Carlos M. Silva

Unified approach to the self-diffusion coefficients of dense fluids over wide ranges of temperature and pressure—hard-sphere, square-well, Lennard–Jones and real substances

Abstract In this work a physically sound approach for calculation of the self-diffusion coefficient of model and real fluids is presented. After carrying out a systematic study focused on the equations and molecular dynamic data available in the literature concerning the self-diffusivities of the hard-sphere (HS), square-well (SW) and Lennard–Jones (LJ) fluids, new models are proposed to represent this transport property. First of all, a free-volume-based equation with only one parameter is used to reproduce the recently published data for the HS fluid. Then it is demonstrated that reasonable models for the SW and LJ systems are liable to arise from the HS model by just introducing an attractive contribution. Finally, it is shown that it is possible to describe the self-diffusivities of real substances in terms of the equations devised for the SW and LJ fluids, which give rise to two-parameter models. These parameters are the diameter and the energy of the considered potential function. Reliable representation is accomplished with the proposed LJ model, the results (AAD=5.45%) being well comprised by the experimental accuracy. The validation involved the largest database ever used (2514 data points).

Chromatographic and spectroscopic analysis of heavy crude oil mixtures with emphasis in nuclear magnetic resonance spectroscopy: A review

The state of the art in the characterization of heavy crude oil mixtures is presented. This characterization can be done by different techniques, such as gas chromatography (GC), high performance liquid chromatography (HPLC), thin layer chromatography (TLC), infrared spectroscopy (IR), Raman spectroscopy, nuclear magnetic resonance (NMR) spectroscopy and mass spectrometry (MS). Nuclear magnetic resonance spectroscopy is the technique of choice due to its capability to provide information on the chemical nature of individual types of hydrogen and carbon atoms in different and complex mixtures of crude oils. The progress made in the interpretation of the NMR spectra with the development of new NMR techniques and different multivariate data analyses could give relevant information about the identification and characterization of hydrocarbons and their physical and chemical properties. These progresses can improve the refining industries operation as a result of the better knowledge on the crude composition that is fed in the refining process, as well as in the prediction of better operating conditions to obtain refined products with desired specifications and in quantities desirable to meet the market demands. The improvement in the refining operation conditions is reflected in economical benefits.

Synthesis and catalytic properties of molybdenum(VI) complexes with tris(3,5-dimethyl-1-pyrazolyl)methane.

The complex [MoO(2)Cl{HC(3,5-Me(2)pz)(3)}]BF(4) (1) (HC(3,5-Me(2)pz)(3) = tris(3,5-dimethyl-1-pyrazolyl)methane) has been prepared and examined as a catalyst for epoxidation of olefins at 55 °C using tert-butyl hydroperoxide (TBHP) as the oxidant. For reaction of cis-cyclooctene, epoxycyclooctane is obtained quantitatively within 5 h when water is rigorously excluded from the reaction mixture. Increasing amounts of water in the reaction mixture lead to lower activities (without affecting product selectivity) and transformation of 1 into the trioxidomolybdenum(VI) complex [{HC(3,5-Me(2)pz)(3)}MoO(3)] (4). Complex 4 was isolated as a microcrystalline solid by refluxing a suspension of 1 in water. The powder X-ray diffraction pattern of 4 can be indexed in the orthorhombic Pnma system, with a = 16.7349(5) Å, b = 13.6380(4) Å, and c = 7.8513(3) Å. Treatment of 1 in dichloromethane with excess TBHP led to isolation of the symmetrical [Mo(2)O(4)(μ(2)-O){HC(3,5-Me(2)pz)(3)}(2)](BF(4))(2) (2) and unsymmetrical [Mo(2)O(3)(O(2))(2)(μ(2)-O)(H(2)O){HC(3,5-Me(2)pz)(3)}] (3) oxido-bridged dimers, which were characterized by single-crystal X-ray diffraction. Complex 2 displays the well-known (Mo(2)O(5))(2+) bridging structure where each dioxidomolybdenum(VI) center is coordinated to three N atoms of the organic ligand and one μ(2)-bridging O atom. The unusual complex 3 comprises dioxido and oxidodiperoxo molybdenum(VI) centers linked by a μ(2)-bridging O atom, with the former center being coordinated to the tridentate N-ligand. The dinuclear complexes exhibit a similar catalytic performance to that found for mononuclear 1. For complexes 1 and 2 use of the ionic liquids (ILs) 1-butyl-3-methylimidazolium tetrafluoroborate and N-butyl-3-methylpyridinium tetrafluoroborate as solvents allowed the complexes to be completely dissolved, and in each case the catalyst and IL could be recycled and reused without loss of activity.

Models for self-diffusion coefficients of dense fluids, including hydrogen-bonding substances

Abstract This work comprises two main and independent purposes: 1. A new four-parameter model is proposed for the description of self-diffusion coefficients of polar, nonspherical and even hydrogen-bonding substances. It gives accurate results over wide ranges of temperature and pressure (AAD=4.45%; 2471 data points). This equation is devised from an extension of a Lennard–Jones fluid model embodying the van der Waals model approach. 2. Generalized correlations are suggested for the estimation of the Lennard–Jones force constants (energy and diameter) of a two-parameter equation. From these it is possible to predict tracer diffusivities with satisfactory results, by means of simple combining rules.

Generalised free-volume theory for transport properties and new trends about the relationship between free volume and equations of state

Abstract In this work, a new generalised model for transport properties is given, the theoretical background of some free-volume equations is revealed and the analysis of various free-volume expressions in the literature is presented. Firstly, we develop the free-volume equation for the self-diffusion coefficient, which embodies the hybrid principles of the real fluids, since it considers both the concepts of molecular free volume and activation energy. Secondly, it is shown that the relevant expressions presented in the literature are particular cases of our generalised equation. With this it is possible to clarify their validity and range of applicability, since the expressions arise by introducing approximations duly established in the fundamental model. Moreover, the relationship between free-volume expressions and the equations of state (EoS) of van der Waals (vdW) type is discussed. Some representative free-volume expressions are given and compared according to the effects of density, temperature and molecular shape. Following this comparison, some expressions are recommended.

Biomass‐Based Polyols through Oxypropylation Reaction

Biomass residues are a potential renewable source for the sustainable production of chemicals, materials, fuels, and energy embodying the so-called biorefinery concept. In this context, agro-forestry and agro-food industry by-products have attracted considerable interest of researchers in academia and industry as a renewable source of polymeric materials. The research developed to date on the valorization of biomass residues by converting them into polyols through oxypropylation is the subject of this review. These bio-based polyols exhibit properties similar to their petrochemical counterparts and, as such, can be used with economical advantage in the production of polyurethanes. The operating conditions of the oxypropylation reaction depend on the biomass and on the desired polyol properties. The discussion of their influence and the economic viability of the process are also presented.

Removal of Anionic Pollutants from Waters and Wastewaters and Materials Perspective for Their Selective Sorption

The presence of some anionic species, such as nitrate, nitrite, chloride, sulfide, fluoride, and cyanide, in water supplies may represent a serious environmental problem. In this work, the main sources and harmful effects of their bioaccumulation on living organisms are reviewed, as well as the most adopted technologies for their uptake. The major advantages and disadvantages of each methodology are also listed. In general, ion-exchange has been elucidated as the most suitable removal process. In view of that the most promising materials used to remove anionic pollutants from aqueous solutions are highlighted in this review. In particular, the major efforts towards the development of low-cost and easily available effective sorbents for water decontamination are covered. For instance, natural waste solid materials and derivatives have emerged as promising low-cost exchangers for selective anions uptake. Besides, a number of structural modifications including the introduction of more suitable surface functional groups or compensation species into the sorbent matrix have been investigated in order to enhance sorbents selectivity and capacity for anionic pollutants. The influence of speciation and removal conditions is also focused.

Supercritical fluid extraction of Eucalyptus globulus bark-A promising approach for triterpenoid production.

Eucalyptus bark contains significant amounts of triterpenoids with demonstrated bioactivity, namely triterpenic acids and their acetyl derivatives (ursolic, betulinic, oleanolic, betulonic, 3-acetylursolic, and 3-acetyloleanolic acids). In this work, the supercritical fluid extraction (SFE) of Eucalyptus globulus deciduous bark was carried out with pure and modified carbon dioxide to recover this fraction, and the results were compared with those obtained by Soxhlet extraction with dichloromethane. The effects of pressure (100–200 bar), co-solvent (ethanol) content (0, 5 and 8% wt), and multistep operation were studied in order to evaluate the applicability of SFE for their selective and efficient production. The individual extraction curves of the main families of compounds were measured, and the extracts analyzed by GC-MS. Results pointed out the influence of pressure and the important role played by the co-solvent. Ethanol can be used with advantage, since its effect is more important than increasing pressure by several tens of bar. At 160 bar and 40 °C, the introduction of 8% (wt) of ethanol greatly improves the yield of triterpenoids more than threefold.

Simulated Moving Bed Strategies and Designs: From Established Systems to the Latest Developments

The simulated moving bed (SMB) technology has evolved significantly since its inception. The appearance of several variants to conventional SMB has led to better performance in productivity and product purity, and lower operating costs, thus creating new opportunities for different applications. This trend is clearly visible in the increasing number of papers and patents published in this field in recent years. This review aims to gather and discuss several modifications and operation modes that have been proposed, from the most well-known Varicol and PowerFeed to the latest Intermittent-SMB and variable external stream systems, giving particular emphasis to innovations and applications of each SMB strategy. SMB valve designs are also covered in this article for their importance in the application of the operation modes discussed, which require ever more flexible units capable of variable switch times, column expandability, zone bypasses, and reduced solvent consumption while still avoiding contamination through line sharing.

Effect of pH and temperature on Hg2+ water decontamination using ETS-4 titanosilicate.

Batch stirred tank experiments were carried out to study the effect of pH (range 2-10, at 294 K) and temperature (277, 294 and 313 K, at pH 5) on the uptake of Hg(2+) ions by ETS-4 microporous titanosilicate. Changes in the pH have a significant effect on the ETS-4 uptake efficiency, the optimal range being 4-6. Due to competition effects, the chemical used to adjust the pH of the Hg(2+) solution also influences the uptake efficiency. Thermodynamic parameters for the uptake of Hg(2+) ions were calculated. For the temperature range studied, the Hg(2+) sorption increased with decreasing temperature.