Showing papers in "Aiche Journal in 2010"
TL;DR: Ceria is a crucial component of automotive catalysts, where its ability to be reduced and reoxidized provides oxygen storage capacity as mentioned in this paper, and because of these redox properties, ceria can greatly enhance catalytic activities for a number of important reactions when it is used as a support for transition metals.
Abstract: Ceria is a crucial component of automotive catalysts, where its ability to be reduced and re-oxidized provides oxygen storage capacity. Because of these redox properties, ceria can greatly enhance catalytic activities for a number of important reactions when it is used as a support for transition metals. For reactions that use steam as an oxidant (e.g., the water–gas-shift reaction and steam reforming of hydrocarbons), rates for ceria-supported metals can be several orders of magnitude higher than that for ceria or the transition metal alone. Because the redox properties of ceria are strongly dependent on treatment history and the presence of additives, there are significant opportunities for modifying catalysts based on ceria to further improve their performance. This article will review some of the contributions from my laboratory on understanding and using ceria in these applications. © 2010 American Institute of Chemical Engineers AIChE J, 2010
273 citations
TL;DR: In this paper, a distributed model predictive control of large scale nonlinear process systems in which several distinct sets of manipulated inputs are used to regulate the process is presented. But, the authors focus on distributed models and do not consider the control actions of the rest of the controllers.
Abstract: In this work, we focus on distributed model predictive control of large scale nonlinear process systems in which several distinct sets of manipulated inputs are used to regulate the process. For each set of manipulated inputs, a different model predictive controller is used to compute the control actions, which is able to communicate with the rest of the controllers in making its decisions. Under the assumption that feedback of the state of the process is available to all the distributed controllers at each sampling time and a model of the plant is available, we propose two different distributed model predictive control architectures. In the first architecture, the distributed controllers use a one-directional communication strategy, are evaluated in sequence and each controller is evaluated only once at each sampling time; in the second architecture, the distributed controllers utilize a bi-directional communication strategy, are evaluated in parallel and iterate to improve closed-loop performance. In the design of the distributed model predictive controllers, Lyapunov-based model predictive control techniques are used. To ensure the stability of the closed-loop system, each model predictive controller in both architectures incorporates a stability constraint which is based on a suitable Lyapunov-based controller. We prove that the proposed distributed model predictive control architectures enforce practical stability in the closed-loop system and optimal performance. The theoretical results are illustrated through a catalytic alkylation of benzene process example. V C 2010 American Institute of Chemical Engineers AIChE J, 56: 2137–2149, 2010
171 citations
TL;DR: In this paper, a new wicking model was proposed based on Darcy's law for flow in porous media, coupled with the mass conservation equation containing an added sink or source term to account for matrix swelling and liquid absorption.
Abstract: The wicking of liquid into a paper-like swelling porous medium made from cellulose and superabsorbent fibers was modeled using Darcy's law. The work is built on a previous study in which the Washburn equation, modified to account for swelling, was used to predict wicking in a composite of cellulose and superabsorbent fibers. In a new wicking model proposed here, Darcy's law for flow in porous media is coupled with the mass conservation equation containing an added sink or source term to account for matrix swelling and liquid absorption. The wicking-rate predicted by the new model compares well with the previous experimental data, as well as the modified Washburn equation predictions. The effectiveness of various permeability models used with the new wicking model is also investigated. © 2010 American Institute of Chemical Engineers AIChE J, 2010
159 citations
TL;DR: The PPR78 approach as mentioned in this paper is a group contribution-based thermodynamic model which combines at constant packing fraction the Peng-Robinson equation of state and a Van Laar-type gE model.
Abstract: The PPR78 approach is a group contribution-based thermodynamic model which combines at constant packing fraction the Peng–Robinson equation of state and a Van Laar-type gE model. This article demonstrates that, using classical mixing rules (linear on b and quadratic on a), the PPR78 model may also be seen as a group contribution method for the estimation of the temperature-dependent kij of the widely used PR EoS. Our model is endowed of 15 groups and it is possible to predict the kij for any mixture containing alkanes, aromatics, naphthenes, CO2, N2, H2S, and mercaptans. This study exhibits the capability of this approach to predict the phase behavior of synthetic petroleum fluids containing components of different volatilities. The many comparisons between calculated and experimental data on natural gases, crude oils, and gas condensates allow concluding that the PPR78 approach is a successful model for phase equilibria calculations of this kind of mixtures. © 2010 American Institute of Chemical Engineers AIChE J, 2010
159 citations
TL;DR: In this paper, a countercurrent moving-bed reducer is proposed to reduce the oxygen carrier with syngas in the cyclic reduction and regeneration of an iron oxide-based oxygen carrier.
Abstract: The syngas chemical looping process co-produces hydrogen and electricity from syngas through the cyclic reduction and regeneration of an iron oxide based oxygen carrier. In this article, the reducer, which reduces the oxygen carrier with syngas, is investigated through thermodynamic analysis, experiments, and ASPEN Plus® simulation. The thermodynamic analysis indicates that the countercurrent moving-bed reducer offers better gas and solids conversions when compared to the fluidized-bed reducer. The reducer is continuously operated for 15 h in a bench scale moving-bed reactor. A syngas conversion in excess of 99.5% and an oxygen carrier conversion of nearly 50% are obtained. An ASPEN Plus® model is developed which simulates the reducer performance. The results of simulation are consistent with those obtained from both the thermodynamic analysis and experiments. Both the experiments and simulation indicate that the proposed SCL reducer concept is feasible. © 2009 American Institute of Chemical Engineers AIChE J, 2010
144 citations
TL;DR: In this paper, the Cubic-Plus-Association (CPA) equation of state is applied to a large variety of mixtures containing H2S, which are of interest in the oil and gas industry.
Abstract: The Cubic-Plus-Association (CPA) equation of state is applied to a large variety of mixtures containing H2S, which are of interest in the oil and gas industry. Binary H2S mixtures with alkanes, CO2, water, methanol, and glycols are first considered. The interactions of H2S with polar compounds (water, methanol, and glycols) are modeled assuming presence or not of cross-association interactions. Such interactions are accounted for using either a combining rule or a cross-solvation energy obtained from spectroscopic data. Using the parameters obtained from the binary systems, one ternary and three quaternary mixtures are considered. It is shown that overall excellent correlation for binary mixtures and satisfactory prediction results for multicomponent systems are obtained. There are significant differences between the various modeling approaches and the best results are obtained when cross association is explicitly accounted for, especially using the cross-association energy from independent experimental studies rather than from combining rules. © 2010 American Institute of Chemical Engineers AIChE J, 2010
140 citations
TL;DR: In this article, a Bayesian regularization method is introduced for performance improvement, through which the effective dimensionality of the latent variable can be determined automatically, and a novel probabilistic strategy is proposed for result combination in different operation modes.
Abstract: This article intends to address two drawbacks of the traditional principal component analysis (PCA)-based monitoring method: (1) nonprobabilistic; (2) single operation mode assumption. On the basis of the monitoring framework of probabilistic PCA (PPCA), a Bayesian regularization method is introduced for performance improvement, through which the effective dimensionality of the latent variable can be determined automatically. For monitoring processes with multiple operation modes, the Bayesian regularization method is extended to its mixture form, thus a mixture Bayesian regularization method of PPCA has been developed. To enhance the monitoring performance, a novel probabilistic strategy has been proposed for result combination in different operation modes. In addition, a new mode localization approach has also been developed, which can provide additional information and improve process comprehension for the operation engineer. A numerical example and a real industrial application case study have been used to evaluate the efficiency of the proposed method. © 2010 American Institute of Chemical Engineers AIChE J, 2010
139 citations
TL;DR: In this article, the authors used NaOH and Ca(OH)2 as alkaline catalysts for lactic acid synthesis from glucose, cellulose, and starch in the presence of reverse aldol condensation and double bond rule of hexose.
Abstract: We investigated the hydrothermal conversion of the carbohydrates including glucose, cellulose, and starch to lactic acid using NaOH and Ca(OH)2 as alkaline catalysts. Both catalysts significantly promoted the lactic acid formation. The highest yield of lactic acid from glucose was 27% with 2.5 M NaOH and 20% with 0.32 M Ca(OH)2 at 300°C for 60 s. The lactic acid yields from cellulose and starch were comparable with the yield from glucose with 0.32 M Ca(OH)2 at 300°C, but the reaction time in the case of cellulose was 90 s. The mechanism of lactic acid formation from glucose was discussed by identifying the intermediate products. Lactic acid may be formed via the formation of aldoses of two to four carbons including aldose of three carbons, which are all formed by reverse aldol condensation and double bond rule of hexose. This implies that carbon–carbon cleavage occurs at not only C3C4 but also at C2C3. © 2010 American Institute of Chemical Engineers AIChE J, 2010
129 citations
TL;DR: In this paper, the transient kinetics of NiO carriers in the temperature range of 600 to 900°C in both a fixed bed microreactor and a fluid bed reactor were investigated.
Abstract: Chemical Looping Combustion technology involves circulating a metal oxide between a fuel zone where methane reacts under anaerobic conditions to produce a concentrated stream of CO2 and water and an oxygen rich environment where the metal is reoxidized. Although the needs for electrical power generation drive the process to high temperatures, lower temperatures (600–800°C) are sufficient for industrial processes such as refineries. In this paper, we investigate the transient kinetics of NiO carriers in the temperature range of 600 to 900°C in both a fixed bed microreactor (WHSV = 2-4 g CH4/h/g oxygen carrier) and a fluid bed reactor (WHSV = 0.014-0.14 g CH4/h per g oxygen carrier). Complete methane conversion is achieved in the fluid bed for several minutes. In the microreactor, the methane conversion reaches a maximum after an initial induction period of less than 10 s. Both CO2 and H2O yields are highest during this induction period. As the oxygen is consumed, methane conversion drops and both CO and H2 yields increase, whereas the CO2 and H2O concentrations decrease. The kinetics parameter of the gas–solids reactions (reduction of NiO with CH4, H2, and CO) together with catalytic reactions (methane reforming, methanation, shift, and gasification) were estimated using experimental data obtained on the fixed bed microreactor. Then, the kinetic expressions were combined with a detailed hydrodynamic model to successfully simulate the comportment of the fluidized bed reactor. © 2010 American Institute of Chemical Engineers AIChE J, 2010
120 citations
TL;DR: In this article, a 1D model is proposed for the particle distribution through the coating thickness at different drying times and conditions, including Brownian diffusion, sedimentation, and evaporation.
Abstract: Key microstructural properties of particulate coatings such as porosity and particle order are established during drying. Therefore, understanding the evolution of particulate distributions during drying is useful for designing coating properties. Here, a 1D model is proposed for the particle distribution through the coating thickness at different drying times and conditions, including Brownian diffusion, sedimentation, and evaporation. Effects of particle concentration on diffusion and sedimentation rates are included. Results are condensed onto a drying regime map which predicts the presence of particle surface accumulation or sediment based on two dimensionless numbers: the Peclet number and the sedimentation number. Cryogenic scanning electron microscopy (cryoSEM) is used to image the transient particulate distributions during the drying of a model system comprised of monodisperse silica particles in water. Particle size and evaporation rates are altered to access various domains of the drying map. There is good agreement between cryoSEM observations and model predictions. © 2010 American Institute of Chemical Engineers AIChE J, 2010
119 citations
TL;DR: Pratsinis et al. as mentioned in this paper proposed a method for the synthesis of mixed, functionalized and/or layered nanomaterials, significantly beyond the simple ones in the market today, with close attention to product handling, safety and health effects.
Abstract: Manufacturing of nanoscale materials and devices offers a number of exciting opportunities for chemical engineers to contribute substantially in product development, process scale-up, and environmental compliance. Current research in nanoscale science and engineering has been directed primarily toward the design and synthesis of (a) materials with passive nanostructures (e.g., nanostructured coatings, dispersion of nanoparticles, as well as bulk nanostructured metals, polymers and ceramics), and (b) active devices with nanostructured materials (e.g., transistors, amplifiers, targeted drugs and delivery systems, sensors, actuators and adaptive structures). Such materials are usually produced through self-assembly of nanoparticles in gasor liquid-phase processes or through compaction of nanopowders. Even though nanoparticles have been in use for a long time (e.g., carbon black, photographic films, fumed silica), the knowledge base for their synthesis and characterization has increased dramatically in the last 30 years with the development of a number of sophisticated scientific instruments, new synthetic processes at the nanoscale, and computational advances. This progress has contributed decisively to the development of effective and quantitative understanding of (a) nanomaterial properties and their dependence on their constituent parts, and (b) synthesis methods on which their fabrication is based, resulting in the discovery of new materials with an array of new functionalities and potential applications with promises of more to come in the future. To date, however, rather little of the aforementioned has been translated into actual industrial products. Most of these exciting discoveries have been made in basic science laboratories with little motivation or capacity to investigate and invent processes for the economic manufacturing of these nanotechnology products; the ‘‘bread and butter’’ of chemical engineers. In fact, little is known about how well these new properties are reproduced during large-scale manufacturing of such nanomaterials. More importantly, there is little understanding on how such manufacturing processes can be designed and operated, while issues arise regarding the controllability of such process operations and sustainability of the product quality, safety and potential hazards for personnel, environment and consumers. The associated risks constitute key issues in developing manufacturing technologies by major industries, a drive that has been part of what is recently referred to as ‘‘green’’ manufacturing. Two of the major challenges are: first, low-cost manufacturing of sophisticated nanostructured materials, and second, the economical and reliable assembly of such materials into functional devices. To meet these challenges a quantitative advancement of the current understanding of the corresponding manufacturing technologies is needed for the synthesis of mixed, functionalized and/or layered nanomaterials, significantly beyond the simple ones in the market today, with close attention to product handling, safety and health effects. Furthermore, assembling of devices with such materials (e.g., synthesis, deposition and processing of multilayers of functional nanoparticles) is needed to meet today’s engineering challenges in energy, mobility, health and life sciences (e.g., nonintrusive medical diagnostics and even cures for chronic and serious illnesses). While most of the current research in nanotechnology is driven by the imperative of scientific questions on the synthesis and characterization of passive nanoscale materials and/or active devices, with minimal attention paid to the Correspondence concerning this article should be addressed to S. E. Pratsinis at sotoris.pratsinis@ptl.mavt.ethz.ch.
TL;DR: In this article, the first kinetics model for supercritical water gasification (SCWG) is presented, which describes the formation and interconversion of individual gaseous species.
Abstract: This article reports the first kinetics model for Supercritical Water Gasification (SCWG) that describes the formation and interconversion of individual gaseous species. The model comprises 11 reactions, and it uses a lumping scheme to handle the large number of intermediate compounds. We determined numerical values for the rate constants in the model by fitting it to experimental data previously reported for SCWG of cellulose and lignin. We validated the model by showing that it accurately predicts gas yields at biomass loadings and water densities not used in the parameter estimation. Sensitivity analysis and reaction rate analysis indicate that steam-reforming and water―gas shift are the main sources of H 2 in SCWG, and intermediate species are the main sources of CO, CO 2 , and CN 4 .
TL;DR: A gas solid Micro Fluidized Bed Reaction Analyzer (MFBRA) was developed to deduce reaction rates and kinetic parameters through measuring time-dependent composition changes of evolved gases from the reactions as mentioned in this paper.
Abstract: A novel gas solid Micro Fluidized Bed Reaction Analyzer (MFBRA) was developed to deduce reaction rates and kinetic parameters through measuring time-dependent composition changes of evolved gases from the reactions. Application of the MFBRA to the decomposition of CaCO(3) powder resulted in an apparent activation energy of 142.73 kJ/mol and a pre-exponential factor of 399,777 s(-1). This apparent activation energy was much lower than the thermogravimetry-measured value of 184.31 kJ/mol, demonstrating a quicker reaction in the MFBRA. This was further verified by CuO reduction in CO, as accelerated by the fast diffusion and high heating rate in the MFBRA. Measurement of pyrolysis of coal and biomass in MFBRA found that the reaction process was completed in about 10 s, a time much shorter than the literature-reported values in larger fluidized bed reactors. By monitoring the release of gas species from reactions at different temperatures, the MFBRA also allowed deeper insight into the mechanism of pyrolysis reactions. (C) 2010 American Institute of Chemical Engineers AIChE J, 56: 2905-2912, 2010
TL;DR: Li et al. as discussed by the authors presented a multiobjective optimization model for the optimal design of amethanol/electricity polygeneration plant, where economic and environmental criteria are simultaneously optimized over a superstructure capturing a number of possible combi-nations of technologies and types of equipment.
Abstract: Pei Liu and Efstratios N. PistikopoulosCentre for Process Systems Engineering (CPSE), Dept. of Chemical Engineering, Imperial College London,London SW7 2AZ, U.K.Zheng LiDept. of Thermal Engineering, Tsinghua University, Beijing 100084, ChinaDOI 10.1002/aic.12058Published online October 1, 2009 in Wiley InterScience (www.interscience.wiley.com).Polygeneration, typically involving co-production of methanol and electricity, is apromising energy conversion technology which provides opportunities for high energyutilization efficiency and low/zero emissions. The optimal design of such a complex,large-scale and highly nonlinear process system poses significant challenges. In thisarticle, we present a multiobjective optimization model for the optimal design of amethanol/electricity polygeneration plant. Economic and environmental criteria aresimultaneously optimized over a superstructure capturing a number of possible combi-nations of technologies and types of equipment. Aggregated models are considered,including a detailed methanol synthesis step with chemical kinetics and phase equilib-rium considerations. The resulting model is formulated as a non-convex mixed-integernonlinear programming problem. Global optimization and parallel computation techni-ques are employed to generate an optimal Pareto frontier.
TL;DR: In this article, an Eulerian multifluid approach coupled with a population balance model (PBM) has been employed to describe the evolution of the bubble size distribution due to break-up and coalescence.
Abstract: Local gas hold-up and bubbles size distributions have been modeled and validated against experimental data in a stirred gas–liquid reactor, considering two different spargers. An Eulerian multifluid approach coupled with a population balance model (PBM) has been employed to describe the evolution of the bubble size distribution due to break-up and coalescence. The PBM has been solved by resorting to the quadrature method of moments, implemented through user defined functions in the commercial computational fluid dynamics code Fluent v. 6.2. To overcome divergence issues caused by moments corruption, due to numerical problems, a correction scheme for the moments has been implemented; simulation results prove that it plays a crucial role for the stability and the accuracy of the overall approach. Very good agreements between experimental data and simulations predictions are obtained, for a unique set of break-up and coalescence kinetic constants, in a wide range of operating conditions. © 2009 American Institute of Chemical Engineers AIChE J, 2010
TL;DR: An in situ characterization method for mechanical and adhesive properties of organic/ceramic composite membranes is built on the basis of nanoindentation technique in this article, where polydimethylsiloxane (PDMS) was used as the separation layer with the support of porous ZrO2/Al2O3 ceramic tubes.
Abstract: An in situ characterization method for mechanical and adhesive properties of organic/ceramic composite membranes is built on the basis of nanoindentation technique in this work. The polydimethylsiloxane (PDMS) was used as the separation layer with the support of porous ZrO2/Al2O3 ceramic tubes. The effects of roughness of the ceramic support and the viscosity of PDMS solution on the mechanical properties of the PDMS separation layer and the interfacial adhesion at the interface were investigated in detail. It was found that when the roughness of the ceramic support increased and the viscosity of PDMS solution decreased, the interfacial adhesion strength of PDMS/ceramic composite membrane increased, but these two variables had little effect on the mechanical properties of the PDMS separation layer. Our results indicate that the mechanical interlocking dominates the adhesion between the PDMS separation layer and the porous ceramic support. V C 2009 American Institute of Chemical Engineers AIChE J, 56: 1584–1592, 2010
TL;DR: The discovery of elongation-induced nano-pore evolution during the external stretching of a novel polyamide-imide nanofiltration hollow fiber membrane in a dry-jet wet-spinning process that simultaneously leads to a decreased pore size but increased pure water permeability is reported.
Abstract: The molecular design of nanoporous membranes with desired morphology and selectivity has attracted significant interest over the past few decades. A major problem in their applications is the trade-off between sieving property and permeability. Here, we report the discovery of elongation-induced nano-pore evolution during the external stretching of a novel polyamide-imide nanofiltration hollow fiber membrane in a dry-jet wet-spinning process that simultaneously leads to a decreased pore size but increased pure water permeability. The molecular weight cutoff, pore size, and pore size distribution were finely tuned using this approach. AFM and polarized FTIR verified the nano-pore morphological evolution and an enhanced molecular orientation in the surface skin layer. The resultant nanofiltration membranes exhibit highly effective fractionation of the monovalent and divalent ions of NaCl/Na2SO4 binary salt solutions. More than 99.5% glutathione can be rejected by the nanofiltration membranes at neutral pH, offering the feasibility of recovering this tripeptide. © 2009 American Institute of Chemical Engineers AIChE J, 2010
TL;DR: In this article, the application of several real-time optimization methods, such as the perturbation and observation method, the gradient method, and the recently proposed multiunit method, for maximizing the power output of MFCs by varying the external resistance is presented.
Abstract: Microbial fuel cells (MFCs) constitute a novel power generation technology that converts organic waste to electrical energy using microbially catalyzed electrochemical reactions. Since the power output of MFCs changes considerably with varying operating conditions, the online optimization of electrical load (i.e., external resistance) is extremely important for maintaining a stable MFC performance. The application of several real-time optimization methods is presented, such as the perturbation and observation method, the gradient method, and the recently proposed multiunit method, for maximizing power output of MFCs by varying the external resistance. Experiments were carried out in two similar MFCs fed with acetate. Variations in substrate concentration and temperature were introduced to study the performance of each optimization method in the face of disturbances unknown to the algorithms. Experimental results were used to discuss advantages and limitations of each optimization method. © 2010 American Institute of Chemical Engineers AIChE J, 2010
TL;DR: In this article, a zero-emissions power plant with high efficiency is presented, where syngas, produced by the gasification of coal, is shifted to produce H2 which in turn fuels stacks of solid oxide fuel cells.
Abstract: A zero-emissions power plant with high efficiency is presented. Syngas, produced by the gasification of coal, is shifted to produce H2 which in turn fuels stacks of solid oxide fuel cells. Because the fuel cells maintain separate anode and cathode streams, air can be used as the oxygen source without diluting the fuel exhaust with nitrogen. This enables recovery of CO2 from the exhaust with a very small energy penalty. As a result, an absorption-based CO2 recovery process is avoided, as well as the production of large quantities of high-purity O2, allowing a high overall thermal efficiency and essentially eliminating the energy penalty for carbon capture. © 2010 American Institute of Chemical Engineers AIChE J, 2010
TL;DR: In this paper, the structural changes of the lipase upon attachment onto carbon nanotubes (MWNTs) were analyzed through circular dichroism and FTIR spectroscopy.
Abstract: Lipase was covalently attached to multiwalled carbon nanotubes (MWNTs). Structural changes of the lipase upon attachment onto MWNTs were analyzed through circular dichroism and FTIR spectroscopy. The conjugate was utilized for the resolution of a model compound (R,S)-1-phenyl ethanol, and the reaction medium was n-heptane. The enzymatic resolutions were carried out at temperatures from 35 to 60°C. The results show that the lipase attached onto MWNTs has significantly affected the performance of the enzyme in terms of temperature dependence and resolution efficiency. The activity of MWNT–lipase was less temperature-dependent compared with that of the native lipase. The resolution efficiency was much improved with MWNT–lipase. MWNT–lipase retained the selectivity of the native lipase for (R)-1-phenyl ethanol. The consecutive use of MWNT–lipase showed that MWNT–lipase had a good stability in the resolution of (R,S)-1-phenyl ethanol. © 2010 American Institute of Chemical Engineers AIChE J, 2010
TL;DR: Gubbins et al. as discussed by the authors proposed a model for hard disk drive lubrication, where polyzwitterionic brushes are polymerized directly onto the mica sheets in a surface force balance (SFB), and the model system exhibits very similar low-sliding friction at pressures as high as 7.55 MPa.
Abstract: N anoconfined fluids — that is, fluids confined between surfaces separated by nanometers — play important roles in many natural and man-made processes and products. One example is hard disk drive lubrication where, as data density has increased exponentially, the distance between the read head and rotating platen has been exponentially decreasing for several decades. This distance is now at 10–12 nm, and in the next generation of disk drives will be at 8 nm; currently, monolayers of lubricant are used to protect disk drives in abnormal situations (e.g., power loss), but in the future it is expected that they will be lubricated at all times, including during read/write operations. Additional examples include the lubrication of microelectromechanical systems (MEMS), and nanoelectromechanical systems (NEMS), and a model for the natural lubrication of synovial joints, all of which can involve moving surfaces separated by distances of the order of nm. The latter exhibit very low-sliding friction at normal pressures up to 5 MPa or more; the model system, consisting of polyzwitterionic brushes polymerized directly onto the mica sheets in a surface force balance (SFB), exhibits very similar low-sliding friction (within a factor of 2 of the natural synovial joints) at pressures as high as 7.55 MPa. These three examples highlight the desirability of being able to lubricate effectively between surfaces moving relative to each other while separated by distances on the order of nm. If the lubricant undergoes a fluid-solid phase transition under nanoconfinement, resulting in a many order of magnitude increase in the effective viscosity and the onset of a nonzero yield stress, then it is clearly not useful as a lubricant. In addition to lubrication at the nanoscale, phase transitions under nanoconfinement are also clearly important in industrial adsorption and catalytic processes (microand mesoporous adsorbents, with pore widths of under 2 nm and 2–50 nm, respectively, are widely used in the chemical, petrochemical, gas processing, and pharmaceutical industries for separations, pollution abatement, and as catalysts and catalyst supports). Additional application areas (e.g., in geology, oil recovery and nanofabrication, including nanotemplating through nanoconfinement) are described in the excellent review article by Gelb et al. Hence, understanding the phase behavior of nanoconfined fluids is key to the rational design and control of many processes and devices, both in the processing industries and in the emerging field of nanotechnology. Specifically, the change in melting temperature as a function of nanoconfinement is an important quantity to understand and predict. Gubbins and coworkers have been leaders in understanding these phenomena
TL;DR: In this paper, a new semicorrelated model was proposed, which incorporates Janssen's wall effects in the calculation of the minimum fluidization velocity, and the model predictions compare favorably to existing correlations and experimental data.
Abstract: Experiments show that the minimum fluidization velocity of particles increases as the diameter of the fluidization column is reduced, or if the height of the bed is increased. These trends are shown to be due to the influence of the wall. A new, semicorrelated model is proposed, which incorporates Janssen's wall effects in the calculation of the minimum fluidization velocity. The wall friction opposes not only the bed weight but also the drag force acting on the particles during fluidization. The enhanced wall friction leads to an increase in the minimum fluidization velocity. The model predictions compare favorably to existing correlations and experimental data. © 2010 American Institute of Chemical Engineers AIChE J, 2010
TL;DR: Interior point methods with the modified CDC strategy for Reduced Hessian Successive Quadratic Programming (RSQP) are incorporated, which was tested through AMPL interface and Aspen Open Solvers interface and related properties are proved.
Abstract: For practical applications, optimization algorithms may converge to the optimal solution unreasonably slowly because of factors such as the poor scaling, ill-conditioning, errors in calculation, and so on. Most improvements during the optimization procedure are made within a small part of the total computation time. To relieve the heavy computational burden, it is necessary to balance the calculation accuracy and computation cost. The traditional termination criteria based on the Karush-Kuhn-Tucker conditions cannot appropriately meet this requirement. Convergence depth control (CDC) strategy for Reduced Hessian Successive Quadratic Programming (RSQP) was presented as an alternative measure in a previous study. This work incorporates interior point methods with the modified CDC strategy, which was tested through AMPL interface and Aspen Open Solvers interface. Related properties are proved. © 2010 American Institute of Chemical Engineers AIChE J, 2010
TL;DR: In this paper, a parametric study was conducted to investigate the influence of various parameters on the gas mixing, and the specularity coefficient for the partial-slip solid-phase wall boundary condition had the most significant effect on gas mixing.
Abstract: Gas mixing in a tall narrow fluidized bed operated in the slugging fluidization regime is simulated with the aid of computational fluid dynamics. In the first part, a parametric study is conducted to investigate the influence of various parameters on the gas mixing. Among the parameters studied, the specularity coefficient for the partial-slip solid-phase wall boundary condition had the most significant effect on gas mixing. It was found that the solid-phase wall boundary condition needs to be specified with great care when gas mixing is modeled, with free slip, partial slip and no-slip wall boundary conditions giving substantial differences in the extent of gas back mixing. Axial and radial tracer concentration profiles for different operating conditions are generally in good agreement with experimental data from the literature. Detailed analyses of tracer back mixing are carried out in the second part. Two parameters, the tracer backflow fraction and overall gas backflow fraction, in addition to axial profiles of cross-sectional averaged tracer concentrations, are evaluated for different flow conditions. Qualitative trends are consistent with reported experimental findings. © 2010 American Institute of Chemical Engineers AIChE J, 2010
TL;DR: In this article, a simulation of a lab-scale fluidized-bed reactor with side-gas injection and filled with 500-600 μm glass beads was performed using the computational fluid dynamics (CFD) code Fluent 6.3, and the results were compared to experimental data obtained using pressure measurements and 3D X-ray computed tomography.
Abstract: Fluidized-bed reactors are widely used in the biofuel industry for combustion, pyrolysis, and gasification processes. In this work, a lab-scale fluidized-bed reactor without and with side-gas injection and filled with 500–600 μm glass beads is simulated using the computational fluid dynamics (CFD) code Fluent 6.3, and the results are compared to experimental data obtained using pressure measurements and 3D X-ray computed tomography. An initial grid-dependence CFD study is carried out using 2D simulations, and it is shown that a 4-mm grid resolution is sufficient to capture the time- and spatial-averaged local gas holdup in the lab-scale reactor. Full 3D simulations are then compared with the experimental data on 2D vertical slices through the fluidized bed. Both the experiments and CFD simulations without side-gas injection show that in the cross section of the fluidized bed there are two large off-center symmetric regions in which the gas holdup is larger than in the center of the fluidized bed. The 3D simulations using the Syamlal-O'Brien and Gidaspow drag models predict well the local gas holdup variation throughout the entire fluidized bed when compared to the experimental data. In comparison, simulations with the Wen-Yu drag model generally over predict the local gas holdup. The agreement between experiments and simulations with side-gas injection is generally good, where the side-gas injection simulates the immediate volatilization of biomass. However, the effect of the side-gas injection extends further into the fluidized bed in the experiments as compared to the simulations. Overall the simulations under predict the gas dispersion rate above the side-gas injector. © 2009 American Institute of Chemical Engineers AIChE J, 2010
TL;DR: In this article, a gas-fluidized bed experiments involving Geldart Group B particles with continuous-size distributions have been carried out and results show that mean particle diameter decreases with increasing bed height, and that wider Gaussian distributions show increased segregation extents.
Abstract: Bubbling, gas-fluidized bed experiments involving Geldart Group B particles with continuous-size distributions have been carried out. Sand of various widths of Gaussian or lognormal distributions were completely fluidized, then axial concentration profiles were obtained from frozen-bed sectioning. Similar to previous works on binary systems, results show that mean particle diameter decreases with increasing bed height, and that wider Gaussian distributions show increased segregation extents. Surprisingly, however, lognormal distributions exhibit a nonmonotonic segregation trend with respect to distribution widths. In addition, the shape of the local-size distribution is largely preserved with respect to that of the overall distribution. These findings on the nature of local-size distribution provide experimental confirmation of previous results for granular and gas-solid simulations. Lastly, an interesting observation is that although monodisperse Geldart Group D particles cannot be completely fluidized, their presence in lognormal distributions investigated still results in complete fluidization of all particles. © 2010 American Institute of Chemical Engineers AIChE J, 2010
TL;DR: In this article, a mixture of 60 wt % Fe2O3 as active phase and 40 Wt % stabilized ZrO2 as support material was used for chemical-looping combustion.
Abstract: Oxygen-carrier particles for chemical-looping combustion have been manufactured by freeze granulation. The particles consisted of 60 wt % Fe2O3 as active phase and 40 wt % stabilized ZrO2 as support material. Ce, Ca, or Mg was used to stabilize the ZrO2. The hardness and porosity of the particles were altered by varying the sintering temperature. The oxygen carriers were examined by redox experiments in a batch fluidized- bed reactor at 800–950°C, using CH4 as fuel. The experiments showed good reactivity between the particles and CH4. NiO was used as an additive and was found to reduce the fraction of unconverted CH4 with up to 80%. The combustion efficiency was 95.9% at best and was achieved using 57 kg oxygen carrier per MW fuel. Most produced oxygen carriers appear to have been decently stable, but using Ca as stabilizer resulting in uneven results. Further, particles sintered at high temperatures had a tendency to defluidize.
TL;DR: In this paper, the quality of gas-solid fluidization of agglomerates of nanoparticles has been greatly enhanced by adding a secondary flow in the form of a high-velocity jet produced by one or more micronozzles pointing vertically downward toward the distributor.
Abstract: The quality of gas–solid fluidization of agglomerates of nanoparticles has been greatly enhanced by adding a secondary flow in the form of a high-velocity jet produced by one or more micronozzles pointing vertically downward toward the distributor. The micronozzles produced a jet with sufficient velocity (hundreds of meters per second), turbulence, and shear to break-up large nanoagglomerates, prevent channeling, curtail bubbling, and promote liquid-like fluidization. For example, Aerosil R974, an agglomerate particulate fluidization (APF) type nanopowder, expanded up to 50 times its original bed height, and difficult to fluidize agglomerate bubbling fluidization (ABF) type nanopowders, such as Aeroxide TiO2 P25, were converted to APF type behavior, showing large bed expansions and homogeneous fluidization without bubbles. Microjet-assisted nanofluidization was also found to improve solids motion and prevent powder packing in an internal, is easily scaled-up, and can mix and blend different species of nanoparticles on the nanoscale. © 2009 American Institute of Chemical Engineers AIChE J, 2010