Effects of Spinning Temperature on the Morphology and Performance

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Effects of spinning temperature on the morphology and performance of PES gas separation hollow fiber membranes Xiangbao Liu, Bing Cao, and Pei Li Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b03990 • Publication Date (Web): 13 Dec 2017 Downloaded from http://pubs.acs.org on December 14, 2017

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Industrial & Engineering Chemistry Research

Effects of spinning temperature on the morphology and performance of PES gas separation hollow fiber membranes Xiangbao Liu, Bing Cao, Pei Li*

College of Materials Science and Engineering, Beijing University of Chemical Technology, Beijing, 100029, China

*Corresponding author. E-mail address: [email protected] (P. Li)

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Abstract In this study, we demonstrated that polyethersulfone (PES) hollow fiber membranes with almost defect-free surfaces could be prepared at low PES concentrations if the hollow fibers were spun at low temperatures using dry-jet wet spinning. A series of hollow fiber membranes were spun at different coagulation batch temperatures (Tc) and dope solution temperatures (Td). Effects of Tc and Td on the membranes’ morphologies and gas separation performances were investigated. Membrane morphologies characterized using Scanning Electron Microscope (SEM) showed that the skin layer thicknesses increased with the decreases in Tc and Td. Moreover, mean surface pore sizes of the membranes, which were evaluated using the gas permeation method, significantly decreased as Tc or Td decreased. PES hollow fiber membranes spun at the lowest temperatures (Tc =7°C, Td = 3°C) showed O2 and CO2 permeances of 18.9 and 53.5 GPU, respectively. And selectivities of O2/N2, CO2/N2, and CO2/CH4 gas pairs were 1.15, 3.26, and 1.24, respectively. After silicone coating, the selectivities increased to 7.24, 47.7 and 39.4, respectively but O2 and CO2 permeances decreased to 4.85 and 31.9 GPU, respectively. To our best knowledge, the gas separation performance was the test among all PES hollow fiber membranes. Keywords Hollow fiber membrane; polyethersulfone; gas separation; spinning temperature; critical concentration; critical temperature

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Introduction Membrane separation processes have received worldwide attention due to their high efficiency, low

power consumption, easy control, small footprint, simple maintenance, and so forth.1 Gas separation membranes have been widely applied in many fields such as the concentration of methane from biogas or natural gas (CO2/CH4), enrichment of O2 or N2 from air (O2/N2), CO2 capture from the flue gas (CO2/N2) and removal of water vapor from natural gas and other gases, etc..2 Although many polymers exhibit appealing gas separation properties, how to manufacture them into asymmetric membranes with high gas permeance and selectivity close to their intrinsic properties is still challenging.3 Today, most asymmetric polymeric membranes are prepared using the non-solvent induced phase inversion method.4 During the phase inversion process, there exists two phase separation mechanisms: nucleation and growth (NG) and spinodal decomposition (SD).5 NG is a slow phase separation process where densely packed membrane structures may form. If a composition of polymer dope enters the metastable region of the phase diagram, the solvent non-solvent exchange rate is slow so that NG takes place. On the other hand, when polymer dope composition crosses the spinodal line, a fast phase separation happens leading to the formation of porous structure.6 To have high selectivity, skin layers of gas separation membranes shall be defect free or have limited defects which can be sealed by silicone rubber.7 Therefore, a highly concentrated polymer dope and low solvent non-solvent exchange rate are preferred to induce NG process at the membrane surface region.6,8 However, a dope with high polymer concentration often causes the formation of a thick skin layer and an integral porous substrate with high resistance. To achieve high gas permeance, some researchers tried to identify a “critical concentration” (CC) that indicates the lowest dope concentration for forming high selective gas separation membranes.9 For instance, Chung et al. correlated the dope viscosities 3



with PES concentrations at 25°C and observed the CC of PES at 35 wt% where the viscosity would increase dramatically when the PES concentration was above this value. Moreover, the PES hollow fiber membranes having an O2/N2 selectivity of 5.8 close to its intrinsic selectivity could be obtained only when the PES concentration was above 35wt%.9 Polymer scaling theory gives a good explanation for the observation of CC.10 As the polymer concentration increases, polymer solution transits form a half diluted solution where polymer chains are untangled to a concentrated solution where polymer chains are tangled. According to Eq (1), the specific viscosity of a tangled polymer solution dramatically increases with the volume fraction of polymer since the exponential increases from 3.9 to 4.7. Thus, CC represents the tangled volume fraction of polymer (Φe).

η sp

( φ ) 3.9  φ* ∝   ( φ * ) 4.7 φ 

φ* < φ < φ e (1)

φe < φ < 1

where ηsp is specific viscosity, Φ- the volume fraction of polymer, Φ*-the overlapped volume fraction of polymer, Φe- the tangled volume fraction of polymer. CC or Φe is a very important parameter which determines the lowest polymer concentration for the occurrence of chain entanglements. During the phase inversion process, polymer chain entanglements would help reduce the solvent non-solvent exchange rate so that induce NG process. However, the key to preparing high performance gas separation membranes is to use a dope with high polymer chain entanglements or viscosity for forming an almost defect-free skin layer and a relatively low polymer concentration for producing a thin selective layer with highly porous substrate. These two requirements are seemed to contradict each other. And it impels us to find a method to lower the CC or Φe of polymer dopes. 4


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It is well known that the relation between apparent viscosity and temperature follows the Arrhenius equation (Eq. (2)).11

E   ηa  ηa ≈ exp   kT   

(2)

where ηa is the apparent viscosity, -the activation energy, k-Boltzmang constant. Eq. (2) demonstrates that viscosity shall significantly increase as temperature decreases. This phenomenon is explained by the free volume theory.10 At low temperatures, solvents have low free volumes and high densities. Thus, energy for moving polymer chain units increases which causes increase in viscosity. The Doolittle equation builds up the relation between apparent viscosity and free volume of solvent as shown in Eq. (3).10

1 ηa ≈ exp  f 

(3)

where f is the solvent free volume. Eq. (2) and (3) tell us that as temperature decreases viscosity of the polymer solution increases and free volume of the solvent decreases. Therefore, CC or Φe shall also decrease with temperature since the volume fraction of solvent decreases with temperature. Previous researches have shown that membrane pore size decreased and the skin layer thickness increased as temperatures of the coagulation bath and dope solution increased.12-14 When the phase inversion process was performed at low temperatures, more selective but less permeable asymmetric ultrafiltration membranes were obtained.15,16 Based on the theoretical deduction and experimental results, we speculate that a polymer dope with low concentration can form almost defect free membranes if the phase inversion process is carried out at sufficiently low temperatures. To prove the speculation, we spun a series of PES hollow fiber membranes from a fixed dope

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composition of PES/NMP/DG at 28/57/15% wt. This dope formulation was used to spin hollow fiber membranes with average pore sizes of 2.3nm at 25°C.17 We varied temperatures of the PES dope solution and coagulation bath from 3°C to 35°C to investigate their effects to the hollow fiber membrane morphologies and the gas separation performances. 2

Experimental

2.1 Materials Polyethersulphone (PES) (Ultrason® E1010; Mw=72,000, Mw/Mn =3.54), was obtained from BASF (Germany). N-methyl-2-pyrrolidone (NMP) with a purity>99% was purchased from Tianjin Da mao Chemical Reagent Factory. Diethylene glycol (DG) was purchased from West long chemical co., LTD. Polydimethylsiloxane (PDMS, Sylgard 184) was supplied by Dow Chemical. All chemicals were used as received. 2.2 Polymer dope solution preparation and viscosity measurements The composition of polymer dope solution was 28/57/15 of PES, NMP, and DG in wt%. The PES pellet was first dried at 120°C in a vacuum oven over night to remove moisture. PES, NMP and DG were neat added to a flask and mechanically stirred till a homogeneous solution was obtained. The dope solution was degassed before the hollow fiber spinning and viscosity measurement. Viscosities of the dope solution at different temperatures including 3°C, 7°C, 15°C, 25°C, 35°C and 50°C were measured using an NDJ-8S Rotational Viscometer (Shanghai Yueping scientific instrument Co.,Ltd, China) with a rotation rate of 6 rpm. To study effect of PES concentration to viscosity at different temperatures, PES solutions with compositions of 30/55/15, 32/54/14, 34/52/14, and 36/51/13 were also prepared. Their viscosities at temperatures of 3°C, 7°C, 15°C, 25°C, 35°C and 50°C were measured. 6


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2.3 Hollow fiber spinning and post treatment methods The PES hollow fiber membranes were fabricated via dry-jet wet spinning as illustrated in Fig. 1, using a spinneret with outer and inner diameters of 1.4 and 0.8 mm, respectively. The degassed dope solution passed through a metal filter to remove particles. The dope solution and bore fluid were then fed into the outer and inner tubes of the spinneret at predetermined flow rates, respectively. After the dope solution and bore fluid exited the spinneret and passed through air-gap and water coagulant bath, solidified hollow fiber membranes were collected by a wind roller. The spinning parameters of different hollow fiber membranes are listed in Table S1. The as-spun fibers were immersed in water for 3 days to remove the residual solvent. The wet hollow fibers were subsequently dried via solvent exchange method to prevent the pore shrinkage of the membrane.18 Specifically, the fibers were first immersed in 3 sequential 30 min methanol baths and followed by a soaking in 3 sequential 30 min n-hexane baths. Finally, the hollow fiber membranes were dried in air for at least one day at room temperature. To seal defects on the membrane outer surface, the hollow fibers were dip-coated in a 3wt% PDMS solution in n-pentane for 3 mins and then dried for 24 h for curing the PDMS.

2.4 Scanning electron microscope (SEM) The surfaces and cross-section morphologies of the hollow fiber membranes were observed using a JEOL JSM-7800F scanning electron microscope. To prepare a smooth surface, the hollow fiber membranes were first fractured in liquid nitrogen and then were freeze-dried. The samples were sputter coated with gold before testing. 2.5 Estimation of the average pore size of the hollow fiber membrane outer surface Before coating with the PDMS solution, average sizes of the defects or pores in the hollow fiber membrane outer surface were estimated using the gas permeation method.18 The postulation is that the 7



gas permeation is governed by the Knudsen diffusion mechanism where the membrane pore size is smaller than the mean free path of the gas molecules. In this case, the gas permeance, J i , is linearly correlated to the average pressure, P , of the membrane feed and permeate sides as described in Eq (4).19

Ji =

2 3

 8RT     πM 

0.5

1 rψ P r 2ψ + RT L P 8uRT L P

(4)

where R is universal gas constant (8.314 J·mol-1·K-1); T is absolute temperature (K); M is molecular weight of gas (g/mol); r is the mean pore radius (m); u is viscosity of gas (Pa.s); LP is the effective length; and Ψ is the surface porosity. Eq (4) can be rewritten in the following form:

J i = K 0 + P0 × P where

K0

2 = 3

 8RT     πM 

0.5

(5)

1 r 2ψ 1 rψ and P0 = . 8uRT L P RT L P

In this work, we measured the N2 permeance (J ) of the hollow fiber membrane at elevated pressures, and then depicted a plot of J versus P. After linear fitting the data, the average pore size was calculated using the values of the intercept (K0) and slope (P0) by Eq (6).

r =

16 3

0.5

 P0  8RT     u  K 0  πM 

The effective surface porosity,

(6)

ψ LP can be obtained from the slope as follows:

ψ 8uRTP0 = LP r2

(7)

2.6 Membranes overall porosity The membrane overall porosity (ε) was estimated by Eq (8) using the method introduced elsewhere.20

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1 1 − ρ ρp ε = m 1 ρm

(8)

where ρp is polymer density (1.37 g/cm3 for PES) and ρm is membrane density which is calculated by measuring the weight of the dried hollow fiber membrane (Wm) and its apparent volume (Vm) using Eq (9).

ρm = where

(

Wm Vm

(9)

)

Vm = π D 2 − d 2 L 4 ; D, d, and L refer to the external/internal diameters, and the length

of the hollow fiber sample, respectively. 2.7 Measurements of gas permeance of the PDMS coated PES hollow fiber membranes Pure gases were used to determine membrane permeance and selectivity. The gas permeation experiments were conducted using pure nitrogen (99.99%), oxygen (99.99%), methane (99.99%) and carbon dioxide (99.99%). The details of the module fabrication, the permeation apparatus and measuring procedure have been described elsewhere.21 The permeation fluxes of gases for silicone-coated hollow fiber membranes were measured at 25°C at a transmembrane pressure of 5 bar.

The permeance, , was calculated by Eq (10):

P Q Q = = L A∆P nDl∆Pπ

(10)

where P is the gas permeability (barrer); L is the thickness of the apparent dense-selective layer

(cm); is the gas permeance (GPU, 1 GPU = 1 × 10−6 cm3 (STP)/cm2 s cmHg); Q is the pure gas flux (cm3/s); n is the number of fibers in one testing module; D is the outer diameter of the hollow fibers (cm); l is the effective length of the modules (cm); and ∆P is the transmembrane pressure (cmHg). The

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ideal separation factor was determined using Eq (11):

αA

P   L =  A P    L B

B

(11)

The effective skin layer thickness was estimated based on the intrinsic oxygen permeability and the oxygen permeance using Eq (12):

L =

PO2

(12)

P    L  O2

2.8 Pure gas permeability measurements of PES Pure gas permeability was tested using a gas permeation cell built in our lab using the standard method reported in literature.22 The permeabilities were tested in a sequence of N2, O2, CH4, and CO2 at 25 °C and 10 atm since the gas permeability of PES is low. Gas permeability was calculated using Eq (13):

P =

273 × 1010 × 760

Vl dP × 1 76 dt × P2 AT × 14.7

(13)

where P is the gas permeability of a membrane in barrer (1 barrer=1×10-10cm3 (STP) cm/cm2 s cmHg); V is the volume of the downstream chamber (cm3); A refers to the effective membrane area (cm2); l is the membrane thickness (cm); T is the operating temperature (K); and P2 is the upstream operating pressure (psia). The gas selectivity is estimated using Eq (14):

αA

= B

PA PB

(14)

where PA and PB are the gas permeability of gases A and B, respectively. 3

Results and Discussion 10


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3.1 Effects of polymer concentration and temperature to the dope solution viscosity In the case of hollow fiber spinning, dope viscosity affects the phase inversion process by controlling the solvent and non-solvent exchange rate.23 Fig. 2 shows the dependence of the logarithmic dope solution viscosities to the reciprocals of the absolute dope solution temperatures for polymer dopes with different PES concentrations. The linear relations between the viscosity and the reciprocal of temperature fit the Arrhenius equation.

Eη η = Ae RT

(15)

where η is absolute viscosity; R is the gas constant; A is a constant; T is absolute temperature; and is flow activation energy. Table S2 lists the parameters of Arrhenius equation as well as the estimated viscous flow activation energies of the dope solutions with the PES concentrations ranging from 28 to 36 wt%. The derivative of the viscosity to temperature can be derived from Eq. (15) and shown below:

∂η = − ∂T

Eη AE η 1 RT e R T2

Fig. 3 shows the relation between Since

(16)

and T at a temperature range of 276K (3°C) to 308K (35°C).

decreases as temperature decreases, dope viscosity tends to increases more rapidly. The

impact of temperature to viscosity is more remarkable for high concentrated polymer dopes. It is because the viscous flow activation energy ( ) is high at high PES concentrations as listed in Table S2. A similar phenomenon has been reported by Qin et al. for the PPESK/NMP system.11 Fig. 4 shows the changes of viscosity with temperature of three PES/NMP/DG polymer dopes. Notable changes in curves’ slopes are observed and the intersections of slopes are defined as “critical temperatures” which are 12, 22, and 25°C, respectively, corresponding to dopes with PES concentrations of 28, 30, and 32wt%. According to polymer scaling theory, we believe that the critical 11



temperatures represent the occurrence of polymer chain entanglements. Below critical temperature, polymer dope enters the region of tangled polymer solution and dope viscosity will increase more rapidly by further decreasing temperature. The results of Fig. 4 tell us the spinning temperatures of the three dopes must be below their corresponding critical temperatures to obtain hollow fiber membranes with limited surface defects. 3.2 Membrane morphologies 3.2.1

The effect of coagulation bath temperature

Section 3.1 indicates that a temperature below 12°C is required to spin defect-free skin layer using the PES/NMP/DG (28/57/15) dope. To study the effect of Tc to the hollow fiber membrane morphology, we first fixed Td at 3°C since it was the lowest dope temperature we could obtain, and varied Tc at 7, 15 and 25°C, respectively, to spin hollow fiber membranes. The SEM images of the cross section and outer surface of hollow fiber membranes are depicted in Fig 5. According to these images, all of the membranes have asymmetric structures, which are composed of sponge-like and finger-like microvoids. As labeled by red circles, the finger-like microvoids gradually increase with Tc and the fibers’ dimensions become larger (wall thickness increases from 280 to 420µm). Moreover, pinholes are only observed on hollow fiber outer surface as Tc increases to 25°C. Strathmann et al.24 claimed that finger-like microvoids usually formed in a fast precipitation process and sponge-like microvoids tended to form in a slow precipitation process. In our case, although all hollow fiber cross-sections have both the sponge-like microvoids and finger-like microvoids, it is clear that a higher Tc produces larger finger-like microvoids. High Tc increases diffusivities of NMP and water by decreasing dope viscosity, chain rigidity and surface tension of dope solution. Consequently, the solvent/non-solvent exchange rate increases and leads to a fast precipitation 12


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of PES. Moreover, binodal curve of the PES/NMP/DG system shall shift to the non-solvent (water) side as temperature increases. It means that polymer dope requires more non-solvent to induce phase separation. Therefore, sizes of the finger-like microvoids increase with the Tc and the overall porosity of the hollow fiber membrane increases too, which causes an increase in the wall thickness of the hollow fiber membranes. When precipitation rate is low, a dense and thick skin layer is formed by the NG mechanism.25 On the contrary, when precipitation rate is high, rapid demixing (spinodal decomposition) dominates membrane forming process. A thin and porous skin layer forms because of the intrusion of water under the driving force of the unbalanced localized stresses from the surface tension, solvent/coagulant agglomeration, volumetric change, and radially convective flows of the outer coagulants.9 When Tc is 7 or 15°C, the dope viscosity is high so that the solvent (NMP) and non-solvent (water) exchange rate is low. The localized stress on the outer surface of the hollow fiber membrane is limited. In addition, the binodal curve moves to the solvent side as temperature decreases. Polymer dope needs less non-solvent to induce precipitation at low temperatures. Therefore, both the driving force and diffusion rate of water decrease at low temperatures. This leads to the formation of dense skin layer. On the other hand, as Tc increases to 25°C, the solvent/non-solvent exchange rate is high and the polymer dope requires more water to induce phase separation. The two factors result in a fast precipitation process and the formation of pores in the membrane surface and large finger-like microvoids in the cross-section of the hollow fiber membrane. Fig. 6 shows the inner surface morphologies of hollow fiber membranes spun at different Tcs. Pore sizes of all hollow fiber membranes are similar and much bigger than those in the outer surfaces. Since bore fluid contains 95% NMP which dilutes the PES concentration in the hollow fiber lumen side, it 13



results in the formation of a highly porous inner surface structure. Fig. 7 shows the outermost region of the hollow fiber membranes with the measured dense layer thicknesses. The dense layer thickness decreases from 1395, 995, to 400Å, respectively, corresponding to the Tc of 7, 15, and 25°C. This observation is similar to the report by Strathmann et al. who claimed that membrane formed in a fast precipitation process tends to have thin skin layer.24 3.2.2

The effect of dope solution temperature

Section 3.2.1 indicates that a low Tc is preferred to form dense and defect-free selective layer. In this section, we fixed Tc at 7°C the lowest maintainable temperature. The influence of Td on the hollow fiber membrane structures was investigated by varying Td at temperatures of 7°C, 15°C, 25°C and 35°C, respectively. Fig. 8 shows the SEM images of hollow fiber cross-sections and outer surfaces. Similar to the effects of Tc, finger-like microvoids become bigger at high Tds. However, all hollow fibers have almost the same dimensions (wall thickness). Microvoids near to the fiber lumen sides are larger than those close to fiber shell side. Note that, the bore fluid contains 95% NMP which dilute the PES dope in the lumen side. Hence, more non-solvent (water) is required for phase separation and the precipitation process is slowed down. There is more time for the polymer lean phase to grow which leads to the formation of large microvoids. Since the nascent fiber contacts a large amount of water in the coagulant bath, the outer layer undergoes solidification in a shorter period, thus limiting the growth of polymer-lean phase. This results in the formation of a membrane with small macrovoids near the outer edge.26 Table S3 shows that the overall membrane porosity increases with Td. This is because a high Td reduces the viscoelasticity of the PES dope solution that favors convective solvent exchange in the instantaneous liquid-liquid phase separation process.16 Consequently, the membranes spun at a high Td 14


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has bigger cavities and higher porosity. Fig. 8 also shows no visible pores on the outer surfaces of all membranes. This indicates that Tc plays a dominate role in the pore formation on the hollow fiber outer surfaces. To determine the pore sizes of the membranes, gas permeation test was applied and the results will be discussed later. Fig. 9 shows the gradually decreased dense layer thicknesses with the increases in Td. The trend is similar to the effect of Tc to dense layer thickness. High Td induces high solvent non-solvent exchange rate, resulting in the formation of a thin skin layer. 3.3 The estimated mean pore size and effective porosity of the hollow fiber outer surfaces Table 1 lists the estimated mean pore sizes and effective surface porosities of all membranes. Since the estimated pore size of fiber A1 is 0.09nm which is smaller than the kinetic diameters of all gas molecules, it indicates that gas transport in this membrane is complicated, thus the method of gas permeation is inapplicable. Hence, the value doesn’t have any physical meaning. For other 6 membranes, the mean pore sizes are bigger than the largest gas molecules CH4 (3.80Å) used in this study. It can be seen that the mean pore size increases with the increase in Tc or Td. A high temperature shall favor the fast phase inversion and results in the formation of large pores on the membrane surface. Another observation is that the effective surface porosity decreases with the increase in Tc or Td. It seems that the number of defects decreases but the average pore sizes increases with the increase in Tc or Td. These results will affect the gas transport properties of the hollow fiber membranes and will be discussed later.

3.4 Gas permeance and selectivity The gas permeance of N2, O2, CH4 and CO2 for uncoated and silicone-coated hollow fiber membranes were measured at 25°C at a pressure difference of 2 bar and 5 bar, respectively. The ideal selectivities of O2/N2, CO2/N2 and CO2/CH4 gas pairs were calculated based on the pure gas permeance 15



data. The results are listed in Table 2 and Table S4. Uncoated membranes exhibited poor selectivity. This indicates the existent of defects on the membrane surfaces. Gas permeance of all four gases increase in an order of A1A3. Note that, the ideal selectivity of the PES polymer for CO2/CH4, CO2/N2 and O2/N2 gas pairs are 37.4, 41.8, and 7.47, respectively as listed in Table S4. The selectivity of Knudsen diffusion for the three gas pairs are 0.60, 0.8, and 0.94, respectively. Therefore, the ideal selectivity listed in Table 2 indicates that gas transport in the three hollow fiber membranes are governed by solution-diffusion, Knudsen diffusion and Poiseuille flow mechanisms. Since the kinetic diameters of N2, O2, CH4, and CO2, are 3.64, 3.46, 3.80, and 3.30 Å, respectively, the estimated mean surface pore sizes of A2 and A3 are bigger than the kinetic diameters of all four gases. With increases in surface pore sizes, the gas transportation is increasingly governed by the Knudsen diffusion and Poiseuille flow mechanism. Hence, the ideal gas selectivities gradually decrease with increasing gas permeance. For uncoated B series hollow fiber membranes, their permeances increase in the same order as their mean surface pore sizes, B1

Effects of Spinning Temperature on the Morphology and Performance

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