Chine Journal of Chemical Engineering, 19(5) 855ü862 (2011)
Integral PV A-PES Composite Membranes by Surface Segregation
Method for Pervaporation Dehydration of Ethanol*
WU Hong ( )1,2, LI Xianshi ( )1, NIE Mingcheng ( ю 1, LI Ben ⡛)1 and JIANG Zhongyi ( )1,**
awfully1 Key Laboratory for Green Chemical Technology of Ministry of Education, School of Chemical Engineering and
Technology, Tianjin University, Tianjin 300072, China
2 Tianjin Key Laboratory of Membrane Science and Desalination Technology, Tianjin University, Tianjin 300072,
China
Abstract A facile surface gregation method was utilized to fabricate poly(vinyl alcohol)-polyethersulfone (PV A-PES) composite membranes. PV A and PES were first dissolved in dimethyl sul
foxide (DMSO), then casted on a glass plate and immerd in a coagulation bath. During the pha inversion process in coagulation bath, PV A spontaneously gregated to the polymer solution/coagulation bath interface. The enriched PV A on the surface was further crosslinked by glutaraldehyde. Scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and energy dispersive spectrometer (EDS) confirmed the integral and asymmetric membrane structure with a den PV A-enriched surface and a porous PES-enriched support, as well as the surface enrichment of PV A. The coverage fraction of the membrane surface by PV A reached up to 86.8% when the PV A content in the membrane recipe was 16.7% (by mass). The water contact angle decread with the increa of PV A content. The effect of co-
agulation bath type on membrane structure was analyzed. The membrane pervaporation performance was evaluated by varying the PV A content, the annealing temperature, feed concentration and operation temperature. The mem-
brane exhibited a fairly good ethanol dehydration capacity and long-term operational stability.
Keywords surface gregation, pha inversion, PV A-PES composite membrane, pervaporation, ethanol
sand
1 INTRODUCTION
It is well known that bio-fuels including bio-ethanol have a number of environmental and economic bene-fits [1]. Among them, absolute ethanol containing 99.5% (by mass) or more ethanol is in the greatest demand [2]. Pervaporation has been demonstrated as an efficient membrane process for ethanol dehydration [3, 4] and as a promising alternative to conventional energy intensive technologies such as distillation [5, 6].
Composite membranes with a thin active layer supported on a porous substrate are often utilized for pervaporation dehydration due to their high permea-tion flux. However, if the hydrophilic active layer and hydroponic substrate do not swell in a coordinated manner, the active layer and support layer would peel off due to the interfacial stress between the two lay-ers exceeding their maximum interfacial adhesion [7]. Therefore, considerable rearches have focud on the structural stability of composite membranes [8, 9], which encompass active layer cross-linking [10], multi-layer arrangement [11, 12], “fusion” [13] and in-terfacial polymerization [14, 15]. For instance, Shao et al. [13] fabricated SPEEK/PMMA-PVDF composite membranes with an integrated skin layer via a “fu-sion” technique. The SPEEK skin layer was prefabri-cated and then the substrate solution was applied to the skin layer to form a porous support layer by the wet pha inversion process. The SP
EEK skin layer was partially dissolved and integrated into the sub-strate layer. The long-term pervaporation operation showed that the structure stability of the resultant membranes was remarkably enhanced.
The surface gregation is a common phenome-non of enrichment of one component at the blend sur-face [16] and has been widely ud in fabricating po-rous membranes, such as ultrafiltration membrane [17 19]. The process can be described as that near the interface between the casting solution and the coagu-lation bath, hydrophilic component is transported more slowly into the casting solution than the hydro-phobic component prior to precipitation, resulting in surface enrichment of the hydrophilic gments [20]. Therefore, polymer chains of the active layer and the support layer formed inter-penetrated network which enhance the structure stability. The flux and paration factor of asymmetric membrane ud in pervaporation are about 0.127 kg·m 2·h 1 and 735 for 90% (by mass) ethanol solution [21].
PV A and PES are widely employed as membrane materials becau of their superior comprehensive properties and membrane-forming property. In this study, PV A as a hydrophilic polymer and PES as a hydrophobic polymer are chon to form an integrated composite membrane via surface gregation. The composition and structure of the resultant composite membrane were p
robed by veral characterization techniques. The pervaporation dehydration of ethanol
news from japaneReceived 2011-06-10, accepted 2011-08-25.
* Supported by the State Key Development Program for Basic Rearch of China (2009CB623404), Program for New Century Excellent Talents in University, the Programme of Introducing Talents of Discipline to Universities (B06006), State Key Laboratory for Modification of Chemical Fibers and Polymer Materials (Dong Hua University).
** To whom correspondence should be addresd. E-mail: zhyjiang@tju.edu
Chin. J. Chem. Eng., Vol. 19, No. 5, October 2011
856performance of PV A-PES composite membrane was investigated in detail. 2 EXPERIMENTAL 2.1 Materials
Poly(vinyl alcohol) (PV A), with an average de-gree of polymerization of 1750f 50, was purchad from Tianjin Kewei Chemical Co. (Tianjin, China). Polyethersulfone (PES) (catalogue no. 6020P, M w 29000) was purchad from BASF Co. (Germany) and dried at 110 °C for 12 h before u. Glutaraldehyde [GA, 50% (by mass)], HCl and absolute ethanol were purchad from Tianjin Guangf
u Fine Chemicals Co. (Tianjin, China). All the chemicals were of analytical grade and ud without further purification. Double- distilled water was ud throughout the experiments. 2.2 Membrane preparation
PV A-PES composite membranes were prepared by pha inversion method [22]. PV A (A) solution [10% (by mass)] and PES (B) solution [25% (by mass)] were prepared by dissolving them parately in DMSO solvent at 80 °C. Then, A, B solutions and dimethyl sulfoxide (DMSO) aqueous solution were mixed at different ratio and filtered to remove air bub-ble until uniformly mixed. Then, this casting solution was poured onto a horizontal glass plate, spread with a casting knife and immediately immerd into a coagulation bath for solidification. The surface crosslinking was obtained by immersing membranes into 2.5% (by mass, GA), 1 M HCl solution for 2 h. Then, the membrane was washed thoroughly with de-ionized water to remove the residues, and dried under t temperature 70, 90, 110, 130, 150 °C. The thick-ness of prepared membranes was about 180 ȝm and there morphology was shown in 3.1. The membranes were designated as PV A(X)-PES(Y) and the composi-tions were prented in Table 1, where X, Y was de-fined as mass fraction of PV A, PES in membrane casting solutions, respectively. Table1 The composition of membrane casting solution for
PV A-PES composite membranes
Composition/g
Membrane
A B DMSO Water
Mass fraction of PV A in dry membrane/%PES 0 36 14 0 0 PV A(1)-PES(17) 5 34 11 0 5.56 PV A(1.5)-PES(16.5) 7.5 33 9.5sm 全称
8.33 PV A(2)-PES(16) 10 32 8 0 11.11 PV A(3)-PES(15) 15 30 5 0
16.67
2.3 Membrane characterization
2.3.1 Scanning electron microscope (SEM)
The cross-ction and surface morphology of as-prepared composite membranes and the element distribution of the membrane were obrved by Phil-ips XL-30M scanning electron microscope instrument operated at 10 kV after being freeze-fractured in liquid nitrogen and sputtered with gold.
2.3.2 Static contact angle measurement
The static contact angles of water on the mem-branes were measured by the ssile drop method us-ing a JC2000C Contact Angle Meter (Powereach Co., Shanghai, China) at room temperature. Water droplets (about 5 mm in diameter) were dropped carefully onto the sample surface. The average contact angle was obtained by measuring the same sample at 12 different sites, and then averaging. The error of measurement for each sample was around ±5%.
2.3.3 X-ray photoelectron spectroscopy (XPS) analysis
X-ray photoelectron spectroscopy patterns of the membranes were recorded using a PHI-1600 diffrac-tometer (American PE Co.) with symmetric reflection geometry. Survey spectra were collected over a range of 0 1100 eV and high-resolution spectra of C 1s peak was also collected. The elemental content on the sur-face of the membrane was obtained from the XPS analysis.
2.3.4 Annealing treatment The as-prepared membranes were allowed to dry completely at room temperature. The membranes were then thermally annealed under specific temperature for 2 h in an oven prior to the pervaporation performance evaluation. 2.3.5 Pervaporation experiment The pervaporation experiments were carried out
in the laboratory using pervaporation apparatus as previously reported and shown in Fig. 1 [23]. The feed solution [90% (by mass) ethanol solution] was supplied
continuously across the surface of a membrane by
pumping. The effective membrane area was about S 25.6 cm 2. The downstream pressure was maintained
at 300 Pa by a vacuum pump. The operating temperature
high sierra
Figure 1 The schematic diagram of membrane permea-tion cell
Chin. J. Chem. Eng., Vol. 19, No. 5, October 2011 857
was 80 °C and the feed flow rate was 60 L·h 1. The permeated vapor was collected in a cold trap with liq-uid nitrogen and was weighed after achieving ambient temperature to gain the permeation flux, J , as defined as follows:
Q
J ST
(1) where Q is the permeate collected in time interval T , and S is the effective membrane area.
Then, the collected permeate was analyzed by HP4890 gas chromatography (GC) equipped with a thermal conductivity detector (TCD) and a column packed with GDX103 (Tianjin Chemical Reagent Co.,
China). The paration factor was calculated according to
W E W E
//P P
F F D (2)
where F W and F E , P W and P E are the mass fractions of water and ethanol in the feed solution and the perme-ate, respectively.
3 RESULTS AND DISCUSSION
3.1 Membrane morphology of PV A-PES compos-ite membrane
SEM images in Fig. 2 showed that PV
A-PES
(a) Ethanol
(b) Isopropyl alcohol
(c) Butanol
(d) Isopropyl alcohol
Figure 2 FESEM images of the cross-ction and surface of PV A(3)-PES(15) membranes prepared in different coagulation bath
Chin. J. Chem. Eng., Vol. 19, No. 5, October 2011
858membranes exhibited the morphology of a typical asymmetric structure and their surface morphology changed greatly with varied coagulation bath. The thickness of compact layer for the composite mem-brane was about 1.2 ȝm when isopropyl alcohol as coagulation bath.
Table 2 showed the solubility parameters of DMSO, water, ethanol, isopropyl alcohol, butanol, PV A and PES which were calculated bad on a sim-ple mixing rule. The clor the solubility parameters of the two organic solvents were, the faster the sol-vent exchange between them. And the morphology of the composite membranes depended strongly on the solvent exchange rate when contacting the coagulation bath. As shown in Fig. 2, sponge-like porous top lay-ers and finger-like porous bottom layer were formed when isopropyl alcohol and butanol were ud as co-agulation medium, while porous surface and finger- like porous support layer were formed when ethanol was as coagulation medium. Similar phenomenon was also reported in the literature [24, 25].
Table 2 Solubility parameters of coagulation medium
Coagulation medium
Solubility parameter (į)/MPa 1/2
DMSO 27.4 water 47.8 ethanol 26.0 isopropyl alcohol
23.8
butanol 23.3 PV A 25.2 PES 20.3
Since the sponge-like layer had the largest mass transfer resistance [26]. To obtain high permeation flux, membranes with finger-like porous were chon as support layers for construction of composite mem-branes. In this study, Isopropyl alcohol was chon as the coagulation medium to obtain a finger-like porous
2013年英语六级support layer with a den top layer to be ud in sub-quent experiments without special explanation.
Surface gregation method was employed to fabricate the integral pervaporation membranes. To interpret the SEM photos, a schematic fabrication process and structure of PV A-PES composite mem-brane was shown in Fig. 3. PV A and PES were physi-cally blended in the membrane casting solution. Upon immersion into the aqueous coagulant bath, PV A spontaneously gregated to all polymer-water interfaces of the forming membrane and lf-organized, creating a hydrophilic brush-like layer. The integral structure could improve the interaction of polymer PV A and PES and then enhanced the membrane stability. 3.2 Water contact angle measurement
Water contact angles were introduced to evaluate the effect of the addition of PV A on the surface hy-drophilicity, and the results were shown in Fig. 4. PES membrane had the highest contact angle, correspond-ing to the lowest hydrophilicity, and the water contact angle for PV A-PES membranes monotonously de-cread with increasing in PV
A content. Furthermore,
Figure 3 Schematic of the fabrication process and ideal structure of PV A-PES composite membrane
Figure 4 PV A-PES composite membrane water contact angle data
Ƶ pha inversion; ƶ solvent evaporation
Chin. J. Chem. Eng., Vol. 19, No. 5, October 2011 859
it could be en that the water contact angles of PV A-PES membranes prepared through pha inver-sion were lower than the solvent evaporation ones, which indicated that the hydrophilic PV A chains were gregated to the membrane surface. Above all, it could be considered as supporting evidence of suc-cessful construction of a hydrophilic membrane surface by surface gregation of P
V A. Incread the loading of PV A in the membrane accordingly incread the hydrophilicity of PV A-PES composite membrane. 3.3 XPS
XPS analysis was employed to quantitatively de-termine the chemical composition of the membrane surface. And the results were shown in Fig. 5. The S element concentration on membrane surface was 3.5%. The near-surface coverage of PV A (ij) was calculated using the Equation (3) [20]:
PES S
PVA 1100%N N D M u
u (3) where N PES is the total mole number of C, O and S in a PES unit, N PV A is the total mole numbers of C, O and S in a PV A unit, and a S is the mole fraction of element
S on membrane surface.
Figure 5 XPS survey scan of PV A(3)-PES(15) membrane surface
C 1s 72.6%; O 1s 23.9%; S 2p 3.5%
utan
It could be calculated that for PV A(3)-PES(15) membrane, the near-surface coverage of PV A was 79.7%, suggesting that the PV A chains which contain hydrophilic OH groups were gregated to mem-brane surface.
3.4 Energy dispersive spectrometer (EDS)
Figure 6 showed the element distribution of C, O, and S at the cross ction of the membrane. The pho-tograph indicated that the content of element S, which existed only in PES, incread obviously with the in-creasing depth under membrane surface. It could be inferred that the S-free PV A chains
was enriched on the membrane surface. Furthermore, the S concentra-tion was incread in an exponential manner and then
reached a plateau under the depth of about 5.6 ȝm. 3.5 The effect of PV A concentration on pervapo-ration performance
The relationship between PV A concentration and pervaporation properties of PV A-PES composite membrane was investigated over the PV A concentra-tion ranging from 0 to 17% (by mass). The permeation flux and paration factor as a function of PV A con-centration were pictured in Fig. 7. It could be en that the paration factor incread with the increasing of PV A concentration. This could be explained by in-creasing PV A content promoted the hydrophilicity of the membrane, and thus facilitated the sorption proc-ess on the upstream membrane surface and enhanced the diffusion of water molecules through the mem-brane. Furthermore, the compact PV A chains also in-cread the paration factor. However, the paration factor had no obvious change after PV A concentration reached 5.6% (by mass) becau it was not dependent on the PV A concentration. Meanwhile, the permeation flux incread with the PV A concentration. This could be interpreted that the incread PV A concentration
was favored to the membrane absorption. Therefore,
water and ethanol were absorbed much faster.
Figure 7
The effect of PV A concentration on the pervapora-tion performance of the composite membrane (dried at 90 °C)
Figure 6 The element analysis of PV A(3)-PES(15) com-posite membrane cross-ction left: S; middle: C; right: O
Chin. J. Chem. Eng., Vol. 19, No. 5, October 2011
8603.6 The effect of annealing temperature on per-vaporation performance
nippon paint
The structure and the morphology of a membrane are crucial to its paration efficiency. Annealing t
emperature, as one important effective strategy, could control the membrane morphology and then changed the membrane performance especially for long-term operation performance. The effect of annealing tem-perature on PV A(3)-PES(15) composite membrane performance was shown in Fig. 8. It could be en that there were significant changes of permeation flux and paration factor with the increa of annealing tem-perature. This could be explained that the increa of annealing temperature resulted in the compression of the polymer chains, which made the diffusion resis-tance for ethanol (kinetic radius of 0.26 nm) was much higher than that for water (kinetic radius 0.13 nm) [27]. Therefore, the paration factor incread and the
permeation flux decread.
Figure 8 The effect of annealing temperature on PV A(3)- PES(15) composite membrane performan
ce
3.7 The effect of water concentration in feed on pervaporation
The permeance Q i and lectivity ȕ could be cal-culated as follows:
0,feed permeate i
i i i i i J Q x p y p J
(4)
water
ethanol
Q Q E
(5)英语儿歌大全
where J i is mass flux of component i (g·m 2·h 1), x i is the mole fraction of the component i in th
e feed liquid, Ȗi is the activity coefficient of component i in the feed liquid, 0,feed i p is the pure component i feed vapor pressure under feed temperature (kPa) and y i is the mole fraction of the component i in the permeate, p permeate is the pressure under the membrane (kPa).
Figure 9 described the effect of water concentra-tion (from 6% to 42%, with a flow rate of 60 L·h 1 at 353 K) in feed on performance of the PV A(3)-PES(15) composite membrane [(a): original data, (b): normal-
ized data]. It could be en that the total permeation flux, ethanol flux and water flux were all incread with the water concentration, while the paration factor had a remarkable decrea due to the membrane swelling.
brisk是什么意思The variation of water flux and total flux could be explained as follows: the water content in the membrane surface incread with the increasing of water concentration, which enhanced the diffusive flux of water through the membrane. Meanwhile, the membrane swelling facilitated the total flux. 3.8 The effect of operating temperature
Figure 10 depicted the effect of operating tem-perature (from 323 K to 353 K) on the permeation flux and paration factor of PV A(3)-PES(15) composite membrane. A flow rate of 60 L·h 1 was kept for
90% (by mass) ethanol aqueous with a downstream pres-sure of 300 Pa. As shown in Fig. 9, both permeation flux and paration factor incread with operation temperature. With the temperature incread from 323 to 353 K, the permeation flux incread significantly from 600 to 1600 g·m 2·h 1, and paration factor considerably incread from 30 to 65, which could be attributed to the favorable effect of temperature on diffusion coefficient and saturation pressure. Diffusion incread with temperature due to higher mobility of water and ethanol molecules. Meanwhile, the satura-
tion vapor pressure incread with temperature and the
(a) Original data
(b) Normalized data
Figure 9 The effect of water concentration in feed on PV A(3)-PES(15) membrane performance
