Eun-Sung Jo ,Xinghai An ,Pravin G.Ingole ,Won-Kil Choi,Yeong-Sung Park ,Hyung-Keun Lee ,*

1 Korea Institute of Energy Research(KIER),71-2 Jang-dong,Yuseong-gu,Daejeon 305343,Republic of Korea

2 Department of Environmental Engineering,Daejeon University,Daejeon,Republic of Korea

3 Department of Advanced Energy and Technology,University of Science and Technology(UST),217 Gajeong-ro,Yuseong-gu,Daejeon 305-350,Republic of Korea

1.Introduction

Membrane gas separation is a developing technology since it has many promising industrial applications like H2/CO separations,SO2/CO2/N2separation,and natural gas promotion[1,2].Especially in CO2capture,membrane processes have attracted great attention due to several merits such as low cost,high energy efficiency,environmental sustainability and simple scaling up[3].According to most researches and studies,polymeric membrane separation is still restricted by the well-known trade-off trend between gas permeability and selectivity[4].Many researches have been carried out to achieve desired separation performance using polymeric membrane thereby meeting technoeconomic requirement of CO2capture[5–8].Our recent pilot plant study[9]showed that CO2removal efficiency and CH4concentration in retentate are increased by post-treatment.Additionally,feed flow rate also has an influence on CO2removal efficiency owing to driving force change.The permeability of a specific gas through the membrane is the product of its diffusivity and solubility.The contribution of diffusivity to permeability is typically small so that a significant increase in permeability insinuates the improvement in solubility[10].

The separation of CO2from CH4is of main importance in numerous industrial processes such as biogas upgrading,natural gas sweetening,and land fill gas purification[11].The use of sustainable and renewable energy resource is required in a view of the fact that of environmental pollution and global warming owing to the emission of greenhouse gases.Several industrial technologies typically engaged for this separation intention containing chemical absorption,pressure swing adsorption(PSA)and the main is membrane separation[12].Membrane separation technology has materialized and concerned rising interest in CO2separation and capture research during the last two decades,owing to its high energy efficiency,simplicity in design and module manufacture and environmental compatibility[2].Diverse membrane categories include polymeric membranes,inorganic membranes,and mixed matrix membranes for the CO2/CH4separation that are available.Herewith in this work we used the polymeric membranes for CO2/CH4mixture gas separation.

One of the most widely investigated glassy polymeric membrane materials for CO2/CH4separation is polysulfone(PSf).Pure-and mixed-gas permeation properties using PSf have been extensively explored for gas separation because of its low price,chemical stability,and mechanical strength[13].It is well known that the polymeric membranes mechanical performance engaged the deformation of a material underneath the influence of an applied force.In the separation process using membrane,the membrane considered as a permselective barrier between two phases and membrane is at the heart of a process.For example,transport through the PSf membrane will take place as a result of the driving force acting on the individual components in the feed[14,15].The gas substances transport transversely through the membrane either by pressure or by concentration difference to obtain a higher permeance,excess pressure will frequently apply to the upstream of the membrane.Herewith present experiments,applied pressure is up to 5 bar and there is no effect on the mechanical properties of the PSf hollow fiber membrane.Compared with CA,PSf has lower CO2permeability and CO2/CH4selectivity but higher plasticization pressure[16].Plasticization pressure has become an important factor in practice due to its effect on membrane selectivity.Membrane with higher plasticization pressure can maintain the selectivity better than that with lower plasticization pressure under high CO2feed concentration or high operation pressure[17].Although various types of PSf-based membranes for CO2/CH4separation have been developed,industrially available membranes are limited.Many studies have been done to overcome those difficulties by changing membrane manufacturing process,formulation,and material modification[18].

Interfacial polymerization(IP)has become an entrench method for the preparation of thin film composite(TFC)membranes for diverse applications[19–21],like ultrathin responsive and enantioselective films[22,23].The major advantages of the IP process involve the formation of an ultrathin active thin film[24],the minimization of macro void fault[25],and the tunable functional groups[26].Many researches have been focused on the preparation of TFC membranes for gas separation using IP technique,and the results showed that IP is also an effective way for the synthesis of gas separation membranes[27].Yunet al.[28]studied the effects of polymerization conditions on membrane structure and performance.

In the view of abovementioned advantages of TFC membranes,we aimed to design and explore TFC hollow fiber membranes(HFM)for the application of CO2/CH4mixture gas separation.To our best knowledge,it is a pioneering work to fabricate polymeric inner-selective TFC hollow fiber membranes in this field.The effects of interfacial polymerization conditions on single gas as well as CO2/CH4mixture gas separation performance of TFC HFM were studied.

The most suitable procedure for TFC HFM fabrication has been identified based on mentioned study.The outcome ofthis study may provide useful insights towards the design of inner-selective PSf-based TFC HFM for next generation of gas separation.

2.Experimental

2.1.Materials

The polysulfone(PSf)ultrafiltration membranes with an average molecular weight cut-off of 6000,were supplied by Guiyang Shidai Huitong Film Technology Co.Ltd.(China).The coating materials used in interfacial polymerization process,1,3–cyclohexanebis–methylamine(CHMA,purity＞99.5%),and trimesoyl chloride(TMC,purity＞99.5%)were purchased from Sigma-Aldrich.N-hexane(99.9%,Fisher Scientific,NJ)was purchased and used as a solvent for TMC.All chemical reagents used for this study were used as received.Deionized(DI)water from Milli-Q ultrapure water purification system(Millipore)was used as aqueous phase solvent.Pure CO2(99.5%,SAFETY GAS,KOREA)and CH4(99.9%,SAFETY GAS,KOREA)were used as carrier gas.The balanced(mixed)gas in a ratio of 30:70 was used.

2.2.Characterization of composite hollow fiber membranes

The chemical characterization of composite hollow fiber membranes was accomplished by ATR-FTIR spectra(Bruker)at 600–4000 cm?1.The surface morphology of fiber surface and cross-section was examined by Scanning Electron Microscopy(SEM,S-4700,HITACHI).The completely dried sample fibers were moisturized with distilled water and fractured by liquid nitrogen.The surface topography of membranes before and after interfacial polymerization was studied by atomic force microscope/Surface Probe Microscope(Nanoman AFM system,Veeco)in tapping mode.Small strip of membranes were placed on a specific sample holder and 3 μm × 3 μm areas were scanned.Mean roughness(Ra),root mean squareZvalues(Rms),and maximum vertical distance between the highest and lowest data points(Rmax)were used to quantify the surface topology of membranes.The mean roughness was specifically used to quantify the difference between various membrane surfaces and was determined by the averages of at least 5 sections of several fibers.

2.3.Preparation of composite membrane by interfacial polymerization

The thin film composite(TFC)membrane was prepared by coating a selective layer in situ on the inside surface of PSf hollow fiber membrane via interfacial polymerization(IP)between organic and aqueous phase monomers as shown in Fig.1.1,3-cyclohexanebis methylamine(CHMA)was selected as the aqueous phase monomer and trimesoyl chloride(TMC)was selected as the organic phase monomer.The operating conditions for fabricating TFC membranes were classified and summarized.By using an inside coating apparatus,the PSf hollow fiber membrane module was firstly equipped in the unit.After that,aqueous solution containing 1,3-cyclohexanebis methylamine(CHMA)was continuously circulated for 10 min at constant flow rate followed by purging for 3 min atconstant flow rate using N2gas to remove excess solution.Then the hexane solution containing TMC in desired concentration was circulated for 5 min at constant flow rate followed by N2purging.The polymerization reaction occurs inside PSf hollow fiber membrane resulting in formation of an ultrathin layer of cross-linked co-polyamide.The composite membrane so obtained was cured in hot air circulation at 70 °C for 10 min in an oven(oven accuracy:0.1 °C)whereby the polymer layer attains chemical stability[29].Table 1 discloses the compositions of aqueous and organic phase solutions,and reaction times used for interfacial polymerization as well.

Fig.1.Reaction scheme by using CHMA and TMC for the preparation thin film composite(TFC)membrane(inside coating)by interfacial polymerization method.

Table 1Composition of aqueous and organic solutions and reaction times used for the preparation of selective layer

2.4.Pure gas permeation

The experimental set-ups for pure gas permeation was illustrated in Fig.2.The experimental conditions are summarized in Table 2.To determine the pure gas permeance,the module was fed with CO2(99.5%,SAFETY GAS,KOREA)and CH4(99.99%,SAFETY GAS,KOREA)towards tube side of membranes,respectively.The permeate side was maintained at atmospheric pressure,and the direction of permeate flow was controlled in counter-current mode for maximum efficiency.Both retentate and permeate flow rates were measured with bubble flow meters with a range of 0.1–0.5 MPa in pressure(pressure gauge error:±0.8%)and 293.15–323.15 K in temperature.

The permeance is described by the following equation:

whereQPis the permeate flow rate through the membrane,△Pis the gas pressure difference across the membrane,andAis the effective membrane area.The unit of permeance is expressed as mol·m?2·s?1·Pa?1or cm3(STP)·cm?2·cmHg?1·s?1in the SI system.However,the more widely used and accepted expression for permeance is gas permeation units(GPU),where 1 GPU=1 × 10?6cm3(STP)·cm?2·cmHg?1·s?1[30].

The ideal selectivity is the ratio of the pure gas permeances as follows:

2.5.Mixed gas permeation

The CO2/CH4mixture gas separation system was illustrated in Fig.2.Experimental conditions and mixture gas compositions are listed in Table 2.The CO2/CH4mixture in a ratio of 70:30 as balanced gas was used.The effects of various operating variables(e.g.,pressure,temperature,and stage cut)were tested.In particular,pressure and stage-cut were controlled by retentate side via a back pressure regulator installed at the retentate side.The operating temperature was kept constant by air circulation inside an oven for balancing with hollow fiber module and feed gas.The permeate side was maintained at atmospheric pressure.The permeate and retentate gas flow rates were measured by bubble flow meters.The compositions of the retentate and permeate streams were determined by a CO2/CH4analyzer(Madur,model:maMoS-400).All measurements were recorded when the system reached its steady state.

The mixture gas permeance can be determined by the following equation[31]

Table 2Experimental condition for single gas and mixture gas separation

wherePiis permeance of component gasi(GPU)in the mixture,Vis total permeate flow rate(cm3·s?1)of componenti,pFandpPare the feed and permeate pressure(kPa),xis logarithmic mean of feed and retentate compositions,and accordinglyxiF,xiRare feed and retentate compositions of componenti.

Since gas mixtures show non-ideal gas effects,individual permeances were calculated using fugacity instead of partial pressure at feed side[32,33].Thus,the permeance in the mixture can be rewritten as follows:

Fig.2.Schematic diagram of gas permeation experiment apparatus.

whereQPis permeate flow rate(cm3·s?1),xiis the permeate side composition of componenti,fFis the fugacity of each component at feed side.yiandziare the composition of componentiat retentate and feed sides,respectively.Fugacity of each gas approaches the ideal behavior in the limit of low pressure because it is related with standard chemical potential differences for different gases arisen solely from internal molecular structures.The Peng–Robinson equation is used for calculating the fugacity coefficients of each component in the mixture since it supports the widest range of operating conditions and the greatest variety of the systems.This equation is easy to use and it often accurately represents the correlation among temperature,pressure,and phase compositions in binary and multi-component systems[25].It is also reported that Peng–Robinson equation yields a good estimate for the non-polar gas and slightly polar gas[34].

The stage-cut is an important factor to determine separation performance of the mixture gas,and it is expressed as follows:

The feed flow rate(ml·min?1)is summation of permeate and retentate flow rates.Therefore,the feed flow rate was controlled by the retentate flow rate.The separation factor for mixture gas is defined as follows:

where π indicates relative change of compositions between feed and permeate sides,CiandCjare compositions of componentiandj,respectively.F and P represent feed and permeate side.

3.Results and Discussions

3.1.ATR-FTIR analysis

The IR spectrum of composite membranes is listed in Fig.3(prepared by CHMA-TMC monomers).It shows a strong amide-I band at 1648 cm?1which is the characteristic of the C=O stretching vibrations of the amide group.A strong characteristic amide-II band,which occurs from the couplings of in-plane N–H bending and C–N stretching vibrations of the C–N–H group,is observed at 1546 cm?1.These two bands(amide-I and amide-II)are characteristics for amides because of their constant position and strong intensities.The amide-II band splits into a multiplet,with peak positions at 1576,1550,and 1545 cm?1.The split in amide-II band is caused by the difference in the dipoles of C–N bond of C(=O)–N–H and C(=O)–N– groups.It is well known that the ν(C=O)frequency shifts with the functional group that bonds directly to the carbon atom.Electron withdrawing substituent's cause an electrostatic stabilization of the C=O group and a shift of the C=O frequency to higher values,while electron donating substituent's destabilize the C=O group.

Fig.3.ATR-FTIR of PSf and thin film composite membranes(a)PD1,(b)PD2,(c)PD3 and(d)PD4 prepared at different conditions.

3.2.SEM analysis

The morphology of composite membranes was observed in scanning electron microscope.Fig.4 displays the effect of monomer concentration on thickness of TFC membrane.The fig.4(a–d)showed the cross section images of TFC membrane along with surface images shown in Fig.4(e–h)of membrane prepared with different CHMA concentrations,and with different TMC concentrations.The surface thickness depends on the monomer concentration and degree of cross-linking.TFC membrane formed when aqueous phase monomer reacts with organic phase monomer.Therefore,increased aqueous phase monomer contributed increasing surface thickness by diffuse with organic phase monomer[35].According to the studies about the effect of organic phase monomer on gas permeance[35–37],the organic phase also increased thickness as the monomer concentration increased as shown in Fig.4.

3.3.AFM image

Fig.5 presents the AFM 3D images of the top surfaces of the TFC membranes.The bar at the bottom of each image indicates the vertical deviations in the sample with the white regions being the highest and the black regions the lowest.The values of mean roughness(Ra),were obtained based on a 3.0 μm × 3.0 μm scan area.The average roughness values have been obtained from depth pro file of the membrane sample area of 3.0 μm × 3.0 μm.The root-mean-square roughness(Rms)of PSf membrane and the TFC membranes(PD1,PD2,PD3,PD4)samples are obtained in between(4.9±0.7)nm and(75.9±2.5)nm,suggesting more rough surface could be generated after the interfacial polymerization of CHMA with TMC.The roughness might be resulted from the fluctuation on the polyamide-organic solvent interface[38].Recently,we found the roughness could be also affected by the competition between the reactions of TMC with amine group of aromatic ring on the film surface[39].

3.4.Effect of aqueous phase concentration and organic phase concentration on CO2/CH4 permeance and selectivity

Experimentally determined values of skin layer thickness,CO2permeance,CH4permeance,and CO2/CH4selectivity for TFC membranes according to different CHMA concentrations are plotted in Figs.6,7 and 8 respectively.Observed skin layer thickness varies slightly around 140 nm in CHMA concentration range of 0.5%–1.0%.In addition,with CHMA concentration increasing,observed extent of crosslinking increases continuously.Besides,as shown in Fig.6,with CHMA concentration increasing,CO2permeance increases whereas CH4permeance increases slowly.Moreover,CO2/CH4selectivity is found to be increased with an increase in CHMA concentration.CO2permeance is less correlated with skin layer thickness(negatively)and crosslinking(positively)while CH4permeance correlates more with skin layer thickness(negatively)and but less with crosslinking(negatively).The relationship between structural properties and performance for TFC membranes prepared with different CHMA concentrations can be explained as follows.Our previous study has proved that the monomers in both aqueous and organic phases play important roles in determining skin layer thickness of TFC membranes[14].However,TMC is fixed in this experiment,which drives IP process to be controlled by the diffusion of CHMA through the aqueous phase into the reaction region.Hence,skin layer thickness is determined by CHMA concentration rather than TMC concentration[14].When the CHMA concentration in aqueous phase is very low(0.5%),there is not enough CHMA to form an integrated network structure[40].Meanwhile,the excess acyl chloride groups are hydrolyzed to form a loose structure with a very low crosslinking extent[40].As the CHMA concentration is further increased,nearly fully crosslinked skin layer gradually forms,contributing to the further increase of the crosslinking extent.Fig.4 shows the representative SEM images of membranes prepared with various CHMA and TMC concentrations.By visual inspection,skin layer thicknesses increase rapidly in high concentration range from 0.5%to 1.0%.Besides,observed extent of crosslinking correlates with TMC concentration moderately in the lower range but strongly in the higher range.These relationships suggest that TMC concentration in organic phase governs skin layer thickness of TFC membrane in the whole range of TMC concentration.Higher TMC concentration facilitates the formation of crosslinked skin layer in smaller TMC concentration range,and vice versa.At a low TMC concentration(0.5%),the reaction rate is very low,which produces limited polymers[41].With increasing TMC concentration from 0.5%to 1.0%,the rate of polymerization reaction becomes higher,which produces a thicker skin layer under the same reaction time[42].Generally,TMC molecules participate in two competitive processes,i.e.polymerization and oxidation.At a low TMC concentration(0.5%),the polymerization reaction rate is low,leading to the oxidation of a large proportion of TMC and yield of–COOH groups.Therefore,a lower crosslinking extent is obtained at a lower TMC concentration.With increasing TMC concentration,the polymerization reaction rate is largely accelerated[43],which results in a larger proportion of TMC reacting with CHMA to gain a more crosslinked polymer.The selectivity for TFC membranes prepared with different CHMA and TMC concentrations are presented in Fig.8.

Fig.4.SEM images of thin film composite membrane(a)PD1,(b)PD2,(c)PD3 and(d)PD4 are the cross section images and(e)PD1,(f)PD2,(g)PD3 and(h)PD4 are the surface images prepared at different conditions.

The mixture gas separation results are shown in Fig.9.Fig.9(a)and(b)present the permeance of CO2and CH4in mixture gas.In the mixture gas,where CO2is permeated more preferentially than CH4through all TFC membranes,since CO2has high quadrupole moment while methane is nonpolar.With the stronger electrostatic interactions between CO2and the membrane,the CH4permeance becomes lower in the mixture system.Moreover,a comparison ofFig.9(a)and(b)indicates that in the system of CO2/CH4mixture,the CO2permeances through the composite membranes are remarkably higher than that of CH4.Fig.9(c)shows the CO2/CH4mixture gas selectivities through all TFC membranes that decrease with increasing stage cut.Besides,the selectivity of PD1 is significantly higher than that of all TFC membranes,but it incessantly decreases while increasing stage cut,compare to other TFC membranes.For example,atstage cut0.1,the CO2/CH4selectivity of PD1 is up to 28,which is obviously higher than that of all TFC membranes.Given that many industrial membrane based separation applications are performed in low stage cut region for to obtain outstanding selectivity.As a result,the TFC membrane PD1 can enhance selectivity for CO2/CH4,especially in the low stage cut region.All these results phenomena proved that the material concentrations used to prepare TFC PD1 shows significantly positive impact on the CO2capacity and CO2/CH4selectivity of the composite membrane.

Fig.5.AFM images of thin film composite membranes(a)PD1,(b)PD2,(c)PD3 and(d)PD4 prepared at different conditions.

Fig.6.Comparison of CO2 permeance on various concentrations at 30°C.

Fig.7.Comparison of CH4 permeance on various concentrations at 30°C.

Fig.8.Comparison of CO2/CH4 selectivity on various concentrations at 30°C.

3.5.Effect of pressure on CO2/CH4 permeance and selectivity

The impact of operating pressure on the gas permeability at five different pressures(0.1 to 0.5 MPa)are shown in Fig.10.As it is observed,permeability of carbon dioxide and methane through all TFC membranes does not significantly change with trans-membrane pressure difference.For both CO2and CH4,it can be interpreted by the fact that the permeability is related to two determinant factors,namely diffusion and solution.Representative CO2/CH4permeance of TFC membranes prepared with different concentrations of aqueous and organic solvents are plotted in Fig.10.With increasing feed pressure,the concentration of CO2in membrane phase increases.Thus,some carriers may be tiedup with the CO2molecules and can combine with excess CO2,resulting in an increase in CO2permeance with increase pressure.However,CO2permeance of TFC membranes prepared with different concentration of monomer varies at a higher pressure range from 0.1 to 0.5 MPa(Fig.10)since the carriers in the membranes are saturated with CO2to their maximum capacity under high pressure[44].

3.6.Effect of temperature on CO2/CH4 permeance and selectivity

Fig.11 present permeance of CO2and CH4through TFC membranes(PD1)prepared in this study according to different temperatures.As temperature increases,gas diffusivity through TFC membranes increases,causing the increase in permeance.Permeance of CH4is more sensitive to temperature through all TFC membranes than that of CO2permeance.As shown in Fig.11,permeance of CO2and CH4are increased by increasing temperature for all TFC membranes studied in our research,as explained above,the permeance of both gases through TFC membranes increase with increasing operating temperature,causing the CO2/CH4selectivity to decrease.

Fig.9.Comparison of(a)CO2 permeance,(b)CH4 permeance and(c)CO2/CH4 selectivity in mixture gas on various concentrations at 30°C.

Fig.10.Effect of pressure on permeance and selectivity for PD1,PD2,PD3 and PD4 thin film composite membranes.

3.7.Comparison with other membranes

The performance of TFC membranes obtained in this work and others reported elsewhere are listed in Table 3[45–50].The TFC-PD1 membrane prepared with 1.0%CHMA and 0.5%TMC showed the best selectivity of 28 whereas TFC-PD2 membrane prepared with 1.0%CHMA and 1.0%TMC showed the best CO2permeance of 37.Moreover,as shown in Table 3,the CO2permeance,CH4permeance and CO2/CH4selectivity of the state-of-the-art polymeric membranes with high performance by physical separation mechanisms in laboratory level(entry numbers 1–6 in Table 3)are close to the trade-off bound.As comparison,CO2permeances of TFC membranes in this work are higher than those by physical separation.TFC membranes with further improved performance can be obtained by the selection of other aqueous monomers along with preparative conditions based on the findings of this work.

4.Conclusions

Four types of TFC membranes have been prepared and investigated to find optimum performance.The results showed that partial pressure of either diffusing gas through supported TFC membranes does not have any effect on permeability.Moreover,permeability of gases increases with increasing temperature.TFC membranes for CO2/CH4separation were prepared using CHMA and TMC by interfacial polymerization on PSf support membrane.The relationships among conditions of skin layer formation,skin layer structure,and membrane separation performance were investigated.Generally,higher CHMA concentration in the aqueous solvent could produce thicker,and less crosslinked skin layers,while lower CHMA concentration in the aqueous solvent could produce thinner,and more crosslinked skin layers.TMC concentration in organic phase determines skin layer thickness,whereas CHMA concentration in aqueous phase governs the crosslinking extent of skin layer.Overall,under the circumstances of forming an integrated skin layer,membranes with high CO2permeance and high CO2/CH4selectivity could be obtained at equimolar concentrations and decreased TMC concentration,respectively.The above culminations have great academic consequence for the controlled preparation of gas separation membranes.

Table 3Comparison of CO2/CH4 separation performance of the membrane obtained in this work with other membranes

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