CO2 Absorption in Polytetrafluoroethylene Membrane Microstructured

Article pubs.acs.org/IECR

CO2 Absorption in Polytetrafluoroethylene Membrane Microstructured Contactor Using Aqueous Solutions of Amines A. Constantinou, S. Barrass, and A. Gavriilidis* Department of Chemical Engineering, University College London, Torrington Place, London, WC1E 7JE, U.K. ABSTRACT: Absorption of carbon dioxide (CO2) in aqueous solutions of monoethanolamine (MEA) and diethanolamine (DEA) was performed in a flat membrane microstructured contactor. The contactor had dimensions 192 mm × 97 mm (length × width). Liquid and gas flowed in channels of 0.2 mm and 0.85 mm depth, respectively, separated by a supported polytetrafluoroethylene (PTFE) membrane of 20 μm thickness containing 0.5−5 μm openings. The function of the membrane was to bring into direct contact the two phases (gas and liquid) without dispersing one phase into the other. Experiments were conducted with 1.6 M MEA and 1.6 M DEA solutions and 20 vol% CO2/N2 inlet concentration, with a fixed inlet molar ratio CO2/amine of 0.5 at room temperature. Substantial CO2 absorption was observed for gas residence time below 0.2 s. A mathematical model with no adjustable parameters was used to simulate the contactor, and experimental results were compared to model predictions in terms of CO2 removal efficiency. The model showed satisfactory agreement with experimental data. Both model and experimental results showed that MEA solution absorbed more CO2 than DEA. CO2 removal increased by increasing the contact area between gas and liquid, using an 8-channel PTFE membrane microstructured contactor. absorbents provided higher absorption flux in the order of MEA/AMP > MEA/DEA > DEA/AMP. Yang et al.11 studied the effects of SO2 on CO2 capture using a hollow fiber membrane contactor in which an aqueous solution of monoethanolamine was used as an absorbent. Experimental results showed that MEA loss per ton captured CO2 increased due to the addition of SO2, resulting in a sharp decrease of CO2 removal and mass transfer rate after several days of operation. Ho et al.12 investigated experimentally and theoretically the absorption of CO2 through a hollow fiber contactor using pure water as an absorbent. They showed that CO2 absorption efficiency in the gas−liquid membrane contactor increased by increasing the liquid absorbent flow rate or decreasing the gas feed flow rate. Lv et al.13 examined the wetting of a polypropylene hollow fiber membrane contactor which leads in the deterioration of CO2 absorption performance during the membrane gas absorption process, using monoethanolamine, methyldiethanolamine, and deionized water as absorbent solutions. They indicated that improving membrane surface hydrophobicity might be an effective way of overcoming wetting problems. Membrane microstructured contactors are the microengineered analogue of membrane contactors and can be used for CO2 capture. They can provide benefits in reducing process development time, saving energy, minimizing environmental impact, and improving process performance. We have previously conducted experimental and theoretical studies using a metallic mesh microreactor to absorb CO2 in sodium hydroxide solution.14 Significant absorption was observed with gas residence times below 1 s. Moreover, model prediction

1. INTRODUCTION Aqueous solutions of alkanolamines are employed for (a) acid gases removal during natural gas sweetening and (b) CO2 capture from power plants and some other important industries such as chemical and petrochemical, steel, and cement production.1 Industrially, more commonly used alkanolamines are monoethanolamine (MEA), diethanolamine (DEA), diisopropalamine (DIPA), N-methyldiethanolamine (MDEA), and 2-amino-2-methyl-1-propanol (AMP).2 The choice of absorbent is mainly based on the absorption capacity, reaction kinetics, and regenerative potential. CO2 is a major greenhouse gas and thus its capture by various techniques has been a research focus in recent years. Conventional techniques, such as packed column absorption3−5 are energy-consuming6 and not easy to operate because of flooding and foaming problems. Hollow fiber membrane contactors are a promising technique for capturing CO27 and allow mass transfer between gas and liquid phases without dispersion of one phase into the other. The efficiency of membrane contactors for CO2 capture has been extensively studied. Kim and Yang8 used polytetrafluoroethylene (PTFE) microporous membrane contactors to capture CO2 from a CO2−N2 mixture with different aqueous amine solutions including 2-amino-2-methyl-l-propanol (AMP), monoethanolamine (MEA), and methyldiethanolamine (MDEA). It was shown that AMP exhibited a high absorption capacity and moderate absorption rate without the high energy requirement of MEA. Paul et al.9 theoretically studied CO2 absorption by different single and blended alkanolamine solvents using a flat sheet membrane contactor. They showed that an aqueous solution of MEA had the highest absorption flux for CO2 among the single amine solutions. Furthermore, by increasing the concentration of MEA and DEA in the blends, the absorption flux of CO2 was increased. Rongwong et al.10 investigated CO2 absorption capability and membrane wetting by single and mixed amine solutions. It was found that the absorption performance was highest for MEA. Mixed amine © 2014 American Chemical Society

Special Issue: Massimo Morbidelli Festschrift Received: Revised: Accepted: Published: 9236

October 13, 2013 February 2, 2014 February 3, 2014 February 18, 2014 dx.doi.org/10.1021/ie403444t | Ind. Eng. Chem. Res. 2014, 53, 9236−9242

Industrial & Engineering Chemistry Research

Article

indicated that the carbon dioxide was consumed within few micrometres from the gas−liquid interface, and the dominant resistance for mass transfer was located in the mesh. We also examined the performance of a silicon nitride mesh contactor for CO2 capture when NaOH and diethanolamine were used as absorbents.15 CO2 removal efficiency was higher when NaOH was used as absorbent rather than DEA. The silicon nitride mesh contactor had the best performance, as compared with other flat membrane contactors. TeGrotenhuis et al.16 studied CO2 absorption in a microchannel polymeric membrane device using diethanolamine solution as an absorbent liquid. They showed that more than 30% of CO2 from a 25 vol % CO2 stream was absorbed in ca. 3 s residence time in 20% diethanolamine solution. In this work, CO2 absorption in solutions of monoethanolamine (MEA) and diethanolamine (DEA) was performed in a membrane PTFE microstructured contactor. Various conditions such as gas flow rates, membrane contact area between the gas and the liquid, different amine solutions, and concentrations were investigated.

2. REACTOR DESIGN AND EXPERIMENTAL CONDITIONS The contactor used in this work consisted of a microstructured membrane placed between two 18 mm thick acrylic plates (S.I.M, UK) with inlet and outlet ports for the fluids. Channels with 0.2 mm and 0.85 mm depth were machined in the acrylic plates forming the areas where liquid and gas flow respectively. The reactor measured 192 mm × 97 mm (Figure 1). Two Viton gaskets 0.5 mm thick (Altec, UK) were placed in 0.4 mm deep grooves machined in the acrylic plates to provide the sealing. The membrane (Figure 2) was made from pure PTFE (Teflon) (Sterlitech, US) laminated onto a polypropylene layer. The pure PTFE was 20 μm thick and contained openings approximately 0.5−5 μm wide as observed by scanning electron microscopy (SEM). The polypropylene layer was 80 μm thick and consisted of holes in an approximately rectangular shape with dimensions of 0.8 × 0.324 mm (Figure 2). The porous area of the membrane where the two fluids came in contact was 5.48 mm × 90 mm. Membrane porosity was ca. 70%. Two pin holes in both plates were used for alignment, while 16 screws were employed for clamping all components together. A picture of the assembled contactor and a schematic of all components of the contactor are shown in Figure 1. To investigate the influence of the gas/liquid contact area, experiments were performed with an 8-channel contactor using the PTFE membrane (Figure 1). More information about the 8-channel contactor can be found elsewhere.14 It had approximately 11.3 times larger contact area (gas/liquid contact area 55.9 cm2), than the single channel contactor (gas/liquid contact area 4.9 cm2). An HPLC pump (Waters 5100) was used to drive the liquid solution in the bottom chamber of the contactor, which contained an aqueous amine solution of 10 wt % MEA or 16.6 wt % DEA. These concentrations were chosen to compare equal molarities of DEA and MEA, which in our case corresponded to 1.62 M. The gas 20 vol % CO2/N2 was controlled by a mass flow controller (Brooks 5850) and flowed above the membrane. The pressure difference between the two phases was controlled at the outlet of the liquid phase by a metering valve (Swagelok). The gas and liquid phase pressures were measured by a digital manometer (Comark; pressure range 0−30 psi). To avoid any liquid getting into the gas

Figure 1. Membrane microstructured contactors: (a) picture of assembled device of the PTFE single channel contactor; (b) picture of the assembled device of the 8-channel PTFE contactor; (c) exploded schematic view of the single channel contactor.

chromatograph (GC) in case of breakthrough of the liquid in the gas, the gas phase outlet passed through a liquid trap. It was then directed to a GC (Shimadzu GC-14B) for carbon dioxide analysis. Experimental data were obtained varying the liquid flow rate in the range 1.66−2.56 mL/min and gas flow rate in the range 160−247 mL/min. These flow rates resulted in residence times (based on the gas/liquid volumes in contact with the membrane area) of 0.102−0.157 s for the gas and 2.31−3.54 s for the liquid, respectively. All the experiments were carried out at room temperature (approximately 20 °C). The CO2 removal efficiency was calculated from 9237

dx.doi.org/10.1021/ie403444t | Ind. Eng. Chem. Res. 2014, 53, 9236−9242


Article

The differential mass balances to describe the concentration profiles of components in the three domains are given below. Mass Balance in the Liquid Phase. The differential mass balance of components in the liquid phase along with the associated boundary conditions are uL

uL

L ∂CCO 2

∂z

L ⎞ ⎛ ∂ 2C L ∂ 2CCO CO2 L ⎜ 2 ⎟ + R CO = DCO + 2 ⎟ 2 2⎜ 2 ∂ ∂ z x ⎝ L ⎠

(6)

L L ⎞ ⎛ ∂ 2C L ∂CAmi ∂ 2CAmi L Ami ⎟ + RAmi ⎜ = DAmi + 2 ∂z ∂x L2 ⎠ ⎝ ∂z

(7)

L L CCO = 0, CAmi = CAmi,in 2

at z = 0,

∂CCO2

at z = L ,

= 0,

∂z

(8)

∂CAmi =0 ∂z

(9)

at x L = δ L , L

L M CCO = mCCO , 2 2

M

∂CCO2 ∂CCO2 ∂CAmi L M = DCO = 0, DCO 2 2 ∂xM ∂x L ∂xL (10)

(which satisfies the flux continuity at the interface) ∂CCO2 ∂CAmi =0 = 0, ∂x L ∂x L

at x L = 0,

Mass Balance in the Membrane. The differential mass balance of components in the membrane, along with the associated boundary conditions are

Figure 2. (a) SEM picture of PTFE membrane with magnification of 5000× (PTFE side); (b) optical image of the laminated part of the PTFE membrane (polypropylene side).

X CO2 = 1 −

M DCO 2

FCO2,out FCO2,in

(1)

where for MEA ⇒ R1 = CH 2CH 2OH,

∂x 2M

=0 (12) L M L , DCO CCO = mCCO 2 2 2

at xM = δM ,

M G G CCO = CCO , DCO 2 2 2

(2)

The overall rate of absorption can be written as

RAmi = 2kAmiCCO2CAmi

(5)

M ∂CCO 2

∂xM

G ∂CCO 2

∂xG

M = DCO 2

M ∂CCO 2

∂xM (14)

(3)

(4)

∂x L

M = DCO 2

Mass Balance in the Gas Phase. The differential mass balances in the gas phase, along with the associated boundary conditions are

19

R CO2 = kAmiCCO2CAmi

L ∂CCO 2

(13)

R2 = H

and for DEA ⇒ R1 = R 2 = (CH 2CH 2OH)

M ∂ 2CCO 2

at xM = 0,

where F is the molar flow rate of CO2. The experimental error in CO2 removal efficiency was assessed to be ± 5 %. For monoethanolamine (MEA) and diethanolamine (DEA) the overall reaction system can be written as17,18 CO2 + 2R1R 2NH ↔ R1R 2NH 2+ + R1R 2NCOO−

(11)

uG

G ∂CCO 2

∂z

G ⎞ ⎛ ∂ 2C G ∂ 2CCO CO2 G ⎜ 2⎟ = DCO + 2 2 ⎟ 2⎜ ∂ z ∂ x ⎝ G ⎠

at z = 0,

3. MATHEMATICAL MODEL A two-dimensional model of the microstructured membrane contactor was formulated, and the following assumptions were made: (1) ideal gas behavior was valid for the gas phase; (2) Henry’s Law was applicable; (3) the system was at steady state; (4) negligible evaporation of water in the gas phase occurred; (5) plug flow in both phases was assumed; (6) membrane pores were gas filled; (7) no change in gas flow rate along channel occurred. The model was divided in three main domains: the gas phase, the membrane, and the liquid phase.

at z = L , at xG = 0,

at xG = δG ,

(15)

G CCO = CCO2,in 2

∂CCO2 ∂z ∂CCO2 ∂xG G CCO 2

(16)

=0

(17)

=0

=

M CCO , 2

(18) G DCO 2

G ∂CCO 2

∂xG

=

M DCO 2

M ∂CCO 2

∂xM (19)

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as the pore size.25 At this point, the meniscus can accommodate the largest pressure difference between the two phases:

The membrane is considered as a homogeneous medium G with DM CO2 = (ε/τ)DCO2, where ε is the membrane porosity with a value of ca. 70% and τ is the tortuosity calculated from24 τ = (2 − ε)2/ε. COMSOL Myltiphysics 3.5.a was used to solve the differential mass balances. A mesh consisting of 52000 elements and 313983 degrees of freedom was used to execute the simulations in Windows XP with Pentium IV 2.93 GHz CPU and 4GB of RAM. At this number of elements the solution was found to be mesh independent. All the parameters used in the calculations are reported in Table 1.

ΔP = PNW − PW =

parameter

a

value

ref −5

1.64 × 10

20

2 DMEA CO2 (m /s)

1.4 × 10−9

18

DLMEA (m2/s) mMEA (dimensionless) kMEA (m3/mol·s) 2 DDEA CO2 (m /s)

7.7 × 10−10 0.76 3.8 1.05 × 10−9

18 21 22 23

DLDEA (m2/s) mDEA (dimensionless) kDEA (m3/mol·s)

4.97 × 10−10 0.8 2.3

23 21 23

2

(m /s)

All the parameters were for T = 20°C.

3.1. Breakthrough Studies. The maximum pressure difference between phases which allowed operation without penetration of one phase into the other was investigated in order to establish the acceptable pressure difference for the two phases to be kept separated. Experiments in flow were performed to find the acceptable operational difference. First, the liquid and gas channels were filled with the absorbent in order to get rid of any gas bubbles in the liquid channel. Then the gas was allowed to flow above the membrane. Using the metering valve in the liquid outlet, the liquid phase pressure was increased until the breakthrough pressure was reached (liquid started entering the gas phase). The gas phase and liquid phase pressures were measured by a digital manometer. The breakthrough pressure was calculated as the difference between the liquid and gas phase pressure. Breakthrough experiments in flow identified that the breakthrough of liquid in the gas phase for DEA or MEA solution occurred at a pressure difference PL − PG of about 180−210 cm H2O. During typical operation (YG = 247 mL/min, YL = 2.56 mL/min) the pressure difference between liquid phase and gas phase was kept at PL − PG ≈ 90 cm H2O. If one considers a single, cylindrical pore, the pressure difference between the two sides of the gas/liquid interface is given by the Young−Laplace equation as

Figure 3. Amount of CO2 removed from the gas phase as a function of gas flow rate, obtained experimentally and theoretically. YCO2/YDEA = 96.4; DEA = 16.6 wt %.

2γ cos θ (20) r where γ is the liquid surface tension, θ is is the contact angle between the wetting phase, and the solid (the membrane material), and r is the pore diameter. The pressure is higher at the side of the nonwetting phase thus:

the CO2 removal as a function of gas flow rate, experimental results are in a very good agreement with model prediction. CO2 removal efficiency decreased by increasing the gas flow rate. The increase of the gas flow rate reduced the residence time in the contactor; hence, it resulted to lower removal of carbon dioxide. In addition, 11−14% of the carbon dioxide contained in the inlet stream was removed within 0.10−0.16 s experimental gas residence time, based on 0.419 cm3 volume of contact area and a gas flow rate in the range of 160−247 mL/ min. These results are comparable with similar research on CO2 absorption in microchannel separated-flow contactors. TeGrotenhuis et al.16 observed that in a microchannel membrane

ΔP =

ΔP = PNW − PW =

2γ cos θ r

(22)

Beyond this, breakthrough occurs by bubble/droplet formation. In our case the nonwetting phase is the liquid phase (amine solution) and the wetting phase is the gas phase (CO2/N2). The theoretical breakthrough of liquid in the gas phase was found to be 538 cm H2O (surface tension γ = 0.066 N/m, pore diameter of 5 μm). There is a discrepancy between the experimental and theoretical breakthrough pressure values. To explain this, one needs to consider that the Young−Laplace equation is valid for a straight pore. For doughnut-shaped pores, similar to the ones which are formed by the fibers in the Teflon membrane, breakthrough is affected by movement of the three-phase contact line as nonwetting pressure increases. Kim and Harriott26 accounted for that with an effective contact angle which results in lower maximum pressures that those predicted by eq 22. It is worth mentioning that in addition to the noncylindrical pores, the PTFE membrane has complex tortuous structure, where the pores are not parallel. Because of these pore wall inclinations, menisci between adjacent pores may interact and merge before they reach their own individual breakthrough limits predicted by the Young−Laplace equation. Finally, the intrusion of solvent into larger pores of the membrane and subsequent enlargement of pores can also result in breakthrough occurring earlier than expected.27 3.2. Model Prediction for CO2 Absorption in Amine Solutions. The predictions of the model are compared with experimental results obtained from carbon dioxide absorption in DEA solution. As can be seen from Figure 3, which shows

Table 1. Values of Parameters Used in the Simulationsa DGCO2

2γ cos(0) 2γ = r r

(21)

For breakthrough of the nonwetting phase to the wetting phase the maximum pressure is reached when the interface reaches its maximum curvature, that is, a hemisphere with radius the same 9239



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system with 100 μm thick liquid and gas layers, using polymeric membranes of 10−50 μm thick, with pore size of 0.1−5 μm, more than 30% of CO2 from a 25 vol% CO2 stream was absorbed in ca. 3 s residence time in 20% DEA solution. Constantinou and Gavriilidis15 found that in a silicon nitride mesh microcontactor of 3210 and 25 μm gas and liquid layers, respectively, with a silicon nitride mesh of 1 μm thickness, approximately 19−23% of the carbon dioxide contained in the inlet stream was absorbed in ca. 0.33−0.52 s gas residence time in 2 M NaOH solution. Although the pores were wetted by NaOH solution in the silicon nitride mesh contactor compared to the nonwetted (pores of the membrane were gas filled) PTFE membrane of the microstructured membrane contactor, the resistance to mass transfer was very small due to the very thin mesh. Furthermore, NaOH provided a faster reaction rate than DEA. 3.3. Influence of Different Amine Solutions on CO2 Removal. Figure 4 shows the CO2 removal for two different

Figure 5. Amount of CO2 removed from the gas phase as a function of gas flow rate, obtained experimentally and theoretically by MEA = 6 wt %, 10 wt %, YCO2/YMEA = 96.4.

concentration less CO2 removal efficiency was obtained. This is because lower MEA concentration provides a lower reaction rate. Since the membrane is gas filled, one would expect mass transfer resistance to exist in the liquid phase. A sharp decrease in CO2 concentration in the liquid phase confirmed that mass transfer resistance exists in the liquid phase as calculated by the model. Rongwong et al.10 performed experiments with different concentrations of MEA and they showed that by increasing the concentration of MEA from 0.25 to 1 M the CO2 flux was increased due to higher reaction rates. Marzouqi and Faiz28 modeled simultaneous absorption of CO2 and H2S using MEA in hollow fiber membrane contactors and they showed that by increasing the MEA concentration from 1 M to 3 M, CO2 removal efficiency was increased due to higher reaction rates. 3.5. Influence of Gas Liquid Contact Area. To examine the influence of the gas/liquid contact area on CO2 removal efficiency, experiments were performed with the 8-channel PTFE contactor (55.9 cm2 gas/liquid contact area) and the single channel PTFE contactor (4.9 cm2 gas/liquid contact area). Figure 6 shows CO2 removal efficiency as a function of

Figure 4. Amount of CO2 removed from the gas phase as a function of gas flow rate, obtained experimentally and theoretically by DEA and MEA solutions. MEA = 10 wt %, DEA = 16.6 wt %, YCO2/YMEA = 96.4, YCO2/YDEA = 96.4.

amine solutions. Both experimental and modeling results show that MEA solution absorbed more CO2 than DEA. More specifically an increase of around 30% on CO2 removal efficiency was observed when MEA (with fluxes between 0.008 and 0.009 mol/(m2·s)) was used compared to DEA. This is because MEA has higher reaction rate constant than DEA. Rongwong et al.10 studied CO2 absorption using a PVDF hollow fiber membrane with three different amine solutions (MEA, DEA, AMP, all of them at 1 M). They showed that CO2 absorption fluxes were increased in the following order MEA > AMP > DEA. Absorption fluxes increased by ca. 45% when MEA (with fluxes around 0.003−0.004 mol/(m2·s)) solution was used compared to DEA solution. Paul et al.9 in their theoretical studies on separation of CO2 by various aqueous alkanolamine solutions in a flat sheet membrane contactor showed that the aqueous solution of MEA had the highest CO2 flux followed by AMP, DEA, and MDEA. They observed an increase on the absorption flux of around 40% (with fluxes around 0.007−0.01 mol/(m2·s)) when MEA solution was used rather than DEA solution. 3.4. Influence of MEA Concentration on CO2 Removal. Experiments were also performed with 6 wt % (0.97M) MEA concentration. Figure 5 shows the comparison of the model with the experimental results for CO2 removal as a function of gas flow rate. As it can be seen from Figure 5, for lower MEA

Figure 6. Amount of CO2 removed from the gas phase as a function of gas flow rate, obtained experimentally and theoretically by the single channel contactor and the eight-channel contactor. MEA = 10 wt %; YCO2/YMEA = 96.4.

gas flow rate for the two contactors. It can be seen that 63−76% of the carbon dioxide contained in the inlet stream was removed within 1.2−1.8 s of experimental gas residence time, while in the single channel contactor 13−19% of carbon dioxide was removed within 0.10−0.16 s experimental gas residence time. Both model and experimental results demonstrated higher CO2 removal for the eight-channel contactor for the same gas flow rate, which is expected, as the contact area and residence time for the eight-channel 9240



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contactor were larger. Kim and Yang8 investigated CO2 absorption using PTFE hollow fibers and various aqueous absorbents. They achieved 85−100% capture of CO2 (depending on liquid flow rate) from a stream of 40 vol % CO2/N2 in 16 s gas residence time in 2 M MEA solution. The PTFE fibers had a pore size of 0.8 μm, porosity of 62%, length of 240 mm, 2.0 mm O.D., 0.9 mm I.D., and a total contact area of 210 cm2. The membrane contact area to gas flow rate ratio (defined as modified gas residence time) was 74 s/cm. This is a more appropriate measure for performance comparison as it accounts for the contact area of the membrane, which is a critical parameter when liquid phase mass transfer resistance is significant, as is the case here. Considering that the liquid-togas flow rate ratio in their work was significantly higher than ours, the eight-channel PTFE contactor, which achieved 76% removal of CO2 with a 1.6 M MEA solution in 1.8 s residence time (21 s/cm modified residence time), showed comparable performance. Hence, a flat membrane contactor configuration offers another alternative for CO2 capture to hollow fiber contactors. Even though the experiments were performed at room temperature, it is worth pointing out that microstructured flat contactors can offer excellent temperature control by incorporating heat exchange plates. The heat exchange surface area per membrane area would be high and it would increase proportionally with membrane contact area, if higher throughput is required. Thus, flat membrane microstructured contactors show great promise for CO2 removal.

L = reaction zone length (m) MEA = monoethanolamine m = Henry’s constant (ratio of liquid to gas concentrations) (-) P = pressure (Pa) R = overall reaction rate (m3/(mol·s)) r = radius of the membrane pore (m) u = velocity (m/s) x = transverse coordinate (m) XCO2 = CO2 removal efficiency, % Y = volumetric flow rate (m3/s) z = axial coordinate (m) Greek Symbols

γ = surface tension (N/m) δ = thickness (m) ΔP = pressure difference (Pa) ε = porosity (-) θ = contact angle (deg) τ = tortuosity (-)

Subscripts

Ami = MEA or DEA CO2 = carbon dioxide G = gas phase In = inlet L = liquid phase W = wetting phase NW = nonwetting phase Out = outlet

4. CONCLUSIONS Carbon dioxide absorption in amine solutions was investigated in a flat PTFE membrane microstructured contactor. The membrane stabilized the gas/liquid interface and kept the two phases separated. Approximately 14% of the inlet CO2 was removed within 0.16 s experimental gas residence time when DEA was used, while 19% was removed when MEA was used for the same gas residence time. A mathematical model with no adjustable parameters was employed to simulate CO 2 absorption. Comparison of experimental data with model predictions, in terms of CO2 removal efficiency, showed good agreement. Further experiments demonstrated that MEA solution absorbed more CO 2 than DEA. Model and experimental results showed higher CO2 removal efficiency when higher MEA concentration was used. CO2 removal efficiency increased by increasing the contact area between gas and liquid, when keeping the gas and liquid flow rates constant, by using an eight-channel PTFE membrane contactor. Comparison of the latter with hollow fiber membrane contactors from literature showed that it had excellent CO2 removal capability.

■

Superscripts

■

G = gas phase L = liquid phase M = membrane

REFERENCES

(1) Wilson, M. A.; Wrubleski, R. M.; Yarborough, L. Recovery of CO2 from Power Plant Flue Gases Using Amines. Energy Convers. Manage. 1992, 33, 325. (2) Kohl, A. L.; Nielsen, R. B. Gas Purification; Gulf Publishing Company: Houston, TX, 1997. (3) Figueroa, J. D.; Fout, T.; Plasynski, S.; Mcllvried, H.; Srivastava, R. D. Advances in CO2 Capture Technology: The US Department of Energy’s Carbon Sequestration Program. Int. J. Greenhouse Gas Control 2008, 2, 9. (4) Yang, H.; Xu, Z.; Fan, M.; Bland, A. E. Progress in Carbon Dioxide Separation and Capture: A Review. J. Environ. Sci. 2008, 20, 14. (5) Abu-Khader, M. M. Recent Progress in CO2 Capture/ Sequestration: A Review. Energy Source Part A 2006, 28, 1261. (6) Li, J. L.; Chen, B. H. Review of CO2 Absorption Using Chemical Solvents in Hollow Fiber Membrane Contactors. Sep. Purif. Technol. 2005, 41, 109. (7) Mansourizadeh, A.; Ismail, A. F. Hollow Fiber Gas−Liquid Membrane Contactors for Acid Gas Capture: A Review. J. Hazard. Mater. 2009, 171, 38. (8) Kim, Y. S.; Yang, S. M. Absorption of Carbon Dioxide through Hollow Fiber Membranes Using Various Aqueous Absorbents. Sep. Purif. Technol. 2000, 21, 101. (9) Paul, S.; Ghoshal, A. K.; Mandal, B. Theoretical Studies on Separation of CO2 by Single and Blended Aqueous Alkanolamine Solvents in Flat Sheet Membrane Contactor (FSMC). Chem. Eng. J. 2008, 144, 352. (10) Rongwong, W.; Jiraratananon, R.; Atchariyawut, S. Experimental Study on Membrane Wetting in Gas−Liquid Membrane Contacting Process for CO2 Absorption by Single and Mixed Absorbents. Sep. Purif. Technol. 2009, 69, 118.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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NOMENCLATURE C = concentration (mol/m3) DEA = diethanolamine D = diffusion coefficient (m2/s) F = molar flow rate (mol/s) k = reaction rate constant (m3/(mol·s)) 9241



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(11) Yang, J.; Yu, X.; Yan, J.; Tu, S. T.; Dahlquist, E. Effects of SO2 on CO2 Capture Using a Hollow Fiber Membrane Contactor. Applied Energy 2013, 112, 755. (12) Ho, C. D.; Sung, Y. J.; Chuang, Y. C. An Analytical Study of Laminar Concurrent Flow Membrane Absorption through a Hollow Fiber Gas−Liquid Membrane Contactor. J. Membr. Sci. 2013, 428, 232. (13) Lv, Y.; Yu, X.; Tu, S. T.; Yan, J.; Dahlquist, E. Wetting of Polypropylene Hollow Fiber Membrane Contactors. J. Membr. Sci. 2010, 362, 444. (14) Constantinou, A.; Gavriilidis, A. CO2 Absorption in a Microstructured Mesh Reactor. Ind. Eng. Chem. Res. 2010, 49, 1041. (15) Constantinou, A.; Barrass, S.; Pronk, F.; Bril, T.; Wenn, D. A.; Shaw, J. E. A.; Gavriilidis, A. CO2 Absorption in a High Efficiency Silicon Nitride Mesh Contactor. Chem. Eng. J. 2012, 207−208, 766. (16) TeGrotenhuis, W. E.; Cameron, R. J.; Viswanathan, V. V.; Wegeng, R. S. Solvent Extraction and Gas Absorption Using Microchannel Contactors. Proceedings of IMRET 3; 3rd International Conference on Microreaction Technology; Springer-Verlag: Berlin, 2000. (17) Astarita, G. Mass Transfer with Chemical Reaction; Elsevier: Amsterdam, 1967. (18) Danckwerts, P. V. Gas−Liquid Reactions; McGraw-Hill: New York, 1970. (19) Versteeg, G. G.; Little, R. J.; Van Swaalj, W. P. M. Kinetics of CO2 with Primary and Secondary Amines in Aqueous Solutions: Influence of Temperature on Zwitterion Formation and Deprotonation Rates. Chem. Eng. Sci. 1991, 47, 2037. (20) Cussler, E. L. Diffusion Mass Transfer in Fluid Systems; Cambridge University Press: Cambridge, 1984. (21) Paul, S.; Ghoshal, A. K.; Mandal, B. Removal of CO2 by Single and Blended Aqueous Alkanolamine Solvents in Hollow-Fiber Membrane Contactor: Modelling and Simulation. Ind. Eng. Chem. Res. 2007, 46, 2576. (22) Rivera-Tinoco, R.; Bouallou, C. Comparison of Absorption Rates and Absorption Capacity of Ammonia Solvents with MEA and MDEA Aqueous Blends for CO2 Capture. J. Cleaner Prod. 2010, 18, 875. (23) Zhang, H. Y.; Wang, R.; Liang, D. T.; Tay, J. H. Theoretical and Experimental Studies of Membrane Wetting in the Membrane Gas− Liquid Contacting Process for CO2 Absorption. J. Membr. Sci. 2008, 308, 162. (24) Lu, G. J.; Zheng, Y. F. Z.; Cheng, M. D. Wetting Mechanism in Mass Transfer Process of Hydrophobic Membrane Gas absorption. J. Membr. Sci. 2008, 308, 180. (25) Butt, H. J.; Graf, K.; Kappl, M. Physics and Chemistry of Interfaces; Wiley-VCH: Weinheim, Germany, 2003. (26) Kim, B. S.; Harriott, P. Critical Entry Pressure for Liquids in Hydrophobic Membranes. J. Colloid Interface Sci. 1987, 115, 1. (27) Barbe, A. M.; Hogan, P. A.; Johnson, R. A. Surface Morphology Changes During Initial Usage of Hydrophobic, Microporous Polypropylene Membranes. J. Membr. Sci. 2000, 172, 149. (28) Marzouqi, Al.M.; Faiz, R. Mathematical Modelling for the Simultaneous Absorption of CO2 and H2S Using MEA in Hollow Fiber Membrane Contactors. J. Membr. Sci. 2009, 342, 269.

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CO2 Absorption in Polytetrafluoroethylene Membrane Microstructured

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