Cyclic Working Capacity Research Articles

Direct air capture based on ionic liquids: From molecular design to process assessment

Direct air capture is a key carbon dioxide removal technology to mitigate climate change and keep the global average temperature rise below 1.5–2 °C. This work addresses for the first time the use of ionic liquids for direct air capture connecting their material design by molecular simulation to process modelling. First, 26 different ionic liquids were designed through quantum chemical calculations and their isotherms were computed to identify those with a positive cyclic working capacity at conditions relevant for direct air capture. Then, the most promising ionic liquids were assessed via process simulations in Aspen Plus. A wide range of operating configurations were screened by modifying the key process variables: air velocity (1 – 3 m/s), solvent mass flow (5 – 50 t/h) and temperature (293 – 323 K), and regeneration pressure (0.1 – 1 bar) and temperature (373 – 393 K). Exergy, energy and productivity were computed to detect optimal operating conditions; moreover, a simplified economic analysis was carried out to highlight the major cost components. The direct air capture system based on [P66614][Im] exhibited the most exergy (5.44 – 16.73 MJ/kg) and energy (15.15 – 35.42 MJ/kg) efficiency for similar productivity (0.5 – 1.3 kg/(m3·h)) thanks to its enhanced cyclic capacity (0.6 – 0.3 mol/kg). The minimum exergy required by [P66614][Im]-based DAC process is slightly better than alkali scrubbing (6.21 MJ/kg) and in line with amine (5.59 MJ/kg) scrubbing. In addition, the assessed DAC process has a theoretical potential to operate in the range of 200 $/tCO2 under reasonable energy and plant expenses. We conclude this work providing guidelines to address future development of direct air capture technologies based on ionic liquids.

Preparation and Testing of Carbon Fiber Reinforced Composites Embedded with Lithium-Ion Polymer Batteries

Vehicle lightweight promises to drive down the maintenance quality and increase energy efficiency in transportation with the safe and strength performance. The use of fiber reinforced polymer composite materials structural battery vehicles is recognized as an essential enabling technology for achieving high bearing capacity and energy storage capacity. At the same time it improves the energy storage capacity, the overall structural energy storage and space utilization of new energy vehicles. In this study we bring an idea to fabricate composites with integrated lithium-ion pouch batteries. The whole preparation process was carried out at room temperature to prevent thermal damage on battery. Experimental results indicate that the cyclic working voltage and capacity of CFRP structure battery are basically consistent with the monomer lithium-ion polymer battery which obtained from Xinwei battery testing equipment. Therefore, the fabrication process does not damage the lithium-ion polymer battery.

Effect of Amine Functionalization of MOF Adsorbents for Enhanced CO2 Capture and Separation: A Molecular Simulation Study.

Different types of amine-functionalized MOF structures were analyzed in this work using molecular simulations in order to determine their potential for post-combustion carbon dioxide capture and separation. Six amine models -of different chain lengths and degree of substitution- grafted to the unsaturated metal sites of the M2(dobdc) MOF [and its expanded version, M2(dobpdc)] were evaluated, in terms of adsorption isotherms, selectivity, cyclic working capacity and regenerability. Good agreement between simulation results and available experimental data was obtained. Moreover, results show two potential structures with high cyclic working capacities if used for Temperature Swing Adsorption processes: mmen/Mg/DOBPDC and mda-Zn/DOBPDC. Among them, the -mmen functionalized structure has higher CO2 uptake and better cyclability (regenerability) for the flue gas mixtures and conditions studied. Furthermore, it is shown that more amine functional groups grafted on the MOFs and/or full functionalization of the metal centers do not lead to better CO2 separation capabilities due to steric hindrances. In addition, multiple alkyl groups bonded to the amino group yield a shift in the step-like adsorption isotherms in the larger pore structures, at a given temperature. Our calculations shed light on how functionalization can enhance gas adsorption via the cooperative chemi-physisorption mechanism of these materials, and how the materials can be tuned for desired adsorption characteristics.

Screening of Ionic Liquids and Deep Eutectic Solvents for Physical CO2 Absorption by Soft-SAFT Using Key Performance Indicators

The efficient screening of solvents for CO2 capture requires a reliable and robust equation of state to characterize and compare their thermophysical behavior for the desired application. In this work, the potentiality of 14 ionic liquids (ILs) and 7 deep eutectic solvents (DESs) for CO2 capture was examined using soft-SAFT as a modeling tool for the screening of these solvents based on key process indicators, namely, cyclic working capacity, enthalpy of desorption, and CO2 diffusion coefficient. Once the models were assessed versus experimental data, soft-SAFT was used as a predictive tool to calculate the thermophysical properties needed for evaluating their performance. Results demonstrate that under the same operating conditions, ILs have a far superior performance than DESs primarily in terms of amount of CO2 captured, being at least two-folds more than that captured using DESs. The screening tool revealed that among all the examined solvents and conditions, [C4 py][NTf2] is the most promising solvent for physical CO2 capture. The collection of the acquired results confirms the reliability of the soft-SAFT EoS as an attractive and valuable screening tool for CO2 capture and process modeling.

A Comparative Assessment of Emerging Solvents and Adsorbents for Mitigating CO2 Emissions From the Industrial Sector by Using Molecular Modeling Tools

Some specific examples are presented showing the possibilities offered by molecular modelling tools to obtain relevant data at process conditions while also gaining molecular insights on the techniques used for CO2 capture and separation. Two different technologies, absorption with amine-based systems and adsorption on porous materials, were explored, using the molecular-based equation of state, soft-SAFT, and Grand Canonical Monte Carlo simulations, respectively. The aqueous monoethanolamine (MEA) system was used as the benchmark for absorption and compared to the performance of eight alternative amine-based systems, while sixteen adsorbents belonging to different families (zeolites, Metal Organic Frameworks, silicas and activated carbons), bare or functionalized with alkylamines, were investigated for the separation of CO2 by adsorption. In addition to obtaining molecular information on the CO2 capture process, the models were further used to examine the CO2 capture performance in terms of cyclic working capacity and energy index as key performance indicators, allowing the identification of promising systems that can improve the current ones to be further evaluated for separation in non-power industries. Results show that for the same total amine mass concentration, non-aqueous amine solvents possess a 5–10% reduction in cyclic working capacity, with a 10-30% decrease in the energy index compared to their aqueous counterparts, due to lower heat of vaporization and sensible heat. It has also been obtained that M-MOF-74 and zeolites NaX and NaY structures offer the best results for adsorption in TSA processes. Some adsorbent structures achieved similar values of energy requirement to the ones obtained for alternative amine-based systems (2-2.5 MJ∙kgCO2-1), although the disadvantage of the TSA process versus absorption should be taken into account. These results confirm the reliability of the molecular modelling approach as an attractive and valuable screening tool for CO2 capture and separation processes.

Adsorption behavior and kinetics of H2S on a potassium-promoted hydrotalcite

Adsorption of H2S and the influence of steam on its adsorption capacity and kinetics were studied on a commercial potassium-promoted hydrotalcite. The sorbent shows a very high cyclic working capacity for H2S compared to CO2 and H2O, even at lower partial pressures and at different operating temperatures ranging between 300 and 500 °C. The operating temperature does not significantly influence the cyclic working capacity for half-cycle times of 30 min. The adsorption mechanism, however, changes at higher temperatures. At lower temperatures (300 °C) a fast adsorption with a fast approach to steady state was observed. At higher operating temperatures, H2S reacts with the hydrotalcite structure, forming strongly bonded sulfuric species on the sorbent. When using dry regeneration conditions, the first cycles in cyclic operation at higher temperatures show a significantly higher adsorption of H2S (especially the first cycle), which cannot be desorbed during regeneration with N2. After the first fast initial adsorption rate a continuous slow adsorption of H2S occurs, probably caused by a surface reaction between H2S and the hydrotalcite structure. This reaction is, however, reversible if steam is used.The adsorption mechanism for H2S and H2O was determined using multiple cyclic experiments comparable to previous studies performed for CO2 and H2O adsorption. It is evident that the adsorption mechanism developed for CO2 on the same sorbents is also valid for H2S, indicating that the developed mechanism is consistent for sour gas adsorption on this type of sorbents. The cyclic working capacity can be significantly increased if steam is used during the regeneration step of the sorbent. The mechanistic model developed for the adsorption of CO2 and H2O was successfully validated with more than 160 different TGA experiments. An operating temperature of 400 °C seems to be optimal to achieve a high cyclic working capacity for H2S, because at higher temperatures the regeneration of the formed sulfuric species seems to be hindered resulting in a significant decrease in the cyclic working capacity.

CO2 and H2O chemisorption mechanism on different potassium-promoted sorbents for SEWGS processes

The sorption kinetics and capacities of CO2 and H2O were investigated for two different potassium-promoted hydrotalcite sorbents and potassium-promoted alumina. Thermogravimetric analysis (TGA) and packed-bed reactor (PBR) breakthrough experiments were performed using sequences of adsorption and desorption steps in different gas mixtures containing CO2 and H2O. Experiments were carried out at an operating temperature of 400 °C with different partial pressures ranging from 0.025 bar to 0.3 bar for CO2 and 0.1 to 0.3 bar for H2O respectively. It was found that a sorption mechanism with different adsorption sites, developed for one of the sorbents, also applies for the other sorbents where capacities are different and depending on the sorbent. From experimental results it was deduced that K2CO3 promotion is mainly responsible for a reactive CO2 adsorption site, which can only be regenerated with steam. The adsorption capacity for this site is enhanced for K2CO3 promoted alumina compared to K2CO3 promoted hydrotalcite. A second adsorption site for CO2, which can be regenerated with N2 is dominant on the K2CO3 promoted hydrotalcite with a high MgO content. This indicates that MgO is probably responsible for the formation of basic sites on the surface of the sorbent, which are relatively easily regenerated at the investigated experimental conditions. The results also show that the sorbent with the highest MgO loading has the highest cyclic working capacity under dry adsorption conditions, whereas the hydrotalcite-based adsorbent with a lower MgO content has the highest cyclic working capacity for CO2 at wet conditions and is therefore the preferred sorbent for sorption-enhanced water-gas shift applications.

Influence of material composition on the CO2 and H2O adsorption capacities and kinetics of potassium-promoted sorbents

Two different potassium-promoted hydrotalcite (HTC)-based adsorbents and a potassium-promoted alumina sorbent were investigated using thermogravimetric analysis (TGA) and different characterization methods in order to study CO2 and H2O adsorption capacity and kinetics. A higher Mg content improves the cyclic working capacity for CO2 due to the higher basicity of the material. The initial adsorption rate for CO2 is very fast for all sorbents, but for sorbents with higher MgO content, this fast-initial adsorption is followed by a slower CO2 uptake probably caused by the slow formation of bulk carbonates. A longer half-cycle time can therefore increase the CO2 cyclic working capacity for sorbents with a higher MgO content. Potassium-promoted alumina has a very stable CO2 cyclic working capacity at different operating temperatures compared to the potassium-promoted HTC’s. Usually a higher operating temperature increases the desorption kinetics for a HTC-based adsorbent, but not for potassium-promoted alumina. HTC-based adsorbents show the highest cyclic working capacity for H2O. The adsorption kinetics for H2O are not influenced by the material composition, indicating that the mechanism behind the adsorption of H2O is different compared to CO2. Depending on the material composition, adsorption of steam at high operating temperatures (>500 °C) results in an irreversible decomposition of carbonate species. Steam can reduce the temperature where usually K2CO3 is irreversibly decomposed resulting in a significantly reduced cyclic working capacity, which is very important concerning the use of these sorbents for sorption-enhanced water-gas shift processes.

Chemisorption of H2O and CO2 on Hydrotalcites for Sorption-enhanced Water-gas-Shift Processes

Thermogravimetric analysis and breakthrough experiments in a packed bed reactor were used to validate a developed adsorption model to describe the cyclic working capacity of CO2 and H2O on a potassium-promoted hydrotalcite, a very promising adsorbent for sorption-enhanced water-gas-shift applications. Four different adsorption sites (two sites for CO2, one site for H2O and one equilibrium site for both species) were required to describe the mass changes observed in the TGA experiments. The TGA experiments were carried out at operating temperatures between 300 and 500°C, while the total pressure in the reactor was kept at atmospheric pressure. Cyclic working capacities for different sites and the influence of the operating conditions on the cyclic working capacity were studied using the developed model. A higher operating temperature leads to a significant increase in the cyclic working capacity of the sorbent for CO2 attributed to the increase in the desorption kinetics for CO2. The model was successfully validated with experiments in a packed bed reactor at different operating temperatures.

CO2 residual concentration of potassium-promoted hydrotalcite for deep CO/CO2 purification in H2-rich gas

Elevated-temperature pressure swing adsorption is a promising technique for producing high purity hydrogen and controlling greenhouse gas emissions. Thermodynamic analysis indicated that the CO in H2-rich gas could be controlled to trace levels of below 10 ppm by in situ reduction of the CO2 concentration to less than 100 ppm via the aforementioned process. The CO2 adsorption capacity of potassium-promoted hydrotalcite at elevated temperatures under different adsorption (mole fraction, working pressure) and desorption (flow rate, desorption time, steam effects) conditions was systematically investigated using a fixed bed reactor. It was found that the CO2 residual concentration before the breakthrough of CO2 mainly depended on the total amount of purge gas and the CO2 mole fraction in the inlet syngas. The residual CO2 concentration and uptake achieved for the inlet gas comprising CO2(9.7 mL/min) and He (277.6 mL/min) at a working pressure of 3 MPa after 1 h of Ar purging at 300 mL/min were 12.3 ppm and 0.341 mmol/g, respectively. Steam purge could greatly improve the cyclic adsorption working capacity, but had no obvious benefit for the recovery of the residual CO2 concentration compared to purging with an inert gas. The residual CO2 concentration obtained with the adsorbent could be reduced to 3.2 ppm after 12 h of temperature swing at 450 °C. A new concept based on an adsorption/desorption process, comprising adsorption, steam rinse, depressurization, steam purge, pressurization, and high-temperature steam purge, was proposed for reducing the steam consumption during CO/CO2 purification.

On the influence of steam on the CO2 chemisorption capacity of a hydrotalcite-based adsorbent for SEWGS applications

Hydrotalcite-based adsorbents have shown great potential for use in sorption-enhanced water-gas-shift applications. A combination of thermogravimetric experiments and breakthrough experiments have been carried out to elucidate the effect of steam on the CO2 cyclic sorption capacity on a K-promoted hydrotalcite-based adsorbent. Different TGA cycles have been designed to study the mass change on sorbents exposed to different sequences of different CO2/H2O/N2 mixtures. Because the complex sorption/desorption and replacement phenomena cannot be explained by TGA experiments only, additional information from breakthrough experiments in a packed bed reactor was used to correlate the observed total mass change in the TGA cycles to the phenomena prevailing on the sorbent.A mechanism has been developed which is able to describe the cyclic working capacity, for both CO2 and H2O under different experimental conditions. It was found that at least four different adsorption sites participate in the sorption/desorption of CO2 and H2O. Two adsorption sites can be regenerated with N2, whereas the other adsorption sites require the presence of H2O or CO2 to be desorbed. Regeneration of the adsorbent with steam leads to a significant increase in the CO2 cyclic working capacity from 0.3 to 0.53mmol/g compared to a dry regeneration with N2 using the same cycle times.

Chemisorption working capacity and kinetics of CO2 and H2O of hydrotalcite-based adsorbents for sorption-enhanced water-gas-shift applications

The adsorption behavior of carbon dioxide and water on a K-promoted hydrotalcite based adsorbent has been studied by thermogravimetric analysis with the aim to better understand the kinetic behavior and mechanism of such material in sorption enhanced water-gas shift reactions.The cyclic adsorption capacity was measured as a function of temperature (300–500°C), pressure (0–8bar) and the cycle time. Both species interact at elevated temperatures with the adsorbent. The history of the adsorbent (pretreatment/desorption conditions) has a profound influence on its sorption capacity. Slow desorption kinetics determine the sorption capacity during cyclic operation, where a high temperature during the desorption and long half-cycle times can increase the cyclic working capacity for both CO2 and H2O significantly. Accounting for the sorbent history and the definition of adsorption capacity are very important features when comparing sorption capacities to values reported in literature. The adsorbent shows very high capacities for H2O compared to CO2 which has not been reported in the literature up to now. The mechanism for H2O and CO2 adsorption seems to be a different one. Whereas H2O adsorption seems to follow the principles of a simple physisorption mechanism, CO2 adsorption can only be explained by a chemical reaction with the adsorbent. Working isotherms (cyclic working capacity at isothermal conditions at different pressures) of both CO2 and H2O were measured up to 8bar total pressure. Higher partial pressures increase the cyclic working capacity of the adsorbent up to 0.47mmol/g for CO2(PCO2=8bar) and 1.06mmol/g for H2O (PH2O=4.2bar) at 400°C after 30min of adsorption followed by 30min of dry regeneration with N2.

Comparison of commercial and new adsorbent materials for pre-combustion CO2 capture by pressure swing adsorption

The IGCC technology (Integrated Gasification Combined Cycle) with pre-combustion CO2 capture is a promising approach for near-zero CO2 emission power plants to be realized in the near future. A key challenge within this technology is the separation of the CO2/H2 gas mixture resulting from the water gas shift reaction that follows the gasification of coal. For the CO2 stream a purity of about 95% is required; additionally a CO2 capture rate of 90% is desired, which implies that both streams, H2 and CO2, are required at rather high purity (∼95%). In contrast to post- combustion capture from power plants, where a large gas stream at low pressure and low CO2 content has to be treated, in pre-combustion capture a gas mixture at up to 40bar has to be separated; therefore an adsorption based process, such as pressure swing adsorption (PSA), constitutes a promising method for CO2 removal from H2.In this work, new materials, namely USO-2-Ni MOF, UiO-67/MCM-41 Hybrid and MCM-41, are characterized in<remove-image> terms of equilibrium adsorption isotherms. Excess adsorption isotherms of CO2 and H2 on these materials are measured at different temperatures (25°C 140°C) and in a wide pressure range (up to 150bar). The experimental data are then described with a suitable isotherm model, in our case Langmuir, Sips and Quadratic. In addition, the cyclic working capacity of CO2 on each material is computed as a further assessment of the suitability of these materials for pre-combustion capture.

MCM-41, MOF and UiO-67/MCM-41 adsorbents for pre-combustion CO2 capture by PSA: adsorption equilibria

Three different adsorbent materials, which are promising for pre-combustion CO2 capture by a PSA (Pressure Swing Adsorption) process, are synthesized, pelletized and characterized. These materials are USO-2-Ni metal organic framework (MOF), mesoporous silica MCM-41 and a mixed material consisting of UiO-67 MOF bound with MCM-41. On these materials, equilibrium adsorption isotherms of CO2 and H2 are measured at different temperatures (25–140 °C) in a wide pressure range (up to 15 MPa). From the experimental data the parameters of different isotherm equations (Langmuir, Sips and Quadratic) are determined, together with the isosteric heats of adsorption. Binary adsorption of CO2/H2 mixtures on USO-2-Ni MOF is additionally measured and compared to predicted values using IAST (Ideal Adsorbed Solution Theory) showing a good agreement. The potential of the materials for the application of interest is evaluated by looking at their cyclic working capacity and compared to those of a commercial activated carbon. From this evaluation especially the USO-2-Ni MOF adsorbent looks promising compared to the commercial activated carbon. For the other two materials a smaller improvement, which is limited to lower temperatures, is expected.

Sorption enhanced reaction process for direct production of fuel-cell grade hydrogen by low temperature catalytic steam–methane reforming

New experimental data are reported to demonstrate that a sorption enhanced reaction (SER) concept can be used to directly produce fuel-cell grade H 2 (<20 ppm CO) by carrying out the catalytic, endothermic, steam–methane reforming (SMR) reaction (CH 4 + 2H 2O ↔ CO 2 + 4H 2) in presence of a CO 2 selective chemisorbent such as K 2CO 3 promoted hydrotalcite at reaction temperatures of 520 and 550 °C, which are substantially lower than the conventional SMR reaction temperatures of 700–800 °C. The H 2 productivity of the sorption enhanced reactor can be large, and the conversion of CH 4 to H 2 can be very high circumventing the thermodynamic limitations of the SMR reaction due to the application of the Le Chetalier's principle in the SER concept. Mathematical simulations of a cyclic two-step SER concept showed that the H 2 productivity of the process (moles of essentially pure H 2 produced per kg of catalyst–chemisorbent admixture in the reactor per cycle) is much higher at a reaction temperature of 590 °C than that at 550 or 520 °C. On the other hand, the conversion of feed CH 4 to high purity H 2 product is relatively high (>99+%) at all three temperatures. The conversion is much higher than that in a conventional catalyst-alone reactor at these temperatures, and it increases only moderately (<1%) as the reaction temperature is increased from 520 to 590 °C. These results are caused by complex interactions of four phenomena. They are (a) favorable thermodynamic equilibrium of the highly endothermic SMR reaction at the higher reaction temperature, (b) faster kinetics of SMR reaction at higher temperatures, (c) favorable removal of CO 2 from the reaction zone at lower temperatures, and (d) higher cyclic working capacity for CO 2 chemisorption at higher temperature.

Influence of Gas-Solid Heat Transfer on Rapid PSA

A simple analytical model of a Differential Pressure Swing Adsorption (DPSA) process on a single adsorbent particle was used to evaluate the effects of gas-solid heat transfer resistance on the cyclic working capacity of the particle. The commonly used assumption of instantaneous thermal equilibrium between the gas and the adsorbent inside an adsorber may not be valid when the gas flow rates are low, the adsorption kinetics is relatively fast, and the PSA cycle times are small.