bluuebunny/crossref_metadata_2025 huggingface dataset:
Molecular thermodynamics of heat‐induced protein unfolding in aqueous media
Authors: Cioci Federico , Lavecchia Roberto
Published: June 2004

The molecular thermodynamic model studied is based on the two‐state mechanism of inactivation, in which only native folded and polymorphous unfolded protein forms are present at equilibrium. The influence of solvent on protein stability is described in terms of perturbation of the protein distribution between the two conformational states. An expression derived for the chemical potential of the protein accounts for conformational changes, ideal mixing effects, and interaction of the protein with the surrounding medium.

Thermal unfolding of lysozyme was then studied in the absence or presence of hydroxylic compounds. Ultraviolet difference spectroscopy was used to monitor the conformational changes induced by heating and to determine the melting temperature of the protein. The additives investigated are ethanol, glycols, and natural osmolytes. Media containing ethanol and glycols destabilized lysozyme, whereas sugars increased the conformational stability of the protein. For all of the systems examined the melting temperature was linearly related to the surface tension of the mixed solvent, supporting the ability of the model to describe the influence of the solvent and composition on lysozyme unfolding. Model predictions agreed fairly well with published differential scanning calorimetric data. The influence of hydroxylic additives on protein's conformational stability does not extend to any special property of these components, but to their ability to perturb the surface tension of water. This model can be used to interpret and correlate thermal unfolding data and to solve the problem of protein stabilization.
Colloidally induced smectitic fines migration: Existence of microquakes
Authors: Mohan K. Krishna , Fogler H. Scott
Published: June 2004

The phenomenon of colloidally induced fines migration is a challenge of both scientific and industrial importance. Its occurrence impacts permeability reduction and alteration of flow pathways in porous media, particulate contaminant migration in groundwater flow, and filtration. The release of smectitic fines is a threshold type of process resulting from discontinuous jumps, called microquakes in the interlayer spacing. There is a critical salt concentration at which these microquakes occur and produce fines migration in the porous media. The changes in the microstructure with decreasing salt concentration were analyzed using X‐ray diffraction. The transition between crystalline and osmotic swelling regime is also shown to depend on the type of cation. A mathematical model developed using colloidal principles predicts the swelling behavior of smectites in aqueous solutions. The model can predict the transition of swelling from crystalline to osmotic regimes and explain the effect of different cations on the transition.
Finite‐element simulation of mixing: 1. Two‐dimensional flow in periodic geometry
Authors: Avalosse Th. , Crochet M. J.
Published: June 2004

Complex mixing flows and mixing parameters are calculated to evaluate mixing quality on the basis of kinematic parameters together with a statistical analysis. The results allow for a comparative evaluation in terms of geometry and flow parameters. The evaluation is limited to two‐dimensional flows with moving boundaries in periodic motion such as the flow in the cross section of a twin‐cam mixer. The finite‐element calculation requires a remeshing procedure for every time step with special techniques to upgrade mixing variables from one time step to the next. The relative mixing quality of single cam and co: or counterrotating cam devices are compared by evaluating segregation scale, length stretch, and efficiency. The numerical results exhibit a good correspondence with their experimental counterpart.
Finite‐element simulation of mixing: 2. Three‐dimensional flow through a kenics mixer
Authors: Avalosse Th. , Crochet M. J.
Published: June 2004

Numerical tools for the analysis of complex 3‐D mixing devices were developed. Calculations are based on the finite‐element method. The flow calculation is combined with that of pathlines for a fairly large number of material points; along these pathlines, relevant kinematic variables, which are then subjected to a statistical treatment, are evaluated. These numerical techniques are applied to the analysis of a Kenics mixer. It is assumed that the geometry is periodic in the axial direction, that is, that the mixer contains an infinite number of blades; the finite‐element analysis may then be completed on a single wavelength of the geometry. To test the accuracy of the three‐dimensional calculation, a Kenics device was mounted in order to mix clays of different colors, and photographs of experimental cross‐sections were compared with numerical results showing concentration maps. A statistical treatment was performed on some 8,000 material points crossing the entry section to evaluate mixing quality.
Steady spontaneous capillary flow in partially coated tubes
Authors: Weislogel Mark M.
Published: June 2004

A spontaneous, steady capillary flow is produced for a liquid index in a circular tube that is partially coated with a surface modifier to create a discontinuous wetting condition between upstream and downstream portions of the tube. As a means of demonstrating that the flow configuration may prove useful as a diagnostic tool in studies concerning capillarity and the physics associated with the moving contact line, average steady velocities are predicted and compared against a large experimental data set that includes the effects of tube dimensions and fluid properties. To access a wider range of tube diameters (0.516–9.88 mm), experiments are performed employing “U‐tubes” tested in the low‐gravity environment of a drop tower, in addition to straight capillary tubes tested horizontally in a laboratory. The sensitivity of the steady capillary flow to surface cleanliness is dramatic.
Photon‐migration measurement of latex size distribution in concentrated suspensions
Authors: Sevick‐Muraca Eva , Pierce Joseph , Jiang Huabei , Kao Jeffrey
Published: June 2004

Frequency‐domain measurements of photon migration were conducted in concentrated polystyrene suspensions to assess their use for on‐line particle sizing in undiluted process streams. Using a numerical inverse algorithm, particle‐size distributions (PSD) and volume fractions of three latex suspensions with differing distributions were recovered from phase‐shift measurements at 15 or less visible wavelengths. Comparison to dynamic light‐scattering measurements show excellent agreement. Since it is the propagation characteristics of multiply scattered light instead of the amount of light detected that is employed to solve the inverse problem, the measurement technique provides a self‐calibrating (and therefore especially suitable) method for on‐line process monitoring of PSD in the chemical‐based industries.
Solid‐phase creep during the expression of palm‐oil filter cakes
Authors: Kamst G. F. , Bruinsma O. S. L. , de Graauw J.
Published: June 2004

For an adequate model of the processes of compressible cake filtration and mechanical expression, permeability and compressibility data are required. Experimental and modeling results of the creep behavior of palm‐oil filter cakes at constant and time‐dependent pressures are presented. Creep curves of palm‐oil filter cakes at constant pressures cannot be modeled with linear viscoelastic models. Modeling with a modified form of the empirical equation of Nutting gives satisfactory results. This modification does not lead to unrealistic values of the porosity at extreme conditions, contrary to the original form of the equation of Nutting. Creep curves at time‐dependent pressures were modeled with two nonlinear viscoelastic models, which describe the time‐dependent creep behavior as a function of the pressure history and creep curves at constant pressures. Modeling with the strain‐hardening model provides the best porosity predictions.
Permeability of filter cakes of palm oil in relation to mechanical expression
Authors: Kamst G. F. , Bruinsma O. S. L. , de Graauw J.
Published: June 2004

Permeability and compressibility data are required for an adequate process model for compressible‐cake filtration and mechanical expression. Experimental and modeling results of the permeability of palm‐oil filter cakes (a highly compressible viscoelastic material) are combined with compressibility data, leading to a model for the expression step. Permeability measurements show that permeability depends strongly on the quantity of fine particles in the cake. Removal of fine particles from the slurry before expression significantly increases the solid‐phase content during expression due to higher permeability. Modeling results of the expression step show that for palm‐oil filter cakes there is a pressure above which the attainable mass fraction of solids becomes independent of pressure. Decrease in specific cake resistance has two effects: a higher mass fraction of solids at the same pressure and a higher pressure at which the mass fraction of solids is not affected further.
Continuous drying of a solid wetted with ternary mixtures
Authors: Vidaurre M. , Martínez J.
Published: June 2004

The influence on drying selectivity of the continuous‐contact mode between a solid wetted with ternary mixtures and a gas stream was theoretically studied. The liquid mixtures, ethanol–isopropanol–water and water–ethanol–acetone, were used. A mathematical model describing a gas‐phase‐controlled process was developed, and the influence of the process variables was studied by simulations. In addition to the inlet composition of the moisture and temperature of the solid, gas composition has the most important effect on selectivity. Small changes of gas composition, either imposed or spontaneous, may modify completely the process trajectory. The extent of these effects depends on the ratio between the flow rates of inlet gas and liquid contained in the solid. Because of their effects on the evolution of temperature and composition, the operating pressure and energy sources other than convection are also useful in controlling the selectivity. Since all these variables determine the composition of the remaining liquid and thereby product quality, their influence should be predictable. The model may be a valuable tool for exploring the process, provided that drying is gas‐phase‐controlled.
Simulation of solids processes accounting for particle‐size distribution
Authors: Hill Priscilla J. , Ng Ka M.
Published: June 2004

A prototype computer program SOLIDS has been developed for the simulation of processes with solids processing steps. In addition to heat and mass balances, the equipment units are modeled with discretized population balance equations. Any transformation mechanisms for the particle‐size distribution (PSD)—nucleation, growth, dissolution, agglomeration, and breakage—can be represented. With the sequential modular code, the PSD can be tracked from unit to unit in a complete plant. To provide maximum flexibility for further development, new or improved equipment models and discretization techniques can be included as additional subroutines. Case studies show that accounting for the PSD can lead to improved equipment design and a more complete description of the interactions among the units in the plant.
Mediating operation of catalytic CSTR to stabilize intermediate steady state
Authors: Gol'dshtein Vladimir , Panfilov Vadim
Published: June 2004

A novel approach for stabilizing the intermediate steady state of a continuous stirred‐tank reactor (CSTR) is proposed by using a special type of periodic forced operation, the so‐called mediating operation. The mediating operation enables new additional steady states for which one of the new intermediate steady states is stable to be produced. Thus the proposed approach employs a periodic forcing for stabilization of a selected steady state by control of a steady‐state multiplicity. The feed flow rate is considered to be a manipulated variable. Changing CSTR multiplicity is investigated for two‐step control inputs. It is shown, analytically, that under two‐step control CSTR can exhibit at most five steady states with a stable intermediate steady state. A constructive procedure is proposed for finding control parameters corresponding to maximal multiplicity, that is, for stabilizing an intermediate steady state by two‐step control.
Two‐flux radiation‐field model for an annular packed‐bed photocatalytic oxidation reactor
Authors: Roupp Gregory B. , Nico Jill A. , Annangi Suneetha , Changrani Rajnish , Annapragada Rao
Published: June 2004

One of the remaining challenges in application of heterogeneous photocatalysis for treatment of air streams containing dilute VOCs is to design a cost‐effective photocatalytic reactor that simultaneously allows efficient contact of the contaminated air and solid catalyst while uniformly irradiating the solid catalyst with light. A pseudohomogeneous model was developed to study effects of system parameters on process performance for a gas–solid lamp‐in‐tube annular‐photocatalytic‐oxidation (PCO) reactor in which the annular space is filled with photocatalyst‐coated packing. In this model the flow field is assumed to be uniform and radial diffusion negligible. Homogeneous reactions are neglected. Heterogeneous reaction rates follow Langmuir–Hinshelwood–Houghen–Watson kinetics with rate parameters extracted from independent experiments. A 1:D “two‐flux” incidence submodel is used to account for the radial UV light distribution throughout the reactor annular space. This submodel requires knowledge of the UV lamp radiant emittance, the optical characterstics of the catalytic thin‐film coating, and the UV irradiance at the outer wall of the reactor and contains only a single adjustable parameter—the mean free path between photon–catalyst interactions. The model was validated with experimental performance measurements for destruction of acetone and isopropyl alcohol in a bench‐scale photoreactor. The validated model can be used to predict the optimum catalyst film thickness for given reactor dimensions, packing shape and size, and VOC abatement problem.
First‐order closure theories for series‐parallel reaction in simulated homogeneous turbulence
Authors: Chakrabarti Mitali , Hill James C.
Published: June 2004

Simple first‐order closure models for covariances of concentration fluctuations, for use in modeling turbulent flow reactors, were tested by direct numerical simulations. Concentration covariances and other statistical functions were evaluated for a series parallel reaction scheme in decaying, homogeneous turbulent flow. The simulations involve solving the unsteady Navier‐Stokes and mass conservation equations by a pseudo‐spectral method in a 643 wavenumber domain, with initially segregated reactants, for an initial turbulence Reynolds number of 29.9. Simulation results show that covariances of concentration fluctuations normalized with respect to mean concentration values are almost constant and that the time dependence of concentration covariances can be estimated if the mean concentrations are known at any one time after the initial time. Predictions of the first‐order closure models of Bourne and Toor, Brodkey and Lewalle, Li and Toor, and Dutta and Tarbell were compared to simulation results. While none of these closures are satisfactory for all the conditions tested, the Brodkey‐Lewalle closure agrees best with the simulations.
Role of coherent structures on reynolds stresses in a 2‐D bubble column
Authors: Mudde R. F. , Lee D. J. , Reese J. , Fan L.‐S.
Published: June 2004

The hydrodynamics of two‐dimensional bubble columns operated in various flow regimes are studied using particle image velocimetry. Both averaged velocity profiles and Reynolds stress profiles are obtained and discussed in relation to large‐scale structures present in the flow. The normal stresses, dominated by large‐scale structures, are an order of magnitude higher than the shear stress. It is found that the contribution from the bubbles to the shear to the normal stresses is negligible. A time series of the flow field is studied, demonstrating that the flow could be split into a low‐frequency contribution due to the vortical structures and a high‐frequency fluctuating part. The latter gives rise to flat normal stress profiles, and the former is responsible for the original form of the normal stress profiles. The shear stress in the smaller columns investigated can be related to the averaged vertical velocity profile according to a Boussinesq approximation. Data on the eddy viscosity are presented.
Modeling and safety verification of discrete/continuous processing systems
Authors: Dimitriadis V. D. , Shah N. , Pantelides C. C.
Published: June 2004

A quantitative, model‐based approach to the safety verification problem for general processing systems operating in the discrete time domain is presented. It is recognized that the operation of most of these systems involves both discrete and continuous characteristics. Therefore, an appropriate modeling framework is proposed, within which models of purely discrete, purely continuous and hybrid systems of arbitrary complexity can be constructed consistently. The models developed can then be incorporated into a safety verification formulation, which allows the identification of potential hazards that may occur while operating such systems, together with the combinations of events that lead to them. Apart from the dynamic process model, the data required for carrying out the analysis include the space of possible disturbances and the set of operating regimes that are considered to be unsafe or undesirable from the operability point of view. The formulation results in a mixed‐integer optimization problem. A number of simple example problems presented illustrate the main ideas of the proposed technique, and the solution of an industrial‐scale case study demonstrates its applicability.
Reaction invariance and stability in catalyst particles for production of Methanol
Authors: Asbjornsen Odd Andreas , Hillestad Magne
Published: June 2004

Industrial practice requires immediate and adequate responses to simple questions asked. For example, does a catalyst particle show any tendency to thermal oscillations in the form of a limit cycle? The root to such cycles is found in the eigenvalues of the Jacobian matrix to the reaction rate vector, applying the concept of reaction invariance, a direct consequence of Avogadro's stoichiometric principle for homogeneous stirred tank reactors. However, this concept does not generally apply to heterogeneous reactor dynamics, because it is found in the heterogeneous transports of heat, reactants and products. The transport is an irreversible phenomenon that usually contributes to shifting the eigenvalues to the left and hence increases the stability. Still, effects of transport mechanisms, both internally and externally, on the catalyst particle are important to assess in such industrial analyses. A practical industrial reactor for methanol production was the subject for stability studies, which concluded that thermal oscillations are not likely to occur. During this study, a number of interesting details were examined such as rank deficiency of the reaction matrix and root loci for the temperature dependence of the Jacobian matrix eigenvalues. A practical consequence of eventual thermal cycling of the catalyst particles is a long‐term degradation of the catalyst efficiency, as seen in the ammonia synthesis.
High‐energy density storage of natural gas in light Hydrocarbon solutions
Authors: Horstkamp S. W. , Starling K. E. , Harwell J. H. , Mallinson R. G.
Published: June 2004

The storage of natural gas in other light hydrocarbons is one procedure for automotive natural gas usage that reduces the requirement of high‐pressure or cryogenic storage. Model solutions of methane in n‐butane, propane, and a liquefied‐bottled‐gas mixture were simulated using the Benedict‐Webb‐Rubin‐Starling equation of state to determine the pressures necessary to maintain a liquid phase with perturbations in both temperature (−1°C to 38°C) and mole fraction (50 to 80 mol% methane). Methane storage in these liquid solutions is between 45 and 93% of an equal “tank” volume of compressed natural gas (CNG) at 21 MPa and 15°C. The simulation results indicate that solutions of this type contain 40 to 67% of the energy of gasoline at pressures that range from 60 to 40% that of CNG at 21 MPa and 15°C.
Unique properties of 30‐μm particles as the catalyst of fluidized‐bed reactors
Authors: Yang Guoqiang , Wei Fei , Jin Yong , Yu Zhiqing , Wang Yao
Published: June 2004

The collapse properties of three kinds of fine particles (19, 30, and 59 μm) are studied with a new type of bed‐collapse technique, “isolated dilute‐phase bed collapse,” to determine the dense‐phase properties under high superficial gas velocities. The properties of the emulsion and bubble phases are investigated by a video camera. The influence of particle size on particle collapse properties is also discussed. When gas velocities are high the 30‐μm particles take the longest to collapse, have the smallest emulsion density, and have largest bubble volumetric fraction. These observations indicate that the 30‐μm particles have a property that the others lack.
Detecting persistent gross errors by sequential analysis of principal components
Authors: Tong Hongwei , Crowe Cameron M.
Published: June 2004

Measurements such as flow rates from a chemical process violate conservation laws and other process constraints because they are contaminated by random errors and possibly gross errors such as process disturbances, leaks, departures from steady state, and biased instrumentation. Data reconcilation is aimed at estimating the true values of measured variables that are consistent with the constraints, at detecting gross errors, and at solving for unmeasured variables. An approach to constructing sequential principal‐component tests for detecting and identifying persistent gross errors during data reconciliation by combining principal‐component analysis and sequential analysis is presented. The tests detect gross errors as early as possible with fewer measuremennts. They were sharper in detecting and have a substantially greater power in correctly identifying gross errors than the currently used statistical tests in data reconciliation.
Comprehensive kinetic model for the low temperature oxidation of hydrocarbons
Authors: Gaffuri Paolo , Faravelli Tiziano , Ranzi Eliseo , Cernansky Nicholas P. , Miller David , d'Anna Andrea , Ciajolo Anna
Published: June 2004

The low‐temperature oxidation chemistry of linear and branched alkanes is discussed with the aim of unifying their complex behavior in various experimental systems using a single detailed kinetic model. New experimental data, obtained in a pressurized flow reactor, as well as in batch‐ and jet‐stirred reactors, are useful for a better definition of the region of cool flames and negative temperature coefficient (NTC) for pure hydrocarbons from propane up to isooctane. Thermochemical oscillations and the NTC region of the reaction rate of the low‐temperature oxidation of n‐heptane and isooctane in a jet‐stirred flow reactor are reproduced quite well by the model, not only in a qualitative way but in terms of the experimental frequencies and intensities of cool flames. Very good agreement is also observed for fuel conversion and intermediate‐species formation. Irrespective of the experimental system, the same critical reaction steps always control these phenomena. The results contribute to the definition of a limited set of fundamental kinetic parameters that should be easily extended to model heavier alkanes.
Analysis of bacterial swimming speed approaching a solid–liquid interface
Authors: Frymier Paul D. , Ford Roseanne M.
Published: June 2004

Previous experimental studies have found that surface interactions significantly affect the transport of motile bacteria through small tubes, along solid surfaces, and through porous media. However, the role that hydrodynamic forces play in the interactions between solid surfaces and motile bacteria remains unclear. In this study, the swimming speeds of populations of Escherichia coli bacteria were measured near (< 10 μm) and far (>10 μm) from a flat glass surface at four ranges of orientations to the surface (0°–45°, 45°–90°, 90°–135°, and 135°–180°). Populations of bacteria close to the surface and moving in the orientation range most perpendicular (0–45°) to the surface experienced the greatest change in the swimming speed when compared to the population in the same orientation range located far from the surface. The decrease in swimming speed experienced by this population was on the same order as that predicted by hydrodynamic models of bacterial swimming near surfaces.
Lagrangian simulation of turbulent particle dispersion in electrostatic precipitators
Authors: Soldati Alfredo , Casal Massimo , Andreussi Paolo , Banerjee Sanjoy
Published: June 2004

Industrial design of electrostatic precipitators is based on the transport theory developed by Deutsch (1922), which assumes that transverse turbulent mixing is effective enough to maintain the concentration profile uniform throughout the cross section (i.e., turbulent diffusivity is assumed infinite). To improve understanding of turbulent particle dispersion under the influence of electrostatic forces, a database on particle trajectories was first generated, based on the flow field from a direct numerical simulation of a plate‐plate precipitator (Soldati et al., 1993). The effect of various parameters, such as particle size, charge and particle migration velocity, on dispersion and collection efficiency was investigated. Results show that particle concentration profiles are not uniform due to finite values of “turbulent diffusion” coefficient. The simulations indicate that the early stages of particle collection are controlled by particle migration velocity, while final stages are controlled by turbulence diffusion mechanisms.
Initial and final stages of compressible filtercake compaction
Authors: Sherwood J. D.
Published: June 2004

Filtration experiments were performed to dewater bentonite slurries at different filtration pressures. The diffusivity of the cake was then determined as a function of void ratio using the measured filtration rates, the final void ratios of the cakes, and the theory of compressible filtercakes. Any filtration process can, in theory, be predicted, once the cake diffusivity is known as a function of void ratio and the void ratio as a function of applied pressure. In particular, the final stage of the filtration process can be modeled and offers an alternative route to determine the cake properties from the experimental results. The final slow approach to equilibrium is difficult to study experimentally; other techniques to determine cake properties are therefore preferable. Final stages of the cake compaction are also markedly slower than predictions based on the intial filtration rates. Possible explanations include a filtercake rheology that is more complicated than assumed in the model or time‐dependent effects caused by exclusion of ions from the bentonite filtercake, but such explanations appear unsatisfactory. Filtration models in which the filtercake void ratio and permeability depend solely on the imposed stress, may therefore be too simple for some materials.
Coupling of thermal cracking with noncatalytic oxidative conversion of Ethane to Ethylene
Authors: Choudhary Vasant R. , Mulla Shafeek A. R.
Published: June 2004

Homogeneous thermal cracking of ethane, which is an endothermic and hence energy‐intensive process, was carried out in the presence of limited O2 under different process conditions (at 600°–850°C; O2/ethane ratio of 0–0.2; H2O/ethane ratio of 0–3.3; and space velocity of 2,000–11,000 h−1). The influence of a sulfur additive in the feed and reactor material on the process performance was also investigated. Noncatalytic oxycarcking of ethane in the presence of limited O2, greatly enhances not only the total conversion of ethane but also the conversion of ethane by purely thermal cracking. This process also occurs at a much lower contact time than that required to achieve the same conversion for the thermal cracking process. Also, both the exothermic oxidative conversion and the endothermic thermal cracking of ethane occur simultaneously, which makes this process very energy‐efficeint, with a drastic reduction in external energy requirement and coke formation. By manipulating the process conditions, particularly the temperature and O2/C2H6 ratio, the overall process can be made almost thermoneutral, mildly exothermic, or mildly endothermic.
Dynamic adsorption and tension of nonionic binary surfactant mixtures
Authors: Siddiqui Faisal A. , Franses Elias I.
Published: June 2004

For the first time, a model for dynamic adsorption and tension of diffusion‐controlled systems has been extended to binary premicellar mixtures of nonionic surfactants of different adsorption capacities and nonideal interactions in the monolayer. Local equilibria between subsurface concentrations and adsorbate densitities are modeled using the nonideal adsorbed solution theory, which describes these two elements. The model shows that larger molecules (those with the smaller adsorption capacity) tend to be preferentially adsorbed at low times, if they have equal adsorption equilibrium constants and diffusivities, and smaller moleules at longer times. This adsorption selectivity is reduced when larger molecules have a much larger adsorption equilibrium constant, or when there are negative deviations from ideality in the monolayer. This model's predictions are compared to tension data for two nonionic surfactants, C12E5 and Triton X‐100, at 25°C. The data are represented well by the diffusion‐controlled model with a finite diffusion‐layer thickness, which describes the faster decrease in tension observed with the bubble surfactometer, compared to data with other techniques. With this model, surface coverages and concentration profiles are calculated, thus elucidating the adsorption selectivity of molecules of different adsorption capacities. Synergistic effect in dynamic tension and adsorption can be predicted. Mild synergism in dynamic tension lowering by the preceding nonionic surfactants is also observed.
Thermodynamics of hydrogen bonding from molecular orbital theory: 1. Water
Authors: Wolbach Jeffrey P. , Sandler Stanley I.
Published: June 2004

To reduce the number of adjustable parameters that appear in the thermodynamic modeling of hydrogen‐bonding fluids, the use of ab‐initio molecular orbital calculations in the form of Hartree–Fock and density functional theories to compute thermodynamic properties are considered. In particular, the enthalpy and entropy changes on hydrogen bonding for water are computed. The theory, the computational methods and the basis sets used, as well as the results for water, are discussed. The results obtained are compared with those of others and with experimental estimates from the literature to determine the validity of the methods that are then used in this extended study. The results obtained here are also used to examine the validity of mixture group contribution results obtained here are also used to examine the validity of mixture group contribution methods for hydrogen‐bonding mixtures, to develop simple estimation procedures for model parameters, and to reduce the number of calculations such as those as those described that need to be done.
Thermodynamics of hydrogen bonding from molecular orbital theory: 2. Organics
Authors: Wolbach Jeffrey P. , Sandler Stanley I.
Published: June 2004

Hartree‐Fock theory and density functional theory were used to compute the enthalpy and entropy changes of dimerization for a number of hydrogen‐bonding compounds. In Part 1, the calculational methods and procedures use for the water dimer are described, and the results obtained are compared with those of others and with experimental estimates in the literature. Here, a variety of organic compounds that can self associate and/or cross associate are considered. The results obtained for the self‐association of these compounds are compared with estimates obtained from various types of experimental data. The results are also used to examine the validity of group‐contribution methods for hydrogen‐bonding mixtures and to test a simple estimation procedure for cross dimerization that can reduce the number of calculations, such as those described here, that need to be done.
Gas‐phase reaction in a trickle‐bed reactor operated at low liquid flow rates
Authors: Castellari A. T. , Cechini J. O. , Gabarain L. J. , Haure P. M.
Published: June 2004

The catalytic hydrogenation of alpha‐methylstyrene to cumene is studied in a laboratory trickle‐bed reactor operated at low liquid flow rates. Under this condition, wetting is not complete, vaporization of the liquid phase may occur, and reaction could proceed via liquid‐solid and gas‐solid catalysis, which increase the hydrogenation rate significantly. To determine and quantify the effect of the gas‐solid catalyzed reaction, we propose the use of post‐packing sections of increasing lengths. The inert bed acts as an absorber of the gas‐phase‐produced cumene. The liquid and gas effluent were monitored. Experimental global rates determined from liquid cumene concentrations varied with the post‐packing length. A simple one‐dimensional model developed shows that theoretical and experimental results agree well.
Modeling of SiO2 deposition in porous Vycor: Effects of pore network connectivity
Authors: Tsapatsis Michael , Gavalas George
Published: June 2004

A mathematical model developed for SiO2 deposition in porous Vycor glass using SiCl4 hydrolysis describes reaction, diffusion and evolution of the pore structure due to accumulation of the solid product. The deposition reaction is described by transient heterogeneous kinetics in terms of the concentrations of silanol and chloride groups in the product layer as well as those of the gaseous reactants. For typical deposition conditions the pseudo steady‐state approximation for surface species could lead to erroneous predictions. Pore structure evolution is modeled by incorporating results of percolation theory. For this purpose the porous glass is represented by a Bethe lattice with coordination number 3 and alternatively by a decorated Bethe lattice in which each bond is replaced by a composite bond consisting of two bonds in series. The second network can capture the effect of pore radius variation along a single pore. For the decorated lattice, pore connectivity interruption at a higher void fraction leads to thinner deposits and shorter deposition times for pore plugging compared to the corresponding ones for the simple lattice.
Explicit algebraic scalar‐flux models for turbulent reacting flows
Authors: Adumitroaie V. , Taulbee D. B. , Givi P.
Published: June 2004

Explicit algebraic scalar‐flux models that are valid for three‐dimensional turbulent flows are derived from a hierarchy of second‐order moment closures. The mathematical procedure is based on the Cayley–Hamilton theorem and is an extension of the scheme developed by Taulbee. Several closures for the pressure–scalar gradient correlations are considered and explicit algebraic relations are provided for the velocity–scalar correlations in both nonreacting and reacting flows. In the latter, the role of the Damköhler number is exhibited in isothermal turbulent flows with nonpremixed reactants. The relationship between these closures and traditional models based on the linear gradientdiffusion approximation is theoretically established. The results of model predictions are assessed by comparison with available laboratory data in turbulent jet flows.
Drag coefficient and velocity of rise of a single collasping two‐phase bubble
Authors: Wanchoo R. K. , Sharma S. K. , Raina G. K.
Published: June 2004

The velocity of rise and the drag of a single vapor bubble collapsing in another immiscible liquid were measured experimentally. During the process of collapse, the dispersed‐phase vapor bubble was transformed to a two‐phase bubble with condensate accumulating at the rear of the two‐phase bubble and vapor at its top. Such a configuration of a two‐phse bubble is commonly known as a drobble. Experimental data for the six pairs of liquids covered a range of drobble (two‐phase bubble) Reynolds numbers from 0.003 to 3,000. Two regimes of drobble movement were encountered. In the first regime (Re < 100), the drobble maintained its sphericity, and the observed drag was less than the solid‐sphere drag predicted by the established solid sphere or Hadamard et al. fluid‐sphere drag models. In the second regime, the drobble was deformed and oscillated; the observed drag departed suddenly from predictions of spherical models and increased with increasing Reynolds numbers. The critical Reynolds number covered a range from 100 to 1,000. Empirical models for drobble velocity and drag coefficient are derived.
Mapping regions with different bifurcation diagrams of a reverse‐flow reactor
Authors: Khinast J. , Luss D.
Published: June 2004

A systematic method is presented for constructing maps of parameter regions with qualitatively different bifurcation diagrams of the reverse‐flow reactor (RFR). These maps are most useful for the rational design and periodic operation of an RFR. The method is illustrated by two examples. The map for a single, exothermic first‐order reaction contains five regions with qualitatively different bifurcation diagrams when the Damköhler number is the bifurcation variable. The high‐temperature branch is isolated in two of these regions. The map for two independent, exothermic first‐order reactions contains seven regions with qualitatively different bifurcation diagrams when the adiabatic temperature rise is the bifurcation parameter. Three stable periodic states exist for some values of the bifurcation variable in five of the regions. In some of these regions the intermediate state cannot be obtained upon a slow change in the concentration of the reactants. In all the examples we studied, a stability exchanges of the pseudohomogeneous model occurred only at the limit points of the bifurcation diagrams. An efficient numerical scheme is presented for computing the loci of the singular points of the RFR model.
Effects of operating conditions on heat removal from polyethylene reactors
Authors: Jiang Yan , McAuley Kim B. , Hsu James C. C.
Published: June 2004

A one‐dimensional nonequilibrium model for multicomponent condensation is used to simulate a vertical single‐pass shell‐and‐tube heat exchanger in an industrial gas‐phase polyethylene reactor system. Starting the calculation at the top of the exchanger, the model can predict temperatures at the bottom of the exchanger within an accuracy of ±5 K as compared to three sets of industrial data. Sensitivities of model predictions were analyzed, including uncertainties associated with physical and transport property estimates, step size, and convergence criterion. Model predictions are not particularly sensitive to the estimation errors of physical and transport properties if K values are calculated using an equation of state applicable to both liquid and vapor phases. Effects of operating conditions on heat removal from polyethylene reactors were investigated for an existing process. It was quantitatively demonstrated why and how severely noncondensable gases impede condensation heat transfer. The level of noncondensable gases and the cooling water temperature are the two most important factors influencing the heat‐removal rate. Replacing a portion of noncondensable gas, such as N2, with a condensable fluid that is inert to polymerization reactions can substantially increase the heat‐removal rate from the reactor, thereby allowing for an increase in polymer production rate.
Generalized approach to NMR analysis of flow and dispersion in porous media
Authors: Seymour Joseph D. , Callaghan Paul T.
Published: June 2004

A generalized approach to the use of pulsed‐gradient spin echo (PGSE) NMR methods for the measurement of flow and diffusion in porous media is presented, in which the fluid dynamics is probed over well‐defined temporal and spatial domains. Various NMR techniques based on PGSE encoding are described in the context of standard theories of dispersion, with reference to Eulerian and Lagrangian coordinate frames. This array of methods provides access not only to the dispersion coefficient and the mean local velocity but also to propagators relevant to spatial and temporal correlations. Methods investigated include flow imaging, average propagator analysis, dispersion measurement, velocity exchange spectroscopy, and flow and disffraction based on scattering analysis. We apply these to a study of flow and dispersion of water in a packed bed of 90.7‐μm‐dia. polystyrene latex spheres. Our measurements of the dependence on Peclet number of dispersion (parallel and perpendicular to the mean flow direction) are in excellent agreement with results reported in the literature. The scattering approach used here has potential for studying complex flow properties involving the interplay between hydrodynamic and structural characteristics of porous media.
Fractal‐like behavior of a mass‐transport process
Authors: Kinoshita Masahiro , Harada Makoto , Sato Yasuo , Tsubata Kyoichi , Nakamura Yasutaka
Published: June 2004

After an aqueous solution containing Sr2+ was flowed through a bed packed with particles of activated charcoal for a very long time, the flow was abruptly switched to deionized water. The variation in ionic concentration at the outlet with time was characterized by the power law, Cout ∼ t−α, for sufficiently large t. This fractal‐like behavior we reported previously has further been studied theoretically and experimentally. A major concern is to examine dependency of α on experimental conditions. Adsorption sites predominate over dead‐end pores or spaces as “trappers” in our case where Sr2+ or Ba2+ is adsorbed on activated charcoal. The experimental results can well be explained by our theoretical model. It has been found, however, that the response curve for Sr2+ deviates from the power law after a very long time. This deviation can be explained by introducing the assumption that there exists a maximum value of the activation energy for the desorption process. The curve for Ba2+, on the other hand, exhibits no such deviation until Cout decreases to the detection limit of the analytical device used.
Thermodynamic behavior of reactive azeotropes
Authors: Okasinski Matthew J. , Doherty Michael F.
Published: June 2004

During process design of reactive distillation systems, there is often uncertainty in the value of the reaction equilibrium constant (Keq), whether it is determined experimentally or calculated from thermochemical data. The effect of the reaction equilibrium constant on the existence and location of reactive azeotropes (constant boiling reactive mixtures) is explored for chemical equilibrium systems with a single chemical reaction. With a known set of starting points, arc‐length continuation was used to track solutions of the equations for reactive azeotropy as a function of the reaction equilibrium constant. The results, portrayed in bifurcation diagrams, reveal that azeotropes may appear or disappear as the equilibrium constant is varied. Results for the esterification of acetic acid with ethanol indicate three distinct regions of phase behavior. The first regime (Keq < 0.449) contains a quaternary saddle reactive azeotrope. At intermediate values of the reaction equilibrium constant (0.449 < Keq <12.5), there are no reactive azeotropes in the system. In the third regime (Keq < 12.5), a minimum‐boiling quaternary reactive azeotrope appears. In addition, for three reported literature values of the reaction equilibrium constant, each one lies in a different regime. Other examples also illustrate the broad taxonomy of reactive azeotropic systems.
Bubble coalescence dynamics
Authors: Stover Richard L. , Tobias Charles W. , Denn Morton M.
Published: June 2004

Coalescence of electrolytically‐generated, 50‐ to 600– μm‐diameter gas bubbles was observed using an optical technique that employs a linear photodiode array to detect interface movement with a resolution of 10−6s. When two bubbles coalesce, the surface energy that is released causes interface velocities of 2 to 4 m/s; these are followed by large‐amplitude damped oblate–prolate oscillations. Within the viscosity range studied, the oscillation period is insensitive to the viscosity and damping is insensitive to surface tension, in agreement with a scaling analysis based on a damped harmonic oscillator. Surface waves are superimposed on the motion. Finite‐element solutions of the free‐surface, nonlinear Navier‐Stokes equations are consistent with the experiments.
Turbulence structure and scalar transfer in stratified free–surface flows
Authors: Nagaosa Ryuichi , Saito Takayuki
Published: June 2004

Direct numerical simulations were used to investigate turbulence structure and scalar transfer across a shear‐free, nonwavy gas‐liquid interface (free surface) in stably stratified turbulent flows. These simulations solve the Navier–Stokes and thermal‐energy conservation equations, using a finite difference approximation. The relation between the turbulence structure in the stably stratified turbulent boundary layer and organized motion near the free surface are discussed. In addition, scalar transfer across a free surface is investigated by solving a passive scalcr conservation equation after both velocity and temperature fields are fully developed. The effects of stable stratification on the scalar transfer process in a region very close to the free surface are also discussed.
Measurement of velocity profiles in reverse‐screw elements of a twin‐screw extruder
Authors: Chandrasekaran Murali , Karwe Mukund V.
Published: June 2004

A laser Doppler anemometer was used to measure velocity components for the flow of a Newtonian fluid in a reversing section of a pilot‐scale corotating twin‐screw extruder. Measurements were made at a fixed axial position in the intermeshing (nip) region and the translational region (áway from the nip). Tangential and axial velocity profiles reached a maximum in the middle of the screw channel, indicating pressure flow. Leakage flow through the gaps between screw flights and backflow toward the hopper were observed only in the nip region. No reversal of flow was detected in the translational region. Measured velocity profiles showed substantially higher shear rates in the reversing section of the screw as compared to those in the forward‐conveying section.
Integral approach to plant linear dynamic reconciliation
Authors: Bagajewicz Miguel J. , Jiang Qiyou
Published: June 2004

An integral method is proposed that performs dynamic data reconciliation on linear systems, in contrast with recent methods that utilize differential algebraic equations. The differential equations representing this system are first rearranged to obtain a system of equations containing only redundant measurements. These equations are formally integrated using polynomial approximations, and the reconciliation is then performed using analytical solutions. A new statistic to detect gross errors is proposed, and a procedure to detect biased measurement is presented.