bluuebunny/crossref_metadata_2025 huggingface dataset:
Graphical methods for reaction distribution in a reactive distillation column
Authors: Lee Jae W. , Hauan Steinar , Westerberg Arthur W.
Published: April 2004

Visual methods of determining how to distribute reaction zones within a binary reactive distillation column were studied using both the Ponchon–Savarit and McCabe–Thiele methods modified to account for the reaction. Two types of reactions are compatible with a totally binary system: isomerization and dimerization. For isomerization with a heavy reactant and a light product, both diagrams indicate the potential for a pinch point if the reaction zone is in the stripping section and no such potential if the zone is in the rectifying section. These same diagrams show that the column will have fewer stages and less usage of utility to yield the same final top and bottom products with the reaction in the rectifying section. The situation reverses if the reactant is the light species for isomerization, suggesting reaction should then take place in the stripping section. The heat effect from the isomerization reaction is considered for four cases depending on whether the reaction is endothermic or exothermic. Even if the heat of the isomerization is dominant, the results do not change: with a heavy reactant and a light product, the separation performance is better when the reaction takes place in the rectifying section. In the dimerization reaction, where two light reactants form a heavy product, the light reactant enriches as we go down the column and the composition profile is reversed in a Ponchon–Savarit diagram with the reaction in the rectifying section even if there is no pinch point. Results on the location of reaction zones are confirmed by rigorous simulations.
Modeling crystal shapes of organic materials grown from solution
Authors: Winn Daniel , Doherty Michael F.
Published: April 2004

The shape of a crystalline organic solid has a major impact on its downstream processing and on its end‐product quality, issues that are becoming increasingly important in the specialty and fine chemical, as well as the pharmaceutical and life science, industries. Though it is widely known that improved crystal shapes can be achieved by varying the conditions of crystallization (such as solvent type and impurity levels), there is far less understanding of how to effect such a change. Until recently, most methods for predicting crystal shapes were based exclusively on the internal crystal structure, and hence could not account for solvent or impurity effects. New approaches, however, offer the possibility of accurately predicting the effects of solvents. Models for predicting crystal shape are reviewed, as well as their utility for process and product design.
Particle‐size analysis in mass crystallization from solutions
Authors: Zelmanov G. , Semiat R.
Published: April 2004

The problem of product‐size distribution exiting a crystallizer is addressed theoretically and experimentally. The distribution function of the particle size is solved and demonstrated experimentally as a function of fines destruction, solids content of feed, particle circulation ratio, feed particle sizes, particle concentration. Theoretical tools are given for estimation of various important parameters such as distribution moments, rate of growth, rate of nuclei production and particle‐size distribution. A good agreement is shown between the theoretical model and the experimental results.
Establishing independence of continuous process product samples
Authors: Christiansen Reed L.
Published: April 2004

New product qualification frequently requires that a manufacturer provide customers with several independent samples for evaluation. For continuous manufacturing processes, equipment residence time distributions (RTDs) introduce serial dependence among the samples. Determining RTD by pulse or step modulation of a tracer or process variable can produce off‐spec material and add costs by limiting the opportunity to conduct other product or process development experiments. A novel method of determining a continuous process' RTD does not risk production of off‐spec material. Through analysis of serial component analytical data, the process' washout curve can be inferred. The theoretical basis for the method is presented and experimental requirements enumerated. Applicability to well‐mixed, laminar flow and convective‐dispersive processes is demonstrated. Experimental validation in a specialty polymer pilot plant resulted in an error of about 5% in mean residence time, when compared to estimates from pulse tests and calibrated level measurements.
Wall effects of laminar hydrogen flames over platinum and inert surfaces
Authors: Andrae Johan C. G. , Björnbom Pehr H.
Published: April 2004

Different aspects of wall effects in the combustion of lean, laminar and stationary hydrogen flames in an axisymmetric boundary‐layer flow were studied using numerical simulations with the program CRESLAF. The importance of the chemical wall effects compared to thermal wall effects caused by heat transfer to a cold wall was investigated in the reaction zone by using different combustion systems at atmospheric pressure. Surface mechanisms include a catalytic surface, an inert surface that promotes radical recombinations, and a completely inert wall used as reference was the simplest possible boundary condition. The analysis of the results show that for the richer combustion case (ϕ = 0.5) the surface chemistry gives significant wall effects, while the thermal and velocity boundary layer gives rather small effects. But for the leaner combustion case (ϕ = 0.1) the thermal and velocity boundary layer gives more significant wall effects, while surface chemistry gives less significant wall effects compared to the other case. As expected, the overall wall effects were more pronounced for the leaner combustion case.
Vapor–liquid equilibrium of aqueous solutions of formaldehyde and methanol
Authors: Albert Michael , Coto García Baudilio , Kuhnert Christian , Peschla Roger , Maurer Gerd
Published: April 2004

A thermodynamic model for phase and chemical equilibrium in formaldehyde–water mixtures was extended to describe vapor–liquid and chemical equilibrium in the binary formaldehyde–methanol system and in the ternary formaldehyde–water–methanol system. Ternary mixtures were extended using new experimental vapor–liquid equilibrium data for the influence of especially small amounts of methanol on the volatility (of small amounts) of formaldehyde in aqueous solutions. New experimental data for the range 333 to 383 K are reported. Some model parameters were fitted to ternary phase equilibrium data, because of cross‐reactions between poly(oxymethylene) glycols and poly(oxymethylene) hemiformals, and strong influence of interactions between those species on the phase equilibrium of the ternary mixture. The new model presents a reliable method for calculating vapor–liquid equilibrium and the conversion of formaldehyde to reaction products [poly(oxymethylene) glycols and poly(oxymethylene) hemiformals] over the whole range of temperature and composition where reliable experimental data are available.
Fluid dynamics in coal liquefaction reactors using neutron absorption tracer technique
Authors: Sakai Naohide , Onozaki Masaki , Saegusa Hitoshi , Ishibashi Hirohito , Hayashi Takashi , Kobayashi Masatoshi , Tachikawa Noboru , Ishikawa Isamu , Morooka Shigeharu
Published: April 2004

Hydrodynamic properties in the coal liquefaction reactors at the Kashima pilot plant, which was constructed based on the NEDOL process, were investigated using the neutron absorption tracer technique. The reactor system is composed of three vessels, each with 1.0 m ID and 11.8 m in height. The gas velocity in the reactors under coal liquefaction conditions was estimated using a reaction simulator that contained reaction rates and vapor–liquid equilibrium. The axial dispersion coefficients in the first and third reactors at superficial gas velocities of 0.06–0.07 m/s were much smaller than those reported for air–water systems under ambient conditions. This suggests that the pilot‐plant reactors operated fundamentally in the homogeneous bubble flow regime.
Direct simulation Monte Carlo method for particle coagulation and aggregation
Authors: Kruis F. Einar , Maisels Arkadi , Fissan Heinz
Published: April 2004

A Monte Carlo simulation technique developed describes dispersed‐phase systems with emphasis on coagulation and aggregation. The method does not use particle trajectories, but is based on the transformation of known collision frequencies into collision probabilities of particle pairs. The particle evolution was computed as a stochastic game, computing the time step after each collision. The simulations were validated by comparing with exact mathematical solutions for aggregation of solid particles and with numerical solutions based on sectional methods for coagulation of droplets. The direct simulation Monte Carlo (DSMC) method is advantageous, because the simulation of complex, multidimensional systems results in very elaborate models when using sectional models and is implemented very easily. Two examples of industrial importance are chemical reaction in coagulating droplets and coating of particles with small solid particles.
Use IAST with MPSD to predict binary adsorption kinetics on activated carbon
Authors: Qiao Shizhang , Hu Xijun
Published: April 2004

A model using the ideal adsorbed solution theory (IAST) coupled with the micropore‐size distribution (MPSD) concept is proposed to describe the multicomponent adsorption equilibrium and kinetics of gases in activated carbon. To overcome the thermodynamic violation, the IAST instead of the extended Langmuir equation was used to calculate the local multicomponent adsorption equilibrium. The micropore size is related to the adsorbate–adsorbent interaction energy by the Lennard‐Jones potential. The overall adsorption isotherm and the diffusion flux of the adsorbed species are the integrals of their corresponding local values over all micropore‐size distribution range accessible by the adsorbate molecules. The size exclusion effect was taken into account in the competition of different sized molecules for a given pore. The model predictions were tested with the adsorption kinetics data of binary gases on Ajax and Norit activated carbon. The results were better than predictions of a previous multicomponent adsorption kinetics model also using the MPSD concept, but with the local multicomponent adsorption isotherm described by the extended Langmuir equation.
Kinetics for the partial oxidation of methane on a Pt gauze at low conversions
Authors: de Smet Carlo R. H. , de Croon Mart H. J. M. , Berger Rob J. , Marin Guy B. , Schouten Jaap C.
Published: April 2004

Intrinsic kinetics of the catalytic partial oxidation of methane were studied at atmospheric pressure in the presence of transport limitations using a single Pt gauze. Catalyst temperature and gas‐phase temperatures were measured directly. CO, CO2, and H2O were the main products at catalyst temperatures between 1,030 and 1,200 K, residence times of 0.02 to 0.2 ms, CH4/O2 ratios between 1.8 and 5.0, and O2 conversions between 9 and 46%. A kinetic model was developed that consists of six reaction steps. Methane adsorption had to be considered as oxygen‐assisted to describe the experimental data, in particular the decrease of the selectivity for CO with increasing space times. Corresponding intrinsic kinetic parameters were obtained from the literature and from regression of the experimental CO selectivities. The calculated conversions and selectivities were obtained using the intrinsic kinetic model, together with a reactor model accounting for no transport limitations quantitatively without any adjustable parameter. Simulations indicate initial CO selectivities between 86 and 96% over the investigated range of conditions, and illustrate the relevance of both surface kinetics and mass transport in CO formation.
Multicomponent hydrogen/hydrocarbon separation by MFI‐type zeolite membranes
Authors: Dong Junhang , Lin Y. S. , Liu Wei
Published: April 2004

Separation of an eight‐component simulated refinery gas mixture including hydrogen (∼84 mol %) and light hydrocarbons (C1–C4, 7.5∼0.3 mol %) by α‐alumina‐supported polycrystalline MFI zeolite membranes was studied at 25–500°C and feed pressures of 0.1–0.4 MPa. The zeolite membrane showed excellent separation properties for rejection of hydrogen from the hydrogen/hydrocarbon mixture at <100°C. At room temperature and atmospheric pressure on both feed and permeate sides, hydrogen permeation rate is almost zero, while the hydrocarbon permeation rate is 2–4×10−4 mol·m−2·s−1. The zeolite membrane outperforms the microporous carbon membrane in terms of both selectivity and permeance for hydrocarbons over hydrogen. At 500°C the zeolite membrane becomes permselective for hydrogen over hydrogen (C1–C4). In the whole temperature range iso‐butane is nonpermeable (with a permeance below the GC analysis limit) through the zeolite membrane. Separation results of the zeolite membrane can be characterized by a solution‐diffusion model considering competitive adsorption of hydrocarbons over hydrogen and configurational diffusion in the zeolite pores. The MFI‐type zeolite membranes showed promise for applications in separation processes for hydrogen concentration/purification from various hydrogen/hydrocarbon mixtures (at lower temperatures) and in membrane reactors for dehydrogenation reactions (at high temperatures).
Characterization of batch mixers using numerical flow simulations
Authors: Jongen T.
Published: April 2004

The objective of this work is to characterize various deformations to which viscous pastes are subjected during their batch mixing and to investigate the influence of mixer configurations and operating conditions. For this, several mixer geometries were modeled in a way representative of the mixing mechanisms taking place. The twin‐blade arrangement, which is typical in batch mixing, was used in this study, as well as planetary mixers. The equations governing the deformation of the fluid were solved in the various mixer geometries, using numerical solution techniques (finite‐element discretization on a fixed mesh, with a fictitious domain method accounting for the internally moving blades). Based on the time‐dependence velocity fields obtained, the type of deformation taking place in various mixers was characterized using relevant kinematic flow classification parameters. These parameters were then related to the mechanical action of the mixer.
Transient fluidization and segregation of binary mixtures of particles
Authors: Marzocchella A. , Salatino P. , Di Pastena V. , Lirer L.
Published: April 2004

Fluidization of binary mixtures of particles belonging to group B of the Geldart classification of powders was studied. Beds tested were prepared by mixing in different proportions particles with almost equal density (≈2,500 kg/m3) and dissimilar size (125 μm silica sand and 500 μm glass beads). Experiments were carried out using a segmented fluidization column equipped with multiple pressure transducers. Experimental procedures included continuous monitoring of pressure drop at different locations along the bed during quasi‐steady or stepwise changes of gas superficial velocity, and characterization of particle‐size distributions in each segment of the fluidization column after fluidization of the bed for given times. Three ranges of gas superficial velocity were recognized for each solids mixture. At low velocity the bed behaves as a fixed bed. At high velocity, it is fully and steadily fluidized. In an intermediate velocity range, transient fluidization takes place: an initially uniform fluidized bed eventually undergoes segregation, giving rise to a defluidized bottom layer rich in the coarser solids and to a “supernatant” fluidized layer where finer particles prevail. The thresholds between these velocity ranges are rather sharp and were characterized as functions of initial bed composition. Rates at which the defluidized solids layer builds up from initially uniform beds, and the ultimate compositions of the defluidized bottom and fluidized top layers are characterized for beds with different compositions at variable gas superficial velocity.
Structural characterization of polyetherimide‐based carbon molecular sieve membranes
Authors: Sedigh Mehran G. , Jahangiri Maryam , Liu Paul K. T. , Sahimi Muhammad , Tsotsis Theodore T.
Published: April 2004

Supported carbon molecular sieve membranes (CMSMs) were prepared by the pyrolysis of a polyetherimide (PEI) polymeric precursor. The membranes were characterized by scanning electron microscopy, energy dispersive spectroscopy, and micropore analysis, using gas adsorption techniques to relate their microstructure characteristics to their transport and separation characteristics. The analysis shows that preparation conditions determine whether the carbon molecular sieve layer forms within or outside the γ‐alumina layer of the substrate. Micropore CO2 and N2 adsorption analysis of both supported and unsupported CMSMs showed a sharp peak at about (3.6–3.8) × 10−10 m using the Horvath‐Kawazoe method. For the supported CMSMs it was observed that consecutive coating/carbonization steps reduced the pore volume in the micro‐ and mesoporous regions, without greatly affecting the volume of pores in the range of (3.5–6) × 10−10 m. The reduction of the pore volume in the micro‐ and mesoporous regions is accompanied by an increase in the separation factor and a decrease in the permeance after each coating/carbonization cycle. Micropore analysis of a membrane, whose performance had degraded, indicated that its volume of pores between (3.6–6) × 10−10 m had drastically decreased from the corresponding value of the as‐prepared membranes.
Experimental measurements of benzene oxidation in supercritical water
Authors: DiNaro Joanna L. , Tester Jefferson W. , Howard Jack B. , Swallow Kathleen C.
Published: April 2004

Oxidation and hydrolysis reactions of benzene in supercritical water were investigated thoroughly using experimental measurements. Little to no reaction occurred without oxygen at temperatures between 530 and 625°C for residence times up to 6 s. Oxidation reactions were studied at temperatures ranging from 479 to 587°C, pressures of 139 to 278 bar, reactor residence times from 3 to 7 s, and with varying initial benzene concentrations from 0.4 to 1.2 mmol/L and oxygen concentrations from 40% of oxygen demand to 100% excess oxygen. To a 95% level of statistical confidence, the oxidation rate was found to be of 0.04 ± 0.07 order in benzene, 0.17 ± 0.05 order in oxygen, and 1.4 ± 0.1 order in water, with an activation energy of 270 ± 10 kJ/mol. The primary oxidation product at all reaction conditions studied was carbon dioxide. Other important oxidation products were carbon monoxide, phenol, and methane. Trace levels of additional light hydrocarbon gases and single‐ and multi‐ringed aromatic species were also detected under some conditions.
Kinetics of the partial oxidation of methane to formaldehyde on silica catalyst
Authors: Arena Francesco , Frusteri Francesco , Parmaliana Adolfo
Published: April 2004

The kinetics of the partial oxidation of methane to formaldehyde (MPO) on a “precipitated” silica catalyst in the 500–800°C range was investigated. The influence of temperature and reaction mixture composition on the density of reduced sites of the catalyst under steady state was evaluated by in situ reaction temperature oxygen chemisorption measurements. The kinetics of the MPO on silica is pseudo first and zero‐order on P and P, respectively, while the density of reduced sites depends upon the square root of the P/P ratio. The rate‐determining step is the activation of C–H bond of CH4 molecules (Ered = 142 kJ·mol−1), while oxygen replenishment is a nonactivated reaction step (Eox≈20 kJ·mol−1). A Langmuir‐Hinshelwood kinetic model, accounting for a two‐site‐dissociative activation mechanism of both CH4 and O2, thoroughly describes the steady state of the silica surface and the reaction kinetics of the MPO in the 500–800°C range.
Structural effects on adsorption of atmospheric gases in mixed Li,Ag–X‐zeolite
Authors: Hutson Nick D. , Yang Ralph T.
Published: April 2004

Silver is known to strongly affect the adsorptive properties of zeolites. In the synthesis of mixed Li,Ag low‐silica X‐type (LSX) zeolite, adding very small amounts of silver and specific dehydration conditions results in a sorbent with enhanced adsorptive characteristics for air separation. The location of the extraframework silver in relation to the aluminosilicate framework is of primary importance for elucidating the effect of silver cations on the adsorptive characteristics of the zeolite. In this work, mixed Li,Ag ion‐exchanged zeolites were synthesized and treated to promote the formation of intracrystalline silver clusters. These samples were structurally characterized using Rietveld refinement of neutron powder diffraction data. Structural characterization revealed the presence of cations in a novel site II* in mixed Li,Ag–LSX zeolites that were vacuum‐dehydrated at 450°C. Cations in this site II* are more interactive with the atmospheric sorbates of interest than silver at the conventional site II location. Vacuum dehydration at 450°C induced thermal migration of Ag+ from site II to site II* and gives rise to the superior properties for air separation.
Boundary effects on a thermophoretic sphere in an arbitrary direction of a plane surface
Authors: Chen Shih H.
Published: April 2004

The thermophoresis of a sphere in a constant applied temperature gradient in an arbitrary direction with respect to a plane surface was analytically studied. The Knudsen number was assumed small to describe the fluid flow by a continuum model with a thermal creep and a hydrodynamic slip at the particle surface. The asymptotic formulas for the temperature and velocity fields in the quasi‐steady situation were obtained by a method of reflections. The plane surface can be a solid wall or a free surface. The boundary effect on the thermophoretic motion was weaker than that on the motion driven by a body force. Even so, the interaction between the plane and particle can be very significant when the gap thickness gets small. For a particle motion normal to a solid wall, plane surface effect reduces the thermophoretic velocity of the particles; however, this solid wall may be an enhancement factor on the particle migration as it is translating parallel to the wall. On the other hand, in the case of a particle migrating close to a free surface due to thermophoresis, the particle velocity can be either greater or smaller than what would exist in the absence of the plane surface, depending on the relative thermal conductivity and surface properties of the particle and its relative distance from the plant. The thickness of the thermophoretic interacting region was also evaluated by considering the thermophoretic mobility. A free surface generally exerts less influence on the particle movement than does a solid surface.
Estimating monotonic functions and their bounds
Authors: Kay Herbert , Ungar Lyle H.
Published: April 2004

A function estimator MSQUID is presented for fitting and bounding noisy data that are known to be monotonic. MSQUID augments a “backpropagation” neural network model with a set of constraints which restricts the model to monotonic functions. Model parameters are estimated using nonlinear, constrained optimization at little increase in computation over standard neural networks. It is proven that MSQUID can estimate any monotonic function and produces more accurate estimates than unconstrained optimization. These monotonic functions and their confidence bounds can be used in many fault detection and diagnosis systems.
On defining the partial control problem: Concepts and examples
Authors: Kothare Mayuresh V. , Shinnar Reuel , Rinard Irven , Morari Manfred
Published: April 2004

Many complex chemical processes having a large number of process variables and poorly understood models can be controlled reasonably well by controlling only a small subset of process variables using an equally small number of manipulated variables. This is the central premise of this article and is referred to as partial control. Knowingly or unknowingly, this idea has been and continues to be applied to successfully control numerous complex industrial processes. Despite its widespread use, partial control has never been explicitly formulated. The partial control problem is defined. A number of terms is introduced such as process variable dominance, modelable responses, practical degrees of freedom, and sufficiency of partial control. The new framework allows incorporation of both engineering‐based decisions and more rigorous theoretical tools to achieve the goals of partial control. A number of practical examples illustrate the applicability of these concepts.
Comparing batch column configurations: Parametric study involving multiple objectives
Authors: Kim Ki‐Joo , Diwekar Urmila M.
Published: April 2004

The principal objective of this study is to advance the state of the art in batch distillation synthesis by providing insights into the behavior of various column configurations, including emerging column designs, using systematic parametric performance indices. Three competing column configurations—the rectifier, the stripper, and the middle vessel column—were compared in terms of various performance indices. Performance indices used include product purity and yield, design feasibility and flexibility, and thermodynamic efficiency. Product purity is related to profitability when yield is constant. Feasibility indices based on the number of plates and the reflux ratio show whether or not such a high‐purity configuration is flexible for changing design and operating conditions. Thermodynamic efficiency provides a measure of effectiveness of heat exchange around the specific column configuration. Trade‐offs between these performance indices are presented.
Simulation of an inert membrane reactor for the synthesis of maleic anhydride
Authors: Pedernera M. , Mallada R. , Meneéndez M. , Santamaría J.
Published: April 2004

A simulation of an inert membrane reactor in the partial oxidation of butane to maleic anhydride in high butane concentrations (2–13%) is presented. A porous membrane was used to distribute oxygen to a fixed bed of VPO catalyst, to which butane was fed. Two different reactor configurations were simulated: a standard inert membrane reactor, in which oxygen permeates inward from the shell side and the catalyst is packed on the tube side; an outward flow inert membrane reactor, in which the catalyst bed is packed on the shell side and oxygen flows outward from the tube side. The model explains the different performances of both types of reactor in terms of the interaction between the oxygen and hydrocarbon concentration profiles and the temperature profiles that are obtained with both reactor configurations.
Influence of preheating rate on kinetics of high‐temperature gas–solid reactions
Authors: Thiers Laurent , Leitenberger Bryan J. , Mukasyan Alexander S. , Varma Arvind
Published: April 2004

The influence of preheating rate on kinetics of high‐temperature reactions in gas–solid systems was investigated by a computer‐assisted electrothermography technique. Two systems were investigated: Nb–N2 and Ti–N2, in the temperature ranges 2,100–2,500 K and 1,350–1,600 K, respectively. The results showed that for both systems, reaction kinetics are enhanced by increasing the preheating rate to the desired reaction temperature. Analysis of the experimental data in the context of the sharp‐interface model showed that the activation energy of the diffusion coefficient does not depend on heating rate, while the preexponential factor is affected. Additional experiments suggest that nonisothermal preheating, even though of short duration, about 10 ms, influences chemical reactivity of the solid reactant by changing its specific surface area.
Phase equilibria for chain‐fluid mixtures near to and far from the critical region
Authors: Jiang Jianwen , Prausnitz John M.
Published: April 2004

A recently established crossover equation of state for pure chain fluids is extended to chain‐fluid mixtures. Far from the critical region, where density fluctuations are small, it reduces to the classic equation of state. Near or at the critical point, the crossover equation of state incorporates contributions form long‐wavelength density fluctuations. Using segment–segment parameters for pure components and one cross parameter k12 for every binary pair as obtained from vapor–liquid equilibrium data remote from the critical region, this crossover equation of state gives vapor–liquid equilibria in good agreement with experiment for binary asymmetric mixtures of n‐alkanes far from, near to, and at the critical point.
Morphological evolution of thick wax deposits during aging
Authors: Singh Probjot , Venkatesan Ramachandran , Fogler H. Scott , Nagarajan N. R.
Published: April 2004

The presence of waxes in crude oil can lead to the formation of wax deposits on the walls of cold subsea pipelines, which restricts flow and can lead to plugging of the pipelines. This problem has recently become more significant as the production wells move further offshore causing the oil to be cooled below the cloud point before reaching shore. Wax deposition was studied in the laboratory under conditions simulating the deposition in a subsea pipeline. Wax deposition is initiated by the precipitation of wax directly on the pipe wall and the formation of a network of' wax crystals (wax‐oil gel) with significant amounts of oil trapped in it. Radial diffusion of the wax molecules from the bulk solution to the gel deposit causes it to simultaneously grow and age with time. The wax molecules diffusing into the gel deposit precipitate near the interface resulting in a faster aging rate of the deposit new the interface than that near the wall. This nonuniform aging of the wax deposit causes the evolution of complex morphologies of the wax deposits. The diffusion of wax molecules into the gel matrix was analyzed theoretically during the growth of the wax deposit. This mathematical model predicted the radial variation of the morphology of the wax deposit observed in the laboratory flow loop experiments along with the deposit thickness as a function of time.
Operational issues in solids processing plants: Systems view
Authors: Wibowo Christianto , Ng Ka M.
Published: April 2004

A systematic approach to deal with the operational issues in solids processing plants is presented. Potential problems in various equipment units are identified and classified according to continuum‐scale phenomena: breakage, adhesion and cohesion, compaction, bulk solids flow problems, fluid‐driven solids flow problems, and segregation. Analysis is performed in the continuum and particle scales so as to identify the causal and opposing effects of each problem. With appropriate mathematical models and empirical correlations, these effects can be related to equipment design, operating conditions, particle attributes, and material properties. The ratio of these competing effects shows whether a potential problem is a real concern and points the direction for taking preventive measures. The action plan may include changing operating conditions or equipment design of a particular unit, or making modifications to upstream equipment units. The method is illustrated with an industrial example: the production of potash.
Aggregated structure of flocculated asphaltenes
Authors: Savvidis Theophylaktos G. , Fenistein Denis , Barré LoïC , Béhar Emmanuel
Published: April 2004

Many petroleum processing problems are related to asphaltene flocculation. A detailed understanding of the colloidal structure of asphaltenes in oild can play a decisive role in improving processing facilities and/or operating conditions. The structure of the flocculated part of the asphaltenes of a crude oil far above the flocculation threshold was studied to understand its links with macroscopic phase separation. Asphaltene filtrations were performed using filter pore sizes from 0.025 to 10 μm. According to small angle X‐ray scattering (SAXS) spectra and density measurements, these extracted asphaltenes in toluene solutions did not have any physical differences. Measurements of nanometric sizes for the dispersed particles after dissolution in toluene show that micron‐size flocculated asphaltenes are strongly aggregated structures. Therefore, filtration experiments separate asphaltenes corresponding powders show spherically shaped aggregates of micron size. SAXS and USAXS techniques were used to investigate the internal structure of the asphaltene powder. The aggregates are clearly a compact organization of asphaltenic material. This dense structure explains why flocculated asphaltenes are subject to sedimentation, which induces the visible macroscopic phase separation.
Laminar flow of four gases through a helical rectangular duct
Authors: Berg Robert F. , Tison Stuart A.
Published: April 2004

Measurements and a model of gas flow through a helical duct of rectangular cross section are reported. Observations of the rate of rise of pressure in a known volume located downstream from the duct yielded molar flow rates of helium, nitrogen, argon and sulfur hexafluoride. The model predicts flow rates from the duct's entrance pressure, exit pressure, and temperature. It accounts for the gas's nonideal equation of state, gas expansion along the duct's length, the increase of kinetic energy near duct's entrance, and slip. After adjusting only one parameter (the duct's height), the model agrees with the data to within 0.2% in the range of Reynolds number 0.08<Re<40. To extend the model to Re = 1,000, the centrifugal effects were accounted for by rescaling Targett et al.'s (1995) numerical calculations. The extended model agrees with the data to within 0.2% in the range 40 < Re < 400 and to within 1% in the range 400 < Re < 1,000.
New correlation for the capture cross section in high‐gradient magnetic separation
Authors: Ebner Armin D. , Ritter James A.
Published: April 2004

A new correlation between the capture cross‐section parameter (λ) and principal variables involved in high‐gradient magnetic separation (HGMS) was developed. It is based on the use of an alternative dimensionless parameter, β, in lieu of the traditional Um/UO, which facilitated the estimation of λ over a much broader spectrum of variables. This new correlation is far more versatile than correlations and approximate models in the current literature, which are only applicable to specific situations in HGMS systems. Overall, this new correlation should be extremely useful in the design, development, optimization and modeling of HGMS systems.
Oxidative coupling of methane on improved bismuth oxide membrane reactors
Authors: Zeng Y. , Lin Y. S.
Published: April 2004

Fluorite‐structured yuria‐bismuth oxide membranes with improved chemical stability were prepared by doping samarium in the lattice. The samarium‐doped yuria‐bismuth oxide (BYS) membranes are in fluorite structure, with high oxygen permeability and good catalytic properties for oxidative coupling of methane (OCM). The oxygen partial pressure dependence of oxygen permeation flax shows a shift of the rate‐limiting step from the suface reaction to the hulk electron‐hole conduction as temperature increases from 750 to 950°C. OCM reactions were conducted in a membrane reactor made of the disk‐shaped BYS membrane. Merhane‐ and oxygen‐containing streams are fed into the opposite sides of the BYS membrane to allow OCM reactions to occur on the membrane surface exposed to methane. The membrane reactor gives C2, yields of 4–11% with C2, selectivity up to 74%. The performance of this membrane reactor is closely related to oxygen permeation properties of the BYS membrane. Experimental results show the effectiveness in improving the C2 yield by increasing the membrane surface area to reactor volume ratio. In this disk‐shaped membrane reactor the highest C2, yield achieved is 17%, with a C2 selectivity of about 80%.
Viscosity of liquid hydrocarbons and their mixtures
Authors: Korsten Hans
Published: April 2004

In the petroleum industry, the viscosity of mixtures containing thousands and millions of chemical species must be predicted over large temperature ranges. Traditional models are empirical based on bulk properties. If such models are applied to mixtures other than those for which they were developed, large errors may occur. New methods proposed calculate the viscosity of petroleum fractions over large temperature ranges, using a new mixing rule reflecting observed positive and negative mixing effects. The temperature range of equations for calculating pure component viscosity is extended to the entire liquid‐phase range. If solid components are dissolved in the liquid, the viscosity increases sharply with further temperature reduction. This effect is captured in the new model by incorporating the freeze point distribution. For converting between absolute and kinematic viscosity, a new equation is proposed for the temperature effect on density.
Critical behavior in quenched random structures: Mean‐field lattice‐gas approach
Authors: De S. , Shapir Y. , Chimowitz E. H. , Kumaran V.
Published: April 2004

A new mean‐field equation‐of‐state model is proposed for predicting the critical behavior of fluids confined in porous, random structures. The approach is based on a lattice‐gas formalism and incorporates effects of both fluid confinement and energetically heterogeneous interactions between fluid molecules and pore surfaces. The model was used to predict a variety of thermodynamic properties in these systems, including the dependence of the confined fluid's critical properties on the porosity and relative strength of fluid–fluid and fluid–pore interaction energies. The study of suface‐energy heterogeneities show that they significantly affect the critical temperature of the confined fluid, at a given porosity, compared to the uniform energy case. Comparison of the model performance with both grand canonical Monte Carlo simulation results and a set of adsorption data in a silica gel suggest that the approach taken here provides a useful analytic method for calculating physical properties in complex systems of this kind.
Mixing in a cross‐flow‐impinging jet reactor
Authors: Nadeau Patrice , Berk Dimitrios , Munz Richard J.
Published: April 2004

The mixing of a tracer in a reactor composed of two opposed jets perpendicular to a main stream is studied both experimentally and theoretically. For the experimental conditions, the gas flow was in a transitional state between laminar and turbulent due to the interaction of the mixed streams. Both the measurements and the mathematical model focus on the fluid flow (gas velocity) and the dispersion of a nonreacting species (tracer concentration) injected through one of the reactor streams. Two models are tested against experimental data: a laminar flow model (giving a time‐dependent solution) and a standard k‐ϵ turbulent flow model. These two models give quite different results, both qualitatively and quantitatively. This study demonstrates the importance of the selection of an appropriate mathematical model and computational domain.
MC simulation of aerosol aggregation and simultaneous spheroidization
Authors: Rosner Daniel E. , Yu Suyuan
Published: April 2004

A companion article by Tandon and Rosner (1999) showed that Monte‐Carlo (MC) simulation methods can generate joint pdfs of particle volume, v, and surface area, a, for coagulating populations of suspended nonspherical particles simultaneously sintering at finite rates. For continuum‐regime Brownian coagulation at times longer than the characteristic coagulation time, a “self‐preserving” asymptotic pdf shape was shown to extend to these bivariate populations, using the rescaled volume v/v̄(t) and rescaled area a/ā(t) as “similarity” variables. This article considers the corresponding problem of coagulation in the “free‐molecule” limit, relevant to the atmospheric pressure flame experiments of Xing et al. This combination of coagulation‐ and sintering‐rate laws again leads to “self‐preserving” rescaled jpdfs, explicitly time‐independent for an isothermal environment, again dependent on a Damköhler‐like number, Damf (ratio of coagulation time to fusion time). While flames are more complex than our idealized simulations, our present pdfs and associated “mixed moments,” capture important features of experimental results as one approaches the flame, especially evolution of a mean particle “shape factor,” mean number of spherules per aggregate, and narrowness of the spread of spherule sizes. This agreement, coupled with the versatility of MC‐methods for multistate variable particle population balances, encourages extensions leading to a versatile, tractable, formalism for the developing field of sol reaction engineering.
Modeling of a novel vibrospring particle‐size distribution analyzer
Authors: Mahgerefteh Haroun , Shaeri Ali
Published: April 2004

The design and modeling of a novel technique for particle‐size distribution analysis are described. In its most basic form, the unit comprises a horizontally held closed‐coil helical spring that is partly filled with the test powder and sinusoidally vibrated in the transverse direction. Particle‐size distribution data are obtained by stretching the spring to known lengths and measuring the mass of the powder discharged from the spring resonant frequency. The spring's vibration characteristics, including the dynamic vibration profile as well as its first harmonic resonant frequency, are predicted using a viscoelastic model, which is in turn validated by comparison with experimental data. Particle‐size distribution data are reported in conjunction with a variety of powders including glass Ballotini, grit stone, sand, and sawdust in the size range 30–800 μm. A typical reproducibility is ±4%.
Novel sampling approach to optimal molecular design under uncertainty
Authors: Tayal Manish C. , Diwekar Urmila M.
Published: April 2004

For a reliable optimal molecular design, imprecision associated with property‐prediction models cannot be neglected. This study presents a novel sampling approach to stochastic optimization, incorporating property‐prediction uncertainty effects in a robust, generalized optimization framework. Detailed uncertainty analysis addresses, through nine case studies, various issues in computer‐aided molecular design under uncertainty. Results indicate that property‐prediction uncertainty can significantly impact the optimal molecular designs. Additional complex cases with nonlinear or black‐box models, nonlinear objective function and constraints, and nonstable distribution for model parameter uncertainty representation highlight the flexibility and versatility of this approach. Sensitivity analysis of uncertainties in the model parameters was also made possible in this generalized framework. Uncertainties, the focus of any future research, were identified through this critical model. Increased computational efficiency of this approach and wider applicability to solve problems involving various kinds of objective functions and constraints, and different forms of uncertainties, is illustrated in the context of polymer design case studies.
Backone family of equations of state: 2. Nonpolar and polar fluid mixtures
Authors: Weingerl Ulrike , Wendland Martin , Fischer Johann , Müller Andreas , Winkelmann Jochen
Published: April 2004

In the BACKONE equations the Helmholtz energy F is written as the sum F = FH + FA + FD + FQ, where FH is the hard‐body contribution, FA the attractive dispersion force contribution, FD the dipolar, and FQ the quadrupolar contribution. In Part 1 of this article (Müller et al., 1996a) the construction of BACKONE and its application to pure fluids were presented. In the extension to mixtures, for each term of F a specific mixing rule is used. This concept requires only one adjustable state‐independent binary mixture parameter. To demonstrate the feasibility of this approach, predictions of phase equilibria, including a liquid–liquid equilibrium and other thermodynamic properties, are given for 20 binary fluid mixtures from the groups: nonpolar + nonpolar; nonpolar + dipolar; nonpolar + quadrupolar; and polar + polar.
Rheooptical determination of aspect ratio and polydispersity of nonspherical particles
Authors: Vermant J. , Yang H. , Fuller G. G.
Published: April 2004

A rheooptical method proposed rapidly determines the aspect ratio and polydispersity of small axisymmetric, nonspherical particles. The time evolution of the average orientation angle of optically isotropic, nonabsorbing particles upon inception of a shear flow was monitored by a polarization modulation method by using dilute suspensions of the particles. Since the orientation angle is a geometric property of the particles, no arbitrary assumptions on the scattering mechanism had to be made to analyze the data. The period of the damped oscillatory response of the orientation angle is related to the average aspect ratio. In the limit of strong flows the damping only depends on dispersion in particle shape. An analytical expression to relate the polydispersity of the equivalent hydrodynamic aspect ratio to the damping function is presented, as well as the applicability of the technique by studying the hydrodynamic aspect ratio and associated polydispersity in a sample containing ellipsoidal hematite particles.
Influence of internal convection during microwave thawing of cylinders
Authors: Basak Tanmay , Ayappa K. G.
Published: April 2004

Numerical simulations were carried out for microwave thawing of 2‐D cylinders of pure materials with internal convection in the liquid regions. Enthalpy formulation of the energy balance equation was used with a superficial mushy region around the melting point. Electric field, energy and momentum balance equations were solved using the Galerkin finite‐element method with the penalty finite‐element formulation of the momentum balance equation. Microwave power absorption, temperature, and stream functions were studied for various cases. For samples of diameter D, thawing was contrasted between samples for 0.032 < D/Dp < 3.73 and 0.10 < D/λm < 1.58. These ratios were computed based on the liquid‐phase penetration depth Dp and wavelength of microwave radiation in the medium λm. In all cases, Pr = 0.5 was used and the Rayleigh number varied from 1.067 × 103 for the smallest diameter to 1.33416 × 105 for the largest sample (D = 2 cm). Thawing was contrasted for MWs being incident from the top and bottom faces of the cylinder and with the thawing dynamics in the absence of convection in the liquid. Our simulations indicate that convection plays a small role for D/Dp ≪ 1 and thawing is independent of the direction of MWs. At intermediate values of D/Dp where a strong maximum occurs in the power, the influence of convection with primary and secondary cell formation in the liquid regions was a strong function of the direction of incident microwaves. In the presence of multiple connected thawed regions convection was suppressed.
Controlling bubble coalescence in a fluidized‐bed model using bubble injection
Authors: de Korte Robert Jan , Schouten Jaap C. , van den Bleek Cor M.
Published: April 2004

Gas–solids fluidized beds are characterized by the presence of gas voids or bubbles causing a mass‐transfer resistance. Enhancing the mass transfer from the bubbles to the emulsion phase by reducing the bubble size can be advantageous for the chemical performance (like the conversion and selectivity) of the reactor. A model using a 2‐D version of the dynamic bubble model of Clift and Grace (1970, 1971) where bubble trajectories are predicted based on an analytic expression for the flow field around a bubble was studied. Bubble coalescence can be reduced by injecting bubbles in certain patterns, which leads to a reduction of the average bubble size higher in the bed. A simple proportional feedback control method to force bubbles on a horizontal line made it possible to generate a bubble injection pattern automatically, leading to smaller bubbles than without the feedback control method. The principle of the controlled bubble injection could lead to improved gas distributor designs.