Carbon Molecular Sieve Adsorbent Research Articles

Biogas upgrading by pressure swing adsorption using zeolite 4A. Effect of purge on process performance

The use of bio-methane as a fuel is an important alternative to reduce the emission of greenhouse gases. Pressure swing adsorption (PSA) processes using carbon molecular sieve adsorbents are industrial state-of-the-art technologies for biogas upgrading. Replacing the CMS adsorbent with a cheaper alternative can be a way to reduce costs for downscaling the technology to make it available to farms operating at low flows and avoiding large methane emissions to atmosphere.We have evaluated the use of zeolite 4A as a selective material that partially excludes (or strongly delays) CH4 adsorption. Adsorption equilibrium and kinetics (at zero-coverage) of pure gases (283, 303 and 323 K) was measured. Adsorption equilibrium was fitted with the multi-site Langmuir model and there is a difference of 3 orders of magnitude in diffusion constants. Binary breakthrough and a two-column PSA experiment were performed to validate a mathematical model. The model was used to determine good operating conditions with this adsorbent material. Bio-methane purity over 98 % was obtained at a recovery biome-thane of ∼ 90 %. These performance indicators were obtained in similar pressure conditions as commercial adsorbents used in industrial PSA units indicating that zeolite 4A is a viable alternative for biogas upgrading processes.

Propane selective carbon adsorbents from phenolic resin precursor

Abstract Novel carbon adsorbents for propane/propylene separation, with an unprecedented adsorption selectivity to propane – the minority component – were prepared from a phenolic resin precursor. The preparation conditions of the carbon molecular sieve adsorbents, such as pre-treatment with phosphoric acid; carbonization and post-treatment with propylene, were carefully investigated concerning their role on the separation performance. The best performing sample, MFF_8, was characterized by SEM, FTIR and small-angle X-ray scattering (SAXS). It was concluded that the pre-treatment with phosphoric acid was critical for obtaining the propane/propylene separation performance – adsorption ratio of ca. 2 at 1 bar and 25 °C; this sample was carbonized at 1100 °C and post-treated with propylene during 12 days. SAXS analysis indicates rod-shaped pores for the MFF_8 sample with a bimodal size distribution with averages of 0.4 nm and 3.7 nm, and HRTEM images show a network of earthworm micropores. The adsorption selectivity of this adsorbent to propane was assigned to the shape and size of the pores and the rigidity of propylene compared with propane for worming through the constriction of the ultra-micropores network. To the best knowledge of the authors, this is the first time a carbon molecular sieve (CMS) adsorbent with rod-shaped pores is reported. This new family of CMS adsorbents show great potential for equilibrium and kinetic based separations of adsorbates displaying different worming performances.

Separation of CO2/N2 mixture by vacuum pressure swing adsorption (VPSA) using zeolite 13X type and carbon molecular sieve adsorbents

Separation of carbon dioxide in flue gases from the combustion equipment is one of the most serious concerns for industries and especially for the power plants which are the main producers of this pollutant gas. From the various separation methods, vacuum pressure swing adsorption (VPSA) process is attracted interest due to its lower energy consumption and high efficiency. A VPSA process was studied for the separation of a mixture of CO2/N2 (80% N2 and 20% CO2). Experiments were performed in an eight-step, four-bed setup using zeolite 13X and carbon molecular sieve (CMS) as adsorbents. Experimental measurements by the bench-scale system have been obtained in a pressure range of 2.7–4.7 bar, cycle times of 360–600 s, the product flow rate of 1–3 (lit/min), and temperature 30 °C. The effects of pressure, cycle time, and product flow rate were investigated. With using 560 s for a cycle time at 3.7 bar, the maximum purity of 97.6% was obtained for zeolite 13X adsorbent.

High performing CMS adsorbent for O2 / N2 separation

The synthesis of a low particle size carbon molecular sieve (CMS) adsorbent was optimized for the O2/N2 separation. The best performing CMS displayed an O2/N2 adsorption kinetic selectivity of 123 and an inverse of apparent diffusion time constant of ca. 9 × 10−2 s−1; these are the highest values reported for an oxygen chemisorption stabilized sample. The adsorbent was prepared from the carbonization of a cellulosic precursor at 1000 °C followed by milling and stabilization steps. The sample structure, morphology and performance were further examined by scanning electron microscopy, thermogravimetric analysis, Fourier transform infrared spectroscopy, CO2 adsorption and adsorption capacity and kinetics of O2, N2, Ar and SF6 at 25 °C. SEM micrographs showed very fine powder particles with stomas. CO2 adsorption isotherms revealed that the CMS adsorbent has a well-developed microporous structure. The obtained results were well above the ones reported on literature for similar conditions opening the door for the preparation of stable carbon molecular sieve adsorbents with extraordinary O2/N2 separation performance.

Pressure Swing Adsorption for Biogas Upgrading with Carbon Molecular Sieve

This work focuses on the study of a pressure swing adsorption (PSA) process for biogas upgrading using a carbon molecular sieve adsorbent. Adsorption equilibrium and diffusion data of pure components were used to predict the multicomponent behavior. To validate the prediction of multicomponent adsorption at different concentrations, breakthrough curve experiments were performed for a gas mixture at different pressures (0.25, 0.5, 1, and 5 bar). Based on basic information, a model was used to predict the performance of a two-column PSA unit. The mixture used as feed was 60% CH4 and 40% CO2 with pressure swings between 5 bar in adsorption mode to 0.1 bar in blowdown. Experimental data demonstrated that the model could describe the PSA performance with good accuracy. We have evaluated the influence of different feed times in the biomethane recovery and purity. Biomethane purity higher than 97.5% with recovery higher than 90% was obtained.

Optimization of a Pressure Swing Adsorption Process for Nitrogen Rejection from Natural Gas

Nitrogen rejection from natural gas is a capital intensive process. The most widely used technology, cryogenic distillation, is uneconomical for gas wells in remote areas or with small production rate. In this work, a detailed simulation and optimization is carried out to explore the feasibility of pressure-vacuum swing adsorption (PVSA) technology for on-site nitrogen rejection at remote/low-throughput gas wells. The designed PVSA unit possesses high mobility at a cost that scales well with low throughput. A nonisothermal and nonisobaric simulation model including detailed description of both micropore and barrier resistances in a kinetically selective carbon molecular sieve adsorbent has been used for process optimization. The process parameters constitute the decision variables of the optimization problem, with recovery and productivity as the objective functions, and industrial pipeline purity specification as an inequality constraint. Starting with a 6-step PVSA cycle, it is shown that a modified 9-s...

Two-Stage Vacuum Pressure Swing Adsorption Using AgLiLSX Zeolite for Producing 99.5+% Oxygen from Air

Oxygen concentrations above 99.5% are required for several applications, mainly in the medical and aerospace fields. Two-stage pressure swing adsorption (PSA) processes, combining kinetic separation with equilibrium separation, have been developed for producing 99+% oxygen from air. Argon and nitrogen are kinetically removed from the air feed using a carbon molecular sieve adsorbent and the remaining nitrogen is removed using a N2/O2 selective zeolite. Despite that, two-stage processes are often unattractive, complex, and energy consuming, requiring two or more compressors/vacuum pumps. Moreover, most of the two-stage units described in literature are unable to reach the required oxygen purity of 99.5%. This work studies three energy-efficient two-stage vaccuum PSA (VPSA) processes, combining an equilibrium based PSA (EPSA) or a kinetic based PSA (KPSA) for the first stage, with a VPSA unit packed with the Ar/O2 selective zeolite AgLiLSX for the second stage, aiming to produce 99.5+% oxygen; the use of ze...

A new carbon molecular sieve for propylene/propane separations

A new carbon molecular sieve (CMS) with a propylene/propane separation factor of approximately 27 was synthesized by a facile pyrolysis process from a gel-type strong acid cation exchange resin. The micropore shrinkage process during pyrolysis was investigated using a new high throughput adsorption technique with 48 parallel cells. This significantly reduced the characterization time. The ratio of propylene/propane adsorption rate in the CMS adsorbent changes from 1 to more than 150 when the final pyrolysis temperature changes from 550 to 1000°C. The best performing CMS pyrolyzed at 850°C was further characterized using a gravimetric adsorption method. The propylene and propane diffusivities are 1.0×10−9 and 1.1×10−11cm2s−1 at 100kPa and 90°C. The high propylene/propane diffusivity ratio of 90 is similar to that in zeolite 4A, while the propylene diffusivity was more than 30 times higher than that in zeolite 4A. An effluent of 90mol% propylene was obtained from a feed of 25mol% propylene during adsorption/desorption tests using the CMS adsorbent pyrolyzed at 850°C in a fixed-bed configuration. The new CMS adsorbent is a promising candidate for industrial scale propylene/propane separations.

Comparison of different molecular sieves for the liquid phase separation of linear and branched alkanes

A carbon molecular sieve adsorbent (CMS-IMP12) obtained from the pyrolysis of a poly(vinylidene chloride-co-vinyl chloride) (PVDC-PVC, Saran™) material was tested and compared with other molecular sieve materials for the separation of a multi-component hydrocarbon mixture. Liquid phase experimental adsorption curves at 303K were obtained in a stirred tank using a model mixture composed by n-heptane, 2-methylheptane, 2,5-dimethylhexane and 2,2,4-trimethylpentane, considering the last one as a non-adsorbing solvent. Materials compared against the CMS-IMP12 were zeolites ZSM-5 (Si/Al=15), ZSM-5 (Si/Al=140), ZSM-22, ZSM-23, Silicalite-1 and Silicalite-2. The CMS-IMP12 adsorbed at least three times the amount adsorbed by the other material tested (i.e. 206 versus 79mg/gads for Silicalite-1) mainly because three components of the mixture (linear, methyl and non-geminal dimethyl alkanes) were adsorbed in higher proportions. None of the other tested materials adsorbed non-geminal dimethyl branched compounds. Regarding the kinetics of adsorption as represented by their second order rate constants, all the materials except the CMS-IMP12 adsorbed the methyl alkane molecule at the same rate as the n-alkane. The slower adsorption rate observed for 2-methylheptane as compared to n-heptane in the CMS-IMP12 may be due to the competition for adsorption sites between the two slow diffusing species: 2-methylheptane and 2,5-dimethylhexane.

Determination of exposure of dispensary drug preparers to cyclophosphamide by passive sampling and liquid chromatography with tandem mass spectrometry

To determine cyclophosphamide exposure to preparers during tablet crushing and subsequent handling by analyzing indoor air collected using a high-performance volatile organic compounds-solvent desorption (VOC-SD) passive air sampler. The passive sampler was taped to the mask over the mouth of the preparer and indoor air was collected during crushing and preparation of cyclophosphamide tablets (Endoxan®). After collection, the carbon molecular sieve adsorbent of the passive sampler was placed in a centrifuge tube, and 1 mL of carbon disulfide was used to elute cyclophosphamide from the adsorbent. Liquid-liquid extraction with 1 mL of water was performed, and the aqueous phase was used as the test solution. Cyclophosphamide concentration was determined by liquid chromatography with ultraviolet and tandem mass spectrometry detection. Cyclophosphamide concentration was detected in the range of 7.6-157.7 ng/sampler. Our results showed that low-level exposure occurred near the mouth of the preparer, which could present risks for long-term exposure, especially if combined with multiple toxic drug exposures. The anticancer drug monitoring methodology described here is a simple exposure assessment that can be used to ensure the safety of hospital pharmacy tablet preparers. Furthermore, since the anticancer drug exposure risk is very high for preparers, preparation should be in hood or with face mask.

Revisiting Transport of Gases in the Micropores of Carbon Molecular Sieves

Single component differential uptake of oxygen and nitrogen in three samples of carbon molecular sieve adsorbent and carbon dioxide and methane in one of the samples has been measured volumetrically at different temperatures over a wide range of adsorbent loading. The results seem to suggest that the early part of the uptake is controlled by a barrier resistance confined at the micropore mouth while the later part is controlled by a pore diffusional resistance distributed in the micropore interior. A dual resistance model is able to capture the observed single component uptake behavior for all the four gases, which is assuredly less tentative than what the current literature may suggest. Moreover, the extracted micropore transport parameters have much stronger dependence on adsorbent loading than that expected from chemical potential gradient as the driving force for diffusion and the assumption of a constant limiting transport coefficient based on prevailing evidence. An approximate way of accounting for...

Ozone reaction with n-aldehydes ( n=4–10), benzaldehyde, ethanol, isopropanol, and n-propanol adsorbed on a dual-bed graphitized carbon–carbon molecular sieve adsorbent cartridge

Ozone reacts with n-aldehydes ( n=4–10), benzaldehyde, ethanol, isopropanol and n-propanol adsorbed on a dual-bed graphitized carbon–carbon molecular sieve adsorbent cartridge. Destruction of n-aldehydes increases with n number and with ozone concentration. In some sampling experiments both generation and destruction of n-aldehydes by ozone are observed. In field experiments the results of sample analysis for n-aldehydes and benzaldehyde are frequently not proportional to sample volume whereas results for toluene and isoprene, and sometimes for total carbon, are. A simple theory is developed to simulate the net result of three processes: the adsorption of compounds from an air stream onto a solid adsorbent, the generation of compounds by reaction of ozone with materials upstream of or on the adsorbent, and the destruction by ozone of pre-existing compounds and compounds adsorbed from the sample stream. The use of distributed volume pairs is recommended as a way to identify loss of sample integrity during air monitoring experiments.

Coalseam methane recovery by vacuum swing adsorption

Coalseam methane gas recovery using pressure and vacuum swing adsorption (PSA/VSA) processes were investigated both theoretically and experimentally. CO2 is more strongly adsorbed and also diffuses faster compared to methane and nitrogen in a carbon-based adsorbent. Hence such adsorbents are suitable for methane recovery. Carbon molecular sieve adsorbents were employed for testing the technical feasibility of the adsorption process. Numerical simulations indicate that the purity of methane gas recovered can be greater than 90% by switching from PSA to VSA operation. The effect of process cycle time, purge pressure and feed flow rates were investigated. Both model predictions and laboratory experiments show promising separation performance. This study shows that a relatively untapped source of methane from coalseam gas can be economically recovered for industrial applications.

A Novel Insight on the High-Temperature Helium Interaction with a Carbon Molecular Sieve

Helium desorption from carbon molecular sieve fibers was measured directly with a sensitive temperature-programmed desorption mass spectrometer with a supersonic molecular beam inlet apparatus. The effects of surface affinity and pore dimension on the peculiar strong interaction of the noble, “nonadsorbable” helium atoms with a carbon molecular sieve adsorbent, at high temperatures (100−400 K), are presented. It is shown that pore dimensions, as sensed by CO2 adsorption kinetics, are a key factor for the evident dramatic change in the interactions of helium with porous surfaces.

Gas chromatography for in situ analysis of a cometary nucleus: II. Analysis of permanent gases and light hydrocarbons with a carbon molecular sieve porous layer open tubular column

Considering the severe constraints of space instrumentation, a great improvement for the in situ gas chromatographic (GC) determination of permanent and noble gases in a cometary nucleus is the use of a new carbon molecular sieve porous layer open tubular (PLOT) column called Carbobond. No exhaustive data dealing with this column being available, studies were carried out to entirely characterize its analytical performances, especially when used under the operating conditions of the cometary sampling and composition (COSAC) experiment of the European Space Agency (ESA) Rosetta space mission to be launched in 2003 for a rendezvous with comet 46 P/Wirtanen in 2011. The high efficiency and speed of analysis of this column at both atmospheric and vacuum outlet column pressure is demonstrated, and the kinetic mass transfer contribution of this carbon molecular sieve adsorbent is calculated. Besides, differential adsorption enthalpies of several gases and light hydrocarbons were determined from the variation of retention volume with temperature. The data indicate close adsorption behaviors on the Carbobond porous layer adsorbent and on the carbon molecular sieve Carboxen support used to prepare the packed columns. Moreover, taking into account the in situ operating conditions of the experiment, a study of two columns with different porous layer thicknesses allowed one to optimize the separation of the target components and to select the column parameters compatible with the instrument constraints. Comparison with columns of similar selectivity shows that these capillary columns are the first ones able to perform the same work as the packed and micro-packed columns dedicated to the separation of this range of compounds in GC space exploration.

Novel family of multi-layer cartridges filled with a new carbon adsorbent for the quantitative determination of volatile organic compounds in the atmosphere

The ability of a new graphitized carbon black adsorbent (Carbograph 5) to retain volatile organic compounds (VOCs) in air has been investigated through laboratory experiments performed by frontal chromatography. An artificial mixture containing 19 different compounds from C 1 to C 7 at levels of 50 ppb (v/v) each was used as eluent. The amounts of VOCs retained by Carbograph 5 were determined by high-resolution GC–flame ionization detection after thermal desorption at 250°C. The retention features of this adsorbent were compared with those provided by Carbograph 1 and Carbograph 2, two graphitic carbons equivalent in specific surface area and adsorption properties to Carbopack B and Carbopack C, respectively. It was found that Carbograph 5 exhibits safe sampling volumes of VOCs much higher than those of Carbograph 1 and is the only hydrophobic material providing performances comparable to those of some molecular sieve adsorbents. Results obtained indicate that Carbograph 5 can be used alone or in combination with other materials. When combined with lighter adsorbents, it allows the quantitative collection of polar and non-polar VOCs with carbon number higher than 3 in air volumes larger than 5 l at any relative humidity. In combination with stronger carbon molecular sieve adsorbents, it makes possible the quantitative collection of components with volatilities ranging from C 2 to C 7 in small samples.

Porous solids for air separation

Recent advances have been made in research on carbon molecular sieve and zeolite adsorbents used in the non-cryogenic production of nitrogen and oxygen. Preparation methods that allow stringent control of porosity, density, and surface properties have resulted in carbon molecular sieve adsorbents with higher capacity and selectivity for nitrogen production. Recently, a new generation of mixed-cation zeolite adsorbents with high N 2/O 2 selectivity and improved stability has been developed for oxygen production.

Separation of methane—nitrogen mixtures by pressure swing adsorption using a carbon molecular sieve

A 60%-40% and a 92%-8% methane nitrogen mixture were separated in a two-bed pressure swing adsorption (PSA) unit using a carbon molecular sieve (CMS) adsorbent. The CMS adsorbent used in this separation is a dual resistance type comprising a barrier resistance on the crystal surface and diffusion resistance in the crystal interior. This dual resistance is modelled using the linear driving force (LDF) PSA model. The parameter Ω of the LDF model, which relates the theoretical and experimental results, was found to be significantly different from that for conventional diffusion-controlled processes. This difference is attributed to the presence of a barrier resistance in the CMS.

An Improved Langmuir Approach for Prediction of Binary Gas Mixture Adsorption Equilibria

Abstract The mixed Langmuir adsorption isotherm has been modified for improved prediction of binary gas mixture adsorption equilibria. The modified method has been applied to predict adsorption equilibria for 52 binary gaseous mixtures on adsorbents like activated carbon, silica gel, and carbon and zeolite molecular sieves for which experimental data are reported in the literature. Predictions by this method are generally better for the binary gas mixtures in systems involving activated carbons, silica gel, and carbon molecular sieve adsorbents compared to that of the mixed Langmuir approach. However, in systems having zeolite molecular sieve adsorbents, predictions are less satisfactory. The higher deviations of predicted data from experimental values in these systems are attributed to the lack of single component adsorption isotherm data at low coverages.

Mass transfer in carbon molecular sieves—an interpretation of Langmuir kinetics

In this paper, the Langmuir kinetics rate expression, which has gained currency in modeling O 2/N 2 mass transfer in carbon molecular sieve adsorbents, is derived, starting from “surface barrier” model of diffusion under a chemical potential gradient; this provides a physical basis for the use of mass-action rate laws to describe diffusion processes.