The design of membrane vapor–gas separation systems

Membrane vapor–gas separation systems are beginning to be applied to a number of gas separation problems in the petrochemical and refinery areas. In this paper, some of the factors that affect the design of these systems are described using, as an application example, the separation of propylene from nitrogen in polyolefin resin degassing vents.     

Potential energy surfaces for the benzene–rare gas systems

A harmonic expansion representation of the intermolecular interaction has been exploited to obtain the potential energy surface (PES) for the C₆H₆–He, –Ne, –Ar, –Kr and –Xe systems in an analytical form. Basic data employed are binding energy, equilibrium distance and long-range attraction predicted by a semi-empirical method for selected configurations of the complexes. For those favorable cases where additional information are available the proposed PESs exhibit features in good agreement with those derived from spectroscopy and scattering experiments and/or ab initio calculations. The availability of realistic PESs expressed in an analytical form opens new perspectives of calculations in molecular dynamics and spectroscopic simulations where the benzene molecule and rare gas atoms are involved.     

Molecular dynamics investigations on Lennard–Jones systems near the gas–liquid critical point

NVT simulations on Lennard–Jones (L–J) systems near the gas–liquid critical point were performed by a direct approach. As a result, the two necessary conditions for simulating the systems in accordance with the thermodynamic limit were proposed: (i) L/ξ≳20 (L: the box-length, ξ: the correlation length), (ii) the total time of evolution, t_E>500 L–J units, for ξ≈3.5. The proposed conditions are probably very close to the sufficient ones. The influence of finite-size effects on pressure and density of small systems was qualitatively predicted. The prediction was confirmed by the simulations but only for L markedly lower than the length of typical critical wave, 2πξ. For L markedly higher, the evolutions were dominated by an effect called here the instability effect. The effect became negligible just when the condition for L/ξ was fulfilled. The ξ₀′ constant for L–J fluid was estimated from direct measurements of ξ to be 0.27±0.02 (L–J units). The thermodynamic parameters of the critical point, obtained from extrapolation, were in agreement with the results of other authors. The β_C exponent was estimated from minimization for a high range of temperatures to be 0.346. A comparison of the efficiency of NVT and NpT methods was also performed and no distinct differences were noted.     

Analysis of polysulfides in drinking water distribution systems using headspace solid-phase microextraction and gas chromatography–mass spectrometry

Sulfide and polysulfides are strong nucleophiles and reducing agents that participate in many environmentally significant processes such as the formation of sulfide minerals and volatile organic sulfur compounds. Their presence in drinking water distribution systems are of particular concern and need to be assessed, since these species consume disinfectants and dissolved oxygen, react with metal ions to produce insoluble metal sulfides, and cause taste and odour problems. The analysis of sulfide and polysulfides in drinking water distribution systems is challenging due to their low concentrations, thermal instability and their susceptibility to undergo oxidation and disproportionation reactions. This paper reports on the development and optimisation of a rapid, simple, and sensitive method for the determination of sulfide and polysulfides in drinking water distribution systems. The method uses methyl iodide to derivatise sulfide and polysulfides into their corresponding dimethyl(poly)sulfides, which are then extracted using solid-phase microextraction in the headspace mode and analysed by gas chromatography–mass spectrometry. Good sensitivity was achieved for the analysis of dimethyl(poly)sulfides, with detection limits ranging from 50 to 240 ng L⁻¹. The method also demonstrated good precision (repeatability: 3–7%) and good linearity over two orders of magnitude. Matrix effects from raw drinking water containing organic carbon (3.8 mg L⁻¹) and from sediment material from a drinking water distribution system were shown to have no interferences in the analysis of dimethyl(poly)sulfides. The method provides a rapid, robust, and reliable mean to analyse trace levels of sulfides and polysulfides in aqueous systems. The new method described here is more accessible and user-friendly than methods based on closed-loop stripping analysis, which have been traditionally used for the analysis of these compounds. The optimised method was used to analyse samples collected from various locations in a drinking water distribution system. Some of the samples were shown to contain inorganic polysulfides, and their presence was associated with high sediment density in the system and the absence of disinfectant residual in the bulk water.     

Phase Equilibria in the Ag–In–Pd System at 700°C

Summary. Phase equilibria in the Ag–In–Pd system were determined at 700°C based on experimental results for 21 alloys. A ternary compound T₁ (with the approximate composition AgInPd₂) was identified by XRD analysis. These data were compared with the results of a CALPHAD-type prediction, based on binary thermodynamic data only and a symmetrical Redlich–Kister–Muggianu model. The experimental results will serve as a basis for refined thermodynamic modeling of the different phases in this ternary system.     

Thermodynamic Investigation of Phase Equilibria in Metal Carbonate–Water–Carbon Dioxide Systems

Summary.  Solubility measurements as a function of temperature have been shown to be a powerful tool for the determination of thermodynamic properties of sparingly-soluble transition metal carbonates. In contrast to calorimetric methods, such as solution calorimetry or drop calorimetry, the evaluation of solubility data avoids many systematic errors, yielding the enthalpy of solution at 298.15 K with an estimated uncertainty of ±3 kJ · mol⁻¹. A comprehensive set of thermodynamic data for otavite (CdCO₃), smithsonite (ZnCO₃), hydrozincite (Zn₅(OH)₆(CO₃)₂), malachite (Cu₂(OH)₂CO₃), azurite (Cu₃(OH)₂(CO₃)₂), and siderite (FeCO₃) was derived. Literature values for the standard enthalpy of formation of malachite and azurite were disproved by these solubility experiments, and revised values are recommended. In the case of siderite, data for the standard enthalpy of formation given by various data bases deviate from each other by more than 10 kJ · mol⁻¹ which can be attributed to a discrepancy in the auxiliary data for the Fe²⁺ ion. A critical evaluation of solubility data from various literature sources results in an optimized value for the standard enthalpy of formation for siderite. The Davies approximation, the specific ion-interaction theory, and the Pitzer concept are used for the extrapolation of the solubility constants to zero ionic strength in order to obtain standard thermodynamic properties valid at infinite dilution, T = 298.15 K, and p = 10⁵ Pa. The application of these electrolyte models to both homogeneous and heterogeneous (solid-solute) equilibria in aqueous solution is reviewed. Received June 26, 2001. Accepted July 2, 2001     

Phase Equilibria in the Ni–Al–W System at 900°C

Summary. Multicomponent Ni-base alloys exhibit good mechanical properties even at elevated temperatures and they are widely used for industrial production of exertion-resistive parts of engines. These properties are mainly determined by the coexistence of a disordered γ matrix with a face centred cubic lattice and cuboidal domains of its ordered γ′ structure. Therefore it is useful to study phase equilibria in Ni-base systems, namely in the regions involving both mentioned phases. One of the conclusions of our recent work on Ni–Al–Cr–W system was a necessity of modification of selected thermodynamic parameters of the ternary Ni–Al–W subsystem in order to achieve a better agreement of our experimental observations with theoretical modelling. This involves new measurements of the microstructure of selected samples of the Ni–Al–W system at 900°C and the comparison of the results with existing literature data in order to confirm our conclusions on higher order system investigated before. It is a first step on the way to an assessment of the Ni–Al–W system, which has not been done before.     

Thermodynamic Investigation of Phase Equilibria in Metal Carbonate–Water–Carbon Dioxide Systems

 Solubility measurements as a function of temperature have been shown to be a powerful tool for the determination of thermodynamic properties of sparingly-soluble transition metal carbonates. In contrast to calorimetric methods, such as solution calorimetry or drop calorimetry, the evaluation of solubility data avoids many systematic errors, yielding the enthalpy of solution at 298.15 K with an estimated uncertainty of ±3 kJ · mol⁻¹. A comprehensive set of thermodynamic data for otavite (CdCO₃), smithsonite (ZnCO₃), hydrozincite (Zn₅(OH)₆(CO₃)₂), malachite (Cu₂(OH)₂CO₃), azurite (Cu₃(OH)₂(CO₃)₂), and siderite (FeCO₃) was derived. Literature values for the standard enthalpy of formation of malachite and azurite were disproved by these solubility experiments, and revised values are recommended. In the case of siderite, data for the standard enthalpy of formation given by various data bases deviate from each other by more than 10 kJ · mol⁻¹ which can be attributed to a discrepancy in the auxiliary data for the Fe²⁺ ion. A critical evaluation of solubility data from various literature sources results in an optimized value for the standard enthalpy of formation for siderite. The Davies approximation, the specific ion-interaction theory, and the Pitzer concept are used for the extrapolation of the solubility constants to zero ionic strength in order to obtain standard thermodynamic properties valid at infinite dilution, T = 298.15 K, and p = 10⁵ Pa. The application of these electrolyte models to both homogeneous and heterogeneous (solid-solute) equilibria in aqueous solution is reviewed.     

Modeling of gas–solid equilibrium of binary systems aided by Frankenstein PSO

Binary systems containing supercritical CO₂ + hydrocarbons were used for modeling gas–solid equilibrium by a combined method including a thermodynamic model and a meta-heuristic algorithm. The Peng–Robinson (PR) equation of state was used in the classical solubility equation. In addition, Wong–Sandler (WS) mixing rules were used, and the van Laar model (VL) was included in order to evaluate the excess Gibbs free energy that appears in these mixing rules. Then, a variant of particle swarm optimization (PSO), called Frankenstein PSO (FPSO) was implemented for minimizing the difference between calculated and experimental solubility values. The results showed that the FPSO algorithm is a very powerful tool for parameter estimation on the PR-WS-VL model with good performance and accuracy, and considerably low deviations. Therefore, values calculated by the combined method (PR-WS-VL + FPSO) are considered accurate enough for physical and engineering calculations, among other uses.     

Structure–reactivity relationship in Diels–Alder reactions obtained using the condensed reaction graph approach

By the structural representation of a chemical reaction in the form of a condensed graph a model allowing the prediction of rate constants (logk) of Diels–Alder reactions performed in different solvents and at different temperatures is constructed for the first time. The model demonstrates good agreement between the predicted and experimental logk values: the mean squared error is less than 0.75 log units. Erroneous predictions correspond to reactions in which reagents contain rarely occurring structural fragments. The model is available for users at https://cimm.kpfu.ru/predictor/.     

Deflagration-to-detonation transition in the gas–liquid-fuel film system

A deflagration-to-detonation transition was experimentally detected for the first time in a channel with a thin wall liquid-fuel film and a gaseous oxidizer using a weak ignition source, which generates no primary shock wave of any significant intensity. In a number of tests, a low-velocity quasi-stationary detonationlike combustion front traveling at an average velocity of 700–900 m/s was recorded; the structure of this front included a leading shock wave and a reaction zone following after a time delay of 80 to 150 μs.     

Phase Equilibria in the System YPO4–K3PO4–KMgPO4

The phase equilibria in the part of the ternary system YPO₄–K₃PO₄–Mg₃(PO₄)₂ over the composition range YPO₄–K₃PO₄–KMgPO₄ were examined and determined by differential thermal analysis, X-ray powder diffraction and microscopic analysis in reflected light. This revised version was published online in August 2006 with corrections to the Cover Date.     

Phase diagrams of a condensed decane–eicosane–cyclododecane system

An n-eicosane–cyclododecane–n-decane system related to eutectic-type systems is investigated by means of differential thermal analysis. The eutectic alloy with melting point of–33.8°C contains 2.8 wt % of n-eicosane, 89.2 wt % of n-decane, and 8.0 wt % of cyclododecane.     

Reinvestigation of Phase Equilibria in the V2O5–ZnO System

A diagram of phase equilibria established in a two-component oxide system V_{2 5}–ZnO has been worked out applying differential thermal analysis and X-ray phase analysis as well as depending on investigations carried out with the aid of high-temperature X-ray attachment and scanning electron microscope linked to an X-ray microanalyser. This revised version was published online in July 2006 with corrections to the Cover Date.     

Photochromic behavior of tetrathienylethene in condensed systems—attempts to control 1,2-dyotropic rearrangement of the closed isomer

Tetrakis(2-methyl-5-methylthiothien-3-yl)ethene (1) exhibits incomplete photochromism in the powder state, KBr pellet, the amorphous state and a polystyrene film. In contrast, 1 in the single crystalline state does not show any photoreactivity. This chromic system involves three possible photon-modes involving the starting open isomer 1, the corresponding closed isomer trans- 2 and the rearranged isomer trans- 3. Unfortunately, efforts to control the interconversions between these isomers, especially the rearrangement of trans- 2 to trans- 3, have not been fruitful. A possible mechanism for photocyclization of 1 to form trans- 2 is also discussed on the basis of the results of density functional theory calculations together with that for the 1,2-dyotropic rearrangement of trans- 2 to give trans- 3.     

Specific features of phase equilibriums in Ln–Ba–Fe–O systems

Formation of five-layered Ln_2–εBa_3+εFe₅O_15–δ phases [exhibiting nanoscale ordering with layer-by-layer location of the cations in the Ln–Ba–(Ln,Ba)–(Ln,Ba)–Ba–Ln perovskite-type structure] has occurred in the Ln–Ba–Fe–O (Ln = Y, Pr, Nd, Sm, Eu, and Gd) systems at 1100°С in air. Partial substitution of iron with cobalt (Ln_2–εBa_3+εFe_5–yCo_yO_15–δ, Ln = Nd, Sm, Eu) has stabilized formation of the ordered structure. The oxygen content in the complex oxides has been determined in air over a wide temperature range by means of high-temperature thermogravimetry and iodometric titration. The change in oxygen content with temperature for the phases with five-layered ordering was significantly smaller than for the disordered phases.     

Phase separation of gas–liquid and liquid–liquid microflows in microchips

Phase separation of gas–liquid and liquid–liquid microflows in microchannels were examined and characterized by interfacial pressure balance. We considered the conditions of the phase separation, where the phase separation requires a single phase flow in each output of the microchannel. As the interfacial pressure, we considered the pressure difference between the two phases due to pressure loss in each phase and the Laplace pressure generated by the interfacial tension at the interface between the separated phases. When the pressure difference between the two phases is balanced by the Laplace pressure, the contact line between the two phases is static. Since the contact angle characterizing the Laplace pressure is restricted to values between the advancing and receding contact angles, the Laplace pressure has a limit. When the pressure difference between the two phases exceeds the limiting Laplace pressure, one of the phases leaks into the output channel of the other phase, and the phase separation fails. In order to experimentally verify this physical picture, microchips were used having a width of 215 μm and a depth of 34 μm for the liquid–liquid microflows, a width of 100 μm and a depth of 45 μm for the gas–liquid microflows. The experimental results of the liquid–liquid microflows agreed well with the model whilst that of the gas–liquid microflows did not agree with the model because of the compressive properties of the gas phase and evaporation of the liquid phase. The model is useful for general liquid–liquid microflows in continuous flow chemical processing.     

Test ofBΣ-interaction potentials in Na 3P-rare gas systems from light scattering

The spectral dependences in Na-rare gas optical collisions $$Na3S + X + hv \to Na3P_J + X$$ have been used to study the long range interaction in Na 3P _J -X systems. For a test of available potentials the fine structure (fs) population ratio and theJ = 3/2 alignment were measured under single collision conditions by analyzing the fluorescence emitted after selective excitation of the molecularBΣ orAΠ states as function of the laser frequencyv. A comparison was made with quantum coupled channels (CC) calculations of thermally averaged spectral profiles using the potential data to be tested as input. In addition, model calculations were performed for a qualitative understanding of the role of the various nonadiabatic couplings involved and the trends in the spectral profiles by the corresponding molecular potentials. In NaAr and NaKr theBΣ-fs wings are seen to depend sensitively on the parameters of the potentials, in particular on the well depth? _ΣII of theBΣ-AΠ difference potentials which is comparable to the spin orbit interaction in these systems; the experimental profiles show, that the actual? _ΣII values are much smaller than those predicted by the model potentials of Düren. In NaKr also theBΣ-alignment wing depends sensitively on? _ΣII caused by a long-range parity dependent rotational coupling, as follows from the CC-analysis. The comparison with the experimental alignment confirms the result from theBΣ-fs wing regarding the actual? _ΣII value.