Hollow Fiber Sorbents Research Articles

Self-regenerative rapid thermal swing adsorption process using structured fiber sorbents for energy-efficient H2O removal

Rapid thermal swing adsorption (RTSA) using hollow fiber sorbents (HFSs) is an emerging separation process that enables significant energy savings. Herein, we propose a self-regenerative RTSA process based on multi-modules of zeolite-13X-impregnated HFSs for energy-efficient H2O removal in various chemical processes. The proposed dehumidification process consumes almost 82% lower energy than the conventional adsorption counterpart for H2O removal because it uses a combination of high heat transfer efficiency and self-regeneration. The steam recovered from the desorption process can be combined with the thermal fluid for H2O desorption from the HFSs, which is referred to as the self-regenerative RTSA process. In addition, the residual sensible heat emitted from the desorption process can be internally integrated into the heating process, which reduces both the sensible heat of the HFSs and desorption heat of H2O. Consequently, the energy evaluation revealed that the newly proposed self-regenerative RTSA process required a total energy of ca. 0.68 GJ t-H2O-1 for H2O adsorption with a heat integration efficiency of 80%. The current work successfully demonstrates an extended application of the RTSA process for dehumidification.

Scalable Formation of Diamine-Appended Metal–Organic Framework Hollow Fiber Sorbents for Postcombustion CO2 Capture

We describe a straightforward and scalable fabrication of diamine-appended metal–organic framework (MOF)/polymer composite hollow fiber sorbent modules for CO2 capture from dilute streams, such as flue gas from natural gas combined cycle (NGCC) power plants. A specific Mg-MOF, Mg2(dobpdc) (dobpdc4– = 4,4′-dioxidobiphenyl-3,3′-dicarboxylate), incorporated into poly(ether sulfone) (PES) is directly spun through a conventional “dry-jet, wet-quench” method. After phase separation, a cyclic diamine 2-(aminomethyl)piperidine (2-ampd) is infused into the MOF within the polymer matrix during postspinning solvent exchange. The MOF hollow fibers from direct spinning contain as high as 70% MOF in the total fibers with 98% of the pure MOF uptake. The resulting fibers exhibit a step isotherm and a “shock-wave-shock” breakthrough profile consistent with pure 2-ampd-Mg2(dobpdc). This work demonstrates a practical method for fabricating 2-ampd-Mg2(dobpdc) fiber sorbents that display the MOF’s high CO2 adsorption capacity while lowering the pressure drop during operation.

CO2 Capture Using PIM-1 Hollow Fiber Sorbents with Enhanced Performance by PEI Infusion

This paper describes hollow fiber sorbents made of the parent polymer of intrinsic microporosity, PIM-1, using a spinning method designed to control the formation of dense barrier layers followed by a simple post treatment with polyethylenimine (PEI) for further improvement of CO2 uptake. The overall process involved solvent-moderated sheath-core spinning with a variation of core/bore flow rates and quench bath temperatures. Dope compositions to promote micropore interconnectivity were optimized by controlling the dope nonsolvent concentration. Under optimized conditions, undesirable skin layers were not formed, avoiding associated mass transfer resistance. The resulting PIM-1 hollow fiber sorbents achieved good CO2 uptake, with maximum equilibrium capacities of 2.8 mmol-CO2/g-fiber. CO2 sorption kinetics were dependent on PEI loading, allowing the designer to balance between capacity and uptake rate, a control feature unavailable with the pure PIM fiber sorbent.

Rapid thermal swing adsorption process in multi-beds scale with sensible heat recovery for continuous energy-efficient CO2 capture

Abstract A rapid thermal swing adsorption (RTSA) process with heat integration is considered to be an energy-efficient method for large-scale CO2 capture. Herein, we propose a new heat-integrated RTSA process for post-combustion CO2 capture as a proof-of-concept based on rationally designed multi-fixed beds by incorporating the internal heat integration concept of the moving bed adsorption process. Our proposed RTSA process consists of multiple beds, each of which undergoes four sequential processes, including adsorption, heating, desorption, and cooling in a continuous cyclic mode. Also, the switching time of adsorption/desorption process is determined by CO2 adsorption/desorption kinetics while that of the heating/cooling process is determined by the heat transfer rate. Our unique RTSA design enables internal heat integration by recovering sensible heat from the cooling process. Furthermore, it can enhance the CO2 cyclic capacity of hollow fiber sorbents with high CO2 recovery by performing multiple adsorption processes simultaneously. Most importantly, our economic analysis based on experiments and rigorous mathematical modeling revealed that the energy demand of the optimized RTSA process is estimated to be approximately 272 kWh/t-CO2 with 58% sensible heat recovery. Such low energy penalty is comparable to that of the most mature amine-based absorption process (271 kWh/t-CO2) due to a combination of internal heat integration and enhanced CO2 cyclic capacity, supporting the feasibility of a RTSA process for post-combustion CO2 capture.

Fabrication of Solution-Cast Polyacrylonitrile Barriers for Hollow Fiber Sorbents Used in CO2 Removal from Flue Gas

Hollow fiber sorbents for gas separation benefit from the use of a heat transfer fluid transported through their bores for thermal moderation and heat recovery. Additionally, for thermal swing proc...

Carbon nanotube silica composite hollow fibers impregnated with polyethylenimine for CO2 capture

The capture of carbon dioxide from flue gases or air using sorption technologies is an adequate means to stop the increase in CO2 emissions that are the main contributor to climate change. Hollow fibers for sorption processes are a promising approach with several improved characteristics such as a decreased pressure drop, lower energy consumption as well as higher CO2 recovery and purity compared to conventional packed-beds. To overcome thermal and chemical limitations of a polymeric matrix we propose carbon nanotube (CNT) based hollow fiber sorbents. Silica particles are dispersed in the CNT network and polyethylenimine is immobilized within the hollow fibers to enhance the sorption performance. Polymeric amines exhibit high CO2 adsorption capacities even at very low partial pressures. The incorporation of silica particles improves the specific surface area of the so-called microtubes from 203.6 m2 g−1 to 283.2 m2 g−1 and yields a CO2 uptake of 1.92 mmol g−1 at 0.15 bar CO2 partial pressure. A gas tight polymer layer coated on the shell side allows the use of a thermal moderator for isothermal operation. Adsorption isotherms and kinetics are presented, discussed and fitted applying appropriate models. The results presented here prove the adaptiveness of the carbon nanotube based hollow fibers and emphasize the possibilities they open up as a platform for different sorption tasks that extend beyond carbon capture.

Multiwall carbon nanotube- zirconium oxide nanocomposite hollow fiber solid phase microextraction for determination of polyaromatic hydrocarbons in water, coffee and tea samples

The purpose of this study was to evaluate the application of hollow fiber solid-phase microextraction (HF-SPME) followed by HPLC-UV to determine the ultra-trace amounts of polycyclic aromatic hydrocarbons (PAHs) as model analytes in complex coffee and tea samples. HF-SPME can be effectively used as an alternative to the direct immersion SPME (DI-SPME) method in complex matrices. The DI-SPME method suffers from serious limitation in dirty and complicated matrices with low sample clean-up, while the HF-SPME method has high clean-up and selectivity due to the high porosity of hollow fiber that can pick out analyte from complicated matrices. As a hollow fiber sorbent, a novel multiwall carbon nanotube/zirconium oxide nanocomposite (MWCNT/ZrO2) was fabricated. The excellent adsorption of PAHs on the sorbent was attributed to the dominant roles of π–π stacking interaction and hydrophobic interaction. Under the optimized extraction conditions, the wide linear range of 0.1–200 μg L−1 with coefficients of determination better than 0.998 and low detection limits of 0.033–0.16 μg L−1 with satisfactory precision (RSD < 6.6%) were obtained. The relative recoveries obtained by spiking the PAHs in water, coffee and tea samples were in the range of 92.0–106.0%. Compared to other methods, MWCNT/ZrO2 hollow fiber solid phase microextraction demonstrated a good capability for determination of PAHs in complex coffee and tea samples.

Optimized Hollow Fiber Sorbents and Pressure Swing Adsorption Process for H2 Recovery

Hollow fiber sorbents have overcome many limitations of packed-bed adsorbers. To fully exploit their potential, holistic design methods are crucial, but overall optimization strategies are lacking and are presented here for the first time. To showcase the methodology, we investigated the separation of H2 and CO2 using hollow fiber sorbents made of zeolites and cellulose acetate. The specific purification costs could be reduced by 13% using the optimization methodology. The purification costs rise from 0.65 €/kg to 0.85 €/kg with increasing product purity from 99% to 99.99%. An increase of the adsorption capacity by 50% only decreases the cost by 5%. Yet, the major conclusion relates to the hollow fiber dimensions, which only change slightly with product requirements and sorption capacities. The optimal inner and outer fiber diameter are in the range of 0.33–0.38 mm as well as 0.80–0.91 mm, respectively. The optimal fiber length varies between 98.7 and 103.1 cm. Thus, we conclude that the identified fiber design is close to a generic optimum.

High capacity polyethylenimine impregnated microtubes made of carbon nanotubes for CO2 capture

Anthropogenic carbon dioxide emissions cause global warming - one of the major challenges society faces today. Carbon capture technologies can mitigate this development and stabilize the global average temperature. However, to reach their potential, energy efficient processes and new materials are required. In this work, we present a new type of hollow fiber sorbents. Highly porous multi-walled carbon nanotube based (MWCNT) microtubes with a high internal surface area were functionalized with polyethylenimine (PEI) via a wet impregnation method. Their tubular geometry opens up the device design space to control gas distribution and residence time. For low PEI fractions in the impregnation solution, the high surface area and porosity of the microtubes is preserved, and the CO2 uptake increases with increasing PEI fractions. We observed a maximum CO2 uptake of 93 mg/gfiber at 20 wt.-% of PEI in the impregnation solution. At higher PEI loadings the CO2 uptake decreases. Due to the thermal and chemical stability of the PEI sorbent and the CNT matrix, we anticipate the applicability of these microtubes for a wide range of separation processes and operation conditions. Such tubular MWCNT geometries may serve as a scaffold platform to immobilize a functional surface coating inside a controlled nanoporosity.

Effect of Post‐Functionalization Conditions on the Carbon Dioxide Adsorption Properties of Aminosilane‐Grafted Zirconia/Titania/Silica‐Poly(amide‐imide) Composite Hollow Fiber Sorbents

AbstractIn this study, 3‐aminopropyltrimethoxysilane (APS) was used as a grafting agent in three solvents with different polarities for post‐functionalization of a ZrO2–TiO2–SiO2/poly(amide‐imide) composite hollow fiber sorbent (Zr–Ti–Si/PAI‐HF) at ambient temperature. The same fiber at two different states: (i) wet (as‐spun) and (ii) dry were used for amine grafting. The influence of the polarity/nature of various solvents such as acetone, ethanol, and toluene on the CO2 sorption characteristics of wet and dry Zr–Ti–Si/PAI‐HF fibers was investigated accordingly. Several parameters such as grafting time (1–4 h), amine concentration (10–50 wt %), and solvent type (acetone, ethanol, and toluene) were systematically varied. The CO2 sorption capacity and breakthrough tests were performed under simulated flue gas conditions for grafted fiber composites. Our results indicate that the CO2 sorption capacity of amine‐grafted Zr–Ti–Si/PAI‐HF strongly depends on both the type of solvent and the state of the fibers (i.e., dryness or wetness). The APS grafting of wet fibers performed in toluene led to high amine loading and CO2 capacity (1.40 mmol g−1), whereas dry fibers gave rise to a relatively low CO2 capacity (0.90 mmol g−1). In contrast, dry fibers functionalized in either acetone or ethanol exhibited higher CO2 capacity compared to the wet fibers prepared in the same solvent. By using ethanol as a solvent, the solvolysis of the Si−O−Si linkages is much faster than the competitive condensation reaction between the silanol groups (Si−OH) of APS moieties and hydroxyl group (OH) of the ethanol, which leads to the lower siloxane formation. In the case of acetone, a dense polymeric network is formed under condensation reaction between an acetone molecule and the amine moieties, thus reducing the amount of primary amine groups available for interacting with the CO2 molecules.

Uncertainty quantification via bayesian inference using sequential monte carlo methods for CO2 adsorption process

This work presents the uncertainty quantification, which includes parametric inference along with uncertainty propagation, for CO2 adsorption in a hollow fiber sorbent, a complex dynamic chemical process. Parametric inference via Bayesian approach is performed using Sequential Monte Carlo, a completely parallel algorithm, and the predictions are obtained by propagating the posterior distribution through the model. The presence of residual variability in the observed data and model inadequacy often present a significant challenge in performing the parametric inference. In this work, residual variability in the observed data is handled by three different approaches: (a) by performing inference with isolated data sets, (b) by increasing the uncertainty in model parameters, and finally, (c) by using a model discrepancy term to account for the uncertainty. The pros and cons of each of the three approaches are illustrated along with the predicted distributions of CO2 breakthrough capacity for a scaled‐up process. © 2016 American Institute of Chemical Engineers AIChE J, 62: 3352–3368, 2016

CO2 Sorption Performance of Composite Polymer/Aminosilica Hollow Fiber Sorbents: An Experimental and Modeling Study

The dynamic CO2 sorption performance of polymer/silica supported polyethylenimine hollow fiber sorbents (CA-S-PEI), focusing on heat and mass transport effects, is investigated experimentally and computationally during sorption of CO2 from simulated, dry flue gases. The effect of the nonisothermality on the sorption performance is investigated by varying the module materials of construction. The heat effects are minimized by using a heat conductive module case with a diameter of 0.25 in., and, accordingly, the breakthrough capacities are increased by 30% over a similar module constructed from less conductive components, thereby improving fiber sorbents utilization efficiency. The sorption kinetics in CA-S-PEI hollow fiber sorbents are investigated in terms of flow rates, module packing fraction, module length, and silica particle size. A mathematical model developed previously is successfully utilized to predict various contributions to the overall mass transfer resistance. In fiber sorbents where the ami...

Poly(amide-imide)/silica supported PEI hollow fiber sorbents for postcombustion CO(2) capture by RTSA.

Amine-loaded poly(amide-imide) (PAI)/silica hollow fiber sorbents are created and used in a rapid temperature swing adsorption (RTSA) system for CO2 capture under simulated postcombustion flue gas conditions. Poly(ethylenimine) (PEI) is infused into the PAI/mesoporous silica hollow fiber sorbents during fiber solvent exchange steps after fiber spinning. A lumen-side barrier layer is also successfully formed on the bore side of PAI/silica hollow fiber sorbents by using a mixture of Neoprene with cross-linking agents in a post-treatment process. The amine loaded fibers are tested in shell-and-tube modules by exposure on the shell side at 1 atm and 35 °C to simulated flue gas with an inert tracer (14 mol % CO2, 72 mol % N2, and 14 mol % He, at 100% relative humidity (RH)). The fibers show a breakthrough CO2 capacity of 0.85 mmol/g-fiber and a pseudoequilibrium CO2 uptake of 1.19 mmol/g-fiber. When tested in the temperature range of 35-75 °C, the PAI/silica/PEI fiber sorbents show a maximum CO2 capacity at 65 °C, owing to a trade-off between thermodynamic and kinetic factors. To overcome mass transfer limitations in rigidified PEI infused in the silica, an alternate PEI infusion method using a glycerol/PEI/methanol mixture is developed, and the CO2 sorption performance is improved significantly, effectively doubling the functional sorption capacity. Specifically, the glycerol-plasticized sorbents are found to have a breakthrough and equilibrium CO2 capacity of 1.3 and 2.0 mmol/g of dry fiber sorbent at 35 °C, respectively. Thus, this work demonstrates two PAI-based sorbents that are optimized for different sorption conditions with the PAI/silica/PEI sorbents operating effectively at 65 °C and the PAI/silica/PEI-glycerol sorbents operating well at 35 °C with significantly improved sorption capacity.

Dynamic CO2 adsorption performance of internally cooled silica‐supported poly(ethylenimine) hollow fiber sorbents

The dynamic adsorption behavior of CO2 under both nonisothermal and nearly isothermal conditions in silica supported poly(ethylenimine) (PEI) hollow fiber sorbents (Torlon®‐S‐PEI) is investigated in a rapid temperature swing adsorption (RTSA) process. A maximum CO2 breakthrough capacity of 1.33 mmol/g‐fiber (2.66 mmol/g‐silica) is observed when the fibers are actively cooled by flowing cooling water in the fiber bores. Under dry CO2 adsorption conditions, heat released from the CO2‐amine interaction increases the CO2 breakthrough capacity by reducing the severity of the diffusion resistance in the supported PEI. This internal resistance can also be alleviated by prehydrating the fiber sorbent with a humid N2 feed. The CO2 breakthrough capacity of prehydrated fibers is adversely affected by the release of the adsorption enthalpy (unlike the dry fibers); however, active cooling of the fiber results in a constant CO2 breakthrough capacity even at high CO2 delivery rates (i.e., high adsorption enthalpy delivery rates). In full RTSA cycles, a purity of 50% CO2 is achieved and the adsorption enthalpy recovery rate can reach ∼72%. Studies on the cyclic stability of uncooled fiber sorbents in the presence of SO2 and NO contaminants indicate that exposure to NO at 200 ppm over 120 cycles does not lead to a significant degradation of the sorbents, but SO2 exposure at a similar high concentration of 200 ppm causes 60% loss in CO2 breakthrough capacity after 120 cycles. A simple amine reinfusion technique is successfully demonstrated to recover the adsorption capacity in poisoned fiber sorbents after deactivation by exposure to impurities such SO2. © 2014 American Institute of Chemical Engineers AIChE J, 60: 3878–3887, 2014

Modeling of rapid temperature swing adsorption using hollow fiber sorbents

The use of novel polymeric hollow fiber contactors loaded with CO2 sorbents has been recently demonstrated experimentally as a new and scalable process configuration for post-combustion CO2 capture. The hollow fiber contactor allows coupling of efficient heat transfer and gas contacting, potentially yielding lower parasitic loads on host power plants compared to traditional contacting strategies using solid sorbents. In this study, a two dimensional mathematical model of a rapid temperature swing adsorption (RTSA) process is developed for the first time to predict polymer-supported amine hollow fiber sorbent performance during post-combustion CO2 capture from flue gas. In particular, this work is focused on developing a single fiber model to simulate a four-step RTSA system accounting for adsorption, heating/desorption, heating/sweeping and cooling steps. The model is validated with experimental breakthrough data obtained from our RTSA system. The sensitivity of the model to parameter values such as gas and water velocity and initial temperatures are evaluated accordingly by considering the effect of these parameters on CO2 concentration and temperature profiles. Furthermore, the CO2 adsorption isotherms obtained experimentally were fitted with the Toth model. In addition, our model was validated against experimental breakthrough profiles. A good agreement was found between experimental and numerical data indicating that our proposed model can describe experimental observation very well. The numerical results obtained from RTSA cycle modeling indicate that under operating conditions considered here, it is possible to achieve high purity and recovery within a cycle time of shorter than 3min. It was also found that there is a trade-off in cycle time, shorter for lower capital cost and longer to enable heat recovery for lower utility costs.

Evaluation of CO2 adsorption dynamics of polymer/silica supported poly(ethylenimine) hollow fiber sorbents in rapid temperature swing adsorption

Rapid temperature swing adsorption (RTSA) using polymer/silica supported amine hollow fiber sorbents is a new post combustion CO2 capture methodology that facilitates CO2 adsorption under nearly isothermal conditions with improved energy efficiency via heat integration. In this work, the dynamic CO2 adsorption characteristics of polymer/silica supported poly(ethylenimine) hollow fiber sorbents (CA-S-PEI-PI) are evaluated in a bench scale RTSA system. Non-isothermal fibers have breakthrough and pseudo-equilibrium CO2 capacities of 0.67mmol/g and 1.03mmol/g at 35°C, respectively, under humid simulated flue gas conditions (100% R.H.). Prolonged exposure of the fiber sorbents to water vapor enabled the breakthrough and pseudo-equilibrium CO2 capacities to increase by 60% and 43%, respectively. Upon the removal of the heat of adsorption by flowing cooling water in the bores of the fiber sorbents, there is a substantial increase in the CO2 breakthrough capacity, reaching 1.16mmol/g using simulated humid flue gases. The breakthrough capacity is found to increase 5% upon increasing the adsorption temperature from 35°C to at 45°C, suggesting improved mass transfer in the fiber sorbent at the higher temperature. The CO2 adsorption and desorption rates are shown to be very rapid, with CO2 breakthrough occurring in less than 72s and the majority of the adsorbed CO2 desorbing in 5min. Extensive cycling studies demonstrate that the CA-S-PEI-PI sorbents have good dynamic swing capacities, stabilizing over 60 cycles. A newly developed rechargeable post-spinning amine infusion technique provides the feasibility of recovering the CO2 adsorption performance of deactivated CA-S-PEI-PI fiber modules, by allowing for straightforward re-infusion of PEI into the deactivated sorbents. Amine-incorporated hollow fiber sorbents have good potential for practical use as scalable, adsorbing heat-exchangers.

Poly(ethyleneimine) infused and functionalized Torlon®-silica hollow fiber sorbents for post-combustion CO2 capture

Organic–inorganic hybrid materials functionalized with amine-containing reagents are emerging as an important class of materials for capturing carbon dioxide from flue gas. Polymeric silica hollow fiber sorbents are fabricated through the proven dry-jet/wet-quench spinning process. In our study, a new technique for functionalizing polymeric silica hollow fiber sorbents with poly(ethyleneimine), followed by a post-spinning infusion step was studied. This two step process introduces a sufficient amount of poly(ethyleneimine) to the polymeric silica hybrid material support to improve the CO2 sorption capacity due to the added amine groups. The poly(ethyleneimine) infused and functionalized hollow fiber sorbents are also characterized by a thermal gravimetric analyzer (TGA) to assess their CO2 sorption capacities.

Dual layer hollow fiber sorbents for trace H2S removal from gas streams

Hollow fiber sorbents are pseudo monolithic materials with potential use in various adsorption based applications. Dual layer hollow fiber sorbents have the potential to allow thermal regeneration without direct contact of the regeneration fluid with the sorbent particles. This paper considers the application of dual layer hollow fiber sorbents for a case involving trace amounts of H2S removal from a simulated gas stream and offers a comparison with single layer hollow fiber sorbents. The effect of spin dope composition and core layer zeolite loading on the gas flux, H2S transient sorption capacity and pore structure are also studied. This work can be used as a guide to develop and optimize dual layer hollow fiber sorbent properties beyond the specific example considered here.

Aminosilane-Functionalized Hollow Fiber Sorbents for Post-Combustion CO2 Capture

Increasing carbon dioxide emissions are generally believed to contribute to global warming. Developing new materials for capturing CO2 emitted from coal-fired plants can potentially mitigate the effect of these CO2 emissions. In this study, we developed and optimized porous hollow fiber sorbents with both improved sorption capacities and rapid sorption kinetics by functionalizing aminosilane (N-(2-aminoethyl)-3-aminoisobutyldimethylmethoxysilane) to cellulose acetate hollow fibers as a “proof of concept”. A lumen-side barrier layer was also developed in the aminosilane-functionalized cellulose acetate fiber sorbent to allow for facile heat exchange without significant mass transfer with the bore-side heat transfer fluid. The functionalized cellulose acetate fiber sorbents were characterized by pressure decay sorption measurements, multicomponent column chromatography, FT-IR, elemental analysis, and scanning electron microscopy. The carbon dioxide sorption capacity at 1 atm is 0.73 mmol/g by using the pres...

Post-spinning infusion of poly(ethyleneimine) into polymer/silica hollow fiber sorbents for carbon dioxide capture

Amine-loaded hollow fiber sorbents for CO2 capture from dilute gas streams are created using a novel post-spinning amine-infusion technique. This technique infuses poly(ethyleneimine) (PEI) into cellulose acetate/mesoporous silica hollow fiber sorbents during the solvent exchange steps after dry-jet, wet-quench, non-solvent induced phase separation spinning. A suitable post-spinning infusing solution was found to be 10% PEI in methanol with an infusion time of 4h. After amine infusion, the 51wt% silica hollow fiber sorbents are demonstrated to have a nitrogen loading of 0.52mmol/g-fiber and a CO2 uptake of 1.2mmol/g-fiber, at equilibrium. Amine-loaded fibers are packaged into a shell-and-tube module and exposed on the shell side to simulated flue gas with an inert tracer (10mol% CO2, 80mol% N2 and 10mol% He at 100% relative humidity; 1atm, 35°C). The fibers are shown to have a breakthrough CO2 capacity of 0.58mmol/g-fiber and CO2 uptake after 20min of 0.92mmol/g-fiber (1atm and 35°C). Under the same conditions, the water uptake was found to be 3.2mmol/g-fiber. The preparation of amine-containing polymeric hollow fibers and demonstration of their CO2 adsorption properties is an important step towards realizing new, scalable process configurations for supported amine sorbents relevant to post-combustion CO2 capture.