Highly efficient helium purification through a dual-membrane system: insights from molecular dynamics simulations

Almost all helium is resourced from natural gas reservoirs. Hence, it is essential to develop new efficient technologies to recover helium from natural gas. In this work, we propose a novel dual membrane system, consisting of C2N (M1) and graphdiyne (M2) membranes, to separate and purify helium from a ternary gas mixture of He/N2/CH4. In this regard, we performed molecular dynamics (MD) simulations to investigate the separation performance of the proposed system. Here, we explored the effect of applied pressure (up to 2 MPa) and the feed composition on the separation performance. The simulation results revealed that in the designed system, the M1 membrane allows He and N2 to diffuse through and prevents CH4 from crossing even at an applied pressure gradient. Next, the M2 membrane only allows He to transfer through and prevents N2 from crossing even at the applied pressure gradient. As a result, the dual membrane system showed a high He permeance of 2.5 × 106 GPU and ultrahigh He selectivity. In addition, the suggested dual membrane system could separate three components simultaneously at the applied pressure of 2 MPa, which implies the outstanding performance of the system. We also analyzed the density map, the van der Waals interactions, and the potential of the mean force calculations to better understand the permeation of gas species across the designed system.

Microstructure and Hydrothermal Stability of Microporous Niobia-Silica Membranes: Effect of Niobium Doping Contents

Methyl-modified niobium-doped silica (Nb/SiO2) materials with various Nb/Si molar ratios (nNb) were fabricated using tetraethoxysilane and methyltriethoxysilane as the silica source and niobium pentachloride as the niobium source by the sol–gel method, and the Nb/SiO2 membranes were prepared thereof by the dip-coating process under an N2 calcining atmosphere. Their microstructures were characterized and gas permeances tested. The results showed that the niobium element existed in the formation of the Nb-O groups in the Nb/SiO2 materials. When the niobium doping content and the calcining temperature were large enough, the Nb2O5 crystals could be formed in the SiO2 frameworks. With the increase of nNb and calcination temperature, the formed particle sizes increased. The doping of Nb could enhance the H2/CO2 and H2/N2 permselectivities of SiO2 membranes. When nNb was equal to 0.08, the Nb/SiO2 membrane achieved a maximal H2 permeance of 4.83 × 10−6 mol·m−2·Pa−1·s−1 and H2/CO2 permselectivity of 15.49 at 200 °C and 0.1 MPa, which also exhibited great hydrothermal stability and thermal reproducibility.

Gas Permeation Characteristics of TiO2-ZrO2-Aromatic Organic Chelating Ligand (aOCL) Composite Membranes

Methyl gallate (MG) and ethyl ferulate (EF) with a benzene ring were separately used as aromatic organic chelating ligands (aOCLs) to prepare two versions of TiO₂-ZrO₂-aOCL composite sols via hydrolysis and polycondensation reactions with titanium(IV) isopropoxide (Ti(OC₃H₇)₄) and zirconium(IV) butoxide (Zr(OC₄H₉)₄). Thermogravimetric and FT-IR analysis of dry gels revealed that aromatic rings were present in the residual organic matter when the gel was fired under nitrogen at 300 °C. In X-ray diffraction (XRD) measurements, the TiO₂-ZrO₂ composite material prepared using these two aOCLs showed an amorphous structure with no crystalline peaks for TiO₂ and ZrO₂. In N₂ adsorption/desorption measurements at 77 K, the TiO₂-ZrO₂ samples using the aOCLs as a template appeared porous with a larger specific surface area than TiO₂-ZrO₂ without aOCL. TiO₂-ZrO₂-aOCL composite membranes were prepared by coating and firing TiO₂-ZrO₂-aOCL sol onto a SiO₂ intermediate layer using an α-alumina porous tube as a substrate. Compared with the TiO₂-ZrO₂ membrane, the TiO₂-ZrO₂-aOCL membranes had higher gas permselectivity. The TiO₂-ZrO₂-EF membrane showed a He permeance of 2.69 × 10⁻⁶ mol m⁻² s⁻¹ Pa⁻¹ with permeance ratios of He/N₂ = 10.6 and He/CF₄ = 163, while the TiO₂-ZrO₂-MG membrane revealed a bit less He permeance at 8.56 × 10⁻⁷ mol m⁻² s⁻¹ Pa⁻¹ with greater permeance ratios of He/N₂ = 61.7 and He/CF₄ = 209 at 200 °C. A microporous TiO₂-ZrO₂ amorphous structure was obtained by introducing aOCL. The differences in the side chains of each aOCL could possibly account for the differences in the microporous structures of the resultant TiO₂-ZrO₂-aOCL membranes.

Review of Membranes for Helium Separation and Purification

Membrane gas separation has potential for the recovery and purification of helium, because the majority of membranes have selectivity for helium. This review reports on the current state of the research and patent literature for membranes undertaking helium separation. This includes direct recovery from natural gas, as an ancillary stage in natural gas processing, as well as niche applications where helium recycling has potential. A review of the available polymeric and inorganic membranes for helium separation is provided. Commercial gas separation membranes in comparable gas industries are discussed in terms of their potential in helium separation. Also presented are the various membrane process designs patented for the recovery and purification of helium from various sources, as these demonstrate that it is viable to separate helium through currently available polymeric membranes. This review places a particular focus on those processes where membranes are combined in series with another separation technology, commonly pressure swing adsorption. These combined processes have the most potential for membranes to produce a high purity helium product. The review demonstrates that membrane gas separation is technically feasible for helium recovery and purification, though membranes are currently only applied in niche applications focused on reusing helium rather than separation from natural sources.

Hydrothermal stability investigation of micro- and mesoporous silica containing long-range ordered cobalt oxide clusters by XAS

This work investigates the hydrothermal stability of cobalt doped silica materials with different Co/Si molar ratios (0, 0.05, 0.10, and 0.25). The resultant materials were characterized by N2 sorption and chemical structures by Raman and X-ray absorption spectroscopy before and after a harsh hydrothermal exposure (550 °C, 75 mol% vapour and 40 h). The cobalt silica materials showed a lower surface area loss from 48% to 12% with increasing Co/Si molar ratio from 0.05 to 0.25 and relatively maintaining their pore size distribution, while pure silica exhibited significant surface area reduction (80%) and pore size broadening. For low cobalt loading sample (Co/Si = 0.05), the cobalt was highly dispersed in the silica network in a tetrahedral coordination with oxygen and a small proportion of Co-Co interaction in the second shell. Long range order Co3O4 was observed when Co/Si molar ratio increased to 0.10 and 0.25. The hydrothermal exposure did not affect the local cobalt environments and no cobalt-silicon interaction was observed by X-ray absorption spectroscopy. The hydrothermal stability of the silica matrix was attributed to the physical barrier of cobalt oxide in opposing densification and silica mobility under harsh hydrothermal conditions.

Microporous niobia-silica membrane with very low CO2 permeability

A sol-gel-derived microporous ceramic membrane with an exceptionally low permeability for CO(2) from gaseous streams was developed and characterized. The sols were prepared from a mixture of niobium and silicon alkoxide precursors by acid-catalyzed synthesis. Microporous films were formed by coating asymmetric gamma-alumina disks with the polymeric sol (Si/Nb=3:1), followed by calcination at 500 degrees C. The membrane consists of a 150-nm-thick layer with a Si/Nb atomic ratio of about 1.5. The single-gas permeance of small gas molecules such as H(2), CH(4), N(2), and SF(6) decreases steadily with kinetic diameter. Hydrogen, helium, and carbon dioxide follow an activated transport mechanism through the membrane. The permeance of CO(2) in this membrane is much lower than that in pure silica, and its behavior deviates strongly from the general trend observed with the other gases. This is attributed to a relatively strong interaction between CO(2) and adsorption sites in the niobia-silica membrane.