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Low-pressure reverse osmosis drying membrane reinforced by polyester web
403
Desalination,64 (1987)403-410
ElsevierSciencePublishersB.V.,Amsterdam-PPrintedin TheNetherlands
LOLI-P"ESSURERjWERSE CSD,OSISfiRYII
OOll-9164/87/$03.50
01987
ElsevierScience PublishersB.V.
404 solvents and additives in a proper proportion then filtered and stilleu; the casting solution was cast into the film on the casting mrchine under controlled temperature and relative humidity, after solvents evaporation, gelation, past treatment and drying, the 2 preparation of the membrane was finished . Measurement of the membrane performance The membrane was fitted into the two stainless steel cells in which the effective area of the each piece of membrane is 40cm2 and the measurements were carried out at these conditions: feed Hangzhou tap water or sodium chloride aqueous, feed rate 12-13.8 l/h. and pressure 1.5MPa (except special note). Conductivity of feed and product water was measured by using conductometer or chlorinity titration. RESILT AND lX%USSION Effects of the composition of the cast solution on the membrane performance The each composition in the casting solution affects the whole membrene performance, respectively and mutually. In other words, the quality of the polymer, solvents and additives, and the interaction of the polymer-solvents-additivecondition the stucture and the performance of the membrane. In the test, we studied the relationship of the kind and quantity of the solvents and additives, and the polymerization degree and viscosity of CTA with the reverse osmosis feature of the membrane (i)
Effect
of
mixed
solvent
system It is well know that acetone
and dioxane are good solvents to CA and CTA, but dissolved content of the CTA in the two solvents is limited 3. According to the principle that substances with similar solubility parameters can dissolve to each other, scme solvents with similar solubility Parameter with CTA were selected to be tested. The results of the test are list in table 1. The results of the test show that the membranes with dioxaneacetone or acetone-dioxance-dichloromethane as solvents have good reverse osmosis performance, and it is still possible to increase further the proportion of the CIA in the casting solution.
405
TABLE 1 Effect of solvents on the membrane performance t state of the casting solution Acetone + dioxance
some fine swelling CIA grains
dissolved Acetone + dichloromethane completely transparent
aPPerance of the membrane
performance ";&m2h)
some undissolved polymer granules smooth,,tough transparent
4.30
91.5
0.55
94.0
smooth few ketone + dissolved 4.52 completely, CIA undissolved dioxance + dichloromethane could be added polymer granules more Acetone + dioxance+ dimethyl sulfoxide
dissolved completely
the wet membrane smooth, but the dry membrane
Re$ytion
a.56
85.6
58.2
By using dichlorcmethane-acetoneas solvents, the pervaporation membrane could be also made, and it was effective to Separate ethanol from water. (ii)Effect of the additive The agents with solubility in solvents and water were chosen as additives, their effects on the membrane can be seen from table 2. In table 2, the membranes with n-propanolacetic acid, methanol-maleic acid or methanol as additive have better reverse osmosis performance lXBLE2 Effect of the additives on the membrane effects the membrane additives Performance Flux Re'ection (mi/ ?X) cm.) Dimethylsulfoxide 20;s 18.6 Formamide+methanol 2.1 91,a Raleic acid+n-propanol 1.1 N-propanol+aceticacid 1.6 ;2*; Formamide 32:4 7.9 Magnesium chloride+ acetic acid 15.3 15.7 Methanol +acetic 0.82 82.0 anhydride
appearance of the membrane
smooth,opalescent smooth,light Opalescent light bluc,good toughness smooth, opalescent white white, light fragile smooth, opalescent
406
Methanol Acetic acid
2.7 0
93.4
opalescent, light fragile transparent, good toughness ,
On the other hand, the orthogonal test found the effects of the in the casting solution. The increase content of the conten of nethanal content in the casting solution almost does not affect the membrane performance, however, it decreases dissolubility of CTA. IO-16% methanal content was proved perferable. The increase of maleic acid and acetic acid would increase the flux of the membrane but decrease the dissolubility of the CTA and the rejection of the membrane. (iii) Effect of polymerization degree and viscosity of the CIA As a main composition, the polymerization degree and viscosity of the polymers should be a studied object. Because cellulose diacetate had been used for many years, the paper just investigated cellulose triacetate. In China, there is not any special factory producing cellulose acetates used for reverse osmosis. We had to deci'de whether a cellulose triacetate is suitable for preparing membrane or not just depending on reverse osmosis experiment and observation (table 3) T-3 Effect of polymerization degree and viscosity of the CTA on the membrane performance membrane appearance f/lgzh) riJ;c
‘ITP
state of casting solutinn
226
dissolved permeated completely seriously
not smooth
23.6
28.2
298
dissolved permeated completely
not smooth
18.2
57.9
309
dissolved permeated completely a few permeated swelling a littl granules
16.5
65.2
undissolved granulars
3.7
90.3
not smooth
16.5 59.2 18.3 28.2
322
CcqJosite of the casting solution with polyester web
344 many swelling not 357 granules permeated
407
rlscoslt~ (sec.) 46
dissolved permeated undissolved not so well little granulars
35.9
a few swelling granules
18.7
dissolved permeated completely little
.*_ 22.5
some swelling granules
permated little
undissolved granulars smooth
permeated undissolved little almost
3.5
92.1
4.0
87.5
3.8
90.2
3.9
86.2
The experiment and obeservation found that the average degree of polymerization of the CIA has influence on the membrane performance and the 320 average degree CTA is preferable as membrane material; the viscosity of the CTA almost does not influence the membrane performance. The effect of the viscosity is that the dissolubility of the CTA d xi:?ses and the touthness of the membrane
inCreSSeS when the viscosity increases. According to the table 3, the CTA S-82855 was selected for preparation of the low-
pressure membrane. Effect of envirorxaental relative humidity on the membrane performance The environmental conditions under which the low-pressure membrane is prepared affected the membrane performance, sometime the effects were so large that the same casting solution may form reverse osmosis membrane or ultrafiltrationmembrane. The concerned factors, such as relative humidity, temperature, atmosphere property and gelation medium and temperature (no annealing) have been tested. Fig 1 shows the effect of the humidity on the membrane performance, I
A +
.,/h;Q-Urn
I
Fig. 1 Effect of relative humidity on the membrane performance
408
In the range of the humidity lower than 65% or higher than 8596, the flux of the membrane is lower and the rejection is higher; otherwise in the range of the humidity from 65% to 8596,the flux of the membrane is higher and the rejection is lower. So Controlling different humidity, the membrane for elifferent purposes could be obtained. It is regarded that the moisture in the atmosphere in the evaporation step has two opposite effects, first it hastens gelation in turn to increase the pore size on the membrane surface, and on the other hand it retards the rate of evaporation in turn to decrease the pore size on the membrane surface 4 . This point can explain the curves on the Fig.1. In the range of high relative humidity or low relative humidity the comprehensive effect of the atmosphere moisture is mainly to decrease the solvent evaporation rate or to increase the gelation respectively. The reverse osmosis performance of the low-pressure membrane (i) Effect of the operation pressure Usually, the flux and the rejection of the CA or/and CTA reverse osmosis membranes decrease as operation pressure decrease, so does the low-pressure. But unlike ordinary CA or/and CX'ARO membrane, even if operation pressure decrease to O.SMPa, the lowpressure membrane developed here still has acceptable flux and rejection. In October, 1986, the CA-CTA,02 type low-pressure mdmbrane was appraised by a special committee. The table IV is the date measured in site. TABLE4 Effect of operation pressure on the membrane performance
~
In the low pressure range (1.5Mpa or less),the CA-CTA type membrane made by the casting machine can remaid stable performance, flux 2.0-5.5ml/cm2.h and rejcetion 90.0-94.@d,in the test and prodution. (ii) The membrane survice life
409 Fig.2
is short time test of the low-pressure membrane service
life.
Fig. 2 Effect of operation time on the membrane performance (iii) Effect of feed concentration on the membrane performance ‘e&a AFt* P R"/ 1
I
I 600 pm
/Jo0
&GO
j.-o
j&a
+
.Lft CerhdhiXw cpyrn) Fig.3 Effect of salt concentration in feed on the membrane perdormance It can be seen from Fig.3 that the flux of the membrane decreases gradually and the rejection does not change almost as the salt concentration in feed increases. Similar result was got when the membrane was fitted into 92OOmm sprial desalters (Table3)
410
TABLE 5 performance of the membrane in desalters eed concentration
flux of single module(T/h) Rejection(%)
1457 1.16
86.54
3400
4400
0.97
0.96
86.87
86.41
Operation pressure: 2.5MPa
CONCLUSION The success of the low-prossure membrane with better reverse osmosis performance depends on the proper composition and proportion of the casting solution and the conditions for making the membrane. The CTA-8285' of 220 s,dioxane-acetone and n-propanolacetic acid et.al are suitable for the membrane material, solvent and additive, respectively. Further, it is much possible to obtain low-pressure membrane with excellent performance by choosing a much reasonable proportion of the casting solution, adding more CTA with higher pp and finding out the best of the preparing membrane conditions and satisfactory support material. The test of the operation conditions shows that in the range of low pressure (about l.OMPa) the CA-CTA,02 type membranes possess effective performance to treat tap water and low-salinity brackish water. The development of the CA-CIA low-pressure membrane lays the foundation of developing low-pressure and high rejection CPA membrane. REFERENCES Ajitk Ghosh and Kamaleshk Sirkar, low-pressure Reverse Osmosis Desalination with Improved Cellulose Acetate Membrane, J.APP. Polym. Sci. V.23, 1979, PP 1291-1307 Liu Yurong, Cai Banxiao et.al, CA-CTAlo, type RO drying Membrane Reinforced with Polyester Web, Technology of Water Treatment, V.11, 1985, PP 25-29 R.M.Kava and A.V.Rao, Indian Journal of Technology, Vol. 20, No.4, 1982 PP 147-149 S.Somrirajan Lecture on Reverse Osmosis, 1983, PP 517
Desalination,64 (1987)403-410
ElsevierSciencePublishersB.V.,Amsterdam-PPrintedin TheNetherlands
LOLI-P"ESSURERjWERSE CSD,OSISfiRYII
OOll-9164/87/$03.50
01987
ElsevierScience PublishersB.V.
404 solvents and additives in a proper proportion then filtered and stilleu; the casting solution was cast into the film on the casting mrchine under controlled temperature and relative humidity, after solvents evaporation, gelation, past treatment and drying, the 2 preparation of the membrane was finished . Measurement of the membrane performance The membrane was fitted into the two stainless steel cells in which the effective area of the each piece of membrane is 40cm2 and the measurements were carried out at these conditions: feed Hangzhou tap water or sodium chloride aqueous, feed rate 12-13.8 l/h. and pressure 1.5MPa (except special note). Conductivity of feed and product water was measured by using conductometer or chlorinity titration. RESILT AND lX%USSION Effects of the composition of the cast solution on the membrane performance The each composition in the casting solution affects the whole membrene performance, respectively and mutually. In other words, the quality of the polymer, solvents and additives, and the interaction of the polymer-solvents-additivecondition the stucture and the performance of the membrane. In the test, we studied the relationship of the kind and quantity of the solvents and additives, and the polymerization degree and viscosity of CTA with the reverse osmosis feature of the membrane (i)
Effect
of
mixed
solvent
system It is well know that acetone
and dioxane are good solvents to CA and CTA, but dissolved content of the CTA in the two solvents is limited 3. According to the principle that substances with similar solubility parameters can dissolve to each other, scme solvents with similar solubility Parameter with CTA were selected to be tested. The results of the test are list in table 1. The results of the test show that the membranes with dioxaneacetone or acetone-dioxance-dichloromethane as solvents have good reverse osmosis performance, and it is still possible to increase further the proportion of the CIA in the casting solution.
405
TABLE 1 Effect of solvents on the membrane performance t state of the casting solution Acetone + dioxance
some fine swelling CIA grains
dissolved Acetone + dichloromethane completely transparent
aPPerance of the membrane
performance ";&m2h)
some undissolved polymer granules smooth,,tough transparent
4.30
91.5
0.55
94.0
smooth few ketone + dissolved 4.52 completely, CIA undissolved dioxance + dichloromethane could be added polymer granules more Acetone + dioxance+ dimethyl sulfoxide
dissolved completely
the wet membrane smooth, but the dry membrane
Re$ytion
a.56
85.6
58.2
By using dichlorcmethane-acetoneas solvents, the pervaporation membrane could be also made, and it was effective to Separate ethanol from water. (ii)Effect of the additive The agents with solubility in solvents and water were chosen as additives, their effects on the membrane can be seen from table 2. In table 2, the membranes with n-propanolacetic acid, methanol-maleic acid or methanol as additive have better reverse osmosis performance lXBLE2 Effect of the additives on the membrane effects the membrane additives Performance Flux Re'ection (mi/ ?X) cm.) Dimethylsulfoxide 20;s 18.6 Formamide+methanol 2.1 91,a Raleic acid+n-propanol 1.1 N-propanol+aceticacid 1.6 ;2*; Formamide 32:4 7.9 Magnesium chloride+ acetic acid 15.3 15.7 Methanol +acetic 0.82 82.0 anhydride
appearance of the membrane
smooth,opalescent smooth,light Opalescent light bluc,good toughness smooth, opalescent white white, light fragile smooth, opalescent
406
Methanol Acetic acid
2.7 0
93.4
opalescent, light fragile transparent, good toughness ,
On the other hand, the orthogonal test found the effects of the in the casting solution. The increase content of the conten of nethanal content in the casting solution almost does not affect the membrane performance, however, it decreases dissolubility of CTA. IO-16% methanal content was proved perferable. The increase of maleic acid and acetic acid would increase the flux of the membrane but decrease the dissolubility of the CTA and the rejection of the membrane. (iii) Effect of polymerization degree and viscosity of the CIA As a main composition, the polymerization degree and viscosity of the polymers should be a studied object. Because cellulose diacetate had been used for many years, the paper just investigated cellulose triacetate. In China, there is not any special factory producing cellulose acetates used for reverse osmosis. We had to deci'de whether a cellulose triacetate is suitable for preparing membrane or not just depending on reverse osmosis experiment and observation (table 3) T-3 Effect of polymerization degree and viscosity of the CTA on the membrane performance membrane appearance f/lgzh) riJ;c
‘ITP
state of casting solutinn
226
dissolved permeated completely seriously
not smooth
23.6
28.2
298
dissolved permeated completely
not smooth
18.2
57.9
309
dissolved permeated completely a few permeated swelling a littl granules
16.5
65.2
undissolved granulars
3.7
90.3
not smooth
16.5 59.2 18.3 28.2
322
CcqJosite of the casting solution with polyester web
344 many swelling not 357 granules permeated
407
rlscoslt~ (sec.) 46
dissolved permeated undissolved not so well little granulars
35.9
a few swelling granules
18.7
dissolved permeated completely little
.*_ 22.5
some swelling granules
permated little
undissolved granulars smooth
permeated undissolved little almost
3.5
92.1
4.0
87.5
3.8
90.2
3.9
86.2
The experiment and obeservation found that the average degree of polymerization of the CIA has influence on the membrane performance and the 320 average degree CTA is preferable as membrane material; the viscosity of the CTA almost does not influence the membrane performance. The effect of the viscosity is that the dissolubility of the CTA d xi:?ses and the touthness of the membrane
inCreSSeS when the viscosity increases. According to the table 3, the CTA S-82855 was selected for preparation of the low-
pressure membrane. Effect of envirorxaental relative humidity on the membrane performance The environmental conditions under which the low-pressure membrane is prepared affected the membrane performance, sometime the effects were so large that the same casting solution may form reverse osmosis membrane or ultrafiltrationmembrane. The concerned factors, such as relative humidity, temperature, atmosphere property and gelation medium and temperature (no annealing) have been tested. Fig 1 shows the effect of the humidity on the membrane performance, I
A +
.,/h;Q-Urn
I
Fig. 1 Effect of relative humidity on the membrane performance
408
In the range of the humidity lower than 65% or higher than 8596, the flux of the membrane is lower and the rejection is higher; otherwise in the range of the humidity from 65% to 8596,the flux of the membrane is higher and the rejection is lower. So Controlling different humidity, the membrane for elifferent purposes could be obtained. It is regarded that the moisture in the atmosphere in the evaporation step has two opposite effects, first it hastens gelation in turn to increase the pore size on the membrane surface, and on the other hand it retards the rate of evaporation in turn to decrease the pore size on the membrane surface 4 . This point can explain the curves on the Fig.1. In the range of high relative humidity or low relative humidity the comprehensive effect of the atmosphere moisture is mainly to decrease the solvent evaporation rate or to increase the gelation respectively. The reverse osmosis performance of the low-pressure membrane (i) Effect of the operation pressure Usually, the flux and the rejection of the CA or/and CTA reverse osmosis membranes decrease as operation pressure decrease, so does the low-pressure. But unlike ordinary CA or/and CX'ARO membrane, even if operation pressure decrease to O.SMPa, the lowpressure membrane developed here still has acceptable flux and rejection. In October, 1986, the CA-CTA,02 type low-pressure mdmbrane was appraised by a special committee. The table IV is the date measured in site. TABLE4 Effect of operation pressure on the membrane performance
~
In the low pressure range (1.5Mpa or less),the CA-CTA type membrane made by the casting machine can remaid stable performance, flux 2.0-5.5ml/cm2.h and rejcetion 90.0-94.@d,in the test and prodution. (ii) The membrane survice life
409 Fig.2
is short time test of the low-pressure membrane service
life.
Fig. 2 Effect of operation time on the membrane performance (iii) Effect of feed concentration on the membrane performance ‘e&a AFt* P R"/ 1
I
I 600 pm
/Jo0
&GO
j.-o
j&a
+
.Lft CerhdhiXw cpyrn) Fig.3 Effect of salt concentration in feed on the membrane perdormance It can be seen from Fig.3 that the flux of the membrane decreases gradually and the rejection does not change almost as the salt concentration in feed increases. Similar result was got when the membrane was fitted into 92OOmm sprial desalters (Table3)
410
TABLE 5 performance of the membrane in desalters eed concentration
flux of single module(T/h) Rejection(%)
1457 1.16
86.54
3400
4400
0.97
0.96
86.87
86.41
Operation pressure: 2.5MPa
CONCLUSION The success of the low-prossure membrane with better reverse osmosis performance depends on the proper composition and proportion of the casting solution and the conditions for making the membrane. The CTA-8285' of 220 s,dioxane-acetone and n-propanolacetic acid et.al are suitable for the membrane material, solvent and additive, respectively. Further, it is much possible to obtain low-pressure membrane with excellent performance by choosing a much reasonable proportion of the casting solution, adding more CTA with higher pp and finding out the best of the preparing membrane conditions and satisfactory support material. The test of the operation conditions shows that in the range of low pressure (about l.OMPa) the CA-CTA,02 type membranes possess effective performance to treat tap water and low-salinity brackish water. The development of the CA-CIA low-pressure membrane lays the foundation of developing low-pressure and high rejection CPA membrane. REFERENCES Ajitk Ghosh and Kamaleshk Sirkar, low-pressure Reverse Osmosis Desalination with Improved Cellulose Acetate Membrane, J.APP. Polym. Sci. V.23, 1979, PP 1291-1307 Liu Yurong, Cai Banxiao et.al, CA-CTAlo, type RO drying Membrane Reinforced with Polyester Web, Technology of Water Treatment, V.11, 1985, PP 25-29 R.M.Kava and A.V.Rao, Indian Journal of Technology, Vol. 20, No.4, 1982 PP 147-149 S.Somrirajan Lecture on Reverse Osmosis, 1983, PP 517