Potassium-promoted Hydrotalcite Research Articles

3D-printing of adsorbents for increased productivity in carbon capture applications (3D-CAPS)

An important driver in the development of adsorption-based CO2 capture technologies is the reduction of cost through increasing the productivity. The 3D-CAPS project aims to increase the productivity (kg CO2/m3hr) of such technologies through structuring, enabled by 3D-printing. This productivity increase would allow for a reduction of the size of the adsorbers and the associated CAPEX and/or energy requirements. Pre-combustion, as well as post-combustion technologies, are investigated using potassium-promoted hydrotalcite (K-HTC) for sorption-enhanced water-gas shift (SEWGS) and amine-functionalized silica (ImmoAmmo) as sorbents, respectively. This contribution presents a technical overview highlighting several aspects of the project ranging from CFD modelling to assess the shape of the sorbents, 3D-printing of the sorbent materials as well as testing of the ImmoAmmo sorbent for post-combustion capture applications. It is discussed how several essential elements of the productivity improvement have been separately proven in the 3D-CAPS project, both by modelling and experimentally: maintained adsorption capacity upon 3D-printing, reduced pressure drop, and improved mass transfer.

Assessment of CO2 capture efficiency in packed bed versus 3D-printed monolith reactors for SEWGS using CFD modeling

Sorption Enhanced Water-Gas Shift (SEWGS) couples the water gas-shift reaction with CO2 adsorption on potassium-promoted hydrotalcite (K-HTC) sorbent material for hydrogen production and CO2 capture. Here, computational fluid dynamics (CFD) models are developed to simulate the adsorption step in SEWGS considering both packed bed and monolith reactors. The goal of this research is increasing SEWGS productivity by using 3D-printed structured bed reactors, as opposed to conventional packed bed reactors, due to the well-known advantages of monoliths, such as less mass transfer limitations and lower pressure drops. The paper presents numerical modeling and simulation work supported by experimental validation in order to compare packed bed and monolith reactors with respect to SEWGS performance. The packed bed multiscale CFD model is validated using breakthrough experimental data. A bench-scale CFD structured bed model is developed and validated based on breakthrough measurements performed by TNO using 3D-printed K-HTC monolith structures. Furthermore, geometry effects on mass transfer efficiency are investigated through CFD modeling for the bench-scale reactor. A scale-up of the monolith model to pilot-scale allows for a proper comparison with the packed bed technology. Monolith reactor model breakthrough predictions show there is a considerable increase in mass transfer rate over the packed bed reactor for the adsorption step in SEWGS, demonstrating promising potential towards enhancing the carbon capture technology.

3d-Printing of Adsorbents for Increased Productivity in Carbon Capture Applications (3d-Caps)

The 3D-CAPS project aims to increase the productivity (kg CO2/m3hr) of adsorbent based carbon capture technologies by an order of magnitude through structuring, enabled with 3D-printing. This productivity increase would allow for a reduction of the size of the adsorbers and the associated CAPEX and/or energy requirements. pre-combustion, as well as post-combustion technologies, are investigated using potassium-promoted hydrotalcite (K-HTC) for sorption-enhanced water-gas shift (SEWGS) and aminefunctionalized silica (ImmoAmmo) as sorbent, respectively. This contribution presents a technical overview highlighting several aspects of the project ranging from CFD modelling to assess the shape of the sorbents, 3D-printing of the sorbent materials as well as testing of the ImmoAmmo sorbent for post-combustion capture applications. It is discussed how several essential elements of the productivity improvement have been separately proven in the 3D-CAPS project, both by modelling and experimentally: maintained adsorption capacity upon 3D-printing, reduced drop reduction, and improved mass transfer .

MODELING OF A RECTANGULAR CHANNEL MONOLITH REACTOR FOR SORPTION-ENHANCED WATER-GAS SHIFT

As CO2 concentration levels in the atmosphere are steadily reaching a point of no return, it is paramount that actions are taken towards mitigating emissions by improving the efficiency of existing processes. Sorption-enhanced water-gas shift (SEWGS) combines the water-gas shift reaction with in-situ adsorption of CO2 on potassium-promoted hydrotalcite (K-HTC). This enables a direct conversion of syngas into separate hot streams of H2 at feed pressure and CO2 at regeneration pressure, making the process attractive for pre-combustion carbon capture and storage (CCS) and reduction of greenhouse gas emissions. The current work is evaluating the high-temperature, high-pressure adsorption step of the SEWGS process enhanced by a novel technology which replaces common packed bed reactors with 3D-printed monolithic structures. This approach would enable an overall increase in process productivity. To this extent, innovative dynamic models based on validated competitive adsorption isotherms were developed in this work. COMSOL Multiphysics was used to develop a 1D computational fluid dynamics (CFD) model of adsorption for a fixed bed reactor in order to verify the model accuracy against existing studies. Subsequently, 2D CFD simulations were developed to describe adsorption inside monolith structures, both free and porous regions. The multi-component adsorption isotherm used in the simulations was validated with published breakthrough capacities for CO2 and H2O at different pressures. Model predictions are in agreement with expected behavior, as monolith reactors provide a more efficient mass transfer, and will be used to enhance the performance of experimental monolith structures used in SEWGS.

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.

An in-situ IR study on the adsorption of CO2 and H2O on hydrotalcites

In-situ IR technique was used to study the reversible adsorption of CO2 and H2O at elevated temperatures on a potassium-promoted hydrotalcite for its use in sorption-enhanced water-gas shift (SEWGS). It was found that mainly bidentate carbonate species are responsible for the reversible (cyclic) adsorption capacity of the sorbent. The presence of H2O can enhance the decomposition of bidentate carbonates bond to the stronger basic surface-sites. The basic strength of the involved adsorption sites for bidentate formation appears to be highly heterogeneous. At higher operating temperatures, reversible formation of bulk carbonates seem to participate in the reversible adsorption for CO2. The presence of H2O on the sorbent can lead to the formation of bi-carbonate, especially at lower operating temperatures of 300 °C. The transient absorbance of the main absorption bands for carbonate species identified during this study can be used in the development of a detailed description of the reversible adsorption/desorption kinetics reported before using thermogravimetric analyses.

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.

Novel alkali-promoted hydrotalcite for selective synthesis of 2-methoxy phenyl benzoate from guaiacol and benzoic anhydride

Esters find several applications such as solvents, flavours and fragrants and intermediates in synthesis of drugs. In the present work, 2-methoxy phenyl benzoate was efficiently synthesized from guaiacol and benzoic anhydride by acylation. A variety of catalysts such as hydrotalcite and alkali-promoted hydrotalcite was synthesized. Potassium-promoted hydrotalcite (K/HT) calcined at 500 °C for 6 h was active, selective and reusable. It was characterized by different techniques. A slurry batch reactor was used to study reaction mechanism and kinetics. 2-Methoxy phenyl benzoate was efficiently obtained with 100% selectivity at guaiacol conversion of 98% over K/HT at 100 °C after 6 h. A power law model with second-order kinetics was fitted to obtain an apparent activation energy of reaction of 21.1 kcal mol−1. The process is clean and green.

Enhancing the low temperature water–gas shift reaction through a hybrid sorption-enhanced membrane reactor for high-purity hydrogen production

The low temperature water–gas-shift reaction (LT-WGS) has been assessed by means of a hybrid sorption-enhanced membrane reactor (HSEMR) that combines both CO2 and H2 removal from the reaction zone. The performance of this reactor has been compared with that obtained by (i) a traditional and (ii) a sorption-enhanced (only CO2 is removed) reactor operating in the same operational conditions. Cu/ZnO–Al2O3 and K2CO3-promoted hydrotalcite materials have been used as a catalyst and CO2 sorbent, respectively. A self-supported Pd–Ag membrane tube has been used in order to selectively separate the H2. The CO2 sorption capacity, in the presence and absence of water vapour, of the potassium-promoted hydrotalcite has been determined by means of breakthrough experiments. The presence of water vapour enhanced the sorption capacity of the hydrotalcite in the experimental conditions used. Concerning the performance of the HSERM, results clearly show that when both CO2 and H2 are removed from the reaction zone, the hydrogen production through the reversible LT-WGS reaction is enhanced compared to either a traditional or a sorption-enhanced reactor, allowing overcoming equilibrium limitations and obtain a pure H2 stream.

High-temperature pressure swing adsorption cycle design for sorption-enhanced water–gas shift

Sorption-enhanced water–gas shift (SEWGS) combines the water–gas shift reaction with in situ adsorption of CO2 on potassium-promoted hydrotalcite (K-HTC) and thereby allows production of hot, high pressure H2 from syngas in a single unit operation. SEWGS is a cyclic process, that comprises high pressure adsorption and rinse, pressure equalisation, and low pressure purge. Here, results are presented of a SEWGS cycle design study, based on recently developed expressions for the interaction of CO2 and H2O with K-HTC. It is shown that during the cycle, steam adsorbs in the rinse step and desorbs during the subsequent reduction in pressure, thereby improving the CO2 purity in the column and thus enhancing the efficiency of the rinse. A parameter study based on numerical simulations shows that the carbon capture ratio depends mainly on the purge steam to carbon feed ratio, whereas the CO2 product purity depends mainly on the rinse steam to carbon feed ratio. An optimisation yields a SEWGS cycle that consumes significantly less steam than cycle designs previously reported in the literature.

Isotherm model for high-temperature, high-pressure adsorption of and on K-promoted hydrotalcite

Sorption-enhanced water-gas shift (SEWGS) combines the water–gas shift reaction with in situ adsorption of CO2 on potassium-promoted hydrotalcite (K-HTC) and thereby allows production of hot, high pressure H2 from syngas in a single process. SEWGS is a cyclic process, that comprises high pressure adsorption and rinse, pressure equalisation, and low pressure purge. In order to design the SEWGS process, the equilibria and kinetics of adsorption must be known for the entire pressure range. Here, a multicomponent adsorption isotherm is presented for CO2 and H2O on K-HTC at 400 °C and 0.5–24 bar partial pressure, that has been derived from integrated experimentally determined breakthrough curves with special attention being given to the high pressure interaction. The experimental results can be well described by assuming that the isotherm consists of a low partial pressure surface adsorption part and a high partial pressure nanopore adsorption part. Surface adsorption occurs at specific and different sites for CO2 or H2O. In contrast, the nanopore adsorption mechanism is competitive and explains the interaction observed in the capacity data at partial pressures over 5 bar. Based on the characteristics of the sorbent particles, a linear driving force relation has been derived for sorption kinetics. Adsorption isotherm and linear driving force kinetics have been included in a reactor model. Model predictions are in agreement with breakthrough as well as regeneration experiments.

High capacity potassium-promoted hydrotalcite for CO2 capture in H2 production

This paper presents an experimental study for a newly modified K2CO3-promoted hydrotalcite material as a novel high capacity sorbent for in-situ CO2 capture. The sorbent is employed in the sorption enhanced steam reforming process for an efficient H2 production at low temperature (400–500 °C). A new set of adsorption data is reported for CO2 adsorption over K-hydrotalcite at 400 °C. The equilibrium sorption data obtained from a column apparatus can be adequately described by a Freundlich isotherm. The sorbent shows fast adsorption rates and attains a relatively high sorption capacity of 0.95 mol/kg on the fresh sorbent. CO2 desorption experiments are conducted to examine the effect of humidity content in the gas purge and the regeneration time on CO2 desorption rates. A large portion of CO2 is easily recovered in the first few minutes of a desorption cycle due to a fast desorption step, which is associated with a physi/chemisorption step on the monolayer surface of the fresh sorbent. The complete recovery of CO2 was then achieved in a slower desorption step associated with a reversible chemisorption in a multi-layer surface of the sorbent. The sorbent shows a loss of 8% of its fresh capacity due to an irreversible chemisorption, however, it preserves a stable working capacity of about 0.89 mol/kg, suggesting a reversible chemisorption process. The sorbent also presents a good cyclic thermal stability in the temperature range of 400–500 °C.

Hydrotalcite as CO2 Sorbent for Sorption-Enhanced Steam Reforming of Methane

Sorption-enhanced reforming of methane is an attractive option for the combined production of electricity and capture of CO2. In this process, the steam-reforming catalyst is mixed with a CO2 sorbent. During the reaction, CO2 is adsorbed, leading to an increase of the hydrogen production rate. Once the sorbent is saturated, it must be regenerated using purge gas, usually steam. The amount of steam needed for CO2 removal from the saturated sorbent determines the system efficiency of the process to a large extent. In this paper, several potassium-promoted hydrotalcite samples, both obtained commercially and prepared in-house, are tested for their suitability as a CO2 sorbent for sorption-enhanced reforming of methane. In particular, the purge gas to adsorbed CO2 ratio is determined under various operation conditions. It is shown that this ratio is still too large under the investigated conditions. Mixed with steam-reforming catalyst the hydrotalcite sorbent can adsorb sufficient CO2 to enhance the CH4 conve...