Purification Of Natural Gas By Using Carbon Molecular Sieve Membrane

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 172085, “Purification of Natural Gases by Use of Carbon-Molecular-Sieve Membranes,” by Subrata Mondal and Kean Wang, The Petroleum Institute, Abu Dhabi, prepared for the 2014 SPE Abu Dhabi International Petroleum Exhibition and Conference, Abu Dhabi, 10–13 November. The paper has not been peer reviewed. Carbon-molecular-sieve membranes (CMSMs) are a promising candidate for natural-gas purification because of their excellent stability, permeation selectivity, and permeability. In this project, two CMSM samples were synthesized at different pyrolytic conditions and examined for separation of N2/CH4 gas pairs. Analysis revealed that both surface diffusion and molecular sieving play important roles in gas-permeation mechanisms, which results in abnormal behaviors in the selectivities of different gas molecules. Introduction Because of its abundance in natural reservoirs and relatively clean-burning nature, natural gas (NG) (CH4>70%) is becoming increasingly important to the global energy supply. Raw NG contains such impurities as carbon dioxide (CO2), hydrogen sulfide (H2S), H2O, and N2, which can result in health hazards, corrosion of equipment, and lower heating value. The concentration of these impurities varies considerably in NGs produced at different locations but should be reduced to certain levels before shipment. Currently, large-scale NG- processing plants employ such technologies as adsorption (e.g., dehydration), absorption (e.g., amine wash for the removal of H2S and CO2), and cryogenic distillation (e.g., N2 removal). Although these technologies are mature and effective, they are expensive, energy-intensive, and environmentally unfriendly. For example, an N2- removal unit (NRU) generally operates at temperatures lower than –150°C, while the amine-wash unit requires constant maintenance and solvent replenishing. Membrane technology offers great advantages for NG processing. It is compact, easy to install or scale up, and requires minimal space, energy input, labor, and maintenance resources. These attributes make it particularly attractive for offshore and remote gas fields. Currently, membrane modules for CO2 removal are commercially available and are the dominant technology in offshore applications even as they become more important in onshore applications. However, improvements are needed to compete with the current technologies in large-scale, inland NG-processing plants.

Directly measured pyrolysis temperature dependence of sorption selectivity for carbon dioxide/methane in Matrimid® polyimide-derived carbon molecular sieve membranes

Multilayer composite membranes composed of carbon molecular sieve membranes sandwiched between stacked zeolite nanosheets

Fabrication and characterization of PPO/PVP blend carbon molecular sieve membranes for H 2/N 2 and H 2/CH 4 separation

A fixed-bed adsorption system was used to remove carbon dioxide in this study. Adsorption isotherms were obtained from a microbalance system and the characteristics of a molecular sieve affected by preparing variables were analyzed. The adsorbents, including a particulate molecular sieve and a molecular sieve membrane, were used to adsorb carbon dioxide in the fixed-bed adsorption system. Since the surface properties of the molecular sieve were affected by the preparation variables, particulate and membrane molecular sieves were synthesized to examine the effects of preparation variables on the adsorption of carbon dioxide. The experimental results showed that the pore size of the molecular sieve has increased with the chain length of quaternary ammonium salts and higher hydrothermal temperature. Adsorption isotherms for carbon dioxide adsorbed by particulate molecular sieve and molecular sieve membrane were favourable. In addition, the breakthrough curve for the lower inlet gas flow rate was closer to that of the particulate molecular sieve under the same amount of outlet fresh air. The results demonstrated that the adsorption system for the staggered alumina carriers coated with molecular sieve membranes is worthy of development.

Carbon molecular sieve membranes derived from hydrogen-bonded organic frameworks for CO2/CH4 separation

Two carbon molecular sieve (CMS) membranes (XS22‐600 and XS23‐600) with closely matched pore‐size distribution were prepared via conditional pyrolysis of the hyperbranched tris(4‐aminophenyl)amine (TAPA)‐based polyimide membranes. The differentiation in their nitrogen contents comes from the dianhydride monomers, 3,3′,4,4′‐benzophenonetetracarboxylic dianhydride (BTDA) and pyromellitic (PMDA). These CMS membranes were characterized using Fourier transform infrared, XRD, XPS, and Raman analysis. XS22‐600 with larger ultra‐micropores has a lower selectivity of 3.6 than XS23‐600 (4.1), implying the molecular sieving mechanism domains in O2/N2 separation. XS22‐600 with higher nitrogen content exhibits a higher selectivity of 19.1 than XS23‐600 (13.1) in CO2/N2 separation. The doped nitrogen atoms exhibit a beneficial effect on CO2 surface diffusing through the CMS membranes. This work provides empirical data for understanding the synergistic effect of selective surface diffusing and molecular sieving in CO2/N2 separation.

This work aims to extend previous studies about performance of gas separation through the carbon molecular sieve membrane (CMSMs) in different conditions by employing statistical analysis and modeling to find the optimal pyrolysis and operating conditions. General D-optimal design is applied to optimize gas permeability and selectivity by implementing five main parameters consisting of “precursor materials”, “blend composition”, “final pyrolysis temperature”, “vacuum pressure” and “operating pressure”. Results from statistical analysis showed that each of these five variables plays a significant role in performance of carbon membranes. Also findings showed that pyrolysis temperature was the dominant factor among the others; whereas operating pressure was almost a neutral one. The optimal condition is the blend composition of 75% Matrimid, pyrolysis temperature at 745.1 °C and vacuum pressure of 1.0E−07 Torr. Under these conditions, the model estimated a CO2/CH4 selectivity of 133.7. These developed models can be employed as a useful technique for gas transport optimization.

Carbon molecular sieve membranes (CMSM) were derived from the pyrolysis of polymeric precursors at 3 different pyrolytic temperatures (600, 700, and 800°C), respectively. The CMSM samples were characterized and tested for the permeation of 4 pure gases (CH4, CO2, N2 and He) in a time-lag rig at different operation temperatures (20 and 0°C). For N2/CH4 gas pair at ambient temperature, CMSM700 showed a good perm-selectivity of ~ 40 with a N2 permeability of ~5.8 Barrer and a CH4 permeability of 0.145 Barrer. Analysis showed that the membrane is working more on sieving mechanism rather than surface diffusion mechanism on this membrane. For CO2/CH4 pair, CMSM700 demonstrated a good selectivity of ~1,000, with a CO2 permeability of ~180 Barrer and CH4 permeability of ~0.1 Barrer. The permeation by surface flow (or surface diffusion) is more significant for strongly adsorptive species CH4 on CMSM800, which revise the trend from N2 selective to CH4 selective. In summary, although the permeation flux is still a bit low for N2/CH4, CMSMs demonstrated superior separation performance for the purification of impurities such as N2 and CO2 in natural gas.

MOF-derived nanocomposites functionalized carbon molecular sieve membrane for enhanced ethylene/ethane separation

Hydrogen is an important energy carrier for the transition to a carbon-neutral society, the efficient separation and purification of hydrogen from gaseous mixtures is a critical step for the implementation of a hydrogen economy. In this work, graphene oxide (GO) tuned polyimide carbon molecular sieve (CMS) membranes were prepared by carbonization, which show an attractive combination of high permeability, selectivity and stability. The gas sorption isotherms indicate that the gas sorption capability increases with the carbonization temperature and follows the order of PI–GO-1.0%-600 °C > PI–GO-1.0%-550 °C > PI–GO-1.0%-500 °C, more micropores would be created under higher temperatures under GO guidance. The synergistic GO guidance and subsequent carbonization of PI–GO-1.0% at 550 °C increased H2 permeability from 958 to 7462 Barrer and H2/N2 selectivity from 14 to 117, superior to state-of-the-art polymeric materials and surpassing Robeson's upper bound line. As the carbonization temperature increased, the CMS membranes gradually changed from the turbostratic polymeric structure to a denser and more ordered graphite structure. Therefore, ultrahigh selectivities for H2/CO2 (17), H2/N2 (157), and H2/CH4 (243) gas pairs were achieved while maintaining moderate H2 gas permeabilities. This research opens up new avenues for GO tuned CMS membranes with desirable molecular sieving ability for hydrogen purification.

Ion-gated carbon molecular sieve gas separation membranes

A comparison of carbon/nanotube molecular sieve membranes with polymer blend carbon molecular sieve membranes for the gas permeation application

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