Polyvinylpyrrolidone Blend Membrane Containing Amine Carrier for

Sep 11, 2014 - This article reports structural characterizations and gas permeation properties of novel CO2-selective cross-linked thin-film composite...
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Novel CO2‑Selective Cross-Linked Poly(vinyl alcohol)/ Polyvinylpyrrolidone Blend Membrane Containing Amine Carrier for CO2−N2 Separation: Synthesis, Characterization, and Gas Permeation Study Arijit Mondal and Bishnupada Mandal* Department of Chemical Engineering, Indian Institute of Technology Guwahati, Guwahati - 781039, Assam, India S Supporting Information *

ABSTRACT: This article reports structural characterizations and gas permeation properties of novel CO2-selective cross-linked thin-film composite poly(vinyl alcohol) (PVA)/polyvinylpyrrolidone (PVP) blend membranes doped with suitable amine carriers. The characterization of the active layer was carried out by thermogravimetric analysis, differential scanning calorimetry, Fourier transform infrared spectroscopy, and X-ray diffraction. Gas streams containing 20% CO2 and 80% N2 by volume were used to study the transport properties of CO2 (CO2 and N2 flux, CO2 permeability, and CO2/N2 selectivity) across the membrane. The effects of active layer thickness (34−87 μm), feed absolute pressure (1.7−6.2 atm), temperature (90−125 °C), and sweep side water flow rate (0.02−0.075 cm3/min) on CO2 transport properties across the membrane were analyzed. The maximum CO2/N2 selectivity of 370 and a CO2 permeability of 1396 barrer were obtained for the composite membrane with 40 μm active layer thickness at 2.8 atm feed side absolute pressure and 100 °C.

1. INTRODUCTION Carbon dioxide emission has been a source of major environmental concern owing to drastic climatic change in recent times.1 Its capture from other gases has wide applications such as mitigating greenhouse gas effects, production of fuel with enhanced energy content, gas purification with the prevention of corrosion problems, etc.2 Conventional matured technologies such as amine absorption has large-scale implementations. However, the limitations of such technologies include high cost and high regeneration energy consumption. New advanced technologies such as polymeric membrane-based technology are potential separation methods because of low energy consumption and compact membrane module leading to enhanced weight and space efficiency. It is also an efficient environmentally friendly alternative compared to other processes.3,4 Polymer membranes such as cellulose acetate, cellulose triacetate, or polyimide are used industrially for CO2 removal. These membranes normally suffer low CO2 permeability as well as low CO 2 /N 2 selectivity. Wind et al. 5 reported a CO 2 permeability of 4 barrers and a CO2/N2 selectivity of 26 for cellulose acetate membranes with the thickness of 50−75 μm at 35 °C and 25 atm. However, most of the industrially useful polymeric membranes are made of common polymeric materials, and separation is governed by a solution−diffusion mechanism. Relative solubility and diffusivity of the components in the gas mixture limits the selectivity of conventional polymeric membrane.6−8 Blending of suitable amine carriers into the polymer matrix can enhance the CO2 transport property by reacting reversibly with the target compound.9−20 Miscibility between polymers and amines is a key factor because it affects membrane morphology, permeability, degradation, thermal stability, etc. Improvement of the thermal © XXXX American Chemical Society

stability of the polymeric membrane is one of the important characteristics in any industrial application because most of the polymeric membranes are affected by high-temperature application. Cross-linking as well as blending of thermally stable polymer matrix may improve thermal stability of the polymer hydrogel. Several investigations have been reported in the literature for improving thermal stability of polymers.21−25 Amine blends have been used as a carrier to enhance CO2 transport.9−20 Poly(vinyl alcohol) (PVA) is an excellent hydrophilic material and useful for many practical applications due to its ease of preparation as well as its excellent filmforming ability, good compatibility with amine, and good thermal stability.26−32 However, its hydrophilicity and thermal stability can be improved by using different cross-linking agents like aldehydes, carboxylic acids, and anhydrides and by blending with a thermally stable polymer matrix.21−25 Pedram et al.33 prepared cross-linked poly(vinyl alcohol) membrane doped with diethanolamine (DEA) on polytetrafluoroethylene (PTFE) support. Glutaraldehyde (GA) had been used as a cross-linking agent. The effective membrane surface area was 4.9 cm2, and the permeation experiment was carried out at room temperature (25 °C). The effects of cross-linking agent content, feed pressure (0.5−7.5 atm), and composition as well as stability on CO2/CH4 transport properties were investigated in both pure and mixed gas experiments. During the gas Special Issue: Energy System Modeling and Optimization Conference 2013 Received: February 11, 2014 Revised: August 26, 2014 Accepted: September 10, 2014

A

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supports (thickness, about 150 μm; average pore size, 0.03 μm) were obtained from Sterlitech, U.S.. Feed gas (20% CO2 and 80% N2) with certified composition was supplied by Vadilal Pvt. Ltd., India and used for the gas permeation tests. 2.2. Membrane Synthesis. The active layer of thin-filmcomposite membranes were synthesized by solution casting methodology on the porous polysulfone support. The membrane synthesis process is mainly divided into two parts. First, the polymer hydrogel was synthesized using formaldehyde as cross-linking agent as well as blending with polyvinylpyrrolidone (PVP). Second, a suitable amine carrier was introduced into the polymer hydrogel. Initially, the membrane was synthesized by varying the degree of cross-linking of poly(vinyl alcohol). Weight percentage of PVA (90 wt %) and KOH (10 wt %) were kept constant for all the membranes. The dry compositions of the membrane were 90 wt % PVA + 10 wt % KOH with 10−80 mol % degree of cross-linking by HCHO. Then the optimum degree of cross-linking with HCHO was selected and the ratio of PVA and PVP was varied keeping the concentration of KOH constant at 10 wt %. Four different compositions of membranes were synthesized by varying the PVA/PVP ratio (PVA/PVP = 1/0.25, 1/0.5, 1/1, 1/2). The dry compositions of the membranes were 72 wt % PVA + 18 wt % PVP + 10 wt % KOH, 60 wt % PVA + 30 wt % PVP + 10 wt % KOH, 45 wt % PVA + 45 wt % PVP + 10 wt % KOH, and 30 wt % PVA + 60 wt % PVP + 10 wt % KOH. Then, blends of PEI and PEHA were incorporated into the cross-linked-PVA-PVP polymer hydrogel, maintaining the similar optimum composition as mentioned in our earlier work.32 Henceforth, the PVA and PVP weight percentage ratio of 1:0.2 along with 60 mol % degree of cross-linking with HCHO was maintained. The detailed composition of the active layer was 41.66 wt % PVA + 8.33 wt % PVP + 10 wt % KOH + 15 wt % PEI + 25 wt % PEHA with 60 mol % degree of cross-linking by HCHO. This composition was based on the weight percent of each component present in the solid polymer film on a dry basis. The active layer solution was prepared following our previously published procedure.20,27 In short, PVA was initially dissolved in water at 95 °C under vigorous stirring. A stoichiometric amount of HCHO and a calculated amount of KOH was added into the PVA aqueous solution to achieve desired degree of cross-linking. The PVA/HCHO/KOH solution was heated at about 95 °C for the time period of 10 h under stirring. The calculated amount of PVP was then added into the PVA/HCHO/KOH solution for the time period of 6 h under continuous stirring. The viscosity of the solution was increased significantly. This increase might be due to the crosslinking of PVA and addition of PVP. A calculated amount of PEI was added into the PVA/formaldehyde/KOH/PVP solution with stirring at around 95 °C for 2 h. Thereafter, a proper amount of aqueous PEHA solution was added into the PVA/formaldehyde/KOH/PVP/PEI solution with stirring. The solution was then centrifuged (Sigma-30K, U.S.) at 13 000 rpm for 35 min before casting. An adjustable micrometer casting knife (GARDCO, Paul N. Gardner, U.S.) was used to control the thickness. The membranes were first dried at room temperature inside a fume hood overnight to remove the surface water. Then they were heated at 125 °C inside an oven over the time period of 9 h to ensure complete cross-linking reaction with PVA and removal of water.

permeation experiments, membrane containing 15 wt % of DEA showed the best performance. The best-yield CO2selective membranes (DEA-PVA/GA (1 wt %)) represented the best CO2/CH4 selectivity of 91.13 and 665 for pure and mixed gas experiments, respectively. Duan et al.34 reported that cross-linked PVA membrane doped with poly(amidoamine) (PAMAM) dendrimer shows selective separation of CO2 from a mixture of CO2 and H2 (80/20 vol % of CO2/H2). The effects of cross-linker concentration (0−50 wt %), membrane thickness (6−400 μm), and temperature (40−60 °C) on separation performance were discussed. CO2 permeance increased with decreasing thickness, but the CO2/H2 selectivity decreased with decreasing thickness. Both permeance and CO2/H2 selectivity increased with increasing temperature. The CO2/H2 selectivity reached a maximum of 32 with CO2 permeance of 3.0 × 10−11 m3 (STP)/(m2 s Pa) at 60 °C under CO2 partial pressure of 5.52 atm using a membrane of 6 μm thickness. Cai et al.9 studied the separation of CO2 from CH4 and N2 mixture using PVA containing poly(allylamine) composite membrane. The CO2 permeance and CO2/N2 selectivity observed were around 24 GPU and 80, respectively, against 0.98 atm feed pressure at room temperature (25 °C).9 Francisco et al.12 studied the effect of different alkanolamines (MEA/AMP/DEA/MDEA) in PVA membranes. Pure CO2 permeance and CO2/N2 selectivity was found to be about 8 GPU and 88, respectively, against the feed pressure difference of 0.5 atm at 24 °C. They have reported that DEA (20 wt %) gives the most satisfactory result among all other alkanolamines.12 Matsuyama et al.14 have reported the CO2 permeance and CO2/N2 selectivity using PVA/PEI blend membrane as 4 GPU and 160, respectively, at CO2 partial pressure of 0.065 atm with 220 μm membrane thickness.14 Although some studies on CO2/N2 separation at low temeprature are available, the literature on CO2/N2 separation using cross-linked PVA membrane containing blended amine carriers at high temperatures is limited. In this study, novel cross-linked PVA membranes blended with polyvinylpyrrolidone (PVP) as well as amines have been synthesized by the solution casting technique. Formaldehyde (HCHO) was used as cross-linking agent. PVP was blended to enhance the thermal stability of the PVA membrane. Blends of primary sterically hindered amine, polyethylenimine (PEI), and primary sterically unhindered amine, pentaethylenehexamine (PEHA), have been used as carriers to enhance the CO2 transport through the membrane. The synthesized membranes were characterized by thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and X-ray diffraction (XRD) to study the weight loss of polymer fraction, glass transition temperature (Tg), melting temperature (Tm), and degree of crystallinity. The CO2 gas transport properties (CO2 flux, CO2 permeability, CO2/N2 selectivity) were obtained against different physical conditions like pressure, temperature, and moisture content of the cross-linked PVA−PVP−amine thin-film composite membranes.

2. EXPERIMENTAL SECTION 2.1. Materials. Poly(vinyl alcohol) (98−99 mol % hydrolyzed powder, Mw = 130 000), polyvinylpyrrolidone (average Mw, 360 000), polyethylenimine (average Mw, 25 000), and pentaethylenehexamine (Mw = 232.37) was procured from Sigma-Aldrich, U.S.. Formaldehyde (37 wt % aqueous solution, Merck, India) and potassium hydroxide (Merck, India) were used without further purification. Microporous polysulfone B

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The cross-sectional view of a field emission scanning electron microscopy (FESEM) photograph of cross-linked-PVA-PVP doped with blended amines (15 wt % PEI + 25 wt % PEHA) membrane on porous polysulfone membrane is shown in Figure 1. It can be seen that the membrane has two layers, viz.,

dry basis was used as feed gas. Pure argon was used as a permeate side carrier gas. Two mass flow controllers (Aalborg, U.S.) were used to control the feed side and permeate side gas flow rates. Both the feed and permeate side gas flow rates were kept at 30 cm3/min. Both the feed and carrier gases were humidified before they entered the permeation cell using two different saturators (Swagelok, U.S.) housed inside the oven. The appropriate amounts of water required for the saturators were pumped using two Varian Prostar 210 pumps (Varian Inc., Palo Alto, CA). The pressure at the feed side and permeate side of the membrane module was maintained by two different backpressure regulators (Swagelok, U.S.). The absolute feed pressure was varied from 1.7 to 6.2 atm, and the absolute permeate pressure was maintained close to atmospheric pressure. Both the retentate and permeate streams leaving the permeation cell were collected in two different water knockout vessels (Swagelok, U.S.), which were kept at ambient temperature. Most of the water vapor condensed inside the water knockout. Partially moisture-free gases from water knockouts were entered through moisture trappers to the gas chromatograph (GC) for composition analysis. Helium was used as the GC carrier gas. The compositions of both the permeate and retentate gas were then analyzed sequentially using a Varian 450 gas chromatograph (Varian Inc., Palo Alto, CA) with one thermal conductivity detector. The GC columns were used as CP7430 capillary columns (Agilent Technologies, Palo Alto, CA) which were the combination of two different capillary columns, one being the CP-Molsieve 5A and the other CP-PoraBOND Q. Each of the membrane permeation measurements were carried out for at least 10−15 h at a specific temperature, pressure, and water flow rate, which allowed for the steady-state permeation. All the gas permeation data were the average of three replicate runs. The CO2/N2 selectivity was obtained from GC analysis. The CO2/N2 selectivity and permeability was defined as follows:

Figure 1. Field emission scanning electron microscopy (FESEM) image of cross-sectional view of cross-linked-PVA-PVP blended with amines (15 wt % PEI + 25 wt % PEHA) active layer thickness around 40 μm on porous polysulfone membrane.

a dense active layer and a porous polysulfone support layer. The dense active layer helps the selective separation of CO2, and the porous polysulfone support layer provides the mechanical support to the membrane. This composite structure minimizes the mass-transfer resistance while maximizing the mechanical strength. Dense active layer thickness of thin-film composite membranes was measured by using a litematic thickness gauge (Mitutoyo, VL-50; Measuring pressure, 0.15N; Japan) with an accuracy of about ±0.2 μm. Initially, the thickness of the polysulfone support layer and then the total thickness of the synthesized composite membrane were measured. The active layer thickness was then calculated from the difference between the total thickness and polysulfone support thickness at various locations of the effective membrane surface area. The dry active layer thickness of different membranes was varied between 30 and 80 μm. The membrane effective area was 51.5 cm2 with the membrane cell dimensions of 8.1 cm inner diameter and 9.1 cm outer diameter. 2.3. Membrane Characterization. The dry membranes were produced based on PVA, KOH, PVP, PEI and PEHA with HCHO as cross-linking agent. These were then characterized via TGA, DSC, Fourier transform infrared (FTIR) spectroscopy, and XRD. The characterizations were done for the active layer only. The detailed protocols of all analyses were followed as per our previous work.27 2.4. Gas Permeation Measurements. The schematic of the permeation measurement setup was reported in our earlier work.27 A stainless steel counter-current permeation module with the specifications mentioned before was used for the permeation study. The module was kept inside a temperaturecontrolled oven (Reico Pvt. Ltd., India). Transport properties of synthesized composite membranes were measured against temperature, feed absolute pressure, and sweep side moisture content. A gas mixture containing 20% CO2 and 80% N2 on a

αij =

yi /yj xi /xj

Pi N = i l Δpi

(1)

(2)

where yi and yj are mole fractions of component i and j in the permeate stream and xi and xj are mole fractions of component i and j in retentate stream; αij is the selectivity of component i over component j. Pi is the permeability (barrer), Pi/l the permeance (GPU), l the thickness of the membrane (cm), Δpi the pressure difference (cmHg), and Ni the flux (10−6 cm3 (STP)/cm2 sec). 1 barrer = 10−10 cm3 (STP) cm/(cm2 sec cmHg) and 1 GPU = 10−6 cm3 (STP)/(cm2 sec cmHg).

3. RESULTS AND DISCUSSION The active layer of the membrane was synthesized using PVA, HCHO, KOH, PVP, PEI, and PEHA via an aqueous route. The total amine concentration was kept constant at 40 wt % along with PVA-to-PVP weight percentage ratio of 1:0.2 and 60 mol % degree of cross-linking by HCHO. 3.1. TGA Characterization. Improvement of thermal stability of PVA membrane after increasing the degree of cross-linking with HCHO is shown in Figure S1 of Supporting Information. It has been observed that the 60 mol % degree of cross-linking with HCHO was the optimum among others, above which liquid polymer hydrogel agglomerate. Addition of C

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PVP to cross-linked-PVA shows drastic change of weight loss compared to cross-linked-PVA membrane, Figure S2 of Supporting Information. It has been observed that PVA/PVP of 1/0.25 shows the optimum ratio (Table S1 of Supporting Information). In this work, the thermal stability and weight loss of crosslinked-PVA-PVP blended with PEI and PEHA membrane was determined by TGA curve and compared with different compositions of PEI and TEPA amine systems studied in our earlier work32 (Figure 2). Each TGA curve showed three main

Figure 3. DSC curves of pure PVA membrane and cross-linked-PVAPVP doped with (15 wt % PEI + 25 wt % PEHA), (15 wt % PEI + 25 wt % TEPA),32 (30 wt % PEI + 10 wt % TEPA),32 and (40 wt % PEI)32 membranes. For all the amine doped membranes, 60 mol % degree of cross-linking by HCHO was maintained.

of the membranes. There is a temperature shift in the region of 40−80 °C for all five membranes. Hence, the Tg of pure PVA membrane and cross-linked-PVA-PVP doped with (15 wt % PEI + 25 wt % PEHA), (15 wt % PEI + 25 wt % TEPA), (30 wt % PEI + 10 wt % TEPA), and (40 wt % PEI) membranes were obtained as 75, 60, 58, 47, and 45 °C, respectively. 3.3. Characterization by FTIR Spectroscopy. The functional groups present in the cross-linked-PVA membranes and cross-linked-PVA-PVP membranes were obtained using Fourier transform infrared spectrometery as shown in Figures S5 and Figure S6 of Supporting Information. Figure 4 shows the FTIR spectra of cross-linked-PVA-PVP blended with PEI and PEHA membrane and compared with different compositions of PEI and TEPA amine systems studied in our previous work.32 The broad peak around 3267 cm−1 for all four membranes was the indication of the presence of hydroxyl group (O−H). The sharp peak at 2912 cm−1 is assigned to the symmetric vibrations of (C−H), predominantly from alkyl groups.28 The sharp frequency at 1650 and 1580 cm−1 is assigned to (CC) stretching and combination of (O−H) and (C−H) bending, respectively.29 The vibrational band observed at around 1419 cm−1 refers to the combination of CH2 bending and CH2 out-of-plane bending.28,29 The band frequency between 1150 and 1085 cm−1 is assigned to the combination of (−C−O−C−) and (C−O) bonds.10,30 For all these membranes it is observed that the intensity of the peak at 1141 cm−1 is well-pronounced, which is evidence of formation of acetal linkage during the cross-linking reaction. The peak at 1100 cm−1 is attributed to the (C−O) stretching.3 3.4. Characterization by XRD. To examine the nature of the crystallinity of polymer membranes, an X-ray diffraction measurement was carried out. The diffraction pattern for the cross-linked-PVA-PVP doped with single amine (PEI) and blended amines (PEI +TEPA and PEI + PEHA) membranes are shown in Figure 5. The PVA polymeric film displays a semicrystalline structure with peaks at 2θ angles of 20°. The peak intensity of all four membranes shows a very nominal change.

Figure 2. TGA curves of cross-linked-PVA-PVP doped with (15 wt % PEI + 25 wt % PEHA), (15 wt % PEI + 25 wt % TEPA),32 (30 wt % PEI + 10 wt % TEPA),32 and (40 wt % PEI)32 membranes. For all the membranes, 60 mol % degree of cross-linking by HCHO was maintained.

steps of weight loss. The initial weight loss observed below 120 °C was caused by the evaporation of absorbed moisture from the atmosphere because of the presence of KOH in the membrane making it more hygroscopic. The second weight loss (204−275 °C) was associated with the removal of hydroxyl groups as well as free amine groups present in the membrane. The final weight loss between 350 to 488 °C was related to the decomposition of the polymer backbones. 3.2. DSC Characterization. The melting temperature (Tm) and glass transition temperature (Tg) were determined by DSC analysis. Improvement of melting temperature as well as glass transition temperature of PVA membrane after increasing the degree of cross-linking with HCHO is shown in Figure S3 of Supporting Information. Addition of PVP to cross-linked-PVA membrane shows dramatic improvement of Tg compared to that of cross-linked-PVA membrane, Figure S4 of Supporting Information. It has been observed that 60 mol % degree of cross-linking and PVA/PVP of 1/0.25 showed the best result among others (Figures S3 and S4 and Table S1 of Supporting Information). Figure 3 represents the DSC curve of pure PVA membrane and cross-linked-PVA-PVP blended with PEI and PEHA membrane, which is also compared with different compositions of PEI and TEPA amine systems reported elsewhere.32 All five membranes show one endothermic peak at around 224 °C for pure PVA membrane and 240 °C for cross-linked-PVA-PVP membranes doped with amines, representing the melting point D

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PEHA (a primary sterically unhindered amine) used as carrier in the present study are shown in Figure 6. The reaction

Figure 6. Chemical structure of polyethylenimine (PEI) and pentaethylenehexamine (PEHA) as carriers.

mechanism between amine (PEI + PEHA) and CO2 in the presence of moisture is similar to that of our earlier work.27,31 For certain applications where higher temperature is required, cross-linking and polymer blending is very essential for improving its thermal stability. The cross-linking mechanism of PVA with HCHO and polymer blending between crosslinked-PVA with PVP are also similar to those of our earlier work32 and are shown in Figures S7 and S8 of Supporting Information. 3.6. Effect of Active Layer Thickness on Separation Performance and Composition Optimization. The effects of active layer thickness on CO2 flux, N2 flux, CO2 permeance, N2 permeance, CO2 permeability, N2 permeability, and CO2/ N2 selectivity were studied using cross-linked-PVA blended with PVP membrane containing blended amines. It is very difficult to maintain exactly the same active layer thickness for all different amine composition membranes using the solution casting methodology. To find the optimum composition, it is necessary to compare CO2 flux, N2 flux, CO2 permeance, N2 permeance, CO2 permeability, N2 permeability, and CO2/N2 selectivity at a fixed thickness for different compositions. It has been found from our earlier work that the cross-linked-PVAPVP doped with blended amines (15 wt % PEI + 25 wt % TEPA) membrane shows the best performance among other compositions at constant operating conditions.32 Hence, in this study, TEPA has been replaced by PEHA through maintaining the same composition like cross-linked-PVA-PVP doped with blended amines (15 wt % PEI + 25 wt % PEHA) membrane. This amine was chosen because of the fact that 1 mol of PEHA contains one extra (−NH) group compared to TEPA (Figure 6), which may enhance the CO2 transport through the membrane via a series of reversible reactions between amine and CO2 in the presence of moisture.27 Three different membranes with different active layer thickness of cross-linked-PVA-PVP doped with 15 wt % PEI and 25 wt % PEHA membranes along with separation performance are given in Table 1 and shown in Figure 7, which are also compared with the three different membranes with different active layer thickness of cross-linked-PVA-PVP doped with 15 wt % PEI and 25 wt % TEPA membranes.32 Feed side and sweep side absolute pressures were maintained constant at around 2.8 and 1.15 atm, respectively. Temperature was kept constant at 95 °C along with constant water flow rate at both sides (feed/sweep, 0.03/0.04 cm3/min). Both feed gas (20% CO2 balance N2 on dry basis) and carrier gas (Ar) flow rates were maintained at 30 cm3/min throughout the experiment.

Figure 4. FTIR spectra of cross-linked-PVA-PVP doped with (15 wt % PEI + 25 wt % PEHA), (15 wt % PEI + 25 wt % TEPA),32 (30 wt % PEI + 10 wt % TEPA),32 and (40 wt % PEI)32 membranes. For all the membranes, 60 mol % degree of cross-linking by HCHO was maintained.

Figure 5. XRD spectra of cross-linked-PVA-PVP doped with (15 wt % PEI + 25 wt % PEHA), (15 wt % PEI + 25 wt % TEPA), (30 wt % PEI + 10 wt % TEPA), and (40 wt % PEI) membranes. For all the membranes, 60 mol % degree of cross-linking by HCHO was maintained.

3.5. Gas Transport and Cross-Linking Mechanism. The detailed CO2 and N2 transport mechanism through the synthesized membrane was discussed in our previous work.27 The CO2 transport is facilitated because of the reversible reactions in the presence of carrier in the membrane in addition to the solution−diffusion mechanism, whereas the N2 transport is driven only by the solution−diffusion mechanism. The membrane synthesized in this work contains both sterically hindered and unhindered primary amines. The chemical structures of PEI (a primary sterically hindered amine) and E

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Table 1. CO2 Flux, CO2 Permeability, and CO2/N2 Selectivity of Different Active Layer Thickness Membranes of Different Amine Compositions [(15 wt % PEI + 25 wt % TEPA)32 and (15 wt % PEI + 25 wt % PEHA)] at Constant Physical Conditionsa active layer composition (wt %) 41.66% PVA + 8.33% PVP + 10% KOH + 15% PEI + 25% TEPA + 60 mol % HCHO 41.66% PVA + 8.33% PVP + 10% KOH + 15% PEI + 25% PEHA + 60 mol % HCHO

a

CO2 flux (× 10−6 cm3(STP)/(cm2 s))

CO2 permeance (GPU)

CO2 permeability (barrer)

CO2/N2 selectivity

34 67 73 40

508 380 353 545

38 22.12 20.58 35.52

1294 1482 1502 1420

180 262 278 336

60

462

26.25

1575

362

75

381

22.02

1651

387

active layer thickness (micron)

ref 32 32 32 this work this work this work

Temperature, 95 °C; feed absolute pressure, 2.8 atm; sweep absolute pressure, 1.15 atm; sweep/feedwater flow rate, 0.05/0.03 mL/min.

Figure 7. (a) Effect of active layer thickness on CO2 and N2 flux: (■) CO2 and N2 flux for (15 wt % PEI + 25 wt % PEHA) and (⧫) CO2 and N2 flux for (15 wt % PEI + 25 wt % TEPA).32 (b) Effect of active layer thickness on CO2 and N2 permeance (GPU): (■) CO2 and N2 permeance for (15 wt % PEI + 25 wt % PEHA) and (⧫) CO2 and N2 permeance for (15 wt % PEI + 25 wt % TEPA).32 (c) Effect of active layer thickness on CO2 and N2 permeability (barrer): (■) CO2 and N2 permeability for (15 wt % PEI + 25 wt % PEHA) and (⧫) CO2 and N2 permeability for (15 wt % PEI + 25 wt % TEPA).32 (d) Effect of active layer thickness on CO2 and N2 selectivity: (■) CO2 and N2 selectivity for (15 wt % PEI + 25 wt % PEHA) and (⧫) CO2 and N2 selectivity for (15 wt % PEI + 25 wt % TEPA).32 Temperature, 95 °C; feed absolute pressure, 2.8 atm; sweep absolute pressure, 1.15 atm; sweep/feed water flow rate, 0.05/0.03 mL/min.

(cm2 sec) and 430 × 10−6 cm3 (STP)/(cm2 sec), respectively (Table 2). The CO2 permeance decreases with increasing active layer thicknesses at a particular composition. This trend is possibly because the mass flux through the membrane is inversely proportional to the membrane thickness. The CO2 permeability and CO2/N2 selectivity is also increased with an increase in active layer thickness at a particular composition. The CO2

It can be seen from Figure 7a that with an increase in active layer thickness the CO2 flux is decreased rapidly with a particular composition. As expected, with increase in active layer thickness, the total mass-transfer resistance increases; hence, the CO2 flux decreases.35−37 At constant active layer thickness of 55 μm for cross-linked-PVA-PVP doped with (15 wt % PEI + 25 wt % PEHA) and (15 wt % PEI + 25 wt % TEPA) membranes, the CO2 flux was 483 × 10−6 cm3 (STP)/ F

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Table 2. CO2 Flux, CO2 Permeability, and CO2/N2 Selectivity of Constant Active Layer Thickness, around 55 Micron Membranes of Different Amine Compositions [(15 wt % PEI + 25 wt % TEPA)32 and (15 wt % PEI + 25 wt % PEHA)] at Constant Physical Conditionsa active layer thickness (micron) 55 55 a

active layer composition (wt %)

−6

CO2 flux (× 10

41.66% PVA + 8.33% PVP + 10% KOH + 15% PEI + 25% PEHA + 60 mol % HCHO 41.66% PVA + 8.33% PVP + 10% KOH + 15% PEI + 25% TEPA + 60 mol % HCHO

3

2

cm (STP)/(cm s))

CO2 permeance (GPU)

CO2 permeability (barrer)

CO2/N2 selectivity

ref

483

28

1535

354

32

430

25.8

1421

230

present work

Temperature, 95 °C; feed absolute pressure, 2.8 atm; sweep absolute pressure, 1.15 atm; sweep/feedwater flow rate, 0.05/0.03 mL/min.

°C, when feed side absolute pressure increased from 1.7 to 6.2 atm along with constant water flow rate at both sides (feed/ sweep, 0.03/0.04 cm3/min), the CO2 flux increased from 408 × 10−6 cm3 (STP)/(cm2 sec) to 689 × 10−6 cm3 (STP)/(cm2 sec) for blended amines of (15 wt % PEI + 25 wt % PEHA) composite membrane, which is higher than that of (15 wt % PEI + 25 wt % TEPA) composite membrane (Figure 8a). Similar observations for CO2 flux through a membrane containing other amine carriers have also been reported in the literature.38−41 The CO2 permeability and CO2/N2 selectivity were also decreased with increase in feed pressure for both the membranes (Figures 8b,c). At low feed gas pressure, the CO2 permeability is high because of contribution of CO2−amine reactions in the presence of the abundance of free amine groups. At higher pressure, the availability of free amine groups becomes less and the CO2 transport is dominated by solution− diffusion mechanism; hence, permeation decreases with increase in feed pressure.39 However, with increase in the feed pressure, N2 permeability mostly does not change throughout the membrane; hence, the CO2/N2 selectivity drops for both the membranes. 3.8. Effects of Sweep Side Water Flow Rate on Separation Performance. The effects of sweep side water flow rate (cubic centimeters per minute) on CO2 and N2 fluxes, permeabilities, and CO2/N2 selectivity were studied at 90 °C using cross-linked thin-film composite membrane containing blended amines (15 wt % PEI + 25 wt % PEHA) and were compared with the cross-linked-PVA-PVP doped with 15 wt % PEI and 25 wt % TEPA membrane.32 The water flow rates of the sweep side were varied from 0.02 to 0.075 cm3/min and those of the of feed side were maintained constant at 0.03 cm3/ min. Feed pressure was kept constant at 2.7, whereas sweep side pressure was maintained at 1.15 atm. Feed gas and carrier gas flow rates were the same as those mentioned before (see section 3.7). Figure 9a−c depicts the effect of sweep side water content on CO2 and N2 permeabilities and fluxes as well as CO2/N2 selectivity for both the membranes. As depicted in these figures, with an increase in the sweep side water flow, the CO2 flux and permeability as well as CO2/N2 selectivity initially increased rapidly and then became constant. The increase in mobility of the CO2−carriers complex occurring through the CO2−carrier reactions in the presence of water vapor might be the probable reason.2,19,27 However, N2 flux and permeability remain almost constant with an increase in sweep side water flow rate. When the sweep side water flow rate is increased from 0.02 to 0.075 cm3/min, CO2 permeability is increased from 545 barrer to as high as 1474 barrer for 15 wt % PEI with 25 wt % PEHA, which is higher than that reported in our previous work32 for blended amines membrane containing 15 wt % PEI with 25 wt % TEPA (Figure 9a). At 0.075 cm3/min

permeability is directly proportional to the membrane thickness.27,32 The N2 permeability is essentially constant with the increase in the membrane thickness, whereas CO2 permeability increases as the thickness is increased; hence, CO2/N2 selectivity goes up.27,36,37 At a constant active layer thickness of 55 μm for cross-linked-PVA-PVP doped with (15 wt % PEI + 25 wt % PEHA) and (15 wt % PEI + 25 wt % TEPA) membranes, the CO2 permeability was 1535 barrer and 1421 barrer, respectively, and CO2/N2 selectivity was 354 and 230, respectively (Figure 7b,c and Table 2). It can be observed from Figure 7a−d that along with a change in active layer thickness the CO2 flux, CO2 permeance, CO2 permeability, and CO2/N2 selectivity has been increased for (15 wt % PEI + 25 wt % PEHA) amine blend composition compared to (15 wt % PEI + 25 wt % TEPA) amine blend at a constant physical condition mentioned above (Table 2). Hence, the cross-linked-PVA-PVP blended with 15 wt % PEI and 25 wt % PEHA has been chosen for the detailed performance study (effect of pressure, temperature, and sweep side water flow rate) via permeation measurement. 3.7. Effects of Feed Pressure on Separation Performance. The effects of feed pressure on CO2 and N2 fluxes and permeabilities as well as CO2/N2 selectivity were studied at 100 °C using cross-linked thin-film composite membrane containing blended amines (15 wt % PEI + 25 wt % PEHA) with the active layer thickness of 40 μm and were compared with the cross-linked-PVA-PVP doped with 15 wt % PEI and 25 wt % TEPA membrane with the active layer thickness of 45 μm.32 Feed side absolute pressure was varied from 1.7 to 6.2 atm, while sweep side absolute pressure was maintained constant at around 1.15 atm for both membranes. The flow rates of the feed gas containing 20% and 80% N2 gas mixtures as well as carrier gas (Ar) were maintained at 30 cm3/min. Feed side water flow rate was maintained at 0.03 cm3/min, and sweep side water flow rate was kept constant at 0.04 cm3/min. The effect of feed pressure on CO2 and N2 fluxes and permeabilities as well as CO2/N2 selectivity for the composite membrane containing blended amines (15 wt % PEI + 25 wt % PEHA) and (15 wt % PEI + 25 wt % TEPA) are shown in panels a, b, and c of Figure 8, respectively. As depicted in this figure, at the low-pressure region the CO2 flux is increased rapidly, whereas it remained almost constant at higher pressure. Formation of a CO2−amine complex could be the probable reason for these phenomena. At higher feed pressure, a greater amount of CO2 is dissolved in the membrane through the formation of a CO2− amine complex; hence, the driving force for CO2 transport is enhanced. At a particular CO2 partial pressure, further enhancement of CO2 transport ceases because of carrier saturation and CO2 flux reaches a constant value. However, with an increase in feed pressure, N2 flux is increased linearly in accordance with the solution−diffusion mechanism.2 At 100 G

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Figure 9. (a) Effect of sweep side water flow rate on CO2 and N2 permeability: (■) CO2 and N2 permeability (barrer) for (15 wt % PEI + 25 wt % PEHA) blend amine membrane and (⧫) CO2 and N2 permeability (barrer) for (15 wt % PEI + 25 wt % TEPA) blend amine membrane.32 (b) Effect of sweep side water flow rate on CO2 and N2 flux: (■) CO2 and N2 flux for (15 wt % PEI + 25 wt % PEHA) blend amine membrane and (⧫) CO2 and N2 flux for (15 wt % PEI + 25 wt % TEPA) blend amine membrane.32 (c) Effect of sweep side water flow rate on CO2/N2 selectivity: (■) CO2/N2 selectivity for (15 wt % PEI + 25 wt % PEHA) blend amine membrane and (⧫) CO2/N2 selectivity for (15 wt % PEI + 25 wt % TEPA) blend amine membrane.32 Temperature, 90 °C; feed absolute pressure, 2.7 atm; feedwater flow rate, 0.03 mL/min.

Figure 8. (a) Effect of feed pressure on CO2 and N2 flux: (■) CO2 and N2 flux for (15 wt % PEI + 25 wt % PEHA) blend amine membrane and (⧫) CO2 and N2 flux for (15 wt % PEI + 25 wt % TEPA) blend amine membrane.32 (b) Effect of feed pressure on CO2 and N2 permeability (barrer): (■) CO2 and N2 permeability (barrer) for (15 wt % PEI + 25 wt % PEHA) blend amine membrane and (⧫) CO2 and N2 permeability (barrer) for (15 wt % PEI + 25 wt % TEPA) blend amine membrane.32 (c) Effect of feed pressure on CO2/N2 selectivity: (■) CO2/N2 selectivity for (15 wt % PEI + 25 wt % PEHA) blend amine membrane and (⧫) CO2/N2 selectivity for (15 wt % PEI + 25 wt % TEPA) blend amine membrane.32 Temperature, 100 °C; sweep/feed water flow rate, 0.03/0.04 mL/min.

sweep side water flow rate, the CO2 flux, N2 flux, and CO2/N2 selectivity reached 543 × 10−6 cm3 (STP)/(cm2 sec), 31.5 × 10−6 cm3 (STP)/(cm2 sec), and 409, respectively, for the membrane containing 25% PEHA, whereas the CO2 flux, N2 flux, and CO2/N2 selectivity reached 417 × 10−6 cm3 (STP)/ (cm 2 sec), 30 × 10 −6 cm3 (STP)/(cm2 sec) and 270, respectively, for the membrane containing 25% TEPA (Figure 9b,c).

3.9. Effects of Temperature on Separation Performance. The effects of temperature on CO2 and N2 fluxes and permeabilities and CO2/N2 selectivity were investigated and also compared with the cross-linked-PVA-PVP doped with 15 wt % PEI and 25 wt % TEPA membrane.32 The temperature was varied from 90 to 125 °C. Feed pressure was kept constant at 2.7, whereas sweep side pressure was maintained at 1.15 atm. H

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The feed and sweep side water flow rate as well as feed gas and carrier gas flow rates were the same as those mentioned in section 3.7. The effect of temperature on CO2 and N2 permeabilities and fluxes as well as CO2/N2 selectivity is shown in panels a, b, and c of Figure 10, respectively, for the membranes containing blended amines having composition of 15 wt % PEI with 25 wt

% PEHA and 25 wt % TEPA. With an increase in temperature, the CO2 permeability and flux as well as CO2/N2 selectivity decreased whereas N2 flux remained almost constant. When the temperature was 125 °C, the CO2 permeability, CO2 flux, N2 flux, and CO2/N2 selectivity reached 400 barrer, 186 × 10−6 cm3 (STP)/(cm2 sec), 34 × 10−6 cm3 (STP)/(cm2 sec), and 94, respectively, for the membrane containing 25 wt % PEHA, whereas the CO2 permeability, CO2 flux, N2 flux, and CO2/N2 selectivity reached 393 barrer, 162 × 10−6 cm3 (STP)/(cm2 sec), 35 × 10−6 cm3 (STP)/(cm2 sec), and 74, respectively, for the membrane containing 25 wt % TEPA (Figures 10a−c). The reduction of water retention at higher temperature might be the probable reason. This in turn affects the mobility and reaction rate of CO2 with carriers, resulting in the decrease in permeability for both membranes. Our permeation experimental data was compared with that of other polymeric membranes, cross-linked-PVA membrane without carriers, and also with our previous work27,32 by the famous upper-bound relationship proposed by Robeson.7 The data of CO2 permeability (barrer) and CO2/N2 selectivity was calculated at a temperature of around 90 °C against 2.7 atm feed absolute pressure with 0.03/0.04 mL/min feed/sweep side water flow rate. CO2 permeability and CO2/N2 selectivity data of the other polymers were taken from Robeson and our previous work,7,27 and the upper bound relationship is shown in Figure 11 and Table 3. The cross-linked PVA with blended

Figure 11. Upper-bound relationship of CO2 permeability and CO2/ N2 selectivity of different polymeric membranes.9,32 See Table 3 for description of membranes 1−11.

amines (15 wt % PEI + 25 wt % PEHA) and (15 wt % PEI + 25 wt % TEPA) composite membrane was found to have the highest CO2 permeability along with the highest CO2/N2 selectivity, which is higher than the upper bound (Figure 11). Flue gas usually contains a large amount of impurities, such as O2, NOx, and SOx apart from CO2. These impurities degrade the amines and are detrimental for the operation of the conventional absorption-based CO2 capture plant.42 To prevent degradation of amines, they are removed before the flue gas flow into the CO2 recovery unit (absorber and stripper units). The aim of this study is to retrofit the existing absorber and stripper units by a membrane separation unit for CO2 separation from flue gas. The thin selective layer of the

Figure 10. (a) Effect of temperature on CO2 and N2 permeability: (■) CO2 and N2 permeability (barrer) for (15 wt % PEI + 25 wt % PEHA) blend amine membrane and (⧫) CO2 and N2 permeability (barrer) for (15 wt % PEI + 25 wt % TEPA) blend amine membrane.32 (b) Effect of temperature on CO2 and N2 flux: (■) CO2 and N2 flux for (15 wt % PEI + 25 wt % PEHA) blend amine membrane and (⧫) CO2 and N2 flux for (15 wt % PEI + 25 wt % TEPA) blend amine membrane.32 (c) Effect of temperature on CO2/N2 selectivity: (■) CO2/N2 selectivity for (15 wt % PEI + 25 wt % PEHA) blend amine membrane and (⧫) CO2/N2 selectivity for (15 wt % PEI + 25 wt % TEPA) blend amine membrane.32 Feed absolute pressure, 2.7 atm; feed/sweep water rate, 0.03/0.04 mL/min. I

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Table 3. CO2 Permeability and CO2/N2 Selectivity of Different Polymers9,32 membrane number in Figure 11 1 2 3 4 5 6 7 8 9 (previous work) 10 (previous work) 11 (25% PEHA + 15% PEI)

CO2 permeability (barrer)

polymer pure PVA without carrier cross-linked-PVA without carrier at 95 °C, 1.65 atm pressure difference and 0.03/0.05 (feed/sweep) water flow rate with 43.8 μm active layer thickness poly[bis(2-(2-methoxyethoxy)ethoxy) phosphazene] PIM-7 modified poly(dimethylsiloxane) PIM-1 poly(trimethylgermylpropyne) poly(trimethylsilylpropyne) cross-linked PVA with blended amines (PAA+AHPD) composite membrane cross-linked poly(vinyl alcohol) membrane blended with polyvinylpyrrolidone/polyethylenimine/ tetraethylenepentamine cross-linked poly(vinyl alcohol) membrane blended with polyvinylpyrrolidone/polyethylenimine/ pentaethylenehexamine

■

membranes contains a small amount of amines and makes such membranes quite prone to early degradation due to these impurities. Hence, gas processing prior to CO2/N2 separation might be required in the case of membrane separation unit. The productivity of the membrane can be improved by using a thinner membrane. Development of a defect-free, thin, selective layer (