material is that PSA membrane has a low flux . This can be anticipated from PSA molecular structure .
271
272 The purpose of this research is to improve the flux of PSA membrane by modification of PSA chemically and to optimize membrane making conditions . Polysulfone amide PSA is obtained by solution condensation polymerization of 3,3' or 4, 4' bisaminophenylsulfone with terephthaloyl chloride at low temperature . The membrane properties from most of aromatic polyamide are very easily deteriorated by the attack of free chlorine, but those from PSA are not . This is related to its molecular structure showed bellow .
n Whole macromolecule forms a large bond by conjugation and the density of electron cloud tends to share equally . This make PSA more stable to oxidation and hydration, as well as the substitution of chlorine on aromatic ring is also became very difficult, Preparation of the membrane The membrane making steps are briefly given bellow . Adequate amount of additives is added to PSA solution, stir the solution until the additives are fully mixed and dissolved, then filter the solution and still for bobble free, pour the PSA solution on a piece of polyester web, and cast the solution into a very thin layer, evaporate the solvent at high temperature for a few minutes, finally put the formed membrane into water at about 25°C . Results and discussion The experiments proceeded in this study include evaporation temperature, evspor*ition period, gelation temper=. .ure, cast solution concentration, additives and modifying agents and dry membrane, as well as parts of operating parameters and the test of resistance free chlorine . 1 . Evaporation temperature It can be seen from Table 1 that the evaporation temperature at 100-110 °C is better than others . The evaporation at higher temperature
273 causes thick skin membrane to form, therefore, the flux o_` the membrane is lower than that at low evaporation temperature . On the other hand, if the evaporation temperature is too low, the evaporation of the solvents with high boiling point is not enough for forming an integrated skin layer, so the rejection of such membrane is very low .
Table 1 . Batch
Effect of evaporation temperature on membrane performance Evaporation temperature ( 00)
Rejection
Flux (ml/cm2 .h)
(%)
1 .23 1 .20
Test condition :
92 .7
10-3 10-4
110
93 .9
1 .24
conductivity of 100-200
100 90 BC
91 .0 51 .0
1 .43
10-5
16 .5
" .47
us/cm Operating pressure : 3 .OMPa Temperature : 25 ° C
10-1 10-2
10-6
140 130
94 .5
feed :
5 .10
tapwater with
2 . Evaporation Period : A lot of work was done here at different evaporation temperature for various period . Obviously, the higher evaporation temperature, the shorter the evaporating period . A two minute evaporation period at 120 0 C was enough for forming a dense skin, while it took 5 minutes at 105 ° C . The blowing in evaporation sharply shortened the evaporation period, and did not change tie membrane performance very much . Table 2 3 and 4 indicate these results . Table 2 .
Batch
Effect of evaporation period on membrane performance (at 125 00)
Evaporation period (min)
Rejection (%)
9-6
6
95 .3
9-5 9-4
5 4
91 .5 90 .4
9-3 9-2 9-1
3 2
90 .4 91 .5 31 .9
1
Flux (MI/cm 0 .8 1 .1 1,1 1 .1 1 .2 1 .1
2.
h)
274 Table 3 . Effect of evaporation period on membrane performance (at 105-110° C) Batch
Evaporation period
Rejection
(min)
(%)
Flux (ml/c,:2 . h)
11-1
10
95 .1
0 .75
11_2
8
93 .4
0 .68
11-3
6
11-4
5
91 .1 95 .6
0 .75 1 .00
11-5
4 2
57 .6
3 .70 16 .60
11-6
7 .5
Table 4 . Effect of evaporation period on membrane performance(blowing) Batch 100-105° C 13-4 13-3 13-2 13-1
Evaporation period 90-95° C (min)
13-8 13-7 13-6 13-5
1 2 3 4 5
100-105 °C
90-95 °C
Rejection Flux Rejection (%) (ml/cm h) (%)
Flux (ml/cm2 .h)
8 .7 0 .98 0 .75 0 .90
0 .83 0 .75 0 .53 0 .75
39 .4 94 .5 93 .8 94 .5 -
92 .4 93 .6 95 .4 96 .0
3 . Additives Two types of additives were used in the experiment : inorganic and organic . The role of inorganics is only for pore-forming agents, but the organics are not only as pore-forming agents, but also as modifying agents . Various inorganic salts have different effect on membrane performance . In general, LiNO 3 , LiCl and Ca(N03 ) 2 are very suitable as membrane additives . It is enough for making the membrane with good performance that the concentration of the additives in cast solution equals one half of that of polymer in cast solution . This is shown in Table 5 and 6 . Prom Table 7, it is found that organic additives are mainly the compounds containing carboxyl, hydroxyl groups, and that containing nitrogen . As an additive, some of them are as good as inorganics . It seems that there is still a big potential for further improvement .
275 Table 5 . Effect of inorganic additives on membrane performance Batch
seat
Rejection (%)
18
CaC12
30
Ca(N03 ) 2 4H 20 86 .5
31
LiCl LiNO 3
32
Table 6 .
Flux (ml/cm2 .h) 1 .27 1 .05
63 .0
1 .15
85 .5 92 .7
0 .90
Effect of concentration of Ca(N03 ) 2 4H20 on membrane
performance Batch
Rejection (%)
Concentration (%)
58
150
57
100
89 .6 89 .8
56
50
91 .0
1 .10 0 .95 0 .90
Table 7 .
Effect of organic additives
Batch
Additive
60-1
tannic acid
41 .6
59-1 52-1
DAPS
58 .5
H5I0I
70-2
Inositol
92 .5 87 .3
13-9 65-1
PEG DC DA
72 .0 92 .9
Rejection (%)
Flux 2 (ml/cm h)
Flux (ml/cm 2 .h) 3 .90 1 .80 1 .65 1 .30 1 .50 0 .85
4 . Resistance to chlorine Resistance of membrane to chlorine is very important because water disinfection by chlorine is widely used in most water treatment technology . If the membrane is poor for resistance to chlorine, the membrane will be deteriorated by chlorine and the service life of the
Operating pressure 3 .0 MPa
* R:
1 .30
90 .0
PSA-52
95 .0 0 .60
0 .60
92 .4
Rejection (%)
F2
86 .6 1 .10 93 .1 0 .80 92 .3 0 .60
B2
0 .80 0 .60
1 .10
F1
97 .0 1 .15
F : Flux (ml/cmN)
0 .60
95 .1 0 .55 97 .1 0 .60
94 .9 0 .55
F4
93 .3 95 .1
R4
92 .0 1 .10 96 .8 0 .75
F3
40
5 540
1 .10 0 .75
B3
28
4 418
88 .6
8
2
0
83 .5 91 .3 90 .6
PSA-27
R• 1
3 245
2 180
1 138
PSA-30 PSA-31 PSA_32
Membrane Performance Batch of membrane
Period (day) Feed conductivity(us/cm) Chlorine concentration in feed(mg/L)
Table 8 . Test of resistance to chlorine for PSA membrane
95 .8 97 .6
97 .0
R5
52
770
6
0 .55 0 .60
0 .75
F5
110
7 1270
26
96 .6 1 .00
R6
135
8 1500
F7
94 .2 1 .00
R7
277 membrane is very short, or the water to be treated by the membrane must dechlorinate first . As indicated in Table 8, the PSA membrane is very good for resistance to chlorine . This may be related to the conjugation of the molecular structure . From this point of view, PSA membrane should be further explored in detail .
5 . Effects of feed pressure and concentration on membrane performance As it can be expected, the flux and rejection of the membrane increased with the increase of pressure . The gradient of flux rising is very slow, this is because the membrane is very dense . The rejection has got a greater rising as increasing pressure from 2 .0 to 3 .0 MP a . This may be the result that more water permeates through the membrane than the salt under higher pressure (Fig .1)
c e J 72'd
N i
30-D BB .*
2.0
86 .0
// O
84.0
Lu 20
3.0
4.0
pressu re ( MQa) Fig . 1 Effect of pressure on membrane performance
It can be seen from Fig .2 that water flux remarns nearly anchange and rejection is improved a lot when the feed concentration is lower than 100 mg/L . On the other hand, water flux decreased nearly 30%, and rejection only increased a little . It can be concluded that the membrane is suitable for desalination of the feed aith high salinity at higher pressure .
6 . SEM photomicrograph of the membrane SEX photomicrograph of cross-section of PSA membrane in Fig .3 reveals the dense structure of both skin layer and substrate of the
278
98 .0 0ac
,a, 9¢0
9ia
1 tonccntraf
o,it"'ft)
Fig .2 Effect of feed concentration on membrane performance
Fig,3 SEM photomicrograph of PSA membrane
membrane . This may be one of the reason for low flux of the membrane . Therefore, to overcome the problem, i,e, to make a looser substrate, is a key factor for making the membrane with high flux .
Conclusion Comparing with other polyamides, PSA can be considered as a good material for making membrane, oweing to its resistance to chlorine and oxidation . This property of PSA is related to the molecular stability of conjugation . The membrane reinforced with polyester web was produced under proper control of cast solution formula and membrane making conditions, The membrane has high rejection and lower flux . There is still a potential for further flux improvement by chemical modification .