Ind. Eng. Chem. Res. 2009, 48, 9215–9223
9215
SEPARATIONS Nanocomposite MFI-Alumina Membranes: High-Flux Hollow Fibers for CO2 Capture from Internal Combustion Vehicles M. Pera-Titus,*,† A. Alshebani,† C.-H. Nicolas,† J.-P. Roume´goux,‡ S. Miachon,† and J.-A. Dalmon† Institut de Recherches sur la Catalyse et l’EnVironnement de Lyon (IRCELYON), UMR 5256 CNRS, UniVersite´ de Lyon, 2 AVenue A. Einstein, 69626 Villeurbanne Cedex, France, and Institut National de Recherche sur les Transports et leur Se´curite´ (INRETS), 25, AVenue Franc¸ois Mitterrand, F-69675 Bron Cedex, France
The transport field accounts for about 35% of CO2 emissions in France, while energy production only involves 16% of the emissions. The strong contribution of transport to the CO2 emission pattern in France is mainly ascribed to the great development of the nuclear field as energy vector. Therefore, in order to meet Kyoto targets, CO2 emissions in vehicles should be drastically reduced in France in the forthcoming decades. To this aim, taking into account a scenario where thermal engines will keep their supremacy as the main propulsion technology at short and mid terms, in addition to increasing more and more energy efficiency, a possibility to reduce drastically CO2 emissions from transport could involve direct CO2 capture and in situ storage from exhaust gases. In this study, we propose the use of high-flux nanocomposite MFI-alumina hollow-fiber membranes recently developed in our laboratory for direct CO2 capture from mobile sources. A critical discussion is provided about the technico-economical feasibility (i.e., CO2 recovery, CO2 purity in the permeate, module volume, energy overcomsumption, and autonomy) of a membrane-based unit for CO2 capture and liquefaction in the special case of heavy vehicles (over 3500 kg) using conventional diesel propulsion standards. 1. Introduction Carbon dioxide (CO2) is regarded as one of the main promoters for climate change, accounting itself for ca. 70% of the gaseous radiative force responsible for anthropogenic greenhouse effect.1 CO2 emissions from man-made activities have been increasing more and more in the past decades, reaching a level of 28 Gtons of CO2/year in 2001.2 Fossil fuel burning for energy production (electricity and heat) is the first world CO2 emission source. According to the IEA-OECD estimates,2 this sector accounted itself for 37% of CO2 world emissions in 2001, with an annual increase about +33% in the period 1990-2002. The second sector in terms of CO2 emissions is transport, involving 19% of world emissions (2001) and showing a rapid increase in the past decade due to the increase of the automobile park. This CO2 emission pattern is, however, inversed in the case of France (see Figure 1). As has been recently pointed out in two exhaustive reports from the French Parliament and Senate3 and from the French Agency of Environment (CITEPA),4 this “French specificity” is mainly attributed to the great development of nuclear energy in this country, providing about 80% of the energy demands. As a matter of fact, CO2 emissions due to fuel burning for power generation only accounted in France for about 16% of total emissions in 2006, while those ascribed to transport corresponded to ca. 35% (25% of which being ascribed to heavy vehicles). The tertiary sector (residential) and the ensemble agriculture-industry involved, respectively, 23 and 26% of the emissions. This particular CO2 emission pattern in France translates into energy-related CO2 emission rates per * To whom correspondence should be addressed. Tel.: +33(0)472445394. Fax: +33-(0)-472445399. E-mail: marc.pera-titus@ ircelyon.univ-lyon1.fr. † UMR 5256 CNRS, Universite´ de Lyon. ‡ INRETS.
inhabitant as low as 1.7 tons of CO2/inhabitant (2003), one of the lowest in Europe (the mean rate for the EU25 in 2003 was 2.4 tons of CO2/inhabitant). Different solutions have been proposed to reduce CO2 emissions from new passenger vehicles to the average level of 120 g of CO2/km by 2012, as requested by the European Decision 2000/1753/EC.5 These solutions can be divided into three main groups: (i) decrease of fuel consumption by increasing energy efficiency of thermal propulsion systems, (ii) switch from petroleum-based energy sources to more sustainable ones, and (iii) CO2 capture, transport, and storage. Figure 2 summarizes the main propulsion possibilities and the required energy sources to mitigate CO2 emissions in vehicles. Better energy savings in thermal systems, as well as the reduction of atmospheric emissions of priority pollutants and greenhouse gases, constitute a major concern for car manufacturers at present. The main strategies involve first the increase of compression rates of conventional thermal systems. The adaptation of thermal engines to other fuels, such as liquefied petroleum gas (LPG) and natural gas vehicle (NGV), is another possibility, allowing prospected reductions up to 25% in CO2 emissions.2 Hybrid systems combining a thermal engine as an
Figure 1. CO2 emissions (2006) in France by sectors according to ref 4.
10.1021/ie9004018 CCC: $40.75 2009 American Chemical Society Published on Web 09/28/2009
9216
Ind. Eng. Chem. Res., Vol. 48, No. 20, 2009
Figure 3. Alumina hollow-fiber support used for zeolite MFI synthesis.
Figure 2. Synthesis of liquid fuels and hydrogen as energy vectors for automobile propulsion.
energy source with an electrical propulsion engine have also been proposed. Some models are already available on the market. Alternative propulsion technologies based on fuel cells using hydrogen as energy vector have long been considered and appear to be a serious option to mitigate CO2 emissions in vehicles at mid term. Hydrogen can be directly obtained from naphthas, or industrially produced from syngas (CO + H2) by steam reforming of coal, oil residues, natural gas, and biomass with subsequent water-gas shift reaction (see Figure 2). Please note that, in this latter case, the overall carbon balance of fuel cells is not zero, since the water-gas shift reaction produces CO2, transferring therefore the CO2 capture problem to the hydrogen generation plant (stationary precombustion CO2 capture). Furthermore, IV generation nuclear reactors are also expected to produce large amounts of hydrogen from water thermolysis or electrolysis for industrial and transport use at long-term. Despite the seductive character of fuel cells, their commercialization does not seem to be immediate. Indeed, their exorbitant costs, as much as 6000-8000 euros/kW compared to those of thermal engines (about 30-50 euros/kW),3 as well as the extremely low volumetric density of hydrogen, make it difficult to devise a large-scale implementation of fuel cells in vehicles before the horizon 2020. Battery-powered electrical vehicles are also often considered as an interesting alternative. The use of these systems is, however, limited due to the insufficient storing capacity of accumulators. Moreover, the overall CO2 balance of accumulators is related to that of the power production (either thermal or others), thus reporting the issue to the power plant. In light of all the above stated considerations, hardly any alternative technology to conventional thermal systems relying on liquid-fuel combustion appears to be competitive at short and mid terms for propulsion in vehicles. Furthermore, even in a scenario characterized by a scarcity of fossil liquid fuels, “unconventional” fuels might be still produced from syngas by Fischer-Tropsch (FT) synthesis (see Figure 2). The important stocks of coal and natural gas (more than 200 years in the case of coal at the current production rates2) ensure the supply of liquid fuels produced via “coal-to-liquids” and “gas-to-liquids” FT processes to the world markets at comparable prices. Similarly, one could also consider biomass as a source of liquid fuels via “biomass-to-liquids” FT process. At this point, and taking into account that liquid-fuel-based thermal engines will not probably lose their supremacy as propulsion technology for at least several decades, the question that arises is: “how can CO2 emissions in vehicles be drastically decreased without significant technological modification of thermal systems, and therefore help mitigating the environmental
impact of transport?” The most reasonable answer is, on the guidance of some recent technical reports,6-8 to proceed with CO2 capture, transport, and sequestration. Several technologies are available or under study for CO2 capture in stationary postcombustion emission sources, such as in power plants (e.g., absorption with amines, cryogenic separation, and pressure- and thermal-swing adsorption9). Nevertheless, none of these technologies appear to be suitable for CO2 capture in mobile sources due to their high-energy costs (>4 GJ/ton of CO2 removed for amine absorption) and large volume requirements. The need of specific postcombustion CO2 capture solutions especially conceived for vehicles, involving reasonable energy costs in terms of power overconsumption and low volume, appears therefore imperative. Using an on-board CO2 capture unit, new vehicles could reduce notably their CO2 emissions without changing the propulsion technology. To meet these objectives, we propose in this work the use of nanocomposite MFI-alumina hollow fibers recently developed in our laboratory,10 highly resistant to thermal shocks, environmentally friendly, and offering high fluxes and promising CO2 separation factors. We provide a critical discussion about the suitability of this material, and the technico-economical feasibility of the solution proposed in terms of energy savings and CO2 emission reduction in the case of heavy vehicles (over 3500 kg). We also address the main improvements in terms of membrane flux and selectivity that should be accomplished before optimizing a unit for CO2 capture and in situ liquefaction in heavy vehicles. 2. Experimental Section 2.1. Synthesis, Physical Characterization, and Mounting of Nanocomposite MFI-Alumina Hollow Fibers. The ceramic hollow fibers used in this study (o.d., 1.65 mm; i.d., 1.44 mm; porosity, 43%), prepared by a wet spinning process following the methodology described in ref 11, were supplied by the Fraunhofer Institute for Interfacial Engineering and Biotechnology (IGB, Germany). Only hollow fibers displaying first bubble points above 120 kPa, corresponding to an average crossing pore size smaller than 0.2 µm, were used for synthesis. A scanning electron microscopy (SEM) micrograph of a raw fiber is shown in Figure 3. The surface-to-volume ratio of the fibers is about 3400 m2/m3, 20 times higher than the value that can be obtained using conventional membrane tubes. The MFI-type zeolite material was synthesized in the porous matrix of the alumina hollow fibers via pore-plugging hydrothermal synthesis following the protocol described in a series of previous studies.10,12-14 After washing and drying, the fibers were calcined at 773 K for 4 h under air flow. X-ray diffraction (XRD) confirmed that pure HZSM-5 was the only zeolitic phase on the fiber after synthesis. The nanocomposite nature of the fibers (i.e., no zeolite film is formed) was inspected by SEM
Ind. Eng. Chem. Res., Vol. 48, No. 20, 2009
9217
Table 2. Effective MFI Thickness (µm) of Fibers F1-F3 Obtained from MS Fittings on Pure H2 and N2 Permeance MFI thickness sample ref
H2
F1 F2 F3
1.26 ( 0.03 2.2 ( 0.2 0.55 ( 0.02
mean thickness 1.2 ( 0.1 2.13 ( 0.05
1.2 ( 0.2 2.1 ( 0.3 0.55 ( 0.02
tion, 10-50%. The contribution of the observed He counterexchange on the measured permeances was taken explicitly into account through a He mass balance. Figure 4. Two nanocomposite MFI-alumina hollow-fiber membranes mounted in their supporting tubes (left) and SEM micrograph of a fiber after synthesis (right) (Micrographs: L. Burel). Table 1. Four Samples Presented in this Work: Sample Reference, Zeolite Mass Uptake, Number of Fibers, and Surface Area from BET sample ref
no. of fibers
wt uptake (mg/g)
BET surf area (m2/g)
F1 F2 F3 F4
1 1 1 2
100 88 108 110
33 35 21
(Hitachi S800, 20 kV) coupled to energy-dispersive (EDX) analysis. Figure 4 shows a SEM micrograph of a MFI-alumina hollow fiber after synthesis. More details about the preparation of this material for CO2 separation can be found elsewhere.10 After synthesis, the as-calcined hollow fibers were immobilized on a supporting dense alumina perforated tube using a homemade low-temperature glaze, as shown in Figure 4. The final ensemble was then mounted in a membrane stainless steel module using graphite cylindrical gaskets (Cefilac-Fargraf) to seal the support tubes to the module. The main characteristics of the fibers prepared in this study are listed in Table 1. Samples F1-F3 consist of a single fiber, whereas sample F4 is made of two fibers mounted in a single module. 2.2. Single Gas Permeance and Mixture Separation Tests. Prior to any transport measurement, the hollow fibers were subjected to in situ high temperature desorption (cleaning) pretreatment at 673 K under 20 Ncm3 · min-1 N2 flow at both sides with a heating ramp of 1 K · min-1 for at least 4 h to remove adsorbed species following the guidelines of a previous study.15 Single gas (N2, H2, CO2; Air Liquide; quality, 99.9999%) permeance tests were carried out in the temperature range 293-723 K using steady-state steps to assess for the temperature behavior of the nanocomposite material. In these tests, the feed pressure was kept close to 125 kPa and the transfiber pressure ca. 3.2 kPa. More details on the setup used to carry out the gas permeation measurements can be found in ref 16. Two gas separations were carried out. The room-temperature separation of n-butane/H2 at low temperature was used for quality testing. This separation was carried out further in Wicke-Kallenbach mode with increasing temperature, the gases being diluted in dry N2 (15% (v/v)) and He (15% (v/v)), respectively. The feed was kept at 104 kPa, while the transfiber differential pressure was kept at 0.4 mbar. The flow rate in the feed and permeate were kept at 80 Ncm3 · min-1 in both sides of the membrane module. The fibers were also tested for separation of CO2/N2 nondiluted mixtures (150 (Ncm3)/min feed, 60 (Ncm3)/min He sweep, and 0.4 kPa transfiber total pressure). The surveyed ranges of the main operational variables were as follows: temperature, 298-723 K; feed pressure, 101-404 kPa; CO2 feed concentra-
In both separations, gas flows and feed compositions were controlled by mass-flow controllers (Brooks, type 5850TR and 5850E). A gas chromatograph (HP 5890) equipped with two thermal conductivity detectors (TCDs) was used to measure feed, retentate, and permeate compositions. The separation factor (Sf) of gas A over gas B (n-butane over H2 or CO2 over N2) was calculated as the permeate-to-feed composition ratio of the first gas, divided by the same ratio of the second one. 3. Results 3.1. Pure Gas Permeance. Figure 5 shows the evolution of single gas permeance of several gases with temperature for the MFI-alumina fiber sample F1. As can be seen, the N2 and H2 permeance values are higher than 1 µmol · m-2 · s-1 · Pa-1 at room temperature, in good keeping with the values found in our previous study.10 This sample shows a particularly high CO2 permeance, with values higher than 2 µmol · m-2 · s-1 · Pa-1 at room temperature. These permeance values translate into a CO2/ N2 ideal permselectivity of about 1.9, higher than the corresponding Knudsen selectivity ≈0.8. The amount of large intercrystalline defects of the synthesized MFI material is low, as inferred from the low viscous contribution to N2 permeance after calcination (lower than 2%), obtained from the slope of N2 permeance with the average pressure (not shown). Figure 6 shows the variation of the H2 and N2 single gas permeance with temperature for samples F1-F3 together with the Maxwell-Stefan (MS) fittings using the following expression:
Figure 5. Evolution of N2, H2, and CO2 single gas permeance through sample F1. Conditions: retentate pressure, 104 kPa; transfiber pressure, 3.2 kPa. The curves are a guide to the eye.
9218
Ind. Eng. Chem. Res., Vol. 48, No. 20, 2009 ∞
N)
csat.FεD0 τl
[
( (
∆H◦ads PR ∆S◦ads exp P° R RT ln ∆H◦ads PP ∆S◦ads exp 1+ P° R RT 1+
) )
]
×
[ ]
exp -
ED RT
(1)
with parameter values taken from refs 10 and 14: R, ideal gas constant (8.314 J · mol-1 · K-1); csat., gas concentration in MFI crystals (5.4 mol · m-3 for both gases); F, MFI density (1700 kg · m-3); ε, porosity of the nanocomposite MFI-alumina structure (0.13); D0∞, Maxwell-Stefan diffusivity at zero coverage (H2, 1.8 × 10-8 m2 · s-1; N2, 0.4 × 10-8 m2 · s-1); τ, tortuosity (1.2); l, effective MFI thickness (m, fitted parameter); PR, retentate pressure (Pa); PP, permeate pressure (Pa); P°, reference to atmospheric pressure (101325 Pa); ∆S°ads, standard adsorption entropy (H2, -43; N2, -50 J · mol-1 · K-1); ∆H°ads, standard adsorption enthalpy (H2, -5900; N2, -13800 J · mol-1); ED, activation energy for diffusion (H2, 2000; N2, 4000 J · mol-1). The values for H2 and N2 surface diffusivities have been chosen after a careful screening of diffusivity values on the guidance of a previous study14 based on MFI-alumina membrane tubes. As can be seen, the permeance of H2 and N2 shows a continuous decrease with temperature, achieving values higher than 3 µmol · m-2 · s-1 · Pa-1 at room temperature in the latter case in the case of sample F4 (double fiber). The MS fittings reflect an effective MFI thickness in the range of 1-3 µm (see Table 2) for the given H2 and N2 surface diffusivities used in
Figure 6. Comparison of H2 (top) and N2 (bottom) single gas permeances through samples F1-F3. Conditions: retentate pressure, 104 kPa; transfiber pressure, 3.2 kPa. The curves correspond to the fittings to the Maxwell-Stefan model.
Figure 7. Evolution of n-butane and H2 molar fluxes with temperature in the separation of a n-butane/H2 equimolar mixture for samples F1 and F2. Conditions: retentate pressure, 104 kPa; transfiber pressure, 0.4 kPa; feed flow, 80 Ncm3/min (15% (v/v) n-butane, 15% (v/v) H2); N2 flow (sweep gas), 80 Ncm3/min. The curves are a guide to the eye. Table 3. Membrane Quality of Fibers F1-F4: Room-Temperature Sf Butane/H2 and Pure H2 Permeance sample ref
Sf butane/H2
H2 permeance (µmol/m2/s/Pa)
F1 F2 F3 F4 (two fibers)
24 27 16 105
1.3 0.9 3.4 1.3
this study. Note that no indication of permeance increase beyond 723 K is observed for either sample and gas. 3.2. n-Butane/H2 Separation. Figure 7 shows the quality of two synthesized hollow fibers (samples F1 and F2) in terms of n-butane/hydrogen equimolar separation. The molar flux is shown in the temperature range of 300-723 K. In both cases, the n-butane flux shows a characteristic maximum at 430 K, as well as a decreasing trend at higher temperatures. The roomtemperature n-butane/H2 separation factors for all samples are shown in Table 3. 3.3. Separation of CO2/N2 Mixtures. Figure 8 shows the variation of CO2 and nitrogen permeance in equimolar mixture as a function of He sweep flow rate for sample F2. The separation factor variation is also shown. As can be seen, the CO2/N2 separation factor and the CO2 flux through the fiber increase with the He flow. A sweep flow of 60 (Ncm3)/min has been selected to carry out further gas separation measurements
Figure 8. Evolution of the CO2/N2 separation factor and CO2 and N2 mixture permeance with He sweep gas flow rate for sample F2 in the separation of an equimolar CO2/N2 mixture. Conditions: retentate pressure, 168 kPa; transfiber pressure, 0.4 kPa; temperature, 298 K. The curves are a guide to the eye.
Ind. Eng. Chem. Res., Vol. 48, No. 20, 2009
Figure 9. Evolution of the CO2/N2 separation factor with the He sweep gas flow for samples F2 and F4 (double fiber) in the separation of an equimolar CO2/N2 mixture. Conditions: retentate pressure, 168 kPa; transfiber pressure, 0.4 kPa; temperature, 298 K; He sweep flow, 60 Ncm3/ min. The curves are a guide to the eye.
Figure 10. Evolution of the CO2/N2 separation factor and mixture CO2 and N2 permeances with temperature for sample F4 (double fiber) in the separation of an equimolar CO2/N2 mixture. Conditions: retentate pressure, 168 kPa; transfiber pressure, 0.4 kPa; temperature, 298 K; He sweep flow, 60 Ncm3/min. The curves are a guide to the eye.
due to a higher sensitivity in the analysis of CO2 concentration in the permeate. Figures 9 and 10 plot the evolution of the CO2/N2 separation factor in the separation of CO2/N2 equimolar mixtures as a function, respectively, of CO2 feed composition (at room temperature), and temperature. As can be seen, the fibers synthesized in this work show CO2/N2 separation factors up to 5.5 at room temperature, 168 kPa feed pressure and equimolar feed composition, decreasing in value with the feed composition. The CO2 mixture permeance reaches a value of ca. 2 µmol · m-2 · s-1 · Pa-1 at 168 kPa and room temperature, while the nitrogen permeance is kept at very low values. Both the CO2/N2 separation factor and the CO2 mixture permeance remain almost unchanged with the feed pressure beyond 101 kPa (not shown). 4. Discussion: Material Properties 4.1. Membrane Morphology. A comprehensive discussion of the nanocomposite hollow-fiber membrane morphology was addressed in a previous study.10 As has been stated above, no zeolite film is observed in the SEM micrograph depicted in Figure 4, most of the MFI material being hosted in the very large cavities. The average Si/Al ratio at about 50 µm depth is
9219
about 13, as measured by EDX analysis, corresponding to smaller pores. 4.2. Single Gas Transport. The samples prepared in this study show pure gas permeance values that are comparable to the highest values reported in the literature on conventional film-like MFI membranes,17 probably due to a comparable MFI effective thickness (lower than 2 µm). Moreover, the permeance shows a continuous decrease with temperature (see Figures 5 and 6), as expected for a nanocomposite architecture.12,13 The differences in the pure H2 and N2 permeation performance at lower temperature for the different samples can be in principle ascribed to a different MFI effective thickness according to the MS fittings (see Table 2). 4.3. n-Butane/H2 Separation. The n-butane/H2 separation data presented in Figure 7 are in good keeping with those already reported on nanocomposite MFI-alumina hollow fibers10 and membrane tubes.12 n-Butane shows a maximum ca. 430 K, and the n-butane/H2 separation factor shows a decreasing trend with temperature from a value of 16-105 at room temperature to 3500 kg weight). Ideally, the CO2 capture system should occupy a low volume and allow the removal of at least 75% of the CO2 in the exhaust gas with a purity of 95% in the permeate without a significant energy overconsumption (
