Efficient methane/nitrogen separation with low-sodium clinoptilolite

The presence of sodium is shown to have a highly detrimental effect on the gas separation ability of clinoptilolite towards methane and nitrogen.     

Plasma reactions of methane in nitrogen and nitrogen/oxygen carriers by matrix isolation FTIR spectroscopy

The plasma-induced reactions of traces of methane in nitrogen and nitrogen/oxygen carriers have been investigated by freezing the products onto a 10 K CsI substrate and performing FTIR analysis on the product mixture. Isotopic substitution studies have been used to assist in identification of reaction intermediates and final products. A combination of low (10 mTorr) and high (2 Torr) pressure discharges has also been used to help in the identification of these products. Oxygen concentration was increased in a stepwise fashion to determine its effect on the reaction product distribution. In the present work, methyl radical was the principal product in low-pressure N₂/CH₄ plasmas, and small amounts of HCN and NH₃ were also produced. In the higher-pressure plasmas, HCN and NH₃ were the principal products. As O₂ was added to the plasmas, CO, H₂O, CO₂, N₂O, NO, O₃, HONO, and HNO₃ were produced in approximately the order shown, i.e., CO was formed in good yield at low oxygen partial pressures, but HNO₃ was produced only in slight yield even at the highest oxygen pressures used in this work. These results are discussed in terms of the development of a plasma device having potential application for destruction of environmentally hazardous materials and how trace organic pollutants might react in such a system.     

Extended UNIQUAC model for thermodynamic modeling of CO2 absorption in aqueous alkanolamine solutions

The extended UNIQUAC model [K. Thomsen, P. Rasmussen, Chem. Eng. Sci. 54 (1999) 1787–1802] was applied to the thermodynamic representation of carbon dioxide absorption in aqueous monoethanolamine (MEA), methyldiethanolamine (MDEA) and varied strength mixtures of the two alkanolamines (MEA–MDEA). For these systems, altogether 13 interaction model parameters are adjusted. Out of these parameters, 11 are temperature dependent.     

Quasilocalization of extra electrons in high density methane and ethane gases

The extent of electron quasilocalization in the high density gases is greater in ethane than in methane. However, the density required for quasilocalization in methane is n/n_c > 0.1, while that in ethane is n/n_c > 0.4, where n_c is the critical density. The mobilities in methane and ethane cross at n = 6 × 10²¹ molecules/cm³, in the respective fluids along their vapor/liquid coexistence curves.     

PSRK group contribution equation of state: comprehensive revision and extension IV,including critical constants and α-function parameters for 1000 components

As a part of an ongoing process, the predictive Soave–Redlich–Kwong (PSRK) group contribution equation of state was extended by the introduction of additional structural groups (F₂, Cl₂, Br₂, HCN, NO₂, CF₄, O₃ and ClNO) and fitting of the corresponding group interaction parameters. Interaction parameters between already existing main groups were also optimized to the growing literature data base. Overall, 75 new parameter sets are given herein, and typical results are presented for various systems. For the sake of completeness, not only the group new interaction parameters but all available PSRK/UNIFAC interaction parameter sets (more than 900) are given as supplementary material. Moreover, the required pure component properties (critical properties, acentric factors, and Mathias–Copeman constants) were revised and are also included for about 1000 components.     

Far-infrared absorption by collisionally interacting nitrogen and methane molecules

Quantum line shape calculations of the rototranslational enhancement spectra of nitrogen-methane gaseous mixtures are reported. The calculations are based on a recent theoretical dipole function for interacting N(2) and CH(4) molecules, which accounts for the long-range induction mechanisms: multipolar inductions and dispersion force-induced dipoles. Multipolar induction alone was often found to approximate the actual dipole surfaces of pairs of interacting linear molecules reasonably well. However, in the case of the N(2)-CH(4) pair, the absorption spectra calculated with such a dipole function still show a substantial intensity defect at the high frequencies (>250 cm(-1)) when compared to existing measurements at temperatures from 126 to 297 K, much as was previously reported.     

Prediction of solid-liquid-vapor phase equilibria of noble gases in nitrogen

The primary objective of this study is to develop an empirical correlation model that is able to predict the solid–liquid-vapour phase equilibria (SLVE) for the ternary system of N₂-Kr-Xe at pressures ranging from 1 to 45 bar and temperatures ranging from 80 to 180 K. The model was based on Peng-Robinson equation of state. To optimize the interaction parameters that are needed in the model, it was first used to correlate the experimental SLVE data found in the literature for the N₂-Kr, and N₂-Xe and Kr-Xe binary systems. When the corresponding interaction parameters were optimized, the model was then expanded to predict the SLVE and construct the phase envelope of the ternary system of N₂ -Kr-Xe.     

Adsorption and diffusion parameters of methane and nitrogen on microwave-synthesized ETS-4

ETS-4 is a titanium silicate developed by Engelhard Corporation, which possesses a small pore network, the size of which can be reduced by heat treatment to improve its kinetic selectivity in nitrogen/methane separation. Most of the reported studies about ETS-4 employ crystals synthesized with conventional heating. Furthermore, information available on the adsorption properties of ETS-4, especially the diffusion properties, is scarce. In this work, Na-ETS-4 crystals have been synthesized by microwave heating and have been exchanged with strontium to obtain Sr-ETS-4 using also microwave heating. This method for obtaining the strontium form of ETS-4 has not been reported before. Both materials have been dehydrated to reduce their pore size. The adsorption and diffusion parameters of nitrogen and methane on these materials, which have not been measured for microwave-synthesized ETS-4 up to the present date, have been estimated by modeling the desorption breakthrough curves of both gases using a fixed bed of ETS-4 crystals. The kinetic selectivity of nitrogen over methane at 298 K of microwave-synthesized Sr-ETS-4 is 26. This value is higher than the maxima reported in the literature for this material.     

Analog calorimetry and UNIQUAC group contributions approaches to the miscibility of pvc with eva copolymers

The miscibility of blends of poly(vinyl-chloride) (PVC) with poly(ethylene-co-vinyl acetate) (EVA) was investigated through analog calorimetry and a group contribution procedure based on the UNIQUAC model. The group contribution parameters quantifying the pair interactions between the structural features of the above polymers were calculated from experimental excess enthalpies of a series of binary mixtures of chlorocompounds, esters and hydrocarbons. Enthalpy data were also collected for the ternary mixtures (2-chloropropane+ethyl acetate+n-heptane) and (2-chlorobutane + methyl acetate+n-heptane), chosen as possible models for the studied macromolecular mixtures. The miscibility window of the PVC-EVA blends is fairly predicted by the group contribution method. It is also acceptably predicted by the enthalpic behaviour of the first ternary set, but only when the latter is calculated with binary data. A slightly narrower miscibility range is predicted by the binary interaction model. The results of these procedures are compared and the higher reliability of the group contribution procedure is emphasized in terms of its capability to reproduce the exact structure of the macromolecules and the non-univocal choice of the model molecules involved in the analog calorimetry approach. This revised version was published online in July 2006 with corrections to the Cover Date.     

Methyl cation affinities of rare gases and nitrogen and the heat of formation of diazomethane

The methyl cation affinities of the rare gases have been calculated at 0 and 298 K by using coupled cluster theory including noniterative, quasiperturbative triple excitations with the new correlation-consistent basis sets for Xe up through aug-cc-pV5Z in some cases. To achieve near chemical accuracy (+/-1 kcal/mol) in the thermodynamic properties, we add to the estimated complete basis set valence binding energies, based on frozen core coupled cluster theory energies, two corrections: (1) a core/valence correction and (2) a scalar relativistic correction. Vibrational zero-point energies were computed at the coupled cluster level of theory at the CCSD(T)/aug-cc-pVDZ level. The calculated rare gas methyl cation affinities (MCA in kcal/mol) at 298 K are the following: MCA(He) = 1.7, MCA(Ne) = 2.5, MCA(Ar) = 16.9, MCA(Kr) = 25.5, and MCA(Xe) = 36.6. Because of the importance of the MCA(N(2)) in the experimental measurements of the MCA scale, we calculated a number of quantities associated with CH(3)N(2)(+) and CH(2)N(2). The calculated values for diazomethane at 298 K are: DeltaH(f)(CH(2)N(2)) = 65.3 kcal/mol, PA(CH(2)N(2)) = 211.9 kcal/mol, and MCA(N(2)) = 43.2 kcal/mol.     

The determination of carbon,nitrogen and oxygen in gases by neutron time-of-flight spectrometry

Carbon, nitrogen and oxygen were determined in gases by time-of-flight spectrometry of prompt neutrons from the respective reactions¹²C(d, n)¹³N,¹⁴N(d, n)¹⁵O and¹⁶O(d, n)¹⁷F, produced by a pulsed beam of deuterons of 2 MeV (for nitrogen) or 3 MeV. The analysis is non-destructive and requires about 15 min. per sample. The relative standard deviation for all three elements was about ±3%. Detection limits, using a total irradiation current of 20 millicoulombs, for carbon, nitrogen and oxygen, respectively, were 6·10⁻⁸ g, 2·10⁻⁷ g and 1.7·10⁻⁷ g per cm² cross-sectional area of irradiating beam.     

Hydroxylation mechanism of methane and its derivatives over designed methane monooxygenase model with peroxo dizinc core

The peroxo dizinc Zn(2)O(2) complex Q coordinated by imidazole and carboxylate groups for each Zn center has been designed to model the hydroxylase component of methane monooxygenase (MMO) enzyme, on the basis of the experimentally available structure information of enzyme with divalent zinc ion and the MMO with Fe(2)O(2) core. The reaction mechanism for the hydroxylation of methane and its derivatives catalyzed by Q has been investigated at the B3LYP*/cc-pVTZ, Lanl2tz level in protein solution environment. These hydroxylation reactions proceed via a radical-rebound mechanism, with the rate-determining step of the C-H bond cleavage. This radical-rebound reaction mechanism is analogous to the experimentally available MMOs with diamond Fe(2)O(2) core accompanied by a coordinate number of six for the hydroxylation of methane. The rate constants for the hydroxylation of substrates catalyzed by Q increase along CH(4) < CH(3)F < CH(3)CN ≈ CH(3)NO(2) < CH(3)CH(3). Both the activation strain ΔE(≠)(strain) and the stabilizing interaction ΔE(≠)(int) jointly affect the activation energy ΔE(≠). For the C-H cleavage of substrate CH(3)X, with the decrease of steric shielding for the substituted CH(3)X (X = F > H > CH(3) > NO(2) > CN) attacking the O center in Q, the activation strain ΔE(≠)(strain) decreases, whereas the stabilizing interaction ΔE(≠)(int) increases. It is predicted that the MMO with peroxo dizinc Zn(2)O(2) core should be a promising catalyst for the hydroxylation of methane and its derivatives.     

Determination of carbon monoxide,methane and carbon dioxide in refinery hydrogen gases and air by gas chromatography

This paper illustrates a method for determining trace amounts of CO, CH4 and CO2 with the detection limit of 0.15, 0.15 and 0.20 microg/l, respectively, in refinery hydrogen gases or in air. A simple modification of a gas chromatograph equipped with a flame-ionization detector is presented. A Porapak Q column, additionally connected with a short molecular sieve 5A packed column and a catalytic hydrogenation reactor on the Ni catalyst have been applied. The principle of the analytical method proposed is the separation of CO from O2 before the introduction of CO to the methanizer. The analytical procedure and examples of the results obtained have been presented. The modification applied makes it possible to use the GC instrument for other determinations, requiring utilization of the Porapak Q column and the flame-ionization detector. In such cases, the short molecular sieve 5A column and the methanizer can be by-passed.     

Thermodynamic study of solubility of B6 hydrochloride in water + co-solvent systems with UNIQUAC model at different temperatures

In this study, due to the importance of the solubility of this vitamin in solvents, based on the UNIQUAC activity coefficient equation, its solubility in the presence of water, acetonitrile, n-propanol and isopropanol has been studied. To investigate the interaction behavior of components in the system, the desired thermodynamic equations were optimized based on experimental results with a combined Genetic + PSO algorithm. Based on the objective function, the value of the theoretical and experimental model was acceptable. Therefore, the optimized model can be significant for formulating and investigating solubility of B6 hydrochloride based on the design of a computer program.     

Effect of nitrogen oxides NOx on gas-phase oxidation of methane by oxygen

We propose a kinetic model for gas-phase oxidation of methane by oxygen in the presence of nitrogen oxides NOx. The model calculations agree satisfactorily with experimental kinetic data provided in the literature. We consider the basic principles for the effect of nitrogen oxides on the rate of the process and the selectivity with respect to methanol and formaldehyde. __________ Translated from Teoreticheskaya i éksperimental’naya Khimiya, Vol. 42, No. 2, pp. 113–118, March–April, 2006.     

Structural and functional model of methane hydroxylase of membrane-bound methane monooxygenase from Methylococcus capsulatus (Bath)

Computer analysis of a wide range of primary sequences showed that -, -, and -peptides of membrane-bound methane hydroxylase contained 2, 7, and 6 transmembrane helices respectively. Conservative amino acid residues participating in complex formation were revealed. The - and -peptides are suggested to contain mononuclear copper ions with the ligand environment mainly consisting of His residues. The Cu sites are located in the hydrophilic region and are responsible for ESR signals. The active site of -peptide in which the activation of O₂ and oxidation of CH₄ occur is localized in the hydrophobic region close to the membrane surface. This site is formed by the amino acid residues of four transmembrane helices and one loop between them and is suggested to be a binuclear Cu—Fe or Fe—Fe center. The Cu site of -peptide transfers electrons to the active site of -peptide, and the Cu site of -peptide is either involved in this process or only stabilizes the protein structure.     

Local and global uncertainty analyses of a methane flame model

Local and global uncertainty analyses of a flat, premixed, stationary, laminar methane flame model were carried out using the Leeds methane oxidation mechanism at lean (phi = 0.70), stoichiometric (phi = 1.00), and rich (phi = 1.20) equivalence ratios. Uncertainties of laminar flame velocity, maximal flame temperature, and maximal concentrations of radicals H, O, OH, CH, and CH(2) were investigated. Global uncertainty analysis methods included the Morris method, the Monte Carlo analysis with Latin hypercube sampling, and an improved version of the Sobol' method. Assumed probability density functions (pdf's) were assigned to the rate coefficients of all the 175 reactions and to the enthalpies of formation of the 37 species. The analyses provided the following answers: approximate pdf's and standard deviations of the model results, minimum and maximum values of the results at any physically realistic parameter combination, and the contribution of the uncertainty of each parameter to the uncertainty of the model result. The uncertainty of a few rate parameters and a few enthalpies of formation causes most of the uncertainty of the model results. Most uncertainty comes from the inappropriate knowledge of kinetic data, but the uncertainty caused by thermodynamic data is also significant.     

Molecular model for solubility of gases in flexible polymers

We propose a model for a priori prediction of the solubility of gases in flexible polymers. The model is based on the concept of ideal solubility of gases in liquids. According to this concept, the mole fraction of gases in liquids is given by Raoult's law with the total pressure and the vapor pressure of the gas, where the latter may have to be extrapolated. However, instead of considering each polymer molecule as a rigid structure, we estimate the effective number of degrees of freedom from an equivalent freely jointed bead‐rod model for the flexible polymer. In this model, we associate the length of the rods with the molecular weight corresponding to a Kuhn step. The model provides a tool for crude estimation of the gas solubility on the basis of only the monomer unit of the polymer and properties of the gas. A comparison with the solubility data for several gases in poly(dimethylsiloxane) reveals agreement between the data and the model predictions within a factor of 7 and that better model results are achieved for temperatures below the critical temperature of the gas. The model predicts a decreasing solubility with increasing temperature (because of the increasing vapor pressure) and that smaller gas molecules exhibit a lower solubility than larger ones (e.g., CH₄ has a smaller solubility than CO₂), which agrees with the experimental data. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 701–706, 2003