Removal from Flue Gas - American Chemical Society

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Ind. Eng. Chem. Res. 2009, 48, 7314–7324

Hollow Fiber Adsorbents for CO2 Removal from Flue Gas Ryan P. Lively,*,† Ronald R. Chance,† B. T. Kelley,‡ Harry W. Deckman,§ Jeffery H. Drese,† Christopher W. Jones,† and William J. Koros† School of Chemical & Biomolecular Engineering, Georgia Institute of Technology, 778 Atlantic DriVe, Atlanta, Georgia 30332-0100, ExxonMobil Upstream Research Company, Houston, TX, and ExxonMobil Research and Engineering, Annandale, NJ

The nation’s pulverized coal infrastructure is aging, and implementation of current retrofit postcombustion capture methods is extremely expensive. This paper describes a technology based on hollow polymeric fibers with sorbent particles embedded in the porous fiber wall to enable postcombustion CO2 capture via a rapid temperature swing adsorption (RTSA) system. The system takes advantage of the hollow fiber morphology by passing cooling water through the bores during sorption to maximize sorption capacities and steam through the bores during desorption to desorb CO2 efficiently. The thin-walled hollow fibers offer the advantage of rapid heat and mass transport. To avoid mass transfer between the core and the fiber sheath, a dense lumen layer is used on the interior of the fiber wall. This system has advantages over competing technologies. Specifically, the fiber sorbent contactor minimizes flue gas pressure drop across the bed, while maximizing sorption efficiencies via rapid thermal cycles and low regenerative thermal requirements. 1. Introduction Coal plants provide the majority of the world’s electrical power1 and are major point sources for greenhouse gas emissions. Developing countries are also rapidly building coal power stations that will add further to atmospheric CO2 levels. Such coal-fired plants provide prime targets for deployment of carbon capture and sequestration (CCS) systems. The most common form of coal power stations are Pulverized Coal (PC) types, which typically produce between 300-500 MW of electrical power and release approximately 6-9 tons CO2 per minute,2 or 2.2-2.4 lbm CO2/kWh. The PC power station infrastructure is aging, and implementing current carbon capture methods is extremely expensive. While integrated gasification combined cycle (IGCC) power stations and natural gas combined cycle (NGCC) power stations offer higher efficiencies, lower emissions, and the potential for lower cost CCS approaches, the PC infrastructure is huge and cannot rationally be replaced by IGCC systems for many decades at best. Thus, CCS retrofits for PC power stations are required for effective mitigation of CO2 emissions. Reducing the cost of capturing CO2 is a main hurdle for CCS development.3 For effective emission control and sequestration, the CO2 should be captured at greater than 90% purity and compressed to roughly 2300 psia for underground storage.4 Economical postcombustion CO2 capture from massive flow rates of low pressure feeds is the greatest challenge to implementation of CCS.5 This paper analyzes a new, potentially low-cost carbon capture system that could be retrofitted onto existing PC plants. Pressure-swing packed bed adsorption has been considered for postcombustion capture; however, the large flow rates and the expense of pressurizing the flue gas to the required bed pressure limits this technology’s use in postcombustion CO2 capture. Temperature swing adsorption in its most common packed bed format cannot be cycled sufficiently frequently to avoid enormous system size and cost. The most prominent * Corresponding author. Tel.: +1 404 385 4717. Fax: +1 404 385 2683. E-mail: [email protected]. † Georgia Institute of Technology. ‡ ExxonMobil Upstream Research Company. § ExxonMobil Research and Engineering.

capture technology is based on liquid-gas column absorption involving chemisorption of the CO2 into aqueous alkanolamine solutions, such as monoethanolamine (MEA) and monodiethanolamine (MDEA).2,3,5 Besides the intensive energy requirements for solvent regeneration, this capture technology suffers from several problems, including the need to handle large amounts of environmentally hazardous waste, equipment corrosion, tower entrainment, and amine solution flooding and weeping.3,6 Recently, work has sought to reduce the footprint of these liquid sorption systems by using hollow fiber membrane contactors.7 Although promising, significant work is still needed in this area since aqueous alkanolamine solutions can preferentially wet polyolefin membranes, causing large mass transfer resistances in the membranes pores.8 Conventional membranes have also been considered for postcombustion carbon capture; however, the low CO2 driving forces and enormous flow rates associated with flue gas make it doubtful that membranes will be used broadly for postcombustion CO2 capture.5 The most popular precombustion carbon capture in IGCC power stations is based on Selexol in liquid-gas adsorption towers.9 Coupled with IGCC, the Selexol process may be economically viable; however, for the low pressures associated with postcombustion capture, Selexol does not perform as well as alkanolamine sorbents5,10 and is limited mainly to new IGCC power stations. 2. Background This paper describes a carbon capture system that avoids many of the deficiencies in the previously mentioned carbon capture approaches. The present work describes the essential elements of a novel hollow fiber-based solid sorbent system with the potential to greatly reduce carbon capture costs in comparison to existing and emerging technologies.11,12 This strategy is based upon creation of a hollow fiber sorbent-based rapid temperature-swing adsorption (RTSA) system with low flue gas pressure drop and regeneration thermal requirements, thus driving down CCS costs. Hollow fiber sorbents are hybrid materials relying upon solid sorbents embedded in a porous polymeric hollow fiber matrix. To our knowledge, this paper is the first description of such a

10.1021/ie9005244 CCC: $40.75  2009 American Chemical Society Published on Web 06/11/2009

Ind. Eng. Chem. Res., Vol. 48, No. 15, 2009

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in the bore of the fibers, and sorbent loading in the fibers. These fiber characteristics are 1200 µm OD, 320 µm ID, 65-75 wt % sorbent loading, a sorbent size of less than 3 µm, and a bed packing of 40-50%. The preferred flue gas/fiber sorbent contact time was estimated to be ∼4 s, and the RTSA cycle time was found to be ∼30 s. These rapid cycles allow for increased sorption efficiency per gram of sorbent and underscore the importance of a sorbent that can rapidly equilibrate. For effective RTSA operation under the given design conditions, 12.5 million fibers in parallel flow configuration were estimated to be needed to adsorb 99% of the CO2 emitted by the power station in one RTSA cycle. This number is reasonable, as large scale commercial hollow fiber membrane systems contain up to 150 million fibers. Figure 1. Schematic of an ideal hybrid polymer-sorbent hollow fiber.

system for CO2 capture, although related approaches that do not include all of the attractive elements considered here have been described by others.13-15 The new system based on polymer-sorbent hollow fibers offers several advantages. First, higher sorption efficiencies can be achieved by utilizing the hollow fiber morphology to supply cooling agents in the bore of the fiber during sorption and heating agents in the bore during desorption. Second, the thin porous walls of the fiber allow for rapid heat and mass transfer times, thereby allowing for more rapid thermal cycles. Third, the hollow fiber sorbents allow for similar CO2 recoveries as packed bed systems, while significantly reducing compression costs due to the parallel channel fiber module arrangement. Finally, heating and cooling agents in the fibers can simply be steam and water, providing an environmentally friendly overall process. The impermeable lumen layer shown in Figures 1 and 2 allows efficient heat transfer with the heat exchange fluid during sorption and desorption without allowing mass exchange. To explore CO2 capture involving hollow fiber sorbents, a conceptually simple two-bed system was considered to enable pseudo-steady-state continuous operation as shown in Figure 3. During sorption, flue gas passes over the fibers in a crossflow heat exchanger arrangement or in a parallel flow arrangement to minimize pressure drop of the flue gas. The first bed, which is actively sorbing CO2, will have liquid water passing through the bores of the fibers while the bed that is actively desorbing will have steam passing through the bore of the fibers. This two-bed system is for illustrative purposes only, as a commercially practical system will require a multibed configuration. A 500 MW PC power station was used to establish an approximate fiber geometry. The characteristics of the flue gas feed for the fiber sorbent system were taken to be those for a pulverized coal power station: 1 M SCFM feed, 15 mol % CO2, at a pressure of 1.1 atm and a temperature of 50 °C. To avoid excessive compression of the large amounts of feed flue gas, a maximum entry pressure of 1.3 atm (achieved by blowers) was used and capture efficiency was set at no less than 90% with the product gas exiting at near ambient pressure. Finally, the mass transfer rate within the highly porous fibers was designed to accommodate the short contact equilibration time required of the flue gas with the fiber. The preferred hollow fiber sorbent characteristics were based on an optimization around the following conditions with respect to the 0.3 atm pressure drop limit: bed dimensions, bed void fraction, fiber OD, fiber ID, fiber porosity, heat transfer times, CO2 contact time with the fibers, water and steam pressure drops

3. Materials and Methodology Polymer-Zeolite Solution Formation. N-Methyl-2-pyrrolidone (NMP) (ReagentPlus 99%, Sigma-Aldrich, Milwuakee, WI) was used as the solvent in the polymer solutions because it is miscible in water and an excellent solvent for cellulose acetate. Methanol (99.8%, ACS Reagent, Sigma-Aldrich) and hexane (ACS Reagent, >98.5%, Baker) were used for the solvent exchange portion of the fiber formation after spinning. Methanol was used to remove excess water from the fibers, and hexane was used to exchange excess methanol from the fibers. This standard procedure replaces high surface tension fluids with lower surface tension fluids to prevent capillary forces from collapsing the pore structure during drying.16 All solvents and nonsolvents were used as-received with no purification or modification. Zeolite 13X (1-3 µm particles, Sigma-Aldrich) was chosen as the sorbent for dispersion into the polymer matrix. Wide angle X-ray diffraction, N2 physisorption, and elemental analysis were used to confirm physical properties in comparison to literature values. Once received, the zeolites were dried at 230 °C to remove potential organic contaminants. After drying, the zeolites were saturated with humid air, thereby filling the pores with water, which was the quench medium. This allowed the sieves to be “passive” fillers during the aqueous bath quench process during fiber spinning. Zeolite 13X was chosen to provide a welldefined basis on which to compare the fiber sorbent platform to traditional CO2 capture methods. In comparison with other sorbents, the capacity of dehydrated 13X for CO2 is high and its heat of sorption (∼36 kJ/mol) is relatively low; however, its hydrophilic nature makes it only useful for dry feeds.17 While it is recognized that a more water-resistant sorbent will ultimately be preferable, 13X allows for a useful discussion of the key concepts for such fiber sorbents. Cellulose acetate (CA) (MW 50 000, Sigma-Aldrich) and polyvinylpyrrolidone (PVP) (MW 55 000, Sigma-Aldrich) were the polymers used in the spinning process. PVP was chosen as a pore former due to its macrovoid suppression properties,18 as well as its pore network formation properties. All polymers were dried in vacuum at 110 °C for one day to remove sorbed atmospheric water and used directly to form solutions. Spin dopes of varying solvent, nonsolvent, and zeolite composition were made to determine the binodal line of the ternary system using the cloud point technique for distinguishing between one-phase or two-phase regimes. Pure polymer solutions were made first to determine the pure polymer binodal; once this was determined, the polymers to liquids ratio was held constant, and the NMP to H2O ratio was held constant as the “passive” zeolites were added in. These sample dopes were

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Figure 2. Sorption and desorption modes in the hollow fiber sorbents.

Figure 3. Damper switching between sorption and desorption modes.

loaded into syringes and extruded into deionized water to qualitatively determine the speed of phase separation. Fiber Formation. The hollow fibers for this work were formed using the well-known nonsolvent phase inversion technique commonly referred to as “dry-wet spinning”.19 Specifically, polymer solutions with suspended sorbent particles and solvents, nonsolvents, and additives for tuning the phase equilibria were extruded through a die (called a spinneret) into a nonsolvent quench bath. A schematic of the spinning setup is shown in Figure 4. The nonsolvent bath causes mass transfer to induce separation and the formation of a porous fiber,20 thereby resulting in a continuous polymer pore network with sorbent particles suspended in the porous polymer network (Figure 1).21 A conventional fiber spinning apparatus was used to create the fiber sorbents. This system emulates industrial fiber spinning systems and is therefore readily scalable. The polymer-sorbent spin dopes were prepared by mixing 80% of the required

Figure 4. Layout of typical fiber spinning apparatus.

amounts of NMP and deionized water into a 1 L glass jar sealed with a PTFE cap. A priming dope to facilitate particle dispersion was made by mixing 20% of the required amounts of NMP and water into a 500 mL glass jar with a PTFE cap, and 20%


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Figure 5. PVDC post-treatment protocol.

Figure 6. Overview of fiber post-treatment system.

of the required amount of dried cellulose acetate and PVP were added to the solution. The solution was mixed on a roller at 50 °C until complete dissolution occurred, usually around 24 h. Zeolite 13X, at equilibrium with ambient humidity conditions, was added to the NMP/H2O mixture and sonicated (1000 W max. horn, Dukane, Leesburg, VA) 3 times for 1 min with 30 s breaks. The prime solution was then added to the sonication solution, and two more sonication cycles were performed. Finally, the remaining dried cellulose acetate and PVP were mixed at high shear rates with an impeller at 50 °C until complete dissolution occurred. Sorption Characterization. To characterize sorption of CO2 in the fiber sorbent as a function of CO2 pressure at constant temperature, a simple pressure decay method developed for polymers is useful.22 The time required to reach equilibrium can also be measured to provide insight into fiber sorbent response times (without thermal moderation in the bore of the fibers). Despite its utility, such sorption kinetic approaches are only useful as preliminary kinetic and equilibrium characterization techniques, since sorption-generated heat effects cannot be effectively mediated inside this sorption system. Moreover, as a batch system, convective mass transfer resistances within the fiber wall cannot be accurately

Figure 7. “Sieve-in-a-cage” morphology.

probed. Finally, since only pure gases can be used, the effects of competitive sorption or humidity in the feed cannot be assessed.

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Table 1. Component Composition and Spin Conditions Used for Fiber Sorbents spin dope component

core dope (wt%)

spinning parameter

CA PVP 13X NMP H2O -

10 4 30 49.3 6.7 -

core flow rate bore flow rate bore composition operating temperature take-up rate air gap

1000 mL/h 250 mL/h 80/20 NMP/H2O 25 °C 11.7 m/min 3 cm

The aforementioned pressure-decay sorption method was used to determine the sorption isotherms and sorption kinetics of the fiber sorbents.23 The sorption cells were immersed in constant temperature oil baths, and zeolite crystal samples were held in a porous stainless steel filter element and capped with aluminum foil while fiber samples were loosely packed in the cell for their respective tests. After the fiber sorbent or crystal samples were loaded, the oil baths were set to 115 °C and vacuum was pulled on the sample cells for one day to completely evacuate the cell and the fiber sorbents. After the drying step, the oil bath was set to 45 or 100 °C and CO2 was introduced to the reservoir and allowed to equilibrate. After thermal equilibrium, the sample valve was opened briefly to introduce the CO2 to the fiber sorbent or crystal sample. The pressure decay over time was recorded for each expansion, allowing for the determination of sorption kinetics and generation of sorption isotherms. Constraints of this simple experiment preclude moderation of local thermal heat effects within the sample cell. Those effects can be avoided in the actual RTSA system. Barrier Layer Formation and Characterization. Creation of the lumen layer via a post-treatment method is straightforward using a latex form of polyvinylidene chloride (PVDC), a very effective barrier polymer. This choice avoids the need for an organic solvent and allows for multiple PVDC “washes” if needed, followed by a drying step to remove the excess aqueous solution (Figure 5). PVDC latex for application in the lumen layer was supplied by SolVin Chemicals (Northwich, UK). This method also has the advantage of being quite tunable: latex concentration, drying gas humidity, length and number of washes, and pressure of the feed latex are all parameters that can be varied. The fiber sorbents were assembled into standard shell and tube modules with a length of 8 in.24 Gas transport properties of the untreated fiber sorbents were characterized with pure N2 and CO2 at bore-side feed pressures of 20-30 psig. Shellside permeate flow rates were measured using bubble flow meters every 45 min until the readings were within 5% of the previous reading. Fibers were post-treated by flowing PVDC through the bores of the fiber at 10 psig using a 500 mL ISCO pump while vacuum was pulled on the shell-side of the modules (Figure 6). The layer was then dried with humid air (50% R.H.) before the next post-treatment. After five post-treatments, the fiber module was dried with dry air flowing through the bore, followed by annealing in a vacuum oven at 90 °C. Treated fibers were characterized with pure N2 and CO2 at bore-side feed pressures of 70-80 psig. Permeate flow rates were measured with downstream pressure transducers (Ametek, Costa Mesa, CA) once steady state was achieved. Scanning Electron Microscopy. Scanning electron microscopy (SEM) was used to evaluate fiber sorbent pore structure, polymer-filler interfaces, and probe for lumen layer defects. Solvent exchanged fibers were soaked in hexane for 2 min,

transferred to liquid N2, and sheared in half using two fine point tweezers. This procedure ensured sharp fiber breaks, and the fibers were then sputter-coated with a 10-20 nm thick gold coating (Model P-S1, ISI, Mountain View, CA) and transferred to a high resolution field emission scanning electron microscope, Leo 1530 (Leo Electron Microscopy, Cambridge, UK). TGA Characterization. TGA (thermogravimetric analysis) was used to analyze the stability of the fiber sorbents in a cyclic thermal environment in a series of experiments described below. A Netzsch STA 409 TGA was used, which could be programmed to control heating and cooling ramp rates and thermal soak times. Liquid nitrogen dried helium gas was set as the TGA purge and protective gas for the inTGA drying (36 h, 115 or 400 °C). As the drying step was completed, the TGA was cooled to 100 °C, and the purge and protective gases were switched using a three-way valve to CO2. For thermal cycling, CO2 gas was used to determine the regenerability and response rates of the sorbents under cyclic conditions. Separate fresh Zeolite 13X samples were run at two activation temperatures, 400 and 115 °C. The 400 °C activation was performed to check the validity of the results from this technique when compared to more traditional sorption characterization methods. The 115 °C activation was performed to analyze the effect of low temperature activation on pure 13X. Finally, the same activation and thermal cycles were performed on the fiber sorbents. CO2 Flow System Characterization. Fixed-bed sorption techniques were used to determine CO2 sorption capacities in a simulated flue gas stream by placing adsorbent in a pyrex tubular reactor (1/4 in. OD) which was then degassed under flowing argon at 115 °C.25 Adsorption experiments were carried out by flowing test gas (10% CO2, balance Ar) through the temperature-controlled adsorbent bed at 20 mL/min at atmospheric pressure. The concentration of CO2 at the reactor outlet was transiently measured by mass spectrometry (ultrahigh vacuum Pfeiffer Vacuum QMS 200 Prisma Quadrupole Mass Spectrometer) until equilibrium was reached. The amount of CO2 adsorbed was calculated by integration of the initial concentration to the final equilibration concentration. Switching of gas flows at the start of an adsorption experiment causes initial dilution of the test gas by the inert purge gas remaining in the system tubing. This dilution effect was measured with blank samples, and the adsorption results were corrected accordingly. 4. Results and Discussion Formation of First-Generation Hollow Fiber Sorbents. To maximize mass transfer rates to the sorbents, a so-called “sieve in a cage” morphology is desirable in which the dispersed particles are not attached at all points on the particle surface to the polymer matrix (see Figure 7). This morphology holds the particle in place while allowing for facile gas bulk diffusion throughout the interconnected pore structure of the fiber without occluding access to the pores of the sorbents. A combination of known factors can be controlled during fiber spinning to allow for the creation of the “sieve in a cage” morphology.26,27 Initial work was carried out to determine a viable spin dope for the proof of concept sorbent, Zeolite 13X. Using standard cloud point techniques,19 the final polymer dope solution chosen was 10 wt % CA/ 4% PVP (pore former)/30% 13X/ 49.3% NMP/6.7% water. In the dry phase, this corresponds to 75 wt % zeolite loading. This solution was chosen because of its rapid phase separation compared to other CA solutions


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Figure 8. (top left): SEM image of fiber sorbent. (top right): 13X dispersion (75 wt %) in cellulose acetate matrix. (bottom): 13X particles exhibiting “sieve in a cage” morphology in cellulose acetate matrix.

Figure 9. Porosity gradient in fiber sorbents. Notice the regions of higher porosity near the bore (left side of figure), and the regions of lower porosity moving toward the exterior of the fiber.

and its high filler loading. Dopes with much higher polymerto-liquid ratios phase separated very slowly and were less ideal for spinning, while dopes with much higher zeolite concentrations could not be spun into a hollow fiber without fracturing. The spin dopes and spinning parameters that gave the best properties are shown in Table 1, and since the optimization to select the preferred parameters relies upon conventional techniques known in the hollow fiber membrane literature, it need not be discussed further here.24 A deep coagulation bath (3 ft. depth) was used to provide more time for the nascent fibers to completely phase separate. Low take up rates were used to produce large fiber OD requirements while high extrusion rates were used to manipulate the die swell of the line to reach the required OD. The bore fluid, composed mainly of NMP, prevented rapid phase separa-

tion at the interior lumen of the fiber. Finally, the air gap was set very low to approach wet-spinning without actually submerging the spinneret, which might cause phase separation in the spinneret annulus. Wet-spinning was desirable to avoid formation of a dense outer resistive skin layer. The optimized fibers were found to be (qualitatively) mechanically strong and able to withstand normal fiber potting procedures. SEM images of these fibers reveal that the fiber design objectives were achieved. As can be seen in Figure 8, the fiber OD and ID are close to the design goal, with the OD being 1100 µm, and the ID being 300 µm as well as good sorbent dispersion seen throughout the porous polymer matrix. The bottom figure shows that the preferred “sieve-in-a-cage” morphology,19 needed for rapid mass transport through the fibers, was achieved. Gas permeation measurements performed on these monolithic hollow fibers showed them to be nonselective with respect to CO2, with a N2 permeation rate of 65 000 ( 21 000 gas permeation units (GPU, or 10-6 cm3-STP/cm2-s-cmHg). This high permeance suggests that the desired open, continuous pore structure has been achieved. A simple bubble test was conducted to determine the largest possible pore size. The shell-side of the module was immersed in water while pressurized nitrogen gas was introduced on the bore-side of the module. The pressure was slowly increased until the first appearance of bubbles was recorded at 4.0 psig. The maximum pore radius was estimated using the Kelvin equation,28 ∆P )

2γ rp

(1)

where ∆P refers to the gas pressure drop, γ represents the surface tension of the water, and rp is the maximum pore radius. The largest possible pore was estimated to be 9.0 µm, a reasonable value from visual inspection of the SEM images. Formation of Barrier Lumen Layer. Another important feature of the fiber sorbent is the creation of a robust lumen barrier

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Figure 10. (top left): SEM of PVDC lumen layer on CA/PVP/13X fiber sorbent. (top right, bottom): SEM close-up of PVDC lumen layer. The layer appears to completely coalesce to the CA layer and the 13X particles contained therein.

Figure 11. (a) From top: Typical hollow fiber membrane operation. (b) Lumen layer bypass in fiber sorbents. (c) Capillary forces during post-treatment were responsible for PVDC capping of fiber tip. (d) Capped fiber tip with no lumen layer bypass.

layer to enable facile heat exchange between bore and fiber wall without significant mass exchange. This layer can be introduced by post-treatment onto the interior of the fiber. For the fiber sorbent RTSA system to operate effectively, the lumen layer must provide a robust barrier to both gas and water in the presence of the continuous thermal cycles. The efficacy of the barrier layer can be probed with permeation techniques, as discussed later. Good adhesion between the sorbent layer and the lumen layer is required,

and the barrier polymer deposition medium must be a poor solvent or nonsolvent for the sorbent layer polymer. Gas permeation was used to evaluate the efficacy of the lumen layer by determining the rate of gas transport through the layer under a fixed driving force between shell and bore-sides of the fiber. In a molecularly perfect lumen layer, the permeation of one gas through the dense polymer layer will be faster than the other in a gas pair. In the porous outer structure of the fiber,


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And RAB =

Figure 12. SEM of PVDC-capped fiber tip. These fibers were used to perform gas permeation experiments.

Figure 13. (a) (top) and (b) (bottom): CO2 sorption isotherms of fiber sorbents, 13X and literature values for CA,33 at 45 °C (top) and 100 °C (bottom). Triangles indicate sorption isotherms for 13X, squares indicate 13X fiber sorbents, and open symbols indicate CO2 flow system results. The dashed line indicates the predictions for 13X fiber sorbents based off a simple weighted average of the CA and 13X sorption capacities.

however, negligible (Knudsen) selectivity is seen due to the minimal gas phase resistances present in the structure. Gas permeability and selectivity are defined as,21 Pi )

(flux)il ∆pi

(2a)

PA PB

(2b)

In the fiber sorbents, however, it is not practical to measure the thickness of the barrier layer, so the gas permeance (Pi/l) is often used to characterize hollow fiber membranes. The gas transport of different pure gases can be used to determine if the barrier layer is defective or not, since any pinholes in the barrier layer allow for a Knudsen diffusion pathway, which would dominate any molecularly selective transport across the lumen layer. Observation of the presence or absence of such Knudsen behavior, therefore, allows one to infer the presence or absence of pinhole defects. The PVDC latex post-treatment method was simple using the as-received latex after dilution with 60 vol % using DI H2O to facilitate flow through the fiber bore at 10 psig. At latex concentrations higher than this, the latex solution was found to block the fiber bore. From Figure 8, above, and Figure 9, below, a porosity gradient extending outward from the bore can be seen, with the fiber porosity decreasing toward the shell-side of the fiber. This porosity gradient is a result of the high concentrations of NMP used as the bore fluid, which maintains the internal nascent fiber in solution while the outside of the fiber is rapidly phase-separating. This porosity gradient provides good uptake at the fiber bore with an intrinsic backstop to the flow of latex (at the operating pressures of 20-30 psig) through the bore at approximately 30 µm, and this can be seen in Figure 10. The latex post-treatment resulted in a very dense barrier layer that does not occupy any additional space within the bore and requires little of the active (sorbing) volume within the sorbent fiber body. The latex moves through the porous region around the bore via capillary forces, thereby allowing every pore to be filled. During the air sweep, excess water in the latex is carried away, causing the surface tension of the evaporating water to draw the polymer particles closer together, while the particle surfaces can interdiffuse to form an intrinsically defect-free layer.29 Carbon dioxide and nitrogen gas permeation were performed on the post-treated sorbent fibers. The dense PVDC layer provides a significant resistance to gas transport, reducing the N2 permeance down to only 3.0 ( 0.3 GPU at 70 psia, while the CO2 permeance was found to be 3.3 ( 0.8 GPU at 70 psia, resulting in a CO2/N2 selectivity of 1.1,29,30 compared to the expected dense polymer selectivity of about 12. Although these permeances are very low in comparison to the untreated fibers (a 99.995% reduction in permeance), they suggest the presence of a few pinhole defects, since a 20-30 µm thick PVDC layer should show a permeance less than 0.01 GPU. Further refinement of the post-treatment protocol should result in such a defect-free lumen layer and is necessary for effective operation of the RTSA system, as a 3.0 GPU permeance is equivalent to 5% steam losses through the barrier layer. An additional issue that had to be overcome with the fiber sorbents involved lumen layer bypass. In a typical selective hollow fiber membrane, the outer selective skin seals against the potting material, thereby forcing feed streams to pass through the selective portion of the fiber (Figure 11a). In a fiber sorbent, however, with the barrier layer on the interior of the fiber, no such seal exists. As such, water and steam that are introduced on the bore-side can bypass through the core structure of the fiber into the shell-side of the manifold. Furthermore, flue gas from the shell-side feed could escape into the water and steam systems (Figure 11b). This problem, if not remedied, would

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Figure 14. CO2 response time comparisons between pure 13X and sorbent fibers embedded with 13X at 45 °C. Closed squares indicate fiber sorbent responses, and closed triangles indicate 13X crystal responses.

Figure 15. TGA results for cellulose acetate/13X fiber sorbents in an air atmosphere.

Figure 16. Thermal cycling of Zeolite 13X in CO2 atmosphere (14.7 psia) after 400 °C activation. The sample temperature is plotted on the right, while the corresponding mass change of the sample is plotted on the left as mmol of CO2 per gram of sorbent (dry basis).

render the RTSA system ineffective. To counter this, a simple method of “capping” the fibers at the potting seals was developed. During the post-treatment method, capillary forces

present at the face of the fiber pull the latex back into the tip of the fiber. This latex was then allowed to dry with an air sweep across the face of the fibers (Figures 11c and 11d). The amount


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Figure 17. Thermal cycling of Zeolite 13X and 13X fiber sorbents in a CO2 atmosphere (14.7 psia) after 115 °C activation with C0 arbitrarily set to be the maximum Zeolite 13X capacity at 45 °C.

of dilution of the latex is important, as more dilute solutions do not exhibit the “capping” effect. The success of this capping technique can be seen in Figure 12 and was confirmed by the ability to perform permeation experiments (an ineffective capping would result in permeation bypass). Hollow Fiber Sorption Characterization. Sorption in the hybrid fibers occurs primarily in the solid sorbents or other sorbent materials embedded in the fiber wall, and sorption uptake can be described in terms of a Langmuir isotherm, Ci )

CH,i ′ bipi 1 + bipi

(3)

where Ci is the amount of sorbate in the molecular sieve, C′H,i is the pore saturation parameter of the molecular sieve sorbent (cc[STP]/cm3 sorbent), bi is the pore affinity for the sorbate (psi-1), and pi is the sorbate partial pressure (psi).31,32 Pressure decay sorption experiments were performed to determine sorption kinetic times and sorption equilibrium in the sorbents. Sorption experiments on the fiber sorbents were compared with literature results for CA,33 adjusted to the measurement temperatures. The fiber sorbent and Zeolite 13X (activated at 115 °C) sorption isotherms at 45 and 100 °C are displayed in Figures 13a and 13b, respectively, and agree well with the flow system results at the low CO2 concentrations. Using the first exposure, sorption kinetics on the sorbent fibers were performed and are shown in Figure 14. The fiber sorbents achieved greater than 95% equilibration in under 4 s. The 13X crystals, however, lagged behind the fiber sorbents in apparent response time because of retarded heat dissipation in the sorption cell for the well-packed crystal bed. While both responses are compromised by the nonisothermicity caused by the intense sorption enthalpy, the results show relatively rapid sorption equilibration, and subsequent TGA results discussed next support this as well. For the TGA studies, a simple three-step heat program was used to confirm the predicted loading of Zeolite 13X in the fiber sorbents. The first two heat steps removed excess water from the zeolites and cellulose acetate, and the final step burned off the cellulose acetate (Figure 15). The final loading of the fiber sorbents was found to be 74.5 wt % Zeolite 13X. To determine the validity of the cyclic TGA experiment, 13X was conditioned under He at 400 °C and then cycled between

100 and 45 °C. The 400 °C mass, with appropriate buoyancy corrections, was taken as the baseline mass, thereby enabling sorption capacities of Zeolite 13X to be calculated. Figure 16 shows the cyclic portion of the thermal program as well as the mass gain/loss versus time. The CO2 capacities match well with values reported in the literature.17 Because of the polymer matrix supporting the zeolites, a 400 °C conditioning is not possible when studying the fibers; therefore, a low temperature activation must be used. However, this is consistent with the planned used of the sieves so this working capacity is relevant. A temperature of 115 °C was used as the baseline/drying temperature under He, and repeated thermal cycles between 100 and 45 °C were performed on the 13X crystals and the fiber sorbents (Figure 17) under CO2. The regenerability of the fiber sorbents can clearly be seen to be associated with the embedded 13X sorbents. Furthermore, the cellulose acetate matrix does not hinder CO2 diffusion into and out of the embedded Zeolite 13X, as can be seen by the near-perfect matching of the mass and temperature signals. While the actual RTSA operation will involve heating and cooling by steam and water, respectively, as opposed to the heated gas environment of the TGA, the TGA studies allow for a “worst-case” approach to heat cycling, since heat transfer from the gas atmosphere to the sorbents will be much slower than the heat transfer between water and the sorbents. The ability of the fiber sorbents to rapidly cycle in CO2 environments, combined with the formation of an effective first generation lumen barrier layer, indicate that hollow fiber adsorbents are one possible solution for postcombustion CO2 recovery. 5. Conclusions This work describes a novel sorption platform based on a hybrid sorbent/polymer hollow fiber that allows for very rapid thermal equilibration and improved sorption efficiencies. This hollow fiber sorbent platform could possibly reduce carbon capture costs by combining the advantages of a traditional packed bed separation (high product recoveries, lower heat requirements than liquid absorbents) while mitigating the negative aspects (long cycle times, large bed pressure drops, etc). Proof-of-concept high loaded hollow fiber sorbents have been successfully spun using cellulose acetate and 13X; sorption characterization confirms the rapid kinetics required by the

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RTSA system. Furthermore, cyclic thermal programs in the TGA confirm the presence of a stable working capacity for the fiber sorbents. A nearly impermeable lumen-side barrier layer has been formulated on the fiber sorbents using a latex posttreatment technique; this layer prevents heat transfer agents in the bore, such as steam and water, from mass exchanging with the shell-side flue gas. There are a number of hurdles to overcome in the development of an economically viable CO2 capture system as described herein. Means must be found to mitigate the harmful effects of water, either by pretreatment or by development of a sorbent with reduced water sensitivity, which is an active area of research in the solid sorbent community.25,34,35 These water-resistant sorbents can be readily added into the polymer matrix of the fibers, which could potentially negate the deleterious effects of water. In addition, success will critically depend on heat management, efficient integration of process heat, and effective reuse of heat associated with the RTSA cycles. Potential hydrodynamic limitations on performance must also be assessed via realistic lab scale modules in combination with state-of-the-art modeling. Efforts are underway to address these issues. Acknowledgment The authors thank ExxonMobil for funding this research. Literature Cited (1) Carbon Dioxide Capture and Storage; 2002; pp 21-37. (2) Cost and Performance Baseline for Fossil Energy Plants. Volume 1: Bituminous Coal and Natural Gas to Electricity, NETL Technical Report Number DOE/NETL-2007/1281, August 2007. http://www.netl.doe.gov/ energy-analyses/pubs/Bituminous%20Baseline_Final% 20Report.pdf. (3) Gough, C. State of the art in carbon dioxide capture and storage in the UK: An experts’ review. Int. J. Greenhouse Gas Control 2007, 2 (1), 155–168. (4) Bachu, S. Sequestration of CO2 in geological media: criteria and approach for site selection in response to climate change. Energy ConVers. Manage. 2000, 41, 953–970. (5) Favre, E. Carbon dioxide recovery from post-combustion processes: Can gas permeation membranes compete with absorption. J. Membr. Sci. 2007, 294 (1-2), 50–59. (6) Kohl, A. L.; Nielsen, R. B. Gas Purification, 5th ed.; Elsevier: Houston, 1997. (7) Yan, S. P.; Fang, M. X.; Zhang, W. F.; Wang, S. Y.; Xu, Z. K.; Luo, Z. Y.; Cen, K. F. Experimental study on the separation of CO2 from flue gas using hollow fiber membrane contactors without wetting. Fuel Process. Technol. 2007, 88, 501–511. (8) Wang, R.; Zhang, H. Y.; Feron, P. H. M.; Liang, D. T. Influence of membrane wetting on CO2 capture in microporous hollow fiber membrane contactors. Sep. Purif. Technol. 2005, 46, 33–40. (9) Riemer, P. The capture of carbon dioxide from fossil fuel power stations. IEA Green House Gas Research, 1993, Report IEAGHG/SR2, London, UK. (10) IAE GHG R&D Programme 10th Anniversary Report; 1991-2001. (11) Lively, R. P. Hollow-Fiber-Based Adsorbent System for CO2 Capture from Flue Gas, Presented at the AIChE Annual Conference, November2008, Philadelphia, PA. (12) Lively, R. P.; Chance, R. R.; Kelley, B. T.; Deckman, H.; KorosW. J. Sorbent Fiber Compositions and Methods of Temperature Swing Adsorption. U.S. Patent Application 12/163140. June 27, 2008. (13) Lee, K. B.; Sircar, S. Removal and Recovery of Compressed CO2 from Flue Gas by a Novel Thermal Swing Chemisorption Process. AIChE J. 2008, 54 (9), 2293–2302.

(14) Merel, J.; Clausse, M.; Meunier, F. Experimental Investigation on CO2 Post-Combustion Capture by Indirect Thermal Swing Adsorption Using 13X and 5A Zeolites. Ind. Eng. Chem. Res. 2008, 47, 209–215. (15) Gilleskie, G. L.; Parker, J. L.; Cussler, E. L. Gas Separations in hollow-fiber adsorbers. AIChE J. 1995, 41 (6), 1413–1425. (16) McKelvey, S. A.; Clausi, D. T.; Koros, W. J. A guide to establishing hollow fiber macroscopic properties for membrane applications. J. Membr. Sci. 1997, 124 (2), 223–232. (17) Chue, K. T.; Kim, J. N.; Yoo, Y. J.; Cho, S. H.; Yang, R. T. Comparison of Activated Carbon and Zeolite 13X for CO2 Recovery from Flue Gas by Pressure Swing Adsorption. Ind. Eng. Chem. Res. 1995, 34 (2), 591–598. (18) Qin, J. J.; Li, Y.; Lee, L. S.; Lee, H. Cellulose acetate hollow fiber ultrafiltration membranes made from CA/PVP 360 K/NMP/water. J. Membr. Sci. 2003, 218, 173–183. (19) Husain, S. Mixed Matrix Dual Layer Hollow Fiber Membranes for Natural Gas Separation. Ph.D. Dissertation, Georgia Institute of Technology, Atlanta, GA, 2006. (20) Li, S. G.; Koops, G. H.; Mulder, M. H. V.; van den Boomgaard, T.; Smolders, C. A. Wet spinning of integrally skinned hollow fiber membranes by a modified dual-bath coagulation method using a triple orifice spinneret. J. Membr. Sci. 1994, 94 (1-3), 329–340. (21) Barrer, R. M., Diffusion and permeation in heterogeneous media. In Diffusion in Polymers; Crank, G. S. P., Ed.; Academic Press: New York, 1968; pp 165-217. (22) Koros, W. J.; Paul, D. R. Design considerations for measurement of gas sorption in polymers by pressure decay. J. Polym. Sci., Polym. Phys. Ed. 1976, 14, 1903. (23) Chandra, P. Multi-component Transport of Gases and Vapors in Poly(Ethylene Terephthalate). Ph.D. Dissertation, Georgia Institute of Technology, Atlanta, GA, 2006. (24) McKelvey, S. A. Formation and characterization of hollow fiber membranes for gas separation (fiber breaks, macrovoids). Ph.D. Dissertation, University of Texas, Austin, Texas, 1997. (25) Hicks, J. C.; Drese, J. H.; Fauth, D. J.; Gray, M. L.; Qi, G.; Jones, C. W. Designing Adsorbents for CO2 Capture from Flue Gas-Hyperbranched Aminosilicas Capable of Capturing CO2 Reversibly. J. Am. Chem. Soc. 2008, 130, 2902. (26) Mahajan, R.; Koros, W. J. Factors Controlling Successful Formation of Mixed-Matrix Gas Separation Materials. Ind. Eng. Chem. Res. 2000, 39 (8), 2692–2696. (27) Moore, T. T. Effects of materials, processing, and operating conditions on the morphology and gas transport properties of mixed matrix membranes. Ph.D. Dissertation, Univ. of Texas, Austin, TX, 2004. (28) Rousseau, R. W. Handbook of Separation Process Technology; John Wiley & Sons: New York; p 917. (29) Brown, G. L. Formation of Films from Polymer Dispersions. J. Polym. Sci. 1956, 22, 423–434. (30) Sweeting, O. J. The Science and Technology of Polymer Films; John Wiley & Sons, Inc.: New York, 1971. (31) Ruthven, D. M.; Farooq, S.; Knaebel, K. S. Pressure Swing Adsorption; John Wiley & Sons, Inc.: Hoboken, NJ, 1994. (32) Yang, R. T. Gas Separation by Adsorption Processes; Butterworth Publishers: Stoneham, MA, 1987. (33) Stern, S. A.; Kulkarni, S. S. Solubility of methane in cellulose acetate-conditioning effect of carbon dioxide. J. Membr. Sci. 1982, 10, 235– 251. (34) Serna-Guerrero, R.; Da’na, E.; Sayari, A. New Insights into the Interactions of CO2 with Amine-Functionalized Silica. Ind. Eng. Chem. Res. 2008, 47 (23), 9406–9412. (35) Liu, J.; Wang, Y.; Benin, A. I.; Jakubczak, P.; Willis, R. R.; LeVan, M. D. CO2/H2O Adsorption Equilibrium and Rates on CuBTC and NiDOBDC. Presented at the AIChE Annual Conference, November, 2008, Philadelphia, PA.

ReceiVed for reView April 1, 2009 ReVised manuscript receiVed June 3, 2009 Accepted June 5, 2009 IE9005244

Removal from Flue Gas - American Chemical Society

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