Improving CO2/CH4 adsorptive selectivity of carbon nanotubes by functionalization with nitrogen-containing groups

Multi-walled carbon nanotubes containing oxygenated groups (O-MWCNTs) have been functionalized with ammonia to improve the adsorption capacity and selectivity of CO₂/CH₄ in gas adsorption process. The effects of oxygen and nitrogen containing functional groups (e.g. hydroxyl and amine), on CO₂ and CH₄ adsorption were studied. The ideal adsorption capacities of MWCNTs were determined using volumetric method at ambient temperature and moderate pressures (from 0.1 to 3.0 MPa). The MWCNTs containing nitrogen groups (N-MWCNTs) showed much higher adsorption capacity of CO₂ and selectivity of CO₂/CH₄ against the O-MWCNTs at different pressures. The highest selectivity was observed at lower pressures at 298 K for the N-MWCNTs. The dynamic adsorption experiments were carried out with a feed containing one to fivefold of CO₂ to CH₄ in a packed bed of N-MWCNTs at 298 K and atmospheric pressure. The breakthrough curves and breakthrough times of CO₂ and CH₄ were determined for the mixed gases. The results indicated high efficiency of the prepared N-MWCNTs in dynamic separation of CO₂ and CH₄.     

Preparation and characterization of polyvinylchloride based mixed matrix membrane filled with multi walled carbon nano tubes for carbon dioxide separation

Poly vinyl chloride/multi wall carbon nano tubes (PVC/MWCNTs) mixed matrix membranes (MMMs) were prepared for gas separation. Raw and functionalized MWCNTs (R-MWCNTs and C-MWCNTs) were utilized in membranes preparation. The C-MWCNT shows better performance compared to raw ones. Membrane (CO₂/CH₄) selectivity was increased from 39.21 to 52.18 at 2 bar pressure by MWCNT loading ratio. The modified membranes with styrene butadiene rubber (SBR-MMMs) showed 63.52 and 34.70 selectivity for (CO₂/CH₄) and (CO₂/N₂) at 2 bar pressure. Mechanical properties analysis exhibited tensile module improvement utilizing blending modification. Increase of feed pressure led to membrane gas permeability decreasing. But gas pair selectivity follows a nearly constant behavior for MMMs and increasing behavior for blend MMMs.     

CO2–CH4 phase equilibria on modified multi‐walled carbon nanotubes using Gibbs excess energy models based on vacancy solution theory

CO₂ and CH₄ equilibrium adsorption are predicted by Excess Gibbs energy models based on vacancy solution theory, for single and binary mixture on Multi‐Walled Carbon Nanotubes (MWCNTs) functioned by –NH₂ group. The experimental data of single gas adsorption isotherms were obtained at moderate pressures and temperatures using the volumetric method in a static gaseous set up. Firstly, the equilibrium pressures related to the adsorbed amounts, for single gases, were correlated on Wilson and Flory–Huggins activity coefficient equations based on vacancy solution theory and the model parameters were determined by fitting the model on the experimental data. Secondly, the pure component parameters were implemented in extended Wilson and Flory–Huggins equations for CO₂ and CH₄ mixture to predict the gas–solid phase equilibria. The results showed fairly good agreement between the experiments and both Gibbs models. Finally, the studied models were compared with the popular model of Extended Langmuir. The results revealed more accurately and precisely prediction of Wilson and Flory–Huggins against Langmuir model for mixed gas of CO₂ and CH₄ on MWCNT–NH₂. © 2011 Canadian Society for Chemical Engineering     

Preparation of graphene/multi-walled carbon nanotube hybrid and its use as photoanodes of dye-sensitized solar cells

This study employed a solution-based method to prepare a 3-D hybrid material comprising graphene and acid-treated multi-walled carbon nanotubes (MWCNTs). The adsorption of MWCNTs on graphene reduces the π–π interaction between graphene sheets resulting from steric hindrance, providing a subsequent reduction in aggregation. Optimal proportions of MWCNTs to graphene (2:1) enabled the even distribution of individual MWCNTs deposited on the surface of the graphene. The hybrid 3-D material was incorporated within a TiO₂ matrix and used as a working electrode in dye-sensitized solar cells (DSSCs). The hybrid material provides a number of advantages over electrodes formed of either MWCNTs or graphene alone, including a greater degree of dye adsorption and lower levels of charge recombination. In this study, DSSCs incorporating 3-D structured hybrid materials demonstrated a conversion efficiency of 6.11%, which is 31% higher than that of conventional TiO₂-based devices.     

Adsorption properties of carbon molecular sieves prepared from an activated carbon by pitch pyrolysis

In this study a heat-treatment process using an activated carbon and coal-tar pitch was developed to prepare carbon molecular sieves (CMSs) for CH₄/CO₂ separation. This process results in a partial blockage of the pores of the activated carbon precursor, so that a reduction in the pore size takes place. Equilibrium CO₂ adsorption measurements at different temperatures, and CO₂ and CH₄ kinetic measurements at different temperatures and feed pressures were carried out using the TEOM technique for a carbon molecular sieve (CMS) prepared by this process (sample CB3) and a commercial CMS (Takeda 3A, sampleT3A). The overall diffusion for CO₂ in sample CB3 was faster than that in T3A and a slightly higher CO₂ adsorption capacity of CB3 was obtained. The transient uptake profiles in both samples at different temperatures and different CO₂ partial pressures were described in some cases by a micropore diffusion model, and in other cases by a dual resistance model. Both equilibrium and kinetic results demonstrate a better CO₂/CH₄ separation performance for the CMS prepared in the present study (CB3) than for the commercial CMS (Takeda 3A), due to the existence of slightly wider pore-mouth openings in sample CB3. This study demonstrates that the process used in this work is an interesting and reproducible approach to prepare CMS for CO₂/CH₄ separation.     

Adsorption and separation of CH4/CO2/N2/H2/CO mixtures in hexagonally ordered carbon nanopipes CMK-5

Adsorption and separation of N₂, CH₄, CO₂, H₂ and CO mixtures in CMK-5 material at room temperature have been extensively investigated by a hybrid method of grand canonical Monte Carlo (GCMC) simulation and adsorption theory. The GCMC simulations show that the excess uptakes of pure CH₄ and CO₂ at 6.0 MPa and 298 K can reach 13.18 and 37.56 mmol/g, respectively. The dual-site Langmuir–Freundlich (DSLF) model was also utilized to fit the absolute adsorption isotherms of pure gases from molecular simulations. By using the fitted DSLF model parameters and ideal adsorption solution theory (IAST), we further predicted the adsorption separation of N₂–CH₄, CH₄–CO₂, N₂–CO₂, H₂–CO, H₂–CH₄ and H₂–CO₂ binary mixtures. The effect of the bulk gas composition on the selectivity of these gases is also studied. To improve the storage and separation performance, we finally tailor the structural parameters of CMK-5 material by using the hybrid method. It is found that the uptakes of pure gases, especially for CO_2, can be enhanced with the increase of pore diameter D_i, while the separation efficiency is apparently favored in the CMK-5 material with a smaller D_i. The selectivity at D_i=3.0 nm and 6.0 MPa gives the greatest value of 8.91, 7.28 and 27.52 for S_CO2/N2, S_CH4/H2 and S_CO2/H2, respectively. Our study shows that CMK-5 material is not only a promising candidate for gas storage, but also suitable for gas separation.     

Development of new carbon honeycomb structures from cellulose and pitch

In this paper the development of a new, low-cost method for the preparation of carbon honeycomb structures for gas adsorption applications is described. The method comprises the impregnation of a petroleum pitch into a cellulose-based corrugated paper. The resultant material has a high carbon content and retains the original structure of the paper, making it suitable for usage in gas flow applications. TEM and SAD studies on the carbonised material suggest the presence of two different types of carbon structures, a disorganised structure and a more organised one. The porosity of the samples was characterised by CO₂ and N₂ adsorption. The results indicated an appreciable narrow microporosity with a high structural stability to high temperatures (presence of the porosity at high temperatures). Finally, the molecular sieve properties of the materials were studied by CH₄ and CO₂ adsorption kinetics and compared favourably with those of a commercial carbon molecular sieve (CMS), indicating their promise for high temperature applications, such as catalyst supports or for gas separations.     

Adsorption of hydrogen sulfide,carbon dioxide,methane, and their mixtures on activated carbon

Abstract

H₂S and CO₂ are acid contaminants of natural gas and biogas, which removal have been studied using adsorption data for monocomponent and binary mixtures. However, equilibrium adsorption data for H₂S?+?CO₂ + CH₄ mixture has not been investigated yet. In this work, H₂S and CO₂ partition coefficients (K) and activated carbon (AC) selectivity (S) for H₂S?+?CO₂ + CH₄ mixture separation at high-pressure and different temperatures were determined. To reach this goal, monocomponent isotherms for H₂S, CO₂ and CH₄ on Brazilian babassu coconut hush AC were experimentally determined at different temperatures and pressures. Then, obtained data were correlated by Langmuir and Tóth models, and multicomponent adsorption was predicted using Extended Langmuir, Extended Tóth and Ideal Adsorption Solution Theory (IAST) methods. Results indicate AC captures approximately 26?wt% of H₂S or CO₂. K values for CO₂ and H₂S reached more than 3 and 26, respectively, depending on the predictive model utilized and were higher for diluted mixtures (high CH₄ content in gas phase). S values for CO₂ and H₂S can reach values greater than 25 for Tóth?+?IAST. Furthermore, selectivity toward H₂S is approximately 5.6 times greater than CO₂. The effect of temperature on multicomponent results indicate K and S values decrease as temperature increases. Therefore, results obtained herein show that is possible to separate H₂S and CO₂ from mixture containing CH₄ using this AC as adsorbent and better separation performance was observed for low H₂S and CO₂ concentrations and lower temperatures.     

Fabrication and modification of cellulose acetate based mixed matrix membrane: Gas separation and physical properties

[Cellulose acetate (CA)-blend-multi walled carbon nano tubes (MWCNTs)] mixed matrix membranes (MMMs), [CA/polyethylene glycol (PEG)/MWCNTs] and [CA/styrene butadiene rubber (SBR)/MWCNTs] blend MMMs were prepared by solution casting method for gas separation applications using Tetrahydrofuran (THF) as solvent. Both raw-MWCNTs (R-MWCNTs) and functionalized carboxylic-MWCNTs (C-MWCNTs) were used in membrane preparation. The MWCNTs loading ratio and pressure effects on the gas separation performance of prepared membranes were investigated for pure He, N₂, CH₄ and CO₂ gases. Results indicated that utilizing C-MWCNT instead of R-MWCNTs in membrane fabrication has better performance and (CO₂/CH₄) and (CO₂/N₂) selectivity reached to 21.81 and 13.74 from 13.41 and 9.33 at 0.65 wt% of MWCNTs loading respectively. The effects of PEG and SBR on the gas transport performance and mechanical properties were also investigated. The highest CO₂/CH₄ selectivity at 2 bar pressure was reached to 53.98 for [CA/PEG/C-MWCNT] and 43.91 for [CA/SBR/C-MWCNT] blend MMMs at 0.5 wt% and 2 wt% MWCNTs loading ratio respectively. Moreover, increase of feed pressure led to membrane gas permeability and gas pair selectivity improvement for almost all prepared membranes. The mechanical properties analysis exhibited tensile modules improvement with increasing MWCNTs loading ratio and utilizing polymer blending.     

Synthesis and characterization of poly(ether-<Emphasis Type="Italic">block</Emphasis>-amide) copolymers/multi-walled carbon nanotube nanocomposite membranes for CO<Subscript>2</Subscript>/CH<Subscript>4</Subscript> separation

One of the effective techniques for improving separation properties of polymeric membranes is incorporation of suitable nanoparticles into their matrices. This study presents the preparation of three types of nanocomposite membranes comprising three grades of poly (ether-block-amide) (Pebax 1074, Pebax 1657 and Pebax 2533) and modified multi-walled carbon nanotubes (MWCNTs) with different loadings (1, 1.5, 2 and 2.5 wt%). The prepared membranes were characterized by field emission scanning electron microscopy (FESEM), attenuated total reflection-Fourier transfer infrared spectroscopy (ATR-FTIR) and X-ray diffraction (XRD). Permeation of CO₂ and CH₄ gases through the prepared membranes was measured at the pressure range of 2-8 bars and 25 °C. The results showed that the incorporation of MWCNTs into the polymers matrices improves CO₂/CH₄ selectivity. Further, Pebax 1074/MWCNT nanocomposite membrane exhibits better performance for CO₂/CH₄ separation compared to the neat Pebax and the two other nanocomposite membranes.     

Adsorption of carbon dioxide, ethane, and methane on titanosilicate type molecular sieves

The separation of carbon dioxide from light hydrocarbons is a vital step in multiple industrial processes that could be achieved by pressure swing adsorption (PSA), if appropriate adsorbents could be identified. To compare candidate PSA adsorbents, carbon dioxide, methane, and ethane adsorption isotherms were measured for cation exchanged forms of the titanosilicate molecular sieves ETS-10, ETS-4, and RPZ. Mixed cation forms, such as Ba/H-ETS-10, may offer appropriate stability, selectivity, and swing capacity to be utilized as adsorbents in CO₂/CH₄ PSA processes. Certain cation exchanged forms of ETS-4 were found to partially or completely exclude ethane by size, and equivalent RPZ materials were observed to exclude both methane and ethane, while allowing carbon dioxide to be substantially adsorbed. Adsorbents such as Ca/H-ETS-4 and Ca/H-RPZ are strong candidates for use in PSA separation processes for both CO₂/C₂H₆ and CO₂/CH₄, potentially replacing current amine scrubber systems.     

Effect of polyether amine canopy structure on carbon dioxide uptake of solvent-free nanofluids based on multiwalled carbon nanotubes

A series of specific solvent-free nanofluids with ionically tethered polyether amine terminated polymers were successfully prepared and evaluated based on multiwalled carbon nanotubes (MWCNTs). The newly synthesized sorbents exhibited enhanced carbon dioxide (CO₂) capture capacities compared to their corresponding polyether amine and pristine MWCNTs. The effects of polyether amine canopy structure such as amine types, Molecular weight (Mw), Ethylene Oxide/Propylene Oxide (EO/PO), viscosity and melting point on CO₂ capture capacities were investigated. It had been demonstrated that the sorbents impregnated with more unprotonated amine groups and higher Mw or EO/PO showed larger CO₂ capture capacities and good stabilities over multiple adsorption–desorption cycles. Last but not least, we also demonstrated that the lower melting point and viscosity were beneficial for the CO₂ uptake.     

Preparation and Characterization of Polycarbonate-Blend-Raw/Functionalized Multi-Walled Carbon Nano Tubes Mixed Matrix Membrane for CO2 Separation

Membrane composed of PC as base of polymer matrix with different ratio of multiwall carbon nano tubes (MWCNTs) as nanofillers and poly ethylene glycol (PEG) as second polymer was prepared by solution casting method. Both raw-MWCNTs (R-MWCNTs) and functionalized carboxyle-MWCNTs (C-MWCNTs) were used in membrane preparation. The MWCNTs loading ratio and pressure effects on the gas transport properties of membranes were examined in relation to pure He, N₂, CH₄, and CO₂ gases. Results showed that the use of C-MWCNT instead of R-MWCNTs in mixed matrix membranes (MMMs) fabrication with base of PC provides better performance and also it increases (CO₂/CH₄) and (CO₂/N₂) selectivities to 27.38 and 25.42 from 25.45 and 19.24, respectively (at 5 wt% of MWCNTs). PEG as the second rubbery polymer was utilized to improve the separation performance and mechanical properties. In blend MMMs, highest (CO₂/CH₄) selectivity at 2 bar pressure increased to 35.64 for PC/PEG/C-MWCNT blend MMMs which was 27.28 for PC/MWCNTs MMMs at 10 wt%. Increase of feed pressure led to gas permeability and gas pair selectivity improvement in approximately all of membranes. Analysis of mechanical properties showed improvement in tensile modules with the increase of MWCNTs loading ratio and use of PEG in prepared MMMs.     

A high CO2 permselective mesoporous silica/carbon composite membrane for CO2 separation

Ordered mesoporous silica/carbon composite membranes with a high CO₂ permeability and selectivity were designed and prepared by incorporating SBA-15 or MCM-48 particles into polymeric precursors followed by heat treatment. The as-made composite membranes were characterized by high-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD) and N₂ adsorption, of which the gas separation performance in terms of gas permeability and selectivity were evaluated using the single gas (CO₂, N₂, CH₄) and gas mixtures (CO₂/N₂ and CO₂/CH₄, 50/50 mol.%). In comparison to the pure carbon membranes and microporous zeolite/C composite membranes, the as-made mesoporous silica/C composite membranes, and the MCM-48/C composite membrane in particular, exhibit an outstanding CO₂ gas permeability and selectivity for the separation of CO₂/CH₄ and CO₂/N₂ gas pairs owing to the smaller gas diffusive resistance through the membrane and additional gas permeation channels created by the incorporation of mesoporous silicas in carbon membrane matrix. The channel shape and dimension of mesoporous silicas are key parameters for governing the gas permeability of the as-made composite membranes. The gas separation mechanism and the functions of porous materials incorporated inside the composite membranes are addressed.     

An experimental evaluation and molecular simulation of high temperature gas adsorption on nanoporous carbon

A combination of experiments and molecular simulations has been used to further understand the contribution of gas adsorption to the carbon dioxide (CO₂) selectivity of nanoporous carbon (NPC) membranes as a function of temperature and under mixed gas conditions. Whilst there have been various publications on the adsorption of gases onto carbon materials, this study aims to benchmark a simulation model with experimental results using pure gases. The simulation model is then used to predict mixed gas behaviour. These mixed gas results can be used in the assessment of NPC membranes as a suitable technology for both carbon dioxide separations from air-blown syngas and from natural gas. The gas adsorption experiments and molecular simulations have confirmed that CO₂ is more readily adsorbed on nanoporous carbon than methane (CH₄) and nitrogen (N₂). Increasing the temperature reduces the extent of adsorption and the CO₂ selectivity. However, the difference between the CO₂ and N₂ heats of adsorption is significant resulting in good CO₂/N₂ separation even at higher temperatures.     

Simulation of CO2/CH4 high pressure separation on microporous activated carbon

Abstract

Pure component adsorption equilibrium of CH₄ and CO₂ on activated carbon have been studied at three different temperatures, 298, 323, and 348?K within a pressure range of 10–2000?kPa. Binary adsorption equilibrium isotherm was described using extended Sips equation and ideal adsorbed solution theory (IAST) model. Experimental breakthrough curves of CO₂/CH₄ (40:60 in a molar basis) were performed at four different pressures (300, 600, 1200, and 1800?kPa). The experimental results of binary isotherms and breakthrough curves have been compared to the predicted simulation data in order to evaluate the best isotherm model for this scenario. The IAST and Sips models described significantly different results for each adsorbed component when higher pressures are set. These different results cause a significant discrepancy in the estimation of the equilibrium selectivity. Simulated and experimental equilibrium selectivity data provided by IAST presented values of around 4, for CO₂/CH₄, and extended Sips presented values of around 2. Also, simulated breakthrough curves showed that IAST fits better to the experimental data at higher pressures. According to the simulations, in a binary mixture at total pressure over 800?kPa, extended Sips model underestimated significantly the CO₂ adsorbed amount and overestimated the CH₄ adsorbed amount.     

A new approach for modeling adsorption kinetics and transport of methane and carbon dioxide in shale

In this article, a new approach is proposed to investigate adsorption kinetics and transport of gases in shale. Due to co-existence of pores with different size in the shale, a set of adsorption processes happened in pores of different sizes are considered. A first-order multi-process model is developed, which can perfectly fit the adsorption kinetic data of CH₄ and CO₂ obtained at different temperatures. The modeling and pore characterization results indicate that an adsorption process happens in micropores/mesopores (<50 nm) and another adsorption process happens in macropores (>50 nm) in the Wufeng shale. Gas diffusion mechanism is dominant in micropores/mesopores, and gas seepage mechanism is dominant in macropores. The effective diffusivity of CO₂ is smaller than that of CH₄, because the adsorption of large amount of CO₂ in the pores hinders its diffusion. The coefficients related to the diffusion and seepage have no obvious trend with temperature.     

CO2/CH4/N2在沸石13X-APG上的吸附平衡

采用磁悬浮热天平测量了CO₂、CH₄与N₂在沸石13X-APG上的吸附等温线,温度为293、303、333和363 K,压力为0～500 kPa。对吸附平衡实验数据采用multi-site Langmuir模型和Sips模型进行拟合,均得到良好的拟合效果,非线性回归得到吸附热等模型参数,可为变压吸附工艺过程的开发提供基础热力学数据。将沸石13X-APG吸附分离性能与文献中报道的吸附材料(如沸石分子筛、活性炭、金属有机骨架材料和介孔硅分子筛)性能相比较。通过比较CO₂、CH₄与N₂吸附容量以及相对分离系数,探讨CO₂/CH₄(垃圾填埋气或者CO₂强化煤层甲烷回收气)体系、CO₂/N₂(燃煤电厂、水泥厂以及焦炭厂烟道气)体系以及CH₄/N₂(煤层气)体系吸附分离的高效材料,为未来二氧化碳吸附捕集和甲烷吸附回收提供基础数据。     

Adsorption dynamics and effects of carbon to zeolite ratio of layered beds for multicomponent gas adsorption

To optimize the performance of an adsorption bed, layered beds of activated carbon and zeolite 5A are used for multicomponent gas separation (H₂: 72.2%, CO₂: 21.6%, CO: 2.03%, CH₄: 4.17%). The adsorption dynamic characteristics were studied experimentally and theoretically using layered beds of activated carbon and zeolite 5A at 8 atm adsorption pressure and 16.67 L/min feed rate. Non-isothermal and non-adiabatic models, based on a linear driving force model and Dual-Site Langmuir adsorption isotherm model, were used. As the carbon ratio increased, the average velocity of CH₄ wavefront became slow, and wavefronts of CO and CO₂ propagated quickly on average.     

Production of carbon molecular sieves from palm shell based activated carbon by pore sizes modification with benzene for methane selective separation

Palm shell based activated carbon prepared by K₂CO₃ activation is used as precursor in the production of carbon molecular sieve by chemical vapor deposition (CVD) method using benzene as depositing agent. The influences of deposition temperature, time, and flow rate of benzene on pore development of carbon molecular sieve (CMS) and methane (CH₄) adsorption capacity were investigated. The parameters that varied are the deposition temperature range of 600 to 1000 °C, time from 5.0 to 60 min, and benzene flow rate from 3.0 to 15 mL/min. The results show that in all cases, increasing the deposition temperature, time, and flow rate of benzene result in a decrease in adsorption capacity of N₂, pore volume and pore diameter of CMS. The BET surface area of CMS (approximately 1065 m²/g) and the adsorption capacity of CH₄ were at a maximum value at a deposition temperature of 800 °C, time of 20 min and benzene flow rate of 6 mL/min. The product has a good selectivity for separating CH₄ from carbon dioxide (CO₂), nitrogen (N₂), and oxygen (O₂).