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.
Abstract
Ammonia (NH
3
) has emerged as a sustainable future fuel for a low‐carbon future. Ideally, the transportation and storage of ammonia in re‐purposed natural gas networks will dramatically ...reduce the economic burden associated with converting to ammonia. However, the highly condensable nature and small kinetic diameter of the ammonia means that this fuel readily permeates through the polymer‐based elastomers and gaskets used in natural gas infrastructure. To mitigate this issue, three kinds of additives have been incorporated into the conventional natural gas elastomer poly(acrylonitrile‐co‐butadiene) (NBR) to reduce ammonia permeance. The first is based on increasing the basicity of the elastomer environment, the second provides chemical reactivity with ammonia to limit transport, and the third uses barrier additives to prevent diffusion. The mechanical properties of the modified elastomers; expressed as the tensile stress at yield and Young's modulus were studied. The creation of a basic environment through the addition of amines resulted in moderate changes in ammonia permeability, with up to 50% reduction of ammonia permeability observed from putrescine added NBR. Chemical additives lowered the permeability by up to 20% with limited impact on the tensile properties. The addition of 3% graphene oxide to NBR had the most significant decrease in ammonia permeability, 80%, but also produced a more rigid material. The investigation shows that ammonia permeability can be significantly reduced in NBR elastomers, through the addition of inexpensive additives in tailored strategies.
Membrane technology holds great potential in gas separation applications, especially carbon dioxide capture from industrial processes. To achieve this potential, the outputs from global research ...endeavours into membrane technologies must be trialled in industrial processes, which requires membrane-based pilot plants. These pilot plants are critical to the commercialization of membrane technology, be it as gas separation membranes or membrane gas-solvent contactors, as failure at the pilot plant level may delay the development of the technology for decades. Here, the author reports on his experience of operating membrane-based pilot plants for gas separation and contactor configurations as part of three industrial carbon capture initiatives: the Mulgrave project, H3 project and Vales Point project. Specifically, the challenges of developing and operating membrane pilot plants are presented, as well as the key learnings on how to successfully manage membrane pilot plants to achieve desired performance outcomes. The purpose is to assist membrane technologists in the carbon capture field to achieve successful outcomes for their technology innovations.
Helium recovery and purification from a natural gas process is increasingly being investigated globally to address rising market demand, as traditional helium sources become depleted. Here, process ...simulations of two types of inorganic membranes were undertaken in Aspen HYSYS to investigate the possibility of recovering and purifying helium from the Nitrogen Rejection Unit (NRU) offgas close to the NRU’s operating temperature. The two membranes were a cobalt-silica membrane that has He/N2 selectivity through molecular sieving and a zeolite membrane that has N2/He selectivity at low temperatures, because of surface diffusion. Both membranes were able to achieve the desired He recovery and purification through a three-membrane-stage process, and for a feed of 4% He, the cobalt-silica membrane could achieve the same separation performance through a two-membrane-stage process above 340 K, because of increasing selectivity with temperature. In contrast, the zeolite membrane could not operate above 220 K, because of the loss of the surface diffusion mechanism. The difference in permeance of the two membranes significantly affected the membrane area, with the cobalt-silica membrane requiring three orders of magnitude more area than the zeolite membrane to recover and purify the same amount of helium. However, the zeolite membrane’s selectivity for N2 meant that the vast majority of the NRU offgas passed through the membrane into the permeate streams. Hence, to ensure a high helium recovery, the permeate streams from the second and third membrane stages must be recycled, resulting in permeate gas throughputs that are orders of magnitude higher than the cobalt-silica membrane process. This placed significant recompression duty on the zeolite membrane process, compared to the cobalt-silica process, and, as such, the zeolite membrane’s power duty for helium separation was at least five times greater than that of the cobalt-silica membrane. Hence, there is a tradeoff between the two inorganic membranes for helium recovery and purification, based on required membrane area and power demand.
Supercritical carbon dioxide (sc-CO2) will plasticize and partially solubilise polymeric membranes, resulting in alteration to the polymer morphology, impacting the gas separation properties. Here, ...cellulose triacetate (CTA) and polyimides, Matrimid and 6FDA-TMPDA membranes were exposed to supercritical CO2 for 2 and 8 h, followed by two depressurization protocols; a rapid depressurization of 12 MPa/min and a slow depressurization of 0.17 MPa/min. The resulting impact on He, N2, CH4 and CO2 permeability as well as the corresponding selectivities were then quantified. Matrimid membranes undergo substantial plasticization in the presence of sc-CO2 resulting in significant increases in permeability and loss of selectivity, irrespective of the sc-CO2 exposure protocol. CTA and 6FDA-TMPDA membranes experience competing phenomenon under supercritical conditions, both demonstrate limited CO2 plasticization which is offset by sc-CO2 partly solubilising the polymers, enabling rearrangement to a more compact morphology. The depressurization protocol strongly impacted the underlying morphology for these two membranes. Rapid depressurization resulted in a higher fractional free volume and greater gas permeability, which is attributed to the sudden expansion of CO2 sorbed in the polymer opening up the morphology. Slower depressurization resulted in a lower fractional free volume and decreased gas permeabilities, because the CO2 gradually desorbs, leaving behind a more dense morphology. The resulting decrease in permeability also corresponded with a significant increase in selectivity. For both CTA and 6FDA-TMPDA membranes the sc-CO2 treatment improved the gas permselectivity relative to the original state. Therefore, exposure to sc-CO2 presents an advantageous process to improve the performance of common polymeric membranes for gas separation.
•Morphology change dependent on whether supercritical CO2 is rapidly or slowly depressurized.•Supercritical CO2 partially solubilised cellulose triacetate and 6FDA-TMPDA membranes.•Competing phenomena observed of plasticization against solubilisation impacting morphology.•Supercritical CO2 exposure significantly improved the permselectivity of CTA and 6FDA-TMPDA.
Abstract
Catalytic solvent regeneration has attracted broad interest owing to its potential to reduce energy consumption in CO
2
separation, enabling industry to achieve emission reduction targets of ...the Paris Climate Accord. Despite recent advances, the development of engineered acidic nanocatalysts with unique characteristics remains a challenge. Herein, we establish a strategy to tailor the physicochemical properties of metal-organic frameworks (MOFs) for the synthesis of water-dispersible core-shell nanocatalysts with ease of use. We demonstrate that functionalized nanoclusters (Fe
3
O
4
-COOH) effectively induce missing-linker deficiencies and fabricate mesoporosity during the self-assembly of MOFs. Superacid sites are created by introducing chelating sulfates on the uncoordinated metal clusters, providing high proton donation capability. The obtained nanomaterials drastically reduce the energy consumption of CO
2
capture by 44.7% using only 0.1 wt.% nanocatalyst, which is a ∽10-fold improvement in efficiency compared to heterogeneous catalysts. This research represents a new avenue for the next generation of advanced nanomaterials in catalytic solvent regeneration.
Polymers of intrinsic microporosity (PIMs) are a promising membrane material for gas separation, because of their high free volume and micro-cavity size distribution. This is countered by PIMs-based ...membranes being highly susceptible to physical aging, which dramatically reduces their permselectivity over extended periods of time. Supercritical carbon dioxide is known to plasticize and partially solubilise polymers, altering the underlying membrane morphology, and hence impacting the gas separation properties. This investigation reports on the change in PIM-1 membranes after being exposed to supercritical CO2 for two- and eight-hour intervals, followed by two depressurization protocols, a rapid depressurization and a slow depressurization. The exposure times enables the impact contact time with supercritical CO2 has on the membrane morphology to be investigated, as well as the subsequent depressurization event. The density of the post supercritical CO2 exposed membranes, irrespective of exposure time and depressurization, were greater than the untreated membrane. This indicated that supercritical CO2 had solubilised the polymer chain, enabling PIM-1 to rearrange and contract the free volume micro-cavities present. As a consequence, the permeabilities of He, CH4, O2 and CO2 were all reduced for the supercritical CO2-treated membranes compared to the original membrane, while N2 permeability remained unchanged. Importantly, the physical aging properties of the supercritical CO2-treated membranes altered, with only minor reductions in N2, CH4 and O2 permeabilities observed over extended periods of time. In contrast, He and CO2 permeabilities experienced similar physical aging in the supercritical treated membranes to that of the original membrane. This was interpreted as the supercritical CO2 treatment enabling micro-cavity contraction to favour the smaller CO2 molecule, due to size exclusion of the larger N2, CH4 and O2 molecules. Therefore, physical aging of the treated membranes only had minor impact on N2, CH4 and O2 permeability; while the smaller He and CO2 gases experience greater permeability loss. This result implies that supercritical CO2 exposure has potential to limit physical aging performance loss in PIM-1 based membranes for O2/N2 separation.
•Simulation of two existing membrane contactors for solvent regeneration by two competing models.•Calculated module lengths for two membrane contactors at different regeneration ...temperatures.•Membrane contactors achieve process intensification compared to a column above 90 °C.•Membrane contactors have lower energy demand than a column above 95 °C.
Membrane gas-solvent contactors are potentially more efficient at undertaking solvent regeneration in carbon capture applications compared to traditional desorber columns. This potential is investigated here through modelling of two experimentally reported membrane contactors based on Teflon AF1600 and polydimethylsiloxane active layers, by a simple approximate model and a one-dimensional mass transfer model. The respective membrane contactors could be operated at temperatures below that corresponding to the vaporisation of the solvent, due to the separation of the solvent and gas phase by the membrane. This required a steam sweep operating under vacuum conditions. The calculated module length for the Teflon AF1600 membrane varied between 8.9–70.2 m with decreasing regeneration temperature. This increase in required length was due to reductions in overall mass transfer coefficient and mass transfer driving force as temperature is lowered. It was determined that at 110 °C a PDMS contactor of length 1.1 m was required to regenerate the solvent, achievable with commercial modules. A comparison of the equipment volume footprint, known as process intensification, revealed that both the Teflon AF1600 and PDMS membranes required a lower volume than a standard packed column when operating at temperatures above 90 °C. Temperatures higher than 95 °C also designated the transition above which membrane contactors have a lower energy duty than the corresponding solvent column approach. This energy duty is a trade-off between the reduction in latent heat required to produce CO2 and regenerate the solvent at lower temperatures, countered by the work duty of the vacuum pump needed to operate the steam sweep at pressures below atmospheric. The investigation demonstrated that membrane contactors are a viable alternative technology for solvent regeneration.
Thermal rearrangement of α-functional polyimide membranes into poly(benzoxazole) improves the permselectivity performance compared to the precursor polymer. This is due to the bimodal cavity size ...distribution generated through the TR process. The cavity volume can be further increased by including segments within the polyimide that undergo degradation at a lower temperature than the TR process. The loss of these segments leaves behind cavity space that can be used to increase gas permeability. This is achieved here for copolymers based on 4,4′-hexafluoroisopropylidene diphthalic anhydride (6FDA) and 3,3′-dihydroxy-4,4’-diamino-biphenyl (HAB) with poly (ethylene glycol) segments, where the PEG segments undergo thermal degradation below the PI to PBO transition temperature. HAB-6FDA-PEG copolymer membranes, with different weight % PEG, had poor permselectivity for CO2-N2 and CO2-CH4 separation. Undertaking thermal treatment to degrade the PEG segments but retaining the PI polymer resulted in an increased fractional free volume of the resulting membrane and higher gas permeability, but a corresponding loss of CO2 selectivity. Producing TR-PBO from the copolymers through thermal rearrangement at 450 °C, improved the gas permeability of the resulting membranes by over an order of magnitude, as well as improving the CO2 selectivity. This was attributed to the degradation of the PEG segments increasing the FFV of the membranes, resulting in over a third of the polymers' morphology being free volume. The resulting TR-PBO membranes formed from copolymers with PEG segment had enhanced permselectivity performance compared to TR-PBO formed from the polyimide homopolymer.
•Novel copolymers of polyimide with polyethylene glycol moieties.•Thermal treatment of the copolymer to produce polyimide and poly (benzoxazole) membranes.•Thermal rearrangement of copolymer precursors to produce membranes with high gas permeabilities.•Fractional free volume control through polyethylene glycol segment of the co-polyimide structure.•Variable gas permselectivity properties of the copolymer membrane based on thermal treatment.
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