bluuebunny/crossref_metadata_2025 huggingface dataset:
Effect of relative solubility on amino acid crystal purity
Authors: Givand Jeffrey C. , Teja Amyn S. , Rousseau Ronald W.
Published: April 2004

The ratio of the pure‐component solubility of a primary solute to that of an impurity in the same solvent has a dramatic impact on crystal purity when lattice substitution is the dominant mechanism of impurity incorporation in the crystal. This is demonstrated for a model system of isoleucine and leucine in which leucine is the impurity. The pure‐component solubility ratios of isoleucine to leucine in mixed solvents were measured and used to select solvents that could affect the lattice substitution of leucine in isoleucine crystals. Addition of electrolytes and/or cosolvents to aqueous solutions of the two amino acids improved the purity of isoleucine crystals obtained by cooling.
Multicomponent thermally coupled systems of distillation columns at minimum reflux
Authors: Fidkowski Zbigniew T. , Agrawal Rakesh
Published: April 2004

A method for calculating the minimum vapor flow in a ternary fully thermally coupled system of distillation columns was developed earlier by Fidkowski and Krolikowski. This method was developed further for fully thermally coupled systems separating four or more components. For multicomponent mixtures having constant relative volatilities and equal latent heats, the method leads to the identification of all of the minimum vapor flow rates in each distillation column that provide the same overall minimum heat duty in the reboiler. The quaternary fully coupled system of columns requires less heat energy than conventional configurations. These savings are often on the order of 20–50%.
Oxygen barriers that use free radical chemistry
Authors: Yang Chuanfang , Cussler E. L.
Published: April 2004

Oxygen barriers made of linoleic acid, ethyl cellulose and a cobalt catalyst can reduce oxygen penetration over 2,000 times compared with an ethyl cellulose barrier of the same thickness. The barrier is effective only when the product of linoleic acid concentration and barrier thickness exceeds a critical value, which is apparently necessary to effectively initiate the reaction. When this condition is met, the time that the barrier is effective can be predicted using a theory that assumes oxygen consumption is diffusion‐controlled. Operating in this condition, the barrier is about 70% efficient, that is, about two‐thirds for the double bonds available in linoleic acid react with oxygen. The results provide a rationale for the design of better oxygen barriers.
Effects of catalyst activity profiles on polyethylene reactor dynamics
Authors: Wells Gary J. , Ray W. Harmon , Kosek Juraj
Published: April 2004

The dynamics and stability characteristics of slurry loop and tank reactors were studied using continuation analysis to examine the effects of catalyst activity profiles on reactor dynamics. The production of high‐density polyethylene through the copolymerization of ethylene with hexene was studied using catalysts of the decay type (for example, TiCl4 on silica, or other Ziegler‐Natta catalysts) and the buildup‐type (for example, chromium oxide catalysts on silica of the Phillips type). Sustained oscillations were found to exist for decay‐type catalysts in loop reactors. For both tank and loop reactors, the buildup‐type catalyst exhibited isolated solution branches (isola) and multiplicity phenomena in addition to sustained oscillations. The sigmoidal shape of the buildup‐type catalyst rate curve is postulated as the reason for the additional multiplicity and isola phenomena. The implications for industrial operating conditions are discussed.
Variables, kinetics and mechanisms of heterogeneous reburning
Authors: Chen Wei‐Yin , Tang Lin
Published: April 2004

The variables, kinetics, and mechanisms of heterogeneous reburning were studied in a flow reactor with a simulated flue gas. The efficiency of heterogeneous reburning depends on the origin of the char, char preparation history and the presence of oxidants, CO2 and O2, and the reducing agent CO, in reburning. Estimated intrinsic rate constants for surface NO reduction in various gaseous environments were compared with those published. In addition to its large surface area, the effectiveness of lignite char appears to be due to its ability to promote two consecutive reactions: (1) the gasification of char by CO2 and O2 for production of CO; (2) the removal of surface oxygen complexes, including those formed after adsorption of NO, by gaseous CO, for the regeneration of reactive sites. Moreover, lignite ash also catalyzes the decomposition of HCN, a major intermediate of NO conversion during gas reburning. These observations suggest that reburning by a mixed fuel containing natural gas and lignite char can be a potentially attractive route for the in–furnance control of nitrogen oxide.
Compressive yield stress from multiple‐speed centrifuge measurements
Authors: Yeow Y. Leong , Leong Yee Kwong
Published: April 2004

Multiple‐speed centrifuge measurement is a standard technique for obtaining the compressive yield stress of concentrated suspensions. Converting the centrifuge data into a compressive yield stress vs. volume fraction curve is an inverse problem. The problem is ill posed in the sense that noise in the data is likely to be amplified if inappropriate data processing methods are used. A method based on Tikhonov regularization is applied to solve this inverse problem. An added complication of centrifuge measurement is the interface between the uncompressed and compressed layers in the centrifuge tube. This interface is not measured and has to be treated as a free boundary. A computation scheme for locating this free boundary is incorporated into the Tikhonov regularization procedure. This new way of processing centrifuge data is assessed by applying it to synthetic data and to laboratory data. Problems caused by small or noisy data sets are discussed.
Determination of the activity of H+ ions within and beyond the pH meter range
Authors: Rodil Eva , Persson Karin , Vera Juan H. , Wilczek‐Vera Grazyna
Published: April 2004

This work confirms that the activities of individual ions measured with ion‐selective electrodes are physically meaningful. The individual activities of the Cl−, Na+ and K+ ions in single‐electrolyte aqueous solutions of HCl, NaOH and KOH were measured at 298.2 K in the range from 0 to 2 molal. In the pH range from 1 to 13, the pH values calculated from the measured activities of the ions are in good agreement with the values obtained with a pH probe. In agreement with previous results for potassium ions, and opposite to the behavior of other cations in 1:1 electrolyte solutions, the K+ and the H+ ions were found to have smaller activities than their conjugate anions. The experimental activity coefficients of the ions were correlated with the Khoshkbarchi‐Vera equation and with the New Hydration Theory, and compared with the predictions given by the Pitzer theory.
Modeling of coalescence in polymer blends
Authors: Lyu SuPing , Bates Frank S. , Macosko Christopher W.
Published: April 2004

Coalescence in polymer blends was modeled by combining population dynamics with three coalescence theories: Smoluchowski, trajectory, and film drainage. Modeling results are compared with our previous coalescence experiments on polystyrene/high density polyethylene blends. Comparison of the Smoluchowski theory (no hydrodynamic interactions) with our experiments reveals that ideal collisions are the main mechanism of coalescence. Coalescence efficiency, however, decreases with increasing shear rate and particle‐size difference. Most interestingly, increasing the ratio of particle to matrix viscosity causes coalescence efficiency first to increase and then to decrease. The effects of particle‐size difference on coalescence efficiency can be modeled by considering changes in trajectory due to hydrodynamic interactions between particles. By considering particle deformation, film drainage theory predicts that coalescence efficiency decreases with increasing shear rate. Comparison with the experiments was only qualitatively correct. However, the prediction that coalescence efficiency decreases with increasing the viscosity ratio by both the trajectory theory and film drainage theory is not consistent with the experimental results. The reasons for the disagreements are discussed in terms of large deformation of colliding particles.
Drying process paths of ternary polymer solution coating
Authors: Dabral Mahendra , Francis L. F. , Scriven L. E.
Published: April 2004

Drying process paths of a ternary solution consisting of a polymer dissolved in a blend of a solvent and a nonsolvent are analyzed. A 1‐D drying model applied to a ternary solution provides composition trajectories followed at different depths in the coating as a function of time, collectively called process paths. The effects of gas‐phase mass‐transfer coefficient, relative diffusivity and relative volatility, and partial saturation of the gas phase with solvents on these process paths are presented. When the solution coating enters the two‐phase region, phase separation may take place, as illustrated by coatings prepared from ternary solutions of cellulose acetate in ketone and methanol. When acetone is the solvent, the coating phase separates. Replacing acetone with methyl ethyl ketone gives homogeneous coatings. Solution phase separation or “blush” is explained in terms of the drying‐process paths and their relationship to the two‐phase immiscible region.
Quantitative framework for reliable safety analysis
Authors: Huang Haitao , Adjiman Claire S. , Shah Nilay
Published: April 2004

The effectiveness of any methodology used to identify hazards in chemical processes affects both safety and economics. To achieve maximum safety at minimum cost, a conservative, but realistic, analysis must be carried out. An approach to hazard identification is proposed based on a detailed process model which includes nonlinear dynamics and uncertainty. A new modeling framework, the region‐transition model (RTM), is developed, which enables the simulation of regions of the operating space through an extension of the hybrid state transition system formalism. The RTM is illustrated on a nonlinear batch reactor with parameter uncertainty. A safety‐verification algorithm identifies regions of the input space (initial conditions and external inputs) which guarantee safe operation. The algorithm is successfully applied to three examples: a tank with overflow and underflow, a batch reactor with an exothermic reaction, and a CSTR with feed preheating.
Hydrogenation of 1,5,9‐cyclododecatriene in fixed‐bed reactors: Down‐ vs. upflow modes
Authors: Chaudhari R. V. , Jaganathan R. , Mathew S. P. , Julcour C. , Delmas H.
Published: April 2004

Performance of trickle‐bed and up‐flow reactors was studied experimentally and theoretically for an exothermic multistep hydrogenation of 1,5,9‐cyclododecatriene (CDT) in n‐decane as solvent over 0.5% Pd/alumina catalyst. Intrinsic kinetics was studied in a batch stirred slurry reactor using the powdered catalyst, and a Langmuir‐Hinshelwood‐type rate model is proposed. Using this rate equation, a trickle‐bed‐reactor model was developed that incorporates contributions of partial wetting and stagnant liquid holdup, in addition to the external and intraparticle mass transfer for the gas‐phase reactant (hydrogen). It was also modified to describe the behavior of the upflow reactor. Experimental data were obtained in both upflow (trickle‐bed) and down‐flow modes at different liquid velocities, pressures, and inlet feed concentrations at 373–413 K. Reactor performance of the two modes was compared in terms of global hydrogenation rate, CDT conversion, selectivity to cyclododecene and the maximum temperature rise observed in the catalyst bed. The conversions and global hydrogenation rate were significantly higher in a trickle‐bed reactor than in the upflow reactor. Similarly, a significant temperature rise in the catalyst bed was observed for the downflow operation compared to the upflow mode, which is explained from wetting characteristics of the catalyst bed. Model predictions for both reactors agreed well with experimental data.
Trickle‐bed reactor model for desulfurization and dearomatization of diesel
Authors: Chowdhury R. , Pedernera E. , Reimert R.
Published: April 2004

Desulfurization and hydrogenation of aromatics in diesel oil were investigated in an isothermally operated trickle‐bed reactor 500 mm long and with a 19‐mm ID, using commercial bifunctional NiMo/Al2O3catalysts. The operating temperature, pressure, liquid hourly space velocity, gas/oil ratio, and the initial concentration of H2S were 300–380°C, 2–8 MPa, 1–4 h−1, 100–500 m3(NTP)/m3and 0–8 vol %, respectively. The increase in pressure and decrease in liquid hourly space velocity lead to deeper desulfurization and hydrogenation of both aromatics. Although a higher gas‐to‐oil ratio and lower initial concentration of H2S enhance the desulfurization reaction, these parameters play almost no role for hydrogenation. Desulfurization increases sharply with increased temperature, but with the same change in temperature the conversion of aromatics increases only up to 360°C, above which it falls sharply. This behavior is explained by the approaching of chemical equilibria by the reversible hydrogenation reactions of the aromatics at higher temperatures. A mechanistic mathematical model was developed for a two‐phase flow reactor, considering both mass transfer and chemical reaction in the reactor. The kinetic equations for desulfurization and for the hydrogenation of mono‐, di‐ and polyaromatics were established. Simulated results are compared satisfactorily with the experimental observations.
Degradation kinetics of polyethylene terephthalate in supercritical methanol
Authors: Goto Motonobu , Koyamoto Hiroshi , Kodama Akio , Hirose Tsutomu , Nagaoka Shoji , McCoy Benjamin J.
Published: April 2004

Degradation of polyethylene terephthalate (PET) in supercritical methanol was investigated to develop a chemical recycling process for waste plastics. Continuous kinetics analysis was applied to the experimental data. A batch reactor was used at 573 K under the estimated pressure of 20 MPa for a reaction time of 2–120 min. PET decomposed to its monomers, dimethyl terephthalate (DMT), and ethylene glycol (EG), by methanolysis in supercritical methanol. PET with a weight‐average molecular weight of about 47,000 was converted to oligomer with that of 3,000 in 300 s and with that of 1,000 in 600 s. The largest yield of DMT was 80 mol % at 7,200 s and that of EG was 60 mol % at 3,600 s. Reaction products were analyzed with size exclusion chromatography, gas chromatography‐mass spectrometry, and reversed phase liquid chromatography. The molecular‐weight distribution (MWD) of the products was obtained as a function of reaction time. The yields of monomer components of the decomposition products, including byproducts, were measured. Continuous kinetics theory was developed to analyze the decomposition behavior. The theory includes MWD change of polymer by random and specific scissions and secondary reactions for monomer components. Change of MWD and monomers as a function of time was simulated by the continuous kinetics.
Solubility of CO2 in H2O+N‐methyldiethanolamine +(H2SO4 or Na2SO4)
Authors: Kamps Álvaro Pérez‐Salado , Rumpf Bernd , Maurer Gerd , Anoufrikov Yuri , Kuranov George , Smirnova Natalia A.
Published: April 2004

The solubility of carbon dioxide in aqueous solutions of methyldiethanolamine (MDEA) sulfate (1 m), in aqueous solutions of MDEA and MDEA sulfate (2 m and 1 m, respectively), and in aqueous solutions of MDEA and sodium sulfate (2 m and 1 m, respectively) was measured at 313 K to 413 K and total pressures up to 10.6 MPa. The experimental results were used to extend and test a model for the thermodynamic equilibrium encountered in the solubility of carbon dioxide in such aqueous solutions.
Atomistic SPH and a link between diffusion and interfacial tension
Authors: Nitsche Ludwig C. , Zhang Weidong
Published: April 2004

The mesh‐free method of smoothed particle hydrodynamics (SPH) is interpreted atomistically to treat the local density of nodes as representing solute concentration, instead of interpolating a concentration field between function values at the nodes. We exploit the conceptual simplicity of tracking little packets (representing supermolecular aggregates) of solute, as in Brownian dynamics simulations, but replace the stochastic generation of diffusive spreading over statistically independent realizations with a purely deterministic summation for extracting the local diffusion velocity using only the instantaneous locations of nearby nodes. Four test problems demonstrate the versatility and accuracy of the atomistic SPH method (ASPH) for mass transfer. Finally, by comparing ASPH with a new Stokeslet‐swarm technique for simulating drop flows (Nitsche and Schaflinger, 2001), a fundamental analogy between diffusion and interfacial tension is exposed.
Experimental study of dynamic wetting in reverse‐roll coating
Authors: Benkreira H.
Published: April 2004

The movement of the dynamic wetting line in reverse‐roll coating was followed in a series of experiments that gave position, XD, angle, θD and the critical metering roller speed, VM, cascade, for the onset of cascade. The data were correlated with operating conditions and compared with plunging‐tape air‐entrainment speeds, V*plunging. It was found that there was hydrodynamic assistance of wetting similar in form but smaller in magnitude to that reported in curtain coating. The correlated data for XD and θD also provide boundary conditions, which can replace the assumptions usually made in the simulations of reverse‐roll coating flows.
Protein nanoparticles formation by supercritical antisolvent with enhanced mass transfer
Authors: Chattopadhyay Pratibhash , Gupta Ram B.
Published: April 2004

In recent years, the supercritical antisolvent (SAS) precipitation technique has emerged as a promising method for the formation of fine particles. Despite its numerous advantages, this technique still cannot be used to produce particles in the sub‐micron range (<300 nm) for many “soft” materials. A significantly improved SAS process can produce particles of controllable size, up to an order of magnitude smaller than those of the conventional SAS process, with a narrower size distribution. Like the conventional SAS technique, this new supercritical antisolvent with enhanced mass transfer technique utilizes supercritical carbon dioxide as the antisolvent, but the solution jet is deflected by a surface vibrating at an ultrasonic frequency atomizing the jet into much smaller droplets. Furthermore, the ultrasound field generated by the vibrating surface enhances mass transfer and prevents agglomeration through increased mixing. The particle size is controlled by varying the vibration intensity of the deflecting surface, which then can be adjusted by changing the power supplied to the attached ultrasound transducer. It is demonstrated by the formation of lysozyme nanoparticles and microparticles. The biological activity of the protein is retained during the processing.
Analytical resolution of a cyclic two‐way chromatographic binary system
Authors: Castel C. , Sardin M. , Simonnot M. O.
Published: April 2004

The focus is on analytical resolution of the mass‐balance relation in the case of a binary ion‐exchange system containing chloride and hydrocarbonate anions involved in a two‐way chromatographic cyclic process under critical operating conditions, that is, feed and elution solutions with the same total normality and percolation volumes. Local equilibrium theory is used to demonstrate the convergence of the system toward a periodic state and to calculate iteratively the instantaneous and asymptotic concentration profiles inside the separation column. One of the main results is that the calculated chloride composition on the inlet side of the column after the elution step is equal to the chloride composition in the feed solution.
Pd membranes formed by electroless plating with osmosis: H2 permeation studies
Authors: Souleimanova Razima S. , Mukasyan Alexander S. , Varma Arvind
Published: April 2004

The synthesis of fully dense Pd‐Vycor glass composite membranes by electroless plating with or without osmosis is reported. Use of osmotic flux allows one to rapidly form thin (a few microns), fully dense palladium films, which have submicron size microstructure. These features significantly enhance hydrogen permeability as compared to the conventional method. By accounting for resistance of the porous support, it is confirmed that for membranes prepared by the same technique, with gradually changing microstructural characteristics, those with smaller palladium crystallites exhibit larger hydrogen flux. Pd‐stainless steel composite membranes were also prepared using electroless plating, either with or without osmosis. Membranes synthesized using osmosis exhibit superior thermal stability.
ZSM‐11 membranes: Characterization and pervaporation performance
Authors: Li Shiguang , Tuan Vu A. , Noble Richard D. , Falconer John L.
Published: April 2004

High‐quality boron and aluminium‐substituted ZSM‐11 membranes were prepared on porous tubular supports to separate alcohols from water by pervaporation. The C1–C3 alcohols were preferentially separated from aqueous solutions through the B‐ZSM‐11 membrane, and the maximum flux was 1.7 kg/m2·h for a 5 wt.% methanol/water mixture. The alcohol fluxes decreased as the carbon number increased. The methanol/water separation selectivity decreased with pervaporation temperature, but other alcohol/water selectivities increased. All selectivities increased with decreasing alcohol feed concentration. In a range of 1 to 50 wt. % alcohol, the separation selectivities at 333 K for the C1–C3 linear alcohols were higher than vapor/liquid equilibrium selectivities. The highest selectivities observed for methanol/water, ethanol/water, 1‐propanol/water, and 2‐propanol/water were 28, 97, 34, and 26, respectively, at 1 wt. % alcohol feed concentrations. The separation selectivities were based on both preferential adsorption of alcohols and differences in diffusion rates.
Optimal temperature control of a steady‐state exothermic plug‐flow reactor
Authors: Smets Ilse Y. , Dochain Denis , Van Impe Jan F.
Published: April 2004

Optimal heat exchanger temperature profiles of exothermic tubular reactors were determined under the assumption of steady‐state and plug‐flow characteristics. The minimum principle of Pontryagin (optimal control theory) was applied in a straightforward analytical sense. To enable a trade‐off between process performance and heat loss, a combined cost criterion was defined. In the first approach of specifying only terminal costs, the optimal control input was of the bang‐bang type that keeps the heat exchanger temperature constant at its maximum or minimum value. Afterwards, the terminal cost criterion was extended with an integral part that accounts for the global heat loss during the process. This integral cost part induced a control of the bang‐singular‐bang type. The desired performance can be met by selecting appropriate weights for terminal and integral costs.
Optimal parameter estimation for the online control of ion milling etch depth
Authors: Klein Eric J. , Ramirez W. Fred
Published: April 2004

The use of optimal parameter estimation for the online regulation of final etch depth in an ion milling process is described. A model‐based control system utilizes a 3‐D surface evolution model to predict dynamic surface profiles and etch depth. A heterodyne laser interferometer is used to measure trench height in situ for calculation of the difference in etch rates between the photoresist layer and the exposed portion of the underlying substrate layer. Adaptive material‐specific etch rate parameters and operating parameters such as ion beam strength and angle of incidence are estimated optimally with an extended Kalman filter. Optimal estimates of the in situ etch depth are calculated from the adaptive model, allowing the stopping time of the milling process to be varied from run to run to regulate final etch depth in the presence of disturbances. The system installed on an ion milling machine shows that the controller is capable of accurately regulating final etch depth in the presence of large process faults.
Phase‐equilibria calculation by group‐contribution perturbed‐hard‐sphere‐chain equation of state
Authors: Elvassore Nicola , Bertucco Alberto , Fermeglia Maurizio
Published: April 2004

A group‐contribution (GC) method was coupled with the perturbed‐hard‐sphere‐chain (PHSC) equation of state (EOS) to predict its characteristic parameters. This model can describe both equilibrium and volumetric properties of regular fluids and chain‐like molecules. The estimation of group contributions for the characteristic volume, surface area, and interaction energy was based only on vapor‐pressure and saturated liquid‐density values of low‐molecular‐weight compounds. It was successful in estimating the EOS parameters of high‐molecular‐weight compounds and chain‐like molecules. Some application results show its reliability but the method was not applied to all classes of compounds. Good estimation of polymer density was obtained by GC PHSC with only the knowledge of their molecular structure. For mixtures, the original model was retained; binary interaction parameters were regressed from vapor–liquid equilibria of binary systems. In most cases, only one binary interaction parameter is enough to describe binary systems in a wide range of temperatures. The same model is extended to the calculation of liquid–liquid equilibria of a system exhibiting both upper critical solution temperature (UCST) and lower critical solution temperature (LCST). In summary, the data required for calculating multicomponent phase equilibria are the molecular structure in term of functional groups and binary interaction parameter values.
Methane hydrate equilibria in silica gels with broad pore‐size distributions
Authors: Smith Duane H. , Wilder Joseph W. , Seshadri Kal
Published: April 2004

Equilibrium pressures for the dissociation of methane hydrates confined in silica gel pores of nominal radii 7.5, 5.0 and 3.0 nm were measured over a wide temperature range, and were observed to be higher than those for bulk methane hydrate. A model is presented that allows the pore radius involved in each equilibrium to be determined from these data, so that the model exactly reproduces the experimental equilibrium pressure. Based on this model, pore volume distributions were reconstructed and found to be in good agreement with those obtained from nitrogen desorption isotherms, indicating that hydrate formed nearly uniformly in the available pores.
Modeling blowdown of cylindrical vessels under fire attack
Authors: Mahgerefteh Haroun , Falope Gboyega B. O. , Oke Adeyemi O.
Published: April 2004

A robust numerical simulation is presented for predicting the risk of rupture during emergency depressurization or blowdown of cylindrical vessels containing high‐pressure two‐phase hydrocarbon mixtures under fire attack. Accounting for nonequilibrium effects between the constituent fluid phases, the model simulates the triaxial transient thermal and pressure stress profiles generated in both the wetted and unwetted wall sections of the vessel. A comparison of this information with the vessel material of construction temperature/yield stress data allows the precise evaluation of the risk of failure and, if applicable, the rupture mode during depressurization. Finally, based on the application of the model to a real system, the subtle differences between the failure risks associated with blowdown under fire as compared to those under ambient conditions are presented and discussed in detail.
Simulation of free‐surface flows induced by partially immersed moving body: Validation
Authors: Jongen T. , Chouikhi S. M.
Published: April 2004

Predictive performance of a free‐surface simulation technique is assessed using the volume‐of‐fluid (VoF) method, in the context of a challenging, yet simple and well‐defined, unsteady‐flow configuration mimicking the in‐mouth kinematic processes occurring during the consumption of liquid foods. An experimental “mouth‐analog” setup consisting of a cavity initially filled with a liquid and agitated by a ram moving periodically was modeled and simulated. Experimental and computational results are compared for a range of liquid viscosities (silicon oil and water) and over a range of processing conditions (low‐ and high‐agitation frequency) that trigger important effects such as surface reconnection and breakup, wall coating, filament formation, and air inclusion. Even with the large mesh and time‐step sizes used in the computations, the main relevant bulk kinematics of this complex transient free‐surface flow can be represented with a Euler volume‐tracking method such as the VoF method.
Experimental analysis of hydrodynamics in axially agitated tank
Authors: Bugay Stephan , Escudié Renaud , Liné Alain
Published: April 2004

Particle image velocimetry (PIV) technique was used to analyze the flow field in a stirred tank induced by a Lightnin A310 axial impeller, focusing on the hydrodynamics, with particular attention to turbulence [production, transport, and dissipation rate of turbulent kinetic energy (TKE)]. The first data of interest are the three components of the mean velocity, which give the overall circulation in the tank. Since turbulence controls mainly the mixing, the six components of the Reynolds stresses were measured. The dissipation rate of TKE was estimated after an experimental evaluation of the different terms of the TKE balance equation at different levels in the tank. A profile close to the impeller is compared to previous empirical modeling where the dissipation rate of TKE was related to the level of TKE and to a macroscale of turbulence that was assumed to be proportional to the impeller.
Screening of cosolvents for a supercritical fluid: A fully predictive approach
Authors: Abaroudi K. , Trabelsi F. , Recasens F.
Published: April 2004

Solubility of solids was modeled in modified supercritical carbon dioxide as a predictive method. The Peng‐Robinson equation of state was used to characterize the ternary systems using the one‐fluid van der Waals rules. Ternary mixture parameters k12, k23, and k13 were obtained from the binary mixtures using Wong‐Sandler mixing rules or the linear combination of Vidal‐Michelsen mixing rules together with UNIFAC as required as a fully predictive tool for the liquid activity coefficients. When this method was applied to predict the solubility of β‐naphthol in CO2 modified with 6% and 10% toluene, values for the predicted solubility were very similar to the measured ones, with deviations of 9.6 and 26.3%, respectively. However, when the method was applied to the data of Simon et al. who examined six common cosolvents, the results were not as good in absolute terms, but the solvents were sorted in about the same order as found experimentally. The method provides quite an approximate guide to judge the most effective cosolvent from several candidates using a fully predictive methodology.
Distributed stream method for tray optimization
Authors: Lang Y‐D , Biegler L. T.
Published: April 2004

Determining the optimum number of trays in a distillation column has been an important process optimization problem for over 50 years, which over the past decade has been addressed successfully as a mixed‐integer nonlinear‐programming (MINLP) problem. But tools for solving MINLPs are not widespread, especially in connection with detailed simulation models. In a differentiable distribution function (DDF), all streams around a column, except top and bottom products, are directed to all of the column trays and the distributed flow rate of entry or exit streams is directed to a specific tray based on the value of its DDF at that tray. It allows the placement of feeds, sidestreams, and number of trays in the column to be continuous variables in the DDF, and in the optimization problem it eliminates the need for integer variables. Instead, the tray optimization problem is formulated as an NLP using the original MESH model. It is critical to deal with trays that have no liquid or vapor flows. To describe this phenomenon properly in the optimization, complementary constraints are formulated and added to the NLP, by taking advantage of a smoothing algorithm developed by Gopal and Biegler for phase equilibrium. Also, since pressure in each tray affects distillation calculations substantially, the pressure drops across each tray depend on the optimal number of trays and a related smoothing function formulation removes the pressure drop on dry trays. Pressure in the condenser or reboiler is then adjusted accordingly. The results of the methods applied to three distillation problems show significant cost reductions in distillation design.
HEN optimizations without using logarithmic‐mean‐temperature difference
Authors: Akman Uğur , Uygun Korkut , Uztürk Derya , Konukman Alp Er Ş.
Published: April 2004

A simple 1‐1 shell‐and‐tube heat‐exchanger model without the logarithmic‐mean‐temperature difference ΔTlm is proposed for heat‐exchanger network (HEN) optimizations. The model consists of two algebraic equations and allows to determine hot and cold outlet temperatures independent of each other. HEN optimization problems are classified as feasible‐ and infeasible‐path formulations, and compared with regard to problems originating from the use of ΔTlm in exchanger energy‐balance equations. The ΔTlm‐free model eliminates numerical failures in computing the logarithmic terms with negative arguments due to possible inconsistent initialization/progress of exchanger interconnection temperatures during HEN optimizations. The ΔTlm‐free model does not require inclusion of approach‐temperature inequality constraints since positive approach temperatures are guaranteed. It also eliminates iterative solution of nonlinear energy‐balance equations in feasible‐path formulations and is represented as linear equality constraints in infeasible‐path formulations for fixed areas, heat‐capacity‐flow rates, and bypass flows. As demonstrated with two representative HEN optimization problems, the ΔTlm‐free model significantly reduces numerical difficulties and decreases computation time in the optimization phase of synthesis/design, retrofit, and flexibility‐analysis of HENs. Thus, it permits handling large HEN problems.
Adaptive parameter estimation for microbial growth kinetics
Authors: Zhang T. , Guay M.
Published: April 2004

An adaptive parameter estimation algorithm for a class of biochemical processes expressed by a nonlinearly parametrized Monod's growth kinetics model is presented. Contrary to conventional least‐square or gradient‐type identification techniques, the proposed parameter estimation algorithm is developed based on Lyapunov's stability theory. A novel class of parameter‐dependent Lyapunov functions is utilized to remove the difficulty associated with estimating the unknown parameters that appear nonlinearly. A persistence of excitation (PE) condition is investigated to guarantee the convergence of the estimation scheme. Simulations are provided to verify the effectiveness of the new approach and the theoretical discussion.
Prediction and quantification of micromixing intensities in laminar flows
Authors: Cerbelli S. , Alvarez M. M. , Muzzio F. J.
Published: April 2004

A new approach presented here predicts and quantifies the nonuniform spatial distribution of passive interfaces in both periodic and aperiodic chaotic mixing processes. Based on a coarse‐grained calculation of line element stretching, it bypasses the direct numerical simulation of continuous interfaces, which is computationally impractical even in simple model flows. The evolution of the structures created by the Lagrangian displacement and deformation process is decoupled into the product of a time‐invariant, spatially‐dependent function expressing the nonuniform density of interface throughout the flow domain and a temporally‐dependent function expressing the exponential growth of the interface, characterized by a global exponent (topological entropy) unambiguously different from the Lyapunov exponent. The Sine Flow agreed excellently with the Journal Bearing, when results from direct tracking of the interface were compared to those using stretching calculations to predict interface density.
CFD of multiphase flow in packed‐bed reactors: I. k‐Fluid modeling issues
Authors: Jiang Y. , Khadilkar M. R. , Al‐Dahhan M. H. , Dudukovic M. P.
Published: April 2004

The Eulerian k‐fluid CFD model was used to simulate the macroscale multiphase flow in packed beds. The geometric complexity of the bed structure is resolved by statistically describing the porosity distribution. The complicated multiphase interactions are computed using the Ergun type of formula developed based on bench‐scale hydrodynamic experiments. The work is presented in two sequential articles. Part I discusses implementation issues of the k‐fluid CFD model for packed beds. The drag exchange coefficients are obtained from the model of Holub et al. for the particle‐fluid interfaces Xks and from the model of Attou et al. (1999) for the gas–liquid interface, Xgl. The effect of particle external wetting on flow distribution was incorporated into the model through the capillary pressure evaluated by either the J‐function of Leverett (1941) for air–water or by the expression of Attou and Ferschneider (1999) for other fluids. In the framework of CFDLIB, the choice of the grid size and boundary conditions are discussed. An appropriate relationship between the section size and variance of the sectional porosity distribution was used for flow simulation. Part II discusses the extensive numerical results, and the CFD model is compared with experimental data in the literature.
CFD of multiphase flow in packed‐bed reactors: II. Results and applications
Authors: Jiang Y. , Khadilkar M. R. , Al‐Dahhan M. H. , Dudukovic M. P.
Published: April 2004

Numerical simulations of multiphase flow using the k‐fluid CFD model described in Part I of this issue are presented for packed beds at various operating conditions. Both steady‐state and unsteady‐state (e.g., periodic operation) feed conditions were studied numerically. Predictions of the k‐fluid CFD model are comparable with the experimental data in the literature for liquid upflow in a cylindrical packed bed. In addition to the mean porosity and the longitudinally averaged radial porosity profile, the variance of the porosity distribution is needed for predicting the probability density function of the sectional flow velocity. In the trickling flow regime, the k‐fluid CFD model provides reasonable predictions of the global liquid saturation and the pressure gradient. Relevant applications of the k‐fluid CFD model are identified in quantifying the relationship between bed structure and flow distribution in various‐scale packed beds. The combined flow‐reaction modeling scheme is proposed through the “mixing‐cell” network concept, in which the k‐fluid CFD simulation can provide the information on sectional flow distribution.
Topology of ternary VLE diagrams: Elementary cells
Authors: Hilmen E. K. , Kiva V. N. , Skogestad S.
Published: April 2004

The classification of ternary vapor‐liquid equilibrium (VLE) diagrams is a key to simple azeotropic distillation analysis. All ternary mixtures reported so far to be occurring in nature can be qualitatively represented by a combination of only four elementary cells. This greatly reduces the number of VLE diagram structures that need to be analyzed in order to reveal the qualitative characteristics of any ternary azeotropic mixture.
Wavelet‐based adaptive grid method for the resolution of nonlinear PDEs
Authors: Cruz Paulo , Mendes Adélio , Magalhães Fernão D.
Published: April 2004

Theoretical modeling of dynamic processes in chemical engineering often implies the numeric solution of one or more partial differential equations. The complexity of such problems is increased when the solutions exhibit sharp moving fronts. A new numerical method is established, based on interpolating wavelets, that dynamically adapts the collocation grid so that higher resolution is automatically attributed to domain regions where sharp features are present. The effectiveness of the method is demonstrated with some relevant examples in a chemical engineering context.
Microwave heating of endothermic catalytic reactions: Reforming of methanol
Authors: Perry W. Lee , Datye Abhaya K. , Prinja Anil K. , Brown Lee F. , Katz Joel D.
Published: April 2004

In an endothermic reaction, such as methanol‐steam reforming, the reaction rate can be limited by the ability to supply heat to the reactor. Heat transfer from the reactor wall normally supplies the required energy in such processes. Drawbacks, such as operating temperature constraints and practical heat‐transfer restrictions, limit this practice. In such situations, microwave heating could provide an effective and efficient method for transferring heat to the catalyst. To explore this possibility, the methanol‐steam reaction performed by conventional heating and microwave heating was studied. Mathematical modeling was performed for 1‐D heat transfer in a single catalyst pellet and for 2‐D heat transfer in a tubular packed‐bed reactor. The single catalyst pellet model indicated that productivity could increase significantly using microwave heating. The 2‐D model showed that microwave heating could minimize radial heat‐transfer effects. Conducting the methanol‐steam reaction in a microwave‐heated reactor experimentally verified the improved productivity.
Synthesis of controlled‐release products in supercritical medium
Authors: Guney Ozge , Akgerman Aydin
Published: April 2004

Controlled‐delivery products have recently received considerable attention. They prolong the drug's therapeutic effect, keeping its concentration between the therapeutic and toxicity limit. The final product must be free of residual solvent, even with nontoxic solvents, to provide its constant property that does not change with natural evaporation of the solvent. A major cost item in the synthesis of controlled‐release drugs is to remove this solvent to acceptable limits. To eliminate the production step involving the organic solvent from the overall process, the drug was introduced into the polymer matrix using supercritical carbon dioxide (scCO2) as the carrier solvent and swelling agent. This study focuses on the impregnation of a biodegradable polymer matrix with 5‐fluorouracil for chemotherapy and β‐estradiol estrogen hormone therapy. Swelling of the polymer matrix, solubility of drug components in scCO2 and the adsorption isotherm/partition coefficients in the presence of scCO2 were studied. To investigate the single‐component supercritical adsorptive synthesis of controlled‐delivery products, accurate experimental techniques to measure drug‐component solubilities in supercritical fluids and adsorption isotherms in the presence of supercritical fluids were developed. Solubility data of these drugs in scCO2 at 35–55°C and 101–220‐bar were reported. The adsorption equilibrium constants/partition coefficients were presented for these drug components on poly‐dl‐lactide‐co‐glycolide (PLGA) at the same conditions. To understand the effect of operating variables on the total drug loading and to predict the drug‐loading breakthroughs, a flow over a flat plate model was developed that accounts for the polymer swelling, the partitioning of the drug component between the supercritical and polymer phases, and the polymer diffusivity. Experimental isotherm data with and without the polymer were incorporated into the model to isolate the system response from the polymer response to determine impregnation efficiencies.
Modeling and extractive properties of quaternary salt in a two‐phase system
Authors: Wu Ho‐Shing , Tseng Ming‐Shu
Published: April 2004

Thermodynamic equilibrium parameters of seven kinds of 1:1 electrolytes of quaternary salts in an organic and aqueous phase, including an extraction constant for ion‐pair and free‐ion, distribution constant, and dissociation constant in an organic solvent/aqueous solution, were determined by a numerical iteration method. Temperature, the kinds of solvents and quaternary salts, and the concentration of quaternary salts were evaluated to understand the thermodynamic properties. The free energies of extraction constant, distribution constant, dissociation constant, and the transfer of dissociated ions and ion pairs for quaternary ions were obtained as well. The amount of coextracted water and thermodynamic parameters were correlated with the solvent property. The equation of hydrophile–lipophile balance for selecting proper quaternary salt was also developed.
New efficient algorithm for solving thermodynamic chemistry
Authors: Carrayrou Jérôme , Mosé Robert , Behra Philippe
Published: April 2004

Modeling thermodynamic equilibrium of complex nonlinear chemical systems with the most used Newton‐Raphson method can lead to nonconvergence. From the mathematical properties of the set of equations, a chemically permitted interval is defined. By imposing this interval, the robustness of the Newton‐Raphson method is increased at a low computing time cost. The new method, the positive continuous fraction, does not depend on the first derivative of the objective function to find the solution. A new algorithm is thus built by the association of this very robust method with the fast Newton‐Raphson method and the definition of the chemically allowed interval. This combined algorithm is very impressive in terms of reliability, robustness, and speed.
Effect of polymer coatings from CO2, on water‐vapor transport in porous media
Authors: Hénon Florence E. , Carbonell Ruben G. , Desimone Joseph M.
Published: April 2004

A variety of perfluorinated polyethers were coated onto surfaces of marble, sandstone and limestone samples from solutions in supercritical carbon dioxide. These polymers make ideal protectants for civil infrastructure by making stone surfaces hydrophobic and preventing penetration and deterioration of the stone by acid rain. The effective diffusivities of water vapor through coated and uncoated stones were measured as a function of polymer applied per unit area of sample. An analysis of the diffusive transport of water through the stones led to estimates of the penetration depths of the polymers and the percentages of blockage of the pores in the coated layers as a function of polymer surface coverage. Penetration depths were seen to strongly depend on the mean size and porosity of the stones. It is important for water‐vapor diffusion to occur through the samples to prevent water condensation inside polymer‐coated structural materials.
Unsaturated liquid percolation flow through nonwetted packed beds
Authors: Liu D. Y. , Wang G. X. , Litster J. D.
Published: April 2004

The unsaturated flow of liquid through packed beds of large particles was studied using six different liquids, all with contact angles greater than 90° on the bed packing (wax spheres of 9, 15 and 19.4 mm diameter). The liquidflow was discrete in nature, as drops for low flow rates and rivulets for high flow rates. For unsaturated liquid flows, the actual percolation velocity, not superficial velocity, should be used to characterize the flow. The percolation velocity did not vary with packed‐bed depth, but was a strong function of liquid flow rate, liquid and particle properties. Effects of liquid and particle properties (but not flow rate) are well captured by a simple correlation between the liquid‐particle fiction factor and Reynolds number based on actual percolation velocities. Liquid dispersion, characterized by the maximum dispersion angle, varies significantly with liquid and particle properties. The tentative correlation suggested here needs further validation for a wider range of conditions.