Behaviors and kinetic models analysis of Li4SiO4 under various CO2 partial pressures

The absorption behaviors of Li₄SiO₄ sorbent under various CO₂ partial pressures and temperatures were investigated through numerical and experimental methods. It was found that Li₄SiO₄ showed poor absorption capacity at high temperatures (>525°C) under CO₂ partial pressure of 5066 Pa. This phenomenon was explained by the thermodynamic results from FactSage5.5 software. Meanwhile, a modified Jander‐Zhang model based on the double‐shell structure of the Li₄SiO₄ sorbent was developed to describe the absorption kinetic behaviors of CO₂ on Li₄SiO₄. The results showed that the modified Jander‐Zhang model could fit the kinetic experimental data well. Furthermore, the influence of steam on CO₂ absorption was also analyzed by the modified Jander‐Zhang model. The results showed that the activation energy in the absorption process with steam was smaller than that without steam, which indicated that the presence of steam could promote the CO₂ diffusion in product layer, therefore, improving the sorption capacity. © 2017 American Institute of Chemical Engineers AIChE J, 63: 2153–2164, 2017     

High-temperature CO2 adsorption over Li4SiO4 sorbents derived from different lithium sources

Li₄SiO₄ is a promising sorbent for high temperature CO₂ capture. It could be synthesized from three different Li sources (LiNO₃, LiOH, and Li₂CO₃) by using the solid state reaction method. The effects of Li sources on the structure and CO₂ adsorption/desorption properties of Li₄SiO₄ sorbents were analyzed in this work. The results showed that Li₄SiO₄ sorbents could be synthesized at a lower temperature by using LiNO₃ and LiOH as the starting materials, which could reduce the sintering during the synthesis process and increase the surface area of synthesized Li₄SiO₄. During the CO₂ adsorption/desorption cycles, Li₄SiO₄ sorbents derived from LiNO₃ and LiOH presented higher initial CO₂ adsorption capacities than those from Li₂CO₃. After 15 cycles, the adsorption efficiency of Li₄SiO₄ derived from LiNO₃ showed no or slight decrease, while that from LiOH rapidly decreased to 20% of the initial value. This was because Li₄SiO₄ derived from LiNO₃ had high surface area and porosity before CO₂ adsorption/desorption cycles, and its surface area even increased after cycles. However, the surface area of Li₄SiO₄ derived from LiOH decreased greatly due to serious sintering. For Li₄SiO₄ derived from Li₂CO₃, its morphology and surface area were almost unchanged before and after CO₂ adsorption/desorption cycles.     

High temperature CO2 capture performance and kinetic analysis of Na4SiO4 ceramics

Alkali silicate-based ceramics sorbents were regarded as particularly suitable materials for CO₂ capture at high temperatures, however, the CO₂ capture behaviors of Na₄SiO₄ had been seldom investigated. In this work, the Na₄SiO₄ ceramics samples were prepared using the one-step synthesized method, and the CO₂ sorption/desorption performances at high temperatures, the thermal stability, and the cycling stability of Na₄SiO₄ were systematically investigated. It was demonstrated that the maximum CO₂ sorption capacity of SO-3 sample reached 19.4 wt% at 725 °C, and the optimal condition of cycling tests were 750 °C for sorption and 800 °C for desorption based on the sorption/desorption capacity and rate, which exhibited good thermal stability and high cyclic stability. Besides, the kinetic analysis results showed that the diffusion process was the rate-determining step of CO₂ adsorption, which was more dependent on temperature than the chemisorption process. The structure and surface morphology variations were also investigated, it was interesting that there was a special “fish scale” surface structure after the sorption process, revealing that the melting phenomenon happened during the chemisorption reaction process. By comparing with common sorbent Li₄SiO₄, the material and CO₂ capture costs of Na₄SiO₄ were much lower. These results proved that Na₄SiO₄ was expected to be a suitable high temperature CO₂ capture material as a good supplement to alkali silicate-based ceramics sorbents.     

Synthesis of highly efficient,structurally improved Li4SiO4 sorbents for high-temperature CO2 capture

Li₄SiO₄ sorbents for high-temperature CO₂ removal have drawn extensive attention owing to their potential application in carbon capture and storage (CCS). The major challenge in the application lies in the poor CO₂ capture performance under realistic conditions of low CO₂ concentrations, owing to the dense structure and poor porosity. In this work, Li₄SiO₄ sorbents were prepared with porous micromorphologies and large contact areas using a variety of organometallic Li-precursors, achieving fast CO₂ sorption kinetics, high capacity and excellent cyclic stability at a low CO₂ concentration (15?vol%). It was found that a high conversion of ~?74% was maintained for pure Li₄SiO₄ even after 100 sorption/desorption cycles. Moreover, by doping with Na₂CO₃ to reduce the CO₂ diffusion resistance, the conversion of the sorbent was further enhanced to 93.2%. The enhancement mechanism of alkali carbonate have been proven here to be ascribed to the formation of the eutectic melt of Li/Na carbonates, the existence and function of which has been confirmed in this study.     

Synthesis,kinetic study,and reaction mechanism of Li4SiO4 with CO2 in a slurry bubble column reactor

Abstract

This study was performed to investigate the synthesis, kinetic and reaction mechanism of Li₄SiO₄ with CO₂ in a slurry bubble column reactor. The Li₄SiO₄ powder sample was prepared via a solid-state reaction. The sample was characterized via X-ray diffraction (XRD) analysis and verified as a single phase. The median diameter of the sample was measured using the laser diffraction and scattering method as about 20?μm. The synthesized sample was suspended in binary molten carbonate of Li₂CO₃–K₂CO₃ having a molar ratio of 38:62. The experimental results show that Li₄SiO₄ in the slurry bubble column absorbed approximately a stoichiometric amount of CO₂. The kinetic study shows that the CO₂ reaction behavior on the Li₄SiO₄ surface was fitted to a double exponential model and the limiting step of the reaction was lithium diffusion. The mass transfer coefficient of CO₂ and rate constant of reaction with Li₄SiO₄ were studied to understand the overall absorption mechanism in the reactor. The resistance for the direct reaction of CO₂ on the Li₄SiO₄ was much smaller than the resistance for the mass transfer of CO₂ to the Li₄SiO₄. We can conclude that the direct contact of CO₂ with Li₄SiO₄ was the main path for the reaction.     

Novel Li4SiO4-based sorbents from diatomite for high temperature CO2 capture

Using inexpensive porous diatomite as silicon source, novel Li₄SiO₄-based sorbents for high temperature CO₂ capture were prepared through the solid-state reaction method at lower temperature (700 °C). Effect of different raw material ratios on CO₂ absorption capacity was investigated. The results showed that CO₂ absorption capacity was dependent on the raw material ratio. When the raw material ratio was 2.6:1, the CO₂ absorption capacity reached 30.32 wt% (83% of the theoretical absorption capacity) in the atmosphere (50 mL/min N₂ and 50 mL/min CO₂). Meanwhile, it was found that the as-prepared Li₄SiO₄-based sorbents from diatomite exhibited good absorption–desorption performance.     

Mass fabrication of Li2TiO3–Li4SiO4 ceramic pebbles with high strength by facile centrifugal granulation method

The bi-phase Li₂TiO₃–Li₄SiO₄ ceramic pebbles have been considered a promising breeder to realize the tritium self-sustainment in the blanket. However, up to now, the reported ceramic pebbles have the disadvantages of low yield, poor crushing load, and loose internal structure, which cannot meet the practical application requirements. In this work, the Li₂TiO₃–Li₄SiO₄ ceramic pebbles with excellent mechanical properties were fabricated successfully via the centrifugal granulation method with the assistance of introducing a spray-drying process, simulating particle trajectory by discrete element software and improving bonding interface between core and shell with ethylene glycol. The composition, microstructure, and inner structure of the Li₂TiO₃–Li₄SiO₄ ceramic pebbles were investigated, respectively, through X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and X-ray computed tomography (CT). It can be found that the employment of the ethylene glycol solution on the surface of Li₂TiO₃ can make the core and the shell combine well. Moreover, the effect of the rolling speed of the Li₂TiO₃–Li₄SiO₄ ceramic pebbles was investigated via discrete element method (EDEM) simulation and experiments. The experimental results displayed that the Li₂TiO₃–Li₄SiO₄ ceramic pebbles sintered at 1100°C for 2 h have a uniform diameter of 1 mm, a good sphericity of 0.97, and an excellent crushing load of 82.4 N, which are superior to those pebbles that obtained by using the traditional wet methods. Moreover, the CT results showed that the appropriate porosity of the core was 3.21% and of the shell was 10.73%. Therefore, the simple centrifugal granulation method can be applied to prepare the Li₂TiO₃–Li₄SiO₄ ceramic pebbles in a large scale and shed a light to investigate the relevant advanced biphasic tritium breeder materials in the future.     

Kinetics of carbon dioxide sorption on potassium-doped lithium zirconate

Potassium-doped lithium zirconate (Li₂ZrO₃) sorbents with similar crystallite but different aggregate sizes were prepared by a solid-state reaction method from mixtures of Li₂CO₃, K₂CO₃, and ZrO₂ of different particle sizes. Carbon dioxide sorption rate on the prepared Li₂ZrO₃ sorbents increases with decreasing sorbent aggregate size. It is the size of the aggregate, not the crystallite, of Li₂ZrO₃ that controls the sorption rate. Temperature effect on CO₂ sorption is complex, depending on both kinetic and thermodynamic factors. A mathematical model based on the double-shell sorption mechanism was established for CO₂ sorption kinetics and it can fit experimental data quite well. Above 500°C, the rate-limiting step of CO₂ sorption is the diffusion of oxygen ions through the ZrO₂ shell formed during the carbonation reaction. Oxygen ion conductivities in the ZrO₂ shell were obtained by regression of the experimental CO₂ uptake curves with the model and are consistent with the literature data.     

High‐Temperature Capture of CO2 on Lithium‐Based Sorbents Prepared by a Water‐Based Sol‐Gel Technique

A convenient water‐based sol‐gel technique was used to prepare a highly efficient lithium orthosilicate‐based sorbent (Li₄SiO₄‐G) for CO₂ capture at high temperature. The Li₄SiO₄‐G sorbent was systematically studied and compared with the Li₄SiO₄‐S sorbent prepared by solid‐state reaction. Both sorbents were characterized by X‐ray diffraction, scanning electron microscopy, nitrogen adsorption, and thermogravimetry. The CO₂ sorption stability was investigated in a dual fixed‐bed reactor. Li₄SiO₄‐G exhibited a special Li₄SiO₄ structure with smaller crystalline nanoparticles, larger surface area, and higher CO₂ adsorption properties as compared with Li₄SiO₄‐S. The Li₄SiO₄‐G sorbent also maintained higher capacities during multiple cycles.     

Tritium and deuterium release behavior of Li2TiO3-0.5Li4SiO4–Pb ceramic

The neutron multiplier lead (Pb) containing Li₂TiO₃-0.5Li₄SiO₄–Pb was prepared, which aimed at the improvement of the tritium release performance of the lithium ceramic without an increase in lithium density. Li₂TiO₃-0.5Li₄SiO₄–Pb and that without Pb were irradiated at Kyoto University Research Reactor (KUR) with the thermal neutron flux of 5.5 × 10¹² n s⁻¹ cm⁻² and the fast neutron flux of 1.2 × 10¹² n s⁻¹ cm⁻². Fast neutrons can trigger the Pb (n, 2n) reaction. The tritium release experiments were conducted using the tritium thermal desorption spectroscopy (tritium-TDS) system. Compared with Li₂TiO₃-0.5Li₄SiO₄, the tritium release rate at the lower temperature range was higher than that for Li₂TiO₃-0.5Li₄SiO₄–Pb. The tritium-TDS spectrum was also shifted to the lower temperature side. The isothermal heating experimental results confirmed that tritium in the form of gas species (HT) was controlled by the diffusion process, while the trapping and de-trapping process contributed to the total tritium release. Further, 3 keV D₂⁺ implantation with the fluence of 2.0 × 10²² D m⁻² and subsequent TDS measurements were performed. It was shown that the D₂ release at the lower temperature side was increased for Li₂TiO₃-0.5Li₄SiO₄–Pb. Meanwhile, HD and HDO release at higher temperature side was significantly decreased.     

Preparation of Li4SiO4-xLi2O powders and pebbles for advanced tritium breeders

Li₄SiO₄ pebbles and Li₂O pebbles have been considered as the potential candidates for tritium breeders. Li₂O exhibits higher lithium density whereas worse lithium loss and hygroscopicity compared with Li₄SiO₄. It is anticipated to obtain an advanced breeder by combining Li₄SiO₄ with Li₂O. The coexistence of Li₄SiO₄ and Li₂O powders could not be obtained by solid state reaction using Li₂CO₃ as lithium source, while the biphasic Li₄SiO₄-xLi₂O (x=0.1, 0.2, 0.3, 0.4) powders were prepared by sol-gel method in this experiment. Meanwhile, the biphasic Li₄SiO₄-xLi₂O pebbles were fabricated by a wet method for the first time. The Li₂O aggregated at the grain boundaries and promoted the grain growth of the Li₄SiO₄. The grain size and the crush load of the Li₄SiO₄-0.3Li₂O pebbles reached 32.3 µm and 46.5 N, respectively.     

Long-term thermal stability of Li4TiO4–Li2TiO3 core–shell breeding pebbles under continuous heating in H2/Ar atmosphere

The long-term thermal stability of tritium breeding materials during service is a key factor to ensure efficient tritium release. In this study, the long-term thermal stability of advanced Li₄TiO₄–Li₂TiO₃ core–shell breeding pebbles under continuous heating in 1%H₂/Ar at 900°C was investigated for the first time. The results show that this core–shell material loses 3.4% Li mass after heating for 30 days, resulting in a reduction in Li density to .415 g/cm³, which is still significantly higher than other breeding materials. The moisture in the sample bed will determine the form of Li volatilization and thus affect the rate of Li mass loss. The core–shell pebbles maintain favorable phase stability during long-term heating, and the grain sizes of the Li₂TO₃ shell and Li₄TiO₄ core after 30 days of heating are 6.5 ± 1.5 and 6.9 ± 2.5 μm, respectively. Moreover, the samples did not crack or collapse during long-term heating and still had a satisfactory crushing strength of 37.61 ± 7.13 N after 30 days of heating. Overall, the high Li density and good thermal stability during long-term heating demonstrate that the Li₄TiO₄–Li₂TiO₃ core–shell breeding pebbles are a very reliable tritium breeding material for long-term service under harsh operating conditions.     

Partial nitridation of Li4SiO4 and ionic conductivity of Li4.1SiO3.9N0.1

The Li content and anion lattice of Li₄SiO₄ were modified to improve ionic conductivity. Li₂CO₃ and Si₃N₄ were mixed in a ratio of Li/Si?=?4.5 and heated in NH₃ at 820?°C, which resulted in the formation of the oxynitride, Li_4.1SiO_3.9N_0.1. Powder X-ray diffraction analyses revealed Li_4.1SiO_3.9N_0.1 and Li₄SiO₄ to be isostructural with a subtle variation in the lattice constants. Diffuse-reflectance absorption spectroscopy, however, showed a significant decrease in the band gap, from 5.6?eV in Li₄SiO₄ to 4.8?eV in Li_4.1SiO_3.9N_0.1. X-ray photoelectron spectra of the Li 1s and Si 2p levels revealed enhanced lattice covalency in Li_4.1SiO_3.9N_0.1 compared to the oxide phase. The ionic conductivity of Li₄SiO₄ and Li_4.1SiO_3.9N_0.1 were measured by ac impedance spectroscopy over the temperature range 100–400?°C. Non-linear fitting analysis of the equivalent circuit revealed that the ionic conductivity of Li_4.1SiO_3.9N_0.1 was approximately one order of magnitude higher than that of Li₄SiO₄.     

Phase and microstructural evolution in lithium silicate glass-ceramics with externally added nucleating agent

Development of lithium disilicate-based glass-ceramics critically depends on use of nucleating agent in the glass matrix. The present study reports the effect of externally added nucleating agent Li₃PO₄ in Li₂O–K₂O–MgO–ZnO–ZrO₂–Al₂O₃–SiO₂ system which is compared with a reference composition (GC1) (SiO₂:Li₂O = 2.16:1) prepared with in situ formed Li₃PO₄. For externally added Li₃PO₄, two compositions were studied. In one case (GC2) before addition of Li₃PO₄, SiO₂:Li₂O ratio in glass was maintained as 2.87:1 and in another case (GC3) SiO₂:Li₂O ratio in glass was maintained same as reference GC1 that is, 2.16:1. The glasses were characterized by using MAS-NMR spectroscopy. Sintering and crystallization behavior of the glass-ceramics was characterized by using XRD, SEM, DTA. Due to in situ formation of Li₃PO₄, GC1 resulted in a dense sample with finer crystals of lithium disilicate. In GC2 and GC3, externally added lithium phosphate, which was in the form of ultrafine aggregated particles, formed flower-like colonies of radially outward crystals. Higher SiO₂:Li₂O ratio in GC2 resulted in lithium disilicate crystals and high viscous glass causing large air entrapment and so less densification. GC3 with higher lithia in glass showed higher densification than GC2 but only lithium metasilicate crystals were formed.     

Synthesis of waste bagasse-derived Li4SiO4-based ceramics for cyclic CO2 capture: Investigation on the effects of different pretreatment approaches

The bagasse, which is a typical agricultural and forestry waste, was employed as the silicon source to produce Li₄SiO₄-based ceramics with superior CO₂ sorption performance. It is found that the presence of impurities elements in the bagasse ash will bring complex side reactions and form side products during the sorbent preparation process, which are extremely harmful to the performance of the synthetic sorbent. Pretreatment has been considered as an effective approach to reduce the impurities elements. In this work, the effects and mechanisms of water/acid washing pretreatments on bagasse ash were firstly detailed investigate and compared. The results indicate that the hydrochloric acid washing pretreatment can effectively remove the residual metals (Ca, Fe, etc.) in the sample in addition to the water-soluble impurities (K, P, S, etc.). The maximum sorption capacity of sorbent synthesized by the acid washing pretreatment is about 0.32 g/g sorbent and the superior performance can be well maintained over 10 cycles. A novel technical route utilizing the ash from biomass power plants for in-situ CO₂ capture has been developed, which has great potential for carbon capture in the future.     

A Kinetic Study of Ethanol Steam Reforming Using a Nickel Based Catalyst

A kinetic study of ethanol steam reforming to produce hydrogen within the region of kinetic rate control was carried out. A Ni(II)–Al(III) lamellar double hydroxide as catalyst precursor was used. H₂, CO, CO₂ and CH₄ were obtained as products. Using the Langmuir–Hinshelwood (L–H) approach, two kinetic models were proposed. The first was a general model including four reactions, two of them corresponding to ethanol steam reforming and the other two to methane steam reforming. When high temperatures and/or high water/ethanol feed ratios were used, the system could be reduced to two irreversible ethanol steam reforming reactions.     

Coupled experimental phase diagram study and thermodynamic optimization of the Li2O–MgO–SiO2 system

A coupled key phase diagram study and critical evaluation and optimization of all available experimental data of the Li₂O–MgO–SiO₂ system was performed to obtain a set of Gibbs energy functions to reproduce all the reliable phase equilibria and thermodynamic data. Differential scanning calorimetry measurements and equilibration/quenching experiments were performed in the Li₂SiO₃–MgO and Li₄SiO₄–Mg₂SiO₄ sections, respectively, using sealed Pt capsules to prevent the volatile loss of Li. According to the present experimental results, Li₂MgSiO₄ is the only compound present in the Li₄SiO₄–Mg₂SiO₄ isopleth, which shows a peritectic melting at 1465 ± 6°C (1738 ± 6 K). The Modified Quasichemical Model, which considers short‐range ordering in the melt, was employed to describe the thermodynamic properties of the liquid phase. The Li₄SiO₄–Li₂MgSiO₄ and Mg₂SiO₄‐rich solid solutions were modeled using the Compound Energy Formalism.     

Improving Physical Adsorption of CO2 by Ionic Liquids‐Loaded Mesoporous Silica

CO₂ sorption capacities of the neat and silica‐supported 1‐butyl‐3‐methylimidazolium‐based ionic liquids (ILs) were measured under atmospheric pressure. The silica‐supported ILs were synthesized by the impregnation‐vaporization method and charactrized by N₂ adsorption/desorption and thermogravimeteric analysis (TGA). Evaluation of the effects of influential factors on sorption capacity demonstrated that by increase of the temperature, flow rate, and the weight percentage of ILs in sorbents, the sorption capacity decreases. Among the sorbents, [Bmim][TfO] and SiO₂‐[Bmim][BF₄](50) had the highest capacity. By increasing the IL portion in SiO₂‐[Bmim][BF₄], the selectivity for CO₂ to CH₄ could be improved. The CO₂‐rich sorbents could be easily recycled.     

Coupled reaction and solvent extraction process to form Li2CO3: Mechanism and product characterization

A coupled reaction and solvent extraction process to produce Li₂CO₃ from LiCl and CO₂ is proposed. The aqueous reaction occurs with difficulty and is not spontaneous at 298 K. To ensure the continuous reaction between LiCl and CO₂ to produce Li₂CO₃, it is necessary to remove HCl. Experiments conducted in an open reactor at room‐temperature showed that the product sample was pure Li₂CO₃ with a particle‐size distribution in two regions and a larger number of small crystals. The large and small crystals were obtained by radial growth in the free aqueous phase and in water‐in‐oil structures with growth space constraints, respectively. Factors influencing the particle‐size distribution and Li₂CO₃ morphology were investigated. A short reaction time, large phase ratio (O/A), low–temperature, and surfactant addition (TritonX‐100) can increase the number of small particle crystals and reduce the number and volume of large particle crystals. © 2013 American Institute of Chemical Engineers AIChE J, 60: 282–288, 2014     

Kinetics of CO2 Adsorption/Desorption of Polyethyleneimine‐Mesoporous Silica

The kinetics of adsorption of CO₂ on solid sorbents based on polyethyleneimine/mesoporous silica (PEI/MPS) was studied by following the mass gain during CO₂ flow. Linear (PEI‐423) and branched (PEI‐10k) polymers were studied. The solid sorbents were synthesized by impregnating the PEI into MPS foam. The kinetics of adsorption was fitted with a double‐exponential model. In contrast, the desorption process obeyed first‐order kinetics. The activation energy of desorption of PEI‐423 was lower than that of PEI‐10k, presumably because the branched polymer required more energy to expose its nitrogen to CO₂. To increase the CO₂ sorption capacity, the MPS was treated with nonionic surfactant materials prior to impregnation with PEI. This also lowered the maximum sorption temperature and desorption activation energies.