Research Article - (2013) Volume 3, Issue 1
Keywords: CO2 /CH4 separation; Cross-linking; Formaldehyde; PVA; DEA; FTMs
Application of polymeric membranes in gas separation has been extensively developed, due to its successful competition with common methods such as pressure-swing adsorption (PSA), cryogenic distillation and absorption. In addition polymeric membrane gas separation has been employed in the various fields such as natural gas sweetening, enhanced oil recovery, landfill gas recovery, hydrogen recovery from nitrogen in ammonia purge gas streams, hydrogen removal from carbon monoxide in synthesis gas and flue gas separation, air separation and removal of volatile organic compounds from effluent streams [1,2]. Commercial polymeric membranes exhibit low throughput in competition with porous materials. Solution-diffusion transport mechanism presented trade off limitation between permeability and selectivity for polymeric membranes [3-6]. In order to overcome the Robeson upper-bound limitation, facilitated transport membranes (FTMs) with the property of high permselectivity can be introduced by employing suitable reactive carriers into the membrane matrix and reversible reaction between the reactive carriers and the target gas (reaction solution-diffusion transport mechanism) . Amineimpregnated membranes have been investigated for CO2 removal owing to the high reacting tendency of CO2 with amine compounds. However, it is still difficult to employ them in industrial application due to their instability .
PVA has the many desirable benefits that make it well-known material for the membrane fabrication such as emulsifying and adhesive properties, environmentally friendly, high tensile strength and flexibility, and hydrophilicity, carrier compatibility, amazing chemicalresistant properties, an excellent film forming, pH stability and good antifouling properties [9-11]. On the other hand, PVA exhibits the poor stability in presence of water. Thus, the cross-linking of PVA polymer is introduced as the competent method for solving this mentioned negative point. PVA can be easily cross-linked with multi-functional compounds, which react with hydroxyl groups. Formaldehyde, melamine–formaldehyde, glutaraldehyde, glyoxal, etc are the most frequently used compounds as the PVA cross-linking agents.
Zou and Winston Ho prepared the novel CO2 -selective facilitated membranes which cross-linked by formaldehyde. They showed that the cross-linking with FA developed the PVA membrane thermal stability [12,13]. Additionally, their results demonstrated the exceptional CO2 permeability with reasonable CO2 /N2, CO2 /CO and CO2 /H2 selectivities. In another work, Park and Lee fabricated the PVA/GA/PEI membranes for CO2 /N2 separation with different PVA/GA blend compositions . Their results revealed that the swelling behavior of cross-linked PVA membranes was dependent on the cross-linking density and chemical functional groups creating by the PVA-GA reaction, such as acetal group, ether linkage, and unreacted pendent aldehydes. Moreover, the CO2 /N2 permselectivities through the PVA/GA membranes were 100,000 (cm3 cm2s-1cmHg) and 80, respectively.
In this work, a novel cross-linked poly(vinyl alchohol) (PVA)/ formaldehyde (FA) membrane comprising secondary alkanolamines (DEA) were prepared. In addition, the different FA/PVA mass ratio (from 1 wt% to 5 wt% ratio%) and different amine concentration (15, 25, 35 and 45 wt%) were investigated for membrane preparation. It must be noted that all the membranes were cast on a hydrophilized microporous PTFE support and evaluated for CO2 /CH4 separation. Furthermore, the effect of feed pressure, DEA concentration, and FA concentration on CO2 /CH4 transport properties were examined.
PVA (99%+ hydrolyzed derived from poly(vinyl acetate); Mw =89,000-98,000) and potassium hydroxide(KOH) were purchased from Sigma Aldrich and used without further purification. Diethanolamine (DEA) with purity of 99% and formaldehyde (FA 37 wt% solution in water) were obtained from Aldrich and used as mobile carrier and crosslinker agent in membrane fabrication, respectively. Deionized (DI) water which was used as the solvent during the membrane fabrication, acquired from Merck. Hydrophilized microporous PTFE (Toyo Roshi Kaisha, Ltd.) membranes were employed as the support. The supports have the porosity of 70% and 0.1 μm mean pore diameter. All gases (i.e., CO2 , CH4 and He with 99.8% purity) were given by Tarkib Alvand Gas Corp., and then were employed in the gas permeation tests.
Amine-mediated membranes were prepared by casting polymer solution onto PTFE and hydrophilized microporous support. The PVA solution (15%wt.) was prepared by mixing deionozed water with poly(vinyl alcohol)(PVA). First of all, PVA was dissolved in water at 60ºC for 12 hr under stirring. Then a certain amount of formaldehyde and potassium hydroxide were added into the polymer to obtain PVA cross-linked solution. The PVA/formaldehyde/KOH solution was heated at about 80ºC for 24 h under stirring. Afterwards, a certain amount of DEA was added into the polymer solution to achieve desired amine concentration. Thereupon, the amine-PVA blend was stirred continuously followed by heating at 60ºC for 12 hr in a capped glass. Subsequently, the solution was left for at least 12 hr in order to degasifying any entrapped air. In the next step, the prepared gel polymer was casted onto the supports by employing a film applicator (Elcometer 3580). Finally, the membrane was dried in ambient air for at least 24 hours at room temperature.
Fourier transform infrared (FTIR): Fourier transform infrared spectroscopy was employed to determine the occurrence of any changes in chemical structure of the PVA and its cross-linked with FA by using a Galaxy series FTIR 5000 spectrometer (Mattson Technology, Fremont, CA) in the range of 1800–500 cm-1 and in transmission mode.
Scanning electron microscopy (SEM): In order to observe the morphology of flat sheet membranes, the films were first fractured with liquid nitrogen, and afterwards sputtered with metallic gold by using BAL-TEC (Liechtenstein, Germany) SCD 005 sputter coater . Surface/cross section of flat sheet membranes were analyzed using a Philips XL30 scanning electron microscope.
Water contact angle measurements: In order to determine the surface hydrophilicity, water contact angle measurement was carried out. The wettability test was performed by advanced contact angle measurement using static sessile drop method by OCA-B (Data Physics) instrument. All measurements were repeated twice and the results were averaged.
Gas permeation measurement
The schematic diagram of gas permeation set-up used in experiments is shown in Figure 1. The circular flat–type permeation cell with surface area of 4.9 cm2 was made from two stainless steel (AISI 316) compartments. The flow of feed and sweep gas was controlled by mass flow controllers (M+W Instrument TM Model D-5111). The feed gas was saturated by humidifier before entering to membrane cell. The sweep gas, helium, was supplied to the permeate side of membrane cell. The pressure of feed gas was controlled by back pressure regulator (Control Air Inc., Model 710-BF) changing in the range of 38 to 266 cmHg. During all experiments, the sweep gas pressure was kept constant at atmospheric pressure. The experiments were carried out at room temperature (298ºK ± 5). The water vapor in retentate and permeate side was captured by knock drum, and the flow rate of gas streams exiting the membrane cell was measured by a precision handmade volumetric flow meter. The composition of dried permeate and retentate streams were determined using a gas chromatograph (Thermo Fisher, Model No. CERES 800 PLUS) equipped with a thermal conductivity detector. It must be noted that all experimental results were reported as the average of three measurements.
FTIR: FTIR spectra of the PVA membrane and its cross-linked with 3wt% of FA are illustrated in figure 2. In the case of pure PVA, the principal absorption peak (υ=980 cm-1) related to C-O(ether), is the major one [16-18]. On the other hand, the FTIR spectrum in PVA/ FA (3wt%) introduced a peak at 1125 cm-1 which is attributed to the –C–O–C– stretch and represents the acetalization reaction during cross-linking with formaldehyde [19,20]. Moreover, wider absorption band of C-O (ether) and C-O-C (acetal ring) formed in PVA/FA crosslinked membrane (υ=1000 to 1200 cm-1) substituted to C-O stretching band (υ=980 cm-1) in pure PVA membrane [17,18,21]. Eventually, the results of FTIR spectra indicated that the FA cross-linker can be employed as the chemical cross-linker among PVA polymer chains.
SEM: Figure 3 (a) depicts the surface of of hydrophilized microporous PTFE. It must be noted that the support morphological specification with pore size=0.1 μm, is in agreement with the supplier claim. Figure 3b and c illustrate the front and back surface of DEAmediated PVA/FA (3 wt%). As can be observed, no macro-voids exist and the homogeneous pattern is created. Furthermore, the microporous PTFE pores were totally filled with the PVA hydrogel. Consequently, a nonporous structure was created due to the penetration of PVA hydrogel in to the pores of support. Figures 3d and e show the cross section of cross-linked membrane. A thin layer of PVA was formed on the top surface of microporous PTFE support.
Water contact angle measurements: The images of wettability test with water are depicted in figure 4. In addition, the values of contact angle achieved from wettability test are given in table 1. It is clear that the contact angle of water on DEA-mediated PVA/PTFE is significantly higher than that of DEA-mediated cross-linked PVA/PTFE due to hydrophilicity reduction. This result is attributed to degree of swelling and cross-linking density. Cross-linking of PVA membrane has been known to decrease the hydrophilicity nature of the membrane.
|Membrane||Right contact angle(º)||Left contact angle(º)||Average Contact angle(º)|
Table 1: Contact angle values (a) Pure PVA and PVA/FA(3wt%)/PTFE membranes.
Gas permeation results
The effects of cross-linker agent content, referred to as the crosslinker/ PVA mass ratio were examined. The cross-linker/PVA mass ratio was ranged from 1 to 5 wt%. It must be noted that it was impossible to incorporate more than 5 wt% of cross-linker in gel matrix resulting from probable side reaction of DEA with unreacted formaldehyde in polymer matrix. The DEA-impregnated PVA/FA membranes with an average ratio of cross-linkers (about 3 wt%) were prepared with different amine concentration of 5, 15, 25, 35 and 45 wt% and the related results are shown in table 2. The results of PVA/PTFE membrane were revealed that membrane containing 15 wt% of DEA was the most favorable membrane during the gas experiments. In general, the higher content of carrier molecules, the higher amount of CO2 , permeates via CO2 - carrier reaction. Although, increasing the amine concentration has unfavorable effect on permeation . The trends of CO2 permeance with DEA concentration depend on the three parameters: CO2 carriers enriching, ionic strength increasing and the degree of swelling [23-32]. During the CO2 –amine reactions, a variety of ionic species such as carbamates, protonated amines and zwitterions, are formed. Indeed, salting-out effect (which attributed to the membrane ionic strength enhancement) causes to decrease both the CO2 solubility and the CO2 –amine complexes diffusivity in the membrane matrix. These ions surrounds the PVA chain segments and this creates a serious problem for free access of CO2 molecules to PVA matrix and finally decrease the physical solution–diffusion of free CO2 in the PVA matrix. Hence, this membrane was selected for further considerations.
|DEA (wt%)||Permeance*||CO2/CH4 Selectivity|
Table 2: The effect of various amine concentrations on DEA-impregnated crosslinked PVA membranes with an average ratio of formaldehyde (about 3 wt%) (Pure gas results, P=152 cmHg).
Figure 5 illustrates the effect of transmembrane pressure on CO2 /CH4 permselectivity of PVA/PTFE containing 15 wt% of DEA membranes with different FA/PVA mass ratios. Generally, CO2 and CH4 permeances decreased with increasing FA/PVA mass ratio, in comparison with uncross-linked membranes, although the selectivities increased considerably. Indeed the gas permeability increased, as the FA content increased. This trend represented that higher FA content led to probable higher cross-linking densities. According to contact angle test, water-swelling degree of cross-linked PVA membranes is reduced. In order to determine the swelling ratio, dry PVA/FA film strips (3×3 cm2) were immersed in water for 48 h at 40ºC to reach equilibrium sorption. The strip was dried under vacuum at ambient temperature overnight and accordingly the weight of dried films was measured. The swelling ratio of PVA/FA membranes is defined by:
where SR denotes the swelling ratio, Wd and Wω denote the weight of the dried and the swollen films, respectively. As it is observed in table 3, the membranes revealed declining water swelling value and this indicates that the high quantity of dialdehyde molecules could form a proper network in order to avoid the polymer solubility in water and this resulted in reduction in membrane free volume and enhancement of water resistance. At higher concentration of FA, the solution became more viscous and this restricted the diffusivity of the reactants. Thus, the packing ability improved due to the reduction of the space among the chains (By polymer acetalization) and accordingly this may led to lower swelling [33-35].
Table 3: Water-swelling ratio of cross-linked PVA/FA membranes as a fuction of cross-linker content.
As shown in figure 5, at low pressure, the CO2 permeance is descended sharply then it is lowered smoothly at higher pressure owing to carrier saturation phenomenon. The CO2 solution into the membrane surface increases, when the pressure difference across the membrane enhances. Thus the formation rate of CO2 –amine complexes increases. Therefore, the most of amine molecules involved in CO2 – amine interaction and this resulted that amine carrier concentration gently reaches saturation state in PVA membrane. Carrier saturation and gradual reduction of gas flux with increasing pressure are generic behavior in facilitated transport membranes [23,26,31,32]. On the other hand, CH4 has no chemical interaction with carrier and its permeance moderately decreased with increasing feed pressure. When the feed pressure increased, CH4 molecules were trapped in polymer matrix and this affected the diffusion transport of them arising from more compressibility and free volume reduction . Tarnsmembrane pressure, carrier concentration and cross-linking agent content are the main parameters which considerably affect the properties of CO2 facilitated transport. The highest selectivity (about 92.72) was shown for PVA/FA(5wt%)/PTFE membrane. Furthermore, the result of water swelling test is in conformity with this outcome relating to the most compact matrix and cross-linking density (Table 3). It is obviously found that CO2 /CH4 selectivity was significantly lowered with increasing cross-linker/PVA mass ratio from 1 wt% to 5 wt%.
DEA mediated cross-linked PVA/FA membranes with several blend compositions (FA/PVA: 1, 3, 5 ratio%) were synthesized and a series of pure gas experiments were carried out to evaluate the effects of amine and cross-linking concentration, pressure differential across the membrane on CO2 facilitated transport. The feed pressures were ranged from 38 cmHg (0.5 bar) to 266 cmHg (3.5 bar). The obtained results are summarized as follow:
• Outcomes from FTIR, SEM and contact angle measurement showed that in PVA/FA cross-linked membranes, a peak at 1125 cm-1 is attributed to the –C–O–C– stretch and represents the acetalization reaction during cross-linking with formaldehyde. Moreover, no macro-voids exist and the homogeneous pattern is created and the cross-linked PVA membranes has less hydrophilicity in comparison with pure ones due to less swelling ration and more polymer crystalinity.
• With increasing cross-linker/PVA mass ratio, CO2 and CH4 permeances were decreased in comparison with uncrosslinked membrane. Although the selevtivities were dramatically enhanced. The highest selectivity was achieved from PVA/ FA(5wt%)/PTFE membrane owing to lowest water swelling as well as the most cross-linking density.