Separation Of SF6 Research Articles

Modulation of pore structure in a microporous carbon for enhanced adsorption of perfluorinated electron specialty gases with efficient separation

Physical adsorption-based separation and recovery of perfluorinated electron specialty gases from N2 contribute to reducing the greenhouse effect and increasing economic value in the electronics industry. Commercial activated carbon materials are promising adsorbents for the practical application due to the industrial-scale production, while they usually characterize an unsatisfactory micropore structures, and the rational modulation to meet the requirement of fluorinated gas adsorption is in demand. In this work, a series of KOH-treated commercial activated carbon (ACK) with KOH/carbon mass ratios of 1 ∼ 4 were systematically characterized and evaluated for adsorptive separation of SF6, NF3, and CF4. Characterization results reveal that KOH treatment largely promotes the modulation of pore structure and results in the extremely narrow pore size distribution at 0.5 ∼ 0.9 nm, highest microporosity of 97.6 % in the optimal ACK1, which are ascribed to etching effect of KOH on carbon framework. Static adsorption results indicate that a simultaneous enhancement in gas uptake and separation selectivity is found in ACK1 compared to the parent AC for all the studied fluorinated gases. Importantly, ACK1 performs a high SF6 uptake of 3.10 mmol·g−1 and the record SF6/N2 selectivity of 683.9 (298 K, 100 kPa) among the reported carbon adsorbents, which is comparable to the top-performing MOFs material. The kinetic adsorption and dynamic breakthrough experiments further verify the fast adsorption rate, excellent separation performance of SF6/N2, and recyclability of ACK1 adsorbent. These findings expand the promising application of commercial AC in the adsorption-based separation of perfluorinated electron specialty gases including SF6, NF3, and CF4.

Porous aromatic frameworks as HF resistant adsorbents for SF6 separation at elevated pressure

The public concern about sulfur hexafluoride (SF6) emission has driven the exploration of related removal processes via adsorptive separation technology. Based on task-specific porous materials, the capture of SF6 achieves lower energy consumption and less carbon footprint. Herein, four HF resistant porous aromatic frameworks (PAF-XJTU-1 to PAF-XJTU-4) with robust CC linkage were prepared for the separation of SF6 in semiconductor etching exhaust. Single-component gas adsorption and IAST selectivity confirmed the separation performance of the four PAFs materials under elevated pressure (10 bar). Moreover, DFT studies revealed the nonnegligible SF6 intermolecular interaction in PAFs at elevated pressure. Under the dynamic scenario, breakthrough experiments proved the effective capture of SF6 in PAF-XJTUs.

Integrated material and process evaluation of metal–organic frameworks database for energy-efficient SF[formula omitted]/N[formula omitted] separation

In this work, we proposed multi-scale screening, which employs both molecular and process-level methods, to identify high-performing MOFs for energy-efficient separation of SF6 and N2 mixture. Grand canonical Monte Carlo (GCMC) simulations were combined with ideal adsorption process simulation to computationally screen 2890 metal–organic frameworks (MOFs) for adsorptive separation of SF6/N2. More than 150 high-performing MOFs were identified based on the GCMC simulations at the pressure and vacuum swing conditions, and subsequently evaluated using the ideal adsorption process simulation. 78 out of 86 MOFs selected for the VSA conditions were able to achieve the 90% target purity level of SF6, but 62 top-performing MOFs selected for the PSA condition could not reach the purity level with a single train PSA configuration. Cascade PSA configuration was proposed and adopted to improve the purity level. We also investigated the effect of vacuum pump and compressor efficiency on the energy consumption of the process. We found that the top-performing MOFs were able to achieve the 90% purity-level of SF6 with 0.10–0.4 and 0.5–1.4 MJ per kg of SF6 for VSA and PSA processes, respectively. Finally, the process-level performance of top-performing MFOs (HKST-1, UiO-67) was evaluated on the basis of the experimental isotherms obtained from the literature, and compared with the other materials reported in the literature (MIL-100(Fe), UiO-66, and zeolite-13X). We found that the results based on the experimental isotherms are in qualitative agreement with the results based on the simulated isotherms.

Formation and dissociation behaviors of SF6 hydrates in the presence of a surfactant and an antifoaming agent for hydrate-based greenhouse gas (SF6) separation

Sulfur hexafluoride (SF6), the most potent greenhouse gas, should be separated from gas mixtures for recycling and for mitigation of global warming. In this study, the formation and dissociation behaviors of SF6 hydrates in the presence of a surfactant (sodium dodecyl sulfate, SDS) and an antifoaming agent (antifoam A concentrate, AAC) were investigated, with a primary focus on kinetic, spectroscopic, and morphological analyses for hydrate-based SF6 separation. The optimum concentrations of SDS and AAC for SF6 hydrates were found to be 250 ppm and 1,500 ppm, respectively. The structure of SF6 hydrates in the presence of SDS and AAC was identified as structure II, indicating that SDS and AAC had no impact on hydrate structure. The formation behaviors of SF6 hydrates were thoroughly examined through gas uptake measurements, visual observation, and in-situ Raman spectroscopy. The addition of SDS 250 ppm significantly accelerated the formation rate of SF6 hydrate and the additional injection of AAC did not inhibit the promoting effect of SDS. Visual observation, temperature profiles, and volume of retrieved gas during the dissociation of SF6 hydrates clearly demonstrated that SDS also had a promoting effect on SF6 hydrate dissociation and its effect was slightly diminished with the addition of AAC, although AAC showed a powerful defoaming effect during the dissociation of SF6 hydrates. The experimental results obtained in this study will be very useful for accelerating the formation rate of SF6 hydrates using SDS and for solving the foaming problem using AAC in the design and operation of the gas hydrate-based SF6 separation process.

SF6 Hydrate Formation in Various Reaction Media: A Preliminary Study on Hydrate-Based Greenhouse Gas Separation.

SF6 hydrate formation behaviors in various reaction media, such as bulk water, porous silica gel, and hollow silica, were investigated for hydrate-based SF6 separation with a primary focus on thermodynamic stability and formation kinetics. The measured three-phase (H-LW-V) equilibria demonstrated that the types of reaction media used in this study had no effect on the thermodynamic stability of SF6 hydrates. The dissociation enthalpy (ΔHd) of SF6 hydrate was measured using a high-pressure micro-differential scanning calorimeter, and it corresponded well with estimates from the Clausius-Clapeyron equation. The unstirred porous silica gel system showed a larger gas uptake and a higher growth rate at the early stage of SF6 hydrate formation. However, the gas uptake and growth rate of SF6 hydrates in stirred bulk water and unstirred hollow silica were significantly increased at a larger temperature driving force or in the presence of sodium dodecyl sulfate. The experimental results obtained in this study will be very helpful for a better understanding of the thermodynamic and kinetic characteristics of SF6 hydrate formed in various reaction media and in surfactant-added solution, and are expected to contribute to further development of the hydrate-based SF6 separation process.

Hierarchically porous polymers containing triphenylamine for enhanced SF6 separation

Porous organic polymers (POPs) have a good potential for gas separation applications owing to their large surface area, structural robustness and chemical tunability. Previous studies have demonstrated the utility of POP-based adsorbents for carbon dioxide separation. However, no study has so far reported on the potential of POPs for capturing sulfur hexafluoride (SF6), which is an extremely potent greenhouse gas. Here we demonstrate that the amine-incorporated POPs synthesized through the co-condensation of dichloroxylene (DCX) and triphenylamine (TPA) for application in SF6/N2 separation. Large surface area and pore volume of DCX-based POPs provide the structural framework for high SF6 adsorption. The incorporation of tertiary amines favors the SF6/N2 selectivity without sacrificing the SF6 adsorption at the point of interest (0.1 bar) according to the combined evaluations of the ideal adsorbed solution theory and idealized vacuum swing adsorption. Further analysis revealed that the tertiary amines increase the isosteric heat of adsorption, a measure of affinity between adsorbate and adsorbent. Undertaking a comparative adsorption kinetics study, we also demonstrated that TPA-containing POPs exhibit much faster SF6 uptake than zeolite 13X, a commonly used commercial adsorbent, owing the co-existence of microporosity and mesoporosity that facilitates the diffusion of adsorbate within the adsorbent. Dynamic breakthrough and chromatographic measurements further confirmed the enhanced SF6 adsorption kinetics of TPA-containing POPs. In overall, we attributed the high SF6 separation performance of TPA-containing POPs to their two main characteristics: i) the presence of large micropores (c.a. 1.2 nm) alongside hierarchical mesoporosity, ii) the selectivity provided by amine functionalities.

Highly selective polymer electrolyte membranes consisting of poly(2-ethyl-2-oxazoline) and Cu(NO3)2 for SF6 separation.

Polymer electrolyte membranes consisting of Cu(NO3)2 and poly(2-ethyl-2-oxazoline) (POZ) were prepared for SF6/N2 separation. It was anticipated that repulsive forces would be operative between the negative charge of water and the F atoms of SF6 when Cu(NO3)2 in the composite was solvated by water, and that the barrier effect of Cu2+ ions would be activated. In fact, Cu(NO3)2 solvated by water in the POZ membrane was observed to have more higher-order ionic aggregates than free ions or ion pairs, as confirmed by FT-Raman spectroscopy. Thus, when Cu(NO3)2 solvated by water was incorporated into the POZ matrix, the N2/SF6 selectivity increased to 28.0 with a N2 permeance of 11.2 GPU at a POZ/Cu(NO3)2 mole ratio of 1:0.7. The coordinative interaction of Cu(NO3)2 with the carbonyl group in POZ was confirmed by FT-IR spectroscopy and TGA, and the film thickness of the membrane was determined from SEM analysis.

Wide Carbon Nanopores as Efficient Sites for the Separation of SF6 from N2.

SF6 and SF6-N2 mixed gases are used widely as insulators, but such gases have high greenhouse gas potential. The separation of SF6 from SF6-N2 mixed gases is an inevitable result of their use. Single-walled carbon nanohorns (CNHs) were used here for a fundamental study of the separation of SF6 and N2. The diameters of the interstitial and internal nanopores of the CNHs were 0.7 and 2.9 nm, respectively. The high selectivity of SF6 over N2 was observed only in the low-pressure regime in the interstitial 0.7 nm nanopores; the selectively was significantly decreased at higher pressures. In contrast, the high selectivity was maintained over the entire pressure range in the internal 2.9-nm nanopores. These results showed that the wide carbon nanopores were efficient for the separation of SF6 from the mixed gas.

High SF6 selectivities and capacities in isostructural metal-organic frameworks with proper pore sizes and highly dense unsaturated metal sites

A series of isostructural M-MOF-74 materials (M=Mg, Co, and Zn) with proper pore sizes (11Å) and highly dense unsaturated metal sites have been examined for the selective adsorption of SF6 over nitrogen. Mg-MOF-74 and Co-MOF-74 show the largest SF6 uptake reported for any adsorbent, including MOFs, at ambient temperature and pressure. Zn-MOF-74 exhibits one of the highest SF6/N2 reported selectivities. These materials were evaluated for two possible SF6 separation processes using five adsorbent criteria, revealing that these materials are promising adsorptive materials for SF6/N2 separation.

Separation of Sulfur Hexafluoride from a Nitrogen/Sulfur Hexafluoride Mixture Using a Polymer Hollow Fiber Membrane

Sulfur hexafluoride (SF6) has been evaluated by the Intergovernmental Panel on Climate Change (IPCC) as the substance with the highest global warming index. Because of its superior insulating and arc clearing capacities, it is commonly used as an insulator in electrical machines. SF6 waste products form in the process of storing, maintaining, and repairing the machines. SF6 emitted into the atmosphere remains for 3,200 years, causing global warming. Release into the mesosphere leads to photolysis and creation of highly toxic and corrosive by-products. A review of the literature related to the retrieval and separation of SF6 using a separating membrane indicates that research on the permeability of the separating membrane material is lacking. Additionally, research on the concentrations of the SF6 waste products and the separation/retrieval with operating conditions with optimal energy efficiency is only in the initial stages. Therefore, this research assessed the permeability of commercialized separation membranes polysulfone (PSf), polycarbonate (PC), and polyimide (PI) using the gases SF6 and N2. Using an SF6/N2 mixture with the same concentration as the SF6 waste products, we studied the separation and retrieval capacities of PSf, PC, and PI separation membranes under varying operating conditions. The permeability tests showed that the selective permeability of N2/SF6 is highest for the PI membrane and lowest for the PC membrane. When the concentrations of SF6 retrieved from the mixture separation process were compared, the PC membrane was found to be the highest, with 95.6 % at 0.5 MPa. The retrieval percentage of SF6 was highest for PSf, with 97.8 % at an operating pressure of 0.3 MPa and a waste production of 150 cm3/min. The retrieval rates and retrieval failure rates have an inverse relationship. In total, 99 % of the supply of SF6 was identified via the retrieval rates and retrieval failure rates, so it could be confirmed that the separation of the SF6/N2 mixture using a macromolecular hollow fiber separation membrane works properly.

Separation of sulfur hexafluoride (SF6) from ternary gas mixtures using commercial polysulfone (PSf) hollow fiber membranes

The separation of sulfur hexafluoride (SF6), a powerful anthropogenic greenhouse gas, was investigated using membrane technology. Permeation measurements in commercial polysulfone (PSf) hollow fiber membranes were performed using single gases (i.e. N2, O2 and SF6) to investigate the effect of temperature and pressure on separation performance. The permeation experiments of ternary gas mixtures (N2/O2/SF6) were also conducted under various operational conditions, including pressure, temperature, stage cut (permeation flow rate/feed flow rate) and gas compositions. The results showed that the SF6 treatment capacity increased with increase in temperature or pressure, but decreased with increasing stage cut and SF6 content in the feed gas mixture. At higher temperatures, the membrane exhibited higher performance for the separation, recovery and enrichment of SF6. A feed with a higher pressure or a lower stage cut resulted in lower SF6 separation and enrichment efficiency, but a higher recovery. The separation of SF6 from a gas mixture with higher contents of SF6 exhibited lower SF6 recovery and enrichment performance. Our current work demonstrated more realistic performance of the commercial PSf hollow fiber membrane for the separation, enrichment and recovery of SF6.

Separation of SF6 from Binary Mixtures with N2 Using Commercial Poly(4-Methyl-1-Pentene) Films

Systematic studies on gas permeation of pure SF6 and N2 as well as their mixture in poly(4-methyl-1-pentene) (PMP) at different temperatures and pressures, using commercially available thin PMP films, are reported in this article. The effective separation of SF6 from binary mixtures with N2 is critical for the proposed replacement of pure SF6, used as an insulating gas in high power industry, by the mixtures of these two gases. This replacement is driven by the fact that SF6 is the most potent greenhouse gas, with a global warming potential of 22,200 times that of CO2. The experiments with a 1:1 mixture of N2 and SF6 revealed the permselectivity of PMP as high as 476 with the corresponding N2 permeability coefficient of 7.6 Barrer. These properties, which are much better than those of other glassy polymers considered for this separation, were not affected by a long-term exposure to SF6, which indicates the excellent resistance of PMP to plasticization by this gas. Using a single stage membrane system utilizing the PMP membrane would allow separating the above gas mixture into a 99% pure SF6 product with the corresponding recovery rate of SF6 greater than 99%.

Separation of SF6 from Gas Mixtures Using Gas Hydrate Formation

This study aims to examine the thermodynamic feasibility of separating sulfur hexafluoride (SF(6)), which is widely used in various industrial fields and is one of the most potent greenhouse gases, from gas mixtures using gas hydrate formation. The key process variables of hydrate phase equilibria, pressure-composition diagram, formation kinetics, and structure identification of the mixed gas hydrates, were closely investigated to verify the overall concept of this hydrate-based SF(6) separation process. The three-phase equilibria of hydrate (H), liquid water (L(W)), and vapor (V) for the binary SF(6) + water mixture and for the ternary N(2) + SF(6) + water mixtures with various SF(6) vapor compositions (10, 30, 50, and 70%) were experimentally measured to determine the stability regions and formation conditions of pure and mixed hydrates. The pressure-composition diagram at two different temperatures of 276.15 and 281.15 K was obtained to investigate the actual SF(6) separation efficiency. The vapor phase composition change was monitored during gas hydrate formation to confirm the formation pattern and time needed to reach a state of equilibrium. Furthermore, the structure of the mixed N(2) + SF(6) hydrate was confirmed to be structure II via Raman spectroscopy. Through close examination of the overall experimental results, it was clearly verified that highly concentrated SF(6) can be separated from gas mixtures at mild temperatures and low pressure conditions.

SF6 Reclaimer from SF6/ N2 Mixtures by Gas Separation with Molecular Sieving Effect

Discussing various methods for SF6 separation from a mixture of low concentrations of SF6 in N2, pressure swing adsorption (PSA) with a suitable kind of synthetic zeolite, which is expected molecular sieving effect, has been selected. This molecular sieving effect, derived from molecular size difference between SF6 and N2, is confirmed by the difference between two equivalent volumes filled with SF6 and N2. Prototype equipment of SF6 separation and liquefaction, about 1 m cube in size and 150 kg in weight, has been assembled and tested. The ability of gas mixture handling is 13 L/min. in average, and the SF6 content is reduced to 0.0% (undetectable level) in separated N2 to exhaust into the atmosphere.