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Some global transport properties of polysulfone capillary UF membranes
Journal of Membrane Science 90 ( 1994) 191-195
Short communication
Some global transport properties of polysulfone capillaiy UF membranes B. Dutr6, G. TrQ$lrdh* Lund University, Foqd Engineering Department, P.O. Box 124, S-22100 Lund, Sweden
(Received January 2 1, 1994; accepted January 25, 1994)
Abstract The hydraulic resistance of polysulfone capillary membranes was measured with different fluids and at different transmembrane pressures. The results showed that the hydraulic resistance was independent of the fluid used but
increased slightly with the transmembrane pressure. This was assumed to be due to membrane compressibility. The capillary membrane used in this work had a very high global volume porosity which means that it was more sensitive to mechanical effects (pressure). The mass transfer coefficient for sodium chloride was also measured. This was done under turbulent flow conditions on both the retentate and permeate side. The values of the mass transfer coefficient found were of the same order of magnitude as for dialysis membranes. This observation is promising for UF applications involving transmembrane diffusive mass transfer such as blood purification and bioreactors. Key wora!s:Ultrafiltration; Capillary membrane; Hydraulic resistance; Mass transfer coefticient; Compressibility
1. Introduction
Membrane performance is very often given in terms of retention properties (which molecular species are retained by the membrane or pass through the membrane) and in terms of transport properties (solute permeability and hydraulic resistance). Capillary UF membranes are “a compromise” between hollow-fibre membranes and tubular membranes. Hollow-fibre, capillary and tubular membranes have diameters of < 0.5,0.5 to 5, > 5 mm, respectively. The hydraulic resistance of UF membranes is one of their most important characteristics. This *Corresponding author.
parameter is a measure of the degree to which solvent flows through the membrane. The hydraulic resistance is given by the well-known law of Darcy: AP J -V-R&
(1)
where J, is the volumetric permeate flux, AP is the transmembrane pressure, R, is the hydraulic resistance of the membrane and Jois the dynamic viscosity of the solvent. The concept of mass transfer coefficient is widely used in chemical engineering. This parameter is a measure of the degree to which molecules diffuse through the membrane from the retentate to the permeate. This parameter is very important for “special” applications of ultrafil-
0376-7388/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved SSDIO376-7388(94)00026-U
B. DutrP, G. Trtigdrdh /Journal
192
of Membrane
Science 90 (1994) 191-l 95
tration such as blood purification and bioreactors. The mass transfer coefficient (k,) is given by the ratio between the transmembrane diffusive mass flux of the solute (jti) and the transmembrane concentration difference (dc) :
membrane was used in a module of our own design able to handle one membrane. This was done to avoid mixing problems on the permeate side during mass transfer measurements. This membrane can withstand a maximum transmembrane pressure of 800 kPa.
jSd k -*-AC
2.2. Chemicals
The mass transfer coefficient given by Eq. (2) is in fact the superposition of three local mass transfer coefficients relating the concentration boundary layers on the retentate side (k,) and on the permeate side (I&), and the property of the membrane itself (I?,,,). (3) In ultrafiltration, the mass flux of the solute (jS) is a superposition of the diffusive transport and convective transport. This is the subject of much discussion in the literature [ 11. Eq. (4) describes this.
For the measurement of the hydraulic resistance, three different kinds of solvent were used: deionised and pre-filtered water at different temperatures (20, 15.6,10.6 and 5.7”(Z), waterglycerol mixtures (14.75,9.625 and 5%) at 20°C and water-sucrose mixtures ( 18, 8.55 and 4.17%) at 20°C. For the measurement of the mass transfer coefficient, aqueous sodium chloride solutions were used. All the chemical substances used were pro-analysi grade: glycerol (Merck, ref. 4094), sucrose (DBH Chemicals, ref. 10274) and NaCl (Merck, ref. 6404).
(4) where kz is the mass transfer coefficient for high flux of microsolute. kz can be related to ksl by Eq. ( 5 ) . The reader interested in further details on the superposition of convective and diffusive mass transfer and the relation between the two mass transfer coefficients (k$ and k,) is referred to ref. 1. /‘+
Jv
ev(Jvlkd)
-1
(5)
2. Experimental
‘J
15
In ~pelmeat8)
Membrane -.
ibl
15
‘-+
outketantate)
2.1. Membranes
Capillary polysulfone UF membranes, type M3/UF80, were supplied by X-FLOW B.V. According to the manufacturer, this membrane has a NMWCO of 80,000 Da, an inner diameter of 1.35 mm and an outer diameter of 2.26 mm. This
Module (item 5) Fig. 1. Experimental rig and module. 1 and 9: jacketed vessels, 2 and 10: pumps, 3: conductivity probe, 4,6, 11 and 12: pressure-measuring systems, 5: module, 7: back-pressure valve, 8 and 13: flow-measuring systems, 14: micro-computer, 15: pressure connections.
B. D&t!, G. Trig&rdh /Journal ofMembrane Science 90 (1994) 191-195
2.3. Equipment
193
tate and the other for the permeate side. In all the experiments, the temperature was controlled ( ~0.2”C) by circulating water from a thermostatic bath through the jackets surrounding the vessels and heat exchanger.
The experimental rig and the module are illustrated in Fig. 1. The experimental rig consists essentially of two closed loops: one for the reten-
(a) ;
2, a!!?! so iiE ‘C Ei=
a
4.5
3
-
oa
o-
‘-
7
4
n
4-
I”
6
3.5 -
3.5
P +
+
s-
t
y
2.5L.J
’ IO
’ 30
J SO
’ 70
_i 90 ,WL
Ilnouaanaar
Trananembrane
pr-
*,
110
130
150
170
IPd
Fig. 2. Membrane hydraulic resistance (R,) versus transmembrane pressure (LIP). A: (0 ) water at 5.7 “C, ( + ) water at 10.6”C, (0)waterat 15.6”C, (A) 14.75%glycerolinwaterat20°C, (x)9.62%glyeerolinwaterat20°C, (V) 5%glycerolinwaterat 20°C. B: (Cl) water at 5.7”C, (+) water at 10.6”C, (0) water at 15.6”C, (A) 13% sucrose in water at 2O”C, (X) 8.55% sucrose in water at 2O”C, ( V ) 4.17% sucrose in water at 20°C.
194
B. Dutrk, G. Triigdrdh /Journal of Membrane Science 90 (1994) 191- I95
3.5
’ 5.2
!
i 5.6
6
6.4 IThousandsl Rap L-1
6.6
7.2
7.6
Fig. 3. Global mass transfer coeffkient for NaCl (b) versus Reynolds number on the permeate side (Re,). Temperature 20°C Reynolds number on the retentate side 5,300 and NaCl concentration on the retentate side 100 g/l. ( •i ) Experiment 1, ( + ) experiment 2.
Before each experiment with a new piece of membrane, the membrane was soaked in water for 24 h at room temperature. 2.4. Methods The permeate loop was not used in the measurements of the hydraulic resistance. The permeate flux was recorded as a function of time until a constant value was obtained. The procedure was repeated at different transmembrane pressures ranging from 10 to 180 kPa (see Fig. 2) and with the different solvents described above. The global volume porosity of the membrane was also measured. This was done by soaking a piece of new or used membrane in water for 24 h at room temperature, After this, the water remaining on the membrane surfaces was removed: the external face of the capillary membrane was wiped with absorbant paper and air was blown into the capillary. Then the capillary membrane was weighed and dried in an oven at
50 ’ C.The dried membrane was finally weighed. This method of measuring membrane porosity is questionable [ 2 ] but it is very easy to perform and the results are reproducible. Finally, the mass transfer coefficient for NaCl was measured. For this, a NaCl solution ( 100 g/ 1) was recirculated in the retentate loop and water was circulated in the permeate loop. The NaCl concentration in the permeate loop (c,) was continuously measured using a conductivity probe. Occasionally, the NaCl concentration in the retentate was checked. Although water may diffuse from the permeate to the retentate, we did not observe any variation in the NaCl concentration in the retentate. Knowing the NaCl concentration ( cP) and the total volume of solvent ( VP) in the permeate loop as functions of time, we can calculate j, using the Eq. (6 ) and then kpl using Eqs. (4) and (5). . _~d(V,c,) IS--A dt
where A is the membrane area and t the time.
(6)
B. Dutr&,G. Triigctrdh/Journal of Membrane Science 90 (I 994) 191- I95
3. Results and discussion
The hydraulic resistance was calculated using Eq. ( 1). The results are presented in Fig. 2 (R, versus dP). This figure shows that R, increases slightly with increasing AP. The values of R, at low AP ( -c20 kPa) are not reliable due to the large errors in the measured values of J, and AP. Neither the solvent’s nature nor the temperature was found to significantly influence the values of R,. These effects are included in the viscosity (see Darcy’s law). The porosity of the membrane used in this work was found to be 81+0.7% for new membranes and 82 + 0.8% for used membranes. These values are the results of 5 determinations and are in good agreement with the value given by the manufacturer (70-80%). Fig. 3 shows the variation of b as a function of the Reynolds number on the permeate side (Rep). The Reynolds number on the retentate side was kept constant at 5,300. As mentioned previously, b is not an intrinsic transport property of the membrane because it depends on the hydrodynamic conditions on both the retentate and permeate side. It is possible to calculate the mass transfer coefficient of the membrane (&) by changing the hydrodynamic conditions [ 31. However, this approach requires numerous experiments and kg is the only mass transfer coefficient of practical interest. The results in Fig. 3 show values of & similar to those of dialysis membranes, although these are thinner than the capillary membrane used in this work. This situation could be explained by the high porosity of the capillary membrane used in this work (8 1%) compared to dialysis membranes (25-60%). 4. Conclusions The capillary membrane studied in this work has hydraulic resistance values similar to other
195
types of UF membranes. The hydraulic resistance of the capillary membrane increases slightly with the applied transmembrane pressure. This is probably a consequence of the high membrane porosity. The mass transfer coefficient of this membrane has not been measured but we have measured a global mass transfer coefficient, taking into account the boundary layer effect on both the retentate and permeate side. The values of this mass transfer coefficient are similar to those of dialysis membranes. This observation is interesting if we bear in mind that dialysis membranes are thinner than the capillary membranes used here. These good diffusive properties are certainly a consequence of the high porosity of the capillary membrane. The capillary membrane studied in this work exhibited interesting performances for both convective and diffusive transport without the disadvantages of hollow-fibre membranes (low maximum pressures, laminar flow, difficulty in handling viscous fluids, etc. ).
5. Acknowledgements This work was supported by The European Community through grant ERBAIR 1CT925 111. The authors acknowledge X-FLOW B.V. for providing capillary membranes,
6. References r11 J.E.
Sigdell, Calculation of combined diffusive and convective mass transfer, Int. J. Organs, ( 1982) 361-372. I21 Z. Morita, H. Ishida, H. Shimamoto, R. Weber and P. Rys, Anion permeability of cellulosic membranes. Part 1. Porosity of water-swollen membranes. J. Membrane Sci., 46 (1989) 283-298. 131 H.D. Spriggs and N.N. Li, Liquid permeation through polymeric membranes, in P. Meares (Ed. ), Membrane Separation Processes, Elsevier, Amsterdam, 1976, pp. 39-80.
Short communication
Some global transport properties of polysulfone capillaiy UF membranes B. Dutr6, G. TrQ$lrdh* Lund University, Foqd Engineering Department, P.O. Box 124, S-22100 Lund, Sweden
(Received January 2 1, 1994; accepted January 25, 1994)
Abstract The hydraulic resistance of polysulfone capillary membranes was measured with different fluids and at different transmembrane pressures. The results showed that the hydraulic resistance was independent of the fluid used but
increased slightly with the transmembrane pressure. This was assumed to be due to membrane compressibility. The capillary membrane used in this work had a very high global volume porosity which means that it was more sensitive to mechanical effects (pressure). The mass transfer coefficient for sodium chloride was also measured. This was done under turbulent flow conditions on both the retentate and permeate side. The values of the mass transfer coefficient found were of the same order of magnitude as for dialysis membranes. This observation is promising for UF applications involving transmembrane diffusive mass transfer such as blood purification and bioreactors. Key wora!s:Ultrafiltration; Capillary membrane; Hydraulic resistance; Mass transfer coefticient; Compressibility
1. Introduction
Membrane performance is very often given in terms of retention properties (which molecular species are retained by the membrane or pass through the membrane) and in terms of transport properties (solute permeability and hydraulic resistance). Capillary UF membranes are “a compromise” between hollow-fibre membranes and tubular membranes. Hollow-fibre, capillary and tubular membranes have diameters of < 0.5,0.5 to 5, > 5 mm, respectively. The hydraulic resistance of UF membranes is one of their most important characteristics. This *Corresponding author.
parameter is a measure of the degree to which solvent flows through the membrane. The hydraulic resistance is given by the well-known law of Darcy: AP J -V-R&
(1)
where J, is the volumetric permeate flux, AP is the transmembrane pressure, R, is the hydraulic resistance of the membrane and Jois the dynamic viscosity of the solvent. The concept of mass transfer coefficient is widely used in chemical engineering. This parameter is a measure of the degree to which molecules diffuse through the membrane from the retentate to the permeate. This parameter is very important for “special” applications of ultrafil-
0376-7388/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved SSDIO376-7388(94)00026-U
B. DutrP, G. Trtigdrdh /Journal
192
of Membrane
Science 90 (1994) 191-l 95
tration such as blood purification and bioreactors. The mass transfer coefficient (k,) is given by the ratio between the transmembrane diffusive mass flux of the solute (jti) and the transmembrane concentration difference (dc) :
membrane was used in a module of our own design able to handle one membrane. This was done to avoid mixing problems on the permeate side during mass transfer measurements. This membrane can withstand a maximum transmembrane pressure of 800 kPa.
jSd k -*-AC
2.2. Chemicals
The mass transfer coefficient given by Eq. (2) is in fact the superposition of three local mass transfer coefficients relating the concentration boundary layers on the retentate side (k,) and on the permeate side (I&), and the property of the membrane itself (I?,,,). (3) In ultrafiltration, the mass flux of the solute (jS) is a superposition of the diffusive transport and convective transport. This is the subject of much discussion in the literature [ 11. Eq. (4) describes this.
For the measurement of the hydraulic resistance, three different kinds of solvent were used: deionised and pre-filtered water at different temperatures (20, 15.6,10.6 and 5.7”(Z), waterglycerol mixtures (14.75,9.625 and 5%) at 20°C and water-sucrose mixtures ( 18, 8.55 and 4.17%) at 20°C. For the measurement of the mass transfer coefficient, aqueous sodium chloride solutions were used. All the chemical substances used were pro-analysi grade: glycerol (Merck, ref. 4094), sucrose (DBH Chemicals, ref. 10274) and NaCl (Merck, ref. 6404).
(4) where kz is the mass transfer coefficient for high flux of microsolute. kz can be related to ksl by Eq. ( 5 ) . The reader interested in further details on the superposition of convective and diffusive mass transfer and the relation between the two mass transfer coefficients (k$ and k,) is referred to ref. 1. /‘+
Jv
ev(Jvlkd)
-1
(5)
2. Experimental
‘J
15
In ~pelmeat8)
Membrane -.
ibl
15
‘-+
outketantate)
2.1. Membranes
Capillary polysulfone UF membranes, type M3/UF80, were supplied by X-FLOW B.V. According to the manufacturer, this membrane has a NMWCO of 80,000 Da, an inner diameter of 1.35 mm and an outer diameter of 2.26 mm. This
Module (item 5) Fig. 1. Experimental rig and module. 1 and 9: jacketed vessels, 2 and 10: pumps, 3: conductivity probe, 4,6, 11 and 12: pressure-measuring systems, 5: module, 7: back-pressure valve, 8 and 13: flow-measuring systems, 14: micro-computer, 15: pressure connections.
B. D&t!, G. Trig&rdh /Journal ofMembrane Science 90 (1994) 191-195
2.3. Equipment
193
tate and the other for the permeate side. In all the experiments, the temperature was controlled ( ~0.2”C) by circulating water from a thermostatic bath through the jackets surrounding the vessels and heat exchanger.
The experimental rig and the module are illustrated in Fig. 1. The experimental rig consists essentially of two closed loops: one for the reten-
(a) ;
2, a!!?! so iiE ‘C Ei=
a
4.5
3
-
oa
o-
‘-
7
4
n
4-
I”
6
3.5 -
3.5
P +
+
s-
t
y
2.5L.J
’ IO
’ 30
J SO
’ 70
_i 90 ,WL
Ilnouaanaar
Trananembrane
pr-
*,
110
130
150
170
IPd
Fig. 2. Membrane hydraulic resistance (R,) versus transmembrane pressure (LIP). A: (0 ) water at 5.7 “C, ( + ) water at 10.6”C, (0)waterat 15.6”C, (A) 14.75%glycerolinwaterat20°C, (x)9.62%glyeerolinwaterat20°C, (V) 5%glycerolinwaterat 20°C. B: (Cl) water at 5.7”C, (+) water at 10.6”C, (0) water at 15.6”C, (A) 13% sucrose in water at 2O”C, (X) 8.55% sucrose in water at 2O”C, ( V ) 4.17% sucrose in water at 20°C.
194
B. Dutrk, G. Triigdrdh /Journal of Membrane Science 90 (1994) 191- I95
3.5
’ 5.2
!
i 5.6
6
6.4 IThousandsl Rap L-1
6.6
7.2
7.6
Fig. 3. Global mass transfer coeffkient for NaCl (b) versus Reynolds number on the permeate side (Re,). Temperature 20°C Reynolds number on the retentate side 5,300 and NaCl concentration on the retentate side 100 g/l. ( •i ) Experiment 1, ( + ) experiment 2.
Before each experiment with a new piece of membrane, the membrane was soaked in water for 24 h at room temperature. 2.4. Methods The permeate loop was not used in the measurements of the hydraulic resistance. The permeate flux was recorded as a function of time until a constant value was obtained. The procedure was repeated at different transmembrane pressures ranging from 10 to 180 kPa (see Fig. 2) and with the different solvents described above. The global volume porosity of the membrane was also measured. This was done by soaking a piece of new or used membrane in water for 24 h at room temperature, After this, the water remaining on the membrane surfaces was removed: the external face of the capillary membrane was wiped with absorbant paper and air was blown into the capillary. Then the capillary membrane was weighed and dried in an oven at
50 ’ C.The dried membrane was finally weighed. This method of measuring membrane porosity is questionable [ 2 ] but it is very easy to perform and the results are reproducible. Finally, the mass transfer coefficient for NaCl was measured. For this, a NaCl solution ( 100 g/ 1) was recirculated in the retentate loop and water was circulated in the permeate loop. The NaCl concentration in the permeate loop (c,) was continuously measured using a conductivity probe. Occasionally, the NaCl concentration in the retentate was checked. Although water may diffuse from the permeate to the retentate, we did not observe any variation in the NaCl concentration in the retentate. Knowing the NaCl concentration ( cP) and the total volume of solvent ( VP) in the permeate loop as functions of time, we can calculate j, using the Eq. (6 ) and then kpl using Eqs. (4) and (5). . _~d(V,c,) IS--A dt
where A is the membrane area and t the time.
(6)
B. Dutr&,G. Triigctrdh/Journal of Membrane Science 90 (I 994) 191- I95
3. Results and discussion
The hydraulic resistance was calculated using Eq. ( 1). The results are presented in Fig. 2 (R, versus dP). This figure shows that R, increases slightly with increasing AP. The values of R, at low AP ( -c20 kPa) are not reliable due to the large errors in the measured values of J, and AP. Neither the solvent’s nature nor the temperature was found to significantly influence the values of R,. These effects are included in the viscosity (see Darcy’s law). The porosity of the membrane used in this work was found to be 81+0.7% for new membranes and 82 + 0.8% for used membranes. These values are the results of 5 determinations and are in good agreement with the value given by the manufacturer (70-80%). Fig. 3 shows the variation of b as a function of the Reynolds number on the permeate side (Rep). The Reynolds number on the retentate side was kept constant at 5,300. As mentioned previously, b is not an intrinsic transport property of the membrane because it depends on the hydrodynamic conditions on both the retentate and permeate side. It is possible to calculate the mass transfer coefficient of the membrane (&) by changing the hydrodynamic conditions [ 31. However, this approach requires numerous experiments and kg is the only mass transfer coefficient of practical interest. The results in Fig. 3 show values of & similar to those of dialysis membranes, although these are thinner than the capillary membrane used in this work. This situation could be explained by the high porosity of the capillary membrane used in this work (8 1%) compared to dialysis membranes (25-60%). 4. Conclusions The capillary membrane studied in this work has hydraulic resistance values similar to other
195
types of UF membranes. The hydraulic resistance of the capillary membrane increases slightly with the applied transmembrane pressure. This is probably a consequence of the high membrane porosity. The mass transfer coefficient of this membrane has not been measured but we have measured a global mass transfer coefficient, taking into account the boundary layer effect on both the retentate and permeate side. The values of this mass transfer coefficient are similar to those of dialysis membranes. This observation is interesting if we bear in mind that dialysis membranes are thinner than the capillary membranes used here. These good diffusive properties are certainly a consequence of the high porosity of the capillary membrane. The capillary membrane studied in this work exhibited interesting performances for both convective and diffusive transport without the disadvantages of hollow-fibre membranes (low maximum pressures, laminar flow, difficulty in handling viscous fluids, etc. ).
5. Acknowledgements This work was supported by The European Community through grant ERBAIR 1CT925 111. The authors acknowledge X-FLOW B.V. for providing capillary membranes,
6. References r11 J.E.
Sigdell, Calculation of combined diffusive and convective mass transfer, Int. J. Organs, ( 1982) 361-372. I21 Z. Morita, H. Ishida, H. Shimamoto, R. Weber and P. Rys, Anion permeability of cellulosic membranes. Part 1. Porosity of water-swollen membranes. J. Membrane Sci., 46 (1989) 283-298. 131 H.D. Spriggs and N.N. Li, Liquid permeation through polymeric membranes, in P. Meares (Ed. ), Membrane Separation Processes, Elsevier, Amsterdam, 1976, pp. 39-80.