Adsorption of propane and propylene onto carbon molecular sieve

Equilibrium data for propane and propylene adsorption on a carbon molecular sieve (CMS) 4A from Takeda are presented in the temperature range 343-423 K and 0-300 kPa pressure. The pellet adsorption loading is 0.9 mol/kg for propane and 1.2 mol/kg for propylene at 100 kPa and 373 K. The equilibrium selectivity for propylene in the low-pressure range are 2.3 (343 K) and 1.7 (423 K). Experimental data were fitted with the Toth and Dubinin models. Zero length column (ZLC) technique has been used to determine the controlling mechanism and estimate the diffusivity parameters. Transport of both hydrocarbons in the pellets is controlled by micropore diffusion. Breakthrough curves were measured in the same temperature range and atmospheric pressure, at the low partial pressure of adsorbate (linear region of the isotherm). Simple models have been used in the simulation of breakthrough curves.     

Adsorption dynamics of carbon dioxide in molecular sieving carbon

The sorption equilibrium isotherm of carbon dioxide at 20 °C on a commercially manufactured carbon molecular sieve has been measured with a variable volume (vacuum to high pressure) volumetric adsorption apparatus. Measurement was taken over the pressure range <10-2000 Torr and the isotherm is characterized by Dubinin-Radushkevich analysis which provides the micropore size distribution. The equilibrium information is subsequently employed to characterize the dynamics of adsorption and it is shown that the uptake of carbon dioxide is Fickian with some deviation from Fickian behavior noted at lower pressures. The derived mobility parameter agrees reasonably well with that predicted by the Darken relation over more than a 200-fold change in pressure.     

Novel carbon molecular sieve honeycomb membrane module: configuration and membrane characterization

A new production concept of carbon molecular sieve membranes modules is described. The supported membrane synthesis process allows for industrial production with uniform and reproducible properties, using flat membranes as a starting precursor. The corresponding membrane modules are made in a honeycomb configuration, leading to high packing density and thermochemical stability. Some of the carbon membranes produced for use in these modules are characterized. Species H₂, O₂, N₂, CO₂ and SF₆ were used in monocomponent adsorption and permeation experiments to obtain adsorption equilibrium and permeation data. High permeabilities were attained in combination with high selectivities. The pore size distribution of the membrane was determined. A number of complementary morphological and structural characterizations were also performed.     

Modification of pore size in activated carbon by polymer deposition and its effects on molecular sieve selectivity

The separation of air for nitrogen production can be carried out by pressure-swing-adsorption over a carbon molecular sieve. The separation is kinetically controlled, since the equilibrium adsorption of both oxygen and nitrogen is very similar, but the adsorption kinetics for oxygen is faster than for nitrogen. Several methods to prepare carbon molecular sieves are reported. In this work, we synthesized a carbon molecular sieve from a commercial activated carbon. After deposition of polyfurfuryl alcohol, these materials were subjected to carbonization at 800°C under an inert atmosphere. All the microporous materials were characterized by analysis of kinetics and equilibrium adsorption data. The molecular sieve performance was assessed by the O₂/N₂ uptake ratio. The material prepared by two depositions has characteristics similar to those of commercial CMS.     

Preparation of molecular sieving carbon from waste resin by chemical vapor deposition

Molecular sieving carbons (MSCs) were prepared from carbonized phenol-formaldehyde resin wastes by the chemical vapor deposition (CVD) of the pyrolyzed carbon from hydrocarbon species. The pore size of the MSCs could be controlled in the range 0.37-0.42 nm by changing the hydrocarbon species pyrolyzed, the pyrolyzing temperature, and the processing time. It is shown that some of the MSCs have an excellent selectivity for separating CO₂ and CH₄, and others for separating C₃H₈ and C₃H₆. As the mechanism for controlling the pore size during CVD processing, we elucidated that the adsorption of hydrocarbon molecules first takes place on the pore surface and then the adsorbed hydrocarbons pyrolyze into carbon. Therefore, the pore size of the MSC can be adjusted by controlling the amount hydrocarbon adsorbed on the phenol-formaldehyde resin char.     

Molecular sieve properties obtained by cracking of methane on activated carbon fibers

Molecular sieve properties of activated carbon fibers modified by cracking treatment with methane are studied herein. The effect of methane treatment on the porous texture of the samples has been studied while varying temperature and time. These materials have been evaluated for their selectivity during CO₂ and CH₄ separation; their uptakes have been compared with non-treated activated carbon fibers (studied previously), which were considered suitable to be used as molecular sieves. Kinetics of CO₂ and CH₄ uptake have also been investigated in this research. The treatment produced materials exhibiting fast kinetics and high selectivity during CO₂ and CH₄ separation; at the same time however, the CO₂ uptake capacity was diminished.     

Temperature- and pressure-dependent transient analysis of single component permeation through nanoporous carbon membranes

Using a transient analysis of permeation, i.e. the time lag method, as a function of temperature and pressure, it is shown that each of the parameters needed to fully evaluate adsorption and diffusion can be obtained in situ. These experiments were conducted on a supported tubular nanoporous carbon membrane, 5.1 cm² in area and prepared by ultrasonic deposition of poly(furfuryl alcohol) onto a porous stainless steel support. The permeation experiments were conducted at temperatures ranging from 25 to 225°C and over a range of pressures from 100 to 700 kPa. Under these conditions the fluxes ranged from 10⁻⁷ to 10⁻⁴ mol/m²/s with permeances ranging between 10⁻¹² and 10⁻⁹ mol/m²/s/Pa. Heats of adsorption were found to be 2.5, 2.21, 3.05 and 2.52 kcal/mol for N₂, O_2, Ar and CO₂, respectively, and generally lower than those reported for granular nanoporous carbons. The apparent activation barriers to diffusion were also found to be low at 2.06, 5.87, 4.12, 5.89 and 2.19 kcal/mol for He, N₂, O₂, Ar and CO₂. These results point to the presence of two parallel pathways for transport — the major one through the nanopores but a second through a few defect pores. Assumed to be on the order of 50 nm in diameter, these defects were calculated to represent a total area fraction of 3.43×10⁻⁹.     

Investigation of porosity of carbon materials and related effects on gas separation properties

Carbon molecular sieving membranes are chemically robust materials with tailorable gas transport properties for O₂/N₂, CO₂/CH₄ and C₃H₆/C₃H₈ separations. Such carbon materials were formed in this study by the pyrolysis of polyimide precursors. The final pyrolysis temperature was varied to alter the carbon structure, which changed the average pore size. Characterization of the porosity of these materials and how this feature changes when pyrolysis conditions are varied could guide the systematic control of these materials. However, the carbon is an amorphous, microporous material, which makes it difficult to characterize compared to crystalline materials. From separation studies of penetrants on these materials it appears that these materials have both ultramicropores (<7 Å) and larger micropores. The ultramicropores are believed to be mainly responsible for molecular sieving while the micropores provide negligible resistance to diffusion but provide high capacity sorption sites for penetrants. Techniques such as wide angle X-ray diffraction and the analysis of carbon dioxide adsorption isotherms using density functional theory were employed to characterize the microporosity of the material. The small dimensions of the key ultramicropores make accurate determination of their pore size distribution difficult. Therefore, to effectively discuss the differences in transport properties when different pyrolysis temperatures are used as well as penetrants with different dimensions, a hypothetical ultramicropore size distribution was used as a tool to discuss and interpret a combination of parameter effects and trends of separation properties.     

Influence of silicon doping on the nanomorphology and surface chemistry of a wood-based carbon molecular sieve

Carbon–silica molecular sieves were prepared by carbonization of Scotch fir (Pinus sylvestris) after impregnation with aqueous waterglass (Na_xSi_yO_z, where x, y and z may take a range of values). Compared to Si-free samples, doping significantly modifies the structure that forms during the carbonization process. For carbonization temperatures between 600 °C and 1000 °C, doped samples shrink less than undoped samples, indicating increased mechanical strength. The specific surface area and pore volume develop in a combined self-activation and chemical vapour deposition (CVD) process. Nevertheless, the presence of the sodium silicates limits self-activation and thus reduces the porosity. Doping drastically reduces the specific surface area, measured both by gas adsorption and small angle X-ray scattering. The latter technique demonstrates that in both doped and undoped samples the specific surface area is isotropic. X-ray photoelectron spectroscopy (XPS) reveals that the spatial distribution of Na and Si atoms within the samples are not identical. The open honeycomb structure, conserved during the heat treatment from the original wood, provides easy access for gas adsorption and separation applications. The ratios of the microporous diffusion time constants of N₂ and O₂ from frequency response (FR) measurements gave separation factors 3.0, 4.3, 2.7 and 1.3 for samples prepared at 600 °C, 700 °C, 800 °C and 900 °C, respectively.     

Sorption of gold and silver on carbon adsorbents from thiocyanate solutions

Sorption recovery of thiocyanate gold and silver complexes on different carbon adsorbents has been studied using model solutions with concentrations 0.08-0.82 mmol/l and 0.16-1.06 mmol/l for gold and silver, respectively. The potassium thiocyanate concentration in these solutions was 0.25 mol/l and the pH of the contacting solution was ∼2. The degree of recovery exceeded 90% for gold and 80% for silver. The separate step-by-step desorption of thiocyanate gold and silver complexes was carried out by varying the initial concentration of thiocarbamide (desorption agent). The degree of recovery of noble metals can be increased up to 95% using basic thiocarbamide solutions (in 0.1-0.2 M NaOH) at the higher temperature of the process (up to 150 °C).     

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.     

A review of the effects of covapors on adsorption rate coefficients of organic vapors adsorbed onto activated carbon from flowing gases

Published breakthrough time, adsorption rate, and capacity data for components of organic vapor mixtures adsorbed from flows through fixed activated carbon beds have been analyzed. Capacities (as stoichiometric centers of constant pattern breakthrough curves) yielded stoichiometric times τ, which are useful for determining elution orders of mixture components. Where authors did not report calculated adsorption rate coefficients k_v of the Wheeler (or, more general, Reaction Kinetic) breakthrough curve equation, we calculated them from breakthrough times and τ. Ninety-five k_v (in mixture)/k_v (single vapor) ratios at similar vapor concentrations were calculated and averaged for elution order categories. For 43 first-eluting vapors the average ratio (1.07) was statistically no different (standard deviation 0.21) than unity, so that we recommend using the single-vapor k_v for such. Forty-seven second-eluting vapor ratios averaged 0.85 (standard deviation 0.24), also not significantly different from unity; however, other evidence and considerations lead us to recommend using k_v (in mixture)=0.85k_v (single vapor). Five third- and fourth-eluting vapors gave an average of 0.56 (standard deviation 0.16) for a recommended k_v (in mixture)=0.56k_v (single vapor) for such.     

Quantification and application of skew of breakthrough curves for gases and vapors eluting from activated carbon beds

Vapor and gas breakthrough curves for packed activated carbon beds are often assumed to be symmetrical, when they are actually more often skewed. This skew explains why adsorption rate coefficients calculated at differing breakthrough fractions may not agree. Three extensive databases of breakthrough curves were analyzed to quantify this skew and the effects of relative humidity (preconditioning and use) on it. Skew results for varieties of chemicals and carbons agreed well and were combined to get a quadratic expression for a defined skew parameter. This expression was combined with a previous observation of the effect of breakthrough fraction on calculated rate coefficient. The combination allows estimation of an adsorption rate coefficient at a desired breakthrough fraction from a rate coefficient known (experimentally or by calculation) at another breakthrough fraction. A sample calculation is given.     

Adsorption of resorcinol and catechol on granular activated carbon: Equilibrium and kinetics

Investigations were conducted in the batch mode for studying the adsorption behavior of resorcinol and catechol on granular activated carbon from a basic salt medium (BSM) at pH≈7.1 and temperature≈30 °C. The isotherm data were correlated with six isotherm models, namely Langmuir, Freundlich, Redlich–Peterson, Radke–Prausnitz, Toth, and Fritz–Schlunder’s using a nonlinear regression technique. It is observed that the catechol isotherm data may be represented by Redlich–Peterson, Radke–Prausnitz, Toth, and Fritz–Schlunder models with similar accuracy (max. dev. 12%). And the resorcinol data may be represented by Freundlich, Redlich–Peterson, Radke–Prausnitz, and Fritz–Schlunder models equally well (max. dev. 15%). Freudlich being a simple model is recommended for resorcinol. At the conditions investigated in this study, catechol is adsorbed to a greater extent than resorcinol. This is due to the compound’s solubility and position of the –OH group on the benzene aromatic ring. The kinetics of adsorption have been found to be diffusion controlled and the value of effective particle diffusion coefficients is of the order of 10⁻¹³ m²/s. Three distinct phases of kinetics—rapid, medium, and slow—have been observed. These results should be useful for the design of adsorbers for removing these pollutants.     

Spectroscopic characterization of contaminants and interaction with gases in single-walled carbon nanotubes

Several spectroscopic techniques have been used to investigate the presence of contaminants in a commercial purified single-walled carbon nanotube (SWCNT) bucky paper, to determine their cleaning procedure in ultra-high-vacuum conditions and to study how impurities influence the interaction between SWCNTs and gas phase molecules. Nickel catalyst particles and sodium-containing species, likely a residual of the surfactant bath, were fully removed only after prolonged (>2 h) annealing at 1270 ± 30 K. Other impurity elements (S and Si) remain in the material as localised clusters that do not interact with the SWCNTs and do not interfere with their properties.A dramatic difference was observed when the Na-contaminated or the Na-free nanotubes interacted with molecular oxygen. O₂ adsorption was strongly altered by the Na traces, which simulated an intense sample oxidation causing a modification of the tube electronic properties. On the contrary, for the Na-free sample the lack of adsorbed oxygen and the stability of the C1s core level after large O₂ doses demonstrated the absence of any chemical bond between SWCNTs and O₂. Similarly, exposures to N₂, H₂O and CO do not have influence on the electronic properties of SWCNTs. Instead, a sizeable effect on the electronic spectra was observed for SO₂, NO and NO₂ adsorption. The sensitivity of the SWCNT electronic spectra to ppb quantities of nitrogen oxides and sulfur oxide undoubtedly foresees applications in the field of toxic gas sensing.     

Determination of pore size distribution and adsorption of methane and CCl4 on activated carbon by molecular simulation

A combined method of grand canonical Monte Carlo (GCMC) simulation and statistics integral equation (SIE) for the determination of pore size distribution (PSD) is developed based on the experimental adsorption data of methane on activated carbon at ambient temperature, T=299 K. In the GCMC simulation, methane is modeled as a Lennord-Jones spherical molecule, and the activated carbon pore is described as slit-shaped with the PSD. The well-known Steele’s 10-4-3 potential is used to represent the interaction between the fluid molecule and the solid wall. Covering the range of pore sizes of the activated carbon, a series of adsorption isotherms of methane in several uniform pores were obtained from GCMC. In order to improve the agreement between the experimental data and simulation results, the PSD is calculated by means of an adaptable procedure of deconvolution of the SIE method. Based on the simulated results, we use the activated carbon with the PSD as the prototype of adsorbent to investigate adsorption. The adsorption isotherms of methane and CCl₄ at 299 K in the activated carbon with the PSD are obtained. The adsorption amount of CCl₄ reaches 20 mmol/g at ambient temperature and pressure. The results indicate that the combined method of GCMC and SIE proposed here is a powerful technique for calculating the PSD of activated carbons and predicting adsorption on activated carbons.