Adsorption of ethane and ethylene on modified ETS-10

Ethylene and ethane adsorption isotherms were measured for Engelhard Titanosilicate (ETS-10) both as made in the sodium form and ion exchanged with different mono-, di-, and trivalent cations. Isotherms for Na-, K-, and Ag-ETS-10 were found to be nearly rectangular indicating very strong adsorption. In contrast, isotherms for Li-, Ba-, and Cu-ETS-10 show substantial curvature. Isotherms for some mixed cation forms (such as Ba/H and La/H) demonstrate the weakest adsorption with substantial adsorption swing capacity between 1 and 200 kPa. Using the ideal adsorption solution theory (IAST), it was determined that ethylene/ethane adsorption selectivity decreases in the order Na>K>Li>Cu≈Ba>Ba/H>La/H, while the adsorption swing capacity (from 1 to 200 kPa) shows an exactly reversed trend. It is thus probable to find the right balance between selectivity and swing capacity in order to design appropriate pressure swing adsorption (PSA) processes for the separation of ethylene and ethane under a wide range of conditions.     

Selective adsorption of carbon dioxide, methane and ethane by porous clays heterostructures

The separation of binary mixture representatives of natural and landfill gases by selective adsorption using adsorbent porous clay heterostructures has been investigated. Using Wyoming clay as a starting material, the solids are prepared by the gallery-templated approach from the polymerization of two silica sources, tetraethoxysilane (TEOS) and phenyltriethoxysilane (PhOS), at several molar ratios. In this way, the surface chemistry and the porosity characteristics of the samples are modified. The adsorbents presented specific surface areas up to 634 m² g⁻¹, micropore-size distributions with maxima near to 0.6 nm, thermal stability up to 400 °C and also hydrophobic characteristics. The selectivity for the separation of various binary mixtures such as CO₂/CH₄, CO₂/C₂H₆ and C₂H₆/CH₄, estimated by a methodology based on the determination of the Gibbs free energy, is discussed.     

Adsorption behavior of carbon dioxide and methane in bituminous coal: A molecular simulation study

The adsorption behavior of CO₂, CH₄ and their mixtures in bituminous coal was investigated in this study. First, a bituminous coal model was built through molecular dynamic (MD) simulations, and it was confirmed to be reasonable by comparing the simulated results with the experimental data. Grand Canonical Monte Carlo (GCMC) simulations were then carried out to investigate the single and binary component adsorption of CO₂ and CH₄ with the built bituminous coal model. For the single component adsorption, the isosteric heat of CO₂ adsorption is greater than that of CH₄ adsorption. CO₂ also exhibits stronger electrostatic interactions with the heteroatom groups in the bituminous coal model compared with CH₄, which can account for the larger adsorption capacity of CO₂ in the bituminous coal model. In the case of binary adsorption of CO₂ and CH₄ mixtures, CO₂ exhibits the preferential adsorption compared with CH₄ under the studied conditions. The adsorption selectivity of CO₂ exhibited obvious change with increasing pressure. At lower pressure, the adsorption selectivity of CO₂ shows a rapid decrease with increasing the temperature, whereas it becomes insensitive to temperature at higher pressure. Additionally, the adsorption selectivity of CO₂ decreases gradually with the increase of the bulk CO₂ mole fraction and the depth of CO₂ injection site.     

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 of methane and ethane into single-walled carbon nanotubes and slit-shaped carbonaceous pores

The adsorption equilibria of methane, ethane and their binary mixture in single-walled carbon nanotubes (SWNTs) and slit-shaped carbonaceous pores were studied by using a Grand Canonical Monte Carlo (GCMC) method. We used a slit-shaped pore for microporous structure of activated carbons and an armchair type of cylindrical pore for SWNTs. Methane was modeled as a spherical Lennard-Jones (LJ) model and ethane as two LJ sites with the unified methyl group. The isotherms of both components in micropore region displayed Type I adsorption by Brunauer et al., which corresponds to unimolecular adsorption. At low pressure the storage capacity of SWNTs for pure components of methane and ethane was higher than that for slit-shaped pore geometries of the same size, and the selectivities of equimolar bulk gas mixture were much higher. GCMC was shown to give good qualitative agreement with Ideal Adsorbed Solution Theory (IAST).     

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.     

Adsorption of lactic acid from fermentation broth and aqueous solutions on Zeolite molecular sieves

The recovery of lactic acid from fermentation broth and aqueous solutions was studied by adsorption on Silicalite molecular sieves. Batch experiments were used to measure the adsorption isotherms of the lactic acid on Silicalite. A linear correlation was found for both solutions. Silicalite showed a higher adsorptive capacity in the case of the aqueous solution than that of the fermentation broth. Henry's constants were estimated as and for the aqueous and broth solutions, respectively. The effect of temperature on adsorption was also studied in batch mode. Henry's constant dependency on temperature was derived from Van’t Hoff's equation. The heat of adsorption was calculated as (29±17) kJ/mol. The kinetics of adsorption was investigated in column studies where the breakthrough and elution curves were measured. The adsorption process was controlled by the internal diffusion in the Silicalite pellets rather than the diffusion through the fluid film around the pellet. The fluid phase resistance was estimated as 21% of the overall resistance. The glucose presence in the fermentation broth had a negligible effect on lactic acid breakthrough curve in the studied range.     

Extraction of ethane from natural gas by adsorption on modified ETS-10

Ethane was extracted from a synthetic natural gas mixture at 298 K and 101.3 kPa using modified ETS-10 as a packed bed adsorbent. Ethane was completely separated from the mixture for a prolonged period until the initial stages of breakthrough. The adsorptive properties of several cation-exchanged forms of ETS-10, a large-pored titanosilicate molecular sieve, were compared. Na-, Ba- and Ba/H-ETS-10 were all found to be selective for ethane over methane. Na-ETS-10 showed the highest Henry's selectivity (α=52) for ethane over methane of the three ETS-10 materials tested. Ideal Adsorbed Solution Theory (IAST) models indicate that significant selectivity should persist at high pressures. Ethane is commonly removed from raw natural gas by energy-intensive cryogenic methods and these results reflect the potential for an alternative and efficient separation process using cation-exchanged ETS-10 as an adsorbent.     

Rational design of shape selective separation and catalysis—II: Mathematical model and computational studies

In our previous work [Gounaris, C.E., Floudas, C.A., Wei, J., 2006. Rational design of shape selective catalysis and separation: I. Concepts and analysis. Chemical Engineering Science, in press, doi: 10.1016/j.ces.2006.09.012], we introduced the concept of molecule projections, referred to as footprints, and briefly described rigorous criteria that define them. We also introduced the concept of strain index and showed how it can be used to identify portals that have the potential of being highly selective between two molecules. In this paper, we present in detail the mathematical formulations for the calculation of footprints and the complete mathematical model and algorithmic framework developed for the calculation of the strain index of any given molecule/portal pair. Thirty-eight molecules of interest, as well as a collection of 123 zeolite structures, involving 217 different windows, have been considered and selectivity results at ambient temperature are presented. A number of commercially interesting examples are studied in further detail, across a wide range of temperatures. It should be noted that the methodology is generic and can be applied for any pair of molecule and portal. The results indicate that there is a great potential in systems involving non-regular portals that are usually not considered in the selection process of a catalyst or molecular sieve.     

Rational design of shape selective separation and catalysis—I: Concepts and analysis

A molecule can enter a zeolite channel when it fits the size and shape of the opening. Zeolites have been used commercially for their ability to admit selectively molecule A but not molecule B. The search and design of zeolites for separation has been based on finding a channel with a diameter that is larger than molecule A, but smaller than molecule B. This method is not sufficiently accurate when the molecule is not a sphere or the channel is not a circle. We present here first a more accurate screening method based on comparing both the major and minor diameters of the molecule projection, called the “footprint”, with the diameters of the channel. Then we present an even better method that is based on the activation energy required to strain and distort a molecule to fit a given channel, thus leading to a lowering of its Boltzmann concentration in this channel. This paper compiles the activation energies encountered among thirty-eight (38) molecules and two hundred and seventeen (217) zeolite channels into a database, and shows how one can use this database to identify the most promising zeolites for the separation of a set of molecules of interest. Such a screening method would help to identify a zeolite that has much lower activation energy for molecule A than for molecule B. The appropriate temperature range for separation would be centered around the optimal temperature T^*, where the degree of selectivity is at a maximum. The separation of the three pentane molecules was selected as an illustration.     

Pith based spherical activated carbon for CO2 removal from flue gases

Serials of pitch based spherical activated carbons (PSACs) were prepared and used as adsorbent for CO₂ adsorption from flue gases. The results indicate that the ultrafine micropores (<1 nm) are effective pores for CO₂ adsorption, and the equilibrium adsorption capacity of CO₂ has a linear relationship with the specific surface area of ultrafine micropores (S_{<1 nm}). The adsorption capacity of CO₂ can obtain 1.12 mmol/g at 15 kPa and 30 °C on one of PSAC sample due to its high S_{<1 nm} (845 m²_/g). Because the molecular CO₂ can be polarized into polar molecules and has four kinds of adsorption configuration, the adsorption selectivities of CO₂ vs. N₂ and O₂ are 86.99% and 69.91%, respectively. When the combined Electric Swing Adsorption and Vacuum Swing Adsorption were applied for CO₂ desorption, about 100% desorption efficiency can be obtained, the desorption rate is twice of that with Temperature Swing Adsorption (TSA) and the energy consumption is only 69% of that with TSA.     

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.     

Experimental and theoretical study of the adsorption of pure molecules and binary systems containing methane, carbon monoxide, carbon dioxide and nitrogen. Application to the syngas generation

This study takes place in the context of the use of a Synthesis Gas in Gas To Liquid process, liquid hydrocarbon production by conversion based on Fischer–Tropsch synthesis. Our aim is the process improvement by a selective recycling of the tail gas. So, we measure pure component isotherms for four gases (CO₂, CH₄, CO, N₂) of the tail gas until 2000 kPa and binary mixture (CO₂–CH₄; CO₂–N₂; CH₄–N₂) equilibria at 303.15 K and 400 and 950 kPa onto a ZSM-5 zeolite. We also predict the binary mixture equilibria by the Ideal Adsorbed Solution Theory (IAST) and the Vacancy Solution Model (VSM, Flory–Huggins and Wilson forms) and we obtain very good results. So not only binary mixture equilibria but also ternary and quaternary mixture adsorption can be predicted. With these data (experimental and simulated), we can conclude that the CO₂ is the most adsorbed component while N₂ is the least one. These two components can be separated from CH₄ and CO which are sent in the Synthesis Gas production step.     

Adsorption of carbon dioxide on hydrotalcite-like compounds of different compositions

The adsorption of carbon dioxide on hydrotalcite-like compounds was investigated. Two different powdered hydrotalcites were used containing the cations nickel and iron. The powdered materials were screened for carbon dioxide adsorption using a thermogravimetric method and it was found that NiMgAl (Sample 1) hydrotalcite has the largest capacity for CO₂, adsorbing 1.58 mmol g⁻¹ at 20 °C, and highest rate of adsorption of up to 0.17 mmol g⁻¹ min⁻¹. This represented an increase of 53% in adsorption capacity, compared with NiMgAlFe (Sample 2). In order to improve the rheological behaviour of hydrotalcite paste for extrusion, hydrotalcite powders were combined with boehmite alumina (70:30 and 50:50 ratios of hydrotalcite:boehmite) before extrusion into pellets suitable for use in a fixed bed adsorber. These pellets were then re-crushed and further tested by thermogravimetric methods. The effects of temperature, composition and pre-treatment of the hydrotalcites on the adsorption of carbon dioxide and nitrogen are reported. At 20 °C, the amount of carbon dioxide adsorbed was between 2.0 and 2.5 mmol g⁻¹ for all the hydrotalcite/alumina samples in this study, although this decayed rapidly with increasing temperature. The results are compared with silica gel as a common sorbent reference, and with literature values. Hydrotalcite/alumina samples have thermal stability and a high adsorption capacity for carbon dioxide over a wide range of temperatures. The composition of the hydrotalcite/alumina pellets investigated in this study has less effect upon the adsorption behaviour compared with the non-calcined hydrotalcite powder, thus allowing a wide choice of pellet compositions to be used.     

Selective adsorption of carbon dioxide over nitrogen on calcined synthetic hectorites with tailor-made porosity

Hectorites were synthesized using 1-butyl-3-methylimidazolium bromide, 1,3-didecyl-2-methylimidazolium chloride and 1-decyl-3-methylimidazolium chloride as synthesis directing agents at condensation temperature for 48 h. Calcinations of synthetic hectorites resulted mesoporous materials with pore diameters ranging from 3.4 to 5.5 nm. The pore diameters depended on the directing agent. Synthetic and calcined hectorites were characterized by powder X-ray diffraction. Adsorption properties of CO₂ and N₂ on calcined hectorites were investigated by volumetric measurements. The adsorption capacity for CO₂ and N₂ decreased with increasing pore diameter. The calcined hectorites showed very good selectivity for CO₂ over N₂ at lower as well as higher partial pressure at 303 K.     

4A分子筛吸附净化异丙醇中微量水的研究

对异丙醇中微量水在4A分子筛上的吸附性能进行了研究。对该体系的吸附平衡数据进行了测定,并用Langmuir、Freundlich模型进行拟合且吻合较好;采用Crank单孔模型对测定的吸附动力学数据进行拟合,得到了290K时的有效扩散系数De=3.539×10-8cm2/s;采用固定床测定了不同床层高度、不同流量及不同初始浓度下的动态透过曲线;这些结果为吸附工艺设计提供了基础数据。     

CH_4/N_2 separation on methane molecules grade diameter channel molecular sieves with a CHA-type structure

Samples of methane molecules grade diameter channel CHA-type molecular sieves(Chabazite-K, SAPO-34 and SSZ-13) were investigated using the adsorption separation of CH_4/N_2 mixtures. The isotherms recorded for CH_4 and N_2 follow a typical type-Ι behavior, which were fitted well with the Sips model(R~20.999) and the selectivity was calculated using IAST theory. The results reveal that Chabazite-K has the highest selectivity(SCH_4/N= 5.5).2 SSZ-13 has the largest capacity, which can adsorb up to a maximum of 30.957 cm~3·g~(-1)(STP) of CH_4, due to it having the largest pore volume and surface area, but the lowest selectivity(S_(CH_4/N_2)= 2.5). From the breakthrough test, we can conclude that SSZ-13 may be a suitable candidate for the recovery of CH_4 from low concentration methane(CH_420%) based on its larger pore volume and higher CH_4 capacity. Chabazite-K is more suited to the separation of high concentration methane(CH_450%) due to its higher selectivity.     

Adsorption equilibria of benzoic acid on silica gel from supercritical carbon dioxide

Supercritical fluids, especially carbon dioxide, are increasingly used as carriers for adsorption-desorption processes, particularly in the pharmaceutical industry. Nevertheless, equilibria data for such processes are rather limited. Therefore, in this work, the adsorption equilibria of benzoic acid onto non-modified silica gel from scCO₂ were evaluated by Supercritical Fluid Chromatography applying the Peak Maxima method. Solubility of benzoic acid in scCO₂ was enhanced by addition of 2-propanol.The effects of modifier content, temperature and pressure on the solute loading were investigated. Experimental data were best described by the cubic Hill isotherm model, which accounted for the change of curvature of the elution profiles observed as concentrations in the mobile phase increased. For the concentration range reached in this study (up to 6 mg/mL), adsorption of benzoic acid was favoured at low modifier contents, high temperatures and low pressures, conditions at which the solvating power of the modified scCO₂ decreased.     

Fabrication and morphological control of three-dimensional cage type mesoporous titanosilicate with extremely high Ti content

TiSBA-1 materials with extremely high Ti content, up to silicon to titanium ratio (n_Si/n_Ti) of 2.4, have been successfully prepared through direct synthesis method by controlling the molar ratio of hydrochloric acid to silicon (n_HCl/n_Si). It has been found that the amount of Ti content and the structure of the TiSBA-1 can easily be controlled by the simple adjustment of n_HCl/n_Si ratio. X-ray diffraction pattern (XRD) of the obtained materials revealed that the materials are highly ordered and possess cubic three-dimensional cage type structure with open windows. N₂ adsorption–desorption measurement confirmed the narrow pore size distribution, high specific surface area (1317–1491 m² g⁻¹), and large specific pore volume (0.68–0.75 cm³ g⁻¹) for all the samples. UV–vis DR spectra of the prepared materials confirmed that Ti atoms are exclusively incorporated within silica framework and occupy the tetrahedral position while the presence of isolated bulk titania could be negligible. Morphologies of the TiSBA-1 materials have been also controlled by simply adjusting the n_Si/n_Ti ratio. With the appropriate Ti content, TiSBA-1 materials can be obtained as regular fine spheres. Moreover, the detailed mechanism on the morphological and phase transition control, and the incorporation of high amount of Ti in the framework of TiSBA-1 materials has been also discussed in detail.     

Permeability and separability of methane and carbon dioxide across meso-porous Mg-Al hydrotalcite and activated carbon media

Hydrotalcite and activated carbon (AC) are known to exhibit certain level of affinity for CO₂. If the materials are constructed for use as an adsorptive film or adsorbent in gas separation application, more of CO₂ can be permeated and separated from the other component gas mixture. In this study, permeability of carbon dioxide across the materials was found to increase almost linearly with increase in the respective gas flow rate but the permeability was nearly independent of pressure, attesting to their meso-porosity. The permeability remained constant at ≈6×10⁻³ mol m/m² s Pa, apparently due to the dominance of Knudsen diffusion mechanism. The separability of gas increased with increase in the inlet flow rate and pressure. Hydrotalcite film deposited on porous alumina substrate demonstrated the highest separability factor of 91, followed by bast AC (separability factor=13) and core AC (separability factor=11). Methane was found to permeate preferentially through the three porous media due to hindered diffusion of CO₂ as a result of the affinitive force attributed to high charged density in the interlayer spacing of Mg-Al hydrotalcite structures that sequestered CO₂.