Simultaneous effect of oxygen impurity and flow rate of purge gas on adsorption capacity of and heel buildup on activated carbon during cyclic adsorption-desorption of VOC.

Irreversible adsorption, or heel buildup, negatively impacts activated carbon performance and shortens its lifetime. This study elucidates the interconnections between flow rate and the oxygen impurity of desorption purge gas with heel buildup on beaded activated carbon (BAC). Nine thermal desorption scenarios were explored, varying nitrogen purge gas oxygen impurity levels (<5 ppmv, 10,000 ppmv, 210,000 ppm (21 %)) and flow rates (0.1, 1, 10 SLPM or 1 %, 10 %, 100 % of adsorption flow rate) during thermal desorption. Results reveal that increasing purge gas flow rate improves adsorption capacity recovery and mitigates adverse effects of purge gas oxygen impurity. Cumulative heel increased with higher purge gas oxygen impurity and lower flow rates. In the least effective regeneration scenario (0.1 SLPM N2, 21 % O2), a 32.8 wt% cumulative heel formed on BAC after five cycles, while the best-case scenario (10 SLPM N2, <5 ppmv O2) resulted in only 0.3 wt%. Comparing the pore size distributions of virgin and used BAC shows that heel initially forms in narrow micropores (<8.5Å) and later engages mesopores. Thermogravimetric analysis (TGA) showed that oxygen impurity creates high boiling point and/or strongly bound heel species. TGA confirmed that higher purge gas flow rates reduce heel amounts but encourage chemisorbed heel formation in oxygen's presence. These findings can guide optimization of regeneration conditions, enhancing activated carbon's long-term performance in cyclic adsorption processes.

Effect of microstructure in mesoporous adsorbents on the adsorption of low concentrations of VOCs: An experimental and simulation study.

This study evaluated the adsorption of five volatile organic compounds (VOCs) on Opoka, precipitated silica, and palygorskite, to elucidate the effect of their pore size on VOCs adsorption. The adsorption capacity of these adsorbents is not only highly correlated with their surface area and pore volume, but also notably improved by the presence of micropores. The variation in adsorption capacity for different VOCs was primarily influenced by their boiling point and polarity. Palygorskite, which had the smallest total pore volume (0.357 cm3/g) but the largest micropore volume (0.043 cm3/g) among the three adsorbents, exhibited the highest adsorption capacity for all tested VOCs. Additionally, the study constructed slit pore models of palygorskite with micropores (0.5 and 1.5 nm) and mesopores (3.0 and 6.0 nm), calculated and discussed the heat of adsorption, concentration distribution, and interaction energy of VOCs adsorbed on different pore models. The results revealed that the adsorption heat, concentration distribution, total interaction energy, and van der Waals energy decrease with increasing pore size. The concentration of VOCs in 0.5 nm pore was nearly three times that in 6.0 nm pore. This work can also provide guidance for further research on using adsorbents with mixed microporous and mesoporous structures to control VOCs.

Graphene oxide enhances thermal stability and microwave absorption/regeneration of a porous polymer.

Microwave regeneration of adsorbents offers several advantages over conventional regeneration methods; however, its application for microwave transparent adsorbents such as polymers is challenging. In this study, hypercrosslinked polymer/graphene oxide (GO) nanocomposites with large surface area and enhanced microwave absorption ability were synthesized. Polymers of 4, 4´-bis ((chloromethyl)-1, 1´-biphenyl- benzyl chloride) were hypercrosslinked through the Friedel-Crafts reactions. GO sheets were synthesized through the Hummer's method. Nanocomposites with different GO contents (1-8 wt%) were synthesized by solution mixing method. Thermogravimetry analysis revealed a large enhancement in the thermal stability of GO-filled nanocomposites compared to pristine polymer. N2 adsorption isotherm analysis showed 7% and 10% reduction in BET surface area and total pore volume of the nanocomposite with 8 wt% GO. Compared to the pristine polymer, the dielectric constant and dielectric loss factor increased from 5 to 17 and 0.05-1.6, respectively, for the nanocomposites with 8 wt% GO. Microwave-assisted desorption of toluene from samples revealed more than 160 ºC and 4 times improvement in the desorption temperature and desorption efficiency, respectively, by addition of 4 wt% GO to the polymer. This study showed the important role of GO addition for efficient microwave-assisted regeneration of polymer adsorbents.

Porous carbon black-polymer composites for volatile organic compound adsorption and efficient microwave-assisted desorption.

Adsorbents with high surface area, thermal stability and microwave absorption ability are highly desired for cyclic adsorption and microwave regeneration processes. However, most polymeric adsorbents are transparent to microwaves. Herein, porous hyper-crosslinked polymers (HCP) of (4,4'-bis((chloromethyl)-1,1'-biphenyl-benzyl chloride)) with different carbon black (CB) contents were synthesized via the Friedel-Crafts reaction. CB was selected as the filler due to its low cost and high dielectric loss and was embedded inside the polymer structure during polymerization. CB-containing composites showed enhanced thermal stability at elevated temperatures, and more than a 90-times increase in the dielectric loss factor, which is favorable for microwave regeneration. Nitrogen physisorption analysis by the Bruner-Emmett-Teller isotherms demonstrated that CB presence in the polymer structure nonlinearly decreases the surface area and total pore volume (by 38% and 26%, respectively at the highest CB load). Based on the characterization testing, 4 wt% of CB was found to be an optimum filler content, having the highest MW absorption and minimal effect on the adsorbent porosity. HCP with 4 wt% CB allowed a substantial increase in the desorption temperature and yielded more than a 450% enhancement in the desorption efficiency compared to HCP without CB.

Mesoporous MCM-41 derived from natural Opoka and its application for organic vapors removal.

Mesoporous silica MCM-41 was synthesized by a facile hydrothermal treatment using sodium silicate extracted from natural Opoka as the Si source. The dynamic adsorption and desorption of organic vapors mixture on the MCM-41 were investigated. Characterization of the textural properties of the samples showed that the sample synthesized with a molar ratio of CTAB/Si = 0.16 possessed the largest specific surface area (988 m2/g) and pore volume (1.02 cm3/g), also uniform pore size distribution centered at 2.8 nm. The adsorption capacity of this sample for organic vapors mixture improved remarkably over raw Opoka and reached 158.5 mg/g at 20 â„ƒ, which is comparable to that of commercial activated carbon. The reusability of the adsorbent was tested by 5 adsorption and regeneration cycles. Obtained results demonstrate that the MCM-41 adsorbent can be easily regenerated by thermal desorption in air, and the cumulative heel on the adsorbent can be markedly reduced by increasing the desorption temperature, making it a promising adsorbent for VOCs abatement.