bluuebunny/crossref_metadata_2025 huggingface dataset:
Analysis of parallelism in modular flowsheet calculations
Authors: Chimowitz E. H. , Bielinis R. Z.
Published: June 2004

An analysis of parallelism for modular process flowsheet calculations is given. The central idea is to examine various algorithms that allow one to decouple and decentralize flowsheet calculations for computation in a multiprocessor environment. Specific algorithms are evaluated which make use of quasi‐Newton updates to converge the flowsheet equations. From the analysis presented and example problems solved, it is clear that parallel processing can be used to enhance the computational speed of modular flowsheet calculations. In particular, various formulations of the simultaneous modular approach are analyzed to show how parallelism may be taken advantage of in flowsheet solutions.
Modeling of bubble column slurry reactors for multiple reactions
Authors: Bukur D. B. , Zimmerman W. H.
Published: June 2004

A model of an isothermal bubble column slurry reactor for an arbitrary reaction network of R independent reactions and S reacting species has been developed. The model accounts for gas velocity variations due to a change in total number of moles during reaction, and for nonuniform catalyst distribution due to gravity settling. It is assumed that the gas phase is in plug flow and the batch liquid phase is unmixed, which is characteristic of pilot plant reactors with high aspect ratios. The minimum number of differential equations required for the complete description of the system is shown to be equal to the total number of independent reactions. A system of two series‐parallel reactions, representative of Fischer‐Tropsch synthesis, was used to study the effect of catalyst dispersion and model parameters on reactor performance. It was found that nonuniform catalyst distribution generally has a negative effect on conversion.
Xylene isomerization using zeolites in a gradientless reactor system
Authors: Ma Y. H. , Savage L. A.
Published: June 2004

A gradientless reactor was used to study the xylene isomerization reaction over a series of cation‐exchanged (H+, Li+, Na+, and K+) ultrastable faujasite and on ZSM5 and silicalite to observe the effects of site modification on the reaction. Adsorption, diffusion, and kinetic parameters were determined in the presence of the reaction; the results generally agreed with the available literature data. The gradientless reactor and associated modeling techniques are shown to be effective tools in investigating catalyst behavior. Precise analytical techniques are required to directly measure the diffusion coefficient in the presence of reaction.
Equilibrium‐limited periodic separating reactors
Authors: Vaporciyan Garo G. , Kadlec R. H.
Published: June 2004

A new unit operation is presented that utilizes a rapid feed pressure swing cycle in a bed packed with catalyst and adsorbent to effect both reaction and separation. This hybrid device combines features of a cyclic‐steady‐state pressure swing adsorber with those of a flow‐forced catalytic reactor. Feed sequences for the periodic separating reactor (PSR) are those of rapid, single‐bed pressure swing adsorbers (PSA). Only the case of extremely fast reactions is considered here. A perturbed reaction‐sorption equilibrium model is formulated and solved for isothermal operation for different equilibrium constants and reaction stoichiometries. The capacity and separation performance for an equilibrium‐limited PSR (EPSR) can be of the same order of magnitude as PSA alone. For reactions involving a single reactant or single product, the principal component in a particular exit stream depends upon both the reaction stoichiometry and feed fraction of the process cycle. The pressure dependency of the reaction equilibrium expression is the cause of separation reversals as parameters are varied.
Mineral dissolution, precipitation, and ion exchange in surfactant flooding
Authors: Krebs R. , Sardin M. , Schweich D.
Published: June 2004

The transient retention of an anionic surfactant below the critical micellar concentration flowing in an argilo‐calcareous sandstone is shown to depend mainly upon: (1) the dissolution of trace amounts of calcium carbonate contained in the sandstone and (2) precipitation and redissolution of the surfactant with calcium ions due to carbonate dissolution or ion exchange on the clays. The adsorpton process on solid surfaces is secondary, due to a high pH resulting from the presence of carbonates. A predictive model is developed. It is compared with typical surfactant breakthrough curves and demonstrates that dissolution of calcite is the main process affecting the transport of surfactant close to the injection point.
Adsorption chromatography of proteins: Determination of optimum conditions
Authors: Yamamoto Shuichi , Nomura Masaki , Sano Yuji
Published: June 2004

A simple and convenient method for predicting optimum chromatographic conditions from linear gradient elution experiments with a small column is presented and verified experimentally. The method was found to be very useful for a rapid survey of the nature of unknown proteins, for the estimation of optimum chromatographic conditions, and for scaling up the separation of proteins in ion exchange and hydrophobic interaction chromatography.

Two applications were successfully carried out on the basis of the proposed method: (1) Separation of a protein mixture of ovalbumin and β‐lactoglobulins A and B with both anion and cation ion exchange columns by means of column switching technique. In this method, the sample is always subjected to the elution process until it is collected as a purified fraction through the two columns. A special valving system is devised for this method. (2) Large‐scale ion exchange chromatography separation of β‐galactosidase. Linear gradient elution of crude β‐galactosidase was carried out with a 30 L ion exchange gel column on the basis of data obtained with a small (23 mL) column.
Effectiveness enhancement and reactant depletion in a partially wetted catalyst
Authors: Harold M. P. , Ng Ka M.
Published: June 2004

A one‐dimensional model is developed to describe reaction between a nonvolatile liquid reactant and a dissolved gas reactant in an isothermal catalytic pellet partially wetted by a flowing liquid film. The kinetics are assumed to be first order and zero order with respect to the dissolved gas and liquid reactant, respectively. The model applies to cases in which there are negligible intraparticle gradients in the direction normal to the wetted surface. A modification of the overall transport coefficients enables the model to approximate cases in which these gradients are important, for both washcoated and uniformly impregnated catalysts. The analytical solutions enable an efficient examination of the interplay between the reaction and several mass transport processes. Conditions are determined for which the catalyst effectiveness is maximized at an intermediate wetting efficiency. It is shown that the maximum is a result of two counteracting processes. As the wetting efficiency is decreased from unity the effectiveness increases if the supply of the gas reactant is more effective on the nonwetted than the wetted part; i.e., effectiveness enhancement. However, if the wetting efficiency is sufficiently reduced, the excess liquid reactant depletes within the pellet, resulting in a decreased effectiveness. A criterion is derived that predicts the minimal activity necessary to initiate depletion of the liquid reactant for a given wetting efficiency. This is useful for determining the conditions for which the common literature assumption of an excess liquid reactant is violated. The model shows good agreement with published data in which the overall rate exhibits a maximum for an intermediate liquid flow rate.
Organic liquids in a bubble column: Holdups and mass transfer coefficients
Authors: öztürk S. S. , Schumpe A. , Deckwer W.‐D.
Published: June 2004

Gas holdups and volumetric mass transfer coefficients kLa are studied in a bubble column of 0.095 m dia. with 50 different gas‐liquid systems comprising pure and mixed organic liquids and various gases. The gas holdups measured in pure organic liquids can be represented by existing correlations but the values in liquid mixtures are high. On the other hand, all mass transfer coefficients can be correlated uniformly by a new dimensionless equation, with 13.3% mean error.
Emulsion polymerization reactor stability: Simplified model analysis
Authors: Rawlings J. B. , Ray W. H.
Published: June 2004

A class of simplified emulsion polymerization models is developed. These models include the distribution of particle sizes and radical numbers, and the coupling of the particle initiation rate to the particle size distribution. An analytical solution to the modeling equations is found from contraction mapping. In addition, necessary and sufficient conditions for local stability are derived. These result in a simple criterion for the onset of oscillations in terms of kinetic rate constants and reactor operating conditions. The results clearly show that radical desorption, chain transfer agents, and certain types of impurities should have a strong influence on reactor stability.
Partial oxidation of CH4 to HCHO over a MoO3‐SiO2 catalyst: A kinetic study
Authors: Spencer N. D. , Pereira C. J.
Published: June 2004

There is considerable economic incentive to convert methane into formaldehyde, which is used in the manufacture of insulating materials and adhesives. This paper discusses the kinetics of a one‐step oxidation of methane with molecular oxygen over a MoO3‐SiO2 catalyst. The space velocity was varied between 2,500 and 10,000 h−1 (at NTP) over a range of temperatures from 848 to 923 K.

Selectivities to formaldehyde between 30 and 89% were observed in the 0–5% methane conversion range, the other principal products being CO, CO2, and H2O. Kinetic analysis indicates that methane is directly oxidized to CO2 and HCHO, with HCHO being further oxidized to CO. The reactions were all found to be zero order in oxygen concentration, with methane and formaldehyde oxidation following overall first‐order rate laws. Kinetic rate constants have been determined and there is good agreement between model predictions and experimental data.
Equilibrium properties of liquids containing supercritical substances
Authors: Campanella E. A. , Mathias P. M. , O'Connell J. P.
Published: June 2004

A two‐parameter, corresponding‐states form for direct correlation function integrals in liquids is applied to many types of systems with one substance being supercritical. This gives a quantitative correlation for volumetric behavior, pure and mixed solvent Henry's constants, and Henry's Law deviations over wide ranges of conditions using only pure‐component information and a single binary parameter. Details of the method and results are given for H2, N2, CO, CH4, and other gases in pure and mixed simple, polar, and aqueous solvents and coal oils. Because of its reliable predictive capability, the present approach can serve as a generator of data for parameter estimation by other models such as equations of state.
Hindered settling of semidilute monodisperse and polydisperse suspensions
Authors: Davis R. H. , Birdsell K. H.
Published: June 2004

A light‐extinction principle was used to measure the fall speeds of the interfaces that develop during gravity sedimentation of monodisperse, bidisperse, and tridisperse suspensions of noncolloidal particles with small particle Reynolds numbers in the semidilute total particle volume fraction range 0.0003 ⩽ c0 ⩽ 0.15. For monodisperse suspensions, the hindered settling velocity of the interface at the top of the suspension was found to be well represented by the correlation of Richardson and Zaki, provided that the isolated particle fall speed was chosen by linear extrapolation of the data to c0 = 0, with 90% confidence intervals on the exponent of n = 5.0 ± 0.1. For dilute monodisperse, bidisperse, and tridisperse suspensions, the hindered settling velocities of the interfaces showed agreement within experimental uncertainty with the theory of Batchelor, which predicts that the settling velocity decreases linearly with increasing particle concentration and which is based upon pairwise particle interactions.
Dispersion‐free solvent extraction with microporous hollow‐fiber modules
Authors: Prasad R. , Sirkar K. K.
Published: June 2004

Extensive studies on dispersion‐free solvent extraction have been carried out using modules made with either hydrophobic or hydrophilic microporous hollow‐fiber membranes. Membrane and boundary layer resistances have been characterized for both kinds of hollow fiber using solvent extraction systems with a wide variation of distribution coefficients and interfacial tensions. It has been found that the Graetz solution for a constant wall concentration describes satisfactorily mass transfer on the lumen side of a hollow‐fiber device. A correlation of the form NSh = [Dh(1 − ϕ)/L]NN appears to provide a close fit to the shell‐side mass transfer coefficient data. The perforamnce characteristics of dispersion‐free extraction in hollow‐fiber modules have been considered against those of commercial packed‐bed extractors. A perspective has been provided on comparative utilities of hydrophobic or hydrophilic hollow fibers for a given solvent extraction problem.
Oxygen transfer between metals and oxygen‐ion conducting supports
Authors: Metcalfe I. S. , Sundaresan S.
Published: June 2004

The charge‐transfer reactions in the three‐phase interfacial region, between a platinum catalyst, an yttria‐stabilized zirconia (YSZ) support, and the gas phase, were studied electrochemically during carbon monoxide oxidation. From electrochemical and reaction‐rate measurements, it appeared that oxygen was being removed from the support by carbon monoxide adsorbed on the platinum, while being supplied to the support by atomic oxygen adsorbed on either the platinum or the YSZ.

Highly disperse Pt/YSZ catalyst outperformed a Pt/Al2O3 catalyst, the rate difference being explained in terms of interfacial reactions. Even in the presence of species such as water and sulfur dioxide, catalysts with YSZ supports outperformed those with alumina supports.
Influence of secondary flows on the stability of chemically reacting systems
Authors: Gatica J. E. , Viljoen H. J. , Hlavacek Vladimir
Published: June 2004

A model is presented for convection and chemical reacton in porous media. An irreversible chemical reaction of arbitrary order is considered. Reactant depletion allows for basic solutions in either the kinetic or the diffusion regime. The cases of forced flow parallel to the lateral walls of the cavity and a closed system are addressed. A linear stability analysis of the basic states is performed and critical values of the thermal Rayleigh number for the onset of natural convection are determined. A dispersion relation is derived and a graphical representation of the linear stability analysis results is provided for typical values of the system parameters. Analytical predictions are verified by results obtained by numerical integration of the complete set of nonlinear partial differential equations. The effect of natural convection is discussed when the basic state is either in the kinetic or in the diffusion regime. For large gradients, associated with the diffusion regime, chemical reaction can drive free convection even for low values of the Rayleigh number. In forced flow systems, natural convection can change substantially the flow pattern of the system.
Effect of nonuniform activity on hydrodemetallation catalyst
Authors: Limbach K. W. , Wei James
Published: June 2004

A model is developed to qualitatively predict the effect of a nonuniform activity distribution on the total quantity of metal deposited in hydrodemetallation catalysts. A fixed catalyst support system is assumed and no restrictions are placed on local or overall catalytic activity in seeking an optimal solution that maximizes the amount of contaminant metal deposition over the useful catalyst life. Results indicate that deposition capacity could be substantially increased by a nonuniform activity profile in which the activity at the catalyst center is higher than that at the outer edge. The shape of the optimum activity profile is presented along with practical design improvements.
Bifurcation behavior of a catalytic reaction due to a slowly varying parameter
Authors: Tsotsis T. T. , Sane R. C. , Lindstrom T. H.
Published: June 2004

Bifurcation problems with slowly varying control parameters are of interest in several research areas, including catalyst deactivation, combustion, and lasers and other nonlinear optical devices. Significant theoretical work has been devoted to the study of these dynamic phenomena, but very little directly relevant experimental information is currently available. Results are presented here from an experimental study of the dynamic behavior of a catalytic reaction system due to a deliberately slowly varied control parameter. Attention is focused on the dynamics due to slow passage through simple limit and Hopf points.
Countercurrent gas‐liquid annular flow, including the flooding state
Authors: Zabaras G. J. , Dukler A. E.
Published: June 2004

The structure of the wavy interface on a falling liquid film is studied for conditions of countercurrent gas flow in order to investigate mechanisms for flooding. Measurements taken just below the liquid feed and at 1.7 m down the tube show that under all conditions, including flooding, the waves propagate only downward and are never of such amplitude as to bridge the tube. These observations are in contrast to speculations in the literature that upward flow of waves or bridging of liquid due to waves cause flooding. In the mechanism suggested, flooding is due to flow reversal in the film just at the liquid entry.
Recovery of uranium from seawater using amidoxime hollow fibers
Authors: Saito Kyoichi , Uezu Kazuya , Hori Takahiro , Furusaki Shintaro , Sugo Takanobu , Okamoto Jiro
Published: June 2004

A novel amidoxime‐group‐containing adsorbent of hollow‐fiber form (AO‐H fiber) was prepared by radiation‐induced graft polymerization of acrylonitrile onto a polyethylene hollow fiber, followed by chemical conversion of the produced cyano group to an amidoxime group. Distribution of the amidoxime group was uniform throughout the hollow‐fiber membrane. The fixed‐bed adsorption column, 30 cm in length and charged with the bundle of AO‐H fibers, was found to adsorb uranium from natural seawater at a sufficiently high rate: 0.66 mg uranium per g of adsorbent in 25 days.
Analytical and preparative electrophoresis in a nonuniform electric field
Authors: Rolchigo P. M. , Graves D. J.
Published: June 2004

A nonuniform electric field is proposed as the basis for the batch and continuous electrophoretic separation of biomolecules. A theoretical analysis shows the advantages of these techniques over conventional separation processes that use a uniform electric field. The mathematical modeling of such processes and its implications for the experimental design of a nonuniform field separator are presented.
Effect of favorable pressure gradients on turbulent boundary layers
Authors: Finnicum D. S. , Hanratty T. J.
Published: June 2004

Large favorable pressure gradients can cause the drag of a turbulent boundary layer on a solid surface to decrease; if large enough, such gradients can cause a return to laminar flow. Scaling relations and a computational model for the viscous wall layer, recently developed in this laboratory are used to interpret this phenomenon.
On nonlinear doubly‐diffusive marangoni instability
Authors: Ho Kwok‐Lun , Chang Hsueh‐Chia
Published: June 2004

Enhancement of simultaneous heat and mass transfer due to surface tension gradient‐induced Marangoni convection is studied. The geometry considered is a thin liquid film between a solid phase and a gas phase. In regions where the two Marangoni numbers are of opposite signs (quadrants II and IV), Orr‐Sommerfeld analysis reveals the possibility of oscillatory roll cells. A nonlinear bifurcation study of the amplitude equations indicates that the finite‐amplitude oscillatory rolls are annihilated by steady rolls slightly beyond the oscillatory stability boundary in a heteroclinic bifurcation. Oscillatory rolls are hence restricted to quadrants II and IV. Finite‐amplitude instability due to subcritical bifurcation of the steady rolls is also shown to be limited to these quadrants. In addition to classifying the bifurcations, the numerical study also provides a useful correlation for single‐component flux enhancement, Nh* = 1 + 0.001324 Nu0.23Pr0.16(NT − N), which is compared to available experimental data.
Analysis of the effect of internal phase leakage on liquid membrane separations
Authors: Borwankar R. P. , Chan C. C. , Wasan D. T. , Kurzeja R. M. , Gu Z. M. , Li N. N.
Published: June 2004

A general physical model of a typical batch extraction system employing liquid surfactant membranes is developed. The model takes into account the continuous‐phase resistance and the interfacial resistance along with permeation through a composite emulsion globule. It also quantifies the loss in extraction efficiencies by leakage of the encapsulated phase due to membrane breakage. The physical model is easily adapted to apply to the case of transport facilitation wherein the solute is reacted in the internal phase to yield products incapable of permeating through the membrane phase. Experimental data on o‐chlorophenol extraction are satisfactorily correlated with the model.
Rapid coal devolatilization as an equilibrium flash distillation
Authors: Niksa Stephen
Published: June 2004

A model for the rapid devolatilization of individual coal particles is developed by analogy with a single‐stage equilibrium flash distillation. In the theory, thermal depolymerization of the coal generates aromatic fragments that are widely distributed in size. Conditions for phase equilibrium determine the partitioning of these fragments into intermediates in the condensed phase, which ultimately form char and light gas, and into tar vapor, which escapes in a stream of light gases. Comparison with data shows that the model accurately correlates the yields of noncondensible gas and tar from bituminous coals over wide ranges of temperature and pressure. The predicted molecular weight distributions of tar are also in agreement with measured distributions, and the model yields a mechanistic basis for their observed insensitivity to temperature. The tendency to form lighter tar at higher pressures is also explained.
A model for molecular weights in vinyl acetate emulsion polymerization
Authors: Lee Chan H. , Mallinson Richard G.
Published: June 2004

The continuous stirred tank emulsion polymerization of vinyl acetate has shown oscillatory behavior in conversion, particle size, molecular weight, and polydispersity during polymerization. The kinetics in vinyl acetate emulsion polymerization are especially complex because of chain transfer to polymer and monomer and the terminal double bond reaction. A nonsteady‐state model to predict the average molecular weights is developed for this polymerization system. Improved results are achieved by inclusion of the effects of polymer chain initiation and termination by absorbed radicals. It is found that the observed oscillations are closely related to the radical diffusion and desorption rates that are involved in a heterogeneous initiation mechanism. The molecular weights and polydispersities are successfully predicted by the model. The nature of the oscillations in these properties is examined.
Combined surface and pore volume diffusion in porous media
Authors: Bhatia S. K.
Published: June 2004

The recent theory for pore volume diffusion in porous media (Bhatia, 1985, 1986) is further extended to include surface diffusion with adsorption‐desorption phenomena proceeding at finite rates. The short‐range memory effects, previously discussed in terms of a correlation between successive pores traversed, are included here and are found to be important for surface diffusion as well. A tortuosity for surface diffusion is defined and found to be larger than 3, in the range of 5 to 7, because of internal correlation effects in the random network structure examined. Under conditions of gradients in pore surface area it is shown that the surface transport equation is

and the relation between the effective surface diffusivity Dse and the pore structure parameters is derived. The equation for pore volume diffusion requires further justification when adsorption‐desorption occurs at finite rates, and is also derived here.
Effect of iron impurity in zinc sulfide concentrates on the rate of dissolution
Authors: Crundwell Frank K.
Published: June 2004

The rate of dissolution of sphalerite is shown to be directly proportional to the concentration of substitutional iron impurity in the solid. This is attributed to the formation of a narrow impurity band within the forbidden band gap of the sphalerite. The impurity band is of iron d‐orbital origin. The transfer of electrons between this d‐orbital band and the oxidant is energetically more favorable than the transfer of electrons between the valence band and the oxidant. A fundamental model combining the electronic structure of sphalerite and semiconductor electrochemistry is presented. An equation is derived that describes the rate of dissolution as being first order for the concentration of iron in the solid and half‐order for the concentration of the oxidant. This is in agreement with the experimental evidence.
Modeling catalytic gauze reactors: HCN synthesis
Authors: Waletzko N. , Schmidt L. D.
Published: June 2004

The performance of the industrial HCN synthesis Andrussow reactor on a Pt gauze catalyst is simulated using rate equations for 13 simultaneous unimolecular and bimolecular surface reactions. Individual rates have been determined experimentally for reactants, intermediates, and products on polycrystalline Pt, and the model therefore contains no adjustable parameters except for reactant flux limits. Predicted selectivities of HCN formation agree quite well with those observed in commercial reactors, and a distinct optimum with feed composition is obtained near that observed experimentally. In addition, behaviors are predicted at compositions, pressures, and temperatures inaccessible to the commercial reactors. Both isothermal and nonisothermal situations are examined. Questions concerning the validity of models, reactant flux limits, and the decoupling of simultaneous surface reactions are considered.
Thermodynamics of solutions with solvent and solute in different pure states
Authors: Kopatsis A. , Salinger A. , Myers A. L.
Published: June 2004

Isoactive‐solvent (ISAC) theory is a thermodynamic framework for the case where the components of a solution exist in different pure states: liquid solvent and gaseous solutes (absorption of gases); solid solvent and liquid solutes (sorption of liquids); and liquid solvent and solid solutes (solubility of mixed electrolytes). Properties of mixed solutes are predicted from known activities of single solutes by assuming that solute‐solute (but not solute‐solvent) molecular interactions are ideal. Equations for an ISAC ideal solution are exact in the limiting case of a solution of identical solutes. The thermodynamic potential is a Legendre transformation of the Gibbs free energy: Ψ = G − μ1n1, where the solvent is component number 1. The ISAC standard state is the real single‐solute mixture.
Lumping nonlinear kinetics
Authors: Astarita Gianni , Ocone Raffaella
Published: June 2004

Lumping of the kinetics of mixtures described by a continuous distribution function of concentration is discussed for the case where individual reactions have nonlinear kinetics. The assumption of independent kinetics, which leads to a paradox, is not used. Functional‐differential equations govern the kinetic behavior of the mixture. Formal solutions are presented for a class of kinetic behavior that includes Langmuir isotherm catalysis and for the special case of bimolecular reactions. A class is defined for which the kinetic functionals can be expanded in a series of integrals for which the kernels are entirely determined. Finally, a thermodynamic analysis is presented to show that the commonly accepted equilibrium theory implies a kinetic approximation of maximal rank for the kinetics at the equilibrium point.
Estimation of the molecular weight distribution in batch polymerization
Authors: Ellis Mark F. , Taylor Tad W. , Gonzalez Victor , Jensen Klavs F.
Published: June 2004

A filtering technique is proposed for on‐line estimation of the temperature, monomer conversion, initiator conversion, and the entire molecular weight distribution in a batch methyl methacrylate polymerization reactor. The technique uses a detailed polymerization model combined with on‐line measurements of conversion, temperature, and the molecular weight distribution, taken at different discrete time intervals. The polymerization model includes a chain‐length‐dependent termination rate constant which allows the prediction of the molecular weight distribution for common free‐radical polymerization conditions. Comparisons between modeling and experimental results show that the polymerization model gives good predictions of the monomer conversion and the molecular weight distribution in the polymerization system. The performance of the estimation scheme is tested for cases of strong gel effect conditions leading to a bimodal molecular weight distribution, and poor initial conditions. Finally, off‐line experimental data are used to test the algorithm under actual reactor operating conditions.
Novel microreactor with quench system for kinetic study of propane pyrolysis
Authors: Rastogi Ashutosh , Svreck William Y. , Behie Leo A.
Published: June 2004

A novel microreactor with a rapid direct quench system was developed to study fast hydrocarbon pyrolysis reactions. Heating of reactants inside the reactor was achieved by a Curie Point Pyrolyser. The propane pyrolysis reaction was studied at four different temperature levels: 600, 700, 800 and 900 °C. Pyrolysis times were varied from 0 to 2 s between 600 to 800°C and from 0 to 1 s at 900°C. The primary reaction products were methane, ethylene, hydrogen and propylene. Using a developed and very effective quench technique increased the production of olefins and reduced the formation of methane. A first‐order kinetic analysis performed on the experimental data between 600 and 700°C yielded an activation energy of 216 kJ/mol, which is in the range of other researchers.
Theoretical study of interfacial transport in gas‐liquid flows
Authors: Back D. D. , McCready M. J.
Published: June 2004

Mass transfer in sheared, concurrent gas‐liquid flows is investigated theoretically using solutions to the unaveraged advection‐diffusion equation. For sufficiently thick films, the resistance to mass transfer is shown to be confined completely within a thin region in the liquid near the interface and mass transfer coefficients are accurately predicted by an improved numerical technique that uses a velocity field derived from an Orr‐Sommerfeld equation with the time‐varying velocity computed directly from measurements of interfacial waves. The mass transfer coefficients are shown to depend on the magnitude and frequency content of the velocity fluctuations normal to the interface. As the film thickness decreases, transfer resistance extends throughout the film and turbulent mixing in the middle of the film controls the transfer rates. For this region, limiting values of transfer coefficients are predicted well by analytical solutions to the advection‐diffusion equation, which assume a laminar flow.
Design of parallel cascade control for disturbance‐rejection
Authors: Yu Cheng‐Ching
Published: June 2004

A new approach is presented to overcome load disturbance problems. Based on the parallel cascade control structure, the primary controller is designed for the servo response, and the secondary controller is designed for the disturbance‐rejection purpose. A perfect disturbance‐rejection controller is derived for the secondary loop. Simulation studies show that improvement in load responses can be achieved for some cases. However, for some other cases, parallel cascade control simply worsens the load responses as a result of interaction between primary and secondary loops. A new interaction measure for load disturbance (γ) is proposed to determine whether improved load responses is achievable using cascade control structure. More importantly, the interaction measure also quantifies the margin of improvement that can be achieved over conventional single‐loop control. A design procedure for improved load responses was proposed. The simulation results show that the new approach offers a simple and effective alternative for disturbance‐rejection.
Study of the adsorption and desorption of multiple adsorbates in a fixed bed
Authors: Huang Chen‐Chia , Fair James R.
Published: June 2004

An experimental and theoretical study was made of the adsorption of ethane/propane mixtures from nitrogen or helium. The study also covered the desorption of the mixtures using a purge of hot nitrogen or helium. The adsorbent was Witco JXC activated carbon. A nonequilibrium, mechanistic model was developed and was shown to represent both adsorption and desorption processes. The model utilized mixture isotherm data predicted by the Flory Huggins form of the vacancy solution model. Both adsorption and hot purge regeneration were found to be intraparticle rate controlled processes, with surface diffusion dominating the intraparticle mass transfer mechanism. The dynamic model can be used for the simulation of a variety of commercial adsorption and desorption conditions.
Theories of precipitation induced by dissolution
Authors: Kopinsky Joel , Aris Rutherford , Cussler E. L.
Published: June 2004

When a porous solid is placed in acid, the solid dissolves at its surface. However, the same solid can precipitate or dissolve in pores inside the solid. The internal precipitation is a consequence of a nonlinear solubility product amplified by fast chemical kinetics. The internal dissolution can be enhanced by unequal diffusion coefficients. Both precipitation and dissolution can occur simultaneously but in different regions, mimicking the patterns of mineral found in teeth.
Surface area evolution of calcium hydroxide during calcination and sintering
Authors: Mai M. C. , Edgar T. F.
Published: June 2004

A simple one‐dimensional mathematical model validated with high‐temperature entrained‐flow reactor data successfully explained dispersed calcium hydroxide particle surface area evolution resulting from concomitant calcination and sintering. An integrated first‐order calcination rate expression and a second‐order sintering rate form accurately predicted time‐resolved surface area generation and degradation. The effects of water vapor and gaseous carbon dioxide concentration on asymptotic specific surface areas and sintering rates were noted. This methodology for predicting sorbent surface area evolution will enable more accurate sulfation modeling efforts in furnace sorbent injection (FSI) applications, where calcination, sintering, and sulfation can all occur on the same time scale.
Phase plane and bifurcation analysis of thin wavy films under shear
Authors: Sheintuch M. , Dukler A. E.
Published: June 2004

A long‐wave equation for film thickness as a function of position is derived for a general case incorporating viscous, surface tension, and interfacial shear effects. The derivation considers both the parabolic and the power‐law velocity profiles. The analysis is aimed at revealing the wave velocity that induces infinitely long (homoclinic) periods as well as substrate thickness and wave peak amplitude. Phase plane analysis shows that at Re ≫ 1, due to time‐scale separation, the homoclinic velocity is near that at the Hopf bifurcation. That enables analytical derivation of the wave characteristics.

Comparison with experimental results in the range of Re‐310–3, 100 with countercurrent gas flow, shows encouraging agreement. At very high Re the wave velocity suggests the onset of turbulence, in agreement with theory. Phase plane analysis predicts also that the wave shape consists of a simple peak with a steep front, with short waves riding on the main wave at low Re.
Slurry reactor studies of homogeneous and heterogeneous reactions
Authors: Fu Chen‐Chen , McCoy B. J. , Smith J. M.
Published: June 2004

A dynamic model is derived for simultaneous, homogeneous and heterogeneous, first‐order reactions in a slurry reactor. The theory is applied to experimental data for the oxidation (with gaseous oxygen) of aqueous sulfite solutions which is first order in oxygen and zero order in sulfite. The homogeneous reaction is very slow at low pH (2–3) where the sulfur exists as dissolved SO2 or HSO3−, even when cobalt is added as a homogeneous catalyst. At higher pH (7–8.5), considerable homogeneous reaction occurs with and without cobalt ions. Here the predominant species are SO3= and HSO3−. Activated carbon is not a heterogeneous catalyst at pH 7 or 8.5, but is responsible for nearly all the reaction at the low pH values. Analysis of the heterogeneous data indicates that the surface reaction between adsorbed oxygen and SO2 controls the rate rather than adsorption of oxygen.
Diffusivity of polymers in dilute solutions undergoing homogeneous flows
Authors: Öttinger Hans Christian
Published: June 2004

In order to describe the translational diffusion of polymers in dilute solution, an elastic dumbbell model is used for which the effects of both hydrodynamic interaction and a nonlinear connector force are treated in a self‐consistently averaged form. For this model, the diffusion tensor in the presence of a homogeneous flow field is obtained from (i) the mass flux caused by concentration gradients, (ii) the average polymer velocity caused by external forces, and (iii) the mean‐square displacement of a polymer caused by the Brownian forces. From the second and third approaches the same expression for the diffusion tensor is found, whereas a different expression is obtained from the first approach. This means that the Nernst‐Einstein equation cannot be generalized to the case of flowing solutions. The model predictions for the diffusion tensors in steady shear flow are discussed in detail.
Capillary pressure: Centrifuge method revisited
Authors: Ayappa K. G. , Davis H. T. , Davis E. A. , Gordon J.
Published: June 2004

The approximate equations of Hassler and Brunner and of van Domselaar are often used to deduce the capillary pressure curve of a porous medium from centrifuge data. The use of these equations restricts the centrifuge method to short samples. Also, these equations require differentiation of data. We report here methods to determine the capillary pressure curve by the midpoint and least‐squares solution of the fundamental equation, relating the average saturation S1 of liquid in the porous medium to the capillary pressure Pc1 at the end of the sample nearest the axis of rotation. The methods do not require differentiation of data and are not restricted to short samples. We introduce and evaluate an approximation based on an exact result derived by Rajan. This new approximation requires the same inputs as do the Hassler‐Brunner and the van Domselaar approximations, but it is accurate over a wider range of sample sizes. In addition, an approximate solution which can be used to estimate capillary pressure curves for long cores is developed.
Catalyst regeneration kinetics in the presence of pore blockage
Authors: Chang Yuli , Perlmutter D. D.
Published: June 2004

A model is developed for fluid‐solid reaction kinetics in a porous solid. The model is based on a convergent‐divergent pore structure and accounts for pore blockage and inside cavities. A numerical study applied the model to the process of coke removal for catalyst regeneration. The calculated results show that in the presence of pore occlusion, the reaction rates are significantly reduced at the beginning of reaction. At higher levels of coke loading the reaction behavior can be especially affected by the volume fraction of the divergent pores. This behavior can be used to select catalysts with pore structures that improve control of the regeneration process.
Fluid mixing with a pipeline tee: Theory and experiment
Authors: Sroka L. M. , Forney L. J.
Published: June 2004

The pipeline mixing of two fluid streams by turbulent jet injection normal to the pipeline has been studied theoretically and experimentally. A simple scaling law for the second moment of the tracer concentration within the pipeline is proposed for the first fifteen pipe diameters down‐stream from the injection point. The similarity solution is derived by assuming that the tracer diffuses in a weak compound jet moving parallel to the pipeline axis. The theoretical results are correlated with all of the available experimental measurements. The results indicate that the second moment of the tracer concentration decreases with increasing jet momentum and distance from the injection point such that M = 0.25. (lm/D)−2 (x/D)−4/3.
Determination of rate constants by the frequency response method: CO on Pt/SiO2
Authors: Li Yao‐En , Willcox David , Gonzalez Richard D.
Published: June 2004

A technique based upon transmission FTIR to obtain rate constants for adsorption and desorption over supported metal catalysts has been developed and tested. The technique requires the use of a sinusoidal perturbation function imposed on steady state linearized adsorption‐desorption kinetics. The measurement of a phase lag between the sinusoidal inlet gas phase forcing concentration and the response of surface coverage, together with a measurement of the maximum amplitudes of the forcing function and surface response enable the calculation of the relevant adsorption and desorption rate constants.

The technique has been successfully applied to the measurement of both adsorption and desorption rate constants for CO adsorbed on a 1% Pt/SiO2 catalyst. The values obtained for these rate constants at 343 K were: Ka = 0.147 s−1 and Kd = 7.28 × 10−3 s−1. A sticking coefficient corresponding to the adsorption of weakly bonded CO on Pt under conditions of high CO surface coverage was obtained.
Analysis of decentralized control structures for nonlinear systems
Authors: Manousiouthakis V. , Nikolaou M.
Published: June 2004

Configuration of feedback loops between input and output variables of a given plant constitutes a subtask of control system synthesis. In the past, this subtask has been tackled through direct decomposition of either the process or the set of input‐output variables. Linear interaction analysis has been used in both approaches to assess and minimize sub‐system interactions. Since most chemical processes exhibit nonlinear behavior, it is evident that a measure is needed for assessment of interactions in the presence of system nonlinearities. In this paper we focus on input‐output variable set decomposition and introduce the notion of nonlinear block relative gain (NBRG) as a nonlinear interaction measure. Both the statistic and dynamic versions of NBRG are discussed, and a computational procedure is presented for their evaluation. Direct simulations on a CSTR verify the interactions predicted by NBRG for different feedback configurations. Moreover, nonsymmetry of the effect of one loop on another, a fact not captured by the linear BRG, is accurately predicted by NBRG.
Phase transition and structural heterogeneity; Benzene adsorption on silicalite
Authors: Guo Chang‐Jie , Talu Orhan , Hayhurst David T.
Published: June 2004

Adsorption equilibria have been gravimetrically measured at 10, 20, 30, 50, and 70°C for benzene on pure crystalline silicalite. Isotherms change from type I to type IV with decreasing temperature, indicating heterogeneous behavior. The transition temperature is determined to be 38 ± 1°C by isobaric experiments. Silicalite is energetically homogeneous since there are no cations present. The observed structural heterogeneity is due to the comparable sizes of benzene molecules and the silicalite pore system. Silicalite is modeled as two types of homogeneous pores where a van der Waals model describes local equilibrium. Model parameters are calculated via nonlinear regressions with prescribed pore distribution determined from crystallographic considerations. Data and model predictions are in good agreement with highly significant parameters.
Pore structure and particle size effects on limestone capacity for SO2 removal
Authors: Zarkanitis Solon , Sotirchos Stratis V.
Published: June 2004

A detailed investigation of the dependence of the transient behavior of limestone particles reacting with SO2 in the presence of oxygen on the pore structure properties of the calcined solid is presented. Experimental reactivity and pore structure data were obtained for two highpurity limestones that yield calcines of qualitatively similar distribution of pore size but of different capacity for SO2 removal. Experiments were carried out over a wide range of particle size and temperature so as to investigate the effects of intraparticle mass transport limitations on the process. The results obtained showed strong dependence of the transient behavior of reacting particles of calcined limestone, and consequently of their sorptive capacity for SO2 removal, on their initial pore size distribution and on the resistance for intraparticle mass transport—which is also determined to a certain extent by the pore size distribution. The experimental results were found to be in agreement with the predictions of a mathematical model for gas‐solid reactions with solid product involving porous solids of distributed pore size. The findings point to the conclusion that the evaluation of limestones proposed for use as sorbents for controlling SO2 emissions should be based on the size distribution and interconnectedness of the pores of their calcines and not only on their average structural properties (porosity and surface area).
Feedforward and feedback control of a solution copolymerization reactor
Authors: Congalidis John P. , Richards John R. , Ray W. Harmon
Published: June 2004

A control system is designed for a copolymerization reactor using a combination of feedforward, ratio, and feedback control to regulate polymer production rate, copolymer composition, molecular weight, and reactor temperature. The solution copolymerization of methyl methacrylate and vinyl acetate in a continuous stirred tank reactor is used as an illustrative example with the kinetic parameters and reactor operating conditions obtained from the literature. The process includes equipment to recycle unreacted monomers and solvent. The recycle stream introduces disturbances to the reactor feed, which perturb the polymer properties. A feedforward control strategy is proposed to counter these disturbances, and its effectiveness is demonstrated using the model. The mathematical model is used to investigate input/output control pairings in order to identify the fundamental nature of the solution copolymerization control problem and to determine the best control system structure. The combined feedforward/feedback strategy is tested on a nonlinear model of the process for set point changes and compensation of unmeasured reactor disturbances. The performance of the control system design was quite good, and such designs have been found successful in plant operations.