Gas-phase Properties Research Articles

Bubble size distributions in a bubble column reactor under industrial conditions

The design of gas liquid processes requires a detailed knowledge of bubble size distributions, since they determine the mass transfer. In this contribution the influence of operating conditions and physical properties of gas and liquid phase on initial and final (“stable”) bubble sizes is discussed. The measurements were performed in a lab scale bubble column for different liquids sparged with nitrogen for pressures up to 50 bars and temperatures up to 175 °C. Bubble size distributions were determined by image processing. Bubbles tend to become smaller with decreasing surface tension, increasing gas density and decreasing liquid viscosity, resulting in reduced stable bubble sizes at increased pressure and also at increased temperature as long as evaporation can be neglected. Impurities in aqueous and organic liquids can severely influence bubble sizes by restraining coalescence. For such systems bubble size distributions in a column mainly depend on the size of the primary bubbles, which are determined by the sparger design.

Electronic structure and spectroscopic properties of electronic states of ScC3 and ScC3−

Theoretical studies on the electronic and thermodynamic properties of several electronic states are presented for the ScC3 molecule and the ScC3− anion employing state-of-the-art techniques that included up to 13 million configurations. The ground and the low-lying electronic states of these two species have been predicted to have C2v ring structures. On the basis of our computed results on the low-lying excited electronic states, we have suggested assignment of the observed anion photodetachment spectra of ScC3−. Thermodynamic properties of reactions involving ScC3 are corrected using the computed gas-phase properties of the molecule and the partition functions. Thermodynamic functions of ScC3− are also determined by fitting the available experimental data with thermodynamic energy cycles. The nature of bonding in the neutral and the anionic carbides has been considered.

Local hydrodynamic properties of gas phase in an internal-loop airlift reactor

The local hydrodynamic properties of the gas phase in an internal-loop airlift reactor were investigated in this study. The hydrodynamic properties including gas holdup, bubble velocity and bubble chord length were measured by dual electrical resistivity probes. The chord length distribution was then transformed to the bubble size distribution by modeling the bubbles as ellipsoids. It was found that the gas holdup increased with decreasing bubble velocity. In addition, most bubbles tended to rise along the riser central axis. Thus, the gas holdup in the axis was higher. The bubble size, bubble velocity and gas holdup were relatively constant in the axial direction of the riser except in the zones near the gas sparger and the gas–liquid separator. The bubble velocity became slower when the bubbles approached the gas–liquid separator. Moreover, the bubble size and bubble velocity for the three-phase system were relatively insensitive to the radial direction compared to those for the two-phase system. It was also found in this study that the bubble rise velocity and bubble size for the three-phase system were lower than that for the two-phase system. However, the gas holdup for the three-phase system were higher than that for the two-phase system due to bubble breakage caused by the solid particles.

Charge redistribution and polarization energy of organic molecular crystals

We present an approach to electronic polarization in molecular solids treated as a set of quantum systems interacting classically. Individual molecules are dealt with rigorously as quantum-mechanical systems subject to classical external fields created by all other molecules and, possibly, external sources. Self-consistent equations are derived for induced dipoles and for atomic charges whose redistribution in external fields is given explicitly by an atom-atom polarizability tensor. Electronic polarization is studied in two representative organic molecular crystals, anthracene and perylenetetracarboxylic acid dianhydride (PTCDA), and contrasted to previous results for systems of polarizable points. The stabilization energies of the neutral lattice, of isolated anions and cations, and of cation-anion pairs are found. Charge redistribution on ions is included. The dielectric tensors of crystals are successfully related to gas-phase properties and provide consistency checks on polarization energies. The procedure is generally applicable to organic crystals in the limit of no intermolecular overlap.

Combined fluctuating charge and polarizable dipole models: Application to a five-site water potential function

We present a general formalism for polarizable electrostatics based on fluctuating bond-charge increments and polarizable dipoles and its application to a five-site model for water. The parametrization is based largely on quantum-chemical calculations and should be easily transferable to other molecules. To examine basis-set effects we parametrized two models from two sets of quantum calculations, using the aug-cc-pVTZ and aug-cc-pVQZ basis sets. We computed several gas-phase and condensed-phase properties and compared with experiment or ab initio calculations as available. The models are quite similar and give condensed-phase properties at ambient conditions that are in reasonable accord with experiment, but evince errors consistent with a liquid-state dipole moment that is slightly too large. The model fit to the aug-cc-pVTZ basis set has a smaller liquid-phase dipole moment and thus gives a somewhat better description of liquid water at ambient conditions. This model also performs well away from room temperature, deviating less than 2% from the experimental density from 0 to 100 °C, and showing good agreement with experimental radial distribution functions, although the temperature of maximum density (∼20 °C) is slightly too high and the model somewhat underpredicts the persistence of the hydrogen-bond network at elevated temperatures.

Molecular Dynamics Simulations of Water with Novel Shell-Model Potentials

The need for a computationally efficient yet physically realistic water model for molecular simulation is longstanding. To account for intermolecular interactions with other molecules, for example the hydration of biomolecules, the model has to be able to adapt itself to different environments. To this end, a number of water models have been proposed that incorporate polarizability with varying degrees of success. We have developed new water models, flexible as well as rigid, on the basis of the shell concept, with three atoms and an extra particle representing the electronic degrees of freedom. This particle is coupled to a dummy position on the bisector of the molecule by a harmonic spring. We named the models SW, or shell water models. To account for anisotropic polarization, the spring force on the shell particle has been modified to depend on the displacement of the shell particle in the molecular coordinate frame. The model is constructed so as to reproduce the vacuum dipole and quadrupole, and the spring constants are chosen such that the polarizability, be it isotropic or anisotropic, is consistent with experimental data. Then the Lennard-Jones parameters are optimized to reproduce the liquid energy and density. The model is evaluated by checking liquid- and gas-phase properties, and the influences of anisotropy and flexibility are discussed.

Ab Initio Computational Study of Environmentally Harmful Gasoline Additives: Methyl tert-Butyl Ether and Analogues

MTBE (methyl tert-butyl ether), a widely used gasoline additive, is recently being scrutinized for potential environmental damage in groundwater and in the atmosphere. This study is a first-level investigation of the basic structure and properties of MTBE and various analogues, as well as an analogous class of amines, as a first step toward understanding the reactions of such species in the environment. The gas-phase structures and properties have been calculated using a variety of ab initio methods, including correlated Hartree−Fock and hybrid density functional theory methodologies. Results are discussed in light of known experimental and environmental phenomena. Accurate fundamental results are projected to aid experimental studies and provide insight into potential mechanisms of action, the latter of which will be the second level of investigation by this group. Eventual proposals of more effective gasoline additives are targeted.

Location of the Negative Charge(s) on the Backbone of Single-Stranded Deoxyribonucleic Acid in the Gas Phase

Multiply-deprotonated, single-strand products of mono and heteronucleotides, prepared in a nanospray external source coupled to an ion trap mass spectrometer, were studied under low-energy collision conditions. Fragmentations enabled the negative charge carried by the selected anions to be located. Comparison of multi-deprotonated 5′-dephosphorylated and 5′-phosphorylated nucleotides showed that the former led to competitive BH and B− losses via isomerization into ion–dipole complexes prior to dissociation, which contrasts with the 5′-phosphorylated nucleoside yielding the competitive HPO3 and nucleobase eliminations. The proposed mechanisms are discussed according to: (i) the charge state of the precursor studied and (ii) the thermochemical gas-phase properties such as the relative acidities of neutrals, which formally constitute the ion–dipole. The formation of various fragment ions from doubly- and triply-deprotonated precursor ions enabled the negative charge to be located, specifically at the terminal phosphodiester groups of the single-strand oligonucleotides. The potential energy is, therefore, minimal for distant charged phosphorylated groups. These findings suggest that a specific conformation is favored in order to minimize coulomb repulsion during the desolvation process. This hypothesis is consistent with the assumed behavior of triply- and quadruply-deprotonated, mono-cationized oligonucleotides, which involves a specific location of the sodium cation, as previously shown.

Fourier Transform Ion Cyclotron Resonance Mass Spectrometry Study of Mesomeric Heterocyclic Betainium Ions

Mesomeric heterocyclic betainium ions derived from 4-aminopyrimidine-2,6-bis(pyridinium) salts, were examined in the gas phase by Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) with electrospray ionisation. The purpose of the study was to clarify the general gas-phase properties and reactions of these novel ions. Collision-induced dissociation (CID) was used to investigate fragment ion formation and fragmentation pathways. The ions have an unusual tripolar structure with one negative and two positive charges that are in cross-conjugation within a common -electron system. Resonance stabilisation within the ions makes them relatively unreactive in ion–molecule reactions under the conditions in the FT-ICR cell. CID spectra showed that the charged pyridinium groups are easily lost as neutral pyridine. Otherwise the fragmentation depends on the substituent on the aniline moiety.

Interface structure studies by atomic resolution electron microscopy, order–disorder phenomena and atomic diffusion in gas-phase synthesized nanocrystalline solids

The paper summarizes recent studies of the structure and atomic diffusion properties of gas-phase synthesized nanocrystalline solids. The atomic structure of interfaces in nanocrystalline solids with vacancy-like free volumes and nanovoids of triple junctions is specifically studied by positron lifetime spectroscopy. The recently studied temperature variation of the positron lifetime indicates a strong temperature dependence of the positron trapping rate of these free volumes. From the investigation of the orientation correlationship of pairs of two adjacent crystallites in n-Pd by atomic resolution microscopy it can be concluded that predominantly high-energy interfaces are present in nanocrystalline metals after gasphase synthesis. Tracer substitutional-diffusion and self-diffusion studied in highly dense nanocrystalline metals demonstrate that the atomic diffusion is similar to that in conventional grain boundaries. The 18O diffusion in the interfaces of n-ZrO 2 is by 3 to 4 orders of magnitude faster than volume diffusion which gives prospects for an increase of oxygen conductivity in nanocrystalline ion conductors. Nanocrystalline ordered intermetallics as, e.g. n-FeAl and n-NiAl can be prepared by gasphase condensation in a partially disordered state. The ordering in n-FeAl occurs at lower temperatures than in n-NiAl which is correlated to the different vacancy migration enthalpies in the two intermetallic alloys.

A Rectilinear Droplet Stream in Combustion: Droplet and Gas Phase Properties

Abstract A basic experiment has been designed to improve the understanding of certain phenomena like droplet interaction occurring in dense sprays. The interaction effect in a stream of monosized ethanol droplets on the drag coefficient (Cd) for non evaporating and reacting conditions is investigated. Also the burning rate is studied. The spacing parameter C (ratio of interdroplet distance to droplet diameter) is the main parameter retained here to represent the interaction effects. Non intrusive optical techniques have been developed to measure, simultaneously, along the droplet path, the droplet size, velocity and mean temperature. The stream is first studied in non evaporating conditions. The data compiled for a large range of parameters (drag coefficient Cd. Reynolds number Re, spacing parameter C) permit to establish a new correlation for the drag coefficient taking into account the spacing parameter. The first experimental results are compared with those previously published. Then the stream is igni...

Operating Pressure and Gas-Phase Properties Effects on the Liquid Flow Nonidealities in Small-Scale Upflow Reactors

Pilot-scale hydrotreatment reactors often operate with gas−liquid upflow. For this type of operation there is a lack of information concerning the nonideal characteristics of the liquid flow, especially at elevated pressures. In this study, the Peclet number and liquid holdup of a pilot-scale upflow reactor were derived by residence time distribution experiments. The experiments were carried out at various operating pressures with different feed gases. The liquid-phase mixing was found to be identical for constant feed volumetric superficial velocities calculated at the system pressure. The liquid holdup diminished when elevated pressures were used. The use of very high gas flow rates during the shift of the operating pressure can intensify the liquid-phase nonidealities.

Determining the gas-phase properties and reactivities of multiply charged ions.

Approaches for analyzing kinetic and thermochemical data from the reactions of multiply charged ions are presented. A method for estimating the electrostatic repulsion in a multiply charged ion is described followed by examples of the potential energy surfaces for two representative reactions of multiply charged ions, proton transfer and nucleophilic substitution (S(N)2). The effect of electrostatic repulsion on reaction barriers is discussed and approaches for extracting thermochemical data from kinetic results are described. For reactions with small intrinsic barriers (i.e. proton transfer), considerable internal electrostatic repulsion is released at the transition state and multiply charged ions exhibit much greater reactivity than singly charged analogs. In contrast, relatively little electrostatic repulsion is released at the transition states of reactions with large intrinsic barriers (i.e. S(N)2) and multiply charged ions with moderate charge separations (>10 A) can exhibit reactivity that is very similar to that of singly charged analogs.

A molecular-dynamics simulation study of water on NaCl(100) using a polarizable water model

We carry out molecular-dynamics simulations of the water–NaCl(100) interface. The study includes the bulk interface at T=298 K and thin physisorbed films for coverages ranging from 0 to 1.5 at T=140 K. We use an efficient SPC/E based fluctuating charge water model to account for polarization effects. The water model is tested calculating cluster, gas, and liquid phase properties of neat water as well as structural and dynamic properties of solutions containing Na+-, Cl−-, and Ca2+-ions. For the bulk water–NaCl(100) system we analyze the surface induced hydration shell structure as well as residence times and the dipole orientation autocorrelation near the surface. At the low temperature we calculate the coverage dependence of the isosteric heat of adsorption, which is compared to available experimental data, including the coverage dependence of the adsorbate structure and dynamics. We note that our simulations support the formation of the (1×1) superstructure seen in helium scattering experiments.

Gas-phase properties and Fukui indices of sulfine (CH 2 SO). Potential energy surface and maximum hardness principle for its protonation process. A density functional study

Gas-phase thermochemical properties of sulfine (CH2SO) and the potential energy surface of its protonation process were studied by the density functional method employing different exchange-correlation potentials. All calculations showed that the most stable protonated isomer is planar with the proton bonded to the oxygen atom in a trans arrangement of the skeleton. Three transition states were located that allow interconversion between the different isomers. Hardnesses and Fukui indices were calculated to follow the reactivity trend along the protonation path and to explain the preference for a particular protonation site on neutral sulfine. Proton affinity, gas-phase basicity and heat of formation values, obtained for the first time fully quantum mechanically, agree well with those derived by a recent mass spectrometry experimental study. Good agreement between density functional theory and previous high-level theoretical and experimental data was also found for the heat of formation of sulfine and its most stable protonated form.

Preparation of N-doped hydrogen-free diamondlike carbon and its application to field emitters

We have studied the electrical, optical and field emission properties of nitrogen gas-phase doped hydrogen-free diamondlike carbon (DLC) films. The N-doped hydrogen-free DLC films were deposited by an alternating layer deposition technique using plasma enhanced chemical vapor deposition, in which deposition of a thin layer of gas-phase doped DLC and CF4 plasma exposure on its surface were carried out alternately. The optical band gap of the DLC films decreases from 1.8 to 1.55 eV with an increase of the [N2]/[CH4] ratio from 0% to 24% because of an increase of the graphite phase (π state). The emission current density and onset field are strongly related to the gas-phase doping concentration in the DLC films. The optimum [N2]/[CH4] flow rate ratio for efficient electron emission was found to be 9%. The onset field and the effective barrier energy at 9% are 7.2 V/μm and 0.02 eV, respectively. The material appears to be modified into a carbon-nitrogen alloy when it exceeds 9%. We have also studied the enhanced field emission characteristics of nitrogen gas-phase doped hydrogen-free DLC films on Mo tip field emitter arrays. The maximum emission current for each pixel increased from 160 μA to 1.52 mA with the addition of a 200 Å thick N doped hydrogen-free DLC coating on the Mo tips.

In-situ control of chemical vapor deposition for fiber coating

Chemical vapor deposition (CVD) is a widely used process for the application of thin films. In this case, CVD is being used to apply a thin-film interface coating (LaAl 11O 18) to single-crystal, monofilament sapphire fibers for the production of ceramic matrix composites. Traditionally, CVD fiber coating has been hampered by problems such as poor coating uniformity, thickness, and chemistry control. To attempt to rectify these problems, a fully-instrumented CVD processing apparatus has been constructed. In addition to more standard sensors such as thermocouples, pressure transducers, and flow meters, sensors capable of monitoring the coating and gas phase properties of interest have been used. Specifically, a quadrupole mass spectrometer and Raman spectrometer are being used to monitor both the gas phase reactants and the coating quality. A fuzzy-logic controller has been developed to control the O 2 flow rate, based on in situ temperature measurements. Mass spectrometer data and corresponding improved temperature control for a deposition are presented. Raman spectra which were taken off-line are also presented.

A comparison of ground- and excited-state properties of gas phase and crystalline ruthenocene using density functional theory

The ground- and excited-state properties of both gas phase and crystalline ruthenocene, Ru(cp)2, are investigated using density functional theory. A symmetry-based technique is employed to calculate the energies of the multiplet splittings of the singly excited triplet states. For the crystalline system, a Buckingham potential is introduced to describe the intermolecular interactions between a given Ru(cp)2 molecule and its first shell of neighbors. The overall agreement between experimental and calculated ground- and excited-state properties is very good as far as absolute transition energies, the Stokes shift and the geometry of the excited states are concerned. An additional energy lowering in the B23 component of the 5a1′→4e1″ excited state is obtained when the pseudolinear geometry of Ru(cp)2 is relaxed along the low-frequency bending vibration.

Modeling the Effect of Unsteady Chamber Conditions on Atomization Process

AP e The response of fluid flow through an orifice to chamK ber pressure oscillations and the evolution of the corp responding liquid jet has been investigated through & the use of a two-dimensional model based on the <£ Boundary Element Method. A series of parametric $ simulations have been performed using this tool to Subscripts evaluate the effect of orifice length, injection condi£ tions, and the amplitude and frequency of chamber g pressure oscillations on the dynamic orifice massflow, j phase lag, and jet surface shape. Results indicate osc substantial differences between the 2-D simulations ss and 1-D theory, particularly in the case where the oo orifice passage is short. Simulations have also been rnin performed to address the velocity profile at the exit max plane under these unsteady injection conditions. pressure drop across the orifice ratio of gas to liquid density surface curvature density surface tension velocity potential phase lag liquid phase properties gas phase properties nodal location oscillation amplitude steady state value far field condition minimum value maximum value

Probing viruses with mass spectrometry.

Mass spectrometry offers a new perspective on the solution and gas-phase properties of viruses. Its broad application to local and global viral structure provides unique insights into many biological processes, including viral-antibody binding, protein-protein interactions and protein dynamics. Mass measuring viral proteins is now routine and since viruses are typically well characterized, in that the capsid proteins and DNA (or RNA) sequences are known, identifying a virus based on the mass of the protein and enzymatic digestion fragments is relatively straightforward. Using mass spectrometry, this paper describes the identification of viral protein post-translational modifications such as myristoylation, phosphorylation and disulfide bridging. Furthermore, complementary data obtained with mass spectrometry and x-ray crystallography demonstrate that viruses are highly dynamic particles whose viral capsid's mobility could, until recently, be inferred only from inherently static spectroscopic methods. Lastly, mass spectrometry has been applied on a global scale via the mass measurement of entire intact viruses. Given the general utility of mass spectrometry, its continuing development should further its application to viral dynamics, structure, function and identification.