Separation Of Binary Gas Mixtures Research Articles

CFD simulation for CO2 separation using diamond-structured thermally rearranged (TR) polymer membrane: Effect of CO2 feed concentration, feed pressure and temperature

Abstract CO2 separation in natural gas purification is important in improving the properties of natural gas such as energy content, volume and corrosivity. Membrane separation using TR-450 polymer membrane is a potential approach to remove CO2 from natural gas efficiently. The study of CO2/CH4 binary gas mixture separation was simulated using a thermally-rearranged (TR) HAB- 6FDA membrane at 450 °C with a partial part of Schwarz Diamond structure because it exhibits advanced mechanical properties with low stiffness for relatively high strength. TR-450 membrane exhibits excellent separation performance that has the potential to exceed the Robeson plot 1991. Computational fluid dynamics (CFD) was used to simulate the fluid behavior within the geometry created using CAD software. The effect of feed pressure, temperature and CO2 feed concentration on the separation performance of TR-450 membrane was also studied. The results indicated that an increase in feed pressure leads to a decrease in CO2 recovery, hence the lower separation performance. It was also found that increasing the feed temperature leads to an increase in CO2 recovery. Concerning the concentration of CO2 in the feed, elevated levels impact the performance of membrane separation. The enhanced membrane separation performance demonstrated by the TR-450 membrane under various conditions underscores the necessity for additional investigation to unveil its potential in industrial gas separation applications.

C2H2/CO2 Separation with a Chain‐Type Zn Pyrazolate MOF

AbstractA novel Zn dipyrazolate metal‐organic framework (MOF), Zn(azbpz) (azbpz2−=4,4′‐azobis(3,5‐dimethylpyrazolate), has been synthesized and structurally characterized using micro‐crystal electron diffraction. The framework contains chain‐type secondary building units comprised of tetrahedral Zn2+ ions bridged by pyrazolate groups, resulting in guest‐accessible channels lined with methyl and azo functional groups. CO2, C2H2, C2H4, C2H6, C3H6, and C3H8 gas adsorption isotherms were measured to investigate the potential of Zn(azbpz) as an adsorbent for selective separation of binary gas mixtures. The data reveal selective adsorption of acetylene over CO2, and column breakthrough experiments demonstrate good performance for C2H2/CO2 separation.

Optimization of the air separation process in single stage cryogenic units

The industrial use of cryogenic air separation units started more than 130 years ago. Cryogenic air separation units produce oxygen, pure nitrogen and argon in liquid and/or gaseous state. Different configurations of these cryogenic plants lead to different quantities of gas and liquid products. In addition, product purity is also affected by the proposed scheme. As a result, the paper analyzes different variants of installations for the separation of binary gas mixtures based on the Linde process. By comparing energy indices and constructive considerations, the separation plant with external air pre-cooling and the use of a single-stage separation column is chosen. Finally, a study is carried out on the processes in the separation column and a calculation of the minimum mechanical work of separation. The real air consumption curve will also be built, with the help of which we shall determine what quantities of O2 and N2 are obtained by air separation at a specific consumption.

Membrane Cascade Type of «Continuous Membrane Column» for Power Plant Post-Combustion Carbon Dioxide Capture Part 1: Simulation of the Binary Gas Mixture Separation

The present paper deals with the complex study of CO2 capture from combined heat power plant flue gases using the efficient technological design of a membrane cascade type of «Continuous Membrane Column» for binary gas mixture separation. In contrast to well-known multi-step or multi-stage process designs, the cascade type of separation unit provides several advantages. Here, the separation process is implemented in it by creating two counter current flows. In one of them is depleted by the high-permeable component in a continuous mode, meanwhile the other one is enriched. Taking into account that the circulating flows rate overcomes the withdrawn one, there is a multiplicative increase in separation efficiency. A comprehensive study of CO2 capture using the membrane cascade type of «Continuous Membrane Column» includes the determination of the optimal membrane material characteristics, the sensitivity study of the process, and a feasibility evaluation. It was clearly demonstrated that the proposed process achieves efficient CO2 capture, which meets the modern requirements in terms of the CO2 content (≥95 mol.%), recovery rate (≥90%), and residual CO2 concentration (≤2 mol.%). Moreover, it was observed that it is possible to process CO2 with a purity of up to 99.8 mol.% at the same recovery rate. This enables the use of this specific process design in CO2 pretreatment operations for the production of high-purity carbon dioxide.

Separation of binary gas mixture in a microchannel with oscillating barriers

The time-dependent flow of a neon–argon mixture in a microchannel interrupted by a row of oscillating barriers is numerically studied using the Direct Simulation Monte Carlo method in a range of Knudsen numbers from 0.1 to 10 and in a wide range of oscillation frequencies. The emphasis of the study is on the effect of mixture separation. It is demonstrated that in addition to a mid-frequency (“resonance”) regime, as discovered in the author's previous works [Kosyanchuk et al., “Numerical simulation of novel gas separation effect in microchannel with a series of oscillating barriers,” Microfluid. Nanofluid. 21, 116 (2017) and Kosyanchuk and Pozhalostin, “Non-stationary rarefied gas flow in a plane channel with a series of oscillating barriers,” Eur. J. Mech.-B/Fluids 92, 90–99 (2022)], two other enhanced separation regimes at very low and at very high oscillation frequencies are present. It is also demonstrated that the effect in the mid-frequency regime degrades with decreasing Knudsen number and is almost absent for Kn values around 0.1. The effect in the high-frequency regime is shown to be dictated both by the high frequency of barrier oscillations and by the high speed of barrier motion, and it is shown that with decreasing Knudsen number, the impact of barriers speed becomes dominant. The effect in the low-frequency regime is present for all Knudsen numbers and significantly depends on the phases of barrier motion, which is not observed in other regimes. The separation factor in the low-frequency regime also increases with the number of barriers but only up to the level of molecular diffusion. It was also shown that in the low-frequency regime, there is a trade-off between the separation factor and the gas flow rate.

Interfacially-confined polyetherimide tubular membranes for H2, CO2 and N2 separations

Capturing CO2 and H2 purification from gas mixtures such as CO2/N2 and H2/N2 are vital in addressing greenhouse gas emission abatement and global energy strategy. Here, we present a systematic investigation of tubular polyetherimide (PEI) membranes prepared by combining dip-coating and multilayer assembly techniques to target CO2 and H2 transports and CO2/N2 and H2/N2 selectivities. The effects of deposition cycle during dip-coating on H2, CO2 and N2 single gas permeations and binary gas separations were evaluated. The membranes prepared by three deposited layers produced gas permeances (10−12 mol m−2 s−1 Pa−1) of 310, 150 and 1 for H2, CO2 and N2 respectively, corresponding to CO2/N2 and H2/N2 permselectivities of 149 and 308. For binary gas mixture separation, an exclusive 100% CO2 and H2 permeate purity was observed irrespective of feed gas concentration. The PEI membrane exhibited a suppression of glass transition temperature and crystallinity compared to the bulk polymer indicating interfacial confinement phenomenon, which was further confirmed by X-ray diffraction and Raman spectroscopic analyses. Due to a subtle change of imide group conformation of the obtained membrane layer, it was inferred that an increase in the polymer amorphicity and interfacial confinement facilitated the exclusive H2 and CO2 transports without much affecting the N2 permeance.

Computational fluid dynamics simulation for dual‐phase membrane module for CO 2 capture: Effect of membrane thickness, temperature and CO 2 feed concentration

CO2 capture and natural gas purification are of paramount significance in power plant applications nowadays. Dual-phase membrane is an effective approach for the enhancement of CO2/CH4 separation efficiency. In this study, CO2/CH4 binary gas mixture separation was simulated by using hydroxide/ceramic dual-phase (HCDP) membranes incorporated in the industrial-scaled tubular membrane module. Computational fluid dynamics (CFD) was utilized as the approach to model the fluid flow within the membrane module. Utilizing the CFD modeling of membrane module, the effects of tubular membrane thickness, operating temperature and CO2 feed concentration on the separation performance of HCDP membrane had been investigated. The results showed that membrane thickness affected the effective surface area of the membrane and CO2 permeance. The greater the membrane thickness, the lower was the membrane separation efficiency. The highest CO2 recovery obtained was 90.12% at 1 mm tubular membrane thickness, with the membrane stage cut ratio of 0.1950. Operating temperature imposed a notable influence on the gas permeance as well. By increasing the operating temperature, CO2 permeances were improved and hence, the separation performance of HCDP membrane was enhanced simultaneously. As for the effect of CO2 feed concentration on the membrane performance, it was found that the overall membrane performance and separation efficiency were better when the CO2 feed concentration was low, due to the higher availability of the adsorption sites on the membrane surface. In conformity with the membrane technology as a cost-effective CO2 sequestration technique, the dual-phase membrane has demonstrated a remarkable potential to be adopted for industrial applications.

Investigations of CO2 Capture from Gas Mixtures Using Porous Liquids.

Porous liquids, a new porous material with fluidity, can be applied in numerous fields, such as gas storage and/or separation. In this work, the separation of binary gas mixtures CO2/N2 and CO2/CH4 with porous liquids was examined by molecular dynamics (MD) simulations. The pure gas adsorption capacity was analyzed with different concentrations of porous liquids. The dependence of the separation effect of a gas mixture on the total pressure and temperature was investigated. Meanwhile, for both CO2/N2 and CO2/CH4 systems, the adsorption and separation effects of porous liquids with a cage:solvent ratio of 1:12 are better than those of 1:91 and 1:170. The results of the spatial distribution function and/or trajectories indicated that porous liquids prefer CO2, leading to the location of CO2 in the channels formed in porous liquids. However, N2 and CH4 are hardly adsorbed into the bulk. The diffusion of gas molecules follows the order of CO2 > N2 (for CO2/N2) and CH4 > CO2 (for CO2/CH4) in the bulk and N2 > CO2 (for CO2/N2) and CH4 > CO2 (for CO2/CH4) at the interface of porous liquids. Upon increasing the concentrations of porous liquids, the working capacities of CO2 show small decreases in CO2/N2 and CO2/CH4 systems, but the sorbent selection parameters are higher in pressure- and temperature-swing adsorption processes. The porous liquid with a cage:solvent ratio of 1:12 is more suitable for the separation of CO2/N2 and CO2/CH4 systems than ratios of 1:91 and 1:170.

Dimensioning of ideal membrane cascade systems for the separation of binary gas mixtures for nuclear fusion applications

In the DEMO reactor highly efficient separation systems are required for the recovery of tritiated species to ensure a continuous re-fuelling of the plasma. For some of these systems, porous inorganic membranes have been proposed due to their promising modularity and cost-effectiveness for gas separation. For instance, zeolite membranes have been considered for the pre-concentration of Q2 in He (Q = H, D, T) in the European helium-cooled pebble bed (HCPB) to decrease the dimensions and costs of Pd/Ag membranes implemented downstream. Due to the similar sizes and masses of Q2 and He, the separation is limited with one membrane, hence a membrane cascade is required to achieve the desired performance requirements. In this paper, a numerical model is presented, which was developed to calculate the number of stages, surface area and power consumption of a membrane cascade using as input parameters the selectivity of the membrane α, permeances Π, pressures-ratio γ and the target separation requirements, the enrichment factor E and recovery fraction R. A sensitivity study was conducted to evaluate the impact of these parameters on the dimensioning of the membrane cascade.The pre-concentration of H2 in He using the zeolite membrane cascade with αH2/He=2 was first used as reference scenario for an inlet flow of Ffinj=8.06×103m3h−1 with E≥20 and R≥90%. The resulting membrane cascade consists of 16 stages, with 2563 m2 and 33.8 MW. For an improved selectivity of α = 10, the membrane cascade would have only 6 stages, with 260 m2 and 6 MW. The sensitivity analysis revealed that the cost-effectiveness and technological relevance of a membrane cascade are highly dependent on α, E and R.

Unlocking CO2 separation performance of ionic liquid/CuBTC composites: Combining experiments with molecular simulations

In this work, we propose a computational methodology based on the state-of-the-art molecular simulations of IL/CuBTC composites composed of ILs having the same cation, 1-n-butyl-3-methylimidazolium ([BMIM]+), and various anions. Using grand canonical Monte Carlo (GCMC) simulations, we predicted CO2, CH4, and N2 uptakes of seven different IL/CuBTC composites and compared the simulation results with our experimental gas uptake measurements to select the most appropriate force field that best represents the experimental results. Motivated from the good agreement between experiments and simulations, we applied the same method to estimate the gas adsorption in two new IL/CuBTC composites which have been synthesized and characterized for the first time in this work. Molecular simulations accurately predicted the experimental gas uptakes of newly synthesized IL/CuBTC composites, validating the transferability of our approach to different types of IL-incorporated CuBTC samples. We also provided a detailed analysis of binary gas mixture separation performances of IL/CuBTC composites and self-diffusion coefficients of gases in the composites performing GCMC and molecular dynamics simulations, respectively. Results showed that IL/CuBTC composites have higher CO2/CH4, CO2/N2, and CH4/N2 selectivities than those of CuBTC, demonstrating a broad potential of these composites for CO2 separation from natural gas and flue gas mixtures. The combination of experiments and molecular simulations that we described in this study will be useful to efficiently screen various IL/MOF composites to unlock their full potential for gas separation applications.

Optimal Design Procedure of Dual-Reflux Pressure Swing Adsorption Units for Nonlinear Separations

In the frame of dual-reflux pressure swing adsorption processes, design strategies applicable to the complete separation of binary gas mixtures are available only in the case of linear adsorption isotherms. Therefore, in this work we propose a simple and efficient design procedure which enables the selection of suitable operating conditions when sharp separation is required and Langmuir adsorption isotherms are involved. Starting from the available strategy for the linear adsorption case, we adapt it to the nonlinear case tuning properly the pressure ratio. Then, the resulting design strategy is validated by application to selected study cases involving different values of key parameters such as different levels of nonlinearity of the isotherms, but also different values of the pressure ratio, feed composition, and selectivity.

Moisture responsive and CO2 selective biopolymer membrane containing silk fibroin as a green carrier for facilitated transport of CO2

The research quest in carbon dioxide (CO2) separation technology through the polymeric membrane is intensive at this time of energy and environmental crisis. But, the pursuit of membranes with high selectivity and flux without trading it off for stability, still exists as a challenge. We report a novel membrane prepared by rational blending of two biopolymers: chitosan (CS) that acts as a matrix and silk fibroin (SF) that aids chitosan as a carrier for facilitated transport. The separation of binary gas mixture (CO2/N2) has been studied at temperature, feed absolute pressure and sweep water flow rate ranging from 60 to 120°C, 2 to 5.93bar and 0.01 to 0.07ml/min, respectively. The optimum CO2 permeance of 140 GPU and CO2/N2 selectivity of 103 for binary gas were observed at 90°C and 0.05ml/min of sweep water flow rate for the membrane containing 45wt% of SF in blend with CS (SF45). The SF45 composite membrane exhibited CO2 permeance of 123 GPU and selectivity of 93 and 60 for CO2/N2 and CO2/H2 respectively, in case of ternary gas mixtures (CO2/N2/H2) at the similar operating conditions. The appreciable CO2 separation performance tunable with temperature and moisture, displayed unprecedented stability for 30 days.

Optimal design of dual-reflux pressure swing adsorption units via equilibrium theory: Process configurations employing heavy gas for pressure swing

Dual-reflux pressure swing adsorption process is theoretically capable of completely separating binary feed gas mixtures into two pure species. The pressure of bed to which the binary gas mixture is fed and the type of gas utilized for pressure swing, results in different process cycle configurations, even if the majority of the previous studies of DR-PSA are restricted to two cycle configurations: that employ heavy gas for pressure swing and deliver feed to the bed operated at either high or low pressure. However, the comparative assessment and the optimal operating pressure ratio of these two process cycle configurations are not well-established. We previously reported an optimal design strategy (that identified a triangular operating zone, inside which, complete separation of binary gas mixtures can be achieved) for one such DR-PSA process cycle configuration. In this work, we report an optimal design strategy for another DR-PSA process cycle configuration: feed to low pressure bed and pressure swing using heavy gas. With respect to previous literature, the equilibrium theory based comprehensive tracking of the characteristic curves and shock transitions during constant and non-constant pressure steps of this specific cyclic process revealed distinct constraints, design parameter values and boundary conditions of the triangular operating zone. Additionally, an in-depth comparative assessment of the impact of process variables (adsorbent selectivity, feed gas composition and, operating pressure ratio) on the design parameters (optimal feed injection position and ratio of pure light reflux to feed rate) and a novel selection criterion is discussed for both of these cycle configurations in order to (i) facilitate the choice of appropriate cycle configuration and (ii) identify the optimal high to low operating pressure ratio range.

2D Squaraine‐Bridged Covalent Organic Polymers with Promising CO2 Storage and Separation Properties

AbstractThree hypothetical 2D porous covalent organic polymers, named SQ‐COPs (i. e., SQ‐COP‐1, SQ‐COP‐2 and SQ‐COP‐3), have been proposed via the combination of linear squaraine unit and heterocyclic molecules (H3B3O3, H3B3N3 and H3C3N3). By using the first principle methods and grand cannonical Monte Carlo (GCMC) simulations, these SQ‐COPs are predicted to possess big pore sizes (13.8‐15.0 Å), large free volumes (5.07‐10.94 cm3 g−1) and high BET specific surface areas (8585‐8938 m2 g−1), which even exceed those of MOF‐210 and PAF‐1. As is expected, these SQ‐COPs show promising gas storage and separation properties of H2, CH4, N2 and CO2 at ambient conditions. Especially at 298 K and 30 bar, their CO2 uptake reaches 804, 575 and 633 mg/g respectively, significantly higher than those of zeolites. Meanwhile, for separation of binary gas mixtures CO2/H2, CO2/CH4, CO2/N2, and CH4/H2, SQ‐COPs are comparable with most of COFs and MOFs. These porous SQ‐COPs materials are not only environmental‐friendly (metal‐free), but also promising candidates for CO2 capture.

Preparation and characterization of multilayer mesoporous γ-alumina membrane obtained via sol-gel using new precursors

AbstractIn this paper, a mesoporous γ-alumina membrane coated on a macroporous α-alumina support via sol-gel method has been reported. A crack-free γ-alumina membrane was obtained by adding PVA to the alumina solution and optimum parameters of roughness, temperature and porosity were achieved. The support was dip-coated in different solutions using two new different solvents with different particle size distributions. Using these two solvents led to the uniform distribution of pore size in the final membrane. The alumina sols showed particle size distributions in the range of 20 to 55 nm which was measured by a DLS Zeta Sizer. X-ray diffraction technique, atomic force microscopy and scanning electron microscopy were used to characterize the membrane layer. XRD and DTA data for the γ-alumina membrane showed its thermal stability up to around 600 °C. The thickness of the mesoporous γ-alumina membrane was about 4 μm with 16 nm of surface roughness and 5 nm pore size. The resultant crack-free mesoporous membrane shows that the membrane preparation procedure was optimum. In this work, it has been investigated the performance of γ-alumina membranes for single gas permeation and separation of binary gas mixtures.

Temperature dependent transition point of purity versus flux for gas separation in Fe/Co-silica membranes

This work investigates the performance of iron cobalt oxide silica membranes for the separation of binary gas mixtures containing H2 and Ar up to 500°C. A series of membranes were prepared by fixing the iron/cobalt molar ratio at 10/90, 25/75 and 50/50. H2 preferentially permeated though the membranes, and H2 purity in the permeate stream increased with temperature for all H2/Ar binary gas mixtures. The fluxes of H2 from binary gas mixtures complied, for the most part, with a temperature dependent transport mechanism, similar to that delivered by single gas permeation. However, it was notable to observe a “transition point” where H2 purity versus H2 flux clearly changed from temperature independent to temperature dependent. This gas separation transition point was also found to be a function of the quality of the membrane. Indeed the best performing membrane (Fe/Co=10/90) also had the highest gas separation transition point at ∼70% H2 purity. This reduced to ∼60% for the medium quality membrane (Fe/Co=25/75) and was at its lowest ∼43% for the lower quality membrane (Fe/Co=50/50). The binary gas fractions therefore affect the H2 fluxes and H2 purity more significantly than that expected in single gas permeation. Therefore, the relationship between membrane quality and gas separation transition point is established for the first time in this work.

An innovative approach for the preparation of confined ZIF-8 membranes by conversion of ZnO ALD layers

An innovative eco-friendly approach has been developed for the synthesis of ZIF-8-based nanocomposite membranes. The method involves the Atomic Layer Deposition (ALD) of ZnO thin films on the grains of a macroporous ceramic support and their subsequent conversion to ZIF-8 using a 2-methylimidazole/methanol solution under solvothermal conditions. Despite applying extremely low quantity of chemicals, reproducible and good quality ZIF-8-based membranes have been obtained, in which the ZIF-8 material was fully confined within the ceramic support macropores, thus leading to enhanced thermo-mechanical strength. A judicious choice of both the deposition conditions of ZnO layers and the solvothermal treatment parameters for their subsequent conversion to ZIF-8 was the key point for validation of this original protocol and evaluation of its potential for up-scaling on tubular industrial supports. The physico-chemical characterization of the ZIF-8/ZnO/α-Al2O3 nanocomposite membranes was completed by a study of their gas transport properties. Reproducible ZIF-8/ZnO/α-Al2O3 nanocomposite membranes were produced and tested in the separation of binary gas mixtures: membranes were found to extract H2 from H2/CO2 and H2/CH4 equimolar gas mixtures with selectivities of about 7.8 and 12.5 respectively, measured at 100°C.

Tuning metal sites of DABCO MOF for gas purification at ambient conditions

Metal–organic frameworks (MOFs) have emerged as new porous materials for capture and separation of binary gas mixtures. Tuning the metal sites in MOF structures has an impact on properties, which enhance affinity of gas adsorption and selectivity (e.g., surface area, cavity, electric field, etc.). The synthesis and characterization of a M-DABCO series (M=Ni, Co, Cu, Zn) of MOFs are described in this study. The experiments were conducted using multicomponent gas mixtures and the Ideal Adsorbed Solution Theory (IAST) was applied to determine the CO2/CH4 selectivity. Experimental adsorption isotherms were fitted with a model equation to evaluate the characteristic adsorption energy (Isosteric, Qst) of this series. The Ni metal in the M-DABCO series reveals the best performance concerning CO2 adsorption and CH4/CO2 selectivity at ambient conditions based on IAST calculations. The combination of characterizations, calculations and adsorption experiments were used to discuss the metal impact on the adsorption sites in the M-DABCO series at ambient conditions.

Optimal design of dual-reflux pressure swing adsorption units via equilibrium theory

An optimal design strategy for dual-reflux pressure swing adsorption units targeted at achieving the separation of binary gas mixtures into two pure components (complete separation) is presented. Based on the equilibrium theory, detailed study of a specific configuration (feed to high pressure bed and pressure swing using heavy gas) reveals that feed location along the adsorption column, capacity ratio of the purge step (ratio between amount of feed to be processed and amount of adsorbent) and light recycle ratio (ratio of pure light reflux to feed rate) are the three most relevant design parameters. With respect to previous literature, two major achievements can be mentioned: (i) the development of an expression to evaluate optimum feed location and (ii) the identification of an operating region inside which complete separation at cyclic steady-state conditions can be established. In the plane defined by the two quantities feed position-capacity ratio of the purge step, such region has triangular shape and optimal conditions correspond to a vertex.

Novel Modification of a Macroporous Stainless Steel Tube by Electroless Ni plating for Use as a Substrate for Preparation of Nanoporous Carbon Membranes

Nanoporous carbon (NPC)/Ni–P composite membranes were prepared on electroless Ni-plating-modified porous stainless steel tubes (PSSTs) by carbonizing polyimide or cellulose acetate precursors at 700 °C in a nitrogen atmosphere. The performance of the membranes was evaluated by single-gas permeation and binary-gas mixture separation. The effects of the substrate with or without modification on gas permeation were studied, and the diffusion mechanism of the gases through the membranes was analyzed. The real separation factors of gas mixtures through the NPC/Ni–P composite membranes were much higher than those through the nanoporous carbon membranes (NPCMs) prepared on the unmodified substrate, especially those for H2/CH4 and H2/CO2. The weight loss rate of polymer precursors was investigated by TG/DTG. The specific surface areas of the membranes and the crystallization of the Ni–P alloy layer were characterized by means of the BET and DSC techniques, respectively, indicating that localized electroless Ni pl...