bluuebunny/crossref_metadata_2025 huggingface dataset:
Model predictive control technique combined with iterative learning for batch processes
Authors: Lee Kwang S. , Chin In‐Shik , Lee Hyuk J. , Lee Jay H.
Published: April 2004

A novel model predictive control technique geared specifically toward batch process applications is demonstrated in an experimental batch reactor system for temperature tracking control. The technique, called Batch‐MPC (BMPC), is based on a time‐varying linear system model (representing a nonlinear system along a fixed trajectory) and utilizes not only the incoming measurements from the ongoing batch, but also the information stored from the past batches. This particular feature is shown to be essential for achieving effective tracking control performance despite model errors and disturbances. In a series of experiments performed on a bench‐scale batch reactor system, the technique was found to deliver satisfactory tracking performance, as expected, overcoming a large amount of model uncertainty and various process disturbances.
Micromixing efficiency of a novel sliding‐surface mixing device
Authors: Rousseaux Jean‐Marc , Falk Laurent , Muhr Hervé , Plasari Edouard
Published: April 2004

A new mixing device, suitable to achieve the mixing of reactants undergoing fast chemical reactions, is studied. In the sliding‐surface mixing device, the chemical reagents are introduced in a confined mixing zone (CMZ), in which high shearing stresses are created in a very small volume to realize rapid micromixing. A system of parallel competing test reactions was used to characterize the micromixing level in the CMZ as a function of feed flow rate, rotation speed, feed pipe position, and a geometric parameter characterizing the CMZ. The influence of these parameters on micromixing efficiency was studied, and a hydrodynamic model was developed. This model successfully predicts micromixing times that were measured at two different flow regimes, two feed points, four recirculation flow rates, three geometric configurations, and various stirrer speeds.
Thermal decomposition of ethylene–comonomer mixtures under high pressure
Authors: Albert Jens , Luft Gerhard
Published: April 2004

Ethylene is unstable above certain limits of pressure and temperature decomposing into carbon, hydrogen, and methane. The influence of comonomers on this limit were studied. Vinyl acetate, acrylic acid, methyl methacrylate, 1‐butene, and 1‐hexene were added in various concentrations to ethylene and by filling the mixture into a preheated autoclave under high pressure (up to 450°C and 150 MPa). As a function of starting pressure and temperature, two reaction pathways can be distinguished: (1) stable (co‐) polymerization and (2) (co‐)polymerization with decomposition. Lowering the boundary between these two pathways seemed to be caused mainly by the thermal stability of the comonomer, for mixtures with vinyl acetate and methyl methacrylate, while an inhibition effect and increase in the limits occur in the comonomer acrylic acid, 1‐butene, and 1‐hexene. The maximum values of pressure, pressure increase, and temperature during decomposition were also influenced by comonomers. The maximum explosion pressure as a function of mixture density has a maximum for pure ethylene. As a function of ethylene concentration, the maximum explosion pressure has minimum values for pure ethylene. At a constant wall temperature and small initial pressures, the maximum rate of pressure increases reached a maximum, while, at high pressures, it decreased again. The position of this maximum depends on wall temperature and comonomer. Adding comonomers to ethylene decreases the maximum temperatures during decomposition reaction.
Distributed parameter approach to the dynamics of complex biological processes
Authors: Lee T. T. , Wang F. Y. , Newell R. B.
Published: April 2004

Modeling and simulation of a complex biological process for the removal of nutrients (nitrogen and phosphorus) from municipal wastewater are addressed. The model developed in this work employs a distributed‐parameter approach to describe the behavior of components within three different bioreaction zones and the behavior of sludge in a secondary settler. Results from a comprehensive experimental validation on an existing pilot‐scale biological‐nutrient‐removal activated‐sludge plant show that model prediction of the components' dynamics is generally good with the exception of acetate in the anaerobic zone and soluble phosphate in the aerobic zone in two experiments. Good results are achieved despite the apparent plant–model mismatch, such as uncertainties with the behavior of phosphorus‐accumulating organisms. Validation of the proposed secondary‐settler model shows that it is superior to two state‐of‐the‐art models in terms of the sum of the square relative errors.
Ligand‐grafted biomaterials for adsorptive separations of uranium in solution
Authors: Hu Michael Z.‐C. , Reeves Mark
Published: April 2004

Many organic molecules, particularly biologicals, contain functional groups (ligands) that actively interact with metal ions in solution by adsorption, ion exchange, or chelation/coordination/complexation. Water‐soluble organics have limitations as reagents for metal‐ion separations from aqueous solutions. However, if the ligand molecule(s) are grafted on to an insoluble matrix, the resulting ligand(s)‐containing product becomes useful for separations applications related to metal recovery or remediation. It was discovered that biomolecules containing a primary amino group, secondary amino group, or hydroxyl group could be grafted into a polyurethane polymeric network via in situ polymerization reactions. With carboxyl groups, grafted material showed good selectivity among a group of divalent metal cations, and a uranium‐binding capacity of more than 10 mg/g of polymer. The material can be regenerated by sodium bicarbonate or sodium carbonate solution and reused. Data from a stirred‐tank reactor showed fast uranium‐binding kinetics, and breakthrough‐elution studies with a packed‐column reactor indicated promising process behavior.
Development of liquid‐phase agitated reactors: Synthesis, simulation, and scaleup
Authors: Samant Ketan D. , Ng Ka M.
Published: April 2004

A systematic procedure is formulated for the development of liquid‐phase agitated reactors. The procedure has three components: synthesis, simulation, and scaleup. Synthesis leads the user to the reactor geometry, agitator type and speed, number and location of feed addition ports, feed addition or residence time, and heat‐transfer policy that would achieve the desired performance objectives. It is based on an analysis of the interplay of reaction and mixing at various length scales, and a knowledge base of the capacities of heat‐transfer equipment. Simulation assesses the performance of the reactors thus synthesized, taking into account detailed flow properties obtained through computational fluid dynamic simulations, experimental data, or a combination of both. Scaleup provides fundamentally based scaleup rules for direct development of these reactors from laboratory scale to production scale. A comparison of these derived scaleup rules with published empirical rules reveals the underlying physics, applicability, and limitations of these empirical rules. Three examples are presented to illustrate the use of this procedure.
Heat transfer from catalysts with computational fluid dynamics
Authors: McKenna Timothy F. , Spitz Roger , Cokljat Davor
Published: April 2004

A computational fluid dynamics software package was used to study heat transfer from spherical particles of different sizes and under different heat‐transfer conditions. It was shown that although the Ranz‐Marshall and other similar correlations are valid in the case where particles do not interact, this is not true for densely packed systems such as those that we find in reactors commonly used in olefin polymerization. It was also demonstrated that convection is in fact not the only means of removing heat from small, highly active particles. Conductive heat transfer between large and small particles present in the same reactor appears to help alleviate problems of overheating and explain why earlier models of heat transfer in olefin polymerization overpredict the temperature rise during early stages of polymerization.
On‐Line runaway detection in batch reactors using chaos theory techniques
Authors: Strozzi F. , Zaldívar J. M. , Kronberg A. E. , Westerterp K. R.
Published: April 2004

In this work nonlinear time‐series analysis using delay coordinate embedding was applied to simulated temperature data from isoperibolic batch reactors to develop an early‐warning detection system of the runaway. In the first part of this study an early‐warning detection criterion, that is, when the divergence of the system becomes positive on a segment of the reaction path, was developed and compared with previous criteria. This criterion has the property of being preserved under phase‐space reconstruction and, hence, it is possible to calculate it when only temperature data are available. This reconstructed divergence is compared with the theoretically calculated divergence using temperature and conversion data. The comparison demonstrates the validity of such an approach finding a new path, lying between the simple use of measured variables and the complex model‐based reconstruction techniques, to assess in advance hazardous situations in chemical reactors.
Simple equation of state for hard‐sphere chains
Authors: Sadus Richard J.
Published: April 2004

A simplified thermodynamic perturbation theory‐dimer framework is used to derive an equation of state for hard‐sphere chains. The accuracy of the equation is tested against simulation data for hard‐sphere chains containing up to 201 hard‐sphere segments. A comparison is also presented with the results of other more sophisticated hard‐sphere chain equations of state. The equation reproduces the compressibility factor of hard‐sphere chains obtained from molecular simulation with a reasonable degree of accuracy. The equation can potentially form the theoretical backbone of an equation of state for real chainlike molecules. From a practical perspective, it has the advantage of being considerably simpler than alternative hard‐sphere chain equations.
Amino‐acid extraction using D2EHPA: New description of equilibrium behavior
Authors: Tulasi G. L. , Kumar Sanjeev
Published: April 2004

A new reaction mechanism to explain the equilibrium behavior of amino‐acid extraction using di(2‐ethylhexyl)phosphoric acid (D2EHPA) is proposed. This mechanism represents the interaction between amino acid and D2EHPA through a set of two reactions, one at the aqueous–organic interface and the other in the bulk of the organic phase. It quantitatively predicts the hitherto unexplained dependence of apparent equilibrium constant on a single parameter—loading ratio (ratio of concentration of amino acid in the organic phase at equilibrium to initial concentration of carrier). At low loading ratios, reaction in the bulk plays an insignificant role, and interfacial reaction alone can explain the equilibrium behavior. At high loading ratios, both reactions need to be considered. The new mechanism is tested for the extraction of two amino acids, phenylalanine and tryptophan, for which the equilibrium data have been published. New data were also obtained for phenylalanine to test it over a much wider range of parameter values. The proposed mechanism can predict all the data very well.
On‐line estimation of the specific growth rate in the bacitracin fermentation process
Authors: Golobič Iztok , Gjerkeš Henrik , Malenšek Jože
Published: April 2004

An on‐line estimation of the specific growth rate of an aerobic fermentation batch process was conducted based on measurements of oxygen or carbon dioxide concentrations in exhaust gas, or the concentrations of dissolved oxygen in the liquid phase in the fermentor. A simple linear model used was upgraded with a mechanism for a time‐varying forgetting factor to enable good tracking of estimated parameters as well during rapid and large changes in the process. Because the measuring methods used are independent of each other and can identify the process well, mutual control of individual sensors is also possible in industrial reactors. The results from the model agree well with experimentally obtained values for bacitracin fermentation in an 80‐m3 and 1.5‐m3 batch bioreactor. The model provides a good basis for robust adaptive regulation of aerobic batch processes.
Bubble nucleation during devolatilization of polymer melts
Authors: Yarin A. L. , Lastochkin D. , Talmon Y. , Tadmor Z.
Published: April 2004

Devolatilization of different types of polymer melts was studied experimentally and theoretically. The experimental work was carried out in a “falling‐strand”‐type apparatus built especially for this study. Polymer samples after devolatilization were examined by scanning electron microscopy. The micrographs obtained showed that strong nucleation of bubble nuclei occurs first in the vicinity of existing primary microbubbles. These bubble nuclei (secondary nuclei) form blisters (microbubbles) growing under appropriate conditions. The growth is controlled by momentum transfer and diffusion. The microbubbles then coalesce leading to formation of large voids and foaming of the polymer strands. The key parameter, the critical bubble radius triggering the process of growth, was assessed from the micrographs. The time dependence of the concentration of the residual styrene (dissolved gas) in polystyrene melt was determined by gas chromatography. A theoretical model of stress‐enhanced secondary nucleation in polymer melts during their devolatilization was developed. It shows that nucleation proceeds due to the growth of initially existing gas bubbles, when a sample is exposed to vacuum. In the vicinity of the bubbles homogeneous nucleation proceeds at an accelerated rate, because it is strongly enhanced by mechanical degradation of polymer macromolecules. The rate of devolatilization was calculated and compared with the experimental data. Theoretical results agreed with the experimental data rather good, although no adjustable parameters characteristic of some of the existing nucleation models were involved.
Raman spectroscopic determination of hydration number of methane hydrates
Authors: Uchida Tsutomu , Hirano Takashi , Ebinuma Takao , Narita Hideo , Gohara Kazutoshi , Mae Shinji , Matsumoto Ryo
Published: April 2004

Hydration numbers of methane hydrates with various water/gas ratios of the sample were measured by Raman spectroscopy. Artificial methane hydrates were formed at temperatures ranging from 273.2 to 278.4 K, and pressures from 3.0 to 7.0 MPa, with a stirring rate of approximately 500 rpm. Under such nonequilibrium conditions, the obtained hydrate samples had a water/gas ratio ranging from 6.4 to 17.4, which was determined by mass measurements. The spectroscopic analysis revealed, however, that the crystallographic hydration number was almost constant at 6.2±0.2, which corresponds to the hydration number estimated from the thermodynamic model. The same measurements were carried out on natural gas hydrate samples obtained from the deep‐sea floor of Blake Ridge. The coincidence of the hydration numbers between natural and artificial samples indicated that the crystallographic hydration number is independent of the formation conditions and the large variation in the water/gas ratio of samples is described by the ratio between pure hydrate and free water.
Onset of solutal marangoni convection in a suddenly desorbing liquid layer
Authors: Kang Kyoung H. , Choi Chang K. , Hwang In G.
Published: April 2004

The onset of surface‐tension‐driven convection in an initially quiescent liquid layer experiencing sudden desorption from a free surface is analyzed using the linear stability theory. The propagation theory was developed for the basic time‐dependent concentration profile, which is strongly nonlinear. Based on this theory, a new set of stability equations is derived in considering the effect of Gibbs adsorption and by neglecting the surface diffusion. It is found that the liquid layer becomes more stable by decreasing the Schmidt and Biot numbers. It is interesting that, during an initial period, the system is stable, depending on the Gibbs effect. The predictions agree favorably with the existing experimental results of triethylamine desorption in water in a wetted‐wall and a liquid‐jet column.
Dynamic behavior of freely rising buoyant solid spheres in non‐Newtonian liquids
Authors: Dewsbury K. H. , Karamanev D. G. , Margaritis A.
Published: April 2004

The results of the hydrodynamic study of rising solid spheres in non‐Newtonian (pseudoplastic) fluids are reported. The hydrodynamic parameters including drag coefficient, trajectory of rise, and terminal velocity were determined. The behavior of free‐rising spheres was compared to that of falling spheres in the same type of fluids to see if any differences exist. It was observed that rising solid spheres behave differently from falling solid spheres at higher Reynolds numbers in non‐Newtonian (pseudoplastic) fluids. At high Reynolds numbers rising solid spheres display a spiraling motion, while falling spheres display a linear trajectory. The angle between the velocity vector and horizontal plane of the spiraling trajectory was 60°. The spiraling trajectory led to a drag coefficient that was more than two times higher for rising solid spheres than that established for falling spheres in the Newton's law range in a non‐Newtonian (pseudoplastic) liquid. The first ever correlation is proposed to calculate the drag coefficient of freely rising light solid spheres in non‐Newtonian (pseudoplastic) fluids.
Riser hydrodynamics: Simulation using kinetic theory
Authors: Neri Augusto , Gidaspow Dimitri
Published: April 2004

Computational fluid dynamics (CFD) for fluidization is reaching maturity (Roco, 1998). It has become common practice to compare time‐averaged solid velocities and concentrations to measurements of fluxes and densities. The dynamic behavior of the riser, however, has not been previously compared to experiments. This article shows that the dynamics of solids flow in the riser is in the form of clusters, but the time‐averaged particle concentrations and fluxes give us the core‐annular flow regime in agreement with measurements. The computed clusters, which are essentially compressible gravity waves, produce major frequencies of density oscillations in agreement with measurements. The model and the CFD code compute granular temperature distributions, agreeing qualitatively with data. For volume fraction around 3–4%, which is the average particle concentration in the riser, the computed viscosity agrees with our experimental measurements.
Effects of microstructure on the compressive yield stress
Authors: Channell Glenn M. , Miller Kelly T. , Zukoski Charles F.
Published: April 2004

The effects of microstructure on the compressive properties of aggregated alumina suspensions are determined by intentionally introducing heterogeneities into the suspension. Suspensions are prepared at a high volume fraction and diluted with low shear hand mixing to a series of initial concentrations. As the initial concentration is increased, larger heterogeneities are introduced, and the suspension becomes more compressible relative to the compressive yield stress of the uniform suspension. A simple model is proposed in which the heterogeneous suspensions compress by rearrangement of the dense aggregates until a critical concentration (ϕc, which coincides with the volume fraction prior to dilution) is reached. Above ϕc, the suspensions consolidate identically to the uniform suspension. With a single fitting parameter (the size of the heterogeneities), the model shows semiquantitative agreement with the experimental data.
Filtration of highly compactible filter cake: Variable internal flow rate
Authors: Lee D. J. , Ju S. P. , Kwon J. H. , Tiller F. M.
Published: April 2004

Conventional analyses used to interpret filtration data assume a constant internal liquid flow rate (qL) and a negligible solid velocity (the constant‐qL approximation). To interpret such data, the upper bounds of errors to estimate αav and kav were analytically derived by the constant‐qL approximation. The solution of the filtration model that incorporates a nonzero solid velocity was then analytically derived. When filtering a highly compactible filter cake, the cake was first compacted toward the filter medium that forms a skin layer and, in doing so, rapidly reaches the steady‐state distribution predicted by the constant‐qL approximation. Except at the first stage of the filtration, the approach of Tiller et al. (1999) is valid for interpreting filtration data of a highly compactible filter cake. Furthermore, the constant‐qL approximation provides the upper limit of errors in terms of estimating cake characteristics regardless of the solid velocity effect.
Adsorption behavior of surfactants and mass transfer in single‐drop extraction
Authors: Chen Liang‐Huei , Lee Yuh‐Lang
Published: April 2004

Three surfactants—SDS, Triton X‐100, and DTMAC—were used to study effects of surfactants on the mass transfer in liquid–liquid extraction. Single‐drop extraction apparatus was used to obtain the mass‐transfer coefficient of acetic acid from dispersed CCl4 droplets to the continuous water phase. The excess mass‐transfer resistance was related to the adsorption behavior of surfactants on the interface. All of these surfactants significantly reduced the mass‐transfer rate. Mass‐transfer coefficient, KR, decreases rapidly with increasing surfactant concentration and reaches a minimum at concentrations less than 0.04 mM. The effectiveness of the surfactants in decreasing the mass‐transfer rate increases in the order: DTMAC<SDS = Triton X‐100. This effectiveness has no direct relation with the ionic types of surfactants, but is affected by the adsorption rate and adsorbed surface concentration on the interface. A slight increase in the mass‐transfer coefficient was found at higher concentrations of Triton X‐100 due to its higher adsorption kinetics. The excess mass‐transfer resistance exerted by the surfactant mainly resulted from the obstruction of the adsorbed barrier layer. The contribution of the inhibition of interfacial flow to the mass‐transfer resistance was considered to be minor.
Crystal growth rates and dispersion for D‐fructose from aqueous ethanol
Authors: Flood Adrian E. , Johns Michael R. , White Edward T.
Published: April 2004

The growth of fructose crystals from aqueous ethanolic solutions was studied using a 1‐L seeded batch crystallizer. The growth kinetics were found to linearly depend on the relative supersaturation of the crystallizing tautomer (β‐D‐fructopyranose). The growth‐rate constant increased slightly with increasing temperature and increasing solvent ethanol content. The growth rates are lower than those for aqueous solutions of comparable supersaturations. Fructose displays significant growth‐rate dispersion (q = 0.35) when crystallized from aqueous ethanolic solutions. The growth‐rate dispersion is independent of solvent composition and temperature within the range studied, although it is slightly higher than is seen in crystallization from aqueous solutions.
SMB enantioseparation: Process development, modeling, and operating conditions
Authors: Lehoucq Sylvie , Verhève Didier , Vande Wouwer Alain , Cavoy Emile
Published: April 2004

The design of a simulated moving bed (SMB) pilot used for chiral drug separation is presented. On‐line concentration measurements allow the observation of detailed concentration profiles along the SMB columns. Based on an extensive set of experimental data, an equivalent countercurrent model was derived. In this model, mass‐transfer kinetics are described by linear driving‐force models, while the equilibrium concentrations are given by modified competitive Langmuir isotherms. The isotherm parameters are estimated using a peak fitting procedure, which starts with initial values given by a simple retention‐time method. The dispersion phenomena are estimated through the variation of the HETP with the fluid‐phase velocity. The good agreement between the model and the real system is demonstrated by several cross‐validation results corresponding to the separation of given racemic mixtures under various operating conditions. The model defines a zone of complete separation and allows optimal operating conditions to be selected.
Flexibility analysis and design of linear systems by parametric programming
Authors: Bansal Vikrant , Perkins John D. , Pistikopoulos Efstratios N.
Published: April 2004

A new, unified theory and algorithms, based on multiparametric programming techniques, for the solution of flexibility analysis and design optimization problems in linear process systems are presented. They are used for the flexibility test and index problems in systems with deterministic parameters, and for the stochastic and expected stochastic flexibility evaluation problems in systems with stochastic parameters. They are computationally efficient and give the explicit dependence of various flexibility metrics on the values of the continuous design variables. The latter feature enables the easy and efficient comparison of design alternatives. It also allows for the compact formulation of design optimization problems that can be solved parametrically to yield the exact algebraic form of the trade‐off curve of economics against flexibility. Key features of the proposed approach are demonstrated through both mathematical and process examples.
Effective extraction of titanium from rutile by a low‐temperature chloride process
Authors: Yang Fenglin , Hlavacek Vladimir
Published: April 2004

The carbochlorination of rutile is carried out at high temperatures (800–1,500°C) in the commercial chloride processes of the pigment industry. Chlorination at high temperatures results in high energy consumption, rapid corrosion of the equipment, pipeline and control system, and agglomeration of solid reactants in the reactor. In a typical chloride process, rutile is used as a primary reactant, Cl2 as a chlorinating agent, and carbon as a reductant. The thermodynamic equilibrium simulation of the reaction system shows that a complete conversion of rutile is feasible at a temperature as low as 200°C. Kinetic and diffusion barriers, however, make the reaction practically impossible at low temperatures. In the study, these barriers were removed by intensifying the rutile‐carbon solid–solid contact. In the low‐temperature chloride process developed, reaction temperatures are reduced to 350–450°C. The conversion rate of the rutile at 385°C is 30% higher than that at 1,000°C. A fluidized‐bed reactor was used for the chlorination process. It was concluded that the formation of an activated TiO2CCl complex on the TiO2/C interface accounts for the gas–solid–solid reaction mechanism.
Thermodynamically consistent modeling of a liquid‐phase nonisothermal packed‐bed reactor
Authors: Syed Faisal H. , Datta Ravindra , Jensen Kyle L.
Published: April 2004

The equilibrium‐limited steady‐state nonisothermal behavior of an integral packed‐bed reactor is described for the liquid‐phase synthesis of ethyl tert‐butyl ether with a model that is inherently consistent with thermodynamics. It involves a coupled one‐dimensional mass‐ and enthalpy‐balance equation with temperature‐dependent thermochemical properties and reaction equilibria. The model accounts for the nonideal nature of the liquid‐phase reaction system by utilizing species activities in the rate expression for the reversible reaction, which is also consistent with thermodynamic equilibrium. Experiments agreed well with theory with all of the kinetic, thermodynamic and other physicochemical parameters being obtained from independent experiments, indicating that the theoretical approach is robust. Further, the results clearly endorse the use of species activities rather than concentration, or mole fractions, in the rate expression for the nonideal liquid‐phase reactions. There is a paucity of such studies in the literature, especially for liquid‐phase nonisothermal packed‐bed reactor analysis.
Newtonian radial entrance flow
Authors: Chatterjee Ajay
Published: April 2004

The laminar flow of a Newtonian fluid is considered through a prototype radial entrance geometry that consists of a circular pipe joined to the bottom plate of a pair of parallel circular plates, a configuration that mimics and idealizes important practical flows. The equations of motion are solved numerically to compute the velocity field as a function of the Reynolds number Re and the geometrical aspect ratio e. This work extends limited previous results on flow development in this entrance geometry. A range of e and Re are considered, and the Re–e parameter space corresponding to flow separation is delineated. The computed results are used to illustrate the role of the aspect ratio in influencing flow development for small e≲1 and are analyzed to quantify the large e asymptotic behavior of various quantities characterizing the developing flow field. The limiting value of aspect ratio beyond which the asymptotic behavior applies is relatively small e≳2. The dependence of these quantities on Re also is analyzed. The results are discussed in light of available experimental measurements and existing analytical results. An important implication of the results in the context of confined laminar impinging jet heat transfer is described. The entrance configuration is contrasted with a companion radial entrance flow (radial source flow); although flow separation occurs in both cases, some basic features are quite distinct. The influence of the elongational character of this radial flow field is highlighted by comparing certain aspects of flow development between this axisymmetric configuration and its 2‐D Cartesian analog.
Gas dispersion in downflowing high velocity fluidized beds
Authors: Cao Chunshe , Weinstein Herbert
Published: April 2004

Measurements are presented of gas mixing in a downflowing high‐velocity fluidized bed using helium tracer detected by an on‐line analyzer. It was found that gas backmixing is very limited in the downer, and the lateral mixing is comparable with that in the riser. A dispersion model is applied to characterize the behavior of the gas mixing in the two‐phase downflowing mixture.
Pricing interprocess streams using slack auctions
Authors: Jose Rinaldo A. , Ungar Lyle H.
Published: April 2004

When plantwide optimization problems must be decomposed and solved in pieces due to numerical complexity or practical concerns, it is natural to view each plant division as a separate profit maximizer. For a division to maximize its profit, however, it must know the prices of interprocess streams. It is claimed that the correct way of finding these prices is to define slack resources over the process streams and price them using slack auctions. When slack auctions terminate properly, it can be shown that the plant divisions' solutions are globally optimal. How slack auctions are related to Lagrangean relaxation and how they can overcome some of the problems that standard decomposition and relaxation techniques have are discussed.
Phase‐equilibrium fractionation of continuous distributions
Authors: McCoy Benjamin J. , Sterling W. Jerome
Published: April 2004

Fractionation of continuously distributed homologous mixtures is described in terms of molecular distributions. The concepts apply to phase equilibrium in vapor–liquid hydrocarbon mixtures and sol–gel polymer mixtures. Models of both differential and finite phase‐equilibrium separations are presented when the partition coefficient for the equilibrium distributions is given. Analytical solutions for the distributions, including a similarity solution for a differential fractionation, can be formed. A differential fractionation solution compares satisfactorily with experimental data.
Separation of light gas mixtures using SAPO‐34 membranes
Authors: Poshusta Joseph C. , Tuan Vu A. , Pape Eric A. , Noble Richard D. , Falconer John L.
Published: April 2004

Continuous SAPO‐34 membranes were prepared on porous alumina tubular supports, and shown to be useful for light gas separations at low and high temperatures. Single‐gas permeances of CO2, N2 and CH4 decreased with increasing kinetic diameter. For the best membrane at 300 K, the He and H2 permeances were less than that of CO2, because He, H2, and CO2 were small compared to the SAPO‐34 pore, and differences in the heat of adsorption determined the permeance order. The smaller component permeated the fastest in CO2/CH4, CO2/N2, N2/CH4, H2/CH4 and H2/N2 mixtures between 300 and 470 K. For H2/CO2 mixtures, which were separated by competitive adsorption at room temperature, the larger component permeated faster below 400 K. The CO2/CH4 selectivity at room temperature was 36 and decreased with temperature. The H2/CH4 mixture selectivity was 8 and constant with temperature up to 480 K. Calcination, slow temperature cycles, and exposure to water vapor had no permanent effect on membrane performance, but temperature changes of approximately 30 K/min decreased the membrane's effectiveness.
Molecular simulation of multicomponent reaction and phase equilibria in MTBE ternary system
Authors: Lísal Martin , Smith William R. , Nezbeda Ivo
Published: April 2004

Reaction and phase equilibria in the isobutene + methanol + MTBE ternary system were studied using the reaction‐ensemble Monte Carlo (REMC) simulation method. The system was modeled at the molecular level by an OPLS force field. No adjustable binary cross‐interaction parameters or mixture data of any kind were used in the simulation model, and only vapor‐pressure data for the pure components was required as input. The REMC method also computes excess internal energies and molar volumes as a biproduct of the simulations. Both the nonreacting and reacting ternary systems were considered over the temperature range of practical interest at 5 bar. Results are compared with the calculations using two conventional thermodynamic approaches: the Wilson and UNIFAC free‐energy models for the liquid phase, together with a truncated virial equation of state for the gas phase in both cases. Computer simulation results were similar to those of the thermodynamic approaches, and they are arguably more accurate.
Multiple steady states in distillation: Effect of VL(L)E inaccuracies
Authors: Bekiaris Nikolaos , Güttinger Thomas E. , Morari Manfred
Published: April 2004

Output multiplicities in heterogeneous azeotropic distillation columns were studied. The accuracy of the thermodynamic description is a key factor that determines if multiplicities can be observed in numerical simulations. The descriptions used in the multiplicity‐related literature are analyzed. The ∞/∞ analysis of Bekiaris et al. (1996) was used to check implications of inaccuracies in the reported thermodynamics on the existence of multiplicities in azeotropic distillation. On this basis, guidelines are derived concerning what features of thermodynamic descriptions need special attention for use in multiplicity prediction and simulation. Secondly, numerical studies on output multiplicities in heterogeneous azeotropic distillation in the literature were compared to the ∞/∞ predictions wherever possible. The ∞/∞ analysis was used to derive the relations between the reported multiplicities and to identify the physical phenomena causing them.
Nonlinear inferential cascade control of exothermic fixed‐bed reactors
Authors: Hua Xiangming , Jutan Arthur
Published: April 2004

A nonlinear inferential cascade control strategy for a tubular fixed‐bed reactor with highly exothermic reaction is presented. Tight control of exit conversion and stabilization of hot‐spot temperature was achieved over a wide range of operating conditions. A multiple cascade structure was developed by lumping the distributed‐parameter system and partitioning it into three subsystems. Practical issues of implementing the control system are addressed, as well as physical insight and assumptions used for model reduction of each subsystem. The direct synthesis approach for nonlinear control systems is used to design the controllers of the important subsystems separately. A lag was added in the primary subsystem, and fast stabilization of the secondary subsystem was implemented. Unknown temperature states and inlet concentration were estimated by a nonlinear observer from only a few temperature measurements. The control problem of the moving hot‐spot temperature was also addressed. Simulation on an industrial phthalic anhydride fixed‐bed reactor showed that the observer can give excellent dynamic tracking of the reactor. The resulting cascade control system can achieve good set‐point tracking and disturbance rejection performance, which is robust in the presence of measurement error and model mismatch, and superior to a single‐loop control system.
Robust steady‐state target calculation for model predictive control
Authors: Kassmann Dean E. , Badgwell Thomas A. , Hawkins Robert B.
Published: April 2004

In practice, model predictive control (MPC) algorithms are typically embedded within a multilevel hierarchy of control functions. The MPC algorithm itself is usually implemented in two pieces: a steady‐state target calculation followed by a dynamic optimization. A new formulation of the steady‐state target calculation is presented that explicitly accounts for model uncertainty. When model uncertainty is incorporated, the linear program associated with the steady‐state target calculation can be recast as a second‐order cone program. This article shows how primal‐dual interior‐point methods can take advantage of the resulting structure. Simulation examples illustrate the effect of uncertainty on the steady‐state target calculation and demonstrate the advantages of interior‐point methods.
Pyrolysis of sawdust in a conical spouted‐bed reactor with a HZSM‐5 catalyst
Authors: Olazar Martin , Aguado Roberto , Bilbao Javier , Barona Astrid
Published: April 2004

The effect was studied of using in‐situ catalyst based on a HZSM‐5 zeolite in flash pyrolysis with an inert gas (N2) of sawdust of pinus insignis in a conical spouted‐bed reactor in the 400–500°C range and for a gas residence time of 50 ms. The use of the catalyst increases the yield of gases and decreases the yields of liquid and char. Likewise, the yield of CO2 decreases, whereas the yield of C4− hydrocarbons increases (15.9 wt. % at 450°C). The catalyst is efficient for partial deoxygenation of the liquid product.
Effect of mass transfer and catalyst layer thickness on photocatalytic reaction
Authors: Chen Dingwang , Li Fengmei , Ray Ajay K.
Published: April 2004

Semiconductor photocatalytic processes have been studied for nearly 20 years due to their intriguing advantages in environmental remediation. A rational approach in determining the effect of mass transfer and catalyst layer thickness during photocatalytic reactions is proposed. The reaction occurs at the liquid–catalyst interface, and therefore when the catalyst is immobilized, both external and internal mass transfer plays significant roles in overall photocatalytic processes. Several model parameters—external mass‐transfer coefficient, dynamic adsorption equilibrium constant, adsorption rate constant, internal mass‐transfer coefficient, and effective diffusivity—were determined either experimentally or by fitting realistic models to experimental results using benzoic acid as a model component. Even though all these parameters are critical to the design and development of photocatalytic processes, they are not available in the literature. The effect of the internal mass transfer on the photocatalytic degradation rate over different catalyst layer thicknesses under two different operating configurations was analyzed theoretically and experimentally verified. It was observed that an optimal catalyst layer thickness exists for substrate‐to‐catalyst illumination.
Multiobjective optimization of an industrial wiped‐film pet reactor
Authors: Bhaskar V. , Gupta Santosh K. , Ray Ajay K.
Published: April 2004

Multiobjective optimization of a third‐stage, wiped‐film polyester reactor was carried out using a model that describes an industrial poly(ethylene terephthalate) reactor quite well. The two objective functions minimized are the the acid and vinyl end group concentrations in the product. These are two of the undesirable side products produced in the reactor. The optimization problem incorporates an endpoint constraint to produce a polymer with a desired value of the degree of polymerization. In addition, the concentration of the di‐ethylene glycol end group in the product is constrained to lie within a certain range of values. Adaptations of the nondominated sorting genetic algorithm have been developed to obtain optimal values of the five decision variables: reactor pressure, temperature, catalyst concentration, residence time of the polymer inside the reactor, and the speed of the agitator. The optimal solution was a unique point (no Pareto set obtained). Problems of multiplicity and premature convergence were encountered. A “smoothening” procedure is suggested to generate near‐optimal operating conditions. The optimal solution corresponds simultaneously to minimum values of the residence time of the polymeric reaction mass in the reactor.
Formation and aging of incipient thin film wax‐oil gels
Authors: Singh Probjot , Venkatesan Ramachandran , Fogler H. Scott , Nagarajan Nagi
Published: April 2004

A fundamental study of the deposition and aging of a thin incipient wax‐oil gel that is formed during the flow of waxy oils in cooled pipes was performed. The solubility of high molecular weight paraffins in naphthenic, aromatic or paraffinic solvents is very low and decreases rapidly with decreasing temperature. This property of the paraffins leads to the formation of gels of complex morphology that deposit on the cold walls of the subsea pipelines during the flow of waxy crudes. This deposition reduces the pipe diameter and decreases the flow capacity of the pipe. These wax‐oil gels contain a large fraction of oil trapped in a 3‐D network structure of the wax crystals that behaves as a porous medium. After the incipient gel is formed, wax molecules continue to diffuse into this structure, thereby increasing its wax content. A model system of wax and oil mixture was used to understand the aging process of the wax‐oil gels, which hardens the wax deposit with time. To understand the physics of the aging process for incipient thin‐film deposits, a series of laboratory flow loop experiments was performed. The aging process was a counterdiffusion phenomenon with a critical carbon number above which wax molecules diffuse into the gel deposit and below which oil molecules diffuse out of the deposit. The aging rate of the gel deposit depends on the oil flow rate and the wall temperature. A mathematical model developed predicted the growth and wax content of the gel deposit on externally cooled pipe walls. The theory agreed with experiments excellently for thin gels.
Novel method for preparing palladium membranes by photocatalytic deposition
Authors: Wu Li‐Qun , Xu Nanping , Shi Jun
Published: April 2004

An ultrathin palladium film on a porous TiO2ceramic membrane was successfully synthesized by a novel photocatalytic deposition (PCD) method. The mechanism was interpreted by the photocatalytic reaction between the surface of the TiO2semiconductor and the Pd(II) solution. Energy dispersive (EDS) analysis and argon permeability of the membrane were used to characterize the deposited Pd. During the PCD process, photoreaction took place with higher efficiency when the pH value of the solution was controlled at the 3—3.5 range. A short irradiation time of 18 min was enough for PCD at a normal reaction temperature. A methanol additive in the solution of about 40 vol. % was optimum for the deposition of Pd. XRD and SEM results on the Pd‐based membrane prepared by PCD indicated that the metal existed in the form of a pure palladium element, with a layer thickness of about 0.1 μm. The gas permeation experiment was conducted in a high‐temperature permeator from 623 to 723 K. The results indicated that there were contributions to the hydrogen flux attributable to both dense and microholes parts of the resulting Pd/TiO2membrane.
Multicomponent space‐charge transport model for ion‐exchange membranes
Authors: Yang Yahan , Pintauro Peter N.
Published: April 2004

A multicomponent space‐charge transport model for ion‐exchange membranes was developed, where the membrane structure was modeled as an array of cylindrical pores with a uniform distribution of fixed‐charge sites on the pore walls. Ion/fixed‐charge site electrostatic interactions, electric‐field‐induced water dipole orientation, ion‐hydration free‐energy changes during ion partitioning, and concentration‐dependent transport parameters were considered in the analysis. The model predicted experimental concentration vs. time data accurately for Donnan dialysis separations with a DuPont Nafion 117 cation‐exchange membrane, where the membrane separated a dilute H2SO4 solution from an aqueous mixture of either Cs2SO4 + Li2SO4 or Cs2SO4 + Na2SO4. Both computer predictions and experimental measurements showed that the alkali metal cation with the larger hard‐sphere radius (lower surface charge density) was selectively absorbed in and transported across the membrane during a multicomponent separation. The cation/cation transport permselectivity was less than the selectivity for equilibrium uptake due to slow ion transport near the pore wall, where discrimination between like‐charge cations was greatest.
Boron‐substituted ZSM‐5 membranes: Preparation and separation performance
Authors: Tuan Vu A. , Noble Richard D. , Falconer John L.
Published: April 2004

ZSM‐5 zeolite membranes with boron substituted into the framework for silicon were prepared on porous stainless‐steel and α‐alumina tubular supports. These membranes had higher n‐C4H10/i‐C4H10 separation selectivities, and effectively separated these isomer mixtures to higher temperatures than membranes with aluminum substituted into the framework. Membranes were prepared with Si/B ratios as low as 12, and the best membranes were prepared from alkali‐free gels. The highest n‐C4H10/i‐C4H10 permselectivity at 473 K was 60, and the highest at 527 K was 24. For most alkali‐free membranes, the n‐C4H10/i‐C4H10 permselectivities and separation selectivities increased with boron content, and membranes on α‐alumina supports had both higher permeances and separation selectivities. Membranes with the same permeances and selectivities can be reproducibly prepared, and they are stable at elevated temperatures.