Aging heat treatments in maraging steels are fundamental to achieve the excellent mechanical properties required in several industries, i.e., nuclear, automotive, etc. In this research, samples of ...maraging 300 alloy were aged using a novel procedure that combines different steps with two atmospheres (nitrogen and water vapor) for several hours. The oxidized surface layer was chemical, microstructural and micromechanically characterized. Due to the thermodynamic and kinetic conditions, these gases reacted and change the surface chemistry of this steel producing a thin iron-based oxide layer of a homogeneous thickness of around 500 nm. Within the aforementioned information, porosity and other microstructural defects showed a non-homogeneous oxide, mainly constituted by magnetite, nickel ferrite, cobalt ferrite, and a small amount of hematite in the more external parts of the oxide layer. In this sense, from a chemical point of view, the heat treatment under specific atmosphere allows to induce a thin magnetic layer in a mixture of iron, nickel, and cobalt spinel ferrites. On the other hand, the oxide layer presents an adhesive force 99 mN value that shows the capability for being used for tribological applications under sliding contact tests.
Mesoporous silica SBA-15 solids have been synthesized at different aging temperature with the goal of obtaining materials with different textural properties. The synthesized samples have been ...evaluated for their CO
2
adsorption capacity. In order to increase the CO
2
adsorption capacity and the CO
2
/N
2
selectivity, the SBA-15 synthesized at different aging temperatures were functionalized via grafting with 3-aminopropyltriethoxysilane (APTES) and via impregnation with polyethyleniminethylenediamine branched (PEI) or tetraethylenepentamine (TEPA). In all cases, the adsorption isotherms of the amine functionalized silica are fitted to the Dualsite-Langmuir model, where physical and chemical adsorption sites are reported. The isotherms reveal that grafted-silicas with APTES displayed the coexistence of both adsorption sites, while the adsorption process of PEI or TEPA impregnated-silicas are mainly governed by chemical interactions. The adsorption isotherms show that the most promising adsorbent for CO
2
capture in terms of CO
2
adsorption capacity and CO
2
/N
2
selectivity is the SBA synthesized at 393 K (aging temperature) and functionalized with TEPA (50 wt%) (SBA-393-50T). This sample reached a CO
2
adsorption capacity of 2.83 mmol g
−1
at 100 kPa and 338 K. The CO
2
adsorption capacity of this material decreased around 5% after the first adsorption/desorption cycle, maintaining a constant value for successive cycles. SBA-393-50T also showed an outstanding CO
2
/N
2
selectivity, increasing significantly as the CO
2
concentration decreases.
This study aimed to evaluate the production of high purity oxygen (90–95%) through experiments in a PSA/VSA unit and develop a mathematical model to describe the dynamic behavior of the process. ...Different operational parameters and the dead volume were investigated for their impact on process performance. The experiments used a laboratory-scale unit filled with beads of a commercial LiX zeolite to obtain breakthrough and PSA/VSA data for model validation. Equilibrium isotherms of pure oxygen and nitrogen were measured at 288, 298 and 313 K for the pressure range of 0 to 3 bar. Single and multicomponent breakthrough curves were obtained at 298 K. Synthetic air (grade 5.0 purity, excluding argon) with a composition of 20% (± 0.5%) O
2
and 80% (± 0.5%) N
2
was used in the PSA/VSA experiments. A novel approach was developed using the mathematical model designed to simulate PSA/VSA cycles to account for the dead volume effects commonly found in units of this type. The model was implemented and solved using gPROMS® software. The simulation data matched well with the experimental data, accurately representing histories of concentration, pressure, temperature, and purity variations during the process. The validated model revealed optimal operating conditions for a VSA unit: 7.5 s adsorption time, 1.5 bar adsorption pressure, 0.1 bar desorption pressure, and a flow rate of 1 SLPM, producing a purity of approximately 94% and a recovery of about 20%. Increasing the adsorption duration negatively affected the oxygen purity but positively influenced process recovery and productivity. Adding an equalization stage improved process recovery by 18.9% for PSA and 14.5% for VSA. Additionally, increased dead volume in the column had adverse effects on purity, productivity, and recovery for both PSA and VSA units.
Hydrothermal stability of Na-LTA shaped with clay binder Moreno, Jimmy D. L.; da S. Pereira, Daniele; Azevedo, Thalita M. ...
Adsorption : journal of the International Adsorption Society,
06/2024, Letnik:
30, Številka:
5
Journal Article
Recenzirano
Gas drying by adsorption on molecular sieves is one of the first unit operations in natural gas processing. Nevertheless, the thermal swings during regeneration combined with the presence of a ...complex gas mixture may lead to early adsorbent fouling. The shaping of adsorbent materials is crucial not only to reduce pressure drop in columns, but also for enhanced thermal and mechanical properties. In this work, synthesized sodium-based zeolite Linde Type A (Na-LTA) in powder was shaped by extrusion into cylindrical pellets using a clay binder followed by calcination at 673 K for 24 h. The produced 2-mm pellets had binder contents of 15 and 20% (w/w). Thereafter, the shaped pellets were subjected to an accelerated aging procedure, which exposes the samples to conditions analogous to those of Temperature Swing Adsorption drying processes with a higher severity. High-resolution water vapor isotherms at 313 K were measured before and after the simulated aging process. Results show that the sample with 20% of binder had an average decrease in water uptake (mol kg
−1
) of 1.2% after aging, whereas the aged zeolite in powder presented a decrease of 8.1, evidencing a protective effect of the clay binder against thermal fouling.
The increasing worldwide demand for energy bound to a strong dependence of fossil fuels has considerably intensified the concentration of greenhouse gases in the atmosphere, reaching alarming levels. ...Among those gases, carbon dioxide is considered the main responsible for global warming due to its higher concentration. In order to mitigate the negative effects of global warming and to reduce emissions, many technologies have been developed in the last decades to separate and recover carbon dioxide (CO2) at different capture scenarios. Adsorption processes rely on the use of highly porous solids such as activated carbons, which are either commercially. Pressure Swing Adsorption (PSA) is a cyclic adsorption process, which allows continuous separation of gas streams. PSA is performed by periodic changes of pressure aiming the optimization of contaminants removal and is considered viable for separation of CO2 from flue gases containing about 5-15% v/v. To achieve a certain performance, a PSA process may consist of several steps, columns and cycle time. One of the most basic configurations comprises four steps: pressurization, feed, blowdown and purge. The performance of a PSA process is usually evaluated by the purity, recovery and productivity reached. This study presents experimental and simulated data obtained from a bench-scale PSA, with a maximum pressure of 6bar for pressurization and feed steps and minimum of 1bar for blowdown and purge steps. The unit was tested with a mixture containing 85% of N2 and 15% of CO2 (on a molar basis). Carbon dioxide and nitrogen breakthrough curves were obtained under typical conditions of post combustion capture. A mathematical-phenomenological model combining momentum, mass and heat balances and using the Linear Driving Force approach (LDF) for mass transport and Langmuir model for equilibrium was applied in this study to simulate the dynamic behavior of the process. The performance tests presented productivity of 15mol h-1 kg-ads-1 and, according to the changes of step time, N2 purity of 97.7%. The model predicted reasonably the breakthrough curves and temperature profiles, with more precision for the latter. The combination of the simulation tool with and experimental PSA unit is very valuable for a deeper understanding of the involved phenomena and helpful with the design of optimized and efficient CO2 adsorption-based capture processes.
CO₂ adsorption on mesoporous silica modified with amine by double functionalization was studied. Adsorption microcalorimetry was used in order to investigate the influence of increasing the nitrogen ...surface density on double functionalized materials with respect to the only grafted materials. The distribution of sites and the rate-controlling mechanism of adsorption were evaluated. A Tian Calvet microcalorimeter coupled to a manometric setup was used to evaluate the energy distribution of adsorption sites and to calculate the thermokinetic parameters from the differential enthalpy curves. CO₂ and N₂ adsorption equilibrium isotherms at 50 and 75 °C were measured with a magnetic suspension balance, allowing for the computation of working capacity and selectivity at two temperatures. With these data, an Adsorbent Performance Indicator (API) was calculated and contrasted with other studied materials under the same conditions. The high values of API and selectivity confirmed that double functionalized mesoporous silica is a promising adsorbent for the post combustion process. The adsorption microcalorimetric study suggests a change in active sites distribution as the amine density increases. Maximum thermokinetic parameter suggests that physisorption on pores is the rate-controlling binding mechanism for the double-functionalized material.
Ion exchange is the reversible exchange of ions in which there is no significant change in the solid structure. Zeolites are aluminosilicates with a defined structure, including cavities occupied by ...cations and water molecules, both with great freedom of movement, which makes cation exchange possible. In this study, small-pore zeolites chabazite (CHA) and clinoptilolite (CLI) were ion-exchanged with potassium. Then, the samples were characterized by N
2
isotherms at 77 K, CO
2
adsorption microcalorimetry at 298 K, and water vapor isotherms at 313 K. A mathematical model was applied to evaluate the adsorption kinetics for water vapor uptakes. Textural analysis showed that the ion exchange with potassium decreased the porosity of both zeolites, but CO
2
microcalorimetric data showed that these samples had higher CO
2
adsorption enthalpy, indicating a greater sorbate-sorbent interaction as compared to the pristine zeolites. Uptake rate curves suggest water diffusion is not appreciably altered after ion exchange. Interestingly, despite the larger size of K
+
cations as compared to Na
+
, effective diffusion time constant is on order of magnitude larger for the potassium-loaded CLI very likely due to the leaching of other contaminants upon ion-exchange.
Special issue on the 13th Brazilian meeting on adsorption Cavalcante, Célio L.; Bastos-Neto, Moisés; Rodrigues, Alírio E. ...
Adsorption : journal of the International Adsorption Society,
2021/10, Letnik:
27, Številka:
7
Journal Article
Zeolites are widely employed in the industrial drying of gases by Temperature Swing Adsorption (TSA). In a typical TSA process, the adsorbent is packed in a fixed bed, which is sequentially subjected ...to a “cold” feed (adsorption) and a hot flush (desorption). Due to the hydrothermal stress, adsorbents may suffer from reduced drying capacity in long-term service. The aim of this work is to assess the impact of thermal aging of two zeolite materials (LTA and CHA) having similar pore openings but different Si/Al ratios. We examined how simulated thermal aging affected porous texture, coke deposition and water vapor adsorption equilibrium and kinetics. Both zeolites showed reduced uptake of probe molecules (N
2
at 77 K and CO
2
at 273 K) and water vapor (303 K) when subjected to simulated thermal aging. In Fourier-Transform Infrared Spectroscopy (FTIR) analyses, only the aged LTA zeolite showed bands related to the presence of aromatic compounds. Water vapor adsorption uptake decreased 28.6% and 18.4% for LTA and CHA at 30 mbar, respectively. Kinetic studies indicate a reduction in water diffusion coefficient after the aging cycles. Although LTA has a significantly higher affinity for water as compared to CHA, the latter is much more resistant to hydrothermal aging with comparatively faster water diffusion.
In view of the promising applicability of adsorption to the capture of CO
2
from post-combustion gases, the use of mesoporous silica functionalized with 3-aminopropyltriethoxysilane (APTES) was ...studied as adsorbent in a fixed bed for CO
2
–N
2
separation under thermal swings. Characterization of the adsorbent performed before and after functionalization indicated that amine grafting was successful. Additionally, CO
2
adsorption on a magnetic suspension balance showed a significant increase in uptake of the APTES-functionalized sample over the support, mainly at low relative pressures. Relatively high values of adsorption enthalpy suggest the occurrence of chemical adsorption attributed to CO
2
bonding with the amines. Breakthrough curves were measured for pure CO
2
, N
2
and the CO
2
/N
2
(15/75% v/v) mixture, which showed good agreement with respect to the uptake of the individual gases, as determined from gravimetric tests. Full CO
2
desorption from the bed required a temperature rise, which suggests that these materials may be suitable for TSA cyclic processes. A temperature of 90 °C was enough for a complete regeneration of the adsorbent during the desorption phase under dynamic conditions. The material showed very stable behavior after 20 successive cycles.
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