Effect of spinning conditions on the structure and the gas permeation properties of high flux polyethersulfone—polyimide blend hollow fibers

Effect of spinning conditions on the structure and the gas permeation properties of high flux polyethersulfone—polyimide blend hollow fibers

DESALINATION Desalination 144 (2002) 121-125 www.elsevier.com/lcmte/desal Effect of spinning conditions on the structure and the gas permeation prop...

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DESALINATION Desalination

144 (2002) 121-125 www.elsevier.com/lcmte/desal

Effect of spinning conditions on the structure and the gas permeation properties of high flux polyethersulfonepolyimide blend hollow fibers G.C. Kapantaidakis

*, G.H. Koops, M. Wessling

Faculty of Chemical Technology, Membrane Technology Group - EMI, University of Twente, PO. Box 217, 7500 AE Enschede, The Netherlands Tel +31 (53) 4892957, Fax +31 (53) 4894611, emails: [email protected], [email protected], M. [email protected] Received

1 February 2002; accepted 19 March 2002

Abstract In this work, the effects of major spinning parameters, such as: polymer concentration, air gap distance, bore fluid composition, and take-up velocity on the structure and the permeation properties of polyethersulfone-polyimide gas separation hollow fibers are discussed in detail. It is shown that a spinning dope starts to exhibit significant chain entanglement at a critical polymer concentration. Fibers spun from this critical concentration exhibit theoretically the thinnest skin layer and minimum surface porosity. The longer the nascent hollow fiber membrane is exposed to a humid air-gap, the higher the water content in the top layer before demixing occurs. This results in higher surface porosity and gas permeance. Better mixing between the polymer solution and the bore liquid is achieved by adjusting the composition of the bore fluid (NMP/H,O). Finally, by increasing the velocity of the take-up drum, the permeance of both CO, and N2 decrease while their permselectivity remains constant. Suitable selection of the spinning conditions results in gas separation hollow fibers with thin skin layers (0.1 urn), macrovoid-free substructure, high permeation rates (CO,: 40-60 GPU) and selectivity coefficients (a CO,&: 40). These results compete directly with the performance of commercial gas separation membranes. Keywords: Polymer blends; Hollow fibers; Dry/wet spinning; Gas separation

*Corresponding

author.

Presented at the International July 7-12, 2002.

Congress on Membranes

and Membrane

Processes

001 l-9 164/02/$- See front matter 0 2002 Elsevier Science B.V. All rights reserved PII: SO0 11-9 164(02)00299-O

(ICOM),

Toulouse, France,

122

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et al. /Desalination

1. Introduction The vast majority of the polymeric membranes that are used in industrial level for the separation of gaseous mixtures are prepared in the form of asymmetric hollow fibers. This geometric configuration offers the advantages of high surface area per unit of module volume, improved separation performance, ease of installation and maintenance as well as process flexibility [I]. Hollow fibers are prepared in spinning equipment using the dry/ wet phase inversion method. In this process,-a suitably formulated polymer solution and a bore liquid mixture are simultaneously pumped to a tube-in-orifice. The extruded fibers pass through an air-gap of specific length and enter a nonsolvent bath where coagulation takes place. The fibers are usually collected and treated subsequently for the removal of residual solvent. Dry/ wet spinning is a complex process since it involves many process variables that influence both the geometrical characteristics and the permeation properties of the hollow fibers [2]. We mention, for example, the concentration of the polymer in the spinning dope, the type and the composition of the bore fluid, polymer extrusion rate, air gap distance, take up velocity, temperature, humidity, type of coagulant etc. For practical reasons hollow fibers are usually spun non-isothermally and under certain tension [3]. Despite the experimental difficulties and theoretical limitations, the ultimate goal of a spinning process is to produce asymmetric hollow fibers with ultra thin skin layer (0.1 urn), minimum or no surface porosity, finely microporous substructure without macrovoids, high permeance values and selectivities intrinsic to the material spun. In this study, blends of polyethersulfoneSumikaexcel (PES) and polyimide-Matrimid 5218 (PI) were used for the preparation of gas separation hollow fibers since they combine the high permeability coefficients of polyimides with the high resistance to plasticization and the low cost of polyethersulfones. This polymer system was studied in the past in the form of dense films and proved to be completely miscible over the

144 (2002) 121-125

whole range of blend composition [4]. Therefore, the scope of this work is to develop PES/PI asymmetric hollow fibers and to examine the effect of the main spinning parameters on their structure and gas permeation properties.

2. Experimental The dry/wet spinning process was used for the development of PES/PI hollow fibers. Pure Nmethylpyrollidone (NMP) was used as a single solvent for the two polymers. NMP is not only benign from a health viewpoint but it is also miscible with water, which is the most preferred coagulation bath medium. Table 1 summarizes the experimental conditions for the 7 different polymer dopes examined here. SEM was used to study the structure and morphology of the hollow fibers. The permeation rates of CO, and N, through uncoated and PDMS coated fibers were investigated in a high pressure permeation set up, based on the variable pressure method. .

3. Results and discussion Fig. 1a shows the effect of PES/PI (l/4) concentration on dope viscosity. A significant increase of viscosity occurs at a critical concentration of about 26 wt%. As shown from Fig. 1b fibers spun from this critical concentration exhibit the highest permeation rates and the thinnest skin layer. When higher concentrations are used fibers exhibit thicker skin layers and more dense substructures, therefore both permeation rates and selectivity values are considerably reduced. Fig. 2a shows the effect of air gap distance on gas permeance while in Fig. 2b a SEM picture of Dope F hollow fibers is presented. As the nascent membrane is exposed longer to the humid atmosphere, the water content in the top layer increases resulting in more porous structures and higher permeation rates. In other words, the utilization of an air gap during spinning could be considered as equivalent to the well-known method of adding

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123

144 (2002) 121-125

Table I Spinning conditions and process parameters Batch code

PESIPI, wt%

Polymer, wt%

Parameters examined

A B C D

4/l l/l l/4 l/l

35 35 35 32

E

l/4

29

F

114

26

G

l/l

30

Air gap: l- 10 cm, coating: PDMS Air gap: l-l 0 cm, coating: PDMS Air gap: l- 10 cm, coating: PDMS Air gap:O-10 cm, take up velocity:5.8-12.5 m/min, bore liquid NMP/H20: 80/20, 90/10, coating: PDMS Air gap:O-20 cm, take up velocity:5.8-12.5 m/min, bore liquid NMP/H20: 70/30,80/20,90/10, bore liquid flowrate: 1.1-2.1 mlimin, coating: PDMS Air gap:6-3 1 cm, take up velocity:5.8-12.5 m/min, bore liquid flowrate: l.lL2.lml/min, coating: PDMS Air gap:5-20 cm, take up velocity: 5.8-12.5 mimin, coating: PDMS

80000

-

= s

s s $j

60000

-

40000

-

20000

-

n

T=40”C.

n=16 mid

.

T=50’C,

n-32

min.’

A

T=60”C,

n=32

min.’

10

. _

. . A

N2. (GPU) C02.

(GPU)

C021N2

Selectivity

I:

.m >

16

r

18

20

22

24

26

28

Polymer concentration,

30

32

34

wt%

20

25

30

Polymer concentration,

35

40

w-t%

Fig. I. Effect of polymer concentration on viscosity (a) and permeation properties (b).

small amounts of water to the dope in order to increase porosity. The chemistry and the composition of the bore liquid affect the performance of PEW1 hollow fibers significantly. Fig. 3a shows that increase of the NMP content in the bore liquid results in higher permeance rates of Dope E hollow fibers. SEM analysis indicates that especially in very high NMP content values (-90 wt%), the polymer solution is diluted considerably causing deformation or irregular shape of the bore side, 3b. From a

thermodynamic viewpoint, the mixing between the polymer solution and the bore fluid is determined by their free energy of mixing, AGn,. Since the entropic contribution of mixing, ASn,,is generally positive the free energy of mixing can be negative only when the enthalpic contribution, mHnl, is as low as possible. The magnitude of AH,,,, is calculated by taking into account the difference of the solubility parameters of the polymer solution and the bore fluid. Similarly, the value of the solubility parameter of the bore liquid depends

GC. Kapantaidakis et al. /Desalination

124

0 0

144 (2002) 121-125

N2, uncoated C02. uncoated N2. coated C02. coated

n

0

Air Gap, (cm)

Fig. 2. Effect of air-gap on the permeance (a) and a SEM photo of Dope F hollow fibers (b).

H 0 A

0.1



N2.(GPU) C02. (GPU) C02/N2 Selectivity

I

I

I

70

80

90

I

NMP Content% Fig. 3. Effect of bore composition on gas permeance of Dope E hollow fibers (a) and bore deformation in high NMP content, i.e. 90 wt%, (b).

on its composition [5]. Therefore, by adjusting the variable of composition, the difference between the two solubility parameters could be relatively low (6: l-2 cal/cm”)“* resulting in low enthalpy values and pronounced mixing between the bore liquid and polymer solution. Finally, Fig. 4 shows the effect of take up velocity on the dimensions, 4a, and the gas permeance,

4b, of Dope E hollow fibers. Increase of take up velocity results in stretching of the produced nascent hollow fiber and therefore in reduction of internal and external diameters. The permeation rates of both uncoated and PDMS coated hollow fibers are subsequently reduced since the substructure becomes denser and the top payer thicker. By adjusting the major process parameters,

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144 (2002)

H 0 A

7.5

10

Take up velocity,

125

121-125

N2. (GPU) C02, (GPU) C021N2 Selectivity

10.0

12.5

15.0

Take up velocity, (m/min)

(mlmin)

Fig. 4. Effect of take up velocity on fiber dimensions (a) and gas permeation properties (b).

hollow fiber with thin skin layers (0.1 pm), microporous and macrovoid-free substructure, high permeation rates (CO,: 4CL-60GPU) and selectivity coefficients (a CO& 3540) have been produced. The morphological and the permeation properties of PES/PI hollow fibers are comparable with commercial membrane units.

Acknowledgements This research was supported through a EC Marie Curie Fellowship, (No HPMFCT-2000-475). References 111 W.S. Ho and K.K. Sirkar (Eds.), Membrane Handbook, Van Nostrand Reinhold, New York, 1992.

4. Conclusions PEYPI blends can be successfully prepared in the form of asymmetric hollow fibers by using the dry/wet spinning process. The structure and the permeation properties of the developed fibers are highly dependent on the spinning conditions. By suitably adjusting major process parameters, such as polymer concentration, air-gap distance, bore-liquid composition and take-up velocity highly permeable, selective and ultra-thin fibers have been produced.

I21 J.H. Kim, Y.I. Park, J. Jegal and K.H. Lee, The effects of spinning conditions on the structure formation and the dimension of the hollow fiber membranes and their relationship with the permeability in dry-wet spinning technology, J. Appli. Pol. Sci., 57 (1995) 1637-1644. [31 T.S. Chung, The limitations of using Flory-Huggins . equation for the states of solutions during asymmetric hollow fiber formation, J. Membr. Sci., 126 (1997) 19-34. [41 K. Liang, J. Grebowicz, E. Valles, F.E. Karasz and W.J. Ma&night, Thermal and rheological properties of miscible polyethersulfone / polyimide blends, J. Polym. Sci., Part B: Pol. Phys., 30 (1992) 465-476. PI D.W. van Krevelen (Ed.), Properties of Polymers, Their Estimation and Correlation with Chemical Structure. Elsevier, Amsterdam, 1978.