bluuebunny/crossref_metadata_2025 huggingface dataset:
Pore‐size effects on activated‐carbon capacities for volatile organic compound adsorption
Authors: Huang Mei‐Chiung , Chou Chia‐Huei , Teng Hsisheng
Published: April 2004

Gas‐phase adsorption of acetone and n‐hexane by activated carbons with different pore structures was investigated. The carbons were prepared from a bituminous coal with KOH activation. Increasing the activation temperature increased both the porosity and pore size. The equilibrium adsorption capacity for the organic compounds increased with the carbon porosity, but not proportionally. The percentage of pore volume utilized showed a decreasing trend with the porosity development for acetone adsorption, while an increasing trend was observed for n‐hexane. By incorporating pore size distribution with the Dubinin‐Radushkevich equation using an inverse proportionality between the micropore size and adsorption energy, the isotherms for adsorption onto different carbons can be well predicted. Simulations indicated that the adsorption energy, which is an inverse function of the micropore size, determines the adsorption capacity. Different effects of porosity development were observed for different adsorbates.
Mathematical modeling of rice cooking and dissolution in beer production
Authors: Davey Malcolm J. , Landman Kerry A. , McGuinness Mark J. , Jin Hong N.
Published: April 2004

When beer is brewed in Asia, rice is traditionally used as an adjunct to provide additional sugars for the fermentation process. Under sufficiently high temperatures, the grain, which is primarily composed of starch, takes up water. The starch undergoes a gelatinization reaction when the moisture content is sufficiently high. Stirring and enzyme activity facilitate the removal of the outer gelatinized rice layers, and this starch dissolves into solution. The starch molecules are subsequently broken down by enzymes. The water uptake, gelatinization, and dissolution processes are modeled using a modified diffusion approach. The mass of starch molecules dissolved in solution as the cooking proceeds is determined. The dissolution process increases the speed of gelatinization, and the gelatinization front speed and the thickness of the gel layer are constant with time. The modeling takes into account different temperature regimes and a distribution of rice size. Such models help understand and optimize the total dissolved solids from this cooking process.
Engineering kinetics for hydrothermal oxidation of hazardous organic substances
Authors: Vogel Frédéric , Smith Kenneth A. , Tester Jefferson W. , Peters William A.
Published: April 2004

Supercritical water oxidation (SCWO) can decontaminate hazardous organic wastes, including mixtures whose constituents vary widely in their susceptibility to oxidation. The SCWO kinetics of complex organic wastes are analyzed using a mathematical model that eliminates the substantial input requirements and computationally demanding mathematics of multistep, component‐specific rate expressions. The approach assumes that SCWO occurs by an infinite set of independent, parallel, first‐order chemical reactions, with a continuous distribution of activation energies. The resulting model, distributed activation energies model (DAEM), was applied by earlier workers to other kinetics problems of comparable complexity, inter alia, decay of molecular defects in solids, and coal pyrolysis. A three‐parameter DAEM is shown to correlate the SCWO kinetics of eight organic wastes, including several complex mixtures. These wastes are: JP‐5 aviation fuel, Velsicol H537 hydraulic fluid, aqueous methanol solutions, NaOH and NH4OH hydrolysates of solid rocket propellants, an orange military dye marker, municipal sewage sludge, and alcohol distillery waste water. The model was separately parameterized for each waste by best fitting its predictions of conversion as affected by residence time and/or temperature to experimental data from various investigators. A satisfying correlation resulted for each case. Illustrative reactor engineering calculations demonstrate DAEM's superiority to a single reaction model in sizing a PFR for SCWO of a complex organic mixture. The times required to achieve 99.99% destruction of different wastes depend strongly on the nature of the waste and differ by about three orders of magnitude from the most labile waste to the most refractory waste.
Influence of swelling and diffusion‐induced convection on polymer sorption processes
Authors: Alsoy Sacide , Duda J. L.
Published: April 2004

Unsteady‐state sorption of a vapor or liquid by a polymer is modeled to include the influence of the moving phase boundary associated with the polymer swelling and diffusion‐induced convection. A formulation presented clearly elucidates the influence of these two effects on the overall sorption process. Numerical solutions of the model equations indicate the errors that can be induced when swelling or convection are neglected. For most polymer‐solvent systems, the influence of the diffusion‐induced convection associated with volume changes in mixing can be neglected in the analysis of sorption processes. In contrast, the correction for the moving boundary or swelling of the polymer phase can be quite significant, particularly when a large step change in solvent concentration is considered.
Transport across a turbulent air‐water interface
Authors: Law C. N. S. , Khoo B. C.
Published: April 2004

Turbulence structure in the immediate vicinity of the air–water interface has a dominant influence on the mass transfer across it. In this work, investigations on near‐surface turbulence and mass‐transfer rate were carried out for two distinct flow conditions: one with turbulence generated from beneath the interface, the other via wind shear from above the interface. Turbulence measurement with respect to the fluctuating interface was performed and Hanratty's β, the gradient of the linear variation of vertical velocity at the interface, was quantified. It is found that incorporating a near‐surface turbulence parameter such as β provides a reasonable model for interfacial mass transfer, applicable to the two vastly different flow conditions studied.
Gas–solid two‐phase flow in FCC riser
Authors: Fan Yiping , Ye Sheng , Chao Zhongxi , Lu Chunxi , Sun Guogang , Shi Mingxian
Published: April 2004

FCC riser performance was examined in a large cold‐riser model made of 0.186‐m‐ID plexiglass. In the prelift zone, distributions of particle velocity and local density in six cross sections were investigated, and the experimental results indicated that the whole prelift zone can be divided into three subzones in the axial direction. In the feedstock injection zone, some new flow parameters were introduced and five flow parameters in five different cross sections were measured. When a nozzle jet was injected, secondary flow of the jet exited in the vicinity of the riser wall due to vortices, and this flow moved forth at first and then merged into the main stream. Based on the gas–solid two‐phase flow characteristics in each zone, the whole feedstock injection zone of the riser can be divided into four subzones. A new axial particle backmixing model and a simplified method for computing the radial distribution of the nozzle jet concentration in a different riser cross section can predict flow parameters of particles and the jet.
Gas and solids mixing in a turbulent fluidized bed
Authors: Du Bing , Fan Liang‐Shih , Wei Fei , Warsito W.
Published: April 2004

Gas and solids mixing characteristics in the bubbling and turbulent regimes of a gas‐solid fluidized bed are examined using helium and phosphor tracer techniques to obtain the gas and solids dispersion coefficients, respectively. The real time, quasi‐3‐D flow behavior is qualified and quantified by the electrical capacitance tomography (ECT) technique. The mixing behavior varies significantly with the flow regimes. As the gas velocity increases, axial and radial gas dispersion coefficients increase in the bubbling regime, reach a peak at Uc, the transition velocity from the bubbling to turbulent regimes, and then decrease. In contrast, as the gas velocity increases, axial and radial solids dispersion coefficients increase in the bubbling regime and continuously increase through Uc to the turbulent regime. Temperature and pressure have little effect on the gas and solids mixing behavior. A small quantity of fine particles is noted to drastically affect the gas and solids mixing behavior in the turbulent fluidized bed. The axial and radial dispersion coefficients of gas and solids reach their maximum when the fines content is about 15%. The ECT measurements illustrate the effect of adding fines on the flow behavior in the bed. A small quantity of fine particles disintegrate the bubble/void phase, significantly modifying bubble/void‐ and emulsion‐phase mixing behavior.
Enantiomer separation in a cascaded micellar‐enhanced ultrafiltration system
Authors: Overdevest Pieter E. M. , Hoenders Marc H. J. , van't Riet Klaas , van der Padt Albert , Keurentjes Jos T. F.
Published: April 2004

The increasing demand for optically pure compounds (enantiomers) stimulates the development of new enantiomer separation processes on an industrial scale. The separation of enantiomers by ultrafiltration of enantioselective micelles was studied in a cascaded system. The feasibility of this separation concept was proven by a cascade of five lab‐scale units and a bench‐scale system using an industrial membrane module. Model calculations show that the separation is not improved greatly at enantioselectivities (ratio of affinity constants) above 10 and the affinity of the enantioselective micelles for the substrate (enantiomers) plays a crucial role in the performance of the separation process. Modeling the separation as a function of the dimensionless affinity number (affinity constant · enantiomer feed concentration) shows that there is an operation window in which the separation satisfies its criterion of high yield and high enantiomeric excess. For countercurrent affinity systems, this implies that any research effort should focus on developing (microheterogeneous) media characterized by weak interactions with the substrate to allow for operation at high feed concentrations, not on high affinity systems.
Scrubbing of fly‐ash laden SO2 in modified multistage bubble column scrubber
Authors: Meikap B. C. , Kundu G. , Biswas M. N.
Published: April 2004

The emission of SO2 from various chemical industries is associated with particulate (fly‐ash), mostly concentrations of particulate laden sulfur dioxide in and around these plants overshoot the danger point. Prediction of fly‐ash laden SO2 removal efficiency is very important for the selection of pollution control equipment. Experimental investigations were conducted on the scrubbing of fly‐ash laden SO2 in the modified multistage bubble column scrubber using water. Experimental results show that almost zero penetration (100% removal efficiency) of fly‐ash laden SO2 can be achieved in this system. A correlation was developed for predicting the percentage collection efficiency of sulfur dioxide in the presence of fly‐ash. Experimental results agreed excellently with the correlation. Enhancement of SO2 collection due to the presence of fly‐ash was also quantified.
Modeling and design of vapor‐phase biofiltration for chlorinated volatile organic compounds
Authors: Den Walter , Pirbazari Massoud
Published: April 2004

A mathematical model was developed for biofilter design and performance prediction with reference to the purification of contaminated gas streams. The model incorporated important aspects such as mass transfer, biodegradation, and adsorption processes. A systematic modeling protocol incorporated the development of a scale‐up strategy based on dimensional analysis and similitude. Trichloroethylene (TCE) was employed as the model contaminant for biofiltration testing and model verification. The biokinetic and adsorption parameters for the contaminant were determined independently from a series of minibiofilter and miniadsorber column experiments, specifically designed to simulate the actual biofilter operational regimes in a miniature scale. Bench‐scale biofilter experiments employing granular activated carbon columns indicated the good predictive capability of the model for the removal of TCE. Dynamic simulation studies were performed to assess the transient‐ and steady‐state behavior of the model under various operating conditions. Model sensitivity was studied to evaluate the influence of adsorption equilibrium, transport and biological parameters on the biofilter dynamics. The results demonstrated that the biofilter performance was greatly influenced by the Monod coefficients and the biofilm thickness.
Coating on a rough surface
Authors: Clarke Andrew
Published: April 2004

Commercial products such as photographic materials and speciality papers manufactured by single or multilayer coating processes, all have some surface topographic structure. Nevertheless, for many materials, a uniform plane approximates their surface, and analyses of dynamic wetting, a fundamental physical process which determines coating process limits, usually assume this condition. It is demonstrated here that under certain combinations of coating process parameters, the maximum coating speed is not predicted even qualitatively by using this assumption. Furthermore, the experiments indicate that a second coating mechanism supersedes complete wetting and promotes extremely high coating speeds.
Modeling and optimizing submerged hollow fiber membrane modules
Authors: Chang S. , Fane A. G. , Vigneswaran S.
Published: April 2004

The filtration behavior of submerged hollow fiber membranes with constant permeate flow operational mode was modeled on two different conditions: the maximum initial flux along the fiber is smaller than the critical flux (Jimax < Jcr); the maximum initial flux along the fiber is greater than the critical flux, but the averaged imposed flux is smaller than the critical flux (Jimax > Jcr but Jmi < Jcr). When the maximum initial flux along the fiber is lower than the critical flux, no particle deposition occurs and the flux distribution can be characterized by a dimensionless parameter ξ = 4LRR. On the other hand, for Jmi < Jcr and Jimax > Jcr, a steady state can be achieved after some initial deposition. The theoretical model shows that the filtration resistance caused by the initial deposition can be affected by Jmi/Jcr and fiber characteristics (L, Ri and Rm) and becomes significant for narrow and long fibers with a high Jmi/Jcr. The models were also used to determine the optimal fiber length and radius for the submerged hollow fiber module. The simulation suggests that the optimal fiber lumen radius for fiber lengths of 0.5–3 m is 0.2–0.35 mm. These results should be relevant to the design of submerged hollow fiber modules.
Adsorption of mixtures containing subcritical fluids on activated carbon
Authors: Do D. D. , Do H. D.
Published: April 2004

A theoretical analysis of adsorption of mixtures containing subcritical adsorbates into activated carbon is presented as an extension to the theory for pure component developed earlier by Do and coworkers. In this theory, adsorption of mixtures in a pore follows a two‐stage process, similar to that for pure component systems. The first stage is the layering of molecules on the surface, with the behavior of the second and higher layers resembling to that of vapor–liquid equilibrium. The second stage is the pore‐filling process when the remaining pore width is small enough and the pressure is high enough to promote the pore filling with liquid mixture having the same compositions as those of the outermost molecular layer just prior to pore filling. The Kelvin equation is applied for mixtures, with the vapor pressure term being replaced by the equilibrium pressure at the compositions of the outermost layer of the liquid film. Simulations are detailed to illustrate the effects of various parameters, and the theory is tested with a number of experimental data on mixture. The predictions were very satisfactory.
Optimization of a simulated moving bed based on an approximated langmuir model
Authors: Yu Hong Wei , Ching Chi Bun
Published: April 2004

Based on the “triangle method,” a shortcut method was developed to establish the possible optimal operating condition of a SMB unit, because the theoretical optimal operating condition was diverted from the actual complete separation region by too many unexpected disturbances. A new CSP was used in the simulated moving bed for the enantioseparation of fluoxetine. Good separation results were obtained. The effects of the difference between m2 and m3 on performance parameters were studied. The results show that this method is useful for establishing the operating conditions. It can be concluded that the new CSP is efficient for the enantioseparation of chiral drugs.
New MILP formulation for instrumentation network design and upgrade
Authors: Bagajewicz Miguel , Cabrera Enmanuel
Published: April 2004

The problem of instrumentation network design and upgrade consists of determining the optimal set of instruments to obtain sufficiently accurate and reliable estimates of variables. An existing formulation of the cost optimal sensor network design and upgrade problem (Bagajewicz, 1997) is in the form of a mathematical programming problem, but it has the disadvantage that the constraints are not stated explicitly in analytical form, forcing therefore a special tree search solution procedure. An alternative MILP formulation is presented, which is useful for small to medium size problems, where all constraints are explicit.
Oxidative coupling of methane in dense ceramic membrane reactor with high yields
Authors: Akin F. T. , Lin Y. S.
Published: April 2004

Oxidative coupling of methane (OCM) was studied with dense tubular Bi1.5Y0.3Sm0.2O3 (BYS) membranes at various temperatures (870–930°C). BYS powders were synthesized by a citrate method. Tubular‐shaped dense membranes of BYS in the fluorite‐type FCC phase structure were fabricated by cold isostatic pressing with green machining. The best one‐pass C2 (C2H4 + C2H6) yield achieved for OCM in the BYS dead‐end membrane reactor was 35% at a C2 selectivity of 54% at 900°C. At the same C2 yield, the membrane reactor mode gives C2 selectivity of over 200% higher than the cofeed mode in the same membrane reactor under similar conditions. The oxygen permeation fluxes through tubular BYS membrane reactors under OCM reaction conditions are approximately 1.5 to 3.5 times higher than those under oxygen permeation conditions with He as the purge. After 6 days of OCM, BYS membrane remained in good integrity with minor phase segregation observed at the reaction side of the membrane.
Measurement and calculation of critical points for binary and ternary mixtures
Authors: Horstmann Sven , Fischer Kai , Gmehling Jürgen
Published: April 2004

The critical lines (temperatures and pressures) for the three binary mixtures ethane + sulfur hexafluoride (SF6), SF6 + propane, and carbon dioxide (CO2) + SF6, and the critical surface for the ternary system CO2 + ethane + SF6 were determined experimentally by a flow method. The binary data were compared to predictions using the predictive Soave‐Redlich‐Kwong (PSRK) group contribution equation of state (EoS). For the system CO2 + SF6, where no phase equilibrium information is available, the critical points were used to optimize the required PSRK group interaction parameters. Using these binary parameters, the ternary critical plane was predicted reliably.
Immunoadsorption model for a novel fluidized‐bed blood detoxification device
Authors: Grovender Eric A. , Cooney Charles L. , Langer Robert , Ameer Guillermo A.
Published: April 2004

A multicompartment Taylor‐Couette flow hemofilter was designed to remove toxins from a patient's bloodstream via immunoadsorption. This device (Vortex Flow Plasmapheretic Reactor, VFPR) treats blood plasma with a fluidized‐bed of small (45–165×10−6 m diameter) particles, while protecting fragile blood cells from lysis. The potential application for the VFPR is dialysis‐related amyloidosis, a disease associated with the systemic accumulation of beta‐2‐microglobulin in patients with long‐term kidney failure. The equilibrium behavior of immunoadsorptive gel beads is characterized experimentally and theoretically using confocal microscopy and the Langmuir adsorption isotherm. The importance of external mass‐transfer resistance within the active compartment is assessed through dissolution studies conducted with benzoic acid particles. These results are combined with mass‐transfer fundamentals to develop a dynamic immunoadsorption model. The modeling results, without the use of adjustable parameters, agree with the experimental data and provide a foundation for further development and eventual application.
Colloid‐Associated contaminant transport in porous media: 1. Experimental studies
Authors: Sen Tushar Kanti , Mahajan S. P. , Khilar Kartic C.
Published: April 2004

Many studies have identified colloid‐associated transport of contaminants as an important mechanism of contaminant migration through groundwater. It is a complex phenomenon in porous media involving several basic processes such as adsorption of contaminants, release and migration of colloidal fines, and entrapment of fines at the pore constrictions. The effects of these basic processes on the contaminant transport are studied. Column experiments are conducted to study the effects of the mobilization and migration of colloidal fines, kaolin on the transport of contaminant, and Ni2+ metal ion through the sand beds containing kaolin particles under both nonplugging and plugging conditions. As reported in literature, colloidal fines can facilitate the contaminant transport when they migrate with the flow. In the absence of migration, kaolin‐sand beds retard Ni2+ transport in comparison to sand beds due to their higher adsorption capacity, but the sand‐kaolin composite bed acts like an inefficient adsorption column with percentage saturation (which is in general below 25%). It is interesting to note that under plugging conditions, which can be induced by using a lower bead size to particle‐size ratio, the breakthrough curves are more flattened and delayed at a higher kaolin content of the bed.
Batch extractive distillation of maximal azeotrope with middle boiling entrainer
Authors: Lelkes Z. , Rev E. , Steger C. , Fonyo Z.
Published: April 2004

Separation of maximum boiling azeotropes using batch extractive distillation (BED) is feasible in a rectifying column with intermediate boiling entrainer. The main difference between BED and solvent‐enhanced batch distillation (SBD) is the application of continuous entrainer feeding to the column or still. Finding a well‐working intermediate boiling entrainer is more difficult than finding a heavy or light entrainer. The test mixture, selected according to didactic viewpoints, is chloroform and ethyl acetate with 2‐chlorobutane. Operation steps (sequencing), limiting flows, and limiting stage numbers are determined by a feasibility study, based on profile maps; the design is validated by rigorous simulation. Results of parametric studies and comparison of SBD and BED, according to effectiveness, are presented. Separation of maximum boiling azeotropes in SBD and BED is feasible with an intermediate boiling entrainer, but the application of continuous feeding (characteristic to BED) makes the process more efficient. Pure‐component A cannot be produced in SBD or BED, but producing component A mixed with entrainer and free of component B is feasible in both processes. The operation steps are similar in the two processes except for continuous feeding of the entrainer in step 2 of BED. With identical specification and comparable operation parameters, BED produces the same products in a shorter time and with a significantly smaller amount of maximum holdup in the still.
Effect of mixing on efficiencies for reactive tray contactors
Authors: Fisher Kevin S. , Rochelle Gary T.
Published: April 2004

The relationship between point efficiencies and Murphree tray efficiencies was studied for systems where reactions take place in the liquid within a tray column. For nonreactive systems, several well‐known mixing models are available for predicting Murphree tray efficiency from point efficiencies. For reactive systems, the magnitude and nature of the effect of mixing on tray efficiency are different. An analytical representation is developed of the mixed‐pool and plug‐flow mixing models for reactive systems with first‐order kinetics, and there are different regimes, depending on the reaction kinetics. For each of these regimes, the effect of mixing on tray efficiency differs in magnitude and arises from different mechanisms. The resulting tray efficiencies predicted for reactive systems using the methods may differ by an order of magnitude or more from the values predicted when standard methods for nonreactive trays are applied to reactive trays.
Sectional and overall reachability for systems with S‐shaped distillation lines
Authors: Hoffmaster Warren R. , Hauan Steinar
Published: April 2004

Total and minimum reflux bounds serve as limiting case scenarios for determining feasibility in azeotropic distillation columns. Specifically, all reachable feed compositions in a staged rectifying or stripping section have been reported as the union of points enclosed by the distillation and pinch‐point curves through a desired product. By eigenvalue considerations of pinch bifurcations and geometric analysis of fundamental region structures, the region of all reachable compositions has seemingly been identified. Additional insights are needed to fully characterize the feasible compositions in a subset of azeotropic mixtures. This article describes how sectional profiles can actually cross pinch‐point curves and generate a region of additional feasibility outside the one previously reported. It also demonstrates how a substantially larger region between the distillation line and pinch‐point curve after the intersection is only accessible by composition profiles going through the extended region. This extended reachable region is explained by evaluating the conditions for a fixed point in composition space. The effect on sectional profiles is illustrated for both an artificially parametric VLE manifold and a nonideal azeotropic system with more than one simple distillation region. Sectional profiles extended beyond the traditional bounds affect coupled sections in a distillation cascade.
Modeling of fouling on packings in absorption columns
Authors: Heberle Arthur , Schaber Karlheinz
Published: April 2004

A model describing the fouling in irrigated columns with random packings was applied successfully to the description of the increase in pressure drop when fouling occurs during the absorption of CO2 in a suspension of Ca(OH)2. It is based on the Darcy–Weisbach law and the channel model. A mass balance around a fouling layer on the packings leads to a time‐dependent equation including the hydraulic radius from the channel model. A removal term is introduced in the fouling model. To approach the experimental results, the removal term had to be extended by an empirical parameter. It can be interpreted as a removal efficiency factor and is obtained by easy numerical fitting.
Free‐surface turbulence and mass transfer in a channel flow
Authors: Tamburrino Aldo , Gulliver John S.
Published: April 2004

Free‐surface turbulence in a fully developed, open‐channel flow was measured for Reynolds numbers of 8,500–45,000. An analysis method of the 2‐D divergence on the free surface has been developed to extract Hanratty's β values, or the velocity gradient into the free surface, from these measurements. Hanratty's β is the parameter that relates most directly to the turbulence effect on the liquid‐film coefficient. Its measurement is a direct measurement of surface renewal. The spatial scales of β were 3 to 5 times smaller than those of the large upwelling events (boils) normally identified as surface renewal. The hypothesis is that the large upwelling events do not have the high‐vorticity gradients associated with large β values. Instead, the locations of high‐vorticity gradients on the free surface will also create the divergence required for high β values, occurring at the edges of a large upwelling event. Because the β frequency spectrum has properties to characterize the liquid‐film coefficient, it was normalized to be determined from its maximum value, the wave number of this maximum value, and a shape factor used to scale the frequency. Measurements of the liquid‐film coefficient from prior studies were also used to characterize the liquid‐film coefficient by measured β values for this nonsheared surface. The larger β scales predominantly influence the liquid‐film coefficient, in contrast to a previous study of a shear‐free surface published by McCready et al. in 1986, where all β frequencies were equally important. Generally, higher frequency turbulence is more significant at a sheared water surface than at a water surface with minimal shear stress.
Experimental validation of 3‐D lagrangian VOF model: Bubble shape and rise velocity
Authors: van Wachem B. G. M. , Schouten J. C.
Published: April 2004

A novel 3‐D computational fluid dynamics model using an advanced Lagrangian interface tracking scheme was studied to find the time‐dependent behavior of gas bubbles rising in an initially quiescent liquid. A novel least‐square approach is used to determine the normal behavior at the interface for an accurate reconstruction and advection of the interface based on mollification of the color function by convolution. The incompressible Navier–Stokes equations are solved using an accurate discretization scheme to obtain the flow field of the gas and liquid phase. Detailed experiments of single rising bubbles of different sizes were performed to compare the shape, rise velocity and pressure signal of the bubble with the performed simulations. The developed Lagrangian volume‐of‐fluid model could accurately track the motion and shape of the gas–liquid interface embedded in a flow field with significant vorticity.
Standing‐wave design of tandem SMB for linear multicomponent systems
Authors: Hritzko Benjamin J. , Xie Yi , Wooley Robert J. , Wang Nien‐Hwa L.
Published: April 2004

The standing‐wave design was extended to achieve any desired split of mixtures containing three or more components in a single‐ring and a tandem two‐ring simulated moving bed (SMB). Mass‐transfer effects were considered in the design for nonideal systems. The separation of a four‐component mixture of glucose, xylose, acetic acid and sulfuric acid was chosen to illustrate the design method. Rate‐model simulations confirmed that the standing‐wave design method could guarantee high purity and high yield. If all the components in a ternary mixture need to be recovered with high purity and high yield, the easier separation should be performed in the first ring of a tandem SMB to achieve the lowest desorbent consumption and the highest product concentration. If only the intermediate component needs to be recovered in high purity, one of the impurities should be allowed to distribute between the two product ports in the first ring to achieve a lower desorbent consumption and a higher product concentration. These strategies also apply to the separation of a mixture containing more than three components.
Optimization of multiple‐fraction batch distillation with recycled waste cuts
Authors: Diehl Moritz , Schäfer Andreas , Bock Hans Georg , Schlöder Johannes P. , Leineweber Daniel B.
Published: April 2004

A strategy for the optimal operation of multiple‐fraction batch distillation processes with waste cut recycling is presented. The processes are modeled by differential algebraic equations formulated in the gPROMS modeling language. A cost function based on economic criteria is developed. Recycling of the waste cuts is formulated as a cyclically repeated batch distillation process, where the waste cuts are recycled at intermediate points in time. The resulting multipoint boundary value optimization problem is efficiently solved by using the dynamic optimization package MUSCOD‐II. A comparison of optimization results for the separation of a ternary mixture shows that considerable gains in profits can be obtained by a complete recycling of the waste cuts. Computation times are in the range of minutes on a standard workstation.
Uncertainty considerations for describing complex reaction systems
Authors: Balakrishnan S. , Georgopoulos P. , Banerjee I. , Ierapetritou M.
Published: April 2004

Models that accurately describe chemical processes are often intricate involving numerous reacting species and reaction steps. For complex reaction mechanisms, output‐species concentration profiles can change dramatically based on the set of values chosen for inputs if they are nondeterministic. A systematic uncertainty analysis can provide insight into the level of confidence of model estimates and aid mechanism reduction. Response surface methods and variants, thereof, require much fewer simulations for the adequate estimation of system uncertainty characteristics. This article focuses on reaction rate constant uncertainty using the stochastic response surface method (SRSM), whereby, uncertain outputs are expressed in terms of a polynomial chaos expansion of Hermite polynomials and engenders such useful properties as the mean and variance and computation of sensitivity information. SRSM determines the uncertainty propagation characteristics very accurately, while using an order‐of‐magnitude fewer model simulations than traditional Monte Carlo techniques. Since uncertainty in kinetic rate parameters largely affects the reduction of kinetic models, a framework of analysis is also developed for mechanism reduction considering uncertainty using sensitivity information from SRSM to create good initial sets of reactions for the efficient solution of a multiperiod optimization problem. Two case studies—an isothermal supercritical wet oxidation process and a nonisothermal H2/CO/air combustion process—elucidate the application of this framework of analysis to complex kinetic mechanisms and illustrate the possible ease of computational burden associated with mechanism reduction under uncertainty.
Mechanistic understanding of degradation in bioerodible polymers for drug delivery
Authors: Larobina Domenico , Mensitieri Giuseppe , Kipper Matt J. , Narasimhan Balaji
Published: April 2004

A new model was developed to understand the mechanism of erosion in bioerodible polymers, which is essential to accurately predict drug release and precisely design controlled release devices. This model takes into account the phenomenon of microphase separation observed for polyanhydrides of certain copolymer compositions, and assumes that erosion is dominated by degradation and, thus, in a system with a fast eroding and a slow eroding species, two rate constants—one for each species—essentially control the evolution of the polymer microstructure. Expressions were derived for the fraction of each monomer released, as well as for the porosity in the system. A partition coefficient accounts for thermodynamic partitioning of a drug into the microdomains. The solutions of the model equations were fitted to experimental data on monomer release kinetics from two polyanhydride systems to obtain the erosion rate constants. Drug release kinetics experiments are compared to the model solution for drug release, and the partition coefficient of the drug is obtained from the fits. The comparisons to the data are promising, while pointing out the limitations of the model. The model does not account for oligomer formation prior to monomer release or for the dependence of the rate constants on parameters such as the degree of crystallinity, the local pH, and the polymer molecular weight.
Enhanced conductivity of fuel cell plates through controlled fiber orientation
Authors: Blunk Richard H. J. , Lisi Daniel J. , Yoo Yeong‐Eun , Tucker Charles L.
Published: April 2004

Manufacturers of polymeric composite bipolar plates have been relying on high loadings (60 to 90 v/o) of discrete and conductive graphite/carbon filler particles to meet fuel‐cell plate conductivity targets. Unfortunately, at these loadings, the plate materials are inherently brittle and result in high scrap rates and the inability to mold thin plates (less than 1 mm) required for high stack volumetric power densities. Low loadings of high‐aspect‐ratio conductive fillers, a fiber/flake alignment process, and a conductive‐tie layer (CTL) were used to simultaneously increase the plate conductivity and toughness. The alignment process reduces the bulk resistance in the current‐flow direction, and the CTL reduces the contact resistance at the plate‐to‐diffusion medium interface. Although a significant reduction in plate resistance is realized, high filler loadings are still required to meet conductivity targets.
Effect of pore‐network connectivity on multicomponent adsorption of large molecules
Authors: Ismadji S. , Bhatia S. K.
Published: April 2004

The effect of pore‐network connectivity on binary liquid‐phase adsorption equilibria using the ideal adsorbed solution theory (IAST) was studied. The liquid‐phase binary adsorption experiments used ethyl propionate, ethyl butyrate, and ethyl isovalerate as the adsorbates and commercial activated carbons Filtrasorb‐400 and Norit ROW 0.8 as adsorbents. As the single‐component isotherm, a modified Dubinin–Radushkevich equation was used. A comparison with experimental data shows that incorporating the connectivity of the pore network and considering percolation processes associated with different molecular sizes of the adsorptives in the mixture, as well as their different corresponding accessibility, can improve the prediction of binary adsorption equilibria using the IAST. Selectivity of adsorption for the larger molecule in binary systems increases with an increase in the pore‐network coordination number, as well with an increase in the mean pore width and in the spread of the pore‐size distribution.
Simulation‐optimization framework for stochastic optimization of R&D pipeline management
Authors: Subramanian Dharmashankar , Pekny Joseph F. , Reklaitis Gintaras V. , Blau Gary E.
Published: April 2004

The simulation‐based optimization framework (Sim‐Opt) uses a twin‐loop computational architecture, which combines mathematical programming and discrete event simulation, to address this problem. This article extends our earlier work to present methods for integrating information from the inner loop (Sim‐Opt time lines, reactive adjustment) and using it in the outer risk‐control loop (Stochastic Optimization loop) to obtain statistically significant improvements in the solutions to the underlying stochastic optimization problem. Two classes of information can be obtained from the inner loop time lines: the first pertaining to portfolio selection and the second resource crowding associated with the chosen operation policy. Methods presented quantify the information on these two classes, and a three‐step heuristic incorporates this information in the outer risk‐control loop to drive the system toward improving solutions with respect to the mean net present value (NPV) of the portfolio and the probability of delivering a positive NPV. This method was used on a pharmaceutical product development case study, consisting of 11 projects, 154 activities, 14 resource types and a 20‐year planning horizon with respect to patent expiration. Basic algorithm engineering efforts are also described to significantly improve the performance of formulation generation, the generation of a heuristic lower bound and the identification of cut families to effectively apply branch‐and‐cut methods.
Kinetics of the reaction of CO2 with aqueous potassium salt of taurine and glycine
Authors: Kumar P. S. , Hogendoorn J. A. , Versteeg G. F. , Feron P. H. M.
Published: April 2004

The kinetics of the reaction between CO2 and aqueous potassium salts of taurine and glycine was measured at 295 K in a stirred‐cell reactor with a flat gas–liquid interface. For aqueous potassium taurate solutions, the temperature effect on the reaction kinetics was measured at 285 and 305 K. Unlike aqueous primary alkanolamines, the partial reaction order in amino acid salt changes from one at low salt concentration to approximately 1.5 at salt concentrations as high as 3,000 mol·m−3. At low salt concentrations, the measured apparent rate constant (kapp) for potassium glycinate is comparable to the values in literature. In the absence of reliable information in the literature on the kinetics and mechanism of the reaction, the applicability of the zwitterion and termolecular mechanism (proposed originally for alkanolamines) was explored. For the zwitterion mechanism, the forward second‐order reaction rate constant (k2) of the CO2 reaction with amino acid salt seems to be much higher than for alkanolamines of similar basicity, indicating that the Bronsted plot for amino acid salts might differ from that of alkanolamines. The contribution of water to the deprotonation of zwitterion seems to be more significant than reported values for aqueous secondary alkanolamines.
Experimental tests of two‐phase fluid model of drying consolidation
Authors: Brown L. A. , Zukoski C. F.
Published: April 2004

Drying of aggregated slurries proceeds through a stage of consolidation prior to bed desaturation. During this period, volume fraction profiles develop within the bed. The magnitude of volume fraction variation is controlled by the rate of mass transfer and the strength of the bed. A two‐phase fluid model for stress balances in conjunction with the compressive yield stress constitutive equation is explored to describe the temporal evolution of bed volume fraction profiles. For this purpose, the compressive rheology of the saturated bed is characterized through independent experiments that fix all adjustable parameters. Excellent agreement is found between model predictions and experimental results.
Synthesis of multicomponent distillation column configurations
Authors: Agrawal Rakesh
Published: April 2004

A systematic procedure presented here draws distillation column configurations to separate an ideal to near ideal n–component mixture into n product streams with each enriched in one of the components. The method synthesizes all feasible basic configurations using n–1 distillation columns with each column having only a condenser at the top and a reboiler at the bottom. It also generates all feasible thermally coupled schemes with classical two‐way liquid and vapor communication between the distillation columns. The method is simple and easy to use. The advantage of such a method is that it could be incorporated in a search algorithm to systematically find an optimum distillation scheme for a given application.
Unsteady‐state residence‐time distribution in perfectly mixed vessels
Authors: Rawatlal Randhir , Starzak Maciej
Published: April 2004

While addressing some difficulties in the modeling of unsteady‐state residence‐time distribution (RTD), the care of perfectly mixed systems was considered. By an extension of the boundary condition domain, the traditional unfeasible concept of fresh and original populations was discarded to achieve a solution valid for all the fluid elements in a single vessel. This result enabled the development of the unsteady‐state RTD for N vessels in series in the form of a joint probability density function. The relationship between the joint RTD and the combined RTD was also established. Numerical simulations were performed under various time varying flow patterns. The proposed solution can be applied to RTD‐based models of chemical reactors operating under unsteady‐state conditions. Potential applications in polymer reaction engineering are briefly outlined.
Oxidative destruction of dichloromethane over protonic zeolites
Authors: López‐Fonseca R. , Cibrián S. , Gutiérrez‐Ortiz J. I. , Gutiérrez‐Ortiz M. A. , González‐Velasco J. R.
Published: April 2004

Various protonic zeolites (H‐Y, H‐ZSM‐5, and H‐MOR) were tested for the total combustion of dichloromethane (DCM) as a model reaction for the catalytic destruction of chlorinated organic pollutants. The order of catalytic activity decreased in the following order: H‐MOR > H‐ZSM‐5 > H‐Y. Strong Brønsted acidity played a determining role in controlling the active catalytic behavior as revealed by TPD of ammonia and IR measurements of adsorbed pyridine. DCM oxidative decomposition gave rise to CO and HCl as main products along with small amounts of CO2and Cl2. In addition, methyl chloride was detected as a reaction intermediate between 250 and 500°C. The presence of water in the feed stream led to an inhibition of the zeolite activity and resulted in a noticeable change in the reaction product distribution. Water effectively promoted the selectivity toward environmentally desirable reaction products, that is, HCl and CO2, and completely suppressed the formation of methyl chloride.
Solidification in heat packs: II. Role of cavitation
Authors: Rogerson Mansel A. , Cardoso Silvana S. S.
Published: April 2004

When the metallic trigger in a commercial heat pack is flexed to initiate solidification, the acceleration of the liquid near the solid surface can cause cavitation and collapse of small vapor bubbles. It is investigated whether such cavitation is responsible for the operation of the metallic trigger in the subcooled sodium acetate solution in a heat pack. Using cavity collapse theory and the results in Part I for the nucleation pressure of a sodium acetate solution, it is shown that cavitation can generate pressures greater than the nucleation pressure of the solution. However, the period of time during which the solution is exposed to such high pressure is not sufficient to lead to nucleation, and hence the metallic trigger does not induce solidification as a result of cavitation. Experimental results, using sonication, tribonucleation, and shock waves, confirm the theoretical findings.
Vaporization ratio in differential ebulliometers for measuring limiting activity coefficients
Authors: Raal J. David , Ramjugernath D.
Published: April 2004

Accurate limiting activity coefficients can be obtained from differential ebulliometry if vapor and liquid holdups in various parts of the equipment are correctly accounted for in calculating the equilibrium liquid composition from the known charge composition. Available procedures require the difficult‐to‐determine dynamic vaporization rates (V/F) or (V/L). Two new procedures proposed determine V/F ( = ϕ): 1. a trial‐and‐error procedure satisfying all three of the equations governing the ebulliometer operation yielding the true liquid equilibrium composition x1, ϕ, and the overall composition in the Cottrell pump, Z1; 2. one procedure based on the measured superheat temperature at the base of the Cottrell pump. A possibly limiting assumption is that local equilibrium exists at this point, but the method has good rigour otherwise. It is, however, very sensitive to measured pressures and temperatures, which need to be of the highest accuracy. The procedure is illustrated for the n‐hexane (1)–ethanol (2) and n‐hexane (1)–ethyl‐acetate (2) systems, and requires liquid density, heat capacity and accurate latent heats over a small temperature interval. Measurements suggest that about 72% of the total vaporization takes place at or before the entrance to the Cottrell pump with only about 28% in the equilibrium chamber for the n‐hexane–ethyl acetate system. For the n‐hexane–ethyl alcohol systems, they are 89% and 11%. The iterative procedure is recommended because it is more reliable and, although computationally extensive, has more rigor.