bluuebunny/crossref_metadata_2025 huggingface dataset:
Product sequences in azeotropic batch distillation
Authors: Ahmad Berit S. , Zhang Yong , Barton Paul I.
Published: April 2004

The formation of multicomponent azeotropes in waste‐solvent streams often complicates separation and recovery of solvents. The implications of applying batch distillation to such a mixture can be studied in a residue curve map. Ahmad and Barton (1996) extend and generalize the theory for ternary residue curve maps applied to batch distillation to homogeneous systems with an arbitrary number of components. These theoretical results are used to develop an algorithm for elucidating the structure of the batch distillation composition simplex for a system with an arbitrary number of components. The batch distillation regions are identified by completing unstable boundary limit sets. The completed boundary limit sets accurately represent the topological structure of the composition simplex and make it possible to extract all product sequences achievable when applying batch distillation. The algorithm is demonstrated on the five‐component system acetone, chloroform, methanol, ethanol, and benzene, which exhibits nine azeotropes. Furthermore, the set of batch distillation product sequences predicted is validated using a commercial dynamic batch distillation column model. Although the problem does not closely approach the limiting conditions of the theory, the predictions agree excellently with the simulations.
Optimizing and controlling the operation of heat‐exchanger networks
Authors: Aguilera Néstor , Marchetti Jacinto L.
Published: April 2004

A procedure was developed for on‐line optimization and control systems of heat‐exchanger networks, which features a two‐level control structure, one for a constant configuration control system and the other for a supervisor on‐line optimizer. The coordination between levels is achieved by adjusting the formulation of the optimization problem to meet requirements of the adopted control system. The general goal is always to work without losing stream temperature targets while keeping the highest energy integration. The operation constraints used for heat‐exchanger and utility units emphasize the computation of heat‐exchanger duties rather than intermediate stream temperatures. This simplifies the modeling task and provides clear links with the limits of the manipulated variables. The optimal condition is determined using LP or NLP, depending on the final problem formulation. Degrees of freedom for optimization and equation constraints for considering simple and multiple bypasses are rigorously discussed. An example used shows how the optimization problem can be adjusted to a specific network design, its expected operating space, and the control configuration. Dynamic simulations also show benefits and limitations of this procedure.
Batch crystallization of a photochemical: Modeling, control, and filtration
Authors: Matthews H. B. , Rawlings James B.
Published: April 2004

Model identification and control were studied for the seeded batch crystallization of an organic, industrial photochemical. The growth and nucleation kinetics are described by empirical power‐law expressions in relative supersaturation, and an additional model describing crystal shape dynamics is proposed. The parameters in the kinetic models are estimated from on‐line measurements of solute concentration and transmittance. Parameter uncertainty is assessed, and optimal experimental design techniques are used to construct experiments to diminish uncertainty. Given the identified model, optimal open‐loop temperature schedules are designed to improve the filtration of final‐time slurries. The profiles minimize the mass of nucleated crystal relative to seed crystal mass, subject to supersaturation and yield constraints. Filtration improvements are verified using constant pressure filtration tests to determine the flow resistance of the filter cake. The best control experiment gives a cake resistance 25% lower than the best identification experiment.
Sol‐gel polycondensation kinetic modeling: Methylethoxysilanes
Authors: Rankin Stephen E. , Macosko Christopher W. , McCormick Alon V.
Published: April 2004

Quantitative kinetic modeling of the condensation of methylethoxysilanes {(CH3)4‐f Si(OC2H5)f} of varying functionality (f) is needed to engineer inorganic polymers, resins, and ceramics. To that end, a kinetic model that accounts for hydrolysis pseudoequilibrium, nearest‐neighbor substitution effects, and unimolecular cyclization reactions in homogeneous ethoxysilane polycondensation is presented. Condensation rate parameters are determined by fitting to 29Si NMR transients. Several important features become evident: (1) the success of the hydrolysis pseudoequilibrium approximation; (2) strong negative substitution effects with unusual dependence on connectivity; (3) a strong kinetic tendency for ring formation, growing with methyl substitution; (4) acceleration of condensation upon methyl substitution; and (5) destabilization of three silicon rings by methyl substitution. The first three observations are consistent with previous findings for ethylethoxysilanes, but the last two are strikingly different.
Study of EOS‐Gex mixing rules for liquid–liquid equilibria
Authors: Escobedo‐alvarado Gonzalo N. , Sandler Stanley I.
Published: April 2004

The performance of two Gibbs free‐energy‐based equation‐of‐state (EOS) mixing rules is tested for the correlation and prediction of liquid‐liquid equilibria (LLE). The LLE of ten binary mixtures were examined using the Peng Robinson EOS combined with the Wong Sandler and MHVl mixing rules with two excess free‐energy models (NRTL and UNIQUAC). Good agreement with low‐pressure experimental data was obtained for nonassociating and self‐associating mixtures using temperature‐independent parameters. However, in the case of aqueous mixtures that exhibit both a lower and an upper critical solution temperature, it was necessary to use temperature‐dependent parameters, as is also true when activity coefficient models are used directly to describe such systems. The Wong Sandler mixing rule was capable of predicting the high‐pressure phase behavior of the systems studied using parameters from correlation of only low‐pressure LLE data, while the MHVl model required the introduction of a binary interaction parameter. Consequently, the MHVl model was useful for the correlation, but not for the prediction of high‐pressure LLE from low‐pressure data.
Modeling of ABS latex coagulation processes
Authors: Chung Chang‐Bock , Park Soo‐Ho , Han In‐Su , Seo Young‐Hyo , Yang Byong‐Tae
Published: April 2004

The mathematical models of acrylonitrile–butadiene–styrene (ABS) latex coagulation processes that show the effect of major operation variables on the particle‐size distribution of recovered resins were derived to provide a guideline for process improvement to reduce the content of fine particles in the product. Kinetic equations for the coagulation and breakup of particles taking place in a turbulent flow field were incorporated into the population balance models for batch and continuous processes. A dimensional analysis for the process models leads to two dimensionless groups, ηc and ηb, that represent the relative intensity of the coagulation and breakup phenomena. The ηc/ηb ratio is designated as a design variable for controlling the particle‐size distribution in the steady‐state operation of the coagulation processes. This role is demonstrated by numerical simulation where increasing the ratio shifted the particle‐size distribution toward large particles. The estimates of ηc/ηb obtained for a set of batch experiments also confirm that fine particles can be reduced by setting the operation variables to increase the ratio.
Description of three‐phase filtration with a novel dimensionless number
Authors: Huizenga P. , Kuipers J. A. M. , van Swaaij W. P. M.
Published: April 2004

Internal filtration in slurry bubble columns offers a possible solution to the filtration problems related to this reactor type. The applicability of the concept has already been demonstrated at full‐scale for wastewater treatment, even though a theoretical description of internal filtration is lacking. Two types of hydrodynamic models were developed for a filtering slurry reactor. Both the model of the surface renewal type and the model based on a force‐balance approach predict that the solids concentration at the axial position of the filter uniquely depends on a single novel dimensionless number. The validity of these theories was demonstrated by measuring filter‐cake resistance as an indirect measure of solids concentration for a system in batch operation with respect to solids. In the experiments the load of solids, permeate flux (J), superficial gas velocity (Ug), particle diameter (dp), kinematic liquid viscosity (vl), and molar mass of the gas were varied.
Intermediate reboiler and condenser arrangement for binary distillation columns
Authors: Agrawal Rakesh , Herron D. Michael
Published: April 2004

The most thermodynamically efficient configuration for adding or removing heat from an intermediate location of an ideal binary distillation column distilling pure products is derived. The optimal policy requires that preconditioning of the feed be part of the overall decision‐making process. The optimal configuration can be determined through the use of two parameters, αIR and αIC, that are solely functions of feed composition. Simple and readily usable heuristics using these parameters are developed that help instantly identify the most efficient selection among (1) totally vaporizing and returning a side‐draw liquid stream from an intermediate location of the distillation column, (2) partially or totally vaporizing a portion of the given saturated liquid feed, (3) partially or totally condensing a portion of the given saturated vapor feed, and (4) totally condensing and returning a side‐draw vapor stream from an intermediate location of the distillation column.
Adsorption of organic solvent vapors on hydrophobic Y‐type zeolite
Authors: Yun Jeong‐Ho , Choi Dae‐Ki , Kim Sung‐Hyun
Published: April 2004

Experimental isotherms and prediction results for adsorption of benzene, toluene, dichloromethane and 1,1‐dichloro‐1‐fluoroethane on hydrophobic Y‐type zeolite are reported. Isotherm shows the type‐V shape according to the classification by Brunauer et al. A simple thermodynamic method is employed to predict the experimental equilibrium data at various temperatures simultaneously. This plain method is based on the assumption that the value of the isosteric heat of adsorption does not depend on temperature for a certain surface loading. The Clausius–Clapeyron equation was used to calculate the isosteric heat of adsorption. To apply the method, only two sets of the experimental isotherm data at two different temperatures are needed. The Clausius–Clapeyron equation with two isotherms provided simple and reliable prediction of adsorption equilibrium relationships at various temperatures. Our results with this method showed that the predicted value agrees well with the experimental data in the range of temperatures for the system tested.
Flow regimes in centrifugal partition chromatography
Authors: van Buel M. J. , van Halsema F. E. D. , van der Wielen L. A. M. , Luyben K. Ch. A. M.
Published: April 2004

Modeling pressure drop, stationary‐phase holdup and separation efficiency in centrifugal partition chromatography (CPC) are hampered by restricted knowledge concerning the flow behavior of the liquid‐liquid two‐phase system. A transparent column (type CPC‐LLN) was constructed to determine the flow behavior as a function of the flow rate, rotational frequency and the properties of the two‐phase system. Two main types of flow regimes, droplets and (broken) jets, are observed although the spraying regime hardly occurred. Droplets occur at low flow rates and low rotational frequencies. The droplet regime changes into the (broken) jet regime when either the flow rate or the rotational frequency increases above a critical value. This critical value is described reliably by a dimensionless number correlation. A flow chart showing the type of flow regime as a function of the Weber number and the centrifugal force enables rational fine‐tuning of operating conditions for given two‐phase systems and equipment.
Structure of Distillation Regions for Multicomponent Azeotropic Mixtures
Authors: Rooks Raymond E. , Julka Vivek , Doherty Michael F. , Malone Michael F.
Published: April 2004

Geometric methods for the conceptual design of nonideal and azeotropic distillation systems have focused primarily on three‐ and four‐component mixtures. A generalization of geometric methods that does not rely on visualization and gives a completely equation‐based approach to determine the structure of distillation regions for homogeneous mixtures is described. This method is useful for mixtures with a large number of components, provided a vapor–liquid equilibrium model is available. The approach is applied to assess feasibility and alternative separations in several examples, including a five‐component mixture with nine azeotropes and a six‐component mixture with seven azeotropes. These methods can be used to rapidly generate process alternatives, and the examples demonstrate that there can be a surprising number of these alternatives for the distillation of azeotropic mixtures.
Liquid‐phase polymerization of propylene with a highly active catalyst
Authors: Samson Job Jan C. , Weickert Günter , Heerze Annelies E. , Westerterp K. Roel
Published: April 2004

An experimental setup and a kinetic study of the polymerization of propylene in liquid monomer with a highly active catalyst are presented. The purification system for monomer, the catalyst injection system, the temperature control system, and the polymerization procedures are described in detail. Further, reproducibility of the experiments is tested, the kinetics are described in the temperature range of 27 to 67°C, and the influence of prepolymerization in cold liquid propylene is investigated.
Desorption by ultrasound: Phenol on activated carbon and polymeric resin
Authors: Rege Salil U. , Yang Ralph T. , Cain Charles A.
Published: April 2004

An experimental feasibility study of using ultrasound to accomplish the dificult desorption of phenol from activated carbon and polymeric resin adsorbents is discussed. The desorption rates of activated carbon were found to significantly increase by ultrasound at 40 kHz and 1.44 MHz. Attrition of the activated carbon due to cavitation could be prevented by operating at a higher frequency and with an intensity below the threshold of the pulverization of carbon. According to the structural stability study of Amberlite XAD‐4 and Dowex Optipore L‐493 resins to withstand the abrasive cavitational effects of ultrasound at 40 kHz, the Dowex Optipore resin was stable under experimental conditions and phenol desorption rates were enhanced significantly with sonication. The ultrasonic desorption rates were favored by decreased temperature, aerated liquid medium, and increased ultrasound intensity. The desorption rates obtained without ultrasound appeared to be limited by pore diffusion, whereas those obtained in the presence of ultrasound were limited by surface reaction. The rate enhancement was due to an increase in diffusive transport within the pores caused by acoustic vortex microstreaming. The activation energy for desorption decreased with an increase in ultrasonic power density, thus making the ultrasound weaken the adsorption bond.
Constrained nonlinear MPC using hammerstein and wiener models: PLS framework
Authors: Patwardhan Rohit S. , Lakshminarayanan S. , Shah Sirish L.
Published: April 2004

Methods of implementing an input‐constrained, nonlinear, model‐predictive controller in latent spaces using partial‐least‐squares (PLS)‐based Hammerstein and Wiener models are discussed. For multiple‐input, multiple‐output (MIMO) systems, the PLS flamework presents a viable alternative for identification and controller synthesis using Hammerstein and Wiener structures. The constraint mappings, which have to be taken into account during controller design in the PLS flamework, are highlighted. PLS‐based Wiener models are well suited for constrained control of nonlinear systems. The use of PLS‐based Hammerstein models for control involves solution of a nonlinear program as a result of the constraint mapping. The proposed approach is demonstrated on a simulated pH‐level control of an acid base neutralization process.
Sound measurement as a means of gas‐bubble sizing in aerated agitated tanks
Authors: Boyd Jonathan W. R. , Varley Julie
Published: April 2004

Bubble size affects mass transfer and mixing hydrodynamics significantly in a gas‐liquid agitated vessel. It is important that accurate and reliable techniques are developed to measure bubble size to improve design and scale‐up of such vessels. Existing bubble‐size measurement techniques are time‐consuming (e.g., photography) or complex and expensive (e.g., ultrasound). The ideal technique, capable of on‐line measurement in any type of liquid, has yet to be developed. It is possible to size gas bubbles from sound measurement within a gas‐sparged agitated tank. A significant amount of the noise, measured close to the impeller, is made up of transient, damped, sinusoidal pressure pulses, which are heard only in the presence of gas bubbles. Analysis of the individual sound pulses in terms of magnitude, frequency and damping, combined with photographic validation, indicates that the sound is caused by bubbles oscillating at frequencies dependent on bubble size. The bubbles are excited into oscillation when formed at the impeller. Sound from other sources (e.g., turbulence) is differentiated from bubble sound. Theoretical modeling and experimental results are used to estimate the sound contribution of a single bubble to a sound spectrum and calculate the bubble‐size distribution at the impeller directly from experimentally measured soundpressure spectra.
Reaction and flow variants/invariants in chemical reaction systems with inlet and outlet streams
Authors: Srinivasan B. , Amrhein M. , Bonvin D.
Published: April 2004

The concept of reaction invariants is extended to include flow invariants of chemical‐reaction systems with inlet and outlet streams. A transformation to normal form is introduced that allows the separation of (1) the evolution of the independent reactions, (2) the influence of inlet streams, and (3) the behavior of the reaction and flow invariants. In the light of this transformation to normal form, model reduction, state accessibility, state reconstruction, and feedback linearizability are analyzed.
Catalytic incineration of VOC containing air streams at very short contact times
Authors: Goralski C. T. , Schmidt L. D. , Brown W. L.
Published: April 2004

A short‐contact‐time catalytic combustor described here is capable of incinerating air streams with low concentrations of volatile organic compounds (VOCs) at conversions exceeding 99.5% for contact times on the order of 5 ms. This is accomplished by adding methane to the VOC‐containing air stream to increase the fuel stream value and then passing the mixture over a platinum‐coated foam ceramic monolith at 900 – 1,400°C. The incineration of air streams containing toluene, chlorobenzene, acetonitrile, and thiophene was examined at concentrations ranging from 500 to 2,000 PPM. Residence time and methane concentration do not affect strongly the outlet concentration of the VOC and conversion. Greater than 99.5% conversion is observed for all compounds examined for residence times ranging from 4 to 12 ms and methane concentrationsfrom 5.5 to 7.0% (80–40% excess air). The mechanism of reaction in this system is primarily heterogeneous, with some homogeneous reactions driven by the heat liberated by the heterogeneous reactions. A simple homogeneous model shows that homogeneous chemistry alone cannot account for complete conversion of the additional methane fuel at the reaction conditions described.
Roles of thermal and radical quenching in emissions of wall‐stabilized hydrogen flames
Authors: Aghalayam P. , Vlachos D. G.
Published: April 2004

A numerical study of the combustion of H2/air mixtures impinging on surfaces has been performed, using detailed chemistry and multicomponent transport, in order to elucidate the roles of surface thermal and chemical quenching in pollutant emissions. For extinguishable fuel/air mixtures, the thermal coupling with the surface has been found to be strong, and surface thermal quenching reduces NOXemissions. However, nonextinguishable mixtures are practically unaffected by changes in surface temperature, except for an increase in NO2at low surface temperatures. It is shown that radical wall quenching can affect emissions for all flames, with H being the most important one primarily for NO2near the surface, due to the reaction NO2+ H ↔ NO + OH. A new method to elucidate the dominant paths for NO formation is also presented.
Operation regimes in catalytic combustion: H2/air mixtures near Pt
Authors: Park Young K. , Bui Pierre‐André , Vlachos Dionisios G.
Published: April 2004

The influence of a platinum catalyst on flammability limits and operation windows for catalytic and catalyst‐assisted homogeneous oxidation is studied, for the first time to our knowledge, as a function of H2in air composition in a stagnation‐point flow geometry. The results show that the coupling between the homogeneous and heterogeneous chemistries leads to relatively easy startup, the coexistence of catalytically (partially) and homogeneously (completely) ignited branches under certain conditions, and the expansion of the fuel‐lean and rich flammability limits. A strategy to attain virtually any desired operation regime is discussed. It is shown that synergism between homogeneous and heterogeneous chemistries leads to higher combustion efficiency and lower fuel emissions at elevated temperatures for fuel‐lean mixtures only. Analysis of the flammability limits indicates that heat generated by surface reactions is primarily responsible for expansion of the flammability limits. Finally, a direct transition to flames occurs over a wide range of composition upon catalytic ignition in the absence of surface heat loss.
Liquid distribution in trickle‐bed reactors
Authors: Saroha Anil K. , Nigam K. D. P. , Saxena Alok K. , Kapoor V. K.
Published: April 2004

Experimental measurements on liquid distribution in a trickle‐bed reactor are presented, as well as the effect of liquid and gas flow on radial liquid distribution. The influence of liquid density and surface tension was also investigated. The model for liquid distribution proposed by Herskowitz and Smith in 1978 was modified to incorporate the effect of particle orientation. Model parameters are correlated with liquid and gas flow rates, as well as liquid surface tension.
Shortcut methods for nonideal multicomponent distillation: I. Simple columns
Authors: Bausa J. , Watzdorf R. v. , Marquardt W.
Published: April 2004

Shortcut methods are a prerequisite for the fast evaluation of alternatives in process design. The rectification body method (RBM) for calculating the minimum energy demand of homogeneous, azeotropic multicomponent distillation processes is presented. The new method incorporates both the classical Underwood technique and certain previously proposed techniques as special cases. It employs all pinch point branches of both column sections and thus requires no a priori selection of active pinch points. It is entirely general and can be applied to any type of split of nonideal and azeotropic mixtures irrespective of the number of components. Hence, it significantly extends previous results and practical applicability. Its features are highlighted through examples of nonideal separations with three, four, and five components.
Active learning from process data
Authors: Raju Gokaraju K. , Cooney Charles L.
Published: April 2004

Much of the prevailing connectionist machine learning research in chemical engineering assumes a one‐way passive relationship between the learner and the application domain. This article investigates a two‐way active relationship between learner and domain. An active relationship is useful and even necessary if the prevailing research is to be successfully applied to real‐world problems involving sparse and strongly biased data. A process development case study is used to illustrate the impact of data quality and quantity and to compare the performance of active learning against conventional passive learning. This study highlights on the need to assess data quality and demonstrates the improvements in the rate of active learning.
Enhancing MTBE rate equation by considering reaction medium influence
Authors: Fité Carles , Tejero Javier , Iborra Montserrat , Cunill Fidel , Izquierdo J. Felipe
Published: April 2004

Rate data are reanalyzed for MTBE synthesis in the liquid phase at 318‐363 K on the sulfonic macroporous resin Bayer K‐2631. The data were obtained in a continuous upflow packed‐bed reactor fed with isobutene (IB)‐methanol (MeOH) mixtures with an IB/MeOH molar ratio ranging from 0.5 to 2.0. Rate equations based on LHHW and ER mechanisms do not predict rate data accurately, because of the influence of the reaction medium on the catalyst activity. A factor evaluating the resin‐medium affinity has been introduced in the rate equations which, as a result, fit the data with a good agreement. Fitted parameters were the overall kinetic constant and the resin solubility parameter. The apparent activation energy agrees with the values quoted in the literature, and the resin solubility parameter and its temperature dependence are plausible.
Simultaneous thermal cracking and oxidation of propane to propylene and ethylene
Authors: Choudhary Vasant R. , Rane Vilas H. , Rajput Amarjeet M.
Published: April 2004

Propane conversion by its simultaneous endothermic thermal cracking and exothermic noncatalytic oxidative conversion topropylene and ethylene in the presence of steam and limited oxygen was investigated at various process conditions (temperature, 635–800°C; C3H8/O2 ratio in feed, 2.0–16.0; H2O/C3H8 ratio, 0.0–2.5; space velocity, 1,170–7,150 h−1; and sulfur additive/C3 H8 ratio, 10−3−10−5). Influence of a sulfur additive (thiophene, CS2, or dimethyl sulfide) in the feed on process performance was also studied. The propylene/ethylene mole ratio in the products is decreased with increasing the temperature but increased with the C3HS/O2 and H2O/C3H8 ratios and space velocity. At all the process conditions, the selectivity for CO, a useful byproduct, is much more than that for CO2. Because the thermal cracking of propane is carried out in the presence of limited O2, not only the total conversion of propane but also the conversion of propane by its thermal cracking alone is increased greatly. Hence this process occurs at a much lower temperature or contact time than that required for achieving the same conversion in the thermal cracking process. Since the endothermic thermal cracking and exothermic oxidative conversion reactions of propane occur simultaneously, both reactions are coupled to make it highly energy‐efficient, with a drastic reduction in external energy requirement and coke formation, and safe to operate. The overall process can be made almost thermoneutral, mildly exothermic, or mildly endothermic by manipulating the temperature and C3H8/O2 ratio in the feed.
Two‐phase flow in porous media: Property identification and model validation
Authors: Kulkarni Raghavendra , Watson A. Ted , Nordtvedt Jan‐Erik , Sylte André
Published: April 2004

The experimental design and analysis of two‐phase dynamic displacement experiments are investigated, with particular attention to incorporating measured saturation distributions. Pressure drops and saturation distributions measured with displacement experiments using multiple flow‐rate injection rates can provide accurate estimates of relative permeability and capillary pressure functions over broad ranges of fluid saturations. Successful reconciliation of measured saturation distributions and pressure drops establishes an experimental validation of the Darcy model for two‐phase flow in porous media.
pH dependence of ion‐exchange equilibrium of proteins
Authors: Bosma J. C. , Wesselingh J. A.
Published: April 2004

The adsorption equilibrium of bovine serum albumin on Q‐sepharose, a strong union exchanger, was studied with batch equilibrium experiments at pH between 4 and 9 and ionic strengths between 5 and 440 mmol/L. Dependence of the adsorption equilibrium on the ionic strength was modeled as an ion exchange reaction. A simplified mechanism of this ion exchange reaction also yielded an expression for the dependence of the equilibrium on the charge of the protein. This model describes the measurements well, using fitted constants with physically realistic values.
Modeling the partial oxidation of methane in a short‐contact‐time reactor
Authors: Deutschmann Olaf , Schmidt Lanny D.
Published: April 2004

Partial oxidation of methane in monolithic catalysts at very short contact times offers a promising route to convert natural gas into syngas (H2 and CO), which can then be converted to higher alkanes or methanol. Detailed modeling is needed to understand their complex interaction of transport and kinetics in these systems and for their industrial application. In this work, the partial oxidation of methane in noble‐metal (Rh and Pt)‐coated monoliths was studied numerically as an example of short‐contact‐time reactor modeling. A tube wall catalytic reactor was simulated as a model for a single pore of the monolithic catalyst using a 2‐D flow field description coupled with detailed reaction mechanisms for surface and gas‐phase chemistry. The catalytic surface coverages of adsorbed species are calculated vs. position. The reactor is characterized by competition between complete and partial oxidation of methane. At atmospheric pressure, CO2 and H2O are formed on the catalytic surface at the entrance of the catalytic reactor. At higher pressure, gas‐phase chemistry becomes important, forming more complete oxidation products downstream and decreasing syngas selectivity by about 2% at 10 bar. Temperature (from 300 to ∼ 1,200 K), velocity, and transport coefficients change very rapidly at the catalyst entrance. The dependence of conversion and selectivity on reactor conditions was examined.
Reactivity of chars and carbons: New insights through molecular modeling
Authors: Bhatia S. K.
Published: April 2004

A new model proposed for the gasification of chars and carbons incorporates features of the turbostratic nanoscale structure that exists in such materials. The model also considers the effect of initial surface chemistry and different reactivities perpendicular to the edges and to the faces of the underlying crystallite planes comprising the turbostratic structure. It may be more realistic than earlier models based on pore or grain structure idealizations when the carbon contains large amounts of crystallite matter. Shrinkage of the carbon particles in the chemically controlled regime is also possible due to the random complete gasification of crystalline planes. This mechanism can explain observations in the literature of panicle size reduction. Based on the model predictions, both initial surface chemistry and the number of stacked planes in the crystallites strongly influence the reactivity and particle shrinkage. Its test results agree well with literature data on the air‐oxidation of Spherocarb and show that it accurately predicts the variation of particle size with conversion. Model parameters are determined entirely from rate measurements.
Condensation of mixed vapors and thermodynamics
Authors: Vuddagiri Srinivas R. , Eubank Philip T.
Published: April 2004

A literal interface between thermodynamics and transport phenomena occurs in the steady‐state condensation/vaporization of mixed vapors/liquids in wetted‐wall towers. Unfortunately, several prominent transport texts assume that the ratio of molar fluxes is the same as the corresponding ratio of mole fractions at the interface for the phase being formed. This is shown to be true only at particular vertical positions corresponding to the first drop of condensate. The thermodynamics of the condensation/vaporization of mixtures is reviewed to understand why the ratio of pure‐component fluxes at the interface differs from that of corresponding equilibrium mole fractions. The dynamics of steady‐state condensation in wetted‐wall towers is also developed based on the film theory of Colburn and Drew. The temperature/composition diagram is used to trace both transport phenomena and thermodynamic variables at various tower heights. The Ackermann factor is corrected allowing more general use of the design equations when the vapor is nonideal. The usual neglect of liquid‐film resistance to heat transfer is examined. Numerical calculations show an unusual rise in the interfacial temperature in a parallel flow tower immediately below the onset of condensation. Possible and impossible sets of temperature and composition profiles are described.
Resolution of glycoproteins by affinity‐based reversed micellar extraction and separation
Authors: Choe Jaehoon , Vandernoot Victoria A. , Linhardt Robert J. , Dordick Jonathan S.
Published: April 2004

Affinity‐based reversed micellar extraction and separation (ARMES) has proven effective in separating glycoproteins from nonglycosylated proteins from natural sources. The ability of ARMES to resolve closely related glycoproteins is of paramount importance if ARMES is to be used in glycoform resolution. It is demonstrated that ARMES can resolve the structurally similar soybean peroxidase (SBP; MW 37 kDa, pI 4.1) and αt‐acid glycoprotein (AGP; MW 43 kDa, pI 3.7), both of which have affinity for Concanavalin A (Con A) (the affinity ligand). SBP was almost exclusively extracted at pH 8 and above, with a separation factor greater than 50 (resolution ∼ 20), far better than was possible using Con A affinity chromatography (R ∼ 0.25, separation factor ∼ 2). Model calculations suggest that differences in affinity measured by an equilibrium‐building assay cannot account for the favorable extraction of SBP over AGP at higher pH. Hydrophobic interactions and/or charge shielding appear to affect partitioning of the lectin ‐ glycoprotein complexes and add greatly to the selectivity of extraction in ARMES, especially at higher pH values.
NMR imaging of pulp suspension flowing through an abrupt pipe expansion
Authors: Arola Darren F. , Powell Robert L. , McCarthy Michael J. , Li Tie‐Qiang , Ödberg Lars
Published: April 2004

Nuclear magnetic resonance imaging was used to measure velocity profiles of water and an aqueous, 0.5 wt. % (consistency) wood pulp suspension flowing through a circular pipe in the vicinity of a 1:1.7 tubular expansion. Flow images are provided for various flow rates and axial positions with respect to the expansion plane. For water, both laminar and turbulent flows were investigated. Pulp suspension flows were investigated upstream and downstream from the expansion plane. The expansion plane imparts shear layer instabilities that disrupt the fiber floe network. The fiber reflocculation length is proportional to the bulk flow rate. We determined a mean reflocculation time of approximately 510 ms for this pulp fiber consistency.
Rational transfer function models for biofilm reactors
Authors: Wik Torsten , Breitholtz Claes
Published: April 2004

Design of controllers and optimization of plants using biofilm reactors often require dynamic models and efficient simulation methods. Standard model assumptions were used to derive nonrational transfer functions describing the fast dynamics of stirred‐tank reactors with zero‐ or first‐order reactions inside the biofilm. A method based on the location of the singularities was used to derive rational transfer functions that approximate nonrational ones. These transfer functions can be used in efficient simulation routines and in standard methods of controller design. The order of the transfer functions can be chosen in a natural way, and changes in physical parameters may directly be related to changes in the transfer functions. Further, the mass balances used and, hence, the transfer functions, are applicable to catalytic reactors with porous catalysts as well. By applying the methods to a nitrifying trickling filter, reactor parameters are estimated from residence‐time distributions and low‐order rational transfer functions are achieved. Simulated effluent dynamics, using these transfer functions, agree closely with measurements.
Dynamically scheduled MPC of nonlinear processes using hinging hyperplane models
Authors: Chikkula Yugender , Lee Jay H. , Ogunnaike B. A.
Published: April 2004

This article deals with the issues associated with designing scheduled model predictive controllers for nonlinear systems within the multiple‐linear‐model‐based control framework. The issues of model set generation from empirical data and closed‐loop application of the generated model set are considered. A method of hinging hyperplanes is proposed as a way to construct a piecewise linear dynamic model, conducive to dynamic scheduling of linear MFC controllers. The design and implementation of dynamically scheduled MFC using a hinge function model are discussed, as well as its advantages. Alternate MFC formulations considered here require more computation, but utilize the hinge function model as a global model. Simulated examples of isothermal CSTRs and a batch fermenter are also presented to illustrate the proposed methodologies.
Design of multistage extractive reaction processes
Authors: Samant Ketan D. , Ng Ka M.
Published: April 2004

Multistage extractive reaction (MSER) processes can successfully improve the yield of and selectivity to the desired product, as well as separation of byproducts. A general and efficient design procedure for stand‐alone countercurrent MSER cascades and process flowsheets presented here determines the range of feasible operation for the MSER process, the number of stages in the MSER cascade, compositions and molar flow rates at each stage, and the yield or product distribution of the reaction system. It is based on the geometric properties of the phase and reaction equilibrium surfaces and the composition profiles of the MSER cascade. First, a key design variable is identified and the fixed points or pinches of the composition profiles are tracked as a function of this variable by using arc‐length continuation, without performing stage‐to‐stage calculations. The location of the fixed points gives the upper or lower bound on the window of the MSER operation. The other bound is determined by the equilibrium behavior of the system. Within the window of operation MSER profiles are guaranteed to be feasible. Then, stage‐to‐stage calculations are performed as an initial‐value problem to complete the design. The results of these calculations provide a comprehensive picture of the trade‐offs in the design of MSER processes and can be used as a basis for screening flowsheet alternatives. The procedure is illustrated with several design examples.
Rapid estimation of rate constants of batch processes using on‐line SW‐NIR
Authors: Bijlsma Sabina , Louwerse D. J. , Smilde Age K.
Published: April 2004

A method is presented for estimating reaction rate constants very rapidly from on‐line short‐wavelength near‐infrared measurements. It can deal with a strong spectral overlap between the individual spectra of the species in the reacting system. For this an iterative curve‐resolution method is combined with a nonlinear fitting procedure. This procedure also makes it possible to assess the quality of the estimated reaction rate constants. Simulations show that the method can be applied in cases with extreme spectral overlap of the individual spectra of the species. The procedure is applied in practice to an example with a strong spectral overlap. Reaction rate constants of the two‐step epoxidation of 2,5‐di‐tert‐butyl‐1,4‐benzoquinone were estimated. This method can lead to the application to on‐line monitoring reaction rate constants.
Volume‐explicit equation of state for hard chains, their mixtures, and copolymers
Authors: Hamad Esam Z.
Published: April 2004

Volume‐explicit equations of states for hard chains, their mixtures, and copolymers were developed. The equations accurately represent the simulation data. They also predict the correct volume at high pressures (close packing), unlike the common pressure‐explicit equations, which predict a physically unattainable volume. Entropic demixing is also predicted by the proposed equations. The developed equation can be used as the repulsive pan in developing volume‐explicit real polymer equations of state or excess Gibbs energy models.
Practical friction‐based approach to modeling multicomponent diffusion
Authors: Zielinski John M. , Hanley Brian F.
Published: April 2004

Although most industrial drying and separation technologies are based on transport of multiple components, a convenient and practical means of describing multicomponent diffusion over wide temperature and concentration ranges has not been developed for polymer solutions or even for simple liquid mixtures. We have been able to demonstrate that all of the diffusion coefficients required to describe molecular transport in multicomponent systems can be easily predicted from self‐diffusion coefficients and thermodynamic information when the mass flux with respect to the mass‐average velocity is expressed in terms of the frictional force experienced by a molecule as it undergoes Brownian motion. This approach enables the flux equations for any combination of materials to be written in terms of experimentally measurable quantities. The model was tested by: (1) predicting literature data for principal diffusion coefficients in a ternary system; (2) applying the diffusion model to predict the drying behavior of three ternary systems; (3) predicting experimental values of the principal and cross coefficients for a ternary organic solvent mixture from self‐diffusion data measured by PFG‐NMR and available thermodynamic data; and (4) examining whether the limiting cases in this model are physically realistic. The applicability of the model to gas transport and polymer interdiffusion is also briefly examined.
Large eddy simulations on the flow driven by a Rushton turbine
Authors: Derksen Jos , Van den Akker Harry E. A.
Published: April 2004

Large eddy simulations were performed on the flow in a baffled stirred tank, driven by a Rushton turbine at Re = 29,000. The simulation procedure consisted of a lattice–Boltzmann scheme for discretizing the Navier–Stokes equations, and a force‐field technique for representing the action of the impeller on the fluid. The subgrid‐scale model was a conventional Smagorinsky model with a Smagorinsky constant cs = 0.12. The uniform, cubic computational grid had a size of about 6 × 106 nodes. The computer code was implemented on a parallel, shared‐memory computer platform. The results on the phase‐resolved average flow, as well as on the turbulence characteristics, are compared with phase‐resolved experimental data. The trailing vortex structure in the vicinity of the impeller was well represented by the simulations.
Deposition of sodium sulfate in a heated flow of supercritical water
Authors: Rogak Steven N. , Teshima Paul
Published: April 2004

Wastes that can be treated by supercritical water oxidation often contain salts. Salts are almost insoluble under supercritical conditions and can result in severe fouling. A simple heat‐ and mass‐transfer model was developed and tested experimentally for sodium sulfate in a fully turbulent flow of water at 25 MPa. This model uses empirical heat‐transfer correlations to estimate mass‐transfer rates. The diffusion coefficient of the salt is calculated from the Stokes‐Einstein relation using a hydrodynamic diameter of 2 to 6 Å. New measurements of solubility showed that the solubility of sodium sulfate decreases by a factor of about 1,000 as the temperature increases from 380°C to 400°C. Salt deposition rates, inferred from the outside temperature of a heated test section, were reasonably close to the model predictions.
Sorption‐enhanced reaction process for hydrogen production
Authors: Hufton J. R. , Mayorga S. , Sircar S.
Published: April 2004

A novel concept called Sorption Enhanced Reaction Process (SERP) for hydrogen production by steam‐methane reformation (SMR) reaction uses a fixed packed column of an admixture of an SMR catalyst and a chemisorbent to remove carbon dioxide selectively from the reaction zone. The chemisorbent is periodically regenerated by using the principles of pressure swing adsorption. The SERP process steps allow direct production of high‐purity hydrogen (>95 mol %) at high methane to hydrogen conversion (>80%) with dilute methane (<5 mol%) and trace carbon oxide (∼50 ppm) impurities at the reaction pressure by operating the reactor at a low temperature of 450°C. A conventional plug‐flow reactor packed with catalyst alone not only needs to be operated at a much higher temperature (>650°C) to achieve the same methane to hydrogen conversion, but produces a much lower purity of hydrogen product (∼75 mol %) with a large quantity of carbon oxide (∼20 mol %) impurities. A novel chemisorbent, which reversibly sorbs carbon dioxide in the presence of excess steam at a temperature of 300–500°C, was developed for application in the SERP and the process is experimentally demonstrated in a bench‐scale apparatus.
Perovskite‐related ZrO2‐doped SrCo0.4 Fe0.6O3‐δ membrane for oxygen permeation
Authors: Li Shiguang , Jin Wanqin , Huang Pei , Xu Nanping , Shi Jun , Hu Michael Z.‐C. , Payzant E. Andrew , Ma Y. H.
Published: April 2004

A typical perovskite La0.2Sr0.8Co0.2Fe0.8O3‐δ (LSCF) membrane and a novel perovskite‐related ZrO2‐doped SrCo0.4Fe0.6O3‐δ (SCFZ) membrane were successfully prepared. The sintered membranes were characterized by an in‐situ high‐temperature X‐ray diffraction, scanning electron microscopy and energy dispersive spectroscopy. Oxygen permeabilities of the two dense membranes were measured in the temperature range of 923 to 1,173 K under a fixed oxygen partial pressure gradient. The results of the oxygen permeation experiment and high‐temperature X‐ray diffraction in argon atmosphere indicate that the perovskite‐related SCFZ membrane has higher oxygen fluxes and is stable than that of the LSCF membrane at elevated temperatures and reduced oxygen partial pressures.
Isolation enhanced principal component analysis
Authors: Gertler Janos , Li Weihua , Huang Yunbing , McAvoy Thomas
Published: April 2004

Principal component analysis (PCA) may reduce the dimensionality of plant models significantly by exposing linear dependences among the variables. While PCA is a popular tool in detecting faults in complex plants, it offers little support in its original form for fault isolation. However, by utilizing the equivalence between PCA and parity relations, all the powerful concepts of analytical redundancy may be transferred to PCA. Following this path, it is shown how structured residuals, which have the same isolation properties as analytical redundancy residuals, are obtained by PCA. The existence conditions of such residuals are demonstrated, as well as how disturbance decoupling is implied in the method. The effect of the presence of control constraints in the training data is analyzed. Statistical testing methods for structured PCA residuals are also outlined. The theoretical findings are fully supported by simulation studies performed on the Tennessee Eastman process.